Donald C. Wyatt

A detailed assessment of numerical flow analysis (NFA) to predict the hydrodynamics of a deep-V planing hull

Over the past few years much progress has been made in Computational Fluid Dynamics (CFD) in its ability to accurately simulate the hydrodynamics associated with a deep-V monohull planing craft. This work has focused on not only predicting the hydrodynamic forces and moments, but also the complex multiphase free-surface flow field generated by a deep-V monohull planing boat at high Froude numbers. One of these state of the art CFD codes is Numerical Flow Analysis (NFA). NFA provides turnkey capabilities to model breaking waves around a ship, including both plunging and spilling breaking waves, the formation of spray and the entrainment of air. NFA uses a Cartesian-grid formulation with immersed body and volume-of-fluid methods. The focus of this paper is to describe and document a recent effort to assess NFA for the prediction of deep-V planing craft hydrodynamic forces and moments and evaluate how well it models the complex multiphase flows associated with high Froude number flows, specifically the formation of the spray sheet. This detailed validation effort was composed of three parts. The first part focused on assessing NFAs ability to predict pressures on the surface of a 10 degree deadrise wedge during impact with an undisturbed free surface. Detailed comparisons to pressure gauges are presented here for two different drop heights, 15.24 cm (6 in) and 25.4 cm (10 in). Results show NFA accurately predicted pressures during the slamming event. The second part examines NFAs ability to match sinkage, trim and resistance from Fridsmas experiments performed on constant deadrise planing hulls. Simulations were performed on two 20 degree deadrise hullforms of varying length to beam ratios (4 and 5) over a range of speed-length ratios (2, 3, 4, 5 and 6). Results show good agreement with experimentally measured values, as well as values calculated using Savitskys parametric equations. The final part of the validation study focused on assessing how well NFA was able to accurately model the complex multiphase flow associated with high Froude number flows, specifically the formation of the spray sheet. NFA simulations of a planing hull fixed at various angles of roll (0, 10, 20 and 30 degrees) were compared to experiments. Comparisons to underwater photographs illustrate NFAs ability to model the formation of the spray sheet and the free surface turbulence associated with planing boat hydrodynamics. Overall these three validation studies provide a detailed assessment on the current capabilities of NFA to predict the hydrodynamics of a deep-V planing hull.

Numerical Simulation of Short Duration Hydrodynamic Impact

Numerical simulations of wedge impact experiments, undertaken by the Naval Surface Warfare Center, Carderock Division, NSWCCD, and more recently by the United States Naval Academy, USNA, Hydromechanics Laboratory, were performed using the computational fluid dynamics code Numerical Flow Analysis, NFA, to assess its capabilities in simulating the short duration hydrodynamic loading associated with free-surface impact. NSWCCD performed experiments using drop heights of 15.24 cm (6 in) and 25.4 cm (10 in), while the Naval Academy used drop heights of: 7.94, 12.7, 15.88, 25.4, 31.75, 38.1, and 50.8 cm (3.125, 5.0, 6.25, 10.0, 12.5, 15.0, and 20.0 in), measured from the keel of the wedge to the calm water surface. Simulations and comparisons were made at heights of 15.24 cm (6 in) and 25.4 cm (10 in) with the NSWCCD data, and 12.5 inches for the USNA data providing for a detailed examination of NFA’s ability to simulate and predict short duration hydrodynamic impacts.Copyright