Lucio Maestrello

On the interaction of a sound pulse with the shear layer of an axisymmetric jet

Abstract The behavior of a sound pulse from a simulated source in a jet is investigated both experimentally and numerically. Both approaches show that in the low and medium frequencies the far field acoustic power exhibits a marked amplification as the flow velocity increases. Experimentally this changes to an attenuation at the higher frequencies which cannot be computed by the numerical model. This amplification is traced to shear noise terms which trigger the instability waves that are inherent within the flow.

NUMERICAL SIMULATION OF BOUNDARY-LAYER EXCITATION BY SURFACE HEATING/COOLING

The concept of active control of growing disturbances in an unstable compressible flow by using time periodic, localized surface heating is studied numerically. The simulations are calculated by a fourth-order accurate solution of the compressible, laminar Navier-Stokes equations. Fourth-order accuracy is particularly important for this problem because the solution must be computed over many wavelengths. The numerical results demonstrate the growth of an initially small fluctuation into the nonlinear regime where a local breakdown into smaller scale disturbances can be observed. It is shown that periodic surface heating over a small strip can reduce the level of the fluctuation provided that the phase of the heating current is properly chosen.

Two-Point Correlations of Sound Pressure in the Far Field of a Jet: Experiment

Correlation of sound pressure between two microphones yield substantially more information than single microphone data; hence, they provide a useful diagnostics tool for jet noise. In the present experiment, the two microphones were located at (R, θ, φ) and (R, θ, φ′), respectively, in spherical polar coordinates. The jet was directed along the polar axis (θ = 0) with the nozzle at the origin. Two sets of space‐time correlation were carried out (1) φ′ = 0; 0 ⩽ θ ⩽ 180°; θ′ = 0, 30°, 45°, 60°, 90°, 120°, 135°; (2) φ′ = 0; 0 ⩽ θ ⩽ 180°; θ′ = 0, 30°, 45°, 60°, 90°, 120°, 135°. Jet diameters were 1, 2, and 3 in. with a range of nozzle velocities from 500 to 1000 ft/sec, and R/D was 166 for the 1‐in. jet. Nineteen carefully phase‐matched microphones were employed. Polar plots of these correlations show a definite cusp at the origin (φ = φ′), unlike polar plots of p2. among other interesting features.

Flowfield and far field acoustic amplification properties of heated and unheated jets

The interaction of an acoustic pulse with the experimentally determined mean flow field of a spreading jet is simulated numerically. The simulation is obtained through solving the Euler equations linearized about the spreading jet. The model reveals a small, sustained oscillation long after the original pulse has passed. This remnant is considered a continual shedding of vortices from the nozzle lip, together with the generation of acoustic ripples. IT is shown that the jet also acts as an amplifier of sound. This amplification is traced to the jets stability characteristics. It is demonstrated that some of the observed differences in the spectra of heated and unheated jets can be attributed to differences in the stability characteristics of the jets.

Simulation of Instabilities and Sound Radiation in a Jet

The inflow and near field of a jet that is excited by an axial mass source located in the potential core are simulated numerically. The simulation takes into account the spreading of the jet. Comparison is made with experimental results for both excited and unexcited jets. It is shown that many of the features observed experimentally are due to the instability of the mean profile (i.e., the large-scale structures) and not due to the turbulence. The instability is shown to generate low-frequency sound. The terms identified by Ribner as the shear noise terms are shown to be responsible for this sound generation.

Quasi-periodic structure of a turbulent jet

Abstract The instantaneous near field pressure fluctuations of an axisymmetric subsonic jet were measured by using a longitudinal and an azimuthal microphone arrays in order to qualitatively determine the behaviors of the quasi-periodic structure within the flow. Statistical analysis is used to explain the characteristic of the pressure signals. In addition to the information obtained by forming the power spectral density, auto- and cross-correlation functions, two types of signals are extracted through a conditional probability analysis to represent the quasi-periodic and the random fine structures within the turbulent jet. The quasi-periodic structure first appears as a rolling up of the mixing layer flow within one nozzle diameter downstream of the exit, then becomes fully developed at approximatelt 3 nozzle diamters downstream with a preferred Strouhal number range 0·3–0·4, and finally disappears beyond the end of potential core. This behavior is also reflected in the variation of the convection velocity. The maxima of the overall sound pressure level and the peak pressure level occur at approximately 1 nozzle diameter from the exit, indicating that the early stage is of significant contribution to the vortex pairing and instability waves.

Coupling between a supersonic boundary layer and a flexible surface

The flexible surface is forced to vibrate by acoustic waves at normal incidence emanated by a sound source located on the side of the flexible surface opposite to the boundary layer. The effect of the source excitation frequency on the surface vibration and boundary layer stability is analyzed. The interaction between a stable two dimensional disturbance of Tollmien Schlichting type with the vibrating surface is also studied

On the interaction of a sound pulse with the shear layer of an axisymmetric jet, III: Non-linear effects☆

Abstract The fluctuating field of a jet excited by transient mass injection is simulated numerically. The model is developed by expanding the state vector as a mean state plus a fluctuating state. Non-linear terms are not neglected, and the effect of non-linearity is studied. A high order numerical method is used to compute the solution. The results show a significant spectral broadening in the flow field due to the non-linearity. In addition, large scale structures are broken down into smaller scales.

Interaction of sound from supersonic jets with nearby structures

A model of sound generated in an ideally expanded supersonic (Mach 2) jet is solved numerically. Two configurations are considered; (i) a free jet and (ii) an installed jet with a nearby array of flexible aircraft type panels. In the later case the panels vibrate in response to loading by sound from the jet and the full coupling between the panels and the jet is considered, accounting for panel response and radiation. The long time behavior of the jet is considered. Results for near field and far field disturbance, the far field pressure and the vibration of and radiation from the panels are presented. Panel response crucially depends on the location of the panels. Panels located upstream of the Mach cone are subject to a low level, nearly continuous spectral excitation and consequently exhibit a low level, relatively continuous spectral response. In contrast, panels located within the Mach cone are subject to a significant loading due to the intense Mach wave radiation of sound and exhibit a large, relatively peaked spectral response centered around the peak frequency of sound radiation. The panels radiate in a similar fashion to the sound in the jet, in particular exhibiting a relatively peaked spectral response at approximately the Mach angle from the bounding wall.

Apparatus and method for jet noise suppression

A method and apparatus for jet noise suppression through control of the static pressure of the jet and control of the rate of entrainment of ambient fluid into the jet downstream of the exhaust nozzle is disclosed and serving to regulate the momentum flux over an extended region of the jet, affecting Reynolds stresses in the jet and the spreading angle of the jet. Static pressure is controlled through a long hollow, porous nozzle plug centerbody which may be selectively vented to ambient conditions, connected to a vacuum source, or supplied with fluids of various densities for injection into the stream. Additionally, sound in the jet may be channeled along the nozzle plug centerbody by injecting coolant such as a cryogenic fluid through the center-body into the jet.