ENGINEERING AND CFD

The helicopter and aeroacoustics group at IAG has simulated the complex aerodynamics and aeromechanics of rotorcraft since the early 1990s. The group added aeroacoustics to its areas of study a couple of years later, using modern-day simulation technology for highly resolved computational fluid dynamics (CFD) solutions, postprocessed to compute noise exposure at arbitrary observer points by means of the Ffowcs Williams-Hawkings approach.

Although most of the sound produced by a helicopter is generated by the main rotor, transport phenomena like shielding and reflection effects at the fuselage can change the noise pattern significantly, depending on the flight state. These effects can be handled by choosing an appropriate integration surface for the postprocessing step, namely surrounding the full aircraft. However, this requires an extremely accurate flow solver providing input data.

The IAG-based group has advanced the decade-long established flow solver FLOWer, integrating higher order accuracy and systematic concentration of spatial resolution in targeted regions, and has enabled results for complete helicopters at certification-relevant flight states within the variance of individual flight tests for the aerodynamic noise. The consideration of the fuselage influence was of paramount importance to meet absolute noise figures with this first principles approach, appropriate for new configurations, a success unrivalled within the international helicopter research community.

The prosperous growth of the group induced a corresponding demand for computing resources to extend the technology to more flight cases and other applications. Consequently, in 2017 the acoustic activities were split from our long-running HELISIM project. The latter is still active and concentrates on aerodynamic and flight mechanic investigations.

Recent CARo results pertain to acoustic interference effects on a compound helicopter with lateral propellers, as seen in this noise carpet, as well as advances including non-deterministic broadband noise for various flight states.

In a different, but related application, the same simulation technology is used for Contra-Rotating Open Rotors (CROR), a prospective propulsion technique for future short- and medium-range aircraft with significantly improved fuel efficiency. However, severely increased noise levels create a demand for acoustic shielding, for example by integration into the empennage of an airliner. The proper positioning is just one aspect of active research in this area.

The simulation technology developed and advanced in the helicopter group at IAG helps to understand such integration and interference phenomena of sound propagation, the influence of relevant parameters, and the underlying physics. The corresponding accurate representation of noise sources is provided by high-fidelity CFD simulations of complex configurations, the foundation for which was and is built within the adjacent HELISIM project. In this combination, ground-breaking research enables the development of helicopters and related rotating machines with reduced noise at equivalent or improved performance.