133,731 research outputs found
Acoustic properties of aircraft wake vortices
No full textThe noise generation by aircraft wake vortices has been studied numerically and experimentally. The numerical study revealed a relation between the circulation Γ, the vortex core size rc and the frequency fa of the peak level in the vortex noise spectra, fa ≈ Γ/(2π rc)2. The experimental data were obtained in measurements at airports applying phased microphone arrays. It has been revealed that sound sources are closely located to the vortex cores. The focused noise spectra of the wake vortices of all measured aircraft types are dominated by two maxima. The second maximum at f = 100 Hz is clearly caused by wake vortices. The origin of the first at 12 Hz has not been identified. Wake vortices were acoustically detected in 80 percent of the flyovers. Lowest detection rates were observed for the newer aircraft types Airbus 319, 320 and Boeing 737-800. A comparison of the wake trajectories obtained by phased microphone arrays and LIDAR revealed that the detection capability of the latter is superior
Aircraft Wake-Vortex Evolution in Ground Proximity: Analysis and Parameterization
No full textField measurement data of 282 wake vortex pairs and respective environmental conditions acquired at Frankfurt Airport by means of lidar, Sodar/RASS, and ultrasonic anemometer are used to analyze wake vortex behavior in ground proximity. Exceptional cases with strong rebounds caused by detached shear layers and obstacles are introduced and estimates of the time needed to clear the runway from wake vortices by advection are provided. The impact of turbulence and crosswind on wake vortex decay proves to be weak, whereas already light crosswind turns out to be sufficient to cause pronounced asymmetric rebound characteristics. Based on the analyses vortex decay and rebound characteristics are parameterized and implemented into the probabilistic two-phase aircraft wake-vortex model. Deterministic and probabilistic prediction skill of the enhanced vortex model are assessed. Comparison to wake predictions out of ground effect indicates that in ground effect (i) the rapid-decay phase progresses slower, (ii) wake vortex evolution can be predicted with improved accuracy, and (iii) fair prediction skill requires only limited environmental data
Wake structure and kinematics in the vortex ring state
No full textHigh-resolution computational simulations of the vortical wake of a rotor operating both near to and within the vortex ring state have been conducted using Brown's vorticity transport model. The nonlinear vortex kinematics of the wake is exposed using three-dimensional visualizations of the simulated flow field. To reveal the vortex dynamics that underpin the highly unsteady flow within the vortex ring state, a rotor with just one blade was modeled. This blade was decoupled aerodynamically from the surrounding velocity field so that it acted merely as a source of trailed vorticity. The investigation identified a significant change in the dominant dynamics of the wake as it swapped fromthe tubular form that is characteristic of hover or very lowspeed descent into the toroidal geometry of the vortex ring state. Initial vortex 'pairing' leads to rotation of vortex filaments away from their original attitude. This phenomenon plays an important role in regulating the downwash that the rotor can produce and thus in precipitating the onset of the vortex ring state. The considerable and persistent coherence of the vortical structure of the wake when in the vortex ring state is revealed, despite these disturbances, as are themechanisms that lead to both small-scale and large-scale wake breakdown events. Simulations show the balance between the vortex pairing and short-wave instability modes to be different in the vortex ring state at high descent speed, where the wake lies above the rotor, compared to in the vortex ring state at low descent speed when the wake lies predominantly below the rotor. This yields subtle differences to the kinematics and structure of the wake in the two cases
A new calculation of the wake of a flat plate
No full textA new method is presented for the calculation of the wake of a finite flat plate. The method is based upon the recent investigations of the boundary layer near the trailing edge, which led to the triple-deck structure. This multi-layered structure has now been extended to the "classical" wake, which in fact is the continuation of the lowest two layers of the triple-deck. With this new numerical formulation an accuracy of 10-3% can easily be achieved.
Far plasma wake of Titan from the RPWS observations: A case study
No full textThe Titan's plasma wake has been investigated using observations from the Radio and Plasma Wave Science (RPWS) instrument onboard the Cassini spacecraft during one Titan flyby on December 26, 2005. The Langmuir Probe and the wideband receiver suggest a strong asymmetry of the plasma wake, which is displaced from the ideal wake. Two distinct structures are identified inbound and outbound of the flyby with significantly different electron number densities (ne). The maximum electron number density reached 14 cm(-3) on the Saturn side, connected to the sunlit ionosphere, while on the opposite side of Saturn observations indicate a density smaller than 2 cm(-3). Other derived parameters of the Langmuir probe analysis suggest also a difference in plasma composition between the two structures, where heavy and light ions dominate the Saturn and anti- Saturn side respectively. The total ion outflow is estimated at 2-7 x 10(25) ions/s assuming a cylindrical geometry for the plasma wake
Ground-based and air-borne lidar for wake vortex detection and characterisation
No full textIn the last two years several ground based and airborne wake vortex campaigns have been performed with the DLR coherent Doppler Lidar. The objectives of those cam-paigns were measurements for comparison with the wake vortex prediction and monitoring system WSVBS, the measurement and description of wakes generated by the new Airbus A380 aircraft and a reference aircraft for the ICAO aircraft separa-tion, and the observation of the influence of different aircraft configurations on the vortex life time in the project AWIATOR
Direct numerical simulation of heat transfer from the stagnation region of a heated cylinder affected by an impinging wake
No full textCopyright © 2011 Cambridge University Press.The effect of an incoming wake on the flow around and heat transfer from the stagnation region of a circular cylinder was studied using direct numerical simulations (DNSs). Four simulations were carried out at a Reynolds number (based on free-stream velocity and cylinder diameter D) of Re = 13200: one two-dimensional (baseline) simulation and three three-dimensional simulations. The three-dimensional simulations comprised a baseline simulation with a uniform incoming velocity field, a simulation in which realistic wake data - generated in a separate precursor DNS - were introduced at the inflow plane and, finally, a simulation in which the turbulent fluctuations were removed from the incoming wake in order to study the effect of the mean velocity deficit on the heat transfer in the stagnation region. In the simulation with realistic wake data, the incoming wake still exhibited the characteristic meandering behaviour of a near-wake. When approaching the regions immediately above and below the stagnation line of the cylinder, the vortical structures from the wake were found to be significantly stretched by the strongly accelerating wall-parallel (circumferential) flow into elongated vortex tubes that became increasingly aligned with the direction of flow. As the elongated streamwise vortical structures impinge on the stagnation region, on one side they transport cool fluid towards the heated cylinder, while on the other side hot fluid is transported away from the cylinder towards the free stream, thereby increasing the heat transfer. The DNS results are compared with various semi-empirical correlations for predicting the augmentation of heat transfer due to free-stream turbulence.German Research Foundatio
Oration Delivered before the Philomathesian and Euzelian Societies : at Wake Forest Institute, July 4, 1835
No full textOration delivered before the Philomathesian and Euzelian Societies at Wake Forest Institute on July 4, 1835, by James C. Dockery
Individual Wake Vortex Separations: Capacity and Delay Impact on Single and Dual Dependent Runway Systems
No full textAirport capacity constraints and growing traffic demand in air transportation cause congestion and delay on the ground and in the air. Conservative wake turbulence separation minima in the approach phase guarantee a minimum of in-flight wake encounters of trailing aircraft. On the other hand, in many situations weather-based separation minima could support more efficient runway utilization. This work estimates delay reduction and capacity gains of DLR`s Wake Vortex Prediction and Monitoring System (WSVBS) through the application of reduced time-based approach separations for a single and dual dependent runway system. The system dynamically adjusts approach separations without compromising safety. Delay calculations are conducted with a dynamic runway queuing model, which is capable to process time varying approach separations. It provides the relevant delay data, where several representative operational scenarios enabled through the application of reduced separations on a single and dual dependent runway system are considered. Capacity profiles are created by processing WSVBS separation data with the actual traffic demand. The results give insight about possible delay reductions as well as related operational impact under given implementation assumptions. It is shown that the WSVBS provides efficiency gains on both runway systems, whereas operational requirements regarding the single runway system need to be taken into account applying reduced individual separations. Regarding the dual dependent runway system, an average hourly delay reduction up to 15 minutes maximum is revealed for the defined scenario setup
The downstream wake response of marine current energy converters operating in shallow tidal flows
No full textThis paper presents findings from an experimental study investigating the downstream wake response from marine current energy convertors operating in various degrees of vertical flow constraint. The paper investigates deep vertically unconstrained sites, mid-depth sites and there is a particular emphasis on shallow tidal stream sites. Shallow tidal resources could be utilised for the deployment of first generation farms. The nature of the downstream wake flow will be a critical factor when determining the farm layout and the wake length is heavily influenced by the flow depth or ratio of rotor diameter to flow depth. A porous actuator disk is used to model the marine current energy convertor and an Acoustic Doppler Velocimeter is used to map the downstream wake. Linear scaling of length ratios suggests mid depth sites of 30-50m will produce the shortest wake lengths and for deeper and shallower sites the wake length increases. It is hoped that these relationships between vertical flow constraint and wake length will help with the layout design of tidal stream farm
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