Characteristic Length in Reynolds Number

[ptrmrk]

I am trying to calculate a Reynolds number for a flat plate (20x15x1ft) in an air flow, in order to determine the drag force. The frontal area is 20x15ft. I don't understand which dimension to use for the characteristic length in the Reynolds number calculation. I think it is the dimension parallel to the flow (1ft) but I'm not sure. However, in this case the wind speed will be such that the Re will be more than 10000. I found a Cd value of 1.18 for this geometry and Re number. Does that sound OK?

Thank you very much for your help.

 

[es335]

I agree. I would say that the car. length should be the 1ft.

And the Cd value sounds fine to me (but then again, I have not worked with drag calc. in a long time).

 

[fvincent]

For such geometry I think a certain average between both frontal dimensions (20 and 15ft) would express the characteristic lenght, not the dimension parallel to the flow.

Just suppose the latter were 1/100000000 ft: could the Reynolds be zero??

What changes the flow (and that's is what Reynolds similitude rules are for) is the frontal area...






fvincent

 

[hacksaw]

you have two types of drag, the one you are interested in depends on the frontal area. The appropriate reynolds number depends on the shorter of the two transverse dimensions.
try marks hand book, in the section dealing with flight,
of course you can resort to CFD...

 

[ptrmrk]

thank you everyone, i appreciate your help