Gust Perturbation Research Articles

Generating controlled gust perturbations using vortex rings.

To understand the locomotory mechanisms of flying and swimming animals, it is often necessary to develop assays that enable us to measure their responses to external gust perturbations. Typically, such measurements have been carried out using a variety of gusts which are difficult to control or characterize owing to their inherently turbulent nature. Here, we present a method of generating discrete gusts under controlled laboratory conditions in the form of a vortex rings which are well-characterized and highly controllable. We also provide the theoretical guidelines underlying the design of gust generators for specific applications. As a case study, we tested the efficacy of this method to study the flight response of freely-flying soldier flies Hermetia illucens. The vortex ring based method can be used to generate controlled gusts to study diverse phenomena ranging from a natural flight in insects to the artificial flight of insect-sized drones and micro-aerial vehicles.

Robust control of gust-induced vibration of highly flexible aircraft

This paper mainly addresses the gust suppression and alleviation of highly flexible aircraft using the model predictive controller (MPC) based on linear parameter-varying (LPV) models. The dynamic behavior of highly flexible aircraft is modeled by a coupled nonlinear aeroelastic and flight dynamic formulation. Conventional trailing-edge flaps along the main wings and tails are deployed to provide the required loads for the wing vibration control and/or longitudinal flight attitude control of the slender vehicle. The coupled dynamic equations are performed with linearization and model reduction about a series of nonlinear equilibria to derive reduced-order LPV models. This work considers two quantities as the scheduling parameters in building the LPV models: the gust-induced angle of attack and the modal magnitude of wing deformation. With the reduced-order LPV model, MPC is designed to minimize the wing vibration and rigid-body motions excited by the time-domain Dryden gust perturbation. The proposed LPV modeling is capable of describing large wing deformations, and the LPV-MPC innovatively previews the scheduling parameter and gust disturbance in the prediction horizon. The closed-loop flight responses of a highly flexible aircraft with gust disturbance are presented in the numerical studies, which demonstrate the effectiveness of controlling coupled vibrations and rigid-body dynamics of such slender vehicles.

Wing Modulation and Aerodynamics of Hoverflies in Gust Perturbations

Wing Modulation and Aerodynamics of Hoverflies in Gust Perturbations

Kutta–Joukowski Theorem for Unsteady Linear Aerodynamics

This paper presents the extension of the Kutta–Joukowski theorem to unsteady linear aerodynamics. Starting from the formulation developed by Theodorsen for the solution of the velocity potential for circulatory flows around thin, rectilinear airfoils, the frequency response function between bound circulation and circulatory lift is derived. To provide a formulation suitable for time-domain applications, two finite-state approximations of the Kutta–Joukowski frequency response function, with different degrees of complexity and accuracy, are also proposed. The validity of the derived unsteady Kutta–Joukowski theorem is verified by correlation with numerical predictions of the circulatory lift given by a validated boundary-element-method solver for potential flows. Furthermore, the proposed finite-state approximations of the unsteady Kutta–Joukowski theorem are applied to problems concerning the circulatory lift response to damped oscillatory airfoil motion and gust perturbation. The higher-accuracy finite-state model provides predictions that correspond perfectly to those obtained by the convolution integrals of the step responses given by the theories of Wagner and Küssner. The results given by the simpler finite-state model derived from the linear approximation of the frequency response function are satisfactory for low-frequency problems, and are compared with those provided by a widely used approximate unsteady version of the Kutta–Joukowski theorem available in the literature.

Unsteady Aerodynamic Response of a Flat Plate Encountering Large-Amplitude Sharp-Edged Gust

This study presents numerical findings of the response of a flat plate to transverse, sharp-edged gusts at a low Reynolds number (). A split velocity method was implemented in a three-dimensional Navier–Stokes solver to simulate a rigid flat plate wing at steady velocity and 0° angle of incidence encountering a sudden sharp-edged gust. The gust perturbations in the flow domain were prescribed to the grid points, and source terms were added to the governing equations to account for the full interaction between the wing and the gust. Two sharp-edged gust profiles are considered: 1) a top-hat profile gust of finite width and 2) a step profile gust of infinite width. The gust amplitude varies from 4 to 120% of the freestream velocity (). The split velocity method was found to be capable of capturing the physical phenomenon of flat plate undergoing sharp-edged gust, and the predicted lift coefficient is found to be in good agreement with that of experiments. The duration and magnitude of the wing–gust interaction were both found to affect the airfoil response. In the step gust encounter, the lift behaves similarly to that predicted by the Küssner’s model for small-amplitude gusts where . The lift buildup is nonlinear for larger gust amplitudes where . Convolution with the computed step gust response shows good agreement with the simulated response during long and large gust encounters. However, both computational fluid dynamics and Küssner-convolution-based methods fail to predict the exit phase from gust and the recovery to a steady-state value.

Wing rapid responses and aerodynamics of fruit flies during headwind gust perturbations

Insects are the main source of inspiration for flapping-wing micro air vehicles (FWMAVs). They frequently encounter wind gust perturbations in natural environments, and effectively cope with these perturbations. Here, we investigated the rapid gust response of flies to instruct the gust stability design of FWMAVs. A novel method to produce impulsive wind gusts that lasted less than 30 ms was designed to observe flies’ rapid responses. Headwind gust perturbations were imposed on 14 tethered fruit flies, and the corresponding wing motions during perturbations were recorded by three high-speed cameras. The numerical simulation method was then applied to analyze aerodynamic forces and moments induced by the changes in wing kinematics. Results shows that flies mainly utilize three strategies against headwind gust perturbations, including decreasing the magnitude of stroke positional angle at ventral stroke reversal, delayed rotation and making the deviation angles in upstroke and downstroke closer (i.e. the wing tip trajectories of upstroke and downstroke tend be closer). Consequently, flies resist increments in lift and drag induced by the headwind gusts. However, flies seem to care little about changes in pitch moment in tethered conditions. These results provide useful suggestions for the stability control of FWMAVs during headwind gust perturbations.

Implementation and verification of gust modeling in an open-source flow solver

A gust simulation and modeling capability will support the aircraft certification process and therefore reduces the development cost. Two different gust modeling techniques are considered in this article: 1) a user-defined boundary condition available in the commercial flow solver of Cobalt and 2) a field-velocity method implemented in the open-source flow solver of FlowPsi. Specifically, this article describes implementation of gust models in the FlowPsi code including a new time-stepping scheme and a new grid-motion capability that could be used in the simulation of responding vehicles to wind gusts. In more detail, the three stage time-stepping scheme, as here developed, provides much better convergence properties than the Crank–Nicolson scheme available in the code. The grid motion using an external code, as here implemented, is a part of larger development plans to make FlowPsi communicate with external processes such as a finite element code. In addition, the FlowPsi code uses a field-velocity approach to simulate the wind gust which offers a reduced computational expense for simulating wind gust responses when compared to Cobalt. Instead of propagating the gust perturbation from the far inflow boundary as used in Cobalt, an artificial velocity profile is imposed on the grid cells to induce the gust effects on the vehicle. This article aims to determine the accuracy of FlowPsi in predicting gust responses when compared to Cobalt which has already been verified to produce accurate results. The two methods in different codes are compared for a number of gust profiles such as one-minus-cosine, step (sharp-edged), and a random gust profile. FlowPsi shows close convergence and agreement to Cobalt predictions for smooth gust profiles such as the one-minus-cosine, however, the code produces a numerical oscillation pattern at low free-stream Mach numbers when modeling gusts with a rapid change of velocity such as the step gust.

Gust Perturbation Alleviation Control of Small Unmanned Aerial Vehicle Based on Pressure Sensor

A gust perturbation alleviation control method based on a real-time pressure sensor is proposed. Pressure measurement provides phase-advance information on external disturbance, while the conventional inertial measurement cannot. Two pairs of pressure sensors embedded on the main wing surfaces are employed to estimate the disturbance of gust-induced rolling moment. The estimated rolling moment is incorporated into a traditional flight controller as an additional feedforward channel. The simulation results show that the additional information on flow field is helpful and that the composite controller’s architecture is more effective for alleviating gust perturbation than the conventional ones.

Interaction of turbulence with the leading-edge stagnation point of a thin aerofoil

An asymptotic model is constructed to analyse the interaction of turbulence generated far upstream with a thin elliptic-nosed solid body in uniform flow. The leading-edge stagnation point causes significant deformation of incident vorticity, and hence our analysis focuses on the region of size scaling with the nose radius close to the stagnation point. Rapid distortion theory is used to separate the flow field generated by a single unsteady gust perturbation into a convective non-acoustic part, containing the evolution of the upstream vortical disturbance, and an acoustic part generated by the interaction of the vorticity with the solid surface, as is typical in gust–aerofoil interaction theory. Using single-frequency gust response solutions, along with a von Kármán energy spectrum, we find the turbulent pressure spectrum generated by homogeneous isotropic turbulence incident from far upstream. Both high- and low-frequency gusts are considered to allow approximations to be found for the turbulent pressure spectra close to the leading edge, and far from the body close to the incident stagnation streamline. Good agreement is shown between the asymptotic results for the near- and far-field leading-edge turbulent pressure spectra and recent experimental findings.

The role of passive avian head stabilization in flapping flight.

Birds improve vision by stabilizing head position relative to their surroundings, while their body is forced up and down during flapping flight. Stabilization is facilitated by compensatory motion of the sophisticated avian head-neck system. While relative head motion has been studied in stationary and walking birds, little is known about how birds accomplish head stabilization during flapping flight. To unravel this, we approximate the avian neck with a linear mass-spring-damper system for vertical displacements, analogous to proven head stabilization models for walking humans. We corroborate the model's dimensionless natural frequency and damping ratios from high-speed video recordings of whooper swans (Cygnus cygnus) flying over a lake. The data show that flap-induced body oscillations can be passively attenuated through the neck. We find that the passive model robustly attenuates large body oscillations, even in response to head mass and gust perturbations. Our proof of principle shows that bird-inspired drones with flapping wings could record better images with a swan-inspired passive camera suspension.

Kinematic strategies for mitigating gust perturbations in insects

Insects are attractive models for the development of micro-aerial vehicles (MAVs) due to their relatively simple sensing, actuation and control architectures as compared to vertebrates, and because of their robust flight ability in dynamic and heterogeneous environments, characterized by turbulence and gusts of wind. How do insects respond to gust perturbations? We investigated this question by perturbing freely-flying honey bees and stalk-eye flies with low-pressure bursts of compressed air to simulate a wind gust. Body and wing kinematics were analyzed from flight sequences, recorded using three high-speed digital video cameras. Bees quickly responded to body rotations caused by gusts through bilateral asymmetry in stroke amplitude, whereas stalk-eye flies used a combination of asymmetric stroke amplitude and wing rotation angle. Both insects coordinated asymmetric and symmetric kinematics in response to gusts, which provides model strategies for simple yet robust flight characteristics for MAVs.

Adaptive Feedforward Control for Gust Load Alleviation

In this paper, an adaptive feedforward control framework is proposed for the suppression of aircraft structural vibrations induced by gust perturbations to increase the resilience of the !ight control law in the presence of the aeroelastic/aeroservoelastic interactions. Currently, aircraft with nonadaptive control laws usually include roll-off or notch lters to avoid aeroelastic/aeroservoelastic interactions. However, if changes in the aircraft conguration aresignicant,the frequencies of the!exible modesof the aircraft may beshifted, andthe notchlterscould become totally ineffective. With the proposed approach, the !exible modes can be consistently estimated in real time via a proven system-identication algorithm. The identied !exible modes information is used in the proposed adaptive feedforwardcontrolalgorithmtoadjusttheparametrizationofthebasisfunctionsinafeedforwardcontroller.Along with the recursive least-squares estimate, the feedforward controller is adjusted, and the structural vibration of the aircraft induced by the gust perturbation can be largely suppressed. An F/A-18 active aeroelastic wing aeroelastic model with gust perturbation based on the linear aeroelastic solver formulation is developed as a test bed to demonstrate the proposed adaptive feedforward control algorithm.

Characteristics of the Diametral Resonant Response of a Shrouded Fan Under a Prescribed Distortion

A comprehensive controlled study of the resonant vibratory structural response of a shrouded fan blade/disk due to known excitation was performed. A full circumferential definition of the inlet velocity field was obtained at five radial locations for three axial spacings and for four unique patterns of distortion and three mass flow rates. Harmonic analyses of the velocity patterns were used to establish a gust perturbation velocity normal to the blade chord. From these spanwise perturbation velocities, a normalized force parameter was established. In-vacuum, nonrotating testing of the fan assembly allowed identification of individual blade frequencies and system modes. This testing used strain gaging and holography to identify mistuning, damping and split factors for diametral patterns of the 3, 4, 5, and 6 diametral mode families. Dynamic strain signatures from rotating rig resonant responses were obtained for inlet flows having 3, 4, 5, and 6 distorted regions to simulate inlet struts. Groups of 1/4-in. rods were used to create these regions of distorted flow. System mode responses to these distortion patterns included occurrences of standing and traveling waves. Trends of gust perturbation force parameter and measured dynamic stress with varying axial gap, mass flow, and loading were established. The data collectively quantify the strong cause and effect relationship between force parameter and measured dynamic stress.