Flow Around a Cylinder

7/29/2019 Flow Around a Cylinder

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Flow Around a Cylinder

2D Flow

In this tutorial, Gambit is used to create and mesh the geometry. Once preprocessingis complete, Fluent will be used to solve the flow problem. Fluent will then be used to

analyze the solution.

This basic 2D tutorial should prepare you to model, mesh, define and analyze morecomplex fluid flow problems.

The methods expressed in these tutorials represent just one approach to modeling, defining andsolving 2D problems. Our goal is the education of students in the use of CAx tools for model-ing, constraining and solving fluids application problems. Other techniques and methods will beused and introduced in subsequent tutorials.

Water flows around a cylinder of radius 0.1 mat a Reynolds number of 20. Solve for pres-sure and viscous drag coefficients and display

the vortcies in the wake of the cylinder.


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Creating Geometry

Begin the problem by creating geometry inGambit.

The computational domain will be a rectan-gle encompasing the 2-D cylinder in thecenter. At a Reynolds number of 20, vortexshedding will be present. It will be neces-sary to create a mesh with higher resolu-tion in the wake of the cylinder to betterresolve these vorticies.

First create the points that will be used tocreate the lines and then faces of the

domain.

Operation > Geometry > Vertex > CreateVertex

Create an arc centered at point G. Use thethird option under Method to specify acenter point and a sweep angle. Create thearc with the following dimensions:

Radius=0.15

Start Angle=90

End Angle=270

Name X Position Y Position

A -2 -1.5

B 2 -1.5

C 2 1.5

D -2 1.5

E 2 -0.15

F 2 0.15

G 0 0


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Creating Geometry

Now draw the straight lines that willcomplete the domain. Connect the

points to create the following linesegments: AB, BE, EF, FC, CD, DA,and the horizontal lines connectingthe new verticies created at the endsof the arc and the points F and E.

Create two faces: a small slender facestretching from the center of the domain tothe far right edge of the rectangle contain-ing a fine mesh to resolve the wake and alarger face surrounding it that will containa rougher mesh.

Operation > Geometry > Face > Form Face

Select the four edges (including the arc)that make the long slender face and clickApply. Then create the larger face thatconsists of all the edges except the smallestvertical edge on the right vertical wall

Now make the cylinder by creating a circleface.

Operation > Geometry > Face > Create

Face

Create a circle of Radius=.05 and clickApply.Now subtract the circle face from the slen-der face. Do not retain the faces them-selves.

Operation > Geometry > Face > BooleanOperations


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Meshing Geometry

Create edge meshes around the circle andright side vertical edges as shown to the

right.

The circle edge should be meshed with aSucessive Ratio = 1 and an Interval Count= 40.

The two edges should be meshed with aLast Length = 0.2 and an Interval Count =17. The mesh should be finer moving clos-er to the X-axis.

Now make a boundary layer around thecylinder.

Operation > Mesh > Boundary Layer >Create Boundary Layer

Create the boundary layer with the follow-ing dimensions:

First Row = 0.0001

Growth Factor = 1.625

Rows = 10


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Meshing Geometry

Mesh the two faces with tri elements.

Operation > Mesh > Mesh Face

The slender face should be meshed with anInterval Size = 0.02. The larger face withan Interval Size = 0.2. The mesh shouldresemble the image shown below.

Finally, specify the solver type and bound-ary conditions.

Solve > Fluent 5/6

Operation > Zones > Specify BoundaryTypes

Create boundary conditions as follows:

Top, bottom, left vertical wall = VelocityInlet

Cylinder = Wall

Right vertical wall (all three edges) =Outflow

Save the Gambit file and export theFluent mesh.

If problems are encountered in creating the

mesh, the completed mesh with boundaryconditions specified can be loaded from thefile Cylinder1_meshed.dbs into Gambitand Cylinder1_meshed.msh into Fluent


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Starting in Fluent

Load the mesh into Fluent.

File > Read > Case...

Check the mesh for errors.

Grid > Check

For this problem, the default Solver set-tings will be sufficient. Ensure that theproper viscous model is selected.

Define > Models > Viscous...

Ensure that Laminar is selected.

Now recall liquid water from the materialsdatabase so that it can be specified in theboundary conditions.

Define > Materials...

Enter the database by clicking on Database.Select water liquid (h2o) in the FluidMaterials list. Click Copy and then Close.

Now move the reference pressure into theflow domain.

Define > Operating Conditions...

Set the Y location to 1 m.


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Defining the Problem

Boundary conditions can now be set.

Define > Boundary Conditions...

Select fluid in the selection menu on theleft and then click on Set... ChangeMaterial Name to water-liquid.

Now click on inlet in the Zones menu andenter the velocity-inlet window. ChangeVelocity Specification Method toComponents and enter a velocity of2.01e-4 m/s (liquid water at Re = 20) next toX-Velocity.

Change the discretization method to ahigher order scheme.

Solve > Controls > Solution...

Change the Discretization for Momentumto 2nd Order Upwind.

The flow domain can now be initialized.

Solve > Initialize > Initialize...

Initialize the flow with the inlet conditions.


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Solving the Problem

Enable the plotting option for residuals andturn off automatic convergence checking.

Solve > Monitors > Residual...

Now change the reference values Fluentuses to calculate coefficients and forces.

Report > Reference Values...

Modify the following fields with the corre-sponding values:

Area = 0.1 m^2

Density = 998.2 kg/m^3

Velocity = 2.01e-4 m/s

Viscosity = 0.001003 kg/m-s

The problem is ready to be iterated.

Solve > Iterate...

Start with 2000 iterations.

If problems are encountered in setting up thisproblem in Fluent, the solved problem can beread in as a Case & Data... from the fileCylinder1_solved.cas.


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Analyzing the Solution

Once Fluent has stopped iterating, theforces and force coefficients on the cylinder

can be calculated.

Report > Forces...

Select the cylinder under Wall Zones andthen click Print. In the main Fluent win-dow a readout of the forces on the cylindershould appear. Use the horizontal scrollbar to pan to the right. Fluent reports thePressure Coefficient is 1.262 and theViscous Coefficient is 0.818. These values

are in very good agreement with coeffi-ceints obtained experimentally which werefound to be 1.25 and 0.8 respectively.

Now display contours of stream functions.

Display > Contours...

Change Contours Of to Velocity... andselect Stream Function in the pull-downmenu just below.

Uncheck Auto Range at the top left of theContours window.

Change Levels to 50.

Enter 0.3006 in the Min field and 0.3012 inthe Max field.

Click Display.


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Analyzing the Results

Stream functions directly around the cylin-der should be displayed. Zoom in on the

cylinder. Two vorticies can be seen in thewake. If desired, re-display more contoursby increasing the Levels in the Contourswindow. It can be seen that these streamlines are very similar to the experimentalimage of a cylinder at a similar Reynoldsnumber (Re = 24.3) shown below.

Photo by Coutanceau & Bouard 1977, from Van Dyke,Milton, An Album of Fluid Motion.


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Procedimento para Execuo em modo transiente o problema de

escoamento sobre um cilindro

Carregar o arquivo com a malha.

1. Abrir o Fluent e selecionar a opo: 2ddp e clicar run;2. Abrir o arquivo salvo com extenso .cas, com o comando:

FileReadCase

Ajuste do modelo

Solve ControlsSolutions..

Selecione a opo second order upwind para momentum e clique OK.

Modelo de escoamento

DefineModelsViscous...

Selecione Laminar e clique OK.

Ajuste dos parmetros do problema transiente

DefineModelsSolve...

Marque a opo unsteady e 2nd-order implicit e clique OK, conforme mostrado na figuraa seguir.


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Propriedades do fluido

Neste tutorial, o fluido utilizado ser a gua no estado lquido, e definida conformemostrado a seguir:

DefineMaterials

Clique em Fluent Database... e na caixa Fluent fluid materials selecionar water liquid.Em seguida clicar em copy e close sequencialmente.

Em seguida mude o fluido na caixa Fluent Fluid Materials para water clique em closepara finalizar esta etapa;


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Condies de Contorno

Em seguida as condies de contorno so definidas conforme apresentam as duas figurasseguintes, seguindo o comando:

DefineBoundary Conditions...

Na caixa Boundary Conditions, selecione as opo fluid para zone e type,respectivamente, e clicar em set. Os parmetros so configurados conforme a figuraseguinte.

Ajuste tambm a velocidade na zona de entrada conforme mostrado abaixo.

As duas operaes de definio de condies de contorno so finalizadas clicando em OK eCLOSE respectivamente.


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Inicializao dos campos de velocidade e presso e ajuste para a animao

O processo de definio da inicializao do clculoem regime transiente definido de acordo com acondio de contorno inicial, conforme mostradona figura a seguir, seguindo o comando:

Solve InitializeInitialize...

Clicar seqencialmente em init e close;

Nesta etapa os resduos a preciso durante aiterao so definidos, seguindo o comando:

Solve MonitorsResiduals...

Ajuste a janela que mostrar o processo de convergncia durante o clculo, e para isto a opo

window deve estar definida como 0, e o cone plot deve ser clicado para o surgimentoda janela, e em seguida clique em OK para finalizar esta etapa; a figura a seguir mostra aetapa descrita.

Agora sero definidas as janelas que mostraro os resultados de velocidade e presso estticaao longo do tempo. As figuras seguintes mostram a seqncia que deve ser seguida paraajustar a janela dos resultados de velocidade e presso.

Solve AnimateDefine...


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ATENO: a visualizao do campo de velocidade ser melhor se desmarcar a opoFilled na ltima janela acima.

importante alterar o nmero da janela para 1 no campo de velocidade e 2 no campo depresso. E, aps o procedimento descrito acima, trs janelas devero estar abertas paramostrar os resultados obtidos simultaneamente ao clculo, com as seguintes barras de status:fluent [0], fluent [1] e fluent [2].

Soluo

Agora a simulao ser iniciada, de acordo com a definio doprocesso de iterao mostrado na figura a seguir. Lembrandoque o tempo total de resultado obtido ser o produto do Timestep size com o number of time steps. Quanto maior onmero de pontos maior ser tempo de processamento econseqentemente maior espao para armazenamento sernecessrio, os comandos para o procedimento so:


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Solve Iterate...

Os seguintes resultados so mostrados durante a obteno dos resultados:


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Visualizao dos resultados

Ao trmino do clculo, os resultados podem ser armazenados em vdeos seguindo oprocedimento a seguir:

Solve Animate Playback

E selecionando a opo velo clicando em write, o um vdeo com todos os resultadosobtidos ser gerado, obtendo uma animao da simulao ao longo do tempo. Para obteno

do mesmo vdeo para presso esttica, o procedimento descrito deve ser repetido.

Flow Around a Cylinder

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