About Compressible Flow

You can simulate the effects of compressible flow when you define the fluid physics for the simulation.

See Also
Defining the Fluid Physics of a Flow Simulation
About Physics in a Flow Simulation

The compressible flow algorithm solves the compressible Navier-Stokes equations in a segregated manner. It is suitable for high-speed flows (Mach number greater than 0.3), where compressibility effects can no longer be neglected.

Compressible flows are frequently encountered in the aerospace industry. The flow over the wing of a commercial aircraft and the jet coming out of an engine are classic examples where a compressible algorithm is required to capture the correct flow physics. Many other applications can also be found in other industries, such as natural gas pipelines and supersonic wind tunnels.

As the Mach number increases, the segregated algorithm faces increasing difficulty in solving the compressible Navier-Stokes equations. The compressible solver can handle Mach numbers up to approximately 2, depending on the configuration.

The robustness of a compressible simulation is sensitive to its initial conditions. The algorithm includes a default initialization method to compute appropriate pressure, velocity, and temperature fields. However, you can override this method with custom values. In this case you should follow these guidelines:

  • When you use a stagnation inlet boundary condition, initialize the flow field so that it is close to the stagnation conditions (pressure set to stagnation pressure, and a small but nonzero velocity).
  • If you are facing convergence issues during the start-up phase, create a multi-step simulation and run the first step with reduced under-relaxation values and a frozen energy equation. After the solution stabilizes, run a second step with the energy equation turned on and regular under-relaxation values.

The compressible flow algorithm is an extension of the incompressible solver, so the two workflows are similar. The main difference is that the incompressible solver assumes the density to be constant, while the compressible solver uses the energy equation in conjunction with an equation of state to compute density fluctuations.

Because of the different nature of the equations being solved, not all boundary conditions are applicable to compressible flow. A velocity inlet can be used but is technically not well suited to simulations with higher speeds; therefore, you should use stagnation inlets and mass flow inlets for high-speed simulations instead.