Turbulence model & y+ for Natural convection
Hi everybody,
I'm simulating a radiator in a room: since a I want to get a very accurate solution about the power generated by the radiator, I'm trying to create a model that let me to predict the laminar-turbulent transition of the boundary layer along the geometry (that is quite complex and close to the real one). Does anybody have experience with natural convection? Maybe I must use Low-Re correction or some complex turbulence model like k-kl-omega or Transition SST (not simply two-equations models)... and i think that y+ should be < 1 (now y+ < 4) Any advice? |
Hello
In my experience in natural convection with transition to turbulent , the best model was the k-e realizable enhanced wall treatment (I compared with experimental resoults). As advice, you should take into account the the thermal boundary layer in the mesh, and maybe, also include radiations models (if you didnīt include it) |
Hi sicfred, thanks for your answer! :)
I have a doubt: k-epsilon model shouldn't be very useful for boundary layer, so probably it can't predict accurately heat transfer (that depends strongly on the type of boundary layer, laminar or turbulent). Anyway I'll try it! ;) |
In terms of turbulence models generally kwsst with a y+=1 mesh are generally more accurate but an RKE model using enhanced wall treatment combined with a y+=1 mesh will not be very different. The important thing is that for accuracy you will need a y+ approaching 1, ideally <5 everywhere. You will inevitably have high aspect ratio cells here but in these cases the fluent can cope since the variation in solution variables is normal to the cell (little variation tangentially). When using these very high aspect ratio cells you may need to run double precision (3ddp) but I would suggest trying single precision first as double will use more memory.
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Thanks foamcfd! :)
What about using II order Upwind instead of I order for the discretization? Does it offer a great advantage? Here below 3 images of the system I should study: http://img441.imageshack.us/img441/7708/radiatore3l.jpg http://img835.imageshack.us/img835/306/radiatore2.jpg http://img268.imageshack.us/img268/4518/radiatore1b.jpg |
Discretization method depends on the mesh you have. Complete hex mesh (aligned with flow) results in first order would little differ from second order results. However for pressure discretization I would go with BWF or Presto for natural convection problems. If you want to see the difference, analyze a case with standard method and have a look at near wall velocity vectors.
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with heat transfer y+ of 4 is probably not good enough. Try to get at least 2 grid points into the linear region (y+ < 5), so shoot for a y+ ~= 2 with a small stretch ratio between cells or go to y+<1 and you can use a bigger stretch ratio. Quote:
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I'm quite sure that it happens: the Rayleigh number is about 10^8 (so in the transition interval). In the lower part the flow is laminar of course, but it increases the velocity and in the upper part of the radiator some tests with smoke underline the turbulences...(ANSYS suggests RNG k-epsilon or k-omega models) some other data: T mean rad= 64 °C T air = 20°C |
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I'll try with a laminar model, too! :)
Another question: this is a closed domain so it's possible that the mass (continuity) could be not conserved... right? |
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ok, I said this because I saw the continuity residuals a bit high (it's difficult to push them under 1)
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Similar question.
Hi guys,
I've got a similar question. I'm running k-omega SST and want to get the best results possible (as this is only a validation case). I'm confused with the following choices regarding the kw-SST: 1. How is the wall resolved when the "low reynolds number" box is not ticked? Is it wall functions? 2. Similarly, how is the flow resolved when this box is ticked? Does this then imply a very dense mesh (y+<1)? It seems that a few other people are also having problems with this, so maybe we can clear this up for good. Thanks. |
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As for how the wall modelling is handled: All of the omega based models use an approach nearly identical to the enhanced wall treatment with the exception of the omega model itself (which is integrated explicitly and does not need to resort to a two-layer approach, the blending of the viscous and log law region are built into the omega equation). |
Low Reynolds number correction in turbulence models refers to the accuracy to capture the viscous layer in your BL (viscous layer = low reynolds number)
Without this corretion you will be able to only capture your log-law region at best. Please correct me if i'm wrong LuckyTran Regards Luke |
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How can I resolve the boundary layer on the wall? I.e. suppose I actually want to have a very fine mesh at the walls and want to directly (don't mean DNS here) compute the flow, how can I do this? Thanks a lot once again. You guys been very helpful. |
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Hello Folks,
Sorry I post here as it passed over a year but I have a problem how to check my Y+ value...in other words, how can I make sure hat I have a fine mesh close to the walls...I wanna solve natural convection... Thank you! |
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