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high and low reynolds numbers - what does it mean

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Old   September 4, 2002, 13:25
Default high and low reynolds numbers - what does it mean
  #1
Bo Busk Jensen
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Hallo all

I am trying to come up with a good explanaiton on what the difference is between a high and a low reylnolds number (in the turbulent flow regime). I know that the reynolds number express the relation ship between inertial and viscous effects. But what exatly does this mean?? If I have a pipe flow of Re = 10,000 and Re = 500,000, what are the differneces of these to types of flows?? Is the turbulence intensity different?? Boundary layer thickness is different?? etc.

Regards

Bo
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Old   September 4, 2002, 16:48
Default Re: high and low reynolds numbers - what does it m
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John Dreese
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Typically, the rate of growth of the boundary layer is inversely proportional to the Reynolds Number. The slower you go, the quicker the boundary layer grows in height. Think of it this way: as the airspeed increases, it tends to slam down the boundary layer for a longer distance... unless of course you have separation or a strong pressure gradient. This is a general statement that doesn't include the detailed differences between laminar and turbulent boundary layer growth.
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Old   September 5, 2002, 03:23
Default Re: high and low reynolds numbers - what does it m
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Lars Ola Liavåg
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Hello Bo

For laminar flows, you might think of it like this: All real flows are in one way or another subject to smaller or bigger perturbations or disturbances (i.e. variations in the momentum/inertial forces), and the viscosity is the factor that acts to restrain these disturbances. Above a certain limit of the Reynolds number, the inertial forces are so large in relation to the viscous ones that the perturbations are not sufficiently damped and will start to grow in stead. The transition from infinitesimal disturbances to finite size fluctuations results in turbulence: The flow breaks down to into substructures (eddies, vortices, slabs or blobs of coherent fluid movement) where the viscous and inertial forces balance each other more closely. These structures continue to break up and interact with other ones until such a balance is reached, after which their remaining mass is entrained in other flow structures.

This process consumes the momentum of an eddy, and even that of other interacting ones. The higher the momentum available in the flow as a whole, the smaller the length scales it can maintain. Thus, for turbulent flows, even a Reynolds number based on the mean stream or far field velocity and a "hardware" length scale carries some information about the range of turbulence length scales and therefore also says something about the mixing properties and kinetic energy dissipation of the system. On the other hand, I suppose there are more appropriate tools to look into the details of those processes. I guess the thumb rule for already turbulent flows is: Higher Re means "more turbulent".

Apart from determining whether the flow is laminar or turbulent, I have had little practical use for this parameter myself. But then perhaps I'm more of an engineer than a researcher.

Regs.

Lars Ola
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Old   September 5, 2002, 06:41
Default Re: high and low reynolds numbers - what does it m
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Tom
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Yes, all the quantities, mean/bulk velocity, fluctuations, pressure gradient, the extent of the logaritmic layer etc. are a function of Re. Only when you scale these statistics and also the disctance to the wall with a proper quantity and make them non-dimensional, the statistics have some common features. To learn more about that you can read the standard textbooks from Townsend, Pope or Schlichting.

Tom
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