Reynolds Number and Turbulence in Atmospheric Boundary Layers

[Martina835]

Hi people! I'm studying the problem of a house in a valley with a wind at 5m/s. If I'd like to compute Reynolds number here and know if the flow is laminar or turbulent which is the charateristic dimension? The height of the house?

 

[mfb]

The height or the width, yes. If they differ too much, then it will need more thought (in doubt, I would take the smaller one).

 

[boneh3ad]

This depends on what portion of the flow you wish to study, as the Reynolds number requires a characteristic length scale and selecting that length scale depends on the geometry of the problem. So what part of the flow are you hoping to check for turbulence? If it is the wake created by the house, then it is likely the width of the house that should be used. If you want to know whether the atmospheric boundary layer itself is turbulent, the answer is almost invariably yes. Even if it is not, Reynolds number is itself meaningless for determining the laminar/turbulent state of an open flow like a boundary layer.

 

[Martina835]

I am studying a 2D geometry so I just have the length and the height! Yes I wish to know if the flow around the house is turbulent...much away from the house (1-1.5km up) I have an unperturbated flow.

 

[boneh3ad]

Okay, but you still haven't really unambiguously described what you mean by "the flow around the house" in this context. Do you mean the incoming boundary-layer flow or the wake behind the house? Like I said before, the wake would indicate the use of some spatial dimension of the house as the characteristic length in the Reynolds number. In this case it would be hard to say whether the height or width would be more appropriate, but I would suspect the height is more appropriate based solely on experience.

Otherwise, if you just want to know if the incoming flow is turbulent, I also said that the answer is essentially just "yes" with an atmospheric boundary layer. An atmospheric boundary layer is essentially always turbulent. Even if it wasn't, Reynolds number does not tell you whether a boundary layer is laminar or turbulent.

I'll also caution you that a 2D flow here will be very irrelevant when comparing to the actual, 3D flow around a house.