effekt of surface roughness very small

Ralf Schmidt

Hi!

We did a simulation of heat transfer from a submerged body into a gas flow (2D, air, inkompr., Re = 10,000 based on body dimension, Tu of the upstream flow = 10%) with differnet surface roughness values and turbulence moddels.

BUT: There is allmost no effect of the wall roughness hight on the Heat transfer. Even a value of 50 µm is resulting in a max. 1% decreased Nu number compared to a smooth surface (k-w sst with yplus between 1 and 5 an also k-e, std wall function yplus 30-80),

What is the reason?? And even if the magnitude of the changes is ok - i would wonder about the direktion. I would expect the heat transfer to be higher for a rough wall!

Any idea??

Ralf