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Cp drops along the wind tunnel

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Old   August 1, 2008, 13:30
Default Cp drops along the wind tunnel
  #1
Francis
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Hi Gents,

I am working on flow along a low speed wind tunnel. When I plot out the Cp along the sidewall, it actually drops down. I am wondering why this happens, is it due to the development of boundary layer or energy dissipation?

Many thanks,

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Old   August 1, 2008, 16:39
Default Re: Cp drops along the wind tunnel
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mettler
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You should make sure you have everything set up correctly and that you grid is good.
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Old   August 1, 2008, 16:52
Default Re: Cp drops along the wind tunnel
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Charles
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Wouldn't you expect a pressure drop if you use energy to force air through a channel? There are subtleties involving boundary layer thickness and divergent tunnel walls as well, but a basic pressure drop sounds kind of realistic to me.
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Old   August 1, 2008, 17:31
Default Re: Cp drops along the wind tunnel
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Francis
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Hi guys,

Thanks for your replies.

The model I am using is quite simple, 2D with straight sidewall, i.e. not a divergent duct. What are the reasons for a basic pressure drop along the sidewall?

Thank you very much,
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Old   August 1, 2008, 18:36
Default Re: Cp drops along the wind tunnel
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otd
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The Cp should be high in the entrance region, dropping to a lower, constant value as the entrance effects diffuse out.

Are you sure you are looking at pressures downstream from the entrance region?
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Old   August 2, 2008, 09:29
Default Re: Cp drops along the wind tunnel
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Paolo Lampitella
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You should be more clear about your b.c. because to have flow through a tube, channel etc. you NEED a pressure drop, that is a pressure gradient opposed to the flow direction, that is the driving force of your flow.

What i understand from your case (a simple straight 2D channel flow)... is that the pressure drop it's a must.
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Old   August 2, 2008, 20:16
Default Re: Cp drops along the wind tunnel
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otd
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You're right. But

His pressure drop should be proportional to the Cp, something like Delta p = rho*Cp* v^2/2 (?).

What he says is that the Cp is dropping along the channel and not approaching a constant value as it should downstream of the entrance region.

My suggestion is that he try a longer channel to make sure he calculates flow downstream from the entrance region.
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Old   August 4, 2008, 03:31
Default Re: Cp drops along the wind tunnel
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otd
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My equation is not correct!

Closer is

Delta p /delta x = Cp * A * rho * V^2 / 2

x is the coordinate that measures increasing channel length

A is (constant) channel cross-sectional area

rho is fluid density

V is bulk velocity (mass flow / (rho * A) )

In the entrance region Cp drops rapidly as the boundary layers form, grow and merge.
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Old   August 6, 2008, 05:46
Default Re: Cp drops along the wind tunnel
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Paolo Lampitella
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The Cp is usually known as:

Cp = 2 * (P - Pref) / (rho * Vref^2)

so your first one was maybe correct but, as stated before, in a straight channel you need a pressure graient so the pressure (and the Cp of course) should not approach a costant value but, as for example in the Poiseuille flow, should be linearly varying (diminishing) from the entrance to the exit.
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