Effects of porous substrates on the structure of turbulent boundary layers

Three different porous substrates (with different pore sizes, s, and permeabilities, K) are used to examine their effect on the structure of boundary layer flow over them. The flow is characterised with single-point hot-wire measurements as well as planar Particle Image Velocimetry. In order to elucidate differences in shallow and deep flows past porous substrate, foams with two different thickness (h) are used (for all three substrates). A wide range of Friction Reynolds number (2000< Retau < 15000) and Permeability based Reynolds number (1<ReK< 50) are attained. For substrates with ReK=1, the flow behaviour remains similar to flow over impermeable smooth walls and as such Townsend's hypothesis remains valid. In contrast, a substantial reduction in velocity disturbances and associated length scales is achieved for permeable (ReK>1) and dense (relative to viscous scales) foam at the thick substrate limit (h/s>10), which leads to the breakdown of outer-layer similarity. As porosity is increased, a thin substrate limit is reached (h/s), and the foam becomes sparse relative to viscous scales (s+ > 100). For such foams, the flow conforms to outer-layer similarity and is more akin to flow over rough surfaces. Such substrates are unable to attenuate velocity disturbances and the dependence of substrate thickness (h/s) on spectral energy content of turbulent fluctuations ceases to exist. The present study shows that transition from thick to thin substrate flow behavior depends not only on thickness-to-pore ratio (h/s) but also on substrate density relative to viscous scales of the flow.Comment: 29 pages, 14 figure

Aeroacoustic investigation of airfoil at near stall conditions

This paper presents a detailed aeroacoustic investigation of a Controlled-Diffusion airfoil at near stall condition. The study aims at answering two research questions: identify the flow mechanism responsible for separation noise for an airfoil near stall conditions and whether the noise is generated by a dipole for airfoil close to stall and can be quantified by Amiet's diffraction theory. The study uses synchronized PIV, RMP and far-field microphone measurements to perform experiments at two chord based Reynolds numbers of about 150,000 and 250,000. The results show that when the airfoil is placed at a higher angle of attack, such as $15^{\circ}$, strong amplification of flow disturbance is seen, resulting in the rolling up of the shear layer in the aft-region of the airfoil, forming large coherent structures. While these rollers play a central role in the increase in noise due to flow separation, the flapping of shear layer does not contribute to the separation noise. The present study conclusively shows that separation noise is dipolar in nature, and that the quadrupolar contribution for low-speed airfoils at near-stall conditions can be neglected. However, the increase in flow disturbances measured close to the trailing-edge of the airfoil implies that the assumption of small amplitude disturbance is no longer valid, which is the central premise of the thin linearized airfoil theory. Outside the frequency range at which flow separation operates, Amiet's theory is able to predict the far-field noise even at high angles of attack.Comment: 45 pages, 25 figure

Bayesian Joint Chance Constrained Optimization: Approximations and Statistical Consistency

This paper considers data-driven chance-constrained stochastic optimization problems in a Bayesian framework. Bayesian posteriors afford a principled mechanism to incorporate data and prior knowledge into stochastic optimization problems. However, the computation of Bayesian posteriors is typically an intractable problem, and has spawned a large literature on approximate Bayesian computation. Here, in the context of chance-constrained optimization, we focus on the question of statistical consistency (in an appropriate sense) of the optimal value, computed using an approximate posterior distribution. To this end, we rigorously prove a frequentist consistency result demonstrating the convergence of the optimal value to the optimal value of a fixed, parameterized constrained optimization problem. We augment this by also establishing a probabilistic rate of convergence of the optimal value. We also prove the convex feasibility of the approximate Bayesian stochastic optimization problem. Finally, we demonstrate the utility of our approach on an optimal staffing problem for an M/M/c queueing model

RFID Based Automatic Shopping Cart

Large grocery stores are nowadays used by millions of people for the acquisition of an enlarging number of products. Product acquisition represents a complex process that comprises time spent in corridors, product location and checkout queues. On the other hand, it is becoming increasingly difficult for retailers to keep their clients loyal and to predict their needs due to the influence of competition and the lack of tools that discriminate consumption patterns. In this article it is presented the proposal of an architecture and solution of an innovative system for the acquisition of products in grocery stores (Intelligent Cart). The Intelligent Cart explores emerging mobile technologies and automatic identification technologies (such as RFID) as a way to improve the quality of services provided by retailers and to augment the consumer value thus allowing to save time and money. Keywords: Automatic Product Identification; Electronic Services; Grocery Stores, RFID, Intelligent car

Aeroacoustic investigation of a ducted tail rotor using the Lattice-Boltzmann Method

Abstract: The present study focuses on the aeroacoustics aspects of a ducted rotor that could be used in helicopter electrically driven tail rotor systems. These systems are axial, low-solidity rotor-stator assemblies operating in low subsonic and transitional regimes at approach. Lattice Boltzmann (LBM) simulations are performed using a variable resolution (VR) mesh strategy that yields a proper convergence after ? 0.35 seconds. The flow behavior, far-field acoustic predictions, and directivity measurements are obtained with the LBM simulations and compared with experiments and RANS simulations done to the same tail rotor system. LBM gives closer predictions to experimentally aerodynamic properties than the RANS simulations. In addition, the direct far-field noise computations show a good overall match with experimental data.Communication présentée lors du congrès international tenu conjointement par Canadian Society for Mechanical Engineering (CSME) et Computational Fluid Dynamics Society of Canada (CFD Canada), à l’Université de Sherbrooke (Québec), du 28 au 31 mai 2023