# UDS, parameters and computations

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 August 15, 2007, 02:42 UDS, parameters and computations #1 hedge Guest   Posts: n/a Hi everyone! I'm using Fluent for simulation of detonation waves. In these simulations I try to use my own chemical heat release model, which is based on UDS. The transport equations for two UDS are solved by fluent, and I only provide the source terms for them. I found the following effect of the problem setup. The results of the simulations (time-evolution of the UDS, actually the values of the UDS at specified time-moment) are strongly dependent on the values of Diffusivity for the UDS and relaxation parameter (on Solver panel). Actually, it drastically changes. If I prefere to use the zero value of diffusivity (which is physical assumption for high speed flows such as detonation), then the solver fails at all. Could you plese advise me or explain what is the problem or how can I solve it. The reference to some paper/book/url will be appreciated! Thank you

 August 15, 2007, 15:23 Re: UDS, parameters and computations #2 Summer Guest   Posts: n/a Hello, Hedge, Regarding the diffusivity, you should determine it based on the transport equation of UDS you want to solve. It has physical meanings. However, in my opinion, the relaxation factor should not affect the solutions. Best wishes

 August 16, 2007, 05:19 Re: UDS, parameters and computations #3 hedge Guest   Posts: n/a Summer, thanks for your reply. According to diffusivity my opinion remains the same - i would prefer to set it as zero (or very small, see table below). This is because of the fact, that I am simulating the high speed detonation waves as inviscid flow, so all the species are only convected with the flow and their diffisivities are setted to zero (inviscid flow, Euler equations). My UDS are convected with the flow as well (their meaning very similar to meaning of species). The only possible way is to set UDS diffusivities to very low value, say 1e-6 kg/s, but the final result is much affected by this small value. According to relaxation parameter for UDS equations. I use the explicit time formulation and all flow equations are solved explicitly. AS i have known from the user gide, the UDS equations are solved in a different way and for their solution the relaxation parameter value is required. To illustrate the influentce of the UDS diffusivity and relaxation parameter, the following table demonstartes the values of my UDS after one time step for different parameters values Diffus. relax.Pram UDS0 0.01 0.1 4e-5 0.01 0.5 3.57e-4 0.03 0.2 9e-5 0 any NaN Probably I have a missunderstanding of some underliing numerics or applicability of the models and setup parameters. It would be greatefull to you if you will explain where I am wrong. Thank you.

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