Thomas C. Fu

Impact of Flow Control Technologies on Naval Platforms (Invited)

This paper highlights flow control technologies and draws a connection between the technologies and US Navy applications, specifically submarines, where possible. This paper does not provide an exhaustive citation listing typical of review papers, but instead focuses on selected applications along with a brief history of submarines. The major theme of the paper is to provide some rationale for which technologies may work in real operational conditions, why technologies have successfully been demonstrated in the laboratory but fail in real operational scenarios, and provide some directions for future research on promising flow control technologies.

Improved Simulation of Ship Maneuvers Using Recursive Neural Networks

An improved Recursive Neural Network (RNN) maneuvering simulation tool for surface ships is described. Inputs to the simulation, cast in the form of forces and moments, are redefined and extended in a manner that more accurately captures the physics of ship motion; the new model is used to extend initial efforts toward RNN surface ship simulations. These extensions include improved formulations of propeller thrust, lift from deflected rudders, and the explicit inclusion of roll and pitch righting moments. Two maneuvers are simulated: tactical circles and horizontal overshoots. Simulation errors for the circles averaged over all maneuvers for such variables as speed, trajectory components and heading were 5% or less. The horizontal overshoot simulation errors were also 5% or less for the same variables with the exception of the transverse trajectory component. The explanation for the latter deficiency is believed to be the result of the exclusion of wind forces acting on the vehicle, which will be the subject of later work.

Radar Measurement of Ocean Waves

Over the past two decades a number of advances have been made in the use of radar systems for the measurement of ocean waves, building on early work at universities and the Naval Research Lab (NRL) to investigate the potential for extracting wave field measurements from the sea clutter seen in shipboard radar images. This early work was the foundation for modern wave radar systems, with hardware systems ranging from commercial off the shelf (COTS) incoherent navigation radar to specially developed, calibrated, coherent instrumentation radar and phased-array systems. Software algorithms and image analysis techniques have also been in constant development, which have evolved from 2D analysis of digitized images into modern techniques performing real-time 3D transformation of high resolution images. Most of these systems are being utilized to measure the directional wave spectra, with some systems also providing wave height estimates and sea surface elevation maps. More recently, the Naval Surface Warfare Center, Carderock Division (NSWCCD) and others have begun to utilize these techniques for shipboard measurement of open ocean waves. All these efforts have led to higher fidelity data, as well as data that were previously unobtainable. In this paper we provide an overview and history of the development of COTS incoherent wave radar systems, analysis techniques, and capabilities, from early characterization of sea clutter return to the latest developments in image inversion and sea surface topography. This review and summary provides a foundation on which to develop analysis techniques for the higher fidelity data, using lessons learned to improve future analysis. While not intending to be exhaustive, this paper seeks to highlight the insights gained from both historical and recent applications of these techniques, as well as the difficulties and issues associated with shipboard measurements such as ship motion, logistical constraints, and environmental factors.Copyright

Shipboard Measurement of Ocean Waves

Over the past several years a number of techniques have been utilized for the measurement of ocean waves from shipboard platforms. These systems have ranged from commercial off the shelf (COTS) navigation radar and Light Detection and Ranging (LIDAR) systems to specially developed in-house instrumentation systems. Most of these systems have been utilized to measure the directional wave spectra around the ship. More recently, the Naval Surface Warfare Center, Carderock Division (NSWCCD) and others have begun to utilize these techniques for shipboard measurement of individual ship generated waves as well as open ocean waves. NSWCCD has used a number of these methods on various Office of Naval Research (ONR) and Naval Sea Systems (NAVSEA) sponsored field tests. These field tests were performed on a variety of naval platforms over a range of sizes, including some fixed platforms, for various sea states. While each of these tests has had individual measurement goals and objectives, the series of tests has also provided an environment for testing and developing new instrumentation and exploring their capabilities. As a result of these efforts, instrumentation has grown in sophistication from qualitative video-based observations of the wave field around an underway vessel to laser and radar based imaging and ranging measurements of free surface dynamics. This work has led to higher fidelity data, as well as data that were previously unobtainable. In this paper we provide an overview of these systems and techniques and summarize the basic capabilities of each method by providing measurement examples/applications. These systems include a shipboard array of ultrasonic distance sensors for measuring directional wave spectra, a COTS wave radar system, and a COTS scanning LIDAR system. While not intending to be exhaustive, this paper seeks to highlight the insights gained from the recent applications of these techniques, as well as the difficulties and issues associated with shipboard measurements such as ship motion and logistical constraints.

Measurement and Modeling of the Motions of a High-Speed Catamaran in Waves

Wetdeck slamming can be defined as a large vertical acceleration event that occurs when ship motions cause an impact between the cross deck and the ocean’s surface. The use of Computational Fluid Dynamics (CFD) and other simulation tools to accurately predict wetdeck slamming loads and ship motions has become the objective of a number of efforts (Hess, et al, 2007; Lin, et al, 2007; Faller et al, 2008; for example). The Sea Fighter, FSF-1, is a high-speed research vessel developed by the U.S. Office of Naval Research (ONR). Christened in 2005, she is an aluminum catamaran propelled by four steerable water jets capable of speeds up to 50 knots. In 2006, Sea Fighter underwent a series of rough water trials to assess its operational profile in high sea states (Fu, et. al., 2007). Along with this assessment, ONR sponsored an effort to obtain full-scale qualitative and quantitative wave slamming and ship motion data. One of these rough water trials took place 18–20 April 2006 as the ship transited from Esquimalt, British Columbia, Canada to San Diego, California, USA. During this trial, the significant wave height ranged from 1.5 to 2.7 m and the ship speed ranged from 20 to 40 knots. This paper describes the results of the effort to characterize the Sea Fighter’s motion in waves. To provide suitable full-scale validation data, the incoming ambient waves had to be characterized. A Light Detecting and Ranging, (LiDAR) system, an array of ultrasonic distance sensors, and several video cameras were used to characterize the incoming wave field. In addition, three fiber optic gyro motion units were deployed to record ship motions. Additionally, a GPS unit was utilized to measure ship speed, pitch, roll, and heading. Several slam and near slam events are discussed over the range of ship’s speed, heading, and sea states tested. Similarities and differences between these events are also noted. Additionally, this data was used to develop a simulation of the Sea Fighter’s motion in waves similar to previous work done utilizing model test data (Hess, et al, 2007; Faller et al, 2008).Copyright

Comparison of Incoherent and Coherent Wave Field Measurements Using Dual-Polarized Pulse-Doppler X-Band Radar

Radar-based remote sensing for measurement of ocean surface waves presents advantages over conventional point sensors such as wave buoys. As its use becomes more widespread, it is important to understand the sensitivity of the extracted wave parameters to the characteristics of the radar and the scatterers. To examine such issues, experiments were performed offshore of the Scripps Institution of Oceanography pier in July 2010. Radar measurements in low wind speeds were performed with a dual-polarized high-resolution X-band pulse-Doppler radar at low grazing angles along with two independent measurements of the surface waves using conventional sensors, a GPS-based buoy, and an ultrasonic array. Comparison between radar cross section (RCS) and Doppler modulations shows peak values occurring nearly in-phase, in contrast with tilt modulation theory. Spectral comparisons between Doppler-based and RCS-based spectra show that Doppler-based spectra demonstrate greater sensitivity to swell-induced modulations, whereas RCS-based spectra show greater sensitivity to small-scale modulations (or generally have more noise at high frequency), and they equally capture energy at the wind wave peak. Doppler estimates of peak period were consistent with the conventional sensors, whereas the RCS differed in assignment of peak period to wind seas rather than swell in a couple of cases. Higher order period statistics of both RCS and Doppler were consistent with the conventional sensors. Radar-based significant wave heights are lower than buoy-based values and contain nontrivial variability of ~33%. Comparisons between HH and VV polarization data show that VV data more accurately represent the wave field, particularly as the wind speeds decrease.

Distribution of Wave Impact Forces From Breaking and Non-Breaking Waves

Impact loads from waves on vessels and coastal structures are highly complex and may involve wave breaking, making these changes difficult to estimate numerically or empirically. Results from previous experiments have shown a wide range of forces and pressures measured from breaking and non-breaking waves, with no clear trend between wave characteristics and the localized forces and pressures that they generate. In 2008, a canonical breaking wave impact data set was obtained at the Naval Surface Warfare Center, Carderock Division, by measuring the distribution of impact pressures of incident non-breaking and breaking waves on one face of a cube. The effects of wave height, wavelength, face orientation, face angle, and submergence depth were investigated. A limited number of runs were made at low forward speeds, ranging from about 0.5 to 2 knots (0.26 to 1.03 m/s). The measurement cube was outfitted with a removable instrumented plate measuring 1 ft2 (0.09 m2 ), and the wave heights tested ranged from 8–14 inches (20.3 to 35.6 cm). The instrumented plate had 9 slam panels of varying sizes made from polyvinyl chloride (PVC) and 11 pressure gages; this data was collected at 5 kHz to capture the dynamic response of the gages and panels and fully resolve the shapes of the impacts. A Kistler gage was used to measure the total force averaged over the cube face. A bottom mounted acoustic Doppler current profiler (ADCP) was used to obtain measurements of velocity through the water column to provide incoming velocity boundary conditions. A Light Detecting and Ranging (LiDAR) system was also used above the basin to obtain a surface mapping of the free surface over a distance of approximately 15 feet (4.6 m). Additional point measurements of the free surface were made using acoustic distance sensors. Standard and high-speed video cameras were used to capture a qualitative assessment of the impacts. Impact loads on the plate tend to increase with wave height, as well as with plate inclination toward incoming waves. Further trends of the pressures and forces with wave characteristics, cube orientation, draft and face angle are investigated and presented in this paper, and are also compared with previous test results.

Real-Time Simulation Based Design Part II: Changes in Hull Geometry

As a second step, and a critical milestone, in the development of a new simulation based design (SBD) tool based on recursive neural network (RNN) technologies the capability to change hull geometries and compute in real-time the new vehicle dynamics has been tested. Previously, this approach was successfully demonstrated for design changes on the ONR Body 1 submarine appendages (sternplane and rudder). The advantages of this RNN based tool are that simulation based design can be performed in a real-time nonlinear simulation (RNS) environment, and this approach enables the fusion of experimental data, when available, with steady Reynolds Averaged Navier-Stokes (RANS) solutions. Building upon the previous work done using RNNs to support submarine simulation based design, the focus of this paper is on the extension of these RNN based approaches, and in particular for the design and modification of hull shape and size, including the computation of non-symmetric hull shapes. As in the previous SBD work on the appendages, a parent training data set for a particular vehicle was used to train an RNN to model how input forces and moments lead to particular output motions. Design changes to the vehicle were then implemented by changing the input force and moment database. As previously shown, appendages can be changed directly by specifying a new geometry and/or lift coefficient. As described herein, the hull geometry can be modified either through the use of other empirical data and/or through the use of steady RANS solutions. The new force and moment database for the hull and the new geometry are used as the input into the RNS based design code to determine the design change impact on vehicle maneuvering. Since only the input force and moment database is changed, no re-training of the RNN is required. As such, the new design simulations can be made in real-time, and the design cycle can, in theory, be shortened significantly. With the results for the hull geometry changes the full utility of this approach can now be defined, and bounds placed on the use of RNN based SBD approaches. This approach resolves the main limitation in RNN technology, namely that it was difficult or impossible to design vehicles using this technology. Now, RNNs can be used not only for vehicle design, but also to determine the result of the design changes in real-time.

Simulation Based Design: A Real-Time Approach Using Recursive Neural Networks

A new simulation based design (SBD) tool has been developed based on recursive neural network (RNN) technologies. This approach permits simulation based design to be performed in a real-time nonlinear simulation (RNS) environment. Further, this approach enables the fusion of experimental data, when available, with steady Reynolds Averaged Navier-Stokes (RANS) solutions. Building upon the extensive work done using RNNs to support Navy submarine simulation and control, a second-generation RNN simulation code has been developed to support submarine simulation based design. Previously, the 1 st generation RNN maneuvering simulation tools were used for the prediction of blind submarine maneuvers in the ONR sponsored Maneuvering Simulation Challenge. A blind maneuver was one for which only the initial conditions and the controls directing the vehicle were provided to the participants. Inputs to the simulation were the controls acting on the vehicle such as propeller rotation speed, rudder and sternplane deflection time histories and the initial conditions. The outputs were time histories of the submarine state variables, the three linear and three angular velocity components. These output data were integrated to recover trajectory and attitude, and differentiated to determine the accelerations acting on the vehicle. Overall, the RNN simulations performed better than any of the other simulation tools including other empirical methods, potential codes and/or vortex tracking methods and unsteady RANS simulations. The focus of this paper is on the extension of these RNN based approaches for use as geometry-to-motion simulation tools. Specifically, a parent training data set for a particular vehicle is used to train an RNN to model how input forces and moments lead to particular output motions. Design changes to the vehicle can then be implemented by changing the input force and moment database. Appendages can be changed directly by specifying a new geometry and/or lift coefficient. The hull geometry can be modified either through the use of other empirical data and/or through the use of steady RANS solutions. The new force and moment database and the new geometry are then used as the input into the RNS based design code to determine the design change impact on vehicle maneuvering. Since only the input force and moment database is changed, no re-training of the RNN is required. As such, the new design simulation can be made in real-time, and the design cycle can, in theory, be shortened significantly. To date, this approach has been used to successfully demonstrate the maneuvering impact of design changes on the ONR Body 1 submarine appendages (sternplane and rudder). This approach resolves the main limitation in RNN technology, namely that it was difficult or impossible to design vehicles using this technology. Now, RNNs can not only be used for vehicle design, but the result of the design changes may be determined in real-time.

A detailed assessment of numerical flow analysis (NFA) to predict the hydrodynamics of a deep-V planing hull

Over the past few years much progress has been made in Computational Fluid Dynamics (CFD) in its ability to accurately simulate the hydrodynamics associated with a deep-V monohull planing craft. This work has focused on not only predicting the hydrodynamic forces and moments, but also the complex multiphase free-surface flow field generated by a deep-V monohull planing boat at high Froude numbers. One of these state of the art CFD codes is Numerical Flow Analysis (NFA). NFA provides turnkey capabilities to model breaking waves around a ship, including both plunging and spilling breaking waves, the formation of spray and the entrainment of air. NFA uses a Cartesian-grid formulation with immersed body and volume-of-fluid methods. The focus of this paper is to describe and document a recent effort to assess NFA for the prediction of deep-V planing craft hydrodynamic forces and moments and evaluate how well it models the complex multiphase flows associated with high Froude number flows, specifically the formation of the spray sheet. This detailed validation effort was composed of three parts. The first part focused on assessing NFAs ability to predict pressures on the surface of a 10 degree deadrise wedge during impact with an undisturbed free surface. Detailed comparisons to pressure gauges are presented here for two different drop heights, 15.24 cm (6 in) and 25.4 cm (10 in). Results show NFA accurately predicted pressures during the slamming event. The second part examines NFAs ability to match sinkage, trim and resistance from Fridsmas experiments performed on constant deadrise planing hulls. Simulations were performed on two 20 degree deadrise hullforms of varying length to beam ratios (4 and 5) over a range of speed-length ratios (2, 3, 4, 5 and 6). Results show good agreement with experimentally measured values, as well as values calculated using Savitskys parametric equations. The final part of the validation study focused on assessing how well NFA was able to accurately model the complex multiphase flow associated with high Froude number flows, specifically the formation of the spray sheet. NFA simulations of a planing hull fixed at various angles of roll (0, 10, 20 and 30 degrees) were compared to experiments. Comparisons to underwater photographs illustrate NFAs ability to model the formation of the spray sheet and the free surface turbulence associated with planing boat hydrodynamics. Overall these three validation studies provide a detailed assessment on the current capabilities of NFA to predict the hydrodynamics of a deep-V planing hull.