# Thread: Reynold Number VS boundary layer

1. Hi, I have a little problem (by the way im new here Hi everybody!)

Im doing a project for my engineering school and im confuse by some of the result

Im using XFOIL and XFLR5, I need to study a naca4312 airfoil. When I enter a Reynold number of 37000 and a mach number of 0.01175, I got a turbulent flow over the succion side

when I enter a re of 10 000 000, I obtain a laminar flow

How come? isnt suppose that higher the renold number is, more turbulent the flow is?

Im i missing something?

Thx!

2.

3. My knowledge of Reynold's number is restricted to dense fluids pumped in boreholes when drilling oil wells. For me the Reynold's number can empirically define a range in which flow transits from laminar to turbulent. That is consistent with what you understand.

My one suggestion (likely wrong) is operator error. The first time I looked at your post, and the second time, I read 10,000, not 10,000,000. Is it possible you keyed in too low a number. I realise that suggestion could seem offensive - "Do you think I don't know how to type in a number correctly" - but in teaching engineering applications to students I am amazed at how often simple and subsequently obvious mistakes such as that are made.

4. When you entered the high Reynolds number did you also enter a correspondingly high Mach number? An Re of 10,000,000 might only be achievable with a Mach number very much higher than .012 and any reasonable chord length. Just guessing; I know very little about airfoils.

Clarification - I'm suggesting that your software back calculates a velocity from the Mach number, finds it not in agreement with the Reynolds number for the given geomtry, and defaults to to the lower velocity, resulting in laminar flow.

5. I checked everything, i did put the good reynold and i did increase the air speed to match the higher renold and it didnt affect my result...

6. Maybe 10,000,000 is out of range. Try a series of lower Reynolds numbers and see if there's a discontinuity in results.

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