The interaction of two opposing,asymmetric curved wall jets

An experimental investigation was made of a two dimensional flow formed by the interaction of two asymmetric turbulent curved wall jets past a circular cylinder. Measurements were made of velocity and turbulence intensity profiles of the two curved wall jets before the interaction, and those of the merged jet after the interaction. The location of the interaction region of the two opposing curved wall jets and the flow direction of the merged jet were found to depend primarily on the ratio of initial momentum fluxes. The velocity and turbulence intensity profiles of the merged jet were similar to those of the plane turbulent jet. However, the growth rate of the merged jet was approximately 1.5 times larger than that of the plane jet. The influence of the momentum flux ratio on the growth rate appeared to be insignificant.List of symbols C _f friction coefficient - h slot height - J _p, J _c initial momentum flux of a power jet and of a control jet, respectively - P, Pa wall static and atmospheric pressure, respectively - Re Reynolds number based on slot height - Re _m local Reynolds number U _m y _m /v - U local mean velocity - U _c velocity along the center line of the merged jet - U _m local maximum velocity of the curved wall jet - u r.m.s. value of velocity fluctuations - u _u friction velocity - U ⁺ U/u_t - x distance along the cylinder surface - x distance along the center line of the merged jet - y _1/2, y _1/2 position of y and y where U = U _m /2 and U = U _c /2, respectively - y ⁺ yu _t/V - deflection angle of the merged jet (Fig. 4) - interaction angle (Fig. 4) - merged jet angle (Fig. 4) - angle measured from the center line of the cylinder (Fig. 4) - interception angle (Fig. 8) - , normalized coordinates, y/y _1/2 and y/y _1/2, respectively     

Turbulence control in wall jets

A new method for the control of mixing of a plane turbulent wall jet has been investigated. A thin wire, mounted in the vicinity of the wall-jet nozzle, changes the formation of the shear-layer structures in the early stages of the development of the wall jet. The wire is operated in two ways: (1) a still wire inhibits the natural shear layer roll-up and reduces the size of the turbulent structures and thereby the mixing; (2) a self-excited oscillating wire introduces large coherent structures and thereby enhances the mixing. The size of these structures does not depend on the shear-layer instability but rather on the wire frequency.Measurements of the mean and fluctuating velocities have been performed with hot-wire anemometry as well as measurements of the skin friction by means of Preston tubes, surface fences and wall hot-wires. The Reynolds number based on the slot width was Re_j=10000.     

Turbulence measurements in a flow generated by the collision of radially flowing wall jets

Early results of an experimental investigation of the abnormally high turbulence level and mixing layer growth rate characteristics found in the upwash regions of aircraft with vertical short takeoff and landing (V/STOL) flows in ground effect are presented. The upwash flow is formed from the collision of two opposing radially flowing wall jets. The wall jets are created in a unique way that allows the upwash to form without any interference due to the source jets. The objective of this work is to systematically characterize the development and structure of the flow. The upwash flow exhibits very large mixing rates compared to turbulent free or wall jet flows. A unique set of two component velocity profiles was taken in the upwash flow field. These measurements include several higher moment terms that appear in the turbulent kinetic energy equations, as well as length scales and intermittency determinations. Measurements were taken' along the axis connecting the two source jets as well as off this axis at six measurement stations above ground. The results provide detailed data on an important class of flows where none existed, and they are expected to significantly improve the computational empirical tools available for predicting V/STOL behavior near the ground.A version of this paper was presented at the 10th Symposium on Turbulence, University of Missouri-Rolla, September 22–24, 1986     

Turbulence measurements in a rectangular mesoscale confined impinging jets reactor

Mesoscale chemical reactors capable of operating in the turbulent flow regime, such as confined impinging jets reactors (CIJR), offer many advantages for rapid chemical processing at the microscale. One application where these reactors are used is flash nanoprecipitation, a method for producing functional nanoparticles. Because these reactors often operate in a flow regime just beyond transition to turbulence, modeling flows in these reactors can be problematic. Moreover, validation of computational fluid dynamics models requires detailed and accurate experimental data, the availability of which has been very limited for turbulent microscale flows. In this work, microscopic particle image velocimetry (microPIV) was performed in a mesoscale CIJR at inlet jet Reynolds numbers of 200, 1,000, and 1,500. Pointwise and spacial turbulence statistics were calculated from the microPIV data. The flow was observed to be laminar and steady in the entire reactor at a Reynolds number of 200. However, at jets Reynolds numbers of 1,000 and 1,500, instabilities as a result of the jets impinging along the centerline of the reactor lead to a highly turbulent impingement region. The peak magnitude of the normalized Reynolds normal and shear stresses within this region were approximately the same for the Reynolds numbers of 1,000 and 1,500. The Reynolds shear stress was found to exhibit a butterfly shape, consistent with a flow field dominated by an oblique rocking of the impingement zone about the center of the reactor. Finally, the spatial auto- and cross-correlations velocity fluctuations were calculated and analyzed to obtain an understanding of size of the coherent structures.     

Turbulence measurements from a plane air jet with buoyancy induced curvature

An experimental investigation of the turbulence structure of a heated plane air jet discharged at various angles into quiescent surroundings is described. Hot-wire anemometry was used to obtain the profiles of mean and turbulent velocities and temperature normal and tangential to the curved path of the flow. Measurements in the buoyancy induced curved region of the jet show the relative influence of the stability induced by both buoyancy and jet curvature on the turbulence structure.     

Heat transfer in a laminar wall jet over a curved surface

The steady state heat transfer characteristics of the wall jet over a curved surface are obtained for constant wall temperature and constant wall heat flux boundary conditions. Both concave and convex curvatures have been considered. Numerical results for the temperature distribution are obtained and solutions for the wall values of the temperature functions have been tabulated for Prandtl number ranging from 0.01 to 100 while the curvature parameter was varied from –0.03 to 0.07.Nomenclature f velocity profile function - h heat transfer coefficient - K thermal conductivity - Nu Nusselt number - Pr Prandtl number - q _w heat flux at the wall - Re Reynolds number - R ₀ surface radius of curvature - T temperature - U characteristic velocity - u velocity component in x direction - v velocity component in y direction - x distance parallel to the surface - y distance normal to the surface - curvature parameter - dimensionless coordinate - dimensionless temperature - dynamic viscosity - kinematic viscosity - fluid density - shear stress - w conditions at the wall - conditions far away from the surface     

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     

MHD laminar wall jet over a curved surface in a variable normal magnetic field

The effects of a magnetic field on jets of an electrically conducting fluid flowing over a curved surface, in the presence of a variable normal magnetic field, are studied. It is noted that the convex surface curvature h and the magnetic interaction parameter m, in small perturbation similarity solution have qualitatively similar effects. They increase the mass flux, decrease the momentum flux, decrease the wall shear and cause an adverse pressure gradient in the flow field.     

The development of an axisymmetric curved turbulent wall jet

An experimental study has been carried out of the low speed Coanda wall jet with both streamwise and axisymmetric curvature. A single component laser Doppler technique was used, and by taking several orientations at a given point, values of the three mean velocities and five of the six Reynolds stresses were obtained. The lateral divergence and convex streamwise curvature both enhanced the turbulence in the outer part of the jet compared with a plane two-dimensional wall jet. The inner layer exhibited a large separation of the positions of maximum velocity and zero shear stress. It was found that the streamwise mean velocity profile became established very rapidly downstream of the slot exit. The profile appeared fairly similar at later downstream positions, but the mean radial velocity and turbulence parameters showed the expected nonself preservation of the flow. Removal of the streamwise curvature resulted in a general return of the jet conditions toward those expected of a plane wall jet. The range and accuracy of the data may be used for developing turbulence models and computational techniques for this type of flow.     

Turbulence modulation in two-phase jets

A simple model is presented which can be used to predict the modifying effect of a dispersed-phase on the turbulence structure of two-phase jets. It is based on Kolmogorov's concept of spectral energy transfer and takes into account the additional energy dissipation resulting from the inability of dispersed-phase particles to completely follow turbulent eddy fluctuations. According to the analysis presented, high-frequency eddies are attenuated preferentially and a reduction of the total rate of turbulent energy dissipation results. Turbulence intensities were also reduced. Good agreement between predictions and experimental findings were obtained.     

Phase-resolved PIV measurements in a transitional plane wall jet: a numerical comparison

The transitional process of a forced plane wall jet is studied both experimentally and numerically. Experimentally, Particle Image Velocimetry (PIV) and laser-sheet/smoke flow-visualization techniques are implemented to provide an overall understanding of the flow features. Numerically, time-accurate computational results are obtained by solving the two-dimensional, unsteady Navier–Stokes equations. Comparison of PIV data and two-dimensional computed results shows excellent agreement in the early stages of transition, demonstrating that the numerical study can be used to complement the experimental one. The results show that, under the influence of external excitation, linear-instability growth is bypassed and a discrete shear-layer vortex is formed in the immediate vicinity of the nozzle exit. This vortex interacts with the boundary-layer vorticity, leading to the formation of another vortex in the inner layer. These two vortices form a vortex couple that for high forcing convects downstream in a stable manner. By adoption of either a no-slip or a slip boundary condition in the numerical computation, it is determined that the flow development is relatively insensitive to the imposed wall-boundary condition. This seems to suggest that the physical mechanism leading to the formation of the boundary-layer vortex is an inviscid rotational one. Received: 14 February 1998/Accepted: 11 August 1998     

Impact of an ideal fluid jet on a curved wall: the inverse problem

This paper analyzes an ideal fluid jet impinging a wall. The usual two-dimensional model of jet flow uses an ideal, incompressible, weightless fluid, and maps this flow in a way that reduces it to a problem of complex analysis that cannot be solved analytically. An efficient procedure is presented here for solving the inverse problem numerically in the case of an arbitrary wall shape, i.e. the design of a wall corresponding to a prescribed velocity (or pressure) distribution. In similar studies, as in airfoil design, important constrains have to be applied to the prescribed distribution in order to ensure the existence of a solution. Not only is this not the case here, but also a constraint must be added to impose the uniqueness of the solution.     

Turbulence measurements in a grid mixing tank

Summary An experiment is described in which a laser-Doppler anemometer was used to measure the spatial decay of turbulence generated by a vertically oscillating grid. In addition to being of theoretical interest, these measurements are necessary for scaling results from mixed-layer entrainment experiments. The main result of the present experiments is the determination of the variation of the length and velocity scales of the grid-induced motion as a function of distance from the grid. The results confirm previous reports that the turbulent kinetic energy (actually, the mean square fluctuating velocity) decreases approximately with the inverse square of the distance from the grid. Power spectra are calculated for the velocity signal and these, along with the raw data, are used to analyze the development of fluid motion with increasing distance from the grid. It is shown that true turbulence is not obtained for depths less than about two mesh spacings from the grid. Problems with background noise in the laser signal are also discussed.

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Sommario Si è utilizzato un laser-doppler per la misura dell'abbattimento della turbolenza generata da una griglia in moto verticale-oscillatorio. Tali misure, oltre a rivestire interesse teorico, tendono a valutare le variazioni delle scale di lunghezza e velocità del moto turbolento in funzione della profondità (distanza dalla griglia). I risultati concordano con precedenti esperimenti con i quali si è dimostrato che l'energia cinetica legata a tale turbolenza decresce con l'inverso del quadrato della distanza dalla griglia.I dati, rilevati con l'ausilio di un calcolatore, sono stati usati per calcolare gli spettri di energia e quindi analizzare lo sviluppo del moto del fluido incrementando la profondità. Si è mostrato che la turbolenza si sviluppa a profondità maggiori di circa due volte la dimensione della maglia che costituisce la griglia.Si sono infine discussi i problemi connessi con i disturbi di fondo di diversa natura presenti nell'acquisizione dati.

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Readers interested in more details, may request them to Dott. Ing. L. Damiani, Ist. di Idraulica e Costruzioni Idrauliche, Facoltà di Ingegneria Università di Bari, Via Re David 200, 70125 Bari.     

Turbulence measurements in a swirling pipe flow

This paper reports laser-Doppler measurements of the mean flow and turbulence stresses in a swirling pipe flow. Experiments were carried out under well-controlled laboratory conditions in a refractive index-matched pipe flow facility. The results show pronounced asymmetry in mean and fluctuating quantities during the downstream decay of the swirl. Experimental data reveal that the swirl significantly modifies the anisotropy of turbulence and that it can induce explosive growth of the turbulent kinetic energy during its decay. Anisotropy invariant mapping of the turbulent stresses shows that the additional flow deformation imposed by initially strong swirling motion forces turbulence in the core region to tend towards the isotropic two-component state. When turbulence reaches this limiting state it induces rapid production of turbulent kinetic energy during the swirl decay.

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PIV measurements of a plane wall jet in a confined space at transitional slot Reynolds numbers

Wall jets are important for a wide variety of engineering applications, including ventilation of confined spaces and cooling and drying processes. Although a lot of experimental studies have been devoted to wall jets, many of these have focused on laminar or turbulent wall jets. There is a lack of experimental data on transitional wall jets, especially transitional wall jets released into a confined space or enclosure. This paper presents flow visualizations and high-resolution Particle Image Velocimetry measurements of isothermal transitional plane wall jets injected through a rectangular slot in a confined space. As opposed to many previous studies, not only the wall jet region but also the recirculation region in the remainder of the enclosure is analyzed. The data and analysis in this paper provide new insights into the behavior of transitional plane wall jets in a confined space and will be useful for the validation of numerical simulations of this type of jets.     

Turbulence characterisation of a confined jet using PIV

The turbulence in the bulk free region of a confined jet is investigated using a simple video-based particle image velocimetry technique. The latter, which relies on the interlacing properties of video filming and simple particle tracking concepts, is suitable for less dense seeding levels and lower flow velocities. The results obtained for the turbulence intensity variation in the jet agree with known data from literature. Spatially resolved integral length scales are calculated, and a relation for Eulerian-type length scales is proposed.     

Concentration measurements in a self-excited jet

The near field of helium-air jets exhausting into an air environment has been experimentally investigated using an aspiration probe and flow visualization. Jets with varying density ratios and Reynolds numbers were studied. Pure helium jets with density ratios of 0.14 were found to display a self-excited behavior characterized by intense mixing. The centerline concentration decay was found to be substantially increased for the self-excited jet. Flow visualization revealed the expulsion of side jets from the potential core region of low density jets. Radial profiles of concentration provide additional evidence that side-jets produce vigorous mixing.     

Turbulence production in flow over a wavy wall

Measurements of the spatial and time variation of two components of the velocity have been made over a sinusoidal solid wavy boundary with a height to length ratio of 2a/λ = 0.10 and with a dimensionless wave number of α⁺ = (2π/λ)(v/u ^?) = 0.02. For these conditions, both intermittent and time-mean flow reversals are observed near the troughs of the waves. Statistical quantities that are determined are the mean streamwise and normal velocities, the root-meansquare of the fluctuations of the streamwise and normal velocities, and the Reynolds shear stresses. Turbulence production is calculated from these measurements. The flow is characterized by an outer flow and by an inner flow extending to a distance of about α^?1 from the mean level of the surface. Turbulence production in the inner region is fundamentally different from flow over a flat surface in that it is mainly associated with a shear layer that separates from the back of the wave. Flow close to the surface is best described by an interaction between the shear layer and the wall, which produces a retarded zone and a boundary-layer with large wall shear stresses. Measurements of the outer flow compare favorably with measurements over a flat wall if velocities are made dimensionless by a friction velocity defined with a shear stress obtained by extrapolating measurements of the Reynolds stress to the mean levels of the surface (rather than from the drag on the wall).