5 research outputs found
A cyclic perspective on transient gust encounters through the lens of persistent homology
Large amplitude gust encounters exhibit a range of separated flow phenomena,
making them difficult to characterize using the traditional tools of
aerodynamics. In this work, we propose a dynamical systems approach to gust
encounters, viewing the flow as a cycle (or a closed trajectory) in state
space. We posit that the topology of this cycle, or its shape and structure,
provides a compact description of the flow, and can be used to identify
coordinates in which the dynamics evolve in a simple, intuitive way. To
demonstrate this idea, we consider flowfield measurements of a transverse gust
encounter. For each case in the dataset, we characterize the full-state
dynamics of the flow using persistent homology, a tool that identifies holes in
point cloud data, and transform the dynamics to a reduced-order space using a
nonlinear autoencoder. Critically, we constrain the autoencoder such that it
preserves topologically relevant features of the original dynamics, or those
features identified by persistent homology. Using this approach, we are able to
transform six separate gust encounters to a three-dimensional latent space, in
which each gust encounter reduces to a simple circle, and from which the
original flow can be reconstructed. This result shows that topology can guide
the creation of low-dimensional state representations for strong transverse
gust encounters, a crucial step toward the modeling and control of airfoil-gust
interactions
Characterization of Aerodynamic Forces on Wings in Plunge Maneuvers
We present experiments and simulations of plunging maneuvers of large amplitude, for velocity ratios of G=1 and 2, defined as the ratio between the peak plunge velocity and the freestream velocity. We explore the effect of the airfoil shape by considering a NACA 0012 wing and a flat plate. The experiments are performed with wings with aspect ratios of 4 and 4.86, whereas the simulations are performed using a model of an infinite-aspect-ratio wing. We report the time evolution of the force coefficients and flow visualizations. A good qualitative agreement is found between experiments and simulations, with small discrepancies in the maximum and minimum lift coefficients observed during the maneuvers and somewhat larger discrepancies during the postmaneuver phase. It is found that the airfoil shape has a small effect on the lift coefficient but a somewhat larger effect on the drag coefficient. We also perform a force decomposition analysis to relate vortical structures to the force on the wings, providing a quantitative measurement of the effect of the leading-edge vortex and trailing-edge vortex on the peak aerodynamic forces.This work was partially supported by the State Research Agency of
Spain (AEI) under grant DPI2016-76151-C2-2-R, including funding
from the European Regional Development Fund and the U.S. Air
Force Office of Scientific Research under grant FA9550-16-1-0508.
The computations were partially performed at the supercomputer
Picasso from the Red Española de Supercomputación in activity
FI-2019-1-0030.Publicad
Mitigation of transverse gusts via open- and closed-loop pitching maneuvers
Unsteady flow conditions present significant challenges to stable flight, and gust rejectionremains a concern for flight control in many modern flight environments. Examples of gustdominated
flight conditions include flight in stormy conditions, aircraft takeoff and landing in
strong crosswinds or ship air wakes, and micro air vehicles in strong shear flow engendered by
urban settings and complex terrain. Improving flight stability during gust encounters relies on an
improved understanding of the flow physics and the development of effective mitigation control
strategies. To this end, the present work seeks to (1) improve our understanding of the unsteady
flow physics of a pitching wing encountering a transverse gust and (2) develop and characterize
successful open- and closed-loop control strategies to mitigate aerodynamic lift transients
induced by the gust using wing pitching input. Classic unsteady aerodynamic theory was used
to construct the open-loop pitch maneuvers and tune the closed-loop controller for closed-loop
control. The dynamical systems treatment of the problem during control design revealed several
important physical features important to vehicle control.
Two sets of wing-gust encounter experiments were conducted using a flat-plate wing model
in a water towing tank. The transverse gust was generated in the center of the towing tank using
a recirculating water jet. Data was acquired using a combination of Particle Image Velocimetry
(PIV), force, and torque measurements. In the first set of experiments, the constructed openloop
pitch maneuvers were implemented as open-loop kinematics in the water towing tank. This
study revealed several findings regarding the change in the flow topology due to pitch actuation,
the necessity of modeling added mass for open-loop pitch maneuver construction, and the increase
in the pitching moment transients due pitch control. This study also demonstrated how
lift-mitigating pitching maneuvers minimized the disturbance to the gust’s flow field, thereby
reducing the momentum exchange between the gust and the wing.
The second set of experiments implemented a proportional control strategy based on classic
unsteady aerodynamic theory using a pitch acceleration input and real-time force measurements.
The closed-loop control experiments spanned upwards and downwards gusts of various strengths
and lift tracking at pre- and post-stall angles of attack. The controller yielded an average rejection
performance of 80% without a priori knowledge of gust strength or onset time and for various
aerodynamic conditions. Reasons for the controller’s success include using lift measurements
directly in control feedback, aerodynamic models that capture the salient physics in the control
design process, and wing pitching as input. Simultaneous time-resolved PIV and force measurements
were used to discover and understand the flow physics underlying the lift transients and
how applying closed-loop control mitigated those transients
Physics of gust response mitigation in open-loop pitching manoeuvres
This paper experimentally investigates the flow field development and unsteady loading
of three force-mitigating pitch manoeuvres during a transverse gust encounter. The
manoeuvres are constructed using varying levels of theoretical and simulation fidelity
and implemented as open-loop kinematics in a water towing tank. It is found that pitch
actuation during a gust encounter results in two important changes in flow topology: (i)
early detachment of the leading-edge vortex (LEV) and (ii) formation of an LEV on the
pressure side of the wing upon gust exit. Each of the pitch manoeuvres is found to mitigate
a significant portion of the circulatory contribution of the lift force while only manoeuvres
with accurate modelling of the added-mass force are found to adequately mitigate the
total lift force. The penalty of aerodynamic lift mitigation using pitch manoeuvres was a
twofold increase in the pitching moment transients experienced by the wing for all cases.
By quantifying changes in the vertical gust momentum before and after the encounter,
lift-mitigating manoeuvres were found to reduce the disturbance to the gust’s flow field,
thereby reducing the momentum exchange between the gust and the wing.https://doi.org/10.1017/jfm.2022.50