Abstract: Abstract
The Shake-The-Box (STB) particle tracking technique, recently introduced for time-resolved 3D particle image velocimetry (PIV) images, is applied here to data from a multi-pulse investigation of a turbulent boundary layer flow with adverse pressure gradient in air at 36 m/s (Re
τ
= 10,650). The multi-pulse acquisition strategy allows for the recording of four-pulse long time-resolved sequences with a time separation of a few microseconds. The experimental setup consists of a dual-imaging system and a dual-double-cavity laser emitting orthogonal polarization directions to separate the four pulses. The STB particle triangulation and tracking strategy is adapted here to cope with the limited amount of realizations available along the time sequence and to take advantage of the ghost track reduction offered by the use of two independent imaging systems. Furthermore, a correction scheme to compensate for camera vibrations is discussed, together with a method to accurately identify the position of the wall within the measurement domain. Results show that approximately 80,000 tracks can be instantaneously reconstructed within the measurement volume, enabling the evaluation of both dense velocity fields, suitable for spatial gradients evaluation, and highly spatially resolved boundary layer profiles. Turbulent boundary layer profiles obtained from ensemble averaging of the STB tracks are compared to results from 2D-PIV and long-range micro particle tracking velocimetry; the comparison shows the capability of the STB approach in delivering accurate results across a wide range of scales. PubDate: 2016-07-23

Abstract: Abstract
A 3D stall-cell flow-field has been studied in a 4.8 aspect-ratio wing obtained by linear extrusion of a laminar NACA64-418 airfoil profile. The span-wise change in the velocity and pressure distribution along the wing has been quantified with respect to the development of cellular structures from
\(8^{\circ }\)
to
\(20^{\circ }\)
angle of attack. Oil-flow visualizations help localizing the regular cellular pattern in function of the angle of attack. Multi-plane stereoscopic PIV measurements obtained by traversing the entire setup along the wing span show that the flow separation is not span-wise uniform. The combination of different stereoscopic fields into a 3D volume of velocity data allows studying the global effect of the stall-cell pattern on the wing flow. Integration of the experimentally computed pressure gradient from the Navier–Stokes equation is employed to compute the span-wise distribution of the mean surface pressure. Comparison of the results with the ones obtained from pressure taps installed in the wing evidences a span-wise periodic loading on the wing. The periodic loading has maxima confined in the stream-wise direction between the location of the highest airfoil curvature and the one of the airfoil flow separation. Estimation of the periodic loading is found within 2–6 % of the sectional wing lift. PubDate: 2016-07-23

Abstract: Abstract
In this manuscript, we investigate the statistical convergence of turbulent flow statistics from finite-record-length time-series measurements. Analytical solutions of the convergence rate of the mean, variance, and autocorrelation function as a function of record length are presented based on using mean-squared error analysis and the consideration of turbulent flows as random processes. Experimental assessment of the statistical convergence theory is presented using 20-kHz laser Rayleigh scattering measurements of a conserved scalar (ξ) in a turbulent free jet. Excellent agreement between experiments and theory is noted, providing validation of the statistical convergence analysis. To the authors’ knowledge, this is the first reported assessment and verification of statistical convergence theory as applied to turbulent flows. The verified theory provides a practitioner a method for a priori determining the necessary temporal record length for a desired statistical accuracy or conversely, accurately estimating the uncertainty of a measurement for a given temporal record length. Furthermore, we propose a new hybrid “multi-burst” data processing scheme based on combined independent ensemble and time-series statistics targeted for shorter-duration time-series measurements. The new methodology is based on taking the ensemble mean of derived statistical moments from many individual finite-duration time-series measurements. This approach is used to systematically converge toward the “expected” value of any statistical moment at a rate of
\(\sqrt M\)
, where M is the number of individual time-series measurements. The proposed multi-burst methodology is assessed experimentally, and excellent agreement between measurements and theory is observed. A key outcome of the implementation of the multi-burst processing method is noted in the estimation of the autocorrelation function. Specifically, an unbiased estimator of the autocorrelation function can be used with much less uncertainty as compared to the biased estimator, which is not the case for single time-series measurements irrespective of record length. The primary outcome of the multi-burst processing scheme is a methodology for achieving high statistical convergence for turbulent flow time-series measurements characterized by limited acquisition time, whether facility or instrument dependent. PubDate: 2016-07-23

Abstract: Abstract
In this work, we conduct experiments to study the interaction between a horizontal free water layer and a planar shock wave that is sliding over it. Experiments are performed at atmospheric pressure in a shock tube with a square cross section (
\(200\times 200\,\hbox {mm}^2\)
) for depths of 10, 20, and 30 mm; a 1500-mm-long water layer; and two incident planar shock waves having Mach numbers of 1.11 and 1.43. We record the pressure histories and high-speed visualizations to study the flow patterns, surface waves, and spray layers behind the shock wave. We observe two different flow patterns with ripples formed at the air–water interface for the weaker shock wave and the dispersion of a droplet mist for the stronger shock wave.
From the pressure signals, we extract the delay time between the arrival of the compression wave into water and the shock wave in air at the same location. We show that the delay time evolves with the distance traveled over the water layer, the depth of the water layer, and the Mach number of the shock wave. PubDate: 2016-07-21

Abstract: Abstract
The possibility of a pneumatic actuator system for controlling the crossflow vortex-induced laminar breakdown is investigated by means of hot-wire measurements. Steady blowing or suction through a spanwise row of periodically arranged orifices initiates a system of longitudinal vortices which reduces the amplitude of the most amplified stationary crossflow vortices. Thus, the onset of high-frequency secondary instability and the following laminar–turbulent transition was shifted farther downstream. All experiments were conducted at the redesigned DLR swept flat plate experiment in the open test section of the 1 m wind tunnel at the DLR in Göttingen. PubDate: 2016-07-19

Abstract: Abstract
We show that aqueous solutions of ammonium thiocyanate (
\(\hbox {NH}_{4}\hbox {SCN}\)
) can be used to match the index of refraction of several transparent materials commonly used in experiments, while maintaining low viscosity and density compared to other common refractive index-matching liquids. We present empirical models for estimating the index of refraction, density, and kinematic viscosity of these solutions as a function of temperature and concentration. Finally, we summarize the chemical compatibility of ammonium thiocyanate with materials commonly used in apparatus. PubDate: 2016-07-13

Abstract: Abstract
Low signal-to-noise in particle image velocimetry (PIV) measurements in systems such as high pressure gas turbine combustors can result in significant data gaps that negatively affect subsequent analysis. Here, gappy proper orthogonal decomposition (GPOD) is evaluated as a method of filling such missing data. Four GPOD methods are studied, including a new method that utilizes a median filter (MF) to adaptively select whether a local missing data point is updated after each iteration. These methods also are compared against local Kriging interpolation. The GPOD methods are tested using PIV data without missing vectors that were obtained in atmospheric pressure swirl flames. Parameters studied include the turbulence intensity, amount of missing data, and the amount of noise in the valid data. Two criteria to check for GPOD convergence also were investigated. The MF method filled in the missing data with the lowest error across all parameters tested, with approximately one-third the computational cost of Kriging. Furthermore, the accuracy of MF GPOD was relatively insensitive to the quality of the convergence criterion. Therefore, compared to the three other GPOD methods and Kriging interpolation, the MF GPOD method is an effective method for filling missing data in PIV measurements in the studied gas turbine combustor flows. PubDate: 2016-07-12

Abstract: Abstract
Vortical structures and dynamics of a Re
h = 2100 elliptic jet impinging upon a flat plate were studied at H/d
h = 1, 2 and 4 jet-to-plate separation distances. Flow investigations were conducted along both its major and minor planes using laser-induced fluorescence and digital particle image velocimetry techniques. Results show that the impingement process along the major plane largely consists of primary jet ring-vortex and wall-separated secondary vortex formations, where they subsequently separate from the flat plate at smaller H/d
h = 1 and 2 separation distances. Key vortex formation locations occur closer to the impingement point as the separation distance increases. Interestingly, braid vortices and rib structures begin to take part in the impingement process at H/d
h = 4 and wave instabilities dominate the flow field. In contrast, significantly more coherent primary and secondary vortices with physically larger vortex core sizes and higher vortex strengths are observed along the minor plane, with no signs of braid vortices and rib structures. Lastly, influences of these different flow dynamics on the major and minor plane instantaneous and mean skin friction coefficient levels are investigated to shed light on the effects of separation distance on the wall shear stress distributions. PubDate: 2016-07-07

Abstract: Abstract
Multilayer nanoparticle image velocimetry (MnPIV) with a refractive-index-matching method is powerful technique for x–y–z (3D) flow measurement, because it can detect the 3D position of fluorescent particles with submicron resolution. In MnPIV, the intensity of fluorescence of a particle is used to estimate its z-position. However, it has been difficult to measure 3D flows around microstructures in water by total internal reflection fluorescence microscopy because of light scattering caused by the different refractive indices of the structures and the working fluid. By using a thermal nanoimprinting technique, we succeeded in fabricating microstructures from a polymer resin whose refractive index is equal to that of water, and we used these microstructures to perform MnPIV in water. As a result of the match between the refractive index of water and that of the microstructures, we were able to perform 3D tracking of nanoparticles around the microstructures in water. PubDate: 2016-06-28

Abstract: Abstract
The effect of image noise on the uncertainty of velocity fields measured with particle image velocimetry (PIV) is still an unsolved problem. Image noise reduces the correlation signal and thus affects the estimation of the particle image displacement. However, a systematic quantification of the effect of the noise level on the loss-of-correlation is missing. In this work, a new method is proposed to estimate the loss-of-correlation due to image noise
\(F_{\sigma }\)
from the autocorrelation function of PIV images. Furthermore, a new definition of the signal-to-noise ratio (SNR) for PIV images is suggested, which results in a bijective relation between
\(F_{\sigma }\)
and SNR. Based on the newly defined SNR, it becomes possible to estimate the signal level and the noise level itself. The presented method is very general because the estimation of
\(F_{\sigma }\)
and SNR works independently of various parameters, including the particle image intensity, the particle image density, the particle image size, the image noise distributions and the laser light-sheet profile. The findings lead to an extension of the fundamental PIV equation
\(N=N_{\mathrm {I}} F_{\mathrm {I}} F_{\mathrm {O}} F_{\Delta }\)
and enable PIV users to optimize their measurement setup with respect to the image noise and not only based on the loss-of-correlation due to in-plane motion, out-of-plane motion and displacement gradients. Furthermore, the new definition of SNR allows for a characterization and comparison of PIV images. The new approaches are validated by using synthetic images, and the predictions are confirmed by using experimental data. PubDate: 2016-06-27

Abstract: Abstract
Comparative wettability studies of graphene are conducted for two different nanofluids with opposite surface potentials of +53 mV (45-nm alumina nanoparticles) and −45 mV (28-nm silica nanoparticles), respectively. Aged graphene surface, which has adsorbed abundant hydrocarbon contaminants, shows weak hydrophobicity of about 90° wetting angles for both nanofluids for the tested volume concentration range from 0 to 10 %. For pristine graphene surfaces, however, the contact angle of alumina nanofluids continually increases from 50° to 70° for the same volume concentration increase, but the contact angle of silica nanofluids shows first increase of up to about 1 % concentration and then remains nearly unchanged with further increasing concentration. Since the nanoparticle–graphene interaction at the solid–liquid (SL) interface is expected to be the most crucial in determining the nanofluid wetting angles, the corresponding surface energy
\(\gamma_{\text{SL}}\)
is examined from elaboration of
\(F_{\text{DLVO}}\)
, the Derjaguin–Landau–Verwey–Overbeek force. The magnitudes of both the repulsive
\(F_{\text{DLVO}}\)
on the alumina nanoparticles and the attractive
\(F_{\text{DLVO}}\)
on the silica nanoparticles show rapid decreases up to 1 % volume concentration and exhibit slower decreases thereafter. The reduced repulsive
\(F_{\text{DLVO}}\)
of the alumina nanoparticle drives the increasing aggregation of nanoparticles on the SL interface with increasing concentration, thus increasing the SL interfacial energy
\(\gamma_{\text{SL}}\)
. On the contrary, the reduced attractive
\(F_{\text{DLVO}}\)
on the silica nanoparticle retards their aggregation on the SL interface with increasing concentration and slows the increase in
\(\gamma_{\text{SL}}\)
, eventually settling on the saturated level of
\(\gamma_{\text{SL}}\)
from a certain concentration onwards. These distinctive behaviors of
\(\gamma_{\text{SL}}\)
are consistent with the measured contact angles that gradually increase with increasing concentration for the positive surface potential (alumina), but initially increase and then settle for the negative surface potential (silica). This phenomenon strongly supports the critical dependence of nanofluid wetting of pristine graphene on
\(F_{\text{DLVO}}\)
in the vicinity of the SL interface. PubDate: 2016-06-25

Abstract: Abstract
A growing number of studies is devoted to anisotropic particles in turbulent flows. In most cases, the particles are assumed to be rigid and their deformations are neglected. We present an adaptation of classical computer vision tools to reconstruct from two different images the 3D conformation of a fiber distorted by the turbulent fluctuations in a von Kármán flow. This technique allows us notably to characterize the fiber deformation by computing the correlation function of the orientation of the tangent vector. This function allows us to tackle the analogy between polymers and flexible fibers proposed by Brouzet et al. (Phys Rev Lett 112(7):074501, 2014). We show that this function depends on an elastic length
\(\ell _\mathrm{e}\)
which characterizes the particle flexibility, as is the case for polymers, but also on the fiber length L, contrary to polymers. PubDate: 2016-06-23

Abstract: Abstract
Temperature field measurements in liquids are demonstrated using zinc oxide (ZnO) thermographic phosphor particles. The particles are added to the liquid as a tracer. Following laser excitation, the temperature-dependent luminescence emission of the particles is imaged and the temperature is determined using a two-colour intensity ratio method. The particle size requirements for accurate temperature tracing in turbulent flows are calculated using a numerical heat transfer model. Particle–water mixtures were prepared using ultrasonic dispersion and characterised using scanning electron microscope imaging and laser diffraction particle-sizing, indicating that the particle size is 1–2
\(\upmu\)
m. The particle luminescence properties were characterised using spectroscopic and particle luminescence imaging techniques. Using 355 nm laser excitation, the luminescence signal is the same in water and in air. However, 266 nm excitation is used to avoid spectral overlap between Raman scattering from water and the detected ZnO luminescence emission. It is shown that 266 nm excitation can be used for temperature measurements in water using mass loads as low as 1–5 mg L
\(^{-1}\)
, corresponding to measured particle number densities 0.5–2.5
\(\times \,10^{12}\)
particles
\(\hbox {m}^{-3}\)
. In this range, the measured intensity ratio is independent of the mass load. The dependence of the intensity ratio on the laser fluence is less pronounced using excitation at 266 nm compared to 355 nm. A single-shot, single-pixel temperature precision of ±2–3
\(^{\circ }\hbox {C}\,(1\sigma )\)
can be achieved over a temperature range spanning
\(50\,^{\circ }\hbox {C}\)
. The technique was applied to a convection experiment to measure the temperature fields in a buoyant thermal plume, demonstrating the suitability of these imaging diagnostics for the investigation of thermal convection and heat transfer. PubDate: 2016-06-22

Abstract: Abstract
The flow structure and lift response of a separated flow over an airfoil that is subjected to an impulsive type of pitching motion are compared to the response produced by a localized pulse disturbance at the leading edge of an airfoil. Time-resolved PIV data are used to obtain the velocity field on the suction side of the airfoil. POD analysis shows that the majority of energy is contained within the first four modes. Strong similarities in the shapes of the POD basis functions are found, especially for the second mode, irrespective of the type of actuation (global or local). The time-varying coefficient of this second POD mode tracks the negative of the lift coefficient or circulation in each case. Basis functions from the localized actuation data were projected on the velocity field of the globally actuated flow to obtain a hybrid set of coefficients. The hybrid coefficients matched reasonably well with the coefficients obtained from the original POD analysis for the globally excited flow. Both types of actuation were found to generate very similar Lagrangian flow structures. The results suggest a certain degree of universality in the POD modes/flow structures for the separated flow over an airfoil, irrespective of the type of excitation. PubDate: 2016-06-21

Abstract: Abstract
The thermocapillary migration of drops in a rectangular cell, with a heated top wall and a cooled bottom wall, was investigated experimentally on the ground. The rectangular test cell was 70 mm high, with a horizontal cross section of 40 mm × 40 mm. In the present experiment, 30 cSt silicon oil was used as the continuous phase, and a water–ethanol mixture was used as the drop phase, respectively. The drops ranged in size from 1.87 to 6.94 mm in diameter and were injected into the continuous phase, where the temperature gradients ranged from 0.193 to 0.484 °C mm−1. In order to measure the temperature distribution of the liquid, a digital holographic interferometry was used, which was non-contact, full-field, and in-situ. The holograms were recorded, and then the corresponding wrapped phase distributions images were numerically reconstructed. The temperature distribution of the continuous phase liquid in the cell had been obtained following the unwrapping. Also, through an algebra layer analysis, the temperature distribution around the drop during the thermocapillary migration was obtained. As a result, the drop was colder than the continuous phase liquid, and a thermal wake existed behind the drop. The influence of convective transport on the drop migration was also investigated for the Marangoni number in the range of 7–174. With the increasing of the Marangoni number, the dimensionless interface temperature difference decreased, which was caused by the convective transport enhanced results in the drop thermocapillary migration velocity becoming decreased. The data were compared with previous space experiments to explain the phenomena of the drop migration. Finally, with the increasing Marangoni numbers, the length of the thermal wake region increased, and the thermal wake region became extended. PubDate: 2016-06-20

Abstract: Abstract
The hydrodynamics of suction feeding is critical for the survival of fish larvae; failure to capture food during the onset of autonomous feeding can rapidly lead to starvation and mortality. Fluid mechanics experiments that investigate the suction feeding of suspended particles are limited to adult fishes, which operate at large Reynolds numbers. This manuscript presents the first literature results in which the external velocity fields generated during suction feeding of early zebrafish larvae (2500–20,000 μm total length) are reported using time-resolved microscopic particle image velocimetry. For the larval stages studied, the maximum peak suction velocity of the inflow bolus is measured at a finite distance from the mouth tip and ranges from 1 to 8 mm/s. The average pressure gradient and the velocity profile proximal to the buccal (mouth) cavity are calculated, and two distinct trends are identified. External recirculation regions and reverse flow feeding cycles are also observed and quantified. One of the unresolved questions in fish suction feeding is the shape and dynamics of the buccal cavity during suction feeding; optical coherence tomography imaging is found to be useful for reconstructing the mouth kinematics. The projected area of the mouth cavity during the feeding cycle varies up to 160 and 22 % for the transverse and mid-sagittal planes, respectively. These findings can inspire novel hydrodynamically efficient biomedical and microfluidic devices. PubDate: 2016-06-16

Abstract: Abstract
In this paper, we assess the local isotropy of higher-order statistics in the intermediate wake region. We focus on normalized odd moments of the transverse velocity derivatives,
\({M_{2n + 1}}(\partial u/\partial z) = {{\overline{{{(\partial u/\partial z)}^{2n + 1}}} }}/{{{{\overline{{{(\partial u/\partial z)}^2}} }^{(2n + 1)/2}}}}\)
and
\({N_{2n + 1}}(\partial u/\partial y) = {{\overline{{{(\partial u/\partial y)}^{2n + 1}}} }}/{{{{\overline{{{(\partial u/\partial y)}^2}} }^{(2n + 1)/2}}}}\)
, which should be zero if local isotropy is satisfied (n is a positive integer). It is found that the relation
\(M_{2n+1}(\partial u/\partial z) \sim R_\lambda ^{-1}\)
is supported reasonably well by hot-wire data up to the seventh order (
\(n=3\)
) on the wake centreline, although it is also dependent on the initial conditions. The present relation
\(N_{3}(\partial u/\partial y) \sim R_\lambda ^{-1}\)
is obtained more rigorously than that proposed by Lumley (Phys Fluids 10:855–858, 1967) via dimensional arguments. The effect of the mean shear at locations away from the wake centreline on
\(M_{2n+1}(\partial u/\partial z)\)
and
\(N_{2n+1}(\partial u/\partial y)\)
is addressed and reveals that, although the non-dimensional shear parameter is much smaller in wakes than in a homogeneous shear flow, it has a significant effect on the evolution of
\(N_{2n+1}(\partial u/\partial y)\)
in the direction of the mean shear; its effect on
\(M_{2n+1}(\partial u/\partial z)\)
(in the non-shear direction) is negligible. PubDate: 2016-06-14

Abstract: Abstract
Mechanical heart valve prostheses are often implanted in young patients due to their durability and long-term reliability. However, existing designs are known to induce elevated levels of blood damage and blood platelet activation. As a result, there is a need for patients to undergo chronic anti-coagulation treatment to prevent thrombosis, often resulting in bleeding complications. Furthermore, recent studies have suggested that the implantation of a mechanical prosthetic valve at the mitral position results in a significant alteration of the left ventricular flow field which may contribute to flow turbulence. This study proposes a bi-leaflet mechanical heart valve design (Bio-MHV) that mimics the geometry of a human mitral valve, with the aim of reducing turbulence levels in the left ventricle by replicating physiological flow patterns. An in vitro three-dimensional particle velocimetry imaging experiment was carried out to compare the hemodynamic performance of the Bio-MHV with that of the clinically established ATS valve. The Bio-MHV was found to replicate physiological left ventricular flow patterns and produced lower turbulence levels. PubDate: 2016-06-13

Abstract: Abstract
This paper reports upon a laser-induced fluorescence visualization and time-resolved particle image velocimetry study to resolve the detailed dynamics associated with Re = 2000 and 4000 circular vortex rings colliding with 30°–75° inclined surfaces. Two-dimensional visualization results show that larger inclination angles lead to increasingly rapid size reduction in the primary vortex-ring core closer to the surface, faster formation of the secondary vortex-ring core, and subsequent ingestion by the former. In contrast, primary vortex-ring core further away from the surface becomes physically larger and incoherent more rapidly, with slower formation and entrainment of the secondary vortex-ring core. Interestingly, a vortex dipole and small vortex-ring-like structure are produced for the largest inclination angle of 75°, possibly due to vortex disconnection and reconnection processes. Results taken along the non-inclined plane show significant bulging of the primary vortex-ring cores when the inclination angle increases from 30° onwards. More importantly, additional vortex cores are observed to entwine with the primary vortex-ring core and provide strong direct evidence for the bi-helical vortex line flow mechanism put forward by Lim (Exp Fluids 7:453–463, 1989). Lastly, the behaviour of the primary and secondary vortex-ring cores further away from the surface is highly sensitive towards the state of the bi-helical lines compressed at that region. Strong compression driven by circumferential flows due to large inclination angles may explain the unique flow structures and behaviour observed for 75° inclination angle here. PubDate: 2016-06-13