NASA Technical Reports Server (NTRS) 20000109955: Wake Vortex Transport in Proximity to the Ground

9 Pages·0.46 MB·English

by  NASA Technical Reports Server (NTRS)

WAKE VORTEX TRANSPORT IN PROXIMITY TO
THE GROUND
David W. Hamilton
and
Fred H. Proctor
NASA Langley Research Center
Mail Stop 156A
Hampton, Virginia 2368 1- 2 1 99
1g th ~igitaAl vionics Systems Conference
7-13 October 2000, Philadelphia, Pennsylvania, AlAA and
IEEE
WAKE VORTEX TRANSPORT IN PROXIMITY TO THE GROUND
roct tor'
Du~~iWd. ~clrnilton'~ indF red
NASA klrzgley Re.~e~ircClze nter, H~rttlpton,V irxirziu
Abstract ;,,,/, altitude above which the ambient wind
velocity becomes constant
A sensitivity study for aircraft wake vortex
;,,, altitude of vortex core
transport has been conducted using a validated large
eddy simulation (LES) model. The study assumes u power function in ambient wind profiles
r,
initial vortex circulation
neutrally stratified and nonturbulent environments
and includes the consequences of the ground. The
Introduction
numerical results show that the nondimensional
lateral transport is primarily influenced by the The National Aeronautics and Space
magnitude of the ambient crosswind and is Administration is making efforts through the
insensitive to aircraft type. In most of the Terminal Area Productivity Program to safely
simulations. the ground effect extends the lateral increase terminal capacity and reduce delays to
position of the downwind vortex about one initial meet the increasing air travel demands.
vortex spacing (h,,) in the downstream direction. Particularly. the Aircraft Vortex Spacing System
Further extension by as much as one h,, occurs (AVOSS) integrates systems that assess the present
when the downwind vortex remains "in ground and short-term weather state and make transport
effect" (ICE) for relatively long periods of time. and decay predictions for aircraft wake vortices [I.
Results also show that a layer-averaged ambient 2. 31. These predictions aim to safely decrease
wind velocity can be used to bound the time for spacing between arriving and departing aircraft
lateral transport of wake vortices to insure safe under instrument meteorological conditions (IMC)
operations on a parallel runway. and effectively increase airport capacity. The initial
goal of AVOSS is to successfully demonstrate a
Nomenclature
capability to increase throughput on a single
h,, initial vortex separation
runway. Currently. capacity is also reduced during
L lateral distance traveled by the downwind
IMC when operations on a parallel runway are
vortex
either shutdown or reduced due to the uncertainties
T nondimensional time: rV,,/h,,
in the positions of laterally shifting wakes.
f time
At time step Presently, independent approaches with lateral
fr
U crosswind component of velocity separations of less that 2500 (-760 m) are not
U,,,,,, ambient crosswind velocity at elevations allowed during IMC. This restriction is due to the
z,,,,, and above danger of potential encounters with drifting
vortices. Recent research has been directed at
layer-averaged ambient crosswind velocity
improving this limitation without jeopardizing
ambient crosswind velocity
safety [4. 5. 6. 71. Schilling [4] concluded that a
initial vortex descent velocity: T, / (2 A h,, )
minimum crosswind magnitude of 2.5 nz s was
initial vortex lateral position
necessary to transport vortices to the parallel
drift model lateral position
runway (-518 m separation) at the Frankfurt
TASS vortex lateral position
International Airport (FIA). This conclusion was
longitudinal. lateral. vertical space
partly based on numerical simulations and assumed
coordinate
a wake vortex lifetime of 180 s. Kopp [5] used a
altitude of initial vortex
continuous-wave lidar at FIA and found that a
'
Kescarch Scientist. Airborne Systems Competency
Kesearch Scicntist, Airhorne Systems Competency. AIM memkr
'
crosswind of 3 would transport a wake lnto the occur under the intluence of turbulence eddies.
171 .\
parallel runway safety area in less than 90 s. Rudls potentially masking the contributions of crosswind
et ul. [6] have observed wakes traveling beyond 500 shear and ground effect. In the present study.
m. in the presence of strong crosswinds and low turbulence effects will be ignored in order to isolate
turbulence. A model that incorporates factors such the ground effect but should be considered for
as mean crosswind. runway separation. and wake future study.
vortex lifetime is needed to insure safe operations
In this study we subject a range of aircraft to
on parallel runways.
varying ambient crosswind conditions. We will use
A wake vortex is considered "in ground these results to quantify the extent of IGE and
effect" (ICE) when its altitude is within one initial present a bound on vortex transport to ensure safe
vortex separation (h,,) above the ground. According operations on a downwind parallel runway.
to Critchley and Foot [8], most aircraft wake
encounters occur between 30-60 m (100-200 ft).
The Numerical Model and Initial Conditions
which for many aircraft is IGE. Wake vortex
behavior IGE has been investigated both The model employed in this study. the
Terminal Area Simulation System (TASS) [23], is a
observationally [5. 6. 9. 10. 1 1. 121 and numerically
compressible. LES model. which has successfully
[4. 7. 13, 14. 15, 16. 171, but most of the research to
demonstrated wake vortex behavior in various
date has only qualitatively described the
meteorological conditions [16. 17. 19. 22. 24. 25.
complexities of this phenomenon. From recent
26. 27. 28. 29. 301.
large eddy simulation (LES) results, Proctor rt ul.
[17] has proposed an empirical model quantifying
the lateral spreading of a vortex pair once the vortex Grid Configuration
has reached its minimum height in ground effect.
For the purpose of examining wake transport.
Their results were limited to cases with little or no
and to save on computational time. the model is
ambient wind shear.
integrated in time using a two-dimensional domain.
Vertical gradients in the ambient crosswind The two-dimensional domain inhibits vortex decay
have been shown to affect wake vortex descent and and prevents three-dimensional instabilities. such as
lateral transport [ 18, 19, 201. Specifically. Crow linking. from developing [31]. Given these
nonlinear shear in the crosswind can reduce the limitations, previous studies have shown that
descent rate. stall. and potentially lead to the rising simulations with the two-dimensional assumption
of a wake vortex. The orientation of the wind shear have given valid results for vortex transport [16. 19.
c'u/(?;'
gradient (i.e. positive or negative) will 24.301.
determine whether the upwind or the downwind
The coordinate system is chosen such that A- is
vortex is more affected [19]. The combination of
along the flight path or longitudinal direction. y is in
wind shear and ground effect. which often coincide.
the cross track or lateral direction and ;i s height
complicate the problem of predicting wake vortex
above ground. The experiments assume a uniform
transport.
one meter grid size. The domain depth is designed
The present study will investigate wake vortex so that two vortex separations (2h,,) exist between
transport in proximity of the ground in order to the initial vortex altitude (z,) and the top of the
determine safe bounds for arriving and departing domain. The numerical grid translates with the
aircraft on a downwind parallel runway. The downwind wake vortex: therefore. allowing the
simulations conducted in this study assume domain width to be independent of crosswind
environments with no ambient turbulence. It should magnitude. In all experiments. the width of the
be noted that turbulence intluences vortex decay domain is fixed at 1 1.5 h,,.
and can have an affect on transport [21]. For
example, Han et ul [22] have examined the
intluence of convective boundary layer turbulence
eddies on wake vortex transport. They determined
that substantial lateral shifting of the vortex can
0 2 4 6 8 10 12
0 0.2 0.4 0.6 0.8 1 1.2
Nondimensional Time (t V,, 1 b,)
Nondimensional Wind Velocity ( IJIV,,)
(b)
Figure 1: Ambient crosswind profiles for TASS 25 1
simulations.
Atmospheric Conditions
The simulations are initialized with a neutrally
stratified and dry atmosphere. The ambient
crosswind is assumed to be horizontally uniform
with its vertical distribution prescribed by the
following power law:
0 2 4 6 8 10 12
Nondirnensional Time (t V,/ b,)
Figure 2: Sensitivity to aircraft type for TASS
> simulated trajectories for (a) calm wind and (b)
for z
Z,O,J crosswind cases. Figure 2b is with wind profile
A", U,, = 2V0 and z, = 1.56,.
6 6
Three wind profiles with contrasting intensities
of shear are chosen for the sensitivity study (see
Fig. 1). Four wind magnitudes are used in each Vortex Initialization
profile and are scaled by the initial vortex descent The initial vortex profile is representative of a
rate of the aircraft tested. i.e. U, ,,,,, IV,, = 1, 2. 4. and post rollup. wake vortex velocity field. The vortex
8. For a typical Boeing 727. these values initialization is the same as that described in Proctor
correspond to a dimensional crosswind velocity of er u1. [17]. The initial vortex parameters for the
U,,,,, = 1.53. 3.06. 6.12. and 12.24 rn .s I. respectively seven aircraft are listed in Table 2 and represent the
[l m s I 2 2 krs]. The parameters for the wind three aircraft weight classes, i.e. small, large. and
profiles are listed in Table I. heavy. Six vortex initialization heights are tested:
z,/h,, = 0.5, 1. 1.5, 2, 4. and 8. The six cases were
Table I. Values for wind profile parameters. chosen to represent vortices initialized in. near. and
out of ground effect [32].
Profile a
- - -. -- - ~&? - -
A 0.4 1 Oh,,
B 0. I I Oh,,
C 0.1 h,,
Table 2. Initial vortex parameters. revealing the insensitivity to aircraft type. The
- - . .. - . --- - - - - - - --- r,, effects of the ground and secondary vortices on
Aircraft 6, (m) V,( m (tn2s -')
S-') vortex motion (Fig. 2a) appear to be less significant
727 26 1.53 250
due to the dominating influence of advection. i.e.
737 2 2 1.49 205
the process of transport by the crosswind velocity
747 50 1.70 534
field. In fact. by T = 8. the lateral position of the
757 30 1.63 307
vortices is an order of magnitude greater than of
F2 8 20 1.28 161
those in calm wind. thus revealing the dominance of
EA 330 37 1.28 378
advection.
DC10-30 40 1.96 493
A "Drift Model" Analysis
An additional 72 simulations have been
conducted examining wake vortex transport near
Sensitivity to Different Aircraft
the ground in a variety of crosswind conditions.
To determine if wake vortex transport is
The simulations involve a mix of the three wind
sensitive to the generating aircraft, two sets of profiles (Fig. I). four wind magnitudes (U,,,,,,). and
experiments are conducted using the seven aircraft six initial vortex altitudes )(.; which were defined
listed in Table 2. Only the downwind (downshear) earlier. A simple relationship for vortex advection.
vortex is analyzed since it is usually the first to
i.e. a drift model. has been calculated for each
impact parallel runway operations.
simulation:
The first experiment examines the lateral
transport of the downwind vortex in an atmosphere
without crosswind (calm wind). A comparison of where I/,, is from Eq. (1 ). In this calculation, ;,,,.
the simulations using conventional nondimensional- the height of the vortex at time t. is taken from the
izations [33] is shown in Fig. 2a. During the initial TASS simulation.
period of the simulation (T < I) the vortices
In order to evaluate the effects of ground and
descend due to their mutual induction and exhibit
the nonlinear interaction with a non-uniform
no lateral displacement. By T = 2, the vortices lose
crosswind profile. the difference is taken between
their vertical motion and spread laterally due to
Eq. 2 and the lateral position predicted by the TASS
ground effect [34]. Between T = 3 and 5. viscous
simulation. The resulting 'drift differences' show
interaction with the ground causes the development
that in nearly all cases. TASS predicts a lateral
of a shear layer with opposite sign vorticity to that
position downwind of the position calculated by the
of the primary vortices. This shear layer separates
drift model (Fig. 3). This downwind extension
from the surface and is transported around the
indicates that transport of the downwind vortex IGE
periphery of each of the vortices [9]. Secondary
is greater than that due to advection alone. The
vortices are generated within this shear layer and
majority of the 'drift difference' curves asymptote
orbit the primary vortices. The circulation from the
close to one h,, (see Fig. 3). Thus. the ground effect
secondary vortices causes the primary vortices to
extends the lateral position of the downwind vortex
rise and bounce. Later. the primary vortices begin
to about one initial vortex spacing.
to oscillate under the influence of the orbiting
secondary vortices. It should be noted that the 2-D A few cases exhibit a 'drift difference'
simulations might exaggerate these oscillations (extension) greater than one h,,. This further
[ 161. However. comparisons with observed vortex extension is attributed to either multiple IGE
behavior have shown good predictions by the 2-D encounters or a prolonged ground interaction.
simulations [16. 24. 301. Figure 4 shows a vortex path revealing a prolonged
ground interaction. The vortex initially sinks and
The second experiment examines lateral
quickly rebounds to a height of 1 .5h,,. then it begins
transport in an atmosphere with crosswind (Fig.
2b). Similar to the calm wind cases, the curves for to oscillate near ;= h,,. Lidar measurements from
the Memphis-95 field study provided observational
lateral transport collapse when nondimensionalized.
I l l . I . .
f D
2 1.5
'e
0 1 2 3 4 5 6 7 8
Nondim ensional T i m e (t V o 1 bo) ( b )
0 1 2 3 4 5 6 7 8
N ondim ensional Tim e (t V ,, I b,,)
i c )
0 1 2 3 4 5 6 7 8
Nondim ensional T i m e (t V . 1 b,,)
Figure 3: Drift differences between TASS prediction and drift model (Eq. 2) for cases with (a) wind
profile "A", (b) wind profile "B", and (c) wind profile "C".
evidence of similar wake paths [35]. Of 80 IGE A "Time-to-Cross" Analysis
cases identified, 8 (10%) displayed a prolonged
A drawback from implementing the results of
ground interaction that is similar to that depicted in
the previous section into an operational system is
Fig. 3.
knowledge of the vortex altitude. A layer-averaged
It appears that the prolonged ground approach is defined and evaluated for a prediction
interaction can be related to the ambient wind shear of vortex lateral transport. rather than estimating the
and the initial vortex altitude. For simulations with height of the vortex. This approach considers
relatively small magnitudes of ambient crosswind neither ground effect nor the vertical trajectory of
' '.
shear gradient, i.e. li2~/?<:' l0 .001 m s there the vortex. and is used to predict the transport of the
was little affect on the vortex descent rate. In these downwind vortex.
cases. a prolonged ground interaction occurred and
The time interval for the downwind vortex to
drift differences were greater than one h,,. On the
travel across a selected distance is compared
other hand. for environments with relatively large
between the layer-averaged prediction and the
magnitudes of the crosswind shear gradient. i.e.
TASS simulation (Fig. 5). The layer-average
(i2~/(".3r' (0 .001 m-' s-'. the descent rate was
/G.
prediction is determined by t=L where is
reduced. For these cases the vortices ascended after
the layer-averaged ambient crosswind velocity
ground interaction. and their drift differences
(averaged between ,- = 0.5h,, and z = ,: + 0.5h,,).
remained bounded by one h,,. The vortex
initialization height can be as important as the
magnitude of shear. When the vortex was
initialized ICE. i.e. ,: = 0.5 h,,. a prolonged ground
..
interaction occurred regardless of the magnitude of Jo.5 h,, .I<.
the ambient wind shear gradient. In these
simulations, the vortex eventually ascended above and where L is an arbitrary distance from the initial
,- = h,,. and the drift difference exceeded one h,,. vortex position. The predictions are determined for
values of L that range from 100 to 800 m by 100 nz
The drift model analysis shows that the ground
increments.
effect extends the lateral position of the downwind
vortex to about one initial vortex spacing (h,,). An empirical relationship for bounding the
relative to transport without ground effect. Further time interval for lateral transport of the downwind
extension beyond one h,, occurs when the vortex ICE is determined from the data in Fig. 5.
downwind vortex either remains in ground effect and is:
for long periods of time or rebounds in ground
effect more than once.
The above relationship (i.e, transport bound. Fig. 5)
represents the minimum time interval for the vortex
to transport across distance L (I, can represent the
separation between parallel runways). The thin line
-
.
in Fig. 5 represents t=L /I/ which is the transport
by simple advection.
This analysis is applied to nine observed wake
vortices and is shown also in Fig. 5. The observed
wake positions were measured by lidar during the
Memphis-95 field experiment [MI. The wind
profiles used in this analysis were from
0 5 10 15 20 25 30 35
measurements near the lidar site. The predicted
Nondirnensional Lateral Position (yh,,)
values for the observed vortices are within or just
Figure 4: Vortex trajectory for the TASS case outside of the transport bound.
with wind profile "A", U- = 4V0, and z, = Ib,,.
Future studies should address the sensitivity of
wake transport to ambient wind shear both in and
out of ground effect. Also. a select number of cases
should be extended to three-dimensions to
determine the effects of decay on wake transport.
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