Advances in experimental techniques and the ever-increasing fidelity of numerical simulations have led to an abundance of data describing fluid flows. This review discusses a range of techniques for analyzing such data, with the aim of extracting simplified models that capture the essential features of these flows, in order to gain insight into the flow physics, and potentially identify mechanisms for controlling these flows. We review well-developed techniques, such as proper orthogonal decomposition and Galerkin projection, and discuss more recent techniques developed for linear systems, such as balanced truncation and dynamic mode decomposition (DMD). We then discuss some of the methods available for nonlinear systems, with particular attention to the Koopman operator, an infinite-dimensional linear operator that completely characterizes the dynamics of a nonlinear system and provides an extension of DMD to nonlinear systems.

Model Reduction for Flow Analysis and Control

Bifurcation control deals with modification of bifurcation characteristics of a parameterized nonlinear system by a designed control input. Typical bifurcation control objectives include delaying t...

/pdf/bifurcation-control-theories-methods-and-applications-2x17nw7jh6.pdf

Bifurcation control: theories, methods, and applications

Bifurcation analysis has previously been applied to the NASA Generic Transport Model (GTM) to provide insight into open-loop upset dynamics and also the impact on and sensitivity of such behaviour to closing the loop with a flight controller. However, these studies have not considered time delay in the system: this arises in all feedback controllers and has specific relevance when remotely piloting a vehicle such as the NASA AirSTAR GTM with ground-based controllers. Developments in the AirSTAR programme, in which a sub-scale generic airliner model will be tested for loss-of-control conditions over long ranges, raise the prospect of increased adverse effects of time delay relative to previous testing. This paper utilises bifurcation analysis, supplemented with time histories, on the GTM numerical model with a LQR-PI controller to evaluate the sensitivity of the closed-loop system stability to time delay. In this paper, the impact of time delays in both a fixed-gain and a gain-scheduled version of the controller is presented in terms of stability of nominal and off-nominal solutions.

Sensitivity of the Generic Transport Model upset dynamics to time delay

Synopsis: From the nostalgic remembrance of the first dynamic flight test this Survey Paper traces several milestones in the history of flight vehicle system identification. A comprehensive account of modern system identification techniques is provided. Several challenging examples bring out the fact that these techniques have reached a high level of maturity, making them a sophisticated and powerful tool not only for research purposes, but also to support the needs of the aircraft industry. This survey paper includes 183 references and provides a consolidated list of publications on the subject.

https://dlr.de/ft/en/portaldata/2/resources/dokumente/ft/jategaonkar/evol_sysid.pdf

Evolution of flight vehicle system identification

Bats have long captured the imaginations of scientists and engineers with their unrivaled agility and maneuvering characteristics, achieved by functionally versatile dynamic wing conformations as well as more than 40 active and passive joints on the wings. Wing flexibility and complex wing kinematics not only bring a unique perspective to research in biology and aerial robotics but also pose substantial technological challenges for robot modeling, design, and control. We have created a fully self-contained, autonomous flying robot that weighs 93 grams, called Bat Bot (B2), to mimic such morphological properties of bat wings. Instead of using a large number of distributed control actuators, we implement highly stretchable silicone-based membrane wings that are controlled at a reduced number of dominant wing joints to best match the morphological characteristics of bat flight. First, the dominant degrees of freedom (DOFs) in the bat flight mechanism are identified and incorporated in B2’s design by means of a series of mechanical constraints. These biologically meaningful DOFs include asynchronous and mediolateral movements of the armwings and dorsoventral movements of the legs. Second, the continuous surface and elastic properties of bat skin under wing morphing are realized by an ultrathin (56 micrometers) membranous skin that covers the skeleton of the morphing wings. We have successfully achieved autonomous flight of B2 using a series of virtual constraints to control the articulated, morphing wings.

A biomimetic robotic platform to study flight specializations of bats

A five-degree-of-freedom dynamic wind-tunnel rig is used in the observation of large-amplitude, stall-related and self-sustaining, pitch oscillations of a model aircraft. These oscillations are investigated on the dynamic wind-tunnel rig during a quasi-steady ramp input to the aircraft model’s elevator surfaces in one-degree-of-freedom pitch mode and in the longitudinal two-degree-of-freedom pitch and heave modes. A mathematical model of the aerodynamics, incorporating the effects of dynamic stall, is proposed. The aerodynamic model is coupled to the test rig equations of motion, which include terms for joint friction, and its parameters are fitted to the experimental data. This latter process is achieved using continuation and bifurcation analysis, which also helped to reveal the influence of friction forces on the oscillatory behavior. The quality of the fit and the use of a phenomenological model allow a possible cause for these oscillations to be proposed.

Investigation of Poststall Pitch Oscillations of an Aircraft Wind-Tunnel Model

A control law design approach that robustifies the nonlinear dynamic inversion method for an aircraft performing nonlinear maneuvers is proposed. The robustness condition is based upon Lyapunov stability theory and multiple time scale dynamic inversion. The algorithm is analytically developed to be robust to a specified level of parameter uncertainty. A smoothed sliding mode control condition is introduced to provide linear stability of a closedloop system and maintain a specified level of command trajectory tracking accuracy. The method is demonstrated via an application to a twin-engined high performance aircraft performing the Cobra maneuver and a 360 degrees stability axis roll at high angle of attack while asymmetrically dropping external stores or experiencing thrust loss of one engine. Simulation results are presented to show the closedloop system dynamics using both the proposed algorithm and the conventional nonlinear dynamic inversion method.

Robust nonlinear dynamic inversion method for an aircraft motion control

The stability augmentation problem for an unstable aircraft with limited control efficiency is considered. To provide the maximum stability region, optimal control algorithms are derived. The design technique developed for a linear multidimensional system is presented in a simple geometrical form allowing the extension to nonlinear systems. The relationship between the stability optimal procedure and the pole assignment method is also established. Qualitative analysis of the closed-loop system dynamics with control constraints is performed. The optimal and conventional control laws are compared in terms of the stability region size of the closed-loop system. Two examples of the considered approach application to the aircraft dynamics are presented.

Maximum stability region design for unstable aircraft with control constraints

Synthetic turbulence modeling for evaluation of ultrasonic cross-correlation flow measurement

The high-incidence aerodynamics of a lightweight jet trainer aircraft model has been investigated using a novel five-degree-of-freedom (DOF) dynamic maneuver rig, recently updated with improved act...

https://arc.aiaa.org/doi/pdf/10.2514/1.C034995

Mikhail Goman

Papers

Investigation of Poststall Pitch Oscillations of an Aircraft Wind-Tunnel Model

Robust nonlinear dynamic inversion method for an aircraft motion control

Maximum stability region design for unstable aircraft with control constraints

Synthetic turbulence modeling for evaluation of ultrasonic cross-correlation flow measurement

Experimental Investigation of Aerodynamic Hysteresis Using a Five-Degree-of-Freedom Wind-Tunnel Maneuver Rig