International Journal of Aeroacoustics

This Part I presents a detailed description of a numerical system built and tested with the final goal of reaching an accuracy of 2–3 dB over a meaningful range of frequencies for airliner engine noise, while having low empiricism and a general-geometry capability. The turbulence is treated by Large-Eddy Simulation with grids of around 1 million points, slightly upwind-biased high-order differencing, and implicit time integration. The code can incorporate boundaries and multi-block grids (thus avoiding the centerline singularity), and capture shocks. The sub-grid scale model is de-activated, because on present grids it strongly interferes with transition in the mixing layer. Without unsteady inflow forcing, the shear-layer roll-up and three-dimensionalization are realistic and reasonably insensitive to the grid. The far-field noise is computed using the permeable Ffowcs-Williams/Hawkings (FWH) formulation without external quadrupoles. The treatment of the disk that closes the FWH surface near the outflow must be approximate, since the turbulent region is unbounded, and is crucial; it benefits from a change of variable from density to pressure, and other mitigating steps. Tests are presented in support of the key elements of the strategy. In a simple isothermal jet, the system is close to the 2–3 dB target both in terms of directivity and of spectrum, up to a Strouhal number of about 1.5. In Part II, the following effects are explored with overall success: jet Mach numbers from 0.3 to slightly supersonic with under-expansion (generating shock cells), jet heating, co-flow, and “synthetic chevrons.”

The numerical system described in Part I (Ref. 1) is applied to a variety of cases which increase difficulty, and progress in the direction of the complete simulation of an airliner engine. The grids have on the order of 1 million points. In many cases, the system meets the 2–3 dB accuracy target both in terms of directivity and of spectrum, up to a Strouhal number of about 1.5. The jet Mach number is varied from 0.3 to slightly supersonic with under-expansion, generating shock cells and greatly increasing side-line noise. For heated jets, the cross-effect between the acoustic Mach number and the temperature is correctly reproduced. Jets placed in a co-flowing stream with velocity up to 60% of the jet's are studied and found to sustain natural transition without unsteady forcing; the noise trends are correct. Finally, “synthetic chevrons” are added by altering the inflow conditions, and found to reduce low-frequency noise while increasing mid-frequency noise. In total, about fifteen meaningfully different cases are presented, and subjected to quantitative comparisons over the direction/frequency space without major failure. The principal limitation of the large-eddy-simulation approach remains its upper limit on frequency.

The paper presents the review of the CIAM/TsAGI investigations of mixer-ejector models, which could be considered to be prototypes of real variable area nozzles for future supersonic aircraft. Acoustic and aerodynamic characteristics of both 2-D and axisymmetrical models were obtained. The influence of nozzle geometry (number of lobes, lobe shape, ejector length, auxiliary inlets in the ejector walls) on its noise and thrust performances is discussed.

The experimental data show that the best mixer-ejector configuration tested provides a noise reduction efficiency of about 3 ∼ 3.5 PNdB per 1% thrust loss at static conditions.

Several experimental, computational and theoretical results devoted to turbulence jet noise are described. All these results were obtained at CIAM in collaboration with the Boeing Company. The selected topics are combined and described from a single point of view. In spite of the many publications in the classical literature devoted to jet noise, many jet noise problems are not completely solved, and most current jet noise prediction methods are still highly empirical and rely heavily on an existing experimental data base. At the same time, many of the available experimental results are not fully reliable and some are even contradictory. The goal of this paper is, therefore, to explore and discuss some of the possibilities and limitations of classical aeroacoustics methods.

The linear stability theory is used to investigate the emergence, in supersonic free shear flows such as mixing layers and fully expanded plane jets, of supersonically travelling instability waves, which do not vanish in the ambient space. It is shown that, at supersonic convective Mach numbers, the slow and fast supersonic modes in the mixing layer as well as the sinuous supersonic mode in the plane jet should lead to Mach wave radiation. Direct numerical simulations are further used to study nonlinear stages of instability development in high-speed mixing layers and jets. They have shown that the formation of oblique shock waves attached to large-scale structures is observed in free shear flows forced by modes with supersonic phase speeds. The relevance of this phenomenon to the noise generation by high-speed jets is discussed.

We present a study of acoustic jets occurring in the vicinity of an orifice in a screen mounted at the end of a waveguide which conducts intense sound. The velocity field is measured by a cooling-power anemometer. We have used an impedance matching principle and demonstrated the ability to generate a strong acoustic jet and to create efficient resonant sound absorbers.

An experimental study of the controlled disturbance field, introduced into the free stream with the help of the local source of disturbances, was carried out. The controlled disturbances were excited in the supersonic boundary layer by using a local disturbance generator designed based on the amount of discharge in the chamber in the plate. This process was accompanied by radiation of controlled disturbances into the free stream. This radiation from the system “discharge-boundary layer” was used as controlled disturbances in the free stream. The acoustic nature of the radiation was obtained. The levels, wave characteristics and modes of the artificial fluctuations have been obtained by hot-wire measurements in the free stream of the supersonic wind tunnel at a Mach number of 2.

A study of the self-sustained quasi-periodic flows near the spike-tipped body at free-stream Mach number M∞ = 6 is carried out. The influence of length and spike cone angle on a flow structure is shown. The character of the pulsation flow mode depending on the geometry of the model is defined.

This paper is a brief review of results of experimental and theoretical studies in the field of acoustics of nonequilibrium gas-plasma media. New acoustical properties of nonequilibrium media caused by the change in sign of the second viscosity and the dispersion coefficients are considered. Such media are acoustically active. Conditions are discussed for generating new nonlinear acoustical structures.