Article
Mechanics
S. Heitkam, K. Eckert
Summary: This work demonstrates that anisotropic drainage in sheared foam leads to convective instability in very long foam channels, with liquid accumulating on one side of the channel when a critical liquid fraction is exceeded. Experimental and numerical simulations show that the critical liquid fraction is smaller than previously thought by a factor of five.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
A. Mishra, G. Mamatsashvili, V. Galindo, F. Stefani
Summary: We investigate the convective and absolute forms of azimuthal magnetorotational instability in a cylindrical Taylor-Couette flow with an imposed azimuthal magnetic field. Our results show that the convective AMRI domain is wider than the absolute AMRI domain, but the absolute instability is more relevant and important for magnetic TC flow experiments. The global AMRI in a TC flow of finite height exhibits a butterfly-type structure, with two dominant absolute AMRI modes giving rise to this pattern in a good agreement with linear analysis.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
N. Yadav, S. W. Gepner
Summary: Numerical analysis was used to study Couette-Poiseuille (CP) flow in the presence of longitudinal grooves. It was found that the grooves can modify the flow, decrease the phase speed of unstable waves, and reduce the distance required for nonlinear effects.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Engineering, Multidisciplinary
Megan Morrison, J. Nathan Kutz
Summary: This study develops a mathematical framework for controlling nonlinear, networked dynamical systems, using dimensionality reduction, bifurcation theory, and model discovery tools to find low-dimensional subspaces for feed-forward control. By leveraging the fact that high-dimensional networked systems have many fixed points, control signals can be computed to move the system between any pair of fixed points. The approach involves fitting a nonlinear dynamical system to a low-rank subspace with the SINDy algorithm, then using bifurcation theory to identify constant control signals for achieving desired outcomes.
IEEE TRANSACTIONS ON NETWORK SCIENCE AND ENGINEERING
(2021)
Article
Mechanics
John P. Koulakis, S. Putterman
Summary: The study investigates the driving force of acoustic field on fluid inhomogeneities, showing that above a certain threshold, the acoustic field can induce instability in the fluid. The locations of acoustic velocity antinodes separate regions of convective stability and instability, and the growth of convective modes with different wavenumbers is influenced by inertia and viscosity.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Scott Morgan, Christopher Davies, Christian Thomas
Summary: The study investigates the control of stationary convective instabilities in the rotating disk boundary layer by modulating the disk rotation rate periodically. The addition of a time-periodic modulation stabilizes the flow and raises the critical Reynolds number for the onset of instabilities. Energy analysis shows a reduction in Reynolds stress energy production and an increase in viscous dissipation across the boundary layer with the time-periodic modulation. Comparisons are made with other control techniques such as distributed surface roughness and compliant walls.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Automation & Control Systems
Pietro Lorenzetti, George Weiss
Summary: This paper presents a novel anti-windup proportional-integral controller for stable multi-input multi-output nonlinear plants. The controller ensures that the integrator state remains within a desired region, satisfying the operational constraints of the problem. Under suitable assumptions, the existence of a small controller gain is proven to achieve closed-loop exponential stability and reference tracking.
Article
Mechanics
Vasily Vedeneev, Nikolay Nikitin
Summary: In this paper, the possibility of absolute instability in a plane unidirectional jet is studied. It is shown that absolute instability can occur for a low velocity at the inflection point or a thin shear layer. The critical Reynolds numbers separating the zones of convective and absolute instability are determined to be a minimum of Re = 315. The results of a direct numerical simulation are in good agreement with the theoretical analysis.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Automation & Control Systems
Ran Wang, Karthikeya S. Parunandi, Dan Yu, Dileep Kalathil, Suman Chakravorty
Summary: This article addresses the problem of learning the optimal control policy for a nonlinear stochastic dynamical system and proposes a decoupled data-based control (D2C) algorithm to solve it. Experimental results show that the performance of this algorithm is nearly optimal, and the training time is significantly reduced compared to other algorithms.
IEEE TRANSACTIONS ON AUTOMATIC CONTROL
(2022)
Article
Mathematics, Interdisciplinary Applications
Hongjuan Wu, Chuandong Li, Zhilong He, Yinuo Wang, Yingying He
Summary: An asymmetric saturated impulsive control scheme is proposed in this paper to achieve lag synchronization of two coupled dynamical systems in the master-slave pattern. Stability analysis and estimation of the domain of attraction are conducted using Lyapunov stability theory, sector nonlinearity model approach, matrix inequality, set invariance, and Schur complement. Sufficient conditions for lag synchronization of two nonlinear dynamical systems under this impulsive control scheme without actuator saturation are provided, with simulation results demonstrating the feasibility and effectiveness of the approach.
CHAOS SOLITONS & FRACTALS
(2021)
Article
Automation & Control Systems
Esmaeil Sharifi, Christopher J. Damaren
Summary: This article proposes two novel passivity-based control design frameworks for hybrid nonlinear dynamical systems involving an interacting mixture of continuous-time and discrete-time dynamics. The Kalman-Yakubovich-Popov (KYP) conditions are derived to characterize dissipativeness for hybrid nonlinear time-dependent dynamical systems, and a hybrid computational algorithm is proposed to solve the resulting equations. Two passivity-based control schemes are then developed, and practical considerations for implementing the algorithms are discussed. The efficacy of the proposed control schemes is assessed using a multi-dimensional system with a hybrid source of actuation.
INTERNATIONAL JOURNAL OF ROBUST AND NONLINEAR CONTROL
(2023)
Article
Computer Science, Information Systems
Igor B. Furtat, Pavel A. Gushchin
Summary: A novel method for studying the instability and stability of equilibrium points in autonomous dynamical systems is proposed, establishing a relationship between various theorems and divergence conditions. The article also considers generalizations of certain theorems and suggests a state feedback control law based on new divergence conditions. Examples are used to demonstrate the efficiency of the proposed method, along with comparisons to existing methods.
Article
Automation & Control Systems
Kim Peter Wabersich, Melanie N. Zeilinger
Summary: This paper introduces a predictive safety filter for nonlinear systems with continuous state and input spaces, which can turn a constrained dynamical system into an unconstrained safe system and be applied to any RL algorithm. Safety is ensured by a continuously updated safety policy based on a data-driven system model and considering state and input dependent uncertainties.
Article
Automation & Control Systems
Manuel Lanchares, Wassim M. Haddad
Summary: This article explores the use of Lyapunov functions in evaluating nonquadratic cost functionals for Ito-type nonlinear stochastic difference equations, showing that the cost functional can be evaluated in closed-form when related to an underlying Lyapunov function ensuring asymptotic stability in probability. By analyzing discrete-time linear and nonlinear stochastic dynamical systems, as well as developing a stochastic optimal control framework, the study establishes connections between stochastic Lyapunov theory and stochastic Bellman theory to guarantee both stability and optimality in the closed-loop nonlinear system.
INTERNATIONAL JOURNAL OF ROBUST AND NONLINEAR CONTROL
(2022)
Article
Mechanics
Jelle Houtman, Sebastian Timme
Summary: This article investigates the impact of an elastic structure on the aerodynamic three-dimensional shock-buffet dynamics and identifies the core area that causes instability. The research results are significant for mitigating the detrimental effects of this phenomenon in supersonic flight.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Petr Kungurtsev, Matthew P. Juniper
JOURNAL OF FLUID MECHANICS
(2019)
Article
Mechanics
Alp Albayrak, Matthew P. Juniper, Wolfgang Polifke
JOURNAL OF FLUID MECHANICS
(2019)
Article
Mechanics
Luca Magri, Matthew P. Juniper, Jonas P. Moeck
JOURNAL OF FLUID MECHANICS
(2020)
Correction
Physics, Fluids & Plasmas
Matthew P. Juniper
PHYSICAL REVIEW FLUIDS
(2020)
Article
Thermodynamics
Hans Yu, Matthew P. Juniper, Luca Magri
Summary: This study developed and tested a physics-based reduced-order model of a ducted premixed flame, where model parameters were learned from high-speed videos. By assimilating experimental data into a level-set solver using an ensemble Kalman filter, an optimally calibrated reduced-order model accurately reproduced complex nonlinear features, matching experiments even after assimilation was switched off. The automatically extracted model parameters were found to match the expected first-order behavior based on physics, showcasing how reduced-order models can be rapidly updated with new data availability without storing the data itself.
PROCEEDINGS OF THE COMBUSTION INSTITUTE
(2021)
Article
Physics, Fluids & Plasmas
Jose G. Aguilar, Matthew P. Juniper
PHYSICAL REVIEW FLUIDS
(2020)
Article
Chemistry, Physical
Filip K. A. Gokstorp, Matthew P. Juniper
Article
Mathematics, Applied
Gunther Waxenegger-Wilfing, Ushnish Sengupta, Jan Martin, Wolfgang Armbruster, Justin Hardi, Matthew Juniper, Michael Oschwald
Summary: A data-driven method for early detection of thermoacoustic instabilities is proposed and shows good performance on experimental data, comparing with state-of-the-art early warning indicators.
Article
Mechanics
Vikrant Gupta, Anagha Madhusudanan, Minping Wan, Simon J. Illingworth, Matthew P. Juniper
Summary: The study uses Navier-Stokes-based linear models to estimate large-scale fluctuations at different positions in wall-bounded turbulent flows, finding that the models perform reasonably accurately when considering the variation of stochastic forcing and eddy dissipation terms with wall distance and length scale.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
S. Demange, U. A. Qadri, M. P. Juniper, F. Pinna
Summary: This study investigates the stability features of spatially spreading heated jets in the viscous regime with real gas effects, using both unsteady two-dimensional axisymmetric simulations and linear analyses. The study confirms that including real gas effects in the stability equations has a strong effect on the growth rate of the global mode. Linear global analyses over the time-averaged states provide a satisfying prediction of the oscillation's frequency and the baroclinic torque obtained from the resulting global mode matches well with that of the simulations.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Denis Busquet, Olivier Marquet, Francois Richez, Matthew Juniper, Denis Sipp
Summary: Numerical investigation of stalling flow around a static airfoil at high Reynolds numbers revealed the existence of stable and unstable modes, including low-frequency stall modes and high-frequency vortex shedding modes. The study also showed how periodic solutions oscillate around steady solutions in phase space and how the disappearance of the limit cycle is due to bifurcations of periodic orbits.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Alexandros Kontogiannis, Scott Elgersma, Andrew J. Sederman, Matthew P. Juniper
Summary: In this paper, we propose a method to solve the problem of joint velocity field reconstruction and boundary segmentation of noisy flow velocity images. The method is tested on synthetic images and experimental images, and its effectiveness and accuracy are demonstrated. The method also provides additional knowledge about the physics of the flow and addresses the shortcomings of other measurement methods.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Acoustics
Matthew P. Juniper, Matthew Yoko
Summary: In this study, we conducted 7000 experiments on an electrically-heated Rijke tube and assimilated the data using Bayesian inference. We developed a 1D pipe flow model for long timescale behavior and several 1D thermoacoustic network models for short timescale behavior. The models were ranked based on their marginal likelihood and the best model was selected for each component. The results showed that the selected model was physically-interpretable, simplified, and accurate across the entire operating regime. Furthermore, the model could be trained on limited data and successfully extrapolated beyond the training set.
JOURNAL OF SOUND AND VIBRATION
(2022)
Article
Computer Science, Artificial Intelligence
Alexandros Kontogiannis, Matthew P. Juniper
Summary: This study proposes a physics-informed compressed sensing (PICS) method for reconstructing velocity fields from noisy and sparse phase-contrast magnetic resonance signals. The method solves an inverse Navier-Stokes boundary value problem and is able to reconstruct and segment velocity fields while inferring hidden parameters. The problem is regularized using a Bayesian framework and Gaussian random fields as prior information. The algorithm developed in this study successfully reconstructs and segments velocity fields from noisy and sparse signals, showing good agreement with fully-sampled high SNR signals.
IEEE TRANSACTIONS ON IMAGE PROCESSING
(2023)
Article
Engineering, Multidisciplinary
Yubiao Sun, Ushnish Sengupta, Matthew Juniper
Summary: We use a physics-informed neural network (PINN) to model and optimize the flow around an airfoil for maximizing lift to drag ratio. The PINN takes airfoil shape parameters as inputs and approximates the Navier-Stokes equations using collocation points. The gradients of the lift-to-drag ratio with respect to the shape parameters are calculated using the differentiability of the PINN.
COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING
(2023)