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

Laminar-to-turbulent transition modelling using SST and SST Gamma Theta models

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

Like Tree2Likes
  • 2 Post By ghorrocks

Reply
 
LinkBack Thread Tools Display Modes
Old   June 8, 2014, 00:35
Default Laminar-to-turbulent transition modelling using SST and SST Gamma Theta models
  #1
New Member
 
Tom
Join Date: Oct 2013
Location: Auckland, New Zealand
Posts: 24
Rep Power: 5
ttu004 is on a distinguished road
Dear All

I am simulating a S814 aerofoil under Low Reynolds number. In wind tunnel experiment, the flow undergoes laminar-to-turbulent (LSB formation) via the separation-induced transition on both upper and lower surfaces.

For my CFX, I tried SST and SST Gamma theta models and revealed that only the SST Gamma Theta model has modeled the LSB on both upper and lower surfaces and is close to the experimental expectation. Comparison of the Turbulent Kinetic energy (TKE) and Velocity Vector plot are attached and has shown pronounced difference. The maximum y+ value for both simulations are approx. 0.55 on the leading edge tip. And the turbulence intensity were kept at approx. 0.07% around the foil.

For SST GammaTheta model, the TKE has started from near half of both surfaces and were matched with the location of LSB when superimposed with vector plot (location of LSB matched well with experiment). This indicates clearly an onset of transition in the boundary layer.
However, the TKE intended to start earlier when SST model was used.

My question are:
1. What is happening in the boundary layer for result from SST model?
2. Is this indicating that i am seeing a natural transition? (not separation-induced transition)
3. Can the SST model simulate cases with the laminar-to-turbulent transition flow (natural transition, separation-induced transition)

Very thankful for any thought and suggestion.

Cheers
Tom
Attached Images
File Type: jpg SSTGammaTheta4TKE.jpg (74.5 KB, 129 views)
File Type: jpg SST4TKE.jpg (54.7 KB, 117 views)
File Type: jpg SST4VelocityVector.jpg (96.3 KB, 106 views)
File Type: jpg SSTGammaTheta4VelocityVector.jpg (97.6 KB, 108 views)
ttu004 is offline   Reply With Quote

Old   June 10, 2014, 08:01
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,832
Rep Power: 100
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
Q1: I do not understand your question. With the fully turbulent simulation the turbulent boundary layer builds from the start of the airfoil, with the transition model it builds from the transition point.
Q2: The laminar sep bubble is a natural transition. The laminar flow separates from the foil and straight away goes turbulent. The turbulent flow reattaches to the foil and that's the laminar separation bubble, and your model is capturing this.
Q3: SST has no transition model. The gamma-theta model is the transition model. If you use SST it will be assumed turbulent everywhere and this will over estimate the flow dissipation before transition.

Your models are showing the expected results: SST+gamma theta captures a transition and a laminar separation bubble, SST alone builds up too much turbulence and has not laminar separation bubble (and is wrong).
chengyu and sainathaero like this.
ghorrocks is offline   Reply With Quote

Old   June 13, 2014, 02:00
Default
  #3
New Member
 
Tom
Join Date: Oct 2013
Location: Auckland, New Zealand
Posts: 24
Rep Power: 5
ttu004 is on a distinguished road
Thanks for the reply. Glenn

The SST is not designed for predicting onset transition but only the SST transitional (Gamma-Theta) model does. That is why I am seeing reduced in lift when SST transitional model is used when the LSBs are modelled (especially, when modelling at low Reynolds number when LSB is enlarged and elongated...).
ttu004 is offline   Reply With Quote

Old   June 13, 2014, 06:43
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,832
Rep Power: 100
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
That is correct.
ghorrocks is offline   Reply With Quote

Reply

Thread Tools
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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Use of k-epsilon and k-omega Models Jade M Main CFD Forum 18 December 1, 2016 08:44
Ansys 12, Fluent, SST Transition models bugra FLUENT 9 February 8, 2016 02:57
SST turbulence with Gamma theta kmgraju CFX 8 December 29, 2013 23:03
Ratio of eddy viscosity to molecular viscosity : Laminar or turbulent flow? JuPa CFX 7 September 9, 2013 07:45
Laminar to Turbulent transition information in Fluent 12.1?? aerobharath FLUENT 0 September 22, 2011 18:29


All times are GMT -4. The time now is 19:02.