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,

You should make sure you have everything set up correctly and that you grid is good.

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.

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,

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?

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.

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.

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.

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.