### 1.2.7. Compressible Flow

CFX can solve for subsonic (less than the speed of sound), transonic (close to the speed of sound), and supersonic (greater than the speed of sound) flows when compressible fluid models are used. Compressible flow is activated in CFX-Pre by using an ideal gas or real fluid or a general fluid whose density is a function of pressure.

When modeling high-speed compressible flow (transonic or supersonic), you may want to use supersonic inlet and/or supersonic outlet boundaries. In this case, you must use the total energy heat transfer model.

A large source of convergence problems arise due to non-physical problem specification, not only in supersonic flows. When you are setting up a problem make sure that the boundary conditions, material properties and geometry are all physically consistent. For instance, for certain internal geometries, there is a maximum achievable mass flow rate. The solver will not produce a converged solution with mass flow boundary conditions specified with a higher value than this limit. Another potentially problematic situation is when an oblique shock wave impinges on a boundary. To begin with, you should try using a slip wall to represent this boundary. Once convergence is achieved, you can switch the boundary to a supersonic Outlet. This may or may not converge, as a domain boundary/shock wave interaction is difficult to model at an Outlet. It may be necessary to move the side boundary in such cases.

For compressible flows, the pressure level should always be set. The pressure can be specified:

- By one of the boundaries in the simulation in the form of a pressure specified inlet, outlet or opening (in transient compressible flows, the initial conditions can set the pressure level).

- If the pressure level is not set at any boundary, it can be set at a particular location using the Pressure Level Information section on the Advanced tab of the Solver Control dialog box.

If the pressure level is not set, the CFX-Solver will set zero relative pressure at node 1. During the course of solution, this may yield negative relative pressures, and if the reference pressure is also small, this may lead to negative absolute pressure values, which is physically inconsistent. The CFX-Solver will then attempt to calculate the density with a negative value of absolute pressure, and will fail. For details, see Setting a Reference Pressure.

#### 1.2.7.1. Mixed Subsonic/Supersonic Boundaries

If you expect the conditions at an Inlet to be mixed subsonic/supersonic, you should use the Mixed flow regime option.

If you expect the conditions at an Outlet to be mixed subsonic/supersonic, there is no way to currently deal with this in CFX. Usually, the way to resolve such a situation is similar to the Outlet boundary problem where inflow occurs (that is, to move the boundary to a position where the conditions are known to be either fully subsonic or fully supersonic).

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