Flow-Induced Lift Forces on a Towed Sphere

Abstract

Abstract An experimental and computational effort to characterize the broadband flow-induced forces on a spherical body that is towed underwater is described. The body itself is the transducer which is comprised of a small geophone encased in a near-neutrally-buoyant sphere, 7.62 cm in diameter. The research described in this paper quantifies the flow-induced unsteady lift force signal as a function of the sphere diameter Reynolds number (215 < Re < 34,290) and the Strouhal number (1.5 < St < 30). It is found that the broadband flow-induced force spectral levels may be separated into two regimes characterized by: 1) unsteady laminar flow in which the forces are proportional to the product of viscosity, mean shear, and area; and 2) turbulent flow in which the unsteady lift forces are proportional to the product of an area and the dynamic pressure of the flow. The low-Reynolds number data were acquired by towing the sphere in various mixtures of glycerine and water. The high Reynolds number lift force spectra, obtained in pure water, are compared to similar data measured previously on a finite-length, right-circular cylinder in cross flow. This comparison indicates that the cylindrical body creates more unsteady side force than does the spherical one, particularly at the lower end of the Strouhal number range. This is also the range of Reynolds number (Re < 3,000) where a simulation of the unsteady laminar flow-induced forces, based on a numerical solution of the Reynolds-averaged Navier-Stokes equations, could be performed. The sphere was held fixed in these simulations, and the resulting forces are thus due to the unsteady flow of fluid around the body itself, with no influence due to free-stream turbulence or tow cable. The computational spectra are found to agree remarkably well with those measured.