
[Sponsors] 
January 30, 2008, 07:42 
Calculation of Cd on axisymmetric body

#1 
Guest
Posts: n/a

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/tecrep/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 Kepsilon. Could any one please help me in this regard? Best wishes Mahbub


January 31, 2008, 18:37 
Re: Calculation of Cd on axisymmetric body

#2 
Guest
Posts: n/a

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 kw 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 ke or RSM or use transitional option of kw model. I strongly recomment you not to use ke model (both standard and even RNG) and standard kw model. If you want to have 3D simulations, using DES form of kw 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 

February 2, 2008, 03:52 
Re: Calculation of Cd on axisymmetric body

#3 
Guest
Posts: n/a

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, OutletPressure outlet, upper velocity inlet, wall, axis. Viscous model: kepsilon, RSM (Standard, wall function), Y+= 8, Pressure velocity coupling PISO, PressureStandard, 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*103) 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 

February 2, 2008, 11:00 
Re: Calculation of Cd on axisymmetric body

#4 
Guest
Posts: n/a

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 

February 4, 2008, 05:27 
Re: Calculation of Cd on axisymmetric body

#5 
Guest
Posts: n/a

Dear ROOZBEH,
Sorry to bother you again. I have used enhanced wall treatment in both Kepsilon 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+ byadapt>boundary>mark>adapt or adapt>Yplus>mark>adapt in FLUENT6.2. I also calculate surface area byreport>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 

February 4, 2008, 05:44 
Re: Calculation of Cd on axisymmetric body

#6 
Guest
Posts: n/a

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 

February 4, 2008, 06:19 
Re: Calculation of Cd on axisymmetric body

#7 
Guest
Posts: n/a

Dear ROOZBEH,
Of course, it's my pleasure! I am sending you the details through email. But, could you please tell me, what is the right way to set Yplus in fluent. I am waiting here to have your massage now. Best wishes, Mahbub 

February 4, 2008, 06:47 
Re: Calculation of Cd on axisymmetric body

#8 
Guest
Posts: n/a

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

February 6, 2008, 03:38 
Re: Calculation of Cd on axisymmetric body

#9 
Guest
Posts: n/a

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 2Daxisymmetric model and try to calculate Cd using kwSST 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 0.150.17). 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 

February 11, 2008, 08:45 
Re: Calculation of Cd on axisymmetric body

#10 
Guest
Posts: n/a

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 

February 12, 2008, 00:53 
Re: Calculation of Cd on axisymmetric body

#11 
Guest
Posts: n/a

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

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
The equivalent axisymmetric body concept  Yu Zhou  Main CFD Forum  0  January 24, 2009 00:01 
The equivalent axisymmetric body concept(hyperbolo  Yu Zhou  Main CFD Forum  0  January 23, 2009 23:54 
The equivalent axisymmetric body concept(hyperbol  Yu Zhou  Main CFD Forum  0  January 23, 2009 23:54 
Area of Axisymmetric Body  MAHBUB  FLUENT  0  January 15, 2008 04:42 
Warning 097  AB  CDadapco  6  November 15, 2004 05:41 