Oscillations of the fluid flow and the free surface in a cavity with a submerged bifurcated nozzle

The free surface dynamics and sub-surface flow behavior in a thin (height and width much larger than thickness), liquid filled, rectangular cavity with a submerged bifurcated nozzle were investigated using free surface visualization and particle image velocimetry (PIV). Three regimes in the free surface behavior were identified, depending on nozzle depth and inlet velocity. For small nozzle depths, an irregular free surface is observed without clear periodicities. For intermediate nozzle depths and sufficiently high inlet velocities, natural mode oscillations consistent with gravity waves are present, while at large nozzle depths long term self-sustained asymmetric oscillations occur.For the latter case, time-resolved PIV measurements of the flow below the free surface indicated a strong oscillation of the direction with which each of the two jets issue from the nozzle. The frequency of the jet oscillation is identical to the free surface oscillation frequency. The two jets oscillate in anti-phase, causing the asymmetric free surface oscillation. The jets interact through a cross-flow in the gaps between the inlet channel and the front and back walls of the cavity.     

Axial plus tangential entry swirling jet

This paper presents an experimental investigation on swirling jets with well-defined initial conditions. The axial, radial, and azimuthal velocity components, with their respective fluctuations were measured using high spatial–resolution particle image velocimetry. These detailed measurements allow the initial conditions of the swirling jets to be established and the jets to be characterized using various swirl number definitions. The significance of each term in the swirl number calculations are quantified, and the effect of the common assumptions and simplifications are examined. The characteristics of the jets in relation to the initial conditions are then investigated and compared with the previous studies using similar characterization parameters. Jets with Reynolds number of approximately 5700 and swirl conditions ranging from a non-swirling reference case to high swirl are studied. General properties of swirling jets such as higher spreading rate, higher centerline velocity decay, and higher turbulence level are observed. When the degree of swirl is sufficiently high, vortex breakdown occurs. A swirl number of 0.94 is recorded for a high swirl case prior to vortex breakdown, much higher than the critical swirl number reported in the literature. This behavior is attributed to the effect of the initial conditions on the swirl number calculation.     

波浪环境中垂直射流紊动特性的实验研究

利用粒子图像测速技术PIV(particle imagevelocimetry)对有限水深规则波浪环境下垂直射流紊动特性进行了实验研究. 应用相位分析法从测量数据中分离出速度脉动项,用4种不同波高的波浪研究波高对射流紊动特性的影响,对紊动量的分布以及大小进行了分析. 结果表明波高对射流的紊动特性有显著影响,并且对流项对波高的变化较紊动扩散项更为敏感,紊动扩散项量值约是对流项的$1/8\sim1/3$, 在时均化的N-S方程中起的作用不可忽略.      

PIV study of large-scale flow organisation in slot jets

The paper reports on particle image velocimetry (PIV) measurements in turbulent slot jets bounded by two solid walls with the separation distance smaller than the jet width (5–40%). In the far-field such jets are known to manifest features of quasi-two dimensional, two component turbulence. Stereoscopic and tomographic PIV systems were used to analyse local flows. Proper orthogonal decomposition (POD) was applied to extract coherent modes of the velocity fluctuations. The measurements were performed both in the initial region close to the nozzle exit and in the far fields of the developed turbulent slot jets for Re ⩾ 10,000. A POD analysis in the initial region indicates a correlation between quasi-2D vortices rolled-up in the shear layer and local flows in cross-stream planes. While the near-field turbulence shows full 3D features, the wall-normal velocity fluctuations day out gradually due to strong wall-damping resulting in an almost two-component turbulence. On the other hand, the longitudinal vortex rolls take over to act as the main agents in wall-normal and spanwise mixing and momentum transfer. The quantitative analysis indicates that the jet meandering amplitude was aperiodically modulated when arrangement of the large-scale quasi-2D vortices changed between asymmetric and symmetric pattern relatively to the jet axis. The paper shows that the dynamics of turbulent slot jets are more complex than those of 2D, plane and rectangular 3D jets. In particular, the detected secondary longitudinal vortex filaments and meandering modulation is expected to be important for turbulent transport and mixing in slot jets. This issue requires further investigations.     

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.     

Mass and momentum transport in the near field of swirling turbulent jets. Effect of swirl rate

Local transport of the flow momentum and scalar admixture in the near-field of turbulent swirling jets (Re = 5,000) has been investigated by using a combination of the particle image velocimetry and planar laser-induced fluorescence methods. Advection and turbulent and molecular diffusions are evaluated based on the measured distributions of the mean velocity and concentration and the Reynolds stresses and fluxes. As has been quantified from the data, the flow swirl intensifies the entrainment of the surrounding fluid and promotes mass and momentum exchange in the outer mixing layer. A superimposed swirl results in the appearance of a wake/recirculation region at the jet axis and, consequently, the formation of an inner shear layer. In contrast to the scalar admixture, the momentum exchange in the inner shear layer is found to be strongly intensified by the swirl. For the jet with the highest considered swirl rate, a substantial portion of the surrounding fluid is found to enter the unsteady central recirculation zone, where it mixes with the jet that is issued from the nozzle. The contribution of the coherent velocity fluctuations, which are induced by large-scale vortex structures, to the turbulent transport has been evaluated based on triple decomposition, which was based on proper orthogonal decomposition analysis of the velocity data sets. For the considered domain of the jet with the highest swirl rate and vortex breakdown, the contributions of detected helical vortex structures, inducing pressing vortex core, to the radial fluxes of the flow momentum and the scalar admixture are found to locally exceed 65% and 80%, respectively.     

Étude d'une ligne de jets impactant une paroi concave par PIV stéréoscopiqueStudy of a row of jets impinging a concave wall by stereoscopic PIV

The aerodynamic study of a row of axisymmetric jets impinging a concave wall is carried out from velocity measurements obtained by the standard and stereoscopic Particle Image Velocimetry. The principle and the specific aspects of the stereoscopic PIV set up, a recent technique of three-dimensional velocimetry, are explained. After a statistical data processing, the three-dimensional structure and the characteristics of multiple jets impinging a concave wall are described with the mean velocity fields and the turbulent values in several planes of the flow. To cite this article: V. Gilard, L.-E. Brizzi, C. R. Mecanique 334 (2006).     

On the analysis of turbulent transport processes in nonreacting multijet burner flows

Measurements of time-resolved velocity characteristics have been obtained with a laser-Doppler velocimeter in the vicinity of a model of an industrial oxy-fuel burner. The burner consists of a central axisymmetric jet surrounded by 16 circular jets, simulating the injection of oxygen in practical burners. The experiments were carried out for isothermal flows and quantify the effect of swirl for 0 ≤ S ≤ 0.9 on the mixing efficiency of the burner assembly. The results show that the present flow develops faster than related coaxial free jets with similar velocity ratios between central and peripheral air streams and, for example, for the nonswirling flow the rate of decay of the centreline velocity increases by a factor of 2. Swirl attenuates the three-dimensional structure typical of multijet flows although the peripheral jets limit the radial spreading of the swirling flow and give rise to increased values of mean shear strain and, therefore, to turbulent production. The existence of zones characterized by large turbulence anisotropy indicate the need to take account of the individual normal stresses in any proposed mathematical model to simulate the flow characteristics. Inspection of the terms in the conservation equation for the turbulent stresses quantify the extent to which interaction of normal stresses with normal strains influences the flow and suggests the likely combined magnitude of turbulent diffusion and dissipation.     

Impact of density stratification on the global mode in a swirling jet: Stochastic modelling and Lagrangian coherent structures

In an experimental investigation, the stochastic dynamics of the global mode in a turbulent swirling jet are considered. From the application of the swirling jet in gas turbine combustors, it was observed that a specific density gradient in the flow leads to a suppression of the global mode. This phenomenon was replicated in a generic swirling jet using an electrical heating coil placed inside the breakdown bubble. In the present investigation, the dynamics of the global mode obtained from PIV and pressure measurements are analysed using a stochastic reduced-order model to describe the instability. The stochastic model is necessary to explain the interaction between the deterministic dynamics of the global mode and the perturbations by the background turbulence. The calibration of the stochastic model provides the amplification rate of the global mode that defines the transition of the flow, dependent on the swirling strength and the density difference. The spatial structure of the global mode is further investigated from Lagrangian coherent structures of the flow field which are computed from the 3D time-resolved velocity field reconstruction based on planar PIV measurements. The Lagrangian visualisations and schlieren visualisations are used to explain the absence of the density effects on the global mode at larger Reynolds numbers. The analysis gives a detailed view of the stochastic dynamics of a hydrodynamic instability in a turbulent flow.     

Particle image velocimetry investigation of the flow-field of a 3D turbulent annular swirling decaying flow induced by means of a tangential inlet

Particle image velocimetry (PIV) has been used in order to measure the three mean components and turbulence intensities of the velocity vector in a swirling decaying flow induced by a tangential inlet in an annulus. This kind of flow motion is found to be very complex, exhibiting three-dimensional and non-axisymmetric characteristics coupled with a free decay of the swirling intensity along the flow path, thereby making it difficult to study. A method allowing the measurement of the three components of the velocity flow-field with a standard PIV system with two-dimensional acquisitions, is presented. The evolution of each velocity component between the inlet and the outlet of the annulus is obtained. Furthermore, the PIV technique is extended to the measurement of turbulent characteristics such as turbulent intensities and dimensionless turbulent energy. The main characteristics of the swirling flow are discussed and the swirl number is estimated as a function of the axial distance from the tangential inlet. Received: 6 July 1998/Accepted: 20 March 1999     

旋流燃烧室内湍流燃烧速度场的实验研究

建立了采用分级进风方式的同轴射流旋流燃烧室实验装置，选用耐高温的氧化铝细粉作为示踪粒子，实现了用三维激光粒子动态分析仪(PDA)测量湍流旋流燃烧的热态瞬时速度场．在分级进风比率和旋流致不同的3组实验工况条件下，得到了气体时均轴向与切向速度、轴向与切向脉动速度均方根值和轴向—切向脉动速度二阶关联量的分布．     

Phase-locked analysis of velocity fluctuations in a turbulent free swirling jet after vortex breakdown

Large-scale organized vortical structures were studied experimentally in a free swirling jet of air experiencing vortex precession (PVC) at ambient conditions. Detailed measurements were performed in the region near the nozzle exit using phase-locked LDV and PIV, at a Reynolds number of Re ?? 24,400 and a swirl parameter S ?? 1.0. The investigation allowed reconstruction of the time-averaged flowfield, with the associated distribution of turbulent fluctuations, the phase-locked structure of the jet and the associated precessing vortex structure. An original joint analysis of power spectra and probability density functions of velocity data led to quantification of the PVC effect on turbulent fluctuations. This analysis showed that the PVC contribution can be properly separated from the background random turbulence, reproducing the results of phase-locked measurements. It is found that the background turbulence in the near field is substantially weaker if compared to the coherent fluctuations induced by vortex precession.     

Dissipation rate estimation from PIV in zero-mean isotropic turbulence

Measuring the turbulent kinetic energy dissipation rate in an enclosed turbulence chamber that produces zero-mean flow is an experimental challenge. Traditional single-point dissipation rate measurement techniques are not applicable to flows with zero-mean velocity. Particle image velocimetry (PIV) affords calculation of the spatial derivative as well as the use of multi-point statistics to determine the dissipation rate. However, there is no consensus in the literature as to the best method to obtain dissipation rates from PIV measurements in such flows. We apply PIV in an enclosed zero-mean turbulent flow chamber and investigate five methods for dissipation rate estimation. We examine the influence of the PIV interrogation cell size on the performance of different dissipation rate estimation methods and evaluate correction factors that account for errors related to measurement uncertainty, finite spatial resolution, and low Reynolds number effects. We find the Re _λ corrected, second-order, longitudinal velocity structure function method to be the most robust method to estimate the dissipation rate in our zero-mean, gaseous flow system.     

Experimental study of heat transfer characteristics due to confined impinging two-dimensional jets

Experimental results are presented for characteristics of impingement heat transfer caused by three slot jets. Experimental values were obtained for the dimensionless distance H = 0.5−3, dimensionless pitch P = 6−16, and Reynolds number Re = 500−8000. For laminar impinging flow, they were compared with numerical results. For turbulent impinging flow, two peaks of the local Nusselt number were obtained behind the second nozzle. The position of the second peak approached the nozzle as the space between nozzle and impinged surface decreased. The average Nusselt number between the central and second nozzles was determined from the ratio P/H and the Reynolds number based on the pitch of the nozzles.     

A study of the flow characteristics in micro-abrasive jets

An experimental study of particle velocities in micro-abrasive jets by using the particle image velocimetry (PIV) technique is presented. It has been found that the particle jet flow has a nearly linear expansion downstream. The particle velocities increase with air pressure, and the increasing rate increases with nozzle diameter within the range considered. The instantaneous velocity profile of the particle flow field in terms of the particle velocity distribution along the axial and radial directions of the jets is discussed. For the axial profile in the jet centerline downstream, there exists an extended acceleration stage, a transition stage, and a deceleration stage. For the radial velocity profiles, a relatively flat shape is observed at a jet cross-section near the nozzle exit. Mathematical models for the particle velocities in the air jet are then developed. It is shown that the results from the models agree well with experimental data in both the variation trend and magnitude.     

Experimental investigation of helicity in turbulent swirling jet using dual-plane dye laser PIV technique

This paper reports a new method of generating two light sheets using a dye laser system and the use of this dual-plane dye laser system to analyse average helicity and energy dissipation in a turbulent swirling flow. The dual-plane PIV system that was used in this study consisted of three cameras and a single frequency Nd:YAG laser, which was used to generate two parallel light sheet planes with differing wavelengths(colour). The method of generating two different light sheet wavelengths using a single laser source is an innovative and new technique. Stereoscopic PIV measurements were obtained in one plane with the use of two CCD cameras, and standard PIV measurements were obtained in the other plane with the use of one CCD camera. The light scattered by the particles on two different light sheets were separated using appropriate optical filters. The measurements obtained were used to estimate the components of the velocity gradient tensor. The tensor components were then used to determine the average vorticity components and helicity quantities of the fluid that was investigated. To determine the average turbulent kinetic energy dissipation, the continuity equation was used to infer the out-of-plane gradient of the out-of-plane velocity. From the analysis of the results, it was found that regions with high helicity were correlated with regions of high turbulent kinetic energy dissipation.     

Numerical and experimental study of turbulent impinging twin-jet flow

The two dimensional impinging circular twin-jet flow with no-cross flow is studied numerically and experimentally. The theoretical predications are carried out through numerical procedure based on finite volume method to solve the governing mass, momentum, turbulent kinetic energy and turbulent kinetic energy dissipation rate. The parameters studied were jet Reynolds number (9.5 × 10⁴  Re  22.4 × 10⁴), nozzle to plate spacing (3  h/d  12), nozzle to nozzle centerline spacing (l/d = 3, 5 and 8) and jet angle (0°  θ  20°). It is concluded that the stagnation primary point moves away in the radial main flow direction by increasing the jet angle. This shift becomes stronger by increasing the nozzle to nozzle centerline spacing (l/d). A secondary stagnation point is set up between two jets. The value of pressure at this point decreases by decreasing Reynolds number and/or increasing the jet angle.

The sub atmospheric region occurs on the impingement plate. It increases strongly by increasing Reynolds number and decreases as the jet angle and/or a nozzle to plate spacing increases. The spreading of jet decreases by increasing nozzle to plate spacing. The intensity of re-circulation zone between two jets decreases by increasing of h/d and jet angle. The increase of turbulence kinetic energy occurs within high gradient velocity.     

PIV Measurements in the Near and Intermediate Field Regions of Jets Issuing from Eight Different Nozzle Geometries

An experimental study was conducted to investigate the effect of nozzle geometry on the mixing characteristics and turbulent transport phenomena in turbulent jets. The nozzle geometry examined were round, square, cross, eight-corner star, six-lobe daisy, equilateral triangle as well as ellipse and rectangle each with aspect ratio of 2. The jets were produced from sharp linear contoured nozzles which may be considered intermediate to the more widely studied smooth contraction and orifice nozzles. A high resolution particle image velocimetry was used to conduct detailed velocity measurements in the near and intermediate regions. It was observed that the lengths of the potential cores and the growth rates of turbulence intensities on the jet centerline are comparable with those of the orifice jets. The results indicate that the decay and spreading rates are lower than reported for orifice jets but higher than results for smooth contoured jets. The jets issuing from the elliptic and rectangular nozzles have the best mixing performance while the least effective mixing was observed in the star jet. The distributions of the Reynolds stresses and turbulent diffusion clearly showed that turbulent transport phenomena are quite sensitive to nozzle geometry. Due to the specific shape of triangular and daisy jets, the profiles of mean velocity and turbulent quantities are close to each other in their minor and major planes while in the elliptic and rectangular jets are considerably different. They also exhibit more isotropic behavior compared to the elliptic and rectangular jets. In spite of significant effects of nozzle geometry on mean velocity and turbulent quantities, the integral length scales are independent of changes in nozzle geometry.     

Reynolds stress modelling of jet and swirl interaction inside a gas turbine combustor

Influences of the inlet swirl levels on the interaction between the dilution air jets and the swirling cross‐flow to the interior flow field inside a gas turbine combustor were investigated numerically by Reynolds stress transport model (RSTM). Due to the intense swirl and jet interaction, a high level of swirl momentum is transported to the centreline and hence, an intense vortex core is formed. The strength of the centreline vortex core was found to depend on the inlet swirl levels. For the higher swirling inlet, the decay of the swirling motion causes strong streamline variation of pressure; and consequently leads to an elevated level of deceleration of its axial velocity. Predictions contrasted with measurements indicate that the stress model reproduces the flow correctly and is able to reflect the influences of inlet swirl levels on the interior flow structure. Copyright © 1999 John Wiley & Sons, Ltd.     

In-Nozzle Measurements of a Turbulent Opposed Jet Using PIV

Turbulent opposed jet burners are an excellent test case for combustion research and model development due to the burners’ compactness, relative simplicity, and the good optical access they provide. The flow-field in the flame region depends strongly on the turbulence generation inside the nozzles, so that realistic flow simulations can only be achieved if the flow inside the nozzles is represented correctly, which must be verified by comparison to suitable experimental data. This paper presents detailed particle image velocimetry (PIV) measurements of the flow issuing from the turbulence generating plates (TGP) inside a glass nozzle. The resulting data is analyzed in terms of first and second moments, time-series, frequency spectra and phase averages. The measurements show how individual high velocity jets emerging from the TGP interact and recirculation zones are formed behind the solid parts of the TGP. Vortex shedding is observed in the jet’s shear layer were high levels of turbulent kinetic energy are generated. Time series measurements revealed periodic pulsations of the individual jets and implied a coupling between adjacent jets. The peak frequencies were found to be a function of the Reynolds-number.