CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > General Forums > Main CFD Forum

Modelled Length Scale in SAS-SST Simulation

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   January 16, 2016, 06:45
Default Modelled Length Scale in SAS-SST Simulation
  #1
New Member
 
Join Date: Oct 2013
Posts: 1
Rep Power: 0
jego is on a distinguished road
Hello,
I have some issues understanding the breakup of turbulent structures into
smaller scales using the SAS-SST Model. I know, that the model uses the
von Karman length scale as an indicator for the local vortex size. So, at the beginning of the Simulation the modelled length scale is much larger than the von Karman length (in unsteady situations). Thus, the QSAS term is activated, the Eddy Viscosity is reduced which leads to a reduced damping effect and the turbulent structures are allowed to break up until the grid limit is reached.
So far so good, but what happens exactly with the modelled scale L, provided by the RANS model. In my understanding while running the simulation, the modelled length scale is also getting smaller when the Eddy Visc. is reduced by the QSAS term. This means, the modelled scale would be reduced until it matches the von karman scale (?).
So, in situations when the local vortex size is significantly larger than the grid resolution, a remaining Eddy Viscosity would exist according to
the local length scale. This leads to my assumption, that higher length scales (Lvk) result in higher remaining eddy viscosity (L/Lvk smaller).
My problem is, that in this situation the flow structures would be well resolved by the grid, so the damping of the eddyy visc. should
be very small. But as I described the progress of turbulent breakup using
the SAS model, the contrary is the case.
Could someone please explain the behaviour of the modelled length L in relation to Lvk in the progress of the Simulation and describe the reduction of the eddy visc.? And does the modelled length scale provided by the RANS model (L=sqrt(k)/(c omega)) match the
actual vortex size after the eddy visc. was reduced by the QSAS-term?


Many thanks in advance four your help
jego is offline   Reply With Quote

Reply

Tags
length scale, sas, sas-sst, scale adaptive, scale resolving

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

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 Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Length scale in k-omega models? MaryBau Main CFD Forum 1 June 1, 2014 11:41
Interface treatment in turbulent model and its length scale. paka OpenFOAM Running, Solving & CFD 0 February 15, 2014 02:02
Turbulent Boundary condition, viscosity ratio and length scale Bollonga FLUENT 80 February 16, 2013 12:06
length scale option rashmi CFX 2 July 24, 2005 23:39
mesh size and length scale Min-Hua Wang Main CFD Forum 2 November 15, 2001 08:37


All times are GMT -4. The time now is 22:19.