# Determination of Reynolds Number

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• Last Post 10 August 2018
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Ananth Narayan posted this 10 August 2018

Hello, I wanted to know how to display overall Reynolds number in CFD Fluent.

Regards Ananth Narayan

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abenhadj posted this 10 August 2018

You cannot input / output directly the device Reynolds number in Fluent.

As you know Re number is defined as: Re=rho*v*L/mu with Re=Reynolds number, rho=fluid density, v=fluid velocity, L=characteristic length, mu=dynamic viscosity.

Which flow configuration you have? The characteric length could be the hydraulic diameter of your Inflow boundary and for external aero might be the chord length

Best regards,

Amine

Raef.Kobeissi posted this 10 August 2018

It depends on the domain you're modelling. Imagine 2 pipes in series (3D), in that case the domain of each pipe has a different Reynolds Number. If it is 1 pipe, then it is calculated using the formule Re:= RHO.V.D/Mu, D is the characteristic length or in this case the hydraulid diameter which is the diameter of the pipe etc...

Raef Kobeissi

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Ananth Narayan posted this 10 August 2018

Hello,

I calculated Re as 111896.7452 ~ 0.1189e5. I made 2d analysis with a circle(cutout) of diameter 0.1 m on rectangular surface with density and viscosity for air as 997 kg/m3 and 0.891e-3 kg/m.s respectively. On attached graph for the above mentioned re number, Cd-drag coeffcient is around 0.5, but on analysis with k omega sst scalable wall function I got drag coefficient as 0.27345. Where might be the mistake. Graph: Search on drag coefficient vs reynolds number sphere

Regards Ananth Narayan

Kremella posted this 10 August 2018

Hello Ananth,

Couple of questions:

• Which turbulence model did you use when you got a Cd of 0.5?
• Could you elaborate a little on your mesh (especially the boundary layer mesh)? Did you ensure that your grid is suitable for the k-omega SST model by ensuring that your y+ ~ 1?
• Can you check the reference values for both the solutions and ensure they are the same?
• Have you performed mesh independence studies?

There is no attachment to your post. Could you please provide appropriate screenshots so we can have a look and help you?

Thank you.

Best Regards,

Karthik