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

Convection velocity of Coherent structures

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   February 1, 2002, 16:27
Default Convection velocity of Coherent structures
  #1
Jongdae Kim
Guest
 
Posts: n/a
From the numerical simulation of wake region of bluff body, I got phase averaged flow fields. Coherent eddies from phase averaging - let's say these as coherent structures - is moving along streamwise direction with some convective velocity.

When I substracted 86% of free stream velocity from the phase averaged flow field, the streamlines of coherent structures look reasonable.

Now, I'd like to calculate more accurate convective velocity explicitly. Because the convecitve velocity of coherent structure is not constant not only along the lateral (or across wind) direction but also steamwise direction.

Please let me know how to calculate it,

At every mointoring points, the convective velocities are calculated based on the correlations between the point and other points. Considering the slope of the line connecting max. correlation points as convective velocity,the convective velocities are more than 90 % of the free stream velocity.

There are two possibilities :

First,the max. position of the correlation of the velocity between two points can not explain the coherent structures from phase averaging.

Second, the data size is not enough to calculate correlation. The velocity data is about one-second long in time. The shedding freq. of the vertical (or across wind direction) is about 48 Hz.
  Reply With Quote

Old   February 4, 2002, 11:54
Default Re: Convection velocity of Coherent structures
  #2
Lionel Larcheveque
Guest
 
Posts: n/a
I had a quite similar problem. I used two points-two times correlations and phase averaged velocity field (2D because I have an homogeneous direction that I use to increase the number of samples of my statistics data) to compute the velocity of coherent structures in a shear layer above a cavity.

1 : classical way : two points-two times velocity correlations and then I look at the time evolution of the correlation peak when distance increase.

2 : I educe my structure using Weiss criterion W (similar to Q or lamda_2 criterion for 2D flows) for each phase (i.e I use phase averaged velocity field as an instantaneous velocity field). I define a region (elliptic area in my problem) surrounding each of the structures at each phase. Then I compute the centroid of Weiss criterion for phase phi and each structure n on the area Omega_(phi,n):

<x_i>_(phi,n)=int_OMEGA_(phi,n)[x_i*W.dOMEGA] / int_OMEGA_(phi,n)[W.dOMEGA]

with W=-.5[ (du/dx)^2 + (dv/dy)^2 + 2*(du/dy)*(dv/dx)]

Instead of using Weiss criterion, it is possible to use rot magnitude with quite correct results.

Hope this helps

Best regards
  Reply With Quote

Old   February 4, 2002, 12:54
Default Re: Convection velocity of Coherent structures
  #3
Jongdae Kim
Guest
 
Posts: n/a
Thank you Lionel. I'll try the method you recommended.

Jongdae
  Reply With Quote

Old   February 5, 2002, 05:04
Default Re: Convection velocity of Coherent structures
  #4
Lionel Larcheveque
Guest
 
Posts: n/a
Just one more remark:

If you use Weiss criterion to compute centroids, you have to use a cutoff as negative value of Weiss criterion means shear and not rotation (as for Q criterion, vortical regions are educed by W > 0). Personally, I set the cut-off to 0.5 (U_inf/L)^2.

For the vorticity version, you can have a look at: Lyn et al., 1995 : A laser-doppler velocimetry study of ensemble-averaged characteristics of a turbulent wake of a square cylinder JFM 304, pp. 285-319

Best regards
  Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

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
UDF error - parabolic velocity profile - 3D turbine Zaqie Fluent UDF and Scheme Programming 9 June 25, 2016 20:08
help with UDF for contact angle based on contact line velocity gandesk Fluent UDF and Scheme Programming 14 October 29, 2012 14:58
how to compute relative velocity from absolute? spk Main CFD Forum 3 July 9, 2010 09:42
Emergency:UDF for a time dependent parabolic velocity zumaqiong Fluent UDF and Scheme Programming 12 March 25, 2010 13:00
Velocity Under-relaxation in SIMPLE type methods Matt U. Main CFD Forum 6 July 4, 2005 06:29


All times are GMT -4. The time now is 11:20.