|
[Sponsors] |
Simple question? On mesh aspect ratio on boundaries |
![]() |
|
LinkBack | Thread Tools | Search this Thread | Display Modes |
![]() |
![]() |
#1 |
New Member
Join Date: Mar 2009
Posts: 17
Rep Power: 17 ![]() |
Hi all,
I keep reading and hearing about problems that mesh elements with high aspect ratio can give. However, I keep seeing that boundary mesh next to the wall have really huge aspect ratios. Canīt that give convergence problems? Or is it that somehow in the boundaries where boundary layers are expected somehow it helps more than worsen the problem? Thanks |
|
![]() |
![]() |
![]() |
![]() |
#2 |
Senior Member
Join Date: Jul 2009
Posts: 357
Rep Power: 19 ![]() |
Generally speaking a poor aspect ratio can cause convergence problems. What you see when you look at a lot of grids is the tradeoff of practical run-times versus aspect ratio - i.e. we accept worse aspect ratios than we might like in order to keep the number of grid points down to a realistic number. We do have one benefit in boundary layers in that the gradients in the streamwise direction are smaller than the transverse gradients, so the large aspect ratio is not as much of a problem as it could be (unless you have to deal with separated flow).
|
|
![]() |
![]() |
![]() |
![]() |
#3 |
Senior Member
John Chawner
Join Date: Mar 2009
Location: Fort Worth, Texas, USA
Posts: 275
Rep Power: 18 ![]() |
TfG: I agree with what agd wrote and add these minor issues. For structured quad/hex grids high aspect ratios in the boundary layer cells are the norm and depending on the solver you can handle ARs of 10,000 or more. However, you still have to keep in mind the effect that those long cells are going to have on accurate representation of the true shape of a curved boundary.
Also, the issue of AR gets more complicated when you consider tri/tet unstructured meshes. A high aspect ratio triangle can have really poor included angles and that will destroy mesh quality. Put those in the boundary layer where you want to resolve large gradients normal to the wall or use a wall function and all bets are off in terms of accuracy. Hope this helps.
__________________
John Chawner / jrc@pointwise.com / www.pointwise.com Blog: http://blog.pointwise.com/ on Twitter: @jchawner |
|
![]() |
![]() |
![]() |
![]() |
#4 |
New Member
Join Date: Mar 2009
Posts: 17
Rep Power: 17 ![]() |
thanks for the info. really helpful
|
|
![]() |
![]() |
![]() |
![]() |
#5 |
New Member
william moore
Join Date: Jan 2013
Posts: 8
Rep Power: 13 ![]() |
My understanding is that high aspect ratio cells are less problematic if they are located within the boundary layer near the wall where the flow velocity normal to wall is generally low. In addition, in order to achieve the recommended y plus of +1 or less for enhanced wall treatment or k-omega SST turbulence models then it is recommended to use at least 10 inflation layers and preferably 15 or higher to provide high resolution inn the viscous sub layer and smooth transition to the turbulent region. For example the model that I am cuurently using has 15 inflation layers for tet prism layer mesh with a max cell skewness of 0.89 and a max aspect ratio of 100 near the walls. The minimum orthogonality is 0.15.
This mesh appears to give much better convergence than model based on using only 5 to 7 inflation layers, with higher y plus values. |
|
![]() |
![]() |
![]() |
![]() |
#6 | |
Senior Member
Meimei Wang
Join Date: Jul 2012
Posts: 494
Rep Power: 16 ![]() |
Quote:
__________________
Best regards, Meimei Last edited by Anna Tian; January 31, 2014 at 05:52. |
||
![]() |
![]() |
![]() |
![]() |
#7 |
Senior Member
Join Date: Jul 2009
Posts: 357
Rep Power: 19 ![]() |
When the flow separates it means that the gradients along the surface can be significant. A large cell aspect ratio simply means you have to be concerned that you are lacking tangential resolution to capture those gradients - so you may get inaccurate results for things like the location of a separation point. It's the same reason why we typically cluster grid points near leading and trailing edges. Unfortunately, separation location is not always so easily predictable, so this is just one more thing you have to consider and possibly justify/correct after making an initial run.
|
|
![]() |
![]() |
![]() |
![]() |
#8 | |
Senior Member
Meimei Wang
Join Date: Jul 2012
Posts: 494
Rep Power: 16 ![]() |
Quote:
Will very large aspect ratio cause any additional problem besides the considerations from the view of discretization error? You know, typically software tutorials on grids quality requirements suggest that the aspect ratio need to be smaller than 5000.
__________________
Best regards, Meimei |
||
![]() |
![]() |
![]() |
![]() |
#9 |
Senior Member
Join Date: Jul 2009
Posts: 357
Rep Power: 19 ![]() |
Clustering is used at leading and trailing edges because at attachment and separation points we want to capture the gradients that show up. The question arises when we don't have separation or attachment at a known location. Then the question is where do we need to have decent tangential resolution? Another issue that presents itself in highly stretched cells, especially in FV schemes, is the reconstruction of the overall gradient. It has been shown that weighted least squares is better than Green-Gauss most of the time - however one of the regions where Green-Gauss seems to be better is in viscous regions where the grid cells are highly anisotropic. So depending on how your solver is reconstructing the gradient there may be some additional issues with highly stretched cells.
|
|
![]() |
![]() |
![]() |
![]() |
#10 | |
Senior Member
Meimei Wang
Join Date: Jul 2012
Posts: 494
Rep Power: 16 ![]() |
Quote:
__________________
Best regards, Meimei |
||
![]() |
![]() |
![]() |
![]() |
#11 |
Senior Member
Join Date: Jul 2009
Posts: 357
Rep Power: 19 ![]() |
RANS models can be used to compute the lift coefficient for stalled airfoils. But as I have stated, the location of the separation point will be affected by the tangential resolution of the grid. Inaccuracies in the location of the separation point can have an impact on the accuracy of the lift calculation. How much of an impact is something that should be determined using your flow solver.
|
|
![]() |
![]() |
![]() |
![]() |
#12 | |
Senior Member
Meimei Wang
Join Date: Jul 2012
Posts: 494
Rep Power: 16 ![]() |
Quote:
__________________
Best regards, Meimei |
||
![]() |
![]() |
![]() |
![]() |
#13 |
Senior Member
Join Date: Jul 2009
Posts: 357
Rep Power: 19 ![]() |
Lift at the onset of stall can be computed using RANS models, but the typical calculation today will generally use a detached eddy version of the basic turbulence models. These will give better results, but the question of accounting for the correct separation/reattachment points still exists. A coarse grid (or a fine grid that is too coarse in the regions of separation and reattachment) can play a role in the accuracy of the lift and drag as well. How much of a role is generally problem dependent, and depends also on what you are trying to get out of your simulation.
|
|
![]() |
![]() |
![]() |
![]() |
#14 | |
Senior Member
Meimei Wang
Join Date: Jul 2012
Posts: 494
Rep Power: 16 ![]() |
Quote:
__________________
Best regards, Meimei |
||
![]() |
![]() |
![]() |
![]() |
#15 |
Senior Member
Join Date: Jul 2009
Posts: 357
Rep Power: 19 ![]() |
||
![]() |
![]() |
![]() |
![]() |
#16 | |
Senior Member
Meimei Wang
Join Date: Jul 2012
Posts: 494
Rep Power: 16 ![]() |
Quote:
__________________
Best regards, Meimei |
||
![]() |
![]() |
![]() |
![]() |
#17 |
Senior Member
Join Date: Jul 2009
Posts: 357
Rep Power: 19 ![]() |
I have run stall simulations for the NACA0012, NACA0015, and NACA63-018, among other profile shapes. I always run my simulations in time-accurate fashion, so I cannot answer your last question.
|
|
![]() |
![]() |
![]() |
![]() |
#18 |
Senior Member
Meimei Wang
Join Date: Jul 2012
Posts: 494
Rep Power: 16 ![]() |
You mean the simulations you ran on that are all transient? Have you tried 3D simulation on that?
__________________
Best regards, Meimei |
|
![]() |
![]() |
![]() |
![]() |
#19 |
Senior Member
Join Date: Jul 2009
Posts: 357
Rep Power: 19 ![]() |
I don't run simulations in steady state mode. I always use a time-accurate solver for my simulations, whether 2D or 3D.
|
|
![]() |
![]() |
![]() |
![]() |
#20 |
Senior Member
Meimei Wang
Join Date: Jul 2012
Posts: 494
Rep Power: 16 ![]() |
I found that the transient and steady-state simulations will give exactly the same results if both of them converge for 3D case. Probably transient simulation is just for a convergence issue.
__________________
Best regards, Meimei Last edited by Anna Tian; February 5, 2014 at 16:29. |
|
![]() |
![]() |
![]() |
|
|
![]() |
||||
Thread | Thread Starter | Forum | Replies | Last Post |
3D Hybrid Mesh Errors | DarrenC | ANSYS Meshing & Geometry | 11 | August 5, 2013 06:42 |
DOES ASPECT RATIO PLAY A PART IN CFD RESULTS? | RIF | Main CFD Forum | 2 | January 5, 2008 13:14 |
ICEM 10 mesh question | DAK565656 | CFX | 6 | May 8, 2007 12:16 |
Aspect Ratio at high turbulent and swirled flows | zago | FLUENT | 0 | May 19, 2004 02:37 |
unstructured vs. structured grids | Frank Muldoon | Main CFD Forum | 1 | January 5, 1999 10:09 |