Engineering topics

Zaman, K. B. M. Q.

Publications and source records attributed to Zaman, K. B. M. Q..

At least 19 records

NASA NTRS2019

Near-Exit Pressure Fluctuations in Jets from Circular and Rectangular Nozzles

This article documents some experimental observations made for near-exit unsteady pressure fluctuations in high-speed jets. These fluctuations are apparently related to ‘trapped waves’ within the jet potential core as investigated and reported recently by other researchers. Round nozzles of three different diameters and rectangular nozzles of three different aspect ratios are studied. The pressure fluctuations manifest as a series of peaks in the spectra. These ‘trapped wave spectral peaks’ are found with all nozzles. Their characteristics and variations with axial and radial distances as well as with jet Mach number are documented. Effects of initial boundary layer state and the presence of a surface nearby as well as the scaling of the frequencies of the spectral peaks are studied and discussed.

Zaman, K. B. M. Q.↗

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

The Effect of Incoming Boundary Layer Characteristics on the Performance of a Distributed Propulsion System

An experimental study is conducted in an effort to advance the understanding of flow physics associated with a boundary layer ingesting, distributed propulsion system. The influence of incoming boundary layer thickness and flow distortions are studied on the flow downstream and the overall performance of the system. The propulsion model, fabricated using additive manufacturing and integrated with electrical fans, is mounted on a flat plate and tested at subsonic speeds. Detailed characterization of the incoming boundary layer and subsequent assessment of the downstream flow field is performed using hotwire anemometry. Modification of the incoming boundary layer is achieved by placing tripping devices, such as rods and vortex generating ramps, near the leading edge of the flat plate. The overall performance of the system for different incoming flow conditions is analyzed by comparing magnitudes of exhaust velocities as well as estimated propulsive power to the corresponding baseline values. For a constant input power to the fans, smaller upstream flow distortions and moderately thickened boundary layers result in marginal changes in the flow field downstream. On the other hand, notable reductions in downstream flow velocities and propulsive power are observed in the case of a significantly thicker and/or distorted incoming boundary layer. It is hoped that this study will serve as a database for this technologically relevant flow field that has not been explored adequately before.

Propulsion↗

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 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 up-per 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 light-weight.

Zaman, K. B. M. Q.↗

NASA NTRS2018

Experimental and Computational Study of Tones Occurring with a Coaxial Nozzle

The source of audible tones occurring with a coaxial nozzle in a range of low Mach numbers is explored experimentally as well as computationally. The hardware is comprised of an inner and an outer nozzle, without a center-body, that are held together by a set of four struts. With increasing jet Mach number (M(sub j)), first a tone occurred at about 2550 Hz around M(sub j)=0.06. At higher values of M(sub j) a tone at 5200 Hz dominated the noise spectra. The corresponding nondimensional frequency, based on effective thickness of the inner nozzle lip and jet exit velocity, turned out to be about 0.2, a value characteristic of Karmann vortex shedding. Thus, vortex shedding from the inner nozzle lip could be linked to the tones. From a comparison of the acoustic wavelengths and the nozzle dimensions, it was inferred that the vortex shedding excited a one-quarter-wave resonance within the divergent section of either the inner nozzle or the outer nozzle. This led to the generation of the sharp tones.

nozzle↗

NASA NTRS2017

An Experimental Study and Database for Tip Vortex Flow From an Airfoil

An experimental investigation of tip vortices from a NACA0012 airfoil is conducted in a low-speed wind tunnel at a chord Reynolds number (Rc) of 4×10(exp 4 ). Data for the stationary airfoil at various angles of attack (alpha) are first discussed. Detailed flow-field surveys are done for two cases: alpha = 10deg with attached flow and alpha = 25deg with massive flow separation. Data include mean velocity, streamwise vorticity, and turbulent stresses at various streamwise locations. For all cases, the vortex core is seen to involve a mean velocity deficit. The deficits in these cases trace to the airfoil wake, part of which gets wrapped up by the tip vortex. Comparison with data from the literature suggests that with increasing Rc, the deficit turns into an excess, with the transition occurring in the approximate Rc range of 2×10(exp 5) to 5×10(exp 5). Survey results for various shapes of the airfoil wingtip are then presented. The shapes include square and rounded ends and a number of winglet designs. Finally, data under sinusoidal pitching condition, for the airfoil with square ends, are documented. All pitching cases pertain to a mean alpha = 15deg, while the amplitude and frequency are varied. Amplitudes of +/-5deg, +/-10deg, and +/-15deg and reduced frequencies k = 0.08, 0.2, and 0.33 are covered. Digital records of all data and some of the hardware design are made available on a supplemental CD with the electronic version of the paper for those interested in numerical simulation.

turbulence↗

NASA NTRS2017

An Experimental Study and Database for Tip Vortex Flow From an Airfoil

No abstract available

Zaman, K. B. M. Q.↗

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 NTRS2017

Increased Jet Noise Due to a "Nominally Laminar" State of Nozzle Exit Boundary Layer

A set of 2-in. diameter nozzles is used to investigate the effect of varying exit boundary layer state on the radiated noise from high-subsonic jets. It is confirmed that nozzles involving turbulent boundary layers are the quietest while nozzles involving a "nominally laminar" boundary layer are loud especially on the high-frequency side of the sound pressure level spectrum. The latter boundary layer state involves a "Blasius-like" mean velocity profile but higher turbulence intensities compared to those in the turbulent state. The higher turbulence in the initial region of the jet shear layer leads to increased high-frequency noise. The results strongly suggest that an anomaly noted with subsonic jet noise databases in the literature is due to a similar effect of differences in the initial boundary layer state.

Zaman, K. B. M. Q.↗

NASA NTRS2017

Increased Jet Noise Due to a "Nominally Laminar" State of Nozzle Exit Boundary Layer

A set of 2-inch diameter nozzles is used to investigate the effect of varying exit boundary layer state on the radiated noise from high-subsonic jets. It is confirmed that nozzles involving turbulent boundary layers are the quietest while nozzles involving a nominally-laminar boundary layer are loud especially on the high-frequency side of the sound pressure level spectrum. The latter boundary layer state involves a Blasius-like mean velocity profile but higher turbulence intensities compared to those in the turbulent state. The higher turbulence in the initial region of the jet shear layer leads to increased high-frequency noise. The results strongly suggest that an anomaly noted with subsonic jet noise databases in the literature is due to a similar effect of differences in the initial boundary layer state.

Zaman, K. B. M. Q.↗

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 NTRS2014

Noise from a Supersonic Round Jet Discharging Into a Duct

In an effort to understand an 'unwanted noise' problem occasionally encountered in ground test facilities, the interaction of a jet flow with a duct is studied in a model scale experiment. While the interaction of subsonic jets was studied earlier, that of supersonic jets is considered in this paper. The effect of the presence of a cylindrical duct in the path of the jet is studied through sound pressure level spectral measurements as well as schlieren flow visualization. When the jet involves screech tones the placement of the duct is found to make only minor effects on the tones themselves as well as on the high frequency noise. However, there is increased energy at low frequencies. The increase in low frequency noise becomes clearer when screech is eliminated from the jet by two small tabs placed at the nozzle exit. It is shown that spectral peaks and increased sound pressure levels occur at frequencies corresponding to the axial acoustic resonance modes of the duct. These peaks persist into the supersonic regime, however, their amplitudes diminish relative to increasing spectral amplitudes at other frequencies with increasing jet Mach number. A wire-mesh screen attached to the end of the duct effectively suppresses such unwanted noise at subsonic as well as supersonic conditions.

Jet Noise↗

NASA NTRS2014

Resonant Interaction of a Rectangular Jet with a Flat-Plate

A resonant interaction between a large aspect ratio rectangular jet and a flat-plate is addressed in this experimental study. The plate is placed parallel to but away from the direct path of the jet. At high subsonic conditions and for certain relative locations of the plate, the resonance accompanied by an audible tone is encountered. The trends of the tone frequency variation exhibit some similarities to, but also marked differences from, corresponding trends of the well-known edge-tone phenomenon. Under the resonant condition flow visualization indicates a periodic flapping motion of the jet column. Phase-averaged Mach number data obtained near the plate's trailing edge illustrate that the jet cross-section goes through large contortions within the period of the tone. Farther downstream a clear 'axis switching' takes place. These results suggest that the assumption of two-dimensionality should be viewed with caution in any analysis of the flow.

Zaman, K. B. M. Q.↗

NASA NTRS2013

Interaction of a Rectangular Jet with a Flat-Plate Placed Parallel to the Flow

An experimental study is carried out addressing the flowfield and radiated noise from the interaction of a large aspect ratio rectangular jet with a flat plate placed parallel to but away from the direct path of the jet. Sound pressure level spectra exhibit an increase in the noise levels for both the 'reflected' and 'shielded' sides of the plate relative to the free-jet case. Detailed cross-sectional distributions of flowfield properties obtained by hot-wire anemometry are documented for a low subsonic condition. Corresponding mean Mach number distributions obtained by Pitot-probe surveys are presented for high subsonic conditions. In the latter flow regime and for certain relative locations of the plate, a flow resonance accompanied by audible tones is encountered. Under the resonant condition the jet cross-section experiences an 'axis-switching' and flow visualization indicates the presence of an organized 'vortex street'. The trends of the resonant frequency variation with flow parameters exhibit some similarities to, but also marked differences with, corresponding trends of the well-known edgetone phenomenon.

Zaman, K. B. M. Q.↗

NASA NTRS2012

Effect of Initial Condition on Subsonic Jet Noise from Two Rectangular Nozzles

Differences in jet noise data from two small 8:1 aspect ratio nozzles are investigated experimentally. The interiors of the two nozzles are identical but one has a thin-lip at the exit while the has a perpendicular face at the exit (thick-lip). It is found that the thin-lip nozzle is substantially noisier throughout the subsonic Mach number range. As much as 5dB difference in OASPL is noticed around Mj =0.96. Hot-wire measurements are carried out for the characteristics of the exit boundary layer and, overall, the noise difference can be ascribed to differences in the boundary layer state. The boundary layer of the quieter (thick-lip) nozzle goes through transition around M(sub j) =0.25 and at higher M(sub j) it remains "nominally turbulent". In comparison, the boundary layer of the thin-lip nozzle is found to remain "nominally laminar". at high subsonic conditions. The nominally laminar state involves significantly larger turbulence intensities commensurate with the higher radiated noise.

Zaman, K. B. M. Q.↗