# Log-Law on Boundary Layer using Fluent

 User Name Remember Me Password
 Register Blogs Members List Search Today's Posts Mark Forums Read

 October 28, 2008, 08:07 Log-Law on Boundary Layer using Fluent #1 Seb Guest   Posts: n/a Hello everyone, I am using Fluent to try to simulate the evolution of a turbulent boundary layer on a flat plate of 30 cm of length with a freestream velocity of 5 m/s (2D-Steady state). The mesh is such as : Dx=1mm and Dy=y+ at the wall I've made a theoritical turbulent boundary profile for the velocity inlet condition. This profil follows the regular laws, as : --> U+=y+ [close to the wall] --> U+=1/0.41*ln(y+)+5 [At the log region] --> The deficient law of Coles farther from the wall I ran some simulations using k-eps model with a wall function until the residuals are small enough (< 10e-8). When I compare the numerical results at several x locations with the log law, it doesn't match at all. Has someone have done something similar ? Thank you ! Seb

 October 29, 2008, 05:34 Re: Log-Law on Boundary Layer using Fluent #2 Bikini Girl with Machinegun Guest   Posts: n/a I've done a bit with the log-law. Remember, Coles' function does not satisfy boundary conditions at the outer edge of the BL. Does that affect your CFD model? Also, Coles' log-law constants are very dodgy. He obtained the values for kappa and B_0 using massaged velocity profile data. Sadly, his values are often still treated as universal constants by noobs.

 October 29, 2008, 06:07 Re: Log-Law on Boundary Layer using Fluent #3 Seb Guest   Posts: n/a Actually I thought that wall functions in the RANS models were made to make the flow to fit the sub-layer law and log-law with the "universal" constants. At the entrance, my theoritical profile is such U+ = 1/0.41 * ln(y+) + 5 [for 10 < y+ < 60] When I get profiles at some location after the simulations, they all fit in a profile such as : U+ = 1/0.41 * ln(y+) + 0.3 [for 10 < y+ < 30] I was wondering if this was coming from the fact that I am simulating a Low Reynolds boundary layer (Rtheta ~ 600). I made a simulation with a higher velocity (and so a different boundary layer at the entrance) at U=100m/s instead of 5m/s and I found the same results...

 November 4, 2008, 11:08 Re: Log-Law on Boundary Layer using Fluent #4 Seb Guest   Posts: n/a After trying different mesh, different profiles at the velocity inlet and different models I found that the best model to simulate the evolution of a turbulent boundary layer is K-EPS --> Realizable --> Enhanced wall treatment. Using this, the profile of velocity fits the log law fairly well; Besides, the first cell doesn't have to be too small (> y+)

 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 [GAMBIT] problems in boundary layer yubaibai88 ANSYS Meshing & Geometry 8 April 19, 2010 21:07 Anthony Haroutunian FLUENT 2 March 26, 2008 03:02 Bob FLUENT 2 August 25, 2007 01:23 Peter FLUENT 0 August 9, 2002 19:41 98.4F Main CFD Forum 1 December 29, 2000 08:23

All times are GMT -4. The time now is 21:13.

 Contact Us - CFD Online - Top