Digital Particle Image Velocimetry Research Articles

Base friction testing methodology for the deformation of rock masses caused by mining in an open-pit slope

Mining in an open-pit slope results in the deformation and instability of rock masses. Correspondingly, it is difficult to quantify the range of internal rock mass damage completely. In this study, a base friction testing method is proposed for the analysis of mining models in an open-pit slope, and new methodology is proposed which combines digital image correlation (DIC) and particle image velocimetry (PIV) techniques to analyze the entire deformation process. A series of base friction tests is performed to reveal the deformation mechanism of the mining model in an open-pit slope, and findings are compared with those obtained from a mining model without an open-pit slope. The test results showed two different types of damage: staged parabolic-like and progressive trapezoidal-like damages. Moreover, the proposed methodology can obtain the entire displacement and velocity vector distributions (from deformation to damage) inside the slope and convert it into data for visualization. This provides a reference for the evaluation and control of field open-pit slopes in similar conditions.

On the Turbulent Drag Reduction Effect of the Dynamic Free-Slip Surface Method

The turbulent boundary layer (TBL) over the hull surface of a water vehicle significantly elevates the drag force on the water vehicle. In this regard, effectively controlling the TBL can lead to a drag reduction (DR) effect and therefore improve the energy efficiency of water transportation. Many DR methods have demonstrated promising DR effects but face challenges in implementation at the scale of engineering application. In this regard, the recently developed dynamic free-slip surface method can resolve some of the critical challenges. It employs an array of freely oscillating air–water interfaces to manipulate the TBL and can achieve a substantial DR effect under certain control conditions. However, the optimal setting of the control parameters that would maximize the DR effect remains unclear. To answer these questions, this study systematically investigates the effects of multiple control parameters for the first time, including the geometric size and curvature of the interface, the frequency of active oscillation, and the Reynolds number of TBL. Digital Particle Image Velocimetry was used to non-invasively measure the velocity and vorticity field of the TBL, and the Charted Clauser method was used to calculate the DR effect. The presented results suggest that the oscillating free-slip interfaces reduce the flow velocity near the wall boundary and lift the transverse vorticity (and the viscous shear stress) away from the wall. In addition, the shape factor of the TBL is elevated by the oscillating interfaces and slowly relaxes back in the downstream regions, which implies a partial relaminarization process induced in the TBL. Up to 36% DR effect was achieved within the current scope range of the control parameters. All of the results consistently suggest that a large DR effect is achieved when the free-slip interfaces oscillate with large Weber numbers. These discoveries shed light on the underlying DR mechanism and provide guidance for the future development of an effective drag control technique based on the dynamic free-slip surface method.

Droplet breakup in airflow with strong shear effect

The deformation and breakup of droplets in airflows is important in spray and atomisation processes, but the shear effect in non-uniform airflow is rarely reported. In this study, the deformation and breakup of droplets in a shear flow of air is investigated experimentally using high-speed imaging, digital image processing and particle image velocimetry. The results show that in airflow with a strong shear effect, the droplet breakup exhibits unique features due to the uplift and stretching produced by the interaction between the deformed droplet and the shear layer. The breakup process can be divided into three stages according to the droplet morphology and the breakup mechanism, namely the sheet breakup, the swing breakup and the rim breakup stages. Theoretical analysis reveals that the swing breakup is governed by the transverse Rayleigh–Taylor instability. A regime map of the droplet breakup is produced, and the transitions between different regimes are obtained theoretically. The stretching liquid film during the droplet deformation and the fragment size distribution after droplet breakup are analysed quantitatively, and the results show that they are determined by the competition of breakup at different stages affected by the shear. Finally, the effect of the droplet viscosity is investigated, and the viscosity inhibits the droplet breakup in a strong shear airflow.

Wake of a Coaxial Corotating Rotor in Hover

A recent paper by Tinney and Valdez (AIAA Journal, Vol. 58, No. 4, 2020, pp. 1657–1667) reports the trade space between near-field acoustics and thrust produced by a laboratory-scale, coaxial, corotating rotor in hover by considering the effect of various index angles and stacking distances between the upper and lower rotor for a range of rotor speeds. The current effort is a follow-up campaign employing high-speed digital schlieren and single camera particle image velocimetry to measure the flowfield produced by the same stacked rotor. Particle image velocimetry measurements quantify the evolutionary behavior of the upper and lower blade tip vortices, and their interactions for various index angles and wake ages for a given rotor speed. Vortex tracking methods quantify the anisotropic wandering motions of the blade tip vortices and reveal patterns that are unique to both the upper and lower rotors. The findings reinforce the notion that the optimal index angle for the thrust produced by a coaxial, corotating rotor is one where the blade tip vortices produced by the upper rotor graze the low-pressure side of the lower rotor. If the miss distance is too small, collisions between the upper rotor vortex and the lower rotor ensue so that any gains in thrust are compounded by blade-vortex-interaction noise penalties.

Knifefish turning control and hydrodynamics during forward swimming.

Rapid turning and swimming contribute to ecologically important behaviors in fishes such as predator avoidance, prey capture, mating and the navigation of complex environments. For riverine species, such as knifefishes, turning behaviors may also be important for navigating locomotive perturbations caused by turbulent flows. Most research on fish maneuvering focuses on fish with traditional fin and body morphologies, which primarily use body bending and the pectoral fins during turning. However, it is uncertain how fishes with uncommon morphologies are able to achieve sudden and controllable turns. Here, we studied the turning performance and the turning hydrodynamics of the black ghost knifefish (Apteronotus albifrons, N=6) which has an atypical elongated ribbon fin. Fish were filmed while swimming forward at ∼2 body lengthss-1 and feeding from a fixed feeder (control) and an oscillating feeder (75 Hz) at two different amplitudes. 3D kinematic analysis of the body revealed the highest pitch angles and lowest body bending coefficients during steady swimming. Low pitch angle, high maximum yaw angles and large body bending coefficients were characteristic of small and large turns. Asynchrony in pectoral fin use was low during turning; however, ribbon fin wavelength, frequency and wave speed were greatest during large turns. Digital particle image velocimetry (DPIV) showed larger counter-rotating vortex pairs produced during turning by the ribbon fin in comparison to vortices rotating in the same direction during steady swimming. Our results highlight the ribbon fin's role in controlled rapid turning through modulation of wavelength, frequency and wave speed.

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.

Centrifuge model studies on desiccation cracking behaviour of fiber-reinforced expansive clay

In this study, randomly distributed fiber reinforcement on the desiccation cracking behavior of expansive clay was investigated. Modeling considerations for the desiccation cracking behavior of unreinforced and fiber-reinforced clay was established using dimensional analysis. A custom-designed setup consisting of a specimen container, heating assembly, and digital image acquisition system was designed, calibrated, and used in the present study. A series of desiccation cracking tests were performed on unreinforced and fiber-reinforced expansive clay in a balanced beam geotechnical centrifuge. The test setup performance was evaluated by conducting tests at varying gravity levels and performing modeling of models. Digital image analysis and particle image velocimetry techniques were used to obtain qualitative information about the cracking of clay under the influence of fiber reinforcement and the varying thickness of the clay layers. The specimens reinforced with fibers exhibited improved cracking resistance than unreinforced clay specimens. The results indicate that the desiccation cracking of clays can be successfully modeled in a geotechnical centrifuge, highlighting the fact that this study is the first-ever such study. This knowledge can be used to study the behavior of critical geotechnical structures, especially clay barriers of landfill cover subjected to desiccation cracking.

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.

CFD modelling of a powder spraying nozzle used for dry coating

The aim of these studies was to examine the suitability of the Euler–Lagrange approach for modelling the powder spraying process performed with a pneumatic nozzle used for dry coating in spout-fluid bed apparatus. Simulation results were validated using the digital PIV (Particle Image Velocimetry) method and measurement equipment constructed by the authors. The validation process led to the determination of the optimal Ansys Fluent solver settings, which ensure a satisfactory accuracy of the simulation results (the average relative error of all simulation points at the level of 10.5%). The experimental results are most accurately reflected by simulation results obtained with the use of the standard k − ε model that has the discrete random walk (DRW) model activated. The detailed analysis of the operation of the spout-fluid bed apparatus allowed for the examination of the influence of hydrodynamic parameters (the mass flow rate of the gas spraying the powder and plasticizer, as well as spouting gas) on the velocity and concentration of the dispersed phase in the device. The scientific investigation covered the analysis of the velocity profiles and the volume fraction of the coating powder in different zones of the column.

The Influence of Valve Leaflet Stiffness Variability on Aortic Wall Shear Stress.

Aortic stenosis is a common cardiac condition that impacts the aorta's hemodynamics downstream of the affected valve. We sought to better understand how non-uniform stiffening of a stenotic aortic valve would affect the wall shear stress (WSS) experienced by the walls of the aorta and the residence time near the valve. Several experimental configurations were created by individually stiffening leaflets of a polymer aortic valve. These configurations were mounted inside an in vitro experimental setup. Digital particle image velocimetry (DPIV) was used to measure velocity profiles inside a model aorta. The DPIV results were used to estimate the WSS and residence time. Our analysis suggests that leaflet asymmetry greatly affects the amount of WSS by vectoring the systolic jet and stiffened leaflets have an increased residence time. This study indicates that valve leaflets with different stiffness conditions can have a more significant impact on wall shear stress than stenosis caused by the uniform increase in all three leaflets (and the subsequent increased systolic velocity) alone. This finding is promising for creating customizable (patient-specific) prosthetic heart valves tailored to individual patients.

A biomimetic fish finlet with a liquid metal soft sensor for proprioception and underwater sensing

Finlets have a unique overhanging structure at the back, similar to a flag. They are located between the dorsal/anal fin and the caudal fin on the sides of the body. Until now, the sensing ability of finlets has not been well understood. In this paper, we design and manufacture a biomimetic soft robotic finlet (48.5 mm long, 30 mm high) with mechanosensation based on printed stretchable liquid metal sensors. The robotic finlet’s posterior fin ray can achieve side-to-side movement orthogonal to the anterior fin ray. A flow sensor encapsulating a liquid metal sensor network enables the biomimetic finlets to sense the direction and flow intensity. The stretchable liquid metal sensors mounted on micro-actuators are utilized to perceive the swing motion of the fin ray. We found that the finlet prototype can sense the flapping amplitudes and frequency of the fin ray. The membrane between the two orthogonal fin rays can amplify the sensor output. Our results indicate that the overhanging structure endows the biomimetic finlet with the ability to sense external stimuli from stream-wise, lateral and vertical directions. We further demonstrate, through digital particle image velocimetry experiments, that the finlet can detect a Kármán vortex street. This study lays the foundations for exploring the environmental perception of biological fish fins and provides a new approach for the perception of complex flow environments by future underwater robots.

Numerical and experimental analysis of poly-dispersion effects on particle-laden jets

This study presents experimental measurements and computational modelling of particle-laden jets for a wide range of Stokes numbers to analyse the effect of polydispersity on particle volume fraction and velocity. The polydisperse two-phase turbulent jet issues from a pipe into a low velocity coflow, resulting in inlet Stokes numbers, Stin, ranging from 0.003 to 25. The particle velocity and volume fraction are experimentally measured using digital particle image velocimetry and planar nephelometry simultaneously. The simulations are conducted using a newly developed model based on the probability density function of the population balance equation (PDF-PBE) in a large eddy simulation framework. A stochastic Monte Carlo method is adopted to solve the PDF-PBE on an ensemble of notional Lagrangian particles, while the method of Stokes binning is employed to explicitly treat inertial effects in a computationally efficient way. There is a satisfactory agreement between the measurements and simulations. A series of monodisperse simulations were also conducted to compare with the polydisperse flows in a two-way coupled flow. The results confirm that in the monodisperse jet, small particles (with Stin≤0.3) closely follow the carrier phase velocity whereas in the polydisperse jet they are affected by the presence of larger particles to reduce their axial velocity decay rate. The opposite trend is observed for large particles (Stin≥5.5), confirming that in the polydisperse jet, the presence of small particles increases the large particles’ radial dispersion that lowers their volume fraction along the centreline compared to the monodisperse jet.

Experimental study of underwater pulse detonation gas jets: Bubble velocity field and time–frequency characteristics of pressure field

Underwater pulse detonation gas jets generated by a detonation tube are experimentally investigated in this study utilizing detonations in explosive gas mixtures to generate pulsating bubbles under water. Three stoichiometric gaseous fuels (methane, hydrogen, and acetylene) are detonated with oxygen under the same filling conditions. Digital particle image velocimetry and wavelet transform techniques are introduced to analyze bubble dynamics and pressure field characteristics by means of which the velocity field of the bubble interface and the time–frequency distributions of the pressure response under water are elucidated, respectively. Motions of the bubble interface, which can now be clearly seen with the oscillations, are indicated in high-speed photographic images. Three main frequency components and their duration are identified: reverberations of water tank, pulsations of the detonation gas bubble, and fluctuations of free water surface. Experimental results show that the reverberation concentrating in high frequencies is due to the detonation wave (DW) and reflected shock waves in the water tank; the pulsations are related to the bubble oscillating periods, which are stronger as the detonation pressure increases; and fluctuation occurs in both the bubble oscillating and floating stages. To explain the directional growth of the detonation gas bubble, an experiment of pulse detonation gas jet in air is conducted where the sudden release of detonation products behind the DW and the subsequent impulsive detonation gas jet are qualitatively presented. Results presented in this paper give in-depth analysis of pulse detonation gas jets and provide a new way to generate pulsating bubbles under water.

Experimental Study of Unsteady Heat Transfer and Phase Changing Process Upon Impacting of Ice Crystals onto Heated Surfaces Pertinent to Aero-Engine Icing Phenomena

Ice accretion on exposed surfaces of aero-engine components has been widely recognized as a significant hazard to aviation safety in cold weathers. Icing process due to the impingement of the supercooled water droplets suspended in the cloud onto the cold surfaces of inlet components of aeroengines have been studied extensively for decades. Since ice particles were believed to simply bounce off from the exposed surfaces of aero-engine components, ice crystals in the clouds were initially considered not to pose a threat to aviation safety. Therefore, the ice accretion process due to the impacting of ice crystals onto the surfaces of hot engine componentes has not been studied until recently. It has been found recently that, tiny ice particles in the cloud may be partial/full melting upon impacting onto the hot surfaces of aero-engine components, such as heated Inlet Guide Vanes (IGV) and various probes. The partially/fully melted ice crystals were found to stick onto the hot surfaces and form thin water film, which would intercept more oncoming ice particles and lead to significant ice accretion over the surfaces of the hot engine components. The ice crystal induced ice accumulation on the critical aero-engine components has been found to cause significant engine performance loss and erroneous data being read from the probes. In the present study, a series of experimental investigations were conducted to elucidate the underlying physics of the dynamic ice accretion process pertinent to ice crystal icing phenomena. A novel ice crystal icing test rig with the capacity of generating controllable amount of ice crystals and flying speed up to 100 m/s was developed in a temperature-controllable environment chamber for the ice crystal icing studies. By using a high-speed imaging system, a digital particle image velocimetry(PIV), and an Infrared (IR) thermal imaging system, a comprehensive experimental campaign was performed to characterize the transient impacting process of ice crystals, dynamic ice accretion and unsteady heat transfer process associated with the impacting of ice crystals onto heated surfaces, in comparison to those due to the impingement of supercooled water droplets. By using an ultra-sensitive force sensor and a high-speed image system, a comparative study is conducted to examine the differences in the transient impinging dynamics of single water droplets, supercooled water droplets, and ice crystals onto solid surfaces with different wettability and stiffness. By upgrading the unique Icing Research Tunnel of Iowa State University (i.e., ISU-IRT) with additional ice crystal icing capability, a set of explorative studies are also conducted to examine the characteristics of the dynamic ice accretion processes over the heated surfaces of an aero-engine Inlet Guide Vane (IGV) model under both ice crystal icing and supercooled droplet icing conditions. The anti-/de-icing performance of a novel hybrid strategy by integrating icephobic coatings and minimized surface heating are also evaluated under both supercooled water droplet icing and ice crystal icing conditions. The new findings derived from the present studies are very helpful to gain further insights into the ice crystal icing phenomena for the development of more effective and robust anti-/de-icing strategies to ensure safer and more efficient aircraft/aero-engine operations in cold weathers.

Examining the Effects of Fluid Velocity Gradients on 4D Digital Holographic PIV/PTV Measurements

4D digital holographic PIV/PTV (4D-DHPIV/PTV) methods have demonstrated theoretical viability due to their relative ease of setup and high spatial resolution (Soria (2018)). This study investigates how velocity gradients related to different flow regimes and their magnitudes affect 3-component–3-dimensional (3C-3D) digital holographic PIV measurement uncertainty. The error introduced by velocity gradients within the interrogation volume is studied by simulating particles in a velocity field, with a given constant velocity gradient superimposed on a uniform flow from which a time-series of hologram pairs are generated and the 3C-3D velocity fields and their errors are determined using 4D-DHPIV/PTV Sun et al. (2020). Hologram pairs are simulated by modelling the propagation and particlediffraction of coherent laser light using the angular spectrum method (Goodman (1996)). The hologram reconstruction then involves direct reconstruction, followed by deconvolution, a particle position refinement and a hologram subtraction step (Sun et al. (2020)). The particle positions obtained from 4D-DHPIV/PTV are then used to resolve particle displacement measurements using 3D cross-correlation digital analysis with a 3D Gaussian fit to sub-pixel resolution (Soria (2006)). The effects of velocity gradients on the displacement uncertainty and bias error have been investigated by undertaking Monte Carlo simulations under a range of velocity gradient environments. Specifically, 5 common velocity gradients have been studied, which included pure strain, pure vorticity and x, y and z-directional shear. The results indicate that the novel 4D-DHPIV/PTV has poorer accuracy and precision in the z-propagation axis, resulting in larger minimum uncertainties and bias errors. The errors in the z axis are also significantly less affected by velocity gradients in the z direction when compared to the effects of x and y directional velocity gradients on x and y errors respectively. Furthermore, the rate of cross-correlation maximum and SNR decrease are approximately 1.36 times slower due to velocity gradients in the z axis than other axes.

The aerodynamics of flying snake airfoils in tandem configuration.

Flying snakes flatten their body to form a roughly triangular cross-sectional shape, enabling lift production and horizontal acceleration. While gliding, they also assume an S-shaped posture, which could promote aerodynamic interactions between the fore and the aft body. Such interactions have been studied experimentally; however, very coarse models of the snake's cross-sectional shape were used, and the effects were measured only for the downstream model. In this study, the aerodynamic interactions resulting from the snake's posture were approximated using two-dimensional anatomically accurate airfoils positioned in tandem to mimic the snake's geometry during flight. Load cells were used to measure the lift and drag forces, and flow field data were obtained using digital particle image velocimetry (DPIV). The results showed a strong dependence of the aerodynamic performance on the tandem arrangement, with the lift coefficients being generally more influenced than the drag coefficients. Flow field data revealed that the tandem arrangement modified the separated flow and the wake size, and enhanced the lift in cases in which the wake vortices formed closer to the models, producing suction on the dorsal surface. The downforce created by the flow separation from the ventral surface of the models at 0deg angle of attack was another significant factor contributing to lift production. A number of cases showing large variations of aerodynamic performance included configurations close to the most probable posture of airborne flying snakes, suggesting that small postural variations could be used to control the glide trajectory.

Performance augmentation mechanism of tandem flapping foils with stroke time-asymmetry

The performance augmentation mechanism of a tandem-foil system undergoing time-asymmetric flapping with unequal up- and downstroke durations (velocities) is investigated at three different phase angles, 0°, 90°, and 180°. Specifically, an asymmetry ratio, ranging from 0 to 0.4, is introduced to quantify the degree of the stroke time-asymmetry and to serve as the primary kinematic parameter of interest that affects the foil performances. Numerical simulations are implemented to predict the force production and to investigate the associated mechanism at different asymmetry ratios and phase angles. Validations are performed using digital particle image velocimetry in water tunnel experiments with two identical 3D printed wings. The results suggest that the foil performances at proper phase angles can be enhanced by stroke time-asymmetry. The force production during in-phase flapping obtains 15% increment while that during counterstroke flapping achieves remarkable enhancements by 2.5 times, as the asymmetry ratio increases from 0 to 0.4. The study also demonstrates that such enhancements are achieved through the changes in foil flapping velocities and foil-vortex interactions between the unequal up- and downstrokes. These findings not only provide insights toward the characteristics of tandem foils which are operated in non-sinusoidal flapping strokes but also offer a reference to the design of efficient wing kinematics for high-performance biomimetic propulsors.

Formation number and pinch-off signals of disc vortex ring based on a Lagrangian analysis

As one fundamental flow structure, the vortex ring behind an impulsively starting disc can vividly characterize the vortex structure around the flapping flight and the drag-based paddling propulsions. In this paper, an elaborate experiment is performed to investigate the formation of the vortex rings (termed as DVR) by using digital particle image velocimetry (DPIV), as well as by investigating the ridges of finite-time Lyapunov exponent (FTLE) and the elliptic Lagrangian coherent structures (LCSs). Experimental results demonstrate that the circulation growth of DVRs satisfies the Logistic law and can be well predicted by a scaling model based on the shear-layer feeding velocity. Applying the criterion originated from the Kelvin-Benjamin variational principle, the formation number of DVRs when DVRs pinch off is found to stay within the range of F∗=3.35~4.1. Then, the pinch-off process of the DVRs is re-examined based on a Lagrangian analysis, and the dynamic signals are detected to coincide with the onset of DVRs pinch-off. Once DVRs pinch off, a trailing pressure maximum appears in the edge of disc and a closed boundary forms to define the region of DVRs based on the FTLE ridges, fluid detrainment by the dynamic lobes is observed, and a saturation phase of the circulation evolution is clearly observed in terms of the elliptic LCSs. Furthermore, the scaling laws, i.e., circulation growth and formation number, of DVR formation suggest a confined formation mode to understand the formation of wake vortices behind a moving body for reference.

Roles of wing flexibility and kinematics in flapping wing aerodynamics

Insects are excellent fliers because of their ability to generate precise wing kinematics and deform their wings for the sufficient production of unsteady aerodynamic forces. Most of the previous studies on the effects of wing kinematics on the aerodynamics have been limited to the use of rigid wings, and leaves the contribution of wing flexibility unknown. Here, an experiment was conducted to investigate the effect of varied sweep duration and timing of rotation on the unsteady aerodynamic characteristics of hovering flexible and rigid wings at Re of 104. This study found that the forces generated by the flexible wing showed a conspicuous phase delay, which was more sensitive to the change in sweep duration than the timing of rotation. The transient negative lift associated with rigid wings undergoing delayed and advanced wing rotations totally disappeared in the flexible case. A digital particle image velocimetry (DPIV) measurement at the middle of stroke revealed a slight difference in the vortical structures surrounding the two wings in terms of proximity to the shed trailing-edge vortices (TEVs). Also, the linearly twisted nature of the flexible wing caused the coherent leading-edge vortex (LEV) to be stabilized throughout the wingspan. This increased the radial limit of the delayed stall from 3.6c in the rigid wing to 4.8c in the flexible wing. In general, the flexible wing with symmetric and delayed wing rotations generated the higher efficiency. The corresponding net force vectors were tilted in an almost vertical direction in comparison to the rigid wing. This is an indication that natural fliers adopt specific wing kinematics in addition to their wing deformation for the upward titling of their net force vector, which will significantly enhance their aerodynamic performance.

Investigations on vortex structures for undulating fin propulsion using phase-locked digital particle image velocimetry

Investigations on vortex structures for undulating fin propulsion using phase-locked digital particle image velocimetry