Alexander K. Stoychev


Last Name

Stoychev

First Name

Alexander K.

ORCID

0000-0001-8996-6327

Organisational unit

09471 - Noiray, Nicolas / Noiray, Nicolas

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2023 - 20245
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Publications 1 - 5 of 5
  • Nonlinear dynamics of an acoustically compact orifice
    Item type: Journal Article
    Stoychev, Alexander K.; Noiray, Nicolas (2024)
    Journal of Sound and Vibration
    This work presents a three dimensional, reduced order model of the dynamics of an acoustically compact aperture, subject to an arbitrary pressure forcing. It provides the time evolution of the velocity profile across the orifice section as function of the dynamical pressure excitation. The volume flow can be deduced therefrom, and can thus provide predictions of the fundamental frequency based orifice impedance. The representation of the nonlinear aperture flow proposed here establishes a direct mathematical relation to the fundamental equations of fluid mechanics. This offers a better understanding of the dominant physical mechanisms governing the system's dynamics and allows for good a priori estimates without supporting experiments. The model assumes that the viscosity induced rotational component of the fluid motion can be reduced to a discontinuity at the in-flow plane of the thin orifice, without significantly influencing the normal velocity profile. This seemingly unconventional assumption is solely targeting the acoustics problem and is validated with direct numerical simulations (DNS) of the aperture flow, using a compressible solver of the Navier-Stokes equations. Apart from the DNS, the model predictions are also validated against well established experimental results from the literature.
  • Synchronization under saturable nonlinearity
    Item type: Journal Article
    Stoychev, Alexander K.; Pedergnana, Tiemo; Noiray, Nicolas (2024)
    Chaos
    In this theoretical work, we introduce a nonlinear gain saturation law representative of the experimentally observed properties manifested by phenomena ranging from aeroacoustic shear layers in self-sustained cavity oscillations to flame heat release rate in thermoacoustic instabilities. Furthermore, this type of saturable gain may be relevant for a wider class of physical systems, such as active laser media in photonics. The nonlinearity discussed herein governs the fullscale behavior of a self-oscillator exhibiting linear loss under large amplitude perturbations, in contrast to the cubic damping and linear gain of the Van der Pol model. A distinctive characteristic of the proposed equation is the simple, well behaved gain term in the slow timescale dynamics.
  • Failing parametrizations: what can go wrong when approximating spectral submanifolds
    Item type: Journal Article
    Stoychev, Alexander K.; Römer, Ulrich J. (2023)
    Nonlinear Dynamics
    Invariant manifolds provide useful insights into the behavior of nonlinear dynamical systems. For conservative vibration problems, Lyapunov subcenter manifolds constitute the nonlinear extension of spectral subspaces consisting of one or more modes of the linearized system. Conversely, spectral submanifolds represent the spectral dynamics of non-conservative, nonlinear problems. While finding global invariant manifolds remains a challenge, approximations thereof can be simple to acquire and still provide an effective framework for analyzing a wide variety of problems near equilibrium solutions. This approach has been successfully employed to study both the behavior of autonomous systems and the effects of non-autonomous forcing. The current computation strategies rely on a parametrization of the invariant manifold and the reduced dynamics thereon via truncated power series. While this leads to efficient recursive algorithms, the problem itself is ambiguous, since it permits the use of various approaches for constructing the reduced system to which the invariant manifold is conjugated. Although this ambiguity is well known, it is rarely discussed and usually resolved by an ad hoc choice of method, the effects of which are mostly neglected. In this contribution, we first analyze the performance of three popular approaches for constructing the conjugate system: the graph style parametrization, the normal form parametrization, and the normal form parametrization for “near resonances.” We then show that none of them is always superior to the others and discuss the potential benefits of tailoring the parametrization to the analyzed system. As a means for illustrating the latter, we introduce an alternative strategy for constructing the reduced dynamics and apply it to two examples from the literature, which results in a significantly improved approximation quality.
  • Superradiant Scattering at Coupled Policeman Whistle Meta-Atoms
    Item type: Conference Paper
    Stoychev, Alexander K.; Guo, Xinxin; Kuhl, Ulrich; et al. (2024)
    2024 Eighteenth International Congress on Artificial Materials for Novel Wave Phenomena (Metamaterials)
    This study introduces an aeroacoustic meta-atom based on a toy policeman whistle and investigates its scattering properties theoretically and experimentally. The manuscript examines the nonlinear scattering at a single atom and, subsequently, at a double unit cell.
  • Nonlinear acoustics of an aperture under grazing flow
    Item type: Journal Article
    Stoychev, Alexander K.; Pedergnana, Tiemo; Noiray, Nicolas (2024)
    Royal Society of London. Proceedings A
    This work presents a mathematical model of a dynamically forced, acoustically compact aperture subject to one-sided mean grazing flow in two or three dimensions. By contrast to other simplified theoretical representations of a grazed aperture, the one proposed in this contribution considers some of the nonlinear effects a reduced order model should naturally inherit from the conservation equations governing the primary system's dynamics. Furthermore, unlike other nonlinear developments, this one is able to reproduce the acoustic forcing amplitude dependence of the fundamental-frequency-based impedance, measured in recent experiments, without relying on empirical parameters. This nonlinear model offers further insight into the dominant physical mechanisms determining the aforementioned behaviour and allows reasonable a priori estimates of the aeroacoustic dynamics of the aperture. This could be used as a building block of more sophisticated systems or for the derivation of even simpler representations suitable for acoustic network modelling.
Publications 1 - 5 of 5