Krishan Ahuja

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• PhD
• Regents Professor at Georgia Institute of Technology

The noise produced by subsonic, internally mixed air jets from a model-scale, axisymmetric nozzle was shown to contain “lip noise” related to vortex shedding from the core-nozzle lip, despite the core-nozzle lip being reasonably thin. The lip noise grew stronger with increased core-stream temperature for a given jet Mach number and was less promine...

The Reynolds number and Mach number are classical quantities used to determine the similarity of aerodynamic flows. Existing studies on the role of Reynolds Number on jet noise show inconsistent results, casting doubt on how small a nozzle diameter can be for the classical jet noise scaling laws to hold. A systematic study was therefore undertaken...

Often, lossless jet-noise data obtained by various researchers in different facilities do not match after accounting for jet operating conditions, measurement distance, and nozzle dimensions using accepted jet-noise scaling laws. There have been a number of explanations for these differences, but the most contentious of these is attributing rig-noi...

In this experimental jet-noise study, the subsonic flow of air through a model-scale, dual-stream, internally mixed nozzle is shown to produce acoustic tones as high as 30 dB above the broadband jet noise which also result in a broadband amplification of jet noise known to occur in acoustically excited jets. The tones are the result of a feedback p...

The howling produced by a model-scale nozzle operated at high-subcritical pressure ratios is shown be related to shock-induced boundary-layer separation just upstream of the nozzle’s exit. A combination of proper orthogonal decomposition (POD) approaches are used to analyze high-speed schlieren (spectral POD) and particle-image velocimetry (snapsho...

Often the measurements from different jet noise studies, which are thought to have been acquired at or corrected to identical jet conditions, do not match when compared to each other. This study looks at the nozzle-exit boundary layer as a possible factor for these differences. The nozzle-exit boundary layer state can easily be changed depending on...

Model-scale jet-noise experiments using a dual-stream, axisymmetric confluent nozzle with an area ratio of about 0.62 were conducted. Farfield acoustic measurements acquired with an extraction ratio (the bypass stream’s total pressure divided by the core stream’s total pressure) of unity and above unity are discussed – both in terms of spectral noi...

Model-scale experiments conducted with an axisymmetric, dual-stream confluent nozzle with an area ratio of 0.62 reveal that the nozzle produces a self-excited flow at certain operating conditions. This work aims to characterize that which arises at high-subcritical pressure ratios with unity extraction ratio (the ratio of the bypass flow’s total pr...

Over the years, there have been numerous studies on determining subsonic jet noise source locations, typically plotted as Strouhal number as a function of distance from the nozzle exit. A comparison of the results of various studies yields a spread of about two nozzle diameters in measured source location. This work examines how boundary layer thic...

View Video Presentation: https://doi.org/10.2514/6.2022-2562.vid Safety earmuffs and earplugs are often worn by personnel exposed to high sound pressure levels, such as flight deck crews on aircraft carriers, in order to prevent noise-induced hearing loss. Personal protective equipment, specifically earplug-style hearing protection, are not typical...

An approach for broadening the frequency range across which duct-mounted half-wave resonators can be considered effective sound absorbers is investigated. This approach involves integrating relatively small diameter space-coiling quarter-wave resonators around the inlet of a half-wave resonator, with the space-coiling resonators designed to absorb...

An experimental facility was recently developed at the Georgia Tech Research Institute to measure the acoustic scattering coefficients of a test article under mean flow, high-temperature, high-amplitude, and higher-order mode propagation conditions. The experimental facility acts like a two-sided impedance tube, in which the measurement section is...

No PDF available ABSTRACT United States Military personnel are exposed to blast overpressure from a variety of sources during training and military operations. While it is known that repeated exposure to high-level blast overpressure may result in concussion like symptoms, the effect of repeated exposure to low-level blast overpressure is not well...

The purpose of this work is to examine the noise source distributions of shock-containing supersonic jets at various pressure ratios corresponding to fully expanded Mach numbers ranging from 1.0 to 1.4 in intervals of 0.2 for various nozzle exit diameters. Source location measurements using a phased array (beamforming), farfield jet noise measureme...

Introduction Military personnel during training and military operations are exposed to a large number of repeated exposures to low-level blast overpressure from a variety of sources. These exposures rarely produce a concussion, but anecdotal evidence from soldiers indicates that it can still cause transient neurological effects. Impulsive acoustic...

This paper deals with the topic of upper surface blowing noise. Using a model-scale rectangular nozzle of an aspect ratio of 10 and a sharp trailing edge, detailed noise contours were acquired with and without a subsonic jet blowing over a flat surface to determine the noise source location as a function of frequency. Additionally, velocity scaling...

In this paper, the complete methodology for measuring the acoustic properties of a test article under uniform heated flow and higher-order mode propagation conditions is developed from the theory of higher-order mode propagation in circular ducts. The major steps in the methodology include modal decomposition based on multiple acoustic pressure mea...

This presentation discusses ongoing research in Infrasound technologies at the Georgia Institute of Technology, Georgia Tech Research Institute (GTRI), the applied research arm of the Georgia Tech. In particular, results of a study that compared a number of commercially available infrasound sensors with several windscreen technologies are presented...

The effect on normal incidence acoustic impedance of a non-circular orifice shape is examined relative to a circular orifice. The impedance of an adjustable porosity perforate, formed from two identical perforates sliding over each other, is measured. As the orifice shape becomes more noncircular, the measured impedance is found to deviate from the...

Georgia Tech Research Institute (GTRI) conducted a flight test of a sub-scale rocket in 2013 outside Talladega, Alabama, to acquire the launch acoustics produced. The primary objective of the test was to characterize the acquired data during a sub-scale launch and compare it with heritage launch data from the STS-1 Space Shuttle flight. Neither lau...

That the peak frequency of the jet noise spectrum in the rear arc at small angles to the jet axis does not change with jet exit velocity in unheated jets and scales with Helmholtz number instead of Strouhal number has been known for over 45 years, but no one has come up with an explanation for this observation. It is demonstrated in this paper for...

That the peak frequency of the jet noise spectrum in the rear arc at small angles to the jet axis does not change with jet exit velocity in unheated jets and scales with Helmholtz number instead of Strouhal number has been known for over 45 years, but no one has come up with an explanation for this observation. It is demonstrated in this paper for...

Hairdryer is one of the devices that is used in everyday life. It is useful and efficient for drying hair in a short time, but most of the people pay less attention to the noise levels of this device. Majority of the hairdryers are capable of producing significantly loud noise which might cause health problems. There are several patents with new id...

Cross-correlation measurements and related coherence spectra for a range of nozzle geometries and jet Mach numbers were studied in an attempt to understand the coherent nature of jet mixing noise from the small-scale structure and that from the large-scale structure using a number of nozzle geometries. Nozzle geometries include: (1) three conical r...

Most university researchers typically end up using small nozzles to study jet noise. The data acquired from many of such existing studies has been used to develop jet noise prediction schemes. However, it is not quite clear as to how small the nozzle can be before such data becomes unreliable. There are two-fold issues here: (1) it may be difficult...

Many theoretical models of shielding by hard surfaces assume the source to be a point monopole source. If one is interested in shielding the noise of a rotor system by interposing a hard surface between the rotor and the observer, can the rotor system really be considered to be a monopole If rotating noise sources are under consideration what is th...

A number of jet noise experiments have been performed over the years in different facilities and the measured lossless spectra do not always match. Some of the researchers have suggested this discrepancy is associated with rig noise. Another school of thought is that this discrepancy could be due to differing nozzle-exit boundary layers. One of the...

In this research, the requirements for microphones to be in the geometric farfield for ducted and unducted rotors have been established. The approach is to acquire acoustic data at various geometric locations relative to the source. This is performed for different rotor configurations and compared with the Inverse Square Law (ISL). This paper produ...

Many theoretical models of shielding by hard surfaces assume the source to be a point monopole source. If one is interested in shielding the noise of a rotor system by interposing a hard surface between the rotor and the observer, can the rotor system really be considered to be a monopole? If rotating noise sources are under consideration what is t...

The objective of the work described in this paper is to use the three-microphone coherence-based signal processing technique to separate the contributions of the small-scale turbulence and the large-scale turbulence from far-field exhaust noise measurements. Both round and rectangular model-scale jets were tested for both subsonic and supersonic Ma...

The primary objective of this investigation is to determine experimentally the sources of jet mixing noise. In the present study, four different approaches are used. It is reasonable to assume that the characteristics of the noise sources are imprinted on their radiation fields. Under this assumption, it becomes possible to analyse the characterist...

The objective of this work was to investigate the applicability of coherence-based, multimicrophone techniques when separating internal or core noise buried in jet-mixing noise as measured in the far-field exhaust of a full-scale aero engine. Research efforts focused on three techniques (1) the coherent output power spectrum method using two microp...

This paper describes a revolutionary, fully-integrated approach for modeling the noise characteristics of maneuvering rotorcraft. The primary objective of this effort is the devel-opment of a physics-based software tool that enables the design of quiet rotors without per-formance penalties. This tool shall accurately predict the rotorcraft flight s...

A compact nonporous windscreen described previously [J. Acoust. Soc. Am. 114, No. 4, Pt. 2, 2323 (2003)] was tested in the field against a soaker hose array to compare performance at infrasonic frequencies. The cylindrically shaped compact windscreen, made of closed-cell polyurethane foam, had dimensions 0.0762 m i.d. x0.2286 m height x0.0127 m wal...

Circulation Control technology is a very effective way of achieving high lift forces required by aircraft during take-off and landing. This technology can also directly control the flow field over the wing. Compared to a conventional high-lift system, a Circulation Control Wing (CCW) can generate comparable or higher lift forces during take-off/lan...

Circulation control technology uses tangential blowing around a rounded trailing edge or a leading edge to change the force and moment characteristics of an aerodynamic body. This technology has been applied to circular cylinders, wings, helicopter rotors, and even to automobiles for improved aerodynamic performance. Only limited research has been...

An oscillating tab actuation scheme was developed as a method to enhance jet mixing over and above that observed by stationary tabs. The initial scheme uses a mechanical voice coil for motion of the tab and requires no high pressure air for actuation. This paper describes experimental work to investigate the effect of oscillating tabs on jet mixing...

This paper documents noise spectra measured in the interior of a large number of automobiles. The original goal was to measure the vehicle interior acoustic amplitudes in the infrasound region, but since the noise was measured in the audible region also, spectra have been analyzed over a frequency range of 0-20 kHz. These experiments were carried o...

Acoustic sensors have been used for battlefield applications since World War I. Acoustic sensors provide several advantages and are increasingly employed in the modern high-tech battlefield. The twenty-first century battlefield calls for cutting-edge technology for military superiority and deployment of state-of-the-art acoustic sensor systems that...

The jet noise reduction potential of Distributed Exhaust Nozzles (DEN) can be significant and they continue to be studied for performance improvement. With an extensive set of static jet noise data completed in a previous work, the effect of forward flight velocity on DEN jet noise is undertaken in the present work. Flight effects are simulated usi...

A unique set of acoustic and fluid dynamic data was obtained for three converging rectangular jets of aspect ratios 1.5, 4.0, and 8.0 and a round nozzle. All four nozzles had the same exit area and were tested in the same facility for static and unheated conditions for a range of subsonic and slightly supersonic Mach numbers. Farfield noise measure...

The aerodynamic characteristics of a Circulation Control Wing (CCW) airfoil have been numerically investigated, and comparisons with experimental data have been made. The configuration chosen was a supercritical airfoil with a 30 degree dual-radius CCW flap. Steady and pulsed jet calculations were performed. It was found that the use of steady jets...

Circulation control wings are a type of pneumatic high-lift device that have been extensively researched as to their aerodynamic benefits. However, there has been little research into the possible airframe noise reduction benefits of a circulation control wing. The key element of noise is the jet noise associated with the jet sheet emitted from the...

Circulation control wings are a type of pneumatic high-lift device that have been extensively researched as to their aerodynamic benefits. However, there has been little research into the possible airframe noise reduction benefits of a circulation control wing. The key element of noise is the jet noise associated with the jet sheet emitted from the...

Circulation Control Wing (CCW) technology is a very effective way of achieving very high lift coefficients needed by aircraft during take-off and landing. This technology can also be used to directly control the flow field over the wing. Compared to a conventional high-lift system, a Circulation Control Wing (CCW) can generate the required values o...

This portion of the report documents the results of an experimental program, which focused on pulsed blowing from the trailing edge of a CCW. The main objective of this study was to assess whether pulsed blowing resulted in more, less, or the same amount of radiated noise to the farfield. Results show that a reduction in far-field noise of up to 5...

This Appendix documents the salient results from an effort to mitigate the so-called flap-edge noise generated at the split between a flap edge that is deployed and the undeployed flap. Utilizing a Coanda surface installed at the flap edge, steady blowing was used in an attempt to diminish the vortex strength resulting from the uneven lift distribu...

The main objective of this paper is to provide guidelines on how best to design and calibrate a highquality jet noise research facility and make high-quality jet noise measurements. Particular emphasis is placed on methodology used to determine if internal noise is dominant in the jet noise spectrum. A section of this document is devoted to clarify...

The purpose of this paper is twofold: First, a set of narrowband noise spectra for an unheated round subsonic jet is produced that can be compared with other nozzles and other facilities as well as for evaluating computational or analytical methods. Second, the subtle differences in dealing with narrowband and 1/3-octave spectra when analyzing jet...

A new signal enhancement method, based upon noise measurements at five farfield microphones, has been developed for separating two different correlated noise sources from farfield measurements contaminated with extraneous noise. Because of the distributed nature of the sources of jet noise at most frequencies, jet noise measured at microphones sepa...

A flexible sound shielding curtain contains a plurality of sound insulating sheet inserts encased within pockets or otherwise secured on the exterior surfaces of the panels of a curtain. The sound insulating sheet inserts may be constructed of a combination of materials selected and configured such that sound impinging upon the curtain is absorbed...

A unique set of acoustic and fluid dynamic data was obtained for three converging rectangular jets of aspect ratios 1.5, 4.0, and 8.0 and a round nozzle. All four nozzles had the same exit area and were tested in the same facility under both static and simulated forward flight conditions, for both heated and unheated jets. Farfield noise measuremen...

The effect of forward flight upon the noise internal to an ejector was investigated in a series of experiments. A nozzle system that simulated a mixer/ejector nozzle was designed and tested under both heated and unheated conditions in GTRI's Anechoic Free-jet Flight-simulation Facility. The nozzle system consisted of a round nozzle surrounded by an...

In a recent investigation, it was found by direct numerical simulation that sound waves at high intensity can induce vortex shedding at the mouths of the resonators of an acoustic liner. Measurements from their numerical simulations indicate that the rate at which kinetic energy is transferred to the shed vortices can be much higher than the viscou...

Feedback phenomenon resulting in acoustic resonance is very common in a range of aeroacoustics problems of current interest. Screech in shock-containing jets, cavity noise, edgetones, jet/collector interactions and howling of ejectors are but a few examples. In most of these problems involving shear layers, there is a match between the frequencies...

The effect on normal incidence acoustic impedance of a non-circular orifice shape is examined relative to a circular orifice. The impedance of an adjustable porosity perforate, formed from two identical perforates sliding over each other, is measured. As the orifice shape becomes more non-circular, the measured impedance is found to deviate from th...

The objective of this study was to acquire acoustic and flow data for numerical validation of impedance models that simulate bias flow through perforates. The impedance model is being developed by researchers at High Technology Corporation, Hampton, Virginia. This report documents normal incidence impedance measurements for a singular circular orif...

This report describes an experimental study conducted to determine the effect of negative bias flow on the sound absorption of a two degree-of-freedom liner. The backwall for the liner was designed to act as a double-Helmholtz resonator so as to act as a hard wall at all frequencies except at its resonant frequencies. The effect of bias flow is inv...

The present work explored the feasibility of controlling the acoustic impedance of a resonant type acoustic liner. This was accomplished by translating one perforate over another of the same porosity, creating a totally new that had an intermediate porosity. This type of adjustable perforate created a variable orifice perforate with non-circular or...

The objective of this study was to acquire acoustic and flow data with hard and lined duct wall sections for % situation of a liner prediction code being developed at NASA LaRC. Both the mean and acoustic flowfields were determined in a cross-plane of the rectangular duct. The test liner was of the locally-reacting type and w as made from a ceramic...

The objective of this study was to acquire detailed acoustic data and limited flow visualization data for numerical validation of a new model of sound absorption by a very narrow rectangular slit backed by a cavity. The sound absorption model is that being developed by Dr. C. K. W. Tam of Florida State University. This report documents normal incid...

This paper describes the results of an Internal Development Program (IDP) No. 97-1 conducted from August 1-October 1, 1996 at the Georgia Tech. Research Institute (GTRI). The IDP program was implemented to establish theoretical relationships and verify the interaction between X-band radar waves and ultrasonic acoustics. Low cost, off-the-shelf (OTS...