Learning Forum

[Melanie Butts]

I need to know whether to use a laminar or turbulent viscosity model, and if I need a turbulent one, which one to pick.

I am simulating the flow created by a 5 bladed room fan. We already took measurements of the air velocity created by the actual fan using an anemometer. We are not sure what the best effective viscosity settings are though, because we are not certain whether our flow is laminar or turbulent. Here is what we have so far.

Using a slow motion setting on a camera, the rpm was measured/counted to vary between 1033 and 1380 rpm. The fan geometry we are using in our simulation is shown below. 

• Width of tips of fan blades (w) - 0.076 m
• Circumference of fan (c) - 1.1 m.
• Kinematic viscosity of air (k) - 0.00001506 m^2/s.
• Based on the rpm and circumference, I calculated the tips of the fan blades to be moving at 19 to 20.9 m/s (v)
• I estimated the Reynolds number using w*v/k
  • Re ranged from 96,000 to 128,000

What would the critical Reynolds number be for a situation like this?

[Keyur Kanade]

Please check following. 

Laminar or turbulent flow? (ansys.com)

Regards,

Keyur

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[ehsan.sadeghi]

a simplest way would be measuring the flow rate in a fan rpm, then based on fan diameter and estimated mass flow rate you can calulate the reynolds number. it should give you a rough estimate of the flow regime. 

[Melanie Butts]

Thank you for your responses.

Keyur Kanade I looked at the link, and it deals with critical Reynolds numbers for internal flow. I should probably clarify that my box domain has pressure boundary conditions on all sides, and no walls. For this situation I would need to use a different critical Reynolds number than can be used for pipe flow, correct?

Thank you

[Melanie Butts]

Ehsan Sadeghi I'm not quite sure what you mean. Are you suggesting than in the equation to calculate the Reynolds number

(v x L x rho)/(mu)

that I take my average axial velocity as v and my diameter as L?

Also, do you have any suggestions for what the critical Reynolds number or transition range would be, given this is external flow?

Thank you

[ehsan.sadeghi]

Exactly. 

For external flat plate flow you may choose 3e5 as a critical transition Re number. In general there is no universal Re number that could define flow regime for all flows.

[Melanie Butts]

Thanks for the clarification. The blades are concave, but I imagine that the critical Reynold's number for a flat plate wouldn't be too far off. 

Thank you for your input!