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August 1, 2008, 14:30 |
Cp drops along the wind tunnel
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#1 |
Guest
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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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August 1, 2008, 17:39 |
Re: Cp drops along the wind tunnel
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#2 |
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You should make sure you have everything set up correctly and that you grid is good.
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August 1, 2008, 17:52 |
Re: Cp drops along the wind tunnel
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#3 |
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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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August 1, 2008, 18:31 |
Re: Cp drops along the wind tunnel
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#4 |
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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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August 1, 2008, 19:36 |
Re: Cp drops along the wind tunnel
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#5 |
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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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August 2, 2008, 10:29 |
Re: Cp drops along the wind tunnel
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#6 |
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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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August 2, 2008, 21:16 |
Re: Cp drops along the wind tunnel
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#7 |
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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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August 4, 2008, 04:31 |
Re: Cp drops along the wind tunnel
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#8 |
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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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August 6, 2008, 06:46 |
Re: Cp drops along the wind tunnel
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#9 |
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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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