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.

So what I’m specifically asking:

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.

Thank you in advance,

Maximus