Engineering topics

Bridges, J. E.

Publications and source records attributed to Bridges, J. E..

8 records

NASA NTRS2019

An Investigation of a Mixer-Ejector Nozzle for Jet Noise Reduction

An experimental study is conducted assessing the performance of an ejector together with an 8:1 aspect ratio rectangular nozzle with the eventual goal of noise reduction for jet engines. Wall static pressure and Pitot probe surveys are conducted to evaluate the performance of the ejector, and sound pressure level measurements are made to assess the impact on noise radiation. It is found that addition of vortex generating tabs at the lip of the nozzle causes large increases in secondary flow entrainment. The baseline ejector (without tabs) often encounters flow resonance with accompanying tones. The tabs have the additional benefit of eliminating those tones. In most cases tried so far, pockets of high-speed fluid remain unmixed. Since jet noise scales as velocity to the eighth power, such ‘hot spots’ defeat the noise reduction goal. In some cases, there is a reduction in noise amplitudes in the mid-frequency range (5-30 kHz), however, an increase occurs on the low frequency end apparently due to flow unsteadiness. This together with a high frequency noise increase caused by the tabs results in minimal reductions in the overall sound pressure level. The focus of ongoing and future efforts is to achieve sufficient mixing and desirable noise reduction while keeping the hard-ware short and lightweight.

Zaman, K. B. M. Q.↗

NASA NTRS2019

Experimental Investigation of a One-Sided Ejector Nozzle

A fundamental exploratory experiment is conducted assessing the performance of a one-sided ejector with the eventual goal of noise reduction for jet engines. The hardware is comprised of an 8:1 rectangular nozzle together with an ejector box whose lower surface is flush with the lower lip of the nozzle. Secondary flow is allowed through a gap between the upper lip of the nozzle and a flap that constitutes the upper surface of the ejector. Wall static pressures and Pitot probe surveys are conducted to evaluate the performance of the ejector with variation of geometric parameters. It is found that addition of vortex generating tabs at the upper lip of the nozzle significantly increases secondary flow entrainment. The entrainment is further enhanced by a divergence of the ejector upper surface. Limited noise measurements are done. The baseline ejector (without tabs) often encounters flow resonance with accompanying tones. The tabs have the additional benefit of eliminating those tones in all cases. However, for the tabbed case, addition of the ejector produces insignificant further noise reduction. This is due to the fact that the flow remains unmixed on the lower half of the ejector. The focus of ongoing and future efforts is to achieve sufficient mixing of the flow so that the exhaust velocities are uniformly low, while keeping the ejector hardware short and lightweight.

Ejectors↗

NASA NTRS2017

Tones Encountered with a Coannular Nozzle and a Method for their Suppression

With multi-stream coannular nozzles, sometimes tones occur that may cause the nozzle to fail noise regulation standards. A two-stream nozzle was studied experimentally and numerically in an at-tempt to identify the sources of such tones and explore remedies. For the given nozzle configuration, sharp tones occurred in a range of low jet Mach numbers. The tones apparently occurred due to a coupling between vortex shedding from the struts, which held the nozzles and the center-body together, with various duct acoustic modes. A leading edge treatment of the struts is shown to eliminate the tones via disruption of the vortex shedding.

vortex shedding↗

NASA NTRS2017

Flow and Noise from Septa Nozzles

Flow and noise fields are explored for the concept of distributed propulsion. A model-scale experiment is performed with an 8:1 aspect ratio rectangular nozzle that is divided into six passages by five septa. The septa geometries are created by placing plastic inserts within the nozzle. It is found that the noise radiation from the septa nozzle can be significantly lower than that from the baseline rectangular nozzle. The reduction of noise is inferred to be due to the introduction of streamwise vortices in the flow. The streamwise vortices are produced by secondary flow within each passage. Thus, the geometry of the internal passages of the septa nozzle can have a large influence. The flow evolution is profoundly affected by slight changes in the geometry. These conclusions are reached by mostly experimental results of the flowfield aided by brief numerical simulations.

septa nozzle↗

NASA NTRS2016

An Experimental Investigation of Jet Noise from Septa Nozzles

Results of an experimental study with a large aspect ratio rectangular nozzle, divided into multiple compartments or septa, as pertinent to distributed propulsion, are presented. Noise measurements at high-subsonic conditions show that the nozzle with the septa is quieter than the corresponding baseline nozzle without the septa. At relatively lower Mach numbers a high-frequency tone is heard. This is shown to be due to Karmann vortex shedding from the trailing edge of the partitions that separate a septum from the adjacent ones. Flowfield measurements for a six septa case show that the cellular flow structure, issuing from the nozzle, goes through a curious coalescence with increasing downstream distance (x) from the nozzle. Adjacent cells pair to yield a three-cell structure by x/D =2, where D is the equivalent diameter of the baseline nozzle. By about x/D =16, both the septa case and the baseline case evolve to yield axisymmetric flowfields.

turbulence↗

NASA NTRS2015

Investigating the Feedback Path in a Jet-Surface Resonant Interaction

A resonant interaction between an 8:1 aspect ratio rectangular jet and flat-plates, placed parallel to the jet, is studied experimentally. For certain locations of the plate relative to the jet, the resonance takes place with a loud accompanying tone. The sound pressure level spectra are often marked by multiple peaks. The frequencies of the spectral peaks are studied as a function of the streamwise length of the plate, its relative location to the jet as well as the jet Mach number. It is demonstrated that the tones are not due to a simple feedback between the plate's trailing edge and the nozzle's exit; the leading edge of the plate also comes into play in the frequency selection. With parametric variation, it is found that there is an order in the most energetic spectral peaks; their frequencies cluster in distinct bands. The 'fundamental', i.e., the lowest frequency band is explained by an acoustic feedback involving diffraction at the plate's leading edge.

Resonance↗

NASA NTRS1987

Roles of initial condition and vortex pairing in jet noise

Sound generation by vortex pairing in circular and elliptic cold-air jets at Mach 0.15-0.35 is investigated experimentally, with a focus on the effects of initial conditions. The results are presented in graphs and interpreted using the theory of vortex sound proposed by Moehring (1978) and vortex-filament models of jet coherent structure. Tripping the nozzle boundary layer is shown to (1) preempt formation of shear-layer vortices, (2) remove the sound they produce in later pairing, and (3) increase the diffusion of coherent vorticity in the vortex rings. Hence pairing noise should not be significant in practical jets, which are initially turbulent.

Bridges, J. E.↗

NASA NTRS1986

Effects of initial condition on circular and elliptic jet noise

Noise produced by cold elliptic jets was studied experimentally using a 4 cm air jet in an anechoic chamber by varying the aspect ratio, nozzle exit boundary layer, and Mach number. Elliptic geometry is found to accentuate the sound produced by pairing of the shear layer vortices in the laminar boundary jet. Calculation of the pressure field produced by the pairing of elliptic vortex rings shows that the ratio of sound intensities produced by the pairing of inviscid elliptic vortex filament rings should be equal to their aspect ratio, the minor axis producing greater sound. Tripping the nozzle boundary layer changes the instability of the shear layer and removes the laminar vortices. If the sound produced by pairing and/or breakdown is considered to be the difference between laminar and turbulent jet noise, the pairing mechanism is seen to be very significant in laminar boundary layer jets.

Bridges, J. E.↗