[Nek5000-users] LES and heat transfer in channel

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

Hi,

thanks for the answers.
Before considering DNS calculations, I'd like to look into the 
Smagorinski model. Extending the turbulent channel example in the nekton 
source, is the following the correct way to set user defined properties 
for the energy equation?

       subroutine uservp (ix,iy,iz,ieg)
       include 'SIZE'
       include 'TOTAL'
       include 'NEKUSE'

       common /cdsmag/ ediff(lx1,ly1,lz1,lelv)
      $              , thermdiff(lx1,ly1,lz1,lelv)

       ie     = gllel(ieg)
       udiff  = ediff(ix,iy,iz,ie)
       utrans = 1.
       if (ifield.eq.2) then
         udiff  = thermdiff(ix,iy,iz,ie)
         utrans = 1.
       endif

       return
       end

To answer some previous questions, the excess heat is subtracted from 
the inflow temperature (via an energy balance between the periodic 
sections).
The flow is driven with a constant flow rate, much like in the jet example.

Markus


nek5000-users at lists.mcs.anl.gov wrote:
> 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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>> Nek5000-users at lists.mcs.anl.gov
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> 
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