Aspect Ratio Rectangular Jets Research Articles

Effect of plasma actuator-based control on flow-field and acoustics of supersonic rectangular jets

We perform a computational study on the effects of localized arc filament plasma actuator based control on the flow field and acoustics of a supersonic 2:1 aspect ratio rectangular jet. Post validation of the baseline jet, effects of control in the context of noise reduction are studied at experimentally guided forcing parameters, including frequencies $St=0.3, 1.0$ and $St=2.0$ with duty cycles of $20\,\%$ and $50\,\%$ . In general, high-frequency forcing reduces noise in the downstream direction, with the actuator signature appearing mostly in the sideline direction. Here $St=1$ , ${\rm DC}=50\,\%$ yields an optimum balance between peak noise reduction (of ${\sim }1.5$ dB) and actuator tones, with control being most effective on the major axis plane that bisects the shorter edges of the nozzle. Shear layer response to the most effective forcing includes generation of successive arrays of mutually interacting staggered lambda vortices, which eventually energize streamwise vortical elements. Causal mechanisms of noise mitigation are further elucidated as follows. First, the control reduces the energy within the supersonic phase speed regime of peak radiating frequencies by redistributing a part of it into a high-frequency band. Second, it enhances azimuthal percolation of energy into the first and second helical modes at frequencies where noise reduction is seen, thus weakening the radiatively efficient axisymmetric mode. Finally, sound-producing intermittent events in the jet are significantly reduced, thereby minimizing the high-intensity acoustic emissions. This small-perturbation-based control strategy results in only minor variations in the mean flow properties. However, reduced production and enhanced convection attenuate turbulent kinetic energy within the spreading shear layer in the controlled jet.

Effect of Plasma Actuator on Velocity and Temperature Profiles of High Aspect Ratio Rectangular Jet

The turbulence jet centerline velocity and temperature decay intensely along the centerline flow direction. Thus, improving it could benefit engineering applications, such as air conditioners. However, active flow control techniques with high-aspect-ratio jets, especially for controlling the temperature, have not been widely investigated. This paper presents the velocity and temperature performance of a high-aspect-ratio rectangular jet controlled by two dielectric barrier discharge plasma actuators located on the longer sides of the nozzle and controlled by high-voltage and high-frequency pulse-width modulation sinusoidal waves. The scanning method was used to cover 362 cases as combinations of working parameters (modular frequency vs. duty vs. phase difference) for the velocity and temperature performances of the jets. Results show that plasma actuators can control both velocity and temperature distribution with minor input power compared with the rectangular jet’s kinetic energy and heat flux. The velocity increased up to 4% and decreased to 11%, measured at the interest position where x/h = 70 on the centerline. There were a 5% increase and a 4% decrease compared to the temperature-based case. Distinctive velocity and temperature distributions were observed under noteworthy cases, indicating the potential of the actuator to create various flow features without installing new hardware on the flow.

Effect of Plasma Actuator on Velocity and Temperature Profiles of High Aspect Ratio Rectangular Jet

Effect of Plasma Actuator on Velocity and Temperature Profiles of High Aspect Ratio Rectangular Jet

Axis switching in low to moderate aspect ratio rectangular orifice synthetic jets

The behavior and performance of synthetic jets is closely tied to the dynamics of vortex rings, which are a dominant feature of the early jet. When a synthetic jet actuator employs a non-axisymmetric orifice, the self-induced deformations of the vortex rings are responsible for changes in the shape, momentum distribution, and entrainment rate of the jet. In this study, axis-switching of vortex rings in finite-span synthetic jets with a range of orifice aspect ratios was experimentally investigated using stereoscopic particle imaging velocimetry, and the results were compared to vortex dynamics simulations based on the Biot-Savart law to gain additional insight.

Empirical modelling of noise from high aspect ratio rectangular jets

Noise measurements have been performed on rectangular jets of aspect ratios ranging from 49 to 987 with the aim of determining the appropriate velocity and length scaling to be used in an empirical noise prediction model. The results have shown that the velocity exponent is a function of the nozzle aspect ratio, decreasing with increasing nozzle aspect ratio. In an effort to establish a general prediction model, the velocity exponent of 7 was chosen as the best compromise to represent all the measured data. The analysis of the noise measurements from high aspect ratio nozzles of varying jet height and width has shown that, for the range of aspect ratios considered, the jet sound power level scales with the nozzle height to the power of 3 and the nozzle width to the power of 1. The derived jet noise scaling has been validated with independent experimental data and shows good agreement.

An experimental investigation of resonant interaction of a rectangular jet with a flat plate

The interaction between an 8:1 aspect ratio rectangular jet and a flat plate, placed parallel to the jet, is addressed in this study. At high subsonic conditions and for certain relative locations of the plate, a resonance takes place with accompanying audible tones. Even when the tone is not audible the sound pressure level spectra are often marked by conspicuous peaks. The frequencies of these peaks, as functions of the plate’s length, its location relative to the jet as well as jet Mach number, are studied in an effort to understand the flow mechanism. It is demonstrated that the tones are not due to a simple feedback between the nozzle exit and the plate’s trailing edge; the leading edge also comes into play in determining the frequency. With parametric variation, it is found that there is an order in the most energetic spectral peaks; their frequencies cluster in distinct bands. The lowest frequency band is explained by an acoustic feedback involving diffraction at the plate’s leading edge. Under the resonant condition, a periodic flapping motion of the jet column is seen when viewed in a direction parallel to the plate. Phase-averaged Mach number data on a cross-stream plane 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 for the time-averaged cross-section of the jet that does not occur otherwise for a non-resonant condition.

Mechanism of axis switching in low aspect-ratio rectangular jets

In this work we systematically study one square jet (AR=1) and four rectangular jets with an aspect ratio of width over height AR=1.5,2,2.5, and 3 respectively using the lattice Boltzmann method for direct numerical simulation. Focuses are on various flow properties on transverse planes downstream to investigate the correlation between the downstream velocity and secondary flow. Three distinct regions of jet development are identified in all the five jets. As the length of the PC (potential core) region maintains about the same, that of the CD (characteristic decay) region strongly depends on the jet aspect-ratio (AR) and Reynolds number (Re). The 45° and 90° axis-switching occur in the CD region, with the former followed by the latter at the early and late stages of the CD region respectively. The half-width streamwise velocity contour reveals that 45° axis-switching is mainly contributed by the corner effect, whereas the aspect-ratio (elliptic) feature affects the shape of the jet when 45° axis-switching occurs. The close examinations of flow pattern and vorticity contour, as well as the correlation between streamwise velocity and vorticity, indicate that 90° axis-switching results from the boundary effect. Specific flow patterns for 45° and 90° axis-switching are identified to reveal the mechanism of the axis-switchings respectively.

Flow visualization of vortex structure in a pulsed rectangular jet

Pulsating jet is visualized using hydrogen bubble method to clarify the vortex nature in the near field of the jet. This study focused on the development in space and time of vortex structures evolution in low aspect-ratio rectangular jet with pulsation. Pulsation means large-amplitude, low-frequency excitation which is expected to increase the mixing and spreading of the jet and to accelerate its transition from a rectangular form to an axisymmetric form. It was deemed appropriate to investigate whether jet characteristics of a pulsating, submerged jet flow can be altered by including pulsations. The difference of the vortex deformation process is discussed in relation to pulsating conditions. Consequently, the pulsation leads to the formation of vortices at regular intervals, which are larger than those occurring in a steady jet. The results show that the streamwise interaction, between leading vortex and trailing vortex rolled up at nozzle lips, strengthens with increasing pulsating frequency. The spanwise drift of the vortex becomes strongly apparent at large amplitude and high frequency conditions. The drifting start position does not change regardless of pulsating condition. The convection velocity of vortex increases at lower frequency and larger amplitude.

Morphology of subsonic rectangular slot jets

This study is motivated by the fact that studies on compressible subsonic rectangular jets are scarce in the literature. Therefore, experiments have been conducted on rectangular jets of aspect ratios 1, 2, 4, and 6, at Mach 0.4 and 1.0. Another highlight of this study is that slots have been used as jet devices rather than nozzles in order to eliminate azimuthal variations of boundary layer growth in the jet device. The flow is characterized in terms of centreline velocity, jet spread, cross-sectional structure, and entrainment behaviour. Results favour the use of low aspect ratio rectangular jets for mixing enhancement.

Frequency characteristics of coherent structures and their excitations in small aspect-ratio rectangular jets using large eddy simulation

Large eddy simulations of air jets from small aspect-ratio (AR) rectangular nozzles are performed with the dynamic subgrid-scale closure. Mean streamwise velocity profiles are in good agreement with experimental data. Results indicate that vortices originating from the longer side of the rectangular jet are dominant compared with that from the shorter side. Furthermore, entrainment is slight in the potential core, and significantly increases in the following vortex roll-up region. However, the jet entrains more with smaller AR. Power spectral density of the streamwise velocity indicates that the oscillations consist of a series of sub-harmonic frequencies, with the predominant frequencies reducing along the axial direction. Analysis shows that among multiple frequencies, there is a characteristic one at f = 0.22 which dominates the near field of the rectangular jet. The characteristic frequency is independent of velocity components, aspect ratios of the jet and locations. Based on this characteristic frequency, calculations with different forced frequencies imposed on the inlet nozzle are carried out. Results indicate that when the forced frequency is approximately equal to the characteristic frequency, development of the coherent structures is the most intense in the near field, and exhibits the strongest entrainment.

Near-field turbulent simulations of rectangular jets using lattice Boltzmann method

We perform large eddy simulation (LES) of the near field of low aspect ratio (AR) rectangular turbulent jets (RTJ) using the lattice Boltzmann method. The computational technique combines a D3Q19 multiple relaxation time (MRT) lattice Boltzmann equation (LBE) with the Smagorinsky model for the subgrid stress. First and foremost, we demonstrate that the MRT-LBE model is more suitable than the widely used single-relaxation-time LBE model for LES of turbulent flows. Then, we proceed to compute four jets with MRT-LBE: AR-1, 1.5, 2, 5; exit velocity u0(m∕s)-60, 39, 60, 23; and Reynolds number Re-184 000, 25 900, 128 000, 14 000. The investigated near-field behavior includes: (1) Decay of mean streamwise velocity (MSV) and inverse MSV; (2) spanwise and lateral profiles of MSV; (3) half-velocity width development and MSV contours; and (4) streamwise turbulence intensity distribution. The simulation results are compared against experimental data. Two unique features of RTJ—the saddle-back MSV spanwise (major axis) profile and axis switching of major axis from spanwise to lateral direction—are investigated. Our simulations show that the jet statistical behavior is more sensitive to inflow velocity and less so to the transverse boundary conditions. Overall, this work demonstrates that the MRT-LBE method is a potentially reliable computational tool for LES of turbulence even at high Reynolds numbers.

Vortex dynamics and entrainment in rectangular free jets

Simulations of low-aspect-ratio, rectangular free jets are presented. The investigations focus on the entrainment and transitional vortex dynamics in compressible (subsonic) jets initialized with laminar conditions, a thin vortex sheet with slightly rounded-off corner regions, and uniform initial momentum thickness. A monotonically integrated large-eddy simulation approach based on the solution of the unsteady flow equations with high-resolution monotone algorithms is used. Inherent uncertainties in the jet entrainment measurement process are addressed using the database from laboratory experiments and simulations. Vorticity geometries characterizing the near flow field of low aspect-ratio (A) rectangular jets are demonstrated, involving: (i) self-deforming and (ii) splitting vortex rings; interacting ring and braid (rib) vortices including (iii) single ribs aligned with corner regions (A [ges ] 2) and (iv) rib pairs aligned with the corners (A = 1); (v) a more disorganized flow regime in the far jet downstream, where the rotational-fluid volume is occupied by a relatively weak vorticity background with strong, slender tube-like filament vortices filling a small fraction of the domain – as observed in fully developed turbulent flows. The near field entrainment properties of low-A rectangular jets are shown to be largely determined by the characteristic A-dependent coupling geometry of interacting rib and ring vortices and by vortex-ring axis-switching times.

Counterflow Thrust Vector Control of Subsonic Jets: Continuous and Bistable Regimes

Fluidic thrust vectoring of a subsonic jet was examined by the asymmetric application of countere ow to the jet shear layers. When countere ow is applied to one side of a 4:1 aspect ratio rectangular jet in the proximity of a control surface, the shear-layer entrainment characteristics are altered asymmetrically giving rise to a cross-stream pressure gradient and e ow vectoring. Thrust vector control up to 20 deg was possible at Mach numbers up to 0.5 using countere ow. Two regimes of e uidic control were identie ed, a continuous regime and a bistable one. During continuous vectoring, proportional control could be achieved between jet response and the pressure conditions established in the countere owing stream. However, under certain circumstances, proportional control was lost leading to jet bistability. Since such bistability is undesirable for aircraft control applications, the phenomenon was studied in detail. A parametric study of the collar geometry was used to minimize jet attachment and expand the operating domain over which proportional control could be maintained.

Self-induced vortex ring dynamics in subsonic rectangular jets

The development in space and time of vortex rings in low aspect-ratio (AR) rectangular jets is investigated. By design, the present studies isolate the self-induced ring dynamics from effects of unsteady events otherwise present upstream and downstream of the rings in developed jets. The simulations show that the vortex rings undergo quite regular self-induced nonplanar deformations, approximately recovering their shape and flatness with axis rotated with respect to their initial configuration. The axis-rotation periods are in good agreement with previously reported data for pseudoelliptic rings, and exhibit nearly linear growth rate as a function of AR. For the larger aspect-ratio case studied (AR=4), bifurcation of the ring due to vortex reconnection into roughly round rings is observed, followed by collision of the split rings and a new reconnection process, suggesting pathways for transition to turbulence based on self-induced vortex deformations and reconnections.

Errata: Time-Averaged Three-Dimensional Flow in a Rectangular Sudden Expansion

An experimental study of a 2:1 aspect ratio rectangular jet that undergoes a 6:1 sudden expansion in area is being conducted to understand the three-dimensional dynamics of the asymmetric vortex rings formed in the confined shear layers and thus to understand entrainment and mixing in this type of configuration. LDV is used to measure three velocity components under forced and unforced conditions. Profound differences are found between the two axes of symmetry: in contrast to a conventional two-dimensional sudden expansion, no time-averaged recirculation is found in the minor axis plane