Invited lectures
- Philip J. MORRIS, Pennsylvania State University, USA
Efficient Hybrid Methods for Computational Aeroacoustics in Complex Environments
Philip J. Morris and Yongle Du
Department of Aerospace Engineering, Penn State University, University park, PA 16802, USA
Predictions of noise generated by turbulent flows require a high fidelity simulation of the unsteady flow in the source region. This region may be only a small part of a larger flow domain that is not directly associated with the noise generation process. Examples include the noise from an installed jet engine or trailing edge noise from a wind turbine. It is computationally inefficient or impossible to perform a detailed flow simulation for the entire aircraft or the entire wind turbine blade. However, these flow environments have an influence on the noise production. This paper describes a hybrid method that permits a high‐fidelity simulation to be embedded in a larger scale lower‐fidelity calculation. The lowerfidelity simulation provides a good estimate of the average flow. It could be determined using any standard RANS code on either structured or unstructured grids. The high‐fidelity simulation is performed for perturbations about this estimated average in a limited region. It is shown how the associated disturbance equations can be written in such a way as to be compatible with any existing unsteady flow solver. Example applications will be given
- Xin ZHANG, University of Southhampton, UK
The stable modelling of sound propagation in conditions with sheared flow
Three linearised perturbation equations have been developed based on a divergence equation, to predict acoustic wave (curl-free) propagation stably and accurately in the time-domain. These equations are designed to overcome Kelvin-Helmholtz instability and Rayleigh–Taylor instability that are numerically developing in sheared flow. Since particle velocity of sound propagation is curl-free, the linearised perturbation equations are developed either directly from a divergence equation or using a divergence operator. In accounting for vortical wave propagation, a curl operator can be conventionally utilised to obtain two-dimensional vortical wave components. Several benchmark cases, including sound propagation in jet flow and shear layer, and modal radiation from semi-infinite unflanged duct with a jet, are employed to validate the proposed models. Stability analyses confirm the stability of the proposed equations in the presence of a background shear flow.
- Oleg RUDENKO, Lomonosov Moscow State University, Russia
Nonlinear Wave Phenomena in Fluids
In the beginning, the difference between (i) truly strong nonlinearity and (ii) strongly expressed weak nonlinearity is discussed. Both types are responsible for waveform distortion, shock fronts formation, and corresponding broadening of the frequency spectra. For example, extra-high-power noise close to jet or rocket engines can be considered as strongly nonlinear wave. On the other hand, strongly expressed nonlinear phenomena are observed when small, but finite amplitude waves travel over sufficiently large distances equal to hundreds or thousands wavelengths. Some nonlinear phenomena significant from the viewpoint of aeroacoustics are reviewed. Among them are transformations of different noise spectra, noise control by high-intensity signal, sonic booms in turbulent atmosphere, standing waves in gas-filled resonators, nonlinear sound absorbers. The attention is focused also on new mathematical models including nonlinear integro-differential equations (IDE) and quadratically cubic nonlinearities. IDE, for example, describe regular and noise waves in atmosphere where the spectrum of relaxation times of different gases should be considered for correct description of shock waves. Some exact and approximate analytical solutions are given
- Blanche DEMARET, Delegate Director for Rotorcraft, Business Development Directorate, ONERA, The French Research Lab
ONERA (Office National d’Etudes et de Recherches Aérospatiales) is a public, scientific and technical
establishment with both industrial and commercial responsibilities, created in 1946, ONERA reports to the French
Ministry of Defence and is financially independent. The expertise of ONERA covers all the scientific disciplines involved in aircraft, spacecraft and missile design. ONERA conducts research in the disciplines and techniques involved in design of an aircraft or spacecraft: aerodynamics, flight dynamics, propulsion, structural strength, materials, optics and laser, acoustics, radar and electromagnetism, electronics, systems, robotics, information processing. The research is focused on federating themes and programmes, such as fluid mechanics and information processing. Onera has contributed to some of today’s most successful aerospace and defence programs.
Rotorcraft aerodynamics and aeroacoustics research at Onera
Noise is a key issue for the public acceptance and the development of rotorcraft missions. The paper will present the status of the art at Onera considering Aerodynamic and aero acoustics tools and solutions to reduce noise:
· from the physical phenomena and numerical simulation tools, such as CFD,
· then comprehensive codes to describe noise emission (focus on rotor noise, Blade vortex interactions)
· the validation through comparisons with experimental data bases is a key point
· example of innovative solutions
· internal noise, description, models and solutions
· how to go further.
- Fabien WLASSOW, Numerical Methods Group, Acoustic Department - EPA6, AIRBUS Operations S.A.S, France
Numerical acoustics in Airbus: current status and future applications
An aircraft manufacturer has to face many challenges related to aero-acoustics in order to produce an aircraft responding to the noise regulation. An overview of the topics addressed at the acoustic department of Airbus will be presented and the contribution of numerical acoustics will be highlighted. An overview of the methods and tools used on a daily basis by acoustics engineers will be detailed with a specific focus on acoustic liner design used to reduce forward fan noise. Then the presentation will focus on future applications where new computational aero-acoustic approaches are seen to be mandatory to help understanding noise mechanisms and to develop low noise technologies. As an illustration, the CFD-CAA strategy developed to predict Open Rotor noise will be presented.
- Dimitris DRIKAKIS, Professor of Fluid Mechanics and Computational Science, Cranfield University, Cranfield MK43 0AL, UK, http://drikakis.com
High-Order, All Mach Number CFD Methods for Complex Aerospace Applications
Dimitris Drikakis, P. Tsoutsanis, AF Antoniadis, Z. Rana, and I. Kokkinakis
We present a computational fluid dynamics (CFD) software suite which comprises a range of computational algorithms in the framework of high-resolution and high-order (physics-based) methods for aerospace applications. The CFD software suite, called Azure, includes the following features: different Riemann solvers; numerical discretisation schemes ranging from 2nd order to 9th-order of accuracy; multi-block structured-grid, as well as fully unstructured and hybrid grid-based capabilities for handling any arbitrary geometry; Reynolds-Averaged Navier-Stokes (RANS), Implicit Large Eddy Simulation (ILES), and Detached Eddy Simulation (DES); multi-physics models. Azure provides a computational platform for studying fundamental flow physics, simulating engineering flows around or inside complex geometries, as well as assessing the accuracy and computational efficiency of different numerical schemes and physics-based models, thus reducing the computational uncertainty at an optimal turnaround computational time. The presentation gives an account of the advancements in the field of high-order methods for turbulent flows ranging from boundary layers to supersonic scramjets and full aircraft configurations.
Advancement in Noise Modelling from Rotating Sources
Dimitris Drikakis and Anastasios Kokkalis, Professor, Hellenic Air Force Academy, Greece
This presentation will provide the outcome of investigations relating to the modelling of noise from rotating sources such as helicopter rotors or aircraft propellers by addressing the noise propagation in transonic/supersonic regimes. It will provide insights on the behaviour of rotating noise sources using a detailed analysis of the retarded time equation and also of the discretisation elements of Emission Surfaces. In particular, a fast and efficient algorithm for the solution of the retarded time equation is defined, which is capable of dealing with sources rotating in both subsonic and supersonic flow regimes. A novel root finder algorithm is developed and tested against the two most common methods exploited during the solution of the Retarded Time Equation, ie the Newton and the Brent methods. The efficiency and accuracy of the numerical methods are compared with the results obtained using a Forward Time solution. Finally, a noise prediction tool developed from these analyses is then tested and applied to the analysis of the UH1H rotor, in strong shock delocalization conditions, a well known and extensively studied case of High Speed Impulsive noise.
- Ethirajan RATHAKRISHNAN, Department of Aerospace Engineering Indian Institute of Technology Kanpur, Kanpur, India
Mixing and Noise Control with Limiting Tab
Supersonic jets normally possess complex shock patterns. Therefore, the role of shock waves in noise generation becomes significant. The main sources of high-speed jet noise are the turbulent nature of the flow, shock–turbulence interaction, flow-induced oscillations of shocks, and resonance effects. Noise from such jets is complex and may even be nonlinearly interdependent. Noise prediction for such flows for a wide range of operating conditions is not possible as yet. An intense discrete acoustic emission termed “screech” or “whistle,” as a consequence of oscillating shock waves within a supersonic jet, usually dominates the noise emitted by a cold model converging jet operated at slightly above choked flow condition.
The primary objective of this investigation is to examine the feasibility of using a cross wire at the nozzle exit as a passive control to achieve mixing enhancement and noise reduction. The Mach numbers 1.6, 1.79, and 2 were specifically chosen to study the cross-wire effect on jets that are screech prone (Mach 1.6 and 1.79 jets) and jet without screech (Mach 2 jet). Even though the Mach 2 jet without control is free from screech, when the wire is introduced, the Mach number downstream of the wire may be such to fall in the range of 1.6 – 2.0 to become screech prone.
This investigation provided an evaluation of the aerodynamic and aeroacoustic effectiveness of passive control in the form of a cross wire at the nozzle exit on the decay and noise characteristics of Mach 1.6, 1.79, and 2.0 jets from convergent–divergent circular nozzles, at different levels of expansion.