Showing papers on "Starting vortex published in 2007"

Flow control with plasma synthetic jet actuators

Abstract: This paper presents an experimental investigation of the characteristics of a plasma actuator design for flow control consisting of an annular electrode in quiescent and flat plate boundary layer flows. In quiescent flow, the circular plasma region produced on actuation was observed to generate a vertical zero-net mass flux (or synthetic) jet, hence the name plasma synthetic jet actuator, the characteristics of which were found to be affected by the actuator operation mode (steady or unsteady). Pulsed operation of the actuator results in the formation of a starting vortex ring that advects ahead of the jet and secondary vortex rings near the actuator surface due to the additional plasma-induced fluid entrainment in the boundary layer. By varying the actuator pulsing frequency, multiple vortex rings were created in the flowfield and the resulting vortex ring interactions were found to increase both the peak velocity and streamwise extent of the jet. The interaction of the actuator with a crossflow was observed to be similar to that seen in conventional or non zero-net mass flux jets with the plasma synthetic jet penetrating into the mean flow. As expected, the influence of the jet on the freestream was found to decrease with increasing mean velocity and the impact on displacement and momentum thickness values diminishes as well.

Direct numerical simulation of stenotic flows, Part 2 : Pulsatile flow.

Abstract: Direct numerical simulations (DNS) of stenotic flows under conditions of steady inlet flow were discussed in Part 1 of this study. DNS of pulsatile flow through the 75% stenosed tube (by area) employed for the computations in Part 1 is examined here. Analogous to the steady flow results, DNS predicts a laminar post-stenotic flow field in the case of pulsatile flow through the axisymmetric stenosis model, in contrast to previous experiments, in which intermittent disturbed flow regions and turbulent breakdown were observed in the downstream region. The introduction of a stenosis eccentricity, that was 5% of the main vessel diameter at the throat, resulted in periodic, localized transition to turbulence. Analysis in this study indicates that the early and mid-acceleration phases of the time period cycle were relatively stable, with no turbulent activity in the post-stenotic region. However, towards the end of acceleration, the starting vortex, formed earlier as the fluid accelerated through the stenosis at the beginning of acceleration, started to break up into elongated streamwise structures. These streamwise vortices broke down at peak flow, forming a turbulent spot in the post-stenotic region. In the early part of deceleration there was intense turbulent activity within this spot. Past the mid-deceleration phase, through to minimum flow, the inlet flow lost its momentum and the flow field began to relaminarize. The start of acceleration in the following cycle saw a recurrence of the entire process of a starting structure undergoing turbulent breakdown and subsequent relaminarization of the post-stenotic flow field. Peak wall shear stress (WSS) levels occurred at the stenosis throat, with the rest of the vessel experiencing much lower levels. Turbulent breakdown at peak flow resulted in a sharp amplification of instantaneous WSS magnitudes across the region corresponding to the turbulent spot, accompanied by large axial and circumferential fluctuations, even while ensemble-averaged axial shear stresses remained mostly low and negative. WSS levels dropped rapidly after the mid-deceleration phase, when the relaminarization process took over, and were almost identical to laminar, axisymmetric shear levels through most of the acceleration phase.

Theory of Concentrated Vortices: An Introduction

Abstract: Equations and laws of vortex motion.- Vortex filaments.- Models of vortex structures.- Stability and waves on columnar vortices.- Dynamics of vortex filaments.- Dynamics of two-dimensional vortex structures.- Experimental observation of concentrated vortices in vortex apparatus.

Mechanisms Influencing the Efficiency of Oscillating Airfoil Propulsion

Abstract: A NACA0012 airfoil undergoing pitching and plunging motion at Re = 20,000-40,000 was simulated using a two-dimensional Navier-Stokes flow solver. Results were compared with experimental measurements in the literature and those from an inviscid analytical method and an unsteady panel method code. Although the peak in propulsive efficiency with Strouhal number demonstrated in the experimental results was predicted by the inviscid methods, it was found to be significantly modified by leading-edge vortex shedding and viscous drag at low Strouhal numbers. The occurrence and influence of vortex shedding is controlled by both the motion of the airfoil (amplitudes and phases of plunging and pitching) and the flapping frequency, which limits the time available for vortex formation and convection over the airfoil surface. Thus, Strouhal number alone is insufficient to characterize the efficiency of flapping-foil propulsion.

Aerodynamics of a NACA4412 airfoil in ground effect

Abstract: The flow characteristics over a NACA4412 airfoil are studied in a low turbulence wind tunnel with moving ground simulation at a Reynolds number of 3.0 x 105 by varying the angle of attack from 0 to 10 deg and ground clearance of the trailing edge from 5% of chord to 100%. The pressure distribution on the airfoil surface was obtained, velocity survey over the surface was performed, wake region was explored, and lift and drag forces were measured. To ensure that the flow is 2-D, particle image velocimetry measurements were performed. A strong suction effect on the lower surface at an angle of attack of 0 deg at the smallest ground clearance caused laminar separation well ahead of the trailing edge. Interestingly, for this airfoil, a loss of upper surface suction was recorded as the airfoil approached the ground for all angles of attack. For angles up to 4 deg, the lift decreased with reducing ground clearance, whereas for higher angles, it increased due to a higher pressure on the lower surface. The drag was higher close to the ground for all angles investigated mainly due to the modification of the lower surface pressure distribution.

Cavitation Influence on von Kármán Vortex Shedding and Induced Hydrofoil Vibrations

Abstract: The present study deals with the shedding process of the von Karman vortices at the trailing edge of a 2D hydrofoil at high Reynolds number. This research focuses mainly on the effects of cavitation and fluid-structure interaction on the mechanism of the vortex generation. The vortex shedding frequency, derived from the flow-induced vibration measurement, is found to follow the Strouhal law provided that no hydrofoil resonance frequencies are excited, i.e., lock-off. For such a regime, the von Karman vortices exhibit strong spanwise 3D instabilities and the cavitation inception index is linearly dependent on the square root of the Reynolds number. In the case of resonance, the vortex shedding frequency is locked onto the hydrofoil eigenfrequency and the spatial coherence is enhanced with a quasi-2D shape. The measurements of the hydrofoil wall velocity amplitude and phase reveal the first torsion eigenmotion. In this case, the cavitation inception index is found to be significantly increased compared to lock-off conditions. It makes clear that the vortex roll-up is amplified by the phase locked vibrations of the trailing edge. For the cavitation inception index, a new correlation relationship that encompasses the entire range of Reynolds numbers, including both the lock-off and the lock-in cases, is proposed and validated. In contrast to the earlier models, the new correlation takes into account the trailing edge displacement velocity. In addition, it is found that the transverse velocity of the trailing edge increases the vortex strength linearly. This effect is important in the context of the fluid-structure interaction, since it implies that the velocity of the hydrofoil trailing edge increases the fluctuating forces on the body. It is also demonstrated that cavitation developing in the vortex street cannot be considered as a passive agent for the turbulent wake flow. In fact, for fully developed cavitation, the vortex shedding frequency increases up to 15%, which is accompanied by the increase of the vortex advection velocity and reduction of the streamwise vortex spacing. In addition, a significant increase of the vortex-induced vibration level is found at cavitation onset. These effects are addressed and thought to be a result of the increase of the vorticity by cavitation.

An inviscid model for vortex shedding from a deforming body

Abstract: An inviscid vortex sheet model is developed in order to study the unsteady separated flow past a two-dimensional deforming body which moves with a prescribed motion in an otherwise quiescent fluid. Following Jones (J Fluid Mech 496, 405-441, 2003) the flow is assumed to comprise of a bound vortex sheet attached to the body and two separate vortex sheets originating at the edges. The complex conjugate velocity potential is expressed explicitly in terms of the bound vortex sheet strength and the edge circulations through a boundary integral representation. It is shown that Kelvin's circulation theorem, along with the conditions of continuity of the normal velocity across the body and the boundedness of the velocity field, yields a coupled system of equations for the unknown bound vortex sheet strength and the edge circulations. A general numerical treatment is developed for the singular principal value integrals arising in the solution procedure. The model is validated against the results of Jones (J Fluid Mech 496, 405-441, 2003) for computations involving a rigid flat plate and is subsequently applied to the flapping foil experiments of Heathcote et al. (AIAA J, 42, 2196-2204, 2004) in order to predict the thrust coefficient. The utility of the model in simulating aquatic locomotion is also demonstrated, with vortex shedding suppressed at the leading edge of the swimming body.

Vortex lock-in phenomenon in the wake of a plunging airfoil

Abstract: The flow over a NACA0012 airfoil, oscillated sinusoidally in plunge, is simulated numerically using a two-dimensional Navier-Stokes solver at a Reynolds number of 20,000. The wake of the airfoil is visualized using a numerical particle tracing method for high reduced frequencies (1.0

Aerodynamics of Low Reynolds Number Plunging Airfoil under Gusty Environment

Abstract: It is known that plunging airfoil can produce both lift and thrust with certain combination of plunging amplitude and frequency. Motivated by our interest in micro air vehicles (MAVs), we utilize a NavierStokes equation solver to investigate the aerodynamics of a flapping airfoil. The roles of the plunging and pitching amplitude and frequency, and Strouhal number are studied. For a symmetric plunging airfoil NACA0012 at zero geometric angle of attack and chord Reynolds number of 2×10 4 , at the same plunging frequency, it can produce either drag or thrust depending on the plunging amplitude. At the considered plunging amplitude (from 0.0125c to 0.075c), the flow history has more influence than the kinematic angle of attack to determine the lift. When drag is produced, the viscous force dominates the total drag with decreasing influence as the plunging amplitude increases. For an airfoil experiencing combined plunge and pitch motion, both thrust and input power increase with the Strouhal number (within the range of 0.03 to 0.5). For the case studied, the thrust is induced by the lift, which approximately follows the curve of the kinematic angle of attack. Leading edge vortex moves downstream and interacts with the trailing edge vortex. We also study the impact of gust on stationary airfoil and flapping airfoil. Within the range of the parameters tested, for stationary airfoil the lift is in phase with the velocity but the drag is slightly out of phase. For flapping airfoil, neither lift nor drag is in phase with the velocity. Nomenclature CD =Drag coefficient per unit span CL =Lift coefficient per unit span CP =input power coefficient CP,mean =time-averaged input power coefficient CT =thrust coefficient CT,mean =time-averaged thrust coefficient c =Chord length

The formation and maintenance of a leading-edge vortex during the forward motion of an animal wing

Abstract: A simple model is presented to explain the observed generation of a quasi-steady vortex at the leading edge of an animal wing that rotates in a horizontal plane about a body-centred axis. Vorticity formed by separation at the leading edge is transported outwards by a spanwise velocity field generated by two sources of spanwise pressure gradient, one induced centrifugally and the other by variations in the vortex size and circulation. The vorticity is then deposited into a trailing vortex system that takes the form of a downward propagating vortex ring. This mechanism appears to apply generally to flying animals but is modelled here for those in hovering flight.

Aerodynamic Performance of a Corrugated Dragonfly Airfoil Compared with Smooth Airfoils at Low Reynolds Numbers

Abstract: An experimental study was conducted to investigate the flow behavior around a corrugated dragonfly airfoil compared with a traditional, streamlined airfoil and a flat plate. The experimental study was conducted at the chord Reynolds number of ReC =34,000, i.e., n the regime where Micro-Air-Vehicles (MAV) usually operate, to explore the potentialnapplications of such bio-inspired airfoils for MAV designs. The measurement results demonstrated clearly that the corrugated dragonfly airfoil has much better performance over the streamlined airfoil and the flat plate in preventing large-scale flow separation andnairfoil stall at the test low Reynolds number level. The detailed PIV measurements near thennoses of the airfoils elucidated underlying physics about why the corrugated dragonflynairfoil could suppress flow separation and airfoil stall at low Reynolds numbers: Instead of having laminar separation, the protruding corners of the corrugated dragonfly airfoil werennfound to be acting as lturbulatorsr to generate unsteady vortices to promote the transitionnnof the boundary layer from laminar to turbulent rapidly. The unsteady vortex structuresnntrapped in the valleys of the corrugated cross section could pump high-speed fluid fromnnoutside to near wall regions to provide sufficient energy for the boundary layer to overcome the adverse pressure gradient, thus, discourage flow separations and airfoil stall.

The merging of two co-rotating vortices: a numerical study

Abstract: The merging of two-dimensional co-rotating vortices is analysed through direct numerical simulations at large Reynolds numbers. It is shown how the Reynolds number affects each of the three phases that characterise this phenomenon. In the first phase, we examine the merging onset and focus on its definition. During the second rapid phase, the contributions of various flow regions upon the dynamics of a vortex are quantitatively studied. These regions are respectively the companion vortex, the filaments and an intermediate zone between vortices and filaments. The third phase is interpreted in terms of an advection diffusion process. Finally the final profile and circulation of the merged vortex is determined: the two thirds of the total circulation of the two initial vortices is contained in the newly formed vortex.

Quantum Turbulence in a Propagating Superfluid Vortex Front

Abstract: We present experimental, numerical, and theoretical studies of a vortex front propagating into a region of vortex-free flow of rotating superfluid 3He-B. We show that the nature of the front changes from laminar through quasiclassical turbulent to quantum turbulent with decreasing temperature. Our experiment provides the first direct measurement of the dissipation rate in turbulent vortex dynamics of 3He-B and demonstrates that the dissipation becomes mutual-friction independent with decreasing temperature, and it is strongly suppressed when the Kelvin-wave cascade on vortex lines is predicted to be involved in the turbulent energy transfer to smaller length scales.

A numerical study of the stabilitiy of helical vortices using vortex methods

Abstract: We present large-scale parallel direct numerical simulations using particle vortex methods of the instability of the helical vortices. We study the instability of a single helical vortex and find good agreement with inviscid theory. We outline equilibrium configurations for three double helical vortices—similar to those produced by three blade wind turbines. The simulations confirm the stability of the inviscid model, but predict a breakdown of the vortical system due to viscosity.

Passive scalar mixing in vortex rings

Abstract: Direct numerical simulation is used to study the mixing of a passive scalar by a vortex ring issuing from a nozzle into stationary fluid. The ‘formation number’ (Gharib et al. J. Fluid Mech. vol. 360, 1998, p. 121), is found to be 3.6. Simulations are performed for a range of stroke ratios (ratio of stroke length to nozzle exit diameter) encompassing the formation number, and the effect of stroke ratio on entrainment and mixing is examined. When the stroke ratio is greater than the formation number, the resulting vortex ring with trailing column of fluid is shown to be less effective at mixing and entrainment. As the ring forms, ambient fluid is entrained radially into the ring from the region outside the nozzle exit. This entrainment stops once the ring forms, and is absent in the trailing column. The rate of change of scalar-containing fluid is found to depend linearly on stroke ratio until the formation number is reached, and falls below the linear curve for stroke ratios greater than the formation number. This behaviour is explained by considering the entrainment to be a combination of that due to the leading vortex ring and that due to the trailing column. For stroke ratios less than the formation number, the trailing column is absent, and the size of the vortex ring increases with stroke ratio, resulting in increased mixing. For stroke ratios above the formation number, the leading vortex ring remains the same, and the length of the trailing column increases with stroke ratio. The overall entrainment decreases as a result.

Sphere-wall collisions: vortex dynamics and stability

Abstract: For moderate Reynolds numbers, a sphere colliding with a wall in the normal direction will lead to a trailing recirculating wake, threading over the sphere after impact and developing into a complex vortex-ring system as it interacts with vorticity generated at the wall The primary vortex ring, consisting of the vorticity from the wake of the sphere prior to impact, persists and convects, relatively slowly, outwards away from the sphere owing to the motion induced from its image The outward motion is arrested only a short distance from the axis because of the strong interaction with the secondary vorticity In this paper, the structure and evolution of this combined vortex system, consisting of a strong compact primary vortex ring surrounded by and interacting with the secondary vorticity, is quantified through a combined experimental and numerical study The Reynolds-number range investigated is (100 < Re < 2000) At Reynolds numbers higher than about 1000, a non-axisymmetric instability develops, leading to rapid distortion of the ring system The growth of the instability does not continue indefinitely, because of the dissipative nature of the flow system; it appears to reach a peak when the wake vorticity first forms a clean primary vortex ring A comparison of the wavelength, growth rate and perturbation fields predicted from both linear stability theory and direct simulations, together with theoretical predictions, indicates that the dominant physical mechanism for the observed non-axisymmetric instability is centrifugal in nature The maximum growth occurs at the edge of the primary vortex core, where the vorticity changes sign Notably, this is a physical mechanism different from that proposed previously to explain the development of the three-dimensional flow of an isolated vortex ring striking a wall

CFD prediction and model experiment on suction vortices in pump sump

Abstract: The sump size is being reduced in order to lower the construction costs of urban drainage pump stations in Japan. As a result of such size reductions, undesirable vortices such as air-entrained and submerged vortices are apt to appear in sumps because of the higher flow velocities. The Turbomachinery Society of Japan (TSJ) Standard S002:2005 states that the appearance of such visible vortices is not permissible for conventional sumps, and experiments with scale models usually have been done to assess the performance of sumps. Such tests, however, are expensive and time-consuming, and therefore, alternative computational fluid dynamics (CFD) methods for evaluating sump performance are desirable. The Research Committee on Pump Sump Model Testing, which is an organization in the TSJ, carried out a benchmark for flows in model sumps. They contributed commercial CFD codes such as “Virtual Fluid System 3D”, “Star-CD 3.22”, “Star-CD 3.26”, and “ANSYS CFX 10.0”. Some of the benchmark results were reported by Matsui, J. at the 23 rd IAHR Symposium in Yokohama, Oct 2006. The remaining results comprise this second paper. The calculated results were compared with experimental ones for flow patterns, locations of vortices, and their vorticity. In the experiments, the critical submergences for flow rates were minutely examined through visual observation with a video camera. The locations of the vortices were obtained by using the laser light sheet visualization method. The velocity and vorticity distribution in the sump were measured by using a PIV method. The following results were obtained. 1) The critical submergence for the air-entrained vortex is almost proportional to the flow rate in the sump. The vortex behavior is unsteady and the duration of the vortex varies greatly. 2) The submerged vortex appears accompanying the air-entrained vortex in the region of low submergences and high flow rates. The critical submergence for the submerged vortex is also proportional to the flow rate. 3) Some CFD codes can predict the visible vortex occurrence and its location for submergence and flow rate conditions with enough accuracy for industrial use. 4) The calculated velocity distribution at the bell entrance qualitatively agrees with the experimental results. However, the agreement is poor in terms of the magnitude and distribution patterns of the vorticity. This difference is caused by the lack of accuracy of the experiment and CFD computation. 5) Predicting the critical submergence for the visible vortices was not imposed in the benchmark. The calculated stream lines and vortex core lines are not able to be used to predict the visible vortices with much accuracy. An additional post-processing such as obtaining the vortex core static pressure and comparing it with ambient pressure for an air-entrained vortex or with the saturated vapor pressure of the water for a submerged vortex would be necessary to predict the visible vortices.

Dynamics and noise emission of vortex cavitation bubbles

Abstract: The growth and collapse of a cavitation bubble forming within the core of a line vortex was examined experimentally to determine how the dynamics and noise emission of the elongated cavitation bubble is influenced by the underlying non-cavitating vortex properties. A steady line vortex was formed downstream of a hydrofoil mounted in the test section of a recirculating water channel. A focused pulse of laser light was used to initiate a nucleus in the core of a vortex, allowing for the detailed examination of the growth, splitting and collapse of individual cavitation bubbles as they experience a reduction and recovery of the local static pressure. Images of single-bubble dynamics were captured with two pulse-synchronized high-speed video cameras. The shape and dynamics of single vortex cavitation bubbles are compared to the original vortex properties and the local static pressure in the vortex core, and an analysis was performed to understand the relationship between the non-cavitating vortex properties and the diameter of the elongated cavitation bubble. Acoustic emissions from the bubbles were detected during growing, splitting and collapse, revealing that the acoustic impulse created during collapse was four orders of magnitude higher than the noise emission due to growth and splitting. The dynamics and noise generation of the elongated bubbles are compared to that of spherical cavitation bubbles in quiescent flow. These data indicate that the core size and circulation are insufficient to scale the developed vortex cavitation. The non-cavitating vortex circulation and core size are not sufficient to scale the bubble dynamics, even though the single-phase pressure field is uniquely scaled by these parameters. A simple analytical model of the equilibrium state of the elongated cavitation bubble suggests that there are multiple possible equilibrium values of the elongated bubble radius, each with varying tangential velocities at the bubble interface. Thus, the details of the bubble dynamics and bubble-flow interactions will set the final bubble dimensions.

Effects of homogeneous condensation in compressible flows: Ludwieg-tube experiments and simulations

Abstract: Effects of homogeneous nucleation and subsequent droplet growth in compressible flows in humid nitrogen are investigated numerically and experimentally. A Ludwieg tube is employed to produce expansion flows. Corresponding to different configurations, three types of experiment are carried out in such a tube. First, the phase transition in a strong unsteady expansion wave is investigated to demonstrate the mutual interaction between the unsteady flow and the condensation process and also the formation of condensation-induced shock waves. The role of condensation-induced shocks in the gradual transition from a frozen initial structure to an equilibrium structure is explained. Second, the condensing flow in a slender supersonic nozzle G2 is considered. Particular attention is given to condensation-induced oscillations and to the transition from symmetrical mode-1 oscillations to asymmetrical mode-2 oscillations in a starting nozzle flow, as first observed by Adam & Schnerr. The transition is also found numerically, but the amplitude, frequency and transition time are not yet well predicted. Third, a sharp-edged obstacle is placed in the tube to generate a starting vortex. Condensation in the vortex is found. Owing to the release of latent heat of condensation, an increase in the pressure and temperature in the vortex core is observed. Condensation-induced shock waves are found, for a sufficiently high initial saturation ratio, which interact with the starting vortex, resulting in a very complex flow. As time proceeds, a subsonic or transonic free jet is formed downstream of the sharp-edged obstacle, which becomes oscillatory for a relatively high main-flow velocity and for a sufficiently high humidity.

Instantaneous Behavior of Streamwise Vortices for Turbulent Boundary Layer Separation Control

Abstract: The present study investigates turbulent boundary layer separation control by means of streamwise vortices with focus on the instantaneous vortex behavior. A turbulent boundary layer is exposed to ...

Boundary Layer Influence on the Unsteady Horseshoe Vortex Flow and Surface Heat Transfer

Abstract: The spatial-temporal flow field and associated surface heat transfer within the leading edge, end-wall region of a bluff body were examined using both particle image velocimetry and thermochromic liquid crystal temperature measurements. The horseshoe vortex system in the end-wall region is mechanistically linked to the upstream boundary layer unsteadiness. Hairpin vortex packets, associated with turbulent boundary layer bursting behavior, amalgamate with the horseshoe vortex resulting in unsteady strengthening and streamwise motion. The horseshoe vortex unsteadiness exhibits two different natural frequencies: one associated with the transient motion of the horseshoe vortex and the other with the transient surface heat transfer. Comparable unsteadiness occurs in the end-wall region of the more complex airfoil geometry of a linear turbine cascade. To directly compare the horseshoe vortex behavior around a turning airfoil to that of a simple bluff body, a length scale based on the maximum airfoil thickness is proposed.

A Reynolds stress closure description of separation control with vortex generators in a plane asymmetric diffuser

Abstract: A way to model the effects of streamwise vortices in a turbulent flow with one homogeneous direction is presented. The Reynolds averaged Navier-Stokes equations are solved with a differential Reynolds stress turbulence model. Assuming that the vortices can be approximated with the Lamb-Oseen model, wall-normal Reynolds stress distributions are calculated, corresponding to the spanwise variances of the estimated velocity distribution downstream of the vortex generators. The Reynolds stress contributions that are due to the vortex generators are added to the Reynolds stresses from the turbulence model so as to mimic the increased mixing due to the vortex generators. Volume forces are applied also in the mean momentum equations to account for the drag of the vortex generators. The model is tested and compared with experimental data from a plane asymmetric diffuser flow which is separating without vortex generators. The results indicate that the model is able to mimic the major features of vortex generator fl...

Wake Vortex Alleviation Using Rapidly Actuated Segmented Gurney Flaps

Abstract: A study to assess the potential for using rapidly actuated segmented Gurney flaps, also known as miniature trailing-edge effectors, for active wake vortex alleviation is conducted. Experiments are performed using a half-span model wing with NACA 0012 shape and an aspect ratio of 4.1. The wing is at an 8.9-deg angle of attack and the chord-based Reynolds number is around 350,000. It is equipped with an array of miniature trailing-edge effectors that extend 0.015 chord lengths perpendicular to the freestream on the pressure side of the wing when deployed and sit just behind the blunt trailing edge when retracted. Measurements of section lift coefficient and the velocity field in the intermediate wake using dynamic particle image velocimetry and a five-hole probe delineate the time-varying perturbation imparted by the miniature trailing-edge effectors upon the trailing vortex while keeping the lift nearly constant. The data are used to form a complete initial condition for a vortex filament computation of the far-wake evolution. Cases at the model-scale and full-scale configurations are computed

Structure of a stratified tilted vortex

Abstract: The structure of a columnar vortex in a stably stratified fluid is studied experimentally and theoretically when the vortex axis is slightly tilted with respect to the direction of stratification. When the Froude number of the vortex is larger than 1, we show that tilting induces strong density variations and an intense axial flow in a rim around the vortex. We demonstrate that these characteristics can be associated with a critical-point singularity of the correction of azimuthal wavenumber m = 1 generated by tilting where the angular velocity of the vortex equals the Brunt-Vaisala frequency of the stratified fluid. The theoretical structure obtained by smoothing this singularity using viscous effects (in a viscous critical-layer analysis) is compared to particle image velocimetry measurements of the axial velocity field and visualizations of the density field and a good agreement is demonstrated.

Helicity in the formation of turbulence

Abstract: Helicity in vortex structures and spectra is studied in the developmental stages of a numerical simulation of the Navier-Stokes equations using three-dimensional visualizations and spectra. First, time scales are set using the growth and decay of energy dissipation, the peak value of vorticity, and the helicity. Then, two stages between the early time, nearly inviscid Euler dynamics with vortex sheets and a final state of fully developed turbulence with vortex tubes, are described. In the first stage, helicity fluctuations develop in Fourier space during a period still dominated by vortex sheets and rapidly growing peak vorticity. At the end of this period the strongest structure consists of transverse vortex sheets with mixed signs of helicity. During the second stage, a dissipative interaction propagates along one of these vortices as the sheets roll each other into vortex tubes.

Modeling the flow past an oscillating airfoil by the method of viscous vortex domains

Abstract: The Lagrangian vortex method for solving the Navier-Stokes equations is applied for numerically modeling the unsteady flow past a wing airfoil executing angular oscillations in a viscous incompressible flow. Formulas relating the unsteady forces on the airfoil and the vorticity field are derived. The calculated results are compared with the experimental data for the NACA-0012 airfoil executing harmonic oscillations in an air flow at the Reynolds number Re = 4.4 × 104.

Numerical modeling of vortex/shock wave interaction and its transformation by localized energy deposition

Abstract: Three-dimensional unsteady Euler simulations are presented for the interaction of a streamwise vortex with an oblique shock of angle β = 23.3° at Mach 3 and 5. The flowfield features are analyzed for weak, moderate and strong interaction regimes. The details of the free recirculation zone at conditions of subsonic and supersonic flow on the vortex axis are considered. The vortex breakdown under conditions of a subsonic vortex core is characterized by a continuous growth and gradual degeneration of the region, unlike the supersonic core condition wherein a steady recirculation zone is achieved. The possibility of using a localized steady and pulsed periodic energy deposition on the vortex axis for stimulating the breakdown is demonstrated for various interaction regimes. It is shown that the formation of a subsonic wake downstream of an energy source lying on the vortex axis contributes to a more significant growth of the dimensions of the recirculation zone compared to the case when the vortex core remains supersonic. The possibility of achieving the effects similar to the steady case is demonstrated by the effect of a pulsed periodic energy source on the flow under consideration for corresponding equivalence parameters.

Plasma induced virtual turbine airfoil trailing edge extension

Abstract: A trailing edge vortex reducing system (11) includes a gas turbine engine airfoil (39) extending in a spanwise direction (SD), one or more spanwise extending plasma generators (2) in a trailing edge region (89) around a trailing edge (TE) of the airfoil (39) The plasma generators (2) may be mounted on an outer wall (26) of the airfoil (39) with first and second pluralities of the plasma generators (2) on pressure and suction sides (46, 48) of the airfoil (39) respectively The plasma generators (2) may include inner and outer electrodes (3, 4) separated by a dielectric material (5) disposed within a grooves (6) in an outer hot surface (54) of the outer wall (26) of the airfoil (39) The plasma generators (2) may be located at an aft end of the airfoil (39) and the inner electrodes (3) flush with the trailing edge base (34) A method for operating the system (11) includes energizing one or more of plasma generators (2) in steady state or unsteady modes