Learning Forum

[cheng-wl]

I am running a simulation for internal flow to try matching the experimental data.  However, I am confused about the wall surface condition setting.  I try to set the roughness height using data from the fluid textbook.  That tends to give a mass flow rate too small compare to experiments.  I reduced it to zero and mass flow rate increased.

Now, it comes to adjusting wall roughness constant.  When I increased it, the mass flow rate decreases, so I reduced that from the default value of 0.5.  I can then matched my experimental data.  However, when I read from the manual, it says 0.5 is the value to be used for a surface with uniform grain.  Otherwise, the value should be 0.5 - 1.0.  So, can a value 0 - 0.5 be used as the roughness constant?  Does that physically mean anything?

[Federico]

There are several resources in our User and Theory Guides on Wall Roughness, for example: 4.18.5. Wall Roughness Effects in Turbulent Wall-Bounded Flows

Wall roughness is a method to model equivalent sand-grain roughness, which in real life is a property of the wall surface finish.