NorthResearch Group

Novel Laser Diagnostics

We have recently developed and characterized a laser based diagnostic technique to characterize unsteady NTE flows.

Molecular tagging velocimetry (MTV) is a powerful alternative to the more common particle imaging velocimetry (PIV)

technique. Traditional PIV often suffers from

nonuniform particle seeding, inaccuracies in

tracking velocities across strong shocks, and a

prohibitive degree of light scattering near

surfaces. The vibrationally excited NO

Monitoring (VENOM) technique is an extension

of two-component MTV using NO(v=1)

generated from the photodissociation of

seeded NO2, where the two sequential

fluorescence images are obtained probing two

different rotational states to provide both

velocity and temperature maps. Velocity and

internal state distributions represent critical

parameters in the characterization of complex

and reacting flowfields. To our knowledge there

is currently no single optical diagnostic

technique capable of simultaneous

measurements of these properties in a plane.

A schematic diagram of the system is shown to the left.

Two ‘read’ lasers pulses, following photolytic production of NO, excite different NO rotational states with a time delay to

allow displacement. Differences bewteen the fluorescence images provides the two-component velocity field. Following

dewarping, the ratio of images can be used to determine the rotational/translational temperature of the flow. Shown to

the right is an image pair of an axisymmetric underexpanded

jet obtained using the VENOM technique. The NO rotational

states have been chosen to provide accurate two-line

temperature determination for this specific flow. Since the

initial image, in practice is time delayed from the well-

characterized ‘write’ grid, it is possible to extract acceleration

from such data. The advantage of using NO2 photolysis in an

MTV experiment resides in being able to overcome the short

fluorescence lifetime of NO in high quenching environments

and in having the possibility of probing the vibrationally

excited photofragment to discriminate from environments

with background NO.

Thermal disturbance of the flow, however, remains a potential

problem when high-photolysis laser powers are employed. An

alternative method to perform these measurements, retaining

the advantages of using vibrationally excited NO without the

significant thermal disturbance of the photolysis approach,

consists in performing tagging of an NO-containing flow by electronic excitation of NO. Electronic excitation of NO in the

gamma bands is followed by the formation of vibrationally excited NO by spontaneous emission and collisional

quenching. Spontaneous emission from the A2+ state of NO favors low-v” states according to Franck-Condon transition

probabilities, peaking at v”=1 with a 28% yield. This results in enough population that can be subsequently probed by

additional electronic excitation probes to track flow motion. This approach is named “invisible ink” and is shown below

to the left. Demonstration of the technique in an underexpanded jet is shown below.

Repetitively Pulsed Hypersonic Test Chamber

We have recently constructed and characterized a repetitively pulsed hypersonic test facility which was developed to

enable laser diagnostic development and exploratory experiments (see below). We have demonstrated pulsed (5-15 ms

pulse length) uniform hypersonic flows at M=2.9, M=3.8, M=4.6 and M=6.2 for extended operation (>8 hours). The pulsed

flow output can be easily synchronized with laser and camera systems using conventional digital delay generators. The

system is particularly well suited to

longtime operation often required for

diagnostic development and/or

optimization, and for the use of toxic

chemical tracers which can be

cryogenically trapped at the modest

flows employed. The pulse-to-pulse

total pressure repeatability was within

1%. The measured exit flow velocity

and temperature, measured using NO

MTV/PLIF, were uniform to within 1.5%

and 3.7% for the M=4.6 and M=6.2

nozzles, respectively.

Recently we have demonstrated the viability of the simultaneous measurement of three components of velocity and planar

temperature across an oblique shock wave by the excitation of two different transitions in the A-X (1, 1) band of

vibrationally excited NO formed by spontaneous emission and collisional quenching from initially excited NO (A 2Σ+).  Three-

component velocity determinations were derived from two-dimensional molecular tagging velocity measurements

employing sequential fluorescence images pair obtained simultaneously by two cameras in stereoscopic configuration.  The

resulting velocity component plots are shown below where the streamwise velocity component in the pre-shock area

results in 760±14 m/s within 2.8% deviation from the predicted value of 739 m/s, which is comparable to our previous

reports of streamwise motion determinations under similar conditions. The post-shock streamwise velocity component

decreases to 672±20 m/s, which is consistent with the calculated value of 657±9 m/s, based on a wedge deflection angle of

16 ± 1 degrees. The out-of-plane velocity measurement in the post-shock resulted in a value of 169 ± 8 m/s, within 10%

deviation from the expected value of 188±10 m/s. The out-of-plane velocity component before the shock wave is consistent

with the expected value of 0 m/s, while the radial velocity component is negligible as predicted in both pre-shock and post-

shock areas. The stereoscopic

VENOM measurements resulted in

an average temperature of 54 K in

the pre-shock area, 6 K higher than

the temperature obtained from NO

PLIF and 20 K higher than the

calculated temperature based on

the nozzle Mach number. However,

the temperature measured

downstream from the shock

resulted in a value of 69±2 K,

showing better agreement with the

predicted temperature of 72±4 K.

Aerothermochemistry

We are currently involved in collaborative projects with Rodney Bowersox of the Aerospace Engineering Department as

part of the National Aerothermochemistry Laboratory (NAL) (formerly as part of the National Center for Hypersonic

Laminar-Turbulent Transition Research) at Texas A&M University. Our research focuses on optical characterization of high

speed (hypersonic) flows; measurement of freestream turbulence, measurement of internal energy distributions and

relaxation of non-thermal equilibrium (NTE) distributions, surface ablation and reactivity, and laser induced NTE driven

turbulence. These measurements require the application of state-of-the art laser diagnostic techniques such as coherent

anti-Stokes Raman spectroscopy (CARS), spontaneous, Raman, and two-line planar laser induced fluorescence (PLIF).

NorthResearch Group

Novel Laser Diagnostics

We have recently developed and characterized a laser based

diagnostic technique to characterize unsteady NTE flows.

Molecular tagging velocimetry (MTV) is a powerful alternative

to the more common particle imaging velocimetry (PIV)

technique. Traditional PIV often suffers from nonuniform

particle seeding, inaccuracies in tracking velocities across

strong shocks, and a prohibitive degree of light scattering

near surfaces. The vibrationally excited NO Monitoring

(VENOM) technique is an extension of two-component MTV

using NO(v=1) generated from the photodissociation of

seeded NO2, where the two sequential fluorescence images

are obtained probing two different rotational states to

provide both velocity and temperature maps. Velocity and

internal state distributions represent critical parameters in

the characterization of complex and reacting flowfields. To

our knowledge there is currently no single optical diagnostic

technique capable of simultaneous measurements of these

properties in a plane. A schematic diagram of the system is

shown to the left.

Two ‘read’ lasers pulses, following photolytic production of

NO, excite different NO rotational states with a time delay to

allow displacement. Differences bewteen the fluorescence

images provides the two-component velocity field. Following

dewarping, the ratio of images can be used to determine the

rotational/translational temperature of the flow. Shown to the

right is an image pair of an axisymmetric underexpanded jet

obtained using the VENOM technique. The NO rotational

states have been chosen to provide accurate two-line

temperature determination for this specific flow. Since the

initial image, in practice is time delayed from the well-

characterized ‘write’ grid, it is possible to extract acceleration

from such data. The advantage of using NO2 photolysis in an

MTV experiment resides in being able to overcome the short

fluorescence lifetime of NO in high quenching environments

and in having the possibility of probing the vibrationally

excited photofragment to discriminate from environments

with background NO.

Thermal disturbance of the flow, however, remains a

potential problem when high-photolysis laser powers are

employed. An alternative method to perform these

measurements, retaining the advantages of using

vibrationally excited NO without the significant thermal

disturbance of the photolysis approach, consists in

performing tagging of an NO-containing flow by electronic

excitation of NO. Electronic excitation of NO in the gamma

bands is followed by the formation of vibrationally excited NO

by spontaneous emission and collisional quenching.

Spontaneous emission from the A2+ state of NO favors low-

v” states according to Franck-Condon transition probabilities,

peaking at v”=1 with a 28% yield. This results in enough

population that can be subsequently probed by additional

electronic excitation probes to track flow motion. This

approach is named “invisible ink” and is shown below to the

left. Demonstration of the technique in an underexpanded jet

is shown below.

Repetitively Pulsed Hypersonic Test Chamber

We have recently constructed and characterized a repetitively

pulsed hypersonic test facility which was developed to enable

laser diagnostic development and exploratory experiments

(see below). We have demonstrated pulsed (5-15 ms pulse

length) uniform hypersonic flows at M=2.9, M=3.8, M=4.6 and

M=6.2 for extended operation (>8 hours). The pulsed flow

output can be easily synchronized with laser and camera

systems using conventional digital delay generators. The

system is particularly well suited to longtime operation often

required for diagnostic development and/or optimization,

and for the use of toxic chemical tracers which can be

cryogenically trapped at the modest flows employed. The

pulse-to-pulse total pressure repeatability was within 1%. The

measured exit flow velocity and temperature, measured

using NO MTV/PLIF, were uniform to within 1.5% and 3.7%

for the M=4.6 and M=6.2 nozzles, respectively.

Recently we have demonstrated the viability of the

simultaneous measurement of three components of velocity

and planar temperature across an oblique shock wave by the

excitation of two different transitions in the A-X (1, 1) band of

vibrationally excited NO formed by spontaneous emission

and collisional quenching from initially excited NO (A 2Σ+). 

Three-component velocity determinations were derived from

two-dimensional molecular tagging velocity measurements

employing sequential fluorescence images pair obtained

simultaneously by two cameras in stereoscopic configuration. 

The resulting velocity component plots are shown below

where the streamwise velocity component in the pre-shock

area results in 760±14 m/s within 2.8% deviation from the

predicted value of 739 m/s, which is comparable to our

previous reports of streamwise motion determinations under

similar conditions. The post-shock streamwise velocity

component decreases to 672±20 m/s, which is consistent with

the calculated value of 657±9 m/s, based on a wedge

deflection angle of 16 ± 1 degrees. The out-of-plane velocity

measurement in the post-shock resulted in a value of 169 ± 8

m/s, within 10% deviation from the expected value of 188±10

m/s. The out-of-plane velocity component before the shock

wave is consistent with the expected value of 0 m/s, while the

radial velocity component is negligible as predicted in both

pre-shock and post-shock areas. The stereoscopic VENOM

measurements resulted in an average temperature of 54 K in

the pre-shock area, 6 K higher than the temperature obtained

from NO PLIF and 20 K higher than the calculated

temperature based on the nozzle Mach number. However,

the temperature measured downstream from the shock

resulted in a value of 69±2 K, showing better agreement with

the predicted temperature of 72±4 K.

Aerothermochemistry

We are currently involved in collaborative projects with

Rodney Bowersox of the Aerospace Engineering Department

as part of the National Aerothermochemistry Laboratory (NAL)

(formerly as part of the National Center for Hypersonic

Laminar-Turbulent Transition Research) at Texas A&M

University. Our research focuses on optical characterization of

high speed (hypersonic) flows; measurement of freestream

turbulence, measurement of internal energy distributions and

relaxation of non-thermal equilibrium (NTE) distributions,

surface ablation and reactivity, and laser induced NTE driven

turbulence. These measurements require the application of

state-of-the art laser diagnostic techniques such as coherent

anti-Stokes Raman spectroscopy (CARS), spontaneous, Raman,

and two-line planar laser induced fluorescence (PLIF).