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AD-NtIS 239 OPTICAL PROPERTIES OF COMPRESSIBLE INHOMOOENEOUS SWEAR 1 LAYVERS RELEVANT TO HIGH POWlER LASERSCU) WASHINGTON UNIV SEATTLE N N CHRISTIANSEN 30 SEP 07 AFOSR-TR UNCLASSIFIED AFOSR-S F/O 9/3 Iii 10MEN hh 1h0 m h h hm h h hom I - ' Hi- ih I N _! I v. Z4 KW lp. UNCL'. -C - F1 E.176 SECURITY CLASSIFICATION OF TH AD-A NPAGE Ia. REPORT SECURITY CLASSIFICA W NI - P UNCLASS IFIlEDI 2a. SECURITY CLASSIFICATION AUTHORITY 3. DISTRIBUTION/AVAILABILITY OF REPORT 2b. DECLASSIFICATION/DOWNGRADING SCHEDULE APPROVED FOR PUBLIC RELEASE DISTRIBUTION IS UNLIMITED 4. PERFORMING ORGANIZATION REPORT NUMBER(S) 5. MONITORING ORGANIZATION REPORT NUMBER(S) AOS NR.tM- F a. NAME OF PERFORMING ORGANIZATION 6b. OFFICE SYMBOL 7a. NAME OF MONITORING ORGANIZATION UNIVERSITY OF WASHING ON ( E f applicable) AFOSR/NA 6c- ADDRESS (City, State, and ZIPCode) 7b ADDRESS (City, State, and ZIP Code) UNIV OF WASH BUILDING 410 SEATLE WASHINGTON BOLLING AFB, DC a. NAME OF FUNDING/SPONSORING 8b. OFFICE SYMBOL 9. PROCUREMENT INSTRUMENT IDENTIFICATION NUMBER ORGANIZATION (If applicable) AFOSR NA AF65r- -c,, 8C. ADDRESS (City, State, and ZIP Code).10. SOURCE.OF FUNDING NUMBERS BUILDING 410 PROGRAM PROJECT TASK WORK UNIT BOLLING AFB, DC ELEMENT NO. NO. NO. ACCESSION NO 6110 F 2307 A2 11 TITLE (Include Security Classification) - (U)OPTICAL PROPERTIES OF COMPRESSIBLE INHOMOGtNEOUS SHEAR LAYERS RELEVANT TO HIGH POWER LASERS.' 12 PERSONAL AUTHOR(S)./ W. CHRISTIANSEN / 13a. TYPE OF REPORT 13b. TIME COVERED 14. DA OF REPORT (Year, Month, Day) [S PAGE COUNT FINAL TECH FROQ/I/83 T_6/30/87 / 9/30/ SUPPLEMENTARY NOTATION 17 COSATI CODES 18 SUBJECT TERMS (CwiV~nue on reverse if necessary and identify by block number) GROUP SUBSHEAR LAYERS, LASER OPTICAL DEGREDATION2 REFRACTIVE INDICES ABSTRAZT (Continue on reverse if necessary and identify by block number) SHEAR LAYERS AND WAKES ARE A MAJOR SOURCE OF OPTICAL DEGREDATION IN FLOW LASERS. THE STRUCTURE OF THESE FLOWS HAS BEEN STUDIED EXPERIMENTALLY WITH SPECIAL ATTENTION GIVEN TO THEIR OPTICAL PROPERTIES. GASES WITH DIFFERENT REFRACTIVE INDICES WERE INVESTIGATED AND THE EFFECTS OF DENSITY RATIO AND MACH NUMBER WERE MEASURED. THE TIME AVERAGED OPTICAL PROPERTIES OF INHOMOGENIOUS SHEAR' LAYERS IS REPORTED HERE WHERIN THE PRINCIPLE FAR FIELD MEASUREMENT WAS THE STREHL RATIO. MODIFICATION OF / THE APPARATUS OF THE SHEAR IS DISCUSSED.(/ JAN F 20 DISTRIBUTION /AVAILABILITY OF ABSTRACT 21 ABSTRACT SECURITY CLASSIFICATIO - OUNCLASSIFIED/UNLIMITED PXIE AS RPT C- DTIC USERS []r,aq.f TED 22a. NAME OF RESPONSIBLE INDIVIDUAL 22b TELEPHONE (Include Area Code) 22c OFFICE SYMBOL DO FORM 1473, 84 MAR 83 APR edition may be used until exhausted SECURITY CLASSIFICATION OF -HIS PAGE All other editions are obsolete UNCLAS I D % U N CL-oI 1!D :' ... ..-- , ;.'..'...,','-. '.k. - '.',. ' '... '. -.. ' ' -? ' ' ' '-. - ' ' ':. * University of Washington AFOS R.' Final Technical Report 2/83-5/31/87 AFOSR Contract # Walter H. Christiansen OPTICAL PROPERTIES OF COMPRESSIBLE INHOMOGENEOUS SHEAR LAYERS RELEVANT TO HIGH POWER LASERS Aerospace and Energetics Research ProgramI i8b 8I Final Technical Report 2/83-5/31/87 AFOSR Contract # Walter H. Christiansen OPTICAL PROPERTIES OF COMPRESSIBLE INHOMOGENEOUS SHEAR LAYERS - RELEVANT TO HIGH POWER LASERS Accession For NTIS GRA&I DTIC TAB Unannounced 5 Justification By Distribution/ _ Availability Codes ~Avail and/or jf copy 1A, o;..5 Final Technical Report 2/83-5/31/87 AFOSR Contract # , Walter H. Christiansen OPTICAL PROPERTIES OF COMPRESSIBLE INHOMOGENEOUS SHEAR LAYERS RELEVANT TO HIGH POWER LASERS ABSTRACT Shear layers and wakes are a major source of optical degradation in flow lasers. The structure of these flows has been studied experimentally with special attention given to their optical properties. Gases with different refractive indices were investigated and the effects of density ratio and Mach numher were measured. The time averaged optical properties of inhomoqeneous shear layers are reported here wherein the principle for field measurement was the Strehl ratio. Modification of the apparatus for low speed measurement, including periodic forcing of the shear is discussed. , - - - . , ,' - . - w, , .' ''r ' '. '.'- -' '. ., ',.- ',,, -,,.... S,- ' ','/ - , ' ,,i ; ' - . -. .''. . . - . - .... ,--' -- .. -1- I. INTRODUCTION Fluid mechanics is involved in many lasing processes and the flow field must he of excellent optical quality so that a near diffraction limited laser beam may he attained. With the general trend of laser development towards shorter wavelenghts, the fluid optics challenge is increased considerahly. In general, the conditioning of the gas laser cavity or external flow effects will continue to he a problem area. The fluid mechanical sources of fluid possible optical difficulties must be carefully examined and understood. It is known that phase distribution as well as, to a lesser degree, amplitude distribution across a coherent beam determines beam quality. We studied basic fluid mechanical properties of compressible shear layers and their effect on the phase distribution of a laser beam. The 2-fD inhomogeneous shear layer is chosen for a number of reasons. It is a simple and well studied flow, at least a low Macn number, M. However, there is no experimental optical data which concentrates on the coherent effects produced by the layer and the extent of the mixing interface on optics. Part of the research involves studying the properties of single two-dimensional shear layers at high Mach numbers and Reynolds numbers appropriate for high power lasers. Exprimentally this involves a systematic A investigation with independent control of density ratio and compressibility effects of the free jets, which has not been done before. The optical quality of each shear layer was measured by examining the farfield diffraction pattern of laser beams passing through the layer. We hoped to under- , stand and to predict compressible shear layer growth rate and optical performance on the basis of this study. Ways of controlling the optical degradation due to these layers has been suggested too. The 2-D layer is now heing ohserved using controlled perturbation techniques which may be used to _%'% %,W ,, '%, ','%, %,,,.,. % % %,,,, , -2- advantage. The structures of these layers, when subjected to small but controlled perturbations, change leading to a rapid change in the mixing rate and probably the optical quality of the layer. In the following section we will discuss our progress during this 'N. grant. II. PROGRESS DURING GRANT PERIOD a. Equipment and Setup We were forced to move the experimental apparatus to a new location early in our program. The new laboratory (location in the AERB) resulted in improved apparatus and larger mass flow capability than from the previous setup. As part of the move, a new and larger capacity gas supply and metering system was constructed. 5. The test gases were ducted away from the optical table area after being discharged by the free jet, otherwise there would he optical interference with the diagnostic systems. The exhaust system carries the spent test gases away from the optical table and discharges them outside the laboratory building. The modest suction required is provided by an ejector in the exhaust ducting. N., Substantial changes were made in the schlieren and interferometer optical diagnostic systems. Both systems were increased to 7.5 cm aperture (from earlier aperture of 4 cm) and the schlieren system was further modified to permit observation parallel and perpendicular to the shear layer simultaneously. The Mach-Zehnder interferometer system uses 6.5 cm beam splitters and mirrors and a new 7.5 cm beam-expanding telescope was built. The equipment includes a ruby laser and another Galilean beam expandingtelescope. 5. -3- Nozzle exit dimensions chosen were 1.4 cm x 1.4 cm for Mach numbers up to 2.0. These represented a compromise between having a realistic nozzle dimension and the prohibitive expenses associated with large test gas mass flows. Reynolds numbers based on the nozzle exit dimensions will vary from 1.6 x 105 at M = 0.5 to 1.4 x 106 at M =2. A sketch of the test section is shown in Fig. 1 for the reader's convenience. h. Observations and Tests During the first part of this program subsonic shear layers were investigated. Shear layer growth rates for jet Mach numbers of 0.1, 0.3 and n.6 were measured using a Mach-Zehnder interferometer. Interferograms using a He-Ne laser source were obtained for jets with various density ratios. Time-averaged optical density (3on=-l) profiles through shear layers were 5 calculated using such data. The results were compared with data available f- in the literature at low Mach numbers and fairly good agreement was. achieved. We have observed a 30% growth rate reduction with the increase of Mach number from 0.1 to 0.6. Higher subsonic Mach number tests have been tried with Mach-Zehnder interferometers, but the optical quality of long exposure interferograms are poor due to loss of optical contrast. Stop action schlieren photographs were taken for various jets of gas at Mach numbers of 0.1, 0.6 and 0.9. The density ratio was varied from 0.66 to 7.2. For the lower Mach number jets coherent structure is not clear. Pictures were also taken simultaneously normal to the shear layer. Time averaged schlieren photographs have provided qualitative measurements of the shear layer growth rates. A reduction in the growth rate of shear layers with increasing Mach number and density ratio was observed. 0 -ii -4- Stop-action interferograms viewed normal to the turbulent interface have been taken for all the test gases. Such data provide near-field phase infromation. A package of software for automated data reduction using digital reading and processing of interferograms on an Apple II computer was developed. With the aid of this program, we were able to calculate the phase degradation of a ruby laser beam as a function of the distance from.4. the exit of nozzle. Papers were published related to the work on this project. The first 0 deals with the fluid mechanical aspects of the program and was presented at the 17th AIAA Fluid Dynamics, Plasma Dyanmics and Lasers Conference in June, 1F84 The second paper deals with the optical properties of the shear layers and was presented at the Chemical Gas Flow Lasers Conference in August, A third paper on computer reduction of the interferograms was published in the Review of Scientific Instruments. Testing of a shearing interferometer as a supplement for the Mach- Zehnder interferometry was initiated then. The shearing interferometer produces an interference pattern between two rays displaced a finite distance apart hy the splitter plate. The interference pattern produced by the flow field does not give optical density, but optical density difference between a preset distance across the flow-field. The test results proved useful, hut additional development was not warranted. Later shear layer growth rates for Mach numbers in excess of 0.9 were measured using a shearing interferometer using a He-Ne laser source. Pulsed schlieren pictures were also taken of these flows. A series of stop action %7 Mach-Zehnder interferograms of shear layers at Mach numbers up to 1.4 were % also taken and some interesting features were observed. But the most consistent results on spreading rates came from long exposure schlieren,j.4 -5- ' measurements. In these measurements, the shear layers spreading rates were seen to decrease significantly with increasing M. A density ratio effect A Al (spreading rates decreasing with increasing X ) was also seen. Large scale coherent structures of the type well known in low speed shear layers were very obvious at M=O.1, were progressively less apparent at M=0.6 and M=U.9 and were apparently absent at M=1.4. The results of shear layer thickness measurements are shown in Figs. 2 and 3. Experiments were carried out to investigate the optical properties of fast shear layers by means of far field diffraction patterns of a circular laser beam resulting from its passage through the layer. The focal plane enerqy distribution of the laser beam is the far field pattern that was measured as part of this program. For these experiments, two different sized parallel HeNe laser beams, one 0.5 cm in diameter and the other 1.0 cm Ṡ.. in diameter, were used to produce the far-field information. Each beam was passed throiqh a rectangular gas jet with cross section of 1.4 cm on a side hounded on three sides by glass walls. After passing through the shear. layer and jet, the beam exited through the glass wall and was demagnified by a telescope for convenience. The modified Airy pattern image was thus produced. The far field images were than compared to the tare data taken in an identical manner, except without the gas jet and shear layer. Optical quality measurements taken normal to the high subsonic flow shear layer were carried out using a cw HeNe light source to obtain time,1' averaged results. The measurements concentrated on Strehl ratio and tilt '5- aberration error. These results were obtained by using a new electronic --.. S W digital image acquisition system. These measurements covering various Mach numbers, M, and density ratios, were published in two masters theses at the University by D. Higgins and T. Blum. j 4% -6- These results coupled with our previous shear layer measurements provided us with values of shear layer width to correlate with density ratio and Mach number. The near-field beam degradation may be related to the farfield intensity if the Strehl ratios are measured. Parameters such as the laser wave number, k, and the index refraction change, An, across the layer are known. The mean shear layer thickness, 6, is known from our fluid mechanical experiments. In the cases where no coherent structure is seen, the scale lengths may he assumed to be roughly proportional to the measured shear layer thickness. Under these conditions we may write the Strehl ratio as Strehl Ratio = exp (-7-2) = exp (-Ak 2 An ) Values of A were found for the range of interest based on our measurements of 6 and Strehl ratio. This correlation relates the far-field performance to fluid mechanical parameters that are defined with respect to the mean thickness of the shear layer. A synopsis of this optical work has been published in the Proceedings of the International Conference on Fluid Mechanics in China c. Low Speed Experimental Setup The optical results in which large scale structures seem to assert an influence require additional testing. Because of experimental difficulties on the size of the apparatus and practical interest in low speed flow, the emphasis has been shifted from a high speed shear layer to the optics of low speed shear layers. This required a transition to a new flow facility which has been built. This new and much larger wind tunnel allows for larger test beams (diameter 5.0 cm). In contrast to what was done earlier, two coplanar flows with velocities U, and U 2 and densities Pand 2 initially %% -7- mix on contact at =. 0 Previously, only one free jet had been used. Now the velocity ratio can be varied. The Mach numbers of the flows are considerably less than one and should have no effect on the shear layer. In addition, each nozzle exit geometry has an aspect ratio of 4:1 in order to provide for better 2D flow characteristics and are enlarged considerably for.5 ease of use. The overall flow consists of free-shear layer mixing between two parallel streams in the presence of walls which bound the streams. This low speed test facility essentially consists of two independent side-hy-side tunnels discharging into a common test section as shown in Fig. 4. Each channel was installed with one honeycomb and three screens to reduce the turhulence level of the freestream. A perforated plate was inserted in one of the channels to produce a velocity difference 5etween the two streams. The nozzles have exit cross sections of 4 x I each. The turhulence level was measured by hot wires to he about 0.5' at the exit plane of the nozzles. A plexiglass test section was fabricated to be 12 long, 4 wide and 2 high. The top and bottom walls were adjustable so that the pressure gradient in the flow direction can be minimized. Under there conditions, the flow field is, on the basis of hot wire and optical measurements, very similar to those of others. Extensive depth hot wire measurements in perturbed flow are in the process of being obtained. Using optically dissimilar gases provides differences in the index of refraction that can be used to study the flow and obtain optical details on the mixing process. A rectangular plenum chamber equipped with six small fans was utilized to supply air flow. Other gases were introduced from pressuri7ed gas hottles for the dissimilar gas mixing layer case with an additional splitter plate. A thin oscillating flap with a width of 10 mm was addei at the end of the splitter plate as a source of external '-'-'.' - - '-'-''.. - ' '--'. . -'' .. '. Y.-. .' ., , .'- .' .-, -'. .% 0 m - 21 -.8- ' perturbation. It was suspended by a music wire under tension such that the flap could he oscillated around its leading edge by two voice coils at frequencies up to 200 cps and up to an amplitude of 2 mm (see Fig. 4). The second part of the preliminary investigation is the inhomogeneous mixing layer. Air-He and air-co 2 mixing layers were chosen, because substantial index-refraction differences exist hetween these gases which are very good for schlieren and shadowgraph pictures. The experiment conditions were kept hasically the same as the air to air mixing layer in order to he comparable. The instantaneous schlieren and shadowgraph pictures with both plan and side view in both inhomogeneous and homogeneous mixing layers have been done. With perturbation, the nature of large coherent structures in the spanwise direction is evident and showed in side and plan view pictures. Our preliminary experimental results confirmed the two-dimensional nature of large streamwise coherent structures in both the homogeneous and inhomogeneous mixing layers. They are essentially a kind of instability wave having a natural frequency and are highly susceptible to external perturbation. This feature may make it possible to improve and finally control the optical properties of the mixing layer. d) Numerical Modeling While the project is largely experimental, numerical simulations of U. plane 2D mixing layers have also been initiated this year. Direct numerical simulations are currently being carried out by solving the nonsteady 20 Euler equations without employing sub-grid scale modeling. Basically, we are applying MacCormack's finite volume methods to solve the Euler equations. Only the explicit scheme has been used at this time. Even for two dimensional flow, such a program is formidable and takes considerable computer time. Our initial results, for an air-air shear layer,,,.-,.,,., _,..,,-,,,..,..,,-,,.....,,....,.,.,..,. :.S :2 -9- with a velocity ratio of 1/2 show a normalized spreading rate of about p This is smaller, but consistent with some experimental observations which are about Mean values of streamwise velocity show a characteristic spreading behavior. It seems that the mean growth of the shear layer % is given quite well even if viscosity is not employed in the basic equations. However, considerable improvements in the code are needed for inhomogeneous flows. We are now computing two air streams with different.5 enthalpies so that the density ratio is 1.1. , S S -10- LIST OF PUBLICATIONS Several recent publications have been wholly or partly supported by this AFOSR Grant. These publications are: 1. Bogdanoff, D.W., The Optical Quality of Shear Layers: Prediction an(j Improvement Thereof, AIAA Journal 22, 58 (1984)
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