July 2013 

NASA/TM–2013-

Iodine Tagging Velocimetry and Mechanism in 

the Hypersonic Near Wake of a MultiPurpose 

Crew Vehicle 

R. Jeffrey Balla 

Langley Research Center, Hampton, Virginia 
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July 2013 

NASA/TM–2013-

Iodine Tagging Velocimetry and Mechanism in 

the Hypersonic Near Wake of a MultiPurpose 

Crew Vehicle 

R. Jeffrey Balla 

Langley Research Center, Hampton, Virginia 
Available from: 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320 

443-757-5802 
1 

Iodine Tagging Velocimetry and Mechanism in the 

Hypersonic Near Wake of a MultiPurpose Crew Vehicle 

R. Jeffrey Balla1

NASA Langley Research Center 

1 Senior Research Scientist, Advanced Sensing and Optical Measurement Branch, MS 493, 

 Adjunct Professor, Physics Dept., Old Dominion University, email:robert.j.balla@nasa.gov 

 Senior member AIAA 

Abstract 

This study demonstrates a new molecular tagging velocimetry(MTV) 

method for velocity measurements of high speed flow. It demonstrates offbody 

Iodine Tagging Velocimetry (ITV) in the hypersonic near wake of a MultiPurpose 

Crew Vehicle(MPCV) model. Experiments are performed in the NASA-Langley 31-

inch Mach 10 air wind tunnel. A 0.5% I2 / N2 mixture is seeded on the leeward 

backshell of the model using a pressure tap. I2 laser-induced fluorescence is 

excited along a 5.5 mm line using an ArF excimer laser near 193 nm. Results 

indicate I2 absorbs at least 2 photons to produce iodine ions and electrons. 

These recombine as the tagged region is displaced downstream to produce 

I (2P3/2 ) whose emission is monitored at 206 nm. Results at P0 = 2.41 MPa (350 

psi), T0 = 990K, and 10 sec transit times produce velocities from 630-820 m/sec 

across the I2 seeded jet at a distance of 38.2 mm (25.5 jet diameters) downstream 

from the jet orifice. Maximum wake jet velocities near the shear layer are 59% of 

freestream velocity. 

I. Introduction

This study represents the third in a series of articles developing new 

instrumentation to measure offbody parameters in the freestream, bowshock, and 

wakes of models in a Mach 10 air windtunnel. Here effort is directed towards velocity 

measurements in near and far hypersonic wakes. These wakes can be highly three 

dimensional, contain densities of 0.03–3.0 x 1017 cm3

, and contain a variety of issues 

including flow separation and reversal, unsteady conditions, shear layers, jets for 

reaction control systems, and high velocity gradients. Current instrumentation is 

inadequate. The result is a lack of data for computational comparison and limited 

understanding of the fundamental flow physics. Decadal reviews of freestream and 

model flowfield instrumentation have been performed in 1992 [1], 1998 [2], and 2009 

[3].

It is well known that current surface methods are indirectly measuring off-body 

flow features which determine many flow field characteristics. New wake 

instrumentation must be capable of quantitative spatially-resolved nonintrusive 
2 

molecular-based and of highest importance offbody measurements at a point, along a 

line, and preferably in a plane. Specific problems of interest as well as problems with 

adapting traditional quantitative subsonic offbody instrumentation to hypersonic wake 

flows have been discussed [4]. Methods include Schlieren, Pitot tubes, hot-wire 

anemometry, and particle methods such as LDV and PIV. All encounter significant 

problems in the low densities, high velocity gradients, and complex flow interactions in 

hypersonic wakes. Most are intrusive and disturb the flow being investigated.

A large number of molecular diagnostic methods are potentially applicable to 

hypersonic wakes. Even a brief discussion of all potential methods let alone their subtle 

variations is beyond the scope of this article. This section concentrates on select 

methods which have been demonstrated or the author concludes might be adapted to 

provide quantitative offbody measurements of velocity in a hypersonic electricallyheated air wake along with their potential advantages and disadvantages. Since the 

primary interest is viability, no consideration is given to quantitative errors. Since the 

test fluid is air at temperatures near or below 300K, diagnostics should be designed 

around atomic or molecular seeds which are chemically nonreactive and do not require 

a heated carrier gas. 

Sodium-based laser-induced fluorescence(LIF) [5] was used to measure velocity, 

temperature and pressure in hypersonic helium. Sodium is useful for nitrogen or other 

inert-gas flows. However, it must be injected with a heated carrier gas, reacts 

explosively with water vapor, and forms oxides in dry air. The resulting heat can perturb 

the flow. 

Raman Doppler Velocimetry can measure velocity, temperature, and pressure 

[6]. RDV is possible in a hypersonic wake but requires a complex multilaser setup. 

Expected low signal-to-noise(S/N) ratio would make this a difficult measurement. 

Velocimetry using RELIEF has the advantage of vibrationally excited oxygen with 

a millisecond lifetime [7]. This is compared to typical molecular tagging velocimetry 

(MTV) methods using diatomics such as nitric oxide (NO) which has a 220 ns natural 

fluorescence lifetime. This allows longer transits between excitation and interrogation 

times resulting in potentially higher accuracy for constant velocity flows. RELIEF is 

currently limited to less than ~700K and greater than ~0.1 Amagat [8]. A RELIEF 

measurement in a 0.001 Amagat (1 Torr) air wake would be challenging. 

The MTV approach is an effective method for offbody velocimetry 

measurements. It uses a single laser to tag a molecular species of interest and a single 

camera to measure fluid displacement using LIF between two locations along with 

transit time. A review of this approach along with its many variations has been 

presented [9]. 

APART is an MTV method which locally creates NO fluorescence using a 

focused ArF excimer in air [10]. This method provides velocimetry with no seeding 

requirement. Since ground-electronic-state NO is produced continually by a chemical 

reaction, laser-induced fluorescence has been observed using a probe laser up to 1 sec 

after the ArF excimer pulse. Neither APART nor RELIEF can be used in hypersonic 

and impulse facilities using pure nitrogen. 

Low wake densities combined with a lack of effective diagnostics based on 

nitrogen or oxygen dictate flow seeding. Since trace seeding is required to minimize 
3 

flow perturbations, the only sufficiently sensitive detection method capable of spatiallyresolved instantaneous measurements is LIF. LIF cross sections are typically 103

 larger 

than Rayleigh cross sections [11], produce high signals, and offer the potential for 

planar measurements. Filters effectively separate fluorescence from scattered light. 

Measurements near and on model surfaces are possible. Flow field perturbation can be 

studied using active and passive seeding methods. Quantum-state-specific chemical 

and vibrational nonequilibrium can be measured and compared with computational 

results. Most are based on nitric oxide planar laser-induced fluorescence (NO PLIF). 

NO is corrosive and a toxic gas with a threshold limit value of 25 ppm. 

Several NO PLIF-based methods have measured velocity in the T2 free-piston 

shock tunnel at the Australian National University. MTV was used to measure the 

velocity profile in a Mach 8.5 laminar, hypersonic boundary layer [12]. Two components 

of velocity were measured in an axisymmetric hypersonic near-wake flow field around a 

model planetary entry vehicle configuration using Doppler shifts [13]. Shapes and 

positions of NO spectral lines at every location in the flow are determined using 

successive T2 runs. Since a single laser shot is acquired during each 300 sec run, 

this approach requires high facility run-to-run reproducibility.

NO PLIF-based MTV has been demonstrated in a pure nitrogen jet in the wake of 

a CEV model in the NASA Langley 31-inch Mach 10 air wind tunnel [14]. Simultaneous 

multiple velocity profiles were obtained using sequentially-imaged fluorescence tagging. 

One advantage of this method is its simplicity and the use of a single laser and camera. 

This method suffers from a short excited state lifetime (< 220 ns) which is further 

reduced by oxygen quenching. This limits transit times, increases uncertainties, and 

dictates high-speed applications. 

Visible I2 LIF diagnostics using the B 3

+

ou X 1

+

g transition near 514.5 nm 

have been reviewed[15]. This method has seen considerable development and broad 

application in a variety of subsonic and supersonic flows. It has demonstrated 

combined density, temperature, and velocity [16] along with planar velocity and 

pressure[17]. Time-averaged values of velocity and temperature have been measured 

in an I2-seeded N2 hypersonic free jet facility at Mach 12 using hyperfine transitions. 

Unfortunately, the time required for these scans is greater than the run time of most fullscale facilities [18]. To overcome corrosion problems associated with large quantities of 

I2 filling the entire flow, McDaniel et al. [19] constructed an aluminum wind tunnel 

painted with flat-black paint along with gaskets and o-ring seals made of Viton for I2

compatibility. Chemical reactivity and toxicity issues connected with large quantities of 

seeded I2 combined with facility redesign and recertification for appropriate gaskets 

make this approach unsuitable for typical large-scale wind tunnels. 

The ultraviolet iodine laser–induced fluorescence (UV I2 LIF) diagnostic uses the 

Cordes band D1

+

u X 1+g transitions near 193 nm. It was developed by LaRC 

researchers specifically for hypersonic wakes. UV cross sections are 30x greater than 

visible cross sections and do not suffer from predissocation. I2 ion-pair unquenchable 

excited states produce fluorescence quantum yields of 1 even at atmospheric pressure 

[20]. Both passive[21] and active[4] seeding are possible. Compared to visible I2 LIF, 

this approach provides higher signals with lower seed densities. The result is 

diagnostics well suited for low-density wake measurements. 
4 

Using local seeding of trace quantities of iodine, traditional problems with iodine 

facility corrosion and toxicity are minimized. Planar flow visualization of 3 x 1012 cm3

iodine in a Mach 6 wake with 1.8 x 1017 cm3

 air was demonstrated using a right circular 

cylinder[21]. Previous studies have demonstrated density [22] and temperature [23] in 

the laboratory along with a discussion of the spectroscopy involved. 

The problem of hypersonic wake time-of-flight velocimetry has been one of 

finding a seed species with sufficiently long fluorescence or chemical lifetime. 

Assuming wake velocities are 1⁄2 the freestream velocity, velocities across the wake 

shear layer range from 300-800 m/sec. These values are in agreement with 

experiments [14] and CFD computations [14,24]. Assuming a goal of 1% error 

spanning 100 CCD pixels, transit times of ~10 sec are required. Typically, molecular 

species are selected. Atomic species are considered unfeasible for time-of-flight 

velocimetry since emission lifetimes are typically < 50 ns [25]. Atom velocimetry has 

been demonstrated using Doppler shifts in arc jets with atomic hydrogen [26] or atomic 

copper [27]. Another approach used a pump laser to create Laser-Enhanced 

Ionization(LEI) of either sodium in a shock tube [28] or strontium ions in a flame [29] 

followed by a probe laser to generate laser-induced fluorescence with appropriate 

delay. 

O’Keefe et al. [30] have shown it is possible to form high-n Rydberg atoms of 

iodine by resonant two-photon excitation of molecular iodine using an ArF excimer 

laser. Results suggest I2 absorbs 2 photons to produce a molecular Rydberg state of 

I2*[R(B2+

g )] in a super-excited state(SES). These converge on the I2

+

 (B 2+

g ) state of 

the ion. The SES predissociates into ground state I(2

P0

3/2) and a high-n Rydberg iodine 

atom I* [R3

P2)] with an average Rydberg lifetime of 325 ± 25 sec lifetime in a zero 

kinetic energy (ZEKE) state. 

Mills et al. [31] proposed that lifetime limitations associated with atomic species 

might be overcome using high-n Rydberg iodine atom I* [R3

P2)] or any similar atomic 

species produced by the mechanism described by O’Keefe et al. [30]. Based on 

calculations of expected signal levels, Mills et al. [31] also proposed using a noble 

gas(krypton 4p5 5s1 [3/2]2 state) metastable state created by two-photon ArF excimer 

laser excitation. 

Although I2 is 8.8x heavier than air, velocity lag or slip is negligible in N2-based 

flows up to Mach numbers < 5 since velocity is equilibrated within ~ 10 collisions [15]. 

Hence no velocity lag is expected for atomic or molecular iodine under the conditions of 

this experiment. 

Efforts are directed toward the development of wake instrumentation for the 

Langley Aerothermodynamic Labs and the 31-inch Mach 10 air wind tunnel in particular. 

Previously, laser Rayleigh scattering was used to measure freestream densities [32]. 

UV I2 LIF was used to measure density [4] of a jet along a line in the wake. The original 

goal of the present study was to explore the concept of Mills et al[31] to use two photon 

excitation of I2 for the production of high-n Rydberg atoms and subsequent velocimetry. 

Serendipitously an alternate iodine-based energy-storage mechanism was discovered 

allowing hypersonic wake velocimetry. Iodine is locally seeded from a pressure port in 

the wake of a CEV model in a Mach 10 flow field. Iodine velocity results are compared 
5 

to previous NO PLIF measurements and CFD computations in the 31-inch Mach 10 

facility [14,24]. 

II. Experimental 

Complete descriptions of various components of the experimental setup including 

the iodine seeding procedure, laser configuration, and CCD camera detector have been 

presented [4,32]. A brief synopsis of previous setups is presented along with a 

complete description of new additions. 

Experiments are performed during Test 466 in the NASA Langley 31-Inch Mach 

10 (31M10) Air Tunnel. It is a unique 12.5-MW electrically-heated blowdown-mode 

facility [33]. The test gas is air which is dried using an activated alumina dryer which 

provides a dewpoint temperature of 232.4 K (- 41 °F) at a pressure of 4.137 MPa (600 

psi). Air is expanded using a three-dimensional contoured nozzle to minimize centerline 

disturbances characteristic of axisymmetric contoured nozzles. The result is a highly 

uniform core flow. The test section is 78.7 cm square. Core flow is 25% of the test 

section, or ± 10 cm about the centerline at low stagnation pressures(Po = 350 psi) and 

increases ± 15 cm as the tunnel wall boundary layer thins at high pressures(Po = 1450 

psi). 

Optical access is provided by three uncoated Corning 7980 windows with 

transmission > 85% at 193 nm. These form three orthogonal walls of the test section. 

Models are positioned on the facility centerline in less than 0.6 seconds using a 

hydraulically-operated injection system mounted on the rear sidewall. 

Facility test times are ~ 60 seconds. The facility typically operates at a single 

stagnation reservoir temperature To = 1,000K (1,350 °F) to prevent air liquefaction. 

Facility stagnation reservoir pressure Po was maintained at 2.41 MPa (350 psia). Flow 

conditions were computed assuming isentropic expansion. 

The experimental setup for line measurements along a laser beam is shown in 

Fig. 1 using ViDI [34]. ViDI is a Langley software tool for interactive 3-D display of the 

facility, test article, data, and computational fluid dynamics prediction. All details of the 

31M10 facility, UV iodine line and LIF (UV I2 LIF) optical setup, sting, strut, and MPCV 

test article are modeled and presented. This allows the experiment to be constructed, 

tested, and refined in a virtual environment before facility testing. The constraints of two 

days of facility setup time and a single day for testing dictated an easy-transport rapidsetup light source. A low-pulse-energy Lambda Physik OpTexPro excimer laser 

operating on argon-fluoride near 193 nm was selected. The output passes through ~ 1 

m of air before it enters the facility. It is turned 90 deg with a mirror and focused into the 

I2 jet 38.2 mm (25.5 jet diameters) downstream from the I2 jet exit (port 4) using an 

uncoated 600 mm focal length Suprasil lens. Focused laser energy is 2.5 mJ, focal spot 

size is ~200 m, and Rayleigh length is ~ 65 cm. Laser alignment assumes the I2 jet 

comes straight back off the model with minimal curvature in a plane normal to the 

freestream flow near the center of the facility. 
6 

Fig. 1 ViDI rendering of UV iodine LIF velocimetry in the 31M10 wind tunnel. 

A. Multipurpose Crew Vehicle Model

Three schematic views of the model are shown in Figure 2. It is an early version 

of the Crew Exploration Vehicle (CEV) or Orion Spacecraft that is based on the Apollo 

geometry. Recently, it has been renamed a MultiPurpose Crew Vehicle(MPCV). It is 4 

inches in diameter (2% scale). The heat shield radius is 1.2 times the diameter, the 

corner radius is 0.05 times the diameter, and the back-shell angle is 33 deg. The model 

is attached to a sting at a 28 deg angle, which was approximately the entry angle of 

attack of Apollo. Pressure orifices are positioned in the front, top, bottom, and rear of 

the model. Essentially, this was a model of convenience used to create a shock jump 

and to evaluate wake and other measurement possibilities. Pressure is measured using 

ports 1,3,5,7, and 15. Port 15 is 19.1 mm from port 3. All ports have a 1.5 mm 

diameter orifice. Forebody temperature is measured by a thermocouple(not shown) 

inside the model and in contact with the forebody near port 7. The model is 

coincidentally rotated 28 degrees counterclockwise off centerline to prevent laser 

scatter off model backshell and sting. The sting is at zero pitch. 
7 

Fig. 2 MPCV– three schematic views 

B. Iodine Seeding 

I2 seeding has been described previously [4]. It is summarized here for 

completeness. I2 is locally heated in a mixing chamber behind the rear tunnel wall to 

improve sublimation. The iodine/nitrogen-carrier-gas mixture traverses ~ 1 meter of 

stainless steel tubing inside the strut and sting. Mixture pressure is 0.138 MPa (20 psi). 

It exits through a 1.5 mm diameter orifice in the model surface at a rear pressure port. 

This internal tubing is heated by conduction and convection by the hypersonic flow. The 

iodine in this tubing equilibrates with the tubing temperature prior to exiting at the rear 

pressure port. Hence, iodine and carrier gas are subject to no abnormal heating. 

Overall, a mixture of 0.5% iodine in nitrogen is seeded from port 4. Iodine is a toxic gas 

with a threshold limit value of 0.1 ppm. Iodine is present in the atmosphere at < 10 ppb. 

The iodine has fully sublimated prior to expansion in the wake jet. There are no known 

chemical reactions between the ground electronic state of molecular iodine and air 

under the temperature and pressure conditions present in a Mach 10 wake generated 

by electrically-heated hypersonic nitrogen or air flows. 

C. Wind Tunnel Velocimetry 

 

For velocity measurements, the laser is operated in broadband mode using a 

stable resonator. Focused laser energy inside the facility is 2.5 mJ. Emission is imaged 
8 

at 90 degrees to the flow with a Nikon 105 mm f/4.5 UV-Nikkor lens(25 mm diameter) 

focused using 193-nm- scattered light. No interference filter is used. Emission is 

observed using a gated, double-intensified ITT Model F4577 CCD camera. A ViDI 

rendering of the MPCV model with sting, iodine jet, and velocity measurement location 

is shown in Figure 3. 

Data is acquired at 30 Hz. Ambient stray light was minimized using a 200 ns 

intensifier gate and extinguishing the room lights. Laser timing jitter was ± 100 nsec. 

Starting location is visualized using Rayleigh scattering from atmospheric pressure 

static air inside the facility. CCD camera gate delay times of 5 and 10 sec are 

introduced using a delay generator triggered using the field index pulse. Ten seconds 

are required to set camera gain. The computer can acquire 371 images (64 Megabytes 

of memory) during a 12.4 second time interval. Six seconds are required to store the 

data. This process is typically repeated twice during a single windtunnel run. 

Fig. 3 ViDI rendering of MPCV model, iodine jet, and velocity measurement location. 

D. Laboratory Spectrally-Resolved Time-dependent Iodine Emission 

Prior to the wind tunnel entry, laboratory experiments were performed to obtain 

insight into the iodine emission mechanism. The experimental configuration has been 

described previously[22]. It is similar to the laser configuration shown in Figure 1. A 

cell with iodine replaces the wind tunnel and a spectrograph replaces the camera. 

Briefly, the broadband excimer laser is focused into an iodine cell evacuated to ~ 0.1 

Torr air. Iodine density is maintained at 1 × 1015 cm-3 using an ice-bath cold finger at 

273K. Emission 90 degrees to the laser beam propagation direction is collected and 

filtered using two 4-in.-diam. by 500 mm focal length lenses, i.e., 1:1 imaging. Emission 

is resolved using an Acton Research Corporation model SP-588 SpectraPro-500i 

imaging spectrograph/monochromator with 200 m slit setting and 3600 line diffraction 

grating blazed at 240 nm. The slit height is 12 mm. Emission is detected with an EMI 
9 

Model 9781B photomultiplier tube, digitized, and stored using a 1 GHz digital 

oscilloscope. Emission is also detected using the intensified gated CCD camera 

described in section II C. 

E. Laboratory Iodine Fluorescence Imaging 

Following the successful wind tunnel entry, an experimental setup similar to 

figure 1 was constructed in the laboratory. A cell with iodine densities equal to those 

described in section II.D replaced the wind tunnel and a home-built four-inch diameter 

four-element lens for increased optical collection in addition to the one-inch Nikon lens 

was used. The lens consists of four 100-mm diameter Suprasil-1 fused silica lenses 

manufactured by ESCO products with the following shape, focal length, center 

thickness and lens-to-lens center spacing characteristics in mm (lens 1-biconvex, +150 

mm, 23 mm, 30 mm; lens 2-plano-concave, -148 mm, 2.97 mm, 30 mm; lens 3-

biconvex, +150 mm, 23 mm, 35 mm; lens 4-plano convex, +250 mm, 30 mm). Focal 

lengths are specified at 248 nm. A series of 5 apertures with inner diameters of 75, 75, 

75, 60, and 32 mm were placed between each successive lens to reduce off-axis rays 

and improve resolution. 

III. Results and Discussion 

A. Windtunnel Velocimetry Images 

Iodine Tagging Velocimetry(ITV) beginning with at least two photon ArF 

excitation of molecular I2 is demonstrated for the first time in Figure 4. This approach 

monitors I 2

P3/2 I 2P0

1/2 emission at 206 nm in the near wake of a MPCV model. 

Figure 4A is the composite of three separate time-averaged images; one 

showing ITV start position using Rayleigh scattering and two showing ITV timeaveraged downstream iodine fluid transit times of 5 and 10 sec. Images at these 

transit times are readily achieved by combining the CCD camera gating and variable 

gain capabilities. Each of the three images in Figure 4A is the average of 371 

instantaneous images. 

Use of UV I2 PLIF to identify the start position is hindered by the fact that the 

edges of the focused laser saturate the Cordes band transitions and broadband 

molecular emission at t=0 sec overwhelms atomic emission. The result is a top-hat 

line image with poorly defined center point. Hence, start position is determined using 

Rayleigh scattering along the line in Figure 4A. Laser beam was angled slightly to 

minimize scatter off the model. The laser is positioned 38.2 mm (25.5 jet diameters) 

downstream of the iodine jet from pressure port #4. Images are cropped to 8.6 mm high 

by 14.8 mm wide. 
10 

Fig. 4. Iodine tagging velocimetry A) time averaged composite images at 0, 5 and 10 

sec transit times, B,C) instantaneous images at 5 sec transit times. 

In Figure 4A, distances from Rayleigh image to first iodine image(3.774 mm, 68 

pixels), second iodine image(7.135 mm, 130 pixels) and distance between iodine 

images(3.361 mm, 62 pixels) yield velocities at the center of the jet of 750, 710, and 670 

m/sec respectively. The mean velocity of 710 m/sec is ~50% of freestream velocity. 

CCD gain was increased to camera maximum (~ 1000x the Rayleigh gain) to visualize 

both iodine images. Emission intensity at 10 sec transit time was multiplied by 3 to 

produce intensities similar to the 5 sec transit time for presentation purposes. Timeaveraged errors are conservatively estimated using full-width at half maximum (FWHM) 

of each image at 10%. Results show velocity decreases with increasing distance from 

the jet. 

To verify repeatability, a second set of data at identical transits were acquired 

during each run. Results are equivalent to those in figure 4. For this proof-of-concept 

test, data was only acquired at a single stagnation condition of 2.41 MPa (350 psi) and 

990K. Fluid conditions at the locations probed in the I2/N2 jet in Figure 4 are 300-400K, 

0.13 x 1017 molecules/ cm3

 N2 and 0.42 torr air [4]. Figure 1 shows that the relative 

positions of the laser beam, iodine jet, and CCD camera field-of-view are fixed since all 

are mounted to the facility. Therefore, images are independent of any bulk facility 

motion due to heating effects during a run. 

Figure 4B is one of the 371 instantaneous velocity images averaged to form 

Figure 4A at 5 sec transit time. Similar unique patterns are observed in each 

instantaneous image. These patterns vary shot-to-shot. Causes of this spatiallylocalized nonuniform emission are discussed in section IV.C below. 

Figure 4C is one of the instantaneous albeit unusual images averaged to form 

Figure 4A. It is presented to show the shot-to-shot extent of the vertical spatial motion 

of the shear layer. It is spatially deformed relative to Figure 4A and 4B. It is included to 

show the capability of this instrumentation for visualizing flow unsteadiness. 
11 

 In figure 4, no light source inside the wind tunnel was available at 206 nm. The 

CCD camera was focused using Rayleigh scattering from 1 atmosphere of air at 193 

nm. Chromatic aberration in the compound lens is responsible for slight image blurring 

for the emission observed at 206 nm. Using a single-intensified CCD camera with 

proper focusing, each point of emission will correspond to ~2 pixels. 

Due to 31M10 facility runtime constraints, no resolved emission studies were 

performed. However, any possible emission above ~ 250 nm in figure 4 would be outof-focus due to chromatic aberration of the compound imaging lens. Data in section 

IV.A and IV.B indicate only long-lived emission at 206 nm is present. Based on all these 

facts, Figure 4 results are attributed to I (2

P3/2)  I (2P0

1/2) emission at 206 nm. 

B. Wake Jet Velocity Spatial Profile 

Visual inspection of the rotated downstream images in Figure 4A indicates a 

considerable velocity gradient at this spatial location. Velocities normal to the exciting 

laser beam at 44 points along a 5.5 mm line for the 10 sec transit time data in Figure 

4A are shown in Figure 5A. The transverse velocity component is not measured. 

Velocities vary from 630 to 820 m/sec(30%) across the 5.5 mm iodine jet and increase 

toward the shear layer. Velocities show only minor variation from -2 to -3.5 mm. As 

described, errors are estimated at conservatively 10%. Profile in Figure 5A can be 

overlaid with textbook shear layer profile near U2 shown in Figure 5B. Iodine does not 

penetrate sufficiently into the shear layer for velocities to flatten as shown in Figure 5B. 

Hence, extrapolation of Figure 5A results suggests gas velocity within and beyond the 

shear layer are > 820 m/sec or > 59% of freestream velocity. Since the wake jet gas 

temperature is ~300K [4], jet Mach number varies from ~ 1.9 to ~ 2.4 across figure 5A. 

Fig. 5. Spatial velocity profile A) Wake jet and B) textbook shear-layer 
12 

C. NO PLIF Velocimetry Comparison 

There have been no velocity measurements under identical experimental 

conditions and at the locations probed in Figure 4. However, NO PLIF has been used 

to visualize and measure jet velocities within a few jet diameters under somewhat 

similar conditions. NO PLIF [14] used a 5-inch diameter Orion CEV model, yaw 

reaction control system(RCS) jet, metal forebody and plastic aftbody, and seeding from 

port 3 at 18 degree angle of attack. The yaw RCS jet nozzle throat diameter was 0.7 

mm (0.0275 in.) with an exit-to-throat area ratio of 22.5. Prenozzle chamber pressures 

were 0.31 and 0.91 MPa (45 and 132 psi). Stagnation pressure was 2.41 MPa(350 psi). 

NO PLIF measurements showed jet centerline velocities varied from 600-800 

m/sec essentially independent of jet pressure 0.31 and 0.91 MPa (45 and 132 psi jets) 

and downstream distances (1 to 15 mm) from jet exit. These measured, averaged 

velocities are similar in magnitude to those observed downstream in Figure 5A. CFD 

calculations [14B, 24] showed similar results and spatial trends to NO PLIF results. It 

was observed that differences between experiment and computation increased with 

increasing downstream distance and near the edges of the jets. Johansen et al. [24] 

performed numerical predictions of a Mars Science Laboratory aeroshell reaction 

control jet interacting with a Mach 10 hypersonic flow. Wake shear layer velocities were 

650-800 m/sec, also in good agreement with figure 5A results. 

Iodine ATV errors are conservatively 10% (20 psi chamber pressure) while NO 

PLIF MTV errors at the furthest downstream distances probed (8 mm at 45 psi and 15 

mm at 132 psi chamber pressure) were at least 30%. The natural NO A state lifetime is 

220 nsec. Quenching in this flow field reduces this to about ~130 ns. NO emission was 

visualized after 500 ns (~4 lifetimes) yielding typically 6-8 pixels of fluid displacement. 

Iodine emission is observed for 5-10 sec or ~ 10-20x the NO A state emission yielding 

68 or 130 pixels of fluid displacement. The result for constant velocity flows is lower 

velocity measurement uncertainty errors are realized with the I2 LIF measurement. Due 

to the different NO LIF measurement conditions, data uncertainties, and measurements 

at different spatial locations, it is concluded that NO LIF velocity measurements cannot 

be compared directly with the iodine PLIF results in Figures 4 and 5A. However, 

ignoring their differences, they are to first order in agreement within 20-30%. This 

suggests the wake flow field created using Po = 350 psi and similar model shapes is 

only weakly influenced by the differences in these experiments. 

IV. Iodine Tagging Velocimetry Mechanism 

The mechanism behind iodine tagging velocimetry (ITV) is not fully understood. 

The term ITV is used to include the possibilities of both molecular and atomic tagged 

states. It is known that molecular iodine is seeded in the wake, excited by an ArF 

excimer laser, and atomic iodine emission at 206 nm is observed. This section 

discusses possible mechanisms and presents the most likely mechanism based on 

literature information and experimental results in the present manuscript. Since 

emission is observed from atomic iodine and emission in the iodine-seeded wake jet is 
13 

observed downstream only where the focused laser overlaps the iodine-seeded jet, 

emission is attributed to iodine and not air constituents. 

Single photon absorption in I2 using an ArF excimer laser occurs near 193 nm via 

the Cordes bands (D1+

u X 1+g ) with a lifetime of 15.5 ns [35], 1 x 10-17 cm2

 cross 

section[36], and fluorescence quantum yield 1 [37]. Later research produced a more 

exact cross section of 2 x 10-17 cm2

 [38]. There are 18 predicted ion-pair electronic 

states arising from I(

1

S) + I+ (

3

P,1D) accessible with ArF laser excitation [39]. All 

should have similar potential curves, large dissociation energies, small vibrational and 

rotational energy-level spacing, large internuclear separations, and ~ 10 ns lifetimes 

similar to D1

+

u. The D’ 3

2g state lifetime produced by D to D’ collisional energy 

transfer is 6.7 ns [40]. 

The iodine emission spanning 190-350 nm at < 1 Torr pressure from singlephoton absorption near 193 nm is well known[20]. Lifetimes of these states are < 15 ns. 

None of these molecular states can account for the velocimetry results where emission 

lasts > 10 sec. Hence single photon absorption cannot account for the observed longlived emission. 

A. Spectrally-resolved Iodine Emission Spectrum 

To obtain insight into the mechanism responsible for long-lived iodine emission, 

laboratory experiments were performed in an iodine cell using a laser configuration 

similar to Figure 1 and described in section II. Iodine is excited with a broadband ArF 

excimer laser near 193 nm. Time-resolved spectra are detected behind a 0.5 m 

spectrograph using a gated intensified CCD camera. An emission spectrum beginning 

at a time delay of 0.3 sec following the laser pulse is presented in Figure 6. Gate width 

is 0.5 sec and 50 lines on the detector were summed. Fluorescence is observed at 

206.16 nm. Similar scans were performed from 220-400 nm. No additional 

fluorescence was detected. 

Fig. 6 Iodine emission spectra delayed 0.3 sec from laser pulse 
14 

B. 206 nm Time-Dependent Iodine Emission 

To obtain insight into the iodine emission mechanism and expand on the results 

in section IV.A, the CCD camera was replaced with a phototube. Tuning the laboratory 

spectrometer to 206.16 nm to observe fluorescence from I (2

P3/2 )  I (2P0

1/2), the timedependent emission profile in figure 7A is observed. 

The first and largest peak in Figure 7A near t = 90 ns is saturated and composed 

primarily of I2 (D1

+

u  X 1+g) emission. D  X emission decays in ~ 26 ns after 

which I (2P3/2)  I (2P0

1/2 ) emission at 206 nm grows to create the second weaker peak 

with an 84 ns induction time measured from the pulse start at 62 ns. Emission 

continues to decay for 1000 nsec. This time constraint is dictated mainly by a digitizer 

with 8-bit dynamic range. Figure 7B shows a plot of the natural log of the signal (Ln) in 

Figure 7A vs time. Using a linear-least-squares straight-line fit spanning 200-500 ns, a 

first –order decay process was observed with a lifetime of 196 ns. 

Initial lifetime measurements for a variety of atomic iodine transitions were first 

reported by Lawrence[41]. Emission from 206 nm and 178 nm originate from the same 

excited state ie. I (2

P3/2 ). Lawrence incorrectly identified this state as I (4P3/2 )*. The 

natural lifetime of excited atomic iodine states with emission spanning 150-206 nm is 

typically 3 to 60 ns [41]. Emission ratio of the 178 and 206 nm lines is 92:1 [41]. 

Emission lifetime of I (2

P3/2 ) at 206 nm is 3.6 ns[41] using the formula Tau(I (2P3/2 ) = 1/ 

 ( A(178 nm) + A(206 nm) ). 

* J. Tellinghuisen, personal communication, 2012 

Fig. 7 A) Atomic iodine emission at 206.16 nm versus time. B) Ln signal from Fig. 7A 

 vs time and linear fit 
15 

Here, A designates the Einstein A-coefficient for spontaneous emission Under 

conditions where oxygen and its deleterious absorption effects can be excluded from 

the emission path, signal levels in Figure 4 can be increased by ~100x by monitoring 

emission at 178 nm. 

Based on the known atomic and molecular iodine emission lifetimes discussed, 

the long-lived emission observed in Figures 4,6, and 7 cannot be attributed to radiative 

properties of iodine atoms. Since the bond dissociation energy for I2 is 148.8 kJ/mole 

[42], energy requirements for 206 nm emission dictate at least a two photon absorption 

process. In addition, the long lifetime in Figures 4, 6 and 7 dictate a process capable of 

storing energy for at least 1 sec(30 sec based on results in section IV.C below). 

C. Iodine Fluorescence Imaging in a Static Gas 

Following the successful wind tunnel velocimetry results, an experimental setup 

similar to figure 1 and described in the experimental section was assembled in the 

laboratory. An iodine cell replaced the wind tunnel. Instantaneous images (200 ns 

shutter) at room temperature in non-flowing iodine at a density of 1 x 1015 cm-3 and 

298K are shown in Figure 8A-D. Images are cropped to show only 7 mm (20%) of the 

imaged line. Single-shot images using the 25 mm diameter aperture Nikkor lens and 

CCD camera delay times from the laser pulse of 0.2, 2, 5, and 10 sec are shown in 

figures 8A-D. Averaged images(371 single shots) using the 100 mm diameter aperture 

home-built lens and CCD camera delay times of 20 and 30 sec are shown in figures 

8E-F. Signal levels between Figures 8A and 8F decrease by ~ 6400x. 

Instantaneous images show a transition from spatially-uniform emission (Figure 

8A) at short delay times to spatially-localized nonuniform emission(Figure 8C, 8D) at 

longer delay times. Based on the results and spectroscopic discussion in section IV.A 

and IV.B, emission in Figure 8 is attributed to I (2

P3/2)  I (2P0

1/2) emission at 206 nm. 

The spatially-localized nonuniform emission at long delay times is a result of low photon 

statistics at long delay times ( < 10 photoelectrons per pixel), detector gain at the 

maximum setting, and a nonadjustable mismatch between the DC levels of the CCD 

camera and digitizer producing a negative baseline. All emission is spread to ~4 pixels 

by the double-intensified CCD camera. 

Averaged images show the improvement in delay times using the 100 mm 

diameter lens. Note that S/N is ~ 6 and ~ 3 in Figures 8E and 8F respectively. 

D. Iodine Tagging Velocimetry Excitation and Emission Mechanism 

The ionization energy of I2 is 9.3074 electron volts(eV) [43]. A two photon 

absorption process using 193 nm radiation has 12.83 eV. This will ionize I2. Data on 

multiphoton ionization of I2 is sparse. Martin et al. [44] provided evidence for a strong 

resonant two photon transition in iodine using an ArF excimer near 193 nm along with 

significant amounts of ions( ~ 1012 cm-3) and therefore electrons. The second photon 

cross section, estimated at ~ 10-16 cm2

, is exceptionally large. Since this cross section 

is 10x the D X cross section, the second absorption process is saturated and this two 

photon process is linear with laser energy. They noted that this state lies well above the 
16 

Fig. 8. Laboratory iodine fluorescence time-delayed images A-D) single-shot images 

using 25 mm aperture Nikon lens, E-F) average images using 100 mm aperture lens. 

atomic ionization threshold and the two-photon process is resonant with the 2

+

g state 

of I2

+

. Two photon excitation of I2 can produce electrons with up to 3.5 eV of 

translational energy. Hewitt et al. [45] used these “hot” electrons to excite V” = 2 in CO2

and V” = 1 in CO.

This paragraph is taken from Hewitt et al[45]. There are five known I2

+

 states that 

lie at or below the energy of 2 photons at 193 nm [46]. These include X 2 3/2g (9.34 eV)

, X 2 1/2g (9.98 eV), A 2 3/2u (11.0 eV), A 2 1/2u (11.81 eV), and B 2  g

+ (12.95 eV). 

The X states are relatively stable with a lifetime of ~ 14 sec [47]. Some of the ions in 

the A states dissociate, and the B state is completely dissociated into I+

 (3P2) and I 

(

2

P3/2) ground state[47]. There are four Z-states in I2

+

 which lie between the 2 and 3 

photon energies of 193 nm photons. In the dissociation limit, they produce I+

 (3P) 

cations and I (2P) atoms with 3.9 eV of translational energy[46]. 

O’keefe et al. [30] showed it is possible to form high-n Rydberg atoms( I*) from 

the neutral dissociation of super excited states of I2 following two photon absorption at 

193 nm. Molecular Rydberg states are considered to be a super excited state(SES) 

when the neutral excited state has internal energy greater than the lowest ionization 

threshold of the molecule. Upon excitation the SES will predissociate into a ground 

state atom I(2

P0

3/2 ), and a high-n Rydberg iodine atom. They showed that high-n 

Rydberg atoms(I*) are formed by predissociation of the optically excited molecular 

Rydberg states I2*[R(B2

+

g )] which converge on the I2

+

[R(B2+

g )] state of the ion. For 

the I*

(R3P2) fragment of this molecular ion, the average Rydberg lifetime is 325 ± 25 

sec. 
17 

Recent measurements have shown that wake pressures in this experiment are 

0.42 Torr [4]. Mean free path is ~0.2 mm. Therefore, this state would be quenched in a 

few collisions corresponding to < 60 ns. This is in reasonable agreement with the 

induction time in Figure 7A. However, assuming 10 ns atomic lifetimes and assuming 

iodine forms in an electronic state which eventually cascades to the I 2

P3/2 state from 

which 206 nm is observed, known emission lifetimes cannot account for the 10 sec 

lifetime in Figure 4. Therefore, under the conditions present in this Mach 10 wake, longlived high-n Rydberg states are not responsible for Figure 4 emission. 

The next paragraphs consider the possibility of radiative emission from 

electronically excited iodine trapped in a cluster. Using a free jet expansion similar to 

the present setup, ArF excimer irradiation of I2/Arn clusters produced I2 in the D’ state 

[48]. For I2 in a cluster, many internal energy modes exist for internal energy 

conversion. Therefore, it is not surprising that emission lifetimes follow the laser pulse 

width of typically 20 ns[48]. I2 in the D’ state was stabilized by the argon cluster and 

shown to persist for 7 sec. However, a second laser was required to generate 

fluorescence from this species. 

Figure 4 results are observed in a N2 jet. I2/N2 clusters are difficult to produce 

and generally require freestream temperatures < 150K. In a free jet expansion with 

rotational I2 temperature of 10K [49], fluorescence from I2 rare-gas clusters of argon, 

krypton, and xenon has been observed[49]. No clustering or fluorescence was 

observed with I2 mixed with nitrogen, helium or neon. Johnson and Levy [50] produced 

I2/N2 clusters by expanding a mixture of 1% N2, 99% helium, with iodine saturated at its 

room temperature vapor pressure at a total pressure of 1000 psi through a 25 m 

supersonic nozzle. Downstream pressure was ~ 0.1 Torr. 

Marshall et al. [51] produced I2/N2 clusters by expansion of a mixture at 300K 

through a Mach 5 nozzle to ~60K in a chemical oxygen-iodine laser (COIL) laser 

experiment. This process removed free iodine and thereby reduced lasing efficiency. A 

cluster-buster wire mesh created a bow shock which raised the temperature to > 160K 

and destroyed the I2/N2 clusters. 

Since I2 seeding in the present study is identical to a previous study[4], it is 

concluded that I2 density in the wake jet of figure 4 is 2.5 x 1014 molecules/cm3

 or ~0.5% 

of jet air density. Wake jet temperatures at the locations probed in Figure 4 are ~ 300-

400K [4]. At these temperatures, I2/N2 clusters cannot exist. I2/N2 stagnation pressures 

were 20 psi; a negligible fraction of reference 50. Excitation of I2/Arn clusters using an 

ArF excimer laser produced emission lifetimes of ~ 10 ns. This is 1000x less than figure 

4 results. It is consistent with the fact that clusters can be thought of as a liquid. 

Hence, they have a large number of energy modes available for internal energy 

conversion. Based on these facts, the present study finds no evidence to support the 

existence of clusters at these wake temperatures. Even if they exist, there is no 

evidence to support the concept of iodine emission within a homogeneous I2 cluster or a 

heterogeneous I2/N2 cluster or any iodine chemical reaction within a cluster producing 

emission observed in Figure 4 on a time scale spanning 10 sec. 

As discussed throughout this manuscript, I2

+

, I+, I-, “hot” electrons and thermal 

electrons are being produced by two or more photon absorption in iodine. The literature 

contains iodine energy level diagrams to aid the reader[37,52]. The possibility of a long-
18 

lived energy storage mechanism involving high-n Rydberg states has been discounted 

due to the pressures in this wake. Since no known radiative properties of neutral or ionic 

iodine atoms or molecules can produce the observed long-lived emission, attention is 

given to chemical processes. They must be capable of storing energy for >10 sec and 

releasing it via a mechanism which produces 206 nm emission. Possibilities based on 

exothermic thermochemical considerations are given in reactions 1-3. I refers to a 

ground electronic state iodine atom and I** refers to a highly electronically excited iodine 

atom. For both I and I**, the energy levels are not specified. M represents a buffer gas 

which can remove excess energy by collisions. Reactions 1 and 3 represent electronion recombination. Reaction 2 represents ion-ion neutralization. 

 I2

+

 + e-  I 2P3/2 + I 1 

 I+

 + I- + M  I 2P3/2 + I + M 2 

I

+

 + e-  I** + M I 2P3/2 + M 3 

In inductively-excited radio-frequency discharges at pressures of 0.5-5 Torr, 

these reactions were postulated[52] as possible mechanisms for producing long-lived 

emission at 206 nm and 804 nm (4

P0

3/2  4

P5/2) which was observed for ~ 600 sec. 

Barnes and Kushnera[52] attributed this long-lived emission to reaction 2.

The large exothermicity of reaction 2 and particularly reaction 3 requires 

collisions from a buffer gas to remove excess energy in the collision complex and allow 

the reaction to proceed. At wake pressures of ~ 0.42 Torr in the present study, the 

probability of collisional stabilization is negligible. Therefore, kinetic considerations tend 

to discount reactions 2 and 3. In contrast to reactions 2 and 3, I2

+

 is a molecule. It has 

more internal energy modes available for energy transfer and can release energy by 

dissociation following the recombination. No collisions are required to stabilize the 

collision complex. 

Hemmati and Collins[37] attributed 206 nm emission observed following ArF twophoton excitation to reaction 1. Additional support came from addition of 0.5 Torr SF6

which effectively scavenged the thermal electrons and quenched the 206 nm emission. 

Literature evidence and discussions in the present study suggest that 206 nm 

emission results from reaction 1. If thermal electrons are required for reaction 1 to 

proceed, this may explain the induction time observed in Figure 7A. 

For the present proof-of-concept test, the primary interest is demonstrating 

viability of the diagnostic in a Mach 10 wake. Results in the present study suggest that 

Figures 4 and 8 are imaging the kinetics of an ion-ion or electron-ion reaction. This 

reaction provides a long-lived chemical storage mechanism capable of releasing 

detectable energy on a time scale > 10 sec. It is hoped the present study will stimulate 

further research to determine which of these reactions is responsible for ITV results 

reported in the present manuscript and further elucidate any potential intermediate 

states in this complex spectroscopic process. It is noted that this two or more photon 

absorption process is excited using only 2.5 mJ, a high divergence (3 mrad vertical x1 

mrad horizontal) unpolarized broadband excimer laser, and 600 mm focal length lens. 
19 

Following emission at 206 nm, iodine atoms are deposited in their first excited 

electronic state I (2

P0

1/2). This is a metastable state with 0.1 sec lifetime [25]. 

Potentially, the I (2

P0

1/2)  I (2

P0

3/2) transition at 1.315 m could be monitored to further 

increase transit time. However, no intensified camera with a gateable infrared 

photocathode and sufficient quantum efficiency currently exists to take advantage of this 

process. The reaction between ground-state atomic iodine and oxygen is endothermic 

and does not occur. Using an I-atom concentration of 2 x 1013 molecules/cm3

, the 

recombination reaction of atomic iodine with itself to form I2 is negligible in 1 Torr air 

[53]. Therefore, addition of a probe laser to excite 2

P3/2 2P0

1/2 at 206 nm, I (2

P0

1/2) I 

(

2

P0

3/2) transition at 1.315 um, or I 2

P3/2 2P0

3/2 at 178 nm could further increase transit 

time at the expense of experimental complexity. 

V. Conclusion 

The first demonstration of time-of-flight based Iodine Tagging Velocimetry(ITV) is 

presented. Experiments were performed along a 5.5 mm line in a 0.5% iodine/nitrogen 

jet in the hypersonic near wake of a capsule entry vehicle model in the NASA-Langley 

31-inch Mach 10 air wind tunnel at 350 psi stagnation pressure and 990K stagnation 

temperature. The original intent of this study was to determine if high-n Rydberg atoms 

of iodine formed by resonant two-photon 193 nm excitation of molecular iodine could be 

used for time-of-flight velocimetry. Unfortunately, at the higher than expected wake 

pressure of 0.42 Torr, collisions quench the high-n Rydberg atoms in < 60 ns. This 

approach may still be viable at significantly lower pressures. However, a study of this 

approach has lead serendipitously to the discovery of an alternate iodine-based energystorage mechanism allowing hypersonic wake velocimetry. 

Results suggest I2 absorbs at least 2 photons to produce I2

+

, I+, I- and electrons. 

I

+

 and I- neutralization and/or the recombination of I2+ and electrons release this energy 

over at least 10 sec as the tagged region is displaced downstream. This process 

produces I (2P3/2 ) whose emission is monitored at 206 nm. Extrapolation of results 

combined with the increased optical collection of a 4” lens developed after the wind 

tunnel experiments suggests wind tunnel fluid displacement times > 10 sec can be 

observed. 

The result is a velocity diagnostic well suited for spatially-resolved velocimetry 

measurements to study the fundamental flow physics of cold-flow hypersonic near and 

far wakes. This velocimetry method has the added advantage of simplicity through use 

of a single laser and camera. Additional research is required to identify the reaction 

mechanism, to understand all the subtleties of this process, and to determine if there 

are other emission pathways which can be used for velocimetry. 

This approach avoids typical toxicity and corrosion problems of iodine using local 

seeding from a single model pressure port with trace quantities (<2.5 x 1014 cm-3) of 

iodine. Iodine is chemically nonreactive, persists in the flow, and follows streamlines. 
20 

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REPORT DOCUMENTATION PAGE Form Approved

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Technical Memorandum

 4. TITLE AND SUBTITLE

Iodine Tagging Velocimetry and Mechanism in the Hypersonic Near Wake 

of a MultiPurpose Crew Vehicle 

5a. CONTRACT NUMBER

 6. AUTHOR(S)

Balla, R. Jeffrey

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NASA Langley Research Center

Hampton, VA 23681-2199

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National Aeronautics and Space Administration

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NASA

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14. ABSTRACT

This study demonstrates a new molecular tagging velocimetry(MTV) method for velocity measurements of high speed flow. It 

demonstrates offbody Iodine Tagging Velocimetry (ITV) in the hypersonic near wake of a MultiPurpose Crew 

Vehicle(MPCV) model. Experiments are performed in the NASA-Langley 31-inch Mach 10 air wind tunnel. A 0.5% I2 / N2 

mixture is seeded on the leeward backshell of the model using a pressure tap. I2 laser-induced fluorescence is excited along a 

5.5 mm line using an ArF excimer laser near 193 nm. Results indicate I2 absorbs at least 2 photons to produce iodine ions and 

electrons. These recombine as the tagged region is displaced downstream to produce I (2P3/2) whose emission is monitored at 

206 nm. Results at P0 = 2.41 MPa (350 psi), T0 = 990K, and 10 μsec transit times produce velocities from 630-820 m/sec 

across the I2 seeded jet at a distance of 38.2 mm (25.5 jet diameters) downstream from the jet orifice. Maximum wake jet 

velocities near the shear layer are 59% of freestream velocity. 

15. SUBJECT TERMS

Hypersonic speed; Iodine tagging velocimetry; Velocity measurement; Wake measurement

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