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Near Surface Characterization of an Impinging Elliptic Jet Array
Abstract
In this study naturally occurring lar ge-scale structures and some turbulence characteristics within an impinging jet array are investigated. The dynamics of a three-by-three elliptic jet array are analyzed relative to the flow structures within the array. With applications to electronic component cooling, low Reynolds number conditions, Re = 300 to 1500, are presented. Two jet aspect ratios are used, 2 and 3, with identical jet hydraulic diameters and jet-to-jet space. The effects of impinging distance are studied in the range of one to six jet hydraulic diameters. Flow visualization and PIV are used for the identification of structures and quantitative analysis. These results are used to evaluate the integrated surface layer vorticity, Gamma* which is shown to depend on the jet aspect ratio and impingement distance. Also, a transport coefficient is presented based on a turbulence velocity and length scales. This coefficient is shown to experience a maximum value versus impingement distance that coincides with the location of axis switching.
... Additionally, they found that the magnitude of this effect increases with increasing nozzle-to-plate distance. The parameters 1 α , 2 α , and 3 α in the non-dimensional investigated heat transfer from an impinging jet array with the same cross-flow schemes. ...
... The other side of the heater was insulated to get the heat transferred only in one direction i.e. onto copper plates. The mass flow rate of the compressed air (1) entering the test section was passed through a settling chamber (2) and was controlled with the help of valves (3) and the pressure regulators (5). The pressure drop across pressure regulators was measured using the pressure gauges (4). ...
... The study of the heat transfer on a target surface has focused mostly on orifice nozzles due to their advantages: requiring little space and providing high free jet shear forces [3,5,[20][21][22][23]. The crossflow in case the jets discharged from modified nozzles becomes less significant than with round nozzles [9,12]. ...
The purpose of this research was to investigate experimentally and numerically the effects of an impinging jet array with entrained air ducts on heat transfer. Straight pipes with 17.2 mm inner diameter (d) and 300 mm length (l) were in a 5 × 5 nozzle array with in-line configuration. The length of air duct (L) was varied among L/d = 2, 4, or 6, and the diameter of the duct (D) satisfied D/d = 2, 4, or 6. The jet-to-surface distance (H) was tested at H/d = 4, 6, or 8; and the jet-to-jet spacing (S) was among S/d = 4, 6, or 8. Reynolds number (Re) in the jets was from 10,000 to 40,000. Conventional impinging jets were also examined for benchmarking. Flow characteristics obtained numerically from v²f turbulence model are also presented to explain the heat transfer mechanisms. The results indicate that heat transfer rates on the target surface became higher when the duct was elongated, and was more widely spread when the duct diameter was expanded. These effects were more marked in cases with narrow jet-to-jet spacing (S/d = 4) and with large jet-to-surface distance (H/d = 8). Moreover, a heat transfer correlation was constructed from the experimental results to predict heat transfer rates within the studied ranges of parameters.
... They have described complex evolution and interaction of vortices, flow instabilities and axis switching. Arjocu and Liburdy [5] conducted PIV experiments to explain axis switching phenomena during jet impingement by array of elliptic jets. Dano et al. [6] reported influence of nozzle configuration on heat transfer of incline array of jet impingement. ...
The present study discusses two equilaterally staggered jet arrangements for uniform cooling of flat surface. The equilateral staggered arrangement consists of a circular jet surrounded by four neighboring chamfered jets at an angle of 45° called as chamfered configuration. An equilateral staggered jet arrangement consisting only of circular jets is considered as the non-chamfered configuration. Large-eddy simulations and Mie-scattering imaging techniques are discussed for heat transfer and flow visualization of equilateral staggered jet configurations. Formation of number of eddies characterizes the flow feature. The turbulence quantities of the jet configurations determine the amount of heat transfer. The eddies are formed due to recirculation which later breaks into smaller parts by the incoming jet fluid. The flow features basically constitute of jet to jet interaction, jet interference and upwash flow. It is also noticed that every jet cools an independent area. However with the inclusion of chamfered jets the region of highest heat transfer shifts away from the jet centerline. This happens because of change in direction of flow of jet due to chamfering. The heat transfer results are discussed in terms of Nusselt number and temperature contours. A maximum variation of 22.8% in average Nusselt number is obtained between both the configurations while varying the gap ratios between 3to 7. An increase of jet-to-jet spacing ratio from 2 to 7 shows improvement in heat transfer by 15.1% and 13.2% for non-chamfered and chamfered configurations respectively.
... En effet, la forme de la bouche de soufflage a un effet très déterminant sur la turbulence et les conditions initiales du jet (le profil de la vitesse en sortie par exemple). Plusieurs catégories de bouches de sorties de jets peuvent être rencontrées dans les applications des jets : les géométries à fentes (Ichimiya et Hosaka (1992)), les bouches à convergent (Huber et Viskanta (1994)), les formes elliptiques (Arjocu et Liburdy (1999)) et bien d'autres types. ...
Self-sustaining sounds related to aero-acoustic coupling occurs in impinging jets when a feedback loop is present between the jet exit and a slotted plate: the downstream-convected coherent structures and upstream-propagating pressure waves generated by the impingement of the coherent structures on the plate are phase locked at the nozzle exit. The upstream-propagating waves excite the thin shear layer near the nozzle lip and result in periodic coherent structures. The period is determined by the convection speed of the coherent structures and the distance between the nozzle and the plate. Simultaneous measurements of the velocity fields and the acoustic waves in a plane jet impinging a slotted plate were performed using time-resolved particle image velocimetry (PIV) and a microphone. A better understanding of the flow physics and the aero-acoustic coupling are obtained thanks to spatio-temporal cross-correlations between the transverse velocity and the acoustic signals. Cross-correlations are calculated using two different methods: classical analysis of the original signals and by developing a pre-whitening technique. The latter method is useful for analyzing small random signals superimposed on a high amplitude pure tone.
... string sNumber = "Null"; string sFinal = "Null"; int iFrameTotal The function is passed the new window // size as integers // Output: None, the viewvolume and viewport are // reshaped by reference. The variables fRb, // fLb, fTb, fBb, and fov are updated //*************************************************************** { printf("Reshaping\n"); VPScale[2] = 1.0; // set the scale for viewport #3 }// end myGlutReshape void myMouse(int button, int state, int x, int y) //=============================================================== // myMouse Function // Purpose: This function will handle the mouse clicking // Function from the "Joe Graphics" code // provided by Dr. Bailey // Input: Function is called when the mouse button // is depressed // Output: None, the Rx and Ry parameters will be updated //*************************************************************** { printf(" iLowerColor = 0; // lower shading bound iUpperColor = 800; // upper shading bound camRotMat[1] = camRotMat[2] = camRotMat[3] = 0.; camRotMat[4] = camRotMat[6] = camRotMat[7] = 0.; camRotMat[8] = camRotMat[9] = camRotMat[11] = 0.; camRotMat[12] = camRotMat[13] = camRotMat[14] prompt sent to terminal alerting the user of // change and a redraw is called. //*************************************************************** { cout << "Changing display style.\n"; ...
Graduation date: 2010 A purpose built facility was designed, built and operated at the Advanced Thermal Hydraulic Research Laboratory, which is located at Oregon State University. This facility was used to heat a plate of Alloy 304 Stainless Steel to 800 °C and then cool the plate using a fine mist of water droplets. During the cooling process, the temperature of the plate was measured using an infrared camera. Using the experimental data an inverse calculation was performed to determine the heat transferred from the test plate. The experimental heat transfer data was used to develop a set of piecewise equations that describe the heat transfer from the test plate with a given flow rate over the four boiling regimes.
... t correlation between the surface temperature and the flow structure, control of heat transfer implies control over the formation and breakdown mechanism of large-scale eddies. For impinging jets, different strategies have been studied to accomplish this. Among these is the adaptation of the shape of the orifice or nozzle from which the jet issues. Arjocu & Liburdy (1999) studied the dynamics of a three-by-three elliptic jet array. Using flow visualizations and PIV, the effects of jet aspect ratio, inter-jet spacing, and orifice-to-impingement-plate on the integrated surface-layer vorticity and turbulent length scales were investigated at low Reynolds numbers (300 to 1500). Although the use of vorticity ...
Thermal imprint and heat transfer on the target surface in multiple impinging jet arrays have been investigated in conjunction with flow pattern and large-scale eddy structure. Surface temperature was measured with liquid crystal thermography (LCT) in a range of jet configurations with hexagonal and in-line orifice arrangements for different combinations of distances between the orifices and between the orifice plate and the impingement surface (where is the orifice diameter). The hexagonal arrangement was considered with two different orifice shapes: sharp-edged and contoured. For selected configurations, the distribution of Nusselt number and its peculiarities were analysed in relation with the underlying eddy structure educed by proper orthogonal decomposition (POD) of the snapshots of fluid velocity measured with particle image velocimetry (PIV). Owing to the breakdown of the jets, heat transfer deteriorates with increasing orifice-to-plate distance. The jet interaction and breakdown become more severe as the jets are placed closer to each other. The large-scale eddies originating from the jet-edge shear layers grow as they are convected towards the impingement plate. Eddies of sizes between 0.2 and 0.3 orifice diameters are shown to break up the jets and cause mixing of fresh and spent fluid, lowering the beneficial temperature gradient. In some configurations, an asymmetric flow pattern is generated with embedded weak eddies on only one side of the diagonal symmetry line, which is reflected in an asymmetrical heat transfer distribution on the impingement plate. For , the Nusselt number shows peak values in and around the jet impingement centres, but relatively uniform distribution of turbulence kinetic energy with local negative energy production close to the impingement surface.
... The shape of a nozzle has a profound effect on the initial velocity and turbulence profiles of the jets issuing from these nozzles. Slot nozzles (Ichimiya and Hosaka, 1992), rosette nozzles (Huber and Viskanta, 1994b), elliptical nozzles (Arjocu and Liburdy, 1999), and other exotic nozzles are not discussed in this thesis; the focus will be on round nozzles only. Figure 2.3 shows sketches of the mean velocity and turbulent kinetic energy profiles for three different round nozzles. ...
Because of their high efficiency and their ability to provide high heat transfer rates, impinging jets are applied for rapid cooling and heating in a wide variety of industrial processes. However, the physical phenomena controlling the heat transfer from impinging jets are to a large degree unknown. The goal of the present project was to gain a better understanding of the interaction between the flow and heat transfer in impinging jet arrays. Experiments were performed in two different configurations: a single impinging jet and multiple impinging jet arrays. LDA and PIV velocity measurements in the single jet were compared: these measurements were aimed at serving as a reference for comparison of the multiple jet features. In a hexagonal and an in-line array of jets PIV was used to provide instantaneous velocity fields over the flow area of interest, what proved to be essential for detecting some salient features of the multiple-jet dynamics. These dynamics were investigated on the basis of POD filtered snapshots of the flow. In both arrays, large scale eddies in the development zone cause the impinging jets to break up or be severely displaced in the out-of-plane direction. A horse-shoe vortex appears around the outer jets of the hexagonal array, whereas the in-line array does not show this feature. This is most likely caused by the higher pitch in the in-line configuration. On the other hand, the flow field in the in-line arrangement appears to be diagonally asymmetrical, even though the symmetry of the nozzle arrangement would imply a symmetrical flow field. This flow asymmetry causes an elliptical heat transfer pattern in the impingement zone of the center jet. The flow asymmetry was also found in numerical simulations of a similar configuration, conducted in a parallel project. Heat transfer measurements were performed in the impingement surface for seven multiple jet arrays using LCT, which provided insight in the influence of different geometrical parameters of the arrays on the heat transfer. Additionally, a non-dimensional correlation was derived linking heat transfer to geometrical and flow parameters. Heat transfer profiles were subsequently compared to velocity and turbulence quantities just above the impingement plate to investigate the effect of the mean flow features on the heat transfer. The apparent correlation between the Nusselt number and the normal contribution to the production of turbulent kinetic energy is coincidental: the cause of the increased Nusselt number in the impingement points of the jets is a combination of an oscillating impingement position due to large vortical structures and a very strong acceleration of the fluid from the impingement point outwards. In the impingement point the boundary layer is very thin and the temperature gradient is very steep, enabling impinging vortices to encapsulate and remove heat in an effective way. The results presented in this thesis should aid in better understanding the role of turbulence and vortical structures in the interaction between adjacent impinging jets, and in the heat transfer from impinging jets. With the advent of a measurement technique capable of capturing instantaneous heat transfer distributions, a potential extension of the present work is a more detailed analysis of the influence of individual vortical structures on impingement heat transfer.
This paper describes the heat transfer and flow characteristics of a single circular laminar impinging jet including buoyancy force in a comparatively narrow space with a confined wall. Temperature distribution and velocity vectors in the space were obtained numerically by solving three-dimensional governing equations for the Reynolds number Re (=u(m)D/v) =400-2000 and the dimensionless space, H (=h/D) =0.25- 1.0. After impingement, heat transfer behavior on the impingement surface is divided into a forced convection region, a mixed convection region, and a natural convection region in the radial direction. The local heat flux corresponding to these three regions was visualized using a thermosensitive liquid crystal. Moreover with the increase in Reynolds number, Re, and dimensionless space, H, the recirculation flow on the impingement surface moves downstream and its volume increases correspondingly. The Nusselt number averaged from r=0 to the minimum point of peripherally averaged Nusselt number Num, was evaluated as a function of Re and H. [DOI: 10. 1115/1.152790]
The characteristics of the impinging heat transfer of a three-by-three square array of submerged, elliptic impinging jets was studied. Low Reynolds number conditions, 300 to 1500, are considered for two different elliptic jet aspect ratios, with the impingement distance ranging from 1 to 6 jet hydraulic diameters. A transient thermochromic liquid crystal method was used to map the local heat transfer coefficient distribution. The results are reported for the unit cell under the center jet and detail the mean heat transfer as well as the characteristics of the spatial variation of the heat transfer coefficient. The average heat transfer is found to depend inversely on the elliptic jet aspect ratio at these low Reynolds numbers. Distributions of the heat transfer coefficient, h, are also used to obtain proper orthogonal decompositions of h which are used to identify major spatially distributed features.
The flow and heat transfer characteristics of confined jet array impingement with crossflow is investigated. Discrete impingement pressure measurements are used to obtain the jet orifice discharge flow coefficient. Digital particle image velocimetry (DPIV) and flow visualization are used to determine the flow characteristics. Two thermal boundary conditions at the impinging surface are presented: an isothermal surface, and a uniform heat flux, where thermocouple and thermochromic liquid crystal methods were used, respectively, to determine the local heat transfer coefficient. Two nozzle geometries are studied, circular and cusped ellipse. Based on the interaction with the jet impingement at the surface, the crossflow is shown to influence the heat transfer results. The two thermal boundary conditions differ in overall heat transfer correlation with the jet Reynolds number. Detailed velocity data show that the flow development from the cusped ellipse nozzle affects the wall region flow more than the circular nozzle, as influenced by the crossflow interactions. The overall heat transfer for the uniform heat flux boundary condition is found to increase for the cusped ellipse orifice.
The flow structure generated by circular and oblate shaped nozzles for an impinging confined 7-by-7 jet array is investigated. Instantaneous velocity fields, obtained from Digital Particle Image Velocimetry (DPIV) along the crossflow direction are analyzed using Proper Orthogonal Decomposition (POD). Also, a vortex detection algorithm is used to locate and quantify the nature of the instantaneous vortices within the flow. The results show that an oblate shaped nozzle when oriented with its major axis aligned with the exhaust flow has flow characteristics resulting in increased turbulent kinetic energy. This has potential for increased surface transport.
This paper summarizes experimental studies of incompressible elliptic jets of different aspect ratios and initial conditions, and effects of excitations at selected frequencies and amplitudes. Elliptic jets are quite different from the extensively studied plane and circular jets - owing mainly to the fact that the azimuthal curvature variation of a vortical structure causes its non-uniform self-induction and hence complex three-dimensional deformation. Such deformation, combined with properly selected excitation can substantially alter entrainment and other turbulence phenomena, thus suggesting preference for the elliptic shape in many jet applications. The dominance of coherent structures in the jet far field is evident from the finding that switching over of the cross-section shape continues at least up to 100 equivalent diameters De. The locations and the number of switchovers are strongly dependent on the initial condition, on the aspect ratio, and, when excited, on the Strouhal number and the excitation level. We studied jets with constant exit momentum thickness θe, all around the perimeter, thus separating the effects of azimuthal variations of θe, (typical of elliptic jets) and of the shear-layer curvature. Also investigated are the instability characteristics, and enhanced entrainment caused by bifurcation as well as pairing of vortical structures. We discuss shear-layer and jet- column domains, and find the latter to be characterized by two modes : the preferred mode and the stable pairing mode - similar to those found in circular jets -both modes scaling on the newly-defined lengthscale De. The paper documents some time- average measurements and their comparison with those in circular and plane jets.
A passive technique of increasing entrainment was found by using a small-aspect-ratio elliptic jet. The entrainment ratio of an elliptic jet was several times greater than that of a circular jet or a plane jet. The self-induction of the asymmetric coherent structure caused azimuthal distortions which were responsible for engulfing large amounts of surrounding fluid into the jet. In an elliptic jet, an interesting feature in the initial stability process is that the thickness of the shear layer varies around the nozzle. The data indicated that instability frequency was scaled with the thinnest initial momentum thickness which was associated with the maximum vorticity. Turbulence properties were also examined and were found to be significantly different in the major- and minor-axis planes.
The problem of a periodic planar arrangement of a large number of co-flowing, interacting jets is investigated. It is shown that this interaction gives rise to strong nearfield oscillations of large-scale spatial coherence and to far-field inhomogeneities. In the experiments performed, the jets were produced behind a flat plate perforated by holes arranged in a square or triangular periodic pattern and placed perpendicular to a uniform flow. At moderate Reynolds numbers, the interaction results in a remarkable low-frequency oscillation of the merging distance of the jets downstream of the plate. A detailed description of the recirculating flow in the cavities between the jets emphasizes the role of the backflow in the cavities on the oscillatory behaviour. This description is supported by measurements of the local fluctuating velocity and pressure, two-point correlation measurements and quantitative flow visualizations. These experimental observations suggest a new formulation for the instability dynamics of such unstable recirculating flows. This formulation, based on the nonlinear delayed saturation of the jet's shear layer instability (NLDS model) predicts successfully the dependence of the oscillation of the merging distance on the jet Reynolds number and on the local geometrical features of the confinement of the jets. Furthermore, it is shown that the diffusion of mass coming from one jet, seeded with an inert dye, gives rise to an exponential diffusion front over a distance corresponding to a few mesh sizes indicating a strong local coupling of the jets. At the scale of the whole jet assembly, the oscillations are organized as large-scale travelling waves, propagating from the boundaries of the domain to its centre. This symmetry-breaking property is discussed and supplemented by a spatio-temporal simulation of an array of coupled oscillators.
Our direct numerical simulations show that a coherent structure (scrC) in an initially fine-scale homogeneous, isotropic turbulent field breeds secondary structures in its vicinity. These are organized. Shaped like concentric spiral threads perpendicular to the axis of scrC, each is found to be highly polarized with the azimuthal vorticity component being dominant. The threads occur in pairs, and the polarization typically alternates between adjacent threads. Above a critical value (~=1000) of Re≡GammascrC/nu (GammascrC is circulation, nu is kinematic viscosity) a small number of circulation-rich threads emerge as a result of the evolution. The secondary structures are of both practical and theoretical importance. For Re>Recrit, the strongest threads excite bending waves on the axis of scrC. For Re>>Recrit, scrC is eventually destroyed. We believe that this feedback phenomenon is of critical importance for the rearrangement of coherent structures (CS) and transition to turbulence in shear flows such as plane, circular, and elliptic jets. Turbulent mixing near scrC is due to entrainment and ejection of fluid by the threads. Local isotropy assumption cannot be applied near a CS, because our results show anisotropy in a layer surrounding scrC. The threads are shown to be the combined result of three mechanisms: (1) azimuthal alignment of small-scale vorticity by the strain rate field of scrC; (2) merger (pairing) and axisymmetrization as in two-dimensional turbulent flows, enabled by the alignment; and (3) polarization by differential rotation.
The dynamics of the jet column mode of vortex pairing in the near field of an elliptic jet was investigated. Hot-wire measurements and flow visualization were used to examine the details of the pairing mechanism of nonplanar vortical elliptic structures and its effect on such turbulence measures as coherent velocities, incoherent turbulence intensities, incoherent and coherent Reynolds, stresses, turbulence production, and mass entrainment. It was found that pairing of elliptic vortices in the jet column does not occur uniformly around the entire perimeter, unlike in a circular jet. Merger occurs only in the initial major-axis plane. In the initial minor-axis plane, the trailing vortex rushes through the leading vortex without pairing and then breaks down violently, producing considerably greater entrainment and mixing than in circular or plane jets.
Late transitional and turbulent flows in the mixing-layer region of a round jet are investigated for a range of Reynolds numbers by using flow-visualization and hotwire techniques. Attention is focused on the vortices in the transition region and the large eddies in the turbulent region. The interaction and coalescence of vortex rings in the transition region are described. The transition region is characterized by a growth of three-dimensional flow due to a wave instability of the cores of the vortex rings. The merging of these distorted vortices produces large eddies which can remain coherent up to the end of the potential-core region of the jet. A conditional sampling technique is used to measure eddies moving near the jet centre-line. These eddies differ significantly from the ring vortices as they are three-dimensional and contain irregular small-scale turbulence. However, when averaged, their structure is similar in cross-section to that of a vortex ring. These sampled eddies contribute greatly to local velocity fluctuations and statistical correlations. The experiments indicate a need for careful consideration of the meanings of terms such as ‘vortex’, ‘eddy’ and ‘turbulent flow’. In particular care must be taken to discriminate between the orderly, easily visualized, vortices in the transition regions of free shear flows and the less clearly visualized, but strong, large eddies in the fully developed turbulent regions.
The available literature in the area of flow structures associated with impinging jet flows reveals that the large scale structures are an important mechanism in enhancing the transport phenomena between the jets and the impingement surface. In this study, naturally occurring structures in jet arrays are investigated in contrast to other studies which have examined externally forced single jets. In this experimental work a three-by-three submerged elliptic water jet array is used. The dynamics of the flow structures of the central jet of the array are investigated. The hydrodynamic conditions are limited to low Reynolds numbers (Re=300 and 1500) and impingement distances from one to five jet hydraulic diameters. Elliptic jet aspect ratios, for this study, are two and three. Particle Image Velocimetry (PFV) is used in the quantitative analysis of the large scale flow structures. The vorticity distribution in the jet and along the impingement surface is calculated and is found to be associated with the influence of the hydrodynamic conditions on the features of the flow structures. In addition, a modified circulation is used to quantify the flow structures strength.
Accurate numerical simulations of vortex rings impinging on flat boundaries revealed the same features observed in experiments. The results for the impact with a free-slip wall compared very well with previous numerical simulations that used spectral methods, and were also in qualitative agreement with experiments. The present simulation is mainly devoted to studying the more realistic case of rings interacting with a no-slip wall, experimentally studied by Walker et
al. (1987). All the Reynolds numbers studied showed a very good agreement between experiments and simulations, and, at Rev > 1000 the ejection of a new ring from the wall was seen. Axisymmetric simulations demonstrated that vortex pairing is the physical mechanism producing the ejection of the new ring. Three-dimensional simulations were also performed to investigate the effects of azimuthal instabilities. These simulations have confirmed that high-wavenumber instabilities originate in the compression phase of the secondary ring within the primary one. The large instability of the secondary ring has been explained by analysis of the rate-of-strain tensor and vorticity alignment. The differences between passive scalars and the vorticity field have been also investigated.
Measurements of mean velocity, spread rate, and merging point location were made to investigate the flowfield generated by four identical jets of air in a square configuration, issuing from axisymmetric nozzles in a common end wall and mixing with the ambient air. The effect of stagnation pressure ratios as well as the nozzle spacings on the flowfield behavior were investigated. The effect of nozzle configurations was also studied. The results show that the nature of the mean velocity profile as well as the mean velocity decay are unaffected by the pressure ratio. The nozzle spacing effect on the mean velocity decay is only marginal. The four jets interact with axis switching at the midpoint between them, which implies better mixing.
The resonance phenomenon which occurs when a high speed turbulent jet impinges on a large flat plate is experimentally studied. The far field noise is also investigated for a wide range of impinging jet speeds. The study involves far field, near field, and surface pressure measurements. A feedback of pressure waves between the nozzle and the plate is found to be the mechanism responsible for the resonance. The feedback loop is formed by downstream traveling large coherent structures, and upstream acoustic waves propagating in the nearby stagnant region around the jet. The upstream acoustic waves are generated by the impingement of large scale structures on the plate. Near the nozzle exit these waves are locked in-phase with the oscillations of the thin shear layer, which is forced to roll into large eddies at the resonance frequency f(r). Furthermore, the experimental data revealed the following interesting results: (1) The total period of the feedback loop is an integer N times the resonance period, N being constant over a resonance frequency stage. A jump from one frequency stage to another (occurring at certain nozzle-to-plate separation distances), takes place through a quantum unit change in N, so as to preserve the phase lock between the two oscillations at the nozzle. (2) As the wavy shear layer evolves into large vortices, a strong interaction develops among the initial instability vortices. This phenomenon is referred to as 'collective interaction', and is believed to be the mechanism responsible for the sharp drop in the passage frequency of the vortices near the nozzle exit. The frequency drops from the high instability frequency f(in) to that of the resonance frequencey f(r).
In a high-speed subsonic jet impinging on a flat plate, the surface pressure fluctuations have a broad spectrum due to the turbulent nature of the high-Reynolds-number jet. However, these pressure fluctuations dramatically change their pattern into almost periodic waves, if the plate is placed close to the nozzle (x0/d < 7·5). In the present study extensive measurements of the near-field pressure provide solid support for the hypothesis that a feedback mechanism is responsible for the sudden change observed in the pressure fluctuations at the onset of resonance. The feedback loop consists of two elements: the downstream-convected coherent structures and upstream-propagating pressure waves generated by the impingement of the coherent structures on the plate. The upstream-propagating waves and the coherent structures are phase-locked at the nozzle exit. The upstream-propagating waves excite the thin shear layer near the nozzle lip and produce periodic coherent structures. The period is determined by the convection speed of the coherent structures, the speed of the upstream-propagating waves as well as the distance between the nozzle and the plate. An instability process, herein referred to as the ‘collective interaction’, was found to be critical in closing the feedback loop near the nozzle lip.
The velocity field of a circular water jet impinging onto a flat plate has been measured using particle image velocimetry, or PIV. The velocity field has been recorded at several instants in time, producing thousands of simultaneous two-dimensional velocity measurements for each realization. The instantaneous velocity, vorticity and rate-of-strain fields reveal the interaction of vortices near the impinging wall within the radial wall jet downstream from the stagnation point. An ensemble average of the instantaneous fields produces a mean velocity field of the jet flow, which reveals many of the processes leading to boundary layer separation and vortex breakaway within the wall jet. The PIV system extracts the velocity measurements using a two-dimensional autocorrelation method, and can obtain thousands of highly accurate velocity measurements within a few minutes. The structure found in these experiments may be similar to the ground level structure of atmospheric microburst phenomena.
Flow Structures in an Impinging Elliptic Jet Array
- S C Arjocu
- J A Liburdy