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 Maximus91 October 19, 2012 10:02

Turbulence model parameters and equations

Dear CFD users,

Right now I´m working on a CFD computation for external flow around a very streamlined car.
I have some questions regarding the turbulence parameters. I´ll first specify what formulas I used to calculate some parameters. FYI, I use FineOpen of Numeca. I will either use the SA or Mentor-SST k –omega (Extended wall) model.

Description of the stream: A free steam around a car driving in the open i.e. on the road. So it is an external flow. The free stream velocity is 25 m/s.

There are a couple parameters I have to specify in the CFD software program which im not complete sure about, being:

Boundary conditions:
for the k-omega model: k and
for the Spalart-Allmaras model: turbulent viscosity

Initial conditions:
for the k-omega model: k and
for the Spalart-Allmaras model: ratio (taken as 1)
So it comes down to calculating/guestimating these parameters. I used the following formulas (all in SI units) to calculate these parameters:

(From Numeca FineOpen manual)
(Also from FineOpen manual)
with being the dynamic (molecular) viscosity
with and
I took (Numeca specifies in the manual that a typical value for this ratio is 1 for external flows). So before I can calculate , I first calculate k.

Turbulent (eddy) viscosity (So before calculating this parameter, I first calculate k, then and then )

Using the above mentioned formulas, I was able to calculate all the needed parameters. Still I’m not sure if these formulas are correct or maybe are not valid in every case.

However, when I just assume these formulas are correct I calculated k, and . This yielded
k = 0.718, and . Do these values make sense in anyway, or are they completely out of range?

The actual reason I’m posting this question is to validate our computation. One year ago, a group performed the same computation for the exact same car. This yielded a drag force of about 40N. The problem we are having now is that the drag force is in the order of 10^4 Newton. Obviously this does not make sense. Right now the mesh I runned in FineOpen contains about 18 million cells with refinements near the car itself, behind the car, beneath the car etc. etc.

1) Can someone verify the method I use, especially the formulas I use
2) What may cause the problem of a way too high drag value?
3) Do the values of k and epsilon make sense?

I know these are a lot of questions, but any help is much appreciated.