Modeling slender bodies with the method of regularized Stokeslets

The motion and flow generated by immersed structures in a fluid in the Stokes regime can be modeled with a variety of different numerical methods. The mathematical structure of the Stokes equations allows one to describe the flow around a three-dimensional object using only information regarding its geometry. This leads to computational techniques such as boundary integral methods or the method of regularized Stokeslets that discretize the surface of the immersed object in the flow. However, when the body in question is slender, a more computationally efficient alternative is to represent the flow by a one-dimensional discretization along the centerline of the object rather than a discretization of the boundary. Using an exact and an asymptotic solution describing the nontrivial three-dimensional fluid flow generated by a slender precessing spheroid, we present a careful analysis of the approximation of the flow using the method of regularized Stokeslets, where the regularized Stokeslets are placed along the centerline of the spheroid. Guidance is presented on how best to choose the numerical parameters within the method of regularized Stokeslets to minimize the error for a given application.     

High-definition PIV analysis on vortex shedding in the cylinder wake

A high-definition analysis based on flow topology is made on the vortex motions under natural and lock-on conditions in the near-wake region of a circular cylinder, where two-dimensional flow fields in the wake-transition regime are measured by a time-resolved PIV system. The Reynolds stress distributions are examined in view of the mean separation streamline and the trajectory ofthe vortex center. It is shown that, by the lock-on, the Reynolds stresses become stronger and their dispositions match well with the shortened wake bubble, indicating perfect synchronization of shedding to the oscillatory forcing flow in the near field, which causes increased lift and drag forces.     

Experimental investigation of the generation of large-amplitude internal solitary wave and its interaction with a submerged slender body

A principle of generating the nonlinear large-amplitude internal wave in a stratified fluid tank with large cross-section is proposed according to the‘jalousie’control mode.A new wave-maker based on the principle was manufactured and the experiments on the generation and evolution of internal solitary wave were conducted.Both the validity of the new device and applicability range of the KdV-type internal soliton theory were tested.Furthermore,a measurement technique of hydrodynamic load of internal waves was developed.By means of accurately measuring slight variations of internal wave forces exerted on a slender body in the tank,their interaction characteristics were determined.It is shown that through establishing the similarity between the model scale in the stratified fluid tank and the full scale in the numerical simulation the obtained measurement results of internal wave forces are confirmed to be correct.     

The effects of vortex breakdown on the static rolling aerodynamics of finned slender body

The static rolling aerodynamics of a finned slender body is numerically studied in this paper.Simulation results show a nonlinear uprising of the rolling moment when the angle of attack is greater than 20°in subsonic flows.Asymmetric vortex break down phenomenon on the"horizontal"rudders is found to be responsible for this phenomenon.By introducing the geometric-equivalent angle of attack and geometric-equivalent sweep angle,the cause of the nonlinear rolling moment characteristics can be explained by the delta wing vortex breakdown analysis.     

Visualization of the large-scale vortex structures in excited turbulent jets

The present work is a part of the more common research aimed at establishing the role of large-scale vortex structures in the mechanism of noise generation by subsonic turbulent jet. The work presents the results of photography and videography of fast non-stationary processes in a circular subsonic jet under lateral acoustic excitation by harmonical source located upstream in a stilling chamber. Jet velocity varied in the range of 40–200 m/s (M=0.12–0.6).     

Modification of the wake behind a circular cylinder by using synthetic jets

Experimental tests were conducted to control the flow around a cylinder by means of unsteady blowing (synthetic jet) through a single slot disposed on the wall of the model. The flow Reynolds number (based on the diameter of the model) wasR _D=10₅. The efficiency of the synthetic jet is quantified in terms of delaying separation and modifying the drag coefficient. The investigations were of three types: measurements of the mean pressure distribution, wall visualizations of the separation line position and measurements of the mean flow-field in the wake.     

Radar monitoring of a wake vortex: Electromagnetic reflection of wake turbulence in clear air

This article deals with X-band radar trial campaigns in 2006 and 2007 at Orly Airport, and in June 2008 at Paris-CDG Airport. An X-band Doppler radar has been deployed to assess short range (inferior to 2000 m) wake vortex monitoring capabilities in all weather conditions (dry and wet conditions). Recorded data have been correlated with electromagnetic and fluid mechanical models of wake turbulences for better and more accurate understanding of roll-up radar cross section (RCS) and Doppler signature.     

Simulating the dynamics of flexible bodies and vortex sheets

We present a numerical method for the dynamics of a flexible body in an inviscid flow with a free vortex sheet. The formulation is implicit with respect to body variables and explicit with respect to the free vortex sheet. We apply the method to a flexible foil driven periodically in a steady stream. We give numerical evidence that the method is stable and accurate for a relatively small computational cost. A continuous form of the vortex sheet regularization permits continuity of the flow across the body’s trailing edge. Nonlinear behavior arises gradually with respect to driving amplitude, and is attributed to the rolling-up of the vortex sheet. Flow quantities move across the body in traveling waves, and show large gradients at the body edges. We find that in the small-amplitude regime, the phase difference between heaving and pitching which maximizes trailing edge deflection also maximizes power output; the phase difference which minimizes trailing edge deflection maximizes efficiency.     

大气中固体燃烧等离子体与微波相互作用的实验研究

 设计制造了含特定组分的化学药剂，利用热力学方法对其在大气中燃烧所产生的等离子体的电子密度进行了理论计算，对该等离子体对2~15 GHz波段微波的透射衰减和反射进行了测试。测试结果表明，该等离子体对该波段微波具有宽波段强吸收和弱反射性能，波段内的平均吸收大于25 dB，反射信号几乎淹没在背景噪声中。并从理论上分析了带电子与中性粒子的造成这一现象的主要原因是碰撞吸收。     

大气中固体燃烧等离子体与微波相互作用的实验研究

设计制造了含特定组分的化学药剂,利用热力学方法对其在大气中燃烧所产生的等离子体的电子密度进行了理论计算,对该等离子体对2~15 GHz波段微波的透射衰减和反射进行了测试。测试结果表明,该等离子体对该波段微波具有宽波段强吸收和弱反射性能,波段内的平均吸收大于25 dB,反射信号几乎淹没在背景噪声中。并从理论上分析了带电子与中性粒子的造成这一现象的主要原因是碰撞吸收。     

Casimir-Polder interaction of atoms with magnetodielectric bodies

A general theory of the Casimir-Polder interaction of single atoms with dispersing and absorbing magnetodielectric bodies is presented, which is based on QED in linear, causal media. Both ground-state and excited atoms are considered. Whereas the Casimir-Polder force acting on a ground-state atom can conveniently be derived from a perturbative calculation of the atom-field coupling energy, an atom in an excited state is subject to transient force components that can only be fully understood by a dynamical treatment based on the body-assisted vacuum Lorentz force. The results show that the Casimir-Polder force can be influenced by the body-induced broadening and shifting of atomic transitions — an effect that is not accounted for within lowest-order perturbation theory. The theory is used to study the Casimir-Polder force of a ground-state atom placed within a magnetodielectric multilayer system, with special emphasis on thick and thin plates as well as a planar cavity consisting of two thick plates. It is shown how the competing attractive and repulsive force components related to the electric and magnetic properties of the medium, respectively, can — for sufficiently strong magnetic properties — lead to the formation of potential walls and wells.     

MeV ion jets from short-pulse-laser interaction with thin foils

Collimated jets of carbon and fluorine ions up to 5 MeV/nucleon ( approximately 100 MeV) are observed from the rear surface of thin foils irradiated with laser intensities of up to 5 x 10 (19)W/cm(2). The normally dominant proton acceleration could be surpressed by removing the hydrocarbon contaminants by resistive heating. This inhibits screening effects and permits effective energy transfer and acceleration of other ion species. The acceleration dynamics and the spatiotemporal distributions of the accelerating E fields at the rear surface of the target are inferred from the detailed spectra.     

Oscillatory modes of interaction of supersonic overexpanded jets with barriers

The self-oscillatory interaction of supersonic jets with barriers has mainly been studied for under-expanded jets. There are only a few experimental studies examining the case of overexpanded jets, with little computational work done in this direction. To fill this gap, we performed numerical simulations of overexpanded supersonic jets with barriers. The calculations were performed by the Godunov method on fine grids using parallel programming techniques. In the course of numerical simulations, the gasdynamic parameters of the jet and the geometric parameter of the barrier were varied. The barrier had the shape of a cylindrical cavity of depth l = (0 − 18)r _a , where r _a is the nozzle exit radius (the case l = 0 corresponds to a flat-end barrier). Based on the results of the numerical simulations, the conclusion on whether the self-oscillation process occurs was drawn and the dependence its characteristics (frequency and amplitude) on the governing gasdynamic and geometric parameters were obtained. Good agreement with experimental data on the fundamental tone frequency was demonstrated. A low-frequency oscillation mode was mostly realized. In this case, the jet experienced periodic suctions into and ejections from the cavity, counter the oncoming jet flow, with the formation of a structure consisting of three discontinuity surfaces (two shock waves and a separating surface contact).     

Large-eddy simulations and vortex structures of turbulent jets in crossflow

Using the method of large-eddy simulation, the 3-dimensional turbulent jets in crossflow with stream-wise and transverse arrangements of nozzle are simulated, emphasizing on the dynamical process of generation and evolution of vortex structures in these flows. The results show that the basic vortex structures in literatures, such as the counter-rotating vortex pair, leading-edge vortices, lee-side vortices, hanging vortices, kidney vortices and anti-kidney vortices, are not independent physical substances, but local structures of the basic vortex structure of turbulent jets in crossflow-the 3-D stretching vortex rings originating from the orifice of the nozzle, which is discovered in this study. Therefore, the most important large-scale structures of turbulent jets in crossflow are unified to the 3-D vortex rings which stretch and twist in stream-wise and swing in transverse directions. We also found that the shedding frequencies of vortex rings are much lower than the one corresponding to the appearance of leading-edge and lee-side vortices in the turbulent jets.     

Plasma jets driven by ultraintense-laser interaction with thin foils

Experimental evidence of plasma jets ejected from the rear side of thin solid targets irradiated by ultraintense (>10(19) W cm(-2)) laser pulses is presented. The jets, detected by transverse interferometric measurements with high spatial and temporal resolutions, show collimated expansion lasting for several hundreds of picoseconds and have substantially steep density gradients at their periphery. The role played by radiation pressure of the laser in the jet formation process is highlighted analytically and by extensive two-dimensional particle-in-cell simulations.     

Experimental dynamics of a vortex within a vortex

We report the experimental dynamics of a new two-dimensional (2D) fluid phenomenon that occurs when an intense, pointlike vortex is placed within a diffuse, circular vortex. Our observations, made using strongly magnetized electron columns to model the 2D fluid, support the analysis performed by Jin and Dubin.     

Effect of small total pulse on development of a wake behind the self-propelled bodies

Experimental studies on the wakes behind the self-propelled bodies of revolution have been analysed, and results of stereometric studies on the co-current flows behind autonomous streamlined and bluff bodies of revolution are presented for the times of their development corresponding to the coordinate system of a distant wake of 2400 and 1200 body diameters, correspondingly. Possible modelling of axisymmetric co-currents by the flows generated by a string at its pulse motion along the axis is examined. It is shown that supposed solutions based on the model of interferential development of wakes behind the self-propelled bodies of revolution can approximate both the known results of experimental studies of wakes behind the self-propelled bodies of revolution and the flows generated by corresponding pulse transportation of a string.