# stochastic model

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 June 28, 2001, 10:27 stochastic model #1 Alex Guest   Posts: n/a Hi everybody, does anybody know how fluent tracks a particle using the stochastic model?what I mean is,what equations are used to track the particle? force balance? how do we know if the particle passed one eddy and is moving on to the next one. if we're using the force balance equ., what is the calculated velocity?instantenous velocity?I'm very much confused. I looked at lots of literatures but I got more confused?!!! thanks in advance alex

 June 28, 2001, 13:47 Re: stochastic model #2 Lanre Guest   Posts: n/a A momentum balance on a particle sets up the equation of motion for the particles solved by FLUENT, i.e., mass of particle x acceleration of particle = SUM (forces acting on particle). The drag force term contains the relative velocity (U_f-U_p) where U_f is mean fluid velocity. However, to model influence of turbulence on the particle U_f is decomposed into u_bar + u'. The instantaneous velocity u' is computed from the local tke in the cell => turbulence intensity, and a randomly generated number (since u' can fluctuate about u_bar). The eddy lifetime, or lifetime of exposure to u', is based on a characteristic time constant (or variable).

 June 28, 2001, 14:14 Re: stochastic model #3 Alex Guest   Posts: n/a thanx Lanre, I understand it much better now.in fluent's mannual I found these explanations : "The instantaneous characteristics of the gas are computed by adding random fluctuations to the mean flow solution from the k-e simulation. Thus, U = Ubar+Cu' where, u'=(2/3k)^1/2 equation 1 where c is random between 1 and -1." do you think there is any mistakes there?I guess it's Ubar that can be calculated from equ. 1.also, you mentioned u' can be calculated from turbulent intensity and randomly generated number.which one is correct?!!! alex

 June 30, 2001, 08:47 Re: stochastic model #4 Alain Guest   Posts: n/a Ubar come from the velocity field calculated by time averaged NS equation. u' is an instataneaous fluctuation which is evaluated from turbulent quantities (basicly eddies lenght scale and turbulent kinetic energy). Try to have a look at "multiphase flows with droplets and particles", C. CROWE and al., CRC press,1998 Best regards Alain

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