CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

Low-Re 2D Airfoil Model

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   March 6, 2022, 02:46
Default Low-Re 2D Airfoil Model
  #1
New Member
 
Join Date: Feb 2022
Posts: 4
Rep Power: 3
kael is on a distinguished road
Hi!
I'm doing the analysis of 2D airfoil in low-Re regime (around 10^5 - 10^6) which the transitional B.L phenomenon is expected. (e.g., laminar separation bubble). I've done some literature study based on this problem. For turbulence model, Transition SST (gamma-Re, theta) seems give better prediction to the aerodynamics forces and the flow physics compared to other turbulence models.

I tried to model the problems in Fluent and compare the results (Cl, Cd, and Cp distribution along chord airfoil) with the exp data. At low aoa, the results match accurately with the exp data but at higher aoa the drag values are way-off.

Based on this, I have some question regarding the correct modelling in Fluent:

1. Steady or Transient? (I got converged solution for the low aoa with steady state solver but at higher aoa the convergence is hard to get, and Cdís discrepancy got lot bigger. My doubt is if there's some unsteady behavior (e.g., separation B.L) is it better to use transient solver?

2. Incompressible or Compressible? (I know for this low re, it can be assumed that the flow is incompressible. My question is should I stick with constant properties or set the ideal-gas properties and let the solver develop itself?

3. Decay of Turbulent Intensity (Tu). Fluent Theory Guide for Transition SST turbulence model give an explanation about the decaying Tu from inlet to the body with respect to streamwise distance downstream of the inlet. Basically saying, I need to set the Tu at inlet to corresponding value to get the actual Tu (exp Wind Tunnel Tu) at the body?

Best Regards,
kael is offline   Reply With Quote

Old   March 7, 2022, 09:05
Default
  #2
Senior Member
 
Lorenzo Galieti
Join Date: Mar 2018
Posts: 342
Rep Power: 11
LoGaL is on a distinguished road
To answer point 1 and 2

1) if the Cl and Cd do not stabilize, you canít say the prediction is way off, it just didnít stabilize. Switch to transient and get time average Cd, Cl

2) low re doesnít mean incompressible. Low Mach means incompressible, and indeed you can have high Re, low Mach scenarios or viceversa. I would calculate your far field Mach number and see if you get something on the order of 0.1-0.2, if yes, then you can stick with incompressible

3 is not clear
LoGaL is offline   Reply With Quote

Old   March 7, 2022, 11:19
Default
  #3
New Member
 
Join Date: Feb 2022
Posts: 4
Rep Power: 3
kael is on a distinguished road
Quote:
Originally Posted by LoGaL View Post
To answer point 1 and 2

1) if the Cl and Cd do not stabilize, you canít say the prediction is way off, it just didnít stabilize. Switch to transient and get time average Cd, Cl

2) low re doesnít mean incompressible. Low Mach means incompressible, and indeed you can have high Re, low Mach scenarios or viceversa. I would calculate your far field Mach number and see if you get something on the order of 0.1-0.2, if yes, then you can stick with incompressible

3 is not clear
Thank you for your responses LoGaL.

1. Cl and Cd values are stabilized, i set the convergence condition for these two parameters so the changes after 10 iterations get less than 0.0001.

2. It's low Mach flow, the freestream flow speed is 1.5m/s, equivalent to Mach 0.03.
kael is offline   Reply With Quote

Old   March 7, 2022, 14:26
Default
  #4
Senior Member
 
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,145
Rep Power: 61
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
3. You need to provide inlet boundary conditions for all transport equations. You are using transitional SST, which means you need to provide profiles of k & omega/epsilon at the inlet just like you need to provide a velocity profile. Turbulence intensity is the term most people use for the sake of discussion, but technically it's k and omega that you need.


You need the k and omega profiles from the wind tunnel data. 99% of the time, that data was never measured. So people just tweak the value of the turbulence intensity at the inlet until their sought-after-parameter matches what was measured.
LuckyTran is offline   Reply With Quote

Old   March 11, 2022, 12:19
Default
  #5
New Member
 
Join Date: Feb 2022
Posts: 4
Rep Power: 3
kael is on a distinguished road
Quote:
Originally Posted by LuckyTran View Post
3. You need to provide inlet boundary conditions for all transport equations. You are using transitional SST, which means you need to provide profiles of k & omega/epsilon at the inlet just like you need to provide a velocity profile. Turbulence intensity is the term most people use for the sake of discussion, but technically it's k and omega that you need.


You need the k and omega profiles from the wind tunnel data. 99% of the time, that data was never measured. So people just tweak the value of the turbulence intensity at the inlet until their sought-after-parameter matches what was measured.
So what do you mean is i should better specify corresponding values of k and omega at inlet rather than specified Tu and turbulent viscosity ratio? i assumed these values will be based on the eqn that fluent gives in fluent user guide right?

and by profile, did you mean that these values are not constant at inlet?
kael is offline   Reply With Quote

Reply

Tags
airfoil, low reynolds, transition model

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
Overflow Error in Multiphase Modelling with Two Continuous Fluids ashtonJ CFX 6 August 11, 2014 15:32
Low Speed Airfoil Mancusi FLUENT 7 April 3, 2014 07:11
Cavitation Simulation of low mach flow over 2D airfoil Harkot FLUENT 3 October 1, 2013 12:04
low reynolds airfoil simulation miguel.soto FLUENT 3 March 3, 2010 14:51
a low Re number model for impinging jets mahesh prakash Main CFD Forum 14 September 3, 1999 17:40


All times are GMT -4. The time now is 22:32.