Articles you may be interested inUltrasonic measurement of the porosity and tortuosity of air-saturated random packings of beads J. Appl. Phys. 93, 9352 (2003); 10.1063/1.1572191 Inverse problem in air-saturated porous media via reflected waves Rev. Sci. Instrum. 74, 2871 (2003); 10.1063/1.1569412 Frame-borne surface waves in air-saturated porous media

New expressions are given that can be used instead of the phenomenological equations of Delany and Bazley. They provide similar predictions in the range of validity of these equations, and in addition are valid at low frequencies where the equations of Delany and Bazley provide unphysical predictions. These new expressions have been worked out by using the general frequency dependence of the viscous forces in porous materials proposed by Johnson et al. [J. Fluid Mech. 176, 379 (1987)], with a transposition carried out to predict the dynamic bulk modulus of air. The model used suggests how sound propagation in fibrous materials can depend both on the diameter of the fibers and on the density of the material.

The acoustical properties of a class of simple porous materials have been studied experimentally and theoretically. Rigid-frame materials containing air-filled pores of uniform cross section were investigated. Two model porous materials were constructed, one with pores of rectangular cross section (0.0146×0.0172 cm) and one with pores of triangular cross section (0.037-cm sides). The characteristic impedance and propagation constant were measured for frequencies between 50 and 4500 Hz and good agreement with exact theoretical predictions was obtained. The exact theoretical expressions for specific pore shapes (e.g., slitlike, square, and triangular) can be used to investigate the assignment of shape factors by various general but approximate theoretical models. It is demonstrated that the model introduced by Attenborough requires a shape factor that is frequency dependent. A theoretical model, appropriate for materials containing pores of uniform cross section, that correctly treats the low- and high-frequency behavior, is presented. Given subsidiary measurements of flow resistivity, porosity, and tortuosity, a single shape factor leads to very good agreement with the exact solutions for all frequencies.

The large increases in activity for gluteus maximus and biceps femoris, which are in accordance with the increase in force production during the propulsive phase, could be considered as instinctive coordination strategies that compensate for potential fatigue and loss of force of the knee extensors (i.e., vastus lateralis and vastus medialis) by a higher moment of the hip extensors.

In this paper, we report the results of our experimental investigation of the pressure distribution on a free-falling wedge upon entering water. Parameters such as the drop height, the deadrise angle and the mass of the wedge are related to the water pressure on the wedge and its dynamic behavior. Existing models that assumed a constant water-entry velocity of the wedge are compared with experimental data. In order to take into account the inherent variation in the velocity of a free-falling wedge, a combination of two models are proposed. This method gives an adequate approximation of the maximum pressures measured.