A computational study of the interaction noise from a small axial-flow fan

Small axial-flow fans used for computer cooling and many other appliances feature a rotor driven by a downstream motor held by several cylindrical struts. This study focuses on the aerodynamic mechanism of rotor-strut interaction for an isolated fan. The three-dimensional, unsteady flow field is calculated using FLUENT®, and the sound radiation predicted by acoustic analogy is compared with measurement data. Striking differences are found between the pressure oscillations in various parts of the structural surfaces during an interaction event. The suction surface of the blade experiences a sudden increase in pressure when the blade trailing edge sweeps past a strut, while the process of pressure decrease on the pressure side of the blade is rather gradual during the interaction. The contribution of the latter towards the total thrust force on the structure is cancelled out significantly by that on the strut. In terms of the acoustic contributions from the rotor and strut, the upstream rotor dominates and this feature differs from the usual rotor-stator interaction acoustics in which the downstream part is responsible for most of the noise. It is therefore argued that the dominant interaction mechanism is potential flow in nature. © 2007 Acoustical Society of America.published_or_final_versio

A computational study of the interaction noise from a small axial-flow fan

Small axial-flow fans used for computer cooling and many other appliances feature a rotor driven by a downstream motor held by several cylindrical struts. This study focuses on the aerodynamic mechanism of rotor-strut interaction for an isolated fan. The three-dimensional, unsteady flow field is calculated using FLUENT®, and the sound radiation predicted by acoustic analogy is compared with measurement data. Striking differences are found between the pressure oscillations in various parts of the structural surfaces during an interaction event. The suction surface of the blade experiences a sudden increase in pressure when the blade trailing edge sweeps past a strut, while the process of pressure decrease on the pressure side of the blade is rather gradual during the interaction. The contribution of the latter towards the total thrust force on the structure is cancelled out significantly by that on the strut. In terms of the acoustic contributions from the rotor and strut, the upstream rotor dominates and this feature differs from the usual rotor-stator interaction acoustics in which the downstream part is responsible for most of the noise. It is therefore argued that the dominant interaction mechanism is potential flow in nature. © 2007 Acoustical Society of America.published_or_final_versio

Equilibrium states of turbulent homogeneous buoyant flows

2002-2003 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Interference between stationary and vibrating cylinder wakes

Author name used in this publication: Y. ZhouAuthor name used in this publication: R. M. C. So2002-2003 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Free vibrations of two side-by-side cylinders in a cross-flow

2001-2002 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

NPY Neuron-Specific Y2 Receptors Regulate Adipose Tissue and Trabecular Bone but Not Cortical Bone Homeostasis in Mice

BACKGROUND: Y2 receptor signalling is known to be important in neuropeptide Y (NPY)-mediated effects on energy homeostasis and bone physiology. Y2 receptors are located post-synaptically as well as acting as auto receptors on NPY-expressing neurons, and the different roles of these two populations of Y2 receptors in the regulation of energy homeostasis and body composition are unclear. METHODOLOGY/PRINCIPAL FINDINGS: We thus generated two conditional knockout mouse models, Y2(lox/lox) and NPYCre/+;Y2(lox/lox), in which Y2 receptors can be selectively ablated either in the hypothalamus or specifically in hypothalamic NPY-producing neurons of adult mice. Specific deletion of hypothalamic Y2 receptors increases food intake and body weight compared to controls. Importantly, specific ablation of hypothalamic Y2 receptors on NPY-containing neurons results in a significantly greater adiposity in female but not male mice, accompanied by increased hepatic triglyceride levels, decreased expression of liver carnitine palmitoyltransferase (CPT1) and increased expression of muscle phosphorylated acetyl-CoA carboxylase (ACC). While food intake, body weight, femur length, bone mineral content, density and cortical bone volume and thickness are not significantly altered, trabecular bone volume and number were significantly increased by hypothalamic Y2 deletion on NPY-expressing neurons. Interestingly, in situ hybridisation reveals increased NPY and decreased proopiomelanocortin (POMC) mRNA expression in the arcuate nucleus of mice with hypothalamus-specific deletion of Y2 receptors in NPY neurons, consistent with a negative feedback mechanism between NPY expression and Y2 receptors on NPY-ergic neurons. CONCLUSIONS/SIGNIFICANCE: Taken together these data demonstrate the anti-obesogenic role of Y2 receptors in the brain, notably on NPY-ergic neurons, possibly via inhibition of NPY neurons and concomitant stimulation of POMC-expressing neurons in the arcuate nucleus of the hypothalamus, reducing lipogenic pathways in liver and/or skeletal muscle in females. These data also reveal as an anti-osteogenic effect of Y2 receptors on hypothalamic NPY-expressing neurons on trabecular but not on cortical bone

An analytically derived shear stress and kinetic energy equation for one-equation modelling of complex turbulent flows

202109 bcvcVersion of RecordPublishe

A Lattice Boltzmann Method Based Numerical Scheme for Microchannel Flows

2008 ASME International Mechanical Engineering Congress and Exposition, IMECE 2008, Boston, MA, 31 October-6 November 2008Lattice Boltzmann method (LBM) has been recently developed into an alternative and promising numerical scheme for modeling fluid physics and fluid flows. The equation is hyperbolic and can be solved locally, explicitly, and efficiently on parallel computers. LBM has been applied to different types of complex flows with varying degree of success, but rarely to micro-scale flow. Due to its small scale, micro-channel flow exhibits many interesting phenomena that are not observed in its macro-scale counterpart. It is known that the Navier-Stokes equations can still be used to treat micro-channel flows if a slip wall boundary condition is assumed. The setting of boundary conditions in LBM has been a difficult task, and reliable boundary setting methods are limited. This paper reports on the development of an algorithm to solve the Boltzmann equation with a splitting method that allows the application of a slip wall boundary condition. Moreover, the fluid viscosity is accounted for as an additional term in the equilibrium particle distribution function, which offers the ability to simulate both Newtonian and non-Newtonian fluids. An LBM based numerical scheme, which is suitable for micro-channel flows, is proposed. A two-dimensional nine-velocity lattice model is developed for the numerical simulation. Validation of the numerical scheme is carried out against micro-channel, micro-tube and driven cavity flows, and excellent agreement is obtained between numerical calculations and analytical solutions of these flows.Department of Mechanical Engineerin

Aerodynamic and structural resonance of an elastic airfoil. due to oncoming vortices

Aerodynamic and structural resonance of an elastic airfoil in a uniform stream with oncoming vortices was investigated experimentally and numerically. An experiment was designed to create two parallel rows of vortices that serve as external excitation for the symmetric airfoil (NACA 0012). The vortices were produced by two identical side-by-side circular cylinders of diameter D in a uniform stream located at a fixed distance ahead of the airfoil. The whole arrangement was placed symmetrically about the midplane in the test section of a wind tunnel. Reynolds numbers ranging from similar to8 X 10(4) to similar to2 X 10(5) were selected because the Strouhal number was essentially constant in this range. A range of D was selected to provide a corresponding range of shedding frequencies f(s) that could lead to both aerodynamic and structural resonance of the airfoil. A hot-wire anemometer and a dual-beam laser vibrometer were used to measure the wake pattern and the vortex convection velocity, and the airfoil response, respectively. The airfoil displacement amplitude at structural resonance increases by many fold compared to its value far away from resonance and the airfoil goes into a limit-cycle oscillation behavior. As D decreases, near-aerodynamic resonance is also observed. These results were used to verify a boundary element method (BEM) developed to treat vortex-airfoil interaction problems. Aerodynamic and structural resonance of an elastic airfoil in a uniform stream with oncoming vortices was correctly replicated by the BEM, thus showing that the BEM model is appropriate for fluid-structure interaction problems