Numerical Investigation of High-Reynolds-Number Air-Ventilated Water Flow under Solid Body with Surface Geometry Variations

Konstantin I Matveev,Jeffrey M Collins

doi:10.3390/fluids6050174

Abstract

Air-ventilated cavities formed under or around the hulls of marine vehicles can reduce water drag. Hull configurations with partial air ventilation where air cavities reattach to body surfaces are of special practical interest, since the required air supply rates to achieve significant drag reduction can be made rather low. However, formation and stability of such air cavities are sensitive to the hull geometry and operational conditions. In this study, an attempt is made to numerically simulate one setup with a partial air cavity that was previously tested experimentally at high Reynolds numbers, above 50 million. A computational fluid dynamics software Star-CCM+ has been employed for numerical modeling. Stable and unstable states of the air-cavity setup, characterized by long and collapsing air cavities, respectively, were modeled at two air supply rates near the stability boundary. Numerical results were similar to experimental data at the optimal water speed for the tested geometry, when a long air cavity was sustained at a minimal air supply rate. For water speeds that were substantially higher or lower than the optimal case, a stable cavity could not be maintained with small air supply rates for the given hull geometry. Numerical simulations demonstrated how alterations of the body surface could help sustain long air cavities across a broader speed range using air supply rates that were similar to the optimal case. These findings suggest that morphing hull surfaces can potentially be used for control of drag-reducing air cavities and expand the viable operating range for their application to marine vehicles.

Highlights

Reducing water drag of marine vehicles is critical for both marine transportation and naval applications, since it can lead to more economically efficient and faster vessels
While there are a variety of air-assisted methods for drag reduction, such as using small air bubbles [1] and thin air layers [2], the topic of the present paper is limited to bulkier air cavities with small air leakage
The present study has demonstrated that 2D computational fluid dynamics (CFD) simulations can adequately predict macroscopic features of high-Reynolds number air-ventilated water flows under a hull with a recess on its bottom

Summary

Introduction

Reducing water drag of marine vehicles is critical for both marine transportation and naval applications, since it can lead to more economically efficient and faster vessels. If stable air cavities of large-area can be maintained at low air supply rates near the solid surface, water drag and vehicle power consumption can decrease significantly. It should be noted that naturally occurring air ventilation under stepped hull surfaces can reduce hydrodynamic drag [3,4], but this method can be realized only on high-speed boats planing on the water surface. One major problem is the complexity of physical phenomena [8,9] related to wall-bounded, turbulent, multi-phase flows, occurring in the presence of gravity and often in unsteady environments. Another issue is the importance of scale effects.

Objectives

Results

Conclusion