[amirfard]

I am going to test the LES turbulent channel flow of OpenFOAM.

In the original test case (OpenFOAM 10) fvConstraints is used. Then I want to retest the problem by just setting the semiImplicitSource for momentum equation (i.e. a body force) in "fvModels". I set the body force according to g= u_\tau^2 * density / H. Here, H is the half channel height. And u_\tau is obtained from the Reynolds number definition for Re=395 as Re_\tau = u_\tau.H/(\nu).

Interestingly, the velocity profile that I obtain is very different from the case of using fvConstaints.

Can anybody explain why and how to set the pressure gradient (or body force) in a turbulent channel flow?

[amirfard]

Originally Posted by amirfard

I am going to test the LES turbulent channel flow of OpenFOAM.

In the original test case (OpenFOAM 10) fvConstraints is used. Then I want to retest the problem by just setting the semiImplicitSource for momentum equation (i.e. a body force) in "fvModels". I set the body force according to g= u_\tau^2 * density / H. Here, H is the half channel height. And u_\tau is obtained from the Reynolds number definition for Re=395 as Re_\tau = u_\tau.H/(\nu).

Interestingly, the velocity profile that I obtain is very different from the case of using fvConstaints.

Can anybody explain why and how to set the pressure gradient (or body force) in a turbulent channel flow?

Solved! In fvConstraints the pressure gradient is automatically adjusted by OpenFOAM to reach the user-defined bulk velocity. If we are not using DNS, this pressure drop is not the same as the theoretical one given in the formula above.
Therefore, if we want to do the test case with fvModels, the user-defined pressure gradient should be the same as what is reported by OF in fvConstraints output log.
In this case, both fvModels and fvConstraints lead to the same u_\tau values.