MicroFlo.pdf

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MicroFlo (CFD) User Guide

6.0


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Contents

1. Introduction.......................................................................................................11.1. Introduction to CFD .............................................................................................................. 11.2. Main Steps in a Typical CFD Analysis ................................................................................. 2

1.2.1. Stage 1: Pre-processing Stage Definition of the Problem........................................ 21.2.2. Stage 2: Solution Stage Solving the Governing Equations ...................................... 21.2.3. Stage 3: Post-processing Stage Analysis of Results ............................................... 2

1.3. Step 0: Preparing to Run MicroFlo....................................................................................... 2

2. Internal Analysis...............................................................................................42.1. Step 1: Settings .................................................................................................................... 4

2.1.1. Turbulence Model........................................................................................................ 52.1.2. Surface Heat Transfer ................................................................................................. 52.1.3. Boundary Conditions ................................................................................................... 62.1.4. Grid Settings ................................................................................................................ 62.1.5. Initial Conditions .......................................................................................................... 62.1.6. Discretisation ............................................................................................................... 62.1.7. Supplies and Extracts.................................................................................................. 7

2.2. Step 2: Select the Room(s) you wish to Simulate................................................................ 82.2.1. Surface Boundary Conditions.................................................................................... 11

2.3. Step 3: Defining Surface Boundary Properties .................................................................. 122.3.1. Supply Diffusers and Extracts ................................................................................... 132.3.2. Fixed Pressure boundaries........................................................................................ 142.3.3. Fixed Temperature .................................................................................................... 142.3.4. Heat ........................................................................................................................... 142.3.5. Porous Baffle ............................................................................................................. 152.3.6. Fan............................................................................................................................. 15

2.4. Room Gains ....................................................................................................................... 152.5. Step 4: Adding Components to the Model ......................................................................... 162.6. Step 5: Gridding the Model ................................................................................................ 19

2.6.1. Generate Grid............................................................................................................ 212.6.2. Add Grid Region........................................................................................................ 212.6.3. Remove Grid Region................................................................................................. 212.6.4. Edit Grid Region ........................................................................................................ 21

2.7. Step 6: Running the Simulation.......................................................................................... 222.7.1. Graphical Display Monitor ......................................................................................... 242.7.2. Cell Monitor ............................................................................................................... 242.7.3. Outer Iterations.......................................................................................................... 242.7.4. Turbulence Model...................................................................................................... 242.7.5. Isothermal.................................................................................................................. 252.7.6. CO

2............................................................................................................................ 25

2.7.7. Variable Control......................................................................................................... 252.7.8. Boundary Conditions File .......................................................................................... 25

3. External Analysis............................................................................................283.1. Step 1: CFD Settings ......................................................................................................... 29

3.1.1. Wind........................................................................................................................... 293.1.2. Turbulence Model...................................................................................................... 303.1.3. Grid Settings .............................................................................................................. 303.1.4. Discretisation ............................................................................................................. 31

3.2. Step 2: Redefine the Model Grid........................................................................................ 313.2.1. Generate Grid............................................................................................................ 323.2.2. Add Grid Region........................................................................................................ 323.2.3. Remove Grid Region................................................................................................. 33

3.2.4. Edit Grid Region ........................................................................................................ 333.3. Step 3: Running the Simulation.......................................................................................... 34


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3.3.1. Graphical Display Monitor ......................................................................................... 363.3.2. Cell Monitor ............................................................................................................... 363.3.3. Outer Iterations.......................................................................................................... 363.3.4. Turbulence Model...................................................................................................... 363.3.5. Variable Control......................................................................................................... 363.3.6. Boundary Conditions File .......................................................................................... 36

4. Post-processing with the MicroFlo Viewer...................................................404.1. Typical Parameter Display Variations with the MicroFlo Viewer........................................ 43

Create AVI .................................................................................................................................. 45

5. Appendix A: Steps to Define Thermal Boundary Conditions fromApacheSim .............................................................................................................46

5.1. Step 1: Define Potential Rooms for MicroFlo Analysis ...................................................... 465.2. Step 2: Export Boundary Conditions .................................................................................. 47

6. Appendix B: Troubleshooting .......................................................................486.1. Difficulty in Achieving Converged Solution ........................................................................ 486.2. Judging When the Run has Reached Completion ............................................................. 48

7. Appendix C: Opening MicroFlo Results from an Earlier Version of the........................................................................................................................498. Appendix D: The wind speed for external simulations................................50


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Table of Figures

Figure 1: MicroFlo Interface................................................................................................................... 3Figure 2: CFD Settings Properties Box ................................................................................................. 5Figure 3: A maximum dimension of 0.2m.............................................................................................. 7Figure 4: A maximum dimension of 1m................................................................................................. 8Figure 5: Selected room plus components............................................................................................ 9Figure 6: Surface level of model.......................................................................................................... 10Figure 7: Selection of Boundary Conditions file generated in ApacheSim / Vista............................... 11Figure 8: Import of MacroFlo openings and subsequent window opening position ............................ 11Figure 9: Dialogue box to define boundary conditions ........................................................................ 12Figure 11: Convective Heat Gains from ApacheSim........................................................................... 16Figure 12: Place Component Dialogue ............................................................................................... 17Figure 13: Model with CFD Components added ................................................................................. 19Figure 14: Model displaying grid scheme............................................................................................ 20

Figure 15: Dialogue box to allow adding a Grid Region...................................................................... 21Figure 16: Interface to allow editing of a Grid Region. ........................................................................ 22Figure 17: CFD Grid Statistics............................................................................................................. 23Figure 18: Simulation Control .............................................................................................................. 23Figure 19: MicroFlo Simulation Control Monitor .................................................................................. 24Figure 20: Simulation in Progress...................................................................................................... 25Figure 21: Simulation Control .............................................................................................................. 26Figure 22: Grid for External Analysis................................................................................................... 28Figure 23: CFD Setting Properties Box for External Analysis ............................................................. 29Figure 24: Grid Settings for External Analysis..................................................................................... 30Figure 25: Dialogue box to allow adding a Grid Region...................................................................... 33Figure 26: Edit Grid Region ................................................................................................................. 33Figure 27: CFD Grid Statistics............................................................................................................. 34

Figure 28: Simulation Control .............................................................................................................. 35Figure 29: CFD Monitor for External Analysis ..................................................................................... 35Figure 30: CFD External Analysis in progress. ................................................................................... 37Figure 31: Simulation Control .............................................................................................................. 37Figure 32: The MicroFlo viewer interface ............................................................................................ 41Figure 33: The comfort conditions dialogue ........................................................................................ 41Figure 34: The particle tracking dialogue ............................................................................................ 42Figure 35: Velocity Vector Slice Figure 36: Velocity Contour Slice ............................................ 43Figure 37: Temperature Contour Slice ................................................................................................ 43Figure 38: Filled Velocity Slice ............................................................................................................ 43Figure 39: Temperature Contour Slice ................................................................................................ 44Figure 40: Temperature Surface Net ................................................................................................. 44Figure 41: The orbit create movie........................................................................................................ 45

Figure 42: The timed create movie...................................................................................................... 45Figure 43: The slice create movie ....................................................................................................... 45


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1. IntroductionThe purpose of this document is to summarise the principal steps in performinga MicroFlo CFD simulation.

1. 1. I nt r oduc t ion t o CF D

Computational Fluid Dynamics (CFD) is concerned with the numericalsimulation of fluid flow and heat transfer processes. The objective of CFDapplied to buildings is to provide the designer with a tool that enables them togain greater understanding of the likely air flow and heat transfer processesoccurring within and around building spaces given specified boundaryconditions which may include the effects of climate, internal energy sources andHVAC systems.

The mathematical simulation of air flow and heat transfer processes involvesthe numerical solution of a set of coupled, non-linear, second-order, partialdifferential equations. MicroFlo uses the primitive variable approach, whichrequires the solution of the three velocity component momentum equationstogether with equations for pressure and temperature, these equations beingknown as conservation equations. The numerical solution is conducted throughthe linearisation and discretisation of the conservation equation set, which

requires the sub-division of the calculation domain into a number of non-overlapping contiguous finite volumes over each of which the conservationequations are expressed in the form of linear algebraic equations, this set offinite volumes is referred to as a grid. The resulting linear algebraic equation setfor the entire domain is then solved in an iterative scheme, which accounts forthe non-linear coupling. The finer the finite volume grid, the closer the solutionof the algebraic equations will represent the original differential equations butthe longer the simulation will take.

In summary CFD involves the numerical solution of the following governingequations:

Momentum Energy Mass continuity Turbulence Scalar/Mass Fraction

The benefits of using CFD include:

Software can be used as a what if tool. Scale up issues are eliminated.


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User testing is safe and not intrusive to the process. Wind tunnel and experimental methods are usually more costly.

Velocity components, pressure, temperature etc available throughout thedomain.

Flow field insight can be gained, which can be very difficult to measure. Design time and costs are usually lower.

1.2. M ain S t eps in a T y pic al CF D A naly s is

1.2.1. Stage 1: Pre-processing Stage Definition of the Problem

Define the model geometry.

Define the computational domain. Define the boundary and initial conditions. Define the grid / mesh. Define all the necessary solver parameters.

1.2.2. Stage 2: Solution Stage Solving the Governing Equations

Inspect the progress of the run. Adjust solver parameter criteria if necessary to achieve convergence.

1.2.3. Stage 3: Post-processing Stage Analysis of Results Visualisation of results and reporting.

The following are the various steps required to perform a MicroFlo CFDsimulation.

1.3. S t ep 0: P r epar ing t o Ru n M ic r oF lo

Things to do:

Create/Open model for internal air flow, external air flow or both (notsimultaneously). The detail of the model may be different e.g. inexternal flow the model may have little detailed information on eachbuilding.

If users have included components for Radiance these will be includedin the CFD model. Care needs to be taken because some IESComponents are very detailed and could result in a much greaternumber of grid cells than would be necessary.

Users can generate MicroFlo boundary conditions for any time from


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ApacheSim. Appendix A summarises the steps necessary to generatethese boundary conditions.

If ApacheSim boundary conditions are not being used users shoulddecide the values of the boundary conditions.

The MicroFlo Interface is shown (Figure 1). MicroFlo is in the CFD Applicationgroup.

Figure 1: MicroFlo Interface

Firstly decide whether an internal or an external analysis is required. Selectfrom the combo box at the right hand side of the main MicroFlo toolbar locatedat the top of the MicroFlo view window. The decision made to the interface willchange to reflect the current option. To make this document clearer theprocedures for performing an internal and external analysis are describedseparately. There is some overlap.

The internal analysis will be described first. An Apache simulation will alsohave been performed and a boundary conditions file created, for details (seeAppendix A).


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2. Internal AnalysisIn addition to studying the air flow through the space, it is possible to model theconcentrations of CO2, moisture and CO, and the local mean age of air through theuse of passive scalars.

The CO, CO2 and moisture calculations are performed automatically when theappropriate boundary condition is set on a supply diffuser (see 2.3) or a source termis introduced via a fluid component (see 2.5). The Microflo viewer display options willbe set appropriately.

The local mean age of air is only calculated for ventilated rooms only, where there isan exchange with the outside. This is a measure of the time a parcel of air has been

in the simulation domain, after allowing for advection and diffusion. Regions of highvalues indicate places of poor ventilation. The air change effectiveness is derivedfrom the local mean age according to chapter 27 of the 2005 ASHRAE Handbook Fundamentals.

There are only two active buttons on the Toolbar that are important when selectingInternal analysis: Settings and Run. Step 1 is to define CFD settings.

2.1. Step 1: Sett ings

On the View Toolbar there is an option called Settings. Users can define thesettings now, or at any time in the modelling process. Select Settings you get thefollowing Properties box (Figure 2):


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Figure 2: CFD Settings Properties Box

Each item has sensible defaults but users can modify this information at any stage

of the modelling process.

The tabs have the following function.

2.1.1. Turbulence Model

There are two types of Turbulence Models:

k-e: the most generally accepted and widely used turbulence model (Default). Thek-e model calculates turbulent viscosity for each grid cell throughout the calculationdomain by solving two additional partial differential equations, one for turbulence

kinetic energy and the other for its rate of dissipation. Constant effective viscosity. This model does not attempt to account for the

transport of turbulence but offers the user a much faster, much more approximatemethod of accounting for turbulence than the k-e model. The turbulent viscosity isassumed constant throughout the calculation domain and it can be defined eitherby specifying an absolute value or a multiplier, which is applied to the molecularlaminar viscosity. This specification of turbulent viscosity is at best approximate butdoes allow a number of scenarios to be investigated for key features, prior to usingthe k-emodel.

2.1.2. Surface Heat Transfer

Two options:


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MicroFlo will calculate heat transfer between solid surfaces and the air (Default).

Users can define the surface heat transfer coefficients.

2.1.3. Boundary Conditions

Set the initial surface temperatures for all wall surfaces and windows (Default 20 Deg.C).These do not need to be set if users are using boundary conditions from Apache.

2.1.4. Grid Settings

Define the default grid spacing and merge tolerance. The merge tolerance enables gridlines that are separated by a distance less than the tolerance to be merged into a singlegrid line to minimise superfluous gridding. Both values default to the last values you

entered.

Ensure Grid merge tolerance is less than or equal to the thickness of the smallestcomponent.For example the walls that are imported using Create multizone space partitions are setup using 0.1m thick components so grid merge tolerance should be less than or equal to0.1m.

2.1.5. Initial Conditions

Define the initial velocity in the x-, y- and z-directions (Default 0.0, 0.0 and 0.0 m/s). The

initial room air temperature can also be set. (Default 20 Deg.C). Quicker convergencemay be achieved if the user chooses an initial temperature close to that of the convergedsolution.

2.1.6. Discretisation

Three options:

Upwind (Default) Hybrid Power Law.

This is an advanced option and is to do with the combined convection-diffusion coefficientsthat result from discretisation of the defining differential equation set. Early attempts toderive CFD solution schemes using the traditional central difference approach todiscretisation were found to fail for flows with high absolute value of Peclet number, due tothe highly non-linear relationship between the transported variable and the transportdistance. The basic remedy for this behaviour is to allow the finite volume cell interfacevalues of the convected properties to take on the upwind grid point values; this method isknown as the upwind scheme. Advanced users who wish to use an alternative schememay opt for the arguably more accurate but more computationally expensive hybrid andpower-law schemes.


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2.1.7. Supplies and Extracts

This sets the number of cell faces across a non-orthogonal inlet. Entering the command

vcomp=on will show the inlet form. Then used as a mesh tool to assist in refining the grid.

Figure 3: A maximum dimension of 0.2m


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The Create multi-zone space partitions button ( ) must be selected if there areinternal partitions within any room or between selected rooms, otherwise they will be

ignored.

In the example model, the larger room in the ground floor is selected. Havingselected the required room, step down to the Body level of decomposition, either bydouble-clicking on the room or clicking on the Move down one level view toolbarbutton.

Once at the Surface level, MicroFlo displays the selected rooms plus all roomcomponents:

Figure 5: Selected room plus components

If components are defined then these will be picked up and used by MicroFlo. Caremust be taken as the greater the detail of the components the greater the number ofcells that will be created, increasing simulation time dramatically without any

significant increase in accuracy. In this example partitions and desktops have beenused but details such as table legs have been excluded.

Components are initially thermally neutral they do not heat or cool the space. InMicroFlo there is the ability to define various attributes for components such astemperature or flux to components; this will be described in a later section.

The second combo box from the left of the view toolbar (located at the bottom of theMicroFlo view window), which is known as the display mode combo box, containsfour modes; Surface, Component, Grid and Slice:

Surface: for entering/modifying surface boundary conditions (Default)


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Component: defining CFD components that exist within the volume of theroom

Grid: to create and modify the CFD finite volume grid Slice: post-simulation analysis of results. N.B. slice mode has been

superseded by the MicroFlo Viewer and now when slice mode is selected theMicroflo Viewer is launched.

On first moving down to the Body level of decomposition, the default display mode isSurface, which allows users to define surface boundary conditions. Note that formulti-zone spaces, it is necessary to define surface boundary conditions separatelyfor each constituent room (i.e. users will need to go back up to the Model level ofdecomposition and move down into each constituent room).

Figure 6: Surface level of model

An additional option is now enabled on the Toolbar for Surface mode operations:

Define Surface Boundary Conditions


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2.2.1. Surface Boundary Conditions

Boundary conditions, resulting from Vista exports, can be imported by selecting ImportBoundary data from the File menu. There is also an option to clear imported boundaryconditions. Please note that any boundaries that overlap with an opening will be removedby this action.

Figure 7: Selection of Boundary Conditions file generated in ApacheSim / Vista

Import Room Gains will import the apache gains for each room.Checking the Import OpeningFlows box generates supply diffusers and extracts, asrequired, for the openings. It is possible to choose where to place them.

Figure 8: Import of MacroFlo openings and subsequent window opening position


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2.3. S t ep 3: Def ining S ur f ac e B o und ar y P r oper t ies

Double clicking on a surface will give a Boundary view and an add boundary

condition option ( ) is available on the Toolbar. Select the Toolbar and thefollowing dialogue box appears (Figure 9):

Figure 9: Dialogue box to define boundary conditions

The following types of boundary conditions that can be added to a surface:

1. Vent boundaries

General Supply Diffuser: constant velocity inflow boundary 2-Way Supply Diffuser: 2-way air supply diffuser 4-Way Supply Diffuser: 4-way air supply diffuser Swirl diffuser Extract: constant velocity outflow boundary Fixed Pressure boundary

2. Solid boundaries

Fixed Temperature: for solid boundaries where temperature is known Heat: for convective heat sources where the heat output s specified


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Solid boundaries can have arbitrary polygonal geometries, but vent boundaries must berectangular. Swirl diffusers are circular.

There are two additional boundaries that can be applied to openings and holes to controlthe flow of air:

Porous Baffle Fan

2.3.1. Supply Diffusers and Extracts

These are boundaries where the flow into and out of the simulation domain is specified.Supply diffusers define the entry of air into the simulation domain. The composition can bespecified, for air from an external source, such as the moisture content and theconcentrations of CO2 and CO as mass fractions. The local mean age of air from an

external source is defined to be zero.

General supply diffusers can be applied to surfaces of arbitrary orientation, but the otherscan only be applied to surfaces aligned with the primary Cartesian axes.Only the flow is required for extracts. They can be placed on surfaces of any orientation.

2.3.1.1. General S upply Diffuser

Specify either the air flow rate (m3/s) or flow velocity (m/s). Specify the X- and Y-Direction Discharge Angles. Specify the air composition and temperature for any external sources. Then draw the supply on the surface.

2.3.1.2. 2-Way S upply Diffuser

Specify either the air flow rate (m3/s) or flow velocity (m/s). Specify the Multiway Discharge Angle which is the angle to the normal and whether

the discharge direction is aligned with the X- or Y- axis when the surface.

Specify the air composition and temperature for any external sources. Then draw the supply on the surface.

2.3.1.3. 4-Way S upply Diffuser

Specify either the air flow rate (m3/s) or flow velocity (m/s). Specify the Multiway Discharge Angle which is the angle to the normal. Specify the air supply temperature, and the water vapour content if required. Then draw the supply on the surface.

2 .3 .1 .4 . S w i r l D i f f u s e r

A swirl diffuser creates a circular flow where the throw can be specified with respect to the radiusvector.


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Specify either the air flow rate (m3/s) or flow velocity (m/s). Specify the angle to the normal (N) and the azimuth angle between the projection

on the discharge vector (v) and the radial vector on the plane of the boundary ( r).

Specify the air composition and temperature for any external sources. Specify a nominal number of vent elements (2x2, 3x3, 4x4, 5x5) to use. Then draw the supply on the surface.Swirl diffusers are generated to second order using the generated mesh and so theactual number of cells may differ from that specified.

2.3.1.5. E xtract

Specify either the air flow rate (m3

/s) or flow velocity (m/s). Then draw the extract on the surface.

2.3.2. Fixed Pressure boundaries

The second type of vent boundary is the fixed pressure boundary where the pressurerather than the flow is prescribed. The gas composition and the temperature of theexternal source are given. The local mean age of air of the external source is defined to bezero.

Specify the external pressure (Pa). Specify the air composition and temperature for any external sources. Then draw the supply on the surface.

2.3.3. Fixed Temperature

To a fixed temperature patch into the surface, first enter:

The patch temperature (C). Then draw the temperature patch on the surface.

2.3.4. Heat

To put heat source patch into the surface, first enter:

Heat Flux (W/m2). Then draw the heat flux patch on the surface.

N

r

v

Figure 10 The velocity vector (v) fora position vector (r), zenith () and azimuth()


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Repeat as necessary on the same surface and change the properties as required.

2.3.5. Porous Baffle

Porous baffles can only be applied to holes. The pressure step for a flow of a givenvelocity through the baffle can be described by Darcys law with an inertial loss term:

Where is the baffle thickness, is the pressure jump coefficient and is thepermeability. The fluids viscosity and density are given by and .

This is essentially a quadratic equation and so the simulation accepts the linear andquadratic coefficients. The curve is cut off at the stationary point , if c2is negative.

The user should:

enter the coefficients c1and c2, then draw the boundary over the appropriate hole.

Since the behaviour of the porous boundary is symmetric irrespective of the direction offlow, it can be drawn on either side of the hole.

2.3.6. Fan

A cubic equation is used to approximate the fan curve for a fan boundary.

The user must specify a target flow rate or target velocity since there can be up to 3solutions for the velocity for a given pressure step. Any value can in principle be chosenfor the coefficients, but it is strongly recommended that a monotonic decreasing function issupplied to obtain a well behaved solution. This is one where the gradient is never greater

than zero ( ), and implies that and . A quadratic or linear function

can also be given.Set the coefficients to zero and set a target velocity if a constant velocity is required,independent of pressure.The user should:

Specify either the air flow rate (m3/s) or flow velocity (m/s).

Enter the coefficients Then draw the boundary over the appropriate hole.

The direction matters here and the boundary must be drawn on the side where the flow isejected. The view shows this directed arrow over the boundary.

2. 4. Room G ains

If you press the Show Room Gains button following the dialogue box (Figure 11) willappear:


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Figure 11: Convective Heat Gains from ApacheSim.

This is useful when checking that convective gains have been included in the model.The gains may be added as heat boundaries on surfaces (on both rooms andcomponents) or component heat sources.

2 .5 . S t ep 4 : A d d i n g C o m p o n e n t s t o t h e M o d e l

There are two types of CFD component: the solid and source. Solid componentsinteract with the modelled fluid via surface boundary conditions, whereas source


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components exert their influence throughout their volume via source terms.It is possible to add pre-defined CFD components and to modify existing

components to incorporate boundary conditions. It is necessary to define CFDcomponents for each constituent room when dealing with a multi-zone space. SelectComponent mode from the display mode combo and select the place componenticon on the Toolbar. Figure 12 is then displayed.

Figure 12: Place Component Dialogue

The first tick box determines whether the component is a solid and determines thebehaviour of the others. Selecting a tick box allows the user the input the relevantdata.

The surface temperature can only be set for solid components.

The heat box determines the total thermal power output of the component. The moisture box is not used for solid components. The user is invited toinput the mass of water vapour generated by the component (in kg/hour orlbs/hour), fluid (or air) components.

The CO2 box is not used for solid components. The total mass of CO2gas (inkg/hour or lbs/hour) generated by the component is required for sourcecomponents that generate CO2.

The CO2 and moisture source terms should balance when there is noventilation. The net sources should be zero when there is no externalventilation.


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Table 1 Component properties for solid and fluid components.

TEMPERATURE HEAT MOISTURE CO2, CO

SOLID C or F W or Btu/h N/A N/A

SOURCE N/A W or Btu/h kg/h or lbs/h kg/h or lbs/h

It should be noted that the values entered for the CO2 orH2O mass fractions aremeaningless and not allowed for solid components as solid boundaries areimpervious to air.

There are a number of pre-defined CFD components:

Radiator

Generic Heat, CO2and moisture source component Generic Solid Heat Source

Having selected a component from the drop list, any associated attributes will bechecked in the attribute list box. Attributes may be added or removed bychecking/unchecking the entry. When an entry is selected and checked, the attributevalue will be displayed below the list box (e.g. Surface Temperature in Figure 12).The scale or actual dimensions of the component can be entered in the dimensiongroup box. Having defined the properties you can then place the component in themodel. Repeat this process to define other components. Attributes of placedcomponents can be changed at a later date by selecting the component - adding,

removing or modifying any required attribute and then clicking on the UpdateSelected Components button. You will notice that the Place Component dialog hasa similar Room Heat Gain display to the Add Surface Boundary Condition dialogdescribed undersection 2.3.8.

In the example model we have to distribute 2.1 kW of equipment gains via components.There are seven desks and seven computers at 300W each. These are all solid heatsources. The result after entering these components is shown below (Figure 13).


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Figure 13: Model with CFD Components added

It is possible to add supply or extract boundary conditions to the constituent surfaces ofcomponents by selecting the required component and moving down through thedecomposition levels of the component in a similar way to adding surface boundaries to aroom surface.

2.6. S t ep 5: G r iddin g t he M o del

Select the Grid mode from the display mode combo box. The finite volume grid usedby MicroFlo is created in the form of a number of regions. Each region can bespaced using the following options:

None no cells Uniform spaced uniformly using the default grid spacing, which can be

defined under Settings.

Increasing Power Law the x-coordinate location of the faces of each grid cellwithin the region increases as the power of the spacing number, which startsat the beginning of the region. So that if i represents the index number of thegrid line counted from the start of the region, the x-coordinate of the i th face iscalculated using the following relationship:

Xi=(region length)(i/n)power+xs

Decreasing Power Law MicroFlo sets the spacing to decrease as the powerof the spacing number counted from the end of the region:

Xi=(region length)[1-(i/n)power]+xs

Symmetric Power Law the x-coordinate of the ith face is calculated usingboth increasing and decreasing powerlaw relationships that meet at the

middle of the region:For i


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Xi=[(region length)/2](2i/n)power+xs

For i >= n/2:

Xi=[(region length)/2][2-(2i/n))power

]+xs

With a default grid spacing of 0.1m the following grid scheme is automaticallycreated:

Figure 14: Model displaying grid scheme.

Resetting the default grid spacing in the CFD Settings (Step 2), allows users torecalculate the default grid. The basic rules for defining the default grid co-ordinatesare:

1. A region end point is placed at the co-ordinate of each object in the modele.g. surface, supply or component. These regions are fixed and shown asred.

2. Regions are uniformly spaced with grid cells using the default grid spacingdefined in the CFD settings. In this case 0.5 m. A smaller spacing wouldgenerate more grid cells. These spacing grid cells are displayed in grey.

The grid setting window to the left hand side of the interface has a number offeatures to allow modifications to the grid. The principal features of this grid interfaceare as follows:

1. X-Grid, Y-Grid and Z-Grid tabs allow users to select the grid scheme in eachof the X, Y and Z dimensions. Only one can be active at a time.

2. Each of these tabs list the end co-ordinate of each grid region in thedimension plus the spacing type used to space the region.


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In addition there are four options above the co-ordinate tabs. These are:

2.6.1. Generate Grid

This option will automatically re-generate the grid based upon the current default gridspacing.

Note all custom grid refinements will be lost when generating the grid.

2.6.2. Add Grid Region

If users select a grid in the Grid tabs and select the add grid region option then Figure 15

appears:

Figure 15: Dialogue box to allow adding a Grid Region.

The end co-ordinate is where the new region needs to be placed in the currently selectedregion. The spacing drop down list enables users to define the spacing throughout thisnew region, using one of the five options described above; None, Uniform, IncreasingPower Law, Decreasing Power Law and Symmetric Power Law.

2.6.3. Remove Grid Region

Grid regions can only be removed that have been added by the user. These are shown as

unlocked. Highlight an unlocked grid region and it will be automatically removed if thisoption is selected.

2.6.4. Edit Grid Region

You can edit the spacing within a grid region via the following dialogue box (Figure 16).


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Figure 16: Interface to allow editing of a Grid Region.

Users have the same five padding options as defined above: None, Uniform, Increasing

Power Law, Decreasing Power Law and Symmetric Power Law. In addition users candefine the minimum spacing to override the default value defined in the CFD Settings.

Note the graphical display of the gridlines is for visual purposes only.

2.7. S t ep 6: Runnin g t h e S im ulat ion

Select the Run simulation icon on the Toolbar. The grid statistics dialog will bedisplayed which shows the available system memory and the amount of memoryrequired for the simulation. If a red cross appears beside the Physical MemoryAvailable entry, you will need to simplify the problem formulation and/or grid to

reduce memory requirements. The CFD Grid Statistics dialog also displays themaximum aspect ratio of cells within the grid and again if a red cross appears nextto this entry, you will need to modify the formulation/grid to reduce this value youcan also use the merge tolerance to eliminate very close grid lines (see Section 2.1).


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Figure 17: CFD Grid Statistics

If you have an existing simulation in your directory then Figure18 will appear next:

Figure 18: Simulation Control

Resume will allow users to restart the simulation from the conditionspaused/stopped the simulation.

Save will allow users to save the file for later use. Delete will remove the file.

Prior to displaying the main simulation control panel, see Figure 19, MicroFlo will


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calculate all the grid cells for the model.

Figure 19: MicroFlo Simulation Control Monitor

Principal features of this simulation control panel:

2.7.1. Graphical Display Monitor

This monitor will graphically display the residual errors, at each iteration, for selectedvariables and for defined cells. This helps users estimate how long a simulation may takeor identify if the simulation is converging or not.

2.7.2. Cell Monitor

Allows users to define which cell or cells to monitor. The cell monitor can be very useful inidentifying problems to do with convergence, see Troubleshooting.

2.7.3. Outer Iterations

Set an upper limit of simulation that will be performed if full convergence is not reached.


Facility to change the default turbulence model.


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2.7.5. Isothermal

If this is checked, the temperature/energy equation will be removed from the solution

scheme.

2.7.6. CO2

If this is checked the solution scheme will solve for CO2 concentration.

2.7.7. Variable Control

Allows users to define the rules for controlling the convergence of individual variables.

2.7.8. Boundary Conditions FileBoundary conditions file will display the name of the boundary conditions file if applied.Now press Run, the simulation will be performed. As the simulation proceeds, theprogress towards convergence is displayed in the CFD Monitor window.

Figure 20: Simulation in Progress

Users can Pause the simulation at any time in order to modify one or more of the CFDcontrol variables to assist convergence. To re-run the simulation you simply press the Runbutton again.


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The simulation will automatically stop when the number of Outer Iterations are reached orwhere the Termination Residual has been reached for all variables i.e. convergence has

been achieved.

Review the results (see Analysing the Results in the next section) at any stage. Click onthe Pause button, the simulation will be suspended, and then click on the Close button,users will be able to review the results by selecting the Slice mode from the display modecombo box.

To restart a simulation, users need to return to the Grid operating mode. Press the Runicon to see Figure 21:


The software has found an existing simulation results file and gives four options:

Resume: re-start most recent simulation from where it was paused.

Save: Allows users to make a copy of this file for archival purposes. Delete: removes current MicroFlo results file and allows users to start from thebeginning.

Cancel: closes the window and does not 'Run' the simulation.

If any changes made to the model need to be incorporated into the simulation, then thelast option, to delete the existing file and carry out the simulation using a regenerated file,should be selected.


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3. External AnalysisExternal air flow is similar, but in some ways simpler, than internal air flow.

To switch from internal analysis to external analysis the MicroFlo windowchanges as follows:

Figure 22: Grid for External Analysis

Depending upon the size of the model a 3D boundary is assigned around themodel and a grid is automatically assigned depending upon the CFD setting(see below). Step 1 is to re-define CFD settings.


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3.1. Step 1: CFD Sett ings

On the View Toolbar select CFD Settings, the following Properties box isdisplayed (Figure 29):

Figure 23: CFD Setting Properties Box for External Analysis

Each item has sensible defaults but users can modify this information at anystage of the modelling exercise.

The tabs have the following function. Note that Turbulence and Discretisationare all the same as the internal analysis.

3.1.1. Wind

There are three parts:

Define Wind Direction Define Wind Velocity Define Exposure type (options: Open country, Suburban and Urban)

The wind model in ASHRAE Handbook Fundamentals (2005) is used. (SeeAppendix D: The wind speed for external simulations.)

The input wind direction and speed are those observed by a weather station, in opencountryside, at a height of 10m.


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There are two types of Turbulence Models available at present:

k-e: the most generally accepted and widely used turbulence model (Default).The k-e model calculates turbulent viscosity for each grid cell throughout thecalculation domain by solving two additional partial differential equations, onefor turbulence kinetic energy and the other for its rate of dissipation.

Constant effective viscosity. This model does not attempt to account for thetransport of turbulence but offers the user a much faster much moreapproximate method of accounting for turbulence than the k-e model. Theturbulent viscosity is assumed constant throughout the calculation domainand it can be defined either by specifying an absolute value or a multiplier,which is applied to the molecular laminar viscosity. This specification of

turbulent viscosity is at best approximate but does allow a number ofscenarios to be investigated for key features, prior to using the k-emodel.

3.1.3. Grid Settings

Define the default grid spacing and merge tolerance. The merge tolerance enablesgrid lines that are separated by a distance less than the tolerance to be merged intoa single grid line to minimise superfluous gridding. Both values default to the lastvalues you entered.

Figure 24: Grid Settings for External Analysis

Ensure Grid merge tolerance is less than or equal to the thickness of the smallestlengthscale required in the simulation.


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MicroFlo will recommend the extensions beyond the model for the external CFDdomain; in the directions upwind, downwind, to the sides of, and above the model.

This is called the Domain Extent. These values can be adjusted by the userdepending on the particular model / flow domain to be run.The recommended values for these extents are based on the building height (h):

Upwind 5 h Downwind 15 h Sides 35 h Above 57 h

3.1.4. Discretisation

Three options:

Upwind (Default) Hybrid Power Law

This is an advanced option and is to do with the combined convection-diffusioncoefficients that result from discretisation of the defining differential equation set.Early attempts to derive CFD solution schemes using the traditional centraldifference approach to discretisation were found to fail for flows with high absolutevalue of Peclet number, due to the highly non-linear relationship between thetransported variable and the transport distance. The basic remedy for this behaviouris to allow the finite volume cell interface values of the convected properties to takeon the upwind grid point values; this method is known as the upwind scheme.

Advanced users who wish to use an alternative scheme may opt for the arguablymore accurate but more computationally expensive hybrid and power-law schemes.

3.2. S t ep 2: Redefine t he M o del G r id

The display mode combo box, which is located on the View toolbar at thebottom of the window, contains two options:

Grid: to create and modify the CFD finite volume grid Slice: post-simulation analysis of results. This option has no meaning

without an available results file, (see Step 4).

The finite volume grid used by MicroFlo is created in the form of a number ofregions. Each region can be spaced using the following options:

None no cells Uniform spaced uniformly using the default grid spacing, which can be

defined under Settings.

Increasing Power Law the x-coordinate location of the faces of eachgrid cell within the region increases as the power of the spacing number,which starts at the beginning of the region. So that if i represents the

index number of the grid line counted from the start of the region, the x-


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coordinate of the ith face is calculated using the following relationship:Xi=(region length)(i/n)

power+xs

Decreasing Power Law MicroFlo sets the spacing to decrease as thepower of the spacing number counted from the end of the region:

Xi=(region length)[1-(i/n)power]+xs

Symmetric Power Law the x-coordinate of the ith face is calculatedusing both increasing and decreasing powerlaw relationships that meetat the middle of the region:

For i = n/2:

Xi=[(region length)/2][2-(2i/n))power]+xs

Reset the default grid spacing in the CFD Settings (Step 2) to recalculate thedefault grid. The basic rules for defining the default grid co-ordinates are:

1. A region end point is placed at the co-ordinate of each object in themodel e.g. surface, supply or component. These regions are fixed andshown as red.

2. Regions are uniformly spaced with grid cells using the default gridspacing defined in the CFD settings. In this case 1.0 m. A smallerspacing would generate more grid cells. These spacing grid cells aredisplayed in grey.

The browser window to the left hand side of the interface has a number offeatures to allow modification to the grid. The principal features of this gridinterface are as follows:

1. X-Grid, Y-Grid and Z-Grid tabs allow users to select the grid scheme ineach of the X, Y and Z dimensions. Only one can be active at a time.

2. Each of these tabs list the end co-ordinate of each grid region in thedimension plus the spacing type used to space the region.

In addition there are four options above the co-ordinate tabs. These are:

3.2.1. Generate Grid

This option will automatically re-generate the grid based upon the current default gridspacing.

Note all custom grid refinements will be lost when generating the grid.

3.2.2. Add Grid Region

Selecting a grid in the Grid tabs and selecting the add grid region option Figure 31

is displayed:


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Figure 25: Dialogue box to allow adding a Grid Region

The end co-ordinate is where you want the new region to be placed in the currently

selected region. The spacing drop down list enables users to define the spacingthroughout this new region, using one of the five options described above; None,Uniform, Increasing Power Law, Decreasing Power Law and Symmetric Power Law.

3.2.3. Remove Grid Region

This allows the removal of grid regions that have been added. These are shown asunlocked. Highlight an unlocked grid region and it will be automatically removed ifselected.

3.2.4. Edit Grid RegionSelect this option to edit the spacing within a grid region via the following dialoguebox (Figure 32).

Figure 26: Edit Grid Region

The same five padding options as defined above: None, Uniform, Increasing PowerLaw, Decreasing Power Law and Symmetric Power Law. In addition users candefine the minimum spacing to override the default value defined in the CFDSettings.

Note the graphical display of the gridlines is visual only.


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3.3. S t ep 3: Runnin g t h e S im ulat ion

Select the Run simulation icon on the Toolbar. The grid statistics dialog will bedisplayed which shows the available system memory and the amount ofmemory required for the simulation. If a red cross appears beside the PhysicalMemory Available entry, you will need to simplify the problem formulationand/or grid to reduce memory requirements. The CFD Grid Statistics dialogalso displays the maximum aspect ratio of cells within the grid and again if ared cross appears next to this entry, you will need to modify theformualtion/grid to reduce this value you can also use the merge tolerance toeliminate very close grid lines (see Section 2.1).

Figure 27: CFD Grid Statistics

If there is an existing simulation in your directory then Figure 34 will appearnext:


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Resume will allow users to restart the simulation from the conditions thatpaused/stopped the simulation

Save will allow users to save the file for later use.

Delete will remove the file.

Prior to displaying the main simulation control panel, see below, MicroFlo willcalculate all the grid cells for the model.

Figure 29: CFD Monitor for External Analysis


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The principal features of this simulation control panel shown in Figure 35 areas follows.

3.3.1. Graphical Display Monitor

This monitor will graphically display the residual errors, at each iteration, for selectedvariables and for defined cells. This helps users estimate how long a simulation maytake or identify if the simulation is converging or not. Note that it is assumed thatexternal air flow is isothermal i.e. uniform temperature.

3.3.2. Cell Monitor

Allows users to define which cell or cells you wish to monitor. The cell monitor can

be very useful in identifying problems to do with convergence, see Troubleshooting.

3.3.3. Outer Iterations

Set an upper limit of simulation that will be performed if full convergence is notreached.


Facility to change the default turbulence model.

3.3.5. Variable Control

Allows users to define the rules for controlling the convergence of individualvariables.

3.3.6. Boundary Conditions File

Will display the name of the Boundary Conditions file if applied.

Now press Run, the simulation will be performed. As the simulation proceeds

progress towards convergence will be displayed in the CFD Monitor window.


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Figure 30: CFD External Analysis in progress.

Pause the simulation at any time in order to modify one or more of the CFD controlvariables to assist convergence. To re-run the simulation simply press the Runbutton again.

The simulation will automatically stop when the number of Outer Iterations arereached or where the Terminal Residual has been reached for all variables i.e.convergence has been achieved.

Results can be reviewed (see Analysing the Results in the next section) at anystage if the user selects pause. After pausing the simulation results will be written

to file. If there is an existying CFD results file when users press the Run icon thefollowing window is displayed (Figure 37):



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The software has found an existing simulation results file and gives four options:

Resume: re-start most recent simulation from where it was Paused. Save: allows users to make a copy of this file for archival purposes. Delete: removes current MicroFlo results file and allows users to start from

the beginning.

Cancel: closes the window and does not 'Run' the simulation.


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3.4.


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4. Post-processing with the MicroFlo Viewer

The MicroFlo viewer application can be used to display your CFD results. Thisoption is similar to the display model viewer button in ModelIT. The user canrotate the model using the orbit button and zoom in and out as necessary toconcentrate on any particular area in the model.

On the upper left hand side of the window, there are various options to selectthat the user can display, for example velocity and temperature contours or filledcontours as required. A combination of vectors and contours can be chosen forthe one image.

On the lower left hand side of the window, the user can select the grid locationsin the x, y and z axes for displaying the slice results. Scrolling up and down thelist of available grid locations can be done using the up and down arrow keys onthe keyboard.

The display settings option can be modified here; the user can change the scaleof the velocity vectors and the maximum displayed value. The range for thevelocity and temperature contours can also be modified here by adjusting theupper and lower values for display.


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Figure 32: The MicroFlo viewer interface

The full list of output variables the MicroFlo viewer displays is listed below:

Velocity vector Velocity contour Temperature contour Pressure contour H2O mass fraction contour CO2 mass fraction contour Local Mean Age of Air contour Filled velocity contour

Filled temperature contour Filled pressure contour Filled H2O mass fraction contour Filled CO2 mass fraction contour Filled Local Mean Age of Air contour Dry resultant temperature contour PMV contour PPD contour Comfort contour Filled dry resultant temperature contour

Filled PMV contour Filled PPD contour Filled comfort contour

The user can specify the comfort conditions for the comfort display options.

Figure 33: The comfort conditions dialogue

The MicroFlo viewer can also be used to create surfaces of temperature,velocity, pressure, H2O and CO2 mass fractions, and the local mean age of air.

These are displayed as virtual nets (see the following page for an example).


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The MicroFlo viewer can display animated particle tracking where the user canspecify the number and length of the particle tracks.

Figure 34: The particle tracking dialogue


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4.1. T y p ic al P ar am et er Dis play V ar iat ions w it h t h e M ic r oF lo V iewer

Figure 35: Velocity Vector Slice Figure 36: Velocity Contour Slice

Figure 37: Temperature Contour Slice Figure 38: Filled Velocity Slice


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Figure 39: Temperature Contour Slice

Figure 40: Temperature Surface Net


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Create AVI

Users can create 3 types of avi to make a video of their CFD results. The 3 types areorbit, timed and slice. It is the users discretion what results are displayed during theavi creation process.

Figure 41: The orbit create movie

Figure 42: The timed create movie

Figure 43: The slice create movie


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5. Appendix A: Steps to Define Thermal BoundaryConditions from ApacheSim

Users may wish to use Boundary Conditions from ApacheSim to improve thequality of the information used by MicroFlo. This section summarises how yougenerate the MicroFlo boundary conditions from ApacheSim.

5.1. S t ep 1: Def ine P ot ent ial Room s f or M ic r oF lo A naly s is

Figure A1: Selection of output variables and rooms.


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In the ApacheSim simulation controller, select Output Options and choose therooms for surface temperature output. This additional information will provide the

boundary condition information for use in MicroFlo.

5 .2 . S t e p 2 : E x p o r t B o u n d a r y C o n d i t i o n s

In Vista select the Export Boundary Conditions options from the File menu. Thefollowing dialogue box appears:

Figure A2: Time selection for boundary conditions

Users define the time on a specific date they want the boundary conditions forMicroFlo. Users can also define the Averaged Duration i.e. the length of tome youwant to average the simulation results.

Users are then asked to name the Boundary Conditions file for use by MicroFlo.


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6. Appendix B: Troubleshooting

6. 1. Dif f ic ult y in A c hiev ing Conv er ged S olut ion

Problem Cause Remedy

Residuals continuouslyincreasing

False time steps set too high Reduce false time steps forvelocities and possiblytemperature

Residuals fail to reduce Oscillating flow pattern

Unrealistic initial values

Internal heat source withoutsink, e.g. radiator in room withadiabatic surfaces

Reduce false time steps forvelocity

Check initial values underSettings

Check that the problem isrealistic. Set cell monitor pointsand check for continuouslyincreasing/decreasing values,which would indicate imbalance

Erratic convergence Unstable f lows, e.g. strong jetsor buoyancy driven plumes

Reduce false time steps forvelocity.

Mass residual reduces veryslowly

Various causes Increase number of inner iterations for pressure

Residuals all reducing steadily

but very slowly

False time step set too low Increase false time steps

If the MicroFlo solver experiences numerical difficulties during the calculation run thefollowing items should be checked and may need to be changed or altered:

Physical model shape Boundary Conditions Make use of sensible initial values Quality of the grid Solver parameters

6 .2 . J u d g i n g W h e n t h e R u n h a s R e ac h e d C o m p l e t io n

1. Check the residual history printed on screen. The sum of the normalizedabsolute residuals should diminish steadily.

2. Check the monitoring cell location for the dependent variables at the user setlocation within the fluid domain. These should stabiles to the convergedsolution.

It is very important that checks are made during the initial stages of the analysis tomonitor progress of the solution run. In cases of solution divergence, the run shouldbe terminated and appropriate adjustments made to the relevant control parameterssuch as relaxation factors, false time steps etc.


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Neglecting this can result in lost time and unproductive simulations. Note, however,that increases in residuals and oscillations in the computed variables during the early

stages of a run are not uncommon and should disappear after a few iterations. Therun should therefore be given sufficient time to stabilise before any judgement ismade on its progress.

7. Appendix C: Opening MicroFlo Results from anEarlier Version of the The following procedure is used to import MicroFlo CFD results that have beencreated in an earlier version of the .

1. Open model in new version of the .2. Enter MicroFlo and select zone(s) to be used for internal analysis.3. Go to File > Translate Output File and choose the .cfd file to be

translated.

4. Click OK to confirm geometry change and then select the filename for thenew .cfd file export.

5. Enter the MicroFlo viewer and select Open File. Choose the new .cfdfile and the available grid slices will appear under the X, Y and Z tabs. AllMicroFlo results can now be viewed as per normal.


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8. Appendix D: The wind speed for externalsimulations.

The model from the ASHRAE Handbook - Fundamentals [2005], (16.3 Airflowaround Buildings) was used to obtain the wind speed (u) profile with height(h).

aa

met

met

met

h

huhu

met

where

met

= 270 m is the Layer Thickness for the meteorological site

(assumed to be of type Country)

meta = 0.14 is the Exponent for the meteorological site (assumed to be

of type Country)

meth = is the measurement height for the meteorological site (assumed

to be 10 m)

The values for the atmospheric boundary layer () and the exponent ( a) aregiven in the table below.

Terrain Type Description Exponenta

Layer Thickness(m)

Country Open terrain with scattered obstructionshaving heights generally less than 10 m,including flat open country typical of meteorological station surroundings

0.14 270

Suburbs Urban and suburban areas, wooded areas, orother terrain with numerous closely spacedobstructions having the size of single-familydwellings or larger, over a distance of at least2000 m or 10 times the height of the structureupwind, whichever is greater

0.22 370

City Large city centres, in which at least 50% of buildings are higher than 21m, over a distanceof at least 2000 m or 10 times the height of thestructure upwind, whichever is greater

0.33 460

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