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Twenty-First Symposium on NAVAL HYDRODYNAMICS
interaction of the hulls creates lifting effects. There are limited data that document off-design characteristics, and lack of understanding as to the underlying physics of the flow features. Off-design conditions frequently occur such as when ships maneuver or advance through a cross current. These situations produce the off-design features defined above and spray and bubble entrainment which are also not well documented. A summary of documented surface-ship studies is provided in (1), including an evaluation of their usefulness for computational fluid dynamics validation.
Herein, the straight-ahead condition is defined as the zero-yaw, i.e., β=0° condition. The other cases, such as maneuvering, cross current, or lift cases (yacht, SWATH, or catamaran), are defined as yaw or β≠0° conditions. It is shown that even for a slender ship, the yaw condition displays many flow features that are common in off-design ship flows. Admittedly, the yaw case is an approximation to the off-design condition, i.e., nominal-wake measurements in the propeller plane of a high-speed FF 1052 combatant (2) display significant differences between zero-yaw tests in a straight basin, yaw tests in a straight basin, and yaw tests in a maneuvering basin (rotating arm). Some of these differences, i.e., axial-velocity contours, were unexpected. Nonetheless, the yaw case has many features of interest that are shared by both design and off-design conditions, it can be performed in a towing tank, and it builds on previous experimental work. This paper is concerned with documentation of the yaw case.
The goals for the present work are to (i) explore and identify the important flow features associated with a yawed body and (ii) document the flow features in sufficient detail for validation of RANS CFD codes. The emphasis is on ‘explore' because the range of yaw-induced changes to the flow field was unknown.
The yaw project is part of an Iowa Institute of Hydraulic Research (IIHR) study concerning free-surface effects on boundary-layer and wake flow. The research and discussions of wave-boundary layer and wake interactions herein, rely upon and compliment previous fundamental studies with flat plates, (3)–(6) and previous work at the IIHR for the 3.048 m Series 60 CB=0.6 in the zero-yaw condition, (7) and (8), which is precursory for the present study. Also of relevance is the recent Series 60 CB=0.6 bow study (9). The yaw study is related to the basic topics of three-dimensional separation (10), wave breaking (11), and vortex-free surface interaction (12) each of which represents a major field of study. Other related work with application to ship flows are provided in (13).
In this study, model-scale experiments are performed in a towing tank with a 3.048 m Series 60 CB=0.6 ship hull. Visualization of the flow is performed with photographs and video for a range of Froude number (Fr) and β. Resistance, sideforce, yaw moment, sinkage, trim, and heel angle are measured for a range of Fr and β. Additionally, wave profiles along the hull and wave elevations are measured for a range of yaw angles and high (0.316) and low (0.16) Fr. For β=10°, detailed mean-velocity and pressure fields are measured at ten axial stations for Fr=0.16 and 0.316 and at several stations in the wake of a breaking wave for Fr=0.316. Finally, and in accordance with established standards and guidelines, an uncertainty analysis is performed for each experiment.
The organization of this paper is as follows. First, a brief description of the experimental facilities and equipment, procedures and conditions, and uncertainty analysis is given. Then, the results are presented with regard to the essential yaw- and wave-induced effects observed in the experiments. This paper is based on Ph.D. thesis research and the results are extensive. Many discussions are abbreviated, however, the complete results are provided in (13). Finally, conclusions from this study are given with recommendations for future work.
OVERVIEW OF THE EXPERIMENTS
Facilities and equipment
A cartesian-coordinate system (Figures 1 and 2) is used that is fixed to the model. The x-axis is coincident with the model centerline, and the y- and z-axes are directed toward the starboard side of the model and upward, respectively, with the origin at the intersection of the waterplane and the FP. Yaw angle is the angle between the model centerline and the tank centerline. For the orientation shown in Figures 1 and 2, the port side is the windward side and the starboard side is the leeward side. Alternately, if the yaw condition approximates a turning ship, the port and starboard sides correspond to the outboard and inboard sides of the turn, respectively.
The IIHR towing tank is 100 m long and 3.05 m wide and deep. Wave dampeners near the free surface on the sidewalls allow for twelve-minute intervals between carriage runs. The carriage is cable driven by a 15-horsepower motor. On board the carriage, a cabinet holds the computer and other instrumentation.
Ship model and geometry
The ship model for the experiments is a 1:40 scale 3.048 m Series 60 CB=0.6 (Figure 1). The Series 60 CB=0.6 is a single-propeller merchant-type ship, a standard for ship-hydrodynamic research, and in particular, was chosen with three other hull types as a representative hull form for the CEP (14). The lines of