# Fluid Mixing

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 March 7, 2005, 14:07 Fluid Mixing #1 Terri Novotny Guest   Posts: n/a Hello, I have a case where I would like to evaluate the mixing of two similar fluids: I have a tank with an inlet and outlet that is filled with "old" water. Through the walls of the tank, there is "new" water coming in. I am interested in how the "new" and the "old" water mix - both have the same physical properties. Thanks, Terri

 March 7, 2005, 17:50 Re: Fluid Mixing #2 np Guest   Posts: n/a Hello, the simplest for you is to create a passive scalar for the 'fresh' water with the same properties as your 'old' water, impose as aboundary layer the flow rate you want to impose into your porous wall an post process the mass fraction of each of the 2 fluids. use a turbulent schmidt number between 0.7 and 1 (classical values). the secondary fluid can be created either by a user defined scalar (but take care of its eddy diffusivity which must be user specified bya udf) or by a secondary specie (multiple species panel). As a passive scalar, you can try to use the temperature. the transport equation is the same, just replace schmidt by prandt number. bye

 March 8, 2005, 07:14 Re: Fluid Mixing #3 Terri Novotny Guest   Posts: n/a Hello np, thank you very much for your answer! It makes sense to me to choose the schmidt number = 1 (i.e. momentum and mass diffusivity are equal), but it is not clear to me why 0.7 would work, too. Thanks, Terri

 March 8, 2005, 11:36 Re: Fluid Mixing #4 np Guest   Posts: n/a Hi Terri, turbulent schmidt number should have a default value arround 0.7 preferably to 1 because when you transport averaged concentrations, you relate the fluctuations of concentration to the reynolds averaged concentrations via a turbulent diffusivity. Experiments have shown that the spectrum of velocity fluctuations and of concentrations fluctuations have not excatly the same shape, so the eddy diffusivity approach will lead to different values for turbulent diffusivity. Same thing with turbulent Prandt number. Depending on the experiments, you can even impose turb. schmidt number to 0.5. Keep 0.7: you'll be safe. Have a look to M. Lesieur or Bird's book on this points. nicolas

 March 8, 2005, 12:49 Re: Fluid Mixing #5 Terri Novotny Guest   Posts: n/a Hi Nicolas, thank you for the information! I have ordered the book "Transport Phenomena" by Bird from the library. In the mean time: if the flow is laminar, I guess that one cannot choose a Schmidt number below 1. Correct? Thanks, Terri

 March 8, 2005, 16:53 Re: Fluid Mixing #6 np Guest   Posts: n/a In fact, all this discussion is about turbulent flows, when you solve rans equations and you have to relate diffusivity to the averaged values. It's a way to model the u'c' terms. If the flow is laminar, RANS asumption are not necessary and you'll have to impose the true mass diffusivity (which is of the order of 10e-9 in liquids). To be more rigorous, you should take into account differentail diffusivities but it's another debate! if your flow is laminar, just take some difusivity of the order of the water one and your mixing will be more convective. have a look to fluent documentation, it's often a good introduction to many problems (and i've no fluent stocks!) np

 March 8, 2005, 20:50 Re: Fluid Mixing #7 Terri Novotny Guest   Posts: n/a Thank you, Nicolas. - Terri

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