Publications
Performance Prediction without Empiricism: A RANS-Based VPP and Design Optimization Capability (2007 Chesapeake Sailing Yacht Symposium)
The use of Velocity Prediction Programs (VPP’s) in sailing yacht design has been standard practice for years. VPP fidelity, however, continues to be limited by the accuracy of aero and hydro force data used to represent a particular yacht. Even the most advanced America’s Cup VPP’s usually derive sail forces from panel or vortex-lattice models, and hull forces from potential flow codes or experiment. Real world effects attributed to viscosity are added using simplified theoretical or empirical models that cannot resolve all the complexity of sailboat physics. This paper describes a new approach for performance prediction and design optimization that relies solely on high-resolution Reynolds-Averaged Navier-Stokes (RANS) computational fluid dynamics. All aero and hydro forces and moments are generated by RANS, and therefore include the real-world flow features of boundary layers, separation, shed vorticity, and turbulence. RANS software and grid model requirements suitable for VPP applications are discussed, and sample aero and hydro solutions included. Examples from America’s Cup design are used to demonstrate the technique’s practicality and accuracy. Finally, since VPP’s require forces from a large number of sailing conditions, the extensive development effort (undertaken through three America’s Cup cycles) to transition state-of-the-art RANS into the practical realm is summarized.
Active and Passive Control of Spar Vortex-Induced Motions (2006 Journal of Offshore Mechanics and Arctic Engineering)
Spars have become an “Industry Solution” for deepwater developments. Vortex-Induced Motion (VIM) of spar platforms in currents remains an important design concern. While strakes are effective at suppressing riser VIM, all three straked classical spars in the Gulf of Mexico have experienced significant VIM events. These are not examples of poor design but indicate a lack of adequate tools for predicting Spar VIM. This paper presents the development and validation of Unsteady Reynolds-Averaged Navier-Stokes (URANS) methods to predict real-world spar VIM behavior. It includes the ability to address rough surfaces and high super-critical Reynolds numbers. The resulting algorithms are used to assess the effectiveness of active and passive control strategies for suppressing spar VIM. Active control consists of injecting high-pressure water tangentially into the boundary layer, and is shown to be extremely effective at reducing drag and VIM amplitudes. Passive control utilizes a sleeve to channel high-pressure stagnation flow into the boundary layer, and is found less effective.
High-Speed Trimaran Drag: Numerical Analysis and Model Tests (2004 Journal of Ship Research)
A numerical technique for high-speed trimaran resistance calculation is developed. The technique is based on the modified viscous-inviscid interaction concept and quasi-linear theory of wave resistance. The key element of this technique, which is called Modified Quasi-Linear Theory (MQLT), is the account of Froude number influence on the ship trim, transom drag, and wetted surface. This influence leads to appearance of a drag component that significantly depends on both Froude number and Reynolds number. This component has traditionally been included in residuary drag in the model test data. The presented preliminary numerical results were obtained with simplifications of the boundary layer theory that are acceptable for slender hulls. The MGLT computations of boundary layers are also compared to Reynolds-Averaged Navier-Stokes (RANS) calculations at model and ship scale Reynolds numbers. An analysis of model-ship scale correlation factor for high-speed slender hulls with transom sterns and diverse mutual position of the trimaran hulls is also done.
Reynolds-Averaged Navier-Stokes in an Integrated Design Environment (2004 Madrid Deseno de Yates)
Reynolds-Averaged Navier-Stokes (RANS) is becoming increasingly important for America’s Cup design. Simpler flow modeling techniques (such as panel methods) have long played a role, but are known to lack accuracy for some critical applications. Since the speed difference between America’s Cup yachts is typically small, high accuracy tools such as RANS are required to fine-tune a design. But while faster computers have made RANS analyses possible, most applications fall far short of being practical. If an America’s Cup designer is to improve boat speed, he or she must analyze hundreds of design alternatives — not the few isolated samples usually associated with RANS. And even if a large number of runs is possible, the measures-of-merit required to rank designs are not obvious RANS outputs like flow detail and predicted drive force. Rather, ranking necessitates quantifying a boat’s speed around the coarse, and therefore requires force balancing within a Velocity Prediction Program (VPP). So two crucial challenges remain for RANS: 1) How can a level of throughput be achieved to rank hundreds of design options; and 2) How can RANS be seamlessly integrated into design tools like VPP’s. This paper describes how these problems were solved for Oracle/BMW Racing during their 2003 campaign. It documents the detailed technologies brought to play, and the steps taken to implement them in the design process. It gives examples from each of the design areas where RANS played a role (hull, appendage, sails, and mast), and demonstrates these advances using specific examples.
International America’s Cup Class Yacht Design Using Viscous Flow CFD (2001 Chesapeake Sailing Yacht Symposium)
The Computational Fluid Dynamics (CFD) tools needed to incorporate viscous effects into design trade-off studies have finally matured to the point where practical guidance can be provided within design cycle turn-around times. The required techniques are known as Reynolds-Averaged Navier-Stokes (RANS) solvers, and saw their first practical application to yacht design during America’s Cup XXX. The team of AmericaOne, for example, utilized RANS for upwind sail design and VPP aerodynamic force predictions. RANS was also used in the keel design process, and to provide “virtual prototyping” for investigating new concepts before building them. This paper presents some of these ground-breaking applications, and demonstrates the types of guidance available to the designer using RANS.
Prediction of Viscous Forces on Oscillating Cylinders by Reynolds-Averaged Navier-Stokes Solver (2000 Journal of International Society of Offshore and Polar Engineers)
An unsteady, three-dimensional, overset-grid, Reynolds-Averaged Navier-Stokes (RANS) method is applied to the problem of vortex shedding behind fixed and moving circular cylinders. The flows selected for study are of particular interest to the offshore industry in that they represent various modes of riser excitation and response. Three distinct sets of simulations are presented with the objective of validating the method: stationary cylinders; cylinders undergoing forced motion; and cylinders free to undergo Vortex-Induced Vibration (VIV). A limited number of high-resolution simulations are also presented to investigate the impact of small, spanwise phenomena such as “braid” vortices.
Hydrodynamic Design of Integrated Propulsor/Stern Concepts by Reynolds-Averaged Navier Stokes Techniques (1998 Practical Design of Ships and Floating Structures)
A new tool has immerged to assist designers with the difficult task of propulsor/stern integration. Viscous flow computational methods, particularly Reynolds-Averaged Navier Stokes (RANS) techniques, have left the realm of academics and entered practical service. When used in conjunction with traditional towing tank tests this new capability has the potential to greatly improve the ship design process. This paper presents a number of practical examples to demonstrate this potential. The first demonstrates the effect of stern shape modifications on the propeller inflow of a traditional single screw product carrier. The second uses RANS to investigate the efficiency of a podded propulsion system with tractor propeller for high-speed wave-piercer hull forms.
Prediction of Viscous Ship Roll Damping by Unsteady Navier-Stokes Techniques (1997 Journal of Offshore Mechanics and Arctic Engineering)
This paper describes a numerical technique for analyzing the viscous unsteady flow around oscillating ship hulls. The technique is based on a general Reynolds-Averaged Navier-Stokes (RANS) capability, and is intended to generate viscous roll moment data for the incorporation of real flow effects into potential flow ship motions programs. The approach utilizes the finite-analytic technique for discretizing the unsteady RANS equations, and a variety of advanced turbulence models for closure. The calculations presented herein focus on viscous and vertical effects without free surface, and utilize k-epsilon turbulence modeling. Series variations are presented to study the effects of frequency, amplitude, Reynolds number, and the presence of bilge keels. Moment component breakdown studies are performed in each case to isolate the effects of viscosity, vorticity, and potential flow pressures.
Computation of Noise Due to Flow over a Circular Cylinder (1996 NASA Workshop on Computational Aero Acoustics)
Noise due to the flow over a circular cylinder at Reynolds number 90,000 and Mach number 0.2 is computed using a two-step procedure. As the first step, the flow is computed using an incompressible, time-dependent Reynolds-Averaged Navier-Stokes (RANS) solver. The resulting pressures are used as input to a two-dimensional frequency domain acoustic solver, and three-dimensional effects are studied using the Lighthill-Curle equation. Grid and time step dependency studies were performed to ascertain the accuracy of the flow computation. Computed acoustic results are compared to experimental values, and agree well over much of the spectrum (although the computed peak values corresponding to 2D acoustic simulations differ substantially from the available experimental measurements). Three-dimensional simulations reduce the 2D noise level by 10dB.
Applied Fluid Technologies Brochure
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