# Specularity coefficient

**2**

Hello,

I am currently simulating a fluidized bed reactor. The flow field is laminar. I am using a Eulerian Eulerian model with algebraic granular temperature formulation, Gidaspow drag model and the typical models for bulk, granular viscosity ecc. As for the boundary conditions at the wall I imposed a specularity coefficient of 0.6.

Now, when I analyze simulation's results I can see that even if I am actually imposing a partial slip boundary condition at the wall, the axial velocity of the solid particles is always equal to zero. Even if I change the values of the specularity coefficient, nothing seems to change. The only way to have an actual slip along the walls is to impose a zero shear stress at the wall.

Could you help me understand why velocity is always equal to zero? Is it wrong?

Thank you

## Comments

200Forum CoordinatorHello,

Specularity coefficient is a measure of the amount of tangential momentum that is imparted by the particles to the walls. So theoretically, a value of 0 means full slip and a value of 1 means no slip conditions.

The specularity coefficient will also have an effect on the volume fraction of particles near the walls.

Since this is a fluidized bed simulation, I am hoping that you are comparing time averaged results.

Can you check if changing the specularity coefficient changes time averaged volume fraction and velocity distributions?

You can use 3 different specularity coefficients to test : 0.006, 0.06 and 0.6 , respectively to get an idea.

Also make sure to have a good ball park estimate of the particle friction factor.

Regards,

Surya

3MemberCurrrenly, I don't have any avaraged result yet. The issue however remains. If I put specularity = 0, I should get a certain velocity along the wall, different from zero. This doesn't happen.