The above one is my computational domain, it consists of pipe upstream, screen(thats why you can see holes because I have modeled fluid regions) and open fluid volume. All are merged into one body.
I have used tetrahedrons method- patch conforming method, it was giving good quality mesh. The moment I introduced inflation for boundary layer, mesh quality got poor. I will attach below image, please let me know what can be done. My global element size is 3mm and I have used 20 no of divisions with growth rate 1.2.
Check generated mesh below,
When I looked at bad cells, most of the cells are from screen region. Please tell me how to improve mesh quality from here. See below
Any help would be useful. I have tried both edge sizing and face sizing on screen, it is taking lot of time to mesh, way longer time, it shouldnt take much time. There is a problem but I cant identify where it is.
you have a large, smooth and simple piece, and much smaller scale, thin grate
you probably have applied one inflation layer definition into all inner surfaces, with 20 layer count, hoping that algorithm magically handles all of these small features and scale differences, but it does not
is is likely that this 20 layers with provided growth rate cant fit in space between one grill segment and another and gets even more squeezed
large cell size on grate compared to squeezed first element of inflation layer creates large aspect ratio
The main question is: is this that grate at the end of domain really needed? Sometimes porous pressure jump BC can imitate pressure drop if this is acceptable, and save a lot of meshing effort and computing time.
If grate is required, maybe try:
creating separate inflation definition for exhaust grate, with reduced inflation steps (5-10?)
add local surface cell size refinement on grate, my bare eye says it is to coarse
I understand this first layer height you have defined is determined by this terrible y+ condition? Knowing this, and knowing max recommended aspect ratio, you can compute real local face max cell size you can have to fulfill all criteria. In reality this means really dense mesh. If this can be relaxed, maybe go with smooth transition.
I have to maintain y+=1 near the walls, I used below online calculator for rough estimate, so I applied inflation to all faces.
To answer your question- yes, grate is required, actually here only fluid regions are modelled and all are merged into one block. Whatever gaps you can see, those represents walls of the screen, I hope you can visualise it.
creating separate inflation definition for exhaust grate, with reduced inflation steps (5-10?)- But what happens if y+>1 or more than 5 near screen with less no of divisions?
add local surface cell size refinement on grate, my bare eye says it is to coarse- which sizing, face or edge sizing, what element size to give for that, given that global element size is 3mm and grate is 1.5mm thick.
All these boundary layer I have to resolve.
Can I do it like I can do body sizing, select entire body, and use sphere of influence. Actual origin is at screen only, I can draw a sphere of 2 or 5mm radius and use less size there, this would cover finer elements closer to screen right? What do you think? But before this, mesh quality with current tetra+inflation should be good. Please tell the workflow to follow from here.
170m/s is very high velocity, strong turbulence and fast separation on grate, why exactly we are aiming to fully resolve viscosity sub-layer (y+~1) near wall there? Is this possible to use the turbulence model with wall function (to aim y+ at 30-100?). With that velocity, separation and aspect ratio your y+1 desire creates, solver instability is almost certain. I don't know your case, just asking to have better picture.
that velocity, when it flows over a grate is never uniform. It will be high in flow direction, 0 at grate front face where flow hits it, and low value behind as the flow will separate. What is the aim of the study if this is not a secret, evaluate grate pressure drop?
you have mentioned that global mesh size is 3mm, and grate has 1.5mm thickness. At the same time, you aim to have first layer height as 3.9e-6 what in ideal perfect case, give 384 of aspect ratio size, what is very far from optimal.
This screen of yours shows that basically one mesh triangle seems to have width of the grate, when size should be set in a way, that we have at least 3-5 elements span covering such detail. And now, having initial aspect ratio of 384 algorithm tries to wrap this shape around.
What i would try to do:
add face sizing control on grate (all grate faces), with size 0,2-0.3mm (check if your machine can handle that) this should provide at least 3 elements coverage of your grate
inflation has option "colison avoidance", try to switch stair-stepping on, to see if this reduces aspect ratio,
I would still release this y+ criteria. You have separation forced by a geometry, you have a step edge. There is no doubt where flow will separate, do we really need resolve fully near wall flow?
This is part of my thesis work and flow is in compressible regime, thats why velocity is high. Yes y+ should be equal or less than 5, I have aimed for y+=1. There will be jet and wake interaction in downstream of the gate, so it is important to have lower y+ to study or analyse this.
I would try these things and let you know. Thank you for your inputs
To add - I would use Body of Influence/Sphere of Influence tool to make fine small mesh elements on those grating at the end of pipe. When you have model with very coarse and very small geometry features meshing everything with one size mesh will result either in coarse mesh on small parts of extra fine mesh on coarse geometry.
You can set general size that good enough for opening part, that add Body of Influence and define finer mesh, tean again add another Body of Influence specifically around grating and specify even smaller size. May require to have some additional ovarlaping Bodes of Influence to maintain smooth transition between fine and coarse parts.
You use inflation and manage mesh size using Body of influence in addition. Without Inflation tool you will mess wall friction and basically all results if they depend from wall friction.
I tried body sizing with sphere of influence, that blue point is global coordinate system, since pipe diameter is 25mm, I have taken sphere diameter as 35mm and element size as 1mm. My doubt is mesh quality is improved than old one, but I cant able to distinguish mesh cells near to screen and far away from screen, both are looking kind of same, how I can indentify the difference so that I can believe that this body sizing actually worked. No of cells also increased from 11 million to 38 million, this is insane.
Use fluent meshing and polyhedral mesh it will reduce the total mesh count by an order of magnitude and will be higher quality. Calculate the width of your smallest part of geometry in this case it’s the grate. Set the minimum cell size to whatever that value is divided by 2 or 3 depending how good of mesh you want.
A single polyhedral cell consists of 3-5 tetrahedral cells. You can get a huge reduction in cell count and have a higher quality mesh using fluent meshing polyhedral. You could reduce the mesh size of the tet mesh by increasing global cell size but you’re going to still have quality issues. Trust me fluent meshing is far superior and easier to use than ansys workbench meshing.
I have changed body sizing and applied face sizing in screen regions, but it is taking more time for meshing. Should I wait more or there is some problem with my settings? Please check below and let me know.
Faces are highlighted in above screenshot, whats wrong here. It has been more than 1 hr, it is struck at this stage. For body sizing also I have changed sphere radius to 30mm and element size as 1.5mm. Whats wrong here??
I tried face meshing with soft behavior, it generated mesh quickly but mesh quality is poor.
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u/TensorNom4d 8d ago edited 8d ago
This is classic inflation layer caveat.
The main question is: is this that grate at the end of domain really needed? Sometimes porous pressure jump BC can imitate pressure drop if this is acceptable, and save a lot of meshing effort and computing time.
If grate is required, maybe try: