9,208 research outputs found
Beam scanning by liquid-crystal biasing in a modified SIW structure
A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium
RIS-assisted Cell-Free MIMO with Dynamic Arrivals and Departures of Users: A Novel Network Stability Approach
Reconfigurable Intelligent Surfaces (RIS) have recently emerged as a hot
research topic, being widely advocated as a candidate technology for next
generation wireless communications. These surfaces passively alter the behavior
of propagation environments enhancing the performance of wireless communication
systems. In this paper, we study the use of RIS in cell-free multiple-input
multiple-output (MIMO) setting where distributed service antennas, called
Access Points (APs), simultaneously serve the users in the network. While most
existing works focus on the physical layer improvements RIS carry, less
attention has been paid to the impact of dynamic arrivals and departures of the
users. In such a case, ensuring the stability of the network is the main goal.
For that, we propose an optimization framework of the phase shifts, for which
we derived a low-complexity solution. We then provide a theoretical analysis of
the network stability and show that our framework stabilizes the network
whenever it is possible. We also prove that a low complexity solution of our
framework stabilizes a guaranteed fraction (higher than 78.5%) of the stability
region. We provide also numerical results that corroborate the theoretical
claims
Intelligent computing : the latest advances, challenges and future
Computing is a critical driving force in the development of human civilization. In recent years, we have witnessed the emergence of intelligent computing, a new computing paradigm that is reshaping traditional computing and promoting digital revolution in the era of big data, artificial intelligence and internet-of-things with new computing theories, architectures, methods, systems, and applications. Intelligent computing has greatly broadened the scope of computing, extending it from traditional computing on data to increasingly diverse computing paradigms such as perceptual intelligence, cognitive intelligence, autonomous intelligence, and human computer fusion intelligence. Intelligence and computing have undergone paths of different evolution and development for a long time but have become increasingly intertwined in recent years: intelligent computing is not only intelligence-oriented but also intelligence-driven. Such cross-fertilization has prompted the emergence and rapid advancement of intelligent computing
Non-equilibrium wall-bounded turbulence and associated noise generation
Abstract : The present study investigates the response of turbulence in a non-equilibrium flows such as transient periodic channel flows and spatially developing boundary layers subjected to pressure gradients. Such a fundamental study is important to understand noise generation in complex wall-bounded turbulent flows. First, to understand the flow dynamics in transient accelerating flows, direct numerical simulations (DNS) of periodic channel flows responding to an impulse acceleration are carried out. The turbulent flow undergoes reverse transition toward a quasi-laminar state, followed by a retransition phase to the new equilibrium state. To reduced simulation cost, the minimal-span methodology is applied and evaluated for simulations of transient flows. Detailed comparisons with a full-span case show that the small-span test case captures the essential dynamics during the transition process despite small, quantitative differences attributed to a slower streak transient growth. A small span is used to characterize accelerating channels with riblets. Results indicate that riblets delay the transition to high Reynolds number state, as it reduces streak meandering. Next, to study non-equilibrium boundary layer flows in the presence of convex wall curvature, DNS simulations over an airfoil (suction side) and a flat plate are compared. Both cases are characterized by matching adverse pressure gradient (APG) along the streamwise direction. For the airfoil boundary layer, existing DNS data obtained by \cite{wu2019effects} of flow around a controlled-diffusion (CD) airfoil is used. For the flat-plate boundary layer, a DNS simulation is carried out, with prescribed pressure gradient distribution that matches that of the airfoil flows in the APG region. Comparison between the two cases shows how the wall curvature affects turbulence in an APG boundary layer, important in industrial applications such as fan flows. Overall, the comparison shows that the boundary layer developments are very similar. This indicates that a flat-plate boundary layer can serve as a low-cost surrogate of an airfoil boundary layer in numerical studies of important features of an airfoil flow. The difference between the two cases represents the effect of a mild convex wall curvature. Specifically, in the region of weak APG, the curvature effect dominates that of the pressure gradient and yields a lower friction coefficient. In high-APG regions (near the trailing edge of the airfoil) the effects of wall curvature and APG appear to interact. Lastly, various existing analytical models are evaluated on their predictions of wall pressure fluctuations, which are essential for noise prediction in non-equilibrium boundary layer turbulent flows that develop on fan blades. Limitations of the existing models are evaluated; new parameters that do not involve the ill-defined wall friction in a boundary layer under strong adverse pressure gradients are proposed. The primary role of the mean velocity logarithmic layer in affecting the overlap range of the wall pressure spectrum is also demonstrated. A new wall pressure spectrum model is proposed and tested in a wide range of boundary layer flows under different Reynolds numbers and zero, adverse and favorable pressure gradients. The test database includes existing experimental data and various DNS flat-plate simulations. The new wall pressure spectrum model is the first generalized model designed for boundary layer flows with a wide range of pressure gradients and Reynolds numbers.Ce mémoire étudie la réponse de la turbulence dans des écoulements hors équilibre, tels que les écoulements transitoires dans un canal périodique et les couches limites se développant spatiallement soumises à des gradients de pression. Une telle étude fondamentale est importante pour comprendre la génération du bruit dans des écoulements complexes turbulents. Premièrement, pour comprendre la dynamique d’écoulements transitoires soumis à une accélération, des simulations directes d’écoulements instationnaires dans un canal périodique soumis à une accélération impulsionnelle ont été réalisées. L’écoulement turbulent subit une transition inversée vers un état quasi-laminaire, suivi par une nouvelle phase de transition vers un nouvel équilibre. Pour réduire le coût de calcul, la méthode de l’envergure minimale du domaine de calcul est appliquée et validée pour de telles simulations instationnaires. Des comparaisons détaillées avec un cas d’envergure complète montrent que la simulation avec une envergure minimale capture l’essentiel de la dynamique de l’écoulement durant la phase de transition et ce malgré quelques petites différences attribuées à la croissance plus lente des tourbillons longitudinaux le long de la paroi (“streaks”). Une envergure réduite est ensuite appliquée à l’étude d’un écoulement accéléré dans un canal avec de micro-sillons ou “riblets”. Les résultats montrent que les riblets retardent la transition du fait qu’ils stabilisent la turbulence de proche paroi. Deuxièmement, pour étudier les couches limites hors équilibre sur une paroi convexe, des simulations directes sur l’extrados d’un profil aérodynamique et d’une plaque plane sont comparées. Les deux cas sont caractérisés par le même gradient de pression adverse dans la direction de l’écoulement. Pour la couche limite sur le profil, on utilise les données existantes de la simulation directe de Wu et al. (2019) autour du profil à diffusion controllée (CD). Pour la couche limite sur la plaque plane, une nouvelle simulation directe a été réalisée avec le même gradient de pression adverse que sur le profil. La comparaison des deux cas montre que la courbure de la paroi convexe peut modifier la turbulence dans une couche limite soumise à un gradient de pression adverse qui est important dans les applications industrielles comme les écoulements dans des ventilateurs. Cependant les modifications restent mineures et la comparaison montre que le développement des couches limite turbulentes dans les deux cas est semblable. Ceci implique que la couche limite sur une plaque plaque sur un domaine réduit peut servir de substitut à celle sur un profil aérodynamique qui requiert un domaine plus grand et des ressources de calcul plus importante. La différence observée entre les deux cas permet d’évaluer l’effet d’une paroi faiblement convexe. Spécifiquement, dans la région de faible gradient de pression adverse, les effets de courbure dominent ceux du gradient de pression et réduisent le coefficient de frottement pariétal. Dans les zones de fort gradient de pression adverse, près du bord de fuite, les effets de gradient de pression et de courbure interagissent. Finalement, la dernière étape a été d’évaluer les différents modèles analytiques de fluctuations de pression pariétale qui sont au centre des prédictions de bruit dans les couches limites turbulentes hors équilibre qui se développent sur les pales de ventilateurs. Les limites des modèles précédents sont évaluées et de nouveaux paramètres ne faisant pas intervenir le frottement pariétal mal défini dans une couche limite à fort gradient de pression adverse sont proposés. Le rôle primordial de la zone logarithmique dans la couche limite turbulente sur le gabarit spectral des spectres de pression pariétale est aussi mis en évidence. Le nou veau modèle de spectre de pression pariétale est ensuite testé sur plusieurs couches limites attachées avec des gradients de pression favorables, adverses, et des écoulements décollés à divers nombres de Reynolds basés sur l’épaisseur de quantité de mouvement. Les données proviennent de bases de données expérimentales et numériques existantes. Des simulations directes supplémentaires ont également été réalisées pour étendre les résultats numériques (notamment sur le profil CD) à des nombres de Reynolds plus élevés. Pour la première fois, un modèle est capable de reproduire les spectres de pression pariétale pour tous ces types d’écoulement
Modelling, Monitoring, Control and Optimization for Complex Industrial Processes
This reprint includes 22 research papers and an editorial, collected from the Special Issue "Modelling, Monitoring, Control and Optimization for Complex Industrial Processes", highlighting recent research advances and emerging research directions in complex industrial processes. This reprint aims to promote the research field and benefit the readers from both academic communities and industrial sectors
The ADMM-PINNs Algorithmic Framework for Nonsmooth PDE-Constrained Optimization: A Deep Learning Approach
We study the combination of the alternating direction method of multipliers
(ADMM) with physics-informed neural networks (PINNs) for a general class of
nonsmooth partial differential equation (PDE)-constrained optimization
problems, where additional regularization can be employed for constraints on
the control or design variables. The resulting ADMM-PINNs algorithmic framework
substantially enlarges the applicable range of PINNs to nonsmooth cases of
PDE-constrained optimization problems. The application of the ADMM makes it
possible to untie the PDE constraints and the nonsmooth regularization terms
for iterations. Accordingly, at each iteration, one of the resulting
subproblems is a smooth PDE-constrained optimization which can be efficiently
solved by PINNs, and the other is a simple nonsmooth optimization problem which
usually has a closed-form solution or can be efficiently solved by various
standard optimization algorithms or pre-trained neural networks. The ADMM-PINNs
algorithmic framework does not require to solve PDEs repeatedly, and it is
mesh-free, easy to implement, and scalable to different PDE settings. We
validate the efficiency of the ADMM-PINNs algorithmic framework by different
prototype applications, including inverse potential problems, source
identification in elliptic equations, control constrained optimal control of
the Burgers equation, and sparse optimal control of parabolic equations
Reformulating aircraft routing algorithms to reduce fuel burn and thus CO2 emissions
During the UN Climate Change Conference (COP26), in November 2021, the international
aviation community agreed to advance actions to reduce CO2 emissions. Adopting more
fuel efficient routes, now that full global satellite coverage is available, could achieve this
quickly and economically. Here flights between New York and London, from 1st December, 2019 to 29th February, 2020 are considered. Trajectories through wind fields from a
global atmospheric re-analysis dataset are found using optimal control theory. Initially,
time minimal routes are obtained by applying Pontryagin’s Minimum Principle. Minimum
time air distances are compared with actual Air Traffic Management tracks. Potential air
distance savings range from 0.7 to 16.4%, depending on direction and track efficiency.
To gauge the potential for longer duration time minimal round trips in the future, due
to climate change, trajectories are considered for historic and future time periods, using
an ensemble of climate models. Next, fixed-time, fuel-minimal routes are sought. Fuel
consumption is modelled with a new physics-driven fuel burn function, which is aircraft
model specific. Control variables of position-dependent aircraft headings and airspeeds or
just headings are used. The importance of airspeed in finding trajectories is established,
by comparing fuel burn found from a global search of optimised results for the discretised approximation of each formulation. Finally, dynamic programming is applied to find
free-time, fuel-optimal routes. Results show that significant fuel reductions are possible,
compared with estimates of fuel use from actual flights, without significant changes to
flight duration. Fuel use for winter 2019–2020 could have been reduced by 4.6% eastbound and 3.9% westbound on flights between Heathrow and John F Kennedy Airports.
This equates to a 16.6 million kg reduction in CO2 emissions. Thus large reductions in
fuel consumption and emissions are possible immediately, without waiting decades for
incremental improvements in fuel-efficiency through technological advances
Recommended from our members
Ensuring Access to Safe and Nutritious Food for All Through the Transformation of Food Systems
Transport Densities and Congested Optimal Transport in the Heisenberg Group
We adapt the problem of continuous congested optimal transport to the
Heisenberg group with a sub-riemannian metric: we restrict the set of
admissible paths to the absolutely continuous curves which are also horizontal.
We get the existence of equilibrium configurations, known as Wardrop
Equilibria, through the minimization of a convex functional over a suitable set
of measures. To prove existence of such minima, that turn out to be equilibria,
we prove the existence of summable transport densities. Moreover, such
equilibria induces transport plans that solve a Monge-Kantorovic problem
associated with a cost function, depending on the congestion itself, which we
rigorously define
- …