Effective Fluid Density Research Articles

What Is a Debris Flood?

AbstractDebris floods have been defined descriptively as mineral and organic sediment‐rich floods, occurring in a steep channel and potentially destabilizing the streambed and banks. While this definition allows one to visualize the process, it does not inform on the mechanics, nor does it recognize different types. We propose to define debris floods as “floods during which the entire bed, possibly barring the very largest clasts, becomes mobile for at least a few minutes and over a length scale of at least 10 times the channel width.” We define the onset of a debris flood by the exceedance of a critical shear stress threshold required to mobilize at least the D84 of bed material. A threefold classification is proposed in which the first type is triggered by the shear stress exceedance. The second is initiated by transition from a debris flow either in the channel or by oblique impact of a debris‐flow‐prone tributary. In this context we highlight the importance of effective fluid density. The third type is associated with outbreak floods from artificial or natural dams. A further subdivision of debris floods is made by using the ratio of the actual shear stress to the critical shear stress, with higher values indicating damaging and finally catastrophic debris floods in which even preexisting channel bank and bed protection is mobilized. This contribution aims to provide a more succinct mechanistic definition of debris floods that can be implemented in hazard and risk assessments for steep streams and rivers.

Acoustic wave propagation in gassy porous marine sediments: The rheological and the elastic effects.

The preceding paper in this series [Mantouka, Dogan, White, and Leighton, J. Acoust. Soc. Am. 140, 274-282 (2016)] presented a nonlinear model for acoustic propagation in gassy marine sediments, the baseline for which was established by Leighton [Geo. Res. Lett. 34, L17607 (2007)]. The current paper aims further advancement on those two studies by demonstrating the particular effects of the sediment rheology, the dispersion and dissipation of the first compressional wave, and the higher order re-scattering from other bubbles. Sediment rheology is included through the sediment porosity and the definition of the contact interfaces of bubbles with the solid grains and the pore water. The intrinsic attenuation and the dispersion of the compressional wave are incorporated using the effective fluid density model [Williams, J. Acoust. Soc. Am. 110, 2276-2281 (2001)] for the far field (fully water-saturated sediment). The multiple scattering from other bubbles is included using the method of Kargl [J. Acoust. Soc. Am. 11, 168-173 (2002)]. The overall nonlinear formulation is then reduced to the linear limit in order to compare with the linear theory of Anderson and Hampton [J. Acoust. Soc. Am. 67, 1890-1903 (1980)], and the results for the damping coefficients, the sound speed, and the attenuation are presented.

Investigation of the effects of the microstructure on the sound absorption performance of polymer foams using a computational homogenization approach

In this paper, a computational homogenization approach is exploited to study the effects of the microstructure of polymer foams on their acoustic properties. A Kelvin cell with partially-open thin membranes is adopted to represent the microstructure of the foam. By applying the homogenization approach, the effective material parameters are obtained based on a microscopic representative volume element (RVE) subjected to different loading conditions. Geometrical properties, including the opening and the thickness of the thin membranes and the cell size, are investigated. It is shown that when the opening of the membranes or the cell size are smaller, the sound absorption performance at low frequencies can be improved, at the expense of the mid-high frequency performance. Moreover, the optimal opening and optimal cell size for best sound absorption performance depend on the target frequency range. The effect of solid properties, including the stiffness and the loss factor, are also discussed. For low-stiffness materials, local resonance of the solid frame greatly affects the effective fluid density and the frame-borne wave, whereas global resonances can be utilized to improve the absorption performance in a specific frequency band.

Two-dimensional model of low Mach number vortex sound generation in a lined duct

The sound generated by a vortex moving across a duct section lined with porous materials and the corresponding vortex dynamics are studied numerically in the present investigation. The combined effects of the effective fluid density, the flow resistance, the length, and the thickness of the porous linings on the vortex dynamics and sound generation are examined in detail. Results show that stronger sound radiation will take place when the length and the thickness of the porous linings are increased or when the effective fluid density is reduced. The flow resistance can only result in stronger sound radiation within a range whose width depends on the above-mentioned system parameters. Such sound amplification cannot be achieved when the initial vortex height gets closer and closer to the duct centerline. The present results also indicate the strong correlation between vortex acceleration and the sound radiation under the actions of the porous linings.

Vortex sound under the influence of a piecewise porous material on an infinite rigid plane

The vortex dynamics and the sound generation by an inviscid vortex in the presence of a finite length porous material on an otherwise rigid plane are studied numerically in the present study in an attempt to understand the sound generation near the surface of a wall lining in a lined duct. The combined effects of the effective fluid density and flow resistance inside the porous material, and the length and thickness of the porous material on the sound generation process are examined in detail. Results obtained demonstrate the sound pressure is longitudinal dipole and show how seriously the above-mentioned parameters are affecting the vortex sound pressure under the influence of the porous material.

Vortex sound in the presence of a wedge with inhomogeneous surface flow impedance

The low Mach number vortex sound scattered by a sharp wedge formed by a rigid and an acoustically softer material is analyzed theoretically in the present investigation. A general expression for the time-varying far field potential due to the incompressible vortex motion around the edge of the wedge is derived to the leading order of magnitude. Results show that the introduction of a softer material increases the far field sound pulse magnitude. Also, it is found that the lower the impedance of the material, the louder the far field sound will be. In addition, it is shown that the effective fluid density inside the porous material is more important that the flow resistance in affecting the far field sound pulse magnitude when a weak vortex is concerned.

Effects of porous boundaries on the dynamics of an inviscid vortex filament

The dynamics of a vortex filament in the proximity of a porous surface are studied theoretically. The porous material is assumed to be characterized by its flow resistance and effective fluid density within its lattice. Results show that the propagation speed of the vortex is reduced upon the introduction of a porous material on to a hard surface. Also, the vortex speed increases with increasing flow resistance at constant effective fluid density. However, this speed increases with the effective fluid density only when the flow resistance is small. At larger flow resistance, a reduction of the speed is observed when the effective fluid density is increased. A similar phenomenon is again observed with the flat surface replaced by a circular cylinder. The study is extended to include the effect of a flow boundary discontinuity where the flat surface is made up of a rigid and a porous surface. Results illustrate that there is a relatively rapid change of vortex acceleration when the vortex moves across the plane of discontinuity, suggesting the possibility of sound generation close to this instant.

Deriving the equations of motion for porous isotropic media

The equations of motion and stress/strain relations for the linear dynamics of a two-phase, fluid/solid, isotropic, porous material have been derived by a direct volume averaging of the equations of motion and stress/strain relations known to apply in each phase. The equations thus obtained are shown to be consistent with Biot’s equations of motion and stress/strain relations; however, the effective fluid density in the equation of relative flow has an unambiguous definition in terms of the tractions acting on the pore walls. The stress/strain relations of the theory correspond to ‘‘quasistatic’’ stressing (i.e., inertial effects are ignored). It is demonstrated that using such quasistatic stress/strain relations in the equations of motion is justified whenever the wavelengths are greater than a length characteristic of the averaging volume size.