The impact of the pitching motion on the structure of the vortical flow over a slender delta wing under sideslip angle

The primary purpose of this investigation is to observe the effect of the pitching motion on the vortical flow structure and bursting of leading-edge vortices over a delta wing under the sideslip angle, β using a dye visualization technique. In the current work, a delta wing with a sweep angle of Λ = 70° was oscillated in upstroke and downstroke direction to be able to discover the influence of pitching motion on the flow characteristics of the delta wing. The values of mean angles of attack were selected as α_m = 25° and α_m = 35°, and the sideslip angle was altered from β = 0 to 16°. The delta wing oscillated with the various periods of T_e = 5 s, 20 s, and 60 s, respectively. Amplitude of motion was adjusted as α_o =  ± 5°. It is found that the pitching motion of the delta wing under the sideslip angle β varies the location of the vortex bursting and vortical flow structure substantially.

     

Visualization of turbulent flow around a sphere at subcritical reynolds numbers

The shear layer evolution and turbulent structure of near-wake behind a sphere atRe= 11,000 and 5,300 were investigated using a smoke-wire visualization method. A laminar flow separation was found to occur near the equator. The smooth laminar shear layers appeared to be axisymmetrically stable to the downstream location of aboutx/d=1.0 atRe=11,000 andx/d= 1.7∼1.8 atRe=5,300, respectively. At Re=11,000, the vortex ring-shaped protrusions were observed with the onset of shear layer instability. Moreover, the transition from laminar to turbulence in the separated flow region occurred earlier at the hiher Reynolds number ofRe=11,000 than atRe=5,300. The PIV measurements in the streamwise and cross-sectional planes atRe=11,000 clearly revealed the turbulent structures of the sphere wake such as recirculating flow, shear layer instability, vortex roll-up, and small-scale turbulent eddies.     

Nature of the source of vortex sound flowing around a cylindrical profile

This paper is devoted to refining the nature of a vortex sound source and validly estimating the parameters of the region of source origination in a wake behind a cylindrical profile depending on the incoming flow velocity and profile diameter. Based on experimental measurements of the rms values of pressure pulsations on the surface and in the wake behind the profile and hydrodynamic laws for 2D fluid flows, the position of the origin of the vortex street in the wake and the size of the region where the street is still irregular are estimated. In this region, the street dimensions and pressure pulsation amplitudes change with distance from the profile. It is found that the maximum of the pressure pulsation in the wake approaches the profile surface in the range of Reynolds numbers (4.7 × 10³?1.5 × 10⁴); the amplitude of pressure pulsations on the profile and vortex sound intensity also increase. Based on the relationship between the source’s position and size and the width of the vortex street, as well as taking into account the decay of vortex circulation in the street with increasing distance to the profile, it is shown that the distance from the source to the surface of the profile should not exceed two gages. It is shown that an obstacle in the wake in the region of its irregularity causes a decrease in pressure pulsations on the profile and attenuation of emitted sound. Sound emission ceases completely when the obstacle comes in direct contact with the region of origination of the vortex street. Theoretical estimates satisfactorily agree with the measurement results.     

Flow visualization behind a streamwise oscillating cylinder and a stationary cylinder in tandem arrangement

Interference is investigated between a stationary cylinder wake and that of a downstream streamwise oscillating cylinder. Experiments were carried out in a water tunnel. A laser-induced fluorescence technique was used to visualize the flow structure behind two inline circular cylinders of identical diameterd. The downstream cylinder was forced to oscillate harmonically at the amplitude of 0.5d and the frequency ratiof _e f _s=1.8, wheref _e is the oscillation frequency of the downstream cylinder andf _s is the vortex shedding frequency from an isolated stationary cylinder. The investigation was conducted for the cylinder center-to-center spacingL/d=2.5 ∼ 4.5. Two flow regimes have been identified, i.e. the ‘single-cylinder shedding regime’ atL/d<-3.5 and the ‘two-cylinder shedding regime’ atL/d>3.5. At smallL/d, the upstream cylinder does not appear to shed vortices; vortices are symmetrically formed behind the downstream cylinder as a result of interactions between the shear layers separated from the upstream cylinder and the oscillation of the downstream cylinder. This is drastically different from that behind two stationary cylinders atL/d<-3.5, where vortices are shed alternately from the downstream cylinder only. AtL/d=4.5, both upstream and downstream cylinders shed vortices. This is true with or without the oscillation of the downstream cylinder. The flow structure is now totally different from that atL/d=3.5. The vortices are shed alternately from the upstream cylinder; a staggered spatial arrangement of vortices occurs behind the downstream cylinder.     

Visualization of flow past a square prism with cut-corners at the front-edge

Flow past a square prism with cut-corners at the front-edge is numerically and experimentally visualized to investigate a mechanism of drag reduction. An adaptive numerical scheme based on the vortex method is implemented for two values of the Reynolds number between 200 and 1,250, and the results are compared with experiments. Experimental visualization techniques include the hydrogen-bubble technique atRe=4,000 and the oil-flow technique atRe=10,000 for a global wake formation, and the aluminum-flake technique for transient flow at the early stage of motion atRe=1,250. A similar reattachment flow pattern is shown in a wide range of the Reynolds number between 200 and 10,000, which implies a possibility of the drag reduction in the Reynolds number being approximately lower than 8,000 unlike the previous findings.     

Three-dimensional array structures associated with Richtmyer-Meshkov and Rayleigh-Taylor instability

A boundary separating adjacent gas or liquid media is frequently unstable. Richtmyer-Meshkov and Rayleigh-Taylor instability cause the growth of intricate structures on such boundaries. All the lattice symmetries [rectangular (pmm2), square (p4mm), hexagonal (p6mm), and triangular (p3m1) lattices] which are of interest in connection with the instability of the surface of a fluid are studied for the first time. They are obtained from initial disturbances consisting of one (planar case, two-dimensional flow), two (rectangular cells), or three (hexagons and triangles) harmonic waves. It is shown that the dynamic system undergoes a transition during development from an initial, weakly disturbed state to a limiting or asymptotic stationary state (stationary point). The stability of these points (stationary states) is investigated. It is shown that the stationary states are stable toward large-scale disturbances both in the case of Richtmyer-Meshkov instability and in the case of Rayleigh-Taylor instability. It is discovered that the symmetry increases as the system evolves in certain cases. In one example the initial Richtmyer-Meshkov or Rayleigh-Taylor disturbance is a sum of two waves perpendicular to one another with equal wave numbers, but unequal amplitudes: a ₁(t=0)≠a ₂(t=0). Then, during evolution, the flow has p2 symmetry (rotation relative to the vertical axis by 180°), which goes over to p4 symmetry (rotation by 90°) at t→∞, since the amplitudes equalize in the stationary state: a ₁(t=∞)=a ₂(t=∞). It is shown that the hexagonal and triangular arrays are complementary. Upon time inversion (t→−t), “rephasing” occurs, and the bubbles of a hexagonal array transform into jets of a triangular array and vice versa. Zh. éksp. Teor. Fiz. 116, 908–939 (September 1999)     

205Tl NMR observation of vortex lattice formation in the high temperature superconductor Tl2Ba2CuO6

We report on the asymmetric field distribution of the triangular vortex lattice in the superconductor Tl₂Ba₂CuO₆ as observed by²⁰⁵Tl NMR atB _a =4.26 T. A penetration depth of =1700±100 Å is obtained.     

Free stream turbulence effect on the flow structure over the finite span straight wing

Flow visualization tests have been performed to examine the structure of the near-wall flow over a low-aspect ratio straight wing installed at various angles of attack a and chord Reynolds numberRe _c=U_∞c/ν=1.76×10⁵. The experiments were carried out at two free-stream turbulence levels, ε=0.1% and ε=1%, the latter one having been achieved using a baffling grid. To visualize the flow, termochromic cholesteric liquid crystals and digital processing of video images were used. At the low turbulence level and α=27°, a flow stall on the lee side of the wing was observed, with a pair of largescale vortices rotating in the wing plane. Simultaneously, no vortex structures were observed on the windward wing surface. It was found the flow patterns on either side of the wing significantly changed with increasing free-stream turbulence level. A separation bubble appeared near the leading edge on the lee side of the airfoil at ε=1%, and large-scale stationary longitudinal vortices originated over the wing windward surface. The number and sizes of the longitudinal structures were found to be dependent on the angle of attack.     

Flow measurement around a cycloidal propeller

Abstract  

An experimental investigation was conducted to study the flow around a cycloidal propeller. Flow fields were obtained using a particle image velocimetry system whose data acquisition was synchronized with the propeller’s angular position. The chord-based Reynolds number was Re _c = u _r c/υ = 1.4 × 10⁴, where u _r is the rotational velocity of the propeller and c is the chord length of the airfoil. Flow characteristics such as mean velocity, vorticity and the RMS value of velocity fluctuation were derived from the measurements. The results demonstrated the presence of a downwash around the propeller during the generation of lift. Detailed observations around each airfoil visualized distinct vortex shedding and reattaching flow at certain phase angles of the propeller.     

Vorticity waves in problems of hydrodynamic stability

The known problem of flow transition near a circular cylinder at Re = 40 from a symmetrical form to the Karman vortex street can be considered as the problem of vortex wave development and intensification. Development of three bundles of vortex waves of low intensity is observed in a wake of a cylinder; these bundles are easily visualized as the structures of relative vorticity $ \bar \Omega $ \bar \Omega = Ω(t ₁) − Ω(t ₀): difference of vorticity Ω at two time moments, t ₀ being fixed. In the field of $ \bar \Omega $ \bar \Omega the alternating structure of quadrupoles is characterized by linear parameter l = h/d: the ratio of the width of the central bundle of vortex waves to the distance between the centers of quadrupoles of a “single sign”. When l = 0.281 is achieved, which coincides with the value of the same parameter of a stable Karman street, the transition from symmetrical streamlining by viscous incompressible liquid to the vortex street occurs.     

柔性旋涡发生器对翼型前缘分离的自适应控制

采用边长为10 mm的三角形柔性和刚性旋涡发生器,安装在二维NACA0018翼型上翼面前缘不同弦长处,用于控制翼型前缘分离流动.实验在低速直流式风洞中进行,以翼型弦长为特征长度的Reynolds数Re=1.1×105,采用单丝热线风速仪测量尾流速度剖面.分别研究柔性和刚性两种材料的三角形旋涡发生器对翼型前缘分离的控制效果.实验结果表明,与刚性旋涡发生器相比,柔性旋涡发生器利用来流能量实现自适应控制,使剪切层下移,从而明显抑制前缘分离.      

Acoustic waves emitted by a vortex ring passing near a circular cylinder

Acoustic waves emitted by a vortex ring moving near a circular cylinder have been studied experimentally and theoretically. The vortex rings used in the experiments had a translational speed ν₀ in the range 26 ⪅ ν₀ ⪅ 58 m/s and a radius of about 4·7 mm comparable in size with the cylinder radius. The acoustic pressure signals were detected by four microphones in the far field, and analyzed by digital methods. The observed pressure p obeys the scaling law p ∞ ν₀³L⁻⁴, where L is the impact distance of the vortex path to the cylinder. The observed sound wave is of dipole radiation type, and the direction of the dipole axis rotates as the vortex position changes relative to the cylinder. The direction of the dipole axis is related to that of the normal to the plane of the vortex ring. The instantaneous resultant force exerted on the cylinder by the vortex motion has also been examined, and the magnitude and the direction determined experimentally as a function of time. The theory of vortex sound predicts that the wave profile is proportional to the second time derivative of the volume flux (of a hypothetical potential flow around the cylinder) through the vortex ring. The observed scaling law and dipole directivity of the pressure are in good agreement with the theoretical predictions. The pressure profiles are calculated by using the observed vortex motion. These profiles also agree well with the observed ones, confirming the validity of the theory.     

A high order Discontinuous Galerkin – Fourier incompressible 3D Navier–Stokes solver with rotating sliding meshes

We present the development of a sliding mesh capability for an unsteady high order (order ? 3) h/p Discontinuous Galerkin solver for the three-dimensional incompressible Navier–Stokes equations. A high order sliding mesh method is developed and implemented for flow simulation with relative rotational motion of an inner mesh with respect to an outer static mesh, through the use of curved boundary elements and mixed triangular–quadrilateral meshes.A second order stiffly stable method is used to discretise in time the Arbitrary Lagrangian–Eulerian form of the incompressible Navier–Stokes equations. Spatial discretisation is provided by the Symmetric Interior Penalty Galerkin formulation with modal basis functions in the x–y plane, allowing hanging nodes and sliding meshes without the requirement to use mortar type techniques. Spatial discretisation in the z-direction is provided by a purely spectral method that uses Fourier series and allows computation of spanwise periodic three-dimensional flows. The developed solver is shown to provide high order solutions, second order in time convergence rates and spectral convergence when solving the incompressible Navier–Stokes equations on meshes where fixed and rotating elements coexist.In addition, an exact implementation of the no-slip boundary condition is included for curved edges; circular arcs and NACA 4-digit airfoils, where analytic expressions for the geometry are used to compute the required metrics.The solver capabilities are tested for a number of two dimensional problems governed by the incompressible Navier–Stokes equations on static and rotating meshes: the Taylor vortex problem, a static and rotating symmetric NACA0015 airfoil and flows through three bladed cross-flow turbines. In addition, three dimensional flow solutions are demonstrated for a three bladed cross-flow turbine and a circular cylinder shadowed by a pitching NACA0012 airfoil.     

Viscous Interaction Between a Vortex Tube and a Rotating Spherical Particle

The transient advection of a cylindrical vortex tube in a viscous incompressible flow field and its interaction with a rotating/spinning spherical particle has been investigated numerically at Reynolds numbers in the range of 20≤ Re≤200 for angular velocities of 0≤Ω≤0.5. The effects of vortex parameters such as size, circulation strength and initial position relative to the particle, on the temporal behavior of the lift and drag forces are studied. Vortex‐sphere interactions bring about major changes in the flow field particularly when coupled with particle rotation. It is observed that the forces acting on the particle are significantly influenced during the time that the vortex core is in the vicinity of the particle. The extent of these local changes are about ±30% in the drag coefficient and about ±200% in the lift coefficient as compared to flow over a rotating solid sphere with no vortex interaction. It is also found that a vortex with core radius between one and two particle diameters creates the strongest temporal variations in the lift and drag coefficients. Furthermore, maximum lift variations occur for the vortex‐particle head on collision, while a vortex with an offset distance of about one diameter from the principal flow axis generates the maximum drag variations.     

Vortex flow patterns of a heaving foil

It is known that an oscillating foil can produce a thrust force through the generation of a reverse Kármán vortex street and this can be expected to be a new highly effective propulsion system. A heaving foil model was made and it was operated within a circulating water channel. The wake formation behind the heaving foil was visualized using PIV method and a dynamic thrust force was measured using a mini 6-axis force sensor based on force and moment detectors. We examined various conditions such as reduced frequency and amplitude in NACA 0010 profile. The vortical patterns in the wake were classified according to the wake mode and force data.     

Thed=2tJ model on a triangular lattice at large doping

We determine the spectra and interactions of quasiparticles in thed=2tJ model on a triangular lattice forJ2t and near 2/3 electron filling. We find coexistence of magnons and quasielectrons atJ=2t and a transition from repulsion among quasielectrons atJ<2t to attraction atJ>2t. The mathematical methods developed here involve graded cosets and are also applicable, in modified form, to the Fermi liquid regime of thetJ model on a square lattice.     

Thermal depolarisation current study of electron irradiated fluorinated ethylene propylene

Thermally stimulated current (TSC) studies have been reported in the co-polymer of tetrafluoro ethylene and hexafluoropropylene films. Depolarisation current peaks are obtained atα ₁,α ₂ andβ relaxation temperatures of the polymer and the detrapping process is explained on the basis of its molecular motion. A cross-over electron energy of 18 keV is observed where the nature of TSC spectra undergoes a remarkable change. This is explained in relation to the surface states in FEP. Five groups of trapping levels, 0.25±0.08, 0.57±0.10, 1.07±0.1, 1.3±0.25 and 2.3±0.4 eV are obtained.     

On vortex streets behind Taylor columns

Computer experiments were performed to explore the flow in the vicinity of a truncated normal flat plate in a rapidly rotating fluid. A Taylor column formed above the flat plate and the vortex shedding in the wake of the Taylor column closely resembled the vortex street behind the solid plate. This is probably the first observation of a Kármán vortex street behind a Taylor column in a computational study.     

Effects of mean flow convection,quadrupole sources and vortex shedding on airfoil overall sound pressure level

This paper presents a further analysis of results of airfoil self-noise prediction obtained in the previous work using large eddy simulation and acoustic analogy. The physical mechanisms responsible for airfoil noise generation in the aerodynamic flows analyzed are a combination of turbulent and laminar boundary layers, as well as vortex shedding (VS) originated due to trailing edge bluntness. The primary interest here consists of evaluating the effects of mean flow convection, quadrupole sources and vortex shedding tonal noise on the overall sound pressure level (OASPL) of a NACA0012 airfoil at low and moderate freestream Mach numbers. The overall sound pressure level is the measured quantity which eventually would be the main concern in terms of noise generation for aircraft and wind energy companies, and regulating agencies. The Reynolds number based on the airfoil chord is fixed at _Rec=408,000${\mathit{Re}}_c=408,000$ for all flow configurations studied. The results demonstrate that, for moderate Mach numbers, mean flow effects and quadrupole sources considerably increase OASPL and, therefore, should be taken into account in the acoustic prediction. For a low Mach number flow with vortex shedding, it is observed that OASPL is higher when laminar boundary layer separation is the VS driving mechanism compared to trailing edge bluntness.     

L 2 andL 3 subshell fluorescence and Coster-Kronig yields forZ=96 from the decay of249Cf

TheL ₂ andL ₃ subshell fluorescence and Coster-Kronig yields forZ=96 have been measured by theK-L x-ray coincidence method with high resolution Si(Li) and Ge(Li) x-ray detectors. The results ν₂=0.650±0.036, ω₂=0.552±0.032, ω₃=0.515±0.034, andf ₂₃=0.188±0.019 agree fairly well with theory. New measurements of theL x-ray singles spectra of²⁴⁴Cm and²³⁸Pu with a high resolution Si(Li) detector permitted reevaluation of the values of ω₂ andf ₂₃ forZ=92 and 94 reported by Byrneet al. The revised values atZ=92 are ω₂=0.529±0.035 andf ₂₃=0.23±0.12, and atZ=94, ω₂=0.523±0.023 andf ₂₃=0.24±0.08. These revised values are in much better agreement with theory and with current experiments.