The effect of slot jet size on the confined transverse slot jet

The current study explored the flowfield characteristics of an induced recirculation zone resulting from a transverse slot jet issuing in a cross flow. The slot jet spanned 95% of the full channel spanwise dimension which resulted in a highly three-dimensional mean flowfield. Experimental investigations were carried out at various jet momentums for two different transverse slot jet thicknesses. Velocity field data were gathered using digital particle image velocimetry and hot-wire anemometry. The momentum ratio of the jet to the channel was found to be the leading parameter for scaling the dimensions of the mean recirculation zone. The jet thickness demonstrated an influence on the level of three-dimensionality, turbulence levels, and integral length scales.     

Evaluation of stochastic particle dispersion modeling in turbulent round jets

ODT (one-dimensional turbulence) simulations of particle-carrier gas interactions are performed in the jet flow configuration. Particles with different diameters are injected onto the centerline of a turbulent air jet. The particles are passive and do not impact the fluid phase. Their radial dispersion and axial velocities are obtained as functions of axial position. The time and length scales of the jet are varied through control of the jet exit velocity and nozzle diameter. Dispersion data at long times of flight for the nozzle diameter (7 mm), particle diameters (60 and 90 µm), and Reynolds numbers (10, 000–30, 000) are analyzed to obtain the Lagrangian particle dispersivity. Flow statistics of the ODT particle model are compared to experimental measurements. It is shown that the particle tracking method is capable of yielding Lagrangian prediction of the dispersive transport of particles in a round jet. In this paper, three particle-eddy interaction models (Type-I, -C, and -IC) are presented to examine the details of particle dispersion and particle-eddy interaction in jet flow.     

Measurement and analysis of the turbulent length scales in jet flows

The characteristic length and time scales of turbulence are reported in some detail for jet flows. The objective of the work is to determine the frequency dependence of these two-point turbulent properties, which are used to model the sources necessary for noise prediction using the acoustic analogy approach. A range of jet flow conditions for single and co-axial configurations are considered so that the effect of Mach number, temperature ratio and nozzle geometry is examined. The frequency dependence of both the fixed and moving frame length scales and the convection velocities for both the turbulence and the Reynolds stress are derived using a two-point complex coherence function. At higher frequencies, the integral scales are found to be strongly isotropic and inversely proportional to the Strouhal number. A frequency-dependent Taylor scale is derived and shown to agree well with the experimental results at the higher frequencies.     

Simulation of sonic boom interaction with shear turbulence

We investigate the interaction of pressure waves with the shear turbulence in a supersonic jet flow obtained from the direct numerical integration of the time-dependent, three-dimensional Euler equations. The resulting self-sustaining turbulent jet flow matches closely the relevant time and length scales of atmospheric turbulence. N-waves, characteristic of sonic booms, are simulated by perturbing the pressure profile and following these perturbations in space and time. The results reproduce most of the wave forms reported in laboratory experiments and in flight test data.     

Round gas jets submerged in water

Water submerged gas jets are characterized by the interplay of inertia and buoyancy forces and can exhibit diverse behaviors ranging from bubbly plumes to stable jets. In this work, direct measurements of the interfacial behavior of water-submerged gas jets, with Mach numbers ranging from subsonic to supersonic, were performed using high-speed digital photography and shadowgraphs. The results indicate that the jets have a preferential pinch-off position that can be attributed and correspond to the location of the maximum streamwise-velocity turbulence fluctuations. A new, experimentally determined jet penetration distance is proposed; the jetting length is taken as the jet length corresponding to a 99% probability of the gas-jet outer boundary being attached to the orifice during the sampling period. Using the shadowgraph analysis and this criterion, we determined that the electro-resistivity probe measurements, previously used to determine the jet penetration length, may be significantly biased. This is attributed to the inability of the probe measurements to distinguish between a continuous gas jet and advecting bubbles. In addition, based on a simple force-balance of the jet cross-section, we introduce a new scaling relationship for the jet penetration distance. This relationship shows that the jetting length scales with the square of the Froude number and compares well with the experimental observations. Finally, measurements of the gas jet boundary acceleration coupled with estimations of the internal gas jet velocity suggest that both Rayleigh–Taylor and Kelvin–Helmholtz mechanisms are nearly equally responsible for governing the jet boundary dynamics.     

Transport, Mixing and Agglomeration of Particles in Turbulent Flows

This paper describes methods and approaches that have been used to simulate and model the transport, mixing and agglomeration of small particles in a flowing turbulent gas. The transported particles because of their inertia are assumed not to follow the motion of the large scales of the turbulence and or the motion of the small dissipating scales of the turbulence. We show how both these behaviours can be represented by a PDF approach analogous to that used in classical kinetic theory. For large scale dispersion the focus is on transport in simple generic flows like statistically stationary homogeneous and isotropic turbulence and simple shear flows. Special consideration is given to the transport and deposition of particles in turbulent boundary layers. For small scale transport the focus is on how the small scales of turbulence together with the particle inertial response enhance collision processes like particle agglomeration. In this case the importance of segregation and the formation of caustics, singularities and random uncorrelated motion is highlighted and discussed.     

Controlling spatio-temporal evolution of natural and excited square jets via inlet conditions

The paper presents numerical investigations of square jets in a wide range of Reynolds numbers with varying inlet turbulence characteristics. The research focuses on flow characteristics depending on inflow turbulent length/time scales and excitation frequencies in case of excited jets. It is found that the parameters of inlet turbulence affect the solutions qualitatively when the Reynolds number is sufficiently low. In these cases the impact of varying the turbulent time scale is considerably larger than changing the turbulent length scale. It was also observed that at sufficiently high Reynolds numbers the jets become quite independent of the inlet turbulence characteristics. This confirms findings of Xu et al. (Phys. Fluids, 2013) concerning weak/strong dependence of the jet evolution on inflow conditions. In case of excited jets the excitation frequencies play an important role and influence the jet behaviour most strongly at lower values of the Reynolds number. For some forcing frequencies a bifurcation occurs at sufficiently large forcing amplitudes. This phenomenon turned out to be independent of the assumed length and time scales of the turbulent fluctuations, both in terms of robustness as well as amplitude.     

Wave influence on turbulence length scales in free surface channel flows

In order to predict sediment movements in coastal environments, the interaction between these particles and turbulence should be better understood. Although previous studies have particularly shown the importance of the turbulence length scales on sediment transport for current flows, few measurements have been made on wave/current flows. The purpose of our experiments is to get a better knowledge on wave action on these characteristic length scales. For this study, in the context of a grid-generated turbulence, we aimed to describe evolution of turbulence macro and micro scales in two kinds of free surface flow. Indeed, current and wave/current flows are studied. Two data analysis techniques are used to estimate these characteristic length scales depending on flow conditions. Whereas a well-known energetic method is used for current flow, a specific analysis based on correlation measurements is lead to describe temporal evolution of turbulence length scale over the wave period. As a main result, we show that the free surface causes a vortex stretching for current flow and that turbulence length scales follow a periodic evolution with a frequency which is twice as the swell period. The turbulence length scales also depend on wave period and amplitude.     

Turbulent Flow and Dispersion of Inertial Particles in a Confined Jet Issued by a Long Cylindrical Pipe

In this work we examine first the flow field of a confined jet produced by a turbulent flow in a long cylindrical pipe issuing in an abrupt angle diffuser. Second, we examine the dispersion of inertial micro-particles entrained by the turbulent flow. Specifically, we examine how the particle dispersion field evolves in the multiscale flow generated by the interactions between the large-scale structures, which are geometry dependent, with the smaller turbulent scales issued by the pipe which are advected downstream. We use Large-Eddy-Simulation (LES) for the flow field and Lagrangian tracking for particle dispersion. The complex shape of the domain is modelled using the immersed-boundaries method. Fully developed turbulence inlet conditions are derived from an independent LES of a spatially periodic cylindrical pipe flow. The flow field is analyzed in terms of local velocity signals to determine spatial coherence and decay rate of the coherent K–H vortices and to make quantitative comparisons with experimental data on free jets. Particle dispersion is analyzed in terms of statistical quantities and also with reference to the dynamics of the coherent structures. Results show that the particle dynamics is initially dominated by the Kelvin–Helmholtz (K–H) rolls which form at the expansion and only eventually by the advected smaller turbulence scales.     

Velocity fidelity of flow tracer particles

Recent developments concerning the unsteady dynamic forces on a spherical particle at finite Reynolds number are reviewed for solid particles and clean micro-bubble. A particle frequency :response function and an energy transfer function are derived for a solid particle or a contaminated micro-bubble in gas or liquid flow. A simple, unified method for estimating the cut-off frequency, or cut-off size, of a solid particle or a contaminated bubble is developed. Particle motion in isotropic turbulence is examined. Responses of the tracer particle to integral length scale structure, to turbulence energy, and to Taylor micro-scale structure are discussed in terms of the particle turbulence diffusivity, the particle turbulence intensity, and the ensemble average of the second invariant of fluid turbulence deformation tensor evaluated on the particle trajectory.The author is grateful to R. J. Adrian and C. Kent for their encouragement and support in writing this paper. This work is supported by the Engineering Research Center (ERC) for Particle Science and Technology at the University of Florida, the National Science Foundation (EEC-9402989), and industrial partners of ERC.     

An optical technique for measuring mean and fluctuating values of particle concentrations in round jets

Measurements of soot particle absorption along a line of sight have been carried out in turbulent jet flames. This makes it possible to determine both the time-averaged and the root mean square values of particle concentrations in these flames. This technique also gives the integral length scales in a plane perpendicular to the axis.     

Nanoparticle dispersion and coagulation in a turbulent round jet

Nanoparticle dispersion and coagulation behaviors in a turbulent round jet were studied in this article. An experimental system was designed to generate a uniformly distributed air–nanoparticle two-phase flow in a turbulent round jet. The particle size distribution (PSD) was measured by a scanning mobility particle sizer (SMPS) in the near field of the jet. The particle diameters were nearly constant in the potential core due to the high carrying velocity and laminar characteristic of the flow but grew larger in the region of high turbulence intensities because the vortex structures in the mixing layer promoted coagulation. Furthermore, the migration property of small-sized nanoparticles forced them to be preserved in the potential core also leading to the diameter increase. The comparison of the particle concentration distributions at different sections indicated that the shear layer is the major region for the mixing of particle-laden stream and ambient air. The particle diameters in the axial direction experienced three stages including a slightly changed stage, an increasing stage and a constant stage. The diameter increase should be attributed to turbulence coagulation.     

Calculation of a turbulent two-phase isobaric jet

A numerical calculation is carried out by the finite-difference method based on proposed equations for a turbulent submerged jet containing an admixture of solid particles. The relative longitudinal particle velocity and the influence of particles on the turbulence intensity are taken into account. The calculated results adequately agree with available experimental data. A turbulent two-phase jet is examined in [1] on the basis of the theory for a variable density jet, assuming equal mean velocities for the gas and particles and not considering the influence of particles on the turbulence intensity. Particles are analogously taken into account by a noninertial gas mixture in [2, 3], and a particle Schmidt number of 1.1 is assumed in [4]. A model is proposed in [5] which takes into account the influence of particles on the turbulence intensity of the gas phase. Problems concerning the initial and main sections of a submerged jet were solved in [6] by the integral method on the basis of this model and the assumed equality of the mean velocities of the gas and particles. Turbulent mixing of homogeneous two-phase flows with allowance made for dynamic nonequilibrium of the phases is considered in [7]. However, the neglect of turbulent transfer of particle mass and momentum led to a physically unrealistic solution for the particle concentration in the far field of the mixture. A two-phase jet is considered in the present work on the basis of the theory of a two-velocity continuous medium [8, 9] with allowance made for turbulent transfer of particle mass and momentum. The influence of particles on the turbulence intensity of the gas phase is taken into account with the model of [5].Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 57–63, September–October, 1976.The author acknowledges useful comments and discussion.of the work by G. N. Abramovich and participants of his seminar. The author sincerely thanks I. N. Murzinov for scientific supervision of the work.     

Effect of external turbulence on the development of a turbulent jet

The effect of external turbulent agitation on jet development has been investigated in [1–3]. The difference of the method employed in the present work lies in the assumption that the turbulence scale of the external flow is substantially larger than the turbulence scales in either the jet or the mixing layer. Utilizing this assumption, it becomes possible to solve separately the energy equations for the turbulence of the external flow and of the jet. Solutions obtained on the basis of this assumption are found to be in qualitative agreement with experimental data.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 24–29, January–February, 1977.     

Turbulence characteristics of radially-confined impinging jet flows

Radially confined, axisymmetric impinging jet flows are investigated by using the standard particle image velocimetry experimental technique. The confinement is achieved by placing a confinement block around a jet, co-axially. The inner diameter of the block is successively varied to nine different values. The inlet-based Reynolds number of the jet is kept constant at 5000. The nine diametric values yielded nine different flows of widely different characteristics. Among other usage, an insight into the flow characteristics can be helpful in designing compact impinging jet applications, as such a radially confined flow is equivalent to passing the pre-impingement jet through a hole perforated in a solid wall (i.e. the jet source can be placed behind a wall). The study has revealed that the flows, in general, form two circulation zones, three mixing layers, and two boundary layers. Based on turbulence characteristics of the five shear layers, overall characteristics of the flows are understood systematically. Mean velocity and various turbulence statistics are also presented, and mechanisms underlying behind their variations are explained. Finally, scaling laws are obtained for the mean velocity and for the turbulence statistics, both in the impingement and in the wall jet regions.     

The generation of nearly isotropic turbulence by means of grids

Grids have been used as a means of generating nearly isotropic turbulence for about fifty years. Even so, there does not appear to be a single document which gives adequate and simple rules for the design of such devices in an airflow installation. This paper attempts to fill this gap by means of a synthesis of experimental data with simple analyses, such that useful design guidelines are derived. Pressure losses, turbulence intensities, spectra, correlation functions and length scales are all examined. The present results are found to agree well with other data published in the literature.     

Experimental investigation on a turbulence generation system with high-blockage plates

An experimental study was conducted to develop and characterize systematically a new turbulence generator system to yield large turbulent Reynolds numbers in a compact configuration. The effect of the geometric parameters of two families of high-blockage plates on the resulting turbulent flow field was systematically studied: one series of plates was characterized by the number and distribution of circular openings; a second series had non-circular opening(s) with different shapes, distribution and position of the opening(s). The plates were placed upstream of a contoured contraction and the near field at the centerline of the resulting turbulent free jet was characterized by hot-wire anemometry in terms of mean axial velocity, turbulence intensity, turbulence length scales and corresponding Reynolds numbers. The plate with a central, non-circular opening produced the best compromise of highest turbulence levels along with excellent uniformity in average velocity and turbulence intensity, as evidenced by scan in the transverse direction. It appears to be the most promising one. By comparison with more traditional approaches to turbulence generation, we increased the turbulent Reynolds numbers based on the integral length scale to values on the order of 1000, which was one of the design objectives. Other plate geometries also yielded intense turbulence, but, in some cases, exhibited spurious frequency peaks in their power spectrum. The turbulent generation approach is to be adapted to combustion studies to reproduce conditions typical of practical system in relatively small experimental set-ups that are well-suited for bench-top experiments.     

Turbulent structure during transition to self-similarity in a round jet

 The developing turbulent region of a round jet was investigated using an improved implementation of digital particle image velocimetry (DPIV). The two-dimensional flow field in planes normal and parallel to the axial velocity was measured at locations between 15 and 30 diameters downstream, for two Reynolds numbers of 5500 and 16,000. The study consisted of instantaneous snapshots of the velocity and vorticity fields as well as measurements of velocity correlations up to third order. In this regime, the Reynolds number had a significant effect on both the instantaneous flow structure and the profiles of mean velocity across the jet. Coherent streamwise structures were present in the jet core for the lower Reynolds number. Additional structures whose evolution was governed by time scales two orders of magnitude larger than the convective scale inside the jet were observed in the entrainment field. The velocity correlations provided further support for the validity of DPIV turbulence measurements. The data was consistent with the equations of motion and momentum was conserved. DPIV measurements of turbulent kinetic energy components agreed with the hot-wire measurements of previous studies. Received: 27 November 1996/Accepted: 14 July 1997