×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Characteristic Length in Reynolds Number

Characteristic Length in Reynolds Number

Characteristic Length in Reynolds Number

(OP)
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.

RE: Characteristic Length in Reynolds Number

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).

RE: Characteristic Length in Reynolds Number


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

RE: Characteristic Length in Reynolds Number


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...

RE: Characteristic Length in Reynolds Number

(OP)
thank you everyone, i appreciate your help

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources