Experimental Investigation of Viscous Roll Damping on the DTMB Model 5617 Hull Form

Abstract

A systematic series of model tests have been performed at NSWCCD to explore the mechanisms of roll damping around a conventional combatant hull form (DTMB model #5617) and an advanced tumble-home hull form (DTMB model #5613-1). Both free roll decay and forced oscillation experiments were carried out in calm water and in waves, over a range of forward speeds. These experimental investigations were performed within the overall context of continuing efforts to advance the capability to assess seakeeping, maneuvering, and dynamic stability characteristics of a surface combatant. Data gathered in these experiments are currently being utilized to develop empirical and analytical roll damping models and to validate the accuracy of simulation programs in the calculation of various components of hydrodynamic forces. This paper will specifically discuss a single-degree-of-freedom free roll decay experiment, with measurements of the appendage lateral force and the associated flow field generated during ship roll motion on the DTMB #5617 model. Using particle-image velocimetry (PIV) measurements, two-dimensional unsteady flow patterns around the bilge keels were performed to study the mechanisms of viscous roll damping due to bilge keels. In addition, lateral forces and moments on the bilge keels, rudders, and propellers have been measured to provide a direct assessment of component roll damping. Analysis for appendage forces and correlation with the measured flow field yield several new important insights into the physical mechanisms of bilge keel roll damping. Flow field observation reveals complex phenomena of viscous flow separations and vortex formation around the bilge keel during different phases of the roll motion cycle. The lateral force on the bilge keels was modeled as the sum of an added mass component and a viscous drag component. The viscous drag coefficients are found to depend strongly on ship forward speed and roll amplitude, but the added mass coefficients are relatively constant for the range of forward speed and roll amplitude investigated.