Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session HG: General Fluid Dynamics I |
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Chair: Bhimsen Shivamoggi, University of Central Florida Room: Hilton Chicago Williford A |
Monday, November 21, 2005 1:20PM - 1:33PM |
HG.00001: Nonlinear Dynamos in Helically Symmetric Pipes Jonathan Mestel, Leszek Zabielski Steady, helically symmetric, pressure-driven flow of a fluid down a helical pipe is considered. Helical symmetry is a generalisation of axisymmetry, and the resulting motion has some similarity with Dean flow. It has been successfully used to model blood flow around three-dimensional arterial bends in the body. If the fluid is electrically conducting, it is shown here that these flows can drive a {\bf dynamo}, {\it i.e.} a magnetic field can be spontaneously generated. This is the only known example of a laminar, pressure-driven dynamo. The growth rates of the field are computed assuming the most unstable mode to be also helically symmetric. Appropriate parameter ranges for the pipe geometry and the magnetic and hydrodynamic Reynolds numbers are identified. Strong field-stretching occurs near the stagnation points in the cross-pipe flow; precisely those low shear regions identified as possible sites for atherosclerotic development in the haemodynamic context. The calculation is continued into the non-linear regime, when the magnetic field reacts back upon the driving flow. Periodic quenching is observed. The field first decays, as the favourable flow conditions are destroyed, but it then regrows as the flow is re-established on a viscous time-scale. [Preview Abstract] |
Monday, November 21, 2005 1:33PM - 1:46PM |
HG.00002: Current Sheet Formation in MHD: Analytical and Numerical Results David Rollins, Bhimsen Shivamoggi Current sheet formation in MHD caused by the magnetic field line sweeping by the plasma flow near a hyperbolic magnetic neutral line is investigated. Analytical and numerical results are given. [Preview Abstract] |
Monday, November 21, 2005 1:46PM - 1:59PM |
HG.00003: Treatment of MHD turbulence with non-equipartition and anisotropy Ye Zhou, W.H. Matthaeus Magnetohydrodynamics (MHD) turbulence theory, often employed satisfactorily in astrophysical applications, has often focused on parameter ranges that imply nearly equal values of kinetic and magnetic energies and length scales. However, MHD flow may have disparity magnetic Prandtl number, dissimilar kinetic and magnetic Reynolds number, different kinetic and magnetic outer length scales, and strong anisotropy. Here we discuss a phenomenology for such ``non-equipartitioned'' MHD flow. We suggest two conditions for a MHD flow to transition to strong turbulent flow, extensions of (i) Taylor's constant flux in an inertial range, and (ii) Kolmogorov's scale separation between the large and small scale boundaries of an inertial range. For this analysis, the detailed information on turbulence structure is not needed. These two conditions for MHD transition are expected to provide consistent predictions and should be applicable to anisotropic MHD flows, after the length scales are replaced by their corresponding perpendicular components. Second, we point out that the dynamics and anisotropy of MHD fluctuations is controlled by the relative strength between the straining effects between eddies of similar size and the sweeping action by the large-eddies, or propagation effect of the large-scale magnetic fields, on the small scales, and analysis of this balance in principle also requires consideration of non-equipartition effects. [Preview Abstract] |
Monday, November 21, 2005 1:59PM - 2:12PM |
HG.00004: The characteristics of sphere wake in freestream turbulent flow Himanshu Tyagi, Rui Liu, David Ting Sphere-turbulence interaction is encountered in many engineering applications. Despite intensive efforts in this field, there is still debate concerning the effect of turbulence on the wake and aerodynamics of a sphere. One possible cause may be the different freestream turbulent flows utilized from one study to another. In the present study a 102~mm sphere supported by strings was subjected to nearly isotropic and homogeneous turbulent air flow at Re~=~5~$\times $~10$^{4}$. The turbulence integral length scale was systematically varied from 30 to~60~mm and the intensity was altered from 5~to~7{\%} by using three perforated plates. In the presence of 5~to~7{\%} turbulence the vortex shedding signal, obtained by the hot-wire measurements, was found to diminish. The shape of the integral length scale contours suggested the presence of horseshoe vortices in the wake boundary region. The opposing shedding of these vortices was indicated in the Reynolds stress contours. The maximum Reynolds stress decreased with increase in integral length scale. The drag coefficient, obtained by loadcell measurements, decreased with increase in turbulence intensity, while the effect of integral length was inconclusive. [Preview Abstract] |
Monday, November 21, 2005 2:12PM - 2:25PM |
HG.00005: Laser doppler velocity measurements of gaseous mixing induced by Richtmyer-Meshkov instability Jean-Francois Haas, Denis Counilh The turbulent mixing of air and SF6 arising from the Richtmyer-Meskov instability is investigated in a shock tube using Schlieren or laser sheet visualization and laser-doppler velocimetry. The shock tube driver, driven and observation sections are 100, 306 and 25 or 30 cm long respectively. The shock tube cross section is 13 by 13 cm square throughout. Initially, the driven and observation sections are filled with air and SF6 at 1 bar and the driver with air at 3.2 bars in order to generate a Mach 1.2 shock wave. Air and SF6 are initially separated with a thin (1 $\mu$m) microcellulose membrane maintained in a plane parallel to the shock by two thin metallic grids, of square mesh spacing 1.8 mm (downstream) and 1.0 mm (upstream). After interaction leading to a Mach 1.3 shock transmitted in SF6, an RMI-induced mixing zone moving at 70 m/s develops to an asymptotic thickness of 1 cm. When the Mach 1.3 reflected shock slows down the flow to -20 m/s, the mixing zone rapidly thickens up to 4 cm when the second reflected (expansion) wave reverses the flow again to 10 m/s. The growth rate is faster for the 30 cm observation section. A two-component laser- doppler velocimeter is used to probe the turbulent velocity field for the 25 cm section. The time evolution of the turbulent kinetic energy at several locations is obtained from many superimposed measurements of the axial and transversal velocities. [Preview Abstract] |
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