Two-dimensional wake vortex physics in the stably stratified atmosphere

The effects of stable stratification on aircraft wake vortices are investigated by means of high-resolution two-dimensional simulations. The simulations elucidate that the vortices first decelerate and then accelerate their descent, where they largely conserve their circulation. However, for very stable stratification the tip vortices may rise again to the flight path. The underlying physical mechanisms are revealed by means of a point vortex method and are examined complementarily by balancing the impulse of the wake vortices. It is shown that the prominant effects, deceleration, detrainment and acceleration, are caused by the kinematic interaction of the vorticity generated by baroclinity and the primary vorticity. Furthermore, it is found that the impulse of the whole system, including the detrained secondary vorticity, is oscillating with the Brunt-Vaeisaelaefrequency which implies that the wingtip vortices themselves do not. Finally, a local shear-number is proposed which takes into account the interaction of primary and secondary vorticity and can describe the instantaneous tendency of wake vortices to accelerate or to decelerate. (orig.)30 refs.Available from TIB Hannover: RR 6341(119) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Wake vortex physics

By means of large-eddy simulations the decay of wingtip vortices and the distribution of the turbine exhaust under the influence of turbulence in a stratified atmosphere is discussed. It is distinguished between atmospheric turbulence and a random turbulent flow originating from the boundary-layer around the aircraft. The ambient turbulence is weak, anisotropic, and decaying with sizes of the most energetic eddies at the order of the separation between the two wingtip vortices. The boundary-layer turbulence in wrapped around the vortices with the maximum intensity at the core diameter. The atmospheric eddies disturb the parallel vortex tubes by advection. Once distorted at that scale, the vortex induced velocities amplify the disturbance according to Crow's instability. The vortex tubes break up after about 1.5 minutes and form rings; the continuous trail of exhaust is reorganized in a row of single puffs. Without any atmospheric turbulence, the wake does not experience a sinusoidal instability but the parallel vortex tubes approach and start to dissolve after 2 minutes when they touch. The dissolution is triggered by the small-scale turbulent friction owing to the boundary-layer turbulence of the aircraft. The exhaust trail remains aligned along the flight track. The numerical data are compared to in-situ measurements. (orig.)Available from TIB Hannover: RR 6341(96) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

The turbulent decay of trailing vortex pairs in stably stratified environments

The decay of trailing vortex pairs in thermally stably stratified environments is investigated by means of large eddy simulations. Results of in-situ measurements in the wakes of different aircraft are used to find appropriate intitializations for the simulation of wake turbulence in the quiescent atmosphere. Furthermore, cases with weak atmospheric turbulence are investigated. It is shown that the early development of the vortices is not affected by turbulence and develops almost identically as in 2D simulations. In a quiescent atmosphere the subsequent vortex decay is controlled by the interaction of short-wave disturbances, owing to the aircraft induced turbulence, and baroclinic vorticity, owing to stable stratification. As a consequence, vertical vorticity streaks between the vortices are induced which are substantially intensified by vortex stretching and finally lead to rapid turbulent wake-vortex decay. When in addition also atmospheric turbulence is present, the long-wave instability is dominantly promoted. For very strong stratification (Fr < 1) it is observed that wake vortices may rebound but lose most of their strength before reaching the flight level. Finally, the simulation results are compared to the predictive capabilities of Greene's approximate model. (orig.)32 refs.Available from TIB Hannover: RR 6341(134) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Short term prediction of the horizontal wind vector within a wake vortex warning system

A wake vortex warning system (WVWS) has been developed for Frankfurt airport. This airport has two parallel runways which are separated by 518 m, a distance too short to operate them independently because wake vortices may be advected to the adjacent runway. The objective of the WVWS is to enable operation with reduced separation between two aircraft approaching the parallel runways at appropriate wind conditions. The WVWS applies a statistical persistence model to predict the crosswind within a 20 minute period. One of the main problems identified in the old WVWS are discontinuities between successive forecasts. These forecast breakdowns were not acceptable to airtraffic controllers. At least part of the problem was related to the fact that the forecast was solely based on the prediction of crosswind. A new method is developed on the basis of 523 days of sonic anemometer measurements at Frankfurt airport. It is demonstrated that the prediction of the horizontal wind vector avoids these difficulties and significantly improves the system's performance. (orig.)15 refs.Available from TIB Hannover: RR 6341(138) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

The decay of wake vortices in the convective boundary layer

The decay of three wake vortex pairs of B-747 aircraft in a convectively driven atmospheric boundary layer is investigated by means of large-eddy simulations (LES). This situation is considered as being hazardous as the updraft velocities of a thermal may compensate the induced descent speed of the vortex pair resulting in vortices stalled in the flight path. The LES results, however, illustrate that (i) the primary rectilinear vortices are rapidly deformed on the scale of the alternating updraft and downdraft regions; (ii) parts of the vortices stay on flight level but are quickly eroded by the enhanced turbulence of an updraft; (iii) longest living sections of the vortices are found in regions of relatively calm downdraft flow which augments their descent. Strip theory calculations are used to illustrate the temporal and spatial development of lift and rolling moments experienced by a following medium weight class B-737 aircraft. Characteristics of the respective distributions are analysed. Initially, the maximum rolling moments slightly exceed the available roll control of the B-737. After 60 seconds the probability of rolling moments exceeding 50% of the roll control, a value which is considered as a threshold for acceptable rolling moments, has decreased to 1% of its initial probability. (orig.)19 refs.Available from TIB Hannover: RR 6341(133) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman