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At least 181 records · Page 10

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NASA NTRS2019

Cavity Ring Down Spectroscopy of Cold Neutral Phenanthrene and Phenanthridine in Supersonic Jets

Here we study the effect of nitrogen insertion on the electronic spectra of polycyclic aromatic hydrocarbon (PAH) molecules measured under laboratory conditions that are relevant for comparison with astronomical data. We examine the case of the substitution of carbon atoms by nitrogen atoms in the benzenoid skeleton of phenanthrene (C 14 H 10 ), a prototype non-compact PAH. The vibronic transitions of the 2 1 A 1 (S 1 )←X 1 A 1 (S 0 ) electronic absorption band system of neutral phenanthrene (C 14 H 10 ) and phenanthridine (C 13 H 9 N) molecules seeded in a supersonic free jet expansion of argon gas are measured in the 315-345 nm region using the cavity ring down spectroscopy (CRDS) technique. Additional measurements of the absorption spectra of the phenanthrene, phenanthridine, and 1,10-phenanthroline (C 12 H 8 N 2 ) molecules isolated in 10-K solid argon matrices are also presented. The results obtained confirm that laboratory spectra of cold, isolated molecules obtained under astrophysically relevant conditions are required for a direct, unambiguous comparison with astronomical observations as well as for the understanding of the physical evolution of the interstellar medium.

Absorption band↗

NASA NTRS1997

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: (1) a free jet and (2) 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.

Fenno, C. C., Jr.↗

NASA NTRS1993

Active control of nonlinear-nonstationary response and radiation of a panel-stringer structure near a supersonic jet

This paper is on the control of nonlinear-nonstationary vibration of an aircraft-type frame-stringer structure and the acoustic radiation resulting from high levels of excitation by a nearby supersonic model jet exhaust. The objective of the control is to reduce the acoustic fatigue and the interior noise in a high-speed aircraft. Control of the structural response is achieved by actively forcing the structure with an actuator at the shock oscillation frequency whose amplitude is locked into a self-control cycle. This controller follows the amplitude modulation that results from the rotation and counter-rotation of the jet column. Results show that the peak level of the power in the structural response due to shock impingement is reduced by a factor of 63, corresponding to a power level reduction of 18 dB. As a result, new broadband components emerge with at least four harmonics contributing to the broadening of the response spectrum. At accelerating and decelerating supersonic speeds, the exhaust from the jet induces higher transient loading on the nearby flexible structure due to the occurrence of multiple shocks from the jet.

Maestrello, Lucio↗

NASA NTRS1983

Interaction of multiple supersonic jets with a transonic flow field

The influence of multiple high pressure, supersonic, radial or tangential jets, that are injected from the circumference of the base plane of an axisymmetric body, on its longitudinal aerodynamic coefficients in transonic flow is studied experimentally. The interaction of the jets with the body flow field increases the pressures on the forebody, thus altering its lift and static stability characteristics. It is shown that, within the range of parameters studied. This interaction has a stabilizing effect on the body. The contribution to lift and stability is significant at small angles of attack and decreases nonlinearly at higher angles when the crossflow mechanism becomes dominant.

Seginer, A.↗

DOE OSTI2021

Transforming Energy through Computational Excellence. Exascale Computing: Combustion; Simulating Effects of Fuel Injection Location in Supersonic Jet Engines

Computational tractable simulations using an adaptive-mesh-refinement solverfor compressible reacting flows help researchers understand how variations in fuel injection location within the supersonic flow cavity impacts combustion efficiency. By identifying the important physical determinants of the combustion processes, this study shows a promising pathway to improving flame stability and combustion efficiency, as well as reducing emissions.

adaptive mesh refinement↗

NASA NTRS1982

On the impingement of supersonic jet on a normal flat surface

An improved method, based on one strip approximation of the method of integral relations which was reported originally by Belov, Ginzburg and Shub (1973), is presented for the calculation of flow parameters in the impingement region of a supersonic, underexpanded jet striking a normal surface located within the first cell. The results are presented for two impingement conditions and found to be in good agreement with the experimental data.

Pamadi, B. N.↗

NASA NTRS2004

Supersonic Jet Exhaust Noise at High Subsonic Flight Speed

An empirical model to predict the effects of flight on the noise from a supersonic transport is developed. This model is based on an analysis of the exhaust jet noise from high subsonic flights of the F-15 ACTIVE Aircraft. Acoustic comparisons previously attainable only in a wind tunnel were accomplished through the control of both flight operations and exhaust nozzle exit diameter. Independent parametric variations of both flight and exhaust jet Mach numbers at given supersonic nozzle pressure ratios enabled excellent correlations to be made for both jet broadband shock noise and jet mixing noise at flight speeds up to Mach 0.8. Shock noise correlated with flight speed and emission angle through a Doppler factor exponent of about 2.6. Mixing noise at all downstream angles was found to correlate well with a jet relative velocity exponent of about 7.3, with deviations from this behavior only at supersonic eddy convection speeds and at very high flight Mach numbers. The acoustic database from the flight test is also provided.

Norum, Thomas D.↗

NASA NTRS1992

Supersonic jet noise reduction by coaxial rectangular nozzles

A physical understanding of noise reduction mechanisms in supersonic, single, and coaxial rectangular jets is quantified and obtained, with emphasis on shock noise reduction. For all conditions, corresponding acoustic measurements for an equivalent round jet are also obtained so that the noise characteristics of the two types of jets can be compared directly to quantify the noise reductions. Comparisons are thus provided for a single rectangular nozzle vs a single equivalent round nozzle, and a coaxial rectangular nozzle vs an equivalent round nozzle. It is shown that different operating conditions and nozzle arrangements for the same thrust, total exit area, and mass flow rate can produce different noise levels. With at least one stream operated supersonically, the coaxial rectangular nozzle operated in the inverted-velocity profile is always quieter than in the normal velocity profile mode for the same thrust, exit area, and mass flow rate. In general, the coaxial rectangular nozzle is shown to be quieter than an equivalent circular nozzle only for those conditions for which both nozzles are operated supersonically.

Ahuja, K. K.↗

NASA NTRS1998

Effect of Swirl on Turbulent Structures in Supersonic Jets

Direct Numerical Simulation (DNS) is used to study the mechanism of generation and evolution of turbulence structures in a temporally evolving supersonic swirling round jet and also to examine the resulting acoustic radiations. Fourier spectral expansions are used in the streamwise and azimuthal directions and a 1-D b-spline Galerkin representation is used in the radial direction. Spectral-like accuracy is achieved using this numerical scheme. Direct numerical simulations, using the b-spline spectral method, are carried out starting from mean flow initial conditions which are perturbed by the most unstable linear stability eigenfunctions. It is observed that the initial helical instability waves evolve into helical vortices which eventually breakdown into smaller scales of turbulence. 'Rib' structures similar to those seen in incompressible mixing layer flow of Rogers and Moserl are observed. The jet core breakdown stage exhibits increased acoustic radiations.

Rao, Ram Mohan↗

NASA NTRS1998

Effect of Swirl on Turbulent Structures in Supersonic Jets

Rao, Ram Mohan↗

NASA NTRS1998

Effect of Swirl on Turbulent Structures in Supersonic Jets

Direct numerical simulation (DNS) is used to study the mechanism of generation and evolution of turbulence structures in a temporally evolving supersonic swirling round jet and also to examine the resulting acoustic radiations. Fourier spectral expansions are used in the streamwise and azimuthal directions and a 1-D b-spline Galerkin representation is used in the radial direction. Spectral-like accuracy is achieved using this numerical scheme. Direct numerical simulations, using the b-spline spectral method, are carried out starting from mean flow initial conditions which are perturbed by the most unstable linear stability eigenfunctions. It is observed that the initial.helical instability waves evolve into helical vortices which eventually breakdown into smaller scales of turbulence. 'Rib' structures similar to those seen in incompressible mixing layer flow of Rogers and Moser are observed. The jet core breakdown stage exhibits increased acoustic radiations.

Rao, Ram Mohan↗

NASA NTRS2010

Coupled Analysis of an Inlet and Fan for a Quiet Supersonic Jet

A computational analysis of a Gulfstream isentropic external compression supersonic inlet coupled to a Rolls-Royce fan has been completed. The inlet was designed for a small, low sonic boom supersonic vehicle with a design cruise condition of M = 1.6 at 45,000 ft. The inlet design included an annular bypass duct that routed flow subsonically around an engine-mounted gearbox and diverted flow with high shock losses away from the fan tip. Two Reynolds-averaged Navier-Stokes codes were used for the analysis: an axisymmetric code called AVCS for the inlet and a three dimensional (3-D) code called SWIFT for the fan. The codes were coupled at a mixing plane boundary using a separate code for data exchange. The codes were used to determine the performance of the inlet/fan system at the design point and to predict the performance and operability of the system over the flight profile. At the design point the core inlet had a recovery of 96 percent, and the fan operated near its peak efficiency and pressure ratio. A large hub radial distortion generated in the inlet was not eliminated by the fan and could pose a challenge for subsequent booster stages. The system operated stably at all points along the flight profile. Reduced stall margin was seen at low altitude and Mach number where flow separated on the interior lips of the cowl and bypass ducts. The coupled analysis gave consistent solutions at all points on the flight profile that would be difficult or impossible to predict by analysis of isolated components.

Chima, Rodrick V.↗

NASA NTRS1997

Large Scale Turbulent Structures in Supersonic Jets

Jet noise is a major concern in the design of commercial aircraft. Studies by various researchers suggest that aerodynamic noise is a major contributor to jet noise. Some of these studies indicate that most of the aerodynamic jet noise due to turbulent mixing occurs when there is a rapid variation in turbulent structure, i.e. rapidly growing or decaying vortices. The objective of this research was to simulate a compressible round jet to study the non-linear evolution of vortices and the resulting acoustic radiations. In particular, to understand the effect of turbulence structure on the noise. An ideal technique to study this problem is Direct Numerical Simulations(DNS), because it provides precise control on the initial and boundary conditions that lead to the turbulent structures studied. It also provides complete 3-dimensional time dependent data. Since the dynamics of a temporally evolving jet are not greatly different from those, of a spatially evolving jet, a temporal jet problem was solved, using periodicity ill the direction of the jet axis. This enables the application of Fourier spectral methods in the streamwise direction. Physically this means that turbulent structures in the jet are repeated in successive downstream cells instead of being gradually modified downstream into a jet plume. The DNS jet simulation helps us understand the various turbulent scales and mechanisms of turbulence generation in the evolution of a compressible round jet. These accurate flow solutions will be used in future research to estimate near-field acoustic radiation by computing the total outward flux across a surface and determine how it is related to the evolution of the turbulent solutions. Furthermore, these simulations allow us to investigate the sensitivity of acoustic radiations to inlet/boundary conditions, with possible application to active noise suppression. In addition, the data generated can be used to compute various turbulence quantities such as mean velocities, turbulent stresses, etc. which will aid in turbulence modeling. This report will be presented in two chapters. The first chapter describes some work on the linear stability of a supersonic round jet and the implications of this for the jet noise problem. The second chapter is an extensive discussion of numerical work using the spectral method which we use to solve the compressible Navier-Stokes equations to study turbulent jet flows. The method uses Fourier expansions in the azimuthal and streamwise direction and a 1-D B-spline basis representation in the radial direction. The B-spline basis is locally supported and this ensures block diagonal matrix equations which can be solved in O(N) steps. This is a modification of a boundary layer code developed by Robert Moser. A very accurate highly resolved Direct Numerical Simulation (DNS) of a turbulent jet flow is produced.

Rao, Ram Mohan↗

NASA NTRS1997

Large Scale Turbulent Structures in Supersonic Jets

Jet noise is a major concern in the design of commercial aircraft. Studies by various researchers suggest that aerodynamic noise is a major contributor to jet noise. Some of these studies indicate that most of the aerodynamic jet noise due to turbulent mixing occurs when there is a rapid variation in turbulent structure, i.e. rapidly growing or decaying vortices. The objective of this research was to simulate a compressible round jet to study the non-linear evolution of vortices and the resulting acoustic radiations. In particular, to understand the effect of turbulence structure on the noise. An ideal technique to study this problem is Direct Numerical Simulations (DNS), because it provides precise control on the initial and boundary conditions that lead to the turbulent structures studied. It also provides complete 3-dimensional time dependent data. Since the dynamics of a temporally evolving jet are not greatly different from those of a spatially evolving jet, a temporal jet problem was solved, using periodicity in the direction of the jet axis. This enables the application of Fourier spectral methods in the streamwise direction. Physically this means that turbulent structures in the jet are repeated in successive downstream cells instead of being gradually modified downstream into a jet plume. The DNS jet simulation helps us understand the various turbulent scales and mechanisms of turbulence generation in the evolution of a compressible round jet. These accurate flow solutions will be used in future research to estimate near-field acoustic radiation by computing the total outward flux across a surface and determine how it is related to the evolution of the turbulent solutions. Furthermore, these simulations allow us to investigate the sensitivity of acoustic radiations to inlet/boundary conditions, with possible appli(,a- tion to active noise suppression. In addition, the data generated can be used to compute, various turbulence quantities such as mean velocities, turbulent stresses, etc. which will aid in turbulence modeling. This report will be presented in two chapters. The first chapter describes some work on the linear stability of a supersonic round jet and the implications of this for the jet noise problem. The second chapter is an extensive discussion of numerical work using the spectral method which we use to solve the compressible Navier-Stokes equations to study turbulent jet flows. The method uses Fourier expansions in the azimuthal and streamwise direction and a 1-D B-spline basis representation in the radial direction. The B-spline basis is locally supported and this ensures block diagonal matrix equations which can be solved in O(N) steps. This is a modification of a boundary layer code developed by Robert Moser. A very accurate highly resolved DNS of a turbulent jet flow is produced.

Rao, Ram Mohan↗

NASA NTRS1979

Experiments of shock associated noise of supersonic jets

The paper examines the aeroacoustics associated with model nozzles operating supersonically. In particular, the shock structure and radiated shock noise of Mach 1.5 and 2.0 nozzles are compared with those of a convergent nozzle over a wide nozzle pressure ratio range corresponding to a fully expanded Mach number between unity and 2.37. The nozzles were operated unheated both with and without a tab for screech tone suppression. The measurements show differences between the shock cell spacing of convergent and convergent-divergent nozzles, and the scaling relation appears to be a function of the exit-to-throat velocity ratio of each nozzle type. The acoustic measurements indicate the extent of the pressure ratio range where a C-D nozzle achieves a noise reduction benefit. At the design point of the Mach 1.5 nozzle, the total integrated sound power from this nozzle is 6 dB less than a convergent nozzle operating at the same pressure ratio and thrust

Seiner, J. M.↗

NASA NTRS1993

The effect of velocity profiles on supersonic jet noise

This paper presents stability calculations made for a shock-free coannular jet, with both the inner and outer flows supersonic, using the instability wave model. The Mach wave emission process is modeled as the noise generated by the large scale turbulent structures or the instability waves in the mixing region. Both the vortex-sheet and the realistic finite thickness shear layer models are considered. The stability calculations have been performed for both inverted and normal velocity profiles. Comparisons are made with the results for an equivalent single jet, based on equal thrust, mass flow rate and exit area to that of the coannular jet. The advantages and disadvantages of these velocity profiles as far as noise radiation is concerned are discussed.

Bhat, T. R. S.↗

NASA NTRS2017

Noise Source Location and Flow Field Measurements on Supersonic Jets and Implications Regarding Broadband Shock Noise

An experiment was conducted in an effort to obtain data that would provide a better understanding of the origins of broadband shock noise (BBSN). Phased array noise source location and two types of flow field data (background oriented schlieren and particle image velocimetry) were acquired on unheated, single-stream jets. Results are presented for one subsonic and four supersonic operating conditions. These data show that BBSN is created primarily in the downstream portion of the shock train with peak BBSN production occurring near where the average size of the turbulent structures is equal to the shockcell spacing. These data tend to validate theories that BBSN is created by turbulent structures that are as large or larger than the shock spacing.

Podboy, Gary G.↗

NASA NTRS1972

Effect on supersonic jet noise of nozzle plenum pressure fluctuations.

Demonstration that pressure fluctuations in the plenum chamber to a supersonic nozzle can strongly increase the noise radiated from the jet plume. The correlation shows that jet noise acoustic efficiency increases from 0.3% to 0.8% (or 4 dB) when the chamber roughness intensity increases from essentially no plenum chamber roughness to 2%. A roughness level of 2% has been observed in some turbojet engines. It is concluded that the reduction or elimination of plenum chamber pressure fluctuations may be an important method of reducing the total noise from jet engines.

Kushida, R.↗