Perform Particle Image Velocimetry Measurements Research Articles

An Anatomically Shaped Mitral Valve for Hemodynamic Testing.

In vitro modeling of the left heart relies on accurately replicating the physiological conditions of the native heart. The targeted physiological conditions include the complex fluid dynamics coming along with the opening and closing of the aortic and mitral valves. As the mitral valve possess a highly sophisticated apparatus, thence, accurately modeling it remained a missing piece in the perfect heart duplicator puzzle. In this study, we explore using a hydrogel-based mitral valve that offers a full representation of the mitral valve apparatus. The valve is tested using a custom-made mock circulatory loop to replicate the left heart. The flow analysis includes performing particle image velocimetry measurements in both left atrium and ventricle. The results showed the ability of the new mitral valve to replicate the real interventricular and atrial flow patterns during the whole cardiac cycle. Moreover, the investigated valve has a ventricular vortex formation time of 5.2, while the peak e- and a-wave ventricular velocities was 0.9m/s and 0.4m/s respectively.

Three-dimensional flow structures and scalar mixing of a sand dune-inspired jet in crossflow

This paper performed particle image velocimetry (PIV) measurements and delayed detached-eddy simulations (DDES) to comprehensively investigate the flow and mixing behaviors of a sand dune(SD)-inspired JICF (jet in crossflow). PIV measurements were conducted in planes parallel to the mid-plane (Z/D = 0, 0.5, and 1), and three velocity ratios (VR = 0.4, 0.8, and 1.2) were studied. The time-averaged and turbulent flow features behind the sand dune-inspired JICF is strongly three-dimensional: the jet tends to move from the mid-plane to the outer planes and can stay attached well to the wall even at Z/D = 1. Then DDES method was applied to reveal detailed three-dimensional flow structures and mixing characteristics of the SD-inspired JICF, and a cylindrical JICF was also simulated as a benchmark for comparison. The horseshoe vortex, arch-shaped shear layer vortices, and streamwise rollers are mainly responsible for the formation of the anti-counter rotating vortex pair (anti-CRVP) observed in the mean flow field of SD configuration. Analysis of concentration distributions indicates that scalar mixing between the jet and the mainstream is highly correlated to the flow features and vortex structures, and the mixing strength of the sand dune-inspired JICF is generally weaker than that of the cylindrical JICF.

Particle Image Velocimetry measurements inside a tapered roller bearing with an outer ring made of sapphire: Design and operation of an innovative test rig

Experimental studies on rolling-element bearing lubrication represent an ambitious engineering challenge. In the present study, the design of a dedicated test rig to perform Particle Image Velocimetry (PIV) measurements on the lubricant inside a tapered roller bearing is presented. Through the usage of a sapphire outer ring, it has been possible to optically access the inside of a bearing without compromising its kinematics. PIV measurements allowed the reconstruction of the lubricant velocity field in the cavity between the cage and the outer ring at different rotational speeds. Results show that as the speed of rotation increases, bubbles due to aeration phenomena tend to appear and modify the behavior of the lubricant considerably.

Velocity distribution in the subchannels of a pin bundle with a wrapping wire (Evaluation of the Reynolds number dependence in a three-pin bundle)

A sodium-cooled fast reactor has been designed to attain a high burn-up core in commercialized fast reactor cycle systems. The sodium-cooled fast reactor adopts a wire spacer between fuel pins. The wire spacer performs functions of securing the coolant channel and mixing between subchannels. In high burn-up fuel subassemblies, the fuel pin deformation due to swelling and thermal bowing may decrease the local flow velocity in the subassembly and influence the heat removal capability. Therefore, understanding the flow field in a wire-wrapped pin bundle is important. This study performed particle image velocimetry (PIV) measurements using a wire-wrapped three-pin bundle water model to grasp the flow field in the subchannel under conditions, including the laminar to turbulent regions. The PIV results confirmed that the normalized flow velocity near the wrapping wire in the low Re number condition was relatively decreased compared to that in the high Re number condition. Meanwhile, in the region away from the wrapping wire, the maximum flow velocity was increased by decreasing the Re number. Accordingly, the PIV measurements using the three-pin bundle geometry without the wrapping wire were also conducted to understand the effect of the wrapping wires on the flow field in the subchannel. The results confirmed that the mixing due to the wrapping wire occurred, even in the laminar condition. These experimental results are useful not only for understanding the pin bundle thermal hydraulics, but also for the code validation.

Experimental investigation of turbulent flow over live mussels

Unionids have been described as ecosystem engineers capable of affecting the local food web as well as the hydrodynamics, passively and actively. In this study, we perform particle image velocimetry measurements to characterize the flow around a live Amblema plicata using natural sediments as tracer particles under two specific flow rates and orientations i.e., facing upstream and downstream, to understand the interaction between the organism and flow surrounding it. The behavior of the mussel is also quantified under all the above experimental conditions using a hall-effect gape sensor that captures valve motion. The flow measurements show clear variations between the two orientations tested, with the presence of a low velocity region behind the mussel extending beyond the tested field of view for the upstream orientation. In contrast, the flow in the wake of the downstream orientation recovers very quickly to the incoming flow strength. The inspection of the vorticity contours around the mussel also highlights clear differences between orientations, with the structure of a shear layer under the upstream orientation changing to a region of concentrated vorticity near the siphons for the downstream cases. The variation of the net flux through the mussel along with the frequency of partial closures captured by the gape sensor measurements illustrate a potential added bioenergetic cost to the mussel feeding when facing downstream, especially at higher flow speeds.

Effects of bottom topography on the spin-up in a cylinder

Motivated by better understanding the long-standing issue of the role of topography on the transport of angular momentum in rapidly rotating fluids, we conducted spin-up experiments in a straight cylinder with a regular pavement of rectangular blocks at the bottom. We perform particle image velocimetry measurements to monitor the decay of the initial differential motion generated by the sudden increase of the container rotation rate. We observe that the re-synchronization time, the so-called spin-up time, is shorter in the presence of topography with a minimum at a particular length scale of the topography pattern. We show evidence of energy transport by inertial waves as well as non-linear mechanisms leading to a scaling of the spin-up time significantly different from the classical E−1/2 in the absence of topography.

PIV measurements in the vicinity of a steam sparger in the PANDA facility

Abstract To study the thermal behaviour, e.g. the stratification build-up and the homogenization, steam is injected through a sparger into a large pool containing 45 m3 of water in the PANDA facility; a scenario which could take place in the wetwell of a BWR in the case of a postulated accident. Depending on the steam flow rates vented in the wetwell two regimes can develop in the water pool (i) a thermal stratification build-up and (ii) thermal mixing or homogenization. If thermal stratification build-up occurs, the temperature of the upper part of the water pool is much higher than the bottom of the pool which would decrease the rate at which steam condenses since the heat sink capability of the pool decreases. Conversely if mixing occurs the pool temperature remains homogeneous throughout the pool and provides a higher heat sink capability for steam condensation. As a complement to temperature recordings, the use of Particle Image Velocimetry (PIV) is needed to validate theoretical models and CFD codes. However, in a two-phase flow of water and condensing steam one is faced with two main challenges. (1) as the steam condenses around the sparger pipe, the temperature increases locally in the pool, consequently, the solubility of air in water decreases and as a result the local heating forces dissolved air out of the water which forms small bubbles rising to the surface. These bubbles might act as additional scattering centres disturbing the velocity measurements in the liquid phase. (2) for high air bubble densities the dense bubble cloud might even block the light passage from the illumination plane to the camera such that the sparger becomes invisible. Thus, a degasification of the water pool is absolutely necessary. To eliminate the bubbles as scattering centres we used Rhodamine-B seeding particles for the PIV measurements. These particles emit light in the red spectrum and with a red filter in front of the camera, only those particles contribute to the velocity calculation. Using degassed water together with Rhodamin-B particles it was finally possible to record velocity fields even in this demanding environment for high steam mass flow rates. The paper contains a description of the methods developed to perform PIV measurements in the vicinity of the sparger and a selection of the results obtained.

PIV measurements of flow around an arbitrarily moving free surface

We present an image preprocessing method for particle image velocimetry (PIV) measurements of flow around an arbitrarily moving free surface. When performing PIV measurements of free surface flows, the interrogation windows neighboring the free surface are vulnerable to a lack, or even an absence, of seeding particles, which induces less reliable measurements of the velocity field. In addition, direct measurements of the free surface velocity using PIV have been challenging due to the intermittent appearance of the arbitrarily moving free surface. To address the aforementioned limitations, the PIV images with a curvilinear free surface can be treated to be suitable for a structured interrogation window arrangement in a Cartesian grid. The proposed image preprocessing method is comprised of a free surface detection method and an image transform process. The free surface position was identified using a free surface detection method based on multiple textons. The detected free surface points were used to transform PIV images of a curvilinear free surface into images with a straightened free surface using a cubic Hermite spline interpolation scheme. After the image preprocessing, PIV algorithms can be applied to the treated PIV images. The fluid-only region velocities were measured using standard PIV method with window deformation, and the free surface velocities were resolved using PIV/interface gradiometry method. The velocity field in the original PIV images was constructed by inverse transforming that in the transformed images. The accuracy of the proposed method was quantitatively evaluated with two sets of synthetic PIV images, and its applicability was examined by applying the present method to free surface flow images, specifically sloshing flow images.

Validation of a numerical 3-D fluid–structure interaction model for a prosthetic valve based on experimental PIV measurements

A numerical 3-D fluid–structure interaction (FSI) model of a prosthetic aortic valve was developed, based on a commercial computational fluid dynamics (CFD) software program using an Arbitrary Eulerian Lagrangian (ALE) formulation. To make sure of the validity of this numerical model, an equivalent experimental model accounting for both the geometrical features and the hydrodynamic conditions was also developed. The leaflet and the flow behaviours around the bileaflet valve were investigated numerically and experimentally by performing particle image velocimetry (PIV) measurements. Through quantitative and qualitative comparisons, it was shown that the leaflet behaviour and the velocity fields were similar in both models. The present study allows the validation of a fully coupled 3-D FSI numerical model. The promising numerical tool could be therefore used to investigate clinical issues involving the aortic valve.

Turbulence statistics and structures of drag-reducing turbulent boundary layer in homogeneous aqueous surfactant solutions

In our earlier work [Itoh et al., Phys. Fluids 17, 075107 (2005)], the additional maximum of the streamwise turbulence intensity near the center of the drag-reducing turbulent boundary layer was found in the homogeneous dilute aqueous surfactant solution which was a mixture of cetyltrimethyl ammonium chloride with sodium salicylate as counterion. In this work, we systematically investigated the influence of the drag-reducing surfactant on the velocity fields of the turbulent boundary layer at various Reynolds numbers Reθ from 301 to 1437 and the drag reduction ratio DR from 8% to 74% under different streamwise locations and concentration and temperature of solutions using a two-component laser-Doppler velocimetry (LDV) system. It was revealed that all data on DR versus the wall-shear rate obtained here were collapsed on a single curve. We verified the existence of the additional maximum of the streamwise turbulence intensity near the center of the boundary layer which appeared at relatively large drag reduction ratios and small Reynolds numbers. It was found that the additional maximum of streamwise turbulence intensity and its wall-normal location were independent of the streamwise location, wall-shear rate, Reynolds number, and drag reduction ratio. The additional maximum could be explained by the bilayered structure model proposed, in which the flow in the near-wall region is in shear-induced structure (SIS) and viscoelastic, whereas the flow in the region away from the wall is in non-SIS and nonviscoelastic. This model was based on measurements of the shear viscosity. We also performed particle image velocimetry measurements, which revealed that the fluctuating velocity vector fields showed two situations, with low and high activity. In low activity, the velocity fluctuations were attenuated largely across the turbulent boundary layer. In high activity, fluctuating velocity vectors were almost parallel to the wall and relatively large in both regions near the wall and the center of the boundary layer, which seemed to be a bilayered structure and supported the bilayered structure model.

PIV measurements of turbulent boundary layer over a rod-roughened wall

The effects of surface roughness on a turbulent boundary layer (TBL) were investigated by performing particle image velocimetry (PIV) measurements. The Reynolds number based on the momentum thickness was about Re θ = 1000–1500. The roughness elements used were periodically arranged two-dimensional spanwise rods, and the ratio of the boundary layer thickness to the roughness height was k/ δ = 0.025. Along the streamwise direction, self-preserving forms of the streamwise mean velocity and the turbulent Reynolds stress were obtained. Spatially developing characteristics of the rough wall TBL were examined. Introduction of the roughness elements augmented turbulent stresses in the region of y < 4–5 k s, where k s is an effective sand roughness height. We found that the effective sand roughness height ( k s) is a more appropriate length scale for representing the extent of roughness effect rather than the roughness height ( k). By using PIV measurements, the effects of surface roughness on the turbulence structure in the roughness sublayer were examined and compared with previous direct numerical simulation (DNS) results. Iso-contours of the mean velocity and Reynolds stress in the roughness sublayer obtained by PIV were in very good agreement with DNS findings. Vortex structures induced by the rod-roughness and strong sweep events were also examined.

MOVEMENT OF TWO CONSECUTIVE TAYLOR BUBBLES IN VERTICAL PIPES

The development of slug flow along vertical pipes is governed by the interaction between consecutive elongated bubbles. It is generally assumed that the trailing bubble's shape and velocity are affected by the flow field in the liquid phase ahead of it. To examine this assumption, a facility is used that allows controlled injection of pairs of Taylor bubbles into vertical pipes filled with stagnant or flowing liquid. An experimental approach is developed to perform particle image velocimetry measurements of the velocity field in front of the trailing Taylor bubble, simultaneously with video imaging of Taylor bubble pairs, to be able to relate the instantaneous parameters (shape and velocity) of the trailing bubble to the instantaneous velocity distribution in liquid ahead of it. Experiments are performed in pipes of two diameters and for a number of Reynolds numbers based on mean water velocity, corresponding to both laminar and turbulent background flows. A model relating the propagation velocity of the trailing Taylor bubble to the local mean centerline velocity of the leading bubble is suggested and verified experimentally. The effect of the velocity fluctuations in the leading Taylor bubble's wake on the instantaneous propagation velocity of the trailing bubble is studied.

PIV measurements of proximal models with different anastomotic angles

The anastomosis glass models are designed and fabricated to investigate the effect of the anastomotic angles (30° to 90° for both forward and backward flow) on the intimal hyperplasia formation. A test rig is designed and built to perform Particle Image Velocimetry (PIV) measurement on the anastomosis models with three Reynolds numbers of 100, 169 and 250. The initial results indicated that there are two low velocity regions, one of them is located at the outer wall of the graft near the joint and the other is located at one diameter down from the joint along the inner wall of the graft. It is found that these low velocity regions are dependent on the Reynolds number and anastomotic angles.