Planar Particle Image Velocimetry Technique Research Articles

Particle image velocimetry study of the flow control effects of singular and multiple curved serration models

This paper reports on preliminary observations of an investigation of flows associated with models that mimic serrations on the leading edge of barn owl feathers. The objective was to use particle image velocimetry measurements to determine the capacity of singular and multiple curved (three-dimensional) serration models to modify the noise-reducing indicators of a narrow-channeled flow past a cylinder. Four models were tested: 3 singular serration models of respective angles of inclination, α = 24°, 27.5° and 31°, and a model consisting of an array of 3 serrations of α = 24°, 27.5° and 31°. Each case was subjected to flow of Reynolds number (based on the serration height and maximum velocity of the flow) of ∼2,000, simulating the flow regime of local flow around barbs of real barn owl flights. A planar particle image velocimetry technique was used to capture the midspan plane velocities to determine the effects of each model. The results show that using singular serration models of inclination angles than 30° may lead to disorganized spatial structures and enhanced turbulence levels. On the other hand, an array of only 3 curved serrations of different geometries can modify the spatial flow structure into a well-ordered one, resulting in a 50% reduction in turbulence intensities. These initial results suggest that under complex flow conditions, the insertion of single and multiple curved serrations can lead to significant flow changes that may result in potential noise modifications.

Tip effects on three-dimensional flow structures over low-aspect-ratio plates: mechanisms of spanwise fluid transport

Three-dimensional flows over low-aspect-ratio rectangular flat plates ( ${A{\kern-4pt}R} = 1.00$ – $1.50$ ) are investigated using tomographic and planar particle image velocimetry techniques. The chord-based Reynolds number is $5400$ , and the angle of attack is fixed at $6^\circ$ . This study reveals for the first time the interplay between spanwise fluid transport and downwash, both originating from the tip effects. Spanwise fluid transport promotes the formation and subsequent coherent development of leading-edge vortices, whereas downwash stabilizes the flow. Specifically, two mechanisms related to spanwise fluid transport are revealed. First, the spanwise fluid transport enhances the intensity of the reversed flow, promoting the shear layer roll-up and vortex shedding. Second, the near-wall spanwise flow interacts with the shed C-shape vortices, thereby strengthening the vortex heads. In particular, through these interactions, spanwise fluid transport can sustain the coherence of the C-shape vortices until the vortex heads split in a regular fashion. Consequently, the C-shape vortices are transformed into novel Þ-shape vortices for the plates of ${A{\kern-4pt}R} \leq 1.25$ , which supplements the previously discovered transformation from C-shape to M-shape vortices for larger ${A{\kern-4pt}R}$ plates. Downstream of this novel vortex-splitting transformation, two fundamental processes contribute to the formation of hairpin vortices. The above comprehensive understanding of complete vortex evolution routine provides valuable insights into the tip effects on the formation of three-dimensional flows over low- ${A{\kern-4pt}R}$ plates.

Counter-flow orbiting of the vortex centre in turbulent thermal convection

We present an experimental study of the large-scale vortex (or large-scale circulation, LSC) in turbulent Rayleigh–Bénard convection in a $\varGamma =\text {diameter}/\text {height}=2$ cylindrical cell. The working fluid is deionized water with Prandtl number ( $Pr$ ) around 5.7, and the Rayleigh number ( $Ra$ ) ranges from $7.64\times 10^7$ to $6.06\times 10^8$ . We measured the velocity field in various vertical cross-sectional planes by using the planar particle image velocimetry technique. The velocity measurement in the LSC central plane shows that the flow is in the single-roll form, and the centre of the single-roll (vortex) does not always stay at the centre of the cell; instead, it orbits periodically in the direction opposite to the flow direction of the LSC, with its trajectory in the shape of an ellipse. The velocity measurements in the three vertical planes in parallel to the LSC central plane indicate that the flow is in the vortex tube form horizontally filling almost the whole cell, and the centre line of the vortex tube is consistent with the so-called ‘jump rope’ form proposed by a previous study that combined numerical simulation and local velocity measurements in the low $Pr$ case (Vogt et al., Proc. Natl Acad. Sci. USA, vol. 115, 2018, pp. 12674–12679). In addition, we found that the oscillation of the local velocity in $\varGamma =2$ cells originates from the periodical orbiting of the vortex centre. Our velocity measurements further indicate that the vortex centre orbiting is absent in $\varGamma =1$ cells, at least in the $Ra$ range of our experiments.

Development of a rapid plasma decontamination system for decontamination and reuse of filtering facepiece respirators.

The COVID-19 pandemic has caused a high demand for filtering facepiece respirators (FFRs), which has brought global challenges in sustaining the supply chain for FFRs. Because respirators are basic personal protective equipment to protect frontline healthcare workers against COVID-19, the chronic, global shortage of N95/N99 masks is one of the most urgent threats to our collective ability to save lives from the coronavirus. The reuse of masks may need to be considered as a crisis capacity strategy to ensure continued availability even though most of the masks are considered one-time use. Moreover, environmentalists warn that single-use masks add to the glut of plastic pollution, threatening the health of oceans and marine life. In this study, we develop a method to decontaminate respirators to reuse filtering facepiece respirators. Samples of SARS-CoV-2 are applied to the 4 × 4 cm2 samples of FFP2 and FFP3 respirator materials. The filtration efficiency of plasma treated samples is measured using a planar particle image velocimetry technique with a neutrally charged polydisperse aerosol particle of NaCl. The measured viral decontamination and filtration efficiencies show that the developed plasma decontamination system can achieve a 4-log reduction for the coronavirus without reducing the filtration efficiency of masks after 5-min plasma exposure. The developed plasma decontamination system demonstrates the feasibility to tackle the acute shortages of FFRs in many countries and their environmental and economic burdens against discarding reusable masks.

PIV measurements using refraction at a solid–fluid interface

Laser light sheet refraction at the fluid flow–window interface is proposed to perform fluorescent particle image velocimetry (PIV) measurements, in particular for cases in which optical access limits the possibilities of illuminating a flow with a planar collimated light sheet. The study of laser sheet propagation in the present measurement method is led by ray tracing to quantify the illumination intensity. To describe the limitations of this developed PIV version (refracted PIV (R-PIV)), we proposed a cross-correlation model (CCM) for analyzing image pairs that is an extension of the models used in the cases of µ-PIV or planar PIV (P-PIV) techniques. This CCM accounts for the R-PIV optical constraints of (1) in-depth and longitudinal inhomogeneities of the light sheet refraction at the location of the object plane or (2) integration in depth of the fluorescence from seeding particles. This R-PIV and the P-PIV techniques are further compared in the case of an academic pipe flow for which the analytic solution is known. Our extended CCM function is finally applied, given the theoretical flow velocity, the flow illumination and the seeding flow parameters, to analyze error levels in determining the real flow velocity field.

PIV study of flow through and over porous media at the onset of inertia

In this paper, laminar flow through and over models of porous media is studied to characterize the flow at the onset of inertia. Horizontal, vertical or mesh arrays of circular rods are used to model the porous medium. The rods are arranged to cover solid volume fraction ranging from 0.03 to 0.49, and to fill approximately three-quarters of the depth of the test section. Each of the porous media models is tested in a refractive-index matched viscous fluid flow driven by pressure. The Reynolds number is also varied over an order of magnitude to cover the regime at which the porous medium flow just transitions from a Darcy to an inertia laminar flow regime. Velocity measurements are then obtained by a planar particle image velocimetry technique. The experimental data confirms that the averaged transition flow is indeed governed by the Brinkman equation if modeled with a new parametrization of the apparent viscosity, and modified with an equivalent permeability that accounts for the effects of inertia. Additionally, the results show that at such inertial flows, the interfacial region is the porous medium zone affected most by changes in Reynolds numbers. The data presented in this work provide tools for analytical and numerical validations, and the prediction of interfacial flow pertaining to isotropic porous media.

Experimental analysis of the flow near the boundary of random porous media

The aim of this work is to experimentally examine flow over and near random porous media. Different porous materials were chosen to achieve porosity ranging from 0.95 to 0.99. In this study, we report the detailed velocity measurements of the flow over and near random porous material inside a rectangular duct using a planar particle image velocimetry (PIV) technique. By controlling the flow rate, two different Reynolds numbers were achieved. We determined the slip velocity at the interface between the porous media and free flow. Values of the slip velocity normalized either by the maximum flow velocity or by the shear rate at the interface and the screening distance K1/2 were found to depend on porosity. It was also shown that the depth of penetration inside the porous material was larger than the screening length using Brinkman’s prediction. Moreover, we examined a model for the laminar coupled flow over and inside porous media and analyzed the permeability of a random porous medium. This study provided detailed analysis of flow over and at the interface of various specific random porous media using the PIV technique. This analysis has the potential to serve as a first step toward using random porous media as a new passive technique to control the flow over smooth surfaces.

A high‐speed tomographic PIV system for measuring zooplanktonic flow

Quantification of the hydrodynamic disturbance of free‐swimming zooplankton provides insight into propulsion as well as organism sensory interactions. Current flow measurement techniques, such as planar particle image velocimetry (PIV), are limited by their two‐dimensional nature. These techniques also are challenged by the small spatial scale of zooplankton, by the high speeds achieved by many zooplankton species, and by zooplankton photosensitivity to a broad range of wavelengths. We present a high‐speed tomographic PIV system using near‐infrared laser illumination that is capable of measuring three‐dimensional velocity vectors in a volume surrounding a plankter. This technique is assessed by recording and analyzing the time‐resolved flow field created by a high‐speed escape of a copepod (Calanus finmarchicus). Persistent body and wake vortices are created by the impulsive momentum transfer to the fluid surrounding the animal. It is shown that some aspects of this flow can be analytically modeled as an impulsive stresslet. Azimuthal asymmetry of the strength and position of the wake vortex is analyzed and attributed to the strong ventral flows created by the metachronally beating swimming legs. In addition, the energy required by a copepod escape jump is estimated by calculating the viscous energy dissipation rate using the spatial gradients of the measured three‐dimensional velocity field. Finally, the challenges and benefits of the tomographic PIV technique are discussed.

An experimental investigation on fluid flow characteristics in a real coolant channel of LP turbine blade with PIV technique

Using a planar particle image velocimetry (PIV) system, the fluid flow characteristics including mean velocity and pressure fields in a real coolant channel of a low pressure (LP) turbine blade have been investigated. To understand the flow characteristics within the real LP turbine blade in detail, the studied channel has a real blade-similar cross section, and the entire channel consists of two dividing walls, three passes, a 180° bend, 2 tip exits and 25 trailing edge exits. At first, the flow structures in the channel are captured by the PIV system at an inlet Reynolds number of 44,000, when all the exits are open. The experimental results exhibit different secondary flow features from the previous investigations, which were conducted mostly in simplified two-pass square or rectangular channels. Secondly, through a statistical post-processing of measured instantaneous velocity fields, the Reynolds-averaged Navier–Stokes (RANS) equations are solved to obtain the mean pressure distributions at several typical cross sections. At last, the effects of the inlet Reynolds numbers and coolant ejection from the tip exit above the 1st pass on the flow characteristics are discussed. From the comparisons of the flow characteristics obtained at different flow conditions, it can be concluded that the flow characteristics are sensible to the inlet Reynolds number and coolant ejection from tip exit.