# Prandtl Number

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 October 22, 1999, 10:22 Prandtl Number #1 Thomas P. Abraham Guest   Posts: n/a Hello Everyone: I have a question for you all. Reynolds number tells us about the degree of turbulence, Grashof number about the degree of buoyancy forces, etc. How about the Prandtl Number? What kind of physics it's telling us? Thanks, Thomas

 October 22, 1999, 11:37 Re: Prandtl Number #2 Patrick Godon Guest   Posts: n/a The Prandtl number measures the relative diffusion rates nu/kappa, where nu is the kinematic viscosity and kappa is the thermal diffusivity. So it gives an estimate of the dominant diffusion process in the flow.

 October 22, 1999, 11:57 Turbulce Prandtl Number? #3 sheng Guest   Posts: n/a I saw a name "turbulence Prandtl Number". I wonder is the turbulence really can diffuse? regards, sheng

 October 22, 1999, 14:33 Re: Prandtl Number #4 Demselles Hollowajiwoak Guest   Posts: n/a Pr=v/a => high Pr=momentum diffusion is dominant low Pr=thermal diffusion is dominant Regards Demselles

 October 23, 1999, 03:01 Re: Reynolds Number??????? #5 Yogesh Talekar Guest   Posts: n/a Is it true that Reynolds numbers tells about degree of turbulance? I think it only tells which one of the forces viz. momentum force and viscous force is dominant. Can anybody explain? Yogesh

 October 23, 1999, 17:56 Re: Turbulce Prandtl Number? #6 Jonas Larsson Guest   Posts: n/a The turbulent Prandtl number is the ratio between turbulent diffusivity of momentum and turbulent diffusivity of heat. The turbulent Prandtl number can in many cases be approximated as a constant, although it in reality varies. By assuming a constant turbulent Prandtl number you can estimate the turbulent transport of heat by using the eddy-viscosity computed with a common turbulence model. This is also what most people do, although there exists turbulence models which solve extra transport equations in order to account for the variation. Sometimes you also see models with algebraic formulas for the turbulent Prandtl number.

 October 24, 1999, 12:03 Re: Reynolds Number??????? #7 Jonas Larsson Guest   Posts: n/a Not entirely true, but you can say that the higher Reynolds number you have the more likely it is that you will have a turbulent flow. A higher Re number also gives a larger range of turbulent scales.

 October 25, 1999, 09:47 Re: Reynolds Number??????? #8 wolfgang schmidt Guest   Posts: n/a I don't agree with this aspects. The reynolds-number is just the ratio of two forces. If it's larger than unity it says momentum forces are larger then viscous forces, if it's lower than unity it's vice versa -that's all nothing less and nothing more. Often -but not always- it allows conclusions about the "turbulence intensity" or the amount of energy needed to keep the fluid flowing in the way it does. Depending of the geometry and fluid conditions the change from laminar to turbulent flow may occure at Reynolds Numbers of 2000, or 3000, or even 100000. But this are only rules of the thumb.

 October 25, 1999, 10:04 Re: Reynolds Number??????? #9 Patrick Godon Guest   Posts: n/a Another important aspect of the Reynodls number is related to the boundary layer. If you have a boundary where a boundary layer can form, then the Reynolds number gives you a first estimate of the size of the boundary layer that will form there. e.g. if Re is very small (e.g. Re=10) then the boundary layer is relatively large, its size is a significant fraction of the size of the whole domain. if Re is very large (Re=1000), then the size of the boundary layer is very small indeed, and you need to have a higher resolution close to the boundary. IN general the size of the boundary layer L is related to the Reynolds number roughly as L proportional to D/sqrt(Re) where D is the size of the whole domain.

 October 25, 1999, 10:31 Re: Reynolds Number??????? #10 Jonas Larsson Guest   Posts: n/a Yep, that's exactly what I said ... a larger Re makes it more *likely* to have a turbulent flow. Note the word likely.

 February 15, 2011, 11:11 what is the range of prandtl number for viscoelastic fluids? #11 New Member   tayyab Join Date: Feb 2011 Posts: 2 Rep Power: 0 what is the range of prandtl number for viscoelastic fluids? could it be near to 1 or 2?? i have seen that for viscoelastic polymer ethylene glycol its range is from 16 to 29.

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