# Modeling Roughness on a channel

Member

Does anyone know of a way to model roughness through the walls of a channel for both laminar and turbulent flow in ansys fluent?

• GermanyForum Coordinator
edited January 2019
This is pure research but generally laminar flows are not influenced by roughness as the streamlines will cover the roughness being stratified. Consequence is the friction factor is independent of roughness. For turbulent flows sticks to omega based models in Fluent and use standard wall resolution as if the wall is smooth.
• GermanyForum Coordinator
edited January 2019
Sans grain height or something equivalent is tout roughness height.
• United KingdomMember
edited January 2019

Not sure if it is suitable:

• Member
edited January 2019

Is this method equitable to roughness heights by using an Ra value?

• GermanyForum Coordinator
edited January 2019
Check paper of Adams for that to relate measured values to sand grain roughness. "A simple algorithm to relate measured
surface roughness to equivalent sand-grain roughness"
• GermanyForum Coordinator
edited January 2019
It should increase
• GermanyForum Coordinator
edited February 2019

As ANSYS-Stuff we can only comment on attached pictures.

• GermanyForum Coordinator
edited February 2019

• Member
edited February 2019

The problem I am facing, is that when I add a roughness factor on the walls, the pressure differential is decreasing rather than increase. The center of the model is an air fluid region with a mass flow rate inlet and pressure outlet. The fluid is surrounded by aluminum walls with a constant heat flux on the top wall and insulated on the left and right wall as well as the bottom. I am not sure why this is the case? I was questioning my mesh being too course and possibly giving invalid results. Or a possible error in the settings:

·      Turbulent: Viscous-Standard K-Epsilon approximation.

·      To model forced convection, the density was set to as a model approximation to assume incompressible ideal-gas

·      Operating pressure: 98,338.2 Pascal (14.2 PSI)

·      Gauge Pressure at outlet: -1112.3 Pascal (-.16 PSI)

• GermanyForum Coordinator
edited February 2019

Which near wall method are you using? Do you exp. data or any correlations?

• Member
edited February 2019

I was using the scalable near wall function.  I am trying to simulate this and will be experimentally be verifying the results. I just don't understand why the wall height i place isn't following the predicted trend for pressure differentials

• Member
edited February 2019

can you tell me what settings were used for this turbulent model? For some reason, my pressure differential is decreasing as the roughness is increasing. I see the K-Epsilon model, realizable model was used, but is there something else I should be aware of?

• GermanyForum Coordinator
edited February 2019
I do not understand your density setting. Regarding viscous model and near wall modelling I recommend using sst model.
• GermanyForum Coordinator
edited March 2019

1/It is up to you to choose the most suitable meshing method. Just fulfill the mesh metric quality requirements: Aspect Ration, Cell Volume Change, Skewness and Ortho Quality.

2/Create an iso-surface based on Mesh>X-Coordinate

• Member
edited June 1

Hi, can you provide reference for this statement please? Thanks.