Trans RINA, Vol 157, Part A3, Intl J Maritime Eng, Jul-Sep 2015
LOW REYNOLDS NUMBER PERFORMANCE OF A MODEL SCALE T-FOIL (DOI No: 10.3940/rina.ijme.2015.a3.336)
J AlaviMehr, M R Davis, J Lavroff, University of Tasmania, Australia SUMMARY
Submerged T-foils are an essential forward component of the ride control systems of high speed ferries. A model scale T-Foil for a 2.5m towing tank model of a 112m INCAT Tasmania high-speed wave-piercer catamaran has been tested for both static and dynamic lift performance. The tests were carried out using a closed-circuit water tunnel to investigate the lift and drag characteristics as well as frequency response of the T-Foil. The model T-Foil operates at a Reynolds number of approximately 105, has an aspect ratio of 3.6 and a planform which is strongly tapered from the inboard to
outboard end. All of these factors, as well as strut and pivot interference, influence the steady lift curve slope ( of the
model T-foil which was found to be 61% of the value for an ideal aerofoil with elliptic loading. The T-foil dynamic performance was limited primarily by the stepper motor drive system and connection linkage. At the frequency of maximum motion of the 2.5 m catamaran model (about 1.5Hz) the model T-foil has approximately 5% reduction of amplitude and 15 degrees of phase shift relative to the low frequency response. Only very small limitations arose due to the unsteady lift as predicted by the analysis of Theodorsen. It was concluded that the model scale T-foil performed adequately for application to simulation of a ride control system at model scale.
NOMENCLATURE a
AR b
CD CL
CLα
Fy Fz h
ḣ ḧ
k L
L/D S
V
ω
1. area,
Dimensionless parameter which determines the location of a point that vertical motion of T-Foil is referenced to
T-Foil Aspect ratio
T-Foil semi-chord length (m) T-Foil drag coefficient T-Foil lift coefficient
D T-Foil drag force (N) f
T-Foil lift-coefficient derivative (
T-Foil Actuation frequency (Hz)
Load-cell output voltage in the horizontal y-axis direction (V)
Load-cell output voltage in the vertical z-axis direction (V)
T-Foil vertical displacement (m) T-Foil vertical velocity (m/s) T-Foil vertical acceleration (m/s2)
T-Foil lift force (N)
T-Foil Reduced frequency (
T-Foil Lift-to-drag ratio T-Foil planform area (m2) Water Flow velocity (m/s)
Vout Potentiometer output voltage (V) Vin α
Stepper-motor input voltage (V) T-Foil pitch angle of attack (degree) T-Foil pitch angular velocity (degree/s) T-Foil pitch angular acceleration (degree/s2)
T-Foil Actuation angular frequency (2 Water density (kg/m3)
INTRODUCTION
A number of large high-speed and lightweight marine vessels have been developed in the last 25 years in order to satisfy fast sea transportation requirements. Catamaran vessels have proved to be particularly popular among different types of high-speed craft due to their large deck
©2015: The Royals Institution of Naval Architects relatively large deadweight, high hydrodynamic
stability and their ability to provide lightweight Ro-Ro vessels. A unique configuration of high-speed wave- piercing catamarans has been developed by INCAT Tasmania [1] with a prominent centre bow located at the vessel centreline between the wave-piercer demihulls.
High-speed catamarans often experience large heave and pitch motions and high motion accelerations due to their hull shape and operating speed. Increases in vessel speed have generally led to an increase in vessel motions, this leading to poor structural
damage while
passenger comfort and potential operating in severe
sea
conditions [2, 3]. A motion control system is therefore required to reduce these large motions and improve the vessel performance.
INCAT Tasmania has applied the use of motion control systems to its high-speed wave piercing catamarans to reduce vessel motions and dynamic structural loads [4, 5]. These motion controls consist of a centre bow mounted T-Foil and active trim tabs located at the stern of the vessel. Figure 1 shows the location of the T-Foil and the trim tabs on the 112 m INCAT Tasmania catamaran vessel [1]. The trim tabs installed on the stern, otherwise known as stern tabs, produce a lift force at the transom of the vessel to keep the vessel on a level trim. Working together, tabs can also control the roll motion of the vessel. The T-Foil installed on the aft section of the centre bow acts to generate a force resisting pitch and heave motion in combination with the stern tabs.
Although some investigations into ship motions as well as motion control systems of INCAT Tasmania vessels have been undertaken through full-scale testing and numerical computations [2-4, 6-9], the mechanisms for the whole motion control system are poorly understood. For optimization of the motion control system further investigation is required to determine the effect of the control system on the ship motions and loads. In
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