# Roughness length, interpolation, unphysical dissipation rate on the ground

 Register Blogs Members List Search Today's Posts Mark Forums Read

 February 14, 2011, 05:37 Roughness length, interpolation, unphysical dissipation rate on the ground #1 New Member   Giorgos Join Date: Feb 2011 Posts: 1 Rep Power: 0 Hello to everybody, I am trying to model a very simple 3-D problem (an empty wind tunnel) with hexahedral cells and I have to apply an experimental wall roughness on the ground equal to 1.53m (full scale). I have used inlet and outlet boundary conditions, symmetry for the lateral planes, inlet at the top boundary (with constant u, k, epsilon), fully rough wall on the ground. Standard k-epsilon High Reynolds turbulence model used with constant profiles for u, k and epsilon derived from the experiment. In star-cd when standard wall functions used for fully rough wall I have two options according to my knowledge: the equivalent sand grain roughness and the roughness length, and it depends on the format of the equation u=u*/κ ln(z/z0) + A. If A=0 then z0 is the roughness length. Also the centre of the first cell near the wall has been selected about 4 times the z0 an extremely high value but it is consistent with the manual. When the steady state simulation finished, I noticed that near the outlet of the domain and inside the first cell near the ground an unphysical behaviour for epsilon using sensors at three positions. One on the ground z=0 where epsilon was zero (?) while it should be infinite, at the centre of the cell (z=dz/2) epsilon had a low value and at the edge of this cell (z=dz) the dissipation value was exactly the same with what I gave at the inlet i.e. the correct one. Do you think that this is normal to happen? Is it a boundary condition problem or an interpolation problem or a user problem? Are there any alternative ways to insert the roughess length e.g. using other near-wall treatment? Thank you in advanced Kind regards Giorgos

 Tags boundary condition, interpolation, roughness length

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post rsin CFX 13 September 3, 2015 12:20 romance CFX 4 October 26, 2009 14:41 Shaohua Wu Main CFD Forum 0 September 16, 2008 23:11 K. Kevala FLUENT 0 July 14, 2004 11:11

All times are GMT -4. The time now is 08:57.