CFD Online Logo CFD Online URL
Home > Forums > FLUENT

Problems with Turbulence Decay Calculation

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

LinkBack Thread Tools Display Modes
Old   March 27, 2010, 06:27
Default Problems with Turbulence Decay Calculation
New Member
Join Date: Mar 2010
Posts: 1
Rep Power: 0
bbilbo is on a distinguished road
Sponsored Links
Hi there,

I've come across a very strange FLUENT behavior when trying to solve a test problem on turbulence decay in 1D incompressible flow.

The parameters are simple: we have a 1D flow of incompressible fluid, Rho = 1.225 kg/m3, U = 1 m/s,
at the inlet I specify k=0.5, Eps = 0.1 to see how turbulence decays with distance (up to 4 m). There is no patricular reason for choosing the above values, just a test.

I specified quite a high value of laminar viscosity (Mu_l =0.01) and obtained successfully a converged solution with k, Eps and Mu_eff decaying with distance. Backward differences are used for convective terms (1st order scheme), standard k-epsilon models, symmetry conditions on side walls, no wall functions required.

PROBLEM: I tried to check the converged solution. In this "classic" flow there is no generation, and at convergence three terms must sum up to zero: convection, diffusion, and dissipation. I've exported cell centered data for k, Eps, Mu_eff and tried to estimate "by hand" each term
(taking finite-volume style differences) and see if they sum up to zero. Surprisingly, the converged to 10-8 solution does not give this consistency: say, convective term Rho*U*(k(i)-k(i-1))/dX =-0.08152, diffusive term
((k(i+1)-k(i))*Mu_eff(i+1/2)-(k(i)-k(i-1)*Mu_eff(i-1/2))/dX^2 is 0.005464935, Dissipation Rho*Eps = 0.090285. This together gives (Diss + Conv - Diff) = 0.00329614, which is nowhere near to 10-8. (Mu_eff at cell boundaries was estimated by linear interpolation).

To put it straight, I dont care for now about physical validity (too high laminar viscosity etc), but trying to figure out - what exactly equation is solved in FLUENT and why its hand calculation is inconsistent with convergence criterion.

Any help ot ideas is highly appreciated, perhaps something is wrong in my checks, so please dont hesitate to point my nose to it
bbilbo is offline   Reply With Quote
Sponsored Links


fluent, turbulence decay

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
Question on Turbulence Intensity Eric FLUENT 1 March 7, 2012 05:30
KOmega Turbulence model from wwwopenFOAMWikinet philippose OpenFOAM Running, Solving & CFD 30 August 4, 2010 10:26
Natural convection - Inlet boundary condition max91 CFX 1 July 29, 2008 20:28
Generalized Math. Form of Turbulence Nomad Main CFD Forum 38 March 22, 2004 10:03

Sponsored Links

All times are GMT -4. The time now is 02:58.