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Publisher: Springer-Verlag   (Total: 2209 journals)

 e & i Elektrotechnik und Informationstechnik       (Followers: 8, SJR: 0.146, h-index: 8) e-Neuroforum Early Childhood Education J.       (Followers: 12, SJR: 0.367, h-index: 12) Earth Science Informatics       (Followers: 3, SJR: 0.245, h-index: 5) Earth, Moon, and Planets       (Followers: 5, SJR: 0.436, h-index: 28) Earthquake Engineering and Engineering Vibration       (Followers: 7, SJR: 0.433, h-index: 17) Earthquake Science       (Followers: 9, SJR: 0.486, h-index: 7) East Asia       (Followers: 7, SJR: 0.165, h-index: 9) Eating and Weight Disorders - Studies on Anorexia, Bulimia and Obesity       (Followers: 7, SJR: 0.289, h-index: 23) EcoHealth       (Followers: 1, SJR: 0.651, h-index: 22) Ecological Research       (Followers: 7, SJR: 0.698, h-index: 38) Economic Botany       (Followers: 8, SJR: 0.666, h-index: 40) Economic Bulletin       (Followers: 4) Economic Change and Restructuring       (Followers: 1, SJR: 0.263, h-index: 6) Economic Theory       (Followers: 6, SJR: 1.857, h-index: 31) Economic Theory Bulletin       (Followers: 1) Economics of Governance       (Followers: 2, SJR: 0.367, h-index: 12) Ecosystems       (Followers: 19, SJR: 1.793, h-index: 83) Ecotoxicology       (Followers: 10, SJR: 1.041, h-index: 53) Education and Information Technologies       (Followers: 154, SJR: 0.207, h-index: 15) Educational Assessment, Evaluation and Accountability       (Followers: 13, SJR: 0.519, h-index: 14) Educational Psychology Review       (Followers: 14, SJR: 1.781, h-index: 52) Educational Research for Policy and Practice       (Followers: 6, SJR: 0.211, h-index: 8) Educational Studies in Mathematics       (Followers: 9, SJR: 0.946, h-index: 27) Educational Technology Research and Development       (Followers: 172, SJR: 1.124, h-index: 45) Electrical Engineering       (Followers: 11, SJR: 0.352, h-index: 17) Electrocatalysis       (SJR: 0.542, h-index: 7) Electronic Commerce Research       (Followers: 3, SJR: 0.636, h-index: 14) Electronic Markets       (Followers: 5, SJR: 0.326, h-index: 5) Electronic Materials Letters       (Followers: 3, SJR: 0.566, h-index: 11) Elemente der Mathematik Emergency Radiology       (Followers: 4, SJR: 0.446, h-index: 22) Empirica       (Followers: 3, SJR: 0.185, h-index: 12) Empirical Economics       (Followers: 8, SJR: 0.5, h-index: 29) Empirical Software Engineering       (Followers: 4, SJR: 2.319, h-index: 33) Employee Responsibilities and Rights J.       (Followers: 2, SJR: 0.21, h-index: 13) Endocrine       (Followers: 4, SJR: 0.659, h-index: 55) Endocrine Pathology       (Followers: 2, SJR: 0.555, h-index: 27) Energy Efficiency       (Followers: 11, SJR: 1.056, h-index: 10) Energy Systems       (Followers: 9, SJR: 0.589, h-index: 5) Engineering With Computers       (Followers: 5, SJR: 0.497, h-index: 26) Entomological Review       (Followers: 3, SJR: 0.128, h-index: 5) Environment Systems & Decisions       (Followers: 2) Environment, Development and Sustainability       (Followers: 28, SJR: 0.319, h-index: 26) Environmental and Ecological Statistics       (Followers: 5, SJR: 0.389, h-index: 29) Environmental and Resource Economics       (Followers: 17, SJR: 1.651, h-index: 46) Environmental Biology of Fishes       (Followers: 3, SJR: 0.486, h-index: 53) Environmental Chemistry Letters       (Followers: 3, SJR: 0.664, h-index: 22) Environmental Earth Sciences       (Followers: 10, SJR: 0.601, h-index: 55) Environmental Economics and Policy Studies       (Followers: 6, SJR: 0.35, h-index: 3) Environmental Evidence Environmental Fluid Mechanics       (Followers: 2, SJR: 0.732, h-index: 23) Environmental Geochemistry and Health       (Followers: 2, SJR: 0.909, h-index: 32) Environmental Geology       (Followers: 11) Environmental Health and Preventive Medicine       (Followers: 2, SJR: 0.388, h-index: 14) Environmental Management       (Followers: 31, SJR: 0.773, h-index: 60) Environmental Modeling & Assessment       (Followers: 11, SJR: 0.413, h-index: 27) Environmental Monitoring and Assessment       (Followers: 9, SJR: 0.671, h-index: 46) Environmental Science and Pollution Research       (Followers: 11, SJR: 0.878, h-index: 42) Epidemiologic Perspectives & Innovations       (Followers: 1, SJR: 1.002, h-index: 14) Epileptic Disorders       (Followers: 1, SJR: 0.669, h-index: 34) EPJ A - Hadrons and Nuclei       (Followers: 1, SJR: 1.435, h-index: 58) EPJ B - Condensed Matter and Complex Systems       (Followers: 3, SJR: 0.749, h-index: 85) EPJ direct EPJ E - Soft Matter and Biological Physics       (Followers: 1, SJR: 0.661, h-index: 57) EPMA J.       (SJR: 0.161, h-index: 4) ERA-Forum       (Followers: 2, SJR: 0.13, h-index: 2) Erkenntnis       (Followers: 11, SJR: 0.62, h-index: 14) Erwerbs-Obstbau       (SJR: 0.173, h-index: 8) Esophagus       (SJR: 0.268, h-index: 9) Estuaries and Coasts       (Followers: 3, SJR: 1.111, h-index: 61) Ethical Theory and Moral Practice       (Followers: 7, SJR: 0.278, h-index: 8) Ethics and Information Technology       (Followers: 176, SJR: 0.363, h-index: 20) Ethik in der Medizin       (SJR: 0.204, h-index: 6) Euphytica       (Followers: 7, SJR: 0.709, h-index: 57) Eurasian Soil Science       (Followers: 2, SJR: 0.271, h-index: 10) EURO J. of Transportation and Logistics       (Followers: 4) EURO J. on Computational Optimization EURO J. on Decision Processes Europaisches J. fur Minderheitenfragen European Actuarial J.       (Followers: 3) European Archives of Oto-Rhino-Laryngology       (Followers: 4, SJR: 0.737, h-index: 37) European Archives of Paediatric Dentistry       (Followers: 1, SJR: 0.446, h-index: 12) European Archives of Psychiatry and Clinical Neuroscience       (Followers: 2, SJR: 1.334, h-index: 62) European Biophysics J.       (Followers: 4, SJR: 0.979, h-index: 53) European Child & Adolescent Psychiatry       (Followers: 4, SJR: 1.269, h-index: 51) European Clinics in Obstetrics and Gynaecology       (Followers: 4) European Food Research and Technology       (Followers: 8, SJR: 0.773, h-index: 49) European J. for Education Law and Policy       (Followers: 5) European J. for Philosophy of Science       (Followers: 4, SJR: 0.165, h-index: 2) European J. of Ageing       (Followers: 8, SJR: 0.49, h-index: 17) European J. of Applied Physiology       (Followers: 7, SJR: 1.044, h-index: 74) European J. of Clinical Microbiology & Infectious Diseases       (Followers: 10, SJR: 0.958, h-index: 74) European J. of Clinical Pharmacology       (Followers: 9, SJR: 0.916, h-index: 69) European J. of Dermatology       (Followers: 7) European J. of Drug Metabolism and Pharmacokinetics       (Followers: 6, SJR: 0.24, h-index: 25) European J. of Epidemiology       (Followers: 17, SJR: 1.946, h-index: 60) European J. of Forest Research       (Followers: 3, SJR: 0.864, h-index: 25) European J. of Health Economics       (Followers: 11, SJR: 0.67, h-index: 25) European J. of Law and Economics       (Followers: 173, SJR: 0.242, h-index: 13)
 Experiments in Fluids    [8 followers]  Follow        Hybrid journal (It can contain Open Access articles)      ISSN (Print) 1432-1114 - ISSN (Online) 0723-4864      Published by Springer-Verlag  [2209 journals]   [SJR: 1.033]   [H-I: 62]
• Three-dimensional flow measurements on flapping wings using synthetic
aperture PIV
• Abstract: We present the results of 3D velocity measurements of the flow fields around a free-flying painted lady butterfly (Vanessa cardui) and a tethered mechanical flapper using Synthetic Aperture PIV (SAPIV). The velocity fields presented for the free-flying butterfly have limited spatial resolution; however, leading edge vortices (LEV) and trailing edge vortices (TEV) can be seen during the downstroke of the butterfly. The results show that SAPIV has potential as a flow analysis tool to obtain whole-field, time-resolved velocities surrounding freely flying insects. The results of a tethered mechanical flapper focus mainly on the LEV and TEV through an entire flapping cycle. The results are compared to velocity measurements taken using traditional PIV techniques. Additionally, force measurements of the lift and thrust generated by the mechanical flapper are compared with the calculated forces from the measured velocity data and circulation in the flow field. The reconstructed visual hull of the butterfly and mechanical flapper is also discussed.
PubDate: 2014-10-15

• Marangoni convection in evaporating meniscus with changing contact angle
• Abstract: In this work, the Marangoni convection in the liquid phase of an evaporating meniscus interface in open air has been studied for varying contact angles. Ethanol undergoes self-evaporation inside a capillary tube of borosilicate glass with internal diameter of 1 mm. The evaporation is not uniform along the meniscus interface pinned at the capillary tube mouth, and this creates a gradient of temperature between the wedge and the centre of the meniscus. It is this temperature difference and the scale (1 mm) that generate a gradient of surface tension that is acknowledged to drive the vigorous Marangoni convection in the meniscus liquid phase. In previous studies of this configuration, the meniscus has mainly been concave and for this reason, other researchers attributed the differential temperature along the meniscus to the fact that the meniscus wedge is closer to the tube mouth and also further away from the warmer liquid bulk than the meniscus centre. The present study investigates concave, flat and convex meniscus by using a syringe pump that forces the meniscus to the wanted shape. With the present investigation, we want to further demonstrate that it is instead the larger evaporation at the meniscus triple line near the wedge that controls the phenomenon. Flow visualization and infrared temperature measurements have been performed. For concave and convex meniscus, the temperature measurements are in line with the predicted trend; the Marangoni vortices for these two menisci shapes spin in the same direction according to the temperature differences along the meniscus. For a flat meniscus, an intriguing experimental evidence has been found: the temperature difference is inverted with respect to concave and convex menisci, but surprisingly, the Marangoni vortices spin in the same direction than for concave and convex menisci.
PubDate: 2014-10-11

• The impact of twine/mesh ratio on the flow dynamics through a porous
cylinder
• Abstract: The impact of the twine/mesh ratio on the flow through a porous hollow cylinder of diameter D has been experimentally investigated at Reynolds number Re = 800 with a surface porosity varying from 0.67 to 0.90. Our porous cylinder models are inspired by aquaculture pens in that they have similar geometries, and porosities, to those nets commonly used within the aquaculture industry. We show that the surface porosity alone is not the key parameter determining the flow topology of the model, but rather a non-dimensional parameter $$\alpha =t^{0.5}D^{0.5}/m$$ (based upon twine thickness t, mesh void m and cylinder diameter D) effectively collapses first-order moments. Three different wake regimes are observed in the flow for different twine/mesh ratios: a laminar flow regime where streamlines pass through the model without significant deformation; a partially occluded flow, where the mean flow is decelerated, and a flow with a fully developed recirculation zone exhibiting a von Kármán vortex street similar to that produced in the wake of a solid cylinder. Our observation that the flow structure depends upon the parameter $$\alpha$$ , rather than simply the surface porosity, is supported by calculated dispersion times of virtual particles released both inside the model and within the wake. The particle distributions display three distinct dispersion behaviours, from nearly linear to a logarithmic decay slower than that of a solid cylinder, thus emphasising the existence of multiple flow regimes and the importance of the relative twine/mesh ratio.
PubDate: 2014-10-07

• Use of PIV to highlight possible errors in hot-wire Reynolds stress data
over a 2D rough wall
• Abstract: Particle image velocimetry (PIV) measurements are carried out in a turbulent boundary layer over a 2D rough surface consisting of transverse square bars. The aim of this work is to investigate a possible cause for the near-wall X-wire measurement errors observed on similar rough surfaces. The PIV measurements do not show the anomalous near-wall deficit of Reynolds stresses as measured with X-wires over the same surface. An extensive flow visualization analysis of the PIV data for a spacing between the roughness elements of p = 7k (k is the roughness element height) shows the occurrence of large-scale inward (sweeps) and outward (ejections) motions with a period of about 10.6δ/U 0 (δ and U 0 are the boundary layer thickness and the free-stream velocity). While these motions dominate the near-wall region and contribute almost equally to the Reynolds shear stress −‹uv›, the mean outward deviation from the mean flow direction is stronger than the inward deviation. Also, when the roughness spacing is reduced to p = 3k, the outward deviation reduces significantly more than the inward deviation. The results support the argument that the outward motions, which can have an instantaneous deviation angle of more than 50° in the case p = 7k, make the X-wire probe inefficient for detecting the ejection events (associated with the outward motions), particularly if the apex angle of the X-wire is not optimized for capturing the strong flow ejections with large deviations. The results explain in part the disparate information on the effect of the roughness on the Reynolds stresses in the outer region of the turbulent boundary layer over rough walls.
PubDate: 2014-10-07

• Experimental study on drag-reduction effect due to sinusoidal riblets in
turbulent channel flow
• Abstract: The drag-reduction effect of a three-dimensional sinusoidal riblet surface is experimentally evaluated in a fully developed turbulent channel flow. The lateral spacing of the adjacent walls of the riblet is varied sinusoidally in the streamwise direction. The obtained maximum total drag-reduction rate is approximately 12 % at a bulk Reynolds number of 3,400. The flow structure over the sinusoidal riblet surface is also analyzed in the velocity field by using two-dimensional particle image velocimetry. The velocity field is compared with the corresponding flow over a flat surface. It is found through pathlines and Reynolds shear stress analyses that the drag-reduction mechanism is similar to those of two-dimensional riblets. A different point is that the present riblet respectively induces a downward and upward flows in the expanded and contracted regions, which prevent vortices from hitting the bottom wall with wider lateral spacing of the riblet. In consequence, the wetted area of the present sinusoidal riblet is smaller than those of two-dimensional riblets, resulting in the high drag-reduction effect.
PubDate: 2014-10-04

• Interference-based overlapping particle tracking velocimetry for fluidized
beds
• Abstract: Fluidized beds are used in a wide number of applications, including power plant boilers and chemical facilities. This study proposes a novel particle tracking velocimetry algorithm for semi-dilute suspensions present in fluidized beds. The proposed algorithm is based on thresholding and profile matching algorithms. Image intensity thresholding is used to find regions which need additional image processing. These regions are then processed using interference-based profile matching algorithm to refine the solution quality. The key idea is to limit heavy profile matching computations only to identified clusters to save as much computation time as possible. The method was tested with simulated data and experimental data. The simulations showed that the proposed method was better than the previous boundary arc detection-based method in noisy conditions and cluster sizes ranging from 2 to 6 particles. The difference between the previous approach and the proposed method was small in cluster sizes larger than six particles, even though the proposed method was still slightly better. In low noise conditions with only two particles, the boundary arc detection could outperform the proposed method, but the difference was small. The method was also tested with experimental data from a small cold model fluidized bed. The velocity distributions obtained from the bed are shown for qualitative evaluation. The velocity distributions were realistic, which suggests the usability of the method. The proposed method was also compared to particle image velocimetry to see if both methods produce similar results. The results were divided into histogram intervals according to image intensity that was proportional to local solids volume fraction. The comparison showed that both methods produced similar results, in particular in the low-intensity range, which supports the ability of the method to produce realistic results also in the semi-dilute range where particles form small clusters.
PubDate: 2014-09-27

• PIV measurements of the unsteady flow structures in a volute centrifugal
pump at a high flow rate
PubDate: 2014-09-25

• Algorithm for soot sheet quantification in a piloted turbulent jet
non-premixed natural gas flame
• Abstract: A novel method to quantify soot sheets from planar images of soot volume fraction is presented and demonstrated in a well-characterised turbulent, non-premixed flame, known as the ‘Delft-Adelaide Flame’. The image processing algorithm presented is based on the adaption and combination of two existing computational methodologies that were presented in the literature. The algorithm starts by identifying the longest line spanning the object. The line is subsequently segmented repetitively to generate ‘anchor points’ that are forced to lie along the centreline of the object. The length of the soot sheet is obtained by fitting straight lines to the anchor points, whilst the average characteristic width of the sheet is determined from the mean thickness of the ellipses that are fitted to the segmented soot sheet. The algorithm employed in this method, which has an uncertainty of $$\sim$$ 11 %, is found to be well suited to extract the characteristic dimension and position information of soot sheet with bend, irregular shape and random orientation. Statistical assessments of these dimensions in this flame reveal that: (1) the characteristic width and length of the soot sheets range from $$\sim$$ 6 to $$\sim$$ 10 mm, and $$\sim$$ 30 to $$\sim$$ 50 mm, respectively, (2) a strong correlation exists between the soot characteristic width and length, and (3) the soot sheets display high sensitivity to local flow dynamics, with measured normalised interaction lengths ranging from 3.4 to 3.9, for the present flame.
PubDate: 2014-09-25

• Effect of linear image processing on the depth of correlation in micro PIV
• Abstract: Abstract This work investigates the effect of selected linear image processing methods on the depth of correlation (DoC) in micro particle image velocimetry using a single camera. In practice, band-pass and high-pass filters (background subtraction) are commonly applied to micro particle image velocimetry images. This work provides analytical models describing the effect of the parameters of low-, high-, and band-pass filters on the DoC and verifies the models by experiments. Furthermore, we propose a scheme that allows computing the weighting function and DoC for more complicated cases numerically.
PubDate: 2014-09-19

• Flow around two tandem square cylinders near a plane wall
• Abstract: Abstract An experimental study has been conducted to investigate the flow around two identical square cylinders in tandem arrangement and placed near a plane wall at a Reynolds number of 6,300. The inter-cylinder spacing ratio was varied from S * = 0.5 to 6, and the cylinder-to-wall gap ratio from G * = 0.25 to 2. Totally, 42 cases were considered to systematically examine the effects of wall proximity and the mutual interference between the two cylinders in the normalized gap–spacing (G *–S *) plane. The flow fields were captured using digital particle image velocimetry, in conjunction with measurements of the fluid forces (drag and lift) acting on the downstream cylinder using a piezoelectric load cell. The results show that the flow is highly dependent on the combined values of G * and S *. Categories relating to G * could be broadly classified as small-gap regime (G * < 0.5) at which periodic vortex shedding from the cylinders is suppressed, intermediate-gap regime (0.5 < G * < 1) where vortex shedding occurs but is under the influence of the wall proximity, and large-gap regime (G * > 1) where the wall effects become negligible. Similarly, the flow interference between the two cylinders can be divided into three basic categories as a function of S *, namely, shielding regime at S * < 1, reattachment regime at 1 < S * < 3, and impinging regime at S * > 3. Variations of force coefficients, amplitude spectra, Strouhal numbers, and Reynolds shear stress with G * and S * are presented to characterize the different flow regimes.
PubDate: 2014-09-19

• Experimental determination of three-dimensional finite-time Lyapunov
exponents in multi-component flows
• Abstract: We present an experimental approach for estimating finite-time Lyapunov exponent fields (FTLEs) in three-dimensional multi-component or multi-phase flows. From time-resolved sequences of particle images, we directly compute the flow map and coherent structures, while avoiding and outperforming the computationally costly numerical integration. Performing this operation independently on each flow component enables the determination of three-dimensional Lagrangian coherent structures (LCSs) without any bias from the other components. The locations of concurrent LCSs for different flow elements (e.g., passive tracers, inertial particles, bubbles, or active particles) can provide new insight into the interpenetrating FTLE structure in complex flows.
PubDate: 2014-09-17

• Simultaneous concentration and velocity field measurements in a
shock-accelerated mixing layer
• Abstract: A novel technique to obtain simultaneous velocity and concentration measurements is applied to the Richtmyer–Meshkov instability. After acceleration by a Mach 2.2 shock wave, the interface between the two gases develops into a turbulent mixing layer. A time-separated pair of acetone planar laser-induced fluorescence images are processed to yield concentration and, through application of the Advection-Corrected Correlation Image Velocimetry technique, velocity fields. This is the first application of this technique to shock-accelerated flows. We show that when applied to numerical simulations, this technique reproduces the velocity field to a similar quality as particle image velocimetry. When applied to the turbulent mixing layer of the experiments, information about the Reynolds number and anisotropy of the flow is obtained.
PubDate: 2014-09-16

• In vitro post-stenotic flow quantification and validation using echo
particle image velocimetry (Echo PIV)
• Abstract: Echo particle image velocimetry (Echo PIV) presents itself as an attractive in vivo flow quantification technique to traditional approaches. Promising results have been acquired; however, limited quantification and validation is available for post-stenotic flows. We focus here on the comprehensive evaluation of in vitro downstream stenotic flow quantified by Echo PIV and validated in relation to digital particle image velocimetry (DPIV). A Newtonian blood analog was circulated through a closed flow loop and quantified immediately downstream of a 50 % axisymmetric blockage at two Reynolds numbers (Re) using time-averaged Echo PIV and DPIV. Centerline velocities were in good agreement at all Re; however, Echo PIV measurements presented with elevated standard deviation (SD) at all measurements points. SD was improved using increased line density (LD); however, frame rate or field of view (FOV) is compromised. Radial velocity profiles showed close agreement with DPIV with the largest disparity in the shear layer and near-wall recirculation. Downstream recirculation zones were resolved by Echo PIV at both Re; however, magnitude and spatial coverage was reduced compared to DPIV that coincided with reduced contrast agent penetration beyond the shear layer. Our findings support the use of increased LD at a cost to FOV and highlight reduced microbubble penetration beyond the shear layer. High local SD at near-wall measurements suggests that further refinement is required before proceeding to in vivo quantification studies of wall shear stress in complex flow environments.
PubDate: 2014-09-16

• Experimental investigation of the fluid–structure interaction in an
elastic 180° curved vessel at laminar oscillating flow
• Abstract: Fluid–structure interaction phenomena are extremely important when laminar flows through elastic vessels such as in biomedical flow problems are considered. In general, such elastic vessels are curved which is why an elastic 180° bend at a curvature ratio $$\delta = D/D_{\rm C} = 0.\bar{2}$$ defines the reference geometry in this study. It is the purpose of this study to compare the results with the steady flow through a 180° rigid pipe bend and to quantify the impact of the fluid–structure interaction on the overall flow pattern and the vessel deformation at oscillating fully developed entrance flow. The findings comprise velocity, pressure, and structure deformation measurements. The vessel dilatation amplitude was varied between 3.75 % and 7 % of the vessel diameter at Dean De and Womersley number Wo ranges of $$327\,\le\,De\,\le\,350$$ and $$7\,\le\,Wo\,\le\,8.$$ The flow is investigated by time-resolved stereoscopic particle-image velocimetry in five radial cross sections located in the elastic 180° bend and in the inlet pipes. The unsteady static vessel pressure is measured synchronously at these cross sections. The comparison of the steady with the unsteady flow field shows a strong change in the axial and secondary velocity distributions at periods of transition between the centrifugal forces and the unsteady inertia forces dominated regimes. These changes are characterized by asymmetric fluctuations of the centers of the counter-rotating vortex pair. The investigation of the impact of the structure deformation amplitude on these fluctuations reveals a significant attenuation at high deformation amplitudes. The spatial motion of the elastic vessel due to the forces applied by the flow exhibits amplitudes up to 15 % of the vessel diameter. Considering the fluid–structure interaction, an amplification of the volume flux amplitude by a factor of 2.1 at the vessel outlet and phase lags up to 30° occur. The static pressure distribution is characterized by a pronounced asymmetry between forward and backward flow with a 40 % higher peak magnitude at backward flow and phase lags of 35°. The results evidence that a strong distortion of the velocity distribution in the bend, which is caused by the oscillating nature of the flow, is reduced as a result of the fluid–structure interaction.
PubDate: 2014-09-16

• Development and application of a point Doppler velocimeter featuring
two-beam multiplexing for time-resolved measurements of high-speed flow
• Abstract: A novel point Doppler velocimeter (pDV) based upon the Doppler global velocimetry principle is presented, which is capable of three-component velocity vector measurements at 100 kHz mean rates over extended time periods. In this implementation, two laser beams are multiplexed to illuminate the flow over alternating time windows, providing for a reduction in the number of sensors required. The implications of this multiplexing paradigm coupled with the fundamental limits set by the optical absorption filter are examined in detail, and uncertainties are predicted via instrumentation modeling and representative synthetic flow data. The results indicate that the multiplexing pDV instrument provides the required temporal and velocity resolution for turbulent shear flows at velocities of nominally 500 m/s. As a demonstration and validation of this time-resolved technique, statistics of three-velocity component measurements in a cold, supersonic, over-expanded jet at jet exit Mach number M j  = 1.4 (design Mach number M d  = 1.65) are presented. Time resolution up to 250 kHz and instantaneous velocity uncertainties between 6.6 and 11.1 m/s were obtained. Comparisons of mean pDV data with laser Doppler velocimetry data are consistent with uncertainty predictions for the technique. The ultimate value of the instrument is exhibited in the analysis of Reynolds stress spectra in the screeching jet, exposing the spatial development of motions at the harmonics of the screech tone, variable phase-coordinated shock motions, and growth of turbulent fluctuations in the developing shear layer of the jet. From the data presented, the screech tone phenomenon is suspected to be linked to the production of radial–azimuthal shear stresses in extended regions beyond the potential core.
PubDate: 2014-09-13

• On the use of the finite-time Lyapunov exponent to reveal complex flow
physics in the wake of a mechanical valve
• Abstract: The finite-time Lyapunov exponent (FTLE) is a popular tool to extract characteristic features of flows that cannot be revealed by other criteria. However, even if the computational cost of computing particle trajectories in space and time has been reduced and optimized in a considerable number of works, the main challenge probably consists in increasing the spatial resolution of Lagrangian coherent structures locally, i.e., where the FTLE field reaches its maximum values. On the other hand, most of experimental data are obtained in planes so that the FTLE field is computed without the out-of-plane particle movements. To investigate which physics the FTLE can capture in flows that are highly three dimensional, the criterion is computed from high-speed stereoscopic particle image velocimetry measurements of the pulsatile flow that develops behind a bi-leaflet mechanical heart valve. The similitude is based on the Womersley number, and experiments are performed for a lower Reynolds number than the physiologic value to obtain sufficiently resolved data in space and time. It is found that the vortex shedding is well captured and that its development can be decomposed into four successive phases. The longest phase occurs near the peak flow rate and exhibits a break of symmetry similar to the one appearing in the wakes of two side-by-side cylinders in the regime when the separation between the cylinders is of the order of their diameter. Specifically, a vortex street with alternating vortex sheddings is observed in a narrow wake behind one of the leaflets, whereas single large vortices develop inside a wide wake downstream of the other leaflet. It appears that these patterns are difficult or even impossible to discern with classical Eulerian vortex identification techniques. The Strouhal numbers of vortex-shedding frequencies, obtained from continuous wavelet transforms and based on the apparent height of the leaflets, are also close to those found in the flow behind two cylinders. By invoking the Taylor hypothesis, an approximate three-dimensional reconstruction of the flow can be obtained and a three-dimensional FTLE field is deduced, which provides a very detailed view of the vortex structures that form and develop in the wakes of the leaflets.
PubDate: 2014-09-04

• Spectral analysis of fluid flows using sub-Nyquist-rate PIV data
• Abstract: Spectral methods are ubiquitous in the analysis of dynamically evolving fluid flows. However, tools like Fourier transformation and dynamic mode decomposition (DMD) require data that satisfy the Nyquist–Shannon sampling criterion. In many fluid flow experiments, such data are impossible to acquire. We propose a new approach that combines ideas from DMD and compressed sensing to accommodate sub-Nyquist-rate sampling. Given a vector-valued signal, we take measurements randomly in time (at a sub-Nyquist rate) and project the data onto a low-dimensional subspace. We then use compressed sensing to identify the dominant frequencies in the signal and their corresponding modes. We demonstrate this method using two examples, analyzing both an artificially constructed dataset and particle image velocimetry data from the flow past a cylinder. In each case, our method correctly identifies the characteristic frequencies and oscillatory modes dominating the signal, proving it to be a capable tool for spectral analysis using sub-Nyquist-rate sampling.
PubDate: 2014-09-04

• Tip vortex structure and aerodynamic loading on rotating wings in confined
spaces
• Abstract: Experiments on a rotating wing in a liquid-filled tank were conducted to determine the minimum required tip clearance to produce data free from wall effects. A rotating wing fixed at an angle of attack of 45° was revolved for two revolutions at Reynolds numbers between 120 and 10,000. Tip clearance was varied from 0.5 to 5 chords by varying wing size, while also varying wing speed to hold Reynolds number constant. Force measurements on the wing, as well as dye flow visualization and particle image velocimetry of the entire tank, were conducted. Tip clearances of 0.5–7 chords were also tested computationally. Results of all measurements show large tip effects for 0.5 chords of tip clearance, and no wall effects for 5 chords of tip clearance at all Reynolds numbers tested. The 3 chord clearance case showed negligible wall effects in both the particle image velocimetry and dye flow visualization for all Reynolds numbers observed. The forces on the 3 chord tip clearance wing indicate wall effects appearing in the second revolution for Reynolds numbers of >1,000. A tip clearance of 5 chords is deemed to be free of wall effects for experiments limited to two wing revolutions within the range of tested Reynolds numbers.
PubDate: 2014-09-02

• Long-range μPIV to
resolve the small scales in a jet at high Reynolds number
• Abstract: The investigation of flows at high Reynolds number is of great interest for the theory of turbulence, in that the large and the small scales of turbulence show a clear separation. But, as the Reynolds number of the flow increases, the size of the Kolmogorov length scale ( $$\eta$$ ) drops almost proportionally. Aiming at achieving the adequate spatial resolution in the central region of a self-similar round jet at high Reynolds numbers ( $$Re_\lambda \approx 350$$ ), a long-range μPIV system was applied. A vector spacing of $$1.5 \eta$$ was achieved, where the Kolmogorov length scale was estimated to be $$55\,\upmu {\rm m}$$ . The resulting velocity fields were used to characterize the small-scale flow structures in this jet. The autocorrelation maps of vorticity and $$\lambda _{\rm ci}$$ (the imaginary part of the eigenvalue of the reduced velocity gradient tensor) reveal that the structures of intense vorticity have a characteristic diameter of approximately $$10 \eta$$ . From the autocorrelation map of the reduced (2D) rate of dissipation, it is inferred that the regions of intense dissipation tend to organize in the form of sheets with a characteristic thickness of approximately $$10 \eta$$ . The regions of intense dissipation have the tendency to appear in the vicinity of intense vortices. Furthermore, the joint pdf of the two invariants of the reduced velocity gradient tensor exhibits the characteristic teapot-shape. These results, based on a statistical analysis of the data, are in agreement with previous numerical and experimental studies at lower Reynolds number, which validates the suitability of long-range μPIV for characterizing turbulent flow structures at high Reynolds number.
PubDate: 2014-08-28

• Stereo particle image velocimetry measurements of perpendicular
blade–vortex interaction over an oscillating airfoil
• Abstract: The aerodynamic interaction of a stream-wise vortex impacting on a NACA 23012 oscillating airfoil was investigated using stereo particle image velocimetry. The experimental rig enabled the study of the aerodynamic effects due to the blade pitching motion in the interaction with the vortex. The experimental study focused on the light dynamic stall regime, which represents a typical condition of the retreating blade of a helicopter in forward flight. Particle image velocimetry was applied to a measurement volume close to the airfoil upper surface in order to obtain the three-dimensional interacting flow field. In particular, the experimental results show that during the airfoil downstroke motion, the vortex impact triggers the stall of the local blade section, indicating that detrimental effects on the blade performance can be introduced by perpendicular vortex interactions.
PubDate: 2014-08-26

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