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Mesh Disappering with Dynamic Mesh

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Old   February 23, 2021, 19:26
Post Mesh Disappering with Dynamic Mesh
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Hello Everyone,

I am simulating somewhat of a unique dynamic mesh using ANSYS fluent 2019 R3, therefore I have had trouble referencing tutorials online and the user guide I had available didn't fully get into the details of the many parameters under mesh methods. I am referencing a previous work to recreate the following conditions, but it is not fully documented.

Basically, I am simulating a 2D symmetric nozzle that has a needle through it with a highly viscous flow (laminar). The needle moves up and down the nozzle with a sin velocity profile and a frequence of 100 Hz. The maximum displacement is 7 Ám. Named sections, the geometry, and my meshing can be viewd in the attached images.

I am using a UDF and ansys in serial mode (parallel processors set to 0) for now. When I display mesh motion, I can observe the wall that is set to move actually oscillates as it is supposed to when using the gernal time and not a reference to flow time.

UDF code with the volocity:

#include "udf.h"
#include "dynamesh_tools.h" 

DEFINE_CG_MOTION(needle_velocity, dt, cg_vel, cg_omega, time, dtime)
	//double t = RG_Get_Double("flow-time");
	cg_vel[0] = sin(2.0*M_PI*100.0*time)*0.0007*M_PI;
	cg_vel[1] = 0;
	cg_vel[2] = 0;

	cg_omega[0] = 0;
	cg_omega[1] = 0;
	cg_omega[2] = 0;
One thing about this UDF is my use of the currect time. Is that right? Since if I choose the flow time, the rigid body motion just moves forward and does not oscillate.

As for the dynamic mesh settings:
  • tme step: 0.0001 s
  • smoothing: diffusion (with a diffusion parameter of 0 and up to 250 iterations)
  • layering: off
  • Rmeshing: local cell (wihth defaul length scale parameretrs)
  • Minimum length scale (m): 6e-8
  • Maximum length scale (m): 5e-5

I set the interior to also be deforming and I am not sure if that is correct. My mesh is fine and with good skewness and orthogonal quality, as well as aspect ratio. The simulation runs with no issues without dynamic meshing. But once I implement dynamic meshing, the mesh disappears and minimal orthogonal quality goes to zero. I tried with a much coarser mesh and I still get the issues. Also, I get the warning that left handed faces are derected in the console.

The issues that I think migh be causing this:
  • Incorrect dynamic mesh zones
  • Incorrect mesh methods
  • Mesh is too fine for the gien time step and settings (18 k cells)

If you follow the image of the mesh that I provided, the following are the settings for my dynamic mesh zones:
  • Interior - deforming
  • Moving needle wall - rigid body (moving with UDF)
  • nozzle wall - stationary
  • symmetry - deforming
  • top needle wall - deforming

I left out the refinement zones that I created and the borders that they have to the rest of the zones. I also defined my interior with a named section, could that be causing an issue too?

I know this is a lot, but I would also appreaciate some guidance in the right direction with some best practices when it comes to dynamic meshing.

Thank You,
Attached Images
File Type: jpg Named Sections.jpg (62.1 KB, 3 views)
File Type: jpg Mesh.jpg (64.8 KB, 3 views)
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