Dynamic tortuosity and bulk modulus in air-saturated porous media

In this paper, the concept of characteristic length introduced in the definition of the dynamic tortuosity by Johnson, Koplik, and Dashen [J. Fluid Mech. 176, 379 (1987)] is extended to express the frequency dependence of the bulk modulus of the saturating fluid at high frequencies. A general phenomenological frequency dependence for this dynamic bulk modulus is obtained using the expression for the dynamic tortuosity. The theoretical predictions for dynamic tortuosity and bulk modulus are compared with experimental results obtained from acoustic measurements on a rigid-frame porous material saturated with air.