441 research outputs found
An implicit algorithm for validated enclosures of the solutions to variational equations for ODEs
We propose a new algorithm for computing validated bounds for the solutions
to the first order variational equations associated to ODEs. These validated
solutions are the kernel of numerics computer-assisted proofs in dynamical
systems literature. The method uses a high-order Taylor method as a predictor
step and an implicit method based on the Hermite-Obreshkov interpolation as a
corrector step. The proposed algorithm is an improvement of the -Lohner
algorithm proposed by Zgliczy\'nski and it provides sharper bounds.
As an application of the algorithm, we give a computer-assisted proof of the
existence of an attractor set in the R\"ossler system, and we show that the
attractor contains an invariant and uniformly hyperbolic subset on which the
dynamics is chaotic, that is, conjugated to subshift of finite type with
positive topological entropy.Comment: 33 pages, 11 figure
On mathematical modelling of insect flight dynamics in the context of micro air vehicles
This paper discusses several aspects of mathematical modelling relevant to the flight
dynamics of insect flight in the context of insect-like flapping wing micro air vehicles (MAVs).
MAVs are defined as flying vehicles ca six inch in size (hand-held) and are developed to
reconnoitre in confined spaces (inside buildings, tunnels etc). This requires power-efficient,
highly-manoeuvrable, low-speed flight with stable hover. All of these attributes are present in
insect flight and hence the focus of reproducing the functionality of insect flight by engineering
means. This can only be achieved if qualitative insight is accompanied by appropriate
quantitative analysis, especially in the context of flight dynamics, as flight dynamics underpin
the desirable manoeuvrability.
We consider two aspects of mathematical modelling for insect flight dynamics.
The first one is theoretical (computational), as opposed to empirical, generation of the
aerodynamic data required for the six-degrees-of-freedom equations of motion. For these
purposes we first explain insect wing kinematics and the salient features of the corresponding
flow. In this context, we show that aerodynamic modelling is a feasible option for certain flight
regimes, focussing on a successful example of modelling hover. Such modelling progresses
from first principles of fluid mechanics, but relies on simplifications justified by the known
flow phenomenology and/or geometric and kinematic symmetries. In particular, this is relevant
to six types of fundamental manoeuvres, which we define as those steady flight conditions for
which only one component of both the translational and rotational body velocities is non-zero
(and constant).
The second aspect of mathematical modelling for insect flight dynamics addressed here
deals with the periodic character of the aerodynamic force and moment production. This
leads to consideration of the types of solutions of nonlinear equations forced by nonlinear
oscillations. In particular, the existence of non-periodic solutions of equations of motion is of
practical interest, since this allows steady recitilinear flight.
Progress in both aspects of mathematical modelling for insect flight will require further
advances in aerodynamics of insect-like flapping. Improved aerodynamic modelling and
computational fluid dynamics (CFD) calculations are required. These theoretical advances
must be accompanied by further flow visualisation and measurement to validate both the
aerodynamic modelling and CFD predictions
Physics-Aware Reduced-Order Modeling of Nonautonomous Advection-Dominated Problems
We present a variant of dynamic mode decomposition (DMD) for constructing a
reduced-order model (ROM) of advection-dominated problems with time-dependent
coefficients. Existing DMD strategies, such as the physics-aware DMD and the
time-varying DMD, struggle to tackle such problems due to their inherent
assumptions of time-invariance and locality. To overcome the compounded
difficulty, we propose to learn the evolution of characteristic lines as a
nonautonomous system. A piecewise locally time-invariant approximation to the
infinite-dimensional Koopman operator is then constructed. We test the accuracy
of time-dependent DMD operator on 2d Navier-Stokes equations, and test the
Lagrangian-based method on 1- and 2-dimensional advection-diffusion with
variable coefficients. Finally, we provide predictive accuracy and perturbation
error upper bounds to guide the selection of rank truncation and subinterval
sizes.Comment: 27 pages, 21 figure
Steady-state oscillations of linear and nonlinear systems
In this paper, an efficient algorithm is developed for the identification of stable steady-state solutions to periodically forced linear and nonlinear dynamical systems. The developed method is based on mapping techniques introduced by Henri Poincare\u27 and the theory of one-parameter transformation groups. The algorithm successfully identifies initial conditions which give rise to strictly periodic orbits. The technique is demonstrated on selected problems associated with linear as well as nonlinear systems
Non-intrusive and structure preserving multiscale integration of stiff ODEs, SDEs and Hamiltonian systems with hidden slow dynamics via flow averaging
We introduce a new class of integrators for stiff ODEs as well as SDEs. These
integrators are (i) {\it Multiscale}: they are based on flow averaging and so
do not fully resolve the fast variables and have a computational cost
determined by slow variables (ii) {\it Versatile}: the method is based on
averaging the flows of the given dynamical system (which may have hidden slow
and fast processes) instead of averaging the instantaneous drift of assumed
separated slow and fast processes. This bypasses the need for identifying
explicitly (or numerically) the slow or fast variables (iii) {\it
Nonintrusive}: A pre-existing numerical scheme resolving the microscopic time
scale can be used as a black box and easily turned into one of the integrators
in this paper by turning the large coefficients on over a microscopic timescale
and off during a mesoscopic timescale (iv) {\it Convergent over two scales}:
strongly over slow processes and in the sense of measures over fast ones. We
introduce the related notion of two-scale flow convergence and analyze the
convergence of these integrators under the induced topology (v) {\it Structure
preserving}: for stiff Hamiltonian systems (possibly on manifolds), they can be
made to be symplectic, time-reversible, and symmetry preserving (symmetries are
group actions that leave the system invariant) in all variables. They are
explicit and applicable to arbitrary stiff potentials (that need not be
quadratic). Their application to the Fermi-Pasta-Ulam problems shows accuracy
and stability over four orders of magnitude of time scales. For stiff Langevin
equations, they are symmetry preserving, time-reversible and Boltzmann-Gibbs
reversible, quasi-symplectic on all variables and conformally symplectic with
isotropic friction.Comment: 69 pages, 21 figure
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