1,720,993 research outputs found
Experimental investigation of impinging sweeping jets
No full textSweeping jets are characterized by an oscillating motion and they are generated by a fluidic oscillator with no moving part [1]. The geometric characteristics of such an oscillator deeply affect the fluid dynamic behavior of the issued sweeping jet. In order to characterize the mean flow field, the oscillating coherent flow field and the turbulent statistics of impinging sweeping jets, phase-locked particle image velocimetry (PIV) measurements are carried out. Several sweeping jet devices have been studied. In particular, three different mixing chamber lengths (2.5w, 3.5w and 4.5w) have been investigated at Reynolds number equal to 6300 and nozzle-to-plate distance (H) ranging between 2 and 10 nozzle width (w).
The flow field, for the case characterized by the longest mixing chamber (4.5w) and shortest nozzle-to-plate distance (2w), is reported in figure 1. These results show a double peak distribution of the time-averaged streamwise velocity component U, caused by the oscillation of the sweeping jet, in agreement with the literature, while the time-averaged vertical velocity component V exhibits the typical behavior of a wall jet. The time-averaged phase correlated kinetic energy (PKE) and turbulent kinetic energy (TKE) maps highlight the regions where the coherent and uncoherent fluctuations are stronger. The observed PKE behavior can be explained considering the oscillating nature of the sweeping jet, while the TKE distribution is mainly caused by the presence of the oscillating jet shear layer
The von Kármán street behind a circular cylinder: Flow control through synthetic jet placed at the rear stagnation point
No full textThe present paper aims at establishing the synthetic jet technology capabilities in controlling the von Kármán street behind a circular cylinder. The circular cylinder, placed in an open-circuit wind tunnel, presents a slot in its rear position, through which the synthetic jet is issued. The Reynolds number, based on the circular cylinder diameter and the free-stream velocity, is equal to 4600 and the von Kármán street is characterized, in the baseline configuration (i.e. without synthetic jet), by a shedding frequency of 16.2 Hz. Several synthetic jet operating conditions are tested. Therefore, the effects of the momentum coefficient (%, 10.8% and 21.6%) and the dimensionless frequency (, 0.98 and 1.96) on the von Kármán street behaviour can be analysed. Instantaneous two-dimensional in-plane velocity fields are measured in a plane containing the synthetic jet slot axis using multigrid/multipass cross-correlation digital particle image velocimetry. These measurements have been used to investigate the mean flow quantities and turbulent statistics of the phenomenon. In addition, the wake extent and behaviour (i.e. symmetric or asymmetric) are analysed as well as the drag coefficient, for each configuration. The extent of the wake region decreases as the momentum coefficient and/or the dimensionless frequency increase, while the symmetric/asymmetric wake behaviour is found to be governed by a different control parameter: the synthetic jet Reynolds number based on its impulse. As regards the drag coefficient, a maximum reduction, of approximately 35%, is found for the configuration at and
Experimental analysis of Rayleigh-Bénard convection in a cylindrical cell by tomographic PIV
No full textThis paper presents an experimental investigation of Rayleigh-Bénard convection in both rotating and non- rotating conditions. Rayleigh-Bénard convection, the fluid flow driven by temperature gradients parallel to the gravity, is relevant to a great variety of physical phenomena, ranging from motions in the atmosphere and the oceans to convection in the interior of planets and stars. In many of these applications, the interplay between a background rotation and the thermal convection has a key role in the fluid dynamic behaviour of the system. In the present work, the time-resolved tomographic particle image velocimetry (PIV) is used to investigate the whole domain of Rayleigh-Bénard convection inside a cylinder with aspect ratio equal to 1/2 at Rayleigh and Prandtl numbers equal to 1.86 ×10^8 and 7.6, respectively. The effects of rotation on the flow dynamics and evolution are also investigated in similar operating conditions at two different Rossby numbers, namely 0.25 and 0.1. The behaviour of the turbulent flow is analyzed both in the time-average and instantaneous evolution with focus on the large scale structures of the flow. Modal decomposition techniques are also used to identify the characteristic modes of the thermal convection
A semi-empirical correlation for the swirl number of swirling jets generated by a radial-type swirler
No full textThis work presents a semi-empirical correlation for the swirl number of a radial-type swirl generator under different conditions of both the Reynolds number and the angle of the swirler channels used to generate the flow tangential velocities. Quantitative velocity measurements are carried out via stereoscopic particle image velocimetry (PIV) to characterize the three-component velocity field of the swirling jets in the proximity of the nozzle exit for four values of the Reynolds number (i.e., 12,800, 20,800, 31,100 and 41,500) and nine swirl angles ranging from 0° to 20°. Such data is used to estimate the values of the swirl number from different theoretical formulations. Semi-empirical formulas are derived based on the mass and angular momentum conservation laws and analytical approximations of the quantities involved in the definition of the swirl number. These formulas elucidate the dependence of the swirl number on the geometrical parameters of the radial-type swirl generator, in particular on the swirl angle, and include coefficients estimated empirically from the velocimetry data, which essentially depend on the exit velocity distribution. The derived correlations may be useful for design and analysis purposes
Effects of the swirl number, Reynolds number and nozzle-to-plate distance on impingement heat transfer from swirling jets
No full textThis paper reports on a parametric study of the impingement heat transfer from swirling jets issuing from a circular nozzle. The swirl velocity component is imparted to the flow via a tangential entry into a recirculation chamber located upstream of the nozzle; the degree of swirl is varied by changing the angle of the channels leading to the chamber with respect to the radial direction. The effects of the dimensionless impingement distance (H/D = 1, 2, 3, 4, 6, 8, 10, 12, 14 with D being the nozzle diameter), the swirl number (S = 0, 0 . 078, 0 . 22, 0 . 37, 0 . 54) and the Reynolds number (Re = 20,800 , 31,100 , 41,500) on the spatial distribution of the heat transfer are investigated by means of infrared thermography and the heated thin foil sensor. The present results show that the swirl number and the impingement distance significantly influence the structure of the heat transfer distribution, while the Reynolds number affects essentially the magnitude. Four distinct ranges with different behaviours can be identified by distinguishing between weakly or non swirling jets (0 ≤S ≤0 . 22) and moderately swirling jets (0 . 37 ≤S ≤0 . 54) and between short and long impingement distances (1 ≤H/D ≤4 and 6 ≤H/D ≤14 respectively). A comparative assessment of the performance of the swirling jets is carried out by focusing on the area-averaged Nusselt number distributions. It is found that in the range of short impingement distances and over relatively small target areas an enhancement of the heat transfer rates can be obtained at a small extent by adding a weak swirl with no detrimental effect on the uniformity of the distribution and at a larger extent by a moderate swirl with a deterioration of the uniformity. For long impingement distances no enhancement is observed, although swirl yields a significant reduction of the non-uniformity. Finally, correlation laws of the area-averaged Nusselt number as a function of the control parameters are derived in the four regimes identified
Experimental determination of the 3-D characteristic modes of turbulent Rayleigh-Bénard convection in a cylinder
No full textThe present paper reports on a time-resolved three-dimensional experimental study of turbulent Rayleigh-Bénard convection inside a cylinder with one-half aspect ratio. The working fluid is water and the Rayleigh and Prandtl numbers are, respectively, and. Measurements are carried out via time-resolved particle tracking velocimetry for a relatively long time (approximately four hours) and due to the limited size of the convection cell (internal diameter of mm) the whole interior of the cylindrical sample is investigated. This allows a proper analysis of the statistical behaviour of the flow across the time. Proper orthogonal decomposition (POD) is used to extract the characteristic modes of the turbulent thermal convection. It is shown that the low-order POD modes are strictly related to the formation of a large scale circulation (LSC) and its organization in a single-roll state (SRS) or a double-roll state. Innovative criteria for the identification of the instantaneous flow state based on the POD analysis are also proposed. Such criteria are proved to overcome the limitations of methods commonly adopted in the previous literature and relying on the analysis of the azimuthal profiles of the temperature or the vertical velocity at three different heights (one quarter, one half and three quarters of the cell height). Compared with the latter methods, the POD-based criteria identify a larger frequency of occurrence of the SRS, which is recognized as the most frequent state of the LSC in the investigated conditions
Flow field features of chevron impinging synthetic jets at short nozzle-to-plate distance
No full textThe flow field of a chevron synthetic jet in impinging configuration at Reynolds number Re=4500, dimensionless stroke length L0/D=28 and nozzle-to-plate distance H/D=2 is experimentally investigated by means of Stereoscopic Particle Image Velocimetry. This study is motivated by the need of explaining the heat transfer behaviour of the chevron synthetic jet highlighting its differences with respect to the circular synthetic jet. Two different experiments are carried out. The first experiment is devoted to the characterisation of the flow field evolution on the impingement plate. The second experiment is a three-dimensional reconstruction of the entire flow field at three characteristic phases. The results show that the circular synthetic jet presents a circular velocity pattern on the plate, while the chevron exit leads to the acceleration of the fluid, thus to a larger impinging velocity with a star-shaped pattern. Furthermore, in both configurations a secondary vortex ring is formed on the wall at a radial position of about 2 diameters. For the circular synthetic jet, the intense turbulence levels are located in two annular regions: the inner region is due to the turbulent wall jet generated by the impinging trailing jet and the outer region is due to the effect of the generation of the secondary vortex ring. For the chevron synthetic jet, the intense turbulence level is produced by the turbulent wall jet along preferential paths. These preferential paths are caused by the star-shaped distribution of impinging velocity and the impinging streamwise coherent vortices. These features characterizing the impinging flow field are fundamental to understand and explain the mechanisms behind the heat transfer behaviour of such devices
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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