1,720,964 research outputs found
Pressure from particle image velocimetry for convective flows: a Taylor’s hypothesis approach
No full textTaylor's hypothesis is often applied in turbulent flow analysis to map temporal information into spatial information. Recent efforts in deriving pressure from particle image velocimetry (PIV) have proposed multiple approaches, each with its own weakness and strength. Application of Taylor's hypothesis allows us to counter the weakness of an Eulerian approach that is described by de Kat and van Oudheusden (2012 Exp. Fluids 52 1089–106). Two different approaches of using Taylor's hypothesis in determining planar pressure are investigated: one where pressure is determined from volumetric PIV data and one where pressure is determined from time-resolved stereoscopic PIV data. A performance assessment on synthetic data shows that application of Taylor's hypothesis can improve determination of pressure from PIV data significantly compared with a time-resolved volumetric approach. The technique is then applied to time-resolved PIV data taken in a cross-flow plane of a turbulent jet (Ganapathisubramani et al 2007 Exp. Fluids 42 923–39). Results appear to indicate that pressure can indeed be obtained from PIV data in turbulent convective flows using the Taylor's hypothesis approach, where there are no other methods to determine pressure. The role of convection velocity in determination of pressure is also discussed
Instantaneous planar pressure determination from PIV in turbulent flow
Full text linkThis paper deals with the determination of instantaneous planar pressure fields from velocity data obtained by particle image velocimetry (PIV) in turbulent flow. The operating principles of pressure determination using a Eulerian or a Lagrangian approach are described together with theoretical considerations on its expected performance. These considerations are verified by a performance assessment on a synthetic flow field. Based on these results, guidelines regarding the temporal and spatial resolution required are proposed. The interrogation window size needs to be 5 times smaller than the flow structures and the acquisition frequency needs to be 10 times higher than the corresponding flow frequency (e.g. Eulerian time scales for the Eulerian approach). To further assess the experimental viability of the pressure evaluation methods, stereoscopic PIV and tomographic PIV experiments on a square cylinder flow (ReD = 9,500) were performed, employing surface pressure data for validation. The experimental results were found to support the proposed guidelines.Aerospace Engineerin
Aerodynamic performance of electro-active acrylic membrane wings
No full textElectro-active materials offer, due to their multivariate compliant nature, a great potential for integrating the lift-producing system and the control system into one. This work presents the first step in describing the aerodynamic performance of such materials and focuses on understanding the nature of such materials in aerodynamic applications. Load measurements are conducted in a wind tunnel for acrylic-based membranes supported in a perimeter-reinforced frame at different prestrains to determine which prestrain offers the widest range of controllability. Furthermore, photogrammetry and load measurements are conducted at zero prestrain for no-voltage and constant-voltage conditions. The resulting data show that, from a controllability standpoint as well as from a lift and aerodynamic performance standpoint, lower prestrains are more favorable. The resulting data also show that, at zero prestrain, the membrane damps most fluid-membrane interaction instabilities resulting in low-level membrane vibrations.</p
Dynamic pitching effect on a laminar separation bubble
Full text linkThe unsteady effect of a periodic pitching motion on the characteristic of a laminar separation bubble on the suction side of a SD7003 aerofoil is investigated by means of time-resolved planar and tomographic particle image velocimetry. The measurements provide information on the separation, transition and vortex roll-up onset as well as the spanwise distribution of vortical structures, for both the dynamic pitching between 4° and 8° and corresponding cases at a static pitch angle. During pitching, a clear hysteresis behaviour is observed for the vortex roll-up position and shedding frequency, showing a strongly delayed recovery of the shear layer with respect to the steady aerofoil case. The development of the shear layer transition exhibits initially 2D Kelvin–Helmholtz rollers that are interrupted, forming ?-shaped rollers, which eventually evolve into 3D arch-shaped hairpin structures. The 3D analysis of undulated rollers allowed the determination of the rollers streamwise spatial separation for both static and pitching aerofoil case
Aerodynamic Experiments on DelFly II: Unsteady Lift Enhancement
No full textParticle image velocimetry measurements and simultaneous force measurements have been performed on the DelFly II flapping-wing MAV, to investigate the flow-field behavior and the aerodynamic forces generated. For flapping wing motion it is expected that both the clap and peel mechanism and the occurrence of a leading edge vortex during the translational phase play an important role in unsteady lift generation. Furthermore, the flexibility of the wing foil is also considered of primary relevance. The PIV analysis shows a strong influx between the wings during the peel but no downward expelling jet during the clap. The force measurements reveal that the peel, oppositely to the clap, contributes significantly to the lift. The PIV visualization suggests the occurrence of a leading edge vortex during the first half of the in- and outstroke, which is supported by a simultaneous augmentation in lift. The early generation of a leading edge vortex during the flex cannot be assessed from the PIV images due to optical obstruction, but is likely to appear since the wing flexing is accompanied with a large increase in lift.Aerodynamics & Wind EnergyAerospace Engineerin
Instantaneous planar pressure determination from particle image velocimetry
No full textForces on flapping or rotating wings, like flapping wings of micro air vehicles or blades of wind turbines are of great interest to engineers. To investigate the ways birds and insects fly, forces created by flapping wings are of importance to biologists. The pressure field, combined with the velocity field, gives a complete description of the (incompressible) flow dynamics. Furthermore the pressure field is the main contributor to the aerodynamic loading of bodies immersed in the fluid. Traditional techniques to determine pressure and forces rely on the determination of surface pressure and integral loads by point pressure and force balance measurements. In situations where it is difficult (or impossible) to instrument the body, using particle image velocimetry (PIV) velocity data to determine forces and pressure poses an interesting alternative to the existing approaches to determine sectional loading.AerodynamicsAerospace Engineerin
The convection of large and intermediate scale fluctuations in a turbulent mixing layer
No full textThe convection velocity of large and intermediate scale velocity fluctuations in a nominally two-dimensional planar mixing layer, and its dependence upon the length scale, is explored by carrying out particle image velocimetry (PIV) experiments. A “global” convection velocity, containing the convection of all the length scales present in the flow, is produced by examining the autocorrelation functions between velocity fluctuations in successive PIV records across the mixing layer. This “global” convection velocity is found to be similar to the mean flow, although fluctuations on the low speed side of the mixing layer on average convect at speeds greater than the mean and fluctuations on the high speed side of the mixing layer are observed to convect at speeds less than the mean. Scale specific convection velocity profiles are then produced by examining the phase difference between the spectral content specific to one wavenumber in streamwise velocity fluctuation traces in successive PIV records, offset by time ?. Probability density functions (pdfs) are produced of this phase difference, which is subsequently converted into a convection displacement, and these show that the convection of single length scale fluctuations exhibits a significant variance, particularly so for larger scale fluctuations and in the high speed side of the mixing layer. Convection velocity profiles are produced from these pdfs using both the mean convection distance and the modal convection distance. It is observed that the convection velocity is relatively insensitive to the length scale of the fluctuations considered, particularly when the mean convection distance is used. A slight sensitivity to length scale is, however, observed for convection velocities based on the modal convection distance. This dependence is primarily observed in the high speed side of the mixing layer, in which smaller length-scale fluctuations convect more quickly than larger length-scale fluctuations. It is also observed that the magnitude of the fluctuation itself affects the convection velocity with larger magnitude fluctuations convecting less rapidly than lower magnitude ones at the largest length scales investigated with this behaviour being reversed at more intermediate length scales.<br/
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
Vortex decay behind a generic wing-flap-jet aircraft model: A time dependent vortex model
No full textThe safety threat posed by vortex wake encounters acts as a limiting factor in meeting the growing demand for airport capacity. To gain more insight in the governing processes in aircraft wakes fundamental research on the formation, persistence and decay of the wake behind aircraft is performed within the EU-funded FAR-Wake project. As part of this project we experimentally determined the influence of a cold jet on the wake behind a wing-flap model -resembling an aircraft in landing/take-off configuration-. The application of a traversing-FOV stereo-PIV system in the TU Delft towing tank leads to an accurate determination of the complete velocity field in a plane perpendicular to the ’flight’ direction for large distance behind the model (up to 100 wing spans).Aerospace Engineerin
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