Spatial perturbation of a wing-tip vortex using pulsed span-wise jets

The separation distance required between transport aircraft to avoid wake vortices remains a limiting factor on airport capacity. The dissipation of the wake can be accelerated by perturbing co-operative instabilities between multiple pairs of vortices. This paper presents the results of a preliminary experimental investigation into the use of pulsed span-wise air jets in the wing tip to perturb a single tip vortex in the very near field. Velocity measurements were made using PIV and hot-wire anemometry. The results demonstrate that the vortex position can be modulated at frequencies up to 50 Hz and, as such, the method shows promise for forcing instability in multiple vortex wakes.     

Three-dimensional measurements of vortex breakdown

Leading-edge vortex formation and breakdown have been measured over a periodically plunging non-slender delta wing at a high angle of attack, using a three-dimensional particle-tracking method. A very rare type of vortex breakdown in the form of a double helix has been captured in the phase-averaged flow at a specific phase of the oscillation cycle.     

Influence of precession on velocity measurements in a strong laboratory vortex

A strong laboratory vortex is generated in a cylindrical cell using a rotating disk and stretched by pumping the fluid out through a hole in the centre of the top of the cell. The velocity field is measured by means of laser Doppler anemometry and Doppler ultrasonic anemometry which are both non intrusive methods. The vortex exhibits a slight precession which induces temporal fluctuations of the velocity at the measurement point. Due to the centrifugal force, the tracers concentrate in a tubular region around the vortex, leading to spatial variations of the measurement counting rate. Under these two effects, the probability density function (PDF) of the one point velocity exhibits a strong non-Gaussian behaviour. In order to access the details of the velocity profile of the vortex in its own system of reference, the influence of the vortex precession, of the spatial variations of the concentration in tracers and of the intrinsic measurement dispersion is investigated and a model is proposed. It allows to recover statistically the characteristics of the vortex and to deduce the trajectory of its centre from the instantaneous velocity profiles. Received: 5 August 1998/Accepted: 20 February 1999     

Reynolds number effects on leading edge vortex development on a waving wing

The waving wing experiment is a fully three-dimensional simplification of the flapping wing motion observed in nature. The spanwise velocity gradient and wing starting and stopping acceleration that exist on an insect-like flapping wing are generated by rotational motion of a finite span wing. The flow development around a waving wing at Reynolds number between 10,000 and 60,000 has been studied using flow visualization and high-speed PIV to capture the unsteady velocity field. Lift and drag forces have been measured over a range of angles of attack, and the lift curve shape was similar in all cases. A transient high-lift peak approximately 1.5 times the quasi-steady value occurred in the first chord length of travel, caused by the formation of a strong attached leading edge vortex. This vortex appears to develop and shed more quickly at lower Reynolds numbers. The circulation of the leading edge vortex has been measured and agrees well with force data.     

Axial static pressure measurements of water flow in a rectangular microchannel

An experimental investigation of water flow through an aluminum rectangular microchannel with a hydraulic diameter of 169 μm was conducted over a Reynolds number (based upon mean velocity and hydraulic diameter) range from 230 to 4,740. Pressure measurements were simultaneously acquired at eight different axial locations within the channel along with pressure measurements in the inlet and outlet ports. The 27 μm pressure taps were more densely packed near the channel entrance in order to study the developing flow region. The average Poiseuille number for laminar flows was 86.4, which is in excellent agreement with the theoretical value of 86.9. The average critical Reynolds number was found to be 2,370. The limited turbulent friction factor data were in good agreement with the Haaland equation. The inlet to the channel was not well rounded and pressure distributions near the channel entrance show a region of pressure recovery. Entrance length and some minor loss coefficient data were not in agreement with theory, but the cause of these deviations were primarily a function of the inlet geometry and pressure recovery in the microchannel rather than a microscale effect.     

Volumetric velocity measurements of vortex rings from inclined exits

Vortex rings were generated by driving pistons within circular cylinders of inner diameter D = 72.8 mm at a constant velocity U ₀ over a distance L = D. The Reynolds number, U ₀ L/(2ν), was 2500. The flow downstream of circular and inclined exits was examined using volumetric 3-component velocimetry (V3V). The circular exit yields a standard primary vortex ring that propagates downstream at a constant velocity and a lingering trailing ring of opposite sign associated with the stopping of the piston. By contrast, the inclined nozzle yields a much more complicated structure. The data suggest that a tilted primary vortex ring interacts with two trailing rings; one associated with the stopping of the piston, and the other associated with the asymmetry of the cylinder exit. The two trailing ring structures, which initially have circulation of opposite sign, intertwine and are distorted and drawn through the center of the primary ring. This behavior was observed for two inclination angles. Increased inclination was associated with stronger interactions between the primary and trailing vortices as well as earlier breakdown.     

Damage smoothing effects in a delocalized rate sensitivity model for metals

It has been long time established that application of damage delocalization method to softening constitutive models yields numerical results that are independent of the size of the finite element. However, the prediction of real-world large and small scale problems using the delocalization method remains in its infancy. One of the drawbacks encountered is that the predicted load versus displacement curve suddenly drops, as a result of excessive smoothing of the damage. The present paper studies this unwanted effect for a delocalized plasticity/damage model for metallic materials. We use some theoretical arguments to explain the failure of the delocalized model considered, following which a simple remedy is proposed to deal with it. Future works involve the numerical implementation of the new version of the delocalized model in order to assess its ability to reproduce real-world problems.     

Some three-dimensional effects during vortex generation at a straight edge

For vortices generated by an impulsively started flow about a straight sharp edge bounded by side-walls, one might expect the vortex-flow in the mid-plane to remain unaffected by the walls for a time. Experiments in water using rectangular nozzles with generally moderate width-to-height ratios showed that a flow was initiated from the walls into the vortex core and towards the mid-plane. This flow set in at the same time as the main flow began. The fastest mass transport took place near the junction between the edge and the walls. This water moved within the vortex axis with an initially constant velocity approximately a third of that of the main flow, independent of the width, the height and the edge-angle within a surprisingly large range of these parameters. A further feature of the wall-near flow is the appearance and growth of a region of vortex breakdown in the core near the wall. In the mid-plane a flow was initiated directed radially outwards from the centre of the vortex. This flow was also short lived, beginning both before the axis became significantly distorted, as well as before any noticeable axial velocity gradient near the mid-plane existed. This radial motion seems thus to be the most sensitive measure of the flow in the mid-plane becoming three-dimensional. During this time the forces associated with the axial and radial flow may be significant. Despite the abovementioned relatively fast secondary flow, the trajectory of the vortex-centre in the mid-plane seems unaffected.     

Thermal-transient effects on pressure measurements

A set of experiments were performed on a flush-mounted strain-gage pressure transducer to determine the nature of erroneous responses induced by two types of thermal transients. The thermal transients studied were a constant step increase in transducer-diaphragm surface temperature, and a constant step increase in diaphragm-surface heat flux. The responses of the transducer to changes in ambient temperature and to temperature gradients in the transducer housing at steady state were investigated also. Analytic models for the transducer were solved under the experimental conditions and compared with the experimental results for the first two transients. These models predicted the response of the transducer quite well for about the first 25 percent of the transient period. Beyond this short time, the transducer behavior was drastically different from the models.     

The effect of spatial wandering on experimental laser velocimeter measurements of the end-wall vortices in an axial-flow pump

In a high Reynolds number axial-flow pump, laser velocimeter (LV) measurements were made to study the size and structure of the end-wall vortex. The time mean measurements show that the core size of the end-wall vortex increased with decreasing tip clearance, which is contrary to existing theory. Observations of cavitation in the vortex showed that the flow was unsteady. The vortices emanating from the smaller clearances were observed to wander or meander spatially and to develop kinks more than the vortices emanating from the larger tip clearances. This observed unsteadiness has a significant effect on the time mean size and velocity distribution of the vortex as measured with the LV employing the field point measurement technique. In order to obtain an estimate of the true size and velocity distribution, computational experiments were conducted which modelled a periodically wandering vortex and the LV measurement process. The computational and experimental results show good agreement, including a broadened and reduced tangential velocity distribution. In this paper, the end-wall vortex LV measurements are presented, and the method of analyzing the vortex wandering is described.     

Investigation of the vortex induced unsteadiness of a separation bubble via time-resolved and scanning PIV measurements

A transitional separation bubble on the suction side of an SD7003 airfoil is considered. The transition process that forces the separated shear layer to reattach seems to be governed by Kelvin–Helmholtz instabilities. Large scale vortices are formed due to this mechanism at the downstream end of the bubble. These vortices possess a three-dimensional structure and detach from the recirculation region, while other vortices are formed within the bubble. This separation of the vortex is a highly unsteady process, which leads to a bubble flapping. The structure of these vortices and the flapping of the separation bubble due to these vortices are temporally and spatially analyzed at angles of attack from 4° to 8° and chord-length based Reynolds numbers Re _c = 20,000–60,000 using time-resolved PIV measurements in a 2D and a 3D set-up, i.e., stereo-scanning PIV measurements are done in the latter case. These measurements complete former studies at a Reynolds number of Re _c = 20,000. The results of the time-resolved PIV measurements in a single light-sheet show the influence of the angle of attack and the Reynolds number. The characteristic parameters of the separation bubble are analyzed focusing on the unsteadiness of the separation bubble, e.g., the varying size of the main recirculation region, which characterizes the bubble flapping, and the corresponding Strouhal number are investigated. Furthermore, the impact of the freestream turbulence is investigated by juxtaposing the current and former results. The stereo-scanning PIV measurements at Reynolds numbers up to 60,000 elucidate the three-dimensional character of the vortical structures, which evolve at the downstream end of the separation bubble. It is shown that the same typical structures are formed, e.g., the c-shape vortex and the screwdriver vortex at each Reynolds number and angle of attack investigated and the occurrence of these patterns in relation to Λ-structures is discussed. To evidence the impact of the freestream turbulence, these results are compared with findings of former measurements.     

Correction of hot-wire measurements in the near-wall region

 A detailed numerical investigation on the corrections needed by hot-wire velocity measurements in wall vicinity was performed. In the case of a perfectly conducting wall the computed results agreed well with available experimental data. At first, the numerical results for the case of an adiabatic wall contradicted previous experimental observations. However, a precise evaluation led to a better understanding of the entire problem. Received: 7 August 1998/Accepted: 7 December 1998     

Instrument effects on stress jump measurements

Stress jumps occur in fluids that possess a viscous contribution to their total stress. On cessation of shear, such a fluid will show an instantaneous loss of stress, while some of the stress will remain and subsequently decay in some manner, either exponential or otherwise. The results in this paper show that filtering, present in most rheometers, will eliminate the stress jump and not allow its measurement. The torque head inertia for a spring-type torque measuring system is found to greatly influence the measurements and a stress jump is not seen. In fact, the material's true stress relaxation behaviour is far from that measured. The force rebalance torque measuring system is found to accurately measure stress jumps.     

Base strain effects on force measurements

Force transducers directly interact with their environment. In this study, a force transducer is attached to the midpoint of a free-free beam and is used to measure the force on a rigid mass that vibrates with the beam. The ratio of force to mass times acceleration is measured for several different masses over a frequency range that includes the first four odd natural frequencies of the beam. Then the force transducer is mounted so that the beam's strain is isolated from the transducer. The tests are repeated. The results dramatically illustrate the effects of base strain on the force measurements. A simple theoretical model is developed that explains the vertical axis shift in the calibration curve. Paper was presented at SEM 1994 Spring Conference and Exhibits.     

Numerical study on the effects of a semi-free and non-uniform flexible filament in different vortex streets

The variable flexibility of a fish body is believed to play a significant role in improving swimming performance.To explore the effect of non-uniform flexibilit...     

Characterising shortwave instabilities on a vortex dipole

The paper presents a method for characterising shortwave instability on a vortex dipole. Such instabilities cause the initially straight vortex lines to deform in a sinuous manner. In order to quantify the phenomenon, it is necessary to (a) characterise the vortex dipole and (b) characterise instabilities developed on it. In this study, the vortex dipole characteristics were quantified by using a nonlinear least squares fit to a Lamb–Oseen vortex profile with the velocity field measured by means of particle image velocimetry. The instability was recorded by capturing images of hydrogen bubbles, which were used to mark the vortex centre line, with a CCD camera. It was characterised by applying a fast Fourier transform and seeking dominant wavenumber components and a representative amplitude based on a range of wavenumbers within a bandwidth of interest. The method was tested on simulated and real data. Using the simulated data, the shortwave instability growth rate was calculated with an uncertainty of better than 1% and the mean wavelength deduced with an uncertainty of 5.3%. Using real data, a constant initial growth rate was deduced in agreement with the established theory. Further work might improve the algorithms so that spatial variations in wavelength and growth rate can be determined.     

Coherent and turbulent process analysis of the effects of a longitudinal vortex on boundary layer detachment on a NACA0015 foil

In this paper, the influence of a single tip vortex on boundary layer detachment is studied. This study offers a preliminary approach in order to better understand the interaction between a propeller hub vortex and the rudder installed in its wake. This configuration belongs to the field of marine propulsion and encompasses such specific problem as cavitation inception, modification of propulsive performances and induced vibrations. To better understand the complex mechanisms due to propeller–rudder interactions it was decided to emphasize configurations where the hub vortex is generated by an elliptical 3-D foil and is located upstream of a 2-D NACA0015 foil at high incidences for a Reynolds number of 5×10⁵. The physical mechanisms were studied using Time Resolved Stereoscopic Particle Image Velocimetry (TR-SPIV) techniques. Particular attention was paid to the detachment at 25° incidence and a detailed cartography of the mean and turbulent properties of the wake is presented. Proper Orthogonal Decomposition (POD) analysis was applied in order to highlight the unsteady nature of the flow using phase averaging based on the first POD coefficients to characterize the turbulent and coherent process in the near wake of the rudder.