CFD Analysis of Free-Fall Ball Penetrometer in Clay

Abstract

The free fall penetrometer (FFP) is used to measure the soil undrained shear strength by penetrating into the soil with its kinetic energy and self-weight. The free fall ball penetrometer (FFB) takes advantages of both FFP and ball penetrometer. However, the soil-FFB interaction is rather complex, which refers to the soil strain rate effect and drag force. Therefore, it is necessary to analyze the forces acting on the FFB to improve its practicability and the accuracy of soil strength measurement. The dynamic penetration of the ball penetrometer in the uniform clay was simulated in this study using the commercial software ANSYS CFX 17.0, which is based on the computational fluid dynamics (CFD) approach. A thin layer element method was proposed to simulate the ball-soil frictional interaction. In the numerical model, the soil was modeled as non-Newtonian fluid incorporating the strain rate effect. The bearing capacity factor (Nb) and drag coefficient (Cd) were calculated during the ball dynamic penetration within the soil. Different ball velocities, soil strengths and densities, interface frictional coefficients and soil strain rate parameters were considered to investigate their effects on the ball strain rate effect. The fitted formulas of the ball bearing capacity factor, the rate effect coefficient and the drag coefficient for the FFB were established based on the present numerical results.