Stability of results and time step ?

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 February 26, 2001, 09:10 Stability of results and time step ? #1 mnasri chokri Guest   Posts: n/a I am simulating equations in spectral variables , the general form of equations is d/dt+2.v.k^2)E(k,t)=T(k,t), the function T(k,t) contains linear and non-linear terms. I have problems with stability, I would like to know which terms may have influence on it, the time step for exemple , or k_min, k_max, k_pic(initial spectrum),the caractristic time, ... thanks in advance.

 February 26, 2001, 13:44 Re: Stability of results and time step ? #2 sylvain Guest   Posts: n/a If you are using the EDQNM form of the transfer term, then you should have : T(K) = int_{P,Q} (a_kpq E(P)E(Q) - b_kpq E(K)E(P)) dPdQ = T_nl - T_l since the linear term T_l is negative, you can increase the stability of your scheme by writing it in an implicit form. This is possible if you are looking to a steady state solution - by forcing the small wave length for example. If you are looking to a unsteady solution - turbulence decay for example - you should use a time scale at least as small as the smallest caracteristic time scale of your problem. In that case, I believe it is the turn over time of the smallest eddy - i.e. the turn over time associate to the Kolmogorov wave length K_kol. So if : K_kol = (epsilon/nu^3)^{1/4} then : dt < 1/ ( 2 nu K_kol * K_kol ) where the dissipation rate epsilon is given by : epsilon = int_{K} 2 nu K * K E(K) dK hope that help

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