Acoustic Saturation in Bubbly Cavitating Flow Adjacent to an Oscillating Wall

Bubbly cavitating flow generated by the normal oscillation of a wall bounding a semi-infinite domain of fluid is computed using a continuum two-phase flow model. Bubble dynamics are computed, on the microscale, using the Rayleigh-Plesset equation. A Lagrangian finite volume scheme and implicit adaptive time marching are employed to accurately resolve bubbly shock waves and other steep gradients in the flow. The one-dimensional, unsteady computations show that when the wall oscillation frequency is much smaller than the bubble natural frequency, the power radiated away from the wall is limited by an acoustic saturation effect (the radiated power becomes independent of the amplitude of vibration), which is similar to that found in a pure gas. That is, for large enough vibration amplitude, nonlinear steepening of the generated waves leads to shocking of the wave train, and the dissipation associated with the jump conditions across each shock limits the radiated power. In the model, damping of the bubble volume oscillations is restricted to a simple "effective" viscosity. For wall oscillation frequency less than the bubble natural frequency, the saturation amplitude of the radiated field is nearly independent of any specific damping mechanism. Finally, implications for noise radiation from cavitating flows are discussed

Numerical Simulation of Flow over an Airfoil with a Cavity

Two-dimensional direct numerical simulation of the flow over a NACA0018 airfoil with a cavity is presented. The low Reynolds number simulations are validated by means of flow visualizations carried out in a water channel. From the simulations, it follows that there are two main regimes of flow inside the cavity. Depending on the angle of attack, the first or the second shear-layer mode (Rossiter tone) is present. The global effect of the cavity on the flow around the airfoil is the generation of vortices that reduce flow separation downstream of the cavity. At high positive angles of attack, the flow separates in front of the cavity, and the separated flow interacts with the cavity, causing the generation of smaller-scale structures and a narrower wake compared with the case when no cavity is present. At certain angles of attack, the numerical results suggest the possibility of a higher lift-to-drag ratio for the airfoil with cavity compared with the airfoil without cavity

An EnKF-Based Flow State Estimator for Aerodynamic Flows

Regardless of plant model, robust flow estimation based on limited measurements remains a major obstacle to successful flow control applications. Aiming to combine the robustness of a high-dimensional representation of the dynamics with the cost efficiency of a low-order approximation of the state covariance matrix, a flow state estimator based on the Ensemble Kalman Filter (EnKF) is applied to two-dimensional flow past a cylinder and an airfoil at high angle of attack and low Reynolds number. For the development purposes, we use the numerical algorithm as both the estimator and as a surrogate for the measurements. Estimation is successful using a reduced number of either pressure sensors on the surface of the body or sparsely placed velocity probes in the wake. Because the most relevant features of these flows is restricted to a low-dimensional subspace/manifold of the state space, asymptotic behavior of the estimator is shown to be achieved with a small ensemble size. The relative importance of each sensor location is evaluated by analyzing how they influence the estimated flow field. Covariance inflation is used to enhance the estimator performance in the presence of unmodeled free stream perturbations. A combination of parametric modeling and augmented state methodology is used to successfully estimate the forces on immersed bodies

On non-autonomous H∞ control with infinite horizon

AbstractWe study the linear H∞ control problem in the infinite-horizon case when the coefficients are time varying and bounded. We pass in a standard way from a Riccati equation to a linear Hamiltonian system of ordinary differential equations, which we study using exponential dichotomies and rotation numbers. In particular, we use the dichotomy concept to define the critical attenuation value

Decomposition of stochastic flows with automorphism of subbundles component

We show that given a $G$-structure $P$ on a differentiable manifold $M$, if the group $G(M)$ of automorphisms of $P$ is big enough, then there exists the quotient of an stochastic flows $phi_t$ by $G(M)$, in the sense that $\phi_t = \xi_t \circ \rho_t$ where $\xi_t \in G(M)$, the remainder $\rho_t$ has derivative which is vertical but transversal to the fibre of $P$. This geometrical context generalizes previous results where $M$ is a Riemannian manifold and $\phi_t$ is decomposed with an isometric component, see Liao \cite{Liao1} and Ruffino \cite{Ruffino}, which in our context corresponds to the particular case of an SO(n)-structure on $M$.Comment: To appear in Stochastics and Dynamics, 201

Progress in Lithotripsy Research

Shock wave lithotripsy (SWL) for the non-invasive treatment of kidney stones was introduced in the United States in 1984. SWL virtually eliminated the need for open surgery to remove kidney stones, and it did not take long for physicians and patients to endorse this revolutionary technology. Early reports told of the efficient removal of even the most troublesome stones without apparent complications, and SWL quickly became the "treatment modality of choice." It was not long, however, before concerned physicians began to report the occurrence of adverse effects in SWL, particularly involving vascular trauma and including cases of severe hemorrhage in the kidney and acute renal failure — significant side effects of serious consequence. Researchers quickly recognized the challenge and opportunity to determine the mechanisms of shock wave action in lithotripsy, and in 1988, the Acoustical Society of America held the first in a series of popular sessions devoted to the topic of shock waves in medicine. The goal of the inaugural session was to improve the fundamental understanding of lithotripsy — to bring better devices and treatments to patients. The goal of this paper is to report on progress in this effort

An update on the human and animal enteric pathogen Clostridium perfringens

Clostridium perfringens, a rapid-growing pathogen known to secrete an arsenal of >20 virulent toxins, has been associated with intestinal diseases in both animals and humans throughout the past century. Recent advances in genomic analysis and experimental systems make it timely to re-visit this clinically and veterinary important pathogen. This Review will summarise our understanding of the genomics and virulence-linked factors, including antimicrobial potentials and secreted toxins of this gut pathogen, and then its up-to-date clinical epidemiology and biological role in the pathogenesis of several important human and animal-associated intestinal diseases, including pre-term necrotising enterocolitis. Finally, we highlight some of the important unresolved questions in relation to C. perfringens-mediated infections, and implications for future research directions

Supersonic Jet Noise from Round and Chevron Nozzles: Experimental Studies

High speed exhaust noise reduction continues to be a research challenge for supersonic cruise business jets as well as for current and future tactical military aircraft. Significant noise reduction may be possible from advanced concepts for controlling instability generated large-scale turbulence structures in the jet shear layer, generally accepted to be the source of aft-angle noise. In response to this opportunity, our team is focused on experimental diagnostic studies and unique instability modeling suited for identifying control strategies to reduce large scale structure noise. The current paper benchmarks the jet noise from supersonic nozzles designed to provide the supporting experimental data and validation of the modeling. Laboratory scale jet noise experiments are presented for a Mach number of Mj = 1.5 with stagnation temperature ratios ranging from Tr = 0.75 to 2. The baseline configuration is represented by a round converging-diverging (CD) ideal expansion nozzle. A round CD nozzle with chevrons is included as the first of several planned non-circular geometries directed at demonstrating the impact on large scale structure noise and validating noise prediction methods for geometries of future technological interest. Overexpanded and underexpanded conditions were tested on both nozzle configurations. The resulting data base provides an opportunity to benchmark the statistical characteristics of round and chevron nozzle data. The current paper examines far field spectra, directivity patterns, and overall sound pressure level dependence comparing observed characteristics with the fine scale turbulence noise and large-scale turbulence structure noise characteristics identified by Tam. In addition, the paper probes the effect of chevrons on the developing flow field and suppression of screech tones. Measurements are also reported from a far-field narrow aperture phased array system used to map the acoustic source distribution on the jet axis. The dominant source region, situated between the end of the potential core and the sonic point, was found to agree with the peak amplitude location of the jet near field wavepackets measured using a unique near field array. This observation supports the cause-effect link between large-scale turbulence structures in the shear layer and their dominant contribution to aft radiated far field noise

Keberkesanan Kaedah Pembelajaran Dalam Talian Ke Atas Kursus Terpilih Bagi Komponen Kesenian Dan Kebudayaan Pusat Kokurikulum Dan Pemajuan Pelajar, Universiti Malaysia Sabah

Susulan penularan wabak COVID-19, institusi pendidikan termasuk Pusat Kokurikulum dan Pemajuan Pelajar (PKPP), Universiti Malaysia Sabah (UMS) telah membuat penyelarasan kurikulum mereka. Pendidikan secara dalam talian adalah kaedah utama penyampaian kurikulum bagi komponen Kesenian dan Kebudayaan dalam Fasa Perintah Kawalan Pergerakan (PKP) hinggalah ke Fasa Pelan Pemulihan Negara (PPN). Selepas lapan minggu pembelajaran dalam talian diadakan, tinjauan persepsi pelajar terhadap kaedah pengajaran dan pembelajaran khususnya komponen Kesenian dan Kebudayaan ini telah dijalankan. Tujuan kajian ini adalah untuk menilai pengalaman baharu pelajar dengan pembelajaran dalam talian dan daya maju pendekatan kaedah pengajaran dan pembelajaran dalam talian bagi komponen Kesenian dan Kebudayaan, Pusat Kokurikulum dan Pemajuan Pelajar. Matlamat kajian ini dapat dicapai dengan meneliti respons daripada 226 pelajar terhadap soal selidik berasaskan kaedah tinjauan. Maklum balas pelajar dalam bentuk skala Likert dianalisis menggunakan kaedah statistik deskriptif. Dapatan kajian menunjukkan bahawa majoriti pelajar berpuas hati dengan pembelajaran dalam talian yang berterusan kerana mereka menikmati akses berterusan ke atas bahan dalam talian, fleksibiliti untuk belajar mengikut keupayaan mereka sendiri, suasana yang selesa dan keupayaan untuk bercakap dengan bebas. Namun, masalah teknikal dan isu peralatan merupakan halangan yang ketara ke atas pembelajaran bidang Kesenian dan Kebudayaan dalam talian semasa pandemik COVID-19. Pembelajaran dalam talian adalah diterima baik oleh pelajar yang mendaftar dalam komponen Kebudayaan dan Kesenian. Namun, penyepaduan pembelajaran talian terhadap kurikulum bidang Kesenian dan Kebudayaan memerlukan strategi yang lebih baik dan pendekatan yang lebih proaktif

On local linearization of control systems

We consider the problem of topological linearization of smooth (C infinity or real analytic) control systems, i.e. of their local equivalence to a linear controllable system via point-wise transformations on the state and the control (static feedback transformations) that are topological but not necessarily differentiable. We prove that local topological linearization implies local smooth linearization, at generic points. At arbitrary points, it implies local conjugation to a linear system via a homeomorphism that induces a smooth diffeomorphism on the state variables, and, except at "strongly" singular points, this homeomorphism can be chosen to be a smooth mapping (the inverse map needs not be smooth). Deciding whether the same is true at "strongly" singular points is tantamount to solve an intriguing open question in differential topology