Viewing problems associated with the measurement of model deformation in cryogenic wind tunnels are discussed. Tests were conducted in the Langley 0.3-Meter Transonic Cryogenic Tunnel to assess viewing capabilities thru the flow field. The effects of condensation and turbulent boundary layers are discussed and a modelling procedure for image degradation is described

Burner, A. W.

Goad, W. K.

Snow, W. L.

English

NASA Technical Reports Server

"SEEING" THROUGH FLOWS IN LANGLEY'S 
0.3-METER TRANSONIC CRYOGENIC TUNNEL 
W. L. Snow, A. W. Burner, and W. K. Goad 
NASA Langley Research Center 
Hampton, Virginia 23665 
133 
https://ntrs.nasa.gov/search.jsp?R=19820024802 2020-03-21T06:15:03+00:00Z
INTRODUCTION 
Cryogenic wind tunnels are being developed to enable testing at realistic 
Reynolds numbers. Such facilities take advantage of the increasing density and\ 
decreasing viscosity associated with cooling. When coupled with high pressure 
operation, these tunnels can provide aeronautical engineers with full scale 
Reynolds number simulation. The high dynamic pressures available in many of these 
test environments coupled with reasonable large and slender models can effect 
considerable deformation. Wing tips are expected to deflect 2 to 3 inches over 30 
inch spans for some configurations in the NTF. The test engineer is faced with the 
dilemma of having unique data on an unknown model geometry unless ancillary measure- 
ments can provide simultaneous topological information. Virtually all of the 
proposed techniques being considered to discern model deformation including stereo 
photogrammetry, moi& topology, or surface LED trackers presume that identifiable 
targets can be viewed clearly and unambiguously thru the flow field. The tests 
described herein were made to shed light on these problems and were conducted in the 
Langley 0.3-Meter Transonic Cryogenic Tunnel. 
134 
IDEALIZED TARGET PROFILES 
For purposes of modelling the problem, it is convenient to represent the ideal 
target as a rectangular distribution of energy (or image density). When viewed under 
less than ideal circumstances the distribution will in general shift and or become 
diffuse as suggested in figure 1. These effects directly influence the accuracy of 
model deformation data. 
Figure 1 
135 
EFFECT OF CONDENSATION OYER AIRFOIL 
Condensation and turbulence are potential problem areas. Cryogenic facilities 
work close to the condensation line to maximize the cryogenic advantage. Figure 2 
shows the effect as saturation.is increased by holding pressure constant and 
decreasing temperature. These areas will be avoidable by effective use of the 
operating envelope of the facility. . As the condensation curve is approached, say 
by holding pressure constant and decreasing temperature, then 'fog' begins to 
appear. This is shown in figure 2 with density increasing from (a) to (d) with case 
(d) representing complete 'white out' as the liquid-vapor curve is crossed. 
% 
SATURATION 
Figure 2 
136 
EFFECTS OF TURBULENT BOUNDARY LAYERS 
Turbulence as suggested in figure 3 will be less forgiving and unavoidable. 
These photos from NACA BM A56B21 show focussed shadowgraphs for still air (a) and 
in the presence of flow induced turbulent boundary layers (b). Boundary layer 
effects will undoubtedly impose limits to image quality and will be assessed for NTF 
during the shakedown testing. 
(b) Tunnel Mach number = 2; missile Mzch Ilumber = 5 
Figure 3 
137 
PHOTOGRAPHY THROUGH THE SIDE WINDOWS 
To quantitatively assess image quality, the Modulation Transfer Function (MTF) 
of the optical designer is used. An image can be decomposed into its two dimension- 
al Fourier spatial frequencies. The MTF is a plot of the relative efficiency of the 
system to pass these frequencies. To measure the response, a logarithmically 
increasing density of bars (Sayce target) is photographed through the flow as shown 
in figure 4. The diffusely illuminated target is viewed through the flow using a 
70,mm camera with telephoto lens. Equipment is housed in sidemounted schlieren 
pods. 
SAYCE TARGET 
\ 
LIGHT 
SOURCE 
- 
01 FFUSER 
AIRFOIL’ 
PLENUM 
fWINOOWS 
4 
%%Z 
Y 
70 mm CaMERA 
WITH TELEPHOTO 
LENS 
Figure 4 
138 
PHOTOGRAPHS OF SAYCE .TARGETS 
An ideal system would resolve even the finest lines. The upper photo in 
figure 5 represents the mildest flow condition encountered in this facility while 
the lower shot represents a temperature induced density increase of only 2.5. 
coarsest grid lines are barely dfscemible even at these modest conditions. 
.The 
Figure 5 
139 
DENSITOMETER TRACES OF SAYCE TARGETS 
Densitometer traces corresponding to figure 5 are shown in figure 6. The top 
trace is for 1.2 atmospheres and 250 K while the lower is for 1.2 atmospheres and 
100 K. The vestiges of a shock can be seen against the Sayce background as a dip! 
in the modulation near the coarse edge of the target. 
Figure 6 
140 
PHOTOGRAPHY THROUGH THE TOP WINDOW 
To more closely approximate the model deformation format, a target was'contact 
printed on stripping film and the film appliqued to the surface of an airfoil. The 
target was photographed using the relay system shown in figure 7. 
FLIP MIRROR 
4 . 
Mmm CAhKRA - n / . - . . . . . . - --.- .r- 
WITH CLOSE-UP - 
ADAPTER 
- 
RELAY LENS 
ASSEMBLY 1 I I 
- r 
’ I 
P 
- \I 
TEST SECTiON C 
\ I \I k 1 
PLENUM 
DICON 
FLOW PERPENDICULAR To 
FIGURE 
Figure 7 
141 
DEFINITION OF THE PloDZJLATION FLJNCTION 
The photos made using the relay system were remarkably crisp. A quantitative 
evaluation was made by plotting the response as a function of spatial frequency. 
The contrast as defined in figure 8 was measured. If Imn is zero then V = 1.0 and 
you have a perfect contrast whereas if Imn equals & there is a complete redistri- 
bution of energy to a dc value, the contrast is zero and results in complete infor- 
mation loss. 
v I J, - $94 
bx + *M-J 
Figure 8 
142 
RESPONSE FJJNCTIONS 
Response functions for two test conditions obtained using both top and side 
viewing configurations are shown in figure 9. Some degradation fs expected with 
increased density but the large dfscrepancy between top and side results were 
puzzling and prompted further invest%gation. The response is defined as the ratio 
of measured to reference modulation. 
1 
.8 
al 
0 
0 
- q 
cl 
El 
0 
Frequency, lines/mm 
I I I 
I 2 3 
Frequency, I i nes/mm 
TOP 0 2% kt’ 182 Al-Ma 
omw !ioAl?% 
Figure 9 
143 
REPEAT OF SIDE WINDOW EXPERIMENT WITH EACH WINDOW TARGETTED 
The discrepancy prompted a return to the pod experiment with targets affixed to 
each window (shown shaded in figure 10) and the camera mounted on a track to allow 
viewing of each target individually at each tunnel condition. The critical station 
proved to be that nearest the camera and outside of the flow. The effect was attri- 
buted to vibration which was progressively worse as the tunnel approached its 
operating envelope. 
01 FFUSER 
AIRFOIL’ 
PLENUM 
Figure 10 
f WI NPOWS 
70 rnri CAMERA 
WITH TELEPHOTO 
LENS 
144 
REYISED RESPONSE FUNCTIONS 
When several independent mechanisms contribute, their MTF's multiply together 
to obtain the system MTF. The transfer function relating to vibration could be 
determined by examining the data for the nearest station to the camera since it was 
outside the flow. When this information was used to correct the earlier results 
(solid symbols) the data became consistent with that for the top shots as shown in 
figure 11. 
SIDE b?FETED FOR VIERATI(XJ) 
0250 K&2 ATM, 
l 
Oo o. 
0120 i(,ko ATM, 
8 
Cl Oo 0 
.2 - 
0 I 1 1 
0 1 2 3 
Frequency, lines/mm 
Figure 11 
145 
MODELLING IMAGE DEGRADATION USING EXPERIMENTAL RESULTS 
The energy distribution for an ideal target can be represented by a step 
function as shown in figure 12. This distribution can be decomposed in a Fourier 
series. Each of these coefficients is tied to a spatial frequency. If the MTF is 
unity for a particular frequency then the corresponding coefficient is unchanged. 
Otherwise it is multiplied by the fractional response thus attenuating its contri- 
bution. If the square wave is reconstructed with coefficients modified by the MTF 
depicted by open squares in figure 11 the result is shown as the attenuated and 
extended curve in figure 12. The energy is redistributed and the contrast reduced. 
Note that this was for a flow-off condition and therefore does not include turbu- 
lence effects. 
8 
6 
Figure 12 
146 
S-Y COMMENTS 
DEGRADATION DUE TO INCREASED DENSITY, THOUGH MEASIJWLE, SHOULD NOT IMPACT THE MODEL 
DEFORMATION APPROACHES. 
EFFECTS OF TURBULENT BOUNDARY LAYERS ARE UNAVOIDABLE AND WILL BE ASSESSED ON SITE. 
VIBRATIONAL DEGRADATION COULD DOMINATE THE PREVIOUS EFFECTS AND WILL BE MONITORED 
AND GUARDED AGAINST BY DESIGN OF CAMERA ENCLOSURES. 
147 


Seeing through flows in Langley's 0.3-meter Transonic Cryogenic Tunnel

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