145,682 research outputs found
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
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
A 3D hybrid simulation study of the electromagnetic field distributions in the lunar wake
No full textAs a consequence of its lack of a thick atmosphere and an ionosphere,
the interaction of the solar wind with the Moon is characterized
by the direct impact of the solar wind on its sunward
hemisphere. This absorption effect produces a near-vacuum in the
wake immediately behind the Moon. The absence of a global magnetosphere
and the low electrical conductivity further leads to the
free passage of the interplanetary magnetic field (IMF) through the
lunar interior. This classic scenario of the solar wind�Moon interaction
was established by the very first plasma measurements in the
lunar environment made by the Explorer 35 spacecraft (Lyon et al.,
1967; Schubert and Lichtenstein, 1974). The wake region is gradually
filled in by the diffusion of solar wind protons into the zone of
density depletion. As described in the early analyses by Michel
(1968) and Whang and Ness (1970), the expansion of the solar wind
plasma into the wake is accompanied by rarefaction waves. At the
same time, the magnetometer experiment on Explorer 35 detected
the existence of a field reduction zone at the wake boundary surrounding
the central region, with magnetic field enhancement up
to a factor of 1.4 in comparison to the value in the ambient solar
wind (Colburn et al., 1967). Some of these features were repeatedly
observed at different distances in the lunar wake by the Win
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
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
Dispersive to nondispersive transition in the plane wake and channel flows
No full textBy varying the wavenumber over a large and finely discretized interval of values, we analyse the phase and group velocity of linear three-dimensional travelling waves both in the plane wake and channel flows to get the transition between dispersive and non-dispersive behaviour. The dispersion relation is computed from the Orr-Sommerfeld and Squire eigenvalue problem by observing the least stable mode, see figure 2, panels (a,b) and the comparison with [1, 2, 4–11, 15, 16]. The group velocity vg is also shown. The Reynolds number varies in the 20-100, 1000-8000 ranges for the wake and the channel flow, respectively, while we consider wavenumbers in the range 0.1-10. The wake basic flow consists of the first two orders of the Navier-Stokes matched asymptotic expansion described in [3, 13, 14]. At low wavenumbers we observe a dispersive behaviour where the phase speed and the group velocity substantially differ. The relevant perturbed solution is amenable to the typical solution belonging to the left branch of the eigenvalue spectrum, see the two examples shown in figure 1 (channel flow: Re = 6000; k = 1; wake Re = 100; k = 0:7). By rising the wave number value, we observe a sharp transition from the dispersive to the nondispersive regime. This transition is located at a critical wave number kd which is a function of the Reynolds number Re, the wave angle _, and the wake downstream observation point x0. Precisely, kd increases with Re and decreases with _ for the wake flow, while these trends are reversed for the channel flow, see tables 1,2. Beyond the wavenumber threshold, the observed least-stable mode belongs to the right branch of the spectrum. The asymptotic solutions in the dispersive region are wall modes for the channel flow , and in-wake modes for the wake flow. This means that, for both the flows, the dispersive behaviour is related to perturbations with high momentum variations (high vorticity) in correspondence to the base flow high-shear region. On the contrary, if k > kd the solutions are central modes for the channel case, and out-of-wake modes for the wake flow. In these cases, the disturbance has high variations outside the base flow high-shear region. To understand the physical mechanism of the dispersive-nondispersive transition we focused on time variation of the wave kinetic energy associated to the convective transport. Figure 2 (c,d) shows the convective term as a function of the wavenumber for the two least stable modes. We observe that the dispersive-nondisperive transition allows waves k > kd to keep the lowest possible temporal variation of kinetic energy, i.e. the lowest decay. This remains true also when all the other more stable modes are considered. In practice nondispersive waves maintain their convective energy with k
Numerical investigation of transitional supersonic axisymmetric wakes
No full textTransitional supersonic axisymmetric wakes are investigated by
conducting various numerical experiments. The main objective is to identify hydrodynamic instability mechanisms in the flow at M=2.46 for several Reynolds numbers, and relating these to coherent structures that are found from various visualization techniques. The premise for this approach is the assumption that flow instabilities lead to the formation of coherent structures. Three high-order accurate compressible codes were developed in cylindrical coordinates for this research: a spatial Navier-Stokes (N-S) code to conduct Direct Numerical Simulations (DNS), a linearized N-S code for linear stability investigations using axisymmetric basic states, and a temporal N-S code for performing local stability analyses. The ability of numerical simulations to deliberately exclude physical effects is exploited. This includes intentionally eliminating certain azimuthal/helical modes by employing DNS for various circumferential domain-sizes. With this approach, the impact of structures associated with certain modes on the global wake-behavior can be scrutinized. Complementary spatial and temporal calculations are carried out to investigate whether instabilities are of local or global nature. Circumstantial evidence is presented that absolutely unstable global modes within the recirculation region co-exist with convectively unstable shear-layer modes. The flow is found to be absolutely unstable with respect to modes k>0 for ReD>5,000 and with respect to the axisymmetric mode k=0 for ReD>100,000. It is concluded that azimuthal modes k=2 and k=4 are the dominant modes in the trailing wake, producing a four-lobe wake pattern. Two possible mechanisms responsible for the generation of longitudinal structures within the recirculation region are suggested
Christmas dinner hosted by Dorothy and Bection Macon for Wake Forest College students
No full textChristmas dinner hosted by Dorothy and Bection Macon for Wake Forest College students: Claude Britt, Bill Shelton, Billie Reynolds, Bill Horton, Don Wilfing, Leary R??d, Bob Savage, Kenneth Royal, Kenneth Fulgrum, Lee Royal, and Mark Alexander. Also in t
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
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