AIAA 45th Aerospace Sciences Meeting and ExhibitJanuary 8-11, 2007, Reno, CA

AIAA Paper 2007–0325Session: Flow Control Experiments

Aero-Optical Interactions, Imaging, andOptimization in Turbulent Separated Flows

Fazlul R. Zubair ∗, Aaron P. Freeman ∗, Siarhei Piatrovich ∗,Jennifer Shockro ∗, Josep Salvans-Tort ∗, and Haris J. Catrakis †

Turbulence and the Dynamics of Flows, Mechanical and Aerospace EngineeringUniversity of California, Irvine CA 92697-3975, USA

Aspects of aero-optical interactions, imaging, and optimization are examined in largeReynolds number compressible separated shear layers using the UC Irvine Aero-OpticsVariable-Pressure Turbulent Flow Facility. Experiments are conducted for unforced sep-arated shear layers as well as for forced separated shear layers using a custom-builtdielectric barrier discharge (DBD) plasma actuator that operates at elevated pressures.The DBD plasma actuator is wall flush mounted upstream of the separation locationand is operated at a test section pressure of p ∼ 3atm. Laser-induced-fluorescence ofthe refractive-index turbulent field at high resolution is conducted by premixing ace-tone vapor with air for the unforced cases. Whole-field shadowgraph imaging of pure-airseparated shear layers is conducted for control off vs. control on cases at various forc-ing frequencies, in order to explore the effects of plasma forcing on the large-scale flowbehavior. In these open-loop experiments, the Reynolds number based on the visualthickness is Re ∼ 6 × 106 and the convective Mach number is Mc ∼ 0.4, with an elevatedtest section pressure of p ∼ 3atm. Evidence is presented indicating the possibility of sup-pression, i.e. disorganization, of large-scale organized structures in the separated shearlayer excited using high-frequency plasma forcing as a means of direct reduction of thelaser aberrations.

1. IntroductionAs discussed in a number of reviews and books on

aero-optical phenomena,1–5 directed energy systemson airborne vehicles, such as high-energy laser prop-agation systems or low-energy laser communicationsystems, are faced with significant challenges associ-ated with the wide range of spatial scales and tem-poral frequencies at the large Reynolds numbers ofaircraft-generated turbulent flows, particularly highly-separated flows. The aero-optical effects associatedwith the wide ranges of scales as well as with thestrong irregularity of turbulent flows pose significantchallenges fundamentally and technologically.6–11

At large Reynolds numbers, the flow structure andresulting laser beam behavior are accessible at presentonly through experiments either in the laboratory or inthe field. This is because theories as well as computa-tions require, at present, modeling of the small scalesfor large Reynolds number conditions. Furthermore,one can appreciate that knowledge of the propagatedbeam behavior only, or of the integrated flow behav-ior only, does not uniquely relate to the aero-opticalinteractions. This is because the optical aberrationsresulting from refractive-index variations integrate theflow structure along the beam propagation path. Thusone needs a means to examine the aero-optical interac-tions throughout the turbulence along the beam path.

∗Graduate Student.†Associate Professor, AIAA Member, Corresponding Author,

E-mail address: catrakis@uci.edu.

In order to overcome the challenges posed by aero-optical effects, fundamental advances are needed aswell as developments of methods for direct reductionof beam aberrations using flow/beam control. In termsof turbulent flow control, two primary approaches canbe identified based on active regularization or disor-ganization of the large-scale flow structure. Pioneeredby Prof. Jumper’s group,12 efforts on active flow con-trol for large-scale flow regularization coupled withadaptive optics provide a promising means to reducethe beam aberrations. Efforts on active large-scaledisorganization are also in progress, with encourag-ing results first presented by Stanek and co-workers,13

which can also in principle be coupled with adaptiveoptics. In addition to these two main types of ap-proaches, one also needs to be able to optimize theflow geometry or turret geometry.

The present work follows on the recent studies14,15

by our group (AIAA Papers 2006-1495 and 2006-3070).The new element is the emphasis on forced separatedshear layers using plasma actuators. The extent towhich the large scales that dominate the aero-opticalinteractions can be suppressed by flow forcing is usefulfor the development of methods for the direct reduc-tion of beam aberrations. In the work presented be-low, we consider plasma forcing by a dielectric barrierdischarge (DBD) actuator and its effects on the com-pressible separated shear layer. We examine the as-sociated aero-optical interactions using shadowgraphsand laser-induced fluorescence imaging of the refrac-tive field.

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AIAA 45th Aerospace Sciences Meeting and ExhibitJanuary 8-11, 2007, Reno, CA

AIAA Paper 2007–0325Session: Flow Control ExperimentsVariable-Pressure Aero-Optics Flow Facility

Fig. 1 Photograph of the UC Irvine aero-opticsvariable-pressure turbulent flow facility.

2. UCI Aero-Optics Variable PressureTurbulent Flow Facility and Imaging

Figures 1 and 2 show the aero-optics variable-pressure flow facility employed for the present studiesand the flow imaging configuration. The facility con-sists of a main pressure vessel that houses the testsection, a reservoir vessel, pulsed laser instrumenta-tion, a high-resolution intensified CCD camera flowimaging system, and Shack-Hartmann wavefront imag-ing instrumentation. This facility enables flow/beamimaging experiments at elevated test-section pressuresin the range 0 <

∼ p <∼ 20 atm. The elevated pressures

are essential to record flow images with higher signal-to-noise ratio in comparison to atmospheric or sub-atmospheric pressures. The resulting image signal-to-noise ratio is larger due to the higher local density ofair molecules as a result of the higher operating pres-sures.

The relatively-large size and variable-pressure capa-bility of the facility have resulted in large Reynoldsnumbers with medium-compressibility conditions:

Re ∼ 106 − 107 , with , Mc ∼ 0.4 , (1)

where the Reynolds number is based on the visualthickness of the maximum imaged transverse extent ofthe flow and Mc is the convective Mach number. Thetest-section pressure is p ∼ 3 atm for these studies.

Fig. 2 Experimental arrangement for laser prop-agation and flow/beam imaging.

3. High-Resolution Laser Imaging ofRefractive-Index Turbulent Field

The present imaging technique utilizes acetone va-por to seed the air at a molecular level in order toconduct direct flow imaging of the refractive fluid in-terfaces. A pulsed ultraviolet laser beam is shapedinto a vertical laser sheet which is then propagatedthrough the quartz windows of the pressure vesseland through the flow test section. A high-resolutiondigital intensified CCD camera captures the visiblelaser-induced fluorescence from acetone vapor molec-ularly premixed in air which captures the refractiveindex field. The use of acetone vapor provides a sig-nificant laser-induced fluorescence signal at elevatedpressures,16 in comparison to other approaches suchas Rayleigh scattering in air, and eliminates the aero-optical intensity streaks that arise in using Rayleighscattering in gases such as ethylene.17 In the presentflow/beam imaging experiments, the use of the acetonevapor ensures purely-gaseous fluid in the test sectionand thus enables the study of purely gas-phase flowand the associated refractive-index variations.

Examples of laser-induced fluorescence images of therefractive field of the separated shear layer, showingdifferent levels of large-scale disorganization vs. or-ganization, are shown in figure 3. The images corre-sponds to streamwise slices through the shear layer.The images shown are able to freeze the instantaneousrefractive-index field since the laser pulse temporal du-ration is <

∼ 10 nsec. In the flow images in figure 3, thecolors shown correspond to the refractive turbulent

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AIAA Paper 2007–0325Session: Flow Control Experiments

interfaces. The white regions located in the bottompart of the flow image correspond to pure ambientair. Freestream acetone/air is visible in the dark blueregions in the upper part of the flow image shown.Intermediate shades of blue correspond to refractive-index values for mixed fluid.

A pulsed Nd:YAG laser was utilized to generate anultraviolet beam as indicated in figure 2. A SpectraPhysics Model INDI 40-10 laser system was utilized asthe illumination source. The laser produced ∼ 50 mJper pulse at a wavelength of 266 nm. This ultravio-let laser wavelength corresponds to frequency quadru-pling, i.e. it correspond to the fourth harmonic of the1064 nm fundamental infrared output of the laser. Asindicated in figure 2, the laser sheet was propagatedthrough the quartz windows of the pressure vessel andinto the large-Reynolds-number single-stream shearlayer in the test section.

For the laser-induced fluorescence imaging, the ver-tical laser-sheet orientation employed corresponds toa streamwise slice of the shear layer, cf. figure 1.The laser-sheet extent corresponding to the illumi-nated part of the shear layer was ∼ 5 cm×10 cm. Thedownstream extent of the imaged shear layer corre-sponded to the laser-sheet width of ∼ 5 cm. Thelarge-scale transverse extent of the imaged shear layerwas ∼ 10 cm. The acetone vapor in the air was ex-cited by the incident ultraviolet laser sheet to generatevisible, in particular blue, fluorescence. An enhancedfluorescence signal resulted due to the elevated test-section pressure of p ∼ 3 atm.

We examine the aero-optical interactions by a com-bined experimental and computational approach inorder to examine the aero-optical aberrations in theflow, i.e. along every location of the beam propaga-tion path, retaining the large-Reynolds-number capa-bility of the experiments while including the benefitsof computations. We confine our studies to the near-field aero-optical distortions for which geometrical op-tics provides a useful approximation,21 whereas far-field optical beam propagation requires Fourier optics.The present combined experimental/computationalapproach can be appreciated by realizing that inpurely-computational aero-optics,8,9, 21–25 as in com-putational fluid dynamics, there are currently limita-tions regarding the Reynolds numbers that are achiev-able even with massively-parallel computers in directnumerical simulation (DNS) techniques. In contrast,whole-field imaging experiments such as in the presentstudy at large Reynolds numbers provide experimentalresults free from any small-scale modeling.

In the present combined computational and exper-imental approach, the imaged refractive-index field isinput to the eikonal equation from the large-Reynolds-number flow imaging experiments. The eikonal equa-tion written in terms of the optical path length

Laser-Induced Fluorescence Imaging of Turbulent Refractive Index Field at Large Re

Fig. 3 Examples of laser-induced fluorescenceimages of turbulent refractive-index fields in sep-arated shear layers.

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AIAA Paper 2007–0325Session: Flow Control Experiments

Fig. 4 Examples of profiles of the interactionoptical path difference (IOPD) for refractive-fieldimages of unforced separated shear layers. Thevertical axis denotes the IOPD, the right-runningaxis denotes the streamwise distance, and the left-running axis denotes distance along the laser wave-front propagation path.

OPL (x, t) and the refractive index n(x, t) is:

|∇OPL (x, t)| = n(x, t) . (2)

With an initial or boundary condition for the shapeof the optical wavefront, the eikonal equation is solvedby a finite-difference scheme. Solutions of the eikonalequation along the individual ray paths correspond tothe OPL integral in terms of the refractive-index field:

OPL (x, t) ≡∫

path

n(`, t) d` ≡∫

path

n(`, t) hn,`|dn| ,

(3)where hn,` denotes the interfacial thickness componentalong the ray direction. The latter expression occurs inthe interfacial-fluid-thickness approach.10 The physi-cal distance along the beam propagation path for eachlight ray is denoted as `. The OPL integral corre-sponds to inverting the eikonal equation.

To gain further insight into the one-to-one aero-optical correspondence as a function of distance zalong the beam propagation path, we consider the in-teraction optical path length (IOPL). In other words,we consider the cumulative integral in terms of therefractive-index field, as a function of the beam prop-agation path distance:

IOPL (z, t) ≡∫ z

path

n(`, t) d` ≡∫ z

path

n(`, t) hn,`|dn| ,

(4)where hn,` denotes the interfacial thickness componentin the ray direction along the beam path.

4. Dielectric Barrier Discharge (DBD)Plasma Actuator for Open-Loop ActiveTurbulent Flow Control

The dielectric barrier discharge (DBD) plasma ac-tuator has been proven to be an effective flow con-trol device.18,19 In our work, we have developed acustom-built DBD plasma actuator for operation inour elevated-pressure flow facility. Photographs of theactuator are shown in figure 5. The DBD was usedto generate high frequency excitation (20 - 70 kHz) tocontrol the separated turbulent shear layer. The actu-ator was oriented with the plasma body force in thesame direction as the freestream, in some of the exper-iments, and opposite to the direction of the freestreamin other experiments.

Results on testing of the DBD plasma actuator, instill air, are shown in figures 6 and 7. Whole-fieldshadowgraphs of the effects of the plasma actuator onthe separated shear layer are shown in figures 8 and9. In the bottom and middle images of figure 8, theDBD plasma actuator is oriented to induce flow down-stream whereas in the images of figure 9 the actuatoris oriented to induce flow upstream. The potential forlarge-scale disorganization, and thus reduction of laseraberrations, is evident based on these images.

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AIAA Paper 2007–0325Session: Flow Control Experiments

Plasma Forcing of Separated Shear Layersfor Direct Reduction of Laser Aberrations

Fig. 5 Photographs of the dielectric barrier discharge (DBD) plasma actuator in the wind tunnel (top),with the plasma actuator on glowing purple (middle), and with the plasma actuator off (top).

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AIAA Paper 2007–0325Session: Flow Control Experiments

Plasma Forcing of Separated Shear Layersfor Direct Reduction of Laser Aberrations

Fig. 6 Plot of the plasma actuator applied voltage as a function of the input supply voltage to thehigh-voltage (HV) power supply at 1atm.

Figure 4.15: Shadowgraph images of the induced flow created by the plasma in stillair with frequencies of 20, 30, and 40 kHz (left to right) at 3 atm.

Figure 4.16: Shadowgraph images of the induced flow created by the plasma in stillair with frequencies of 50, 60, and 70 kHz (left to right) at 3 atm.

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Fig. 7 Shadowgraph images of induced flow created by plasma in still air with frequencies of 20kHz,30kHz, and 40kHz (left to right) in the test section of the facility at a pressure of 3atm.

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AIAA Paper 2007–0325Session: Flow Control Experiments

Fig. 8 Experimental whole-field shadowgraphs produced by propagating and imaging a 10-inch diameteroptical beam through turbulent separated shear layers at Re ∼ 6× 106, p ∼ 3atm, and Mc ∼ 0.4 (bottom).Top: Unforced separated shear layer. Middle and bottom: Forced shear layers using DBD plasma ac-tuator, oriented for induced downstream flow, with evidence of large scale regularization (middle) anddisorganization/suppression of large-scale structures (bottom).

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AIAA Paper 2007–0325Session: Flow Control Experiments

Fig. 9 Forced shear layers with the DBD plasma actuator oriented upstream rather than downstream.

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AIAA 45th Aerospace Sciences Meeting and ExhibitJanuary 8-11, 2007, Reno, CA

AIAA Paper 2007–0325Session: Flow Control Experiments

The present open-loop flow forcing experiments areaimed toward direct reduction of the laser aberrationsby suppression of large-scale organized structures inthe separated shear layer. The excitation is gener-ated using high-frequency plasma forcing applied justprior to the separation location of the shear layer. Itis expected that high-frequency forcing provides ex-citation of the small scales26 and, thereby, disorgani-zation/suppression of the large scale structures of theseparated shear layer.13 This approach provides thepromise of direct reduction of the laser aberrations dueto the suppression of the large scale structures. It isimportant to quantify the relative effects of the forcingon the large vs. small scales, in order to compare thecontrol off vs. control on behavior. Future work onthese studies will include closed-loop flow control aswell as adaptive optics control, i.e. coupled adaptiveflows and adaptive lasers.

4. Conclusions and ImplicationsAspects of aero-optical interactions, imaging, and

optimization were examined in large Reynolds num-ber compressible separated shear layers using the UCIrvine Aero-Optics Variable-Pressure Turbulent FlowFacility. Experiments were conducted for unforcedseparated shear layers as well as for forced separatedshear layers using a custom-built dielectric barrier dis-charge (DBD) plasma actuator that operates at ele-vated pressures. Whole-field shadowgraph imaging ofpure-air separated shear layers was conducted for con-trol off vs. control on cases at various forcing frequen-cies, in order to explore the effects of plasma forcingon the large-scale flow behavior. Evidence has beenpresented indicating the possibility of suppression, i.e.disorganization, of large-scale organized structures inthe separated shear layer excited using high-frequencyplasma forcing as a means of direct reduction of thelaser aberrations.

AcknowledgementsThis work is part of a research program on turbu-

lence and the dynamics of flows with support by theAir Force Office of Scientific Research monitored byLt. Col. Dr. Rhett Jefferies. We are grateful for theadvice of Prof. Eric Jumper, Col. Demos Kyrazis, andRudy Martinez (AFRL/DE, NM).

References1Gilbert, K. G. and Otten, L. J., “Aero-optical phenomena,”

Progress in Astronautics and Aeronautics, Vol. 80, AmericanInstitute of Aeronautics and Astronautics, 1982.

2Sutton, G. W., “Aero-optical foundations and applica-tions,” AIAA J., Vol. 23, No. 10, 1985, pp. 1525–1537.

3Andrews, L. C. and Phillips, R. L., Laser beam propaga-tion through random media, SPIE Optical Engineering Press,Bellingham, WA, 1998.

4Jumper, E. J. and Fitzgerald, E. J., “Recent advances inaero-optics,” Prog. Aerospace Sci., Vol. 37, 2001, pp. 299–339.

5Catrakis, H. J., “Turbulence and the dynamics of fluid in-terfaces with applications to mixing and aero-optics,” RecentResearch Developments in Fluid Dynamics Vol. 5 , TransworldResearch Network Publishers, ISBN 81-7895-146-0, 2004, pp.115–158.

6Liepmann, H. W., “Deflection and diffusion of a light raypassing through a boundary layer,” Douglas Aircraft CompanyReport SM-14397, Santa Monica, CA.

7Sutton, G. W., “Effects of turbulent fluctuations in anoptically active fluid medium,” AIAA J., Vol. 7, 1969, pp. 1737–1743.

8Truman, C. R. and Lee, M. J., “Effects of organized tur-bulence structures on the phase distortion in a coherent opticalbeam propagating through a turbulent shear flow,” Phys. FluidsA, Vol. 2, 1990, pp. 851–857.

9Tsai, Y.-P. and Christiansen, W. H., “Two-dimensional nu-merical simulation of shear layer optics,” AIAA J., Vol. 28, 1990,pp. 2092.

10Catrakis, H. J. and Aguirre, R. C., “New interfacial fluidthickness approach in aero-optics with applications to compress-ible turbulence,” AIAA J., Vol. 42, No. 10, 2004, pp. 1973–1981.

11Fitzgerald, E. J. and Jumper, E. J., “The optical distortionmechanism in a nearly incompressible free shear layer,” J. FluidMech., Vol. 512, 2004, pp. 153–189.

12Siegenthaler, J. P., Jumper, E. J., and Asghar, A., “Apreliminary study in regularizing the coherent structures ina planar, weakly-compressible, free shear layer,” AIAA 41st

Aerospace Sciences Meeting & Exhibit, AIAA 2003-0680 , Reno,NV, January 2003.

13Stanek, M., Sinha, N., Seiner, J., Pearce, B., and Jones,M., “High frequency flow control: suppression of aero-optics intactical directed energy beam propagation and the birth of anew model (part I),” AIAA 33rd Plasmadynamics and LasersConference, AIAA 2002-2272 , Maui, HI, June 2002.

14Catrakis, H. J., Garcia, P. J., and Nathman, J. C., “Cu-mulative aero-optical interactions along laser beam propagationpaths: experiments and computations,” 44th AIAA AerospaceSciences Meeting and Exhibit, AIAA 2006-1495 , Reno, Nevada,January 2006.

15Zubair, F. R., Garcia, P. J., and Catrakis, H. J., “Aero-optical interactions along laser beam propagation paths in com-pressible turbulence,” AIAA 37th Plasmadynamics and LasersConference, AIAA 2006-3070 , San Francisco, California, June2006.

16Lozano, A., Yip, B., and Hanson, R. K., “Acetone: a tracerfor concentration measurements in gaseous flows by planar laser-induced fluorescence,” Exps. Fluids, Vol. 13, 1992, pp. 369–376.

17Dimotakis, P. E., Catrakis, H. J., and Fourguette, D.C. L., “Flow structure and optical beam propagation in high-Reynolds-number gas-phase shear layers and jets,” J. FluidMech., Vol. 433, 2001, pp. 105–134.

18Corke, T. and Post, M., “Overview of Plasma Flow Con-trol: Concepts, Optimization, and Applications,” AIAA 43rd

Aerospace Sciences Meeting and Exhibit, AIAA 2005-0563 ,Reno, Nevada, 2005.

19Samimy, M., Adamovich, I., Webb, B., Kastner, J., Hile-man, J., Keshav, S., and Palm, P., “Development and charac-terization of plasma actuators for high-speed jet control,” Exps.Fluids, Vol. 37, 2004, pp. 577–588.

20Kyrazis, D., “Optical degradation by turbulent free shearlayers,” Optical Diagnostics in Fluid and Thermal Flow , editedby S. S. Cha and J. D. Trolinger, SPIE, Orlando, Florida, 1993,pp. 170–181.

21Jones, M. and Bender, E. E., “CFD-based computer sim-ulation of optical turbulence through aircraft flowfields andwakes,” AIAA 32rd Plasmadynamics and Lasers Conference,AIAA 2001-2798 , Anaheim, CA, June 2001.

22Elghobashi, S. E. and Wassel, A. T., “The effect of turbu-lent heat transfer on the propagation of an optical beam across

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supersonic boundary/shear layers,” Intl J. Heat Mass Transfer ,Vol. 23, 1980, pp. 1229–1241.

23Yahel, R. Z., “Turbulence effects on high energy laser beampropagation in the atmosphere,” Appl. Opt., Vol. 29, 1990,pp. 3088–3095.

24Mani, A., Wang, M., and Moin, P., “Computation of op-tical beam propagation through numerically simulated turbu-lence,” Bull. Am. Phys. Soc., 2003.

25Tromeur, E., Garnier, E., Sagaut, P., and Basdevant, C.,“Large eddy simulations of aero-optical effects in a turbulentboundary layer,” J. Turbulence, Vol. 4, No. 5, 2003, pp. 1–22.

26Wiltse, J. M. and Glezer, A., “Direct excitation of small-scale motions in free-shear flows,” Phys. Fluids, Vol. 10, 1998,pp. 2026–2036.

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