Vortex Flow Field Research Articles

Discrete-element modeling of shock compression of polycrystalline copper

Shock compression of polycrystalline copper was numerically investigated by use of a discrete-element model to highlight underlying heterogeneous and nonequilibrium processes at the grain level. The average diameter of model grains was 12 \ensuremath{\mu}m. Results show highly transient vortical flow fields and strong particle velocity dispersion that are consistent with the experimental results of Mescheryakov and his associates. Characteristic times for these phenomena were on the order of acoustic propagation times across the grains. The number of vortices increased with shock strength, but their size decreased almost inversely. Ejection of copper particles from the back free surface of the specimen was also observed. The cause of ejection is grain boundary cracking.

Flow Computations Around Delta Wings Using Unstructured Hybrid Grids

The capability of an unstructured hybrid grid method to compute the compressible Navier-Stokes equations is discussed for vortical flows over slender delta wings at low subsonic, high-alpha conditions as a basic study of off-design aerodynamics of high speed civil transport configurations. Computations of these flows require high resolutions of the boundary layers on the wing and of the vortical flow region above the wing. The unstructured hybrid grid, which is composed of prismatic grid in boundary layers and tetrahedral and pyramidal grids for the remaining region, is used to treat such viscous flows. The compressihle Navier-Stokes equations are solved on the hybrid grid by a cell-vertex, upwind finite volume method. The lower-upper-symmetric Gauss-Seidel implicit time integration scheme is used for the convergence acceleration to steady state. The numerical accuracy of the present method is discussed by comparing with the experimental data for the same configurations. It is demonstrated that the present hybrid grid method is efficient and accurate enough to predict the vortical flow fields over the delta wings

Research on the algorithm of binary image cross-correlation for unsteady flow field measurement

In the present paper, the algorithm of Binary Image Cross-Correlation (BICC) was developed to measure the unsteady flow field. A vortex flow field was used to test the algorithm by numerical simulation. The results show that BICC is an effective algorithm for particle identification from consecutive images, the accurate velocity vector field can be obtained. The real velocity field in a valve chamber was measured by BICC in this study. From the full-field velocity information, the pressure and vorticity fields were also extracted by post-processing.

A cyclone flow cell for the investigation of gas-diffusion electrodes

Electrochemical gas–liquid reactions can be efficiently carried out at porous gas diffusion electrodes (GDE). These electrodes are simultaneously in contact with a gas phase and a liquid phase. For the design and scale-up of electrochemical reactors based on these GDE their macrokinetic behaviour (i.e., the interaction of reaction and internal mass transport phenomena) must be investigated under well defined external mass transfer conditions and controlled wetting conditions. To meet these requirements, a novel cyclone cell has been designed in which two vortex flow fields are realised on either side of a horizontally positioned GDE. The external mass transfer coefficients in this cell are determined from limiting current measurements for the oxidation of Fe(CN6)4−.

斜め衝撃波がランプ型並行噴射方式の超音速混合流れ場へ与える影響についての研究

The structure of supersonic mixing flow field in ram/scramjet combustor was investigated numerically and experimentally. One type of parallel injection method with a ramp injector was selected and the effects of oblique shock impingement on the mixing layer generated from the ramp had been studied. The wedge angle of the shock-generator and the distance from the injection plane to the leading edge of it were varied parametrically. The pattern of shock waves and the flow pattern on the wall surface were visualized in the experiments. The numerical results of those flow pattern images had a good agreement with the results. Also, comparisons of static pressure on the wall surface showed good agreement qualitatively. Numerical results captured the structures of vortical flow field clearly. It was shown that the impingement of oblique shocks made streamwise vorticies strongly and the wedge angle of the shock-generator was more effective to enhance vorticity of the streamwise vorticies.

Bathtub Vortex Profile and Flow Field Model with Circulation.

A new flow field model around a bathtub vortex was proposed, which was based on an exact solution of the Navier-Stokes equations called an expanding vortex flow. Velocity and pressure distributions were numerically calculated assuming that the downward velocity near the vortex center is proportional to a distance from the free surface and that the axial distribution of circulation is uniform. Radial distribution of circulation was found to be almost constant at a distance from the vortex center under some conditions, which was similar to the experimental result. Gas core profile, i.e. gas core radius and gas core depth, was analytically calculated with the new flow model. Gas core radius was defined to be a radial position where radial distribution of pressure is half. Gas core depth was calculated from the Bernoulli's equation. Analytical gas core profiles showed good agreement with experimental results. Formation of the gas core and the stagnant region around the vortex center was analytically discussed.

A flux-limiting scheme for solution of the eikonal evolution equation

A finite-difference scheme is derived for numerical solution of the eikonal scalar-field front propagation equation. The numerical scheme is implemented for the description of the dynamics of a flame surface propagating through a vortical flow field. Attributes of the numerical scheme are its accuracy and numerical stability when large velocity fluctuations in the flow are present, and the absence of explicit artificial viscosity.

Surface Reflective Visualizations of Shock-Wave/Vortex Interactions Above a Delta Wing

The surface reflective visualization (SRV) system is used to investigate the vortical flowfield above a 65-deg sweep, sharp leading-edge, noncambered delta wing in a high subsonic flow. Configurations of M oo -α are considered that both do and do not exhibit vortex breakdown above the wing, with and without the presence of a terminating shock-wave system above the wing. The SRV system provides a new opportunity to investigate the behavior of vortex breakdown in the high subsonic regime as well as the spanwise distribution of the terminating shock-wave system above the wing. Two classes of vortex breakdown, asymmetric and symmetric, are observed.

97/04011 Methane combustion in multi-stratified vortical flow field

97/04011 Methane combustion in multi-stratified vortical flow field

Methane combustion in multi-stratified vortical flow field

A two-step reaction model is used to analyze the interaction between (1) a fuel-air multi-stratified vortical mixing layer and (2) strong pressure waves caused by rapid combustion of the mixed region. During the 1st stage induction period where no heat release occurs, a large-scale vortical structure evolves, generating a wide contact surface and extended mixing between fuel and air. During the ensuing 2nd stage exothermic reaction, the locally premixed fuel-air rapidly burns and produces strong pressure waves. The results show that: (1) the 2nd stage exothermic reaction starts at the core of each vortex where “reactivity index = (Y CH 4)(Y O 2) 2 exp(−Ze/T ∗” is high; (2) then the generated strong pressure waves promote the reaction at the outermost stratified region of each vortex; (3) spot-like flames formed in the premixed region propagate toward the surrounding unburned region, due to conventional defraglation mechanism; (4) on the other hand, delayed combustion starts in the braid region where mixing is highly inactive; (5) finally, after the consumption of premixed reactants, a diffusion flame with a low mass consumption rate prevails.

Numerical Simulation of Secondary Flows in Channels Driven by Applied Lorentz Forces

The effect of applied magnetohydrodynamic (Lorentz) forces on the flowfields in saltwater plane channel flow has been investigated by performing large-scale computer simulations. Direct numerical simulations of three-dimensional, time-dependent secondary flow of an electroconducting fluid driven by Lorentz forces caused by applied electromagnetic fields have been performed. The computational model consists of a two-dimensional doubly periodic configuration of magnet and electrode microtiles flush-mounted on the lower surface of the channel. The resulting flowfields for low Reynolds number simulations reveal that the applied downward Lorentz force formed over the area between the magnet/electrode pairs causes the velocity perturbations to be confined to a very thin layer in the wall-normal direction. Thus, the design objective of localizing the Lorentz force close to the wall is achieved. Lateral components of the Lorentz force also contribute significantly to the formation of the secondary vortical flowfield. Simulations were also carried out with a pulsatile Lorentz force.

Dynamics of small, spherical particles in vortical and stagnation point flow fields

The transport in vortical and stagnation point flow fields is analyzed for particles across the entire range of density ratios, based on the Maxey–Riley equation [Phys. Fluids 26, 883 (1983)] without history effects. For these elementary flow fields, the governing equations simplify substantially, so that analytical progress can be made towards quantifying ejection/entrapment trends and accumulation behavior. For a solid body vortex, the analysis shows that optimal ejection or entrapment occurs for all density ratios, as the difference between inward and outward forces reaches a maximum for intermediate values of the Stokes number. The optimal Stokes number value is provided as a function of the density ratio. Gravity is shown to shift accumulation regions, without affecting the entrapment or ejection rates. For a point vortex flow, the existence of up to three different regimes is demonstrated, which are characterized by different force balances and ejection rates. For this flow, optimal accumulation is demonstrated for intermediate Stokes numbers. The stagnation point flow gives rise to optimal accumulation for heavy particles, whereas light particles do not exhibit optimal behavior. The analysis furthermore indicates that nonvanishing density ratios give rise to a finite Stokes number regime in which the particle motion is oscillatory. Above and below this regime, the motion is overdamped.

Premixed flame propagation in a high-intensity, large-scale vortical flow

The propagation of a premixed flame through a large-scale vortical flow field is studied numerically by solving a front propagation equation governing the evolution of a scalar field whose zero-level surface defines the location of a self-propagating interface. The flame front is considered to propagate normal to itself with a constant speed, and the density variation across the flame is considered to be zero. Average burning rates are calculated for large velocity fluctuation intensities, as the formulation allows naturally for the formation of pockets of unburned gas downstream from the reaction front. The propagation rate of the corrugated flame front is found to vary periodically with time, with an average that varies linearly with velocity fluctuation intensity at large intensities. A mechanism is identified for the decreasing sensitivity of the average burning rate to increases in the fluctuation intensity, over an intermediate range of intensities, in the zero-viscosity limit.

Ignition and Flame Propagation in Methane-Air Multi-Stratified Vortical Flow Field

Abstract The objective of this study is to analyze the flame propagation under the interaction between fluid mechanics and chemical kinetics in a counter shear layer. First, a large-scale Kelvin-Helmholtz instability is triggered by initially-imposed linear perturbations to form a multi-stratified vortical layer. Second, mixed reactants in such a stratified vortex are ignited by local energy addition where flame propagation under the effect of instability is observed. Three different ignition locations are selected to examine combustion efficiency. The ignition in a vortex core shows a 1.7 times C02 productivity in comparison with the ignition in a braid region: The most advantageous ignition position for C02 productivity is the vortex core region where the mixture strength is high and the flammable mixture spreads rapidly. Once formed, a wrinkled diffusion flame propagates through the multi-stratified mixture, destroying the initially existing vortical structure. In addition, a Lewis number analysis in t...

Study on Sound Generating Mechanism Using Vibrational Energy and Sound Energy. 2nd Report. Relationship between Structural Intensity Vortex Flow and Sound Field.

Most of the sound generated by vibrating structures is structure-borne sound. The purpose of this study is to investigate the generating mechanism of the structure-borne sound using vibrational energy and sound energy. The analytical model is a rectangular plate which is one of the basic elements of structures. The plate is excited sinusoidally at one point on the plate and proportional damping is taken into consideration. The dynamic response of the plate is calculated using the finite element method to obtain vibrational energy and sound energy. This paper describes the relationship between sound energy generated by a vibrating plate and vibrational energy flow patterns at different excitation points. As a result, sound radiation efficiency varies under the condition that generates the vortex flow patterns of the vibrational energy, while it is constant under other conditions. Sound energy under the influence of the vibrational energy flow can be estimated from sound radiation efficiency and two kinds of energy ratios. The influence of the vibrational energy flow patterns on sound field can be expressed by the phase distribution of complex displacement on the vibrating plate.

Clear Water Scour at Circular Piers: a Model

The three-dimensional quasi-steady vortex flow field around circular piers in a quasi-equilibrium scour hole under a clear water regime has been investigated experimentally. The main characteristic features of the flow, to be modeled, are a relatively large secondary vortex flow within the scour hole and skewed velocity distributions along the circumference of the pier. The flow field has been divided into a number of parts according to the flow characteristics. The components of quasi-steady velocity for different parts have been theoretically expressed in the form of equations satisfying the continuity equation and the requirements of fitting an individual profile of the measured velocity components. Using the suitable functions, the equations of the velocity components derived for different parts have been combined and matched at the junctions of these parts to get a single equation for each velocity component. The measured data have been utilized to determine the coefficients and exponents used in these equations through curve fittings. The proposed flow model, which corresponds closely with the observations, can be utilized to simulate the flow field in prototype.

Three-dimensional vortex flow field around a circular cylinder in a quasi-equilibrium scour hole

The three-dimensional flow field around a circular cylinder in a quasi-equilibrium scour hole under a clear water regime has been investigated experimentally. The distribution patterns of tangential, radial and vertical components of velocity upstream and longitudinal component of velocity downstream of a cylinder have been presented in this paper. The flow simulation results show the existence of a helical motion of fluid around the cylinder.

Spatiotemporal chaos in a model of Rayleigh-Bénard convection.

We present a numerical investigation of mean-flow induced spiral turbulence in a generalized Swift-Hohenberg model in two dimensions. We show the existence of a spatiotemporal chaotic state comprised of a large number of rotating spirals in a large aspect ratio system. This state is not predicted by classical theory. We calculate the spatial correlation functions for the order parameter, vorticity, and mean flow field in order to characterize more quantitatively the spiral chaos state. Our simulations show that there is a power-law behavior in the temporal dynamics of spiral defect chaos. Our study suggests that this spiral defect state occurs for low Prandtl numbers and large aspect ratios. We use as global variables the spectra entropy and the magnitude of the vorticity to characterize the transition from the global parallel roll state to the localized spiral state. Unfortunately we cannot determine whether this transition is gradual or sharp.

Surface reflective visualization system study to vortical flow over delta wings

Evaluation of the surface reflective visualization (SRV) system is conducted using both experimental and numerical simulation techniques. Experimental measurements are made with a spherical five-hole probe utilizing a local look-up calibration algorithm. These data are used to determine the location of the primary vortex core as well as the spanwise integrated density gradient. The numerical simulation technique employs a fast ray tracing algorithm and schlieren system simulation to determine the integrated density gradient distribution over the surface of the wing. The initial numerical flow solution used in the simulation is generated via a computational code based on a finite volume discretization of the three-dimensional conservation law form of the Euler equations. Both the experimental and numerical validation procedures support initial interpretations of SRV images of the compressible vortical flowfield above the lee side of the delta wing. The experimental results of the probe explorations supported the geometric interpretation of the images. The numerical simulation demonstrated that the SRV technique can, indeed, be expected to visualize the embedded crossflow shock existing at transonic flow conditions. Nomenclature cr = root chord, 120 mm / = focal length / = illumination n = refractive index yle = local spanwise distance from root chord to leading edge Zh = length of integration path K = Gladstone-Dale constant

Shock wave/vortex interaction in a flow over 90-deg sharp corner

Two-dimensional unsteady Euler computation with adaptive mesh refinement (AMR) algorithm is performed to study the evolution process of a vortical flowfield generated near the corner. Due to the growth of the Kelvin-Helmholtz instability, a shear layer emanated from the corner rolls up to form discrete vortices.