A turret/fairing assembly for laser applications was designed and tested. Wind tunnel testing was conducted using flow visualization techniques. The techniques used have included the methods of tufting, encapsulated liquid crystals, oil flow, sublimation and schlieren and shadowgraph photography. The results were directly applied to the design of fairing shapes for minimum drag and reduced turret buffet. In addition, the results are of primary importance to the study of light propagation paths in the near flow field of the turret cavity. Results indicate that the flow in the vicinity of the turret is an important factor for consideration in the design of suitable turret/fairing or aero-optic assemblies

Vankuren, J. T.

Walterick, R. E.

English

NASA Technical Reports Server

FLOW VISUALIZATION TECHNIQUES 
'IN THE AIRBORNE LASER LABORATORY PROGRAM 
R. E. WALTERICK 
J. T. VAN KUREN 
537 
https://ntrs.nasa.gov/search.jsp?R=19800017111 2020-03-21T02:53:30+00:00Z
I INTRODUCTION 
The design and testing of a three-dimensional structure, such as 
a turret/fairing assembly for laser applications, is a complex empiri- 
cal problem. The attendant flow field is characterized by large scale 
turbulent structures, which are difficult to map without some form 
of flow visualization. 
Toward this end, wind tunnel testing has been done in the Airborne 
Laser Laboratory (A.L.L.) program, using flow visualization techniques. 
The techniques used have included the methods of tufting. encapsulated 
liquid crystals, oil flow, sublimation and schlieren and shadowgraph 
photography. 
The results have been directly applied to the design of fairing 
shapes for minimum drag and reduced turret buffet. In addition, the 
results are of primary importance to the study of light propagation paths 
in the near flow field of the.turret cavity. Depending on the cavity 
azimuth and elevation angles this can involve a propagation path 
through shock patterns, s e arated flow regions, shear layers, or a p 
combination of all three. 
Therefore, the flow in the vicinity of the turret is an important 
factor for consideration.in the'dessgn of suitable.turret/fairing or 
aero-optic assemblies. 
This presentation is in chronological order of wind tunnel tests. 
The different methods of flow visualization for each test are listed 
and discussed based on accompanying photographic figures. 
538 
SECTION II 
RESULTS AND DISCUSSION 
a. The first wind tunnel test covered in this presentation was termed 
the "Transonic Ten Pin Phase I Test" conducted in the Air Force Flight 
Dynamics Laboratory/Trisonic Gasdynamics Facility (hereafter referred to 
as AFFDL/TGF) from September to December, 1971.l 
The flow visualization techniques used consisted of tufting and 
the use of encapsulated liquid crystal. Tuft results (Fig. 1) for Mach 
numbers (M,) of 0.55 and 0.60 indicate a large turbulent wake behind 
the on-gimbal turret, extending downstream beyond the right hand edge of 
the photographs. The tufts on the nose of the model are lying flat and 
unperturbed indicrrtint a region of steady attached flow. Those farther 
downstream and not within the wake region are in unsteady motion indi- 
cative of a turbulent boundary layer. 
The use of encapsulated liquid crystals (temperature sensitive 
material) to detect boundary layer transition and regions of high heat 
transfer (turbulent flows) is shown in figure 2. Warmer temperatures 
are revealed by blue-green color variations and cooler temperatures by 
red shades. A grit strip was applied to the nose of this particular 
model and existence of the resultant turbulent boundary layer is shown 
by the blue-green shades of the strips. 
b. The next test covered was referred to as the "AMES I Test" 
conducted in the NASA Ames Research Center 14 ft wind tunnel during 
January and February, 1972.2 
A shadowgraph is shown in Figure 3. The view is looking down on the 
turret with the turret cavity oriented 120 degrees downstream from the 
wind axis. A multiple shock system is apparent on the turret at an azimuth 
539 
angle of 80 degrees. The formation of this shock system is dependent on 
the cavity azimuth angle (Fig. 4). 
c. Now we turn to a different test and a lesser practiced form of flow 
visualization. The test was called "Transonic Ten Pin Phase II" and 
was run in the AFFDL/TGF during May through August, 1972.3 The flow visua- 
lization method used was sublimation of freon crystals. A timed sequence 
of photographs (Fig. 5) taken of an on-gimbal turret on a flat plate 
shows the areas where higher heat transfer rates1 occur. By definition 
these areas include the characteristic vortices on the turret and those 
shed from the turret/flat plate intersection. 
Included in the flow visualization techniques for this test were oil 
flow and schlieren photography. Figure 6 is a schlieren photograph of an 
on-gimbal turret on simulated aircraft fuselage at M,= 0.90. Due to the 
large model frontal area to test section area the flow is choked as evidenced 
by the strong shocks on the model nose and on the aft section of the model. 
Also evident in this schlieren are the turret turbulent wake and the buildup 
of the fuselage boundary layer. 
The same configuration at the lower Mach number of 0.75 is shown in 
Figure 7 along with an oil flow at the same conditions. The flow features 
discussed in the last figure are still there only the shocks are weaker. 
The turret shock and resultant separation is confirmed by the oil flow which 
abruptly ends at mid turret. Several vortices on the turret appear as well 
as the large vortex behind the turret. This large vortex is just one of a 
pair of counter-rotating vortices that exist behind the turret. As will be 
seen later this vortex pair rapidly gives way to fully turbulent flow within 
three turret diameters downstream of the turret. 
540 
d. The technique of spraying oil on a model, as opposed to painting, was 
used at the Air Force Academy's Transonic Wind Tunnel during September, 
1972. 4 The objective of this test was to develop minimum drag fairings. 
Oil flow visualization was used to map separated flow regions which 
contribute to the overall drag of the configuration. The fairing desig- 
nated VJC-6 (Fig. 8) shows some separation at the rear of the fairing as 
with all aft fairing designs. The aft portion of the turret is also in 
separated flow as is again the case with most turret/fairing assemblies. 
e. Another entry into the Ames 14 ft wind tunnel with the same turret 
was termed the tlAmes II Test" conducted from October to November, 1972. 
In this test the visualization technique of oil flow was applied. Figure 9 
shows a turretlfairing combination designed by General Dynamics referred to 
as the full forward and partial aft fairing. The fence apparatus on the 
full forward portion of the fairing was designed to produce fully turbulent 
flow over the turret and eliminate adverse acoustic phenomena within the 
turret cavity. The oil was applied by a spray technique which resulted 
in a uniform "speckling" of the model surface. Hence, any separated flow 
regions would remain speckled and those of attached flow would streak. As 
can be seen the entire cutout region is in separated flow except for a 
small portion of the turret crown. Otherwise flow on the fairing is attached. 
An AFFDL design (Fig. 10) named the FDL T-2 fairing consisting of a 
turret with rear fairing only was also tested. Again the cutout region 
(this time symmetrical) is in separated flow. Flow on the turret itself 
separates at mid turret. As seen in the previous shadowgraph a shock is 
located in this region. Therefore we can attribute flow separation to a 
shock-boundary layer interaction. 
541 
I - 
f. The Air Force Academy was used in January, 1976 for testing of a 
different fairing concept now being flown on the A.L.L. KC-135 aircraft 
(Fig. 11). This involved a fairing designed with simple geometric shapes 
that was both higher and wider than the turret irself. The objective 
behind this was to obtain a more definite flow reattachment after separa- 
tion from the turret. The reattachment point now occurs within the cutout 
region between the turret and fairing. It is interesting to note the 
effect that turret cavity orientation has on the reattachment point. 
On the cavity side reattachment is delayed, while on the non-cavity side 
reattachment is early. Also evident from this figure are the characteristic 
separation at the faiqing rear as well as the diverging wake of the turret/ 
fairing assembly. 
!s- Returning to the NASA Ames 14 ft wind tunnel for further A.L.L. Cycle 
III/IV tests during October, 1976 we see the use of tufts on a 3/lOths 
scale model mounted to a flat plate (Fig. 12).5 The unsteady flow in the 
cutout region is evident from the blurred images of the tufts, indicating 
several oscillations of the tufts within the camera exposure time setting. 
The flow is attached and smooth further back on the fairing. A composite 
sketch (Fig. 13) of two photographs reveals the attached flow on the rear of 
the fairing and on the forward portion of the turret. 
h. In December of 1977 the full scale on-gimbal turret plus Cycle III/IV 
fairing was flight tested at Edwards Flight Test Center in California. 
Flow visualization consisted of tufting the turret, fairing and a large 
portion of the fuselage.6 Photographs (Fig. 14) were then taken from a 
chase plane. These reveal a significant region of unsteady flow in the 
turret/fairing cutout. The tufts in this region have either been removed 
or frayed due to the violent unsteady flow. 
542 
In 1978 during a flight from Kirtland AFB, New Mexico to Wright- 
Patterson AFB, Ohio some oil flow studies were performed on the turret/ 
fairing assembly (Fig. 15). The flow patterns correspond well with small 
scale oil flows. However, flow details are not apparent due to the higher 
viscosity oil used and its sparse application. A closeup of the cutout 
region (Fig. 16) shows attachment only one turret diameter downstream 
from the fairing leading edge. 
i. The A.L.L. Cycle III/IV fairing was used in conjunction with a similar 
on-gimbal turret in a test in support of the B-52 Short Range Applied 
Technology (SRAT) program. The test was conducted during August-September, 
1978 in the AFFDL/TGF.' Oil flow studies were made with the turret/fairing 
assembly mounted just upstream of the large vertical stabilizer (Fig. 17). 
Flow patterns on the turret and fairing are similar to previous ones. 
The only observable differences are a spreading of the turret/fairing wake 
and larger flow separation at the rear of the fairing/fuselage juncture. 
j. Finally a test was run from April to May, 1979 in the AFFDL/TGF in 
support of the Advanced Airborne Demonstrator (AAD) program.' Flow visuali- 
zation was by oil and detailed photographs were obtained. An interesting 
look at the flow about an on-gimbal turret mounted to fuselage (Fig. 18a) 
shows the double vortex pattern behind the turret. The turret cavity is at 
60 degrees azimuth to the wind axis, hence the downstream location of the 
lower vortex member. Separation on the turret is distinct as well as the 
wake formation and spreading. The coelostat turret (Fig. 18b) exhibits the 
same flow patterns except for the location of the vortex pair on the turret 
itself and a less divergent wake. 
The coelostat turret plus an aft fairing (Fig. 19a) with cutout region 
and small radius leading edges show the retention of the vortex pair. In 
543 
addition flow separation off of the leading edges and at the rear of the 
fairing exists. Partially filling the cutout region (Fig. 19b) and in- 
creasing the radii of the leading edges eliminates flow separation. 
However, there still is separation at the fairing trailing edge. 
544 
III CONCLUSION 
Although all six methods of flow visualization have been used in 
the A.L.L. wind tunnel testing program it is perhaps easy to state that 
the most useful, in terms of the amount of information gained as well as 
the minute flow details revealed, have been the oil flow and schlieren 
photography. Tuft studies are helpful but too coarse to reveal the small 
detail of any large scale structures. Their use should be restricted to 
determining separated flow regions and unsteady flow regions. 
Encapsulated liquid crystal use should probably be restricted to 
determination of transition location and shock location. 
Sublimation techniques, again being of a coarse nature, are best 
used to reveal the location of regions of high heat transfer as in vortices. 
Shadowgraph photography produces results identical to schlieren 
photography but its application is more restricted than that of schlieren. 
Finally oil flow and schlieren photography are easy methods to apply. 
Large quantities of data can be collected by these methods and the flow 
details are exceptionally clear. 
545 
REFERENCES 
1. Van Kuren, J.T. and Golden, W.J. "Transonic Ten Pin Test Phase I - 
Basic Flow Studies and Preliminary Propagation Results, "AFFDL-TM- 
75-87-FX, December, 1972 revised June, 1975. 
2. Van Kuren, J.T. afid Otten, L.J., "Acoustic Phenomena of Open Cavity 
Airborne Cassegrainian Telescopes, "AFFDL-TM-73-54-FX, May, 1973. 
3. Van Kuren, J.T. and Conner, W.R., "Transonic Ten-Pin Test Phase II- 
Configuration Studies and Open Port Pressure Fluctuations, "AFFDL- 
TM-73-159-FXM, September, 1972. 
4. Van Kuren, J.T. and Conner, W.R., "Fairing Design for Fuselage Mounted 
Turret in Transonic Flow, "AFFDL-TM-73-115~FXM, September, 1973. 
5. Van Kuren, J.T., Unpublished Data. 
6. Otten, L.J., Unpublished Data. 
7. Thomas, J.P., "Test of Turret/Fairing Configurations for the B-52 
SRAT Program", AFFDL-TM-FXM-79-21, February, 1979. 
a. Walterick, R.E., Unpublished Data. 
546 
M, = 0.55 
%a =0.60 
FIGURE 1 
ON-GIMBAL TURRET TUET F'LOW 
547 
Mm = 0.55 
.--. 
. 
M, = 0.65 
FIGURE 2 
ON-GIMBAL TURRET LIQUID CRYSTAL RESULTS 
548 
.’ 
Q 
. 
‘, i: 
. ,. .,‘: . 
’ 
FIGURE 3 
ON-GIMBAL TURRET SHOCK PATTERN, M, = 0,75 
AZ =O” 
El-O0 FLOW 
..z 
AZ = 700 
El =O” 
AZ = 40° 
El= O” 
AZ = 1200 
El = O” 
FIGURE 4 
ON-GIMBAL TURRET SHOCK PATTERN 
VARIATION WITH AZIMUTH ANGLE, M,= 0.75 
550 
FIcXJRE 5 
ON-GIMBAL TURRET SUBLIMATION, Mm= 0.75 
FIGURE 6 
ON-GIMBAL TURRET SCHLIEREN PHOTOGRAPH, M, = 0.90 
FIGURE 7 
ON-GIMBAL TURRET SCHLIEREN AND 
OIL FLOW PHOTOGFtABS, M, = 0.75 
553 
FIGURE 8 
OIL FLOW VK-6 FAIRING, Q = 0.66 
FIGURE 9 
GENERAL DYNAMICS CYCLE II TURRET/FAIRING OIL FLOW 
I 
.I------.. 
FIGURE 10 
AFFDL T-2 FAIRING OIL FLOW 
- 
FIGURE 11 
OIL FLOW CYCLE III/IV FAIRING, & = 0.77 
FIGURE 12 
TUFT FLOW CYCLE III/IV FAIRING, M, = 0.50 
. 
M =0.5 00 
Re/L=2.3540 6 t-T -’ 
FIGURE 13 
A.L.L. 3/lOths SCALE MODEL TUFT’ lIlAGRAM 
FIGURE 14 
FULL SCALE FLIGHT TUFT FLOW 
FIGURE 15 
FULL SCALE FLIGHT O'LL FLOW 
‘C ..- 
i 
M,= 0.55 _ _. 
Maa = 0.70 
FIGURE 17 
B-52 SRAT/CYCLE III/IV FAIRING OIL FLOW 
563 
ON-GIMBi*TUiRET 
b. 
COELOSTAT TURRET 
FIGURE 18 
TURRET OIL FLOWS, &, = 0.75 
564 
FULLa&JTOUT 
b. 
CUTOUT PAkTIALLY FILLED 
FIGURE 19 
COELOSTAT TURRET/FAIRING OIL FLOW, IQ, = 0.75 
565 


Flow visualization techniques in the Airborne Laser Laboratory program

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