Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session M24: Compressible Flows II: Experimental Methods |
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Chair: Michael Hargather, New Mexico Institute of Mining and Technology Room: 319 |
Tuesday, November 26, 2013 8:00AM - 8:13AM |
M24.00001: IR thermography measurements on roughness induced transition. Francesco Avallone, Ferry F.J. Schrijer, Gennaro Cardone The current investigation focusses on the effect of surface protrusions and indentations on the occurrence of transition. The objective of this work is to characterize the roughness-induced transition in the hypersonic Ludwieg tube of Delft University of Technology at Mach 7.5 and a unit Reynolds number ranging from $8\cdot 10^{6}m^{-1}$ to $14\cdot 10^{6}m^{-1}$. The tests are carried out on a 200 mm long, 5$^{\circ }$ compression ramp on which the 1 and 2 mm high tripping elements were positioned at 30 and 60 mm from the leading edge. This configuration resulted in a non-dimensional trip height $k/\delta $ ranging from 0.5 to 1 where k is the trip height and $\delta $ the laminar boundary layer thickness at the trip location. Measurements were performed for isolated and distributed three-dimensional roughness elements having various shapes (cylinder, square, swept ramp, half sphere) and dimensions. Additionally two dimensional cavities and three dimensional cavities are tested and their effectiveness is compared to the protruding elements. The flow field downstream of the roughness elements is analyzed by means of quantitative infrared themography and schlieren visualization. Finally some first results obtained using particle image velocimetry are presented. [Preview Abstract] |
Tuesday, November 26, 2013 8:13AM - 8:26AM |
M24.00002: Supersonic Jet Mixing with Vibrational Non-Equilibrium Heath H. Reising, Utsav KC, Philip L. Varghese, Noel T. Clemens A new study has been initiated to study the effect of vibrational non-equilibrium on turbulent mixing and combustion. This work is relevant to high-speed, high-temperature environments, such as scramjet combustors, where shocks and mixing can lead to high degrees of vibrational non-equilibrium. In this experimental study, a new facility has been developed that consists of a perfectly-expanded axisymmetric Mach 1.5 turbulent air jet issuing into an electrically heated co-flow of air for precise control of the temperature and thus vibrationally-active population. This hot flow can be brought into non-equilibrium when the co-flow fluid is rapidly mixed with the colder supersonic jet fluid. Effects of the non-equilibrium can be isolated by replacing the nitrogen in the flow with argon. The degree of non-equilibrium in the jet shear layers is quantified by using high-spectral resolution time-averaged spontaneous Raman scattering centered on the Stokes-shifted $Q$ branch line of N$_{\mathrm{2}}$ at 607 nm. In this first phase of the study, the effect of non-equilibrium on the mixing field will be investigated, but future work will focus on H$_{\mathrm{2}}$-air combustion. Planar Rayleigh thermometry is utilized to investigate the effects of vibrational non-equilibrium on the turbulent structures and thermal dissipation field. [Preview Abstract] |
Tuesday, November 26, 2013 8:26AM - 8:39AM |
M24.00003: Resolving turbulence in hypersonic flows using PIV Owen Williams, Tue Nguyen, Alexander J. Smits Measuring turbulence in hypersonic flow using PIV is made difficult by high dynamic range requirements and low flow density, which leads to stringent particle sizing requirements to avoid filtering the turbulent signal. Particle frequency response is usually measured using a strong shock and conventionally taken to be representative of the entire flow. A particle dynamics model is used to demonstrate how measured frequency responses depend on shock strength in hypersonic flow, due to changes in particle drag associated with finite inertia, compressibility and slip. A method is presented to extrapolate freestream shock responses to other positions in the flow with disturbance levels comparable to turbulent motions. This method leads to estimates of the variation in frequency response and Stokes number; highlighting regions of the flow which might suffer from filtering. We will demonstrate practical improvements to particle response and the resulting change in turbulent filtering. [Preview Abstract] |
Tuesday, November 26, 2013 8:39AM - 8:52AM |
M24.00004: Background-Oriented Schlieren Characterization of Explosions Cynthia Romo, Michael Hargather Characterizing the energy release from large explosions is a difficult process using traditional point-pressure gages. The background oriented schlieren technique is used here to provide large-field-of-view visualization of the shock wave propagation from large-scale explosions. This technique is used to allow field-measurements of blast wave properties instead of traditional point-wise measurements. By analyzing the shock wave propagation Mach number the peak overpressure and overpressure duration are estimated for different explosions. The technique is applied to the visualization of encased explosions, including car bombs, to estimate the amount of shock energy lost to the fragmentation and acceleration of the casing. Comparisons are made to un-encased explosions. The optically measured data is compared to experimental data recorded using piezoelectric pressure transducers. Scaling relationships are examined to determine scalability of encased explosions. [Preview Abstract] |
Tuesday, November 26, 2013 8:52AM - 9:05AM |
M24.00005: Quantitative schlieren measurement of shock wave pressure profile Jesse Tobin, Michael Hargather Quantitative schlieren imaging is used to measure the pressure profile of a shock wave in air. The quantitative schlieren technique uses a weak lens calibration object to relate pixel intensity values in schlieren images to a known refractive index gradient. The refractive index gradient is converted to a density gradient, which in turn is converted to a pressure distribution using an approximated local air temperature. A high-speed digital camera is used to record schlieren images of shock wave propagation. Post-processing of the image record determines the changes in pixel intensity, and thus the density and pressure distributions across the shock front. The calculated pressure profile is compared to measurements performed using a piezoelectric pressure transducer. The quantitative schlieren measurement approach is benchmarked using a laminar flat plate free-convection boundary layer. [Preview Abstract] |
Tuesday, November 26, 2013 9:05AM - 9:18AM |
M24.00006: Characterization of Magnetohydrodynamic (MHD) Shock Sensor using Schlieren Imaging Owen Rockwell, Michael Hargather Schlieren imaging is used to quantitatively determine the speed and pressure duration of a shock wave traveling through air. The high-speed quantitative schlieren images are then used to characterize a new magnetohydrodynamic (MHD) shock sensor. This device uses the air density and particle velocity changes across a shock wave to determine the shock velocity via the distortion of a magnetic field. Using Faraday's law of electromagnetic induction, the shock velocity and pressure can be interpreted from a change in potential across the electrodes within the device. This principle along with the assumption that the shock wave is traveling through the undisturbed air allows for the calculation of shock velocity. Piezoelectric pressure gauges are used for comparison to measure the pressure pulse magnitude and duration. [Preview Abstract] |
Tuesday, November 26, 2013 9:18AM - 9:31AM |
M24.00007: Quantitative image processing of high-speed Schlieren of a hot supersonic jet Tobias Ecker, Donald R. Brooks, K. Todd Lowe, Wing F. Ng Understanding the physics of noise generation from hot supersonic jets is indispensable in the effort of jet noise reduction. This study describes the analysis of time-resolved Schlieren images obtained in a hot supersonic jet with Mach wave radiation (``crackle''). Proper orthogonal decomposition (POD) is commonly used with large quantitative experimental and numerical datasets. Recent research has shown application of POD post-processing with flow visualization techniques in order to extract valuable information on the large-scale turbulent structures in the flow. POD of the intensity distributions of the Schlieren images were performed to reveal organized structures in the outer shear layer, while mode evolution may be reconstructed in the images taken at over 86,400 frames per second. Albeit the screeching modes are more than twice as energetic than the next order of modes for NPR = 3, TTR = 2.5, a number of modes of similar energy density were found to exhibit convective cyclical structures. [Preview Abstract] |
Tuesday, November 26, 2013 9:31AM - 9:44AM |
M24.00008: Turbulence measurements in high-speed flows using the Focusing Laser Differential Interferometer Matthew Fulghum, Gary Settles The Focusing Laser Differential Interferometer (FLDI) was invented by Smeets at ISL in the 1970s, and was used recently by Parziale in the CalTech T5 shock tunnel. It is a relatively-simple, non-imaging common-path interferometer for measuring refractive signals from transition and turbulence, and it has a unique ability to look through facility windows, ignore sidewall boundary-layers and vibration, and concentrate only on the signal near a pair of sharp beam foci in the core flow. Benchtop experiments using a turbulent helium jet in air demonstrate focusing ability, frequency response, unwanted signal rejection, and ease of use. The FLDI is then used to measure freestream turbulence intensity and spectra in the PSU supersonic wind tunnel at Mach 3, with results compared to hot-wire-anemometer data. A special feature of the FLDI instrument used here is the replacement of traditional fixed Wollaston prisms with variable Sanderson prisms for laser-beam separation and recombination. Research sponsored by AEDC Hypervelocity Tunnel 9. [Preview Abstract] |
Tuesday, November 26, 2013 9:44AM - 9:57AM |
M24.00009: Synthetic streak images (x-t diagrams) from high-speed digital video records Gary Settles Modern digital video cameras have entirely replaced the older photographic drum and rotating-mirror cameras for recording high-speed physics phenomena. They are superior in almost every regard except, at speeds approaching one million frames/s, sensor segmentation results in severely reduced frame size, especially height. However, if the principal direction of subject motion is arranged to be along the frame length, a simple Matlab code can extract a row of pixels from each frame and stack them to produce a pseudo-streak image or x-t diagram. Such a 2-D image can convey the essence of the large volume of information contained in a high-speed video sequence, and can be the basis for the extraction of quantitative velocity data. Examples include streak shadowgrams of explosions and gunshots, streak schlieren images of supersonic cavity-flow oscillations, and direct streak images of shock-wave motion in polyurea samples struck by gas-gun projectiles, from which the shock Hugoniot curve of the polymer is measured. This approach is especially useful, since commercial streak cameras remain very expensive and rooted in 20$^{\mathrm{th}}$-century technology. [Preview Abstract] |
Tuesday, November 26, 2013 9:57AM - 10:10AM |
M24.00010: Plenoptic PIV: Towards simple, robust 3D flow measurements Brian Thurow, Tim Fahringer In this work, we report on the recent development of plenoptic PIV for the measurement of 3D flow fields. Plenoptic PIV uses a plenoptic camera to record the 4D light-field generated by a volume of particles seeded into a flow field. Plenoptic cameras are primarily known for their ability to computational refocus or change the perspective of an image after it has been acquired. In this work, we use tomographic algorithms to reconstruct a 3D volume of the particle field and apply a cross-correlation algorithm to a pair of particle volumes to determine the 3D/3C velocity field. The primary advantage of plenoptic PIV over multi-camera techniques is that it only uses a single camera, which greatly reduces the cost and simplifies a typical experimental arrangement. In addition, plenoptic PIV is capable of making measurements over dimensions on the order of 100 mm x 100 mm x 100 mm. The spatial resolution and accuracy of the technique are presented along with examples of 3D velocity data acquired in turbulent boundary layers and supersonic jets. [Preview Abstract] |
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