Jet Exit Mach Number Research Articles

Effect of increasing bypass ratio on supersonic co-flowing jet with finite lip thickness

The article presents the investigation on the effect of increasing bypass ratio on a supersonic co-flowing jet with finite lip thickness for a jet exit Mach number of 2.0. Co-flowing jets of differing bypass ratios (BR = ms/mp, where ms and mp are the mass flow rate of secondary and primary jets, respectively) 3.69, 9.0, and 16.0 were employed to explore the impact of high bypass secondary jets on the primary jet. A single-jet BR 0 is used for comparison. The supersonic core lengths of the single jet and the co-flow jet for BR 3.69, BR 9.0, and BR 16.0 were metrics for quantifying the mixing. The surrounding jet enhances the wake dominance in the nozzle lip, and the supersonic core length shortens. High bypass co-flowing jets are extremely efficient at shortening the supersonic core length compared to the low bypass counterpart, and hence, the jet mixing increases. The radial Mach number profiles quantitatively represent different zones in the co-flowing jet. The variation of these zones with bypass ratios highlights the dynamic interaction between the primary and secondary jets. The physical reason for apparent jet mixing and flow categories has been discussed based on considerations about changes in the flow field and shock structure.

Effect of velocity ratio and Mach number on thin lip coaxial jet

Abstract The effect of nozzle lip thickness and velocity ratio on coaxial subsonic jet mixing, at different Mach numbers, has been studied experimentally and numerically. Decay of coaxial subsonic jets emanating from coaxial nozzles of lip thickness 0.7, 1.7 and 2.65 mm with velocity ratio (VR) from 0.2 to 1.0 at primary jet exit Mach numbers of 0.6, 0.8 and 1.0 has been studied. Free jet without co-flow (VR0) was also studied for comparison. Jet centerline Mach number decay, turbulence and velocity variation in the radial direction are analyzed. The results show that mixing the coaxial jet at a low-velocity ratio is better than a high-velocity ratio, at all Mach numbers of the present study. The nozzle lip thickness has a significant influence on the secondary jet. Mixing of the jet in the presence of VR0.2 coaxial jet is found to be the highest. Characteristic decay of Mach 0.8 and 1.0 jet for lip thickness 1.7 and 2.65 mm is faster than lip thickness 0.7 mm. For a given lip thickness, increasing of velocity ratio is found to retard the mixing between primary and secondary jets.

Jet noise predictions by time marching of single-snapshot tomographic PIV fields

This work combines the latest advancements in time marching of 3D vector fields from tomographic particle image velocimetry, with an adapted version of Lighthill’s formulation, for the prediction of far-field jet noise. Three-dimensional velocity vector fields of the jet flow are first reconstructed from a tomographic volume of 4times3times9.5 D_{j}^3, with D_{j} = 5 cm being the jet-exit diameter. (The jet-exit Mach number M_{j} ranges from 0.10 to 0.20.) The obtained vector fields are then used as input to a recently developed procedure for the time marching of the vorticity field, which relies upon the vortex-in-cell methodology. This yields time series of each three-dimensional velocity field, from which the far-field pressure is computed via Lilley’s acoustic analogy (through evaluation of the Lighthill’s stress tensor). It is shown that the estimate of the far-field noise spectrum compares well with the spectrum measured directly from a far-field microphone in the anechoic A-tunnel facility of TU Delft, in the Strouhal number range from approximately 1 to 12.Graphical

Lip thickness effect on high bypass co-flowing jet mixing

PurposeThe purpose of this study is to analyse numerically and experimentally the effects of lip thickness (LT) and bypass ratio on co-flowing nozzle under subsonic and correctly expanded sonic jet decay at different Mach numbers.Design/methodology/approachCo-flowing jets from co-flowing nozzles of different LTs, 0.2, 1 and 1.5 Dp (where Dp is the primary nozzle exit diameter = 10 mm), with an annular gap of 10 mm at main jet exit Mach numbers 0.6 have been studied experimentally and the other cases have been performed numerically. The co-flowing jet with 2 mm LT was used for comparison.FindingsCo-flowing jet axial pitot pressure decay, axial static pressure decay, axial velocity decay, radial velocity decay and streamline velocity contours were analyzed. The results illustrate that the mixing of the co-flowing jet with profound LT is prevalent to the co-flowing jet with 2 mm LT, at all Mach numbers of the current study. Also, the LT of the co-flowing jet has a strong impact on jet mixing. Co-flowing jets with 10 mm and 15 mm LT with a constant co-flow width of 10 mm experience a considerably advanced mixing than co-flowing jets with 2 mm LT and a co-flow width of 10 mm.Practical implicationsThe application of bypassed co-flow jet is in turbofan engine operates efficiently in modern civil aircraft.Originality/valueAll subsonic jets are considered correctly expanded with negligible variation in axial static pressure. However, in the present study, static pressure along the centerline varies sinusoidally up to 9% and 12% above and below atmospheric pressure, respectively, for primary jet exit Mach number 1.0. The sinusoidal variation is less for primary jet exit Mach numbers 0.6 and 0.8 in static pressure decay.

Numerical investigation of critical lip thickness of subsonic co-flowing jet

Purpose The effect of increasing lip thickness (LT) and Mach number on subsonic co-flowing Jet (CFJ) decay at subsonic and correctly expanded sonic Mach numbers has been analysed experimentally and numerically in this study. This study aims to a critical LT below which mixing enhances and above which mixing inhibits. Design/methodology/approach LT is the distance, separating the primary nozzle and the secondary duct, present in the co-flowing nozzle. The CFJ with LT ranging from 2 mm to 150 mm at jet exit Mach numbers of 0.6, 0.8 and 1.0 were studied in detail. The CFJ with 2 mm LT is used for comparison. Centreline total pressure decay, centreline static pressure decay and near field flow behaviour were analysed. Findings The result shows that the mixing enhances until a critical limit and a further increase in the LT does not show any variation in the jet mixing. Beyond this critical limit, the secondary jet has a detrimental effect on the primary jet, which deteriorates the process of mixing. The CFJ within the critical limit experiences a significantly higher mixing. The effect of the increase in the Mach number has marginal variation in the total pressure and significant variation in static pressure along the jet axis. Practical implications In this study, the velocity ratio (VR) is maintained constant and the bypass ratio (BR) was varied from low value to very high values for subsonic and correctly expanded sonic. Presently, commercial aircraft engine operates under these Mach numbers and low to ultra-high BR. Hence, the present study becomes essential. Originality/value This is the first effort to find the critical value of LT for a constant VR for a Mach number range of 0.6 to 1.0, compressible CFJ. The CFJs with constant VR of unity and varying LT, in these Mach number range, have not been studied in the past.

Effects of counterflow jet on the performance of a generic rocket

Effects of a jet emanating from the nose of a generic rocket in the opposite direction of the incoming freestream have been studied to determine the effects of the jet in reducing the drag and the heat loads on the rocket. The effects are comparatively evaluated for different jet exit Mach numbers from ideally expanded nozzles, designed using the method of characteristics, operating at a total temperature of 500 K and at total pressures with predetermined ratios of the freestream total pressure. Axisymmetric computations were conducted with the generic rocket flying at a Mach number of 6.0 and 40,000 ft altitude with zero-degree angle of attack. Results indicate that a jet generated by the Mach 1.5 nozzle, operating at a total pressure corresponding to 1/4th of the freestream total pressure, resulted in the best combined overall drag and temperature reduction. The area-averaged surface temperature and drag were reduced by 52%, and 22%, respectively, in comparison to the body without the flow control.

Effect of Counterflowing Jet on Supersonic Slender-Body Configurations: A Numerical Study

Previous studies have demonstrated that the use of counterflowing supersonic jets operating in the long penetration mode (LPM), directed from the nose of blunt bodies against a supersonic/hypersonic freestream, can greatly reduce the drag and heat loads on the vehicle. However, the benefits of LPM for slender bodies of revolution operating at low supersonic speeds, applicable to supersonic transport applications, remain largely unknown. This Paper conducts a systematic parametric study for the effects of a counterflowing jet on two slender body models (cone-cylinder and quartic body of revolution, respectively) operating at a freestream Mach number of 1.6, using time-accurate computations performed with an unstructured Navier–Stokes solver. Different ranges of jet flow rates, jet exit Mach numbers, nozzle exit diameters, and jet-to-base diameter ratios are examined systematically. The jet-to-base diameter ratio is determined to be the most important parameter to facilitate the establishment of the LPM regime. Depending on the geometry of the slender body, longer jet penetration (thus larger shock-standoff distance) associated with the LPM does not always result in larger drag reduction. Furthermore, current results indicate that inclusion of an LPM jet is likely to cause large oscillations of surface pressure and drag on these slender geometries.

Validation of Jet Noise Simulations and Resulting Insights of Acoustic Near Field

A hybrid lattice Boltzmann method–very-large-eddy simulation (LBM-VLES) solver for high-speed nonisothermal subsonic flows is used to simulate the unsteady jet flow exhausting from a single axi-symmetric nozzle, as well as the associated noise spectra and directivity. The jet exit Mach number and temperature ratio are set according to three representative operating conditions from the NASA SMC000 experimental campaign. The far-field noise is computed through a Ffowcs Williams and Hawkings analogy applied to a fluid surface encompassing the jet plume. Both time- and frequency-domain formulations are used, the latter in combination with an azimuthal Fourier transform of the linear source terms to analyze the contribution of the different azimuthal components. A resolution study is carried out for both aerodynamic and acoustic results. The near- and far-field results confirm that the underlying flow features and noise mechanisms are fully represented by the numerical solution. A wavelet decomposition technique is applied to analyze the source mechanisms for a heated core case. This is achieved by separating the coherent flow motion from the chaotic perturbations in the turbulent flows. Finally, a frequency-domain integral formulation is used to analyze the acoustic far-field of the two segregated components.

Influence of bypass ratio on subsonic and correctly expanded sonic co-flowing jets with finite lip thickness

The effects of bypass ratio on co-flowing subsonic and correctly expanded sonic jet decay have been studied experimentally. Co-flowing jets with lip thickness 1.0 Dp (where Dp is the diameter of primary nozzle and is equal to 10 mm) with bypass ratios of around 0.7, 1.4, and 6.4 at primary jet exit Mach numbers 0.6, 0.8, and 1.0 have been analyzed. A single free jet equivalent to primary nozzle of the co-flowing nozzle was considered for comparison. Primary jet centerline total pressure decay, spread, and static pressure variation were investigated. The results show that the mixing of the high bypass ratio co-flowing jet with lip thickness 1.0 Dp is superior to low bypass ratio co-flowing jet. Both lip thickness and bypass ratio have a strong influence on the co-flowing jet mixing. Bypass ratio 6.3 experiences a significantly higher mixing than bypass ratio 0.7 and 1.4. Selected jets were also investigated computationally. The computations capture the salient flow physics and reproduce well with the experiments.

Development and application of a point Doppler velocimeter featuring two-beam multiplexing for time-resolved measurements of high-speed flow

A novel point Doppler velocimeter (pDV) based upon the Doppler global velocimetry principle is presented, which is capable of three-component velocity vector measurements at 100 kHz mean rates over extended time periods. In this implementation, two laser beams are multiplexed to illuminate the flow over alternating time windows, providing for a reduction in the number of sensors required. The implications of this multiplexing paradigm coupled with the fundamental limits set by the optical absorption filter are examined in detail, and uncertainties are predicted via instrumentation modeling and representative synthetic flow data. The results indicate that the multiplexing pDV instrument provides the required temporal and velocity resolution for turbulent shear flows at velocities of nominally 500 m/s. As a demonstration and validation of this time-resolved technique, statistics of three-velocity component measurements in a cold, supersonic, over-expanded jet at jet exit Mach number M j = 1.4 (design Mach number M d = 1.65) are presented. Time resolution up to 250 kHz and instantaneous velocity uncertainties between 6.6 and 11.1 m/s were obtained. Comparisons of mean pDV data with laser Doppler velocimetry data are consistent with uncertainty predictions for the technique. The ultimate value of the instrument is exhibited in the analysis of Reynolds stress spectra in the screeching jet, exposing the spatial development of motions at the harmonics of the screech tone, variable phase-coordinated shock motions, and growth of turbulent fluctuations in the developing shear layer of the jet. From the data presented, the screech tone phenomenon is suspected to be linked to the production of radial–azimuthal shear stresses in extended regions beyond the potential core.

Analyzing the impact of the inlet temperature on the acoustic noise production form a supersonic jet using large eddy simulations

Non-ideal expanded supersonic jets emerging from a convergent-divergent nozzle produce three different types of noise, i.e., the shock-associated broadband noise, the screech noise, and the turbulent mixing noise. Interesting to note that the screech tone outside of the nozzle was exposed mostly in downscaled laboratory experiments, while under realistic conditions the exhaust jet of a gas turbine engine does not show this phenomena. Apart from a geometric scaling difference, usually a lower temperature is employed in experimental studies. It is believed that the screech tone occurs due to self-excitation of the shear-layer in a feedback-loop. Acoustic waves generated by vortical structures interacting with a shock are propagating upstream within the subsonic region of the shear-layer or outside of it. They eventually hit the nozzle's lip and excite instabilities at a certain frequency. The compressible Navier-Stokes equations are simulated numerically by using Large Eddy Simulation approach. The effect of supersonic jet operation temperature onto the associated noise is investigated. The jet-exit Mach-number is 1.56, while the total temperature ratios considered at the inlet plane of the nozzle are 1.27, 2.05, and 3.65. The differences in the near-field acoustics will be presented in each of the cases and the flow-acoustic interaction will be analyzed and quantified.

Experimental Investigation on Supersonic Jet Screech Tones

In order to investigate the characteristic of the supersonic jet screech tones, an experimental bench of the supersonic jet was designed and a free field noise signal acquisition system was established. Effects of the nozzle size and jet Mach number on jet noise sound field distribution was analyzed, through the result comparison of supersonic jet noise experimental measurement. Results indicate that the field distribution of supersonic jet screech tones is characterized with very strong directivity. Peak value of the screech tones decrease and occurrence frequency of the screech tones increase with the decreasing jet exit Mach number; occurrence frequency of the screech tones decrease with the increasing nozzle size, but the peak value change very less. The experimental measurement of supersonic jet noise provides mechanism research of sound production with data supports and references; and also provides the numerical modeling of supersonic jet noise with validation criteria.

Numerical study comparing RANS and LES approaches on a circulation control airfoil

A numerical study over a nominally two-dimensional circulation control airfoil is performed using a large-eddy simulation code and two Reynolds-averaged Navier–Stokes codes. Different Coanda jet blowing conditions are investigated. In addition to investigating the influence of grid density, a comparison is made between incompressible and compressible flow solvers. The incompressible equations are found to yield negligible differences from the compressible equations up to at least a jet exit Mach number of 0.64. The effects of different turbulence models are also studied. Models that do not account for streamline curvature effects tend to predict jet separation from the Coanda surface too late, and can produce non-physical solutions at high blowing rates. Three different turbulence models that account for streamline curvature are compared with each other and with large eddy simulation solutions. All three models are found to predict the Coanda jet separation location reasonably well, but one of the models predicts specific flow field details near the Coanda surface prior to separation much better than the other two. All Reynolds-averaged Navier–Stokes computations produce higher circulation than large eddy simulation computations, with different stagnation point location and greater flow acceleration around the nose onto the upper surface. The precise reasons for the higher circulation are not clear, although it is not solely a function of predicting the jet separation location correctly.

Characteristics of Annular Mixing Layer in High Subsonic Jet

Experimental studies were conducted on an annular mixing layer which evolved from freely expanding adiabatic circular jet produced by a conical nozzle. The mixing layer region shrouding the potential core was explored at four different jet-exit Mach numbers - 0.5, 0.6, 0.7 and 0.8. Pressure measurements were carried out by means of a miniature Pitot probe attached to an automated traverse. Mean velocity profiles were obtained from the radial distribution of pressures across the annular mixing layer thickness at various axial locations. The iso-velocity curves, plotted in longitudinal plane, exhibited that the velocity in the line of the nozzle lip was always about 60% of the nozzle exit velocity for the entire range of Mach numbers studied. The mixing layer thickness, assessed in terms of vorticity thickness, was found to grow linearly with the longitudinal distance downstream of the nozzle. The mixing layer growth rate was found to decline with increasing Mach number. However, the mixing layer was found ...

Spatially Resolved Heat Transfer Rates in an Impinging Circular Microscale Jet

An experimental study of a submerged 125-μm circular microscale jet impingement is presented. These jets flows are associated with low exit Reynolds number and a correspondingly high, subsonic Mach number. Detailed distributions of heated and adiabatic wall temperature, and local and average Nusselt number variations are presented for five laminar exit Reynolds numbers in the range of 690 to 1770 at three nozzle-to-surface spacings of 2, 4, and 6 times the nozzle diameter. The corresponding jet exit Mach numbers range between 0.26 and 0.63. An infrared radiometer is used in conjunction with a heated-thin-foil technique to measure detailed surface temperatures. Results indicate that the adiabatic surface temperature distribution is relatively insensitive to nozzle-to-surface spacing within the parameter range studied. With an increase in Reynolds number, the adiabatic surface temperature decreases significantly near the stagnation point. The average Nusselt numbers are higher compared to Martin's correlation [1] for large Reynolds numbers. A similar observation has been reported previously in a numerical study on microscale jet impingement [5] and attributed to compressibility of the flow and the possible existence of a slip flow.

Assessment of Parameters Influencing the Prediction of Shear-Layer Mixing

A multi-site team has completed an investigation of modeling high speed mixing layers using the computational methods currently being applied to predict HSCT nozzle flow fields. The objectives of this investigation were to: (1) calibrate the codes used by the various team members against benchmark experimental data, and (2) assess the accuracy of the Navier-Stokes codes in calculating turbulent flows having flow characteristics similar to those of HSCT engine nozzles. Two flow geometries were investigated using the five codes of NASTAR, PAB3D, GIF3D, NASTD, and NPARC. The first was the heated supersonic round jet of Seiner. For this configuration, with a jet exit Mach number similar to that of the primary flow from mixer chutes, three nozzle flow temperatures were investigated with the five codes. Using the same grid, boundary conditions, and k-e turbulence model (in the mixing region), very similar results were obtained for all codes, but the solutions did not agree well with the experimental velocity and temperature profiles. Further calculations using different turbulence models, compressibility corrections, and axisymmetric dissipation corrections improved the agreement with experimental data, but the corrections are not universally applicable. The second configuration was the two-dimensional supersonic mixing layer of Goebel & Dutton. For the flow case examined, with two supersonic streams, the five codes again produced very similar results using the same grid, boundary conditions, and turbulence model. The agreement with experimental data was better than for the Seiner round nozzle.

Use of Linearized Euler Equations for Supersonic Jet Noise Prediction

The use of linearized Euler equations for direct prediction of supersonic jet noise is explored. It is shown that a high-order numerical scheme coupled with proper boundary treatment can produce a stable solution nearly free from reflections. Results are verified against analytical results for sound radiated by instability waves. Applicability of this approach to real jets is explored by taking the inflow disturbances to be random in time and comparing the computed sound field to the experimentally measured one. Nomenclature D = jet exit nozzle diameter e = total energy per unit volume Me = jet exit Mach number p = pressure Ue = jet exit velocity u = axial velocity v = radial velocity w = azimuthal velocity p = density pe = jet exit density

Studies on Twin Non-Parallel Unventilated Axisymmetric Jets

Twin non-parallel jet configurations occur in many practical devices like burners, combustion chambers, and also in the area of fluidics etc. Despite the importance of such configurations, studies on the interaction of twin non-parallel jets are very limited. The present work is an attempt to investigate the mixing of two axisymmetric jets obliquely oriented towards each other. Experimental studies were made on the interaction of twin intersecting axisymmetric jets issuing from two unventilated convergent nozzles. The nozzles of exit diameter (D) 10 mm were set on a common end wall with their axes intersecting each other at half angles (α3) of 0°, 5°, 10° and 15°. The centre-to-centre spacing (S) of the nozzles, non-dimensionalized, as S/D, was 3.1. The jet exit Mach number (Me) studied was 0.2. The results indicate that the near field characteristics are strongly influenced by α3. However, the potential core of the individual jets and the far field characteristics of the twin jet flow field, that is beyond downstream distances of 40 nozzle diameters, are not significantly influenced by α3. The cross-section of the jet just downstream of the combining point is approximately elliptic. The axis switching phenomenon normally associated with non-circular jets is observed in the jet flow field. The spread of the combined jet is more in the transverse direction than in the spanwise direction. Entrainment of the ambient fluid is found to be more in the case of twin parallel jets (α3 = 0°). The entrainment decreases with increasing α3. The self-preserving nature of the combined jet is almost independent of the initial geometric conditions. The combined jet is axisymmetric with regard to the normalized velocity and length scales.

The feedback loop in impinging two-dimensional high-subsonic and supersonic jets

The instabilities in a supersonic impinging jet are investigated by solving the two-dimensional Euler equations using the piecewise parabolic method (PPM) and Roe's linearized Riemann solver. The predicted shock cell spacing agrees well with the observed and theoretical values. The frequency and nature of the dominant instabilities are found to be a function of the impingement distance. Two instability modes are possible: a symmetric (or varicose) mode and an asymmetric (or sinuous) mode. For two given jet exit Mach numbers ( M = 0.98 and 1.29), the energy in, and frequency of, these modes are functions of impingement distance, leading to an integral staging due to an acoustic feedback loop. The predicted frequencies of the fundamental symmetric and asymmetric instabilities agree with the theoretically allowed values. The staging of predicted frequencies that occurs in experimental work is also predicted.

In-flight imaging of transverse gas jets injected into compressible crossflows

The development of and results from an airborne flight-test experiment to study nonreacting gas jets injected transversely into subsonic and slightly supersonic crossflows is presented. Freestream/crossflow Mach numbers in the study ranged from 0.6 to 1.2, and planar laser-induced fluorescence imaging of an iodine-seeded nitrogen jet was used to visualize the jet flow. Time-dependent images were obtained with a high-speed intensified video camera synchronized to the laser pulse rate. The entire experimental assembly was configured compactly inside a unique flight-test fixture mounted under the fuselage of NASA Dryden's F-104G research aircraft, which served as a flying wind tunnel. Recorded transverse gas jet images were then digitized to allow analysis of jet trajectory, spreading, and mixing characteristics. Comparisons of experimental results with model predictions in this transonic crossflow regime show reasonable agreement. Nomenclature AJ = cross-sectional area of jet nozzle at exit A6 = cross-sectional area of jet nozzle at transducer location; see Fig. 4 Mj = jet exit Mach number