DNS of blasius flow for trasition

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

 October 15, 2009, 19:05 DNS of blasius flow for trasition #1 New Member   Join Date: Oct 2009 Posts: 29 Rep Power: 9 I try to do DNS of blasius flow for trasition. I presume I put the correct unstable modes and boundary condition, including the code, and disturbance equations. But I can only see TS waves damping downstream. I follow the work of Fasel. But my question is, since the streamwise velocity magitude along constant y actually decreases in the downstream direction (i.e. as the boundary layer grows), why do the waves grow in the first place. I am intrigued, and must have missed something. Can someone in this field explain to me about it using simple logic. For instance how do I treat pressure values, i.e. I just use the pressure to enforce divergence free flow. and obviously for bl dp/dx=0. So do I need to enforce p_upstream_edge=p_dowstream_edge?. But already in the base flow u/u_infinity=1 ouside the boundary layer. TAW

 October 19, 2009, 03:22 #2 Member   Edison Join Date: Mar 2009 Posts: 40 Rep Power: 9 (1) "why do the waves grow in the first place." I want to ask the same question!

 October 19, 2009, 09:47 #3 Member   Join Date: Mar 2009 Posts: 55 Rep Power: 9 I am not quite sure why the disturbance grow. But depending on the stability charecteristic of the flow these disturbances can amplify or die down along the flow. When the amplitude of TS waves are small compared to the base flow, you can use linearised stability equation to find the growth characeteristic, since the growth depends on linear characeteristic.

 October 19, 2009, 21:23 #4 New Member   Shyam Join Date: Apr 2009 Posts: 29 Rep Power: 9 It might be related to the boundary layer instability used in estimating the laminar to turbulence transition zone. This can be mathematically analyzed by linearizing the NS equations for the boundary layer, and then doing a fourier analysis to estimate its stability characteristics. This, I think, will provide the stability characteristics of the boundary layer flow. Please correct me if I am wrong.

 October 19, 2009, 21:49 #5 New Member   Join Date: Oct 2009 Posts: 29 Rep Power: 9 Thank you for the above responses, I do take both replies correct. It is true the LST actually provides the stability of the flow to infintismal amplitude disturbances, and I was able to obtain this result as in the published papers on stability planes. That solution is an eigenvalue solution method, and the result is in wavenumber-Re space. My problem, is when I wanted to reproduce this result with FV-DNS. Many publications have successfully reproduced this. For instance, in DNS the outflow bc is the source of the problem, as it creates unphysical wave reflections, and I did some absorbing condition for that. But still, I only see a damping wave, for the most unstable frequencies, and I said that is reasonable if the stream wise velocity is decellarating downstream. But it is not the case, and I was wondering what I have missed. Right now I am investigating if the absorbing bc has contribution on the unexpected damping. Regards, TAW

 October 20, 2009, 03:42 #6 Member   Join Date: Mar 2009 Posts: 55 Rep Power: 9 Correct me if I am wrong, but it is in accelerating flow that you may see dampening of turbulence generation. Positive acceleration in effect increased transition region. In decelerating flow transition is more easily induced. I am speaking in terms of the physics of transition process.

 October 20, 2009, 04:24 #7 New Member   Join Date: Oct 2009 Posts: 29 Rep Power: 9 Hi SKK, Thanks for the reply. I think what you said is correct; that for instance favorable pressure gradient dp/dx

 Tags blsius flow, dns, transition

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post goodegg Main CFD Forum 12 January 22, 2013 12:47 diaw Main CFD Forum 104 February 16, 2006 06:44 youngan CFX 0 June 30, 2003 02:32 Pravir Kumar Rai FLUENT 0 February 19, 2003 15:03 ff_fan Main CFD Forum 1 September 12, 2002 02:28

All times are GMT -4. The time now is 14:02.