Ganesh Raman

• Illinois Institute of Technology

The Rayleigh criterion has long been considered the standard criterion for angular resolution of imaging systems, including microphone arrays. However, advanced beamforming methods have been developed in the past 20 years that have dramatically improved the angular resolution of microphone array systems. In this investigation, the angular resolutio...

Several advanced beamforming methods have been developed in the past 20 years that have dramatically improved the angular resolution of microphone array systems. Meanwhile, the Rayleigh criterion has long been considered the standard criterion for angular resolution of such systems. In this investigation four microphone arrays were constructed as s...

An experimental comparison of four different methods of nearfield acoustic holography (NAH) for planar acoustic sources is presented in this study. The four NAH methods considered in this study are based on: (1) spatial Fourier transform, (2) equivalent sources model, (3) boundary element methods and (4) statistically optimized NAH. Two dimensional...

This paper evaluates the performance of various regularization parameter choice methods applied to different approaches of nearfield acoustic holography when a very nearfield measurement is not possible. For a fixed grid resolution, the larger the hologram distance, the larger the error in the naive nearfield acoustic holography reconstructions. Th...

Related reviews are given of (1) international wind turbine noise regulations and standards and (2) modern experimental methods in aeroacoustic measurement with sample applications. For the latter, specific emphasis has been placed on three measurement techniques: (1) single microphone measurements, (2) phased microphone array measurements, and (3)...

The use of nearfield acoustic holography (NAH) for locating and sizing leaks in building envelops has been previously presented [J. Acoust. Soc. Am. 137, 2233 (2015); J. Acoust. Soc. Am. 137, 2325 (2015), and J. Acoust. Soc. Am. 136, 2172 (2014)]. Standard NAH algorithms are fairly limited in the distance from the sources at which reconstruction ca...

Monitoring of aircraft in the skies surrounding an airport is important to ensure the safety of the aircraft as well as quality of life for nearby residents. The traditional method of tracking aircraft position involves the ubiquitous rotating active radar, and the traditional method of monitoring aircraft noise is the single omnidirectional microp...

Aircraft noise is a form of environmental noise pollution that is a cause for resident complaints, especially near larger airports. Noise monitoring is usually performed by placing single microphones at various locations in neighborhoods that are near airports. Single omnidirectional microphones, however, record every sound wave that is incident on...

A crack on a building wall and sound generated inside the building using an artificial sound source were simulated using Matlab. Various noise types affecting the pure sound signal, such as background noise in the room, were simulated. The forward problem of sound propagation from the crack surface to a virtual microphone array was simulated using...

The acoustic properties of a wall jet over a hard-walled backward-facing step of aspect ratios 6, 3, 2, and 1.5 are studied using a 24-channel microphone phased array at Mach numbers up to M=0.6. The Reynolds number based on inflow velocity and step height assumes values from to . Flow without and with side walls is considered. The experimental set...

Building infiltration is a significant portion of the heating and cooling load of buildings and accounts for nearly 4% of the total energy use in the United States. Current measurement methods for locating and quantifying infiltration in commercial buildings to apply remediation are very limited. In this talk, the development of a new measurement s...

This paper experimentally investigates the noise from a large wind turbine (GE 1.5 MW) with a compact microphone array (OptiNav 24) using advanced deconvolution based beamforming methods, such as DAMAS and CLEAN-SC beamforming algorithms, for data reduction. Our study focuses on the ability of a compact microphone array to successfully locate both...

Nearfield acoustic holography (NAH) has been a successful tool in localizing and quan- tifying the acoustic source strengths in many fields such as plate vibration analysis. This paper explores the possibilities of aeroacoustic noise source localization using NAH. Build- ing leakage detection using this tool is used as a test case in this work. Thr...

Previously it was shown that a compact microphone array was suficient to resolve noise sources on a wind turbine while using advanced deconvolution algorithms. It was found that deconvolution approach for the mapping of acoustic sources (DAMAS) and deconvolution through Linear Programming (LP) were efficient in locating the aerodynamic noise from t...

In the United States excess air infiltration is estimated to account for approximately 23% and 14% of the total amount of energy used for heating and cooling in all residential and commercial buildings, respectively. The overall goal is to use a non-intrusive acoustic phased array with beamforming techniques to remotely detect specific envelope lea...

Small wind turbines, which are increasingly being used near residential areas, sound louder than their larger counter parts due to their close proximity to dwellings. Previous noise measurements on small scale wind turbines were performed using single microphones which only provide an overall estimate of the total noise emitted from the wind turbin...

The main goal of this study was to separate and evaluate the noise contribution of each sub-component of a small scale wind turbine drive train that contributes to the overall mechanical noise inside the nacelle of a small (8 kW) wind turbine. The experiments were performed in a drive train simulator that housed the drive train of a Viryd 8000 wind...

Air infiltration in residential and commercial buildings of the United States are estimated to cost approximately 23% of the total amount of heating energy used (for cooling, this estimate is approximately 14%). The aim of the current work is to apply non-intrusive acoustic techniques to detect and quantify specific envelope leaks in buildings. Noi...

Remote acoustic source localization techniques are demonstrated for complex machines. Complex machines include wind turbines (blades and drive train) as well as office machines. While the technique can be applied to any complex machine our data and analysis in this paper focus on wind turbines. In order to reduce wind turbine noise it is essential...

A new deconvolution strategy for aeroacoustic beamforming based on linear programming (LP) is introduced. Results of an aeroacoustic test show that it produces virtually the same results as DAMAS in much less time. DAMAS, DAMAS2 and LP display superresolution, which is expressed in terms of the Sparrow resolution limit from optics. CLEAN-SC and TID...

Landing gear noise is a key component of airframe noise during aircraft approach. In the present study we experimentally investigate a simplified landing gear model consisting of two cylinders in a tandem configuration. The cylinders are located at the exit of a jet flow facility operating at low subsonic speeds. The goal of this study is to use a...

There is a critical need to develop reliable high control authority flow control actuators for high speed applications. This paper introduces a novel high control authority actuator in the form of a miniature shock tube actuator. Two types of shock tubes were developed, namely, a single driver and a multiple driver shock tube. Various characterizat...

Locating the dominant noise sources on a wind turbine is an important problem in designing and developing low noise wind turbines. Previously very large microphone arrays were used to locate these sources. The primary focus of this paper is to show that using a compact and mobile microphone array with advanced beamforming algorithms, the noise sour...

The knowledge of noise source locations on a wind turbine is crucial for designing low noise wind turbines. It is also important to monitor the noise from the wind turbine, be it mechanical or aerodynamic noise, to monitor the health of the wind turbine. Currently compliance standards for wind turbine noise are based on single microphone measuremen...

One of the challenges that one faces when developing and designing low noise wind turbines is knowledge of components that radiate the most noise. Typical measurement techniques using a single microphone will result only in the overall sound pressure level radiated by the wind turbines. When it comes to locating sources of noise on a wind turbine,...

The role of microphone phased arrays as a tool to detect flow separation is investigated. A backward-facing step is used to create a separated flow, thereby inducing a region of elevated free-stream turbulence downstream of the step. Velocity measurements and a flow visualization technique are used to complement microphone measurements. It is shown...

The use of an unsteady CFD simulation to provide data for a full acoustic analysis of a wind turbine has been demonstrated. The OVERFLOW solver was employed to predict the near-field conditions. The data from the CFD simulation was sampled to provide the input for the PSU-WOPWOP Ffowcs Williams-Hawkings solver. PSU-WOPWOP was employed to predict th...

In the present work, we study stability characteristics of synthetic profiles prototypical of coannular jets, using a spatial, inviscid and compressible formulation of linear stability theory. The focus of our study is to understand the effect of the wake of the nozzle that separates the primary jet from the secondary jet, on the characteristics of...

Jet and cavity flows have been of great interest both for basic science and engineering applications. In order to effectively control these flows several groups have undertaken the development of flow control actuators. The aeroacoustics research group within the Fluid Dynamics Research Center at the Illinois Institute of Technology has been involv...

Wind energy is one of the oldest and cleanest forms of energy and the most developed of the renewable energy sources. Noise made by wind turbines is of key interest for many reasons. The noise made by a machine is a measure of its inefficiency. Wind turbine that are associated with excessive noise and vibrations are wasting energy and are prone to...

A rod in crossflow is a technique known for its simple and effective means of suppressing cavity tones. Although several hypotheses have been put forward regarding its working principle, no validated explanation exists. In the present study we investigate whether the cylinder, through its wake, changes the stability characteristics of the shear lay...

There is a critical need to develop reliable high control authority flow control actuators for high speed applications. This paper introduces one such actuator namely, the multiple driver shock tube actuator. The objective of this paper is to characterize and compare the result of the multiple driver shock tube actuator with that of a single driver...

The present work evaluates the connection between shear layer dynamics and unsteady pressures produced within a resonating cavity. The shear layer dynamics are considered for the baseline cavity as well as a cavity with a spanwise oriented cylinder located upstream of the cavity. The cylinder is used as a suppression device on account of its simpli...

In coannular jets the nozzle that separates high stream flow from the low stream flow, leaves its wake. By studying the stability characteristics of the synthetic hybrid profiles, prototypical of the mean velocity profiles downstream of the inner nozzle exit, we aim to shed light on the influence of this wake on the development of the co-annular je...

The main contribution of the present paper is the design, development, testing and experimental validation of ultrasonic powered resonance tube actuators. These flow control actuators potentially contribute lower levels of self noise. A detailed study of the nearfield of these actuators was undertaken with the goal of determining optimal placement...

Careful studies of wind turbine installations are necessary to assess their acoustic characteristics as wind turbine noise may impede extensive use of wind energy. Oerlemans and Schepers conducted a study, measuring noise on two turbines, a GE 2.3 MW prototype test turbine with a rotor diameter of 94 m, tower height of 100 m and a GAMESA G58 850 kW...

A new actuator for aerodynamic now control applications is described and evaluated in this paper: the piezo-fluidic oscillator. This actuator is a fluidic device based on wall attachment of a fluid,jet and modulated by piezoelectric devices. The piezo-fluidic oscillator successfully decouples the operating frequency from the now characteristics of...

A rod in cross flow is a technique known for its simple and effective means of suppressing cavity tones. Although several hypotheses have been put forward regarding its working principle, no validated explanation exists. In the present study we investigate whether the cylinder, through its wake, changes the stability characteristics of the shear la...

The purpose of this paper is to examine nonlinear processes occurring during the coupling of complex twin jet plumes using higher-order spectral methods. Inadequacies of linear spectral analysis when closely spaced multiple screech sources exist in complex configurations are demonstrated. Cross-bicoherence is used for identifying the nonlinear inte...

The Hatmann whistle, a high efficiency acoustic device with various sounding advantages, differs from other tone generators due to its high acoustic output capability and working principles. The advantages of the whistle include purely aeroacoustic sound generation, high sound quality due to intense tonal content and high amplitude, and tunable fre...

The present work is an ongoing effort to shed light on the suppression mechanism of the rod in cross flow as an actuator for cavity tone suppression. Control cases with similar configuration but contrasting suppression results are chosen to illustrate important aspects of the flow field such as presence of the recirculation bubble close to the fron...

Hartmann discovered the resonance tube phenomenon in 1918. Although researchers have conducted extensive studies on this topic during the intervening 90 years, no single resource lists, analyzes, synthesizes and interprets the vast body of findings. This review offers a consolidated resource tracing development of the Hartmann tube from discovery t...

Flow control has the potential to improve the efficiency of aerospace systems. The design of a flow control system requires knowledge of flow phenomena including information on the evolution of naturally occurring disturbances in the flow. Natural flow disturbances can evolve at various frequencies and growth rates and can assume various mode shape...

The use of a rod in cross flow is a well known technique known for its simple and effective means of suppressing cavity tones. Despite several studies regarding its performance as an actuator, there is still no consensus in scientific community regarding the mechanism by which it suppresses cavity resonance. Several hypotheses have been put forward...

A number of previous experiments have shown that high frequency excitation can lead to dramatic suppression of cavity ∞ow resonances typically associated with modern aircraft weapons bays. However, despite all the documented efiectiveness of this method, there is a lack of clear understanding of the physics driving the suppression mechanism. In thi...

Understanding instability modes in impinging jets is important for the design of advanced vertical takeoff and landing aircraft. Our initial experimental observations of helical modes in subsonic impinging jets were contrary to expectations based on published stability theory results. This paper focuses on the curious phenomenon of the occurrence o...

Sound propagation through regions of non-uniform temperature distribution in a gas is studied numerically. The main objective of this study is to determine the impact of temperature gradients on the sound wave parameters and to evaluate the effectiveness of using regions of hot gas as sound barrier. Such regions of hot gas (low acoustic impedance)...

This work is an experimental study of high subsonic jet impingement tone suppression. We begin by documenting the characteristics of the impingement tone for various Mach numbers and standoff (nozzle exit to ground plate) distances. The results revealed frequency staging and the presence of two types of impingement tones. A novel feature of our wor...

The microfluidic oscillator is a new microscale actuator developed for flow control applications. These patented devices can produce a 325-mu m-wide oscillating gas jet at high frequencies (over 22 kHz) and very low flow rates (similar to 1 l/min or similar to 1 g/min). Furthermore, microfluidic oscillators have no moving parts; the jet oscillation...

In military aircraft weapon bays, experience has shown that an open cavity at moderately high speeds can have a profound effect on internally carried weapons, their suspension equipment, separation characteristics, and the structural loads on the parent aircraft. Acoustic resonance causes high pressure, fluctuating loads in and near the bay, high e...

Configurations that exhibit fluid flow over twin open cavities occur fairly commonly in practical applications in the aircraft industry such as in weapons bays of modern aircraft and in landing gear wells of both commercial and military aircraft. This paper presents the results of ongoing research in the area of twin-cavity interaction using metric...

This paper compares the effectiveness of zero- low- and high-frequency flow control methodologies when applied to a generic weapons bay cavity. Using the High Frequency Excitation Active Flow Control for Supersonic Weapons Release (HIFEX) generic weapons bay model the actuators were tested at Mach 0.85 and 1.19. The zero-frequency actuator tested w...

In this paper we focus on understanding the behavior of twin nozzles of complex geometry in various yaw orientations. To the best of our knowledge there are no published studies addressing the effects of nozzle orientation on the coupling of twin jets of complex exit geometry. We study the behavior of 1) uniform-exit rectangular nozzles, 2) single-...

The powered resonance tube (PRT) actuator and its variants are new developments in active flow control (AFC) technology. The PRT is attractive because it has no moving parts and can produce acoustic tones that have amplitudes greater than 150dB over a large frequency bandwidth. The first part of this paper deals with the resonance characteristics o...

The Powered Resonance Tube (PRT) actuator, which is an adaptation of the Hartmann whistle, was selected for bandwidth enhancement. The operation and near field features of this actuator were characterized both experimentally and computationally. An improved analytical theory was developed for the prediction of the frequency of actuation. The analyt...

In July 2005, the AFRL program Flow Control Analysis Development (FlowCAD) tested the High Frequency Excitation Active Flow Control for Supersonic Weapons Release (HIFEX) generic weapons bay model in the Boeing's Polysonic windtunnel facility. The 10% scaled weapons bay with an L/D of 5 was tested at Mach 1.82. Several flow control devices were tes...

This paper examines instability mode switching in various supersonic jet configurations that involve resonant acoustics. Resonant acoustics includes situations where flow instabilities are enhanced by feedback. The pressure spectra in such situations are rich in multiple modes, and mode switching can occur rather unpredictably. Our experiments reve...

A novel approach to suppressing jet-cavity interaction tones using miniature fluidic devices was investigated. It was observed that the fluidic devices had dimensions ranging from 0.5 to 1 mm and had no moving parts and could provide oscillatory flow of waveforms at frequencies up to 3 KHz. It was also observed that at the upstream end of the cavit...

The aeroacoustics of coupled twin jets of complex geometry was investigated. The twin convergent nozzles, with an aspect ratio of 7 with spanwise oblique, exits operated over the fully expanded Mach number range from 1.3 to 1.6, was also studied. It was shown that the inter-nozzle spacing (s/h) was varied from 7.4 to 13.5. The results show that cou...

Several studies have proved that the geometry of an oscillating acoustic resonator strongly influences its resonance frequencies and the nonlinear standing pressure waveform generated within the cavity. The research presented herein uses a quasi-one-dimensional numerical model to solve the acoustic field and is validated by comparing with experimen...

The present paper addresses both active-flow-control actuator technology development and the demonstration of the effectiveness of actuators that could be easily integrated into practical aircraft applications. The actuator used is an adaptation of the Hartmann oscillator. Demonstration experiments that illustrate the effectiveness of this actuator...

1) Standing waves with maximum pressures of 188 kPa have been produced in resonators containing ambient pressure air; 2) Addition of structures inside the resonator shifts the fundamental frequency and decreases the amplitude of the generated pressure waves; 3) Addition of holes to the resonator does reduce the magnitude of the acoustic waves produ...

The fluidic oscillator is a new micro-scale actuator developed for flow control applications. These patented devices can produce a 325-μm wide oscillating jet at high frequencies (over 22 kHz) and very low flow rates (~1 L/min or ~1 g/min). Furthermore, micro fluidic oscillators have no moving parts - the jet oscillations depend solely on the inter...

PRT actuators, which consist of a high pressure air supply jet, a resonance tube, and an integration slot, are capable of producing high amplitude pressure oscillations. The traditional approach requires a very long resonance tube when low frequencies are desired. A Helmholtz based PRT offers a method of substantially reducing the resonance tube le...

The screech frequency and amplitude, the shock spacing, the hydrodynamic-acoustic standing wave spacing, and the convective velocity of large organized structures are measured in the nominal Mach number range of 1.1 less than or = Mj less that or = l0.9 for supersonic, underexpanded jets exhausting from a circular, a rectangular and an elliptic noz...

Interaction of sound waves with glow discharge plasma was studied experimentally, as a continuation of the work reported earlier [1]. The main thrust of this investigation was to determine the effectiveness of using glow discharge plasma as a sound barrier in aerospace applications. The present study focused on the determination of the angular depe...

A dissonant acoustic resonator with a conical shaped cavity was tested in four configurations: (A) baseline resonator with closed ends and no blockage; (B) closed resonator with internal blockage; (C) ventilated resonator with no blockage; and (D) ventilated resonator with an applied pressure differential. These tests were conducted to investigate...

A high bandwidth powered resonance tube (PRT) actuator potentially useful for noise and flow control applications was developed. High bandwidth allows use of the same actuator at various locations on an aircraft and over a range of flight speeds. The actuator selected for bandwidth enhancement was the PRT actuator, which is an adaptation of the Har...

This paper focuses on neutral gas temperature measurements in glow discharge plasma in air. The present is a continuation of our earlier work that investigated the applicability of glow discharge plasma as a sound barrier in aerospace applications 1,2,3. In these earlier studies, the sound plasma interaction was studied theoretically, numerically a...

The symmetric and antisymmetric coupling modes of jets emanating from twin single beveled nozzles were studied with regard to its effects on the jet nearfield. The nozzles used were two dimensional with an aspect ratio of 6.6. The variation of the instability modes along the downstream axial direction was also studied using both the linear and non-...

The purpose of this paper is to understand non-linear processes occurring during the coupling of twin jet plumes using higher order spectral methods. To the best of our knowledge, most previous studies have used linear spectral analysis to document coupling and have focused on identifying configurations and conditions that produce coupling, without...

It is expected that with the projected increase in air travel aeroacoustics research will be very critical in meeting increasingly stringent aircraft noise certification standards. In the United States aeroacoustics research has steadily progressed toward enhanced safety, noise benefits and lower costs. This report provides a brief summary of selec...

Optimizing resonator shapes for maximizing the ratio of maximum to minimum gas pressure at an end of the resonator is investigated numerically. It is well known that the resonant frequencies and the nonlinear standing waveform in an acoustical resonator strongly depend on the resonator geometry. A quasi-Newton type scheme was used to find optimized...

The value of dimensionless attenuation coefficient is an important factor when numerically predicting high-amplitude acoustic waves in shaped resonators. Both the magnitude of the pressure waveform and the quality factor rely heavily on this dimensionless parameter. Previous authors have stated the values used, but have not completely explained the...

Nonlinear acoustic standing waves of high amplitude have been demonstrated by utilizing the effects of resonator shape to prevent the pressure waves from entering saturation. Experimentally, nonlinear acoustic standing waves have been generated by shaking an entire resonating cavity. While this promotes more efficient energy transfer than a piston-...

An experimental study was conducted to investigate the coupling of twin jets of complex geometry. The study was motivated by the need for nozzles with spanwise oblique exits on modem stealth aircraft and the possibility of plume coupling that could result in structural damage. Although several studies have examined aspects of twin jet coupling, ver...

An active noise suppression apparatus and method for reducing jet engine noise and buffeting. A plurality of resonant cavities are formed in a structural member disposed on or adjacent to a jet engine exhaust nozzle. Each resonant cavity opens toward an output slot. A compressed air chamber is used to supply a pressurized, supersonic jetstream airf...

There has been a recent surge of interest in powered resonance tube actuators for flow control applications. Additional features of powered resonance tube actuators (both experiments and simulations) are presented in this paper. A Powered Resonance Tube (PRT) is a device based on aeroacoustics principles, capable of producing intense perturbation l...

The first section of the results presented herein was conducted on an axisymmetric resonator configured with open ventilation ports on either end of the resonator, but otherwise closed and free from obstruction. In the remaining section we present the results of a similar resonator shape that was closed, but contained an axisymmetric blockage centr...

A fundamental study of resonant tones produced by jet-cavity interaction over a wide range of flow conditions covering both subsonic and supersonic speeds is described. Two significant findings emerge. For the jet-cavity configurations investigated, a suitably defined reduced frequency parameter allows for a global classification of all jet-cavity...

The reflection of sound waves by glow discharge plasma was studied theoretically. The main thrust of the investigation was to determine the effectiveness of using glow discharge plasma as a tool in flow control applications. Plasma offers a unique possibility of flow control by remotely introducing energy into the gas medium. In can be generated in...