Distributed aerodynamic sensing is a key requirement for future fly-by-feel UAV systems. Inspired by mechanosensory systems found in flying animals, this paper investigates the use of a bio-inspired optically tracked flexible pillar sensor array for aerodynamic sensing and stall detection on a washed-out NACA0012 aerofoil. Experiments were conducted in a low-speed water tunnel, with flow at chord-based Reynolds Re=70×103 and the pillar sensors set to measure local flow conditions. Sensor calibration and dynamic characterisation were performed prior to testing. Time-resolved flow visualisation measurements were used to validate sensor response and investigate local flow phenomena. The results demonstrated that flexible pillar sensors can capture early indications of stall through monitoring of spanwise mean deflection, flow reversal events associated with incipient and fully separated flow, and characteristic low-frequency oscillations. The findings demonstrate the potential of distributed bio-inspired sensor arrays to enhance stall detection and enable real-time aerodynamic monitoring in future fly-by-feel UAV systems.

Bio-inspired flexible pillar-like wind-hairs show promise for the future of flying by feel by detecting critical flow events on an aerofoil during flight. To be able to characterise specific flow disturbances from the response of such sensors, quantitative PIV measurements of such flow-disturbance patterns were compared with sensor outputs under controlled conditions. Experiments were performed in a flow channel with an aerofoil equipped with a 2D array of such sensors when in uniform inflow conditions compared to when a well-defined gust was introduced upstream and was passing by. The gust was generated through the sudden deployment of a row of flaps on the suction side of a symmetric wing that was placed upstream of the aerofoil with the sensors. The resulting flow disturbance generated a starting vortex with two legs, which resembled a horseshoe-type vortex shed into the wake. Under the same tunnel conditions, PIV measurements were taken downstream of the gust generator to characterise the starting vortex, while further measurements were taken with the sensing pillars on the aerofoil in the same location. The disturbance pattern was compared to the pillar response to demonstrate the potential of flow-sensing pillars. It was found that the pillars could detect the arrival time and structural pattern of the flow disturbance, showing the characteristics of the induced flow field of the starting vortex when passing by. Therefore, such sensor arrays can detect the “footprint” of disturbances as temporal and spatial signatures, allowing us to distinguish those from others or noise.

Flexible wingtip extensions matched to the Cauchy and Reynolds numbers of a peregrine falcon's primary feather in flight have been tested in differing configurations and compared to rigid ones for reference. The wingtip configurations were attached to the end of a symmetric (NACA 0012) aerofoil and were tested at 5 degrees and 10 degrees angles of attack and Reynolds numbers of 70 k and 90 k. Time resolved particle image velocimetry (TR-PIV) was used to study the dynamics of the individual vortices. The results show that, at increased angle of attack the configuration with C -type variation of the free length of the winglets is spreading the vorticity into spanwise and vertical directions, generating a circular multi -core vortex arrangement. In contrast, for the case of winglets of the same length (I -type configuration) a continuous vortex sheet is formed which rolls up into a single core, dislocated outboards and upwards from the original tip -vortex location. This remains the case even for larger angle of attack. It is concluded that - besides the known reduction of induced drag - the former configuration is also beneficial for a more rapid disintegration of the tip -vortex in the wake, while the latter shows less instability. This let us speculate that the latter could be relevant for reducing the tip -noise at higher angle of attack such as for Owls, who hunt during night and have adapted to fly silently.

A feasibility study using an event-based camera for the detection of plane-wise motion of a 2D array of bio-inspired sensing pillars to detect generated flow events. EBCs show potential for future applications where PIV may have been classically used. They have the benefit of only generating data when an event happens and with comparatively cheaper setups, they hold great potential for flow monitoring. EBCs collect pixel x and y locations, as well as pixel contrast change data at each time there is an event and can be trained on regions of interest. Comparative experiments were carried out in the labs at City, University of London, to investigate the feasibility of using EBCs for the detection of generated large flow structures in both air and water. The disturbances were produced by a gust generator, an upstream cylinder and an air gun for a variety of controlled situations, such as disturbances caused by gusts, crossflows and wake downstream oscillations. Some results were also compared to known PIV results from our lab to see how EBCs can detect a known flow structure over the sensing pillars. The results presented demonstrate the capability of the pillars and EBC to detect large flow structures. These initial results show promise for future applications of flow monitoring, such as real-time flow monitoring via a machine learning algorithm that is trained to detect certain flow patterns over the sensing pillars. This would be an intermediate step into a fully automated flow control loop.

Owls have evolved unique wing features that enable silent flight, a crucial adaptation for their nocturnal hunting strategy. This study investigates the role of leading edge combs, also known as serrations or barbs, in reducing noise emissions and enhancing aerodynamic performance. Previous research has demonstrated the positive effects of serrations on both noise suppression and aerodynamic efficiency on rectangular wings. However, the aerodynamic mechanism by which serrations contribute to noise suppression on swept wings remains unexplored. We speculate herein that this is specifically affecting the tip vortex noise at varying angles of attack, inspired from animal studies with removed serrations. Experiments using particle image velocimetry (PIV) and flow visualization techniques are employed and show the flow turning induced by serrations and its subsequent effect on tip vortex formation, which results in lower vortex strength and core size, linked towards reduced vortex-induced noise emission. The results demonstrate the passive and three-dimensional flow control dependent on inflow conditions and offer an insight into the mechanism by which serrations contribute to silent flight, particularly during the final stages of hunting. It is also suggested that the serrations work in similar fashion to other swept wing flow control devices in attenuating the spanwise velocity component and increasing stall AoA.

The silent flight of barn owls is associated with wing and feather specialisations. Three special features are known: a serrated leading edge that is formed by free-standing barb tips which appears as a comb-like structure, a soft dorsal surface, and a fringed trailing edge. We used a model of the leading edge comb with 3D-curved serrations that was designed based on 3D micro-scans of rows of barbs from selected barn-owl feathers. The interaction of the flow with the serrations was measured with Particle-Image-Velocimetry in a flow channel at uniform steady inflow and was compared to the situation of inflow with freestream turbulence, generated from the turbulent wake of a cylinder placed upstream. In steady uniform flow, the serrations caused regular velocity streaks and a flow turning effect. When vortices of different size impacted the serrations, the serrations reduced the flow fluctuations downstream in each case, exemplified by a decreased root-mean-square value of the fluctuations in the wake of the serrations. This attenuation effect was stronger for the spanwise velocity component, leading to an overall flow homogenization. Our findings suggest that the serrations of the barn owl provide a passive flow control leading to reduced leading-edge noise when flying in turbulent environments.

Ducted wind turbines for residential purposes are characterized by a lower diameter with respect to conventional wind turbines for on-shore applications. The noise generated by the rotor plays a significant role in the overall aerodynamic noise. By making modifications to the blade sections of the wind turbine, we can alter the contributions of aeroacoustic noise sources. This study introduces innovative wind turbine blade designs inspired by owl wing characteristics, achieving significant noise reduction without compromising aerodynamic performance. A three-dimensional scan of an owl wing was first employed to derive a family of airfoils. The airfoils were employed to modify the blade of a referenced wind turbine airfoil section at various positions on the blade span to determine a blade operating more efficiently at the tip-speed ratio of the original one. While maintaining the same aerodynamic performance, the bio-inspired profiles show a more uniform pressure coefficient distribution, considerably decreasing in the noise level. Furthermore, this study makes considerable progress in ducted wind turbine design by obtaining an 8 dB noise reduction and a 12% improvement in sound pressure level. An in-depth aerodynamic examination shows a 6.4% rise in thrust force coefficient and optimized power coefficients, reaching a peak at a tip speed ratio of 8, demonstrating improved energy conversion efficiency. The results highlight the dual advantage of the innovative design: significant noise reduction and enhanced aerodynamic efficiency, offering a promising alternative for urban wind generation.

Herein a novel bio-inspired morphing wing UAV demonstrator developed under the ESA project BIOUAS-HALO-21. The design is inspired by the peregrine falcon's morphology and offers unique capabilities like full-span morphing wings for variable lift and roll control, avian-like wing folding for compact stowage, and the ability to tube launch. This research paves the way for future UAVs with mission-adaptive wings, offering potential applications in various fields, including extreme manoeuvrability, gust mitigation, and autonomous flight control. The modular design facilitates further development for diverse UAV applications, with initial targets including reconnaissance UAVs for SAR and HADR missions.

Previous research has identified the capabilities of artificial whisker sensor arrays as an innovative method of tracking and evaluating flow events and disturbances. In this work, a new approach is put forward, focusing on achieving enhanced sensitivity and improved performance of fibre-optic whisker sensors, based on the principle of fibre Bragg Grating (FBG) based optical stress sensing. Its performance is evaluated against more simplistic approaches, and previously demonstrated methods of optically tracking the tips of whisker sensors. The study has found the performance and sensitivity of these FBG-based sensors to be very satisfactory, with sufficient sensitivity to bending stresses to enable using Cross-Correlation (CC) and multilateration techniques (as evaluated in prior studies) to produce reliable Direction of Arrival (DoA) and velocity estimations, at a typical SNR of around 2 dB. The system has shown the capability for correction of potentially disruptive variations in temperature, allowing for effective measurement of the key hydrodynamic disturbances under study, irrespective of the local environmental conditions.

Purpose Covert feathers on avian wings can show dynamic pop-up behaviour in rapid succession as a reaction to turbulent gusts. The purpose of this paper is to understand the possible flow control mechanism induced during such dynamic motion cycles. A model aerofoil is designed with suction side spanwise control of rows of bio-inspired flaplets. Design/methodology/approach A NACA 0012 aerofoil is equipped with a spanwise row of eight flaplets at 80% chord, connected to pneumatic actuators and can be deployed to max 15° in a prescribed open–hold–close manner. The model is placed in a water tunnel and flow measurements are done in the wake of the flaps during a cycle using particle image velocimetry. Findings During opening, boundary layer flow is sucked into the void space between the wing surface and the flaplet, which induces backflow underneath the flaplet and traps the fluid inside. This fluid is expelled downstream during closure, which generates a forward directed jet as seen by the formation of a vortex-ring like structure with higher axial momentum. The entrainment of the jet leads to the re-energising of the boundary layer flow further upstream. Originality/value This paper presents a furtherment of understanding of the action of pop-up feathers for separation control. The actuation of the bio-inspired flaplets shows a flow vectorising effect which can be used for active separation and gust control. In the case of incipient separation, flaplet action can act to re-attach the flow because of the jet entrainment effect.

A novel approach for sensing and characterising the flow over an aerofoil is introduced. Arrays of flexible wind-hair-like sensors distributed over an aerofoil, which are tracked remotely using high-speed imaging and processing, acting as "digital tufts", are used to provide real-time readings of local flow information with high temporal resolution. The use case presented in this paper has the sensors embedded within the suction side of a NACA0012 aerofoil and tested in a wind tunnel for varying angles of attack in static and dynamic tests. The time-averaged signals were able to provide information pertaining to the free-stream velocity and instantaneous angle of attack. The capability of the sensor type to provide temporal flow information is also explored. The sensors were used to detect low-frequency oscillations, which are pre-cursory to stall. These are hypothesised to be linked to breathing modes of the laminar separation bubble, causing a shear-layer flapping observed on the sensors. Such low-frequency oscillations were also detected shortly before separation in the ramp-up studies.

The utilization of Underground Pumped Storage Power Systems (UPSP) addresses the growing need for energy storage in the face of increasing intermittent energy sources. Simultaneously, the closure of mining activities has resulted in vast underground spaces potentially becoming available for alternative purposes. This paper explores the potential of repurposing abandoned mines, particularly coal mines, as lower reservoirs for UPSPs. The challenges associated with employing abandoned mines as lower reservoirs are multifaceted. The foremost challenge stems from limited knowledge about the current state of the mines due to post-mining processes, such as weathering, dissolution, hydration, leaching, swelling, slacking, subsidence, creeping along faults, gas migration, and precipitation, along with corrosion and deterioration of the support elements. This study documents and discusses the various processes related to cyclical pumping and discharge within the context of UPSPs, encompassing hydraulic discharge processes, cyclic loading, dry and wet processes, as well as fatigue and thermal stress. These processes significantly impact the safety, productivity, and stability of the lower reservoir. To address these challenges, the paper presents different numerical solutions available to comprehend and mitigate cyclical processes in abandoned mines. Finally, it explores the economic feasibility of repurposing mines as lower reservoirs and the conditions required are examined, including favorable rock mass properties, reduced land acquisition costs, the necessity of permanent water pumping, and the potential income from excavated rock as a revenue source in case of new excavations. This research contributes to the understanding of utilizing abandoned mines for UPSPs, highlighting the challenges associated with the use of coal mines as lower reservoirs and presenting several main processes to prevent safety and productivity issues.

A novel approach for characterising specific flow phenomena unique to incipient stall in real-time on small scale UAVs and aerodynamic systems operating at low to medium Reynolds numbers is introduced. Flexible pillar sensors emanating from the suction side of an aerofoil of length approx. 3% of chord length are installed on a NACA0012 and tested at City, University of London's wind-tunnel facilities. The sensors are tracked in real-time using a high-speed camera in a simulated 'on-board' position, acting as ‘digital tufts’, and the results subsequently processed. The results show the pillars reacting to specific flow phenomena that are unique to incipient stall, and scale with angle of incidence. Namely, these are low-frequency oscillations of local streamwise velocity components which are hypothesised to be resulting from quasi-periodic vortex shedding/shear layer flapping from the breathing of the laminar separation bubble over the suction side.

Abstract A novel approach for detecting characteristic flow signatures precursory to stall along aerofoils is introduced. It uses arrays of flexible wind-hair like sensors distributed around the aerofoil which are tracked remotely using high-speed imaging and processing. The sensors act as “digital tufts" providing real-time readings of local velocity information with a high temporal resolution. Such sensors are integrated into a NACA0012 aerofoil and tested in a wind-tunnel for varying angles of attack in static tests and dynamically in a ramp-up test. For the static tests, the mean values of the sensor signals provide information on local free-stream velocity and angle of incidence. The fluctuating part of the signals show that at angles approaching separation prominent low frequency oscillations are detected, the magnitudes of which scale with the angle of incidence. These are hypothesised to be linked to breathing modes of the Laminar Separation Bubble causing a shear-layer flapping observed on the sensors. Such low-frequency oscillations were also detected short before separation in the ramp-up studies. As the high-speed cameras are mounted in a simulated “on-board" position, the sensing method could be used for early stall warnings in small-scale UAVs with integrated on-board object tracking cameras.

Previous behavioural research on live sea lions has shown that they are able to detect the direction of oncoming vortices, even when impacting contralaterally. These experiments showed that the whisker system and the animal's neural processing is seemingly able to detect the Direction of Arrival (DoA) from just one side of the heads vibrissal pads. Therefore, temporal differences between whisker stimulation is a likely method for determining the angle. Herein, a theoretical model is presented based on multilateration, and tested by experimental studies on a 2D array of bio-inspired whiskers with regular spacing, and a 3D array of bio-inspired whiskers on a model head of a sea lion, as used in our previous studies. The results show that arrays of whiskers can in principle work as antennae to determine the DoA. This detection of the DoA is achieved by cross-correlation of triplets of whiskers, and Time Difference Of Arrival based multilateration, a method similar to signal processing in modern communication systems and other source localization applications. The results on the 2D array are conclusive and clearly support the hypothesis, while increased uncertainties were found for the 3D array, which could be explained by structural shortcomings of the experimental model. Possible ways to improve the signal are discussed.

The advances of many future engineering applications rely on effective cooling techniques. Beyond the traditional thermal management solutions, the design potential of unsteady impingement cooling is still under-explored. As a combined experimental and numerical study, this paper reports new findings on high-amplitude intermittent impingement cooling with controlled unsteady patterns. Specifical attention was paid on the intermittent flow close time ratio. The experimental work involved unsteady cooling performance measurement with a small-scale water tunnel system. Unsteady Reynolds Averaged NavierStokes Simulation (URANS) was conducted to illustrate the unsteady flow physics, and to evaluate the cooling performance at a wider range of flow conditions (average Reynolds number 2800 < Re-m < 10,000, pulsating frequency 0.1 Hz < f < 2 Hz, close time ratio 0.2 < gamma < 0.8). Both experimental and numerical data confirm a remarkable improvement of overall cooling efficiency by high-amplitude intermittent impingement flow. Especially around the wall jet region, the enhancement can reach as high as 50%. The generation and interaction of vortex rings break the development of thermal boundary layer, and enhance the generation of near wall turbulence, especially for the wall jet region. Saving in coolant consumption with high-amplitude intermittent impingement cooling technique in practice is also demonstrated. The novel concept presented in this paper can be applied to a wide range of applications including electronic cooling, deicing, gas turbine blade cooling, etc. (c) 2021 Elsevier Ltd. All rights reserved.

14 Vortex dynamics around the body of a female swimmer was investigated for 15 several successive underwater Dolphin kicks. This is a typical motion sequence 16 of swimmers after they have pushed off the wall. The method of Computational 17 Fluid Dynamics (CFD) was used in combination with a digital reproduction of 18 the kinematics of the body surface to investigate the unsteady flow phenomena 19 involved therein. The results showed the formation of larger vortices near the 20 swimmer’s body, part of it being used to enhance thrust generation at the legs 21 (vortex re-capturing). At each downstroke, a vortex ring was shed into the wake 22 forming a street of vortices behind the swimmer. Further downstream these rings 23 re-combined into streamwise oriented parallel vortex tubes. Within the motion 24 cycle a distinct variation of drag and thrust force were observed as a 25 characteristic footprint of the kinematics. Mean drag force over the complete 26 cycle was about twelve times higher during dolphin kick when compared to 27 passive, gliding swimming. Maximum mean thrust was reached in 3 motion 28 cycles after the swimmer has pushed off the wall and remained constant from 29 thereon. 30 [199 words] 31

During the pull-out maneuver, Peregrine falcons were observed to adopt specific flight configurations which are thought to offer an aerodynamic advantage over aerial prey. Analysis of the flight trajectory of a falcon in a controlled environment shows it experiencing load factors up to 3 and further predictions suggest this could be increased up to almost 10g during high-speed pull-out. This can be attributed to the high maneuverability promoted by lift-generating vortical structures over the wing. Wind-tunnel experiments on life-sized models together with high fidelity simulations on idealized models, which are based on taxidermy falcons in different configurations, show that deploying the hand-wing in a pull-out creates extra vortex-lift, similar to that of combat aircraft with delta wings. The aerodynamic forces and the position of aerodynamic center were calculated from Large Eddy Simulations of the flow around the model. This allowed for an analysis of the longitudinal static stability in a pull-out, confirming that the falcon is flying unstably in pitch with a positive slope in the pitching moment and a trim angle of attack of about 5$^\circ$, possibly to maximize responsiveness. The hand-wings/primaries were seen to contribute to the augmented stability acting as `elevons' would on a tailless blended-wing-body aircraft.

An aeroacoustic investigating looking into the effect of changing the inter-spacing of passive self-oscillating trailing edge flaplets has been carried out using a NACA 0012 aerofoil. Clear differences can be observed between each of the different cases. Where a reduction in low frequency noise, an increase in high frequency noise and a reduction in turbulence intensity in the wake can be seen for each case. As the inter-spacing is reduced, the differences become more prominent, when compared to the reference aerofoil.

This paper presents an acoustic study of a standard NACA 0012 aerofoil with additional self-oscillating passive flaplets deployed from the trailing edge for self-noise reduction, putting special emphasis on the potential reduction of tonal noise generated by the periodic shedding of vortices from the trailing edge. The flaplets protruding out of the trailing edge act as rectangular thin cantilever beams exited by the surrounding flow to oscillate in their dominant flexural bending mode. The noise attenuation performance is studied for different deployment length, width and inter-spacing to find the most dominant contributions at chord based Reynolds numbers Rec from 100,000 to 900,000 and three geometric angles of attack αg=0°,10° and 15°. It was observed that all flaplet configurations oscillate and thereby reduce the tonal noise. The range of the highest noise reduction in the low frequency range scales with the Strouhal number based on chord length, whereby this range can be set specifically by how far the flaplets protrude out of the trailing edge. This determines the free vibrating length and therefore the natural frequency of the oscillators. Laser-based measurements of the flaplet oscillations confirm the occurrence of a lock-in mechanism observed in previous studies, which effectively describes the oscillating flaplets as pacemaker to keep the fundamental instabilities of the flow in their linear state. It is concluded that this contributes mainly to the tonal noise reduction while the more broadband noise reduction stems from the geometry of the flaplets acting as ‘slitted-serrations’. A larger width of the flaplets is most effective in the low-frequency range, while narrower flaplets are best addressing high-frequency noise components. The results further show that a smaller inter-spacing also benefits the noise reduction. The study paves the way for novel morphing techniques to target specific noise ranges during different flight manoeuvres.