Encyclopaedia Index

### (a) When to use BFCs

Simulations are often required of flows around bodies of arbitrarily curved shapes, such as ships, airplanes or automobiles; or within ducts having complicated curved boundaries, such as pipe bends or centrifugal-compressor passages; or of flows with internal discontinuities such as curved shock waves, flame fronts or phase- change boundaries.

The body-fitted-coordinate (BFC) feature of PHOENICS is designed for use in such simulations.

Before beginning the introduction to the BFC feature, however, it is useful to point out that PHOENICS does possess simpler means of handling curved boundaries, etc, and that these may even be prefer- able on grounds of ease of set-up and of computational economy. Three such means will now be mentioned.

### Alternative 1

The use of porosity factors to block out portions of a cartesian (or cylindrical polar) grid is recommended in many situations, especially for domains containing internal obstacles with discontinuous boundary shapes.

An example would be an auditorium, with rows of steps as the lower boundary and the seats (and people) as the blockages. The representation of the steps by blockages is very straightforward, although it should be said that the control cells which disappear under the steps are wasted, unless the flow beneath the floor requires to be simulated.

### Alternative 2

Porosity factors greater than unity can be used as grid-expansion factors, provided that the rate of expansion set by them is not too rapid.

When gross rather than detailed aspects of the flow are of interest, this method is a highly-recommended one. An example in the Input Library is the one-dimensional compressible flow in a convergent-divergent nozzle, with an outlet pressure which is high enough to cause a shock wave to appear in the divergent section.

Another typical application is to the prediction of the flow of effluent in a bay. For this a 2-dimensional analysis can be used for the horizontal directions (x and y) and the variation of the vertical dimension of the domain necessary to encompass the variation of sea bottom with position can be represented by appropriate settings of the east-face, north-face and cell-volume porosities.

### Alternative 3

The parabolic option of PHOENICS is especially useful for the analysis of the growth of shear layers, boundary layers,jets and wakes.

The lateral dimensions of the domain can be expanded with downstream direction to accommodate this growth by means of the parameters AZXU and AZYV (note that BFC=T is not permitted for parabolic simulations).