## 232 Outlet

The surface of the solution domain through which the flow exits may be defined as the outlet. Normally the outlet boundary condition implies that gradients normal to the outlet boundary are zero for all the variables except pressure. If the direction normal to the outlet boundary is denoted by y, the outlet boundary conditions can be expressed as dS

It is not necessary to specify pressure at the outlet boundary.

Use of the outlet boundary condition is not appropriate when the gradients at the outlet boundary are not zero or when conditions downstream of the outlet boundary may influence flow within the solution domain (see Fig. 2.4, which shows inappropriate use of outlet boundary conditions). For such cases, it may be necessary to enlarge the solution domain to ensure that all the gradients vanish at the outlet boundary of the enlarged domain. Alternatively, a pressure boundary condition may be used (as discussed in the previous section). For the exit boundaries, static pressure is defined

instead of total pressure. For outflow through the boundary, this pressure is used to calculate the outflow velocities. If an inflow occurs through such a boundary, the kinetic energy of the incoming fluid is then assumed to be negligible.

For multiphase flows, it is possible to encounter a boundary of the solution domain through which one of the phases exits the domain but not the other (for example, the top surface of the bubble column reactor). Special boundary conditions need to be developed to represent such cases. These are discussed in Chapter 5.

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