A single-axis, five-beam, three-component laser velocimeter (LV) system was used in a major experiment. Satisfactory results were obtained with the LV system in the juncture flow. Limited optical access to the tunnel proved to be a problem for the three component LV system in determining the third component

Kubendran, L. R.

Meyers, Jim F.

English

NASA Technical Reports Server

N90-12525
THREE-COMPONENT LASER DOPPLER VELOCIMETER
MEASUREMENTS IN A JUNCTURE FLOW
L. R. Kubendran and J. V. Meyers
Langley Research Center
Hampton, Virginia
389
https://ntrs.nasa.gov/search.jsp?R=19900003209 2020-03-20T00:54:48+00:00Z
INTRODUCTION
In a wing-fuselage juncture the oncoming turbulent boundary layer on the
fuselage surface experiences steep adverse pressure gradients as it approaches the
wing leading edge. These gradients cause the boundary layer to separate ahead of the
leading edge, resulting in a vortex which rolls up and trails downstream in the
juncture. The use of laser velocimetry facilitated making detailed measurements near
the non-planar surfaces forming the juncture, and within near-separating flows close
to the trailing edge of the wing (refs. 12, 13, and 14).
The experiment was carried out in the Low-Turbulence Pressure Tunnel (LTPT) at
the NASA Langley Research Center. The LTPT is a single return, closed-circuit wind
tunnel which can be operated at pressures from 0.I to I0 atmospheres. It is capable
of operating at Mach numbers from 0.05 to 0.50, and unit Reynolds numbers from
300,000 to 49,000,000/m. The turbulence level is very low because of the nine
turbulence reduction screens and because of the 17.6"1 contraction ratio. The test
section is 0.91 m (3 ft) wide, 2.29 m (7.5 ft) high, and 2.29 m (7.5 ft) long.
Figure 1 gives the details of the experimental setup.
16 in\ J
Flat splitter plate _\_\k t°
;y.. ii!
1 \ _Vx_ i_..__ -_-
i xb...--___ /--l_ In chord
Flow _ Wing
_ 6 _ wR/Ft - O.7 x 10
Measurement
stations
Figure I
390 i
THREE-COMPONENT LDV SYSTEM USED IN LTPT
OF e_
The single-axis, five beam optical configuration uses the standard two-color,
two-component beam pattern with the two green beams arranged in the horizontal plane
and the blue beams arranged in the vertical plane (ref. 12). The green beams are
used to measure the U component and the blue beams to measure the V component. A
third green beam is placed along the optical axis bisecting the angles between the
original two green beams. The addition of this beam creates two additional fringe
patterns, from which the W component is obtained. Bragg cells are used in the two
outer green beams to separate the three signals obtained from the three green fringe
patterns. (Fig. 2.)
Figure 2
ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAp_
391
OPTICAL ACCESS THROUGH SIDEWALL WINDOW
The optical access to the model was limited to a single window of diameter 0.75
m. This window was surrounded by a drum 0.978 m in diameter by 0.711 m in depth.
This further complicated the design of the laser velocimeter (LV) system, resulting
in a reduced cross-beam angle. Also, an off-axis backscatter collection mode had to
be used in order to reduce the effect of background reflections from the splitter
plate. (Fig. 3.)
392
v%'V V
Figure 3
ORIGINAL PAGE
BLAC,,K AND WHITE PHOTOGRAPH
_: 1 : : ÷=:=- :
E
w
|
-_ITzMODEL MO_TED IN TEST SECTION
The LV system described here resulted in a sample volume of diameter 80
micrometers and a length of 120 micrometers. In order to make measurements very
close to the juncture, the LV beam system was oriented at an angle of ii degrees with
respect to the horizontal wing. Flare from the juncture surfaces restricted the
closest measurement distance to 3.5 mm from the vertical splitter plate, and to 1.0
mm from the horizontal wing. Tridecane, a kerosene derivative, was successfully used
as the seeding material in this experiment (ref. 14). (Fig. 4.)
Figure 4
ORI@IN/_L PAGE"
BLACK AND WHITE PHOTOGRAPH
CT-@__
393
COMPARISONOF LV DATA WITH HOT-WIRE RESULTS
In order to determine the accuracy of the LV data, measurements made at
stations located farthest from the juncture have been compared here with the two-
dimensional turbulent boundary layer results from Klebanoff's hot-wire measurements
over a flat plate with zero pressure gradient (ref. 34). Even though these stations
were chosen to minimize the influence of the juncture, some three-dimensional effects
are expected to be present in the LV data. The mean velocity component U shown in
figure 5 compares very well with the hot-wire results.
U
Uoo
75% chord, z = 1.18 in. (3 crn)
1.2--
o o
.6 - 0 LV data
• 4 - Klebanoff's hot-wire data
(flat plate, dp/dx = O)
.2
I I I I I ! I_ I I I
0 .2 .4 .6 .8 1.0 1.2 1.4 1.6 1.8 2.0
y16
Figure 5
394
MEANVERTICALVELOCITY
In a flat plate boundary layer, the mean velocity component V is non-zero
because of the displacement effect of the boundary-layer growth; it is directed away
from the plate and has small magnitude. The values of mean velocity component V
measured by the LV system satisfy this criterion. (Fig. 6.)
75% chord, z = 1.18 in. (3 cm)
V
Uoo
.06 -
.04 -
.02 --
0
--o02 --
--iO 4 --
-. 06
0
© LV data
V: Small and positive in
2-D flat plate boundary layers
oo o o-o-o-o-o-o-o0 0 0
I I I
.2 .4 .6
I I ! I I
.8 1.0 1.2 1.4 1.6
y/5
I I
1.8 2.0
Figure 6
395
SPANWISE VELOCITY
The mean velocity component W is expected to be zero in a two-dimensional flow.
But the flow in the juncture can still influence the measurements at the location
where the data are being compared (fig. 7). This can partially explain the non-zero
distribution of the W component.
7.5% chord, z = i. 18 in. (3 cm)
•06 -
•O4
.02
W
0
Uoo
-.02
-. 04 -
-. 06
0
0
-0000 0 0 0
0 0
0
0
0 0
LV data
Expected 2-D results
I I I I I I I I I I
.2 .4 .6 .8 1.0 1.2 1.4 1.6 1.8 2.0
y/6
Ffgure 7
396
=
m
i
_=
CHORDWISE VELOCITY FLUCTUATIONS
In the case of the turbulence intensity, u/u_ (where u is the RMS fluctuating
component of velocity in the x direction), the LV data obtained inside the boundary
layer compare well with Klebanoff's hot-wire results (ref. 34). The value outside of
the boundary layer is higher than the freestream turbulence intensity of 0.04
percent, obtained from hot-wire measurements made in the same facility. The value is
also higher than the LV resolution limit of I percent. This can be due to the
following reason: hot-wire measurements normally filter out the very low frequency
tunnel oscillations (< 2 Hz) whereas in the case of LV measurements, no such
filtering is carried out. This will result in the higher-than-normal distribution of
turbulence intensity u' as measured by the LV system. (Fig. 8.)
• 12
.10
.O8
N
U
-- .06
Uoo
•04
.02
75% chord, z = 1.18 in. (3 cm)
0 LV data-- Kleban0ff
0 (flat plate)
o
_ 0
- 0 0 0 0 0
I I 1 I I I I II I_"--I---P-I I I I I ! I
0 .2 .4 .6 .8 1.0 1.2 1.4 1.6 1.8 2.0
)'/6
Figure 8
397
VERTICAL VELOCITY FLUCTUATIONS
Comparison of the turbulence intensity, _/u_ (where _ is the RMS fluctuating
component of velocity in the y direction), with Klebanoff's results (ref. 34) within
the boundary layer is very good, but outside of the boundary layer the turbulence
level is high. This is due to the fact that the lowest turbulence level that could
be measured with the present LV system (or any other LV systems) is about 1 percent.
(Fig. 9.)
398
•12
.I0
•O8
V
-- .06
Uoo
.04
0
75% chord, z = 1.18 in. (3cm)
-00° 0
_ 0 0 0
1 _ _ !
.2 .4 .6 .8 1.0 1.2 1.4
Figure 9
0 LV data
Klebanoff
(flat plate)
I
1.6
?
2.0
iSPAN'WISE VELOCITY FLUCTUATIONS
The overall distribution of the turbulence intensity, w/u= (where w is the RMS
fluctuating component of velocity in the z direction), is much higher than
Klebanoff's reference profile (Ref. 34). Part of it can be attributed to the three-
dimensional effects of the juncture. Also, the narrow cross-beam angle between the
laser beams that were used to obtain w-related components resulted in lower signal-
to-noise ratio; this could have introduced some inaccuracies in this and other w-
related components (fig. i0).
N
W
n
U
oO
•12 -
75% chord, z = 1.18 in. (3cm)
B
.10
.08
•06
•04
.02
0
m
_ O0 ° 0
0 0
- 0 0 0 0 0 0
- 0
i
0 LV data
Klebanoff
(flat plate)
i I J I , I
.2 .4 .6 .8 1.0 1.2 1.4 1.6 1.8 2.0
y/6
Figure I0
399
TURBULENT SHEAR STRESSES
There is excellent agreement between LV measurements of the Reynolds stress u-'_
and Klebanoff's hot-wlre results (Ref. 34). The accuracy of w-related Reynolds
stresses u--_and vw was not as good because of the reasons detailed earlier.
In conclusion, a single-axis, five-beam, three-component laser veloc_meter
system has been used in a major experiment. Satisfactory results have been obtained
with the LV system in the juncture flow. Limited optical access to the tunnel proved
to be a problem for a three component LV system in determining the third component
(W). (Fig. II.)
•003
. OO2
•001
-- 0
- 001
75% chord, z = 1.18 in. (3 cm)
0
-. 002 -
-.003 [
0
, I , I , I , l l I , I
.2 .4 6 .8 1.0 1.2
y/5
Figure 1 l
0 LV data
-- Klebanoff
(flat plate)
, I , I i I ,
1.4 1.6 1.8
0
I
2.0
4OO


Three-component laser Doppler velocimeter measurements in a juncture flow

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