Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session BJ: Compressible Flows |
Hide Abstracts |
Chair: S. Girimaji, Texas A&M University Room: Salt Palace Convention Center 250 D |
Sunday, November 18, 2007 10:34AM - 10:47AM |
BJ.00001: A midsummer-night's shock wave Michael Hargather, Thomas Liebner, Gary Settles The aerial pyrotechnic shells used in professional display fireworks explode a bursting charge at altitude in order to disperse the ``stars'' of the display. The shock wave from the bursting charge is heard on the ground as a loud report, though it has by then typically decayed to a mere sound wave. However, viewers seated near the standard safety borders can still be subjected to weak shock waves. These have been visualized using a large, portable, retro-reflective ``Edgerton'' shadowgraph technique and a high-speed digital video camera. Images recorded at 10,000 frames per second show essentially-planar shock waves from 10- and 15-cm firework shells impinging on viewers during the 2007 Central Pennsylvania July 4th Festival. The shock speed is not measurably above Mach 1, but we nonetheless conclude that, if one can sense a shock-like overpressure, then the wave motion is strong enough to be observed by density-sensitive optics. [Preview Abstract] |
Sunday, November 18, 2007 10:47AM - 11:00AM |
BJ.00002: Belt-snap and towel-snap shock waves Gary Settles, Michael Hargather, Michael Lawson, Rory Bigger Traditional simple means of generating shock waves are examined by high-speed imaging. A leather belt is folded upon itself at mid-length and the ends are grasped firmly in each hand. When pushed together a loop forms, and when quickly pulled apart the loop closes rapidly, producing a sharp ``crack'' similar to the cracking of a whip (Shock Waves 8(1), 1998). The towel-snap mimics whip cracking by causing the towel end to rotate supersonically. We investigated these phenomena using a high-speed digital camera (10k and 30k frames/sec, 4 microsec exposure) and a sensitive schlieren optical system of 1m aperture. Results show that compression of the air between the two rapidly-approaching leather belt bands first causes a spherical shock wave to form near one hand. The compression then runs along the belt length toward the other hand at supersonic speed, producing an oblique shock wave that is responsible for the audible crack. In the towel-snap, shock waves are visible from tip motion in open air as well as from the compression due to snapping the towel against a surface. There are no known useful applications of these simple phenomena, but they do address how weak shock waves can be generated by muscle power alone. Several other related examples are also mentioned. [Preview Abstract] |
Sunday, November 18, 2007 11:00AM - 11:13AM |
BJ.00003: High-speed visualization of underwater explosions and shock wave behavior Matthew Madalis, Michael Hargather, Gary Settles Underwater explosions differ in several ways from those traditionally observed in air. They create a high-temperature, high-pressure gas bubble in addition to a shock wave, whereas in air a fireball and a shock wave form. Various underwater shock wave phenomena are considered here, including shock interaction with a free boundary, shock reflection from a blunt object, and shock propagation through a thin wall. Shock speed is also measured optically. Observation of these underwater phenomena is provided by simple shadowgraph or schlieren methods coupled with a Photron APX-RS high-speed digital camera. Small explosive charges on the order of 1 gram are suspended and detonated in a transparent-walled water tank with a liquid capacity of approximately 0.15 cubic meter (40 gallons). [Preview Abstract] |
Sunday, November 18, 2007 11:13AM - 11:26AM |
BJ.00004: A numerical investigation of polygonal converging shock waves Veronica Eliasson, William D. Henshaw Numerical simulations of cylindrically converging shock waves were performed. The converging shocks impinged upon a set of zero to sixteen regularly spaced cylindrical obstacles. For more than two obstacles, the resulting diffracted shock fronts formed polygonal-shaped patterns near the point of focus. For three obstacles, a triangular-shaped shock was observed during the last stages of the focusing process. The triangle was subjected to regular reflection, and thus the shape remained unchanged until it had focused. Guderley's self-similar solution for the radius, R, as a function of time, t, can be expressed as $R=\varepsilon _0 \left( {t_c -t} \right)^\alpha $. The self similar exponent, $\alpha $, was close to the expected value of unity for the triangular case. For the square-shaped shock undergoing Mach reflection, the self-similar exponent was found to be $\alpha $=0.844, which agrees well with Guderley's solution for cylindrical shocks ($\alpha $=0.834). Also, the maximum pressure and temperature as a function of number of obstacles were studied. The highest maximum pressure and temperature occurred with no obstacles. This should be expected, since in all other cases, part of the flow is reflected by the obstacles and never reaches the focal point. [Preview Abstract] |
Sunday, November 18, 2007 11:26AM - 11:39AM |
BJ.00005: On spherical and cylindrical shock-wave implosion D.I. Pullin, H.G. Hornung Is spherically or cylindrically symmetric shock implosion universal? No, but almost. [Preview Abstract] |
Sunday, November 18, 2007 11:39AM - 11:52AM |
BJ.00006: On oscillating shock layers of supersonic impinging jets Martin Rein, Konstantin Klinkov Shock layers of supersonic jets impinging on a plate are studied by means of multi-exposure photography and synchronized pressure measurements performed at the stagnation point on the plate. At certain nozzle-to-plate distances the bow shock formed ahead of the plate oscillates strongly. The pressure variations are strongly correlated with the oscillating shock. Peak frequencies are determined by a spectral analysis of the pressure signal. The spectra show a characteristic dependence of the peak frequencies on the nozzle-to-plate distance. The behavior of the primary peaks depends strongly on the characteristics of the supersonic jet (over/underexpansion). Frequencies of secondary peaks that decrease about inversely with the nozzle-to-plate distance, depend only little on the characteristics of the jet. In addition, low frequency bursts that occur for certain nozzle-to-plate distances are traced to the formation and destruction of a recirculation bubble. [Preview Abstract] |
Sunday, November 18, 2007 11:52AM - 12:05PM |
BJ.00007: Stabilizing effect of compressibility in high-Mach number turbulence: One possible explanation Sharath S. Girimaji In high gradient Mach number mixing layers, it is generally accepted that compressibility effects lead to reduced levels of kinetic energy and lower levels of mixing (the so-called Langley curve). It has been pointed out by several authors (Sarkar, 1995) that the physical mechanism responsible for the reduction is also present in homogeneous shear turbulence. In an attempt to study this mechanism in isolation, we perform rapid distortion analysis of homogeneous flow subject to large strain. The investigation reveals that several regimes of turbulence behavior are possible depending on the gradient Mach number. In some regimes, the turbulence levels are higher than in comparable incompressible flow. The stabilizing regime is also identified. In this talk we will discuss the underlying physics of stabilizing and destabilizing regimes and present a simple dynamical model that captures the quintessential aspects of the observed behavior. [Preview Abstract] |
Sunday, November 18, 2007 12:05PM - 12:18PM |
BJ.00008: Modeling Compressible Turbulent Flow past a Blunt Nosed Body with Spinning Base. Igbal Mehmedagic, Donald Carlucci, Siva Thangam Flow over blunt-nosed cylinders that are spinning about their axis and subject to wall heating is analyzed with applications to projectile design. In this study, computations are performed using an anisotropic Reynolds-stress model to study compressible flow past spinning projectiles subject wall heat flux. The model utilizes a phenomenological treatment of the energy spectrum and diffusivities of momentum and heat to include the effects of rotation, wall heat transfer and compressibility. The time-averaged equations of motion and energy are solved using the modeled form of transport equations for the turbulence kinetic energy and the scalar form of turbulence dissipation with an efficient finite-volume algorithm. The experimental results of Carlucci {\&} Thangam (2001) are used to benchmark isothermal flow over spinning cylinders. The model is applied for several test cases to validate its predictive capabilities for capturing the effects of swirl, wall heat transfer and compressibility. Compressible flow past single rigid cylinder as well as that of flow past cylinders with a free-spinning are analyzed and compared with experimental data available in the literature. Applications involving the design of projectiles are summarized. [Preview Abstract] |
Sunday, November 18, 2007 12:18PM - 12:31PM |
BJ.00009: A thermal lattice-Boltzmann model for polyatomic gases Xiaobo Nie, Xiaowen Shan, Hudong Chen Kinetic theory based methods in fluid mechanics have recently shed new light to both the theoretical research of the subject and its practical applications. Much progress has also been made to the methods itself, especially in handling of compressible fluid dynamics. Here, using well-established results, we rigorously derive a kinetic model for polyatomic gases. The macroscopic recovery of correct hydrodynamics from the corresponding lattice-Boltzmann model is theoretically proved by a Chapman-Enskog expansion and numerically validated by simulating sound waves, heat transfer and the Sod-Riemann problem. [Preview Abstract] |
Sunday, November 18, 2007 12:31PM - 12:44PM |
BJ.00010: Shaping converging shock waves Malte Kjellander, Veronica Eliasson, Nils Tillmark, Nicholas Apazidis Converging shock waves can be used for generation of high temperatures and pressures. The optimal shape of a shock is the completely circular, which is unstable for stronger shocks. The instability distorts the wavefront and plane sections appear. The new shape is an asymmetric wave that does not focus to a distinct point. This ability to produce plane surfaces can be used to stabilize the shocks. Numerical calculations show that symmetric polygonal shock waves are stable and behave like circular shocks, in the sense that the Mach number increase similarly. An exception is the triangular shock, for which the Mach number approaches an asymptotic value. Experiments have been made to verify the analytical and numerical results. The work has been conducted in a shock tube designed for studies of converging shock waves. The tube creates converging cylindrical shocks which are shaped by small objects placed inside the test section. One square and one triangular configuration have been investigated. Experiments show a different behaviour between the two. While the square shock experiences a periodical transformation, the triangular shock keeps its original shape and orientation. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700