1,721,008 research outputs found
Large and small scales in a turbulent orifice round jet: Reynolds number effects and departures from isotropy
Full text linkAn experimental investigation of the near field of a turbulent orifice jet is performed using high resolution Particle Image Velocimetry, aiming to highlight effects on the flow field due to changes in Reynolds number. The attention is focused onto departures from isotropy for large and small scales, by considering statistics of mean square velocity and velocity derivatives and specifically the non-dimensional ratios of such quantities. The results compare well with available literature data and pointed out that the effects of Reynolds number on large scales are usually small and limited to a region ranging less than seven-ten diameters from the jet outlet. For small scales, such Reynolds number dependence is extended up to ten-fifteen diameters. Farther from the jet exit, Reynolds number dependence almost disappears and all data approach similar asymptotic behaviors. On the other hand, velocity and some velocity derivative statistics clearly show that neither large nor small scale statistics strictly follow the isotropy condition; nonetheless, differences from that condition are limited to a factor which is almost constant in the whole measured field. In order to provide a link between such large and small scale departures from isotropy, a relation among mean square velocity ratios and mean square derivative ratios is proposed and proved to be well verified in the measured region and interval of Reynolds numbers. This relation allows deriving small scale derivative ratios, which are difficult to measure experimentally or to obtain numerically, due to high resolution requirements, from large scale velocity ratios, which are achieved much easier
Experimental investigations on mixing evaluation in non-circular sharp edge nozzles
No full textAn experimental study using Particle Image Velocimetry (PIV) on free jets issuing from sharp edge nozzles having
different cross-section geometries is reported: elliptical, square, rectangular and triangular. The mixing efficiency of each nozzle is
quantified at large and small scales. In respect to the large scales contribution, the definition of mixedness is used, while for smallscales, the entropy generation concept is employed. These quantities are computed to evaluate the mixing efficiency in the near field
and interaction zone (
Investigation on the effect of horizontal and vertical deflectors on the near-wake of a square-back car model
No full textThe near-wake flow field downstream of a square-back car model is analyzed to investigate on the combined effect of horizontal and vertical deflectors. Aerodynamic coefficients and near-wake velocity fields are presented for a range of deflectors size and slant angles, for Reynolds number Re ≈ 250,000 based on free stream velocity and model height. Mean velocity fields and turbulence quantities are shown to shed light onto the interactions taking place within the wake. The underlying rationale is to couple a decrease of the wake extent along the vertical direction, achieved by reducing the size of the top separated region, to a simultaneous reduction of the fluctuations in the horizontal plane induced by the lateral longitudinal vortices. Vertical deflectors height is reported to play a key role in this scenario markedly affecting the resulting drag and lift coefficients. Analysis of velocity fields points out the existence of an optimal vertical spoiler size, in the presented set-up equal to 2/3 of model height, which features the overall best performance compared to other tested configurations. The beneficial effect of combined spoilers is found to be more relevant when horizontal and vertical deflectors feature a moderate (20°) slant angle with respect to free stream direction
Propagation of perturbations and meandering in a free surface shallow water jet
Full text linkA shallow water free-surface turbulent jet is investigated at Froude numbers variable between about 0.15 and 0.6 and a Reynolds number equal to 5000, with a jet height-to-width aspect ratio variable from 0.5 to 1.1. In these experimental conditions, the flow is unstable and develops local transverse instabilities, which are amplified to a global meandering motion, forcing the jet to oscillate orthogonally to its axis. Instantaneous and averaged velocity fields are obtained by means of high-density, correlation-based time-resolved Particle Tracking Velocimetry. From the average point of view, the present jet configuration resembles a confined jet condition similar to that of wall jets, but with additional relevant three-dimensional effects, retaining self-similar properties, with a net co-flow and some ambient fluid entrainment. For increasing Froude numbers, such configuration is moved downstream and the jet spreading is delayed. The high resolution in space and time of present measurements allows to locally detect and following perturbed patterns and to derive amplitude and frequency of oscillation of the global meandering motion, the former increasing with axial distance and decreasing with Froude number, the latter doing just the opposite. The velocity of propagation of perturbed patterns is also investigated in comparison to the jet mean velocity, the local celerity of propagation of fluctuating velocity fields being derived. Results indicate that axial fluctuations propagate downstream similarly to transverse fluctuations along the orthogonal direction, thus showing a close coupling among the two motions and the onset of a self-sustained mechanism driven by the mean flow, responsible for the generation of the macroscopic meandering motion. This effect is in agreement with numerical predictions and is emphasised as the Froude number increases
Experimental investigations on secondary structures in a fully developed turbulent jet
No full textThe effect of streamwise vorticity on entrainment in jet flows has been the subject of various works and is related to many applications involving effective fluid mixing. In this work the near-field of a fully developed turbulent round jet was investigated at different Reynolds numbers ranging from ≈ 3000 to ≈ 30000 by means of time-resolved particle image velocimetry. Acquisitions were carried out on transverse planes at different downstream positions, in order to visualize the evolution of vortical structures and to assess their features. Results from average velocity fields and rms show a strong impact of crosswise vortical structures on radial velocity distributions as downstream distance increases. Velocity correlation functions analysis exhibits a decrease of crosswise structures size as the Reynolds number increases, reaching an asymptotic behaviour which may be also observed in TKE distributions, which feature high levels in the outer radial area. Entrainment rates calculations supported by spectral analysis confirm the decisive role of streamwise vorticity in the control of the entrainment process as well as its dependence on Reynolds number. Entrainment rates approaching an asymptotic state versus Reynolds number suggests a connection between the effectiveness of this process and crosswise vortical structures size
Effect of windshield configuration and pilot position on motorcycle performances
No full textThe effect of three windshield shapes on the aerodynamic performance of a racing-type motorcycle is investigated experimentally via global wind-tunnel force balance measurement and detailed local wake analysis by Particle Image Velocimetry. Results are compared to two driver's stances as well as to a baseline case. Results are dependent mainly on the windshield length as coupled to the driver positioning. Full-size windshield is characterized by a quite uniform wake structure and reduced level of velocity fluctuations and turbulence and is associated to improved stability, in spite of a relative increase of drag. When the additional safety and driver's protection from weather conditions are considered, the use of a full-size windshield appears an effective solution for commercial motorcycles
Investigation on vortex dynamics downstream moving leaflets by means of Robust Image Velocimetry
No full textExperimental investigation on the flow field and performances of a Savonius-Darrieus wind turbine
No full textIn this paper, measurements performed on a vertical axis wind turbine (VAWT) model are reported. The turbine is a combination of Savonius and Darrieus rotors to optimize performances in medium-low wind regimes. To this end, the turbine is positioned into a wind-tunnel to derive instantaneous and averaged velocity fields by means of Particle Image Velocimetry (PIV). PIV data allow to derive specific fluid flow phenomena on each single and on combined geometrical configurations. These results are also compared with electrical measurements to determine performances, efficiency and the best working conditions
Heavy particles in the near field of a turbulent jet
Full text linkThe behaviour of aluminium spherical particles of density ρp=2700 Kg/m3 dispersed in a turbulent round water jet, at Reynolds number Re≈15000 and exit Stokes number St0=0.013 is investigated via Particle Image Velocimetry up to x/D=16 from the nozzle exit. Average and fluctuating velocity fields are analysed and compared to the unladen jet condition. Generally, particles are reported not to behave like tracers, in particular outside the region of the jet potential core. Velocity fluctuations are reported to differ from the fluid fluctuations in particular at the jet centreline and shear layers. In particular, axial velocity fluctuations appear to be dominated by the fluid phase whereas a tendency towards an increase of radial fluctuations downstream of the jet exit is reported for particles. The complexity of this scenario suggests a selective particle response with respect to the flow dynamics
Investigation of vortex dynamics downstream of moving leaflets using robust image velocimetry
No full textThe interaction of a sudden flow through a rectangular slot with moving leaflets, hinged at its border, was investigated experimentally in a Plexiglas vessel. This configuration resembles schematically some key features of many biological flows, e.g. in sea-animal propulsion, where the moving flaps control the flow, optimizing thrust, or in heart valves, where leaflets prevent backflow. Therefore, the comprehension of the basic mechanisms of the flow-structure interaction and of the features of the flow is of interest in a wide range of applications. Although some detail of the phenomenon could depend on the specific leaflet design, material and forcing, the objective of the present work is to investigate the overall dependence of the flow field on the leaflet arrangement. Specifically, three leaflet configurations have been tested at Reynolds number Re = 2,000 and Strouhal number St = 0.2: two symmetrical leaflets, two non-symmetrical leaflets, one being twice as wide as the other, and a single leaflet. Velocity fields were obtained using Robust Image Velocimetry in order to accurately resolve the structure of the vorticity field. The dynamics of the opening leaflets, the vorticity fields and the features of the vortices generated during the leaflet opening were investigated and compared in the different leaflet configurations. Advantages in the opening time, maximum aperture and closing time were observed in the two-leaflet non-symmetrical case in comparison to the other configurations
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