TAGGED: #fluent-#cfd-#ansys
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July 8, 2024 at 4:17 pmEthan SklarSubscriber
Hello, I am currently working on a 3D simulation in Ansys Fluent that involves external flow over a finned projectile at high Mach Numbers (>5). I have several issues that I keep running into:
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1. When I am constructing my mesh, I have my max element size set to 0.3m, I then input an element size face sizing feature for the projectile specifically and reduce the element size to 0.1m. I then input a body sizing (for an extruded box that was created in designmodeler in the wake) for wake refinement, also setting the element size to 0.1m. I then try to add an inflation feature for the projectile and every time I receive an error that says a stairstep mesh was created in some locations and this reduces my othogonal quality to extremely low, unacceptable, values. Do you know why this keep occurring or how I can fix it?
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2. I have established my "setup" settings as density-based and transient, as I understand those are better for high mach number, compressible flow. Is this true? When I use pressure-based and steady, the simulation converges as opposed to density based where it typically does not. For my model I am currently using k-epsilon realizable, yet I have also used k-omega SST, do you know which may be better for my application?
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3. For my boundary conditions, I have set up a fairly large enclosure around the projectile. Previously, I had several named selections, being inlet, outlet, walls, and projectile. Yet I read that for high mach number flows, it is better to define the entire enclosure as pressure far-field and enter in a mach number for the flow. I now have two named selections, being pressure far field for the entire enclosure, and the projectile. Is this correct? Is there a different way that I should be establishing my boundary conditions?
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4. For most of the simulations I have run so far, my residuals have started low, and then increased largely to the point where most of the time, the calculations fail and a "floating point" error appears. Do you have any suggestions for how to go about defining specific mesh characteristics, or specific setup settings for my particular case? I previously tried running pressure-based, steady simulations on this same projectile and the contours that I obtained were not detailed at all and only appeared very close to the projectile surface.
5. I am trying to determine center of pressure of the projectile that is oriented in the XY plane. I set Z = 0 yet the CP x value is no where near where the expected CP should be. Do you have any tips for how to go about this?Â
Any assistance that can be provided would be greatly appreciated.
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Thank you.
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July 9, 2024 at 1:40 pmFedericoAnsys Employee
Hello Ethan,Â
let me provide some suggestions to your questions. I am assuming that you are using Fluent's Watertight Geometry Workflow.
1) Rather than using a separate box for the wake of the projectile, I would suggest you use a Body of Influence (BOI). A BOI is a local mesh setting that allows you to control the mesh size locally. The BOI option is set from the Add Local Sizing task enables the generation of a local body sizing that can be smoother than what might be possible by using multiple bodies with shared topology options activated in SpaceClaim or the Form New Part option in Design Modeler. When using a Body of Influence, it's important to note that the body used is not divided into mesh elements but serves only as a reference for controlling mesh size.
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July 9, 2024 at 1:46 pmFedericoAnsys Employee
2) The Density Based solver is particularly suitable for high-speed flows where shock waves might be present, as it can capture the shock waves and expansion fans accurately. What type of solution initialization are you using? Full MultiGrid (FMG) initialization would be appropriate for your case.
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July 9, 2024 at 1:48 pmFedericoAnsys Employee
3) Yes, pressure farfield with projectile (wall) boundary conditions is the preferred way for this sort of problem.
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July 9, 2024 at 1:51 pmFedericoAnsys Employee
4) Solution convergence will be highly dependent on mesh quality. Hence, I would recommend you work on the mesh and achieve at least minimum orthogonal quality > 0.1 before your bring your model to Fluent solution. Also, as mentioned in 2), you should use the density-based solver with a good quality mesh to ensure you resolve your flow accurately in this regime.
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July 9, 2024 at 1:52 pmFedericoAnsys Employee
5) Center of pressure will be dependent on your surface pressure solution on your projectile, so I recommend you make sure that your solution is correct, following some of my suggestions above, before working on this.
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