136,871 research outputs found
Modification of three-dimensional transition in the wake of a rotationally oscillating cylinder
No full textA study of the flow past an oscillatory rotating cylinder has been conducted, where the frequency of oscillation has been matched to the natural frequency of the vortex street generated in the wake of a stationary cylinder, at Reynolds number 300. The focus is on the wake transition to three-dimensional flow and, in particular, the changes induced in this transition by the addition of the oscillatory rotation. Using Floquet stability analysis, it is found that the fine-scale three-dimensional mode that typically dominates the wake at a Reynolds number beyond that at the second transition to three-dimensional flow (referred to as mode B) is suppressed for amplitudes of rotation beyond a critical amplitude, in agreement with past studies. However, the rotation does not suppress the development of three-dimensionality completely, as other modes are discovered that would lead to three-dimensional flow. In particular, the longer-wavelength mode that leads the three-dimensional transition in the wake of a stationary cylinder (referred to as mode A) is left essentially unaffected at low amplitudes of rotation. At higher amplitudes of oscillation, mode A is also suppressed as the two-dimensional near wake changes in character from a single- to a double- row wake; however, another mode is predicted to render the flow three-dimensional, dubbed mode D (for double row). This mode has the same spatio-temporal symmetries as mode A
Aircraft Wake Vortex Deformation in Turbulent Atmosphere
No full textLarge-scale distortion of aircraft wake vortices appears to play a crucial role for aircraft safety during approach and landing. Vortex distortion is investigated based on large eddy simulations of wake vortex evolution in a turbulent atmosphere. A vortex identification method is developed that can be adapted to the vortex scales of interest. Based on the identified vortex center tracks, a statistics of vortex curvature radii is established. This statistics constitutes the basis for understanding, modeling, and exploiting mitigation effects of vortex distortion on wake vortex encounters
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
Wake Vortex Scenarios Simulation Package for Take-Off and Departure
No full textThe WakeScene-D software package (Wake Vortex Scenarios Simulation Package for Departure) has been developed for comprehensive airspace simulations of take-off and departure. WakeScene-D consists of modules that model traffic mix, aircraft trajectories, meteorological conditions, wake vortex evolution, and potential hazard area. The software package estimates the probability to encounter wake vortices in different traffic and crosswind scenarios using Monte Carlo simulation in a domain ranging from the runway to an altitude of 3000 ft above ground. A comparison to measured vortex tracks of about 10,000 departures from runway 25R of Frankfurt airport indicates good agreement of global wake vortex transport characteristics in ground proximity. The standard departure situation employing a two-minute aircraft separation is compared to scenarios with reduced departure separations and various crosswind conditions. Comprehensive sensitivity analyses have been conducted which are briefly recapitulated. Effects related to departure route combinations and wind direction sectors are reported in more detail. Finally, an advanced scenario with an asymmetric crosswind criterion is introduced
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.
Prediction of dynamic pairwise wake vortex separations for approach and landing
No full textDesign and performance of the Wake Vortex Prediction and Monitoring System WSVBS are described. The WSVBS has been developed to tactically increase airport capacity for approach and landing on single runways as well as closely-spaced parallel runways. It is thought to dynamically adjust aircraft separations dependent on weather conditions and the resulting wake vortex behavior without compro-mis>ing safety. Dedicated meteorological instrumentation and short-term numerical terminal weather prediction provide the input to the prediction of wake-vortex behavior and respective safety areas. LIDAR monitors the correctness of WSVBS predictions in the most critical gates at low altitude. The WSVBS is integrated in the arrival manager AMAN of DLR. Performance tests of the WSVBS have been accomplished at Frankfurt airport in winter 2006/07 and at Munich Airport in summer 2010. Aircraft separations for landings on single runways have been compared employing the concepts of either heavy-medium weight class combinations or dynamic pairwise separations where individual aircraft type pairings are considered. For the very conservative baseline setup of the WSVBS the potential capacity gains of dynamic pairwise operations for single runways appear to be very small. On the other hand, the consideration of individual aircraft types and their respective wake characteristics may almost double the fraction of time when radar separation could be applied
Development of a radar simulator for monitoring wake vortices in rainy weather
No full textA simulator for the evaluation of the radar signature of
raindrops within wake vortices is presented. Simulated
Doppler spectrum of raindrops within vortices let to think that it could be a potential criterion for identifying wake vortex hazard in rainy weather
The wake vortex prediction and monitoring system WSVBS Part I: Design
No full textThe design of the Wake Vortex Prediction and Monitoring System WSVBS is described with all its components and their interaction. The WSVBS has been developed to tactically increase airport capacity for approach and landing on closely-spaced parallel runways. It is thought to dynamically adjust aircraft separations dependent on weather conditions and the resulting wake vortex behaviour without compromising safety. Dedicated meteorological instrumentation and short-term numerical terminal weather prediction provide the input to the prediction of wake-vortex behaviour and respective safety areas. The prediction tools employ a number of conservative aircraft parameter combinations that represent the aircraft weight categories medium and heavy. Predictions of the time when all approach corridors along the final approach do not overlap with safety areas determine aircraft separations for follower aircraft of categories medium and heavy. As a safety net a LIDAR monitors the correctness of WSVBS predictions in the most critical gates at low altitude
Investigation of wind turbine flow and wake
No full textThis paper is dedicated to the investigation and analysis of wind turbine wake. An experimental work is undertaken in wind tunnel on a horizontal axis wind turbine model. The velocity field in the wake is measured using PIV with phase synchronization in order to relate velocity and vortices to the rotating blades. The tip vortices are investigated in successive azimuthal positions of the rotor. A specially developed algorithm based on the circulation maximum detects the positions of the vortex cores and permits to use conditional averaging technique. The analysis of obtained velocity fields enables to determine the vortex core diameter, the swirl velocity distribution and the vortex diffusion as functions of the vortex age. The quality of obtained results permits to use them as reference for the validation of numerical computations
- …
