NASA/TM-2000-210121

Flow Visualization by Elastic Light

Scattering in the Boundary Layer of a

Supersonic Flow

G. C. Herring and Mervin E. Hillard, Jr.

Langley Research Center, Hampton, Virginia

August 2000
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NASA/TM-2000-210121

Flow Visualization by Elastic Light

Scattering in the Boundary Layer of a

Supersonic Flow

G. C. Herring and Mervin E. Hillard, Jr.

Langley Research Center, Hampton, Virginia

August 2000
Available from:

NASA Center for AeroSpace Information (CASI) National Technical Information Service (NTIS)

7121 Standard Drive 5285 Port Royal Road

Hanover, MD 21076-1320 Springfield, VA 22161-2171

(301) 621-0390 (703) 605-6000

Acknowledgments

We are pleased to thank R. W. Gregory and W. E. Lipford for help with the optical setup, K. Stacy for image processing

and video production, J. Thibodeaux for overseeing the model preparation, and the UPWT operations staff.

The use of trademarks or names of manufacturers in this report is for accurate reporting and does not constitute an

official endorsement, either expressed or implied, of such products or manufacturers by the National Aeronautics and

Space Administration.
Abstract

We demonstrate instantaneous flow density visualization of the

boundary layer region of a Mach 2.5 supersonic flow over a flat plate

that is interacting with an impinging shock wave. Tests were performed in the Unitary Plan Wind Tunnel (UPWT) at NASA Langley

Research Center. The technique is elastic light scattering using

10-nsec laser pulses at 532 nm. We emphasize that no seed material of

any kind, including water (H2O), is purposely added to the flow. The

scattered light comes from a residual impurity that normally exists in

the flow medium after the air drying process. Thus, the technique

described here differs from the traditional vapor-screen method, which

is typically accomplished by the addition of extra H2O vapor to the airflow. The flow is visualized with a series of thin two-dimensional light

sheets (oriented perpendicular to the streamwise direction) that are

located at several positions downstream of the leading edge of the

model. This geometry allows the direct observation of the unsteady

flow structure in the spanwise dimension of the model and the indirect

observation of the boundary layer growth in the streamwise dimension.

Introduction

Langley Research Center’s (LaRC) Unitary Plan Wind Tunnel1 (UPWT) is one of three nearly identical supersonic flow facilities in the U.S. that has been heavily used in the development of supersonic

aircraft and missiles since the 1950’s. In addition to schlieren, the only other standard method for offbody flow visualization in these tunnels is vapor-screen2–4 visualization, sometimes referred to as laser

light sheet visualization. In this method, water (H2O) is injected into the facility to increase the light

scattering cross section of the flow medium. An Argon-ion laser beam is then swept through the flow

near the model by using a scanning mirror to direct the beam into the tunnel test section. The scattered

light that visualizes flow structures is then recorded on videotape for later data analysis. These images

are acquired at the standard video rate (i.e., integrated over 1/30-sec); thus, short-lived supersonic flow

structures will be washed out when imaged over this relatively long 1/30-sec window. Also, it is wellknown that large amounts of H2O in the flow medium can significantly modify the flow parameters. For

example, the increased drag on a model or the increased pressure reading of a pitot tube are two typical5

indications used to detect strong condensation effects in transonic wind tunnels. Additionally, freestream stagnation conditions are known3,6 to be altered up to ∼10% after the condensation of large

amounts of H2O in the UPWT. Flow visualization without H2O addition can minimize these problems.

In this report, we discuss flow visualization at the UPWT by using elastic light scattering off the dry

flow medium with high-intensity, short-pulse lasers. This type of flow imaging complements the capabilities of the currently used methods of schlieren and vapor screen. The elastic light scattering method

is truly noninvasive (i.e., no probes or excessive H2O in the flow and no optical excitation of the flow

medium). In addition, this technique can capture high-speed temporal structure and image it with good

spatial resolution. With Nd:YAG lasers (Neodymium : Yittrium Aluminum Garnet, i. e., Nd3+ : Y3 Al5

O12), the flow can be imaged in 10−8 sec, which effectively freezes the flow down to scales of 0.01 mm.

Most flow visualization reported before ∼1990 involved visualizing the scattered light from some type

of seed material (i.e., not a component of typical air), where the question arises of “how well does the

seed material disperse and follow the airflow?” At the molecular level, Rayleigh scattering does not suffer from this potential problem.
2

Laser-based elastic light scattering for two-dimensional flow visualization of low-speed fluids consisting of heavy molecules was demonstrated7,8 about 15 years ago by using visible wavelengths. This

technique was extended9 to supersonic airflow by using the relatively larger scattering cross section at

ultraviolet (UV) wavelengths. More recently, this method has been tested10 in large-scale flow facilities

and subsequently used for experimental studies11 of the dynamics of fuel-oxidizer mixing at Mach 6.

Another attraction of elastic light scattering, in the absence of cooling-induced condensation12–15 of the

flow constituents, is that currently available high-power lasers are intense enough to illuminate the flow

at the molecular level without seeding any foreign material into the flow. This situation allows for quantitative off-body flow field density measurements16,17 with high spatial resolution in two dimensions.

Many laboratory-type demonstrations of elastic light scattering have been made over the last five years.

For example, density, temperature, and velocity imaging17–19 in one and two spatial dimensions has

been demonstrated.

Even with the relatively weak scattering at the visible wavelength of 532 nm, scattered light levels

are strong enough to visualize the flow (without seeding) at Mach 2.5 inside the UPWT at NASA LaRC.

We conjecture that a normally occurring low-level impurity in the flow of this particular wind tunnel

has a large scattering cross section compared to molecular air. We are able to visualize the flow without

the injection of H2O into the wind tunnel because the light scatter by this impurity is strong. The scattering signals are strong enough to visualize flow features with a single 10-nsec laser pulse, so these data

represent instantaneous images with the flow frozen in time. We present 2-dimensional images of a flat

plate boundary layer flow that is perturbed with an impinging shock wave. This work extends previous

laboratory work on boundary layer visualization to a large-scale facility that is often used by the aeronautical industry.

The shock wave/boundary layer interaction model that we study in this work was originally

designed and built for another project (unpublished work by Ira J. Walker, Lockheed Engineering and

Sciences, 1993). We originally worked with this model to obtain nonintrusive off-body stimulated

Raman data for comparison to surface pressures. During the course of the Raman work on this model,

we noticed some anomalous intensity noise on one of the Raman laser beams. We postulated that this

extra intensity noise might be due to beam steering from turbulent, unstable, or unsteady flow near the

model. To help analyze the Raman data, we decided to use some tunnel time to obtain Rayleigh images

of the boundary layer. However, the Rayleigh images provide useful flow visualization; thus, we

present them in this report.

To illustrate the widespread potential of this type of flow visualization, we also discuss onedimensional flow visualization in the supersonic vortices above a delta wing. This vortex-related work

was performed prior to the shock wave/boundary layer work and in the same wind tunnel at Mach 2.8. It

was this unanticipated vortex visualization that prompted us to try the boundary layer visualization.

Experimental Setup

Unitary Plan Wind Tunnel

Figure 1 shows a schematic of the experimental configuration used for the shock wave/boundary

layer interaction model. We use a Q-switched Nd:YAG laser that is frequency doubled to wavelength

λ = 532 nm and that delivers 120 mJ per laser pulse (10 pulses/sec) into the test section of the wind tunnel. By using a 91-cm focal length cylindrical lens, the beam is focused into a thin, vertical light sheet

that is 1 cm high. The thickness of this sheet along the free-stream flow direction is about 200 μm and is

approximately constant over the 38-cm width of the model. The viewing angle θ is 30 deg, and the laser

beam is linearly polarized in the free-stream direction. The detector is a Princeton Applied Research
3

intensified-charge-coupled device (ICCD) camera that images the laser beam region with either an f/4.5

Nikon or a 75 mm Fujinon zoom lens.

All images shown in this report were acquired with the bottom edge of the laser sheet positioned

about 1.2 cm above the lower surface of the model. Thus, we are probing the boundary layer region

from 1.2 to 2.2 cm above the lower plate surface at five streamwise locations. This height was as close

as we could put the laser beam to the surface without generating significant scatter from the surface

because we made only a quick attempt to aperture the beam as it transited from the laser to the wind tunnel. This lack of effort was dictated by the short time available to execute this work. With longer

planning and setup time, we would be able to better reduce the scattered light and get the beam

significantly closer to the surface, if required for thinner boundary layers. However, for the flow and

model combination studied here, 1–2 cm from the surface is close enough to observe the boundary layer

height that varies from near zero at the leading edge to about 2–3 cm at some downstream locations.

This work was performed in the 1.2 by 1.2-meter test section 2 at the UPWT of LaRC. The freestream flow conditions are Mach 2.5 and unit Reynolds number 6.5 × 106

 per meter. For these tests, the

flow medium is dry air. A schematic of the shock wave/boundary layer interaction model is shown in

figure 1. Figure 2a shows the model mounted in the test section, while figure 2b shows the view that we

see from our camera, with the Fujinon zoom lens adjusted for a relatively wide field of view. The dark

protrusions in the bright light sheet illustrate the unsteady edge of the boundary layer, where lowdensity boundary layer fluid mixes into the denser free-stream fluid. In figure 2b, the laser beam is

located 7.6 cm downstream of the shock impingement point. Replacing the Fujinon lens with the Nikon

f/4.5 lens allows us to see an image, under higher magnification, of one half of the field of view of figure 2b. Figure 3 shows an example of the light sheet with the higher magnification. The schematic of

figure 4 shows the five locations on the model where light-sheet data were acquired. For our 2.5-cmdiameter lenses, the solid angle of collection is about 5 × 10−4

 sr.

Laboratory Calibration

We reproduced our wind tunnel setup in the laboratory after the tunnel tests were completed to calibrate the absolute strength of the scattered signals from the UPWT. In the laboratory setup, we used the

same beam geometry, laser powers, and signal collection geometry as in the facility. We used small

sample pieces of the schlieren quality windows from the UPWT to reproduce beam intensity reductions

due to reflections. A low-speed flow of bottled Nitrogen (N2) at the exit of an 8-cm diameter pipe was

used to create a dust-free sample region. Using this setup, we measured the magnitude of the molecular

Rayleigh scattering at 1 atm and 300 K on our camera. Since the two setups are so similar, these laboratory data are used to estimate the strength of the UPWT facility signals relative to the molecular air signals in the laboratory.

Results and Discussion

Laboratory Calibration

Comparing the laboratory signals from dust-free N2 to the signals from the UPWT facility, we conclude that the free-stream flow from the facility produces a scattered light level that is about 500 times

larger than what is expected from Rayleigh scattering from pure N2 or air. Inspection of the images

obtained in the free stream (see fig. 5a) shows that the scattered light level is uniform on all scales that

we can resolve (from the width of the model to the limit of our resolution, about 300–500 μm) with the

Nikon lens. Therefore, the scattered light signals that we detect are likely from a normally occurring
4

impurity that is uniformly distributed throughout the airflow. We are definitely not directly detecting

the N2 and oxygen (O2) molecules of the airflow.

Nature of the Scattering Species

Scatterers whose size is much less than the light wavelength λ produce Rayleigh scattering, while

scatterers whose size is much greater than λ produce Mie Scattering. These two types of scattering have

different magnitudes and angular dependencies.

If the flow constituent responsible for the scattered signal is not from air molecules, we need to

determine what it is, how uniformly it is mixed with the air, and how well it follows the supersonic

flow. Oil is known to be in the flow (e.g., oil accumulates on the test section windows after running the

tunnel at high Reynolds number [1.3 × 107

 per meter] for long periods of time). Several possible impurities are (1) dust or dirt particles; (2) small clusters, or large droplets of condensed oil; (3) clusters or

ice crystals of residual H2O; and (4) clusters of argon (Ar), carbon dioxide (CO2), N2 or O2 molecules.

The arguments, detailed below, concerning the nature of the scattering centers and our conclusions

based on these arguments are summarized in table 1. The possibilities are listed in order of increasing

probability for contributing to the observed signal, as determined by our subjective decision based on

the comments in the right-side column. Because of the relatively large static temperature of 150 K and

relatively small static pressure of 0.05 atm in the test section free-stream flow, we strongly believe that

it is unlikely that the air molecules (except CO2) are clustering or condensing significantly. The temperature of 150 K is well above the boiling point for N2, O2, and Ar for a pressure of 0.05 atm.

Using some qualitative observations of the data images, we can eliminate some of the possible

sources of scatter that are listed above. If one ignores the expected variation due to the nonuniform

Table 1. Summary of Potential Scattering Species in LaRC’s UPWT, Along With 

Qualitative Estimates for Occurrence of Each Species

Possible

scatterer

Chance of

occurrence

Comment

N2 clusters Low Not likely at 150 K and 0.05 atm

O2 clusters Low Not likely at 150 K and 0.05 atm

Ar clusters Low Not likely at 150 K and 0.05 atm

Mixed molecule 

clusters

Low Literature does not contain much discussion 

on mixed species clusters.

Dust or dirt Medium a) Data clearly shows dia. ≤ 200 mm.

b) Data also suggests dia. ≤ 86 nm.

c) UPWT contains no filters; very small 

(<10 μm) dust is possible.

CO2 clusters Medium We estimate 3000 CO2-CO2 collisions in one 

nozzle transit.

H2O clusters Medium We estimate 3000 H2O-H2O collisions in one 

nozzle transit (i.e., [H2O] ≈ [CO2] for dew 

point = 244 K).

Oil clusters High Oil condenses on windows for long runs at 

large Reynolds numbers.
5

beam profile, the images that are obtained in the free-stream flow show uniform light scattering over

large regions of flow. The very bright scatter from room air dust that typically saturates many adjacent

detector pixels is almost completely absent when the tunnel is running. We also note that the large signal from typical dust at atmospheric pressure in the test section (with no flow) is about 30 times larger

than the impurity signal that we detect at Mach 2.5 flow conditions. Large particle scatterers cannot produce the relatively small and uniform signals that we see in the free-stream flow data. Thus, we do not

put large (>> λ ) dirt, dust, or oil particles high on our list of likely scatterers, but we cannot rule out

very small (≤ λ) sizes of these species.

Using some details of the collection geometry, we can estimate10 the minimum density of the scatterers. We assume that there must be at least one scatterer or more per volume element that is imaged

onto each camera pixel because the free-stream images are so uniform. The camera pixels are

11.5 × 25 μm in size and the magnification of our collection lens is 1/16. Since the laser sheet is about

200 μm thick, we estimate that each camera pixel images 3 × 10−5

 cm3. The reciprocal of this number,

3.3 × 104

 cm−3, is the minimum number density that the scattering species must have to obtain one

imaged scatterer per camera pixel and produce the uniform signal that is observed.

Next, we can use this minimum number density to estimate the maximum size of the scatterers.

Recall that the free-stream signals are about 500 times that of dust-free room air and that the size of

room air molecules is about 0.1 nm. Assuming that the scattering process is Rayleigh, rather than Mie,

the scattering cross section is proportional to the particle diameter to the 6th power, thus allowing one to

write the equality,

[500] Nr [0.1 nm]6 = Ni

 [D]6,

where Nr is the room air molecular density, Ni

 is the wind tunnel impurity number density, and D is the

maximum possible size of the impurities in units of nm. Thus, we obtain D = 86 nm for the maximum

size of the free-stream impurity scatterers.

Since the laser light wavelength λ is 532 nm, we cautiously conclude from this maximum size that

the scattering process is in the Rayleigh (λ >> D) regime. However, note that we had to assume that the

process was Rayleigh to use the fact that the signal is proportional to the 6th power of the particle diameter. Mie scattering (λ << D) signals will vary much less than the 6th power of diameter. Thus, the argument of the last paragraph is somewhat circular, and we lose some confidence in the conclusion that the

scattering process is Rayleigh. We cannot definitively rule out Mie scattering from particles (≈ few μm

in diameter), where several of these particles could potentially fit in the sample volume of each camera

pixel to yield the uniform images that we observe.

Residual Water

The typical dew point of the flow is about 244 K, depending on the outside weather. With this small

amount of H2O in the flow and the 10-m length of the expansion nozzle, we roughly estimate only 3000

H2O-H2O collisions during the expansion. Assuming that clustering occurs under these conditions, that

every collision sticks (i.e., 3000 molecules per cluster), and that there is 10−7

 Torr vapor pressure of

H2O at 150 K, we then estimate an H2O cluster density of 2 × 106

 cm−3. This density is two orders of

magnitude larger than the previous estimate for the minimum scatterer density. Thus the estimated H2O

cluster density is consistent with our observation of uniform scattering on our smallest resolved spatial

dimensions, as calculated in the preceding two paragraphs. We cannot use this calculation to rule out

H2O as the scatterer. However, we present a last argument that implies that H2O condensation is

unlikely to contribute to our signals.
6

The following argument contains three unproven assumptions and should be taken as our opinion of

the most reasonable scenario, but not necessarily as rigorous fact. Consider that the scatterer is not H2O.

Assume that the scatterer is mixed such that it makes up a constant mole fraction of the flow density.

We compare the brighter (larger density) free-stream signals to the dimmer (smaller density) boundary

layer signals. For most of the data, the bright regions of the free stream are measured to be about 2.7

±0.6 times brighter than the dark regions of boundary layer material that is mixing upward. This measurement is the average of 15 ratio measurements made from 15 different individual video frames. The

uncertainty of ±0.6 is the statistical (1 σ = 68% confidence) error for the sample of 15 ratios. The 15 different frames were chosen from light sheet locations 2–5 of figure 4 and there is no obvious trend in the

measured ratio with location. Stray background light from window scatter and the ICCD dark current

are approximately accounted for in each individual frame by measuring the signal levels in pixels just

below the edge of the laser beam and subtracting this level from the total signal level in the laser beam

regions of interest. This approximate method for subtracting stray light and dark current backgrounds is

used since it is not possible (due to leaks) to evacuate the UPWT test section to a density that is much

lower than the static density during the run, which would allow a more accurate background subtraction

on a pixel by pixel basis.

It is useful to compare this measurement with the expected density ratio. Assuming isentropic

expansion throughout the nozzle, we can calculate the free-stream density. If we assume constant pressure and that the boundary layer regions are frictionally heated back to the stagnation temperature

(temperature recovery = 1), then we also know the boundary layer density. The calculated ratio of freestream to boundary layer density is the ratio of the stagnation temperature to the free-stream temperature or about 325 K/150 K = 2.2, in agreement with the measurement of 2.7 ±0.6. Thus the observed signals are proportional to the two densities (free-stream and boundary layer) that we know with some

degree of confidence. This result is consistent with the assumption that the impurity that is responsible

for the scattered signal is uniformly mixed and follows the flow reasonably well. For example, if the

flow density changes by a factor of two, then we expect that the impurity density and the scattering signal will also change by a factor of two.

On the other hand, consider that the scatterer is condensed H2O that is partially or completely evaporated in the hot boundary layer and undergoes increased clustering when cooled to the free-stream temperatures. Although not impossible, it is a remarkable coincidence if the increase in signal is also a

factor of two — due to an increase of the clustering rate of the cool free-stream relative to the warm

boundary layer. Thus, we also believe, although less confidently than for the other possibilities, that it is

unlikely that H2O is clustering or condensing just enough to provide the observed scattering signal that

seems to be proportional to density.

Determination of the Scattering Species

Based on these arguments, we believe that the strongest candidate for our signal source is small

clusters of oil in the flow. Unfortunately, we can not specify with absolute confidence what is contributing to our scattered light signal. The somewhat speculative nature of some of the previous arguments is

less than satisfactory, and more work is necessary to identify the scatterer unambiguously.

In the next three sections, we shall assume that a uniform (i.e., constant mole fraction) oil impurity

in the flow consists of small clusters that accurately follow the flow and make the only contribution to

our scattered light signal. Thus, the signal is proportional to the total flow density. Hence, regions of

bright signal indicate higher airflow densities and dimmer signals indicate smaller airflow densities. In

figures 2b, 3, and 5, the small, cloud-like dark regions that protrude upwards into the narrow bright band

of the laser light sheet are eruptions of hot (∼325 K) boundary layer material. These hotter regions are
7

just starting to mix upward into the colder (∼150 K) and denser free-stream flow that is more brightly

illuminated by the laser light sheet. If we are wrong, and the scattering species is H2O, for example, it

will not significantly change any of the discussion of the flow visualization that follows.

Visualization of Shock/Boundary Layer Region

We have visualized the flow in two-dimensional sheets perpendicular to the streamwise direction,

concentrating on the boundary region (i.e., 1–2 cm above the surface) between the free-stream flow and

the boundary layer. We initially used the Fujinon zoom lens to obtain relatively wide fields of view. An

example is the image of figure 2b, which shows a segment of the laser light sheet that is about 30 cm

long. We have also imaged narrower fields of view by using the f/4.5 Nikon lens, where the imaged

region is shown in figure 3.

All images presented in this report do not have the stray background contributions subtracted. However, the signal-to-background ratio is sufficiently large to display clear flow structure.

One example of a raw data image is shown in figure 3a, which shows a magnified view from the

central region of the model that is centered on the centerline of the model. The width of this field of

view is about 15 cm, i.e., less than half of the 38 cm-width of the model. Figure 3b shows the same data

after the contrast has been enhanced for display purposes. This is the same field of view that is displayed

in figure 5. The purpose of figure 3 is to convince the reader that we have not unreasonably altered the

raw images with our image enhancement process. Details of the image enhancement of the original 8-bit

images are as follows. Linear contrast stretching and image smoothing are both used. The linear contrast

stretching technique involved choosing two gray levels (here 20 and 100) in the original image and linearly mapping each gray level value within this specified range onto the entire gray level range of 0

to 255. All gray level values that fall below and above the specified range are clipped to zero (black) or

255 (white), respectively. The image-smoothing technique reduces high-frequency noise by replacing

the gray level at each pixel with the average of the gray levels in a 3 by 3 neighborhood.

To sequentially probe different locations, we have qualitatively visualized the growth of the turbulent boundary layer thickness as a function of distance downstream from the leading edge of the model

by translating the model in the streamwise direction. A two-dimensional composite of the model and

five laser light sheet locations are shown in figure 4. The five locations are (1) 5 cm upstream of the

leading edge, i.e., the free stream; (2) 16 cm downstream of the leading edge, i.e., about 1/2 the streamwise distance down the shock-generating ramp; (3) 22 cm downstream of the leading edge, i.e., the

shock impingement point; (4) 5.1 cm farther downstream of the impingement point; and (5) 7.6 cm

downstream from the impingement point.

Figures 5a–e show eight successive single-shot images obtained for each of the five different

streamwise locations that are shown in figure 4 and described in the preceding paragraph. In other

words, figure 5a shows eight sequential images from position 1, the free-stream location of figure 4; figure 5b shows eight sequential images from position 2, 1/2 way down the ramp and so forth. Each individual image of figure 5 is a 10-nsec shot of flow field and hence gives an instantaneous picture of the

upper boundary layer structure. At each location, the eight different images were acquired at a 10-Hz

repetition rate. The data at each location were typically acquired about 2–5 minutes after the preceding

upstream location. All images of figure 5 are enhanced in the same manner as figure 3b.

The net result of figure 5 is to show both the instantaneous flow structure that is transverse to the

streamwise direction in each image and the growth or decay of the boundary layer with streamwise

location over the entire set of images. Location 2 is the furthest upstream position where we are able to
8

see the first hints of transverse structure at this height off the surface. All positions upstream of this

location give images that show a uniform light sheet and are indistinguishable from images at the freestream location; we interpret this to mean that the boundary layer thickness in smaller than 1.2 cm,

which is the distance from the lower edge of the laser sheet to the lower surface of the model. Hence, we

have not illustrated any locations between the leading edge and location 2 in figures 4 and 5.

At location 2 and farther downstream, we see the roughly bimodal structure that is shown throughout figures 5b–e. By bimodal, we mean that the bright regions of the laser sheets usually display about

twice the signal level as the darker cloud-like structures that protrude up into the bottom of the sheet.

We interpret these darker regions as hot boundary layer gas mixing upward into the brighter regions of

colder free-stream flow. The images of figure 5 vividly illustrate the spanwise inhomogeneity of the

boundary layer on an instantaneous timescale. Moving downstream, the maximum height of these structures occurs near location 3, the shock impingement point. Farther downstream at locations 4 and 5, the

maximum height of these regions decreases, as shown in the figure.

Thus, we observe a boundary layer thickness that increases with increasing downstream distance

from the leading edge and reaches a maximum height at the position (i.e., location 3 in fig. 4) where the

shock impinges on the lower plate. Downstream from this location, the boundary layer thickness begins

to decrease, with farther increasing distance downstream. This variable boundary layer height is consistent with typical descriptions20–22 of shock/boundary-layer flow fields, where the boundary layer separates from the surface near the shock impingement point and reattaches farther downstream. This

behavior is also in agreement with schlieren data obtained on this model. At the impingement point,

schlieren data show a maximum boundary layer thickness of about 2 cm, consistent with the maximum

height (∼2.2 cm or larger) of the flow structures in figure 5.

The centers of the light sheets of the figure 5 images correspond to the center of the model. Inspection of all figure 5 images shows that the hot boundary layer protrusions into the cold free-stream flow

seem to occur more often on the left-hand side of the laser sheet than on the right-hand side. Note that

attenuation of the laser beam, as it transits the test section, is negligible and thus does not play a role in

this observation. This observation is confirmed by reviewing about 10,000 total images (spanning 2–

3 hours of run time) that were acquired during the entire test. At any location that shows the mixing

structures, we always see a thicker boundary layer, on the average, on the right-hand side, as compared

to the left-hand side. We believe that, for whatever reason, this observation suggests that the boundary

layer is growing asymmetrically over the bottom plate for these particular setup and run conditions, thus

illustrating a spanwise inhomogeneity of the boundary layer in a time-integrated sense. This kind of

observation is not possible with the standard schlieren data because schlieren signals are averaged over

a line of sight.

Note that the shock structure was too weak to detect with this particular combination of laser intensity, wavelength, and viewing angle.

One-Dimensional Visualization

During the stimulated Raman experiments that immediately preceded this flow visualization work,

we had visualized this same boundary layer region with the laser beam focused to a narrow line that

crossed the model at the same locations as the sheets shown in figure 4. Thus, we imaged narrow lines

of flow field that are perpendicular to the streamwise direction and parallel to the bottom plate. The

laser energy/pulse was about 40–50 mJ, and the 532 nm light was focused to minimum diameter of

about 200 μm at the centerline of the model. The line of laser light was placed 8 mm above the surface
9

of the lower plate. The scatter by the flow medium was large enough that the line of laser light could be

seen with the naked eye in the near forward and backward directions.

The results of this one-dimensional variation of flow visualization are in qualitative agreement with

the results of the sheet imaging described previously and are illustrated with reproductions of hand

sketches (that we made while observing, by naked eye, the beam in real time) in figure 6. The data were

recorded in this manner only because we were in the middle of Raman experiments and not able to

acquire images with the ICCD camera. Three streamwise locations are shown in figure 6, corresponding

to positions 1, 3, and 4 of figure 5. In the free stream, the beam brightness was uniform in both time and

the spanwise direction. At shock impingement point 3, the beam became completely invisible inside the

model boundaries but remained steadily bright outside the model boundaries. Farther downstream at

position 4, the beam appeared intermittent in both time and the spanwise dimension inside the model,

but remained steadily bright outside the model.

These results are consistent with both the schlieren and sheet-imaging results described previously.

At the impingement point, the low-density boundary layer has grown thick enough (and has also

separated from the surface) to completely overlap the beam and reduce the scattered signal from regions

inside the model. At downstream position 4, the boundary layer has decreased just enough that some of

the largest hot regions of boundary layer material protrude up into the beam to create several dark spots

in the beam across the width of the model. The spatial intermittence of the laser beam rapidly varies

with time, illustrating the high-speed temporal dependence of the boundary layer height as a function of

spanwise location. This temporal behavior is washed out in most conventional space-averaged or timeaveraged schlieren images.

Delta Wing Vortex Visualization

To illustrate the general applicability of this technique, we briefly describe the results of flow visualization of the vortices above a delta wing in a Mach 2.8 supersonic flow. This work was done in the

UPWT prior to the boundary layer visualization. The angle of attack is 12 deg. The energy/pulse is

40 mJ. In the delta wing work, the 532-nm laser beam is not focused into a sheet but is again focused

into a line (perpendicular to the model centerline) with a 120-cm focal length spherical lens. Thus only

line images perpendicular to the free-stream direction can be visualized.

The geometry is shown in figure 7, where the free-stream flow is out of the plane of the paper in the

end view depictions of figures 7a and 7b, and is from left to right in the side view depiction of figure 7c.

The line of the focused laser beam is perpendicular to the free-stream flow, and the flow was visualized

by naked eye observation in the near forward direction, as illustrated in figure 7a. The results were

recorded with hand sketches and are shown in figure 7. In the free-stream flow, i. e., upstream of the

model, the laser beam appeared as a uniform and steadily bright line where the beam crossed the tunnel

test section (as shown in fig. 7a). The model was translated along the direction of the flow until the

beam was located about 6 mm above the delta wing, as shown in the side view of figure 7c. Primary and

secondary vortices were clearly observed as abrupt “dark holes” in the laser beam, as shown in

figure 7b. We emphasize that this visualization was done without any seeding or purposeful increase in

the normal amount of H2O (dew point = 244 K) in the flow.

This flow visualization was performed during the same time frame as that used for the acquisition of

both nonintrusive Raman data and five-hole-probe velocity data (unpublished work, James E. Byrd,

Lockheed Martin, 1993). The probe was mounted such that it could be translated, along the streamwise

direction, from far downstream of the trailing edge of the model to the position just upstream of the

trailing edge that contained the laser beam. The probe was used as a reference marker for the placement
10

of the crossing position of the Raman beams by locating the crossing inside of the vortex at a known

probe position. On one day of Raman data acquisition, we backed the probe far downstream of the

model after positioning the Raman beams. On a second day, the probe tip was left just upstream of the

trailing edge after positioning the Raman beams. We believe that we observed a significant difference in

the transverse position of the vortex between these two longitudinal probe positions. Two problems

(i.e., the limited tunnel time and the naked eye detection with the awkward geometry of observing along

a line in the near forward direction of the laser beam) hindered this observation. The less-than-ideal

nature of this observation leaves us uncertain of the tentative conclusion that the presence of the probe

near the trailing edge moves the position of the vortex. However, this observation at supersonic velocities is consistent with a similar report23 of a seven-hole-probe that influenced the position of a vortex at

subsonic speeds.

Summary

We have demonstrated the visualization of the boundary layer regions of an inlet model in the normal unmodified test gas of a Mach 2.5 free-stream flow in NASA-Langley’s UPWT facility. Flow

visualization has been performed previously with schlieren or by seeding large amounts of extra H2O

into the flow. We have shown that one can use 532 nm to visualize the supersonic flow in a large-scale

facility because of the relatively high light scattering cross section of an impurity contained in the dry

air flow. With the caveat that the impurity is uniformly mixed and follows the flow, the signal is proportional to total density, and the images presented here are representative of the total flow density.

Although the nature of the impurities that provide our scattering signal are not known with complete

confidence at this time, there is evidence that suggests that the size of these impurities is less than

86 nm. We believe that the two most likely possibilities for the light scatterers that we observe are oil or

residual H2O.

Our geometry allows us to observe the nonuniform flow structure in the dimension transverse to the

free-stream flow direction. We also observe the growth of the boundary layer height due to an impinging shock wave. This work illustrates the potential of future laser-based, instantaneous, nonintrusive

flow visualization of boundary layer regions in this (or a similar) facility. In future work, a geometry

orthogonal to the one used in this work would allow the instantaneous flow visualization of the boundary layer, including transition from laminar to turbulent flow, in a streamwise direction on an instantaneous timescale. Quantitative measurements, if possible in the UPWT, will require more detailed work

of calibrating the raw data in the wind tunnel itself and account for stray light backgrounds and other

possible systematic errors. We have also demonstrated the visualization of the vortices generated by

supersonic flow over a delta wing using the same technique as used in the boundary layer work.

For future work, we suggest tests to investigate the nature of the scattering signal that we are

observing in the UPWT facility. Precise knowledge of the scattering species would give more confidence in any aerodynamic conclusions that are drawn from work based on the noninvasive flow visualization technique described in this report.
11

References

1. C. M. Jackson, Jr., W. A. Corlett, and W. J. Monta, “Description and Calibration of the Langley Unitary Plan

Wind Tunnel,” NASA TP-1905 (November 1981).

2. W. L. Snow and O. A. Morris, “Investigation of Light Source and Scattering Medium Related to Vapor-Screen

Flow Visualization,” NASA TM-86290 (December 1984).

3. O. A. Morris, W. A. Corlett, D. L. Wassum, and C. D. Babb, “Vapor-Screen Technique for Flow Visualization

in the Langley Unitary Plan Wind Tunnel,” NASA TM-86384 (July 1985).

4. I. McGregor, “The Vapour-Screen Method of Flow Visualization,” J. Fluid Mech. 11, 481–511 (1961).

5. R. M. Hall, “Studies of Condensation Effects on Airfoil Testing in the Langley 0.3-Meter Transonic Cryogenic

Tunnel.” NASA TP-2509 (January 1986).

6. P. P. Wegener and L. M. Mack, “Condensation in Supersonic and Hypersonic Wind Tunnels,” in Advances in

Applied Mathematics, Vol. 5, edited by H. L. Dryden and Th. von Kármán, Academic Press, NY, pp. 307–447

(1958).

7. M. C. Escoda and M. B. Long, “Rayleigh Scattering Measurements of the Gas Concentration in Turbulent

Fields,” AIAA J. 21, 81–84 (1983).

8. B. Yip, D. C. Fourguette, and M. B. Long, “Three-Dimensional Gas Concentrations and Gradient Measurements in a Photoacoustically Perturbed Jet,” Appl. Opt. 25, 3919–3923 (1986).

9. M. Smith, A. Smits, and R. Miles, “Compressible Boundary-Layer Density Cross Sections by UV Rayleigh

Scattering,” Opt. Lett. 14, 916–918 (1989).

10. B. Shirinzadeh, M. E. Hillard, and R. J. Exton, “Condensation Effects on Rayleigh Scattering Measurements in

a Supersonic Wind Tunnel,” AIAA J. 29, 242–246 (1991).

11. B. Shirinzadeh, M. E. Hillard, R. J. Balla, I. A. Waitz, J. B. Anders, and R. J. Exton, “Planar Rayleigh Scattering Results in Helium-Air Mixing Experiments in a Mach-6 Wind Tunnel,” Appl. Opt. 31, 6529–6534 (1992).

12. J. V. Becker, “Results of Recent Hypersonic and Unsteady flow Research at the Langley Aeronautical Laboratory,” J. Appl. Phys. 21, 619–628 (1950).

13. P. P. Wegener, “Nucleation of Nitrogen: Experiment and Theory,” J. Phys. Chem. 91, 2479–2481 (1987).

14. P. P. Wegener, Chapter 4, in Nonequilibrium Flows, Part 1, edited by P. P. Wegener, Marcel Dekker, Inc, NY,

pp. 163–243 (1969).

15. B. Shirinzadeh, M. E. Hillard, A. B. Blair, and R. J. Exton, “Study of Cluster Formation and its Effects on Rayleigh and Raman Scattering Measurements in a Mach 6 Wind Tunnel,” AIAA paper # 91-1496, 22nd Fluid

Dynamics, Plasma Dynamics, & Lasers Conference, Honolulu, HI (24–26 June 1991).

16. B. Shirinzadeh, R. J. Balla, and M. E. Hillard, “Quantitative Density Measurements in a Mach 6 Flow Field

Using the Rayleigh Scattering Technique,” 1995 ICIASF, Wright-Patterson AFB, OH (18–21 July 1995).

17. R. G. Seasholtz, “Instantaneous 2D Velocity and Temperature Measurements in High Speed Flows Based on

Spectrally Resolved Rayleigh Scattering,” AIAA paper # 95-0300, 33th Aerospace Sciences Meeting, Reno,

NV (1995). 

18. R. B. Miles, J. N. Forkey, and W. R. Lempert, “Filtered Rayleigh Scattering Measurements in Supersonic/

Hypersonic Facilities,” AIAA paper # 92-3894, 17th Aerospace Ground Testing Conference, Nashville, TN

(6–8 July, 1992).

19. G. Grünefeld, V. Beushausen, and P. Andresen, “Planar Air Density Measurements Near Model Surfaces by

Ultraviolet Rayleigh/Raman Scattering,” AIAA J. 32, 1457–1463 (1994).

20. U. Domröse, E. Krause, and M. Meinke, “Numerical Simulation of Laminar Hypersonic Shock-Boundary

Layer Interaction,” Z. Flugwiss. Weltraumforsch. 20, 89–94 (1996).
12

21. F. S. Alvi and G. S. Settles, “Physical Model of the Swept Shock Wave/Boundary-Layer Interaction Field,”

AIAA Journal 30, 2252–2258 (1992).

22. Henckels, A. F. Kreins, and F. Maurer, “Experimental Investigation of Hypersonic Shock-Boundary Layer

Interaction,” Z. Flugwiss Weltraumforsch 17, 116–124 (1993).

23. J. T. Kegelman and F. W. Roos, “The Flowfields of Bursting Vortices Over Moderately Swept Delta Wings,”

AIAA-90-0599, 28th Aerospace Sciences Meeting, Reno, NV (1990).
13

(a) Top view.

(b) Side view.

Figure 1. Diagram of experimental arrangement for flow visualization of flat plate boundary layer region showing

views from the top and side. The laser beam, collection optics, and camera are fixed relative to the wind tunnel test

section. Different longitudinal positions along the model are viewed by translating the model inside the test section along the free-stream flow direction. (Figure not to scale.)

Laser beam

30°

ICCD

Model Windows

Mach 2.5

free-stream

flow

91-cm cylindrical lens

Shock-generating ramp

Laser light

sheet at

location 2 of fig. 4

and fig. 5b

Upper

plate

Lower

plate

Strong shock

Interaction of shock

and boundary layer
14

(a) Photograph of shock boundary layer interaction model in UPWT test section.

(b) ICCD image of boundary layer region.

Figure 2. Shock-boundary layer interaction model in the UPWT test section and ICCD image of the boundary

layer region (illuminated by laser light), using a wide field of view.

Upper plate

Laser light sheet

(~1 cm off lower surface)

Shock-generating ramp

Lower plate

Scatter off ramp on

upper plate

Scatter off lower plate

downstream of laser beam

Laser light sheet

(~1 cm tall with lower edge

~1 cm off lower plate)

Lower plate of model

directly below light sheet

Reflection of light sheet

off of lower plate

upstream of laser beam
15

(a) Raw image.

(b) Contrast-enhanced version.

Figure 3. The raw and contrast-enhanced version of same image.
16

Figure 4. Illustration of model and five laser light sheets, with each sheet shown at a different streamwise position.

The five locations are (1) 5 cm upstream of leading edge (i.e., the free stream); (2) 16 cm downstream of the leading edge (i.e., about 1/2 way down the shock-generating ramp on the upper plate); (3) 22 cm downstream (i.e., the

shock impingement point); (4) 5.1 cm farther downstream of impingement point; and (5) 7.6 cm downstream from

impingement point. Single pulse images (for each of five locations) are shown in figure 5.

Mach 2.5 free stream

Light sheet locations

Inlet ceiling

Inlet floor
17

(a) Location 1.

Figure 5. Successive single pulse (i.e., instantaneous) sheets of flow structure as a function of distance downstream from leading edge of model. Locations 1 through 5, described in figure 4, respectively correspond to parts

(a) through (e) of this figure. All images are with the same magnification as in figure 3. For each location we show

eight successive laser shots at 0.1 sec temporal separation.
18

(b) Location 2.

Figure 5. Continued.
19

(c) Location 3.

Figure 5. Continued.
20

(d) Location 4.

Figure 5. Continued.
21

(e) Location 5.

Figure 5. Concluded.
22

(a) Free-stream position. (Location 1)

(b) Impingement point position. (Location 3)

(c) Position 7.6 cm downstream of impingement point. (Location 5)

Figure 6. Sketch of line imaging made from visual observations of boundary layer region from a shock-boundary

layer interaction model. Three illustrated positions (free stream, impingement point, and 7.6 cm downstream of

impingement point) correspond to (a), (c), and (e) of figure 5. The narrow focused beam is 8 mm above model

surface.

Mach 2.5 Flow

Bright beam

across

test section

Model

Laser beam

Leading edge

Mach 2.5 Flow

Beam

disappears

inside model

Model

Laser beam

Leading edge

Mach 2.5 Flow

Intermittent

beam

inside model

Top view of model

(imagine "invisible" top plate

to see laser beam)

Model

Laser beam
23

(a) Uniform intensity of beam in free stream.

(b) Primary and secondary vortices.

(c) Side view.

Figure 7. Sketch of line imaging made from visual observations of the supersonic flow field of the vortices above

a delta wing. The uniform intensity of beam in the free stream is illustrated in (a), while the four gaps in the beam

show low density regions of the two primary and two secondary vortices in (b). The side view of (c) shows the

approximate location of the beam relative to the wing.

Test section windows

Naked eye observation direction

Laser

beam

Mach 2.8 free-stream flow is

out of the plane of the paper

Laser

beam

Delta wing

Secondary vortices

Primary vortices

Laser beam out of page

Mach 2.8 12°

Free-stream

flow

Delta wing
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August 2000 Technical Memorandum

Flow Visualization by Elastic Light Scattering in the Boundary Layer of a

Supersonic Flow WU 522-31-61-01

G. C. Herring and Mervin E. Hillard, Jr.

L-17960

NASA/TM-2000-210121

We demonstrate instantaneous flow visualization of the boundary layer region of a Mach 2.5 supersonic flow over a

flat plate that is interacting with an impinging shock wave. Tests were performed in the Unitary Plan Wind Tunnel

(UPWT) at NASA Langley Research Center. The technique is elastic light scattering using 10-nsec laser pulses at

532 nm. We emphasize that no seed material of any kind, including water (H2O), is purposely added to the flow.

The scattered light comes from a residual impurity that normally exists in the flow medium after the air drying process. Thus, the technique described here differs from the traditional vapor-screen method, which is typically

accomplished by the addition of extra H2O vapor to the airflow. The flow is visualized with a series of thin twodimensional light sheets (oriented perpendicular to the streamwise direction) that are located at several positions

downstream of the leading edge of the model. This geometry allows the direct observation of the unsteady flow

structure in the spanwise dimension of the model and also allows the indirect observation of the boundary layer

growth in the streamwise dimension.

Rayleigh scattering; Boundary layers; Supersonic flow 28

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