Statistical Properties of Turbulent Free Jets Issuing from Nine Differently-Shaped Nozzles

This paper reports the centerline evolutions of turbulent statistical properties in nine air jets issuing from differently-shaped nozzles into still air surroundings. All nozzles of investigation have nominally identical opening areas or equal equivalent diameters (D _e ) and their aspect ratio (AR) varies from AR = 1 (circle) to AR = 2.5 (isosceles triangle). Present measurements were made at the Reynolds number (based on D _e ) of approximately 15,000. Results show that the loss of jet-axisymmetry at the exit generally causes the mean velocity decaying faster, and the fluctuating intensity growing, in the near field, thus indicating the increased overall entrainment rate. It is also shown that a change of shape of the nozzle exit does not affect the asymptotic decay rate of the centreline velocity in the far field. The near-field structure of the isosceles-triangular jet is deduced to be most three-dimensional, compared with the circular counterpart from smooth contraction being least. These discrepancies, however, weaken as the downstream distance x is increased. Beyond x/D _e  = 20–30, the normalized velocity spectra for all jets of small AR collapse well, indicating similar statistical behaviors over a wide range of scales in the central region. Indeed, sufficiently downstream from the exit, insignificant differences occur in jets’ velocity probability density function (PDF), the related skewness and flatness factors, and also in their Taylor and Kolmogorov microscales. It is demonstrated that all the length scales grow approximately linearly with x at x/D _e  ≥ 20.     

The effects of acceleration in jets: kinematics of the near field vortices

Direct and large-eddy simulations (DNS/LES) of accelerating round jets are used to analyze the effects of acceleration on the kinematics of vortex rings in the near field of the jet (x/D < 12). The acceleration is obtained by increasing the nozzle jet velocity with time, in a previously established (steady) jet, and ends once the inlet jet velocity is equal to twice its initial value. Several acceleration rates (α = 0.02–0.6) and Reynolds numbers (Re _D = 500–20000) were simulated. Acceleration maps were used to make a detailed study of the kinematics of vortex rings in accelerating jets. One of the effects of the acceleration is to cause a number of new primary and secondary vortex merging events that are absent from steady jets. As the acceleration rate α increases, both the number of primary merging events between rings and the axial position where these take place decreases. The statistics for the speed of the starting ring that forms at the start of the acceleration phase for each simulation, agree well with the statistics for the “front” speed observed by Zhang and Johari (Phys Fluids 8:2185–2195, 1996). Acceleration maps and flow visualizations show that during the acceleration phase the near field coherent vortices become smaller and are formed at an higher frequency than in the steady jet, and their (mean) shedding frequency increases linearly with the acceleration rate. Finally, it was observed that the acceleration decreases the spreading rate of the jet, in agreement with previous experimental works.      

Near-field instability of variable property jet in normal gravity and microgravity fields

The near-field instability of variable property jets of air, CO₂, and He, issued into the ambient air, has been investigated experimentally within normal gravity and microgravity fields. The density ratio to the ambient air is unity for air jets, more than unity (1.53) for CO₂ jets, and less than unity (0.14) for He jets, respectively. The ratio of kinematic viscosity to the ambient air is unity for air jets, less than unity for CO₂ jets (0.53), and more than unity for He jets (7.75), respectively. The jet velocity is varied from 0.4 to 1.8 m/s and then the jet Reynolds number varies from 60 for Helium jet to 2,000 for CO₂ jet, while the Richardson number varies from negative to positive values. The motion of the jet is visualized using a laser tomographic method and recorded by a high-speed digital video camera with 250 frames/s. The result shows that the instability of the jet is intensified when Re > 800 while it is weakened at Re < 800 at the microgravity field, indicating that the viscosity plays an important role in weakening the instability. Under a normal gravity field, the buoyancy also becomes important. In order to quantify the instability criteria, the quantity of the instability is introduced, which consists of the Kelvin–Helmholtz instability, buoyancy effect and viscous effect. When the ratio of the sum of Kelvin–Helmholtz and buoyancy forces to viscous force exceeds a certain value, around 12 in the present study, the jet becomes unstable even when Re < 800. These results reveal that the instability of variable property jets is influenced by the Kelvin–Helmholtz instability, the viscous effect and the buoyancy effect.     

The stability of low Reynolds number round jets

Multigrid cross-correlation digital particle image velocimetry (MCCDPIV) is used to investigate the stability and structure of low Reynolds number axisymmetric jets. The in-plane velocities, out-of-plane vorticity and some of the components of the Reynolds stress tensor are measured. Two Reynolds numbers based on the orifice outlet diameter are examined (680 and 1,030) at two different positions: one close to the orifice, ranging from 2D ₀ to 5D ₀ (D ₀ is the orifice diameter); and the other further from the orifice, ranging from 10D ₀ to 14.4D ₀. The results show that the lower Reynolds number jet (Re=680) is marginally unstable in the near-orifice region and is best described as laminar. Further downstream some intermittent structures are observed in the jet, and the growth in integrated turbulent kinetic energy with axial position indicates that the jet is also unstable in this region. For the higher Reynolds number jet (Re=1,030) the increasing size and intensity of vortical structures in the jet in the near-orifice region observed from the MCCDPIV data and the growth in integrated turbulent kinetic energy indicate that the jet is unstable. Further downstream this jet is best described as transitional or turbulent. From flow visualisation images in the near-orifice region it seems that, for both Reynolds numbers, shear layer roll-up occurs when the jet exits the orifice and enters the quiescent fluid in the tank, resulting in vortical structures that appear to grow as the jet proceeds. This is indicative of instability in both cases and is consistent with previous flow visualisation studies of low Reynolds number round jets. Discrepancies observed between the flow visualisation results and the MCCDPIV data is addressed. On the basis of flow visualisation results it is generally assumed that round jets are unstable at very low Reynolds number, however the present work shows that this assertion may be incorrect.     

A phase averaged PIV study of circular and non-circular synthetic turbulent jets issuing from sharp edge orifices

An extensive experimental study using Particle Image Velocimetry (PIV) on synthetic jets issuing from different orifice shapes is reported. All data are phase and time averaged to derive mean velocity, half-velocity width and rms velocity profiles in the near field of the jet (0 < X/D < 7), at a Reynolds number around 10,000. Different non-circular orifice shapes as rectangular, square, elliptic and triangular are considered and results are compared to those of the circular orifice in order to investigate the effect of asymmetry on the turbulent flow field in view of mixing enhancement. The measurements are carried out on two orthogonal planes to capture three dimensional features of non-circular jets. Results show highest velocity decay rate for elongated orifices, especially the rectangular one, in comparison to the circular one, both in phase and time-averaged plots. Time averaged results show higher velocity decay rate of synthetic jets in comparison to those of continuous ones. It is also observed that, for X/D > 5, only profiles of circular and square jets become partially self-similar. For synthetic jets, higher turbulence content is measured for all orifice shapes at the centerline and close to the orifice exit in comparison to continuous jets.     

Measurements of air entrainment by vertical plunging liquid jets

This paper addresses the issue of the air-entrainment process by a vertical plunging liquid jet. A non-dimensional physical analysis, inspired by the literature on the stability of free jets submitted to an aerodynamic interaction, was developed and yielded two correlation equations for the laminar and the turbulent plunging jets. These correlation equations allow the volumetric flow rate of the air carryunder represented by the Weber number of entrainment We _n to be predicted. The plunging jets under consideration issued from circular tubes long enough to achieve a fully developed flow at the outlet. A sensitive technique based on a rising soap meniscus was developed to measure directly the volumetric flow rate of the air carryunder. Our data are compared with other experimental data available in the literature; they also stand as a possible database for future theoretical modelling. Received: 2 November 2000/Accepted: 13 November 2001     

Compressible spreading rates of supersonic coaxial jets

The compressible spreading rates of two supersonic coaxial jets were studied experimentally. The center jet had a fully-expanded Mach number of 3, and the outer jet of M = 1.8. The geometries of the center jet were circular and rectangular with two configurations, both with a 3∶1 aspect-ratio. The convective Mach numbers were varied in the range between 0.25 < M _c< 2.25. This was accomplished by varying the density ratio between the center jet and the coaxial jet, as well as the velocity ratio. The experimental compressible spreading rate was determined using Schlieren photography and total-pressure measurements. The spreading rate of the center circular jet decreased with increasing convective Mach number until it reached a constant value of 0.2 to 0.3 of the incompressible spreading rate for M _c>1.4. The rectangular jets exhibited a similar drop, at the same range of M _c, but their spreading rate was higher relative to the circular jet in the entire convective Mach number range.     

The spatial modulation of a forced triangular jet

An investigation of the possibility of controlling the evolution of jets into the far field is presented. Driven by practical concerns the study examined a highly turbulent jet flow. To enhance controllability of the flow evolution the virtues of non-circular nozzles and active flow excitation were combined. The study examined an air jet, Re _de =8000, average turbulence intensity 1.8%, issuing into stagnant room air out of a triangular nozzle, which had piezoceramic actuators mounted on the flat sides. The evolution of the jet flow field was examined over the range of 0X/D30.Small amplitude, single mode, excitation with frequency as the varying parameter was found to be ineffective for controlling the far field evolution. In contrast, excitation of the jet with non-integer and counter propagating azimuthal modes yielded marked changes in the jet streamwise evolution. The most notable changes in the far field were the transition of the cross section from round to elliptical, increased jet cross sectional area based on half centerline velocity contour, asymmetric threedimensional flow, and an increase in the entrainment rate. The entrainment of ambient air by the jet increased slightly more than twofold for non-integer and counter propagating azimuthal modes, compared with the unexcited jet, and only a 50% increase in the entrainment for single, integer, mode excitation.While excitation of the jet with modes m=0 and 1 resulted in symmetric evolution of the jet in the x-y-t space, excitation at non-integer and counter propagating modes resulted in time-dependent asymmetric motion. The near field induced jet column motion is controlling the far-field evolution of the examined jet.The authors would like to thank Dr. S. Ragab for useful discussions, and Dr. R. Kriz for access to the Scientific Visualization Laboratories, both from the ESM department at Va. Tech. The opportunities to discuss the research with Dr. M. G. Mungal from Stanford University are highly appreciated     

Experimental investigation of liquid jet injection into Mach 6 hypersonic crossflow

The injection of a liquid jet into a crossing Mach 6 air flow is investigated. Experiments were conducted on a sharp leading edge flat plate with flush mounted injectors. Water jets were introduced through different nozzle shapes at relevant jet-to-air momentum–flux ratios. Sufficient temporal resolution to capture small scale effects was obtained by high-speed recording, while directional illumination allowed variation in field of view. Shock pattern and flow topology were visualized by Schlieren-technique. Correlations are proposed on relating water jet penetration height and lateral extension with the injection ratio and orifice diameter for circular injector jets. Penetration height and lateral extension are compared for different injector shapes at relevant jet-to-air momentum–flux ratios showing that penetration height and lateral extension decrease and increase, respectively, with injector’s aspect ratio. Probability density function analysis has shown that the mixing of the jet with the crossflow is completed at a distance of x/d _j  ~ 40, independent of the momentum–flux ratio. Mean velocity profiles related with the liquid jet have been extracted by means of an ensemble correlation PIV algorithm. Finally, frequency analyses of the jet breakup and fluctuating shock pattern are performed using a Fast Fourier algorithm and characteristic Strouhal numbers of St = 0.18 for the liquid jet breakup and of St = 0.011 for the separation shock fluctuation are obtained.     

Measurements on supersonic free jets

Summary An experimental study of round supersonic air jets discharging into quiescent air is described. The initial stagnation enthalpy of the jets was equal to that of the atmosphere. Most of the experiments concerned a fully expanded jet with initial Mach number M _I =1.74. From the experimental results the turbulent Prandtl number and the turbulent coefficient of momentum transfer could be calculated in a large part of the mixing zone of the jets. A mixing parameter K has been introduced and calculated for the jets. The results of this investigation are compared with those obtained for low-speed jets.     

Experimental investigation of a free-surface turbulent jet with Coanda effect

The deviation of a jet from the straight direction due to the presence of a lateral wall is investigated from the experimental point of view. This flow condition is known as Coanda jet (from the Romanian aerodynamicist Henry Marie Coanda who discovered and applied it at the beginning of XXth century) or offset jet. The objective of the work is to detail the underlying mechanisms of such a phenomenon aiming to use it as a flow control method at polluted river flows mouth. To do this, a large laboratory free-surface tank with an incoming channel has been set up and velocity field measurements are performed by Optical Flow methods (namely Feature Tracking). Preliminary tests on the well-known free jet configuration without any marine structure (i.e. lateral wall) are performed to allow comparison with free jet scaling and self-similar solutions. The presence of the free-surface gives rise to centerline velocity decay which is lower than in free unbounded plane or circular jets due to the vertically limited ambient fluid entrainment. In the second part of the paper, the effect of a lateral wall on the jet configuration is examined by placing it at different lateral distances from the jet outlet. The resulting velocity fields clearly show an inclined Coanda jet with details which seems to depend on the lateral wall distance itself. The analysis of self-similarity along the inclined jet direction reveals that for wall distances larger than 5 jet widths this dependence almost disappears.     

Particle image velocimetry investigation on mixing enhancement of non-circular sharp edge nozzles

An experimental study using Particle Image Velocimetry (PIV) on free jets issuing from different orifice plate (OP) nozzles is reported. Mean velocity, turbulence intensity and higher order profiles relevant for large and small scale mixing are considered in the near field and interaction zone (0 < X/D < 20). This is done to determine mixing enhancement due to rectangular, squared, elliptic and triangular nozzles in comparison to circular nozzle results in two orthogonal planes. The effect of Reynolds number on the differences among the nozzle shapes is also considered by performing measurements just after laminar–turbulent transition (Re = 8000) and in the fully turbulent regime (Re = 35,000). The results at low Reynolds number show two classes of jets, i.e. at one side, those closer to axial-symmetric conditions, as circular, square and triangular jets, whereas on the other side those with elongated nozzles as rectangular and elliptic. The reason for the different behavior of the latter is connected to the phenomenon of axis-switching which allows a rearrangement of turbulence over the different velocity components and directions. However, for the highest Reynolds number investigated, all nozzles show similar behavior especially in the jet far field (X/D > 10), thus suggesting a significant Reynolds number dependence of the results.     

A flow field study of an elliptic jet in cross flow using DPIV technique

The digital particle image velocimetry (DPIV) technique has been used to investigate the flow fields of an elliptic jet in cross flow (EJICF). Two different jet orientations are considered; one with the major axis of the ellipse aligned with the cross flow (henceforth referred to as a low aspect ratio (AR) jet), and the other with the major axis normal to the cross flow (henceforth referred to as a high aspect ratio jet). Results show that the vortex-pairing phenomenon is prevalent in the low aspect ratio jet when the velocity ratio (VR)3, and is absent in the high aspect ratio jet regardless of the velocity ratio. The presence of vortex pairing leads to a substantial increase in the leading-edge peak vorticity compared to the lee-side vorticity, which suggests that vortex pairing may play an important role in the entrainment of ambient fluid into the jet body, at least in the near-field region. In the absence of vortex pairing, both the leading-edge and the lee-side peak vorticity increase monotonically with velocity ratio regardless of the aspect ratio. Moreover, time-averaged velocity fields for both AR=0.5 and AR=2 jets reveal the existence of an unstable focus (UF) downstream of the jet, at least for VR2. The strength and the location of this focus is a function of both the velocity ratio and aspect ratio. In addition, time-averaged vorticity fields show a consistently higher peak-averaged vorticity in the low aspect ratio jet than in the high aspect ratio jet. This behavior could be due to a higher curvature of the vortex filament facing the cross flow in the low aspect ratio jet, which through mutual interaction may lead to higher vortex stretching, and therefore higher peak-averaged vorticity.Nomenclature A nozzle or jet cross-sectional area - AR aspect ratio, defined as the ratio of the nozzle cross-stream dimension to its streamwise dimension, =H/L - D characteristic jet diameter (for circular jet only) - D_h hydraulic diameter, =4A/P - D_major major axis of an elliptic nozzle - D_minor minor axis of an elliptic nozzle - H cross-stream dimension of the nozzle - L streamwise dimension of the nozzle - P perimeter of the nozzle - Re_j jet Reynolds number, =V_jD/ - VR velocity ratio, =V_j/V - V_j mean jet velocity - V mean cross-flow velocity - x downstream distance from jet center - X cross-plane vorticity - kinematic viscosity     

Helium jets discharging normally into a swirling air flow

The characteristics of helium jets injected normally to a swirling air flow are investigated experimentally using laser Doppler and hot-wire anemometers. Two jets with jet-to-crossflow momentum flux ratios of 0.28 and 12.6 are examined. The jets follow a spiral path similar to that found in the swirling air flow alone. Swirl acts to decrease jet penetration, but this is being counteracted by the lighter jet fluid density which is being pressed towards the tube center by the inward pressure gradient. Consequently, in spite of the large variation in momentum flux ratio, jet penetration into the main flow for the two jets investigated is about the same. The presence of the jet is felt only along the spiral path and none at all outside this region. Upstream of the jet, the oncoming swirling flow is essentially unaffected. These characteristics are quite different from jets discharging into a uniform crossflow at about the same momentum flux ratios, and can be attributed to the combined effects of swirl and density difference between the jet fluid and the air stream. Finally, the jets lose their identity in about fifteen jet diameters.List of symbols C mean volume concentration of helium - C _j mean volume concentration of helium at jet exit - c fluctuating volume concentration of helium - instantaneous volume concentration of helium - c RMS volume concentration of helium - D _j jet nozzle diameter - D _T diameter of tube - F flatness factor of c - J = _j U _j ² / _a U _a ^{gn 2} jet-to-crossflow momentum flux ratio - P(c) probability density function of c - r radial coordinate measured from tube centerline - R = D _T /2 radius of tube - Re _j = D _j U _j / _j jet Reynolds number - S = = tan swirl number - Sk skewness of c - instantaneous axial velocity - u RMS axial velocity - U mean axial velocity - local average mean axial velocity across tube - U _j jet exit velocity - U _a overall average mean axial velocity across tube - instantaneous circumferential velocity - w RMS circumferential velocity - W mean circumferential velocity - x axial coordinate measured from exit plane of swirler - x ₁ axial coordinate measured from centerplane of normal jet - y normal distance measured from tube wall - _j jet fluid kinematic viscosity - _a air density - _j jet fluid density - vane angle (constant)     

Selective control of flow coherence in triangular jets

The triangular jet was investigated for use as a passive device to enhance fine-scale mixing and to reduce the coherence of large-scale structures in the flow. The suppression of the structures is vital to the enhancement of molecular mixing, which is important for efficient chemical reactions including combustion. The sharp corners in the jet injector introduced high instability modes into the flow via the non-symmetric mean velocity and pressure distribution around the nozzle. Both aerodynamic and hydrodynamic flows showed the difference between the flow at the corner (vertex) and at the flat side. While highly coherent structures could be generated at the flat side, the corner flow was dominated by highly turbulent small-scale eddies. The flow characteristics were tested using hotwire anemometry for mean flow and turbulence analysis, and flow visualization in air and water.List of symbols D inlet duct diameter - D _e equivalent diameter - D _i inside diameter - E _v velocity fluctuation energy - f _F forcing frequency - f _j preferred mode frequency - L length - Re Reynolds number - R _e equivalent radius (same area) - r _0.5 jet half-width - R _1.2 cross-correlation factor - r radial coordinate (circular duct) - St _e most energetic Strouhal number - St _j preferred mode Strouhal number - U _m centerline (maximum) velocity in radial u-profile - U ₀ jet exit velocity - u local axial mean velocity - x axial coordinate - X ₁ axial position of first of two hot-wires for axial cross-correlation - + y _F lateral coordinate at flat side of triangular duct - - y _V lateral coordinate at vertex side of triangular duct - (E _V)_j preferred mode energy - X axial distance between hot-wires - r radial distance between two hot-wires (circular jet) - y lateral distance between two hot-wires (triangular jet) - P/P pressure amplitude - momentum thickness - time     

Smoke-wire flow visualization of the near-wake region behind a circular disc at low Reynolds numbers

The flow structures in the near field of the unducted wake region behind a circular disc for annular flow at low Reynolds numbers were studied by smoke-wire flow visualization technique. A twisted-dual-wire was employed to perform the time evolving visualization. Three typical characteristic flow modes: Q-tip, open-top toroid, and closed toroid, were identified in the near disc region. For Reynolds number between 130 and 390, the Q-tip flow mode which subject to a periodic up-down oscillatory motion was observed. The open-top toroid mode which experiences the expelling vortex shedding was found for Reynolds number between 390 and 455. The free separation surface turns around and merges to the central axisymmetric axis to form the conventionally observed toroidal recirculation bubble for Reynolds number higher than 455. The closed toroid mode exhibits both expelling and shear-layer vortex sheddings. With the identified flow modes at low Reynolds numbers, the recirculation contours, recirculation length, and the shedding frequency in each mode were measured and discussed.List of symbols B.R. blockage ratio (=D ² /D _a ² ) - D _a outer diameter of annular jet, 30 mm - D diameter of circular disc, 20 mm - f frequency of vortex shedding, Hz - L _r axial length of recirculation zone - R radius of circular disc, 10 mm - u _a average exit velocity of annular jet - ₀ stream function with value of zero - mass density of annular flow - u average axial velocity - r radial coordinate, originated from center of circular disk - r ₀ radial coordinate of the boundary of the recirculation zone - Re _a Reynolds number of annular jet based on the disc diameter - Z axial coordinate, originated from center of circular disk - w _max maximum half-width of the recirculation zone - St Strouhal number (=fD/D _a )     

The near field behavior of turbulent gas jets in a long confinement

The present investigation reports on the near field behavior of gas jets in a long confinement and points out the differences between this type of jet flow and those of free jets and jets in a short confinement.The jet, with a diameter of 8.73 mm, is aligned concentrically with a tube of 125 mm diameter; thus giving a confinement area ratio of 205. The arrangement forms part of the test section of an open-jet wind tunnel and this gives a confinement length-to-jet diameter ratio of 1,700. Experiments are carried out with carbon dioxide, air and helium/air jets at different jet velocities. Mean velocity and turbulence measurements are made of the jet near field using a one-color, one-component laser doppler velocimeter operating in the forward scatter mode. In addition, the turbulent shear field of an air jet is examined in more detail using hot-wire anemometers.In view of the long confinement, the presence of the jet is not being felt immediately at the tunnel exit. Consequently, the air column inside the tunnel is first compressed by the jet and then slowly pushed out of the tunnel. This behavior causes the jet to spread rapidly and to decay quickly. As a result, an equilibrium turbulence field is established in the first two diameters of the jet. This equilibrium field bears striking similarity to that found in self-preserving, turbulent free jets and jets in short confinement and is independent of jet fluid densities and velocities. In terms of these characteristics, the near field of jets in a long confinement is very different from that found in free jets and jets in short confinements.     

Numerical study of unsteady compressible flow driven by supersonic jet injected into elliptical cell with small exit hole

The unsteady behavior of flow driven by a jet suddenly injected into an elliptical cell is numerically studied by solving the axisymmetric two-dimensional compressible Navier–Stokes equations. This system is a model of laser ablation of a certain duration followed by a discharging process through the exit hole at the downstream end of the cell. The parameters for the calculations are the exit diameter of the cell, the Mach number and duration of the injected jet. The injected jet becomes a traveling plume approaching the downstream end of the elliptical cell and discharges from the cell through an exit hole. The plume generates and interacts with a shock wave in the elliptical cell. The unsteady flow properties downstream of the cell are found to be attenuated by the combination of the phenomena occurring in the cell and at the exit. Monitoring the velocity at the exit hole is used to clarify the characteristics of the flow and apply them to applications in pulse laser ablation. The results show that the vortex in the plume head with the same radius as the exit diameter (i.e., D_e/D_j = 2.7, where D_e is the exit diameter and D_j is the injected jet diameter) causes a relatively constant velocity at the exit for about 10 μs. In the downstream flow characteristics, the suddenly injected jet makes a single or double peak in the velocity variation outside the cell depending on the combination of parameters. This suggests that a single laser pulse might generate two beams through the exit hole of a cell, which could increase the efficiency of beam generation with the combination of an elliptical cell and the laser ablation. It is also found that the wave form of the variations and their level are roughly determined by the durations of the jet and the exit diameters of the cell exit, respectively. PACS 51.35.+a; 47.40.Nm     

Heat transfer due to impinging co-axial jets and the jets’ fluid flow characteristics

This investigation had multiple goals. One goal was to obtain definitive information about the heat transfer characteristics of co-axial impinging jets, and this was achieved by measurements of the stagnation-point, surface-distribution and average heat transfer coefficients. These results are parameterized by the Reynolds number Re which ranged from 5000 to 25,000, the dimensionless separation distance between the jet exit and the impingement plate H/D (4–12), and the ratio of the inner diameters of the inner and outer pipes d/D (0–0.55). The d/D = 0 case corresponds to a single circular jet. The other major goal of this work was to quantify the velocity field of co-axial free jets (impingement plate removed). The velocity-field study included both measurements of the mean velocity and the turbulence intensity.It was found that the variation of the stagnation-point heat transfer coefficient with d/D attained a maximum at d/D = 0.55. Furthermore, the variation of the local heat transfer coefficient across the impingement surface was more peaked for d/D = 0 and became flatter with decreasing d/D. This suggests that for cooling a broad expanse of surface, co-axial jets of high d/D are preferable. On the other hand, for localized cooling, the single jet (d/D = 0) performed the best. In general, for a given Reynolds number, a co-axial jet yields higher heat transfer coefficients than a single jet. Off-axis velocity peaks were encountered for the jets with d/D = 0.105. The measurements of turbulence intensity yielded values as high as 18%.     

Space–time correlation measurements of high-speed axisymmetric jets using optical deflectometry

An optical deflectometry system is used to provide unique space–time correlation measurements at two positions separated by varying axial distances within a high-speed jet shear layer. The measurements were made for both pure air and for helium/air mixture jets at Mach numbers M=0.9 and M=1.5. The jets issue from round nozzles and the sensing volumes at the two measurement positions consist of small light filaments along spanwise lines that are tangential to the annular jet shear layer. Applying this technique to obtain measurements detailing the level of correlation, spectral content, and convection velocity for jet flows in these flow regimes near the end of the potential core is particularly important in the understanding and prediction of jet noise. Measurements near the end of the potential core along the jet lip line exhibit distinct cross-correlation curves for the pure air jet cases. However, helium/air mixture jets display much lower levels of correlation and little evidence of large-scale structure in the measured spectra. It is believed that the thick visual density gradients dominated by smaller scales throughout the shear layer of the helium/air mixture jets effectively mask the large-scale structure, thus, reflecting a limitation of this optical deflectometer. Finally, a decrease in normalized convection velocity with helium addition is observed.