CFD Online Logo CFD Online URL
Home > Forums > Software User Forums > ANSYS > CFX

SST turbulence model question in ANSYS CFX

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

LinkBack Thread Tools Search this Thread Display Modes
Old   January 9, 2015, 07:59
Default SST turbulence model question in ANSYS CFX
New Member
Siddharth Kulkarni
Join Date: Oct 2010
Location: Birmingham, UK
Posts: 6
Rep Power: 14
drsidd10 is on a distinguished road
Hello everyone

I have simulated a tidal turbine blade geometry using SST turbulence model for steady state analysis.

I was wondering what is the difference in results between SST steady state analysis and SST transient for the same geometry?

Kind regards

drsidd10 is offline   Reply With Quote

Old   January 9, 2015, 09:35
Senior Member
JuPa's Avatar
Join Date: Jun 2012
Location: Britain
Posts: 361
Rep Power: 13
JuPa is on a distinguished road
My understanding is:

There isn't really a difference in turbulence models between steady state and transient. There is a difference in the conservation equations solved due to the addition of the transient terms in the continuity, momentum and energy equations.

In transient runs steady RANS equations become unsteady URANS equations. The SST model is applied just the same to RANS as is URANS.

Edit: I've just re-read the SST turbulence equations. It turns out I'm slightly wrong. The SST turbulence model does contain unsteady terms. Namely
d(rho k) / dt and d(rho omega) / dt
Where rho is density, k is turbulence kinetic energy, omega is turbulence dissipation rate and t is time.

In steady state these partial derivatives go to zero. The rest of what I wrote sounds correct.
JuPa is offline   Reply With Quote

Old   January 18, 2015, 05:38
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,362
Rep Power: 139
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
As Mr CFD says the only real modification to the equations is the addition of the normal transient term.

But an important distinction is implicitly changed - RANS assumes that there is a turbulent flow component which averages out over time to zero, and a mean flow component. In steady state this separation of turbulent and mean flow components is simple. But in transient flows the distinction is not straight forward. You have to assume that the simulated, resolved flow represents the mean flow and that the turbulent component is on a much faster time scale so it can still average out over time to zero. This results in some issues for transient flows:
1) If the time step size in the simulation is in the turbulent time scales then you cannot make a clear distinction between mean flow and turbulent time scales and the RANS assumption becomes less appropriate; and
2) Some flows do not have clear distinctions between flow and turbulent time scales. Vortex shedding is an example - is the vortex street turbulent or mean flow? There are many other examples. When there is no clear distinction then you should consider LES and related models.
ghorrocks is offline   Reply With Quote


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
Wrong calculation of nut in the kOmegaSST turbulence model FelixL OpenFOAM Bugs 27 March 27, 2012 09:02
Low Reynolds k-epsilon model YJZ ANSYS 1 August 20, 2010 13:57
Fluent 12 k-w SST turbulence model DarrenC FLUENT 0 December 13, 2009 08:33
Turbulence model for CFX moving mesh songxguan CFX 7 June 28, 2009 21:05
Urgent: How to model a stationary sphere in a pressure driven flow using Ansys CFX? farhan OpenFOAM Running, Solving & CFD 1 April 14, 2009 14:34

All times are GMT -4. The time now is 22:20.