Mean Flow and Turbulence Measurements in a Turbulent Free Cruciform Jet

The results of measurements of all three components of the mean velocity vector, the Reynolds normal and primary shear stresses and the mean static pressure in a turbulent free jet, issuing from a sharp-edged cruciform orifice, are presented in this paper. The measurements were made with an x-array hot-wire probe and a pitot-static tube in the near flow field of the jet. The Reynolds number, based upon the equivalent diameter of the orifice, was 1.70 × 10⁵. In addition to the quantities measured directly, the mean streamwise centreline velocity decay, the jet half-velocity widths, the jet spreading rate, the mean streamwise vorticity, the mass entrainment rate, the integral momentum flux and the one-dimensional energy spectra have been derived from the measured data. The results show that the mean streamwise centreline velocity decay rate of the cruciform jet is higher than that of a round jet issuing from an orifice with the same exit area as that of the cruciform orifice. The mean streamwise velocity field changed shape continuously from a cruciform close to the orifice exit plane to circular at 12 and half equivalent diameters downstream. The mean streamwise vorticity field, up to about three equivalent diameters downstream of the orifice exit plane, consists of four pairs of counter-rotating cells, which are aligned with the four edges in the centre of the cruciform orifice.     

Mean flow characteristics of a turbulent plane jet with buoyancy induced curvature

An experimental investigation of heated vertical and inclined plane air jets discharged into quiescent surroundings is described. A unique feature of this data is that Pilot tube measurements were used to define the mean trajectory of the inclined jets so that subsequent hot-wire traverses could be made normal to the curved path. While the mean velocity and temperature profiles are self-similar for the range of exit conditions studied, other aspects of the mean jet development depend on the exit Reynolds and Froude numbers, or the discharge angle. It is noted that variations between this study and other published data suggest further measurements of this flow situation are needed, with particular attention to specific features of the jet apparatus and ambient surroundings, and to the exit Reynolds number. Presently with Dept. of Mechanical Engineering, University of Alexandria, Alexandria, Egypt     

An experimental study of nonrheometric flows of visco-elastic fluids: I. flow into a re-entrant tube

A laser anemometer has been used to study the developing flow both upstream and downstream from the entry plane in a re-entrant tube geometry. A 0.75% polyacrylamide/water solution was used and Reynolds numbers (based on wall conditions in the fully developed downstream flow) in the range 100–500 were obtained in 1.82-cm and 2.40-cm-diameter tubes.The shear stress-shear rate relationship for the fluid was measured using a cone and plate geometry in conjunction with a Weissenberg rheogoniometer. Theoretical fully developed velocity profiles were calculated numerically from these measurements. The measured fully developed velocity profiles were found to be in excellent agreement with those calculated.Velocity profiles measured at the tube entry plane showed the pronounced wall region distortion typically predicted by recent numerical solutions of the flow of purely-viscous fluids through an abrupt tube contraction.It was found that the major velocity rearrangements were achieved within only a few diameters (both upstream and downstream) of the entry plane. In particular, the velocity distribution near the tube wall varied negligibly over the relatively longer distance (many diameters) that it took for the centreline velocity to achieve its fully developed value. Entry lengths were found to be only about half those for purely-viscous fluids.Calculation of the time of flight along the central streamline confirmed that the major rearrangements of velocity suffered by the fluid occurred over a relatively short time period. This indicates that hereditary integral constitutive equations may have to be used in theoretical analyses of this type of flow situation.     

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.     

A novel formula to describe the velocity profile of free jet flow

Having a potential core, the velocity profile in initial zone of incompressible submerged jet flow is different from that in fully developed region. In the former researches, the two regions were studied separately, even a short part between the two regions being considered as a transition region. The velocity profile in fully developed region looked as a Gaussian distribution, which is valid when jet initial region is comparatively short. But when the size of initial zone is long enough not to be able to be neglected, especially for large-size exit, this kind of assumption is not acceptable. Based on the analysis of flow structure of jet flow, a new velocity profile formula of submerged jet flow was proposed, which unites the initial, transition, fully developed regions of jet flow via modifying Gaussian distribution with a radial adjusting coefficient. For the round jet with the medium or high range of Reynolds number, the radial adjusting coefficient is a power function of reciprocal of jet distance. And then some literature experimental data were applied in verification, and the new formula exhibited a good calculation result. This work opened that the jet flow velocity profile at any site along the flow distance can be described via a same formula.     

Flow structures of a separating,reattaching, and recovering boundary layer for a large range of Reynolds number

Flow structures of a separating, reattaching, and recovering boundary layer over a smoothly contoured ramp are examined for an order of magnitude range of Reynolds number, which is achieved by increasing the wind tunnel pressure by up to 8 atm and varying the tunnel speed by a factor of three. The study was performed using instantaneous velocity vector plots and two-point correlations measured by PIV. The present paper discusses length scales of large-scale motions of the flow and roller vortices in the separated shear layer and their dependence on the Reynolds number or the flow geometry. Two-point correlation profiles show that the Reynolds number effects on the correlations vary strongly from region to region within the flow. Downstream of reattachment, correlation profiles show that the inner portion of the boundary layer recovers more rapidly than the outer portion, where excessively active eddies still persist. The Reynolds number effects seen at reattachment diminish in the recovery region, and the correlation profiles start to take on ordinary boundary layer characteristics.     

Effect of bubbles on the turbulence near the exit of a liquid jet

The objective of this paper is to examine the effect of bubbles on the turbulence levels of a water jet. Simultaneous measurements of the axial and radial velocity components were taken in a bubbly jet with a Laser Doppler Velocimeter (LDV) and then compared to the velocities of a single phase jet at the same liquid flow rate. Mean bubble diameters ranged from 0.6 to 2 mm and the void fractions were up to about 20%. The liquid Reynolds numbers were from 5,000 to 10,000 approximately. The measurements extended to from an axial distance of 4–12 cm. It was observed that bubbles did not affect significantly the average velocity profiles in the jet. However bubbles increased the turbulence intensities in the core of the jet near the jet exit. The increase in turbulence intensities was more pronounced at lower Reynolds numbers and at higher void fractions.     

Direct Numerical Simulation of Subsonic Round Turbulent Jet

Direct numerical simulation(DNS) of spatially developing round turbulent jet flow with Reynolds number 4,700 was carried out. Over 20 million grid points were used in this simulation. Fully compressible three-dimensional Navier–Stokes equations were solved. High order explicit spatial difference schemes and Runge–Kutta time integration scheme were used to calculate derivatives and time marching, respectively. Non-reflecting boundary conditions and exit zone techniques were adopted. Some refined computational grids were used in order to capture the smallest turbulent structures near the centerline of the jet. Low level disturbance were imposed on the jet inflow velocity to trigger the developing of turbulence. Turbulent statistics such as mean velocity, Reynolds stresses, third order velocity moments were obtained and compared with experimental data. One-dimensional velocity autospectra was also calculated. The inertial region where the spectra decays according to the k^− 5/3 was observed. The quantitative profiles of mean velocity and all of the third order velocity moments which were difficult to measure via experimental techniques were presented here in detail. The jet flow was proven to be close to fully self-similar around 19 jet diameters downstream of jet exit. The statistic data and revealed flow feature obtained in this paper can provide valuable reference for round turbulent jet research.     

Three-dimensional flow structure in highly buoyant jet by scanning stereo PIV combined with POD analysis

The flow characteristics and the structure of highly buoyant jet of low density fluid issuing into a stagnant surrounding of high density fluid is studied by scanning stereo PIV combined with proper orthogonal decomposition (POD) analysis. The experiment is carried out at Froude number of 0.3 and Reynolds number of 200, which satisfies the inflow condition due to the unstable density gradient near the nozzle exit. An increase in the maximum mean velocity occurs and the vertical velocity fluctuation is highly amplified near the nozzle exit, which suggests the influence of inflow due to the unstable density gradient. The POD analysis indicates that the vertical velocity fluctuation is the major source of fluctuating energy contributing to the development of the highly buoyant jet. The examination of the POD modes show that the longitudinal structure of the vertical velocity fluctuation is generated along the jet axis having the opposite sign of velocity fluctuation on both sides of the jet axis. The vertical scale of the POD mode decreases with increasing the mode number and results in the frequent appearance of cross-flow across the buoyant jet. The reconstruction flow from the POD modes indicates that the vortex structure is caused by the highly sheared layer between the upward and downward velocity and the inflow is induced by the vortex structure. The magnitude of the vortex structure seems to be weakened with an increase in the distance from the nozzle and the buoyant jet approaches to an asymptotic state in the further downstream.     

The near-field characteristics of circular jets at low Reynolds numbers

An experimental study has been undertaken to investigate the effect of Reynolds number on the near-field region of circular turbulent air jets. Measurements were made using a two-component Laser Doppler Anemometer, and included mean velocity, turbulence intensity, skewness factor, flatness factors and power spectrum. Measurements were taken up to 10 nozzle exit diameter in the downstream direction for different exit Reynolds numbers in the range of 1400 to 20000. The Reynolds number was found to have a strong effect on the jet flow behavior in the near-field region; the centerline velocity decays faster (decay constant = 6.11 for Re = 19400, = 1.35 for Re 1430) and the potential core gets shorter with decreasing Reynolds number. Profile measurements of the skewness and flatness factors indicate that the jet flow becomes more intermittent with decreasing Reynolds number. Power spectrum measurements of the streamwise fluctuating velocities reflects the high energy content of the high Reynolds number jet. It also reveals that there is greater energy at the higher frequencies with increasing Reynolds number.     

Near Field Round Jet Flow Downstream from an Abrupt Contraction Nozzle with Tube Extension

Round air jet development downstream from an abrupt contraction coupled to a uniform circular tube extension with length to diameter ratio L/D?=?1.2 and L/D?=?53.2 is studied experimentally. Smoke visualisation and systematic hot film velocity measurements are performed for low to moderate Reynolds numbers 1130?<?Re _b ?<?11320. Mean and turbulent velocity profiles are quantified at the tube exit and along the centerline from the tube exit down to 20 times the diameter D. Flow development is seen to be determined by the underlying jet structure at the tube exit which depends on Reynolds number, initial velocity statistics at the tube exit and the presence/absence of coherent structures. It is shown that the tube extension ratio L/D as well as the sharp edged abrupt contraction influence the initial jet structure at the tube exit. For both L/D ratios, the presence of the abrupt contraction results in transitional jet flow in the range 2000?<?Re _b ?<?4000 and in flow features associated with forced jets and high Reynolds numbers Re _b ?>?10⁴. The tube extension ratio L/D downstream from the abrupt contraction determines the shear layer roll up so that for L/D?=?1.2 flow visualisation suggests the occurrence of toroidal vortices for Re _b ?<?4000 whereas helical vortices are associated with the transitional regime for L/D?=?53.2. Found flow features are compared to features reported in literature for smooth contraction nozzles and long pipe flow.     

Experimental investigation of the unsteady structure of a transitional plane wall jet

 A laminar wall jet undergoing transition is investigated using the particle image velocimetry (PIV) technique. The plane wall jet is issued from a rectangular channel, with the jet-exit velocity profile being parabolic. The Reynolds number, based on the exit mean velocity and the channel width, is 1450. To aid the understanding of the global flow features, laser-sheet/smoke flow visualizations are performed along streamwise, spanwise, and cross-stream directions. Surface pressure measurements are made to correlate the instantaneous vorticity distribution with the surface pressure fluctuations. The instantaneous velocity and vorticity field measurements provide the basis for understanding the formation of the inner-region vortex and the subsequent interactions between the outer-region (free-shear-layer region) and inner-region (boundary-layer region) vortical structures. Results show that under the influence of the free-shear-layer vortex, the local boundary layer becomes detached from the surface and inviscidly unstable, and a vortex is formed in the inner region. Once this vortex has formed, the free-shear-layer vortex and the inner-region vortex form a vortex couple and convect downstream. The mutual interactions between these inner- and outer-region vortical structures dominate the transition process. Farther downstream, the emergence of the three-dimensional structure in the free shear layer initiates complete breakdown of the flow. Received: 8 November 1995/Accepted: 6 November 1996     

The predominant effect of stroke length on velocity profiles at the exit of axisymmetric synthetic jet actuators

This paper addresses the effect of stroke length on velocity profiles at the exit of axisymmetric synthetic jet actuators. An actuator with a “short-orifice” nozzle geometry was studied (the length of the sharp-edged orifice nozzle being always shorter than the stroke length). The investigation was carried out using hot-wire experiments and numerical approaches in the range of the Reynolds number from 400 to 5600 and a dimensionless stroke length from 1.5 to 86 (i.e. the Stokes number was from 16 to 102). To quantify the velocity profiles at the actuator exit, the ratio of the two characteristic velocities was used. The former one was evaluated as the time- and spatially-averaged orifice velocity and the latter as the time-mean centerline velocity. It was found that this average-to-centerline velocity ratio depends mainly on the dimensionless stroke length, with a minor dependence on the Reynolds number. An approximation function of this predominant relationship has been proposed. The results of both experimental and numerical approaches are in reasonable agreement showing variation of average-to-centerline velocity ratio in the range of 0.7–1.3.     

Characteristics of strongly-forced turbulent jets and non-premixed jet flames

Previous researchers have demonstrated that strong pulsations of the fuel flow rate can significantly reduce the flame length and luminosity of laminar/transitional non-premixed jet flames. The physical mechanisms responsible for these changes are investigated experimentally in acoustically-forced jet flows where the peak velocity fluctuations are up to eight times the mean flow velocity. Both reacting and non-reacting flows were studied and Reynolds numbers, based on the mean flow properties, ranged from 800 to 10,000 (corresponding to peak Reynolds numbers of 1,450–23,000), and forcing frequencies ranged from 290 to 1,140 Hz. Both the first and second organ-pipe resonance modes of the fuel delivery tube were excited to obtain these frequencies. An analysis of the acoustic forcing characteristics within the resonance tube is provided in order to understand the source of the high amplitude forcing. Flow visualization of jets with first resonant forcing confirms the presence of large-scale coherent vortices and strong reverse flow near the exit of the fuel tube. With second-resonant forcing, however, vortices are not emitted from the tube as they are drawn back into the fuel tube before they can fully form. Increased fine-scale turbulence is associated with both resonant cases, but particularly at second resonance. The power spectra of the velocity fluctuations for a resonantly pulsed jet show the presence of an inertial subrange indicating that the flow becomes fully turbulent even for mean-Reynolds-number jets that are nominally laminar. It is shown that these pulsed jet flows exhibit strong similarities to synthetic jets and that the Strouhal number, based on the maximum velocity at the fuel tube exit, is the dominant parameter for scaling these flows. The Strouhal number determines the downstream location where the coherent vortices breakdown, and is found to provide better collapse of flame length data (both current and previous) than other parameters that have been used in the literature.     

Experimental study of a non-viscometric flow: kinematics of a viscoelastic fluid at the exit of a cylindrical tube

In the scope of a study on non-viscometric flows of viscoelastic liquids, the authors have studied die-swell at the exit of a cylindrical tube. The work presents the determination of axial velocity profiles, by laser anemometry, from the nearly parabolic profile in the tube (compared to the Poiseuille curve computed with the Le Roy-Pierrard model) to the flat profile in the jet.The main conclusions are that the axial velocities are modified at more than one diameter upstream in the die and rigid body flow is attained at about one radius downstream.     

LDA measurements in the turbulent round jet

In the present paper, LDA was used to measure the velocity field of turbulent round air jet flows. Two cases were investigated; a jet issuing vertically upward and freely in the laboratory surrounding environment, and a jet issuing vertically upward but out of wall section setting flush horizontally at the nozzle exit. Data were collected for three exit Reynolds numbers of 1.32 × 10⁴, 2.64 × 10⁴ and 3.96 × 10⁴, which correspond to exit velocities of 10 m/s, 20 m/s and 30 m/s respectively. For each Reynolds number, profile measurements of the mean velocity, turbulence intensity, skewness and flatness factors were made at 8 downstream stations up to 30 nozzle-exit diameter. The relative influence of using a wall at the jet exit plane on the jet behavior and characteristics is the objective of the present study. The experimental results indicate that the wall, placed at the exit plane, limits the interaction of the jet flow with the surroundings, and consequently results in a reduction in the velocity spread rate, kinematic momentum flux, and kinematic mass flux. Further, the flatness and skewness factors distributions across the jet flow registered relatively higher values in the outer region of the jet when the wall was used. This indicates a more intermittent behavior of the jet flow in that region due to the existence of the wall.     

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.     

Effect of particle size on a two-phase turbulent jet

The effect of particle size on two-phase turbulent jet flow structure is studied in the present experimental investigation. Polystyrene solid particles of 210, 460, and 780 μm were used. The particles' mass loading ratios ranged from 0 to 3.6. The flow Reynolds number was 2 ‘ 10⁴, which was based on the pipe nozzle diameter and the fluid-phase centerline velocity at the nozzle exit. A two-color laser-Doppler anemometer (LDA), combined with the amplitude discrimination method and the velocity filter method, was employed for measurement. The measurement range of the jet flow was from the initial pipe exit to 90D downstream. Results are presented for the mean velocities of particle and fluid phases, the flow's turbulent intensities and the flow's Reynolds stresses. The energy spectra and the correlation functions of the two-phase jet flow were also obtained by using another one-component He-Ne LDA system.     

Experimental study of parallel and inclined turbulent wall jets

An experimental study of the flow field in a two-dimensional wall jet has been conducted. All measurements were carried out using hot-wire anemometry. The experimental facility has a rectangular slot nozzle of high aspect ratio l/b = 100 (where l and b are the length and height slot, respectively). Mean velocities and Reynolds stresses were determined with three nozzle Reynolds numbers (Re = 1 × 10⁴, 2 × 10⁴ and 3 × 10⁴) and four different inclination angles between the wall and the flow velocity at the nozzle (β = 0°, 10°, 20° and 30°). Results indicate that all wall jets are self-preserving in the developed region. Normal to the wall two regions can be identified: one similar to a plane free jet and the other similar to a boundary layer. Downstream the interaction between these two regions creates a mixed or third region. The logarithmic region increases with the distance from the nozzle and with the Reynolds number. For the inclined wall jet, the spreading rate expressed in terms of jet half-width or maximum velocity decay with respect to the streamwise distance, asymptotes to a linear law. The streamwise locations where the jet becomes self-similar are farther from the exit than in parallel wall jet. The slope of both half-width and maximum velocity decay in the developed region are affected by both wall jet inclination angle and nozzle exit Reynolds number.     

应用二维激光测速系统研究面流消能的紊动特性

本文应用二维激光测速系统对面流消能工下游典型流态的紊动特性进行了系统的试验研究,得到了不同来流条件下面流的时均流速、紊动强度及雷诺应力等沿主流与回流区的分布特征;计算并绘制了流线图,由此分析了淹没面流时底旋滚长度和佛劳德数的关系。对紊动能量的转换过程及耗散特征进行了计算分析,初步探明了消能机理。