Flow Field Of Impinging Jet Research Articles

Sınırlandırılmış Çarpan İkiz Jetlerin Akış Yapısı ve Isı Transferi Karakteristikleri Üzerine Bir İnceleme

An investigation into the heat transfer performance and flow characteristics of circular twin jets was conducted using both numerical and experimental methods. The jets emanated from the lower surface and were confined to impinge perpendicularly on the upper surface. To simulate the bidimensional impinging twin jet flow field with powerful turbulence and heat transfer, Realizable k-ε and Standard k-ω turbulence models were employed. Reynolds numbers ranging from 30000 to 50000 were used in both simulations and experiments, with jet-to-jet spacing ranging from 0.5 to 2 and nozzles to target impingement plate spacing in the same range. Sub-atmospheric pressure regions were detected on the impingement surface in experimental measurements for spacing up to 1, whereas numerical results suggested that they could be found at all nozzle to target impingement plate spacings studied. A correlation was discovered between the peaks in coefficients of heat transfer on the target surface and the sub-atmospheric regions in pressure distributions.

Analysis of the Dominant Structures of an Impinging Round Jet at M=0.9

Large-eddy simulation of subsonic round jet impinging on a flat circular plate is performed at Mach number 0.9 and Reynolds number 25,000, based on the diameter and centerline velocity of the jet for two impingement distances h of 6r0 and 8r0 using high-order compact finite difference scheme. The complex flow phenomena associated with the impinging jet flowfield are studied. The power spectral density (PSD) of instantaneous pressure reveals a discrete impingement tone. Dynamic mode decomposition (DMD) of the velocity and pressure is also done for both cases to extract the most dominant modes of the flow. The fundamental impingement tone frequency obtained from the PSD of pressure is identical to that of the first dominant pressure mode obtained from DMD. Both axisymmetric and semihelical modes are present for the h=8r0 case, but only axisymmetric modes are there in the h=6r0 case, and it is noted that the modes corresponding to the fundamental tone have an axisymmetric structure. Also, one dominant mode showing the wall jet structures denoting the liftoff of fluid from the wall after impingement at some radial distance away from the stagnation region is present in both impingement distance cases.

An innovative application of multiple impinging jet flow field in fuel cells

AbstractThe flow field in a fuel cell is expected to distribute the reactants as uniformly as possible over the active plate, support the reasonable pressure drop across the channel and maximize the mass transfer through the catalyst layers. To simultaneously accomplish these requirements, an innovative multiple impingement jet flow field (MIJFF) is proposed in this study. A three‐dimensional thermo‐fluid simulation is used to evaluate the proposed idea and compare its performance against the commonly used parallel field flow (PFF). The domain of calculations includes a channel with multiple impingement jets linked to a porous gas diffusion layer under low Reynolds flow conditions. The results reveal that the suggested MIJFF design significantly increases the transport of the reactant gases through the catalyst layer. The penetration depth into the catalyst layer in the MIJFF arrangement is higher than that of the PFF setup and the use of the catalyst layer is optimized, which in turn can lead to a reduction of the activation drop. Compared to a PFF design under similar operating conditions, the mean Nusselt number is shown to increase by a factor of about 3.5 in the MIJFF setup. Furthermore, the temperature is more uniformly distributed in the MIJFF pattern which results in more effective distribution of the reactant gases over the active surface. The current study shows that under equal pressure drop conditions, the MIJFF exhibits a much higher performance than a PFF channel design. That is while the corresponding flow rate for the MIJFF is much lower than that of PFF.

Dynamics and Response to Control of Single and Dual Supersonic Impinging Jets

An impinging jet produces a highly unsteady flowfield which results in very high noise levels, harmful structural vibrations, and significant loss of lift and stability in applications such as the short takeoff and vertical landing aircraft. An understanding of the flowfield and implementation of an effective flow control technique are necessary to reduce these adverse effects. The presence of multiple impinging jets leads to the generation of a fountain flow, which further adds to the complexity of the flowfield dynamics. In the present study, flowfields of a single supersonic round jet (Mach 1.5) and dual impinging jets (consisting of a Mach 1.5 supersonic jet and an additional sonic jet) are explored. A microjet-based control method is applied to attenuate flow unsteadiness by modifying shear layer instability characteristics of the supersonic jet. Passive and active influence of the microjet hardware on acoustics are examined over a range of impingement distances (2.25D–10D) with a fine resolution (D/16) to obtain detailed resonance characteristics. Based on the resonance tone behavior as a function of impingement distances, detailed flowfield characteristics at certain impingement distances are investigated with the help of schlieren visualization, near-field acoustics, and unsteady surface pressure measurements. The effectiveness of the microjet flow control method in reducing noise is also evaluated in the flowfield of single and dual impinging jets. For single and dual impinging jets configuration, relative to their respective baseline cases, the microjet control on dual impinging jets shows better noise attenuation of overall sound pressure level within short impingement distances ().

Flow and heat transfer characteristics of turbulent swirling impinging jets

• PIV to resolve the time-averaged flow field of turbulent swirling jets. • Flow field of free and impinging swirl jets are compared. • Flow features correlated to heat transfer characteristics. • Vortex breakdown examined at Re = 11,600–35,000 at low and high swirl. This paper addresses both the upstream and downstream flow domain in turbulent impinging jets, which have widespread applications in cooling, heating, and drying. The role of impingement on swirl induced vortex breakdown as well as the interplay between heat transfer (at the impingement plane) and the downstream flow domain are analysed. Particle Image Velocimetry (PIV) is used to resolve the flow field in a range of jets, covering three Reynolds numbers (Re = 11,600, 24,600, and 35,000), two swirl numbers (S = 0.3 and 0.74), and two impingement distances (H/D = 2 and 4) as well as free jets. With regard to the upstream flow field at low swirl (S = 0.3), both free and impinging jets do not exhibit vortex breakdowns. However, for high swirl (S = 0.74), vortex breakdown in the upstream is observed at Re = 11,600 and Re = 24,600, but not Re = 35,000. Impingement distance affects the size, position, and strength of the recirculation bubble (vortex breakdown). The recirculation bubble is more spherical and stronger at Re = 11,600 in contrast to the Re = 24,600. Near-field impingement (H/D = 2) significantly affects the size and shape of the bubble. In free jets, the width of the vortex breakdown bubble at Re = 11,600 spans −0.4 < r/D < 0.4, but shrinks to −0.2 < r/D < 0.2 at Re = 24,600. Correlating the velocity field near the impingement plane with the heat transfer characteristics shows that at low swirl conditions (S = 0.3), that do not lead to upstream vortex breakdown, the spatial extent of a recirculation zone stabilised onto the impingement plane reduces in size at a higher Reynolds number for both H/D = 2 and 4. However, higher swirl conditions (S = 0.7) which typically lead to upstream vortex breakdown, do not coincide with the appearance of any downstream recirculation zones that stabilise onto the impingement plane.

Flowfield Characteristics of a Supersonic Jet Impinging on an Inclined Surface

The study of the flowfield and noise characteristics of supersonic impinging jets is important as these configurations can be found in many engineering applications, such as short takeoff and landing vehicles, hot surface cooling mechanisms, cold gas dynamic spray processes, and turbomachinery systems. This study experimentally investigates the flowfield characteristics of supersonic jets impinging on an inclined surface with nozzle-to-plate distances of and ( is the jet hydraulic diameter) and nozzle pressure ratios () of 3.7 and 5.9. The two-dimensional two-component velocity fields were acquired in the central plane of the test nozzle and along the 45° inclined impingement surface by using a planar particle image velocimetry (PIV) technique. The flow characteristics of supersonic impinging jets, such as mean velocity and turbulent kinetic energy, were computed from the acquired PIV velocity vector fields, and the statistical profiles were compared to study the effects of the impingement surface and on the flow characteristics. The obtained statistical results have shown the presence of shock cells near the nozzle outlet, oblique plate shocks near the impingement, and tail shocks along the inclined surface. In addition, spatial two-point cross-correlations of turbulent velocity were computed using the PIV velocity vectors to study the sizes, shapes, and orientation of turbulent flow structures in the impinging jets. Finally, a proper orthogonal decomposition analysis was performed on the collections of velocity snapshots extracting the coherent flow structures of a supersonic jet impinging on an inclined surface.

Unsteady velocity measurements of model-scale supersonic exhaust jets in military-relevant configurations

Of the utmost importance is the need to better understand the high temperature, high velocity flow fields generated by military tactical aircraft during “run up” and take-off that gives rise to extremely hazardous conditions for personnel and equipment within the vicinity of the aircraft. The present study aims to fill the need for high frequency, two velocity component measurements throughout the flow fields produced by university-scale supersonic jets exhausting from nozzles in configurations relevant to practical, full-scale application. Specifically, this work focuses on studying the supersonic jets operating in two basic configurations: horizontal, free jets and jets impinging normal to a ground plane reminiscent of current short-takeoff and vertical landing aircraft. Experiments are conducted at nozzle operating conditions similar to those of full-scale aircraft. Both mean velocities and turbulence components are measured in both flow fields using a laser Doppler velocimeter. Axial components of the mean flow and turbulence are measured in the free jet. In the single impinging jet flow field two-component mean velocity and turbulence components are measured in the jet plume, impingement region, and outwash flow. Free jet velocity measurements show good consistency with 50% increase in jet Reynolds number. Turbulence intensities up to 15% of the mean jet exit velocity are observed at the nozzle exit plane. Laser Doppler measurements in the outwash of an impinging jet show turbulent fluctuations produce unsteady velocities well above the mean value. Two-component impinging jet unsteady velocity spectra show a distinct peak at the same frequency as the impingement tone observed in prior impinging jet acoustic field measurements.

Noise characteristics and flow field of an impinging jet on a conical obstacle

The noise characteristics and flow field of a low-speed impinging jet on a conical obstacle have been numerically simulated using the kinetic energy transport subgrid-scale model of the large-eddy simulation method. Noise measurement is carried out to validate the proposed simulation method. The effects of the impinging distance on the development, separation and diffusion of vortices on the back of the conical obstacle are investigated. The jet structure is better preserved and the vorticity value becomes larger as the impinging distance increases. Simulation results of the noise spectrums and overall sound pressure level (OASPL) agree well with the experimental data. The noise spectrums are analyzed and combined with simulation results for the flow field. When the impinging distance is small, the main acoustic sources with the broadband characteristic consist of the dipole source produced by pressure fluctuation on the surface of the obstacle, and the quadrupole source produced by vortices. As the impinging distance increases, the quadrupole source becomes the major acoustic source with low-frequency characteristics. In addition, the OASPL of the impinging jet is obtained at different impinging distances.

半円筒内面への衝突噴流の壁面せん断応力の測定

A jet was issued from an exit with aspect ratio 16 opened out by the convex of the semicylinder and it impinged onto the semicylinder inside. The impinging jet flow field was controlled with two pieces of semicircular plates which were installed parallel to sandwich the long side of the exit. The static pressure and the shear stress on three places of the circumference direction of the impingement surface were measured when the distances between two pieces of semicircular plates were changed. The static pressure values in the stagnation region become higher than the values for the absence of semicircular plates. The static pressure and shear stress measured in three places of the circumference direction of the inside becomes the same values in the stagnation region along the span direction, respectively.

Parametric analysis of a round jet impingement on a heated plate

In this study, the flow and heat transfer characteristics of a round air jet have been experimentally investigated in details using two techniques: Particle Image Velocimetry (PIV) and the Laser Doppler Velocimetry (LDV). The measurement of the mean velocity components are compared, and agree well with the experimental data obtained by Baydar (1999). The distributions of the velocity, turbulence quantities and temperature profiles are analyzed in the main characteristic regions of the jet where the heat transfer occurs. Parametric variations were conducted to produce information about the influences of the Reynolds number (Re = 1000, 2000, 3000), the distance between the pipe exit and the flat impingement plate (h/d = 1 and h/d = 2) and the temperature of the plane (Tp = 22 °C, 54 °C, 96 °C) on the impinging jet flow field.

Characterization of Near-Field Spray of Nongelled- and Gelled-Impinging Doublets at High Pressure

Atomization mechanism of gelled propellants in an impinging jet flowfield is significantly different from that of nongelled liquid propellants and is not clearly understood. This study explores the effect of liquid fluid properties such as viscosity and surface tension on the liquid sheet breakup with a special emphasis on the effect of ambient pressure. A rheologically matched non-Newtonian fluid that is nonreactive and nontoxic is used as a simulant for the gelled hypergolic propellants. Near-field spray characteristics such as the sheet formation and breakup length of the liquid sheet are experimentally determined using shadowgraph. Various sheet breakup regimes have been identified for both nongelled and gelled simulants over a range of flow conditions. For all fluids, the breakup length is found to decrease as the ambient pressure increases. Near-field imaging and its analysis show that the ambient pressure affects jet surface dynamics before impingement by increasing the jet surface disturbance length scale and sheet dynamics after impingement by shortening the surface wavelength, resulting in shorter breakup length with the increase of ambient pressure.

Investigation of impinging jet resonant modes using unsteady pressure-sensitive paint measurements

At given nozzle to plate spacings, the flow field of high-speed impinging jets is known to be characterized by a resonance phenomenon. Large coherent structures that convect downstream and impinge on the surface create strong acoustic waves that interact with the inherently unstable shear layer at the nozzle exit. This feedback mechanism, driven by the coherent structures in the jet shear layer, can either be axisymmetric or helical in nature. Fast-response pressure-sensitive paint (PSP) is applied to the impingement surface to map the unsteady pressure distribution associated with these resonant modes. Phase-averaged results acquired at several kHz are obtained using a flush mounted unsteady pressure transducer on the impingement plate as a reference signal. Tests are conducted on a Mach 1.5 jet at nozzle to plate spacings of \(h/D_{j} = 4\, \text{ and}\, 4.5\). The resulting phase-averaged distribution reveals dramatically different flow fields at the corresponding impingement heights. The existence of a purely axisymmetric mode with a frequency of 6.3 kHz is identified at \(h/D_{j} = 4.5\) and is characterized by concentric rings of higher/lower pressure that propagate radially with increasing phase. Two simultaneous modes are observed at \(h/D_{j} = 4\) with one being a dominant symmetric mode at 7.1 kHz and the second a sub-dominant helical mode at 4.3 kHz. Complimentary phase-conditioned Schlieren images are also obtained visualizing the flow structures associated with each mode and are consistent with the PSP results.

Studies on Microjet Control Effectiveness in High-Temperature Supersonic Impinging Jets

This study examines the influence of jet temperature on the impinging jet flowfield and compares the effectiveness of microjet control for a Mach 1.5 ideally expanded jet. The tests were performed on a 36.2-mm-diam jet of air, heated to stagnation temperatures up to 1200 K. The jet impinged normally onto a traversable ground plane oriented between 1.5 and 12 throat diameters from the nozzle exit. Findings indicate that the impingement tones persist and generally become more prominent at higher temperatures, and the lift loss levels increase. Microjet injection produced substantial reductions of overall noise levels and considerable lift recovery, although the effectiveness is slightly lower for elevated jet temperatures. In the nozzle-to-ground range resulting in the highest pressure fluctuation levels on the lift plate, reductions reached 9 dB for the 1200 K jet and 10 dB for the cold jet. There was consistent behavior regarding the suppression of near-field and far-field overall sound pressure levels, with maximum reductions for the 811 K jet of 7 and 8 dB, respectively. The lift loss at small nozzle-to-ground distances for elevated jet temperatures increased by up to 30% compared to the cold jet, and microjets were able to reduce lift loss by as much as 34% for the 811 K jet.

Convective heat transfer by a row of confined air jets from round holes equipped with triangular tabs

Experimental study was conducted to investigate the heat transfer produced by single row of impinging jets inside a confined channel with different tab orientations of the triangular tabs at the jet exits. The effects of the tab number, tab orientation angle and tab penetration length on the jet impingement heat transfer behaviors were experimentally tested under nozzle-to-plate spacing of 1–3 diameters and jet Reynolds number of 10,000–20,000. A numerical computation was also carried out on the simulation of impinging jet flow fields to reveal the trends and differences observed in the heat transfer measurements. The results show that the presence of tabs increases the jet core velocity and induces array pairs of vortices, and hence enhances the heat transfer in the impingement region over the no-tab case. The tabs oriented upwards or downwards at 45° can better improve the heat transfer than the normal inclination case. The heat transfer enhancement produced by the tabs increases with the tab penetration length for the given range of 0.12–0.23 diameter. For the arrays of 4–12 triangular tabs, the array of 8 tabs seems to produce more effective heat transfer enhancement.

Role of coherent structures in supersonic impinging jets

This paper describes the results of a study examining the flow field and acoustic characteristics of a Mach 1.5 ideally expanded supersonic jet impinging on a flat surface and its control using steady microjets. Emphasis is placed on two conditions of nozzle to plate distances (h/d), of which one corresponds to where the microjet based active flow control is very effective in reducing flow unsteadiness and near-field acoustics and the other has minimal effectiveness. Measurements include unsteady pressures, nearfield acoustics using microphone and particle image velocimetry. The nearfield noise and unsteady pressure spectra at both h/d show discrete high amplitude impinging tones, which in one case (h/d = 4) are significantly reduced with control but in the other case (h/d = 4.5) remain unaffected. The particle image velocimetry measurements, both time-averaged and phase-averaged, were used to better understand the basic characteristics of the impinging jet flow field especially the role of coherent vortical structures in the noise generation and control. The results show that the flow field corresponding to the case of least control effectiveness comprise well defined, coherent, and symmetrical vortical structures and may require higher levels of microjet pressure supply for noise suppression when compared to the flow field more responsive to control (h/d = 4) which shows less organized, competing (symmetrical and helical) instabilities.

Large Eddy Simulations of Supersonic Impinging Jets

Supersonic impinging jet flow fields contain self-sustaining acoustic feedback features that create high levels of tonal noise. These types of flow fields are typically found with short takeoff and landing military aircraft as well as jet blast deflector operations on aircraft carrier decks. The United States Navy has a goal to reduce the noise generated by these impinging jet configurations and is investing in computational aeroacoustics to aid in the development of noise reduction concepts. In this paper, implicit large eddy simulation (LES) of impinging jet flow fields are coupled with a far-field acoustic transformation using the Ffowcs Williams and Hawkings (FW-H) equation method. The LES solves the noise generating regions of the flow and the FW-H transformation is used to predict the far-field noise. The noise prediction methodology is applied to a Mach 1.5 vertically impinging jet at a stand-off distance of five nozzle throat diameters. Both the LES and FW-H acoustic predictions compare favorably with experimental measurements. Time averaged and instantaneous flow fields are shown. A calculation performed previously at a stand-off distance of four nozzle throat diameters is revisited with adjustments to the methodology including a new grid, time integrator, and longer simulation runtime. The calculation exhibited various feedback loops which were not present before and can be attributed to an explicit time marching scheme. In addition, an instability analysis of the heated jets at both stand-off distances is performed. Tonal frequencies and instability modes are identified for the sample problems.

Numerical investigation of the flow behavior of an isothermal impinging jet in a room

The impinging jet concept has been proposed as a new ventilation strategy for use in office and industrial buildings. The present paper reports the mean flow field behavior of an isothermal turbulent impinging jet in a room. The detailed experimental study is carried out to validate the numerical simulations, and the predictions are performed by means of the RNG k− ε and SST k− ω model. The comparisons between the predictive results and the experimental data reveal that both of the tested turbulence models are capable of capturing the main qualitative flow features satisfactorily. However, it is worth to mention that the predictions from the RNG k− ε model predicts slightly better of the maximum velocity decay as jet approaching the floor, while the SST k− ω model accords slightly better in the region close to the impingement zone which is crucial for spreading of the jet inside the room. Another important perspective of this study is to investigate the influence of different flow and configuration parameters such as jet discharge height, diffuser geometry, supply airflow rate and confinement from the surrounding environment on the impinging jet flow field with the validated model. The obtained data are presented in terms of the jet dimensionless velocity distribution, maximum velocity decay and spreading rate along the centerline of the floor. The comparative results demonstrate that all the investigated parameters have certain effects on the studied flow features, and the diffuser geometry is found to have the most appreciable impact, while the supply airflow rate is found to have marginal influence within the moderate flow range.

Relationship between vortex and sound in under-expanded supersonic impinging jets

SUMMARY The study of an under-expanded supersonic jet impinging on a flat plate by using large-eddy simulation is reported. A third-order upwind compact difference and a fourth-order symmetric compact scheme are employed to discretize the nondimensional axisymmetric compressible Favre-filtered Navier–Stokes equations in space, whereas the third-order Runge–Kutta method with the total variation diminishing property is adopted to deal with the temporal discretization. The numerical simulation successfully captures the shock wave and vortex structures with different scales in the flow field. Waves with high and low frequencies traveling forward and reflecting back, and sound sources in different locations can be observed. By comparison with the frequency of the impinging tone from the experiment, it can be deduced that the change of pressure and swirling strength in the shear layer, pressure change on the impinging plate, and vortex merging in the jet shear layer are interdependent with the impinging tone. The effects of nozzle lip thickness on the impinging jet flow field have been investigated. The results show that the values of pressure fluctuation and vortex swirling strength in the shear layer near the nozzle have an extremum with the variation of the nozzle lip thickness. The results provide a theoretical foundation for the design of supersonic nozzles. Copyright © 2011 John Wiley & Sons, Ltd.

Flowfield and Noise Characteristics of Twin Supersonic Impinging Jets

A way of controlling the acoustic and aerodynamic characteristics of ideally expanded impinging twin jets has been proposed. As a means of control, microjets were used at the nozzle exit. The effect of the controlling method on the flowfield and acoustic field was investigated. Particle image velocimetry was used as the main tool for flowfield diagnostics

Experimental and computational investigation of supersonic impinging jets

The results of an experimental and computational study of a moderately underexpanded axisymmetric supersonic jet issuing from a converging nozzle and impinging on a ground plane are presented. The goal of this work is to develop a better understanding of the impinging jet flowfield, which is of significant practical interest because of its presence in short takeoff and vertical landing (STOVL) aircraft during hover as well as in other aerospace-related and industrial applications. The experimental measurements include flow visualization, surface-pressure distributions, and velocity field data obtained using particle image velocimetry (PIV). The experimental data, especially the velocity field measurements, were used to verify the accuracy of computational predictions. Computational results obtained using two different turbulence models produced almost identical results