On the estimation of time dependent lift of a European Starling during flapping

We study the role of unsteady lift in the context of flapping wings in birds' flight. Both aerodynamicists and biologists attempt to address this subject, yet it seems that the contribution of the unsteady lift still holds many open questions. The current study deals with the estimation of unsteady aerodynamic forces on a freely flying bird through analysis of wingbeat kinematics and near wake flow measurements using time resolved particle image velocimetry. The aerodynamic forces are obtained through unsteady thin airfoil theory and lift calculation using the momentum equation for viscous flows. The unsteady lift is comprised of circulatory and non-circulatory components. Both are presented over wingbeat cycles. Using long sampling data, several wingbeat cycles have been analyzed in order to cover the downstroke and upstroke phases. It appears that the lift varies over the wingbeat cycle emphasizing its contribution to the total lift and its role in power estimations. It is suggested that the circulatory lift component cannot assumed to be negligible and should be considered when estimating lift or power of birds in flapping motion

Reduced-order Dynamic Modeling and Robust Nonlinear Control of Fluid Flow Velocity Fields

A robust nonlinear control method is developed for fluid flow velocity tracking, which formally addresses the inherent challenges in practical implementation of closed-loop active flow control systems. A key challenge being addressed here is flow control design to compensate for model parameter variations that can arise from actuator perturbations. The control design is based on a detailed reduced-order model of the actuated flow dynamics, which is rigorously derived to incorporate the inherent time-varying uncertainty in the both the model parameters and the actuator dynamics. To the best of the authors’ knowledge, this is the first robust nonlinear closed-loop active flow control result to prove exponential tracking control of a reduced-order actuated flow dynamic model, which formally incorporates input-multiplicative time-varying parametric uncertainty and nonlinear coupling between the state and control signal. A rigorous Lyapunov-based stability analysis is utilized to prove semiglobal exponential tracking of a desired flow field velocity profile over a given spatial domain. A detailed comparative numerical study is provided, which demonstrates the performance improvement that is achieved using the proposed robust nonlinear flow control method to compensate for model uncertainty and uncertain actuator dynamics

On Progress in Exploring Controlled Viscous Limit-Cycle Oscillations in Modified Glauert Airfoil

The paper reports on the progress in the development of a novel robust, nonlinear flow control technology that employs an array of synthetic-jet actuators (SJAs) embedded in 2-DOF, elastically mounted, optimized Modified Glauert (MG) airfoil design in order to control limit cycle oscillations (LCO) at low subsonic flow regimes. The focus here is on the conceptual design of the wind energy harvesting system that employs, e.g., a piezoelectric device to extract energy from plunging LCO, with the closed-loop controller being capable to sustain the required LCO amplitudes over a wide range of wind speeds. The current high-fidelity studies first include validation of the static-airfoil aerodynamic predictions against results obtained from the concurrent experimental campaign. Next, a set of parametric 1-DOF and 2-DOF numerical analyses examine open-loop and closed-loop LCO control strategies that employ the ability of MG airfoil to sustain LCO at subcritical velocities due to natural separation-induced flutter

Mechanisms of airfoil noise near stall conditions

The focus of this paper is on investigating the noise produced by an airfoil at high angles of attack over a range of Reynolds number Re≈2×10⁵–4×10⁵. The objective is not modeling this source of noise but rather understanding the mechanisms of generation for surface pressure fluctuations, due to a separated boundary layer, that are then scattered by the trailing edge. To this aim, we use simultaneous noise and surface pressure measurement in addition to velocimetric measurements by means of hot wire anemometry and time-resolved particle image velocimetry. Three possible mechanisms for the so-called “separation-stall noise” have been identified in addition to a clear link between far-field noise, surface pressure, and velocity fields in the noise generation

Leading Edge Blowing to Mimic and Enhance the Serration Effects for Aerofoil

Copyright: © 2021 by the authors. Leading edge serration is now a well-established and effective passive control device for the reduction of turbulence–leading edge interaction noise, and for the suppression of boundary layer separation at high angle of attack. It is envisaged that leading edge blowing could produce the same mechanisms as those produced by a serrated leading edge to enhance the aeroacoustics and aerodynamic performances of aerofoil. Aeroacoustically, injection of mass airflow from the leading edge (against the incoming turbulent flow) can be an effective mechanism to decrease the turbulence intensity, and/or alter the stagnation point. According to classical theory on the aerofoil leading edge noise, there is a potential for the leading edge blowing to reduce the level of turbulence–leading edge interaction noise radiation. Aerodynamically, after the mixing between the injected air and the incoming flow, a shear instability is likely to be triggered owing to the different flow directions. The resulting vortical flow will then propagate along the main flow direction across the aerofoil surface. These vortical flows generated indirectly owing to the leading edge blowing could also be effective to mitigate boundary layer separation at high angle of attack. The objectives of this paper are to validate these hypotheses, and combine the serration and blowing together on the leading edge to harvest further improvement on the aeroacoustics and aerodynamic performances. Results presented in this paper strongly indicate that leading edge blowing, which is an active flow control method, can indeed mimic and even enhance the bio-inspired leading edge serration effectively.UK Engineering and Physical Sciences Research Council research grant (Grant No. EP/N018737/1) “Quiet Aerofoils of the Next-Generation”; The third author would like to thank the Israel Science Foundation (Grant No. 207162) for the funding of her time to be spent in this project

Unsteady aerodynamics loads during flapping flight of birds; case study : starling and sandpiper

Flapping wing is one of the most widespread propulsion methods found in nature. However, the current understanding of the bird aerodynamics is incomplete. The role of unsteady motion in the flow and its contribution to the aerodynamics is still an open question. The current study deals with the estimation of unsteady aerodynamic forces on freely flying birds through analysis of wingbeat kinematics and near wake flow measurements using long duration time-resolved particle image velocimetry. Two bird species have been investigated, the starling and sandpiper. Using long-time sampling data, several wingbeat cycles have been analyzed in order to cover both the downstroke and upstroke phases of flight. Lift and drag were obtained using the momentum equation for viscous flows and were found to share a highly unsteady behavior. The two birds show similar behavior during the downstroke phase of flight, whereas the sandpiper was shown to have a district signature during its upstroke phase. The contribution of the circulatory lift component is shown to be significant when estimating lift (or power) of birds in flapping flight. Moreover, the unsteady drag term was found to have a crucial role in the balance of drag (or thrust), particularly during transition phases. These findings may shed light on the flight efficiency of birds by providing a partial answer to how they minimize drag and maximize lift during flapping flight.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016

On the manipulation of flow and acoustic fields of a blunt trailing edge aerofoil by serrated leading edges

This paper employs serrated leading edges to inject streamwise vorticity to the downstream boundary layer and wake to manipulate the flow field and noise sources near the blunt trailing edge of an asymmetric aerofoil. The use of a large serration amplitude is found to be effective to suppress the first noise source—bluntness-induced vortex shedding tonal noise—through the destruction of the coherent eigenmodes in the wake. The second noise source is the instability noise, which is produced by the interaction between the boundary layer instability and separation bubble near the blunt edge. The main criterion needed to suppress this noise source is related to a small serration wavelength because, through the generation of more streamwise vortices, it would facilitate a greater level of destructive interaction with the separation bubble. If the leading edge has both a large serration amplitude and wavelength, the interaction between the counter-rotating vortices themselves would trigger a turbulent shear layer through an inviscid mechanism. The turbulent shear layer will produce strong hydrodynamic pressure fluctuations to the trailing edge, which then scatter into broadband noise and transform into a trailing edge noise mechanism. This would become the third noise source that can be identified in several serrated leading edge configurations. Overall, a leading edge with a large serration amplitude and small serration wavelength appears to be the optimum choice to suppress the first and second noise sources and, at the same time, avoid the generation of the third noise source

Leading edge serrations for the reduction of aerofoil separation self-noise

This paper presents an experimental investigation into the use of LE serrations for the reduction of trailing edge self-noise, at least for the NACA-65 aerofoil family. It is shown that the leading edge serrations are able to reduce the self-noise in a low frequency range at small and negative angles of attack. The exact mechanism of this reduction is still not completely discovered, but the LE serrations are discovered able to modulate the mean velocity field and turbulent velocity spectrum in that range of frequencies, as well as to dampen the effect of the angle of attack on the pressure field and to reduce its coherence. We emphasise that this paper represents work in progress and further investigations are still necessary in order to completely understand the dynamics behind this reduction

Unsteady aerodynamics of single and tandem wheels

The major unsteady aerodynamic forces and major physics of a generic single wheel and tandem wheels are studied for the first time using wind tunnel tests. The wind-tunnel tests are performed in the 2.1 m × 1.5 m wind tunnel at the University of Southampton. The tandem-wheel configuration consists of two in-line wheels that can be tested at different inter-axis distances and various installation angles. A vibration test is performed in situ on the model assembly to validate the unsteady-load measurements. Mean and unsteady aerodynamic loads and on-surface pressures are measured. Particle Image Velocimetry is used to acquire the velocity fields in the wake downstream of the model and surface oil-flow technique is used to identify the flow features on the surface of the wheels. Proper Orthogonal Decomposition is also used to characterise the wake in terms of unsteady fluctuations. The results of the experiments on the tandem wheels show that higher values of inter-axis distance correspond to slightly higher total mean drag coefficients and remarkably lower drag coefficient RMS values. Higher installation angles are associated with higher mean drag coefficients but generally lower fluctuations of the force coefficients. Non-zero mean lift coefficients are found for low inter-axis distance configurations at zero installation angle. The flow on the single wheel and on the front wheel of the tandem wheels is affected by laminar-turbulent transitional features. The vortical structures past the tandem wheels consist of four vortices that detach from the tyre shoulders of the front wheel and interact with the rear wheel. The study and obtained databases contribute to the general understanding of the complex flow and help to improve engineering predication of the gear aerodynamic loads

An implementation of an aeroacoustic prediction model for broadband noise from a vertical axis wind turbine using a CFD informed methodology

This paper presents an enhanced method for predicting aerodynamically generated broadband noise produced by a Vertical Axis Wind Turbine (VAWT). The method improves on existing work for VAWT noise prediction and incorporates recently developed airfoil noise prediction models. Inflow-turbulence and airfoil self-noise mechanisms are both considered. Airfoil noise predictions are dependent on aerodynamic input data and time dependent Computational Fluid Dynamics (CFD) calculations are carried out to solve for the aerodynamic solution. Analytical ow methods are also benchmarked against the CFD informed noise prediction results to quantify errors in the former approach. Comparisons to experimental noise measurements for an existing turbine are encouraging. A parameter study is performed and shows the sensitivity of overall noise levels to changes in inflow velocity and inflow turbulence. Noise sources are characterised and the location and mechanism of the primary sources is determined, inflow-turbulence noise is seen to be the dominant source. The use of CFD calculations is seen to improve the accuracy of noise predictions when compared to the analytic ow solution as well as showing that, for inflow-turbulence noise sources, blade generated turbulence dominates the atmospheric inflow turbulence