calculating Reynolds number in wind tunnel

mostafagemy

when calculating a Reynolds number for an obstacle in wind tunnel
Re = air density * air velocity * D / Dynamic viscosity
D here is the characteristic length of the immersed body ( frontal length ) 2 dimensional problem
Or the Hydraulic diameter which relates the tunnel dimensions to the characteristic length of the immersed body ?

boneh3ad

It depends entirely on for what reason what you are calculating the Reynolds number. In general, you won't be using any tunnel dimensions, as almost all interesting quantities are going to be related to the model you are using. Usually the idea is to use the most relevant characteristic length for what you are studying, be it the diameter of your object, the chord of your object, the boundary layer thickness, roughness height or any other number of things.

mostafagemy

really , I need to verify a model on a program ( flow over a square cylinder with different angles of attack to calc the drag and lift coefficients on it ) . I looked alot for others work about this problem but what I found was only about 0 and 45 angle of attack , so if u can support me with other web sites , plots or books name , it will be a great help. And thanks for your reply.

boneh3ad

That would be hard to do without any background on what you a trying to do (and saying you are just verifying a model doesn't really tell me much). For your purpose, the relevant Reynolds number is most likely going to be based on the length of one side of your cross section. I suppose the other option would be finding an effective diameter (similar to the hydraulic diameter of a square tube). I would look into the literature on the subject to figure out what the normal convention is in this case.

For example, perform a search over at JFM:
http://journals.cambridge.org/action...ournal?jid=FLM

mostafagemy

hi boneh3ah ,
I had a network connection problem yesterday

the attached photo is about the model I calculate the Cd & Cl for it.

I'm testing the variation of many parameters on Cd & Cl ( Air speed, square side dimension , tunnel height) also, the shape ( square , triangle)

the fixation points are in the middle of each side ( four fixation points ) & they are also the points which I measure the reaction forces on them.

I'm using Comsol multiphysics program for simulation problem.
thanks alot

Attached Thumbnails

 

boneh3ad

Like I said, do a search through the literature. I remember seeing a JFM article about calculating lift and drag on a square cylinder, but I personally am not all that familiar with this particular flow problem so I can't tell you all that much.

What I can tell you is that your simulation appears to be steady and laminar based on the wake and this is a highly unsteady phenomenon.

mostafagemy

thanks alot for your help boneh3ad