Peak Tangential Velocity Research Articles

Nature of Wakelike and Jetlike Axial Tip Vortex Flows

The mechanisms responsible for the wakelike and jetlike axial flows of a tip vortex were investigated experimentally at Re = 3.07 x 10 5 . Both square and round tips were tested. Along the wing tip, a pocket of a higher- than-freestream, or jetlike, fluid was continually present, regardless of the tip condition and airfoil angle of attack α. Its maximum velocity increased with α and always exhibited a peak value around the trailing edge. In the near field, this jetlike fluid pocket was entrained by the shear layers and the wing wake, and it resulted in a wakelike axial flow for smaller α. For higher α, the jetlike fluid pocket was, however, surrounded by the shear layers that, in turn, protected it from the destructive effects of the wing wake and remained jetlike. The switchover angle at which the axial flow switched from wakelike to jetlike appeared around 7 deg, regardless of the tip condition, which also coincided with the angle at which the lift-to-drag ratio was a maximum. Finally, the round-tip-produced vortex was more concentrated and had a higher peak tangential velocity and core circulation but a smaller core radius compared with those of the square tip.

Simulated WSR-88D Velocity and Reflectivity Signatures of Numerically Modeled Tornadoes

Abstract Low-altitude radar reflectivity measurements of tornadoes sometimes reveal a donut-shaped signature (low-reflectivity eye surrounded by a high-reflectivity annulus) and at other times reveal a high-reflectivity knob associated with the tornado. The differences appear to be due to such factors as (i) the radar’s sampling resolution, (ii) the presence or absence of lofted debris and a low-reflectivity eye, (iii) whether measurements were made within the lowest few hundred meters where centrifuged hydrometeors and smaller debris particles were recycled back into the tornadic circulation, and (iv) the presence or absence of multiple vortices in the parent tornado. To explore the influences of some of these various factors on radar reflectivity and Doppler velocity signatures, a high-resolution tornado numerical model was used that incorporated the centrifuging of hydrometeors. A model reflectivity field was computed from the resulting concentration of hydrometeors. Then, the model reflectivity and velocity fields were scanned by a simulated Weather Surveillance Radar-1988 Doppler (WSR-88D) using both the legacy resolution and the new super-resolution sampling. Super-resolution reflectivity and Doppler velocity data are displayed at 0.5° instead of 1.0° azimuthal sampling intervals and reflectivity data are displayed at 0.25-km instead of 1.0-km range intervals. Since a mean Doppler velocity value is the reflectivity-weighted mean of the radial motion of all the radar scatterers within a radar beam, a nonuniform distribution of scatterers produces a different mean Doppler velocity value than does a uniform distribution of scatterers. Nonuniform reflectivities within the effective resolution volume of the radar beam can bias the indicated size and strength of the tornado’s core region within the radius of the peak tangential velocities. As shown in the simulation results, the Doppler-indicated radius of the peak wind underestimates the true radius and true peak tangential velocity when the effective beamwidth is less than the tornado’s core diameter and there is a weak-reflectivity eye at the center of the tornado. As the beam becomes significantly wider than the tornado’s core diameter with increasing range, the peaks of the Doppler velocity profiles continue to decrease in magnitude but overestimate the tornado’s true radius. With increasing range from the radar, the prominence of the weak-reflectivity eye at the center of the tornado is progressively lessened until it finally disappears. As to be expected, the Doppler velocity signatures and reflectivity eye signatures were more prominent and stronger with super-resolution sampling than those with legacy-resolution sampling.

Experimental Investigation of Plain- and Flapped-Wing Tip Vortex

Particle image velocimetry was used in a low-speed wind tunnel to investigate and characterize wing tip vortex structures. A rectangular wing of a subsonic wall interference model was used as a vortex generator in two different configurations: 1) plain wing and 2) flapped wing with the trailing-edge flap extended at 20 degrees. Vortex flow quantities and their dependence on angle of attack at Reynolds numbers of 32:8 � 103 and 43:8 � 103 were evaluated. Assessment of measured data reveals that the peak values of tangential velocities, vortex strength, and vorticities are directly proportional to the angle of attack. The vortex core radius value grows slowly as the angle of attack is increased. Both plain and flapped configurations showed similar trends. The peak tangential velocities and circulation almost doubled when the flapped configuration was used instead of the plain wing.

Effects of a Movable Tip Strake on Wake Vortex Structure

The impact of a tip-mounted strake, set at different deflection angles relative to the main-wing chord plane on the near-field wake vortex structure, and the associated lift-induced drag were investigated experimentally at Re = 191, 000. Force-balance aerodynamic forces were also obtained to supplement the wake vortex measurements. The results show that in addition to its substantial influence on the aerodynamic performance, the presence of the strake also led to an earlier completion of the rolling up of the tip vortex, compared with the baseline wing. The tip vortex had a reduced peak tangential velocity and core axial velocity but an increased core radius, compared with the baseline wing. These vortex core flow quantities increased with increasing strake angle δ. The core and total circulation increased above and remained below the baseline-wing value for δ > 5 deg and δ 5 deg.

Tip Vortex Control via a Tab Deflecting at Higher Harmonic Frequencies

The impact of a trailing-edge tab, actuated at different start times and higher harmonic frequencies, on the size and strength and the position of a tip vortex generated by an oscillating NACA 0015 wing was investigated. The results show that for all the tab control cases tested, the peak tangential velocity of the tip vortex was found to be a weak function of the tab motion, whereas the core radius and circulation were strongly dependent upon the tab deflection. The 2/revolutions higher harmonic tab control actuated during pitch up produced the largest reduction in the peak tangential velocity and the greatest increase in the core radius and also provided the most promising blade-vortex-interaction suppression. Similar control effectiveness was also observed for 3/revolutions higher harmonic tab control motion but to a lesser extent. The 4/revolutions higher harmonic tab control was, however, most effective during the pitch-down flow-reattachment process.

Effectiveness of Dynamically Deflected Tab Control of a Tip Vortex

The control of the tip vortex generated by an oscillating NACA 0015 wing via a dynamically deflected trailing-edge tab, actuated at 29 different start times, amplitudes, and ramp-up rates, was investigated. The upward tab deflection was found to be more effective in weakening the peak tangential velocity and vortex strength than the downward deflection, regardless of the start time and ramp-up rate. A pair of corotating vortices was persistently noticed during pitch-up. An earlier and faster tab deflection (actuated during pitch-up) led to a larger reduction in the core vortex flow quantities. The downward tab deflection was, however, more effective in displacing the vortex position and in diffusing the vortex. The later and slower the downward tab actuation, the larger the vortex displacement and diffusion that took place. The largest reduction in the vortex strength was obtained with a start time at the beginning of pitch-down. The effectiveness of the tab control was also evaluated via the hysteretic behavior of the vortex core flow quantity and the potential suppression of the blade-vortex-interaction noise.

Impact of short-span strip on oscillating-wing tip vortex

The impact of 12 spoiler–tab configurations, of different heights and widths, on the tip vortex generated by an oscillating NACA 0015 wing was investigated experimentally. For an oscillating wing equipped with a spoiler, the peak tangential velocity and core and total circulation were greatly reduced compared to a tab, regardless of its width, while the core radius remained largely unaffected with its center displaced vertically above the baseline wing. The most noticeable impact of a spoiler with a reduced height was its potential in alleviating the blade–vortex interaction (BVI) strength. Meanwhile, the largest favorable impact on the critical vortex flow parameters was achieved via a 25%-span spoiler–tab combination with a height of 5 and 2.5% chord, respectively. A contrary effect on the BVI suppression, especially during pitch-up, was, however, observed. The impact on the BVI can be improved by reducing the height of the spoiler at the expense of unfavorable change in the vortex strength and displacement.

Tip Vortex Control via an Active Trailing-Edge Tab

The effect of a movable trailing-edge tab on a tip vortex generated by an oscillating NACA 0015 wing was investigated at Re = 1.86 × × 10 5 by using a miniature triple hot-wire probe. Both upward and downward tab deflections, actuated with different start times and durations, were tested. The downward tab motion was most effective in displacing the vortex position. The earlier the actuation the larger the displacement. The upward tab motions were, however, found to be more effective in diffusing the tip vortex and rendered a significantly reduced peak tangential velocity and core circulation, compared to an uncontrolled baseline wing. The earlier and longer the upward tab actuations the lower the vortex core strength and peak tangential velocity. The vortex core radius was rather insensitive to the tab motion. The upward tab deflection, however, produced less induced drag compared to a downward deflection.

Oscillating-Wing Tip Vortex with Passive Short-Span Trailing-Edge Flaps

Tip vortex flow behind an oscillating NACA 0015 wing with passive short-span trailing-edge flaps was investigated experimentally at Re = 1.81 x 105. The spoiler was found to be very effective in diffusing the tip vortex, in terms of the reduction of peak tangential velocity ν θpeak and vortex strength, compared to a baseline wing, and also led to additional vertical upward displacement of the vortex center. No significant influence on the vortex size was observed, however. The spoiler-induced negative camber effects also translated into slightly reduced bound circulation, total lift, and lift-induced drag. For a tab, the dynamic increase-decrease in the vortex size and strength persisted and had values larger than for a baseline wing. No substantial change in ν θpeak , compared to that for a baseline wing, was noticed. The axial core velocity was always wakelike, and the vortex center was shifted relatively outboard and vertically downward below the baseline wing. The tab also led to an increase in the lift and drag forces. The symmetric flap provided a compromise in the critical vortex-flow quantities between the spoiler and tab cases.

Reynolds Stress Model in the Prediction of Confined Turbulent Swirling Flows

Strongly swirling vortex chamber flows are examined experimentally and numerically using the Reynolds stress model (RSM). The predictions are compared against the experimental data in terms of the pressure drop across the chamber, the axial and tangential velocity components, and the radial pressure profiles. The overall agreement between the measurements and the predictions is reasonable. The predictions provided by the numerical model show clearly the forced and free vortex modes of the tangential velocity profile. The reverse flow (or back flow) inside the core and near the outlet, known from experiments, is captured by the numerical simulations. The swirl number has been found to have a measurable impact on the flow features. The vortex core size is shown to contract with the swirl number which leads to higher pressure drop, higher peak tangential velocity, and deeper radial pressure profiles near the axis of rotation. The adequate agreement between the experimental data and the simulations using RSM turbulence model provides a valid tool to study further these industrially important swirling flows.

Effect of Winglet Dihedral on a Tip Vortex

The effect of winglet dihedral (-87.5 ≤ δ ≤ 87.5 deg) on the near-field tip vortex flow structure behind a swept and tapered NACA 0015 wing was investigated at Re=1.81 × 10 5 . The winglet dihedral led to a significantly reduced (increased) peak tangential velocity and vorticity (core radius) compared to a baseline wing. The values of core circulation (core axial velocity) could be below or above the baseline-wing value, depending on the winglet angle, and exhibited a local maximum (minimum) at δ=0 deg. The inner region of the tip vortex flow exhibited a self-similar behavior for a wing with and with no winglet. The lift-induced drag was always reduced by the addition of a winglet. The negative dihedral was more effective in reducing the lift-induced drag compared to the corresponding winglet of positive dihedral. A large discrepancy in the lift-induced drag was, however, observed between the wake integral method and the classical lifting-line theory.

Tip Vortex Behind a Wing Undergoing Deep-Stall Oscillation

The flow structure of a tip vortex in the near field of a NACA 0015 wing with an effective aspect ratio of 5.04 undergoing a deep-stall oscillation with a(t) = 18 deg + 6 deg sin ωt at Re = 1.86 × 10 5 was investigated. The wing oscillation imposed a strong discrepancy in contour shapes and magnitudes between the pitch-up and pitch-down phases of the oscillation cycle. The vortex was more organized and nearly axisymmetric for x/c > 0.5 during pitch-up than during pitch-down. The peak tangential velocity and vorticity and the strength and size of the vortex increased with a(t), except in the vicinity of α max , and had higher values during pitch-up than during pitch-down. The axial flow was always wake like and the velocity deficit decreased with α(t), while exhibiting a sharp increase and decrease on the upstroke and downstroke in the vicinity of α max , respectively. The tangential velocity decreased slightly with the downstream distance. The vortex size increased rather significantly with x/c during pitch-down while remaining virtually unchanged during pitch-up. The inner region of the vortex exhibited a self-similar structure, similar to that of a stationary wing. The induced drag increased with a(t) and had a local maximum at 20 deg during pitch-up.

An experimental study on hurricane mesovortices

Mesovortices in the eyewall region of a hurricane are intriguing elements of the hurricane engine. In-situ measurements of them are sparse, however, and our understanding of their overall role in the physics of a hurricane is incomplete. To further understand their dynamics an experimental apparatus using a homogeneous fluid (water) has been constructed to emulate the lower tropospheric flow of the hurricane eye/eyewall region.For experimental configurations possessing a central aspect ratio less than unity, a primary and secondary circulation similar to the in flow layer of an intense hurricane, and a similar radius-to-width ratio of the curvilinear shear layer bordering the eye and eyewall region, the flow supports two primary quasi-steady vortices and secondary intermittent vortices. The vortices form through Kelvin–Helmholtz instability of the curvilinear shear layer bordering the slowly upwelling fluid in the centre and the converging fluid from the periphery. The primary vortices are maintained by convergence of circulation from the periphery and merger of secondary vortices spawned along the shear layer.The horizontal flow field is measured using a particle image velocimeter. Despite the relatively strong secondary circulation through the parent vortex the horizontal flow is found to be approximately uniform in the direction parallel to the rotation axis. The peak tangential velocity is found to occur in the mesovortices and is roughly 50% greater than the parent vortex that supports them. The measurements provide insight into recent observations of excessive wind damage in landfalling storms and support the hypothesis that intense storms contain coherent vortex structures in the eyewall region with higher horizontal wind speeds locally than the parent hurricane.

Velocity distributions in a hydrocyclone separator

The internal three-dimensional flow field in a hydrocyclone was studied using laser velocimetry. Seven axial planes were investigated for three different inlet flow rates and three independent and different rejects rates. Results at each measurement plane showed that the measured tangential velocity profile behaves like a forced vortex at the region near the air core, and like a free vortex in the outer portion of the flow. The peak nondimensional tangential velocity decreases as the distance from the inlet region increases, however, the peak dimensional tangential velocity increases as the distance from the inlet region increases. The nondimensional peak tangential velocities are approximately equal for all of the flow rates. The magnitude of the tangential velocity increased in the inner forced vortex region as the rejects rate was increased. Backflows exist in the axial velocity profile near the inlet region, but these reversed flows disappear in the exit region. The dimensional vorticity is proportional to inlet flow rate and decreases with increasing rejects flow rates.

Effect of a Stick on the Flow Field in a Cyclone and the Pressure Drop Reduction Mechanism

Measurements of the field in a cyclone separator with and without a reducing pressure drop stick (Repds) showed that the Repds reduces the peak tangential velocity, the axial velocity gradient, and the radial gradients of static and total pressure and reverses the axial static pressure gradient. These changes reduce the energy consumed by the rotating kinetic energy, the internal friction, the turbulent kinetic energy, and the drag of the negative pressure difference. The results are used to discuss why the separation efficiency remains high while the pressure drop is reduced. The results also show that a 24% short flow occurs near the cyclone entrance. Analysis of the changes in the field and the pressure drop due to the thin stick shows that the Repds increases the pressure drop in the outer vortex zone and reduces the pressure drop in the inner vortex zone. Therefore the pressure drop reduction with the Repds is due to its wake vortex, which leads to the hypothesis that the pressure drop in...

Delayed onset of progressive dystonia following subacute 3-nitropropionic acid treatment in Cebus apella monkeys.

Delayed abnormal movements can be observed in patients with acute neurologic insult after a prolonged period of apparent neurologic stability. To reproduce such a secondary neurologic manifestation in primates, the present experiment investigated whether systemic administration of subacute 3-nitropropionic acid (3NP), a mitochondrial toxin, could induce abnormal movements that were delayed and progressive over time. Four Cebus apella monkeys received systemic 3NP injections until acute neurologic signs manifested. The monkeys were regularly video-recorded and rated for abnormal movements for up to 15 weeks after the cessation of 3NP treatment. Five to 6 weeks after the 3NP treatment, monkeys displayed a significant increase in dyskinesias compared with pretreatment conditions. Over time the chorea attenuated, whereas the dystonic movements increased in intensity and severity which was characterized by a delayed decrease of peak tangential velocity. The intensity of abnormal movements and extent of affected body regions observed in each monkey were consistent with the size of basal ganglia hypersignal as documented by T2 sequence on magnetic resonance imaging. Thus, more severe motor impairments were associated with large magnetic resonance image abnormalities. This novel primate model may be particularly useful for studying the structural changes underlying delayed and progressive manifestations of abnormal movements with the ultimate goal of facilitating the evaluation of novel therapeutic strategies.

Pointing in 3D space to remembered targets

The accuracy of visually guided pointing movements decreases with speed. We have shown that for movements to a visually defined remembered target, the variability of the final arm endpoint position does not depend on movement speed. We put forward a hypothesis that this observation can be explained by suggesting that movements directed at remembered targets are produced without ongoing corrections. In the present study, this hypothesis was tested for pointing movements in 3D space to kinesthetically defined remembered targets. Passive versus active acquisition of kinesthetic information was contrasted. Pointing errors, movement kinematics, and joint-angle coordination were analyzed. The movements were performed at a slow speed (average peak tangential velocity of about 1.2 m/s) and at a fast speed (2.7 m/s). No visual feedback was allowed during the target presentation or the movement. Variability in the final position of the arm endpoint did not increase with speed in either the active or the passive condition. Variability in the final values of the arm-orientation angles determining the position of the forearm and of the upper arm in space was also speed invariant. This invariance occurred despite the fact that angular velocities increased by a factor of two for all the angles involved. The speed-invariant variability supports the hypothesis that there is an absence of ongoing corrections for movements to remembered targets: in the case of a slower movement, where there is more time for movement correction, the final arm endpoint variability did not decrease. In contrast to variability in the final endpoint position, the variability in the peak tangential acceleration increased significantly with movement speed. This may imply that the nervous system adopts one of two strategies: either the final endpoint position is not encoded in terms of muscle torques or there is a special on-line mechanism that adjusts movement deceleration according to the muscle-torque variability at the initial stage of the movement. The final endpoint position was on average farther from the shoulder than the target. Constant radial-distance errors were speed dependent in both the active and the passive conditions. In the fast speed conditions, the radial distance overshoots of the targets increased. This increase in radial-distance overshoot with movement speed can be explained by the hypothesis that the final arm position is not predetermined in these experimental conditions, but is defined during the movement by a feedforward or feedback mechanism with an internal delay.

Riluzole Reduces Incidence of Abnormal Movements but Not Striatal Cell Death in a Primate Model of Progressive Striatal Degeneration

Riluzole has been shown recently to increase life expectancy in patients with amyotrophic lateral sclerosis. A number of experimental studies also suggest that this compound may be a neuroprotectant. We have investigated in baboons whether riluzole would protect striatal neurons from a prolonged 3-nitropropionic acid (3NP) treatment and ameliorate the associated motor symptoms. In animals receiving 3NP and the solvent of riluzole, 12 weeks of high-dose 3NP treatment resulted in the appearance of persistent leg dystonia and significant increases in the incidence of three categories of abnormal movements and in the dyskinesia index in the apomorphine test (0.5 mg/kg im). Quantitative assessment of these behavioral deficits using a video movement analysis system demonstrated a significant decrease in locomotor activity and peak tangential velocity in 3NP-treated animals compared to controls. Histological analysis showed the presence of severe, bilateral, striatal lesions, localized in both caudate and putamen. Cotreatment with riluzole (4 mg/kg ip, twice daily) significantly reduced the dyskinesia index (−35%,P<0.02) in the apomorphine test. In the quantitative behavioral analysis, riluzole significantly ameliorated the decrease in peak tangential velocity (P<0.02) but not the decrease in locomotor activity observed after 3NP. Comparative histological analysis of the two groups of treated animals did not demonstrate a clear neuroprotective effect of riluzole. The present study suggests that one potential therapeutic interest for riluzole in neurodegenerative disorders may reside in the reduction of motor symptoms associated with striatal lesions.

Effect of swirl on combustion in a short cylindrical chamber

Combustion of a swirling, stoichiometric, and homogeneous mixture of natural gas and air in a short cylindrical chamber has been studied experimentally and simulated numerically. Each mixture was given a steady-state swirling motion by a rotating roughened disc before being ignited at the center of the chamber. By using discs of differing roughness and by varying the disc speed, the intensities of swirl and turbulence could be varied independently so that the effects on combustion of mixture turbulence and swirl-induced buoyancy could be separately examined. Combustion rate and overall chamber heat transfer were inferred from chamber pressure-time records. High-speed schlieren photography showed the effect of swirl on the early flame kernel. With given swirling angular momentum, increased turbulence level always reduced burning duration and increased total heat transfer rate. With given turbulence level, increasing the swirl intensity from zero first decreased, then strongly increased, the burning duration. The swirling Reynolds number (based on chamber radius and peak tangential velocity) at which combustion duration was minimized was in the range 30,000–40,000. At high Reynolds number buoyancy forces appear to have a strongly inhibiting effect on flame propagation.

Perpendicular blade vortex interaction

The interaction between a streamwise vortex and a spanwise blade was studied in incompressible flow for blade-vortex separations between ±1/8 chord. Three-component velocity and turbulence measurements were made from 4 chord lengths upstream to 15 chord lengths downstream of the blade using miniature four-sensor hot-wire probes. The interaction of the vortex with the blade causes an almost immediate loss in vortex core circulation. Downstream of the blade the core becomes embedded in the blade wake and begins to grow rapidly. Core radius increases and peak tangential velocity decreases but circulation remains roughly constant. True turbulence levels within the core are much larger downstream than upstream of the blade. Outside of the core the interaction produces a substantial region of turbulent flow associated with the blade and vortex generator wakes. Overall, perpendicular blade vortex interaction substantially alters the flow and produces a much larger and more intense region of turbulent flow than that presented by the undisturbed vortex. These results have significant implications for the prediction of both impulsive and broadband helicopter noise.