[roi247]

Hello everybody!

I'm running LES to simulate a turbulent flow in a rectangular channel. Basically, there is a under estimation (prediction) of velocity magnitude close to the wall compared with some experimental data using Preston tube. Even after a relatively long distance, the velocity is still not stable(The velocity contour keeps changing after a long distance).


MESH
Mesh were just made to guarantee yplus<=1 at the first cell. But I can only make it at the 3/4 distance from the inlet. At the inlet, the yplus is pretty large because the shear velocity and wall stress are large.

FLUENT SETUP
Transient unsteady-2nd-order-bounded
large-eddy-simulation les-subgrid-smagorinsky
materials/fluid water-liquid
Boundary conditions
top surface specified shear stress 0 0 0
velocity-inlet inlet 0.495
Bottom Wall and side walls no-slip boundary condition
Solution methods scheme(simple) pressure(second order) Momentum (Bounded central difference) transient formulation(bounded second order implicit)
Pressure outlet
solve/monitors/residual/convergence-criteria 1e-06 1e-06 1e-06 1e-06
solve/set/time-step 0.003
solve/dual-time-iterate 1500 120

RESULTS
I have the data from an experiment. The comparison of the wall shear is always under estimation. The best one is with error around 7% at the wall shear of 3/4 distance from the inlet. And it keeps decreasing along the channel shown in the figure below.
This is the along channel velocity contour

In the other plot of velocity change along the channel bottom, it indicates the velocity close the wall keeps decreasing.

Question:
1. Why I cannot choose roughness in the LES? So it doesn't matter what the material of the wall is?


(20160309) My case is end up steady(doesn't change with time step), and I'm increasing the length of the channel. Hope the velocity profile would be end up horizontal(fully-developed)

(20160310) The velocity and wall shear near the wall keeps decreasing. I mean the mean velocity at the cross section is stable, but the velocity close to the wall is decreasing. It becomes more serious when I decrease the cell length and increase the channel length. So maybe pressure outlet is not good for this case.


Rui