Unsteady loading of hydrofoils normally immersed in a turbulent boundary layer

 

Brief Description 

Introduction

For ships and submarines, constraints relating to control, serviceability and efficiency dictate that propulsion and control equipment be compact and located at the stern of the vessel. This equipment is then affected by the turbulent flow about the vessel afterbody. At the stern, the hull boundary layer and embedded wakes have had the full vessel length to develop and may be further thickened due to afterbody adverse pressure gradients. Consequently, control surfaces and propulsion devices may be partially or fully immersed within the afterbody flow and be subject to unsteady loading and hence be a source of vibration and noise production[1]. For these effects to be minimised insight into the flow physics and excitation spectra are required. Such information would enable more rigorous analysis and design for optimisation of control surface and propeller structural response. 

The problem of unsteady airfoil loading has received considerable attention since the initial work by Theodorsen (1935) and von Karman and Sears (1938), for an oscillating flat plate, and Sears (1941) for a thin airfoil encountering a sinusoidal gust. Much of this work is motivated by aeronautical applications involving prediction of radiated noise as well as unsteady loading. Detailed reviews are provided in recent work by Howe (2001 and 2002), Mish and Devenport (2006a and 2006b) and Glegg and Devenport (2009 and 2010). These recent studies refer to a significant body of work in which enhancements to the Sears linearised inviscid theory have been made. In some cases use has been made of modern computational capabilities to numerically solve various inviscid formulations. Despite the extensive development of theoretical models there are relatively few complimentary experimental studies (e.g. McKeough and Graham, 1980 and Jackson et al., 1973). Mish and Devenport note this as a motivation for their extensive wind tunnel experiments additional to numerical studies. These experimental investigations have typically involved two or three-dimensional airfoils immersed in grid generated turbulence with lift spectra measured directly or derived from surface pressure measurements.

Objectives

  • To investigate the unsteady loading of hydrofoils normally immersed, partially or fully, within a flat plate turbulent boundary layer

 

  • To investigate the flow physics and the effect of hydrofoil geometry on the unsteady loads

 

  • To acquire an experimental dataset suitable for comparison with LES simulations

 

  • To develop scaling rules and recommendations for designers to optimise control surface design for minimum vibration and noise production

 

Scope of the work

Measurements of hydrofoil lift and drag spectra would be made for a range parameters and model geometries. Tests would be made at several Reynolds numbers and a range of incidences. Models of differing span (and/or chord) would be tested to investigate the effect of varying immersion. Other geometric parameters of interest may include details of root and tip terminations and means of articulation.

Which SEA program would benefit and how? Final outcome and impact (eg TRL increase?)

SEA1000 and Collins Sustainment projects could benefit from this research

 


[1] This is in contrast to aircraft where despite similar Reynolds numbers and boundary layer thicknesses propulsion equipment maybe located outside the boundary layer/or exploit boundary layer diversion and control surfaces don’t have the size restrictions of analogous maritime platforms