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Modeling Moving Walls in CFD:
Transient Simulation with Moving Meshes
 
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Why should a CFD engineer care?

Very often a CFD simulation will require the movement of two bodies in relation to each other. Classic examples of these include high speed elevators moving down a shaft, overtaking automobiles or objects that are in relative motion to other bodies or the stationary frame.

The main problem with these types of analyses is the issue surrounding mesh deformation and skewing of mesh elements. Simply put, if you have two walls within the same domain moving with a relative velocity V, it is easy to see how after a certain time Δt, mesh elements near these surfaces would have either squeezed or stretched. In cases where the movement (and mesh deformation) is small, the results might still be accurate. In others cases however (computational aeroacoustics is a good example), it might be crucial that the mesh size does not exceed a certain length, so as to capture the wavelengths of interest. In these cases then, if the mesh is skewed or stretched beyond a certain length, the validity of the results does come into question.

There are alternatives of course which include setting up dynamic adaptive meshing, or setting up the mesh elements at the start of the simulation such that the distortions experienced during the solution do not affect the accuracy. While this is doable, the time and effort required to achieve this is at least twice as much when compared to setting up a good stationary mesh in the first place.

In using ANSYS, CFD engineers at CADFEM UK CAE Ltd. are able to tap into the Sliding Mesh capability to circumvent the above problems.

How does it work?

Consider the scenario where we want to know the flow field around two objects as they approach each other. Instead of keeping the model as a single domain, we can split the model such that each object is contained within a separate domain. Then in ANSYS CFX using Regions of Specified Motion, we can assign each domain a velocity V1 and V2, while also having them speak to each other via a Domain Interface. As the Figure 1a-b show, using multiple domains with sliding meshes removes the concern of mesh skewing altogether. The only constraint we have to adhere to is keeping an equal mesh on either side of the Domain Interface, so that information is passed through between the domains seamlessly.

 
Single Domain showing highly skewed meshed elements   Multiple Domains with unskewed elements

 

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