Digital Particle Image Velocimetry System Research Articles

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.

Investigation of resist filling profile evolution in microimprint lithography

In micro-/nanoimprint lithography, the quality of the imprinted patterns can be reflected by their final profile shapes. The evolution of resist filling profiles was investigated through numerical simulations and a visualization experiment. A numerical model based on computational fluid dynamics was built to predict the resist filling process. Meanwhile, a 3D defocusing digital particle image velocimetry system was developed to capture the spatial coordinates of the fluorescent tracer particles at different filling times and reconstruct the filling profiles according to the particles' coordinates. The three-dimensional filling profiles of the resist can help understand this microflow phenomenon and describe the resist filling modes. A comparison of filling profiles for a single mold geometry and a single initial thickness revealed good consistency between the model prediction and experiment. The critical range of conversion from single-peak filling mode to double-peak filling mode was determined, which will provide qualitative information for optimizing mold geometries and process parameters.

An experimental study on a suction flow control method to reduce the unsteadiness of the wind loads acting on a circular cylinder

An experimental investigation was conducted to assess the effectiveness of a suction flow control method for vortex-induced vibration (VIV) suppression. The flow control method uses a limited number of isolated suction holes to manipulate the vortex shedding in the wake behind a circular cylinder in order to reduce the unsteadiness of the dynamic wind loads acting on the cylinder. The experimental study was performed at Re ≈ 3.0 × 104, i.e., in the typical Reynolds number range of VIV for the cables of cable-stayed bridges. In addition to measuring the surface pressure distributions to determine the resultant dynamic wind loads acting on the test model, a digital particle image velocimetry system was used to conduct detailed flow field measurements to reveal the changes in the shedding process of the unsteady wake vortex structures from the test model with and without the suction flow control. The effects of important controlling parameters (i.e., the azimuthal locations of the suction holes in respect to the oncoming airflow, the spanwise spacing between the suction holes, and the suction flow rate through the suction holes) on the wake flow characteristics, the surface pressure distributions, and the resultant dynamic wind loads were assessed quantitatively. While a higher suction flow rate and smaller spanwise spacing between the suction holes were beneficial to the effectiveness of the suction flow control, the azimuthal locations of the suction holes were found to be very critical for reducing the fluctuating amplitudes of the dynamic wind loads acting on the test model using the suction flow control method. With the suction holes located at the proper azimuthal locations on the test model (i.e., at the azimuthal angle of θ = 90° and 270° for the present study), the characteristics of the wake flow behind the test model were found to change significantly along the entire span of the test model, even though only a limited number of the isolated suction holes were used for the suction flow control. The detailed flow field measurements were correlated with the measured surface pressure distributions and the resultant dynamic wind loads acting on the test model to gain further insight into the fundamental mechanism of the suction flow control method for VIV suppression.

An experimental study of flow fields and wind loads on gable-roof building models in microburst-like wind

An experimental study was conducted to quantify the flow characteristics of microburst-like wind and to assess the resultant wind loads acting on low-rise, gable-roof buildings induced by violent microburst-like winds compared with those in conventional atmospheric boundary layer winds. The experimental work was conducted by using an impinging-jet-based microburst simulator in the Department of Aerospace Engineering, Iowa State University. Two gable-roof building models with the same base plan and mean roof height, but different roof angle, were mounted over a homogenous flat surface for a comparative study. In addition to measuring the surface pressure distributions to determine the resultant wind loads acting on the building models, a digital particle image velocimetry system was used to conduct flow field measurements to reveal the wake vortex and turbulence flow structures around the building models placed in the microburst-like wind. The effects of important parameters, such as the distance of the building from the center of the microburst, the roof angle of the building, and the orientation of the building with respect to radial outflow of the oncoming microburst-like wind, on the flow features such as the vortex structures and the surface pressure distributions around the building models as well as the resultant wind loads acting on the test models were assessed quantitatively. The measurement results reveal clearly that when the building models were mounted within the core region of the microburst-like wind, the surface pressure distributions on the building models were significantly higher than those predicted by ASCE 7-05 standard, thereby induced considerably greater downward aerodynamic forces acting on the building models. When the building models were mounted in the outflow region of the microburst-like wind, the measured pressure distributions around the building models were found to reach a good correlation with ASCE 7-05 standard gradually as the test models were moved far away from the center of the microburst-like wind. It was also found that both the radial and vertical components of the aerodynamic forces acting on the building models would reach their maximum values when the models were mounted approximately one jet diameter away from the center of the microburst-like wind, while the maximum pressure fluctuations on the test models were found to occur at further downstream locations. Roof angles of the building models were found to play an important role in determining the flow features around the building models and resultant wind loads acting on the test models. The flow field measurements were found to correlate with the measured surface pressure distributions and the resultant wind loads (i.e., aerodynamic forces) acting on the building models well to elucidate the underlying physics of flow-structure interactions between the microburst-like winds and the gable-roof buildings in order to provide more accurate prediction of the damage potentials of the microburst wind.

Unsteady aerodynamics and flow-induced vibrations of a low aspect ratio rectangular membrane wing with excess length

In this study, an experimental investigation of a low aspect ratio rectangular membrane wing with excess length was presented at angles of attack ranging from 0° to 25° at Reynolds number of 48,700. Time-accurate measurements of membrane deformation using Digital Image Correlation system, and velocity field measurements with Digital Particle Image Velocimetry system were carried out while normal forces of the wing were measured by helping a load-cell system. The experimental results showed that the camber of membrane wing with excess length excited the separated shear layer and this situation increased the lift coefficient relatively more as compared to membrane wings without excess length. At the angles of attack of α=6–10° and α=19–22°, dominant vibrational modes were obtained as a spanwise mode of two, whereas dominant spanwise modes were three at α=11–18° and α=23–25°. Moreover, the membrane wings with excess length exhibited small separated regions since the shear layer was closer to the membrane wing surface although camber of the wing increased. Consequently, it was shown that in membrane wings with low aspect ratio, unsteadiness included tip vortices and vortex shedding, and the combination of tip vortices and vortex shedding caused complex unsteady deformations of these membrane wings.

A PIV dynamic velocity range enhancement approach using ROI option of imaging sensors

The ability to interact with the readout mechanism of contemporary image sensors offers new possibilities that can be utilized to improve the performance of digital particle image velocimetry (DPIV) systems. For instance, an intelligent selective region-of-interest (ROI) option can increase the frame rate of a camera, which would allow flow assessment of higher speeds thus ameliorate the dynamic velocity range (DVR) of a DPIV system. However, ROI increases the frame rate by narrowing down the field-of-view (FOV) of the PIV system, which prevents embracing the complete field of flow. To overcome this constraint and maintain the full FOV, we propose a technique, where a fixed height ROI strip is slid over a sensor array and partial flow fields corresponding to each ROI position are obtained. Then, the overall flow field is reconstructed by stitching these partial flow fields. We demonstrate an application of the proposed method by a series of experiments with laminar flows inside a rectangular microchannel. Specifically, we show that by applying the proposed technique the maximum assessable flow velocity by an ordinary DPIV system can be increased at least 4 times without any extra hardware investment.

Unsteady characteristics of inlet vortices

An experimental study of the unsteady characteristics of inlet vortices has been conducted using a high-frame rate digital particle image velocimetry system. The results revealed the formation of a pair of counter-rotating inlet vortices for the no-wind configuration and one single inlet vortex when there was crosswind. In all measurement planes, from near the ground to the inlet, evidence of vortex meandering with quasi-periodicity was found. The vortex meander is dominant in the direction of the crosswind, and its amplitude increases with crosswind velocity. The proper orthogonal decomposition analysis of the instantaneous velocity field suggested that the most energetic mode was a helical displacement wave, corresponding to the first helical mode. Similarities with the meandering of the trailing vortices from wings were noted. The present results also suggest that the unsteady characteristics of the focus of separation formed on the ground might be responsible for the unsteady nature of the inlet vortex.

Fish and robots swimming together: attraction towards the robot demands biomimetic locomotion

The integration of biomimetic robots in a fish school may enable a better understanding of collective behaviour, offering a new experimental method to test group feedback in response to behavioural modulations of its 'engineered' member. Here, we analyse a robotic fish and individual golden shiners (Notemigonus crysoleucas) swimming together in a water tunnel at different flow velocities. We determine the positional preference of fish with respect to the robot, and we study the flow structure using a digital particle image velocimetry system. We find that biomimetic locomotion is a determinant of fish preference as fish are more attracted towards the robot when its tail is beating rather than when it is statically immersed in the water as a 'dummy'. At specific conditions, the fish hold station behind the robot, which may be due to the hydrodynamic advantage obtained by swimming in the robot's wake. This work makes a compelling case for the need of biomimetic locomotion in promoting robot-animal interactions and it strengthens the hypothesis that biomimetic robots can be used to study and modulate collective animal behaviour.

Numerical Simulation and Experimental Study on Resist Filling Behavior in UV-nanoimprint Lithography

The resist filling behavior is crucial to the quality of the final imprinted pattern in NIL. A numerical model was built to predict the resist filling process. The effect of the duty ratio of recessed feature on the cross-sectional profile of imprinted resist and the filling mode of resist from double-peak to single-peak mode were analyzed. A three-dimensional defocusing digital particle image velocimetry system was set up to verify the correctness of simulation results. The research results will help to understand the resist filling mechanism and provide useful instruction for selection of process parameters and mold structure optimization.

A simple, inexpensive system for digital particle image velocimetry (DPIV) in biomechanics

Functional morphology and biomechanics seek to reveal the mechanistic bases of organismal functions and the physical principles involved at the phenotype-environment interface. Characterization of fluid flow (air or water) within and around organismal structures is an example of this approach. Digital particle imaging velocimetry (DPIV) has been exploited in a variety of biological systems to visualize fluid flow associated with animal movement. DPIV employs particles suspended in air or water that are illuminated by a laser light sheet and recorded with a high-speed video camera. Software tracks particle movement across a specified number of video frames, generating vector diagrams showing patterns of fluid flow through time. As powerful as DPIV methods are, they are limited in application by the high cost and complexity of the equipment required. In this article, we describe a simple DPIV system that substitutes widely available, inexpensive consumer components for scientific-grade equipment to achieve low cost (<$1,000 total) and high accuracy (total error calculated to be approx. 6%, as compared with 5% in professional systems). We have employed this system successfully in our studies on the fluid dynamics of chemosensory tongue-flicking in snakes. This system can be used for research and teaching in labs that typically cannot afford the expense or commitment of a traditional DPIV apparatus and is particularly suited for obtaining preliminary data required to justify further grant and institutional support.

Modulation of leading edge vorticity and aerodynamic forces in flexible flapping wings

In diverse biological flight systems, the leading edge vortex has been implicated as a flow feature of key importance in the generation of flight forces. Unlike fixed wings, flapping wings can translate at higher angles of attack without stalling because their leading edge vorticity is more stable than the corresponding fixed wing case. Hence, the leading edge vorticity has often been suggested as the primary determinant of the high forces generated by flapping wings. To test this hypothesis, it is necessary to modulate the size and strength of the leading edge vorticity independently of the gross kinematics while simultaneously monitoring the forces generated by the wing. In a recent study, we observed that forces generated by wings with flexible trailing margins showed a direct dependence on the flexural stiffness of the wing. Based on that study, we hypothesized that trailing edge flexion directly influences leading edge vorticity, and thereby the magnitude of aerodynamic forces on the flexible flapping wings. To test this hypothesis, we visualized the flows on wings of varying flexural stiffness using a custom 2D digital particle image velocimetry system, while simultaneously monitoring the magnitude of the aerodynamic forces. Our data show that as flexion decreases, the magnitude of the leading edge vorticity increases and enhances aerodynamic forces, thus confirming that the leading edge vortex is indeed a key feature for aerodynamic force generation in flapping flight. The data shown here thus support the hypothesis that camber influences instantaneous aerodynamic forces through modulation of the leading edge vorticity.

Application of Wavelet Analysis in the DPIV Information Processing for the River Model Test

It’s important to measure the horizontal distribution of the 2D velocity field for model verification and scheme selection at model test in the tidal estuary. Digital particle image velocimetry (DPIV) has become an accepted technique for the measurement of two- and three component planar velocities in a wide variety of fluid flows, which has advantages of high speed and nil interference. However, sometimes DPIV images are overexposure, uneven distribution of the gray values, and obscurity on the edges of the particles, which will affect the accuracy of flow data by inversion. Wavelet transform has good characteristics in time-frequency localization, and it has been widely used for image preprocessing, image compression, image restoration, image feature extraction and pattern recognition. A wavelet-based multi-resolution technique was applied to analyze the 2D measurement results of a DPIV system in this paper. In the different stages: boundary treatment, noise reduction, identification and elimination of the error vector, which are applicable to the processing of the DPIV information for getting the velocity vectors with higher resolution and precision through the continuous wavelet transform of velocity data.

Characterization of the wind loads and flow fields around a gable-roof building model in tornado-like winds

An experimental study was conducted to quantify the characteristics of a tornado-like vortex and to reveal the dynamics of the flow-structure interactions between a low-rise, gable-roof building model and swirling, turbulent tornado-like winds. The experimental work was conducted by using a large-scale tornado simulator located in the Aerospace Engineering Department of Iowa State University. In addition to measuring the pressure distributions and resultant wind loads acting on the building model, a digital Particle Image Velocimetry system was used to conduct detailed flow field measurements to quantify the evolution of the unsteady vortices and turbulent flow structures around the gable-roof building model in tornado-like winds. The effects of important parameters, such as the distance between the centers of the tornado-like vortex and the test model and the orientation angles of the building model related to the tornado-like vortex, on the evolutions of the wake vortices and turbulent flow structures around the gable-roof building model as well as the wind loads induced by the tornado-like vortex were assessed quantitatively. The detailed flow field measurements were correlated with the surface pressure and wind load measurements to elucidate the underlying physics to gain further insight into flow-structure interactions between the gable-roof building model and tornado-like winds in order to provide more accurate prediction of wind damage potential to built structures.

Validation of CFD and Thermal Hydraulics Codes by Digital Particle Image Velocimetry

Abstract Experimental validation of thermal hydraulics and CFD codes of scaled down models and full models of Nuclear Reactor components is very important for reactor system designers and reactor safety experts for optimal design and providing best safety features of the reactor systems. Over the period of time several techniques have been developed for this purpose. Among them Digital Part icle Image Velocimetry (DPIV), a laser based optical technique, is the recent, innovative, promising and flexible enough one so that it can be adapted to a large variety flows. By this technique test time is reduced significantly. DPIV is a non-intrusive optical measurement technique, which allows capturing several thousand velocity vectors within large flow fields instantaneously. DPIV technique is being used in very wide and different applications, from micro flows over combustion to supersonic flows for both industrial needs and research. 2-D DPIV facility having non-intrusive, instantaneous flow visualization with high spatial and temporal resolution features has been developed. It is used for measurement of flow velocity vectors at many (e.g. thousands) points in a flow field simultaneously and it provides instantaneous vector fields and displays velocity vectors in rea l-t ime. DPIV combines flow visualization with accurate quantitative velocity data. 2-D Velocity vectors are obtained by analysis of displacement of tracer particles on images in a known time. The development of visualizat ion tools are powerful enough to at tack the complexity of a resulting flow field but also flexible enough so that they can be adapted to a large variety of different flow. DPIV system has Laser light source, Laser beam delivery system, High speed camera interface, DPIV image acquisition & analysis software, CCD camera, computer controlled synchronizer, seeding particles & PC. This system will enhance our capability of thermal hydraulics, neutronics and mult iphysics modelling by experimental support & actual measurement of parameters, in addition to codes we had with us. Application of DPIV is must for the flow in the separation and reattachment regions where it is directly related to drag and heat transfer coefficients and for Vortex shedding. This facility is being used for basic research of code verification, code development, design optimizations and safety analysis.

Time-resolved vortex wake of a common swift flying over a range of flight speeds

The wake of a freely flying common swift (Apus apus L.) is examined in a wind tunnel at three different flight speeds, 5.7, 7.7 and 9.9 m s(-1). The wake of the bird is visualized using high-speed stereo digital particle image velocimetry (DPIV). Wake images are recorded in the transverse plane, perpendicular to the airflow. The wake of a swift has been studied previously using DPIV and recording wake images in the longitudinal plane, parallel to the airflow. The high-speed DPIV system allows for time-resolved wake sampling and the result shows features that were not discovered in the previous study, but there was approximately a 40 per cent vertical force deficit. As the earlier study also revealed, a pair of wingtip vortices are trailing behind the wingtips, but in addition, a pair of tail vortices and a pair of 'wing root vortices' are found that appear to originate from the wing/body junction. The existence of wing root vortices suggests that the two wings are not acting as a single wing, but are to some extent aerodynamically detached from each other. It is proposed that this is due to the body disrupting the lift distribution over the wing by generating less lift than the wings.

An experimental study of a bio-inspired corrugated airfoil for micro air vehicle applications

An experimental study was conducted to investigate the aerodynamic characteristics of a bio-inspired corrugated airfoil compared with a smooth-surfaced airfoil and a flat plate at the chord Reynolds number of ReC = 58,000–125,000 to explore the potential applications of such bio-inspired corrugated airfoils for micro air vehicle designs. In addition to measuring the aerodynamic lift and drag forces acting on the tested airfoils, a digital particle image velocimetry system was used to conduct detailed flowfield measurements to quantify the transient behavior of vortex and turbulent flow structures around the airfoils. The measurement result revealed clearly that the corrugated airfoil has better performance over the smooth-surfaced airfoil and the flat plate in providing higher lift and preventing large-scale flow separation and airfoil stall at low Reynolds numbers (ReC < 100,000). While aerodynamic performance of the smooth-surfaced airfoil and the flat plate would vary considerably with the changing of the chord Reynolds numbers, the aerodynamic performance of the corrugated airfoil was found to be almost insensitive to the Reynolds numbers. The detailed flow field measurements were correlated with the aerodynamic force measurement data to elucidate underlying physics to improve our understanding about how and why the corrugation feature found in dragonfly wings holds aerodynamic advantages for low Reynolds number flight applications.

Flow-Field Measurement of Embedded Streamwise Vortex Generated from Passive Flow-Control Devices

Detailed flow-field measurements were performed downstream of a single vortex generator (VG) using a Stereo Digital Particle Image Velocimetry system. The passive flow-control devices examined consisted of a low-profile VG with a device height, h, approximately equal to 20 percent of the boundary-layer thickness, Δ, and a conventional VG with h ≈ Δ. Flow-field data were taken at twelve cross-flow planes downstream of the VG to document and quantify the evolution of embedded streamwise vortex. Key parameters including vorticity, circulation, trajectory, and half-life radius—describing concentration, strength, path, and size, respectively—of the device-induced streamwise vortex were extracted from the flow-field data. Peak vorticity and circulation for the low-profile VG decayed exponentially to the distance downstream from the device. The device-height normalized vortex trajectories for the low-profile VG, especially in the lateral direction, followed the general trends of the conventional VG.

Experimental study of a pickup truck near wake

Abstract The present study focuses on the aerodynamics of pickup trucks. The flow about a generic pickup truck has been documented experimentally. The main objective is to gain a better understanding of the pickup truck aerodynamics through mean and unsteady pressure measurements as well as detailed flow field measurements using particle image velocimetry (PIV). Of particular interest and complexity are the symmetric and asymmetric separated vortex flows which develop behind the cab and the tailgate. A secondary objective is to obtain a comprehensive experimental data set for validation of computational fluid dynamics (CFD) models. The experiments were conducted at Reynolds numbers in the range of 5–8.5×105 in an open return wind tunnel equipped with two-frame digital PIV system. The mean pressure results of the pickup truck show that the pressure outside the tailgate is higher than inside the tailgate suggesting that the tailgate reduces aerodynamic drag. The root-mean-square (rms) values of pressure fluctuations at the cab back is very low and increases significantly towards the back of the bed and the tailgate top edge. Pressure fluctuation spectra indicate a spectral peak at a Strouhal number of ∼0.07. Velocity field measurements of the near wake flow show a relatively quiet region behind the cab and no recirculating flow region in the symmetry plane behind the tailgate. Instead there is a strong downwash at the symmetry plane behind the tailgate. This downwash is due to the existence of a pair of counter-rotating vortices downstream of the tailgate. The downwash promotes attached flow behind the tailgate, thus resulting in a pressure recovery that reduces drag.

A color-coded backlighted defocusing digital particle image velocimetry system

Defocusing digital particle image velocimetry (DDPIV), as a true three-dimensional (3D) measurement technique, allows for the measurement of 3D velocities within a volume. Initially designed using a single CCD and 3-pinhole mask (Willert and Gharib in Exp Fluids 12:353–358, 1992), it has evolved into a multi-camera system in order to overcome the limitations of image saturation due to multiple exposures of each particle. In order to still use a single camera and overcome this limitation, we have modified the original single CCD implementation by placing different color filters over each pinhole, thus color-coding each pinhole exposure, and using a 3-CCD color camera for image acquisition. Due to the pinhole mask, there exists the problem of a significant lack of illumination in a conventional lighting setup, which we have solved by backlighting the field-of-view and seeding the flow with black particles. This produces images with a white background superimposed with colored triple exposures of each particle. A color space linear transformation is used to allow for accurate identification of each pinhole exposure when the color filters’ spectrum does not match those of the 3-CCD color camera. Because the imaging is performed with a multi-element lens instead of a single-element lens, an effective pinhole separation, de, is defined when using a pinhole mask within a multi-element lens. Calibration results of the system with and without fluid are performed and compared, and a correction of the effective pinhole separation, de, due to refraction through multiple surfaces is proposed. Uncertainty analyses are also performed, and the technique is successfully applied to a buoyancy-driven flow, where a 3D velocity field is extracted.

Development of Squeeze Flow in Mechanical Heart Valve: A Particle Image Velocimetry Investigation

Fluid between the reducing flow channel of the valve occluder and the orifice wall tends to be squeezed out of the flow channel, causing a high-speed flow. The squeeze flow is accompanied by a sharp local pressure drop, which may result in potential cavitation phenomenon in a mechanical heart valve (MHV). Limited experimental investigation has been conducted into the flow physics of this squeeze flow phenomenon, which is likely to be the origin of MHV cavitation. We used a pulsatile test loop simulating physiologic flow conditions and an actual-size transparent MHV model for flow visualization. A digital particle image velocimetry (DPIV) system incorporated with a microscope was applied to observe flow within a narrowing channel. A triggering mechanism was designed so that the DPIV system could be timed to capture images when the valve occluder was near its closing position. A series of images within the channel from 1.4 to 0.1 mm were captured. As the gap between the tip of the valve occluder and orifice wall becomes narrower, evidence of high-speed jet flow becomes more apparent. When the flow channel is reduced to around 0.1 mm, flow velocity of up to 2 m/s was noted. A sudden increase in high-speed jet flow causes a corresponding reduction in local pressure, and is a likely source for potential cavitation.