Particle Image Velocimetry System Research Articles

Characterization of sand convective motions at a vertical wall subjected to long-term cyclic loading

By conducting a two-dimensional experimental study, this paper aims to enhance the understanding of the mechanism of sand convective motions in the vicinity of a wall subjected to long-term cyclic lateral loadings. The experimental tests were conducted in a rectangular sandbox with a transparent front-wall, through which the process of sand particle motions could be recorded by using a high-resolution digital camera. The images were processed with a high time-resolved PIV (Particle Image Velocimetry) system. Based on the experimental data, this work (1) presents the sand flow field in the convective zones; (2) provides means to describe the convection mechanism; (3) proposes the relationships between the loading conditions and dimensions of the region with intense sand movement; and (4) elaborates the similarity of the sand flow velocity structure within the sand convective zones.

Formation Mechanism of Wall Jet Generated by Plasma Actuators in Quiescent Air

Flow control technology using dielectric barrier discharge plasma actuators stimulated by a sinusoidal alternating current (AC) high-voltage power supply has been studied explosively during the last two decades in the aerospace area for promoting the aerodynamics performance of the airplane. For the purposes of uncovering the controlling mechanism and enhancing the control effect of plasma actuators, the formation mechanism of the wall jet produced by plasma actuators in still air is studied by using a high-frequency particle image velocimetry system, a pressure-field microphone, and a high-accuracy phase-lock image freezing Schlieren technique. The discharge of the plasma actuator is mainly involved in the streamer discharge and the glowlike discharge during one sinusoidal AC period. It is found that the acoustic streaming flow generated by the plasma actuator lifts the airflow, and the body force generated by the plasma actuator pushes the airflow, during the streamer discharge stage. In addition, the airflow is drawn by the entrainment and pushed by the induced body force, respectively, during the glowlike discharge stage. Based on the body force and acoustic streaming flow, the influences of the plasma actuator on quiescent air are summarized as lift–push–push behavior.

Development of a Model for the Separation Characteristics of a Deflector Wheel Classifier Including Particle Collision and Rebound Behavior

Deflector wheel classifiers are widespread in industry for the separation of powders into fine and coarse powders. Even though this separation process has been known for quite some time, it is not yet fully understood, and existing models fail to precisely predict the separation characteristics. Due to the high throughput of deflector wheel classifiers, it is greatly beneficial to estimate the separation characteristics before the experiment. Here, the developed model critically examines the usual assumptions, such as ideal airflow, neglection of particle–wall and particle–particle interactions, or spherically-shaped particles. First, the investigation of the air flow using a Particle Image Velocimetry (PIV) system showed significant differences to the assumed ideal flow field, then particle sphericity and its influence on the interaction between the particles and the paddles of the deflector wheel was investigated and compared with particle rebound behavior on a static wall. Surprisingly, comminuted glass behaves similarly to comminuted limestone in multiple aspects and not like glass beads. To determine the number of particle–particle collisions, Discrete Element Method (DEM) simulations were performed. The aforementioned aspects found application in the model and the separation behavior was well-estimated.

Study of electromagnetically driven flows of electrolytes in a cylindrical vessel: Effect of electrical conductivity, magnetic field, and electric current

The paper reports on the flow in a cylindrical vessel filled with an electrolyte and driven by a Lorentz force generated by the interaction of a dc current and the magnetic field of a permanent magnet. Using a two-dimensional (2D) particle image velocimetry (PIV) system, we analyze the velocity field in an axial-oriented central plane. In particular, two different electrolytes are used as working fluids: Potassium chloride and Sodium bicarbonate. For each electrolyte, three different values for electric current are explored: 3, 4, and 5 mA; as well as, two different magnets are used. The first magnet partially covers the diameter of the cavity; whereas the second one is larger and three times more intense. Velocity fields obtained for electrolytes and magnets are compared. We found that, the highest the conductivity of the electrolyte, the highest a parameter related to the kinetic energy of the velocity field in the plane. For the square magnet, this parameter increases nearly linear as a function of the current; in contrast, for the rectangular magnet, it slightly decreases. Additionally, 3D numerical simulations are performed using the COMSOL Multiphysics software. The electric scalar potential magnetohydrodynamic (MHD) formulation is implemented to model the flow behavior. The computed numerical results reproduce the main characteristics of the flow.

Drag reduction in cylindrical wake flow using porous material

Due to its unique pore structure, porous materials have the potential to be used in the fields of acoustic noise reduction and flow drag reduction control. In order to study their effects and mechanism of drag reduction on the flow around a circular cylinder, experiments are conducted in a low-speed wind tunnel with low turbulence intensity. The drag forces acting on a circular cylinder model are measured using wind tunnel balance when porous materials with different permeability are applied within different intersection angles on the trailing-edge and leading edge, and the flow fields are visualized with a particle image velocimetry system with high time resolution. The method of dynamic mode decomposition (DMD) is also used for reduced-order analysis of the vorticity field in the wake of the cylinder. The measured drag forces and wake flow fields are then compared with those of a smooth cylinder, and the results show that porous materials laid on the trailing-edge can reduce drag, when a porous material with 20 pores per inch is laid within 270° on the leeward side, the best effect of the drag reduction ratio of 10.21% is reached. The results of flow visualization indicate that after the porous material is applied, the vortex region in the wake of the cylinder is expanded; both the frequency of vortex shedding and the magnitude of vorticity fluctuation decrease; the Reynolds-shear-stress decreases significantly, and both indicate that vorticity is dissipated earlier. The results of DMD analysis show that porous materials can effectively relax the energy of vortices in different modes.

Experimental investigation of R744 transcritical flow and mixing in the two-phase ejector

In this work, the laboratory test rig for visualisation of the CO2 ejector is shortly presented. The internal flow structures in the premixing chamber and expansion cone are visualised using two different measurement arrangements, i.e. Particle Image Velocimetry (PIV) system and high-speed camera system. Both systems allow to determine some characteristic features of the R744 flows and propose some important quantities (i.e. expansion and mixing angles) describing expansion and mixing processes. Potentials of those preliminary results for field validation data are also pointed out.

2D-3C PIV measurement of the near wake of a ducted propeller

The wake dynamics of the ducted propeller is complicated due to the interaction between the nozzle and the propeller. The development of the tip and hub vortices and the contraction of the slipstream in the near wake are analyzed using the time-averaged velocities measured by the towed underwater stereo particle image velocimetry (SPIV) system. The PIV measurement results are in a two-dimensional plane which contains three velocity components (2D-3C), and the representative features of the ducted propeller wake are captured: The decelerated axial velocity area appears near the interface between the accelerated flow and the free stream. The duct accelerates the evolution process of the propeller wake as a faster decrease of the axial velocity in the downstream is observed. The vorticity magnitude the tip vortex structures are smaller than that of the open propeller at the same downstream positions. The wake energy of the ducted propeller diffuses faster owing to the earlier break down of wake vortex structures.

Investigating Shear-Layer Instabilities in Supersonic Impinging Jets Using Dual-Time Particle Image Velocimetry

Flow unsteadiness in supersonic impinging jets due to the aeroacoustic feedback can lead to practical problems like ground erosion in short takeoff and vertical landing aircraft, high noise levels, and high unsteady structural loading. The shear-layer instabilities of the jet play an important role in the flow dynamics. A full study of these instabilities requires time-resolved measurements of the flow velocity field. However, experimentally obtaining time-resolved velocity fields, derived from particle image velocimetry (PIV) techniques, has its challenges in high-speed flow applications due to the simultaneous requirement of high spatial and temporal resolution. A novel approach is presented in this paper, which uses time-unresolved dual-time digital PIV coupled with dynamic mode decomposition (DMD) analysis (DT–DPIV–DMD) of the velocity field pairs and applies it to study the dynamics of the shear-layer instabilities in supersonic impinging jets. DT–DPIV–DMD involves the acquisition of a pair of instantaneous velocity fields separated by a small time delay using two covisual PIV systems. The time delay must be sufficiently small to provide the necessary bandwidth to capture all the relevant dynamics of the flow being investigated. This is followed by DMD analysis of all acquired digital PIV velocity field pairs. A large-eddy simulation of an underexpanded impinging jet at Reynolds number 60,000, with nozzle-to-wall distance was first used to investigate the required measurement scales for the application of DT–DPIV–DMD. Using the optimum measurement scales, the technique was then experimentally demonstrated using DT–DPIV–DMD for two supersonic impinging jets cases and validated with acoustic measurements.

Active failure characteristics and earth pressure distribution around deep buried shield tunnel in dry sand stratum

The stability of the tunnel face is a key issue in shield tunnel engineering. In this paper, a series of 1 g model tests were conducted in dry sand for various buried depth ratios H/D = 2, 3, 4, 5, and 6 to study the failure characteristics, earth pressure distribution and the soil arching effect. Each test provided a measurement of the earth pressure with the face withdrawal displacement. And the soil deformation and the failure characteristics were investigated by Particle Image Velocimetry (PIV) system. The development of the failure characteristics was discussed for the buried depth ratios H/D = 3. The relationships between the failure characteristic and buried depth ratios were investigated. The variation of limit support pressure, soil stress distribution at different depths and earth pressure around the tunnel are analyzed. The soil arching effect around the tunnel at deep burial conditions is revealed. With the withdrawal displacement increasing, the tunnel face is in close contact with the support panel at first, and then gradually separated from the support panel. The larger the H/D is, the smaller the increment of horizontal soil stress is. It could be concluded that the increase of tunnel depth directly leads to the soil arching effect.

Effect of Natural Fracture Characteristics on Proppant Transport and Placement in an Irregular, Nonplanar Fracture

Summary Hydraulic fracturing is recognized as the primary stimulating technique to enhance recovery in unconventional reservoirs. Connecting natural fractures and filling proppant are crucial elements to achieve the effective development of the reservoirs. Many studies have investigated the interaction characteristics between hydraulic fracture and natural fracture and proppant transport in complex fracture systems. However, few studies show how natural fracture affects proppant migration and placement. In this work, we experimentally explore proppant transport in an irregular, nonplanar fracture containing hydraulic fractures and a natural fracture. The objective is to identify the influence of natural fractures on proppant transport and placement. In this paper, proppant transport in a nonplanar fracture with bends is studied experimentally by a laboratory size slot, and the bending section represents a natural fracture. A high-resolution particle image velocimetry (PIV) system is used to detect the instantaneous velocity field in the complex pathway to understand particle transport behaviors. Proppant placement is quantitatively evaluated by the proppant coverage area as the proppant bed reaches the equilibrium state. Also, the effects of natural fracture width and height, intersection angle with hydraulic fracture, and relative location with the inlet on proppant placement are investigated. The results identify that the natural fracture would significantly increase the complexity of proppant transport and lead to a discontinuous and irregular proppant bed in the complex fracture. The vorticity flow appears around the bend and resuspends deposited proppants through the bending section. The small natural fracture width and height increase flow erosion and are not conducive to proppant settlement, resulting in no proppant. The more the bending angle, the lower the coverage area and bed height. When the natural fracture is close to the inlet, a depleted region without proppants would be formed around the wellbore. The multiple linear regression method developed two reliable models expressed by four normalized parameters to predict the bed coverage areas. The experimental results and laws provide ways to quantitatively analyze natural fracture characteristics' effect on proppant transport and placement in unconventional reservoirs and fissured formations.

Two-Dimensional Flow Field Measurement Method for Sediment-Laden Flow Based on Optical Flow Algorithm

This paper proposes a novel particle image velocimetry (PIV) technique to generate an instantaneous two-dimensional velocity field for sediment-laden fluid based on the optical flow algorithm of ultrasound imaging. In this paper, an ultrasonic PIV (UIV) system is constructed by integrating a medical ultrasound instrument and an ultrasonic particle image velocimetry algorithm. The medical ultrasound instrument with a phased sensor array is used to acquire acoustic echo signals of particles in water and generate two-dimensional underwater ultrasound images. Based on the optical flow field, the instantaneous velocity of the particles in water corresponding to the pixels in the ultrasonic particle images is derived from the grayscale change between adjacent images under the L-K local constraint based on the optical flow field, and finally, the two-dimensional flow field is obtained. Through multiple sets of experiments, the proposed algorithm is verified. The experimental results are compared with those of the conventional cross-correlation algorithms. The results show that the L-K optical flow method can not only obtain the underwater velocity field accurately, but also has the advantages of good smoothness and extensive suitability, especially for the flow field measurement in sediment-laden fluid than conventional algorithms.

Analysis of Swirl Number Effects on Effusion Flow Behavior Using Time-Resolved Particle Image Velocimetry

Abstract The analysis of the interaction between the swirling and cooling flows, promoted by the liner film cooling system, is a fundamental task for the design of turbine combustion chambers since it influences different aspects such as emissions and cooling capability. In particular, high turbulence values, flow instabilities, and tangential velocity components induced by the swirling flow deeply affect the behavior of effusion cooling jets, demanding for dedicated time-resolved near-wall experimental analysis. The experimental setup of this work consists of a non-reactive single sector linear combustor test rig scaled up with respect to engine dimensions; the test section was equipped with an effusion plate with standard inclined cylindrical holes to simulate the liner cooling system. The rig was instrumented with a 2D time-resolved particle image velocimetry system, focused on different field of views. The degree of swirl for a swirling flow is usually characterized by the swirl number, Sn, defined as the ratio of the tangential momentum flux to axial momentum flux. To assess the impact of such parameter on the near-wall effusion behavior, a set of three different axial swirlers with swirl number equal to Sn = 0.6–0.8–1.0 were designed and tested in the experimental apparatus. An analysis of the main flow field by varying the Sn was first performed in terms of average velocity, root mean square, and Tu values, providing kinetic energy spectra and turbulence length scale information. In a second step, the analysis was focused on the near-wall regions: the strong effects of Sn on the coolant jets were quantified in terms of vorticity analysis and jet oscillation.

Dynamic Stall Inception and Evolution Process Measured by High-Frequency Particle Image Velocimetry System in Low Specific Speed Impeller

Abstract Stall in centrifugal pumps is a complicated flow phenomenon, which is detrimental to the pumps' safety and stable operation. Using a high-frequency particle image velocimetry (PIV) system (f = 10k Hz) and a bench-scale refractive index matching experimental setup, two measurement methods are introduced to observe the dynamic stall inception and evolution. In the first method, the flow rate was continuously reduced at an interval of 0.005Qd, and the experiment was carried out under stable flow rate condition. It shows the flow adjacent to the blade suction side gradually evolved from the flow separation into a broken vortex. The stall vortex moved toward the impeller's inlet and continuously grew, and resulted in significant changes in the main flow direction at the channel inlet. The formation and development of the other vortex structures in channel were closely related to the stall vortex at the inlet. The second method is the dynamic flow rate measurement, and the results show that the stall is not caused by the increase in the relative inflow angle. It was obtained that the velocity value in the stall channel near the suction side rapidly decreased; however in the nonstall channel, the velocity value increased at the channel inlet. By analyzing the velocity distribution in both flow channels before and after the stall, the mechanism of alternating stall is well explained. Meanwhile, it was obtained that the stall was more likely to originate from the flow separation near the blade suction side for low specific speed impeller.

Characteristics of forced flow past a square cylinder with steady suction at leading-edge corners

We experimentally investigate the characteristics of a dynamic wake and of flow separation for a square cylinder with steady suction at its leading-edge corners. The wind tunnel experiments were conducted at a Reynolds number of 5946, and suction slots were manufactured symmetrically at the leading corners of the square cylinder. Steady suction was characterized with a suction momentum coefficient Cμ varying from 0.0227 to 0.3182. A time-resolved particle image velocimetry system was used to evaluate the control of leading-edge suction at different Cμ. Next, the measurements were analyzed by applying a proper orthogonal decomposition (POD) to study the control effectiveness. The POD results suggest that the first four modes of wake vortex shedding are transformed in controlled cases and that periodic Karman vortex shedding is suppressed. The results also show that, even with a very small momentum coefficient, the steady suction at the leading-edge corners stabilizes the cylinder wake. The wake region becomes longer and narrower in comparison with the baseline case. In addition, modifications of separation flow were visualized. At quite small Cμ, flow separation at the leading-edge corners is considerably suppressed. Upon increasing the suction momentum coefficient to 0.1364, flow separation at the leading edges is almost eliminated. Finally, we estimate the effect of drag reduction due to the leading-edge suction.

Experimental Investigation and Validation on Suppressing the Unsteady Aerodynamic Force and Flow Structure of Single Box Girder by Trailing Edge Jets

In the present investigation, a wind tunnel experiment was performed to evaluate the effectiveness of the trailing edge jets control scheme to mitigate the unsteady aerodynamic force and flow structure of a single box girder (SBG) model. The flow control scheme uses four isolated circular holes for forming the jet flow to modify the periodic vortex shedding behind the SBG model and then alleviate the fluctuation of the aerodynamic force acting on the test model. The Reynolds number is calculated as 2.08 × 104 based on the incoming velocity and the height of the test model. A digital pressure measurement system was utilized to obtain and record the surface pressure that was distributed around the SBG model. The surface pressure results show that the fluctuating amplitude of the aerodynamic forces was attenuated in the controlled case at a specific range of the non-dimensional jet momentum coefficient. The Strouhal number of the controlled case also deviates from that of the original SBG model. Except for the pressure measurement experiment, a high-resolution digital particle image velocimetry system was applied to investigate the detailed flow structure behind the SBG model to uncover the unsteady vortex motion process from the SBG model with and without the trailing edge jets flow control. As the jet flow blows into the wake, the alternating vortex shedding mode is switched into a symmetrical shedding mode and the width of the wake flow is narrowed. The proper orthogonal decomposition was used to identify the energy of the different modes and obtain its corresponding flow structures. Moreover, the linear stability analysis of the flow field behind the SBG model shows that the scheme of trailing edge jets can dramatically suppress the area of unsteady flow.

Effects of single synthetic jet on turbulent boundary layer

The turbulent boundary layer (TBL) is actively controlled by the synthetic jet generated from a circular hole. According to the datasets of velocity fields acquired by a time-resolved particle image velocimetry (TR-PIV) system, the average drag reduction rate of 6.2% in the downstream direction of the hole is obtained with control. The results of phase averaging show that the synthetic jet generates one vortex pair each period and the consequent vortex evolves into hairpin vortex in the environment with free-stream, while the reverse vortex decays rapidly. From the statistical average, it can be found that a low-speed streak is generated downstream. Induced by the two vortex legs, the fluid under them converges to the middle. The drag reduction effect produced by the synthetic jet is local, and it reaches a maximum value at x + = 400, where the drag reduction rate reaches about 12.2%. After the extraction of coherent structure from the spatial two-point correlation analysis, it can be seen that the synthetic jet suppresses the streamwise scale and wall–normal scale of the large scale coherent structure, and slightly weakens the spanwise motion to achieve the effect of drag reduction.

Wind tunnel study on the wake characteristics of a wind turbine model subjected to surge and sway motions

An experimental study was performed in a uniform incoming flow wind tunnel to investigate the effect of surge and sway motions on the wake characteristics of a wind turbine model. The platform surge and sway motions can be simulated at prescribed amplitudes and frequencies through a translation stage mounted on the base of the turbine model. The flow fields of near, intermediate, and far wakes of the turbine model were characterized by a particle image velocimetry system. Results show that the mean velocity profiles during the whole surging and swaying process of the downwind wakes were almost unchanged with that of a base-fixed turbine; however, the maximum mean velocity differences appeared near the blade tips for both motions compared with the base-fixed cases. Little difference was found between the mean velocity profiles of the turbine model while moving in different directions, for both the surge and sway motions. Later, the characteristics of instantaneous velocity profiles were investigated, and the lateral offset of the instantaneous velocity profiles for the swaying turbine at the two ends of the sway motion was observed. The offset distance was very close to sway amplitudes within the near wake, and it gradually decreased until it disappeared as the downstream distance increases. This work may provide an instrumental guide in the further research on the wake of floating offshore wind turbines.

Two-time interval method to circumvent particle image velocimetry dynamic range

In the velocity range outside the velocity-dynamic range of a PIV (particle image velocimetry) system, the velocity measurements are known to result in noisy velocity vectors due to erroneous detection of correlation peaks, especially in the low-velocity region. Here a simple method is proposed that uses two different timings of a pulsed laser, in conjunction with a median-test of residuals based on the velocity data, to resolve both low and high velocities in a flow field. This method detects the erroneous vectors and replaces them with the correct vectors via a comparison of two sets of PIV data, procured simultaneously with an appropriate time-delay. The validation of the proposed method is demonstrated via a comparison of the present experimental data with previous experiments and DNS (direct numerical simulations) of a round turbulent jet.•The method uses two-time intervals to resolve two sets of velocities such that each set falls within the dynamic range of PIV corresponding to the time interval. (Dynamic range heavily depends on time interval).•A quantity called normalised residual median is defined based on “universal outlier detector” which is used to consolidate the two sets of data.•The strength of the method lies in its ability to obtain instantaneous entrainment velocity even in an unsteady flow.

Quantifying the spatiotemporal evolution of the turbulent horseshoe vortex in front of a vertical cylinder

The spatiotemporal evolution of the turbulent horseshoe vortex (THV) in front of a cylinder vertically mounted on a hydraulically smooth flat-bed was physically modeled in a large water flume. A particle image velocimetry (PIV) system with upward-illumination was, in particular, employed for the junction flow visualization. The examined Reynolds number was varied from 1.28 × 104 to 1.08 × 105, which is above the threshold of turbulent transition for a junction flow. Based on the PIV measurements, the characteristic features were presented for both the time-averaged and the instantaneous flow fields in a sheet flow at the upstream wall-cylinder junction. Statistical analyses on the experimental data are performed to characterize the spatial-temporal evolution of THV strength. The normalized THV strength in the time-averaged flow field increases first and then approaches a constant value with increasing the ratio of cylinder diameter to water depth. Two alternating patterns, i.e., the regular oscillation and the random wandering, are identified for the quasi-periodic oscillating behaviors of the instantaneous THV. It is found that the cumulative distribution curves for the normalized instantaneous THV strength can be described by the Weibul distribution. The present results provide a physical insight and quantitative characterization for the spatiotemporal evolution of THV, which is critical for predicting the associated wall shear stresses.

Modification of subcritical cylinder flow with an upstream rod

In the current research, a small circular rod is placed upstream of the main cylinder. This is used to change the incoming flow and, thus, impact the vortex shedding process of the wake as well as the resultant aerodynamic forces. The experiments are carried out in a wind tunnel with a subcritical Reynolds number of Re = 32 000. The sectional pressure distribution around the main cylindrical model is obtained to calculate unsteady aerodynamic forces. The experimental results show that the gap ratio markedly affects the aerodynamic forces exerting on the main cylinder. The optimal gap ratios for drag reduction are found to be G/D = 1.5 and 2.0, where the average drag and unsteady lift forces exerting on the main cylinder are suppressed. In addition to pressure measurements, we also apply the high-speed particle image velocimetry system to obtain flow characteristics of the cylinder wake as well as the gap flow. Two different modes of gap flow are found, i.e., the cavity mode and the wake impingement mode, and the transition value between them is around G/D = 1.5. Moreover, at the appropriate gap ratio, the wake vortex shedding pattern of the main cylindrical model changes from antisymmetric to symmetric.