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

LES with dynamic model

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

Reply
 
LinkBack Thread Tools Display Modes
Old   July 19, 2008, 11:24
Default LES with dynamic model
  #1
Nabi
Guest
 
Posts: n/a
Hi; I am solving a turbulent channel flow with complex geometry by LES. To avoid the averaging in the homogeneous plane, I have used a Lagrangian Dynamic Model of Meneveau (1996). For initial condition, Cs=0.16 is chosen. I recognized that the value of Cs varies between 0 and 1. The high values of Cs causes to remove the turbulence fluctuations (makes the flow laminar). To avoid high values for Cs I clipped the Cs when it exceeds 0.16. I have now a turbulent channel, but the velocities are over predicted. I have some questions: 1. Is it usual in dynamic model to get the Cs higher than ~0.16 in some regions of the domain? 2. Is it a correct way to clip the Cs when it becomes large? Thanks

  Reply With Quote

Old   July 19, 2008, 12:09
Default Re: LES with dynamic model
  #2
pankaj saha
Guest
 
Posts: n/a
Hi,

I am also working on turbulent channel flow. Could you share some aspect?

What initial condition you are using ? i hope you are using periodic b.c. then what is the mean driving pressure gradient.

----------------------------------------------------------

As , for your quearry, i can tell , what i am using.

First--You do localised volume averaging of cs over the test filter cell. Then you use clip.

everybody use clip to avoid numerical instability.

thanks
  Reply With Quote

Old   July 19, 2008, 12:30
Default Re: LES with dynamic model
  #3
Nabi
Guest
 
Posts: n/a
Thanks pankaj saha for help. I am using periodic b.c. in stream and span-wise directions. For the normal direction, no-slip in the bottom and free sleep at the top. The bottom is not flat. I am solving the flow over a wavy bed. I use a Cartesian grid with ghost cell immersed boundary condition. The initial condition is randomly by using sin and cosine for u, v and w. The flow is driven by pressure gradient to make the bulk velocity unit. As I mentioned, I am using Lagrangian Dynamic model. In Lagrangian Dynamic Model by using ramp function for I_LM, we avoid the negative viscosity. The problem arises only from the high value positive viscosity.

Can you please let me know, which kind of Dynamic Model you use? Plane averaging or Lagrangian?

  Reply With Quote

Old   July 19, 2008, 12:37
Default Re: LES with dynamic model
  #4
pankaj saha
Guest
 
Posts: n/a
I am using Localized volume average. It is not palne average.

See, From formulae you will get, cs. Then do the ageraging over the neighbouring test volume cell. Like evering time you will get 27 cell average value.

Could you please post, the expresion you are using for the 3 componeent of velocity with random function....menas the for u, v, w .

Thanks.--

If you have any problem, i will send you a paper, which told how to do vol.average
  Reply With Quote

Old   July 19, 2008, 12:46
Default Re: LES with dynamic model
  #5
Nabi
Guest
 
Posts: n/a
Thanks pankaj for reply. I initialise as follow:

do k = 0,kmax+1

do j = 0,jmax+1

do i = 0,imax+1

u(i,j,k) = 0.01 * sin(i+j*k+0.) + 1.0 ! 1.0 is the bulk velocity

v(i,j,k) = 0.01 * sin(i+j*k+1.)

w(i,j,k) = 0.01 * sin(i+j*k+2.)

p(i,j,k) = 0.0

enddo enddo enddo

Can you please send me the paper for volume averaging of cs. Thanks

  Reply With Quote

Old   July 19, 2008, 14:39
Default Re: LES with dynamic model
  #6
pankaj saha
Guest
 
Posts: n/a
Physics of fluids, A5(12), pp-3186-3196.

Zang,Street, Koseff.

---------------------------------- If, you dont have, any access to p.o.f, then tell me your mail id, i will send you a copy.

here , in appendix, the vol, avg technique is there.

------------------------------------------------------ Doubt: You, use 0, 1, 2 in the three component of velocity. What is that?

Also, you did not use, coordinate distance. Means, only the i,j,k----No, x,y,z is there.?

Could you write in details?

Also, are you supplying driving pressure gradient by itarative technique or simply a body force term??
  Reply With Quote

Old   July 20, 2008, 00:37
Default Re: LES with dynamic model
  #7
Nabi
Guest
 
Posts: n/a
Thank you to send me the paper. I have access to download it. I used i, j, k to make a random field of velocity in the domain. If you use x, y and z, in this case for two points near together, you will have not much differences in their velocities. Because you have no other source to apply the perturbations (because of periodic boundaries) the turbulence will maybe die for relatively low Reynolds numbers. If we apply i,j and k for all three components with the same function (here sin) we will have the same perturbation for all three components. To make a difference between the velocities components, I added arbitrary numbers 1, 2 and 3. After giving a random field, you need to apply the boundary conditions. For pressure gradient, I used a body force. The body force is calculated from the bulk velocity. In any time step, I considered " fx=(Ubulk0-Ubulk)*cof ". Ubulk0 is the desired and Ubulk is the calculated bulk velocities. Because the difference is not very high to balance fx, you need to multiply it by a coefficient. For example I use 100. Don't forget to use Ubulk0 a little higher that the desired value to have a body force in case you reach Ubulk0. Determining value for cof and Ubulk0+eps is a try and error problem. If you have large fluctuations, fx will maybe oscillates, because Ubulk will be not constant in time. In this case you need to calculate the average Ubulk in time. Thanks

  Reply With Quote

Old   July 20, 2008, 15:34
Default Re: LES with dynamic model
  #8
pankaj saha
Guest
 
Posts: n/a
fx=(Ubulk0-Ubulk)*cof

I hope, you are adding 'fx' in streamwise momentum equation.

see, for any Re, Ubulk0 has a desired value. So, it is fixed before the code is run. for e.g--Re=4000=Ubulko*H/v So, IF you take h=1 and Ubulk0=1 v=1/4000 is the setting condition for code.

Now, for any time you will get Ubulk, from code and it will give fx, thereafter.

So, when Ubulk0=Ubulk then fx=0, so, the code does not get any body force. You, are saying so you are taking an eps. How, you set that one . That means, code is not calculating eps, rather you supply it manually.

i did it, by some formuallae----like below.

p_new=p_old(1+cof((Ubulk0/Ubulk)**2-1)

First, you take a guess value of p_old, Also, Ubulk0, is known from Re. Ubulk is calculated , everytime.

And your body force term, p_new, will be updated at everytime step. when, you reach , Ubulk=Ubulk0, then after, P_new=P_old...so, no need of trial and error.

You just put this, P_new to your x-momentum equation.

try this--this is more scientific.

  Reply With Quote

Old   August 4, 2010, 12:01
Default QUICK Scheme
  #9
Member
 
Join Date: Mar 2009
Posts: 46
Rep Power: 8
mmahdinia is on a distinguished road
Hi,

I wanted to know if anyone has used QUICK scheme in LES despite it is too dissipative. According to Mittal and Moin (1997) the QUICK is too dissipative for reactive flows and generated noise flows and there central schemes should be used. But for ordinary flows it is nearly sufficient.

I would appreciate very much it if anyone could tell me about any resent journal articles in which QUICK (or B-QUICK) is used.

Sincerely,
Maani
mmahdinia is offline   Reply With Quote

Reply

Thread Tools
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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Undefined symbol error after compiling a new LES model jabhiji OpenFOAM Programming & Development 7 March 6, 2015 05:36
How to use dynSmagorinsky model correctly ? panda60 OpenFOAM 120 November 20, 2014 16:07
A bug in using dynamic LES models arslantuf OpenFOAM Bugs 6 December 8, 2009 17:34
LES and combustion model Margherita Cadorin CFX 0 October 29, 2008 06:24
Algebraic Dynamic LES Model noName FLUENT 0 March 24, 2005 18:34


All times are GMT -4. The time now is 06:25.