3 results
On the generation of lift forces in random soft porous media
- P. MIRBOD, Y. ANDREOPOULOS, S. WEINBAUM
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- Journal:
- Journal of Fluid Mechanics / Volume 619 / 25 January 2009
- Published online by Cambridge University Press:
- 25 January 2009, pp. 147-166
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In this paper, we examine the generation of pressure and lift forces in a random soft fibrous media layer that is confined between two planar surfaces, an infinite horizontal lower boundary and a horizontal inclined upper boundary, in the lubrication limit where the characteristic thickness of the fibre layer H ≪ L the length of the inclined surface. The model for the fibre layer is a Brinkman equation and the Darcy permeability Kp is described by the widely used Carman–Kozeny equation for random porous media. Two cases are considered: (a) an inclined upper boundary which slides freely on top of a stationary fibre layer which is firmly attached to the lower boundary and (b) an inclined stationary upper boundary with an attached fibre layer in which the horizontal lower boundary slides freely in its own plane beneath it. Superficially, the problems appear equivalent to the classical problem for a slider bearing where the solutions for the pressure distribution and lift force are independent of which boundary is moving. In this problem there is an optimum compression ratio k = h1/h2 = 2.2, where h1 and h2 are the heights at the leading and trailing edges, for maximum lift force. However, this symmetry is lost if the intervening space is filled with a soft porous fibrous material since the Brinkman equation is not invariant under a transformation of coordinates in which the inherently unsteady problem in case (a) is transformed to a steady reference frame in which the inclined upper boundary is stationary and the horizontal boundary with the adhered fibre layer moves below it. Although in the steady reference frame case (a) now appears to resemble case (b), the solutions are strikingly different and depend critically on the value of the dimensionless fibre interaction layer thickness . For α ≪ 1 the solutions for both cases approach the classical solution for a slider bearing. For α ≫ 1 we show, using asymptotic analysis that the solutions diverge dramatically. In case (a) the pressure and lift force increase as α2 and asymptotically approach a limiting behaviour for large values of α, first predicted in Feng and Weinbaum (J. Fluid Mech., vol. 422, 2000, p. 288), while in case (b) the pressure and lift force decay as α−2 since the inclined upper boundary is screened by the fibre layer and the amount of fluid dragged through the fluid gap decreases as α increases and vanishes for α ≫ 1. The solution in case (a), where the inclined upper boundary moves, is of particular interest since it reveals the potential to generate enormous lift forces using commercially available inexpensive soft porous materials provided the lateral leakage at the edge of the planform can be eliminated through the use of a channel with impermeable sidewalls as first proposed in the work by Wu, Andreopolous and Weinbaum (Phys. Rev. Lett., vol. 93, 2004, p. 194501). The behaviour is illustrated for both a toboggan sliding in such a channel and a larger planform that might be useful in commercial transportation.
Dynamic compression of highly compressible porous media with application to snow compaction
- Q. WU, Y. ANDREOPOULOS, S. XANTHOS, S. WEINBAUM
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- Journal:
- Journal of Fluid Mechanics / Volume 542 / 10 November 2005
- Published online by Cambridge University Press:
- 25 October 2005, pp. 281-304
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A new experimental and theoretical approach is presented to examine the dynamic lift forces that are generated in the compression of both fresh powder snow and wind-packed snow. At typical skiing velocities of 10 to 30ms$^{-1}$ the duration of contact of a ski or snowboard with the snow will vary from 0.05 to 0.2s depending on the length of the planing surface and its speed. No one, to our knowledge, has previously measured the dynamic behaviour of snow on such a short time scale and, thus, there are no existing measurements of the excess pore pressure that can build-up in snow on this time scale. Using a novel porous cylinder–piston apparatus, we have measured the excess pore pressure that would build-up beneath the piston surface and have also measured its subsequent decay due to the venting of the air from the snow at the porous wall of the cylinder. In further experiments, in which the air is slowly and deliberately drained to avoid a build-up in pore pressure, we have been able to separate out the force exerted by the ice crystal phase as a function of its instantaneous deformation. A theoretical model for the pore pressure relaxation in the porous cylinder is then developed using consolidation theory. Dramatically different dynamic behaviour is observed for two different snow types, one (wind-packed) giving a steady continuous relaxation of the excess pore pressure and the other (fresh powder) leading to a piston rebound with negative pore pressure. A feature of the rebound is the apparent debonding of sintered ice crystals after maximum compression. This behaviour is described well by introducing a debonding coefficient where the debonding force is proportional to the expansion velocity of the medium. The experimental and theoretical approach presented herein and the previous generalized lubrication theory for compressible porous media, have laid the foundation for understanding the detailed dynamic response of soft porous layers to rapid deformation.
The structure of weakly compressible grid-generated turbulence
- G. BRIASSULIS, J. H. AGUI, Y. ANDREOPOULOS
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- Journal:
- Journal of Fluid Mechanics / Volume 432 / 10 April 2001
- Published online by Cambridge University Press:
- 22 June 2001, pp. 219-283
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A decaying compressible nearly homogeneous and nearly isotropic grid-generated turbulent flow has been set up in a large scale shock tube research facility. Experiments have been performed using instrumentation with spatial resolution of the order of 7 to 26 Kolmogorov viscous length scales. A variety of turbulence-generating grids provided a wide range of turbulence scales with bulk flow Mach numbers ranging from 0.3 to 0.6 and turbulent Reynolds numbers up to 700. The decay of Mach number fluctuations was found to follow a power law similar to that describing the decay of incompressible isotropic turbulence. It was also found that the decay coefficient and the decay exponent decrease with increasing Mach number while the virtual origin increases with increasing Mach number. A possible mechanism responsible for these effects appears to be the inherently low growth rate of compressible shear layers emanating from the cylindrical rods of the grid. Measurements of the time-dependent, three dimensional vorticity vectors were attempted for the first time with a 12-wire miniature probe. This also allowed estimates of dilatation, compressible dissipation and dilatational stretching to be obtained. It was found that the fluctuations of these quantities increase with increasing mean Mach number of the flow. The time-dependent signals of enstrophy, vortex stretching/tilting vector and dilatational stretching vector were found to exhibit a rather strong intermittent behaviour which is characterized by high-amplitude bursts with values up to 8 times their r.m.s. within periods of less violent and longer lived events. Several of these bursts are evident in all the signals, suggesting the existence of a dynamical flow phenomenon as a common cause.