Optical Diagnostics for High-speed Flows

Optical diagnostics for high-speed flows

Richard B. Miles 1,n

Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA

a r t i c l e i n f o

Article history:Received 25 June 2014Accepted 11 September 2014Available online 11 October 2014

Keywords:DiagnosticsHigh speed flowsFlow imaging

a b s t r a c t

Since 2000 there has been a revolution in diagnostics of high-speed air flows. The foundations for thisrevolution were laid over the past few decades, but with the development of new short pulse and pulseburst laser technologies, higher laser powers and higher pulse energies, new high-speed cameras, betterlaser control and improved detection and laser delivery methodologies, many very effective newcapabilities have been developed. Newly developed methods for molecular tagging velocimetry providehigh fidelity visualization of transport properties and may be extended to simultaneous temperaturemeasurements. Rapid field imaging with frequency tunable pulse burst lasers shows instantaneous flowstructure and complex boundary and mixing interactions. Extending these pulse burst concepts to sweptvolumetric imaging is very promising for full volumetric data collection. Fast wavelength modulationspectroscopy follows real-time flow variation, and three-dimensional particle imaging extends particleimaging velocimetry to volumetric data acquisition.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction and retrospective

The use of optical diagnostics for the study of high-speed flowsdates back to the 1800s when shadowgraphs and schlieren yieldedimages of the bow shocks, Mach disks and other structures associatedwith high-speed projectiles and supersonic flows [21] (see Fig. 1).Notable progress in optical diagnostics through the twentieth centurythat did not involve laser technologies primarily focused on furtherdevelopment of schlieren and shadowgraph for high sensitivity andhigh-resolution imaging of flow structure. These approaches providedgood resolution of large-scale structures but suffered from integrationover the full optical path length, so details of turbulent boundarylayers, curved and unsteady shocks and mixing structures were notwell resolved. The introduction of an electron beam [30] overcamethat problem since electron beams could be spatially collimated andelectronically swept, providing luminous cross sections of shock andboundary layer structure. However, electron beams are limited to lowdensity flows due to electron scattering, and they are very difficult tointegrate into a test facility. Focusing schlieren [55] provided a methodfor imaging flow structure over a reduced integrated path length inhigher density flows.

Since the invention of the laser in 1960, the continuing evolution ofoptical flow diagnostics has been driven in large part by ever-increasing laser and camera capabilities. The very first laser invented,the pulsed ruby laser, provided high energy and excellent coherence,

which enabled the development of interferometric methods tomeasure flow field properties, such as flow velocity using seedparticles [54] and the imaging of boundary layer structure usingdensity variations [16]. It was not until after the invention of thefrequency tunable laser in 1966 [50,48] that atomic and molecularspectroscopy could be utilized for diagnostics. The tunable dye lasersonly operated efficiently in the visible portion of the spectrum wherethe air is highly transparent, so for these new applications, flowseeding became important. Initially seeding with sodium providedplanar imaging of flow cross sections [33] and enhanced schlieren [4].These advances were accomplished by tuning the laser either onto aresonance or near a resonance and utilizing the laser-induced fluor-escence for imaging planar cross sections or the enhanced index ofrefraction for higher sensitivity schlieren. Tuning the lasers providedmethods for imaging and measurement of velocity fields by takingadvantage of the Doppler shift associated with the motion of the gas[58]. Due to the reactivity of sodium with air, these experiments werecarried out in either helium or nitrogen flows. Imaging and inter-ferometry in these early experiments were done with conventionalhard film.

Molecular iodine was subsequently used for laser-induced flowimaging [29] since it has spectral features throughout the visible anddoes not react with air. Later, tunable ultraviolet lasers becameavailable through frequency up conversion of nanosecond laser-driven pulsed dye lasers, and nitric oxide [41] and acetone [25]became the preferred molecular species for seeding. CCD array andintensified CCD cameras became available and provided high sensi-tivity, time gating and convenient data processing capabilities.

The possibility of using nonlinear optical methods for flowdiagnostics became credible with higher energy, frequency tunable

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n Tel.: 1 60 925 8 5131.1 Robert Porter Patterson, Professor of Mechanical and Aerospace Engineering,

Fellow AIAA.

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pulsed lasers and led to temperature and species measurements byCoherent Anti-stokes Raman Scattering (CARS), temperature andvelocity measurements by Laser Induced Thermal Anemometry(LITA) andMolecular Tagging Velocimetry (MTV). The CARS approach[27,18] brings two or three pulsed laser beams together at a pointand, through a resonant nonlinear interaction, generates a new laserbeam whose intensity is determined by the properties of the gas atthat point. CARS has the capability of measuring species concentra-tions as well as temperature. For that reason it is also of great interestfor combustion studies. Originally CARS required that at least one ofthe lasers be scanned in frequency to acquire the data, and thatmeant that CARS could only be used for time averaged measure-ments. Broadband and later dual broadband CARS [14] solved thatproblem by replacing the scanned laser with a broadband laser andseparating the multiple CARS signal frequencies that were simulta-neously generated with a spectrometer. This allowed CARS to capturefull spectral data over a limited band in a single pulse.

LITA [9] was a similar local measurement approach. It used apair of focused lasers to produce a localized thermal grating intothe air through nonlinear mixing, and a probe laser to follow themotion of the acoustic waves created by that grating as theyinterfered with each other. This provided a local measure of thetemperature though the speed of sound and a measure of theflow velocity through frequency offsets associated with the flowmotion.

MTV [20] introduced a line or array of lines into the flow andtracked them in time as they moved, providing a measure of boththe velocity and the flow velocity structure. The first MTV conceptimplemented in unseeded air was Raman ExcitationLaserInduced Electronic Fluorescence (RELIEF) [34], which used threelaser beams two to drive the oxygen into the vibrational statethrough stimulated Raman excitation, and one to interrogate thedisplaced line or pattern by laser-induced fluorescence. Its greatfeature was that it did not require seeding of the air with othermolecular species and produced negligible perturbation. It workedwell because of the relatively long lifetime of the oxygen vibra-tional state (many microseconds even in humid air). It was limitedby the complexity of the laser systems. Its success led to thedevelopment of other approaches including laser-induced OzoneTagging Velocimetry (OTV) [44], which was also used as a tag inunseeded air. In this case, the ozone was created by a chemicalreaction following laser-induced dissociation of molecular oxygen.The motion was tracked by subsequent laser-induced dissociationof the ozone and imaging of the fluorescence from the excitedmolecular oxygen fragment. Other MTV approaches for air devel-oped before the 2000 used seed molecules and included biacetyl[17] and water vapor [5]

Single mode, frequency tunable lasers utilizing injection lock-ing enabled the development of molecular, atomic and etalonfiltered technologies, permitting strong suppression of backgroundscattering [35], imaging of air temperature, velocity and density(Filtered Rayleigh Scattering [36]) and velocity imaging by Dopplershifted particle imaging through an iodine filter (Doppler GlobalVelocimetry [31]) as well as velocity and temperature imaging ofDoppler shifted Rayleigh scattering through an etalon [49]. Singlemode tunable diode lasers derived from the communicationsindustry and augmented by wavelength modulation technologyhave also opened the door to diagnostic methods for air based ondirect absorption spectroscopy using very weak near infrared linesin molecular oxygen [43]. This concept has been successfullyimplemented for density, velocity and temperature measurementsbased on the measurement of extinction, line shifts and linebroadening.

Particles have been used for centuries to observe flows, but thedevelopment of laser provided a method for quantitative measure-ment through instantaneous holographic imaging and other inter-ferometric methods. Much early work focused on Laser DopplerAnemometry (LDA) [13] with continuous lasers for one or twocomponent point measurements of flow velocity, in which two laserbeams intersected at the sample point and the scattering of theparticle as it moved through the interference pattern which wascreated provided the measure of velocity. With four crossing beams,two velocity components could be measured. The development ofhigh power nanosecond pulsed lasers enabled imaging of timefrozen particle fields and this led to particle imaging velocimetry(PIV) [1], where the two-dimensional velocity field was measured bythe displacement of the particles captured with double pulsed lasersystems. Digital PIV [56] was enabled by the development of high-resolution CCD cameras and eliminated the need for hard film.

Thus at the beginning of the twenty-first century manycapabilities existed for optical diagnostics of high-speed flows.Since that time further development and implementation of thesecapabilities have occurred and laser technology has significantlyadvanced, enabling new approaches. In addition to achievinghigher pulse energy, better reliability and higher efficiency lasers,optical fiber technologies, new cameras, frequency tunable pulseburst lasers and sub-picosecond lasers have opened up newpossibilities for diagnostics. With these tools major advances havebeen made in high-speed imaging, molecular flow tagging, wave-length modulation spectroscopy, Particle Imaging Velocimetry,Coherent Antistokes Raman Scattering and Rayleigh scattering.

2. Imaging

Laser Rayleigh scattering is the strongest non-resonant lightscattering process available for air measurements, but the lowscattering cross section of air molecules has made its use for high-speed diagnostics challenging and only recently practical withhigh energy pulsed lasers and high sensitivity, time gated cameras.It is best applied in free jet facilities where background scatteringcan be minimized. An important application of Rayleigh scatteringin an free jet of air was the evaluation of the Mariah II/RadiativelyDriven Hypersonic Wind Tunnel concept [39]. Those tests wereundertaken for the validation of computational models of anelectron beam heated hypersonic ground test facility and wereconducted at Sandia National Laboratory using their 1 MW Hawkelectron beam facility [28]. The configuration for the tests isshown in Figs. 2 and 3. The 1 MW electron beam is steered andfocused into the nozzle from downstream using a carefullycontoured magnetic field and the Rayleigh imaging is performedwith a frequency doubled Nd:YAG laser focused to a thin sheetalong the center line of the flow at the exit of the nozzle, providing

Fig. 1. Image of an underexpanded sonic jet taken with schlieren photography [26]

R.B. Miles / Progress in Aerospace Sciences 72 (2015) 3036 31

an image of the instantaneous density cross section. As theelectron beam deposits energy into the core of the flow insidethe nozzle, the total enthalpy is more than doubled. The enthalpydeposition leads to a reduced density in the core of the flow, andthe ensuing density profile together with spatially localizedtemperature and velocity measurements provide a quantitativemeasure of the energy deposited. Fig. 4 shows the Rayleigh imagesof the evolution of that density profile starting before the electronbeam is turned on and continuing until equilibrium is reached300 s later. The exit jet is an over expanded supersonic free jet atthe beginning of the energy addition. For accurate measurementsof the density, images such as these need to be carefully calibratedand background noise must be subtracted [3].

Filtered Rayleigh Scattering enhances the capability of laserRayleigh scattering through the use of an atomic or molecular gaswith a strongly absorbing sharp spectral line to filter the scattering.A cell with that gas is placed in front of the camera, and a narrowline width laser tuned in the vicinity of the strong absorption line isused to illuminate the flow. The filter suppresses backgroundscattering, but permits light that has been frequency shifted bythermal, acoustic or convective motion to pass through. By scanningthe laser, this filter can provide for measurement of temperatureand pressure [38]. Single shot imaging of temperature fields can beacquired if the pressure is known [37]. The ability of FilteredRayleigh Scattering to capture cross-sectional cuts of boundarylayer structure in supersonic and hypersonic flows has produceddetailed images of boundary layer behavior in the vicinity of shock-induced separation [6]. Image contrast is greatly enhanced byseeding the flow with about 1% of CO2 gas [45], which condenses,forming a nanoscale particle fog in the cold core of the flow. The

CO2 condensation highlights the outer portion of the boundarylayer where the temperature rises to the sublimation temperature.

This imaging capability is further enhanced by the pulse burstlaser [22]. Because it is based on a master oscillator poweramplifier design (MOPA) the laser is naturally single frequency(single mode) and frequency tunable over a limited range due tothe diode pumped Nd:YAG master oscillator. The master oscillatoroperates as a continuous laser and only after pre-amplification isthe laser beam temporally chopped into a pulse burst and passedthrough the power amplifiers. This feature makes it especiallyuseful for applications such as Filtered Rayleigh Scattering thatrequire narrow line width and tunability over a limited range.

The very rapid response of the CO2 nanoparticle sublimationand condensation provides a clear set of images of the timeevolving shock wave boundary layer structure as shown in Fig. 4,where the pulse burst laser has been used to acquire sequentialimages of the boundary layer driven fluctuations of a separatedshock in the vicinity of a 151 ramp at 2 s intervals. By tuning thelaser wavelength relative to the iodine filter absorption edges, highand low velocity features can be highlighted as shown in thefigure. Images were taken with a MHz rate camera with 30 framesof on board storage designed by Princeton Scientific Instruments.

The initial design of the pulse burst laser had a limited timewindow of 100 s or so over which the pulses could be generatedbased on the gain time of the flashlamp pumped power amplifiers.Recent work has significantly extended that range throughprecision-controlled diode pumping of the power amplifiers, andincreased the overall energy of the pulse burst. These advanceshave enabled the dynamic imaging of lower frequency boundarylayer instabilities and flow phenomena and extended the utility ofthe pulse burst laser to processes such as combustion ignition thatoccur over longer time intervals [51]. Extension of the spectralfrequency range of the pulse burst laser has been achieved withthe addition of optical parametric oscillators/amplifiers and fre-quency up conversion crystals, and now the capability for rapidplanar laser-induced fluorescence imaging of nitric oxide [19,2]and other molecular species has been demonstrated. By combiningthe pulse burst laser with fast beam sweeping technologies, andlenslet array cameras, it is now being extended to full three-dimensional data acquisition [52].

3. Advances in molecular tagging

Molecular flow tagging has developed significantly since 2000.The Air Photolysis And Recombination Tracking (APART)

Fig. 3. Time sequenced Rayleigh images of the density at the exit of the MARIAHII/RDHWT nozzle before and during the electron beam energy addition.

Fig. 2. Setup for the Rayleigh experiments on the electron beam coupled MARIAH II/Radiatively Driven Hypersonic Wind Tunnel at Sandia National Laboratory.

R.B. Miles / Progress in Aerospace Sciences 72 (2015) 303632

Fig. 4. Filtered Rayleigh Scattering, 500 KHz rate images of a shock wave boundary layer interaction upstream of a 15 degree wedge. The presence of CO2 condensatenanoparticles in the low temperature core of the flow provides the contrast. Columns 2 and 3 are with the laser tuned to highlight high and low velocity features. (Flow isfrom right to left.)


technique [10] uses a UV laser to dissociate oxygen and form nitricoxide, which is later imaged by laser-induced fluorescence. If nitricoxide is seeded into the flow or already present from combustionprocesses, then it can be used for molecular tagging in high-speedenvironments by taking advantage of the lifetime of the laser-induced fluorescence [11]. Due to quenching, the fluorescencelifetime is short, but at the low densities and high speedsassociated with hypersonics, the lifetime is long enough to allowa time-gated camera to image the delayed fluorescence and thusthe velocity profiles. The Vibrationally Excited NO Monitoring(VENOM) technique uses photodissociation of seeded NO2 toproduce vibrationally excited NO, which is subsequently imagedby LIF after a delay [51]. The VENOM method also yields a measureof the temperature from the NO rotational spectrum.

The availability of high pulse energy femtosecond lasers hasenabled Femtosecond Laser Electronic Excitation Tagging (FLEET)[32]. In this case a 150 fsec, 2 mJ, Ti:sapphire laser is focused intoair and dissociates nitrogen molecules throughout the focal zoneby a high-order nonlinear interaction. The nitrogen atoms that areformed by this dissociation recombine over an 100 s timeinterval, forming nitrogen molecules in the electronically excited Bstate. Those molecules fluoresce in the red and near-infraredportion of the spectrum, and that fluorescence can be imagedwith a time gated, high sensitivity camera. This process isdiagrammed in Fig. 5. Since the nitrogen atoms are only formedthrough the focal region of the excitation laser, they are initiallyformed along a straight line and act to tag that region. The time-delayed image shows the location to which each segment alongthat line has moved in the time interval between tagging andinterrogation. An important aspect of FLEET is the continuedluminosity of the tagged region for tens of microseconds. Thusby using a fast sequentially gated camera, the evolution of the

tagged elements in the three-dimensional flow can be followed inreal time. Fig. 6 shows images of FLEET lines written at the nozzleexit and imaged at one microsecond time delay intervals showingthe flow at sequential heights above the exit of an over-expandedMach 2.6 air jet. The shape of each line gives the instantaneousvelocity profile at that location and the displacement from theoriginal line gives a quantitative measure of the velocity. FLEETmay also have the capability to measure temperature along theline by relying on the second positive ultraviolet and other spectralfeatures from molecular nitrogen that are emitted at the time oftagging. This emission lasts only a few nanoseconds and appar-ently comes frommolecules that are excited but not dissociated bythe femtosecond laser pulse [15]

4. Advances in single point measurements

The most important advances in the measurement of singlepoint properties have involved the further development of CARStechnologies. As noted earlier, CARS has the capability of capturingtemperature and species information at the point where the lasersintersect. The CARS process can be separated into an initial step,which drives the selected molecules in the sample volume into acoherent oscillating state using a pair of lasers, and a probe thatscatters coherently from the driven molecules in the volume, isfrequency and phase shifted by this coherent process, and producesthe CARS beam. For high-speed air applications, CARS is useful formeasurements of temperature and nonequilibrium conditions at apoint [47]. For combustion and SCRAM jet applications, its ability tosample species is of central importance. Much recent work hasfocused on methods to suppress background, improve single shotperformance, and increase the sample acquisition rate. The mostdifficult background signal is from a similar third-order nonlinearprocess involving electronic resonances that occur simultaneouslywith CARS. The coherence associated with that background processis very short lived, so methods that use femtosecond lasers to takeadvantage of the longer coherence lifetime associated with thedesired process have been developed. Very short-pulsed lasers havethe great advantage of producing very high intensities with lowenergy pulses. The nonlinear process that leads to the CARS signalrequires high intensity pulses, so with femtosecond lasers operationat high repetition rates becomes possible with practical lasersystems. A very successful approach to background suppressionand high-resolution signal generation uses femtosecond lasers todrive the coherence and a time delayed picosecond laser probeoptimized in shape and delay to suppress the nonresonant back-ground [42]. The femtosecond lasers couple to all the appropriatemolecular states of interest and the bandwidth of the picosecondprobe laser is broad enough to enable broadband CARS. Thisapproach now promises to allow kHz rate measurements oftemperature [40].

5. Advances in integrated path measurements

Recent work has demonstrated the utility of the wavelengthmodulation spectroscopy in oxygen for measurements in windtunnels [23,24], and this approach has the distinction of successfuldevelopment for flight testing as part of the Hypersonic Interna-tional Flight Research Experimentation (HIFiRE) 1 experimentalpackage [8,7]. The wavelength modulation approach is seeingwide applications for combustion systems where the measure-ment of water vapor and carbon monoxide are also of interest. Thisapproach uses diode lasers and is particularly attractive because ofthe low power requirements and efficient packaging associatedwith those laser systems.

Fig. 5. Energy level diagram for molecular nitrogen showing the recombinationpath for nitrogen atoms leading to B to A first positive emission. Dissociation is by ahighly nonlinear interaction driven by the 800 nm, 150 fsec laser.

Fig. 6. FLEET lines written at the exit of a vertical Mach 2.6 overexpanded air jet.Lines are imaged at sequential 1 microsecond time intervals following the taggingof a straight line just above the exit.


6. Advances in particle imaging velocimetry

PIV has proven to be a very effective method for the measure-ment of velocities in a plane, and that success has motivatedrecent work extending it to the measurement of three-dimensional velocity vectors, volume fields and to its applicationin hypersonic facilities. These efforts have been facilitated by thedevelopment of high pixel density cameras, the increasing pulseenergy available for the illumination of larger volumes and thedevelopment of the pulse burst laser. Stereoscopic PIV [46] uses apair of cameras to follow each particle's motion in three dimen-sions. This requires that the laser sheet be made relatively thick sothat the particles are not lost as they move in the out-of-planedirection. The accuracy of the measurement is limited in the out-of-plane direction viewing angle separation of the cameras.Another approach to capture three-dimensional PIV and simulta-neous volumetric data is based on a repeatedly swept laser. Theconcept for this was demonstrated in water [12], but with newpulse burst laser and high-speed camera technologies, it has beenextended to air flows [53]. In this case the laser is operated at500,000 pulses per second and rapidly swept through the volume.Images are captured using a DRS Hadland Ultra68 intensified high-speed camera which yields 68 frames with 220 220 pixelresolution in 136 s. By cycling twice, the cameras capture twotime displaced volumetric data sets containing information on thedisplacement of all the particles within the scanned volume.Reduction of the data provides the full three-dimensional flowfield velocity. Extension of PIV to hypersonic flows has also been apriority and a difficult task due to the requirement for very smallparticles in order to avoid problems with particle lag and scatter-ing from walls, which obscures the PIV signal in just the regionwhere the data are the most important. Recent success has beenachieved ([57]) at Mach 7.4 through careful seeding and maskingof wall scattering, and proper selection of data analysis algorithms.

7. Summary

The field of high-speed diagnostics of air has added many newconcepts and expanded previously existing approaches during thelast decade or so, leading to the potential for detailed measure-ments of highly complex flows. Many of these new approacheshave been enabled by new developments associated with laserand camera technologies. For example, these include the fre-quency tunable pulse burst lasers, high pulse energy femtosecondlasers, and multiple image storage fast camera systems. Otheradvances reflect continued development of methods that werepreviously proven, but that are now becoming more versatile andare being demonstrated as reliable instruments for flow fieldmeasurements. The first incorporation of an optical diagnosticinto hypersonic flight occurred during this time, and doubtlessthat is just a taste of what we can expect over the next decade. Thereduction of laser cost, size and weight makes transportablesystems more available, providing new opportunities to moveconcepts that have been proven in small-scale laboratory settingsout into the field. The scale of even very complex systems is beingreduced to a size that may be practical for flight within the nextfive years or so.

Acknowledgments

The Air Force Office of Scientific Research under Dr. JohnSchmisseur has supported the recent work at Princeton. Over thepast several decades, the development of advanced laser diagnos-tics has been strongly supported by the Air Force Office of

Scientific Research. That support has led to the successful imple-mentation of these diagnostics in laboratory facilities and has laidthe foundation for many of the new advances reported here.

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Optical diagnostics for high-speed flowsIntroduction and retrospectiveImagingAdvances in molecular taggingAdvances in single point measurementsAdvances in integrated path measurementsAdvances in particle imaging velocimetrySummaryAcknowledgmentsReferences

Optical Diagnostics for High-speed Flows

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