Dual-camera system for high-speed imaging in particle image velocimetry

Particle image velocimetry is an important technique in experimental fluid mechanics, for which it has been essential to use a specialized high-speed camera. However, the high speed is at the expense of other performances of the camera, i.e., sensitivity and image resolution. Here, we demonstrate that the high-speed imaging is also possible with a pair of still cameras.Comment: 4 pages, accepted by Journal of Visualization (see http://www.springerlink.com

Near-wall velocity measurements by Particle-Shadow-Tracking

We report a new method to measure the velocity of a fluid in the vicinity of a wall. The method, that we call Particle-Shadow Tracking (PST), simply consists in seeding the fluid with a small number of fine tracer particles of density close to that of the fluid. The position of each particle and of its shadow on the wall are then tracked simultaneously, allowing one to accurately determine the distance separating tracers from the wall and therefore to extract the velocity field. We present an application of the method to the determination of the velocity profile inside a laminar density current flowing along an inclined plane

An Improved Algorithm For Unmixing First‐Order Reversal Curve Diagrams Using Principal Component Analysis

First‐order reversal curve (FORC) diagrams of synthetic binary mixtures with single‐domain, vortex state, and multi‐domain end members (EMs) were analyzed using principal component analysis (FORC‐PCA). Mixing proportions derived from FORC‐PCA are shown to deviate systematically from the known weight percent of EMs, which is caused by the lack of reversible magnetization contributions to the FORC distribution. The error in the mixing proportions can be corrected by applying PCA to the raw FORCs, rather than to the processed FORC diagram, thereby capturing both reversible and irreversible contributions to the signal. Here we develop a new practical implementation of the FORC‐PCA method that enables quantitative unmixing to be performed routinely on suites of FORC diagrams with up to four distinct EMs. The method provides access not only to the processed FORC diagram of each EM, but also to reconstructed FORCs, which enables objective criteria to be defined that aid identification of physically realistic EMs. We illustrate FORC‐PCA with examples of quantitative unmixing of magnetic components that will have widespread applicability in paleomagnetism and environmental magnetism

Wake Development behind Paired Wings with Tip and Root Trailing Vortices: Consequences for Animal Flight Force Estimates

Recent experiments on flapping flight in animals have shown that a variety of unrelated species shed a wake behind left and right wings consisting of both tip and root vortices. Here we present an investigation using Particle Image Velocimetry (PIV) of the behaviour and interaction of trailing vortices shed by paired, fixed wings that simplify and mimic the wake of a flying animal with a non-lifting body. We measured flow velocities at five positions downstream of two adjacent NACA 0012 aerofoils and systematically varied aspect ratio, the gap between the wings (corresponding to the width of a non-lifting body), angle of attack, and the Reynolds number. The range of aspect ratios and Reynolds number where chosen to be relevant to natural fliers and swimmers, and insect flight in particular. We show that the wake behind the paired wings deformed as a consequence of the induced flow distribution such that the wingtip vortices convected downwards while the root vortices twist around each other. Vortex interaction and wake deformation became more pronounced further downstream of the wing, so the positioning of PIV measurement planes in experiments on flying animals has an important effect on subsequent force estimates due to rotating induced flow vectors. Wake deformation was most severe behind wings with lower aspect ratios and when the distance between the wings was small, suggesting that animals that match this description constitute high-risk groups in terms of measurement error. Our results, therefore, have significant implications for experimental design where wake measurements are used to estimate forces generated in animal flight. In particular, the downstream distance of the measurement plane should be minimised, notwithstanding the animal welfare constraints when measuring the wake behind flying animals

Velocity field measurements of cavitating flows

A particle Image Velocimetry (PIV) system has been developed to study the microfluid mechanics of cavitating flows. Planar PIV was used to examine the non-cavitating flow in the thin boundary layer near a hydrofoil surface for the cases of a naturally developing boundary layer and a boundary layer stimulated to turbulence by roughness near the foil leading edge. PIV was also used to examine the flow near the surface of individual cavitation bubbles and incipient attached cavitation. A system was devised to create a single nucleus in the flow upstream of a hydrofoil, and planar PIV was used to study the flow around the resulting traveling cavitation bubble. Velocity vectors were determined close to the solid surfaces and the gas/liquid interfaces of the bubbles. Seeding of the flow with particles did not result in the addition of active cavitation nuclei.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47070/1/348_2004_Article_BF00189302.pd

On the effects of surface morphology on the structure of wall-turbulence

Experiments were conducted in the fully-rough regime on surfaces with large relative roughness (h/δ ≈ 0.1) generated by regularly distributed LEGO™ bricks of uniform height, arranged in different configurations. Measurements were made with high resolution PIV on six different frontal solidities, λF, at fixed plan solidity, λP. Results indicate that the spatial underlying structure of the turbulence across the different surface morphologies is universal in both its shape and orientation in relation to the flow velocity. Harpin packets inclination with respect of the wall is also found to be consistent not only across the different wall surfaces but also when compared to previous studies on smooth walls. Slices of two-point correlations for both streamwise and wall-normal velocity fluctuations and Reynolds shear stresses present a good collapse across the entire y/δ range for all wall morphologies

Automatic eduction and statistical analysis of coherent structures in the wall region of a confine plane

This paper describes a vortex detection algorithm used to expose and statistically characterize the coherent flow patterns observable in the velocity vector fields measured by Particle Image Velocimetry (PIV) in the impingement region of air curtains. The philosophy and the architecture of this algorithm are presented. Its strengths and weaknesses are discussed. The results of a parametrical analysis performed to assess the variability of the response of our algorithm to the 3 user-specified parameters in our eduction scheme are reviewed. The technique is illustrated in the case of a plane turbulent impinging twin-jet with an opening ratio of 10. The corresponding jet Reynolds number, based on the initial mean flow velocity U0 and the jet width e, is 14000. The results of a statistical analysis of the size, shape, spatial distribution and energetic content of the coherent eddy structures detected in the impingement region of this test flow are provided. Although many questions remain open, new insights into the way these structures might form, organize and evolve are given. Relevant results provide an original picture of the plane turbulent impinging jet

A common polymorphism in the oxygen-dependent degradation (ODD) domain of hypoxia inducible factor-1α (HIF-1α) does not impair Pro-564 hydroxylation

BACKGROUND: The hypoxia-inducible factor (HIF) transcription complex, which is activated by low oxygen tension, controls a diverse range of cellular processes including angiogenesis and erythropoiesis. Under normoxic conditions, the α subunit of HIF is rapidly degraded in a manner dependent on hydroxylation of two conserved proline residues at positions 402 and 564 in HIF-1α in the oxygen-dependent degradation (ODD) domain. This allows subsequent recognition by the von Hippel-Lindau (VHL) tumor suppressor protein, which targets HIF for degradation by the ubiquitin-proteasome pathway. Under hypoxic conditions, prolyl hydroxylation of HIF is inhibited, allowing it to escape VHL-mediated degradation. The transcriptional regulation of the erythropoietin gene by HIF raises the possibility that HIF may play a role in disorders of erythropoiesis, such as idiopathic erythrocytosis (IE). RESULTS: Patients with IE were screened for changes in the HIF-1α coding sequence, and a change in the ODD domain that converts Pro-582 to Ser was identified in several patients. This same change, however, was also detected at a significant frequency, 0.073, in unaffected controls compared to 0.109 in the IE patient group. In vitro hydroxylation assays examining this amino acid change failed to reveal a discernible effect on HIF hydroxylation at Pro-564. CONCLUSION: The Pro582Ser change represents a common polymorphism of HIF-1α that does not impair HIF-1α prolyl hydroxylation. Although the Pro582Ser polymorphism is located in the ODD domain of HIF-1α it does not diminish the association of HIF-1α with VHL. Thus, it is unlikely that this polymorphism accounts for the erythrocytosis in the group of IE patients studied

Turbulent thermal diffusion in a multi-fan turbulence generator with the imposed mean temperature gradient

We studied experimentally the effect of turbulent thermal diffusion in a multi-fan turbulence generator which produces a nearly homogeneous and isotropic flow with a small mean velocity. Using Particle Image Velocimetry and Image Processing techniques we showed that in a turbulent flow with an imposed mean vertical temperature gradient (stably stratified flow) particles accumulate in the regions with the mean temperature minimum. These experiments detected the effect of turbulent thermal diffusion in a multi-fan turbulence generator for relatively high Reynolds numbers. The experimental results are in compliance with the results of the previous experimental studies of turbulent thermal diffusion in oscillating grids turbulence (Buchholz et al. 2004; Eidelman et al. 2004). We demonstrated that turbulent thermal diffusion is an universal phenomenon. It occurs independently of the method of turbulence generation, and the qualitative behavior of particle spatial distribution in these very different turbulent flows is similar. Competition between turbulent fluxes caused by turbulent thermal diffusion and turbulent diffusion determines the formation of particle inhomogeneities.Comment: 9 pages, 9 figure, REVTEX4, Experiments in Fluids, in pres

Large-Scale Streamwise Vortices in Turbulent Channel Flow Induced by Active Wall Actuations

© 2017, Springer Science+Business Media B.V., part of Springer Nature. Direct numerical simulations of turbulent flow in a plane channel using spanwise alternatively distributed strips (SADS) are performed to investigate the characteristics of large-scale streamwise vortices (LSSVs) induced by small-scale active wall actuations, and their role in suppressing flow separation. SADS control is obtained by alternatively applying out-of-phase control (OPC) and in-phase control (IPC) to the wall-normal velocity component of the lower channel wall, in the spanwise direction. Besides the non-controlled channel flow simulated as a reference, four controlled cases with 1, 2, 3 and 4 pairs of OPC/IPC strips are studied at M = 0.2 and Re = 6,000, based on the bulk velocity and the channel half height. The case with 2 pairs of strips, whose width is Δz+ = 264 based on the friction velocity of the non-controlled case, is the most effective in terms of generating large-scale motions. It is also found that the OPC (resp. IPC) strips suppress (resp. enhance) the coherent structures and that leads to the creation of a vertical shear layer, which is responsible for the LSSVs presence. They are in a statistically steady state and their cores are located between two neighbouring OPC and IPC strips. These motions contribute significantly to the momentum transport in the wall-normal and spanwise directions showing potential for flow separation suppression