[Nek5000-users] LES and heat transfer in channel

[nek5000-users at lists.mcs.anl.gov]

Hi Markus,

How do you subtract off the excess heat ?

The std way would be something like:


       subroutine userq  (ix,iy,iz,eg)
       include 'SIZE'
       include 'TOTAL'
       include 'NEKUSE'
       integer e,eg

       common /fluxa/  flux_area,gamma_t

       e  = gllel(eg)
       qvol = -vx(ix,iy,iz,e)*gamma_t

       return
       end


Where gamma_t is chosen to balance your net influx through q"
at the surface.  Here, what one is effectively doing is (correctly)
postulating:

     T(X,t) = @(X,t) + gamma * x

where gamma is a constant and theta is periodic:  @(X,t) == @(X+L,t)

Then, nek solves for the periodic function @.   I guess w/ your
recycle bcs you're doing the same thing -- in which case you wouldn't
need the qvol bit above, assuming you subtract off gamma*L from
your incoming temperature.



Also, do you have fixed flow rate?

Also -- you might want to use PN-PN (lx2=lx1, etc), as we (Stefan)
found this to give much better results for LES.

Also, it could be that filtering really does not work well as an SGS
for the passive scalar eqn.  (It has the right look and feel for
momentum equation, particularly when one considers the effects on
the spectra; but I've not delved very deep in the case of temperature
yet...).


Paul



On Mon, 8 Feb 2010, nek5000-users at lists.mcs.anl.gov wrote:

> Hi,
>
> I am running a channel LES simulation (periodic in streamwise and spanwise, 
> length=width=9*channel height) in nek with the following parameters:
> -LES model: Filter last 3 modes, 5% filter weight; lx1=14; ld1=20
> -Piece of the rea file
> "
>   1.00000         DENSITY
>   0.31250E-04     VISCOS
>   1.00000         RHOCP
>   1.00000         CONDUCT
>   4.00000           p99=3 ==> dealiasing turned on
> T      IFFLOW
> T      IFHEAT
> T      IFTRAN
> T T F F F F F F F F F  IFNAV & IFADVC (convection in P.S. fields)
> F F T T T T T T T T T T  IFTMSH (IF mesh for this field is T mesh)
> F      IFAXIS
> F      IFSTRS
> F      IFSPLIT
> F      IFMGRID
> F      IFMODEL
> F      IFKEPS
> F      IFMVBD
> T      IFCHAR
> "
> -Reynolds number based on 2*channel height: 12800
> -Constant heat flux on walls
> -The streamwise boundary is a recycling one, I set it up similar to the 
> turbJet example, but I added temperature recycling. To avoid "over-heating", 
> I subtract the added thermal energy from the temperature when recycling
> -Some material parameters are set in the .usr file:
> "
> param(8)   = param(2)/0.71 ! Prandt=0.71
> cpfld(2,1) = param(8)      ! conductivity
> "
>
> When I compare the spanwise averaged and time averaged Nusselt number on a 
> wall, it is fairly constant with streamwise direction (which is good), but 
> about 50% higher then the experimental one from the correlation
> Nu=0.022*Re^0.8*Pr^0.5
> This might be because I am only 2.5 flow throughs away from the initial 
> condition, but I wanted to make sure that everything is right before putting 
> in more computing hours.
> Does the above look OK? Is there another explanation for the heat transfer 
> coefficient being too high?
>
> Thanks,
> Markus
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