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A review is given of outdoor schlieren and shadowgraph imaging, beginning with an historical perspective. The optical principles of the sunlight shadowgraph method and schlieren observation by background distortion are discussed. Examples and illustrations are given of the visualization of outdoor thermal convection, combustion, and explosion pheno...
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... being refracted with a component to the left, in which direction the air density increases in this particular example. Some experiments can only be performed outdoors, where conventional optical apparatus is unavailable. This is the case with large explosions, which must then depend upon background distortion to reveal shock wave motion, etc. Fig. 6 is an outstanding example. Following the above-ground nuclear test ban treaty some decades ago, it still remains possible to approximate nuclear blast effects by setting off very large conventional chemical explosions. The scale of the one shown in Fig. 6 is indicated by the trees at bottom center, which are outlined and dwarfed by the ...
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... which must then depend upon background distortion to reveal shock wave motion, etc. Fig. 6 is an outstanding example. Following the above-ground nuclear test ban treaty some decades ago, it still remains possible to approximate nuclear blast effects by setting off very large conventional chemical explosions. The scale of the one shown in Fig. 6 is indicated by the trees at bottom center, which are outlined and dwarfed by the giant fireball. The hemispherical shock wave is clearly seen by distortion of the cloudy background sky. In the Fall of 1997, 50 years after an airplane first "broke the sound barrier," two jet-powered cars vied to accomplish the same feat on the ground. ...
Citations
... In gases and air, a linear relationship exists between the gases' refractive index and its density [24]. In other words, factors like temperature and gas composition change the density of the gas and cause a shift in its refractive index that leads to its visualization using the Schlieren method [25][26][27]. Its applicability strength is an ability to monitor gas flows non-intrusively, ...
Electrostatic rotary bell sprayers (ERBSs) are widely used in the automotive industry. In ERBS, atomization is facilitated using centrifugal forces which disintegrate the paint film inside the cup into droplets at the cup edge. The droplets are then transported by the flow of a shaping air (SA) and electrostatic forces to a target surface; the characteristics of these droplets dramatically influence the quality of a painted surface and the painting transfer efficiency. In the current paper, a novel Schlieren-based visualization of the shaping air in the absence of paint droplets was performed during a qualitative investigation to delineate shaping air flow behavior and its interaction with droplets and droplet transport. An infrared thermographic flow visualization (IRFV) method and droplet size measurement were used to complement the Schlieren data for providing insight into shaping air-droplet interactions. The results demonstrated the impact of different operating conditions on the SA flow pattern, and the influence SA has on the secondary atomization and transport of droplets. Hence, these experimental methods combine with a useful tool for optimizing SA configurations that improve spray quality, droplet transport, and the efficiency of ERBS operations.
... The idea of studying refractive index fields through the distortion induced in a background pattern reappeared from time to time [7,8]. Then, from the end of the 1990s years, background distortion methods flourished. ...
This paper aims to describe several application of Schardin’s schlieren #2 in heat and mass transfer studies. Quantitative information can be obtained by different data processing such as Fourier Transform, windowed Fourier Transform or correlation algorithms of the type used in PIV (Particle Image Velocimetry). Experimental validation as well as a comparison with results obtained by different experimental methods and/or data processing will be presented and discussed.
... Norman & Light herein describe the visualization of blade tip vortices from a 0.16-scale Sikorsky S-76 rotor using a shadowgraph setup with large retro-reflective background screens. A review of schlieren applications in outdoor environments is given by Settles (1999), including the visualization of heat plumes, jet exhaust gases, thermal boundary layers as well as aerodynamic and explosiondriven shock waves in front of natural backgrounds. ...
The vortices produced by helicopter rotor blades dominate the aerodynamics of the rotor wake. Their interactions with the rotor blades cause sound and structural vibrations and have detrimental effects on the aircraft performance. Current research strives to better understand and reduce the effects of these blade-vortex interactions (BVI). Sub-scale model rotor tests – and an increasing number of full-scale investigations – are performed to develop measures against the interaction effects. Active rotor control concepts – such as active twist actuation – have the potential to effectively reduce the sound and vibrations of helicopter rotors.
The present thesis focuses on the experimental investigation of active twist for the reduction of BVI effects on a model rotor. Results of a large-scale smart-twisting active rotor (STAR) test under hover conditions are described. This test investigated the effects of individual blade twist control on the blade tip vortices. The rotor blades were actuated with peak torsion amplitudes of up to 2° and harmonic frequencies of 1 - 5/rev with different phase angles. Time-resolved stereoscopic particle image velocimetry (PIV) and background-oriented schlieren (BOS) measurements were carried out to study the effects of active twist on the strength and trajectories of the tip vortices between 3.6° and 45.7° of vortex age.
The analysis of the vortex trajectories revealed that the 1/rev active twist actuation mainly caused a vertical deflection of the blade tip and the corresponding vortex trajectories of up to 1.3% of the rotor radius R above and -1%R below the unactuated condition. An actuation with frequencies of 2/rev and 3/rev significantly affected the shapes of the vortex trajectories and caused negative vertical displacements of the vortices relative to the unactuated case of up to 2%R within the first 35° of wake age. The 2/rev and 3/rev actuation also had the most significant effects on the vortex strength and altered the initial peak swirl velocity by up to -34% and +31% relative to the unactuated value. The present aerodynamic investigation reveals a
high control authority of the active twist actuation on the strength and trajectories of the trailing blade tip vortices. The magnitude of the evoked changes indicates that the active twist actuation constitutes an effective measure for the mitigation of BVI-induced sound on helicopters. The majority of available studies on BVI – including the STAR experiments – are based on sub-scale model rotor tests. It is challenging to correctly downscale the multitude of underlying effects contributing to BVI. Full-scale investigations with innovative, whole-field vortex visualization techniques are, consequently, required for the validation of the model test results. The present work thus also targets the advancement of optical measurement techniques for the
investigation of vortices on full-scale helicopters. A series of flight tests were carried out to improve a reference-free variant of the BOS technique and demonstrate its vortex visualization capabilities for various test conditions. The goal of the main flight test was the quantitative measurement of the main rotor tip vortex system of a full-scale BO105 helicopter under maneuvering flight. The tip vortices of the helicopter were simultaneously visualized from different perspectives by a multi-camera BOS setup consisting of ten individual cameras. Based on this data set, a three-dimensional reconstruction of the main rotor vortex system of a maneuvering helicopter was realized for the first time. The flight test results thus demonstrate the potential of the BOS measurement technique for quantitative vortex investigations on full-scale helicopters
under realistic flight conditions.
... These methods make use the effect of changes in the density of overexposed medium on the value of the refractive index. [3,4]. These methods relatively easily allow to obtain a lot of relevant information on the flow field that can be used to analyze the physics of the phenomenon. ...
The paper presents experimental studies devoted to the convection phenomenon within the steel charge of mixed porosity. Such charges constitute bundles of hollow long elements such as pipes or rectangular sections which are heat treated. A significant portion of the gas phase in the volume of the charge makes that natural convection of the gas occurring within the individual elements may have an effect on the course of heating. To the tests the Schlieren method was used which is one of the optical visualization methods applied to the analysis of the flow phenomena in the transparent and non luminous media such as air or water. The tested samples have the form of porous charge beds made from pipes and rectangular profiles. During the experiments the samples were heating up for the constant heat flux rate. The direction of flux was vertical, from the bottom to the top.
... The tests consisted in the visualization of the convective movements of gas within sections being heated, for which the Schlieren method was employed. Alongside with the interference and shadowgraph methods, it is classified to the group of optical convection phenomenon examination techniques that make use of the effect of variations in the density of the transparent medium on the value of the index of refraction [18,19]. The Schlieren method, is a technique relying on the angular deflection of light rays passing through a transparent fluid region that is characterized by the inhomogeneity of the index of refraction, n. ...
... The optical measuring system relying on the Schlieren system allowed tests to be carried out in a broad range of configuration of experimental heat exchange systems, which were characterized, at the same time, a lower risk of the incorrect interpretation of obtained results. The core of the employed optical measurement technique is the utilization of relationship between the optical properties of the medium and its thermophysical parameters [6][7][8][9]. The optical properties of the medium are defined by changes in the index of refraction. ...
The article concerns the selected application problem of gradient optical technique for the analysis of development of the thermal boundary layer. The recording of thermal boundary layer development was carried out using the modified Schlieren system. The modifications introduced to the optical measuring systems allowed the extension of the information carrier measuring range along with a reduction in the susceptibility of the systems to misalignment. As the information carrier, a collimated 25-mW He–Ne laser beam was used. The recording of the distribution of optical density of information carrier around the experimental model under analysis was carried out using a CCD matrix with the image acquisition card. The experimental model under consideration was a converging air channel in the form of a space enclosed by two inclined heating surfaces. The measurements of convective heat transfer were carried out for different angles of inclination (18degrees, 30degrees and 45degrees) and thermal conditions (320 K, 370 K and 490 K) of the heating surfaces. The obtained results of experimental analysis were approximated with the power relationship Nux = C(Rax)^a
... There have been several attempts to take schlieren photography and shadowgraphy out of the laboratory into the great outdoors (Refs. [5][6][7][8][9][10][11]. Full-scale and in situ experiments are the key to clarifying Mach number and Reynolds number effects on vortex structure iteration, farfield interference effects, and complex wake flows. ...
Background-oriented schlieren (BOS) methods suited for large-scale and in-flight testing are presented with special emphasis
on the detection and tracing of blade tip vortices in situ. Retroreflective recording and photogrammetric epipolar analysis
for the computation of the vortices’ spatial coordinates in the wind tunnel are described. Feasibility and fidelity of referencefree BOS in conjunction with natural formation backgrounds and related evaluation methods are discussed, additionally, illustrating their simplicity and robustness. Results of successful image acquisition from a chaser aircraft are presented allowing vortex wakes to be identified at a wide range of flight attitudes, including complex maneuvers.
... In fact, BOS has been demonstrated in real flight tests using a conventional CCD camera, a forest as a background and the sun as an illumination source.[ (Raffel, Richard and Meier 2000), (Sommersel, et al. 2008), (Settles 1999)] As will be seen, the image distortion measured in a BOS experiment is directly related to the wavefront distortion, a fact which we seek to exploit to obtain quantitative measurements of the wavefront distortion. In this regards, BOS has the potential to outperform Shack-Hartmann wavefront sensors with respect to spatial resolution, field-of-view and cost. ...
... In fact, BOS has been demonstrated in real flight tests using a conventional CCD camera, 12 a forest as a background 13 and the sun as the source of illumination. [9][10][11][12][13][14] As will be seen, the image distortion measured in a BOS experiment is directly related to the wavefront distortion, a fact which we seek to exploit to obtain quantitative measurements of the wavefront distortion. In this regards, BOS has the potential to outperform Shack-Hartmann wavefront sensors with respect to spatial resolution, field-of-view and cost. ...
The fundamental principles of Background Oriented Schlieren (BOS) imaging are conducive for the measurement of optical wavefront distortions imposed by turbulent flows. This work explores the initial development of a wavefront sensor based on BOS. The advantages of a BOS based wavefront sensor over a competing device, such as a Shack-Hartmann wavefront sensor, is the ability to measure large aperture wavefronts with potentially high spatial resolution in an economical fashion. An analytical analysis which incorporates the imaging function requirement of the sensor illustrates the ability to measure local wavefront tilts with accuracies on the order of 20 microradians. The analysis finds that under imaging conditions, sensitivity is mainly a function of the lens f-number, which allows the background to be positioned further away from the test section, with the lens focal length only playing a minor role. These ideas were tested using experiments conducted on a cone mounted in a Mach 2.0 wind tunnel. Wavefront measurements agreed qualitatively with the distortion expected from an analytical model of the conical shock's density field. Turbulent boundary layers and Mach wave radiation from the tunnel walls were also detected. Future experiments will calibrate the measurement with a known distortion source. Overall, the concept of a BOS based wavefront sensor is shown to be a valid and viable option for wavefront measurements, particularly for aero-optics studies in medium to large-scale environments.
... Kindler et al. also presented a statistical approach for quantifying the effectiveness of potential backgrounds. Settles (2001Settles ( , 1999b reviewed outdoor schlieren and shadowgraph imaging in detail. Outdoor schlieren observation by background distortion was examined in particular, and several examples were given (blast wave seen against clouds, jet aircraft shock waves, and exhaust seen against a background tree-line or the sun, etc.). ...
... It was also shown that BOS is actually Schardin's canonical schlieren method #1 (Schardin 1942). Readers are encouraged to see the pre-BOS historical survey in Settles (1999b), which is omitted here. ...
... The sensitivity of this system, i.e., the smallest density gradient that can be detected, is a function of the optical geometry shown in Fig. 1, the camera capability, and the strength of the schlieren object. For a given schlieren object, BOS sensitivity is influenced by the distance of the background to the object, Lt, and the distance L from the background to the camera (Settles 2001;Meier 2002;Settles 1999b;Goldhahn and Seume 2007). Camera lens focal length and pixel size also affect the ability to detect small disturbances (Goldhahn and Seume 2007). ...
The background-oriented schlieren (BOS) flow visualization method has the potential for large-scale flow imaging outside the
laboratory by using natural backgrounds instead of the artificial patterns normally used indoors. The natural surroundings
of an outdoor test site can sometimes be used as such a background, subject to criteria of fine scale, randomness and contrast
that are developed here. Some natural backgrounds are more appropriate than others for a given application. Backgrounds used
here to visualize both high- and low-speed schlieren disturbances include a sunlit cornfield and a backlit grove of trees.
A range of image post-processing methods is considered for qualitative BOS. It is found that high sensitivity and a broad
measuring range are in conflict here, much as they are in traditional schlieren instruments. Applications of natural-BOS include
explosive characterization, firearms and artillery testing, chemical and natural-gas leak detection, and related phenomena.
