Vortex Descent Research Articles

Large-Eddy Simulation of Wake Vortices at Tokyo/Haneda International Airport

A large-eddy simulation (LES) of wake vortices in ground effect is conducted under the influence of airport hangar buildings to investigate the airport-specific behavior of wake vortices. We consider a landing at Tokyo/Haneda International Airport in Japan, where a wake of hangar buildings covers a landing path and touchdown zone under particular wind conditions. We combine an LES code and the Weather Research and Forecasting numerical weather prediction model to retrieve realistic wind conditions around the airport. The behavior of wake vortices is then investigated in the retrieved wind field using the adaptive mesh capability of the LES code for capturing the tiny vortex core of the wake vortex. At the crosswind on the order of a vortex descent speed, the hangar wake reduces the crosswind near the runway, enhancing the windward vortex’s rebound. The effect is alleviated by doubling the crosswind to twice the vortex descent speed. On the other hand, the wind fluctuations caused by the hangar buildings only slightly affect the wake vortex decay.

Inverted conical methane/air flame shape transformation under acoustic excitation

An inverted conical plane-symmetrical premixed methane–air flame under acoustic excitation was investigated experimentally. The flame was a conical one stabilized by a thin transverse rod. The Reynolds numbers were varied within 400–3000, and the fuel equivalence ratios were 0.8–1.4. Such a flame can take the shape of two fundamentally different types: M—when the flame front attaches both to the nozzle edge and stabilization rod; V—only to the stabilization rod. The transition conditions from one front configuration to another (M–V and V–M) were studied under various excitation parameters. Acoustic disturbances with a frequency of up to 420 Hz and an amplitude of longitudinal velocity pulsations up to 10% of the mean flow velocity excited the flame. New experimental data were obtained on the conditions corresponding to the M–V and V–M transitions. Qualitative and quantitative parameters of the hysteresis effect at which M–V and V–M transitions are realized, similar to the case of swirling flames, were revealed. The frequencies at which the acoustics most strongly affects the M–V and V–M transitions conditions were found. The hysteresis deterioration for M–V and V–M transitions was shown for the first time. It was noted that the conditions for the hysteresis deterioration are close to the theoretical frequency of the descent of vortices from the nozzle edge. The local maxima of the characteristic Strouhal numbers corresponding to the features of the minimum divergence of the flow velocities at the M–V and V–M transitions were determined.

Long-lived contrails and convective cirrus above the tropical tropopause

Abstract. This study has two objectives: (1) it characterizes contrails at very low temperatures and (2) it discusses convective cirrus in which the contrails occurred. (1) Long-lived contrails and cirrus from overshooting convection are investigated above the tropical tropopause at low temperatures down to −88 °C from measurements with the Russian high-altitude research aircraft M-55 Geophysica, as well as related observations during the SCOUT-O3 field experiment near Darwin, Australia, in 2005. A contrail was observed to persist below ice saturation at low temperatures and low turbulence in the stratosphere for nearly 1 h. The contrail occurred downwind of the decaying convective system Hector of 16 November 2005. The upper part of the contrail formed at 19 km altitude in the tropical lower stratosphere at ∼ 60 % relative humidity over ice at −82 °C. The ∼ 1 h lifetime is explained by engine water emissions, slightly enhanced humidity from Hector, low temperature, low turbulence, and possibly nitric acid hydrate formation. The long persistence suggests large contrail coverage in case of a potential future increase of air traffic in the lower stratosphere. (2) Cirrus observed above the strongly convective Hector cloud on 30 November 2005 was previously interpreted as cirrus from overshooting convection. Here we show that parts of the cirrus were caused by contrails or are mixtures of convective and contrail cirrus. The in situ data together with data from an upward-looking lidar on the German research aircraft Falcon, the CPOL radar near Darwin, and NOAA-AVHRR satellites provide a sufficiently complete picture to distinguish between contrail and convective cirrus parts. Plume positions are estimated based on measured or analyzed wind and parameterized wake vortex descent. Most of the non-volatile aerosol measured over Hector is traceable to aircraft emissions. Exhaust emission indices are derived from a self-match experiment of the Geophysica in the polar stratosphere in 2010. The number of ice particles in the contrails is less than 1 % of the number of non-volatile aerosol particles, possibly because of sublimation losses and undetected very small ice particles. The radar data show that the ice water content in convective overshoots is far higher than measured along the flight path. These findings add insight into overshooting convection and are of relevance with respect to hydration of the lower stratosphere.

Wind Impact on Single Vortices and Counterrotating Vortex Pairs in Ground Proximity

Wall-resolved large eddy simulations are employed to investigate the behaviour of wake vortices and single vortices in ground proximity at a variety of wind conditions. The six considered strengths of wind, ranging between 0.5 and 4 times the initial wake vortex descent speed, w 0, include practically and theoretically significant wind speeds. A crosswind of 0.5 w 0 may lead to windward stall posing a potential hazard to subsequently landing aircraft, whereas theoretical considerations predict that at 4 w 0 the rebound of the luff vortex is completely suppressed. The same range of wind speeds is also used to investigate the effects of headwind and diagonal wind in order to discriminate between effects of environmental turbulence increasing with wind speed and the direction of the wind shear. The study has been complemented by a number of single vortex computations in order to differentiate between effects related to the mutual interaction of the vortex pair and the individual vortices with the turbulent boundary layer flow. It is shown that vortex ascent, descent, rebound and decay characteristics are controlled by (i) the interaction of the vortices with secondary vorticity detaching from the ground, (ii) the redistribution of vorticity of the boundary layer which is altering the path of the primary vortices by mutual velocity induction, and (iii) the interaction of the vortices with the environmental turbulence.

Trace gas transport in the 1999/2000 Arctic winter: comparison of nudged GCM runs with observations

Abstract. We have compared satellite and balloon observations of methane (CH4) and hydrogen fluoride (HF) during the Arctic winter 1999/2000 with results from the MA-ECHAM4 middle atmospheric general circulation model (GCM). For this purpose, the meteorology in the model was nudged towards ECMWF analyses. This nudging technique is shown to work well for this middle atmospheric model, and offers good opportunities for the simulation of chemistry and transport processes. However, caution must be used inside the polar vortex, particularly late in the winter. The current study focuses on transport of HF and CH4, initialized with satellite measurements from the HALOE instrument aboard the UARS satellite. We have compared the model results with HALOE data and balloon measurements throughout the winter, and analyzed the uncertainties associated with tracer initialization, boundary conditions and the passive tracer assumption. This comparison shows that the model represents some aspects of the Arctic vortex well, including relatively small-scale features. However, while profiles outside the vortex match observations well, the model underestimates HF and overestimates CH4 concentrations inside the vortex, particularly in the middle stratosphere. This problem is also evident in a comparison of vortex descent rates based upon vortex average tracer profiles from MA-ECHAM4, and various observations. This could be due to an underestimate of diabatic subsidence in the model, or due to too much mixing between vortex and non-vortex air.

Probabilistic Two-Phase Wake Vortex Decay and Transport Model

A new parametric wake vortex transport and decay model is proposed that predicts probabilistic wake vortex behavior as a function of aircraft and environmental parameters in real time. The probabilistic two-phase wake vortex decay model (P2P) accounts for the effects of wind, turbulence, stable stratie cation, and ground proximity. The model equations are derived from the analytical solution of the spatiotemporal circulation evolution of the decaying potential vortex and are adapted to wake vortex behavior as observed in large-eddy simulations. Vortex decay progresses in two phases, a diffusion phase followed by rapid decay. Vortex descent is a nonlinear function of vortex strength. Probabilistic components account for deviations from deterministic vortex behavior inherently caused by the stochastic nature of turbulence, vortex instabilities, and deformations, as well as uncertainties and e uctuations that arise from environmental and aircraft parameters. The output of P2P consists of cone dence intervals for vortex position and strength. To assign a dee ned degree of probability to the predictions reliably, the model design allows for the continuous adjustment of decay parameters and uncertainty allowances, based on a growing amount of data. The application of a deterministic version of P2P to the Memphis wake vortex database yields favorable agreement with measurements.

Tracer‐based determination of vortex descent in the 1999/2000 Arctic winter

A detailed analysis of available in situ and remotely sensed N2O and CH4 data measured in the 1999/2000 winter Arctic vortex has been performed in order to quantify the temporal evolution of vortex descent. Differences in potential temperature (θ) among balloon and aircraft vertical profiles (an average of 19–23 K on a given N2O or CH4 isopleth) indicated significant vortex inhomogeneity in late fall as compared with late winter profiles. A composite fall vortex profile was constructed for 26 November 1999, whose error bars encompassed the observed variability. High‐latitude extravortex profiles measured in different years and seasons revealed substantial variability in N2O and CH4 on θ surfaces, but all were clearly distinguishable from the first vortex profiles measured in late fall 1999. From these extravortex‐vortex differences we inferred descent prior to 26 November: as much as 397 ± 15 K (1σ) at 30 ppbv N2O and 640 ppbv CH4, and falling to 28 ± 13 K above 200 ppbv N2O and 1280 ppbv CH4. Changes in θ were determined on five N2O and CH4 isopleths from 26 November through 12 March, and descent rates were calculated on each N2O isopleth for several time intervals. The maximum descent rates were seen between 26 November and 27 January: 0.82 ± 0.20 K/day averaged over 50–250 ppbv N2O. By late winter (26 February to 12 March), the average rate had decreased to 0.10 ± 0.25 K/day. Descent rates also decreased with increasing N2O; the winter average (26 November to 5 March) descent rate varied from 0.75 ± 0.10 K/day at 50 ppbv to 0.40 ± 0.11 K/day at 250 ppbv. Comparison of these results with observations and models of descent in prior years showed very good overall agreement. Two models of the 1999/2000 vortex descent, SLIMCAT and REPROBUS, despite θ offsets with respect to observed profiles of up to 20 K on most tracer isopleths, produced descent rates that agreed very favorably with the inferred rates from observation.

Correction to “Tracer‐based determination of vortex descent in the 1999/2000 Arctic winter” by J. B. Greenblatt et al.

Correction to “Tracer‐based determination of vortex descent in the 1999/2000 Arctic winter” by J. B. Greenblatt et al.

On the origin of polar vortex air

Forward and backward three‐dimensional stratospheric polar vortex trajectory computations between fall and spring for each of the years 1992–1993 through 1999–2000 in the Northern Hemisphere and the years 1992–1999 in the Southern Hemisphere have been carried out. We find that the forward and backward trajectories give very different pictures of polar vortex descent. The backward trajectories show a complex distribution of parcels in which one population originates in the upper stratosphere and mesosphere and experiences considerable descent in the polar regions, while the remaining parcels originate at lower altitudes of the middle and lower stratosphere and are mixed into the polar regions during vortex formation. The forward trajectory calculations do not show this second population since the parcels in the forward calculation originate within the protovortex. The amount of descent experienced by the first population shows little variability from year to year, while the computed descent and mixing of the remaining parcels show considerable interannual variability due to the varying polar meteorology. Spring methane values reconstructed from Halogen Occultation Experiment (HALOE) fall data and the back trajectories compare fairly well with typical HALOE spring values. These results imply that using a comparison between prewinter and postwinter tracer profiles to estimate the amount of descent over the fall‐to‐spring time period might result in unrealistically large values of descent, since the fall profiles are not representative of the same air as the spring profiles. Additional back trajectory calculations of descent in the lower stratosphere over 3‐month time periods from spring to midwinter show that very little mixing occurs at these times and altitudes in the Northern Hemisphere during winters when the vortex is undisturbed, and in the Southern Hemisphere for all years studied. Thus, for this shorter time period, tracking constant mixing ratios of tracers within the vortex should give reasonable estimates of descent amounts, except for years when there are meteorological disturbances in the Northern Hemisphere.

Numerical Study of Wake Vortex Decay and Descent in Homogeneous Atmospheric Turbulence

Numerical simulations are performed to isolate the effect of ambient turbulence on the wake vortex decay rate within a neutrally stratified atmosphere. Simulations are conducted for a range of turbulence intensities by injecting wake vortex pairs into an approximately homogeneous and isotropic turbulence field. Consistent with field observations, the decay rate of the vortex circulation increases clearly with increasing levels of ambient turbulence. Based on the results from the numerical simulations, simple decay models for the vortex pair are proposed as functions of nondimensional ambient turbulence intensity, nondimensional radial distance, and nondimensional time. For strong atmospheric turbulence, the model predictions are in reasonable agreement with the observational data. For weak turbulence with stable stratification, the model, based on turbulence dissipation alone, underestimates circulation decay with consistent overestimation of vortex descent, unless stratification effects are included

Canopy Deposit and Off-Target Drift in Forestry Aerial Spraying: The Effects of Operational Parameters

ABSTRACT A Markov chain process type of Lagrangian model is used to predict the deposition and drift loss of pesticide droplets from aerial spraying of a coniferous forest. The model includes (a) the effects of atomizer droplet spectrum, (b) droplet evaporation, (c) atmospheric turbulence, (d) inertial impaction of droplets on foliage, and (e) forest type characteristics. Variations in atmospheric stability class are also included. Agreement of theory with field trial results is good provided the effective release height'' of droplets from the aircraft wake vortex is properly chosen. For a typical forestry spray application, the model prediction (neutral stability) gave a 10.8% (mass) drift of the active ingredient past 400 m downwind of the spray line, while experimental results gave 5% drift (effective release height 10 to 15 m above ground level). For the stable and unstable cases, the agreement was 51% (model) and 50% (expt.) and 37% (model and 42% (expt.) for release heights of 30 to 35 m and 20 to 25 m respectively. These effective release heights compare well with experimental observations. The depth of vortex penetration into the canopy is the dominant factor in determining drift. Neutral conditions allow deep penetration while stable and unstable conditions promote elevated effective droplet release heights, the first due to buoyancy effects, the other due to vortex break-up caused by turbulence. For a fixed release height, variation in atmospheric stability class produces minor effects. For reduced off-target drift in forestry spraying (coniferous forests) the droplet emission spectrum should be in the 15 jum to 55 yim range (after evaporation) and spraying should be done in conditions as near neutral as possible with aircraft having high vortex descent rates.

Turbulent Trailing Vortices in Stratified Fluids

The effects of stable atmospheric-density stratification, vortex core size, and turbulent scale on the descent of aircraft-trailing vortices are investigated using numerical solutions of a second-order closure turbulence model. A Boeing 747 vortex descent is simulated numerically and compared to reported measurements of descent distance, velocity, and circulation profiles. It is concluded that the pair was halted in its descent by a diffuse region of countersign vorticity primarily outboard and above the vortex cores. It is shown that the core size and turbulent macroscale have significant effects on vortex behavior through their influence on turbulence production, diffusion, and dissipation.

Aircraft Vortex Studies in Forest Aerial Spraying

ABSTRACT FIELD tests were conducted to reproduce the con-ditions of spruce budworm spraying carried out in Eastern Canada. For a Grumman TBM aircraft, generating droplets with a VMD of 60 M, the finer spray droplets were found to be caught in vortices generated by the aircraft. The trajectories of the vortices were followed photographically. The descent rate of the vortices, above tree-top, was reduced by atmospheric instability. The in-itially high vortex descent rates were found to approach the theoretically predicted values from the Rankine vortex model using elliptic wing loading.