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Maxime Gauthier May 9, 2011 09:59

Questions regarding Particle Tracking and Rotating Frame of reference
Good morning all,

I am currently trying to simulate the passage of fish through a hydraulic turbine passage using particle tracking in CFX v13.0. My goal is to get a rough idea of the number and severity of impacts between fish and turbine components in order to estimate fish suvivability. My simulation is steady-state, and my domain includes 2 subdomaines: 1 guide vane passage (non-rotating frame of reference) and 1 turbine blade passage (rotating frame of reference), with a stage-type interface between the two.

The problem I am seeing is that that the particles tracks behave oddly in the blade passage (rotating frame): the particle tracks do not follow the fluid streamlines: they do not "move" towards the blade passage outlet, in fact they seem to "die out" soon after entring the rotating blade passage (the particle tracks end in the middle of the flow field).

Any hints as to what I am doing wrong?

Some more details concerning the simulation:
-Prototype-sized domain (24 guide vanes, 5 runner blades, Throat Diameter = 8.85 m, 65.5 rpm speed, 23785 kg/s mass flow at inlet, Ave. Stat. Pressure = MPa at outlet)
-Continuous Fluid: Water
-The fish are modeled with "Particle Transport Solid" morphology, with an approx. diameter of 0.12 m and and 0.63 cross-section area factor;
- Fluid Pairs (in both sub-domains): 1-way coupling, with Drag Coefficienct ~ 0.44 and Virtual Mass Force ~ 0.5;
- Inlet Conditions: particles injected at the inlet, 25 parcels with ~ 22.5 kg/s particle mass flow, Zero Slip Velocity;
- At the Walls, I set equation-dependent wall interaction with, with Restitution Coefficients : parallel coefficient = 1 and perpendicular coefficient = 0.25;
- I set the Expert Parameter PT TRACKS IN ABSOLUTE FRAME = t
- I set other forces, erosion models, heat transfer, etc. to None;

Maxime Gauthier

Maxime Gauthier May 9, 2011 15:07

Well, I found at least one thing I was doing wrong: I forgot to edit the Particle Termination Control, these where still set to the defaults and the default maximum tracking Length was still set to 10 m, which was not enough. Raising it to 100 m solved my problem.

Now I have a new issue: I want to evaluate the number of impacts and the severity of the impacts of particles on the walls, and I found that the Particle Boundary Vertex Variable: Wall Stress seems like a good candidiate. Plotting this variable along with the Particle paths, I found that that the Wall Stress was > 0 Pa at wall boundary vertexex closest to where the particles hit the walls; To count the impacts, all I have to do is count the amount of vetexes where wall stress > 0 Pa.

The second aspect is the severity of the collisions (by which I mean how hard the particle (i.e. )fish will hit the wall, trying to ultimatly determine how many fish will survive). The magnitude of the wall stress might be a good indicator of the collision severity, but I have trouble finding the equation that CFD-Post uses to calculate the Wall Stress ( I cannot find it in the documentation). Does anyone know the equation for the Wall stress with Particle tracking?


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