K. B. M. Q. Zaman

Synthetic Jets in Cross-Flow

Results of an experimental investigation on synthetic jets from round orifices with and without cross-flow are presented. Jet Reynolds number up to 46,000 with a fully turbulent approach boundary layer, and Stokes number up to 400. are covered. The threshold of stroke length for synthetic jet formation. in the absence of the cross-flow, is found to be Lo /D approximately 0.5. Above Lo /D is approximately 10, the profiles of normalized centerline mean velocity appear to become invariant. It is reasoned that the latter threshold may be related to the phenomenon of saturation of impulsively generated vortices. In the presence of the cross-flow, the penetration height of a synthetic jet is found to depend on the momentum- flux ratio . When this ratio is defined in terms of the maximum jet velocity and the cross-flow velocity. not only all data collapse but also the jet trajectory is predicted well by correlation equation available for steady jets-in-cross-flow. Distributions of mean velocity, streamwise vorticity as well as turbulence intensity for a synthetic jet in cross-flow are found to be similar to those of a steady jet-in-cross-flow. A pair of counter-rotating streamwise vortices, corresponding to the bound vortex pair of the steady case, is clearly observed. Mean velocity distribution exhibits a dome of low momentum fluid pulled up from the boundary layer, and the entire domain is characterized by high turbulence.

Synthetic Jets in Cross-flow. Part 1; Round Jet

Results of an experimental investigation on synthetic jets fro m round orifices with and without cross -flow are presented. Jet Reynolds number up to 46 ,000 with a fully turbulent approach boundary layer, and Stokes number up to 400, are covered. The threshold of ‘stroke length’ for synthetic jet formation, in the abse nce of the cross -flow, is found to be L0 /D � 0.5. Above L0 /D � 10, the profiles of normalized centerline mean velocity appear to become invariant. It is reasoned that the latter threshold may be related to the phenomenon of ‘saturation’ of impulsively ge nerated vortices. In the presence of the cross -flow, the penetration height of a synthetic jet is found to depend on the ‘momentum -flux ratio’. When this ratio is defined in terms of the maximum jet velocity and the cross -flow velocity, not only all data c ollapse but also the jet trajectory is predicted well by correlation equation available for steady jets -in -cross -flow. Distributions of mean velocity, streamwise vorticity as well as turbulence intensity for a synthetic jet in cross -flow are found to be si milar to those of a steady jet -in -cross -flow. A pair of counter -rotating streamwise vortices, corresponding to the ‘bound vortex pair’ of the steady case, is clearly observed. Mean velocity distribution exhibits a ‘dome’ of low momentum fluid pulled up fro m the boundary layer, and the entire domain is characterized by high turbulence.

Effect of a wedge on coannular jet noise

Directional noise attenuation for a coannular jet, caused by a wedge in the fan stream, is studied with data from three different facilities. The effect, first observed with small -scale convergent nozzles at UCI , involves noise attenuation on the side away from the wedge. The trend is confirmed with data from a lar ge facility. However, several anomalies are noted. In particular, the wedge is found to be ineffective with nozzles having parallel flow lines near the exit. In the latter case, the wedge simply causes a lateral spreading of the flow. In the convergent cas e, the wedge also diverts the flow downward forming the thicker annular layer underneath that is key to the noise attenuation seen on the ground.

Excess Broadband Noise Observed with Overexpanded Jets

Results of an experiment on the characteristics of an excess noise occurring with convergent-divergent nozzles in the overexpanded regime are presented in this paper. Data are obtained with five convergent-divergent nozzles and a convergent nozzle, all having the same exit diameter. The results clearly establish that, relative to the convergent case and at same pressure ratios, the convergent-divergent nozzles are noisier in the low Mach number range of the overexpanded regime. This is evidenced from the directivity patterns as well as the overall radiated sound power. The excess noise is broadband in nature and is found to be more pronounced with nozzles having a higher design Mach number or larger half angle of the divergent section. It appears to occur when a shock resides within the divergent section and results from random unsteady motion of the shock.

Subsonic jet noise reduction by microjets - a parametric study

The effect of injecting tiny secondary jets (‘μjets’) on the radiated noise from a subsonic primary jet is studied experimentally. The μjets are injected on to the primary jet near the nozzle exit with variable port geometry, working fluid and driving pressure. A clear reduction in the overall sound pressure level in the direction of peak noise radiation is observed that improves with increasing μjet pressure. It is found that smaller diameter ports with higher driving pressure, but involving less thrust and mass fraction, can produce better noise reduction. A collection of data from the present as well as past experiments is examined in an attempt to correlate the noise reduction with the operating parameters. The results indicate that the OASPL reduction, as monitored at a shallow angle, correlates with the ratio of μjet to primary jet driving pressures normalized by the ratio of corresponding diameters (pμd/pj D). With gaseous injection, the spectral amplitudes decrease at lower frequencies while an increase is noted at higher frequencies. The amplitude ‘crossover’ is thought to be at least partly due to shock-associated noise from the underexpanded μjets themselves. Such crossover is not seen with water injection apparently because the flow in that case is incompressible and there is no shock-associated noise. Centerline velocity data show that larger noise reduction is accompanied by faster jet decay as well as significant reduction in turbulence intensities. While a physical understanding of the dependence of the noise reduction on pμd/pj D remains unclear, given this correlation, an analysis explains the observed dependence of the effect on various other parameters.

SYNTHETIC JETS IN CROSS -FLOW. PART II: JETS FROM ORIFICES OF DIFFERENT GEOMETRY

The flow fields of synthetic jets in a cross -flow from orifices of different geometry are investigated. The g eometries include a straight, a tapered, a pitched and a cluster of nine orifices, all ha ving the same cross -sectional area through which the perturbation is discharged into t he cross -flow. The ‘strength’ of the jet from the tapered or ifice in comparison to that from the straight one is found to be only slightly enhanced. The flow field from the clu ster of orifices, when viewed a few equivalent diameters downstream, is similar to that from the single or ifice. However, the penetration is somewhat lower in the former case due to the increased mixing of the distributed jets with the cross -flow. The pen etr ation for the pitched configuration is the lowest , as expected. The jet trajec tories for the straight and pitched orifices are well represented by correlation equ ations available for steady jets -in -cross -flow. Distributions of streamwise velo city, vorticity as well as turbulence inte nsity are documented for various cases. In additio n, distributions of phase -averaged velocity and vorticity for the cylindrical and the clustered orifices are presented pr oviding an insight into the flow d ynamics.

Noise from a Jet Discharged into a Duct and its Suppression

This study addresses unwanted high intensity noise sometimes encountered in engine test facilities. Model-scale experiments are conducted for a round jet discharged into a cylindrical duct. In most cases, the unwanted noise is found to be due to longitudinal resonance modes of the duct excited by the random turbulence of the jet. When the ‘preferred mode’ frequency of the jet matches a duct resonant frequency there can be a locked-in ‘super-resonance’ accompanied by a high intensity tone or ‘howl’. Various techniques are explored for suppression of the unwanted noise. Tabs placed on the ends of the duct are found ineffective; so are longitudinal fins placed inside the duct. Arod inserted perpendicular to the flow (‘howl stick’) is also found generally ineffective; however, it is effective when there is a super-resonance. By far the most effective suppression is achieved by a wire-mesh screen placed at the end of the duct. The screen not only eliminates the super-resonance but also the duct mode spectral peaks. Apparently the screen works by dampening the velocity fluctuations at the pressure node and thereby weakening the resonant condition.

Effect of Wedge-Shaped Deflectors on Flow Fields of Dual-Stream Jets

The effect of wedge-shaped fan flow deflectors on the mean and turbulent flow-fields of dual-stream jets is investigated. Several wedge-shaped deflector concepts were used to create asymmetry in the plume of a dual-stream jet issuing from a scaled down version of the NASA Glenn ‘5BB’ bypass-ratio 8 turbofan nozzle. The deflector configurations comprised internal and external wedges with and without a pylon. Some external wedges incorporated local extensions of the fan nacelle. All the deflectors reduced radial velocity gradients, magnitudes of peak Reynolds stresses, and peak turbulent kinetic energy beneath the jet centerplane, with an increase above the jet centerplane. A correlation was obtained between the maximum radial velocity gradient and the peak turbulent kinetic energy in the dominant noise source region.

A Quantitative Comparison of Leading-edge Vortices in Incompressible and Supersonic Flows

When requiring quantitative data on delta-wing vortices for design purposes, low-speed results have often been extrapolated to configurations intended for supersonic operation. This practice stems from a lack of database owing to difficulties that plague measurement techniques in high-speed flows. In the present paper an attempt is made to examine this practice by comparing quantitative data on the nearwake properties of such vortices in incompressible and supersonic flows. The incompressible flow data are obtained in experiments conducted in a low-speed wind tunnel. Detailed flow-field properties, including vorticity and turbulence characteristics, obtained by hot-wire and pressure probe surveys are documented. These data are compared, wherever possible, with available data from a past work for a Mach 2.49 flow for the same wing geometry and angles-of-attack. The results indicate that quantitative similarities exist in the distributions of total pressure and swirl velocity. However, the streamwise velocity of the core exhibits different trends. The axial flow characteristics of the vortices in the two regimes are examined, and a candidate theory is discussed.

Shock-Induced Boundary-Layer Separation in Round Convergent-Divergent Nozzles

The location of shock-induced boundary-layer separation inside the divergent section of convergent–divergent nozzles is studied experimentally. Pressure-sensitive paint technique is used with nozzles of different design Mach numbers in the range 1.4–2.8. Nozzle pressure ratios in the range of 1.12–4.91, corresponding to a “jet Mach number” range of 0.4–1.7 and a Reynolds number range of 0.35×106–0.58×107, are covered in the experiment. As it is well-known, one-dimensional nozzle flow theory grossly overpredicts the throat-to-shock-location distance at a given nozzle pressure ratio. A correlation from the literature based on rocket nozzle databases is also found to be inadequate for these nozzles of lower design Mach number typical of aircraft applications. For the parametric range covered, a simple correlation for the shock location distance is found. All data collapse in a cluster when plotted as a function of the ratio of jet Mach number to design Mach number. A curve-fit equation representing the avera...