Jet Shear Layer Research Articles

Simulation of shear layers interaction and unsteady evolution under different double backward-facing steps

ABSTRACT High-order accurate schemes are employed to numerically simulate the interaction of a supersonic jet and a co-directional supersonic inflow. A double backward-facing step model is proposed to investigate the interaction between the jet shear layer and the supersonic inflow shear layer. It is found that due to the interaction of the shear layer, a secondary jet is injected into the recirculation zone at the intersection of the two shear layers. The secondary jet produced by the interaction of the two shear layers has a periodicity because of shear layers interaction. The distinction in the shape of double backward-facing steps will induce changes in the period of the secondary jet. The analysis and discussion of the periodicity of the secondary jet are mainly focused in this letter.

A study of noise reduction mechanisms of jets with fluid inserts

The fluid insert technology is an active noise reduction technique that has been shown to successfully reduce supersonic jet noise in model scale nozzles. The goal of this paper is to better understand the noise reduction mechanisms of fluid insert jets using Doak's Momentum Potential Theory and Spectral Proper Orthogonal Decomposition applied to an LES database. It is found that the use of fluid inserts results in a lower convection speed of the wavepacket-like coherent structures in the jet shear layer resulting in a shallower Mach wave radiation angle. It is also shown that the use of fluid inserts results in a reduction of length scales of the turbulent fluctuations in the shear layer, which correlates correctly with the observed noise reductions.

Solution of cavitating compressible flows using Discontinuous Galerkin discretisation

A methodology for modelling cavitating flows using a high-order Adaptive Mesh Refinement (AMR) approach based on the Discontinuous Galerkin method (DG) is presented. The AMR implementation used features on-the-fly adaptive mesh refinement for unstructured hybrid meshes. The specific implementation has been developed for the resolution of complex multi-scale phenomena where high accuracy p-adaptive discretisations are combined with an h-adaptive data structure. This approach accommodates the fine spatial resolution for the interface discontinuities and the shock waves observed in compressible cavitating flows. The Tait equation of state is used for the modelling of the liquid phase while an isentropic path is assumed for the liquid/vapour mixture. Second order spatial and a third order non-oscillatory temporal discretisation are used for the integration of the mass and momentum conservation equations, in order to resolve the flow structures responsible for the formation of cavitation bubbles and the resulting compression waves. Assessment of the developed methodology is performed for the one-dimensional advancement of a compressible liquid-vapour interface and the symmetric collapse of a spherical vapour bubble.Following, results obtained with the developed multi-scale modelling AMR approach has revealed a complex bubble collapse mechanism near a rigid wall, providing evidence of processes that have been unknown before due to reduced resolution and dissipative nature of past simulations. The impinging jet accompanying the collapse of a bubble near a wall, was found to induce vortical structures, which result to the formation of a secondary cavitation of a wall-attached bubble at the vicinity of the impingement jet shear layer. At the final stages of the initial bubble collapse, the impinging jet was found to penetrate the centre-line of the wall bubble inducing its partial collapse. This secondary collapse results to a rich spatial structure of shock waves, interacting with the secondary bubbles. Moreover, the calculated pressure level are found to be much higher than those reported from previous methodologies.

Interactions of large-scale structures in the near field of round jets at high Reynolds numbers

A possible interaction between vortical structures in a round jet shear layer, viz. vortex rings and streamwise vortices, is explored following Davoust et al. (J. Fluid Mech., vol. 709, 2012, pp. 408–444). These authors reported a radial organization of streamwise vorticity in a jet at high diameter-based Reynolds number ( ), contrary to the classically observed azimuthal organization. They hypothesized that the observed weaker vortex rings in such jets could be deformed by streamwise vortices and further reoriented and stretched in the streamwise direction. As this hypothesis was based on the observations of one configuration of a jet flow, our study aims at assessing it by varying the key parameter, i.e. the relative strength of the vortex rings and the streamwise vortices, through forcing, along with various jet configurations. We first analyse a low-Mach-number, transitional jet, using high-speed stereo particle image velocimetry in a cross-sectional plane at two jet exit diameters from the nozzle exit. The axisymmetric mode is acoustically excited at various amplitudes to increase the strength of the rings relative to streamwise vortices, at a Strouhal number ( ) of 0.49, the most energetic frequency in the unforced jet. Starting from a radial array in the unexcited jet, a gradual shift towards an azimuthal configuration is obtained with increasing excitation level. Quantification of the relative strengths of the streamwise vortices and vortex rings confirms that a radial array is observed whenever the streamwise vortices are more intense than the rings, and conversely for the azimuthal configuration. We then extend the analysis to other jet cases in terms of , and state of the exiting boundary layer. We observe that the correlation between the radial or azimuthal organization of streamwise vortices and the relative strengths of the vortical structures holds systematically, confirming the possibility of the proposed interaction. A detailed analysis of the forced jets also sheds light on some interesting effects of acoustic excitation on the vortical organization in round jets.

A scale-dependent coherence analysis of turbulent round jets including the effects of shear layer manipulation

Two-point hot-wire measurements were conducted to assess the scaling of coherent structures in turbulent round jets. A scale-dependent coherence analysis was applied to datasets consisting of two-point synchronized axial velocity measurements with varying radial separations between the hot-wire positions while the reference hot-wire was fixed at the centreline and two off centreline positions in different datasets. Measurements were carried out at ReD=10000, 20000, and 50000 and a range of axial locations (3 ≤ x/D ≤ 25). The scale-dependent coherence analysis reveals a hierarchical structure of eddies over the axial range 15 ≤ x/D ≤ 25 for λx/y0.5 < 3, where λx is the axial wavelength and y0.5 is the jet half width. While these eddy structures are not geometrically self-similar at the centreline, away from the centreline they approach self-similarity status. In the jet shear-layer where the eddy structures are self-similar, a stochastic aspect ratio of ▪ ≈ 3.4 was found at ReD=10000. The aspect ratio increases to ▪ ≈ 3.9 at ReD=20000, and ▪ ≈ 4.7 at ReD=50000. These values are significantly smaller than the aspect ratio of ▪ =14 observed in turbulent boundary layers. The eddy structure aspect ratio is not affected by the axial distance from the nozzle (within x/D=15−25) or shear layer manipulation by control rings. However, the stochastic radial span of the largest eddies was found to change due to shear layer manipulation and change in the axial location.

Design of a novel vortex-based feedback fluidic oscillator with numerical evaluation

ABSTRACT In this study, a new vortex-based fluidic oscillator was designed whose performance was numerically evaluated by solving the URANS equations. The oscillator consists of a primary chamber and secondary chamber, separated by a mid-throat and connected by two feedback channels. The jet passing through these chambers forms two pairs of vortices inside the primary and secondary chambers. These vortices roll onto the jet shear layer, and gradually disturbs the jet symmetry. As a result, the vortices grow and shrink asymmetrically, activating the feedback flow to make a harmonic oscillation. The collision of the oscillating jet with the mid-throat traps a part of the jet shear layer in the primary chamber, which forms a strong vortex, returning the jet toward the other side of the mid-throat. The vortex inside the secondary chamber causes the jet to attach to one side of the secondary chamber, deviating the outlet jet to the other side. To compare the performance of the new oscillator with the original one, URANS equations were solved for two-dimensional models of both oscillators with the same grid generation and boundary conditions. The new oscillator had a higher oscillation frequency, a lower pressure drop, and a higher output momentum flux.

Evolution of the flow structures in an elevated jet in crossflow

The characteristic flow features of an elevated square jet in crossflow (EJICF) are studied numerically using large eddy simulation. The effect of jet to crossflow velocity ratio, also called velocity ratio (VR), on the flow field of an elevated jet in crossflow (EJICF) is investigated. All the computations are carried out at a Reynolds number (Re) of 20 000, based on the outer width of the stack (d) and free stream crossflow velocity (U∞), for four different velocity ratios (VR), namely, 0.5, 1.0, 1.5 and 2.0. The stack used in this study has an aspect ratio h/d = 7. The shear-improved Smagorinsky model has been used to account for the subgrid scale stress while solving the filtered three-dimensional unsteady Navier-Stokes equations. The modes of shedding in the stack wake are analyzed using both instantaneous and phase-averaged data. It is found that at a low jet to crossflow velocity ratio (VR = 0.5), the stack wake exhibits two different modes of shedding, symmetric and antisymmetric, similar to the wake of a wall mounted finite-size cylinder. At higher velocity ratios (VR ≥ 1), the stack wake shows the presence of only antisymmetric modes of shedding. It is also found that velocity ratio (VR) has a profound effect on the source of vorticity of the jet shear layer structures near the upwind side. At VR = 0.5, the upstream sides of the jet shear layer structures are found to draw their vortices from the outer surface of the stack boundary layer. At higher VR, they seem to be fed by the vorticity of the boundary layer developed on the inner wall surface of the stack. Both jet vortices and stack wake vortices are found to be present in the jet wake, and the shedding frequency of both the jet wake and the stack wake is found to be same. The spatial evolution of the counter-rotating vortex pair in the case of an EJICF is found to be similar to that of a JICF.

Turbulence and entrainment in an atmospheric pressure dielectric barrier plasma jet

AbstractParticle image velocimetry, laser‐induced fluorescence, and computational modeling are used to quantify the impact of plasma generation on air entrainment into a helium plasma jet. It is demonstrated that discharge generation yields a minor increase in the exit velocity of the gas. In contrast, the laminar to turbulent transition point is strongly affected, attributed to an increase in plasma‐induced perturbations within the jet shear layer. The temporal decay of laser‐induced fluorescence from OH is used as an indicator of humid air within the plasma. The results show that plasma‐induced perturbations increase the quenching rate of the OH‐fluorescent state; indicating that shear‐layer instabilities play a major role in determining the physicochemical characteristics of the plasma.

Shear layer of inclined jets in crossflow studied with spectral proper orthogonal decomposition and spectral transfer entropy

Several classes of vortical structures are present in a jet in crossflow, and the present paper sets out to investigate one of them: jet shear layer unsteadiness. An inclined jet in crossflow that is relevant to discrete hole film cooling applications is considered, with focus on two velocity ratios, r=1 and r=2. Highly resolved large eddy simulations (LES) described and validated in previous work are employed. Power spectra show that shear layer oscillations contain a broader range of frequencies than suggested by previous studies, and two spectral techniques are used to investigate this behavior. Spectral proper orthogonal decomposition (SPOD) shows modes that encompass the hole and the shear layer, suggesting their motions are linked. Then, we develop and employ a technique to establish causality between signals in different locations called spectral transfer entropy (STE), which is adapted from information theory. The results show that unsteadiness in the hole affects the spectral content of the shear layer, confirming the importance of resolving the plenum and the hole in simulations with realistic jet inflow, such as film cooling configurations.

Study on three-dimensional flow structures of a sweeping jet using time-resolved stereo particle image velocimetry

The three-dimensional flow structures that were induced by a sweeping jet emitted from a sweeping jet actuator to an open space were examined experimentally using stereo and two-dimensional PIV (particle image velocimetry) measurements. A new method was proposed to obtain phase-averaged flow field based on proper orthogonal decomposition, which is more robust to local flow fluctuations than existing methods. The jet Reynolds number (Re) was fixed to 8000 and the jet exit had a square shape with a hydraulic diameter (Dh) of 10 mm. The three-dimensional time-averaged flow fields of the sweeping jet were first reconstructed using multiple two-dimensional flow fields along various planes. In the time-averaged flow field, the jet flow distribution generally showed a V-shape pattern, which consisted of two inclined jet columns with an angle of approximately 90°. As the jet traveled away from the jet exit, the shape of jet column also changed from an elliptical shape to a circular shape because of the confinement effect of the jet exit. Stereo-PIV measurements were then conducted at two cross-sectional planes with distances of 3Dh and 8Dh away from the jet exit. In the plane 3Dh from the jet exit, one pair of counter-rotating vortices formed during the switching period when the jet swept from one side to the other. This is because the jet switching motion can enhance the jet shear layer, as in the case of a jet in crossflow. On the other hand, on the plane 8Dh away from the jet exit, the counter-rotating vortices dissipated quickly owing to the strong turbulence and weak switching motion.

Passive jet noise reduction using contoured inserts

Successful supersonic jet noise reduction requires an approach that simultaneously (1) breaks up coherent structures in the jet shear layer turbulence, (2) reduces convection velocity of the acoustic sources, and (3) weakens the shock structure in the plume. If any one of these is done in isolation the result is typically a shift in frequency content and not an overall reduction in source amplitude. Using a set of contoured inserts placed within the expansion section of the nozzle allows all three of these design goals to be addressed. The contour weakens the shock structure while simultaneously generating strong streamwise vorticity the enhances mixing in the jet shear layer. Assuming that future high-performance aircraft would possess a variable area ratio nozzle with independent exit and throat area controls, multi-objective design optimization has yielded a design that maximizes noise reduction with minimal thrust penalty in a variable area ratio framework. Model-scale acoustic data are presented to demonstrate the efficacy of the design.Successful supersonic jet noise reduction requires an approach that simultaneously (1) breaks up coherent structures in the jet shear layer turbulence, (2) reduces convection velocity of the acoustic sources, and (3) weakens the shock structure in the plume. If any one of these is done in isolation the result is typically a shift in frequency content and not an overall reduction in source amplitude. Using a set of contoured inserts placed within the expansion section of the nozzle allows all three of these design goals to be addressed. The contour weakens the shock structure while simultaneously generating strong streamwise vorticity the enhances mixing in the jet shear layer. Assuming that future high-performance aircraft would possess a variable area ratio nozzle with independent exit and throat area controls, multi-objective design optimization has yielded a design that maximizes noise reduction with minimal thrust penalty in a variable area ratio framework. Model-scale acoustic data are presented to d...

Flow field characteristics of a supersonic jet influenced by downstream microjet fluidic injection

Mixing characteristics of a supersonic jet influenced by a Downstream Microjet Fluidic Injection (DMFI) system are numerically investigated. The DMFI system is built on the observation of a previous experimental study that utilized transverse fluidic injection from four equally spaced injection ports placed on an injection tube at a distance downstream of a 1.5 Mach number nozzle. The measurements from these previous experiments demonstrated thickening and mixing enhancement of the jet shear layer as a result of fluidic injection. The current numerical study examines the underlying physics of the flow field, as well as the effectiveness of the DMFI system at smaller mass flow rate ratios compared to those utilized in the previous experiments. Results indicate good agreement with the trend observed by the experimental study, and considerable improvement in enhancement of the jet mixing is observed. The observed mixing enhancement is attributed to the presence of the microjet tube and fluidic injection and the consequent generation of streamwise vortices, as well as the natural separating bow shock due to the transverse flow injection. DMFI system is shown to enhance the early mixing, resulting in attenuation of the downstream turbulence production. Furthermore, the DMFI system is demonstrated to be an effective method of mixing enhancement for supersonic jets with a potential for reducing the jet noise radiation.

Shock Motion and Flow Structure of an Underexpanded Jet in the Helical Mode

A large-eddy simulation is performed to study the shock motions and flow structures of an underexpanded free jet. The jet is characterized by a nozzle pressure ratio of 3.4 and a fully expanded Mach number of 1.45. At this condition, the dominant instability mode of the jet is the helical (C) mode. The numerical results of mean velocity fields, shock structures, and the screech frequency are in good agreement with the experimental results. A single helical vortex appears in the jet shear layer, which coincides with the helical mode. At the fourth shock cell, the dominant acoustic source location for the screech tone, the shock interacts with vortices and oscillates periodically. The flow structures associated with the generations of the screech tone and the second harmonic are analyzed by the proper orthogonal decomposition. The coherent structures with the azimuthal wave number and 2 are related to the screech tone and its second harmonic, respectively.

Numerical study of a fully confined supersonic slot impinging jet from bleed system

A supersonic slot jet issuing from a bleed system and impinging into a plenum is numerically investigated to deepen our understanding of the flow field and heat transfer of a fully confined impinging jet. The air of the slot jet is forced to exhaust from a plenum in unidirection, resulting in a fully confined configuration. It is of significant practical interest because of its presence in the bleed systems of a scramjet or other related applications. Three primary factors are surveyed, i.e., impingement angle, impingement distance, and back pressure at the plenum exit. The results show that the supersonic slot jet spontaneously generates a plate shock standing on the impingement wall and a strong jet shock on the right side of the jet core. On the left side, the jet shear layer can directly impinge the wall because of the pressure self-adaption effect of a recirculation region. The jet shock and jet shear layer result in two peaks of Stanton number. As the impingement angle increases, the jet shock weakens gradually and moves toward the impingement region of the shear layer until the shock disappears. And thus a single heat peak occurs in place of the previous two peaks. When the downstream flow of the plenum is choked, a wall jet pattern is exhibited because of the formation of a large scale recirculation region. The walls suffer severe aerodynamic heating as the plenum back pressure increases. In particular, the impingement wall endures the maximum thermal load at a certain back pressure at which the jet reduces to the sound speed. The plenum should be controlled to keep the back pressure below this threshold so that the slot walls Stanton number won't increase further. And the plenum should be designed as high as possible to relieve its thermal load.

Hybrid LES-RANS study of an effusion cooling array with circular holes

A multi-row effusion cooling configuration with scaled gas turbine combustor conditions is studied numerically, using a novel wall-proximity-based hybrid LES-RANS approach. The distribution of the coolant film is examined by surface adiabatic cooling effectiveness (ACE). Simulation results have shown that the accuracy of cooling effectiveness prediction is closely related to the resolution of turbulent flow structures involved in hot-cold flow mixing, especially those close to the plate surface. The formation of the coolant film in the streamwise direction is investigated. It is shown that the plate surface directly downstream the coolant holes are covered well by the coolant jets, while surface regions in between the two columns of the coolant holes could not be protected until the coolant film is developed sufficiently in the spanwise direction in the downstream region. More detailed study has also been carried out to study the time-averaged and time-dependent flow fields. The relation between the turbulent flow structures and coolant film distribution are also examined. The Kelvin–Helmholtz instability in the upper and lower coolant jet shear layer, is found to have the same frequency of around 8000 Hz, and is independent of the coolant hole position. Additionally, it is suggested by the spectral coherence analysis that those unsteady flow structures from the lower shear layer are closely related to the near wall flow temperature, and such effect is also independent of the coolant hole position.

Measurements of the single-point and joint turbulence statistics of high subsonic jets using hot-wire anemometry

This paper describes a systematic and comprehensive hot-wire investigation into the turbulent statistics of low-, moderate- and high-speed subsonic jets. Experiments were performed to obtain the one-point and two-point statistics of a single-stream, unheated jet turbulence field over a broad region of the jet plume. Results show that hot-wires can be used to measure both the one-point and two-point statistics of the high turbulence intensity, noise-producing regions of unheated, compressible, subsonic jets. For the two-point measurements, probe pairings are performed over the three orthogonal axes. Analysis of the experimental data reveals four main conclusions: (1) both the statistical and joint moments of the turbulence scale well with the local jet shear layer half-width; (2) a simple relationship exists between the statistics of the velocity fluctuations and the square of the velocity fluctuations; (3) a simple relationship exists between the longitudinal and transverse length-scales, and (4) a semi-empirical model has been developed to predict the cross-correlation coefficients, power spectral density, frequency-dependent length-scales and coherence decay of the turbulent velocity field. From the second and third conclusions, it is shown that, in the locations near an eddy’s centre of rotation (i.e. the midpoint of the jet shear layer), the turbulence statistics can be described as quasi-homogeneous and quasi-frozen. The joint statistical moments, therefore, can be inferred simply from single-point tests. These results will help to develop models for predicting jet mixing noise, highlighting the situations in which the simplifying assumptions are inadequate.Graphical abstract

Time-resolved particle image velocimetry measurements of a single impinging jet in the upper plenum of a scaled facility of high temperature gas-cooled reactors

In this study, the flow characteristics of a single isothermal jet discharging into the upper plenum of a 1/16th scaled facility of high temperature gas-cooled reactors was experimentally investigated. Time-resolved particle image velocimetry (TR-PIV) measurements were performed in the central plane of the jet flow and the upper plenum for various jet Reynolds numbers ranging from 3, 413 to 12, 819. From the acquired TR-PIV velocity snapshots, statistical characteristics of the jet flows such as the mean velocity, root-mean-square fluctuating velocity, and Reynolds stress were computed and presented. Furthermore, two-point velocity cross-correlations, spectral analysis, and squared coherent functions were performed on the TR-PIV velocity vector fields. The estimated integral length scales were found to linearly increase downstream and along the jet shear layers in the axial direction, while the squared coherent functions remained close to unity for small spatial separations and lower frequencies up to 50 Hz for points at y/Dj=1 and 2.

On the effects of square-fractal turbulators on the flow field generated by a synthetic jet actuator

In this work the effects due to the introduction of a 3-iterations square fractal grid inserted at the exit section of a synthetic jet actuator are experimentally analysed and discussed. The contributions related to the secondary iterations are assessed comparing the results with those obtained inserting a single-square grid (obtained by removing the 2nd and the 3rd iteration bars). The flow field generated by an axisymmetric synthetic jet is also investigated as a reference.The synthetic jet is produced under three different dimensionless stroke lengths, L0/D=7.1,19.2 and 50.0, covering flow regimes ranging from the nearly pure vortex to the nearly continuous jet. Results are reported both in terms of time-averaged and phase-averaged statistics. Notwithstanding the different nature of the jet flow, fractal turbulators confirm the interesting feature of an extended range of turbulence production, as already documented for continuous jets.Phase-averaged results show that the vorticity field generated by the presence of the grid interacts with the vortex produced in the jet shear layer, causing its azimuthal distortion. This phenomenon can be prevented in the single-square grid case at lower L0/D values, but it is still clear when the fractal one is located at the nozzle exit section.Regardless of the inlet conditions, the outer vortex position scales with the stroke length. However, the introduction of the grid shows an effect on the lifetime of the vortex, with a faster dissipation in the fractal grid case.

Effects of the angle of impact on the aeroacoustic feedback mechanism in supersonic impinging planar jets

Three planar impinging supersonic jets of infinite extent are simulated using compressible large eddy simulations in order to study the effects of the angle of impact on the flow and acoustic fields of the jet. At the exit of a nozzle of height h, they are ideally expanded and have an exit velocity uj, yielding a Mach number of 1.28 and a Reynolds number of 5 × 104. They impinge on a flat plate at a distance 5.5 h from the nozzle lips with angles of 60°, 75°, and 90° between the jet direction and the plate. Mean velocity flows and snapshots of density, pressure, and vorticity are first shown. The mean convection velocity of the turbulent structures in the jet shear layers is then determined. The sound pressure levels are computed, and several tones due to the presence of a feedback mechanism are found to establish between the nozzle lips and the flat plate. They agree well with the corresponding measurements and with the classical model of the feedback mechanism. Moreover, when the angle of impact deviates from 90° to 75°, a jump from the third to the fourth mode of the feedback mechanism and a reduction in intensity are noted. By applying a Fourier decomposition to the near pressure fields, hydrodynamic–acoustic standing waves are found for each dominant tone frequency. Moreover, as suggested by amplitude fields and velocity spectra in the jet shear layers, the feedback mechanism seems to establish mainly along the lip that is farther away from the plate when the impact angle is not normal. This jump from the third to the fourth mode is similar to the jump observed experimentally for an angle of impact of 90° when the nozzle-to-plate distance increases from 5.5 h to 5.85 h. Finally, for an angle of impact of 60°, it is seen that none of the modes of the feedback persists in time, but that several modes randomly establish during short periods of time. These rapid switches between different modes lead to several tones that are less energetic on average and centered on St = 0.25.

Shock oscillation in highly underexpanded jets**Project supported by the National Natural Science Foundation of China (Grant No. 11602028) and the Science and Technology Project of General Administration of Quality Supervision Inspection and Quarantine of China (Grant Nos. 2017QK119 and 2017QK188).

The oscillatory motions of shocks in highly underexpanded jets with nozzle pressure ratios of 5.60, 7.47, 9.34, and 11.21 are quantitatively studied by using large eddy simulation. Two types of shock oscillations are observed: one is the Mach disk oscillation in the streamwise direction and the other is the shock oscillation in the radial direction. It is found that the Mach disk moves quickly in the middle of the oscillatory region but slowly at the top or bottom boundaries. The oscillation cycles of Mach disk are the same for different cases, and are all dominated by an axisymmetric mode of 5.298 kHz. For the oscillation in the radial direction, the shocks oscillate more toward the jet centerline but less in the jet shear layer, and the oscillation magnitude is an increasing function of screech amplitude. The cycles of the radial shock oscillation switch randomly between the two screech frequencies for the first two cases. However, the oscillation periodicity is more complex for the jets with high nozzle pressure ratios of 9.34 and 11.21 than for the jets with the low nozzle pressure ratios of 5.6 and 7.47. In addition, the shock oscillation characteristics are also captured by coarse mesh and Smagorinsky model, but the coarse mesh tends to predict a slower and weaker shock oscillation.