Light beams are altered by refractive index changes; flow induced refractive index changes provide the impetus for conventional visualization techniques such as schlieren and shadowgraph. Unfortunately effects related to the flow can be masked by refractive index inhomogeneities external to the test section. A simple shadowgraph scheme was used to assess the flow quality of the Langley 0.3 meter Transonic Cryogenic Tunnel. When the penetration tubes were evacuated the quality of the shadowgraph improved dramatically

Burner, Alpheus W.

Goad, William K.

Snow, Walter L.

English

NASA Technical Reports Server

N90-12528
RECENT FLOW VISUALIZATION STUDIES
IN THE 0.3-m TCT
Walter L. Snow, Alpheus W. Burner, and William K. Goad
Langley Research Center
Hampton, Virginia
412
https://ntrs.nasa.gov/search.jsp?R=19900003212 2020-03-20T00:54:56+00:00Z
SCHEMATIC OF TYPICAL FLOW VISUALIZATION SETUP IN 0.3-M TCT
Light beams are altered by refractive index changes and indeed flow induced
refractive index changes provides the impetus for conventional visualization
techniques such as schlieren and shadowgraph. Unfortunately effects related to the
flow can be masked by refractive index inhomogeneities external to the test section.
For high-pressure cryogenic facilities this is especially true in the interface
between the region of test and the more benign environment where equipment is housed.
Since the refractive index is directly proportional to density, it depends upon
pressure and temperature through the ideal gas law. Even mild temperature
inhomogeneities can be detrimental when coupled with high pressures as occurs in the
plenum. The simple shadowgraph scheme depicted in figure 1 was used to assess the
flow quality of the Langley 0.3-Meter Transonic Cryogenic Tunnel. The light source
was a pulsed (15 microsecond) 75-watt xenon arc lamp focused onto a 0.51 mm diameter
pinhole. Light was collimated with an f/8 mirror with a 1.22-m focal length. Data
were recorded on film with a 70mm format reflex camera.
Top view - No evacuation
Pressure
shell
Equipment pod
Windows
Equipment pod Light
Folding source
Plenum mirror Window
Test section
Figure i
413
REPRESENTATIVESHADOWGRAPH RESULTS PRIOR TO IMPROVEMENTS
The simple shadowgraph scheme depicted in figure i was used to characterize the
flow quality of the Langley 0.3-Meter TCT with results shown in figure 2. This
collage of photos was taken at Mach 0.65 conditions with stagnation pressure and
temperature as indicated. The completely uniform light field which characterizes the
pretest condition becomes increasingly mottled and jumbled as temperature decreases
and pressure increases. This behavior is typical of earlier optical experiments
conducted in this facility (see refs. 41, 42, and 43).
5.0
Moo = 0.65, no evacuation
Pt' atm 2.5
1.4
180 225 270
Tt, OR
Figure 2
ORIGINAL PAGE.
BLACK AND WHITE PHOTOGRAPH
414
!MODIFIED SHADOWGRAPH SYSTEM TO EXCLUDE PLENUM EFFECTS
Several of the tests devised to determine the source of the image degradation
are described in reference 19. The crucial test involved installation of tubes to
penetrate the plenum. The tubes could be purged or evacuated to allow direct viewing
of the test section. Figure 3 shows the same layout as figure i with the hatched
area designating the region impacted by the isolation tubes. A bellows type of
arrangement was required to accommodate the large differential expansion experienced
in the structure of the facility. Details of the assembly are found in the
reference.
Top view--With evacuation in equipment pods (cross hatched)
Equipment pod
Pressure shell
Penetration assembly I I
(both sides) L_
Plenum ---_
Flow
Test section
Equipment pod
Windows
Folding
mirror Window
Mirror
Light
sou rce
Figure 3
415
EFFECT OF PLENUM EVACUATION ON SHADOWGRAPH RESULTS
When the penetration tubes were evacuated the quality of the shadowgraphs
improved dramatically. Results for the worst test condition of figure 2 (Tt - 180 R,
Pt - 5 atm, Mach 0.65) are shown in figure 4. Residual "graininess" in the evacuated
case is believed to be due to the flow field itself. Earlier difficulties were
brought about by density inhomogeneities in the regions of strong temperature
gradients especially in those areas under high pressure. Provisions for evacuated
paths to and from the test section should be seriously considered in future flow
visualization efforts for facilities of this type. In scaled up cryogenic facilities
such as the National Transonic Facility equipment is completely housed within the
plenum area (ref. 42). Much of the critical optical path would be in the controlled
environment of the equipment enclosures. Nevertheless, regions near windows where
extreme temperature gradients occur should be evacuated if possible to lessen the
effects of convection induced mixing.
No evacuation ..... With evacuation
Figure 4
ORIGINAL PAGE
1_4LAC,K AND WHITE PHOTOGRAPH
416
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