Shock Cell Spacing Research Articles

Particle image velocimetry of supersonic jets with micro vortex generators

The flow field of supersonic jets with micro vortex generators (MVGs) is investigated using Particle Image Velocimetry (PIV) to shed light on the noise reduction mechanisms associated with such devices. Recently, MVG nozzles have been developed as a new noise reduction technology through the collaboration between the University of Cincinnati and the Naval Research Laboratory. MVGs were implemented on model scale nozzles representative of GE F404 engine nozzles, and noise reductions up to −6 dB have been observed in both microphone measurements and LES simulations. The PIV measurements of the flow field reveal that the MVGs alter the shock cell spacing and strengths in the jet plume and substantially reduce the mean axial velocity at the jet axis. In addition, streamwise vortices generated by MVGs enhance shear layer mixing and redistributes turbulence along the shear layer to narrower regions. The measured flow field quantities are extracted to explain flow field changes on the noise reductions observed in the acoustic far field.

Empirical Closure Model for Coupling Mode Prediction in Supersonic Rectangular Twin Jets

An empirical closure model of rectangular twin jets’ screech and coupling has been developed to predict their coupling mode at a given flow condition. It builds on Powell’s original screech closure model (Powell, A., “On the Mechanism of Choked Jet Noise,” Proceedings of the Physical Society, Section B, Vol. 66, No. 12, 1953, pp. 1039–1056) of feedback waves produced by the interactions of large-scale structures (LSSs) with each shock cell. Powell’s work is expanded to include nonuniform shock-cell spacing, overexpanded flow regimes, and twin jet configurations. The feedback wave interference pattern strength at the nozzle exit is postulated to determine the realized coupling mode. The required empirical parameters are the shock-cell streamwise locations and the convective velocity of the LSSs. The model was validated using data from an experimental investigation of rectangular twin jets at overexpanded and underexpanded conditions. The coupling mode is correctly predicted for 11 of the 12 nozzle pressure ratios tested. Two datasets from the literature (Jeun et al., “Aeroacoustics of Twin Rectangular Jets Including Screech: Large-Eddy Simulations with Experimental Validation,” AIAA Journal, Vol. 60, No. 11, 2022, pp. 1–21 and Raman, G., and Taghavi, R., “Coupling of Twin Rectangular Supersonic Jets,” Journal of Fluid Mechanics, Vol. 354, Jan. 1998, pp. 123–146) were also examined, and the model made accurate predictions of the coupling mode for all seven reported cases. The response of the twin jets to flow control at various artificially imposed frequencies and excitation modes strongly supports the model predictions, highlighting their usefulness as a guide for the implementation of active flow control in twin jets.

Optical-acoustics source analysis of supersonic jet noise reduction using micro vortex generators

A new supersonic jet noise reduction technology has been developed using Micro Vortex Generators (MVGs) by the collaboration between the University of Cincinnati and the Naval Research Laboratory. MVGs are used on model scale nozzles that are representative of GE F404 engine nozzles. Noise reductions up to −10 dB have been observed in both laboratory measurements and LES simulations at conditions related to take off in the overexpanded regime. Analysis of the acoustic field and flow field using Schlieren visualization reveal the noise reduction mechanisms associated with MVGs. Direct visualization of the changes in shock cell spacing, Large Coherent Structures (LCS) formation, and their convective velocity are identified and those changes modify the downstream propagating hydrodynamic waves and the upstream propagating acoustics waves. Spectral Proper Orthogonal Decomposition (SPOD) is utilized to examine the flow sources at frequencies associated with the noise components observed in the acoustic spectra to explain the noise reduction mechanisms of MVGs.

A unifying theory of jet screech

This paper describes the mechanism underpinning modal staging behaviour in screeching jets. An upstream-propagating subsonic guided-jet mode is shown to be active in all stages of screech. Axial variation of shock-cell spacing manifests in the spectral domain as a series of suboptimal peaks. It is demonstrated that the guided-jet mode may be energized by interactions of the Kelvin–Helmholtz wavepacket with not only the primary shock wavenumber peak, but also suboptimal peaks; interaction with suboptimals is shown to be responsible for closing the resonance loop in multiple stages of jet screech. A consideration of the full spectral representation of the shocks reconciles several of the classical models and results for jet screech that had heretofore been paradoxical. It is demonstrated that there are multiple standing waves present in the near field of screeching jets, corresponding to the superposition of the various waves active in these jets. Multimodal behaviour is explored for jets in a range of conditions, demonstrating that multiple peaks in the frequency spectra can be due to either changes in which peak of the shock spectra the Kelvin–Helmholtz wavepacket is interacting with, or a change in azimuthal mode, or both. The absence of modal staging in high-aspect-ratio non-axisymmetric jets is also explained in the context of the aforementioned mechanism. The paper closes with a new proposed theory for frequency selection in screeching jets, based on the observation that these triadic interactions appear to underpin selection of the guided-jet mode wavelength in all measured cases.

Effect of tab parameters on the near-field mixing characteristics of a Mach 1.5 elliptic jet

The effect of delta-tabs with its apex leaning downstream on the jet mixing characteristics of a Mach 1.5 elliptic jet of aspect ratio (AR) 3.37 is reported in this paper. The delta-tabs were placed along the minor and major axes of the elliptic nozzle, referred to as tab configuration C1 and C2, respectively. For each tab configuration, four different orientation angles varying from θ= 90∘ to 150∘ were studied, where θ= 90∘ is the tab positioned normal to the nozzle exit and θ= 150∘ is the tab oriented in the downstream direction at the nozzle exit. The tab effects on jet manipulation were investigated via shadowgraph visualization, and the mean Pitot pressure was measured along the jet centerline and along the orthogonal planes of the jet. The non-dimensonalized supersonic core length Lc*, average shock-cell spacing Ls¯*, and the axis-switching location xas* were examined in detail to understand the jet characteristics of the natural and manipulated jets. The mixing capability of the manipulated jets quantified based on the reduction in supersonic core length ΔLc* depends strongly on the ideally expanded jet Mach number Mj, tab configuration (C1 and C2), and θ. As such, the ΔLc* for the manipulated jet at θ= 135∘ is the largest, revealing the optimal tab orientation for jet mixing enhancement, as also observed for the circular jet [Zaman et al., “Control of an axisymmetric jet using vortex generators,” Phys. Fluids 6(2), 778–793 (1994)]. The relationship between ΔLc* and ΔLs* is presented to understand the correlation between the jet mixing and noise characteristics of the manipulated jet. Discussion is conducted based on the dependence of ΔLc* and ΔLs* with θ, which provides useful insight into the jet flow physics. With suitable placement of tabs C1 and C2, the axis switching could be either stopped or augmented. The superior mixing characteristics of an elliptic jet relative to a circular jet, which were found in previous works on the supersonic over-expanded jet [Kumar and Rathakrishnan, “Characteristics of a supersonic elliptic jet,” Aeronaut. J. 120(1225), 495 (2016)] prevail in the supersonic jet under-expanded jet as well.

Multi-Schlieren CT Measurements of Supersonic Microjets from Circular and Square Micro Nozzles

Instantaneous three-dimensional (3D) density distributions of a shock-cell structure of perfectly and imperfectly expanded supersonic microjets escaping into an ambient space are measured. For the 3D observation of supersonic microjets, non-scanning 3D computerized tomography (CT) technique using a 20-directional quantitative schlieren optical system with flashlight source is employed for simultaneous schlieren photography. The 3D density distributions data of the microjets are obtained by 3D-CT reconstruction of the projection’s images using maximum likelihood-expectation maximization. Axisymmetric convergent-divergent (Laval) circular and square micro nozzles with operating nozzle pressure ratio 5.0, 4.5, 4.0, 3.67, and 3.5 have been studied. This study examines perfectly expanded, overexpanded, and underexpanded supersonic microjets issued from micro nozzles with fully expanded jet Mach numbers Mj ranging from 1.47 - 1.71, where the design Mach number is Md = 1.5. A complex phenomenon for free square microjets called axis switching is clearly observed with two types “upright” and “diagonal” of “cross-shaped”. The initial axis-switching is 45° within the first shock-cell range. In addition, from the symmetry and diagonal views of square microjets for the first shock-cells, two different patterns of shock waves are viewed. The shock-cell spacing and supersonic core length for all nozzle pressure ratios are investigated and reported.

Cold Flow Measurements of Supersonic Low Aspect Ratio Jet-Surface Interactions

In streamlined airplane configurations, additional noise sources can be created from interactions between the jet flow and surfaces on an aircraft’s body. During takeoff and landing procedures the ground is close enough to already cause jet-surface interference. To assess this interaction, the presence of a flat plate impinging on a supersonic jet of a low aspect ratio (2:1) rectangular nozzle of equivalent exit diameter, De = 20.65 mm, is studied from the minor and major axis orientation. The impact of the plate, 30De in length to resemble an aircraft carrier deck, is studied at supersonic nozzle pressure ratios (NPRs) of 2.5–4.5 for a low jet temperature ratio of TR = 1.1. Streamwise particle image velocimetry (PIV) data was taken to extract average velocity and turbulence kinetic energy (TKE) of the flow. Plate offset (h) distances of h/De = 0, 1, 2, and 3 from the nozzle lip are studied to assess trends related to shock cell spacing, potential core length, and shear layer development relative to a free jet configuration. The surface offset from the nozzle is shown to vary flow properties and even increase screech tones particularly at h/De = 1 and 3, while the in between distance of h/De = 2 is shown to reduce them. These off-design conditions causing screech are of particular interest in this study.

Flow and acoustics of an isothermal supersonic 2:1 rectangular jet with an adjacent surface

In integrated airframe/propulsion system configurations, additional noise sources can be created from the interactions between the jet flow and the adjacent airframe surfaces. Another situation of surfaces of close proximity occur during takeoff and landing when the ground is close enough to cause the jet-surface interference. The impact of a plate oriented parallel to the axis of a 2:1 rectangular supersonic jet (De = 20.65mm) at the minor axis side is studied. Plate offset (h) distances of h = 0, 1, 2, and 3 equivalent diameters from the nozzle lip to the surface are included. The impact of the plate is studied at nozzle pressure ratios (NPRs) of 2.5–4.5 when the design Mach number is 3.67, at a temperature ratio of TR = 1.1. Mean and turbulent flow field data are acquired using streamwise particle image velocimetry (PIV). Trends of shock cell spacing, potential core length, and shear layer development are analyzed. Near and far field data are taken and correlated with the flow field details. The offset from the nozzle is shown to vary flow and acoustic properties and impact screech tones.

Jet noise reduction using co-axial swirl flow with curved vanes

Experimental studies are carried out to reduce the jet noise using co-axial swirlers in the form of curved vanes fixed in an annular passage. The swirl numbers considered for the present work ranged from 0 to 1.31, and the corresponding swirl vane angles ranged from 0 to 60°. The nozzle pressure ratios studied ranged from 1.8 to 6. The acoustic far field study at subsonic conditions revealed the presence of transonic tones for the non-swirl jet. However, swirl eliminates the transonic tones and a weak swirl is most efficient for noise reduction at subsonic conditions. The centerline total pressure measurements indicate the reduced core length for the swirl jets compared to the non-swirl jets. At supersonic conditions, the non-swirl jet emits the highest noise at all the emission angles compared to the swirl jets. The swirl jets are free from screech tones, and have lower amounts of shock associated noise, even at high nozzle pressure ratios. The centerline total pressure measurements and schlieren visualization studies show that shock cell spacing and the number of shock cells are reduced in the swirl jets compared to the non-swirl jet.

Passive scalar mixing studies to identify the mixing length in a supersonic confined jet

Supersonic jet with a co-flow, closely bounded by walls is known as supersonic confined jet. Supersonic confined jet is encountered in practical devices like the supersonic ejector. Mixing of the primary and the secondary fluid inside the confined passage is complex. From a design perspective, it is necessary to have an accurate knowledge of the mixing length (L-MIX). Tracers that do not actively participate in the flow behavior but rather mark the fluids such that they faithfully follow the fluid motion are known as passive scalars. Passive scalars help in the understanding the progression of mixing amidst interacting flows. In this work, we have performed passive scalar mixing studies in a supersonic confined jet for different operating conditions using an existing low area ratio (AR = 3.7) rectangular supersonic gaseous ejector. Air is used as the working fluid in both the primary and the secondary flow. The design Mach number of the primary flow nozzle (M-PD = 1.5-3.0) and the total pressure of the primary flow (P-OP = 4.89-9.89 bar) are varied during the experiments. Using the planar laser-induced fluorescence (PLIF) technique and acetone as the passive scalar, L-MIX is determined. A 266 nm Nd-YAG laser with a repetition rate of 8 Hz is used to excite the acetone molecules in the flow field, and the emitted fluorescence is captured by an ICCD camera. A new method is proposed to study the passive scalar distribution from the acetone PLIF images through digital image processing. Spatial Scalar Fluctuations Intensity (SSFI or psi ) is a parameter defined at every transverse section along the flow direction. Based on the variation of psi along the jet, the location of L-MIX can be identified. L-MIX is defined as the length from the supersonic nozzle exit where psi first attains a value of 0.05. For the first time, L-MIX is quantified in a supersonic confined jet. L-MIX values are observed to be in the range of 3H to 6H for the cases under study, where H is the height of the confined passage. The behavior of L-MIX is closely dependent on the nozzle operating conditions. The values of L-MIX are found to be reduced by 17.67% for the over-expanded flows and increased by 15.76% for the under-expanded flows from the perfectly expanded condition. This study also provides other supersonic confined jet characteristics like the potential core length (L-PC) and the shock cell spacing (S-x) of the primary supersonic jet. Compared to the supersonic free jet, values of L-PC and S-x are found to be different in the supersonic confined jet.

Parametric experimental studies on mixing characteristics within a low area ratio rectangular supersonic gaseous ejector

We use the rectangular gaseous supersonic ejector as a platform to study the mixing characteristics of a confined supersonic jet. The entrainment ratio (ER) of the ejector, the non-mixed length (LNM), and potential core length (LPC) of the primary supersonic jet are measures to characterize mixing within the supersonic ejector. Experiments are carried out on a low area ratio rectangular supersonic ejector with air as the working fluid in both primary and secondary flows. The design Mach number of the nozzle (MPD = 1.5–3.0) and primary flow stagnation pressure (Pop = 4.89–9.89 bars) are the parameters that are varied during experimentation. Wall static pressure measurements are carried out to understand the performance of the ejector as well as to estimate the LNM (the spatial resolution is limited by the placement of pressure transducers). Well-resolved flow images (with a spatial resolution of 50 μm/pixel and temporal resolution of 1.25 ms) obtained through Planar Laser Mie Scattering (PLMS) show the flow dynamics within the ejector with clarity. The primary flow and secondary flow are seeded separately with acetone that makes the LNM and LPC clearly visible in the flow images. These parameters are extracted from the flow images using in-house image processing routines. A significant development in this work is the definition of new scaling parameters within the ejector. LNM, non-dimensionalized with respect to the fully expanded jet height hJ, is found to be a linear function of the Mach number ratio (Mach number ratio is defined as the ratio of design Mach number (MPD) and fully expanded Mach number (MPJ) of the primary jet). This definition also provides a clear demarcation of under-expanded and over-expanded regimes of operation according to [MPD/MPJ] > 1 and [MPD/MPJ] < 1, respectively. It is observed that the ER increased in over-expanded mode (to 120%) and decreased in under-expanded mode (to 68%). Similarly, LNM decreased (to 21.8%) in over-expanded mode and increased (to 20.4%) in under-expanded mode. Lengthening of LPC by 139% and a reduction of 50% in shock cell spacing have also been observed for specific flow conditions. The details regarding experimentation, analysis, and discussions are described in this article.

Gas dynamics and flow characteristics of highly turbulent under-expanded hydrogen and methane jets under various nozzle pressure ratios and ambient pressures

The current study used large eddy simulations to investigate the sonic and mixing characteristics of turbulent under-expanded hydrogen and methane jets with various nozzle pressure ratios issued into various ambient pressures including elevated conditions relevant to applications in direct injection gaseous-fuelled internal combustion engines. Due to the relatively low density of most gaseous fuels such as hydrogen and methane, DI requires high injection pressures to achieve suitable mass flow rates for fast in-cylinder fuel delivery and rapid fuel-air mixing. Such pressures typically form an under-expanded fuel jet past the nozzle exit. Test cases of hydrogen injection with nozzle pressure ratio (NPR) of 10 issued into quiescent air with pressure P∞ ≈ 1, 5 and 10 bar were simulated. Direct comparison between hydrogen and methane jets with NPR = 8.5 and P∞ ≈ 1 was also made. The effect of ambient pressure on features of transient development of the near-nozzle shock structure and tip vortices (vortex ring) was investigated. It was observed that at constant NPR, higher ambient pressure resulted in slightly faster formation of the Mach reflection and shorter Mach disk settlement time. Different mechanisms were observed between hydrogen and methane with regards to transient formation of their initial tip vortex rings. It was found that the initial transient tip vortices of hydrogen jets may also contribute to the flow instabilities at the boundary of the intercepting shock and, unlike for methane, promote fuel-air mixing before the Mach reflection. It was also shown that the near-nozzle shock structure was only affected by NPR regardless of the ambient pressure. Furthermore, no flow recirculation zone was found just downstream of the Mach disk, a finding comparable to all previous experimental investigations. Also, it was observed that a locally richer mixture was created for jets with higher NPR or with higher ambient pressure at constant NPR. Based on the results of the current study, correlations were proposed for the shock cell spacing and jet tip penetration of highly under-expanded jets issued from millimetre-size circular nozzles.

An experimental investigation of the flow dynamics of streamwise vortices of various strengths interacting with a supersonic jet

The results of an experimental study involving the introduction of streamwise vorticity into a supersonic jet are presented. Both streamwise and cross stream PIV measurements of a baseline jet and three vortex generator configurations of varied penetration were acquired in the overexpanded, ideally expanded, and underexpanded regimes. Streamwise vortex pairs were shown to persist no further than two diameters downstream independent of initial magnitude. Integration of the modulus of streamwise vorticity was shown to better correlate with secondary flow field modifications than maximum values. Reductions in shock cell spacing and downstream turbulence, and increases in initial spread rate and upstream integrated turbulence were correlated with increases in streamwise vorticity for all operating conditions. Streamwise vorticity was shown to achieve opposite effects on shock strength in the over/under expanded regimes due to introduction of secondary shock structures in the underexpanded case. Additionally, limited effect on potential core length was quantified.

Investigation of Broadband Shock Noise from a Jet Near a Planar Surface

Many current and future aircraft designs rely on the wing or other aircraft surfaces to shield observers on the ground from the engine noise. However, the available data showing how surfaces interact with a jet to shield and/or enhance the jet noise are currently limited. Therefore, far-field noise data and background-oriented schlieren images were acquired for a round jet, operating in the overexpanded, ideally expanded, and underexpanded supersonic flow regimes, near a planar surface to investigate how airframe surfaces might affect the shock-cell structure in the jet plume and the broadband shock noise produced. These data show that broadband shock noise is produced by the relatively weak shocks far downstream of the nozzle exit; consequently, a surface will be effective at reducing broadband shock noise only if it is long enough to shield the noise produced by shocks. Furthermore, the presence of a surface very near the edge of an underexpanded jet increases the shock-cell spacing, pushing the shock cells farther downstream. Conversely, the surface has a minimal affect on the shock cells in an overexpanded jet.

Fluidically Enhanced Chevrons for Supersonic Jet Noise Reduction

An experimental and computational investigation of a combination of fluidic injection and chevrons (fluidically enhanced chevrons) for supersonic jet noise reduction was performed. Previous studies have shown that chevrons underperform in overexpanded flow due to reductions in effective penetration. Consequently, this study focuses on improvements gained by fluidic enhancement in the overexpanded regime. Acoustic results indicated the fluidically enhanced chevrons outperform the chevrons, with additional overall sound pressure level reductions of 2 dB in the upstream direction and nearly 1.5 dB in the downstream direction. Spectral results indicated that primary benefits were in shock and low-frequency noise. A small high-frequency penalty was the only observed detriment. Time-averaged velocity and turbulence quantities as well as shock cell spacing, were computed by large-eddy simulation and then compared to particle imaging velocimetry and shadowgraph results. These flowfield measurements indicated that the fluidically enhanced chevrons modify the flowfield by reducing the shock cell spacing and shock strength and increasing the upstream jet half-width, peak and integrated turbulence values, and shear-layer thickness. These modifications were caused by the introduction of streamwise vortices with over four times the magnitude of the unenhanced chevrons. Noise reductions were attributed to flowfield modifications through the use of several theoretical models, such as Powell’s screech formula.

Density measurements for rectangular free jets using background-oriented schlieren

AbstractAn experimental study incorporating the use of the Background-Oriented Schlieren (BOS) technique was performed to measure the density field of a rectangular supersonic jet. This technique is easier to set up than conventional schlieren since the optical alignment involving the various mirrors, lenses and knife-edge is replaced by a background pattern and a single digital camera. The acquired images which contain information of density gradients in the flow are solved as a Poisson equation and further processed using deconvolution and tomographic algorithms to generate a 3D domain which contains information about the actual density. 2D slices can then be extracted to quantitatively visualise the density along any required planes. The results from supersonic axisymmetric jets are used for validation of the code; these show excellent agreement with pre-validated CFD data. The results for a rectangular supersonic jet are then obtained. These show good agreement with the CFD data, in terms of shock-cell spacing and overall structure of the jet. The technique has proved useful for investigating axis-switching, a phenomenon generally associated with non-axisymmetric jets.

Experiments on free and impinging supersonic microjets

The fluid dynamics of microflows has recently commanded considerable attention because of their potential applications. Until now, with a few exceptions, most of the studies have been limited to low speed flows. This experimental study examines supersonic microjets of 100–1,000 μm in size with exit velocities in the range of 300–500 m/s. Such microjets are presently being used to actively control larger supersonic impinging jets, which occur in STOVL (short takeoff and vertical landing) aircraft, cavity flows, and flow separation. Flow properties of free as well as impinging supersonic microjets have been experimentally investigated over a range of geometric and flow parameters. The flowfield is visualized using a micro-schlieren system with a high magnification. These schlieren images clearly show the characteristic shock cell structure typically observed in larger supersonic jets. Quantitative measurements of the jet decay and spreading rates as well as shock cell spacing are obtained using micro-pitot probe surveys. In general, the mean flow features of free microjets are similar to larger supersonic jets operating at higher Reynolds numbers. However, some differences are also observed, most likely due to pronounced viscous effects associated with jets at these small scales. Limited studies of impinging microjets were also conducted. They reveal that, similar to the behavior of free microjets, the flow structure of impinging microjets strongly resembles that of larger supersonic impinging jets.

Numerical prediction of supersonic jet screech frequency

Screech frequency is predicted using three different approaches, which make use of different quantities measured from the same computed flow field. The three different approaches are based on shock cell spacing in the imperfectly expanded supersonic jet, the wavelength of standing wave formed at the edge of the shear layer due to interference between downstream propagating hydrodynamic instabilities and upstream propagating acoustic waves, and from a spectral analysis of near-field pressure fluctuations. The computed flowfield for underexpanded and overexpanded axisymmetric screeching jets are obtained by solving the unsteady Navier-Stokes equations using a higher order Weighted Essentially Non-Oscillatory (WENO) discretization along with a subgrid scale Large-Eddy Simulation (LES) model.

Acoustic properties associated with nozzle lip thickness in screeching jets

Imperfectly expanded supersonic jets generate discrete frequency sound known as screech tones. In the low supersonic Mach number, the screech phenomenon is axisymmetric. In this study the effect of nozzle lip thickness on the axisymmetric jet screech is investigated both by screech scattering computation and screeching jet simulation. The conservative form of the axisymmetric Euler equations in generalized coordinates are used to simulate a scattering problem and the Reynolds-Averaged Navier-Stokes equations with the modified Spalart-Allmaras turbulence model is employed for screeching jet simulation. Numerical results of scattered directivity pattern in scattering computation will be reported. Influence of the nozzle lip thickness on the momentum thickness, shock-cell spacing and screech tone wavelength is examined.

Effects of Counterflow on the Aeroacoustic Properties of a Supersonic Jet

The influence of counterflow on the mixing and acoustic characteristics of a Mach 1.4 rectangular jet operated at on- and off-design conditions were studied experimentally for different levels of counterflow. The results show that counterflow significantly enhances shear-layer mixing and reduces the jet potential core length under all operating conditions. Significant changes in both shock-cell spacing and strength were ohserved when counterflow was applied to nonideally expanded jets. Consequently, screech tones were either reduced or totally eliminated, and broadband shock-associated noise was shifted to higher frequencies. In the underexpanded mode, a Mach disk was formed at certain levels of counterflow, which substantially weakened the subsequent periodic shock-cell structure and reduced the broadband shock-associated noise and the overall sound pressure level (OASPL) by as much as 3 dB. Interestingly, it was also discovered that a jet operating at overexpanded conditions could be decelerated nearly isentropically by applying the proper amount of counterflow. This modification led to a 4 dB reduction in OASPL. Based on the present study, it is suggested that counterflow warrants further investigation as a potential noise reduction technique.