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Recent Lidar-based Wake Vortex Measurements at DLR

VIEWS: 35 PAGES: 5

									           Recent Lidar-based Wake Vortex Measurements at DLR

                                Stephan Rahm, Igor Smalikho and Rudolf Simmet
                                                    DLR
                                              Münchner Str. 20
                                               82234 Wessling
                                                  Germany
                                           stephan.rahm@dlr.de




Abstract

In the last two years several ground based and airborne wake vortex campaigns have been performed
with the DLR coherent Doppler Lidar. The objective of those campaigns was the measurement and
description of the wakes generated by the new Airbus A380 aircraft for the ICAO landing separation as
well as the influence of different configurations on the vortex life time in the project AWIATOR.


Introduction

The investigation of aircraft wake vortices by means of coherent Doppler lidar is rather actual. In the
boundary layer atmosphere these measurements can be performed with an ground based lidar. At this
type of measurement the lidar performs an elevation scan with the LOS perpendicular to the vortex axis
(picture 1a). The scanning speed can be lower because the vortex is moving only slowly in the lidar
coordinate system. The advantage of this setup is also a generally high backscatter coefficient in Central
Europe with a high SNR of the lidar signal resulting in high quality vortex measurements. The drawback
however are the properties of the boundary layer itself. The atmosphere near the ground is either layered
at calm wind conditions and low turbulence or it is turbulent with a significant influence of the turbulent
cells to the vortex. If the vortex is generated in proximity to the surface the topology an surface roughness
will result in some interesting effects that are pretty difficult to predict.

In the free atmosphere the conditions are in general much more homogeneous compared to the boundary
layer. However the main drawback here is mostly the lack of aerosol particles resulting in a pretty poor
SNR making good lidar measurements impossible. This restriction can be overcome by using smoke
generators at the generating aircraft or by flying in a hazy layer of the atmosphere or under contrail
conditions. Here the big issue is to obtain these rare condition in the usually very limited time slot of the
aircraft availability. At this kind of measurements the Doppler lidar was installed in the DLR research
aircraft Falcon F20 in down looking configuration (picture 1b). In this case the geometry for the vortex
measurement is better suited because the two vortices are separated according to the LOS of the lidar.
The scanning speed needs to be higher due to the virtually higher dynamic movement of the vortex in the
lidar coordinate system.




                                      s can ing range               sc anin g ra nge
                       l idar


Figure 1: scanning range for ground based (left 1a) and airborne (right 1b) wake vortex measurements
Lidar system and measurement geometry

The lidar system consists of a wind tracer transceiver from Lockheed Martin former CTI. It is operating at
2.02 micron wavelength with 1.5 mJ energy and a repetition rate of 500 Hz.

The DLR custom build scanner in front of the transceiver consists of two Silicon wedges that can be
turned independently by two stepper motors. Thus it is possible to address each LOS direction inside a
cone with +/- 30° opening angle. Such a scanner is pretty compact and - compared to a mirror scanner -
insensitive to vibration.

The data system follows the strategy of early digitizing that means the heterodyning signal is directly
digitized together with a timestamp. All other housekeeping parameter are also stored in their original data
format together with a timestamp in a separate computer. This computer also controls the scanner. This
strategy offers maximal flexibility in the offline data processing. Especially the correction of time and
frequency jitter of the transmitted pulse, or the sorting out of pulses where the seeding was not optimal.
The correction of any offsets or systematical errors of housekeeping parameters is also easily possible.
Thus at wake vortex measurements with a maximum range of 2 km the data rate is roughly 250 MByte /
minute that can be handled by a commercial available computer with SCSI hard disk.
At ground based wake vortex measurements the lidar is placed ideally in 1000 m distance normal to the
flight path of the wake vortex generating aircraft. This distance is a tradeoff between a good SNR and high
resolution requiring a close range and high volume covered at a higher range. The elevation angle ranges
there from 15° to 50° depending on the altitude of the generating aircraft.

At airborne wake vortex measurements the lidar is installed in the Falcon F20 aircraft in a down looking
configuration. Consequently this aircraft is flying 600 to 1000 m higher than the generating aircraft(s).
Again this distance turns out to be the optimal compromise between resolution and cross section scanned.
The higher flight level does relax security issues of operating several planes close together. The accuracy
of navigation is here a major issue. At 1000 m range the measuring aircraft (Falcon F20) has to be right
above the vortex pair with an accuracy of better than +/- 400m horizontally. Taking into account that the
vortex is drifted by crosswind, a pretty bad visibility of an older vortex even if seeded by smoke or marked
by contrail, and the aircraft velocity of 100 - 200 m/s it is obvious that the navigation is very critical. In case
the vortex of the generating aircraft ins seeded by smoke in order to improve the SNR the time of the
smoke generator is limited to a total of 10 to 20 minutes depending on the smoke generator. Depending to
the velocity of the generating aircraft, the required vortex ages, the velocity range of the chasing aircraft
with the lidar, and the clearance of the flight control several approaches are possible in order to get the
maximum measurements out of the very limited time of smoke. Where possible the generating aircraft will
fly in the same direction as the current wind to avoid any drifting of the vortex beside the trajectory. The
Falcon can fly either the opposite direction if the measurements shall cover a wide range of vortex ranges.
Or the Falcon is flying in the same direction with a different speed for a higher density of measurements
per vortex age. Data processing takes place offline in a four stage processing described in several papers.
For a high quality of the results also in circulation strength the homogeneity of the backscatter coefficient
has to be monitored closely. Therefore an automated processing is not advisable. The data products of
the processing are the center of the vortex together with the circulation.


A380 vortex measurements

The measurements for the new airbus A380 started in April 2005. In a first attempt the DLR lidar container
was located on the roof of an Airbus building in Toulouse, France looking towards the glide slope of the
airport Blagnac. Here the measurements were quite infrequent because it was performed during the
normal flight test program of Airbus that means a few landings a week at this time. Those set of
measurements seemed to prove the value of Doppler lidar measurements for the characterization of wake
vortices so that consequently the lidar container was moved to Istres military airport near Marseille where
a huge lot of measurements were performed in different altitudes in order to study the wake vortex at an in
ground effect as well as at higher altitudes where the vortex does not hit the ground. Sodar and RASS
wind profiler were used to probe the atmosphere in terms of wind and eddy dissipation rate (EDR). To get
a comparison with existing aircrafts most of those measurements were done in that way that the over
flights of the A380 alternated with a Boing 777 and later with a Boing 747-400 chartered from Lufthansa in
order to obtain comparable results in the same atmosphere in terms of turbulence and cross wind.
Because the sun has a very significant influence to the turbulence in summer most measurements took
place either in the morning or evening. Those measurement were observed by a member of Eurocontrol
and/or the FAA. End of December 2005 those ground based measurements were finished. Due to
requests from the "international A380 wake vortex steering group" two more series of vortex
measurements were performed in spring and summer 2006 in Oberpfaffenhofen, Germany.

The behavior of a wake vortex pair generated by an A380 in cruising altitude is also a point of
consideration. Here the question is how far sinks a vortex down and how does it affect smaller aircrafts
flying at nearby flight levels. The first and mayor problem at those altitudes is the normally low backscatter
at cruising altitude. All available smoke generator did also not work in this altitude so that meteorological
conditions had to be used where sufficient backscatter was present. Consequently contrail conditions had
to be searched. In March 2006 the first flights were performed. At this day conditions were absolutely
optimal. In cruising altitude at 10000 ft. a moist layer of hazy air was found in between the high cloud
layers where the SNR was incredibly high and contrails were also present so that the navigation was not
too difficult. At this day the A380 and an A340-600 were flying in parallel so that the wake vortex results
could be compared against each other. The vortex of both generating aircrafts were measured from
directly behind the aircrafts to a distance of 25 nm that corresponds to a vortex age of roughly 4 minutes.
Unfortunately did the very tight test plan of the A380 not allow to prolong the measurements for more than
90 minutes. A second series of flights were done in June 2006. Here the A380 was flying parallel to a 747-
400 chartered from Lufthansa. Here the Meteorological conditions were not optimal. The prediction where
long contrails occur seems to be very difficult. Another fact is also that the two outer contrails originating
from the four engines are roled up in the vortex. As the air gets mixed inside the vortex the aerosols are
vanishing pretty fast so that the lidar signal becomes pretty weak at an older vortex.

The results of all A380 related vortex measurements are confidential at least until a final decision about
the landing separation from ICAO, FAA, EUROCONTROL is made. Currently some ground based
measurements at Tarbes, France are going on with the focus on measurements in a calm atmosphere.

Awiator A340-400 measurements

Another interesting topic is if the static or dynamic setting of Flaps can have an influence on the vortex
strength or lifetime. The idea is that instabilities are introduced in the wake vortex that cause the rapid
decay phase to start earlier. Experiments on a model in a water tank do not answer all questions. Again
the Doppler lidar is currently the only instrument that can probe the wake vortex in the atmosphere
generated by a real aircraft. The influence of the atmosphere to the wake vortex has to be as small as
possible. Consequently those measurements took place above the boundary layer. As generating aircraft
a, A340-300 from airbus with smoke generators on both wings was used. This aircraft was flying in
approx. 3000 m altitude so that the smoke generators were still working but also high enough to be
outside the boundary layer. The lidar was again installed into the DLR Falcon F20 looking downward. In
total 4 sets of flights were performed on two days.
Figure 2: Circulation normalized to the initial circulation over vortex age for four different configurations

Three different flap configurations have been tested against one standard setting. At those flights the
A340 and the Falcon with the lidar were flying in opposite direction. At a flight level below the A340 a small
Metro Swenningen from NLR equipped with a video camera was flying directly under the Falcon. This way
a correlation of the vortex parameters obtained from the lidar measurement with the optical appearance of
the smoke seeded wake vortex pair is possible. Figure 2 shows the result of the circulation strength over
the vortex age for the four different configurations. Shown are the single measurements as well as a mean
curve derived from all eight single measurements for each configuration.


Acknowledgements

The wake vortex measurements on the Airbus A380 were performed under Airbus contract.
The A340-300 measurements were funded by the European Union AWIATOR program contract no.
G4RD.CD200200836.


Summary

In this paper an overview was given about the wake vortex measurements done from DLR from 2005 to
2007 under different projects / contracts. At all measurement the LOS of the lidar was perpendicular or
close to perpendicular to the vortex axis. The position and the circulation strength was estimated from the
lidar signal.


Literature
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