BTP Final Report

8/3/2019 BTP Final Report

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Abstract

Wind tunnels are used for investigating new types of trains at high Reynolds

numbers. Such a wind tunnel is to be set up by CRR (Centre for Railway

Research), IIT Kharagpur. The purpose of this project is to model the proposedwind tunnel in Gambit, test it with high wind speeds in Fluent and calculate the

wall shear stress and total pressure drop across the tunnel length and check

their compliance with the theoretically calculated values. This total pressure drop

is required to design the fan used for providing the indraft.

1



INDEX

Contents Page no.

1. Introduction 3

2. Procedure 11

3. Results and Discussions 13

4. Scope for further work 31

5. References 32

2



Introduction

The science of Wind Engineering, which deals with measurements of actual wind flows to predict the

forces transferred to engineered structures and machines, relies heavily on wind tunnels. Wind tunnels

are used for a variety of reasons such as to test prototypes early in design cycles, or to record a large

amount of data, but the most important feature of wind tunnels is their ability to accurately recreatethe full complexity of fluid flow, employing the entire knowledge that man has gained on fluid

dynamics, with minimal effort by the user, which allows us to predict, within reasonable bounds, the

fluid behaviour in real world scenarios. They are indispensable in the testing phase of new designs in

locomotives, ships, aeroplanes, space vehicles, skyscrapers, and missiles.

Two main layouts considered for wind tunnels are closed loop and open. In a closed loop system, the

air is re-circulated; in an open system the air is used only once and released into the surroundings on

completing the cycle. Both layouts have their own pros and cons. The open system has a much lower

capital investment, but requires larger, more powerful fans. The closed loop system requires a larger

capital investment, but uses less powerful fans because the loop maintains the net circuit pressure,

hence having lesser operating cost. Also, it leads to superior flow quality in the test section and lesser

noise pollution. For these reasons, a closed loop wind tunnel is preferred in our case.

The main components of the proposed wind tunnel in order from intake to outlet are the axial fan, first

diffuser, U-shaped duct with corner vanes, settling chamber, contraction, test section, second diffuser,

and another U-shaped duct with corner vanes.

Axial Fan:

3



There are many different fans that can be used in the wind tunnel. Electric axial fans are preferred for

their low cost and efficiency in producing high wind velocities. The disadvantage of using axial fans

is they impart a small amount of tangential velocity that has to be rectified with counter-rotational

vanes and/or additional flow conditioners. The cost of the added flow conditioning for the axial fan is

much less than the cost of blowers, which produce a more uniform velocity distribution. Axial fanspeeds can be regulated either by adjusting the rotational speed or the pitch of the blades.

The Diffusers:

The diffuser is used to reduce the wind speed velocity while minimizing losses. Using a diffuser

before the settling chamber decreases the speed of the air flow for screens and flow straighteners, thus

minimizing the power losses through the wind tunnel as they are a function of the cube of flow

velocity.

Total Pressure loss Coefficient, K L=α1 – α2/ (A1/A2)2 – (p2 – p1)/ (0.5ρU12)

For the first diffuser a taper angle of 1.7 was chosen, as it would decrease wind velocity, while ̊

preventing boundary layer separation. The total loss in the diffuser section is the sum of the frictional

losses along its surface area and the expansion losses. Assuming constant material friction factor the

frictional loss can be calculated using the following equation:

Diffuser frictional loss coefficient, K f = (1 – 1/ (A1/A2)2) x (f/8sinθ)

K f = Diffuser Frictional Loss Coefficient

f = Diffuser Friction Factor

q = Diffuser Expansion Angle

The expansion loss equation is empirical and depends on the diffuser cross sectional area, and it

equivalent conical angle. The total pressure loss is the sum of the frictional and expansion loss

components.

Δpd = (K f + K e) x (0.5ρV2)

The Diffuser minimum length is given by:

Ld = R i ((A1/A2)2 - 1)/ (tan θ)

Ld = Diffuser Minimum LengthR i= Inlet Hydraulic Radius

The diffusers are kept octagonal in shape. It is done to reduce flow turbulence at the sharp corners by

reducing their angles as much as possible. An ideal cross-section would be circular, but due to

logistical limitations a regular octagon is selected for the cross sectional geometry.

4



First Diffuser

Second Diffuser

5



U-shaped regular octagonal ducts with corner vanes are located in between the first diffuser and

settling chamber, as well as between the second diffuser and the outlet. The objective of these ducts is

to turn the flow’s direction 180 degrees causing the least disturbance possible. Corner vanes are

provided for this purpose. They help guide the air along the 90 deg turn without preventing eddy

formation.

First Diffuser

Second U-Duct

6



Settling Chamber:7



In the settling chamber the flow velocities are straightened parallel to the centre axis and made

uniform. In this section the irregularities caused by fan (imparting lateral velocities to the flow) and

the minor inconsistencies from expansion are smoothed out. In this proposed design there is one

turbulence mesh screen and a honeycomb screen. The honeycomb is used to straighten the flow with

minimal losses. The mesh screens are used to create a uniform velocity profile, it does this because

the flow resistance of the wire is proportional to the speed squared, slowing the faster flow regions

more than the slower flow regions. By incorporating a contraction following the settling chamber

fewer number of mesh screens are required in the settling chamber to achieve flow uniformity. It is

suggested by Mehta and Bradshaw that to straighten the flow without impeding it there should be at a

minimum approximately 25,000 cells in the honeycomb, which for this wind tunnel correlates to a

hydraulic cell diameter of 0.1713 inches. For the screen pressure loss calculations a screen mesh

factor of 1.3 for average circular wire was assumed and used in

The function of the settling chamber is to produce a parallel, spatially uniform, steady stream of air. In

general, a honeycomb screen is used to straighten the flow and suppress lateral turbulence (caused by

the fan and expansion in the diffuser), while mesh screens are used to reduce spatial variations in the

axial velocity and reduce axial turbulence, thus creating a uniform velocity profile. That can be done because the flow resistance of the wire is proportional to the speed squared, slowing the faster flow

regions more than the slower flow regions. While screens do act to turn the flow normal to the plane

of the screen, they are not as effective a flow straightener as honeycomb, which is why the two are

used in combination. By incorporating a contraction following the settling chamber fewer number of

mesh screens are required in the settling chamber to achieve flow uniformity. In the calculation, only

one mesh screen with attributes:

Aperture, ψ = 0.52 mm, Wire Diameter, Dw = 0.31 mm, and Screen Open Area Fraction, β = 0.39.

The screen axial turbulence reduction factor fu, is defined as being the ratio between the downstream

and upstream root-mean-square ( RMS ) of the instantaneous velocity

---- (1)

f u depends on the overall pressure drop coefficient, K L, which is defined as:

---- (2)

---- (3)

---- (4)

---- (5)

K o is the pressure loss coefficient due to screen open area

ReW is Reynolds number based on wire diameter

β is fractional screen open area

8



ψ is aperture

DW is wire diameter

9



Contraction:

In addition to improving circuit efficiency by reducing losses in the settling chamber, a contraction is

used to improve flow uniformity and reduce the turbulence intensity in the working section. It

increases the incoming fluid velocity while minimizing pressure losses, flow separation, and flow

variability. A contraction ratio of 13 is employed and the profile is of a straight line.

Test Section:

The test section is a square duct with chamfered corners and houses the train model. The wind speed

in the inlet of the section is to be kept at 100 m/s.

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Procedure

The modelling was done in Gambit as per the dimensions mentioned in the schematic diagram. The

Cooper meshing scheme is used for volume meshing of the diffusers, contraction, and test section

using hex/wedge elements. For using the Cooper meshing scheme it is important that the non source

faces/side faces be mappable and the source faces/end faces are not previously meshed. The corner

components of the U-ducts did not follow the first condition and hence Tet/Hybrid meshing scheme

was used with t-grid type meshing for meshing the U-ducts. The Boundary layers were assigned

keeping in mind the minimum cell length adjacent to the wall as calculated earlier.

In Fluent, the viscous model has been chosen as realizable k-ε model with default model constants

and ‘near wall treatment’ being the standard wall functions.

The 2 transport equations for this model are:

For turbulent kinetic energy k,

And for dissipation ε,

The turbulence specification method is chosen as intensity and hydraulic diameter with turbulentintensity, defined as u’/u with u’ = (2k/3)1/2. For a fully developed pipe flow, we have Turbulent

Intensity, I = 0.16 (ReDH)-1/8 and Hydraulic Diameter, DH = 4Acs/P. The calculated values are arranged

in Table 1 in results and discussion.

Under residual monitors, 0.001 is added as the absolute convergence criteria for z-velocity, k and ε.

Residuals are like the average errors obtained in iteration and when they reduced beyond 0.001 the

solution was said to have converged.

Sometimes as the program is iterating, changes in the solution are too aggressive, and adversely affect

convergence. Reduced under-relaxation factors damp out changes in the solution as the iterations

progress, often leading to better overall convergence. Under the present circumstances, the solution

does not vary strongly, hence the under-relaxation factors are kept intact.

For the wall zone, the roughness height is given according to Table 1.2 and roughness constant as 0.5.

The simulation was started from the last component, the 2nd U-duct, with its outflow meeting the inlet

fan. The outflow was fixed at 0 Gauge pressure and the inlet was assigned a velocity-inlet Boundary

condition with velocity calculated from continuity equation as the velocity of air entering the test

section is 100 m/s.

From the solution, the pressure at the inlet of the U-duct was found out. The pressure will be same as

that of the outlet of the Diffuser preceding it. So to solve the diffuser, the pressure conditions wereassigned from the previous solution. A velocity was assumed for the inlet and on solving, the outlet

11



velocity was matched with the inlet velocity of the following component. It was a hit and trial

approach, and required repeatedly solving the mesh to finally acquire the desired outlet velocity

within reasonable error.

The other components were solved in a similar manner. The process involved two parts:

1. Use the outflow pressure condition to specify the pressure at the exit. This pressure was

obtained from solving the previous component.

2. Assign a velocity value at inlet and see if the velocity at outlet on solving matched the inlet

velocity of the previous component.

12



Results and Discussions

Calculating minimum cell length near wall boundaries:

Before modelling the turbulent air flow in the hexagonal duct, we need to ensure that the viscous sub-

layer and part of the transition layer of the flow are excluded from the analysis as the flow is notturbulent in those regions.

In the viscous sub layer, (valid for 0 < y+ < 5-7)

u+ = u/u* = yu*/ν = y+

where, u* = (τw /ρ)1/2 = friction velocity

u = mean turbulent velocity

y = normal length to the wall

y+ and u+ are non dimensional parameters

for y+> 30, we have the semi logarithmic equation,

u+ = 2.5ln(y+) + 5.0

2.5 = 1/k where k = von karman constant =0.4187

The region between y+ = 5-7 and y+= 30 is the buffer/transition region. According to the following

figure, we can see that the log law can be employed beyond the point of intersection of the two

extrapolations. According to the Fluent manual, the log law is employed beyond y+ = 11.225.

13



u+ = u/u* = 11.225 (yu*/ν)

For a given duct with length L, Hydraulic Diameter DH, with fluid density ρ, and fluid velocity U:

Static pressure drop, Δp = f x L/DH x 0.5ρU2

and Δp x Acs = τw x Asurface

so, for the test section, L=1500 cm , Dh = 147.217 cm, U = 100 m/s, ρ = 1.225 kg/m3, f=0.0085 (from

Table 2)

therefore, Δp = 530 Pa, and τw = 13 Pa

therefore, y = (νρ/11.225τw)u = 1.2 e-5 m = 1.2 e-2 mm

hence we should keep the length of the cell closest to the boundary layer at least 1.2 e-2 mm. Keeping

a safety factor of 10, we make the minimum cell length 1.2 e-1 mm. Hence we can bypass the

calculations for other components.

Hydraulic Diameter and Turbulent Intensity:

14



After modelling in Gambit, the perimeter and Cross sectional area is calculated. The Hydraulic

diameter being 4A/P is calculated and the velocity (area weighted average) at the various cross

sections is taken from Fluent. From these values the values for Reynolds number and turbulent

intensity are found, which are then used through iterations.

Sr.No.

Component Cross-sec.Area,Acs

(cm2)

Perimeter, P (cm)

Hyd.Dia.,DH

(cm)

VelocityV (m/s)

Re. no.(x109)

Turb.Intensity, TI =0.16(ReDH)

-1/8

(x100) (%)

1 FirstDiffuserinlet

81741 1056 314.4 120.38 2.59 1.065

FirstDiffuserOutlet

254558 1836.80 554.35

29.94 1.14 1.180

2 First U-Curve inlet

254558 1836.80 554.35

29.67 1.13 1.182

First U-Curveoutlet

254558 1836.80 554.35

33.29* 1.26 1.165

3 Contractioninlet

254558 1836.80 554.35

7.97 0.30 1.395

ContractionOutlet

19600 532.55 147.217

100.45 1.01 1.198

4 TestSectioninlet

19600 532.55 147.217

100.19 1.01 1.198

TestSectionoutlet

19600 532.55 147.217

101.43 1.01 1.198

5 SecondDiffuserinlet

19600 532.55 147.217

100.78 1.01 1.198

SecondDiffuseroutlet

81741 1056 314.4 25.44 0.55 1.293

6 Second U-Curve inlet

81741 1056 314.4 25.99 0.56 1.290

Second U-Curveoutlet

81741 1056 314.4 26.6 0.57 1.287

Table 1

From the above table we have the values of turbulent intensity and hydraulic diameter for all

boundary conditions in Fluent modelling.

*The meshed screen reduces spatial variations in the axial velocity and reduces axial turbulence, thus

creating a uniform velocity profile. In this process it considerably reduces the velocity magnitude.

15



The specifications of the considered meshed screen are:

Aperture, ψ = 0.52 mm, Wire Diameter, Dw = 0.31 mm

From equation 5, =0.39

From equation 4, k 0 = 2.88

From equation 3, k L = 3.2From equation 2, with n=1, f u = 0.23

Therefore, with output velocity being 7.97 m/s, the input velocity is found to be 7.97/0.23 = 34.35

m/s.

The honeycomb screen acts as a flow straightener, so it straightens the flow but doesn’t alter the

velocity magnitude.

16



Roughness ratio and Friction factor:

For a turbulent flow, friction factor, f = f (Re, ε/DH)

ε = relative roughness of inner duct wall

ε/DH = roughness ratio

for ordinary concrete, ε = 0.3-1 mm

As the wind tunnel is made of polished concrete, we take the lower limit, ε = 0.3 mm

Using the Moody Diagram, we can calculate the friction factor at the flow boundaries of each

component. For initial calculation of velocity, the default value of friction factor is assumed. Once the

factors are calculated, the iterations are again done using the values.

Sr.

No.

Component Hyd.

Dia.,DH

(cm)

Roughness

ratio,ε/DH (x 10-5)

Re. no.

(x10

9

)

Friction factor

1 First Diffuser 314.4 9.54 2.59 0.01152 First U-Curve 554.35 5.41 1.13 0.01083 Contraction 554.35 5.41 0.30 0.01084 Test Section 147.21

70.20 1.01 0.0085

5 SecondDiffuser

147.217

0.20 1.01 0.0085

6 Second U-Curve

314.4 9.54 0.56 0.0115

Table 2

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18



First U-Duct:

20



21



Contraction:

22



23



24



T est Section:

25



26



Second Diffuser:

27



28



Second U-Duct:

29



30



Calculations of pressure drops across the individual components:

Sr.No.

Components

Pstatic

(inlet)(MPa)

Pstatic

(outlet)(Pa)

ΔPstatic

(Pa)Pdynami

c

(inlet) (Pa)

Pdynamic

(outlet) (Pa)

ΔPdyn

(Pa)ΔPtotal

(Pa)WallShearStress(Pa)

1 FirstDiffuser

-262.1

7087 -7349 8884.6

1094.86

7789.7

440.7 4.04

2 First U-Curve

4838.9

3951.9

887 629 999.6 -370.6

516.4 1.26

3 Contraction

3951 -4827 8778 51.8 8562.8 -8511

267 0.43

4 Test

Section

-

4827

-5401 574 7960 7986 -26 548 12.9

5 SecondDiffuser

-5407

560 -5467 6697 531 6167 700 1.69

6 Second U-Curve

560.5

0.0 560.5 515.4 666.7 -151.3

409.2 0.79

-2016.5

4898 2881.3

21.11

Table 3* All pressure values are Gauge Pressures** All pressure values are Area Weighted Averages

The total pressure drop across the tunnel is found out to be 2881 Pa and the Wall Shear Stress is 21.11

Pa. These specifications can be now used to make the inlet fan.

31



Scope for improvements

1. The contractor surface is kept straight in the present design, but a 5th order polynomial curve

works best and minimises exit flow irregularities as well as boundary layer separation and

energy loss. Such a model can be modelled in Gambit.

2. The present calculations have been done only for an empty tunnel. The calculations for atunnel with the train model in the test section haven’t been done.

3. For high wind speed testing, it is also prudent to attach sensors in the wind tunnel that

measure the temperature of the body in the tunnel.

32



References

1. Harold Sherwood Boudreau: “design, construction, and testing of an open

atmospheric boundary layer wind tunnel” – University of Florida dissertation

2. G. Johl, M. Passmore and P. Render: “Design methodology and performance of

an indraft wind tunnel” – The Aeronautics Journal, Sep 24, pg 465-473

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