hello,

I posted a topic 'plz help' on 10th August…. May be I was imprecise in the explanation of my CFD problem. I would like to give details more accurately.

Actually, I am doing benchmarking of an AIAA paper. The paper includes CFD analysis of a simple cone cylinder configuration body whose base dia =3.7 inch. They have performed the analysis with Mach 2.5 and Re=1.2 million. The results in the paper lists the drag coefficient, normal force coefficient etc. No other information of the flight conditions is provided.

To generate the same simulation, I have made a similar grid (which is size independent). I have used pressure far field B.C as used in paper. I have set the Mach no. value in Fluent but I cannot set the Reynolds no. since Fluent has no option for a user to enter the value…. In order to get the same coefficients over the body, I must match my (Reynolds & Mach) with that of AIAA paper….So, I have set a combination of (pressure and temperature that will give me same Reynolds=1.2 million & Mach2.5) in pressure far field BC.

Unfortunately, I didn't get the same results. Cd AIAA = 0.17 Cd mine = 0.09

My problem has nothing to do with wind tunnels' data. But AIAA paper has compared its results with wind tunnel's and they match.

I made a combination like this:

M = 2.5 = V /√(γRT) ……………eqn1

γ =1.4

R = 287 (SI)

L = 37inch Re = 1.2*106 = ρVL/μ ……….…eqn2

μo = 1.75*10-5 (sea-level) To = 273 K (absolute) μ = μo(T/To)1.7……………&hel lip;……eqn3

I supposed sea-level value for temperature 'T'=288.16K. Then from eqn 3, I get 'μ' …...also from eqn 1, I get 'V' Now, put 'μ' and V in eqn 2……we get 'ρ'. Since, P = ρRT, we get a pressure.

This pressure and temperature gives (M=2.5, Re 1.2 million).

Look the definition of CD, (Frition Force normalized by the free stream KE), then ask yourself, how the programme calculates the friction force, Let us assume for the moment that you are proficient in both the science of Fluid Mechanics and The computational techniques used to solve the Navier Stokes equations, then you ought to know about the law of the wall My only recommendation to you right now is to check the values of Y+ and compare it with those recommended for the wall treatment.

Can you check if you are using the correct factor to make drag dimensionless ? Its possible that your Cd is 'D/(rho S U^2)' instead of '2D/(rho S U^2)'? (If the AIAA paper has done it this way).

-Dominic

Are you comparing the correct drag forces? Drag is made up of two parts - pressure and friction. Is the drag coefficient you're comparing to include both or only one of the two? Beyond that you should make sure (as already posted) that your grid resolution at the solid boundary is sufficient.

y+ value along the major part of the body is 8. Maximum value is 106. I have used k-e realizable with standard wall function.

Cd of my simulation includes the contribution of frictional as well as pressure drag. Cd values in AIAA paper also include contribution from both drag forces.

It is quite likely that this is not a sea level condition. Did they give a "Q" value? Can you post the AIAA paper #? You may have to dig into the references to find the test data.

Sea leval Mach 2.5 gives a Q of ~ 9250psf very very very high even for a missle.

A typical design Q of 1000 occurs at around 51Kft, a Q of 2500 (perhaps a missle) occurs at around 32kft

Running at sea level may be your problem - Andy

To Andy

wat do u mean by Q?

wat does Q denote?

Q is a designation for the dynamic pressure.

that paper # is AIAA-2000-0391

Q (dynamic pressure) is not provided in the paper. i understand that the simulations are not carried out at sea-level values.

how shd i get the operating values?

Try contacting the authors?