# Some questions on Boussinesq and AdvectionDiffusionBGK Dynamics

Hi,
I’m working on BoussinesqThermalProcessor3D and have studied the BoussineqThermal3D case.
I have some doubts.

1. For the parameter “gravity” for BoussinesqThermalProcessor3D, the example gave RayleighBenardFlowParam.getLatticeGravity(). But RayleighBenardFlowParam.getLatticeGravity()= getDeltaT() * getDeltaT() / getDeltaX(), it seems that it is just the non-dimension parameter for the gravity. So, for the real gravity, should I use 9.8 * RayleighBenardFlowParam.getLatticeGravity() instead?

2. For the parameter “dir” for BoussinesqThermalProcessor3D, in my opinion, it represents the direction vector for the gravity constant, for the real three dimensional should it be (0,0,1) or (0,0,-1)? I saw the example employed (0,0,1). And when I tried (0,0,-1), it did not work.

3. We know that Boussinesq approximation is F= - beta * g * (T-T0), where beta is the coefficient of thermal expansion and usually beta=1/T0 [K]. But in the source code, I found F= gravity * dir / deltaTemp * (T-T0), where deltaTemp=HighTemp-LowTemp. Why do we use deltaTemp=HighTemp-LowTemp here, while not beta=1/T0?

4. For AdvectionDiffusionBGKDynamics, I think its omega is obtained from kappa= nu/Pr. But when I’m using LES for the flow field, we also have kappa_sgs=nu_sgs/Pr_sgs. Is there any AdvectionDiffusion Dynamics that can read omega_total from (kappa + kappa_sgs), just like the omega_total in the flow field?

with best wishes,
steed188