Turbulent Reynold Number (Re_y)

Jo_Sharon · October 30, 2015, 07:31

Hey all,

I'm pretty new in CFD. In fact I've just finished a few tutorial on Fluent CFD. What I want to ask is:
What is the definition of Turbulent Reynold Number (Re_y)?
Is it different from common reynold number?

I'm actually working on simulation of simple 3D mixer (steady state, moving reference frame). The tank is rectangular with 6 blades turbine mixer. I want to evaluate the performance of the mixer. For now, I just use water as the working liquid. Just to see how the water move with the current mixer setting (70 RPM, 70 degree pitch).

How do I evaluate the mixer performance?
I'm guessing that Turbulent Reynold Number (Re_y) is Reynold Number (Re) across the tank. So if all part of liquid in the tank achieve turbulent Re (10000), then the mixing is good. But that just a guess. So please if any of you would be kind enough to help.

Thanks

LuckyTran · October 30, 2015, 08:02

What exactly does your 3D mixer do? Does it mix temperature? Different species? Does it stir up gold coins in your safety deposit box? You need to develop criteria based on what it is you are trying to accomplish. Often performance is normalized by the cost (work needed to drive the mixer), but there could be other costs. Performance is subjective, even when quantified.

Reynolds number has very little to do with performance...

For any Reynolds number you should always ask what is the velocity and length scale. Given velocity, length, and momentum diffusivity one can calculate a different type of Reynolds number.

There are many types of turbulent Reynolds numbers. The one used in fluent is based on u = sqrt(k) and distance to the nearest wall. You could also have turbulent Reynolds number based on large eddy size, based on Taylor macro-scale, Taylor micro-scale, Kolmogorov scale, etc.

Jo_Sharon · October 30, 2015, 17:51

3D mixer I mean I simulate the mixer with 3D geometry. I can't use 2D Axysymmetric since the tank is rectangular.

Right now I'm not mixing anything since I'm just studying. But in the future, I want to be able to simulate CaO (Powder) mixing.

Secondly, please inform me what books you need to learn CFD from the beginning. Or maybe it's already in another thread. I'll check it out.

Thanks

October 30, 2015, 08:02		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,676 Rep Power: 66	What exactly does your 3D mixer do? Does it mix temperature? Different species? Does it stir up gold coins in your safety deposit box? You need to develop criteria based on what it is you are trying to accomplish. Often performance is normalized by the cost (work needed to drive the mixer), but there could be other costs. Performance is subjective, even when quantified. Reynolds number has very little to do with performance... For any Reynolds number you should always ask what is the velocity and length scale. Given velocity, length, and momentum diffusivity one can calculate a different type of Reynolds number. There are many types of turbulent Reynolds numbers. The one used in fluent is based on u = sqrt(k) and distance to the nearest wall. You could also have turbulent Reynolds number based on large eddy size, based on Taylor macro-scale, Taylor micro-scale, Kolmogorov scale, etc. Last edited by LuckyTran; October 30, 2015 at 18:33.

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October 30, 2015, 07:31	Turbulent Reynold Number (Re_y)	#1
Jo_Sharon New Member Join Date: Oct 2015 Location: Indonesia Posts: 2 Rep Power: 0	Hey all, I'm pretty new in CFD. In fact I've just finished a few tutorial on Fluent CFD. What I want to ask is: What is the definition of Turbulent Reynold Number (Re_y)? Is it different from common reynold number? I'm actually working on simulation of simple 3D mixer (steady state, moving reference frame). The tank is rectangular with 6 blades turbine mixer. I want to evaluate the performance of the mixer. For now, I just use water as the working liquid. Just to see how the water move with the current mixer setting (70 RPM, 70 degree pitch). How do I evaluate the mixer performance? I'm guessing that Turbulent Reynold Number (Re_y) is Reynold Number (Re) across the tank. So if all part of liquid in the tank achieve turbulent Re (10000), then the mixing is good. But that just a guess. So please if any of you would be kind enough to help. Thanks

October 30, 2015, 17:51		#3
Jo_Sharon New Member Join Date: Oct 2015 Location: Indonesia Posts: 2 Rep Power: 0	3D mixer I mean I simulate the mixer with 3D geometry. I can't use 2D Axysymmetric since the tank is rectangular. Right now I'm not mixing anything since I'm just studying. But in the future, I want to be able to simulate CaO (Powder) mixing. Secondly, please inform me what books you need to learn CFD from the beginning. Or maybe it's already in another thread. I'll check it out. Thanks