Week 9- Unsteady moving zones Approach

Aim:

• To compare the results of turbomachinery component with Moving Reference Frame Approach and Moving Mesh Approach.

Take Away:

By the end of this article, one will be able to,

• Compare the results for both the approaches.
• Simulate the study with the help of two approaches.
• Plot the results and contours for better data inference and visualization respectively.

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Overview of Moving Reference Frame:

Problems that involve multiple moving parts cannot be modeled with the Single Reference Frame approach. For these problems, you must break up the model into multiple fluid/solid cell zones, with interface boundaries separating the zones.

Zones that contain the moving components can then be solved using the moving reference frame equations, whereas stationary zones can be solved with the stationary frame equations.

The manner in which the equations are treated at the interface lead to two approaches:

1) Multiple Moving Reference Frame:

- Multiple Reference Frame model (MRF).

- Mixing Plane Model (MPM)

2) Sliding Mesh or Moving Mesh.

Both the MRF and mixing plane approaches are steady-state approximations, and differ primarily in the manner in which conditions at the interfaces are treated.

The sliding mesh model approach is, on the other hand, inherently unsteady due to the motion of the mesh with time.

Multiple Reference Frame model (MRF):

The MRF model is, perhaps, the simplest of the two approaches for multiple zones. It is a steady-state approximation in which individual cell zones can be assigned different rotational and/or translational speeds.

The flow in each moving cell zone is solved using the moving reference frame equations. If the zone is stationary (ω">ωω = 0), the equations reduce to their stationary forms.

At the interfaces between cell zones, a local reference frame transformation is performed to enable flow variables in one zone to be used to calculate fluxes at the boundary of the adjacent zone.

It should be noted that the MRF approach does not account for the relative motion of a moving zone with respect to adjacent zones (which may be moving or stationary); the mesh remains fixed for the computation. This is analogous to freezing the motion of the moving part in a specific position and observing the instantaneous flow field with the rotor in that position. Hence, the MRF is often referred to as the “frozen rotor approach.”

While the MRF approach is clearly an approximation, it can provide a reasonable model of the flow for many applications. For example, the MRF model can be used for turbomachinery applications in which rotor-stator interaction is relatively weak, and the flow is relatively uncomplicated at the interface between the moving and stationary zones. In mixing tanks, since the impeller-baffle interactions are relatively weak, large-scale transient effects are not present and the MRF model can be used.

Another potential use of the MRF model is to compute a flow field that can be used as an initial condition for a transient sliding mesh calculation. This eliminates the need for a startup calculation. The multiple reference frame model should not be used, however, if it is necessary to actually simulate the transients that may occur in strong rotor-stator interactions, as the sliding mesh model alone should be used.

Sliding Mesh Model/Moving Mesh Model:

The sliding mesh model is a special case of general dynamic mesh motion wherein the nodes move rigidly in a given dynamic mesh zone. Additionally, multiple cells zones are connected with each other through non-conformal interfaces.

As the mesh motion is updated in time, the non-conformal interfaces are likewise updated to reflect the new positions each zone. It is important to note that the mesh motion must be prescribed such that zones linked through non-conformal interfaces remain in contact with each other (that is,“slide” along the interface boundary) if you want fluid to be able to flow from one mesh to the other. Any portion of the interface where there is no contact is treated as a wall.

Reference: Ansys Fluent Guide.

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Simulation Procedure:

There are two approaches to this problem; 1) Steady State and 2) Unsteady State

1) Steady State Approach:

• Import the geometry file provided.

General Tab:

Physics Tab:

• Viscous Model:

• Cell Zone Condition:

T = 60/N

T= 60/300 = 0.2secs (minimum time required for one single rotation)

The above calculation is for unsteady state solution.

• Boundary Condition:

Inlet and Outlet are pressure-inlet and pressure-outlet respectively.

• Hybrid Initialization.

2) Unsteady

The only change here is change the 'Time' to unsteady in genelar tab.

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Resuts:

1) Steady State:

• Residual:

• Mass Flow Rate:

• Pressure Contour:

• Velocity Contour:

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2) Unsteady State:

• Residual:

• Mass Flow Rate:

• Pressure Contour:

• Velocity Contour:

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