Active control of the accordion modes of a submerged hull

Active vibration control of a submerged hull is presented. A submarine hull can be idealised as a ring stiffened finite cylinder with applied fluid loading. At low frequencies, rotation of the propeller results in discrete tones at the blade passing frequency and its harmonics. The low frequency axial and radial vibration modes of the submerged body can result in a high level of radiated noise. Global hull modes are difficult to attenuate since passive control techniques such as damping materials are not practical due to size and weight constraints. This work investigates active vibration control of a submarine hull for attenuation of the structural and acoustic responses. Based on a feedforward algorithm at tonal frequencies, active vibration suppression of the axial and radial hull displacements are investigated. The effect of the various control arrangements on the structure-borne radiated noise is examined. Numerical simulations of the control performance are presented.

On the application of the light-attenuation technique as a tool for non-intrusive buoyancy measurements

This article describes the application of the light-attenuation technique as a tool for measuring dilution occurring in buoyancy-driven flows. Whilst this technique offers the experimental fluid dynamicist the ability to make rapid synoptic buoyancy measurements non-intrusively, its successful application requires careful selection of chemical dye, dye concentration, illumination and optics. After establishing the advantages offered by methylene blue as a dyeing agent, we assess the accuracy of buoyancy measurements made using this technique compared with direct measurements made with density meters. Density measurements obtained using light-attenuation differ from those obtained using the density meter by typically less than 3%. It is hoped that this article will provide useful advice with regards to its implementation in the field of buoyancy-driven flows. © 2011 Elsevier Inc.

Shock Propagation and MPT Noise From a Transonic Rotor in Nonuniform Flow

One of the major challenges in high-speed fan stages used in compact, embedded propulsion systems is inlet distortion noise. A body-force-based approach for the prediction of multiple-pure-tone (MPT) noise was previously introduced and validated. In this paper, it is employed with the objective of quantifying the effects of nonuniform flow on the generation and propagation of MPT noise. First-of-their-kind back-to-back coupled aero-acoustic computations were carried out using the new approach for conventional and serpentine inlets. Both inlets delivered flow to the same NASA/GE R4 fan rotor at equal corrected mass flow rates. Although the source strength at the fan is increased by 38 dB in sound power level due to the nonuniform inflow, far-field noise for the serpentine inlet duct is increased on average by only 3.1 dBA overall sound pressure level in the forward arc. This is due to the redistribution of acoustic energy to frequencies below 11 times the shaft frequency and the apparent cut-off of tones at higher frequencies including blade-passing tones. The circumferential extent of the inlet swirl distortion at the fan was found to be two blade pitches, or 1/11th of the circumference, suggesting a relationship between the circumferential extent of the inlet distortion and the apparent cut-off frequency perceived in the far field. A first-principles-based model of the generation of shock waves from a transonic rotor in nonuniform flow showed that the effects of nonuniform flow on acoustic wave propagation, which cannot be captured by the simplified model, are more dominant than those of inlet flow distortion on source noise. It demonstrated that nonlinear, coupled aerodynamic and aero-acoustic computations, such as those presented in this paper, are necessary to assess the propagation through nonuniform mean flow. A parametric study of serpentine inlet designs is underway to quantify these propagation effects. © 2013 American Society of Mechanical Engineers.

Shock propagation and MPT noise from a transonic rotor in non-uniform flow

One of the major challenges in hig4h-speed fan stages used in compact, embedded propulsion systems is inlet distortion noise. A body-force-based approach for the prediction of multiple-pure-tone (MPT) noise was previously introduced and validated. In this paper, it is employed with the objective of quantifying the effects of non-uniform flow on the generation and propagation of MPT noise. First-of-their-kind back-to-back coupled aero-acoustic computations were carried out using the new approach for conventional and serpentine inlets. Both inlets delivered flow to the same NASA/GE R4 fan rotor at equal corrected mass flow rates. Although the source strength at the fan is increased by 45 dB in sound power level due to the non-uniform inflow, farfield noise for the serpentine inlet duct is increased on average by only 3.1 dBA overall sound pressure level in the forward arc. This is due to the redistribution of acoustic energy to frequencies below 11 times the shaft frequency and the apparent cut-off of tones at higher frequencies including blade-passing tones. The circumferential extent of the inlet swirl distortion at the fan was found to be 2 blade pitches, or 1/11th of the circumference, suggesting a relationship between the circumferential extent of the inlet distortion and the apparent cut-off frequency perceived in the far field. A first-principles-based model of the generation of shock waves from a transonic rotor in non-uniform flow showed that the effects of non-uniform flow on acoustic wave propagation, which cannot be captured by the simplified model, are more dominant than those of inlet flow distortion on source noise. It demonstrated that non-linear, coupled aerodynamic and aeroacoustic computations, such as those presented in this paper, are necessary to assess the propagation through non-uniform mean flow. A parametric study of serpentine inlet designs is underway to quantify these propagation effects. Copyright © 2011 by ASME.

Low attenuation of GHz Rayleigh-like surface acoustic waves in ZnO/GaAs systems immersed in liquid helium

Low attenuation of Sezawa modes operating at GHz frequencies in ZnO/GaAs systems immersed in liquid helium has been observed. This unexpected behaviour for Rayleigh-like surface acoustic waves (SAWs) is explained in terms of the calculated depth profiles of their acoustic Poynting vectors. This analysis allows reproduction of the experimental dispersion of the attenuation coefficient. In addition, the high attenuation of the Rayleigh mode is compensated by the strengthening provided by the ZnO layer. The introduction of the ZnO film will enable the operation of SAW-driven single-photon sources in GaAs-based systems with the best thermal stability provided by the liquid helium bath. © 2013 American Institute of Physics.

A novel MPT noise prediction methodology for highly-integrated propulsion systems with inlet flow distortion

Embedded propulsion systems, such as for example used in advanced hybrid-wing body aircraft, can potentially offer major fuel burn and noise reduction benefits but introduce challenges in the aerodynamic and acoustic integration of the high-bypass ratio fan system. A novel approach is proposed to quantify the effects of non-uniform flow on the generation and propagation of multiple pure tone noise (MPTs). The new method is validated on a conventional inlet geometry first. The ultimate goal is to conduct a parametric study of S-duct inlets in order to quantify the effects of inlet design parameters on the acoustic signature. The key challenge is that the mechanism underlying the distortion transfer, noise source generation and propagation through the non-uniform flow field are inherently coupled such that a simultaneous computation of the aerodynamics and acoustics is required. The technical approach is based on a body force description of the fan blade row that is able to capture the distortion transfer and the MPT noise generation mechanisms while greatly reducing computational cost. A single, 3-D full-wheel unsteady CFD simulation, in which the Euler equations are solved to second-order spatial and temporal accuracy, simultaneously computes the MPT noise generation and its propagation in distorted mean flow. Several numerical tools were developed to enable the implementation of this new approach. Parametric studies were conducted to determine appropriate grid and time step sizes for the propagation of acoustic waves. The Ffowcs-Williams and Hawkings integral method is used to propagate the noise to far field receivers. Non-reflecting boundary conditions are implemented through the use of acoustic buffer zones. The body force modeling approach is validated and proof-of-concept studies demonstrate the generation of disturbances at both blade-passing and shaft-order frequencies using the perturbed body force method. The full methodology is currently being validated using NASA's Source Diagnostic Test (SDT) fan and inlet geometry. Copyright © 2009 by Jeff Defoe, Alex Narkaj & Zoltan Spakovszky.

Review of computational aeroacoustics for application in electronics cooler noise

A review of computational aeroacoustics (CCA) was made for application in electronics cooler noise. Computational aeroacoustics encompasses all numerical methods where the purposes is to predict the noise emissions from a simulated flow. Numerical simulation of the flow inside and around heat sinks and fans can lead to a prediction of the emitted noise while they are still in the design phase. Direct CCA is theoretically the best way to predict flow-based acoustic phenomena numerically. It is typically used only for low-frequency sound prediction. The boundary element method offers low computational cost and does not use a computational grid, but instead use vortex-surface calculations to determine tonal noise. Axial fans are commonly used to increase the airflow and thus the heat transfer over the heat sinks within the computer cases. Very detailed source simulations in the fan and heat sink region coupled with the use of analogy methods could result in excellent simulation results with a reasonable computational effort.

Trailing edge noise theory for rotating blades in uniform flow

This paper presents a new formulation for trailing edge noise radiation from rotating blades based on an analytical solution of the convective wave equation. It accounts for distributed loading and the effect of mean flow and spanwise wavenumber. A commonly used theory due to Schlinker and Amiet (1981) predicts trailing edge noise radiation from rotating blades. However, different versions of the theory exist; it is not known which version is the correct one and what the range of validity of the theory is. This paper addresses both questions by deriving Schlinker and Amiet's theory in a simple way and by comparing it to the new formulation, using model blade elements representative of a wind turbine, a cooling fan and an aircraft propeller. The correct form of Schlinker and Amiet's theory (1981) is identified. It is valid at high enough frequency, i.e. for a Helmholtz number relative to chord greater than one and a rotational frequency much smaller than the angular frequency of the noise sources.

Airfoil flow and noise computation using monotonically integrated large eddy simulation and acoustic analogy: Effect of the grid resolution.

A new scalable Monotonically Integrated Large Eddy Simulation (MILES) method based on the Compact Accurately Boundary-Adjusting high-REsolution Technique (CABARET) has been applied for the simulation of unsteady flow around NACA0012 airfoil at Re = 400,000 and M = 0.058. The flow solution is coupled with the Ffowcs Williams-Hawkings formulation for far-field noise prediction. The computational modeling results are presented for several computational grid resolutions: 8, 16, and 32 million grid cells and compared with the experimental data available.