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Calculation of Cd on axisymmetric body

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Old   January 30, 2008, 07:42
Default Calculation of Cd on axisymmetric body
  #1
Mahbub
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Dear Sir, I am trying to investigate the Drag coefficient on the axisymmetric body (pod) using FLUENT and compare my result with that of the technical report' http://www.dt.navy.mil/hyd/tec-rep/v...ts/viscous.pdf It says that the experimental value of Cd = 0.003208 for L/D = 4 and Re= 2*10^7. But I found Cd = 0.0038 using RSM and K-epsilon. Could any one please help me in this regard? Best wishes Mahbub

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Old   January 31, 2008, 18:37
Default Re: Calculation of Cd on axisymmetric body
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ROOZBEH
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Dear Mahbub;

Your problem may be related to the numerical grid. To predict an exact viscous force, you should be sure that the grid is fine enough near the wall. It is related to the kind of wall treatment you used in the turbulence model. If you have used the wall function approach, the Y+ should be in the range of 30 to 300. But if you have used the enhanced wall treatment or if you have used the transitional option of k-w model, the Y+ should has values below 5. Your problem also may be related to the prediction of the separation in the rear part of the body. Note that if you have flow separation as a result of the positive pressure gradient, you have to use the enhanced wall treatent in k-e or RSM or use transitional option of k-w model. I strongly recomment you not to use k-e model (both standard and even RNG) and standard k-w model. If you want to have 3D simulations, using DES form of k-w model will lead to a very exact result about the prediction of separation. This approach is a hybrid RANS/LES approach.

with the best regards for your graduate studies, Roozbeh
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Old   February 2, 2008, 03:52
Default Re: Calculation of Cd on axisymmetric body
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Mahbub
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Dear ROOZBEH,

Thank you very much for your great suggestion. I am now trying according to your suggestion. For your kind information, I would like to give the details: Re = 2*10^7, .,L/D=4, Surface Area = 7.45. Density =1000, viscosity =0.001, velocity=6.0096. B.C in 2d simulation: Inlet- velocity Inlet, Outlet-Pressure outlet, upper- velocity inlet, wall, axis. Viscous model: k-epsilon, RSM (Standard, wall function), Y+= 8, Pressure velocity coupling -PISO, Pressure-Standard, Momentum= 2nd order etc.

Using these setting I found pressure coff. Cvp=1.12*10^-3(actual 7*10^-4), Viscous coff.=2.75*10^-3 (actual 2.4*10^-3) and Cd= 3.39*10^-3 (Exp. result=3.208*10-3) So in my result Pressure coefficient should be decrease to reach the exact result. Could you please give me any specific suggestion? I hope your kind assistance. Best wishes,

Mahbub

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Old   February 2, 2008, 11:00
Default Re: Calculation of Cd on axisymmetric body
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ROOZBEH
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Dear Mahbub;

If you have separation in the flow field, using wall function is a big mistake. You should use a grid resulting the Y+ in order of 1 and also apply the enhanced wall treatment.

best regards, ROOZBEH
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Old   February 4, 2008, 05:27
Default Re: Calculation of Cd on axisymmetric body
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Mahbub
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Dear ROOZBEH,

Sorry to bother you again. I have used enhanced wall treatment in both K-epsilon and RSM and refine the near wall mesh and achieve Y+ = 1.5 (maximum) according to your suggestion. But, my result diverse from the experimental one (Cd=0.003028). I have found Cd= 0.0055 (pressure co.=0.00188 + viscous co. = 0.00364) by using RSM.

I reduce Y+ by---adapt>boundary>mark>adapt or adapt>Y-plus>mark>adapt in FLUENT6.2. I also calculate surface area by---report>surface integral>area>pod>compute and write it as reference area.

I am in great trouble with this work for more than two months. What is my problem? Furthermore, I have calculated Cd on the sphere using the same axisymmetric setting, but this also gives the wrong result. I have found Cd= 0.041(Cvp=0.018 + Cf=0.023) at Re=6*10^5, but some paper says the result is 0.1. Have you any more suggestion for me? Thanks again for your kind assistance.

Mahbub

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Old   February 4, 2008, 05:44
Default Re: Calculation of Cd on axisymmetric body
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ROOZBEH
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Dear Mahbub;

If you want, you can send me your exact geometry and experimental observations. I may have some times to simulate your case. Could you please tell me that what's your name and where are you from?

regards, ROOZBEH

email: mousavi.roozbeh@gmail.com
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Old   February 4, 2008, 06:19
Default Re: Calculation of Cd on axisymmetric body
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Mahbub
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Dear ROOZBEH,

Of course, it's my pleasure! I am sending you the details through e-mail. But, could you please tell me, what is the right way to set Y-plus in fluent. I am waiting here to have your massage now. Best wishes,

Mahbub
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Old   February 4, 2008, 06:47
Default Re: Calculation of Cd on axisymmetric body
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ROOZBEH
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hi;

You can generate a numerical grid that leads to obtain suitable Y+. It is a trial and error task.

Roozbeh
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Old   February 6, 2008, 03:38
Default Re: Calculation of Cd on axisymmetric body
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Mahbub
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Dear ROOZBEH,

I sent you three files of my work, did you get it? Now, the solution of my problem depends on you, I have tried all of the ways as my knowledge. My problem may be setting the reference area. For validitation, I have simulated the incompressible flow over a sphere in 2D-axisymmetric model and try to calculate Cd using k-w-SST model enabling transition flow for Re=2*10^6. I consider radius=0.5. In the setting report>reference value>area----- If I put area=3.14(surface area=4*pi*r^2) then I found Cd=0.05.(But some paper says it is 0.15-0.17). On the other hand, If I put area=0.785(area of circle=pi*r^2) then Cd=0.174(which is similar to that). So, may I put the area=0.785 as reference area? But, If I compute area using fluent: report>surface integral>area compute----- then It shows area=3.14. So, I am in dilemma--- Have you any suggestion in this regard? Best wishes,

Mahbub

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Old   February 11, 2008, 08:45
Default Re: Calculation of Cd on axisymmetric body
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Jim
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Hi Mahbub,

May I ask you where did you find a CAD model of the geometry you're studying? Is that your own model or is it an iges file from a database?

Thank you,

Jim
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Old   February 12, 2008, 00:53
Default Re: Calculation of Cd on axisymmetric body
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Mahbub
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Dear Jim,

I found the vertices from a thesis paper by Apurva Gupta, the university of Texas at Austin. Thank you.

Mahbub
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