The development of the large-scale structures in round impinging jets exiting long pipes at two Reynolds numbers

Measurements of the two-point and two-time correlation of the fluctuating wall pressure were performed in radial wall jets formed by impinging jets exiting a long pipe with Reynolds numbers of 23,300 and 50,000, and nozzle-to-plate distances of 2.0 diameters. The results from the two flows were compared to examine how the three-dimensionality and the development of the large-scale structures in the near field of the radial wall jet changed as the Reynolds number of the impinging jet varied. The measurements indicated that the large-scale structures were more two-dimensional, more periodic, and more prominent in the higher Reynolds number flow.This revised version was published online in November 2004 with corrections to the acknowledgement.     

Wall jet similarity of impinging planar underexpanded jets

Velocity profiles and wall shear stress values in the wall jet region of planar underexpanded impinging jets are parameterized based on nozzle parameters (stand-off height, jet hydraulic diameter, and nozzle pressure ratio). Computational fluid dynamics is used to calculate the velocity fields of impinging jets with height-to-diameter ratios in the range of 15–30 and nozzle pressure ratio in the range of 1.2–3.0. The wall jet has an incomplete self-similar profile with a typical triple-layer structure as in traditional wall jets. The effects of compressibility are found to be insignificant for wall jets with Ma < 0.8. Wall jet analysis yielded power-law relationships with source dependent coefficients describing maximum velocity, friction velocity, and wall distances for maximum and half-maximum velocities. Source dependency is determined using the conjugate gradient method. These power-law relationships can be used for mapping wall shear stress as a function of nozzle parameters.     

Variational boundary integral approach for asymmetric impinging jets of arbitrary two‐dimensional nozzle

We consider asymmetric impinging jets issuing from an arbitrary nozzle. The flow is assumed to be two‐dimensional, inviscid, incompressible, and irrotational. The impinging jet from an arbitrary nozzle has a couple of separated infinite free boundaries, which makes the problem hard to solve. We formulate this problem using the stream function represented with a specific single layer potential. This potential can be extended to the surrounding region of the jet flow, and this extension can be proved to be a bounded function. Using this fact, the formulation yields the boundary integral equations on the entire nozzle and free boundary. In addition, a boundary perturbation produces an extraordinary boundary integral equation for the boundary variation. Based on these variational boundary integral equations, we can provide an efficient algorithm that can treat with the asymmetric impinging jets having arbitrarily shaped nozzles. Particularly, the proposed algorithm uses the infinite computational domain instead of a truncated one. To show the convergence and accuracy of the numerical solution, we compare our solutions with the exact solutions of free jets. Numerical results on diverse impinging jets with nozzles of various shapes are also presented to demonstrate the applicability and reliability of the algorithm.     

高压水射流冲击煤体的力学特征

以质量守恒与动量守恒定律为基础, 建立了高压水射流冲击煤体的力学模型。运用此模型分析了高压水射流在冲击煤体的过程中, 未破水体、破碎水体、煤体的破碎区与扩孔区的力学特征, 利用严格的力学守恒关系得出高压水射流冲击煤体的简化常微分方程组。将理论计算结果与现场实验和数值模拟结果进行对比, 结果表明:理论计算结果与数值模拟结果和实验结果基本一致。此模型具有明确的力学意义, 且能够反映真实的冲击过程。     

Computation of normal impinging jets in cross-flow and comparison with experiment

The PHOENICS code has been used to model the flow field surrounding subsonic and underexpanded jets impinging on a ground plane in the presence of a cross-flow, for cases with both a fixed ground plane and a ‘rolling road’. The standard k-ε turbulence model is used, without correction factors. It is confirmed that this overpredicts the free jet entrainment rate; the wall jet spreading rate is slightly underpredicted but the initial thickness is too high. Agreement with experiment is, nevertheless, much better than for previous calculations, showing the importance of the extent of the grid used. The ground vortex formed in cross-flow is shown to move with varying effective velocity ratio and with rolling road operation in the same manner as experimentally observed. Ground vortex self-similarity is also accurately predicted with the numerical modelling.     

Numerical simulation of an impinging jet on a flat plate

Two-dimensional normal impinging jet flowfields, with or without an upper plate, were analysed by employing an implicit bidiagonal numerical method developed by Lavante and Thompkins Jr. The Jones–Launder K–? two-equation turbulent model was employed to study the turbulent effects of the impinging jet flowfield. The upper plate surface pressure, the ground plane pressure and other physical parameters of the momentum flowfield were calculated at various jet exit height and jet inlet Reynolds numbers. These results were compared with those of Beam and Warming's numerical method, Hsiao and Chuang, and others, along with experimental data. The potential core length of the impinging jet without an upper plate is longer than that of the free jet because of the effects of the ground plane, while the potential core length of the impinging jet with an upper plate is shorter than that of the free jet because of the effects of the upper plate. This phenomenon in the present analysis provides a fundamental numerical study of an impinging jet and a basis for further analysis of impinging jet flowfields on a variable angle plate.     

Effect of jet-to-plate spacing in laminar array jets impinging

The rate of heat transfer from a plate due to impinging of an array of jets was investigated. The effect of jet-to-plate spacing in a confined array of impinging laminar square jets was investigated numerically through the solution of Navier Stokes and energy equations. The simulation is carried out for the jet-to-plate spacing between 2 B and 20 B and for jet-to-jet spacing of 4 B, where B is the jet width. Five in-line jets subjected to across-flow were used in this investigation. Also, six different ratios of jet to cross-flow velocity are simulated (0.5, 1.0, 2.5, 5, 7.5 and 10) for the jet Reynolds number of 200. The predicted results show a formation of one or two ground horseshoe vortices between the jets. In addition, a horseshoe vortex forms at different position between the orifice and impinging plates due to the interaction of two jets before they combine. The number of the ground horseshoe vortex and its size are strongly affected by the jet-to-plate spacing and by jet to cross-flow velocity ratio. The effect of jet-to-plate spacing and jet to cross-flow velocity ratio on heat transfer is presented and discussed.     

Time resolved investigations on flow field and quasi wall shear stress of an impingement configuration with pulsating jets by means of high speed PIV and a surface hot wire array

The effects of jet pulsation on flow field and quasi wall shear stress of an impingement configuration were investigated experimentally. The excitation Strouhal number and amplitude were varied as the most influential parameters. A line-array with three submerged air jets, and a confining plate were used. The flow field analysis by means of time resolved particle image velocimetry shows that the controlled excitation can considerably affect the near-field flow of an impinging jet array. These effects are visualized as organization of the coherent flow structures. Augmentation of the Kelvin–Helmholtz vortices in the jet shear layer depends on the Strouhal number and pulsation magnitude and can be associated with pairing of small scale vortices in the jet. A total maximum of vortex strength was observed when exciting with Sr = 0.82 and coincident high amplitudes.Time resolved interaction between impinging vortices and impingement plate boundary layer due to jet excitation was verified by using an array of 5 μm surface hot wires. Corresponding to the global flow field modification due to periodic jet pulsation, the impact of the vortex rings on the wall boundary layer is highly influenced by the above mentioned excitation parameters and reaches a maximum at Sr = 0.82.     

Heat transfer from an obliquely impinging circular, air jet to a flat plate

A series of experiments was conducted for the measurement of local convective heat transfer coefficients for an obliquely impinging circular air jet to a flat plate. In the experiments, the oblique angles selected were 90°, 75°, 60° and 45°, with 90° being a vertical jet. Two different Reynolds numbers of 10,000 and 23,000 were considered for the purpose of comparison with previous data available in the literature. Another parameter varied in the measurements was the dimensionless jet-to-plate distance, L/D. Four values of L/D(2, 4, 7, and 10) were considered in the experiments. The experiments were conducted using the preheated wall transient liquid-crystal technique. Liquid-crystal color changes were recorded with a video system. Local convective heat transfer coefficients were obtained through the surface transient temperatures that were related to the recorded color information. Detailed local heat transfer coefficients were presented and discussed in relation to the asymmetric wall jet upon impingement of the jet flow. Results of experiments show that, for a given flow situation, the point of maximum heat transfer shifts away from the geometrical impingement point toward the compression side of the wall jet on the axis of symmetry. The shift is more pronounced with a smaller oblique angle (larger jet inclination) and a smaller jet-to-plate distance. Comparisons of experimental results with existing heat transfer data for both obliquely impinging jets and vertical impinging jets are made. The effect of oblique angles on heat transfer was assessed.     

Experiments on free and impinging supersonic microjets

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

Large-Eddy Simulations of Forced Three-Dimensional Impinging Jets

Large-eddy simulations of the flow field around twin three-dimensional impinging jets were carried out to simulate the near-ground hover configuration of a vertical takeoff and landing (VTOL) aircraft. Both the impinging jet and the upwash caused by the collision of the wall jets are modeled in this study. The evolution of the vortical structures in the impinging jet flow field, due to the introduction of axisymmetric and azimuthal perturbations at the jet exit, has been investigated. The vortical structures formed in the jet shear layer due to azimuthal forcing, show significant three-dimensional vortex stretching effects when compared to the structures formed during axisymmetric forcing. Breakdown of the large-scale structures into smaller vortices also occurs much earlier during azimuthal forcing. When compared to the upwash formed during axisymmetric forcing, the azimuthally perturbed jet forms an upwash that is less coherent and results in a weaker upload or lift-off force on the aircraft undersurface. Comparison with available experimental data indicates good agreement for the centerline velocity decay, the wall pressure variation and the phase speed of the vortical structures.     

Experimental investigation of an axisymmetric, impinging turbulent jet. 2. Scalar field

In this, the second part of a two-part study of an impinging air jet, measurements of mean and rms concentrations and concentration probability density functions obtained using a Mie scattering technique are reported. Results in the wall jet are in good agreement with earlier data obtained using laser Raman spectroscopy, although differences in the spreading rate of the wall jet do occur, most likely due to buoyancy. The data demonstrate the influence of the recirculation zone, identified in the first part of the study, on the mixing field in causing low levels of jet fluid to persist to large distances from the surface. This finding has important consequences for many mass transfer applications of impinging jets.     

Numerical simulation of two‐dimensional laminar slot‐jet impingement flows confined by a parallel wall

Two‐dimensional laminar incompressible impinging slot‐jet is simulated numerically to gain insight into flow characteristics.Computations are done for vertically downward‐directed slot‐jets impinging on a plate at the bottom and confined by a parallel surface on top. The behaviour of the jet with respect to aspect ratio (AR) and Reynolds number (Re) are described in detail. The computed flow patterns for various AR (2–5) and for a range of jet‐exit Reynolds numbers (100–500) are analysed to understand the flow characteristics. The transient development of the flow is also simulated for AR = 4 and Re = 300. It is found that the reattachment length is dependent on both AR and Reynolds number for the range considered. The correlation for reattachment length is suggested. The maximum resultant velocity V_rmax and its trajectory is reported. A detailed study of horizontal velocity profile at different downstream locations is reported. It is found that the effect of Reynolds number and AR is significant to the bottom wall vorticity in the impingement and wall jet regions. Copyright © 2007 John Wiley & Sons, Ltd.     

Predictive Capabilities of an Improved Cubic k–ε Model for Inert Steady Flows

Through an improved ε transport equation, a major quality enhancement of the cubic k–ε model, earlier developed in[13], is obtained. The ε-equation of [13],yielding good results for wall-bounded and rotating flows, is combined with the one derived by Shih et al. [20], which produces good results for free shear flows (e.g. the plane jet–round jet anomaly is resolved).Results are presented for the following flows: fully developed stationary and rotating channel and pipe, backward-facing step, sudden pipe expansion, smooth channel expansion and contraction, plane and round jet. Heat transfer predictions in turbulent impinging jets are also discussed. Accurate results are obtained for the mean flow quantities for all test cases, without case dependent model tuning. This revised version was published online in July 2006 with corrections to the Cover Date.     

Swirling and Impinging Effects in an Annular Nonpremixed Jet Flame

The effects of swirl and downstream wall confinement on an annular nonpremixed flame were investigated using direct numerical simulation (DNS). Fully three-dimensional parallel DNS was performed employing high-order numerical methods and high-fidelity boundary conditions to solve governing equations for variable-density flow and finite-rate Arrhenius chemistry. Three swirl numbers have been examined: 0 (without swirl), 0.4 and 0.8, while the effects of downstream wall confinement have been examined for swirl numbers 0 and 0.4. Results have been presented in terms of instantaneous and time-averaged flow quantities, which have also been analysed using energy spectra and proper orthogonal decomposition (POD). Effects of swirl on the fluid dynamic behaviour of the annular nonpremixed flame were found to be significant. The fluid dynamic behaviour of the flame is greatly affected by the interaction between the geometrical recirculation zone (GRZ) near the jet nozzle exit due to the annular configuration, the central recirculation zone (CRZ) associated with swirl, the unsteady vortical structures in the jet column due to the shear instability, and the downstream wall confinement. Depending on the degree of swirl, the GRZ near the burner mouth and the CRZ may co-exist or one zone may be overwhelmed by another. At a moderate swirl number, the co-existence leads to a flame with strong reaction attached to the burner mouth; while at a high swirl number, the CRZ dominates over the GRZ. The precessing vortex core was observed to exist in the swirling flow fields. The Nusselt number distribution of the annular impinging flames differs from that of round impinging jets. The POD analysis revealed that wall effects on the flow field are mainly associated with the higher mode numbers.     

Experimental investigation of the interaction between a plane wall jet and a parallel offset jet

The dual-jet flow generated by a plane wall jet and a parallel offset jet at an offset ratio of d/w = 1.0 has been investigated using Particle Image Velocimetry (PIV). The particle images are captured, processed, and subsequently used to characterize the flow in terms of the 2D velocity and vorticity distributions. Statistical characteristics of the flow are obtained through ensemble averaging of 360 instantaneous velocity fields. Also presented is a time series of instantaneous flow fields to illustrate the dynamic interaction between the two jets. Results reveal that the near field of the flow is characterized by a periodic large-scale Karman-like vortex shedding similar to what would be expected in the wake of a bluff body. The existence of the Karman-like vortices results in periodic interactions between the two jets; in addition, these vortices produce noticeable impact on the jet outer layers, i.e., the free shear layer of the offset jet and the wall boundary layer of the wall jet. A schematic of vortex/shear layer interaction is proposed to illustrate the flow pattern.     

A comparison of flow visualisation and wall pressure measurements for a jet impinging on a plane surface

Several studies of jets impinging on a plane surface have already been made. This paper suggests a new approach to studying impingements of jets. Comparisons have been made between visualisation results and wall pressure measurements. It is shown that only one of these two techniques is sufficient for characterising the flow nature near the wall. Visualisations can be sufficient for determining the location of wall pressure maxima and minima. Detachments and reattachments of the flow are thus located, and the main characteristics of a jet impinging on a plane wall can be shown by simpler experiments such as the spreading over method. Received: 15 October 1998/Accepted: 27 July 1999     

The preheated Airy wall jet

The Airy jet is a wall-bounded flow belonging to the similarity class of the well known free jet but, in contrast to the latter, its far field behavior is an algebraically decaying rotational flow. The velocity and temperature distributions of a preheated Airy jet flowing over an insulated wall are investigated using both analytical and numerical methods, and are compared with those of the classical (preheated) exponentially decaying wall jet. For the same value of the dimensionless skin friction parameter, the maximum of the similar velocity profile of the Airy jet exceeds that of the classical wall jet by approximately 20%. The dimensionless temperature along the insulated wall scales for large values of the Prandtl number with Pr^2/3 for both jets, while for small values of the Prandtl number the temperature scales with Pr^1/3 for the Airy jet and goes to 1 for the classical wall jet.This work is dedicated to Michael B. Glauert who passed away on June 14, 2004     

Predictions of axisymmetric and two-dimensional impinging turbulent jets

The k − turbulence model and a version of a second-moment closure, modified to include the effect of pressure reflections from a solid surface, have been used as the basis of predictions of the flow that results from the orthogonal impingement of circular and two-dimensional (2-D) jets on a flat surface. Comparison of model predictions has been made with velocity measurements obtained in the stagnation and wall jet regions of the impinging flows. Results, in general, confirm the superiority of the Reynolds stress transport equation model for predicting mean and fluctuating velocities within the latter regions of such flows. In particular, modifications to the second-moment closure to account for the influence of the surface in distorting the fluctuating pressure field away from the wall successfully predict the damping of normal-to-wall velocity fluctuations throughout the impinging flows. In contrast, results derived from the eddy-viscosity-based approach do not, in general, accurately reproduce experimental observations.     

Numerical modeling of multi micro jet impingement cooling of a three dimensional turbine vane

This paper reports a numerical investigation on the prediction of the thermal and hydrodynamic flow fields of multi micro jet impingement cooling of three dimensional turbine vanes. A three dimensional vane is modeled with an in-line array of impinging jets of diameters 0.5 and 0.25 mm. The numerical model consists of the steady, Reynolds-Averaged Navier–Stokes equations and the Kω SST Turbulence model. The governing equations are solved using a finite volume method. The crossflow mass velocity (G _c ) to jet mass velocity (G _j ) ratio, and the average and local heat transfer distributions are analyzed with varying mass velocity and jet-to-target spacing. It is found out that a significant decrease in crossflow ratio occurs with the smaller diameters. Due to the lower crossflow and higher exit velocities of the smaller jets, the penetration into the crossflow is much higher. Moreover, at a constant mass flow, the use of micro-jets enhanced the overall average heat transfer coefficient by 63%, while at a fixed pressure drop across the vane instead of the mass flow, the smaller diameters will still yield an enhancement of 34.3% in the overall average heat transfer coefficient.