PELTI Flight Report for RF07, 19 July 2000

   The objective of this flight was to assess the ambient/LTI relationship,
with slightly more emphasis on sea salt than dust.  A similar flight on
21 July will emphasize dry dust, since a high-altitude dust storm is forecast
for that time.  We flew four (4) one-hour legs, each with TAS and FSSP
measurements to evaluate the inside/outside difference.  A pair of legs
at 30 m was designed to see how the size-dependent efficiency changes
with turbulence in the LTI.


 UTC
14:57   Takeoff, ENE 
15:03:20   Setup and adjustment of flows at 30 m

15:24:28 to
15:29:26   Common-inlet calibration
           Turned to East

15:31 to
16:20   Level at 30 m - LTI Laminar 120 lpm

15:36:50 to
16:20      APS Data Period  (distribution variable due to rain)

15:59   Turned to Southeast as soon as we cleared the Antilles chain
16:03   Brief rain; turned south to avoid more

16:27 to
17:20   Level at 30 m - LTI Turbulent at 200 lpm

16:28:25 to
17:20      APS Data Period (distribution fairly constant)

17:20 to
17:39   Sounding to 3000 m 

17:39 to
17:43   Level at 3000 m 

17:43 to
18:43   Level at Dust Altitude (2700 m) - LTI Laminar

17:44:38 to
18:43      APS Data Period (7 micrometer peak variable)

18:43 to
18:57   Sounding to Salt Altitude (230 m)

18:57 to
19:38   Level at Salt Altitude (230 m) - LTI Laminar

18:57:50 to
19:38      APS Data Period (distribution variable)

19:38   Common-inlet calibrations

19:40:05 to
19:55(?)   APS Data Period

20:04   Return to STX


Notes:

The sizing modules were all calibrated on 18 July with PSL and glass beads of
various sizes.  This showed us that the APSs were not all sizing exactly the
same.  One was off by 2 channels and one by 1 channel.  We will correct the
sizes henceforth for this sizing error.  The cause of it is a mystery, since
all the APS sample flows seemed to be well-controlled when checked with a
reference flowmeter.  The flight was delayed an hour while we cleaned all
the tubing that had become coated with the calibration beads.

We started and ended the flight with common-inlet calibrations, and noted a
slight decrease in the sensitivity of the LTI APS (relative to the CAI APS)
during the flight.

We again had difficulty getting the DU computer to correctly compute the LTI
sample flow, which introduces uncertainty into how isokinetic the LTI was.
We tried to use thermal mass flowmeter data to determine the correct LTI
sample flow and adjusted the suction flow accordingly to what we believe
was with in a few percent of isokinetic. 

The Wing-mounted FSSP-300 gave about 2 orders of magnitude more counts below
0.5 micrometers than the internal 300, so it could not be used as a way to
look for dust layers during profiling.

The "dry dust layer" was not as dry as we had expected, with an RH varying
between 50 and 60%. 

Dust particles seemed to bounce back into the stream from the diffuser walls:
the nephs behind the solid diffuser and the LTI were nearly identical in the
dust layer.  The CAI neph was still considerably lower, however.


Commentary:

   This flight was an excellent comparison of the inlets under several sets of
conditions, and we should have ample TAS data against which to test our
inside/outside questions.  The dust leg was particularly interesting, because
the volume peak at 7 micrometers changed size throughout the leg, while the
mode at 3 micrometers was much more constant.  This is pretty strong evidence
that the larger mode is a real one, and not only an artifact of enhancement
by the curving streamlines in the LTI.  This same mode was present in the sea
salt legs, as well, which appear to be a combination of a typical sea salt
mode and a dust mode at a larger size.  The SEM analyses will give us a solid
answer to this issue, if they see a lot of volume at 7 micrometers which is
different from that at 3 micrometers.

   The turbulence didn't seem to make much difference in the LTI performance
relative to the other inlets in the MBL.  This turbulence that is observed at
200 lpm sample flow is very mild relative to that observed when there is no
suction or when we flew with a sharp leading edge on the LTI.  I suspect that
this turbulence begins farther back in the LTI, where the air has already
slowed enough that the danger of large-particle loss is greatly reduced.  It
may well be that we can operate the LTIs in this slightly-turbulent range and
achieve higher sample flows (and use less suction) than what is required to
produce a large flow of entirely-laminar air.

Barry Huebert
31 July 2000