CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > General Forums > Main CFD Forum

Cp drops along the wind tunnel

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   August 1, 2008, 14:30
Default Cp drops along the wind tunnel
  #1
Francis
Guest
 
Posts: n/a
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,

  Reply With Quote

Old   August 1, 2008, 17:39
Default Re: Cp drops along the wind tunnel
  #2
mettler
Guest
 
Posts: n/a
You should make sure you have everything set up correctly and that you grid is good.
  Reply With Quote

Old   August 1, 2008, 17:52
Default Re: Cp drops along the wind tunnel
  #3
Charles
Guest
 
Posts: n/a
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.
  Reply With Quote

Old   August 1, 2008, 18:31
Default Re: Cp drops along the wind tunnel
  #4
Francis
Guest
 
Posts: n/a
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,
  Reply With Quote

Old   August 1, 2008, 19:36
Default Re: Cp drops along the wind tunnel
  #5
otd
Guest
 
Posts: n/a
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?
  Reply With Quote

Old   August 2, 2008, 10:29
Default Re: Cp drops along the wind tunnel
  #6
Paolo Lampitella
Guest
 
Posts: n/a
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.
  Reply With Quote

Old   August 2, 2008, 21:16
Default Re: Cp drops along the wind tunnel
  #7
otd
Guest
 
Posts: n/a
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.
  Reply With Quote

Old   August 4, 2008, 04:31
Default Re: Cp drops along the wind tunnel
  #8
otd
Guest
 
Posts: n/a
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.
  Reply With Quote

Old   August 6, 2008, 06:46
Default Re: Cp drops along the wind tunnel
  #9
Paolo Lampitella
Guest
 
Posts: n/a
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.
  Reply With Quote

Reply


Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
how to consider perforated wind tunnel wall in CFD simulation littlelz CFX 4 June 11, 2009 06:51
Cp drops along the wind tunnel Francis FLUENT 0 August 1, 2008 14:31
Simulating a Göttingen type wind tunnel in FLUENT Chris FLUENT 3 December 4, 2006 18:33
Virtual Wind Tunnel in FLUENT ND FLUENT 0 April 7, 2006 08:43
Wind Tunnel Website now online Mike Worthey Main CFD Forum 0 June 6, 2000 03:27


All times are GMT -4. The time now is 18:56.