Discussion Forum

Hi

indeed it would be nice to have a good tutorial illustrating the real meaning of these CFD specific "engineering number" such as Reynolds, & Nusselt, but also Eckert, Euler, Froude, Grashof, Mach, Prandtl, Strouhal, Weber, or Lift, Drag, Pressure, Temperature ... ratios and all the others (and not all are truly dimensionless !)

I have regularly problems to identify what is a "specific length" for CFD, probably that is also why COMSOL proposes the Reynolds "cell" number (that I'm understanding as mesh element size related R# ) and not a model generic R#

Back to the other issue: average T gradient along a direction, I believe this could be done via an average & projection coupling operator, but I do not really understand how to link it to the R#.

If you want to use the R# as a "filter" for the integration (you multiply your variable by *(100<R#)*(R#<1000), then probably its worth to do the average manually too, as else the average would probably be taken on the full area/volume and not the "filtered" region. To be tested out

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Good luck
Ivar

I am trying to calculate the average Nusselt # over a flat plate vs. Reynolds #. Thus I need to take the average of the derivative of temperature in the y direction at the plate for the various Reynolds #'s. Can anyone help me out with this?

In the end, I want to plot Nusselt # vs. Reynolds #. Thus I need to take the average and then multiply by a constant. I am trying to verify some of the empirical formulas for Nusselt #, and Reynolds #. I need to make sure I can do this before I give it to my undergrads as a study in simple introduction to forced convection

Calculating Nu using gradients is not recommended in Comsol. Instead you can use the reacf(T) operator to calculate the heat flux and use it in the Nu calculations, which is the most accurate method.

Danial
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Comsol 4.1
Ubuntu 10.04.1

I am trying to calculate the average Nusselt # over a flat plate vs. Reynolds #. Thus I need to take the average of the derivative of temperature in the y direction at the plate for the various Reynolds #'s. Can anyone help me out with this?

In the end, I want to plot Nusselt # vs. Reynolds #. Thus I need to take the average and then multiply by a constant. I am trying to verify some of the empirical formulas for Nusselt #, and Reynolds #. I need to make sure I can do this before I give it to my undergrads as a study in simple introduction to forced convection

Calculating Nu using gradients is not recommended in Comsol. Instead you can use the reacf(T) operator to calculate the heat flux and use it in the Nu calculations, which is the most accurate method.

Danial
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Comsol 4.1
Ubuntu 10.04.1

Could you please explain in detail? AN step by step instruction would be great. Thank you.

Hi,

do you have an example in how to calculate the nusselt number , with based on Incropera´s book?

Thanks