Particle Image Velocimetry Technique Research Articles

Aerothermal characteristics past a matrix structure inside a rectangular serpentine passage

A matrix structure can offer a high heat transfer and improved structural rigidity to the gas turbine's aerofoils. The earlier matrix studies are limited to presenting aerothermal behavior within the matrix structures. Thus, there is a scope to investigate the aerothermal characteristics downstream of a matrix geometry. The present experimental effort seeks to characterize the detailed flow and heat transfer downstream of the matrix and at the bend for Re = 800 and 6500. The heat transfer and flow fields are captured in various planes using liquid crystal thermography and stereoscopic particle image velocimetry techniques. The results show that the matrix ejects a highly turbulent vortical flow in the form of two co-rotating vortices, which further diffuse and merge in downstream. The matrix offers a very high average augmentation (Nu¯/Nu¯0) in the first pass for Re = 800 (varies from 12 to 4), but it shows a poor heat transfer (Nu¯/Nu¯0 < 1) at the bend region, which can generate local hot spots for Re = 800. In contrast, for Re = 6500, the matrix offers a stable augmentation throughout the first pass and bend region, i.e., Nu¯/Nu¯0≈ 2.4–1.

Self-similarity of sand bed scour by an oblique impinging plane jet under different Reynolds numbers

The present study investigated the effects of varying Reynolds number (Re = 6,000, 8,000, 10,000 and 12,000) of a plane oblique impinging water jet on an erodible sand bed under a fixed impinging angle (θ = 40°) and a fixed impinging distance (h = 0.202 m). Time evolution of the scour hole profile was recorded using a video camera, and the flow fields in the scour hole at equilibrium state were measured using Particle Image Velocimetry (PIV) technique. The results reveal that both the scour hole profile and the flow structure in the scour hole under different Reynolds numbers are mostly self-similar. Specifically, the distributions of streamline, velocity, average kinetic energy and Reynolds shear stress at equilibrium state are analyzed in detail. It is found, among the first according to the authors' best knowledge, that all these flow quantities exhibit spatial self-similarity, when normalized with proper length and/or velocity scales. Furthermore, the Reynolds shear stress on the sand bed profile has two peak values. The positive peak near the impingement point is similar in magnitude (between 4 and 6 Pa for different Reynolds numbers), while the negative peak in the bed vortex region differs.

Investigation of a Near-Field Cylinder Wake in the Subsonic, Transonic, and Supersonic Regimes

The near wake of a circular cylinder in a high-Reynolds-number regime is investigated using the particle image velocimetry technique for an inflow Mach number spanning 0.3 to 2. The mean flow and turbulent kinetic energy field for different Mach number cases are presented. Furthermore, the influence of the Reynolds and Mach number effects is examined based on the mean separation point along the cylinder surface, which is extracted from complementary schlieren visualization measurements, and the literature. The wake width for the supersonic inflow case is found to be an order of magnitude smaller physically than the subsonic case with an identical cylinder diameter. Additionally, the swirl strength criterion is used to identify the eddies present within the wake, and the eddy size and convection velocity are characterized based on the spatiotemporal correlations. The result suggests that the mean wake features for the subsonic and supersonic cases are very different. Moreover, the interaction of eddies generated at the cylinder’s top and bottom shear layers leads to the large-scale features present in the wake, and the differing paths followed by the eddies between the subsonic and supersonic cases are responsible for the variation in the observed wake width.

Ferrofluid drops-based actuator in a narrow gap

Directional transport of a liquid is an important issue in microfluidic systems and application purpose. Here, through combining the ideas of pressure-driven gas bubble-induced acoustic streaming flow and magnetic field-deformed ferrofluid drop, we study the ambient flow induced by an oscillating ferrofluid drop as an in situ actuator in a millimeter-sized gap environment. A drop squeezed by two parallel glass sheets, under the influence of a magnetic field, is discovered to undergo multimodal oscillations. The particle image velocimetry technique helps us to reveal the vortex-typed flow structure surrounding the oscillating drop. The shape changes of drop are found including the circular, elliptical, triangular, inverse-triangular, and circular shapes. We employ the numerical front-tracking method and analytical mixed-mode model to elucidate a drop-driven flow. We find that the pulsating, translational, and quadrupole mode oscillation of the drop is capable to describe most features of the flow distribution. Furthermore, we demonstrate an in situ pump by applying a spatially non-uniform pulsating magnetic field onto the arranged ferrofluid drops. The ferrofluid drop-based in situ pump shows the ability to produce a flow rate of 108 μl/min, which should be a great help in microfluidic pumping.

Wall-proximity effects on vortex-induced vibrations of a circular cylinder

Vortex-induced vibrations (VIV) of an elastically supported circular cylinder (of diameter D) undergoing a wall interference are investigated for a mass-damping ratio 0.31, Reynolds numbers 1.77 × 103–1.24 × 104, and wall-cylinder gap height ratios e* = e/D = 0.1–1.6. Measurements of vibration responses and vortex shedding frequencies are done using a laser vibrometer and a hotwire anemometer, respectively, while particle image velocimetry techniques are employed to capture flow fields. This investigation aims to assimilate the effect of e* on vibration response, frequency response, VIV range, quality factor of VIV, added mass, added damping, lift force, and flow structure. A smaller e* is observed to increase the added mass, reduce the maximum vibration amplitude, and shorten the VIV range while enhancing the quality factor of VIV. The increase in added mass and the decrease in the maximum vibration amplitude in the VIV range are dramatic when e* is decreased from 0.6. Decreasing e* significantly impacts the flow structures and vortex shedding modes, resulting in intensified asymmetry in pressure distributions. The interference between the wall and the cylinder is further reflected by increasing time-mean lift forces and decreasing fluctuating lift forces with decreasing e* < 0.6.

Flow Turbulence and Pressure Fluctuations in a Hydraulic Jump

Turbulence and pressure fluctuations are key elements in the bed protection design of hydraulic structures. However, their roles in a hydraulic jump are not yet fully understood, and the nature of their relationships are not conclusive. In order to better understand the relationships between pressure fluctuations and flow characteristics of a hydraulic jump downstream of a weir, detailed measurements of flow kinematics using the nonintrusive techniques of particle image velocimetry and bubble image velocimetry and pressure using voltage-type pressure gauges were carried out in this study. The physical modeling of the hydraulic jump was carried out using the simultaneous measurements of pressure and turbulent flow properties. The distributions of flow properties, such as water level and velocity, were assessed in each case. Based on the measurements, the correlations between the pressure fluctuations and the variables were investigated by coupling the statistical values of the variables at the same points. The analysis results show that the water level and turbulence intensity are the main factors influencing the pressure fluctuations in the hydraulic jump. Using these factors, an empirical formula and dimensionless numbers are proposed to show that the pressure fluctuations depend on the bubble flow behavior.

FRP spiral strip-confined concrete columns: Stress-strain behavior and size effect

It is well-known that the hoop rupture strain of fiber-reinforced polymer (FRP) jacket in an FRP-confined concrete column is commonly much lower than that obtained from coupon tests. To improve the strain efficiency of the FRP jacket, FRP spiral strip wrapping scheme has been proposed. In the present study, totally 24 circular column specimens [including 16 FRP spiral strip-confined concrete (FSSCC) column specimens] of four different sizes, with a diameter ranging from 100 mm to 400 mm, were tested under uniaxial compression. The particle image velocimetry (PIV) technique was adopted to obtain the hoop and axial strains as well as the strain distributions of the test specimens. The test results showed that the FRP hoop rupture strains measured by the PIV technique in the FSSCC columns are close to that from the coupon tests. The size effect in terms of compressive strength and stress-strain behavior is obvious in the test FSSCC columns. An existing stress-strain model for FRP-confined concrete was then modified to predict the test results. Finally, a field application of the FRP spiral strip wrapping technique completed recently was also briefly introduced.

The Momentum Transfer Mechanism of a Landslide Intruding a Body of Water

Landslide-generated waves occur as a result of the intrusion of landslides such as mud flows and debris flows into bodies of water such as lakes and reservoirs. The objective of this study was to determine how the momentum is transferred from the sliding mass to the body of water on the basis of theoretical analysis and physical model experiments. Considering the viscoplastic idealization of natural landslides, the theoretical model was established based on the momentum and mass conservation of a two-phase flow in a control volume. To close the theoretical equations, slide thickness and velocity passing through the left boundary of the control volume were estimated by lubrication theory, and the interaction forces between the slide phase and water phase, including hydrostatic force and drag force, were given by semiempirical equations fitted with experimental data obtained using the particle image velocimetry (PIV) technique. The near-field velocity fields of both the sliding mass and the body of water, as well as the air–water–slide interfaces, were determined from the experiments. The theoretical model was validated by comparing the theoretical and experimental data of the slide thickness and slide velocity, as well as the momentum variations of the two phases in the control volume.

Greenwater due to plunging breaking wave impingement on a deck structure. Part 1: Experimental investigation on fluid kinematics

This study experimentally investigates the kinematics of greenwater resulting from plunging breaking wave impacts on a deck structure. Two impingement scenarios, characterized by different breaking points, were considered. The ensemble-averaged flow fields were obtained by applying both PIV (particle image velocimetry) and BIV (bubble image velocimetry) techniques to 50 repeated tests. The study examines flow patterns, velocity fields, and derived quantities from velocity data to identify differences in flow behaviors, variations in velocity magnitude, and potential distribution of peak impact pressures. The presented results provide useful information and indication for assessing concentrated loadings and enhancing our understanding of hydrodynamic behaviors. Additionally, the validity of applying Ritter dam break solutions to describe the horizontal velocity distribution of greenwater flows is examined, and prediction equations based on self-similar horizontal velocity distribution are derived.

Coupled motion characteristic of standard flag and inverted flag in tandem arrangement

In this paper, the dynamics of the flow-induced flutter of standard flag and inverted flag in tandem arrangement were experimentally studied in wind tunnel. The kinematics of the flapping flags was investigated using high-speed camera and the flow field around the flags were examined using particle image velocimetry technique. The experimental results show that the critical velocity of inverted flag and standard flag in tandem were both decreased due to the flow perturbation between two flags. In particular, the hysteresis loop of the tandem standard flag was decreased significantly compared with single standard flag. Hence, the flow velocity range for flag flapping is broadened widely as compared to that of a single flag. By the action of the flapping flags, flow around the tandem flags exhibited severe turbulence. Small-scale vortices and Kelvin–Helmholtz instabilities could be observed. These findings will provide theoretical support for the mechanism design based on flag fluttering and overcoming the inherent limitations of the single flag in practical application.

Assessment of pegged geogrid (PG) pullout performance in coarse-grained soils using PIV analysis

The paper examines the efficacy of using small, welded metal plates (pegs) in enhancing the pullout resistance of commercially available geogrids. Soil deformations and particle displacements at the soil-geogrid interface and in the vicinity of the pegs were investigated using the Particle Image Velocimetry (PIV) technique. Effects of the soil particle size, overburden pressure, and the height and locational arrangement of the pegs were examined. Results indicate the effectiveness of pegs in enhancing geogrid pullout resistance, which increases significantly with peg height and the distance from the pullout load. Meanwhile, the PIV images showed larger soil strains and particle displacements in coarser soils and under greater overburden pressures. Results also indicate that taller pegs are more effective in engaging the soil adjacent to the reinforcement layer, resulting in reduced particle displacements and increased passive resistance of a larger volume of soil, leading to greater pullout resistance. Findings of the study can be beneficial in practice where pegged-geogrids of reduced length could be used to meet design requirements while addressing space limitations and/or excavation costs, and thereby, help with the feasibility and cost-effectiveness of MSE walls and reinforced slopes in related projects.

Flow dynamics and heat transfer characterization of confined multiple jets impinging on a complex surface

Submerged and confined multiple jet impingement is widely implemented in cooling applications since it provides high heat transfer coefficients and uniformity over the target plate. Its performance depends on several variables that make it complex and difficult to control. To understand the physical phenomena and characterize the flow field, an in-depth study, using Particle Image Velocimetry (PIV) technique and an heat flux sensor, is conducted in this study. The PIV provides relevant data, but the accuracy of the measurements depends on an effective experimental setup and a careful selection of the most appropriate tracer particles. Therefore, this work presents the purpose-built experimental apparatus and comprises an analysis of the efficiency of different seeding particles. The results demonstrate that olive oil particles are appropriate to track turbulent flows since particles with about 1 μm diameter are obtained by the seeding generator. PIV measurements highlight the complexity of the jet flow impinging on a step surface, which induces a strong flow reversal that affects the jet flow development and the interaction with the adjacent jets. The large-scale structures induced in the vicinity of the target plate are captured by the PIV, as well as the strong fountain flows generated between the adjacent jets. Compared with the flat geometry, the turbulence intensity at the central jet is around 25% higher for the 1 D step, while for the 2 D step, this increase reaches 7.5%. The increased turbulence intensity leads to an heat transfer enhancement. For the 2 D step plate, the Nusselt number recorded is 25% greater than the flat plate. Through this study, relevant insights for several engineering applications that use multiple jet impingement are provided, highlighting that the combination of PIV and heat flux sensors are appropriate to characterize the jet’s flow dynamics and the heat transfer of this complex process.

Turbulent dispersion of a passive scalar in a smooth-wall turbulent boundary layer

The scalar dispersion of a ground-level point-source plume in a smooth-wall turbulent boundary layer is experimentally investigated using simultaneous particle image velocimetry and planar laser-induced fluorescence techniques. In the near-source region, the viscous sublayer is observed to trap dye, while in the far field, the half-width, vertical profiles and peak decay of the mean concentration and concentration variance exhibit self-similar behaviour and collapse with empirical relations. Full two-dimensional maps of the turbulent scalar fluxes show a net transport direction of upward and towards the incoming flow, with the vertical profiles collapsing well with Weibull-type exponential functions and the decay of peaks following power laws. Using the first-order gradient transport to model the turbulent scalar fluxes, maps of the anisotropic turbulent diffusivity tensor and an effective turbulent diffusivity coefficient are calculated. The streamwise and wall-normal turbulent scalar fluxes are driven dominantly by the wall-normal concentration gradient. The turbulent Schmidt number, relating the turbulent diffusivity and the turbulent (eddy) viscosity calculated using the Boussinesq hypothesis, varies with wall-normal position with values of the order of unity in the logarithmic layer.

Application of PIV for Virtual Mass Coefficient Measurement

The particle image velocimetry (PIV) technique is employed for the measurement of the virtual mass of a submerged object, an important parameter in the two-fluid model, particularly so for reactor thermal-hydraulic and safety analyses. Instead of carrying out the measurement through traditional transient processes that mix steady-state drag, virtual mass force, and Basset force, a new PIV approach is developed for steady-state flows through the integration of the fluid kinetic energy around the object. The Basset force, an inseparable transient force in viscous flows, is eliminated in the new approach, making virtual mass quantification possible. This new method has been applied to the virtual mass measurement of a solid cylinder, and although the measurement uncertainty from the flow’s random fluctuations is substantial, the results are very encouraging. The results suggest that the existence of drag force in viscous flow affects the virtual mass, as the flow field is different from the ideal potential flow. When the measurement method was applied to the quantification of air bubbles, no reliable data were obtained due to complications from bubble lateral motions and deformation. Further study is needed for the PIV method to be employed for bubble virtual mass force measurements under steady-state flow conditions.

Marangoni flow induced in a waterbody by the impact of a raindrop

In this study, the Particle Image Velocimetry (PIV) technique is employed to investigate the transient flow that arises when a freshwater drop impinges onto the surface of a water body. Our measurements show that in addition to the impact of the drop itself, surfactant and salt concentration play a crucial role in determining the resulting flow on and near the surface. The latter contributions arise from surface tension gradients that arise from merging the drop’s water with the water on the surface, which reduces the concentrations of salt and surfactant. As the surface tension of water falls with decreasing salt concentration, the decrease in salt concentration leads to a Marangoni flow, which moves radially outward from the impact area. On the other hand, the reduction in surfactant concentration causes a reverse Marangoni flow, which flows towards the impact point as the surface tension of water increases with decreasing surfactant concentration. The overall direction and strength of the flow depend on the Weber number and the surfactant and salt Marangoni numbers. Experiments indicate that when the surfactant Marangoni number dominates, the radial flow due to the drop’s impact quickly reverses direction giving rise to a jet-like flow from the impact point in the downward direction. In contrast, when the salt Marangoni flow dominates, the radial flow persists for a longer duration, and the diameter of the region free of PIV particles on the surface is larger than for the freshwater case. The height of the water column increases with increasing Weber and surfactant Marangoni numbers but decreases with salt Marangoni number. The strength and direction of the surface flow are crucial for enhancing the mixing of dissolved gases in the near-surface layer into the water body.

Effect of oxygen concentration on the velocity in buoyant turbulent diffusion flames

To better understand fire dynamics of buoyant turbulent diffusion flames under different oxygen concentrations (OCs, volumetric basis), we have recently conducted measurements on the flame temperature, radiation, and soot temperature and volume fraction under normal and reduced OCs. In the present study, particle image velocimetry (PIV) technique was employed to investigate the effect of OC (20.9%, 16.8%, and 15.2%) on the velocity field of the same flames with a theoretical heat release rate (HRR) of 15 kW. The statistics of the velocity field, such as mean and standard deviation of velocity, turbulent kinetic energy, Reynolds stress, and turbulent viscosity, were investigated and compared among different OCs. Results show that OC has minimal effect on the velocity distribution at heights close to the flame length. However, at lower heights above burner (HABs), turbulent mixing is reduced under reduced OCs, and the velocity shows an increased value at the flame centerline but with a narrower profile along the radial direction. Finally, a comparison with previous data was conducted and analyzed to further explore the fire dynamics. This study improves the physical understanding of related fire dynamics under reduced OCs and provides a comprehensive dataset for turbulent fire modeling validation.

Heat transfer enhancement in laminar water flow through a square channel by streamwise ultrasound irradiation

This research focuses on the heat transfer of laminar water flow in a square channel under 28–120 kHz and 60–120 W ultrasonic waves, irradiated along the streamwise direction. The experiment was conducted at Reynolds numbers of 400–2000, and the uniform surface heat flux of the pipe was set as 3.1–12.9 kW/m2. In this study, ultrasound-induced streaming was investigated using a particle image velocimetry (PIV) technique, and cavitation bubbles were detected using a digital camera. From the results, the maximum heat transfer enhancement was 155% with 28 kHz and 120 W ultrasound at q = 12.9 kW/m2, Re = 400, and x/Dh = 2.58. The Nu ratio, which represents the capability of ultrasound-induced heat transfer enhancement, was also determined. A maximum Nu ratio of 2.017 was achieved with 28 kHz and 120 W ultrasound at q = 7.2 kW/m2 and Re = 400 or St = 28.16 × 104. With a relatively low Re, the poorest heat transfer was observed at St values ranging from 40 × 104 to 120 × 104. This study provides a predictive formula for the Nu ratio, with deviations ranging from 6.89 to 13.2%. Furthermore, the PIV analysis suggested that acoustic streaming of Eckart type appears under the ultrasound irradiation and its velocity enhancement effect becomes maximal under the conditions for the maximum Nu ratio. It is also confirmed that the ultrasound irradiation can enhance the nucleation of bubbles at the pipe wall; ultrasound-induced dynamics of these nucleated bubbles are expected to further disturb near-wall liquid motion and contribute to heat transfer enhancement.

Reconstruction of missing flow field from imperfect turbulent flows by machine learning

Obtaining reliable flow data is essential for the fluid mechanics analysis and control, and various measurement techniques have been proposed to achieve this goal. However, imperfect data can occur in experimental scenarios, particularly in the particle image velocimetry technique, resulting in insufficient flow data for accurate analysis. To address this issue, a novel machine learning-based multi-scale autoencoder (MS-AE) framework is proposed to reconstruct missing flow fields from imperfect turbulent flows. The framework includes two missing flow reconstruction strategies: complementary flow reconstruction and non-complementary flow reconstruction. The former requires two independent measurements of complementary paired flow fields, posing challenges for real-world implementation, whereas the latter requires only a single measurement, offering greater flexibility. A benchmark case study of channel flow with ordinary missing configuration is used to assess the performance of the MS-AE framework. The results demonstrate that the MS-AE framework outperforms the traditional fused proper orthogonal decomposition method in reconstructing missing turbulent flow, irrespective of the availability of complementary paired faulty flow fields. Furthermore, the robustness of the proposed MS-AE approach is assessed by exploring its sensitivity to various factors, such as latent size, overlap proportion, reconstruction efficiency, and suitability for multiscale turbulent flow structures. The new method has the potential to contribute to more effective flow control in the future, thanks to its characteristic that eliminates the requirement for complementary flow fields.

Experimental Study of Flow Kinematics and Impacting Pressures on a Suspended Horizontal Plate by Extreme Waves

The flow kinematics and impacting pressures on a suspended horizontal plate under extreme waves were investigated experimentally. Three different stages of extreme waves, unbreaking, incipient breaking, and broken, were separately generated using a dispersive focusing method. The flow field kinematics around the plate during the slamming process was measured using a combination of particle image velocimetry and bubble image velocimetry techniques. We found that for aerated areas, there are significant differences in flow patterns under different conditions. The velocity distribution in aeration areas is more discrete. The slamming peak on the upper surface is influenced greatly by the aeration effect, resulting in the maximum slamming peak of the unbreaking case being 3.8 kPa, which is 0.41 times larger than that of the incipient-breaking case and 1.12 times larger than that of the broken case. However, for the area below the plate, the slamming force and flow evolution under different types of breaking exhibit similarity.

Experimental study on the wake control of a square cylinder mounted with dual rigid/flexible splitter plates in the subcritical regime

The wake control of a bluff body is vital to the safety of structures in the field of coastal and ocean engineering. The quantitative statistics wake control and flow field in the subcritical regime are relatively unexplored. The purpose of this research is to find an optimum arrangement for the wake control of a square cylinder mounted by dual flexible/rigid splitter plates and uncover the physical mechanisms. The Reynolds number based on the side length of the square cylinder is 4.2×103≤Re≤1.0×104. Splitter plates with two thicknesses were adopted, which represents the flexible and rigid splitters, respectively. The splitter length was considered over the range l/L=1∼2 with an increment of Δl/L=0.25. The force exerting on the bluff body and the instantaneous velocity field were measured by the triaxial load cell and particle image velocimetry techniques. The results indicate the modulation of the flapping splitter as well as the splitting effect of the tip vortices determine the lift suppression and the gap flow and streamlined trailing play a vital role in drag reduction. The maximum drag reduction and lift suppression reach ηmax=58.3% and 90.6%, respectively. The flexible splitter outperforms its rigid counterpart, especially at high Reynolds numbers, which is beneficial for those structures in marine engineering. Given the hydrodynamic characteristics and sensitivity to incoming flow conditions, the optimum arrangement is the flexible splitter configuration with splitter length of l/L=1.25∼1.5.