Chryssostomos Chryssostomidis

On the formation of vortex streets behind stationary cylinders

The formation of vortex streets behind stationary cylinders is found to be caused by an absolute instability in the wake immediately behind the cylinder. The inviscid Orr–Sommerfeld equation is used together with measured profiles at Reynolds numbers of ( a ) Re = 56 when the absolute instability provides a Strouhal number of 0.13; and ( b ) Re = 140000 providing a Strouhal number of 0.21, both in agreement with experimental values. At the subcritical Re = 34 the instability is of the convective type; i.e. the disturbance decays, being convected away once the external disturbance is removed, in agreement with experimental observations. Finally, the instability of the mode which causes a symmetric array of vortices is shown to be always of the convective type.

A second generation survey AUV

Odyssey class autonomous underwater vehicles (AUVs) are designed to be small, high performance survey platforms. The logistical complexities of operating off of oceanographic vessels or in hostile environments, such as the Arctic, make a small vehicle with minimal support requirements extremely attractive. Although built for great depths and endurances of up to two days, Odyssey class vehicles are small by the standards of existing AUVs. This paper describes Odyssey II, the second generation of Odyssey class AUV, and presents the results of under-ice field trials in New Hampshire and the Arctic.

AUV guidance with chemical signals

A great variety of marine animals navigate using chemical signals to locate food sources, mates and spawning grounds. The sensory systems and behaviors exhibited by these animals are well adapted for robust operations in turbulent and chemically complex environments. The algorithms used by an animal to rapidly extract guidance information from a discontinuous and complex chemical signal are presently unknown and are the topic of current research. We have developed a small benthic robot that is being used as an algorithm test bed for ideas in biological chemosensing behavior. The robot is designed to mimic the basic features of a lobster relevant to chemical sensing and associated behaviors. We present the rational for this research, the design of the vehicle and initial results.

System identification of open-loop maneuvers leads to improved AUV flight performance

The authors focus on demonstrating a simple design procedure for the Odyssey III autonomous underwater vehicle (AUV) flight control system. This procedure can be carried out quickly and routinely to maximize vehicle effectiveness. A hydrodynamic model of the vehicle was first developed from theory and bench-top laboratory tests. Using this initial model, a controller was developed from basic principles. Then, using this initial controller to reach a desired typical operating condition, a very compact set of open-loop maneuvers was performed in the field. The vehicle model was optimized using the Nelder-Mead simplex method, and a revised controller was then implemented and tested successfully

Analysis of various all-electric-ship electrical distribution system topologies

As advances in technology mature, the need is evident for a coherent simulation of the total electric-drive ship to model the effect of new systems on the overall performance of the vessel. Our laboratory has been developing an integrated architectural model in a physics-based environment which analyzes ship variants using a standard set of metrics, including weight, volume, fuel usage and survivability. This paper discusses advances in the model including the use of operational scenarios, incorporation of a survivability metric, and streamlining the performance of model. The model is employed herein to compare two possible distribution system topologies: a ring bus and a breaker-and-a-half. The ring bus is heavier and larger but more survivable. Fuel usage is equivalent in the two variants.

Curved shape reconstruction using multiple hypothesis tracking

Panoramic sweeps produced by a scanning range sensor often defy interpretation using conventional line-of-sight models, particularly when the environment contains curved, specularly reflective surfaces. Combining multiple scans from different vantage points provides geometric constraints necessary to solve this problem, but not without introducing new difficulties. Existing multiple scan implementations, for the most part, ignore the data correspondence issue. The multiple hypothesis tracking (MHT) algorithm explicitly deals with data correspondence. Given canonical observations extracted from raw scans, the MHT applies multiple behavior models to explain their evolution from one scan to the next. This technique identifies different topological features in the world to which it assigns the corresponding measurements. We apply the algorithm to real sonar scans generated specifically for this investigation. The experiments consist of interrogating a variety of two-dimensional prismatic objects, standing on end in a 1.2-m-deep freshwater tank, from multiple vantage points using a 1.25 MHz profiling sonar system. The results reflect the validity of the algorithm under the initial assumptions and its gradual performance degradation when these assumptions fail to characterize the environment adequately. We close with recommendations that detail extending the approach to handle more natural underwater settings.

Space reservation for shipboard electric power distribution systems

The constant demands for increased power in all modern navy ships places ever greater need for attention to efficient and well positioned power cabling. The focus of this paper is the characterization of the power distribution system of an all-electric modern combatant. The ensuing mathematical model is suitable for use in the very early stages of design. This includes the use of Set-Based Design during the concept phase of design. In this version of the paper we describe the method we developed to reserve the necessary space to accommodate the shipboard electric power distribution system. Future versions of this paper will include how to compute such metrics as system weight, and total area and volume in each compartment and a vulnerability index to be used to determine a figure of merit to compare different power distribution designs.

Adding simulation capability to early-stage ship design

The Navys early-stage ship design tools do not currently include an inherent simulation capability. Under Navy direction, the Electric Ship Research and Development Consortium (ESRDC) has worked to develop a simulation tool that can be used to determine functionality of ship systems at the early stages of design. This paper describes the current capabilities of the simulation tool and the process and status of the efforts to integrate this tool with the Navys design tools.

Design of a notional ship for use in the development of early-stage design tools

In this paper, current early-stage design tools are used to produce a notional ship that includes leading-edge weapons and sensors. These new systems stress the capabilities of current design tools and demonstrate the need for tools that can address the increasingly integrated, powerful and heat-producing nature of future payloads. The data produced in this process are shown to be the required input to new design tools under development, thus establishing the link between the existing state of the art and tools that provide more advanced capability necessitated by advances in ship system technology. A framework for a semi-automated template-based system arrangement tool is then presented.

System-level analysis of chilled water systems aboard naval ships

A thermal management simulation tool is required to rapidly and accurately evaluate and mitigate the adverse effects of increased heat loads in the initial stages of design in all-electric ships. By reducing the dimension of Navier-Stokes and energy equations, we have developed one-dimensional partial differential equation models that simulate time-dependent hydrodynamics and heat transport in a piping network system. Besides the steady-state response, the computational model enables us to predict the transient behavior of the cooling system when the operating conditions are time-variant. As a demonstration case, we have performed a thermal analysis on a realistic naval ship.