Large eddy simulation of flow past free surface piercing circular cylinders

Proceedings of the Seventh International Conference on Hydroscience and Engineering, Philadelphia, PA, September 2006. http://hdl.handle.net/1860/732Flows past a free surface piercing cylinder are studied numerically by large eddy simulation (LES) at Froude numbers up to FrD=2.0 and Reynolds numbers up to ReD=1.105. A two-phase FCT-VOF method is employed to simulate the air-water interface. The effects of the free surface on vortex structure in the near wake are particularly investigated. The loadings on the cylinder at various Reynolds and Froude numbers are also studied. Computation results show the free surface inhibits the vortex generation in the near wake, and this effect is stronger at higher Froude numbers, nevertheless, it is attenuated as the Reynolds number increases. It is also found that the strong three dimensionality in the near wake at higher Froude numbers affects the force distribution dramatically along the cylinder, whereas variation in the Reynolds number has no significant effect

Direct Numerical Simulation Of Particle Saltation In Turbulent Channel Flow

This paper numerically investigates particle saltation in a turbulent channel flow having a rough bed consisting of 2-3 layers of densely packed spheres. The Shield’s Function is 0.065 which is just above the sediment entrainment threshold to give a bed-load regime. The applied methodology is a combination of three technologies, i.e., the direct numerical simulation of turbulent flow, the combined finite-discrete element modelling of the deformation, movement and collision of the particles, and the immersed boundary method for the fluid-solid interaction. It is shown that the presence of entrained particles significantly modifies the flow profiles of velocity, turbulent intensities and shear stresses in the vicinity of a rough-bed. The quasi-streamwise aligned streaky structures are not observed in the near-wall region and the particles are distributed randomly on the rough-bed owing to their large size. However, in the outer flow region, the turbulent coherent structures recover due to the weakening rough-bed effects and particle interferences. First and second-order statistical features of particle translational and angular velocities, hydrodynamic forces and moments, together with sediment concentration and volumetric flux density profiles, are presented. Several key parameters of the particle saltation trajectory are calculated and agree closely with published experimental data. Time histories of the hydrodynamic forces exerted upon a typical saltating particle, together with those of the particle’s coordinates and velocities, are presented. A strong correlation is shown between the abruptly decreasing stream-wise velocity and increasing vertical velocity at collision which indicates that the continuous saltation of large grain-size particles is controlled by collision parameters such as particle incident angle, local bed packing arrangement, and particle density, etc

Machinability and Optimization of Shrouded Centrifugal Impellers for Implantable Blood Pumps

This paper describes the use of analytical methods to determine machinable centrifugal impeller geometries and the use of computational fluid dynamics (CFD) for predicting the impeller performance. An analytical scheme is described to determine the machinable geometries for a shrouded centrifugal impeller with blades composed of equiangular spirals. The scheme is used to determine the maximum machinable blade angles for impellers with three to nine blades in a case study. Computational fluid dynamics is then used to analyze all the machinable geometries and determine the optimal blade number and angle based on measures of efficiency and rotor speed. The effect of tip width on rotor speed and efficiency is also examined. It is found that, for our case study, a six-or seven-bladed impeller with a low blade angle provides maximum efficiency and minimum rotor speed

Creating Real-Time Aeroacoustic Sound Effects Using Physically Informed Models

Aeroacoustics is a branch of engineering within fluid dynamics. It encompasses sounds generated by disturbances in air either by an airflow being disturbed by an object or an object moving through air. A number of fundamental sound sources exist depending on the geometry of the interacting objects and the characteristics of the flow. An example of a fundamental aeroacoustic sound source is the Aeolian tone, generated by vortex shedding as air flows around an object. A compact source model of this sound is informed from fluid dynamics principles, operating in real-time, and presenting highly relevant parameters to the user. A swinging sword, Aeolian harp, and propeller are behavior models are presented to illustrate how a taxonomy of real-time aeroacoustic sound synthesis can be achieved through physical informed modeling. Evaluation indicates that the resulting sounds are perceptually as believable as sounds produced by other synthesis methods, while objective evaluations reveal similarities and differences between our models, pre-recorded samples, and those generated by computationally complex offline methods

Numerical Simulation Of A Three Blade Marine Current Turbine

Renewable energy usually refers to those natural sources of energy which are possible to use without diminishing the resource and the current European target is to source 20% of its energy from renewable sources by 2020. The marine current turbine (MCT) is an exciting proposition for the extraction of renewable tidal and marine current power. It is gaining momentum as a viable technology and is currently the subject of much attention and research. However, the numerical predication of the power performance of a marine current turbine under a free surface is difficult due to its complex geometry, fluid-structural interactions and ever-changing free surface interface. In this paper, an immersed boundary method, first introduced by Peskin (1972) to simulate blood flow around the flexible leaflet of human heart, was used to couple the simulation of turbulent fluid flow with a solid using a three-dimensional finite volume in-house LES code, CgLes (Thomas and Williams (1997)). Extensive validation work was carried out to prove the reliability and accuracy of the coupled code which can be found in the work of Ji et al. (2012). A conservative level set method, proposed by Olsson and Kreiss (2005) was adopted to track the free surface and then validated respectively by various studies and a coupled simulation was proposed for marine current turbines operating under free surface waves. The power coefficients of a horizontal axis marine current turbine (MCT) with different rotating speeds are calculated and compared against experimental data. It is found that the method is in general agreement with published results and provides a promising potential for more extensive studies on MCT’s together with other applications

Influence of curvature distribution smoothing on the reduction of aerofoil self-noise

Purpose – The paper aims to investigate the influence of smooth curvature distributions on the self-noise of a low Reynolds number aerofoil and to unveil the flow mechanisms in the phenomenon. Design/methodology/approach – The paper performed Large Eddy Simulation (LES) approach to investigate the unsteady aerodynamic performance of both the original aerofoil E387 and the redesigned aerofoil A7 in a time-dependent study of boundary layer characteristics at Reynolds number 100,000 and Angle of Attack 4-degree. The aerofoil A7 is redesigned from E387 by removing the irregularities in the surface curvature distributions and keeping a nearly identical geometry. Flow vorticity magnitude of both aerofoils, along with the spectra of the vertical fluctuating velocity component and noise level, are analysed to demonstrate the bubble flapping process near the trailing edge and the vortex shedding phenomenon. Findings – The paper provides quantitative insights about how the flapping process of the laminar separation bubble within the boundary layer near the trailing edge affects the aerofoil self-noise. It is found that the aerofoil A7 with smooth curvature distributions presents a 10% smaller laminar separation bubble compared to the aerofoil E387 at Reynolds number 100,000 and Angle of Attack 4-degree. The LES results also suggests that curvature distribution smoothing leads to a 6.5% reduction in overall broadband noise level. Originality/value – This paper fulfils an identified need to reveal the unknown flow structure and the boundary layer characteristics that resulted in the self-noise reduction phenomenon yielded by curvature distribution smoothing

Investigation of the formation and evolution of over-tip shock waves in the pressure-driven tip leakage flow by time-resolved schlieren visualization

Time-resolved schlieren visualization and transonic wind tunnel are used to investigate tip leakage flows (TLFs) over several generic blade tip models. Focus is on the generation and evolution of the over-tip shock waves in the clearance region. A multi-cutoff superposition technique is developed to improve the schlieren system for better visualization. Unsteady flow structures, such as over-tip shock oscillation, shear-layer flapping, and vortex shedding, are revealed by Fourier analysis and dynamic mode decomposition. To predict the generation and decaying of over-tip shocks, a simplified model is proposed by analogizing the shock system to be an N-shaped sawtooth wave. The results show that (1) the proposed model is able to capture the main features of the generation and decaying of over-tip shock waves. The processes of shock generation, decaying, and fading-out are dominated by the mean background flow, the shock state, and the flow fluctuations, respectively. Adding extra coming flow fluctuations can be an efficient way to control the evolution of over-tip shock system. (2) The shock-oscillating frequency is kept the same with the shear-layer flapping, and shock waves with a given oscillating frequency range is constrained to a specific position range. This is termed the “lock-in effect,” which is also observed in TLFs over contoured blade tips. The non-uniformity generation and the nonlinear propagation of shock waves are responsible for this effect. Constrained by this effect, the evolution of over-tip shock waves is separated into four discrete phases. Thus, this effect can be applied for the control of TLFs