The performance of an internally cooled silicon window in a high heat flux environment has been characterized in the laboratory. The article under test was convectively/radiatively heated with a large-area-flame oxy- acetylene torch, and cooled by circulating water through internal channels. Heating rates ranging from 5 to 120 W/cm2 were achieved over the surface of the window and generated thermal gradients in the window that exceeded flight levels by an order of magnitude. This gave us a measure of window performance under stressing conditions. Thirty one heating tests were conducted to measure the thermal and optical efficiency of the windows. The degree of surface temperature uniformity was derived from midwave infrared images of the test window surface collected on a two- dimensional array, InSb camera. Optical wavefront distortion was measured with an infrared shearing interferometer. Data was collected on both a video tape recorder and a digital data acquisition system before, during, and after the period of window heating. Experimental data on two windows along with theoretical predictions are presented in this paper. The theoretical code took a given heat flux distribution into the window and predicted the surface temperature distribution, and the change in the window dimensions and index of refraction. Experimental data and theoretical predictions compared well.
The surface temperature uniformity of an internally cooled nosetip and window has been characterized in a laboratory environment. The articles under test were conductively heated by an electrical resistance heater and then cooled by circulating a liquid through internal passages. Data was collected before and during the cool-down. Temperatures were derived from the images collected with an InSb MWIR starring focal plane array camera. All of the nosetips and windows tested, performed as designed.
Internally liquid cooled apertures (windows) installed in a full size forebody have been characterized under high heat flux conditions representative of endoatmospheric flight. Analysis and test data obtained in the laboratory and at arc heater test facilities at Arnold Engineering Development Center and NASA Ames are presented in this paper. Data for several types of laboratory bench tests are presented: transmission interferometry and imaging, coolant pressurization effects on optical quality, and coolant flow rate calibrations for both the window and other internally cooled components. Initially, using heat transfer calibration models identical in shape to the flight test articles, arc heater facility thermal test environments were obtained at several conditions representative of full flight thermal environments. Subsequent runs tested the full-up flight article including nosetip, forebody and aperture for full flight duplication of surface heating rates and exposure ties. Pretest analyses compared will to test measurements. These data demonstrate a very efficient internal liquid cooling design which can be applied to other applications such as cooled mirrors for high heat flux applications.
Diffraction effects produced by internally liquid cooled silicon windows have been characterized. Experimental point-spread function data for three window designs are presented. Measurements were made using a single wavelength (3.39 micrometers ) laser as well as a broadband blackbody source operating at various temperatures. From these data, angular distribution of energy and the ratio between the contained energy of the central and higher order lobes were derived. Comparisons to analytically generated curves show excellent agreement.
A simple technique has been developed to record both the high and low frequency fluctuations contained within an aerooptically distorted 2-D point spread function. A collimated beam of light from an Nd:YAG laser, operated at 1.064 jim, was passed through a high velocity wrbulent flow field and imaged on the focal plane of a 128 x 128 array CCD camera (60 jun square pixels). Extremely short duration (50 nsec) pulses from the laser, synched with a high speed (92 frames per second), video data acquisition system captured one pulse per frame and effectively froze all motion in the flow. Post test averaging of single pulsed frames made it possible to visualize the full range of frequencies contained in the Fourier plane. Investigations are currently being made into the mathematical relationship between these high resolution images and the 2-D power spectrum for the refractive index fluctuations. The technique has the advantages of being non-intrusive and capable of acquiring multiple samples in a short period of time to insure statistical validity. Experimental measurements were performed on the center of a turbulent mixing shear layer (—8 mm thick), generated by back-to-back supersonic nitrogen/argon gas nozzles, with a mean flow velocity of ''380 rn/sec. Calculations were made using horizontal scans through several frames from a single test run. Turbulent scale lengths down to 0.6 mm were resolved.
Characterization of small scale structures within high speed turbulent flow fields requires instrumentation that is capable of acquiring high speed data at rates exceeding one megahertz. From experimental studies performed by the Teledyne Brown Engineering (TBE) Experimental Aero-Optics Group in conjunction with SY Technology, it has been observed that structures within a high speed turbulent flow have a limited lifetime. With the development of the Ultranac computer controlled high speed camera, the collection of high speed images was possible. The camera was capable of 8 to 24 short sub-microsecond exposure times and fast MHz frequency frame rates, all of which was variable and could be set independently for each frame recorded by the camera. An application for this system was demonstrated using a collimated beam of HeNe laser light to record shadowgraphs of turbulent flow structures generated by TBE's Aero-Optic Simulator (AOS). Argon gas was exhausted at a low speed from one nozzle and neon gas was exhausted at a higher speed from the other nozzle to give a calculated shear layer flow velocity of approximately 450 m/s. Frame-by- frame comparisons were made and flow structures were observed to persist for periods on the order of a microsecond. Based on experience from this preliminary demonstration, improvements for future experiments have been suggested. These tests clearly demonstrate the potential of the Ultranac camera to aid in the characterization of high speed turbulent flows.
Experiments were conducted by Teledyne Brown Engineering (TBE), that simultaneously recorded flow field shadowgraphs and imaging data, to investigate the relationship between high velocity turbulence and aero-optic image distortion. A laboratory based Dual Nozzle Aero-Optic Simulator (DNAOS) was used to produce a turbulent flight level aero-optic environment similar to that encountered by a hypersonic vehicle. An Nd:YAG laser, operating at 1.064 micrometers , was expanded to 5 mm, collimated, and directed through the turbulent flow field to serve as a point source at infinity. On the opposite side of the flow field, the beam was split into two components and directed towards two 60 micrometers square pixelated, 128 X 128 CCD array cameras. One camera had a bare focal plane and was used to record the turbulence induced scattering field (shadowgraph), while the other had a 3.4 m focal length lens to image this field, producing a point-spread-function (PSF) on the CCD array. A 50 nsec duration laser pulse at a frequency of 92.5 Hz (frame rate of the CCD cameras) was recorded by each of the cameras and the data was digitized by a high speed data acquisition system. The shadowgraphs and imaging data were compared frame-for-frame to determine the similarities between the flow field events and the image distortion. Based on this analysis, a procedure has been proposed to numerically transform shadowgraphs to obtain pseudo-images that could be compared to experimentally recorded images.
An experimental investigation was undertaken to examine the aero-optic performance of multi- aperture windows for use in hypersonic endo-atmospherical vehicles. A series of imaging tests was conducted through a two-dimensional flat plate model of a multi-aperture window that was incorporated into the Teledyne Brown Dual Nozzle Aero-Optic Simulator (DNAOS). This simulator brought two high-velocity gas streams together in an enclosed test region to form an approximate Mach 2 mixing/shear layer, creating the turbulent properties found in hypersonic flight. The same series of tests was conducted looking through a monolithic flat plate window. The images recorded through both window schemes were analyzed to determine image distortion and results were compared to demonstrate the various optical phenomenon associated with multi-aperture windows.
Mixing/shear layer turbulence is the major contributor to optical degradation effects experienced by a windowed hypersonic vehicle. A critical component in the prediction of these aero-optic effects, is the distribution, relative sizes, and velocities of the turbulent structures found within the mixing/shear layer. Previous attempts have had difficulty in measuring these high frequency, small scale turbulent properties. Therefore, a novel non-intrusive optical technique called the fiber optic flow monitor, was developed. This device was used in conjunction with a dual nozzle aero-optic simulator to experimentally determine turbulent flow properties and investigate their relationship to image distortion. The flow field studied was a dual species mixing/shear layer that had a mean flow velocity of approximately 430 m/s with a calculated mean turbule size of 0.7 mm. It was observed that the turbulent structures redistributed incident collimated energy into unique patterns of light. By monitoring these patterns, it was possible to measure several flow field properties. Data, gathered from this technique, was used to compute a statistical distribution of turbule velocities that was compared to theoretical predictions and image distortion parameters. Close correlation between experimental and theoretical values confirms that the technique provides a non- intrusive method of accurately characterizing small scale, high velocity turbulent structures.
A nonintrusive experimental method, utilizing two Quad Cell detectors and a cross beam correlation (CBC) technique, was applied to Teledyne Brown Engineering (TBE) to investigate the relationship between fluctuating optical properties and image distortion caused by high velocity turbulence. A laboratory based Dual Nozzle Aero-Optic Simulator (DNAOS) was used to produce a mixing/shear layer that simulated a flight level aero-optic environment. Two Quad Cells were used to simultaneously measure the centroidal shift of two orthogonal laser beams that were located in a plane normal to the direction of flow and were coincident at only one point within the mixing/shear layer. The 2D angular deviations of the laser beams were calculated from the recorded centroidal fluctuations. From this data, the cross correlations could be calculated to determine the turbulence induced optical deviation (wavefront distortion) and density related index of refraction fluctuations for the flow field. The experimentally measured angular deviations were found to compare reasonably well to theoretically predicted values. This demonstrates that Quad Cell CBC can provide a nonintrusive method of accurately characterizing the fluctuating optical properties resulting from small scale hypersonic turbulent structures.
A nonintrusive optical technique has been developed that measures the velocity components of high velocity mixing/shear layers. Within the high velocity turbulent media, flow structures exist that can randomly concentrate and/or redistribute incident light into unique intensity patterns. By observing the motion of these patterns over short time intervals, it is possible to deduce the velocity of the flow. A series of laboratory experiments was conducted to demonstrate the technique using the Teledyne Brown Engineering Dual Nozzle Aero-Optic Simulator (DNAOS). A binary gas, classical mixing/shear layer with a mean flow velocity of approximately 460 m/sec was generated for the tests. Two independent Q-switched Nd:YAG laser beams (1.064 micrometers ) were colinearly aligned, directed through the flow, and then recorded with a high speed CCD square array. Each laser was fired once during a camera frame, with a measured time delay of 1.36 microsecond(s) between the two laser pulses. The frames were taken at 92.5 Hz and stored for post-test analysis. By identifying the projected flow structure patterns and measuring the displacement of the patterns as recorded by the camera, two dimensional velocity components were calculated. These values were in fair agreement with mean flow velocities predicted with an empirical flow field prediction code.
A unique experimental apparatus has been designed and constructed to characterize aero-optical distortions related to
the turbulent flow conditions experienced by a windowed hypersonic vehicle. Using this apparatus, a series of imaging tests
was conducted with a classical mixing/shear layer traveling at approximately 600 mIs. The experimental setup consisted of a
collimated 0.84 mm laser diode point source that was passed through the flow field and imaged onto a CCD array. During a
one second stable flow period, 92.5 frames of images were collected. Several runs were made with the lasr diode operating in
both continuous and pulsed (40is duration) modes. These images were used to investigate several effects such as, image blur,
jitter, and strehl loss. For long integration periods, the image experienced an average image blur circle size increase of
approximately 24 times from the "wind-off" case. The pulsed runs showed an increase in jitter of approximately 36.4 j.trad.
In addition, during continuous runs, a strehl ratio of approximately 0.0026 was observed. These and other preliminary results
correlated well with theoretical predictions.
An investigation has been undertaken which utilizes nonintrusive optical interferometric techniques to visualize the turbulent structure found in a high-velocity flow field and thereby characterize the resulting optical distortion. Experiments were conducted on a 7.68 mm by 7.68 mm cross section of a high velocity, dual gas, mixing/shear layer, and the preliminary results are presented. The experimental apparatus consisted of a dual beam Mach-Zehnder interferometer with a customized high-speed CCD camera data acquisition system. A series of time varying images of the gas flow were captured and digitized with the interferometer configured in both a finite and an infinite fringe mode. By correlating the initial tare run wavefront (gas off condition) to any subsequent distorted wavefront (gas on condition), the turbulent flow field structure and the relative phase shift across the test region was analyzed. Both classical and nonclassical approaches were taken in analyzing the interferometric data to obtain an understanding of the high velocity flow field. In addition, the experimental results were compared to theoretical predictions for RMS wavefront distortion.
A single-jet supersonic mixing/shear layer simulator in which two high speed video data acquisition systems simultaneously sample the flow region has been used to characterize the structure of turbulence in a high-speed mixing/shear layer and its degradation effect on an optical test beam. The experiment imaged a point source through the flow field. Correlations are shown to exist between the turbulent structure of the flow field and specific aerooptic distortions, offering the foundations for real time error correction of optical degradations associated with hypersonic flight.
This paper presents a simple, nonintrusive technique to separate mechanical jitter and aero-optic jitter from the combined measured jitter. This technique has the advantages of being relatively simple to implement, deriving information from the imaging data, and giving insight into the nature of the flow-field turbulence. The method employs variable collection apertures and a subtraction algorithm designed to separate aero-optic jitter from mechanical jitter. Several levels of vibration and aero-optic effects were measured and then separated in the posttest data processing.
Recent wind tunnel tests have been used to establish baseline characteristics of the effect of small particle impacts on
two sapphire window plates. These two window plates have been exposed to wind tunnel flowfields of approximately Mach
10 at a simulated altitude of 50,000 feet. Test conditions indicate a free stream velocity, of approximately 4800 ft/sec.
Within the wind tunnel flowfield, small particles were carried along and have appeared as impact sites on the exposed surface
of each window plate. Examination of the impacted sites indicate that the particles were composed of Deirin (a hard acetal
homo-polymer) or stainless steel, both of which resulted in small but measurable damage to each window plate.
Characterization of the plate surface appearance and impact damage was conducted at two locations: 1) the University of
Alabama in Huntsville, (UAH), Optical Measurements Laboratory (OML) and, 2) Oak Ridge National Laboratories (ORNL),
Optical Component Characterization Laboratory (OCCL).
The growing need for computational codes to optimize windowed hypersonic vehicle designs has required a more
complete understanding of aero-optical effects produced in the flight environment. Teledyne Brown Engineering (TBE) has
determined the mixing/shear layer created by the cooling of a window on a hypersonic vehicle to be one of the largest
contributors to aero-optical distortion. A novel experimental setup is currently being developed that effectively reproduces
this mixing/shear layer and thus its significant aero-optical effects. In addition, various test techniques have been devised to
investigate these phenomenon, as well as to produce simulated flight data required for validation of the aero-optic optimizing
codes.
A simple, non-intrusive technique to separate mechanical jitter and aero-optic jitter from the combined measured jitter is
presented. This technique has the advantages of being relatively simple to implement, deriving information from the imaging
data, and giving insight into the nature of the flow field turbulence. This method employs variable collection apertures and a
subtraction algorithm designed to separate aero-optic jitter from mechanical jitter. Several levels of vibration and aero-optic
effects were measured and then separated in the post-test data processing.
Interferometric visualization of the turbulent structure of a simulated high velocity mixing/shear layer has been achieved using a pulsed laser shearing interferometer. This technique has the advantages of being simple to implement, relatively insensitive to vibration, and capable of recording many interferograms during each test run. Visualization of the structure of these flowfields enables experimental determination of the turbule dimension, distribution, and relative spatial density. This makes possible the calculation of the correlation length and other flow parameters for more accurate experimental predictions of the aero-optic effects such as image jitter, boresight error, blur growth, and transmission loss.
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