Valve Wall Contact
Dynamic meshing allows for moving parts without the need of overset meshing which introduces interpolation errors at the component/background interface. For simulations that have wall-to-wall contact, dynamic meshing allows for contact detection to close the gaps and prevent fluid flow in these regions. This feature was added in Ansys Fluent 2021 R1.
A good use case for dynamic meshing is rigid body motion where boundaries or internal walls move relative to each other. The motion of the walls can be specified directly through the use of a UDF (User-Defined Function) or it can be a result of the passive solution using the 6-degree of freedom (6DOF) solver which will move the wall due to forces from the fluid solution instead of prescribed motion by the user.
Figure 2: Velocity Contour (Close up)
Figure 3: Velocity Vectors
A specific type of dynamic mesh is called layering. This technique takes advantage of hex or prism cells and removes an entire row at a time. The video below shows how the mesh is deformed near the valve seat wall. Layering avoids the use of complex re-meshing techniques and is therefore very robust. This technique works well for purely translational degrees of freedom such as piston or valve motion.
Figure 4: Mesh Layering
Dynamic meshing allows for CFD to model a very wide range of fluid flow applications without any motion restriction to wall boundaries or solid objects.
Water Turbine
The 6DOF solver is ideal for freewheeling turbomachinery whose rotational speed is a passive solution from the flow field. Applications of this model include: wind and water turbines, aircraft engine turbines, and automotive torque converters.
Below is an example of a water turbine with the water velocity set to 3 m/s. The turbine is initially held fixed and then released at t = 0 s.
Figure 5: Animations
Figure 6: Animations
There is very high acceleration for the first few moments (0 < t < 0.05 s) when the torque on the turbine is highest due to being held fixed and allowing the flow field to reach steady state. Then the turbine acceleration overshoots the toque imposed on it from the fluid and causes a brief deceleration. Once the rotation of the turbine and the flow field begin to rotate together, there is a steady acceleration until about 12 RPM where steady state operation is reached with the turbine rotating.
Figure 7: Angular Velocity vs Time
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