[nasir.iitd]

Hi everybody,
I already posted my problem earlier but did not get any
reply.
I’m working with RANS followed by LES. My complete domain is rotating. I've set the following BC.
Cell zone => Frame motion > amount of rotation in rad/s > specified rotational axis.
Velocity Inlet => Component>Absolute>cylindrical coordinate>Axial velocity in m/s>Radial and tangential vel. set to 0 m/s >angular vel. Set to 0 rad/s (as frame motion is set).(Snapshots for vel inlet and outlet attached).

Outlet => Pressure outlet>gauge Pr. set 0 >normal to boundary>radially equilibrium pr. distribution>back flow set to 0(as no back pr. in the domain).

Wall => stationary wall (as the frame is rotating by default the wall is also rotating).

Are my settings o.k.? With these settings I simulated the flow but KE decay profile, tangential vel. and radial vel. profiles are not good as expected. There is abrupt rise in KE profile along axial distance near the inlet.

In my case y+ distance is 6.5 i.e. first grid point from wall lying in buffer layer and Re No. is above 50,000. Can LES run give reliable result?
Any suggestion is appreciated.

[RodriguezFatz]

Originally Posted by nasir.iitd

In my case y+ distance is 6.5 i.e. first grid point from wall lying in buffer layer and Re No. is above 50,000. Can LES run give reliable result?
Any suggestion is appreciated.

As I learned from the books: no.
You should either have y+=1 or an y+ high enough to use wall modeled LES. For the first case ( y+=1 ), there are also quite strong restrictions for x+ and z+. If you don't meet all three of them, your results will probably be wrong.
Some approximate numbers:

• wall resolved LES: y+ <=2, z+<=15, x+<=50
• wall modeled LES: y+ about 30-150, z+ 100-300, x+ 100-600

Numbers differ between publications.

[nasir.iitd]

Dear RodriguezFatz,
Thanks a lot for the valuable information provided. My domain is cylindrical pipe. So y+ is radial, x+ is peripheral and z+ is axial as I have chosen z as axis of rotation and axial length. Is this idea correct? I have refined the mesh to keep y+=1.3 and x+ & z+ also changed accordingly as you informed. Can you please tell whether boundary condition and cell zone settings are correct or not for rotating pipe flow?
Awaiting for the reply eagerly.

[RodriguezFatz]

Using the numbers from above, you need "x" to be the direction of your flow! As I get your last post, you switched "z" and "x".
I have no idea about the boundary settings. But a flow takes like 100 diameters to develop inside a pipe. The mesh will be insanely large.. !?
Normally (without the swirl) people use like 5d long pipes with peridic boundary conditions. I don't know how to do this in a swirling flow...

[nasir.iitd]

Thanks a lot for your help.