Experimental Characterization of the Downwash Generated by an Insect-Like Flapping-Wing System

T HE flow produced by a bio-inspired flapping wing is unsteady, three-dimensional, and dominated by separated flow and strong vortices. To better understand which structures in this complicated flow are most responsible for the production of lift and drag, several canonical problems have been designed to model portions of a natural wing stroke. These include both twoand threedimensional transient, reciprocating, and quasi-steady variations of pitching, plunging, translating, and rotatingwings. The rotatingwing model is designed to represent the translational phase of an insect wing stroke. In this model, the wing rotates about its root in a propeller-likemotion at a fixed angle of attack. The spanwisevelocity and pressure gradients that exist on an insect wing due to forward/aft sweep about the wing root are preserved, along with the threedimensionality induced by the root and tip vortices. The result is a relatively simple flowfield that preserves the most important characteristics of the translational phase of an entomological wing stroke. Some of the earliest rotating-wing experiments were performed by Usherwood and Ellington on hawkmoth wings at Reynolds numbers O 10 [1,2]. In these experiments, the wing’s lift coefficient was found to decrease as Reynolds number increased from 10,000 to 50,000, and it was postulated that this change in lift production was due to the formation of aweaker leading-edge vortex (LEV) at higher Reynolds numbers. Later, Ozen and Rockwell used particle image velocimetry (PIV) to characterize the steady-state flow structure on a low-aspect-ratio rotating plate at fixed angles of attack between 30 and 75 deg. They observed a stable LEV for a range of Reynolds numbers between 3600 and 14,500 [3]. However, other experiments focusing on the start of a rotating wing accelerating to Reynolds numbers between 10,000 and 60,000 revealed an LEV that formed and shed early in the wing stroke, resulting in a high-lift transient, after which lift dropped to about half of themaximum value [4–6]. At lower Reynolds numbersO 1000 , flow visualizations and PIV have demonstrated spanwise flow on a rotatingwing, and an attached LEV during wing acceleration that later burst over the outboard half of the wing during deceleration [7–9]. The objective of the work presented here is to identify the formation, structure, and possible separation of the leading-edge vortex at the beginning of thewing stroke (i.e., within the first 90 deg of wing rotation) as well as after long convective times (i.e., for wing strokes greater than 90 degrees including multiple revolutions). To this end, flow visualization is performed for threewing revolutions to analyze the vortex structure and the location of the burst point. In addition, unsteady lift and drag measurements are acquired for two revolutions to relate the flow structure to the aerodynamic forces produced by the wing.

In this study, we investigate the response of a swirling annular jet flow and flame to transverse acoustic excitation. Characterizing this response is a necessary step towards understanding velocity-coupled transverse combustion instabilities in lean, premixed flames. These effects are investigated using smoke visualization, particle image velocimetry (PIV), and high-speed flame imaging. This study particularly focuses on the effects of excitation on unsteady vortex development in the shear layers, as well as the effects of high-amplitude acoustics on the time-averaged flow field. First, the time-averaged characteristics of the flow are discussed under both nominal and forced conditions. The shape of the flow changes significantly at high forcing amplitudes as a result of changes in the vortex breakdown structure. Next, the shear layer dynamics with and without acoustic forcing are considered. The shear layers are visualized using smoke visualization and PIV, and the result of vortex rollup on the flame is imaged using high-speed imaging of the flame. We hypothesize that the convectively unstable shear layers and absolutely unstable vortex breakdown bubble play different dynamical roles in controlling the flame response to excitation. The unsteady vortex breakdown bubble is primarily important through its impact on the time-averaged flow field upon which perturbations evolve. It really only changes character at high acoustic forcing amplitudes, resulting in significant variations to the time-averaged flame and flowfield. The shear layer rollup, responding at the frequency of acoustic forcing, creates large-scale wrinkles on the flame and is the main driver of flame response.

Visualization of the vortex wake of a flying animal provides understanding of how wingbeat kinematics are translated into the aerodynamic forces for powering and controlling flight. Two general vortex flow patterns have been proposed for the wake of hovering hummingbirds: (1) The two wings form a single, merged vortex ring during each wing stroke; and (2) the two wings form bilateral vortex loops during each wing stroke. The second pattern was proposed after a study with particle image velocimetry that demonstrated bilateral source flows in a horizontal measurement plane underneath hovering Anna’s hummingbirds (Calypte anna). Proof of this hypothesis requires a clear perspective of bilateral pairs of vortices. Here, we used high-speed image sequences (500 frames per second) of C. anna hover feeding within a white plume to visualize the vortex wake from multiple perspectives. The films revealed two key structural features: (1) Two distinct jets of downwards airflow are present under each wing; and (2) vortex loops around each jet are shed during each upstroke and downstroke. To aid in the interpretation of the flow visualization data, we analyzed high-speed kinematic data (1,000 frames per second) of wing tips and wing roots as C. anna hovered in normal air. These data were used to refine several simplified models of vortex topology. The observed flow patterns can be explained by either a single loop model with an hourglass shape or a bilateral model, with the latter being more likely. When hovering in normal air, hummingbirds used an average stroke amplitude of 153.6° (range 148.9°–164.4°) and a wingbeat frequency of 38.5 Hz (range 38.1–39.1 Hz). When hovering in the white plume, hummingbirds used shallower stroke amplitudes ( $$ \bar{x} $$ = 129.8°, range 116.3°–154.1°) and faster wingbeat frequencies ( $$ \bar{x} $$ = 41.1 Hz, range 38.5–44.7 Hz), although the bilateral jets and associated vortices were observed across the full kinematic range. The plume did not significantly alter the air density or constrain the sustained muscle contractile frequency. Instead, higher wingbeat frequencies likely incurred a higher metabolic cost with the possible benefit of allowing the birds to more rapidly escape from the visually disruptive plume.

A modern approach to graduate flight dynamics, stability, and control courses

Radiofrequency ablation with a vibrating catheter: A new method for electrode cooling

The use of large-scale particle image velocimetry (PIV) is proposed for cycling aerodynamic study to advance the general understanding of the flow around the rider and the bike, leading to new strategies for cycling aerodynamic drag reduction in the future. The investigation concentrates on the measurement of the wake velocity and its relation to the aerodynamic drag of stationary models in wind tunnels and of transiting models in the field. In the first part of this work, PIV measurements are conducted in a wind tunnel to capture the wake flow topology of a full-scale cyclist model and determine the cyclist aerodynamic drag. In-house built seeding systems are employed to inject Helium-filled soap bubble (HFSB) tracers upstream of an elite time-trial cyclist replica. The obtained flow topology compares well among different experimental repetitions and with literature, demonstrating the robustness of the PIV measurement approach. The aerodynamic drag is obtained by a so-called PIV wake rake approach, which relies on the conservation of momentum in a control volume surrounding the model. Comparison of the PIV wake rake aerodynamic drag against that of a force balance demonstrates that a drag accuracy of the latter below 1% is possible. The PIV wake rake measurements are conducted in a plane downstream of the bike’s rear wheel to avoid shadows and optical blockage. At this distance from the athlete, however, investigation of the separated and reverse flow regions, that are the main driver of the aerodynamic drag, is not possible. In the second part of this dissertation, therefore, robotic volumetric PIV measurements are conducted to retrieve the velocity description close to the cyclist. The near-wake of the cyclist limbs is presented, which somehow resembles that of isolated bluff bodies, such as cylinders, featuring a recirculation region bounded by two shear layers. The size of the recirculation region, however, is not only governed by the width of the limb, but also by the coherent vortical structures emanating from these limbs near the limb junctions (e.g. elbows and knees). Moreover, interaction of the limbs with the wakes of the upstream body parts also plays a role in the local wake properties. In addition to the measurement of the cyclist’s near wake at typical race speed, also the cyclist Reynolds number effects are investigated to understand how to reduce the aerodynamic drag by dedicated skinsuits designs in the future. This is achieved repeating the robotic volumetric PIV measurements in a wide range of freestream velocity. While reductions of the wake width are observed on both lower leg and arm with increasing free-stream velocity, the wake of the upper leg follows an opposite trend increasing in size at higher velocity. These variations of wake width with increasing freestream speed are related to the behaviour of the local drag coefficient, indicating a drag crisis behaviour on both leg and arm. The distribution of the so-called critical velocity upon these body segments is discussed, as it determines the freestream speed where a minimum value for the drag occurs. The third, and last part of this work, is dedicated to the development of quantitative flow visualisation and drag determination of cyclists in the field. This so-called Ring-of-Fire system allows, among others, aerodynamic studies that are practically impossible in the wind tunnel, such as model accelerations and model curved-linear trajectories. A tomographic PIV wake rake is employed to measure the flow around a simplified transiting bluff body, a towed 10 cm sphere. These scaled experiments serve as a proof-of-concept of this novel measurement system. The aerodynamic drag is obtained invoking the control volume momentum balance in a frame of reference moving with the object. The expression for the time-average drag consists of three terms, a momentum, Reynolds stress and pressure term, which are individually evaluated at increasing distance downstream of the sphere. It is shown that the aerodynamic drag is most accurately evaluated when the contribution of the momentum term dominates the overall drag and that the PIV pressure evaluation can be avoided five sphere diameters into the wake. The latter largely simplifies the data reduction procedures of the Ring-of-Fire. Finally, the present system estimates the aerodynamic drag with an accuracy of 20 drag counts. This is evaluated from repeated model passages in a range of Reynolds numbers in which the model’s drag coefficient is constant. This resolution is comparable to other aerodynamic drag measurement field techniques. It is rather poor, instead, in comparison to force balance measurements in wind tunnels. In contrast to the latter drag measurement techniques, the Ring-of-Fire also provides information about the flow yielding advanced insights into cyclist aerodynamics in the future.

Understanding the mechanisms and physics of flame stabilization and blowoff of premixed flames is critical toward the design of high velocity combustion devices. In the high bulk flow velocity situation typical of practical combustors, the flame anchors in shear layers where the local flow velocities are much lower. Within the shear layer, fluid strain deformation rates are very high and the flame can be subjected to significant stretch levels. The main goal of this work was to characterize the flow and stretch conditions that a premixed flame experiences in a practical combustor geometry and to compare these values to calculated extinction values. High resolution, simultaneous particle image velocimetry (PIV) and planar laser induced fluorescence of CH radicals (CH-PLIF) measurements are used to capture the flame edge and near-field stabilization region. When approaching lean limit extinction conditions, we note characteristic changes in the stretch and flow conditions experienced by the flame. Most notably, the flame becomes less critically stretched when fuel/air ratio is decreased. However, at these lean conditions, the flame is subject to higher mean flow velocities at the edge, suggesting less favorable flow conditions are present at the attachment point of the flame as blowoff is approached. These measurements suggest that blowoff of the flame from the shear layer is not directly stretch extinction induced, but rather the result of an imbalance between the speed of the flame edge and local tangential flow velocity.

The wake vortices generated by a wind turbine or a wind farm could interference with passing-by flying vehicles. A wind turbine wake study using engineering wake modelling, CFD, LIDAR field measurement and piloted flight simulation are carried out at University of Liverpool in cooperation with CAA, UK. A modified Kocurek wind turbine wake vortex model has been developed to simulate wind turbine wakes. It has been validated on the MEXICO wind turbine with the PIV wind tunnel measurements and the full CFD wake simulation results. This wake vortex model was applied to a WTN250 wind turbine, which has been installed near the East Midlands Airport, UK, where field measurements of wind turbine wake using LIDAR were carried out. The LIDAR measurement data were compared with the wake velocity fields generated by different wake modelling methods. The WTN250 wind turbine wake velocities generated by the Kocurek wake vortex model were integrated into an aircraft flight dynamic model to simulate a wind turbine wake encounter scenario, designed for a light aircraft approaching an airport, where a wind turbine was installed. The severity of the wind turbine wake encounter was investigated using piloted flight simulations. The simulation results suggest that the wake generated minor upsets on the aircraft and resulted a severity rating of B if only the disturbances caused by wake velocity deficits were taken into account.

Wake effects in wind farms can cause significant power losses. In order to reduce these losses layout and control optimization can be applied. For this purpose, simple and fast prediction tools for the wake flow are needed. In the first part of this work, five analytical wind turbine wake models are compared to small-scale turbine wind tunnel measurements. The measurements are conducted at several downstream distances, varying the ambient turbulence intensity and upstream turbine blade pitch angle. Furthermore, an adjustment of a recently developed wake model is proposed. Subsequently, the adjusted model is found to perform best throughout all test cases. In the second part, the performance of an aligned downstream turbine is modelled based on the predicted wake flow using a Blade Element Momentum method with guaranteed convergence. In order to consider the non-uniform inflow velocity a mean-blade-element-velocity method is developed. Moreover, a blockage effect correction is applied. A comparison to wind tunnel measurement data shows that the wake velocity as well as the combined power of two aligned turbines are fairly well predicted. Additionally, the presented analytical framework of wake and downstream turbine performance modelling proposes several model improvements for state-of-the art wind farm simulation tools.

The entrained solid particles in multiphase flows present significant challenges for fluid handling systems, particularly due to the erosion of internal surfaces. This erosion affects the life, reliability, and safety of pipelines and other flow systems. A key factor influencing erosion is the particle impact velocity, which determines the extent of material degradation over time. However, accurately predicting these velocities remains a challenge due to the complex spatial and temporal distribution of particles within the flow. Despite extensive research, a universally applicable and highly reliable model for predicting impact velocities in multiphase flows has yet to be established. Many existing erosion prediction models rely on computational fluid dynamics (CFD) simulations. However, these models often include assumptions that fail to accurately capture the intricacies of multiphase flows, especially in geometrically complex regions such as pipe bends. The need for improved predictive models is critical in industries where erosion related failures lead to increased maintenance costs and potential safety hazards. The current study focuses on examining particle velocities in a liquid-solid multiphase flow through a 90-degree pipe bend using a commercially available CFD software package that incorporates a discrete phase model. The accuracy of the CFD predictions was validated through experimental testing using particle image velocimetry (PIV), which provided a direct comparison of simulated and experimental velocity data under similar flow conditions. The PIV experimental setup consisted of a transparent glass elbow through which a recirculating water-solid mixture was pumped. To ensure high image clarity, glass beads with a diameter of 10 microns were used at a concentration of 1%, alongside fluorescent tracer particles that enhanced visualization during imaging. To assess the performance of the CFD model across varying flow conditions, both simulations and experiments were conducted at three different velocities: 0.37 m/s, 0.56 m/s, and 0.75 m/s, with a maximum volumetric flow rate of 6 gallons per minute (GPM). These velocities were selected based on the limitations of the experimental setup and to provide a representative range of flow conditions. The comparison of the CFD and PIV results demonstrated strong agreement, confirming the reliability of the CFD based approach for predicting particle velocities. The result of this study demonstrates the potential of CFD simulations to accurately model particle impact velocities in multiphase flows. Improved prediction of impact velocities can contribute to the development of more robust erosion prediction models, ultimately enhancing the design and maintenance of fluid transport systems. By refining computational techniques and incorporating experimental validation, future research can further improve the accuracy of erosion assessments and mitigate the risks associated with particle-induced wear.

The wind turbines with a flanged-diffuser shroud-so called “wind lens turbine”-are developed as one of high performance wind turbines by Ohya et al. In order to investigate the flow characteristics and flow acceleration, the paper presents the flow velocity measurements of a long-type and a compact-type wind turbines with a flanged-diffuser shroud by particle image velocimetry. In the case of the long type wind turbine, the velocity vectors of the inner flow field of the diffuser for turbine blades rotating and no blades rotating are presented at Reynolds number, 0.9×105. Furthermore the flow fields between with and without rotating are compared. Through the PIV measurement results, one can realize that the turbine blades rotating affects as suppress the disturbance and the flow separation near the inner wall of the diffuser. The time average velocity vectors are made on the average of the instantaneous velocity data. There are two large vortices in downstream region of the diffuser. One vortex behind the flange acts as suck in wind to the diffuser and raise the inlet flow velocity. Another large vortex appears in downstream. It might be act as blockage vortex of main flow. The large blockage vortex is not clear in the instantaneous velocity vectors, however it exists clearly in the time average flow field. The flow field around the wind turbine with a compact-type flanged-diffuser shroud is also investigated. The flow pattern behind the flange of the compact-type turbine is the same as the long-type one. It means that the effect of flow acceleration is caused by the unsteady vortices behind the flange. The comparison with CFD and PIV results of meridional time-average streamlines after the compact-type diffuser is also presented.

ILLER1 has called attention to a seeming paradox in the vortex theory for hovering rotors. In the usual application of vortex theory for the limiting case of an infinite number of rotor blades to propellers, the induced velocities in the wake are small compared to the forward velocity and the distributed vortices in the wake may be assumed to move downstream at the propeller forward velocity for lightly loaded propellers. Theodorsen showed,2 however, that in the application of the Goldstein theory for optimum circulation distribution on propellers with a finite number of blades, it is necessary, for more heavily loaded propellers representative of practical designs, to take into account the induced velocities in arriving at vortex geometry in the wake. Theodorsen also gave an approximate method for computing slipstream contraction, but for the cases of propellers in cruise flight which he considered, the slipstream contraction was of the order of 1%; the analysis specifically excluded static thrust conditions. For propeller cruise operating conditions it is well known3'4 that the results of vortex theory for an infinite number of blades are essentially identical to the momentum theory of the actuator disk; this remains true for the case of nonconstant blade circulation if both theories are applied to differential annular strips on the blades. It is also true when induced wake rotation is introduced into both theories. A particular result of practical significance in the theories (neglecting wake rotation) is that the induced axial velocity at the propeller vl is one-half the velocity in the ultimate wake, v2. In the propeller case, with negligible slipstream contraction, this follows from the fact that the vortices are assumed to be uniformly distributed over the circumference of a cylinder of approximately constant diameter that has a semi-infinite length viewed from the plane of the propeller and an infinite length viewed from a transverse plane in the ultimate wake, giving rise to a factor of two in the calculated induced velocities. It should be noted, however, that in the momentum theory of the actuator disk it is assumed that the flow velocity in the wake is constant for constant disk loading. In the vortex theory, this is a conclusion arrived at from analyses of the induced flowfield in the vortex cylinder. (Yet another approach is to represent the acutator disk by a doublet layer; this gives results equivalent to vortex theory.) With large slipstream contractions, such as occur for the static thrust condition of a propeller or for a hovering helicopter rotor, it is no longer valid to make this particular calculation since the wake vortices cannot be considered to lie on cylinders of constant diameter in the vicinity of the propeller plane. Yet, even in this case, vortex theory for an

ILLER1 has called attention to a seeming paradox in the vortex theory for hovering rotors. In the usual application of vortex theory for the limiting case of an infinite number of rotor blades to propellers, the induced velocities in the wake are small compared to the forward velocity and the distributed vortices in the wake may be assumed to move downstream at the propeller forward velocity for lightly loaded propellers. Theodorsen showed,2 however, that in the application of the Goldstein theory for optimum circulation distribution on propellers with a finite number of blades, it is necessary, for more heavily loaded propellers representative of practical designs, to take into account the induced velocities in arriving at vortex geometry in the wake. Theodorsen also gave an approximate method for computing slipstream contraction, but for the cases of propellers in cruise flight which he considered, the slipstream contraction was of the order of 1%; the analysis specifically excluded static thrust conditions. For propeller cruise operating conditions it is well known3'4 that the results of vortex theory for an infinite number of blades are essentially identical to the momentum theory of the actuator disk; this remains true for the case of nonconstant blade circulation if both theories are applied to differential annular strips on the blades. It is also true when induced wake rotation is introduced into both theories. A particular result of practical significance in the theories (neglecting wake rotation) is that the induced axial velocity at the propeller vl is one-half the velocity in the ultimate wake, v2. In the propeller case, with negligible slipstream contraction, this follows from the fact that the vortices are assumed to be uniformly distributed over the circumference of a cylinder of approximately constant diameter that has a semi-infinite length viewed from the plane of the propeller and an infinite length viewed from a transverse plane in the ultimate wake, giving rise to a factor of two in the calculated induced velocities. It should be noted, however, that in the momentum theory of the actuator disk it is assumed that the flow velocity in the wake is constant for constant disk loading. In the vortex theory, this is a conclusion arrived at from analyses of the induced flowfield in the vortex cylinder. (Yet another approach is to represent the acutator disk by a doublet layer; this gives results equivalent to vortex theory.) With large slipstream contractions, such as occur for the static thrust condition of a propeller or for a hovering helicopter rotor, it is no longer valid to make this particular calculation since the wake vortices cannot be considered to lie on cylinders of constant diameter in the vicinity of the propeller plane. Yet, even in this case, vortex theory for an

Research on bio-inspired flapping-wing micro-aerial vehicles (MAVs) has experienced a steady growth over the past two decades. In particular, experiments on insect flight dynamics may provide new solutions for various challenges ranging from morphological design to force control mechanisms. A significant amount of research in this area is focused on modeling and simulation of such dynamics; however, mass of the wings and corresponding inertia effects are often ignored for simplification purposes. In this paper, the MAV is considered to be a structure with three rigid bodies, i.e., a main body and two wings. Wing strokes are limited within the body's transverse plane, though each wing can also passively pitch around its lateral axis. Using the Lagrangian, a dynamic multibody model of this system is developed to ≪1≫ analyze the significance of wing mass in flight dynamics and ≪2≫ simulate flight control experiments. The employed control approach is based on investigated relationships between mechanical impedance properties of the wing pitch joints and average values of aerodynamic forces. The results suggest that the wings' mass and mechanical impedance properties of the joints can be optimized together to enhance lift/thrust production. In addition, simulations of various flight maneuvers with the optimized model and proposed control approach always demonstrate an agile and stable behavior.

The influence of the wing root junction on the laminar separation bubble forming on the suction surface of a semispan NACA 0018 wing cantilevered from the wind tunnel test section wall is studied using surface flow visualisations, particle image velocimetry and surface pressure measurements at a chord Reynolds number of 125 000 and an angle of attack of 6 $^\circ$ . The test section wall boundary layer upstream of the wing is turbulent, and the spanwise influence of the junction on the separation bubble extends well beyond the test section wall boundary layer thickness. Substantial three-dimensionality is seen in the separation bubble flowfield near the wing root, where earlier transition and a reduction in separation bubble thickness is observed. In contrast with the wing tip, earlier transition and a reduction in separation bubble length occurs near the wing root. Outside of the junction affected region, the separation bubble is similar to separation bubbles forming on two-dimensional geometries, and displays mild spanwise waviness. The transition process away from the end affected regions is characterised by the formation of spanwise roll-up vortices that are shed in a nearly two-dimensional manner across the span. The analysis of the results shows that, near the wing root, the increased level of perturbations leads to earlier vortex roll-up and spanwise flow contributes to more rapid vortex breakdown. The results in the wing root region are complemented by the analysis of data from Toppings and Yarusevych (J. Fluid Mech., vol. 929, 2021, A39) in the wing tip region to provide a more holistic outlook on the laminar separation bubble topology and dynamics on the entire finite wing.