Mehmet Bahadir Alkislar

On the use of microjets to suppress turbulence in a Mach 0.9 axisymmetric jet

We have experimentally studied the effect of microjets on the flow field of a Mach 0.9 round jet. Planar and three-dimensional velocity field measurements using particle image velocimetry show a significant reduction in the near-field turbulent intensities with the activation of microjets. The axial and normal turbulence intensities are reduced by about 15% and 20%, respectively, and an even larger effect is found on the peak values of the turbulent shear stress with a reduction of up to 40%. The required mass flow rate of the microjets was about 1% of the primary jet mass flux. It appears that the microjets influence the mean velocity profiles such that the peak normalized vorticity in the shear layer is significantly reduced, thus inducing an overall stabilizing effect. Therefore, we seem to have exploited the fact that an alteration in the instability characteristics of the initial shear-layer can influence the whole jet exhaust including its noise field. We have found a reduction of about 2 dB in the near-field overall sound pressure level in the lateral direction with the use of microjets. This observation is qualitatively consistent with the measured reduced turbulence intensities.

Supersonic Cavity Flows and Their Control

A detailed experimental study of supersonic, Mach 2, flow over a three-dimensional cavity was conducted using shadowgraph visualization, unsteady surface pressure measurements, and particle image velocimetry. Large-scale structures in the cavity shear layer and visible disturbances inside the cavity were clearly observed. A large recirculation zone and high-speed reverse flow was revealed in the cavity. In addition, supersonic microjets were used at the leading edge to suppress flow unsteadiness within the cavity. With a minimal mass flux (blowing coefficient B c = 0.0015), the activation of microjets led to reductions of up to 20 dB in the amplitudes of cavity tones and of more than 9 dB in the overall sound pressure levels. The microjet injection also modified the cavity mixing layer and resulted in a significant reduction in the flow unsteadiness inside the cavity as revealed by the shadowgraphs and the velocity-field measurements.

The effect of streamwise vortices on the aeroacoustics of a Mach 0.9 jet

The role of the streamwise vortices on the aeroacoustics of a Mach 0.9 axisymmetric jet is investigated using two different devices to generate streamwise vortices: microjets and chevrons. The resultant acoustic field is mapped by sideline microphones and a microphone phased array. The flow-field characteristics within the first few diameters of the nozzle exit are obtained using stereoscopic particle image velocimetry (PIV). The flow-field measurements reveal that the counter-rotating streamwise vortex pairs generated by microjets are located primarily at the high-speed side of the initial shear layer. In contrast, the chevrons generate vortices of greater strength that reside mostly on the low-speed side. Although the magnitude of the chevrons axial vorticity is initially higher, it decays more rapidly with downstream distance. As a result, their influence is confined to a smaller region of the jet. The axial vorticity generated by both devices produces an increase in local entrainment and mixing, increasing the near-field turbulence levels. It is argued that the increase in high-frequency sound pressure levels (SPL) commonly observed in the far-field noise spectrum is due to the increase in the turbulence levels close to the jet exit on the high-speed side of the shear layer. The greater persistence and lower strength of the streamwise vortices generated by microjets appear to shift the cross-over frequencies to higher values and minimize the high-frequency lift in the far-field spectrum. The measured overall sound pressure level (OASPL) shows that microjet injection provides relatively uniform noise suppression for a wider range of sound radiation angles when compared to that of a chevron nozzle.

Structure of a screeching rectangular jet: a stereoscopic particle image velocimetry study

The unsteady velocity field generated by an underexpanded jet has been investigated using stereoscopic particle image velocimetry (PIV). A 4:1 aspect ratio converging–diverging rectangular nozzle designed to operate at a fully expanded condition of

Characteristics of the Shock Noise Component of Jet Noise

M=1.44

Aeroacoustic Properties of Supersonic Cavity Flows and Their Control

was used. The nozzle was operated at off-design conditions to generate imperfectly expanded jets with intense screech tones. Phase-locked PIV measurements show the spatial and temporal evolution of the three-dimensional jet with high fidelity. In addition to the globally averaged mean and turbulence velocity field data, the phase-averaged data for the velocity and vorticity fields were also obtained. The turbulence quantities were resolved into contributions from the periodic and random motions. The deformation of the periodic spanwise structures results in the formation of strong streamwise vortices that appear to govern the mixing of the jet. It is shown that the presence of coherent vorticity of significant strength, in addition to the shock cell strength, is largely responsible for determining the screech intensity.

Structure of Supersonic Twin Jets

The characteristics of the flow and the noise of shock-containing jets have been studied for nearly three decades. It is now established that broadband shock-associated noise is generated by the interaction of the downstream-convecting coherent structures of the jet flow with the shock cells in the jet plume. The analyses of farfield data have been carried out with the total measured noise, which contains both the turbulent mixing noise and shock noise. In this study, these two components are first separated and extracted from the total spectra, obtained with convergent nozzles. The decomposition is made possible by a recently developed scaling methodology for turbulent mixing noise, which collapses perfectly the mixing noise spectra from jets at all velocities but at a fixed temperature ratio. The characteristics of the shock component alone are investigated. The intensity and the spectra of the shock-associated noise are not affected by jet temperature but mainly controlled by the jet Mach number. However, the shock intensity for the total noise does not scale as the fourth power (shock exponent) of 2 2 D j M M − , but has a weak dependence on jet temperature ratio and the radiation angle; the value of the exponent decreases with increasing jet temperature and increasing inlet angle. This finding is contrary to conventional belief that the intensity is independent of the jet temperature and the observer angle. For the shock component alone, there is no discernible trend for the variation of the exponents with angle and temperature. It is not straightforward to collapse the shock spectra. It is also established for the first time that nonlinear propagation effects are manifested at lower radiation angles, where the shock component is dominant. The physical phenomenon that triggers the onset of nonlinear propagation for the shock noise could not be identified. The characteristics of the correlation functions at the lower radiation angles for subsonic and supersonic jets are different, attesting to the different noise generation mechanisms.

Stereoscopic PIV Measurements of a Screeching Supersonic Jet

A detailed experimental study of supersonic, Mach 2, flow over a 3D cavity was conducted using shadowgraph, particle image velocimetry (PIV), and unsteady surface pressure measurements. Large-scale structures in the cavity shear layer and visible disturbances inside the cavity were observed. The PIV data reveals the highly unsteady nature of the entire flowfield and the presence of a large recirculation zone with reverse velocities as high as 40 % of the freestream velocity. Supersonic microjets at the leading edge are used to control the cavity flow and suppress resonance in the cavity. Using minimal mass flux through the microjets, overall sound pressure level (OASPL) was reduced by greater than 9 dB with tonal reductions greater than 20 dB. The PIV data reveals that microjet injection modifies the cavity shear layer and results in a significant reduction in the unsteadiness of the cavity velocity-field.

Aeroacoustics of a Mach 0.9 Jet with Chevron-Microjet Combination

The flow characteristics of coupled oscillatory motion of M j = 1.5 supersonic twin jets are described using the particle image velocimetry measurements. As a result of self-excitation caused by screech, sinuous oscillation of individual jets is observed. The coupling between the jets, characterized by the presence of coherent large-scale eddies, results in a symmetric mode with respect to the midplane separating the two jets. A control technique using microjets placed at the nozzle exit is used to suppress the unsteady flow

Flowfield and Noise Characteristics of Twin Supersonic Impinging Jets

The effect of acoustic feed back on global flow response is illustrated through an example of a rectangular screeching jet operating at a nominal Mach number of 1.69. Using a stereoscopic Particle Image Velocimetry, the detailed flow characteristics within a screeching cycle are obtained with fidelity. To resolve the “bias” errors inherent with standard PIV image processing technique, a novel mesh-free and high spatial resolution scheme is implemented to yield accurate velocity measurements in a complex three-dimensional supersonic flow. The axis-switching phenomenon that arises due to unusual mixing enhancement in the minor axis plane of a rectangular jet is vividly displayed. Strong streamwise vortex structure in the jet shear layers, enhanced by the inherent instability of the shear layer, is reported.