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Farfield Size and Shape

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Old   August 7, 2014, 09:17
Default Farfield Size and Shape
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Travis Carrigan
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I had an someone ask me about my best practices for defining a farfield. What's a good size? What's a good shape? I thought I'd share my response with the community.

Depending on whether you are generating a structured or unstructured grid, both the shape and the size of the farfield domain can have an effect on the solution.

First, the size. I've found that for an airfoil or a wing you'll want to have a farfield that has a radius of at least 100 reference chord lengths. This will help ensure that the flow field instability caused by the wing does not propagate beyond the farfield boundary. If you take a simple airfoil example and compute the coefficient of lift while changing the farfield size, you'll notice it changing to a certain point at which any changes are negligible.

Second, the topology. The topology can often be dictated by the choice in mesh type, structured or unstructured. If you're creating a structured domain, it's relatively easy to generate an o-type or c-type topology. I've found the o-type to be very easy to generate and good at computing the coefficient of lift. However, it's not that great for computing the coefficient of drag due to the lack of wake resolution (see this video, A c-type grid is much better at computing the coefficient of drag due to the topology's inherent ability to capture the wake.

Lastly, the shape. Similar to my comments above, the shape is also often dictated by the choice in the mesh type. If a structured mesh is desired, you'll find creating a hemispherical farfield or a c-type farfield more natural. For unstructured, you can create any farfield shape you desire. I've traditionally used hemispherical farfields because they are easy to construct, have a constant radius, and it's much easier to specify the boundary conditions for the solver. Most solvers have a farfield type boundary condition that you can assign to the entire farfield domain.
Travis Carrigan
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Pointwise, Inc.
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Old   April 15, 2016, 19:29
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Could you tell what is the meaning of farfield and farfield resolution?
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