## Fluids

#### Determination of Reynolds Number

• Ananth Narayan
Subscriber

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

• DrAmine
Ansys Employee

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

• Raef.Kobeissi
Subscriber

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...

• Ananth Narayan
Subscriber

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

• Karthik R

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?