Phase averaged velocity field in an axisymmetric jet subject to a sudden velocity decrease

An unsteady transient axisymmetric turbulent jet was studied experimentally. The initial flow perturbation consisted of a sudden and large decrease in the ejection velocity. The temporal evolution of the mean and fluctuating unsteady velocity field was measured by using X hot-wire probes. In the jet far field, adaptation of the externally imposed unsteadiness to the local jet time scale is confirmed quantitatively. The main features of the phase averaged velocity field are presented and comments are made about the instantaneous state of the turbulence energetics. Transient mean radial velocities are deduced and an important increase of the instantaneous rate of entraining external fluid into the jet is found. Finally, we show that the pressure effect due to radial impusle terms plays an important role in the propagation of the mean perturbation. The longitudinal adaptation of the perturbation time scale driven by the local jet time scale provides a turbulent flow that is intermediate to quasi-static flows and rapidly distorted flows.We wish to thank Professor H. Fiedler and Professor M. Wolfshtein for their helpful comments about this work. We have benefited greatly from discussions with Dr. H.J. Nuglisch, Professor E.K. Longmire and Dr. A. Sevrain and of the technical support of G. Couteau and J.F. Alquier.     

Unsteady effects in mechanically-excited turbulent plane jets

Two methods of mechanically exciting a plane turbulent free jet are described; periodic perturbatin of the nozzle exit velocity, and forced oscillation of a small vane located in the het potential core. Hot-wire measurements obtained by conditional sampling techniques indicated that the flow fields of the two jets are substantially different although they have the same Strouhal number of 0.0032. While the mean flow development of the pulsed jet can be described adequately by a quasi-steady model, the vane-excited jet exhibits unsteady effects which depart significantly from quasi-steady approximations such as increased entrainment, amplification of excitation and non-linear effects in the form of the presence of high harmonics. The constancy of momentum flux has been examined in both the steady and unsteady jets     

Reynolds number effects on the behavior of a non-buoyant round jet

The behavior of a non-buoyant circular water jet discharged from a contraction nozzle was experimentally investigated. In this experiment, the Reynolds number of the jet, based on the mean velocity results obtained by particle image velocimetry (PIV), ranged from 177 to 5,142. From the experimental results, we found that the cross-sectional profile of the axial velocity for a laminar flow near the nozzle did not show a top-hat distribution, whereas the profiles with Reynolds number higher than 437 were almost top-hat. The length of the zone of flow establishment (ZFE) was found to decrease with increasing Reynolds number. The measured centerline velocity decayed more rapidly and, consequently, approached the theoretical equation earlier near the nozzle as the Reynolds number increased. The decay constant for the centerline velocity of the turbulent cases was relatively lower than that discovered in theory. It is assumed that this probably resulted from the use of the contraction nozzle. Verifying the similarity of the lateral velocity profiles demonstrated that the Gaussian curve was properly approximated only for the turbulent jets and not for the laminar or transitional flows. The jet half width seldom grew for the laminar or transitional flows, whereas it grew with increasing axial distance for the turbulent flows. The spreading rates for the turbulent flows gradually decreased with increasing Reynolds number. The normalized turbulence intensity along the jet centerline increased more rapidly with the axial distance as the Reynolds number increased, and tended to the constant values proposed by previous investigators. The Reynolds shear stress levels were also found to increase as the Reynolds number increased for the turbulent jets.     

Velocity measurements in a precessing jet flow using a three dimensional LDA system

 The turbulent, three dimensional and time dependent flow field of a precessing jet is investigated. In the present case the jet precession is generated by mechanically rotating a round jet inclined relative to the axis of rotation. A conditional flow visualisation technique is used to complement three dimensional laser Doppler velocity data, time-averaged and phase-averaged at the frequency of precession. The conditional phase-averaging technique enables phase-averaged velocity contours and vectors to be obtained which reveal flow patterns and structures within the flow field. Time-averaging of the velocity data shows that these structures are significant in that they generate a reverse flow (recirculation) region between the jet and its spinning axis. They are found also to cause a rapid decay of the mean velocity. The characteristics of the precessing jet found here are compared with previous investigations of the same flow and with investigations of other turbulent jets. Received: 17 March 1995/Accepted: 7 December 1995     

入口速度比对射流拟序结构的影响

为了深入了解湍流流动机理以及湍流拟序结构发现过程的影响因素，本文采用大涡模拟方法对不同入口射流伴流速度比的平面湍射流流动进行了数值模拟。采用分步投影法求解动量方程，亚格子项采用标准Smagorinsky亚格子模式模拟，压力泊松方程采用修正的循环消去法快速求解，空间方程采用二阶精度的差分格式，在时间方向上采用二阶精度的显式差分格式。模拟结果给出了平面射流中湍流拟序结构的瞬态发展演变过程，分析了入口速度比对射流拟序结构发展演化过程及宏观流场形态的影响。为进一步研究射流拟序结构及其在湍流流动中的作用提供了基础。     

Simultaneous measurements of reactive scalar and velocity in a planar liquid jet with a second-order chemical reaction

This paper presents a new experimental approach for simultaneous measurements of velocity and concentration in a turbulent liquid flow with a chemical reaction. For the simultaneous measurements, we developed a combined probe consisting of an I-type hot-film probe and an optical fiber probe based on the light absorption spectrometric method. In a turbulent planar liquid jet with a second-order chemical reaction $(\mathrm{A}+\mathrm{B}\rightarrow \mathrm{R})$ , streamwise velocity and concentrations of all reactive species are measured by the combined probe. The turbulent mass fluxes of the reactive species are estimated from the simultaneous measurements. The results show that the influence of the chemical reaction on the turbulent mass flux of the reactant species near the jet exit is different from its influence in other regions, and the turbulent mass flux of the product species has a negative value near the jet exit and a positive value in other regions.     

Investigation of flows induced by subsonic turbulent jets and their relation with the effect of static pressure reduction in a jet

The interaction between turbulent jets, both swirling and nonswirling, and the ambient medium is studied on the basis of the results of measurements and numerical simulation. It is shown that the turbulent flow and the swirl give rise to induced ejection flow toward the jet. The mechanism of the jet action on the ambient medium is connected with a decrease in the static pressure in the jet, which, in turn, is due to either the flow swirl or the fluctuating flow in the mixing layer, when the static pressure reduces owing to the presence of velocity fluctuations. The former rarefaction mechanism is predominant in swirling jets and the latter predominates in jets without swirling. It is shown that the ambient medium inflow into the jet due to the rarefaction is independent in nature of the mechanism of the lowered pressure generation and that it is the kinetic energy of the jet that is the energy source for the induced flow.     

Layered structure of turbulent plane wall jet

This paper investigates the layered structure of a turbulent plane wall jet at a distance from the nozzle exit. Based on the force balances in the mean momentum equation, the turbulent plane wall jet is divided into three regions: a boundary layer-like region (BLR) adjacent to the wall, a half free jet-like region (HJR) away from the wall, and a plug flow-like region (PFR) in between. In the PFR, the mean streamwise velocity is essentially the maximum velocity, and the simplified mean continuity and mean momentum equations result in a linear variation of the mean wall-normal velocity and Reynolds shear stress. In the HJR, as in a turbulent free jet, a proper scale for the mean wall-normal flow is the mean wall-normal velocity far from the wall and a proper scale for the Reynolds shear stress is the product of the maximum mean streamwise velocity and the velocity scale for the mean wall-normal flow. The BLR region can be divided into four sub-layers, similar to those in a canonical pressure-driven turbulent channel flow or shear-driven turbulent boundary layer flow. Building on the log-law for the mean streamwise velocity in the BLR, a new skin friction law is proposed for a turbulent wall jet. The new prediction agrees well with the correlation of Bradshaw and Gee (1960) over moderate Reynolds numbers, but gives larger skin frictions at higher Reynolds numbers.     

Computations for a jet impinging obliquely on a flat surface

A SIMPLE-C algorithm and Jones-Launder k-ε two-equation turbulence model are used to simulate a two-dimensional jet impinging obliquely on a flat surface. Both the continuity and momentum equations for the unsteady state are cast into suitable finite difference equations. The pressure, velocity, turbulent kinetic energy and turbulent energy dissipation rate distributions are solved and show good agreement with various experimental data. The calculations show that the flow field structure of the jet impinging obliquely on a flat surface is strongly affected by the oblique impingement angle. The maximum pressure zone of the obliquely impinging jet flow field moves towards the left as the oblique impingement angle is decreased.     

Modeling Of A Stable Boundary Layer With A Low-Level Jet

An improved three-parameter model of turbulence is applied to study the characteristics of a flow of a stably stratified boundary layer with formation of a jet in its lower part. The results obtained show that the turbulent Prandtl number increases with increasing thermal stability, which agrees with the results of laboratory experiments and actual observations in the atmosphere. It is also shown that the gradient Richardson number is nonmonotonic in time in the range below the maximum jet velocity.     

Numerical analysis of steady turbulent triple jet flow with temperature difference

The numerical predictions confirm a new classification of flow patterns of triple jet interaction. The addition of side jets increase the rate of decrease of the centreline velocity for the flow of type A and decreases in the other cases. The effect of various types of flow on the rate of decrease of the velocity, the turbulent kinetic energy and the temperature in the combined region are detailed. Several correlations are proposed.     

The turbulent/non-turbulent interface at the outer boundary of a self-similar turbulent jet

The flow at the outer boundary of a submerged self-similar turbulent jet at Re=2᎒³ is investigated experimentally by means of combined particle image velocimetry (PIV) laser-induced fluorescence (LIF) measurements. The jet fluid contains a fluorescent dye so that the LIF data can be used to discriminate between the jet fluid and the ambient fluid. The axial velocity, Reynolds stress, and vorticity are determined relative to the jet boundary. The results are compared against the conventional profiles, and the results of a direct numerical simulation of the turbulent far-wake behind a flat plate. The results show a sharp transition between rotational and irrotational fluid at the fluid interface, and the existence of a layer of irrotational velocity fluctuations outside the turbulent region.     

On the analysis of an impinging jet on ground effects

Laser Doppler measurements and flow visualization are presented for a turbulent circular jet emerging into a low-velocity cross-stream and, then, impinging on a flat surface perpendicular to the jet-nozzle axis. The experiments were performed for a Reynolds number based on the jet-exit conditions of 6 × 10⁴, a jet-to-crossflow velocity ratio of 30 and for an impingement height of 5 jet diameters and include mean and turbulent velocity characteristics along the two normal directions contained in planes parallel to the nozzle axis. The results, which have relevance to flows found beneath VSTOL aircraft in ground effect, show the presence of a complex 3-D scarf vortex formed around the impinging jet. In zones where measurement data are not available, the flow details are numerically-visualized using a solution of the finite difference form of the fully threedimensional Reynolds-averaged Navier-Stokes equations, incorporating the turbulence viscosity concept. The turbulent structure of the flow is affected by flow distortion at the impinging zone, which results in an unconventional behaviour of the dimensionless structure parameters that determine the empirical constants in engineering models of turbulence. The relative magnitude of the terms involved in the transport equations for the turbulent stresses is quantified from the experimental data in order to assess the importance of these effects and show the extent to which the turbulent structure of the impingement zone is affected by extra rates of strain.     

PIV measurements of a turbulent jet issuing from round sharp-edged plate

An experimental investigation is presented of a turbulent jet issuing from a round sharp-edged orifice plate (OP) into effectively unbounded surroundings. Planar measurements of velocity were conducted using Particle Image Velocimetry (PIV) in the near and transition regions. The Reynolds number, based on the jet initial diameter and velocity, is approximately 72,000. The instantaneous and mean velocities, Reynolds normal and shear stresses were obtained. The centerline velocity decay and the half-velocity radius were derived from the mean velocity. It is revealed that primary coherent structures occur in the near field of the OP jet and that they are typically distributed asymmetrically with respect to the nozzle axis. Comparison of the present PIV and previous hot-wire measurements for the OP jet suggests that high initial turbulence intensity leads to reduced rates of decay and spread of the mean flow field and moreover a lower rate of variation of the turbulence intensity. Results also show that self-similarity of the mean flow is well established from the transition region while the turbulent statistics are far from self-similar within the measured range to 16 diameters.     

An experimental study of a radial wall jet formed by the normal impingement of a round synthetic jet

The flow field of a radial wall jet created by the impingement of a round synthetic jet normal to a flat surface was characterized using hot-wire anemometry. In the synthetic wall jets the width of the outer layer was observed to increase linearly with the radial distance along the wall, while the local maximum velocity varied inversely. The synthetic wall jet exhibits self-similar behavior as distinguished by the collapse of the mean and rms velocity profiles when normalized by the outer layer scaling variables. Increasing the actuator driving amplitude at a fixed frequency (i) increased the growth rate of the outer layer, and (ii) decreased the decay rate of the local velocity maximum. The flow field of the synthetic wall jet was dominated by vortical structures associated with the actuator driving frequency, and harmonics connected with the interaction of the produced vortex structures. For the actuator conditions investigated, neither the classical laminar nor fully turbulent analytical solutions for continuous wall jets were amenable to modeling the synthetic wall jet profile due to the transitional and unsteady nature of the synthetic wall jet.     

Measurement of velocity and velocity derivatives based on pattern tracking in 3D LIF images

Pattern tracking in consecutive 3D LIF images based on least squares matching (LSM) of grey levels has been developed recently for velocity and velocity gradient measurements. The shortcomings of this method are clearly shown. The present article presents an improvement on this method by introducing a local multi-patch (LMP) technique through the LSM approach. The method is validated using the flow field of a turbulent channel flow obtained by direct numerical simulation (DNS) and a synthetic image with grey-level patterns. The results show that LMP matching allows the determination of the velocity and the velocity gradient fields with high accuracy including the second derivatives. Measurements of a round non-buoyant jet are presented which demonstrate the good performance of the method when applied under laboratory conditions. This method can also be applied on two-dimensional images provided that the flow is strictly two-dimensional. Received: 7 July 1999/Accepted: 13 November 1999     

Population,characteristics and kinematics of vortices in a confined rectangular jet with a co-flow

Vortex behavior and characteristics in a confined rectangular jet with a co-flow were examined using vortex swirling strength as a defining characteristic. On the left side of the jet, the positively (counterclockwise) rotating vortices are dominant, while negatively rotating vortices are dominant on the right side of the jet. The characteristics of vortices, such as population density, average size and strength, and deviation velocity, were calculated and analyzed in both the cross-stream direction and the streamwise direction. In the near-field of the jet, the population density, average size and strength of the dominant direction vortices show high values on both sides of the center stream with a small number of counter-rotating vortices produced in the small wake regions close to jet outlet. As the flow develops, the wake regions disappear, these count-rotating vortices also disappear, and the population of the dominant direction vortices increase and spread in the jet. The mean size and strength of the vortices decrease monotonically with streamwise coordinate. The signs of vortex deviation velocity indicate the vortices transfer low momentum to high-velocity region and high momentum to the low velocity region. The developing trends of these characteristics were also identified by tracing vortices using time-resolved particle image velocimetry data. Both the mean tracked vortex strength and size decrease with increasing downstream distance overall. At the locations of the left peak of turbulent kinetic energy, the two-point spatial cross-correlation of swirling strength with velocity fluctuation and concentration fluctuation were calculated. All the correlation fields contain one positively correlated region and one negatively correlated region although the orientations of the correlation fields varied, due to the flow transitioning from wake, to jet, to channel flow. Finally, linear stochastic estimation was used to calculate conditional structures. The large-scale structures in the velocity field revealed by linear stochastic estimation are spindle-shaped with a titling stream-wise major axis.     

Time-resolved velocity and concentration measurements in variable-viscosity turbulent jet flow

This paper addresses the ability to reliably measure the fluctuating velocity field in variable-viscosity flows (herein, a propane–air mixture), using hot-wire anemometry. Because the latter is sensitive to both velocity and concentration fluctuations, the instantaneous concentration field also needs to be inferred experimentally. To overcome this difficulty, we show that the hot-wire response becomes insensitive to the concentration of the field, when a small amount of neon is added to the air. In this way, velocity measurements can be made independently of the concentration field. Although not necessary to velocity measurements, Rayleigh light-scattering technique is also used to infer the local (fluctuating) concentration, and, therefore, the viscosity of the fluid. Velocity and concentration measurements are performed in a turbulent propane jet discharging into an air–neon co-flow, for which the density and viscosity ratios are 1.52 and 1/5.5, respectively. The Reynolds number (based on injection diameter and velocity) is 15400. These measurements are first validated: the axial decay of the mean velocity and concentration, as well as the lateral mean and RMS profiles of velocity and concentration, is in full agreement with the existing literature. The variable-viscosity flow along the axis of the round jet is then characterized and compared with a turbulent air jet discharging into still air, for which the Reynolds number (based on injection diameter and velocity) is 5400. Both flows have the same initial jet momentum. As mixing with the viscous co-flow is enhanced with increasing downstream position, the viscosity of the fluid increases rapidly for the case of the propane jet. In comparison with the air jet, the propane jet exhibits: (1) a lower local Reynolds number based on the Taylor microscale (by a factor of four); (2) a reduced range of scales present in the flow; (3) the isotropic form of the mean energy dissipation rate is first more enhanced and then drastically diminishes and (4) a progressively increasing local Schmidt number (from 1.36 to 7.5) for increasing downstream positions. Therefore, the scalar spectra exhibit an increasingly prominent Batchelor regime with a ~ k ^?1 scaling law. The experimental technique developed herein provides a reliable method for the study of variable-viscosity flows.     

Pulsed hot-wire measurements in two- and three-dimensional wall jets

Pulsed Hot-Wire Anemometry (PHWA) measurements are performed in well defined two- and three-dimensional turbulent wall jets. For the two-dimensional wall jet the objective is to study reported differences between conventional Hot-Wire Anemometry (HWA) and Laser Doppler Anemometry (LDA) results. In the three dimensional wall jet, new improved data are provided, employing a measuring technique suitable for highly turbulent flows. This, since only hot-wire results previously have been published for this flow. The pulsed wire results show good agreement with existing Laser Doppler anemometer data in the two-dimensional wall jet, both reporting significantly higher turbulence levels in the outer region of the flow than hot-wires do. The hot-wire anemometer errors generally increase with increasing local turbulence intensity and since the three-dimensional wall jet has a higher turbulence level than its two-dimensional equivalent, the new pulsed hot-wire results improve the information available for the turbulence field in this flow significantly. Received: 29 January 1998/ Accepted: 19 February 1999     

An Experimental Investigation of the Turbulence Structure of a Lifted H2/N2 Jet Flame in a Vitiated Co-Flow

Measurements of mean velocity components, turbulent intensity, and Reynolds shear stress are presented in a turbulent lifted H₂/N₂ jet flame as well as non-reacting air jet issuing into a vitiated co-flow by laser doppler velocimetry (LDV) technique. The objectives of this paper are to obtain a velocity data base missing in the previous experiment data of the Dibble burner and so provide initial and flow field data for evaluating the validity of various numerical codes describing the turbulent partially premixed flames on this burner. It is found that the potential core is shortened due to the high ratio of jet density to co-flow density in the non-reacting cases. However, the existence of flame suppressed turbulence in the upstream region of the jet dominates the length of potential core in the reacting cases. At the centreline, the normalized axial velocities in the reacting cases are higher than the non-reacting cases, and the relative turbulent intensities of the reacting flow are smaller than in the non-reacting flow, where a self-preserving behaviour for the relative turbulent intensities exists at the downstream region. The profiles of mean axial velocity in the lifted flame distribute between the non-reacting jet and non-premixed flame both in the axial and radial distributions. The radial distributions of turbulent kinetic energy in the lifted flames exhibit a change in distributions indicating the difference of stabilisation mechanisms of the two lifted flame. The experimental results presented will guide the development of an improved modelling for such flames.