# CFL influence on turbulence decay

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 May 26, 2014, 18:30 CFL influence on turbulence decay #1 New Member   KS5 Join Date: Mar 2012 Location: US Posts: 15 Rep Power: 14 Hi, I am working on a spectral element code, flow in a sudden expansion pipe. I ran a simulation with polynomial order N=5 at Reynolds number Re= 2000, I found turbulence inside my pipe which stays there forever and this matches with the real physics and some literature. for same Re, I increased the order of my polynomial from N = 5 to N = 7 this in turn increased my CFL number, This time the turbulence decayed completely and became steady flow. So I reduced the time step DT to get the CFL number same like that of the 1st simulation (N=5) and I found that the turbulence occurs but takes a little more longer time to decay I reduced the time step Dt much more lower to have a CFL = 0.036 and found that the whole pipe became turbulent. Could some tell me what is the effect CFL number on laminar to turbulent transition ?? why did the turbulence decay when I increased my order from N=5 to N =7 ?

 June 1, 2014, 11:50 #2 Senior Member   N/A Join Date: Mar 2009 Posts: 189 Rep Power: 17 This is a classic problem with high-order methods. When you increase the order, you are removing most of the dissipation which can trigger turbulence. This is a well-known problem with spectral methods. The fix that has been suggested is to perform simulations on a coarse grid with say p=3 or p=5 and let the turbulence develop. Interpolate the obtained results to the grid for p=7 and continue simulations. infikamal5 likes this.

 June 1, 2014, 12:02 #3 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,777 Rep Power: 71 Are you performing DNS? If so, are you sure that you used time and space steps enough small to resolve physical scales? No matter in a CFL value if the time step is at the level of the Kolmogorov time scale, otherwise the effect is in an implicit filtering. infikamal5 and pswpswpsw like this.

 June 2, 2014, 04:23 #4 New Member   KS5 Join Date: Mar 2012 Location: US Posts: 15 Rep Power: 14 Thanks Harishg and FMDenaro !!!

June 5, 2014, 09:10
#5
New Member

Shawn
Join Date: Mar 2012
Posts: 21
Rep Power: 14
Quote:
 Originally Posted by harishg This is a classic problem with high-order methods. When you increase the order, you are removing most of the dissipation which can trigger turbulence. This is a well-known problem with spectral methods. The fix that has been suggested is to perform simulations on a coarse grid with say p=3 or p=5 and let the turbulence develop. Interpolate the obtained results to the grid for p=7 and continue simulations.
I am wondering if increasing the total time would help. You suggestion mentioned a coarse grid, which in return might produce larger time step and larger total time, if CFL and iteration steps are not changed.

Moreover, if high-order as high as p=7 methods removing most of the dissipation that would trigger turbulence, is it less realistic for higher order methods?

Best,
Shawn

 June 5, 2014, 09:22 #6 Senior Member   N/A Join Date: Mar 2009 Posts: 189 Rep Power: 17 The idea is that the initial condition has some sort of perturbation to trigger turbulence similar to the physical occurrence of turbulence due to some instability in the flow. So it is not about being realistic. It is about tripping the flow to become turbulent. Experiments use trip wires and we need some form of numerical trip wire.

June 5, 2014, 10:23
#7
New Member

Shawn
Join Date: Mar 2012
Posts: 21
Rep Power: 14
Quote:
 Originally Posted by harishg The idea is that the initial condition has some sort of perturbation to trigger turbulence similar to the physical occurrence of turbulence due to some instability in the flow. So it is not about being realistic. It is about tripping the flow to become turbulent. Experiments use trip wires and we need some form of numerical trip wire.
I got what you mean. Yes it is. Thank you for your response.

 Tags cfl number, spectral method, turbulence