[Nek5000-users] LES and heat transfer in channel

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

Hi Markus,

it's hard if something is wrong with your setup or if you're running into other issues.

I would start with a simple DNS of turbulent heat transfer in a channel e.g. similar to Hawamura et. al Journal of Head and Fluid Flow, 1998.

Let's say you do the following simulation:
Re_tau = 180
Pr = 0.71
Domain: (12.8/2/6.4) where you're reference length is the channel half-height
Resolution: 24x12x24 elements using N=9 (this should be pretty well resolved)
Thermal BC: uniform heat-flux heating

- How do the Nek results compare to the results of Hawamura?

I think that's a good way of testing your setup and after doing this exercise you're sure that everything works correct (assuming you get the right answers). 
Then the next step is to use a much coarser resolution together with a SGS-model (e.g. the simple filtering and/or dynamic Smagorinski).

- How do the LES results compare to the DNS results?   

I would be not too surprised if you cannot get good results in your LES. It's well known that turbulent passive scalar mixing behaves quite different and a flow SGS model will not do a very good job in this case.


Cheers,
Stefan


On Feb 9, 2010, at 4:34 AM, 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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