# Computational Fluid Dynamics (CFD)

CFD (Computational Fluid Dynamics) code enables to predict steady flows (drag and lift) and slow transient flows like heating and defrosting.

Aero-Acoustic is the engineering field dealing with noise generated generally by a turbulent fluid flow interacting with a vibrating structure. This field differs from pure acoustic domain where the object is the propagation of acoustic pressure waves, including reflections, diffractions and absorptions, in a medium at rest.

A classification of Aero-Acoustic problems can be made using the following three categories:
• External wind noise transmitted to the inside through a structure: In the automotive industry, a pillar, side mirror and windshield wipers noise are typical problems of this category.
• Internal flow noise transmitted to the outside through a structure: Examples of this class of problems are exhaust, HVAC and Intakes noises.
• Rotating machines noise: Axial and centrifugal fans are noisy components that bring with them many interesting Aero-acoustic problems.
The necessary ingredients to perform direct Aero-Acoustic numerical simulation are implemented in a single numerical code and they are:
• Compressible Navier Stokes: Able to propagate pressure waves; and therefore take into account in a single simulation the flow and the noise including all possible cavity modes.
• Fluid structure coupling: Able to treat the problems involving a turbulent flow, one side of the structure and the noise radiation on the other side.
• Transient turbulence modeling: Unlike the Reynolds Averaged Navier Stokes (RANS) method that make the assumption that flow is a combination of a steady state and turbulent fluctuations. Aero-acoustic noise is directly linked to the small scale turbulent fluctuations and strongly time dependent.
• Acoustic boundaries with set impedance: This is a critical point of a good Aero-Acoustic simulation. Boundaries need to be able to perform tasks, such as giving a free field impedance to an inlet with fixed velocity, setting a specific impedance at the outlet of a duct to make sure long wavelength stay trapped inside, treat exterior air impedance effect on a vibrating structure and be used to model absorbing materials (carpet and foams) that are used to coat many components.
• Large Eddy Simulation Turbulence modeling: The noise induced by turbulent structures is taken into account properly. Unfortunately, the turbulent structures that are simultaneously active any given time range from the full size of the problem to the microscopic Kolmogorov size.

These components are needed to perform Aero-Acoustic simulations with no particular assumptions on the flow (except of course, the use of a turbulence model), the fluid structures coupling or the vibrations.