C. Chryssostomidis

A robust and accurate outflow boundary condition for incompressible flow simulations on severely-truncated unbounded domains

We present a robust and accurate outflow boundary condition and an associated numerical algorithm for incompressible flow simulations on unbounded physical domains, aiming at maximizing the domain truncation without adversely affecting the flow physics. The proposed outflow boundary condition allows for the influx of kinetic energy into the domain through the outflow boundaries, and prevents un-controlled growth in the energy of the domain in such situations. The numerical algorithm for the outflow boundary condition is developed on top of a rotational velocity-correction type strategy to de-couple the pressure and velocity computations, and a special construction in the algorithmic formulation prevents the numerical locking at the outflow boundaries for time-dependent problems. Extensive numerical tests for flow problems with bounded and semi-bounded physical domains demonstrate that this outflow boundary condition and the numerical algorithm produce stable and accurate simulations on even severely truncated computational domains, where strong vortices may be present at or exit the outflow boundaries. The method developed herein has the potential to significantly expedite simulations of incompressible flows involving outflow or open boundaries, and to enable such simulations at Reynolds numbers significantly higher than the state of the art.

Optimization of a z-source DC circuit breaker

DC faults may cause severe disruptions in continuity of service to vital loads in a shipboard integrated power system, hence detection, isolation, and protection against such faults must be incorporated in both medium-voltage DC (MVDC) and low-voltage DC (LVDC) systems. Here we consider the effectiveness of existing z-source breakers and propose several new designs more appropriate for fault detection in MVDC and LVDC systems. In particular, we perform an optimization study that aims to minimize dissipation and weight and we identify the key parameters for use in MVDC and LVDC systems. Preliminary verification and validation studies are also included.

A NEW PARADIGM FOR SHIP HULL INSPECTION USING A HOLONOMIC HOVER-CAPABLE AUV

The MIT Sea Grant AUV Lab, in association with Bluefin Robotics Corporation, has undertaken the task of designing a new autonomous underwater vehicle, a holonomic hover-capable robot capable of performing missions where an inspection capability similar to that of a remotely operated vehicle is the primary goal. One of the primary issues in this mode of operating AUVs is how the robot perceives its environment and thus navigates. The predominant methods for navigating in close proximity to large iron structures, which precludes accurate compass measurements, require the AUV to receive position information updates from an outside source, typically an acoustic LBL or USBL system. The new paradigm we present in this paper divorces the navigation routine from any absolute reference frame; motions are referenced directly to the hull. We argue that this technique offers some substantial benefits over the conventional approaches, and will present the current status of our project.

Notional all-electric ship systems integration thermal simulation and visualization

This work presents a simplified mathematical model for fast visualization and thermal simulation of complex and integrated energy systems that is capable of providing quick responses during system design. The tool allows for the determination of the resulting whole system temperature and relative humidity distribution. For that, the simplified physical model combines principles of classical thermodynamics and heat transfer, resulting in a system of three-dimensional (3D) differential equations that are discretized in space using a 3D cell-centered finite volume scheme. As an example of a complex and integrated system analysis, 3D simulations are performed in order to determine the temperature and relative humidity distributions inside an all-electric ship for a baseline medium voltage direct current power system architecture, under different operating conditions. A relatively coarse mesh was used (9410 volume elements) to obtain converged results for a large computational domain (185m×24m×34m) containing diverse equipment. The largest computational time required for obtaining results was 560 s, that is, less than 10 min. Therefore, after experimental validation for a particular system, it is reasonable to state that the model could be used as an efficient tool for complex and integrated systems thermal design, control and optimization.

Design of an Inspection Class Autonomous Underwater Vehicle

Autonomous Underwater Vehicles (AUVs) have become ever more common in ocean science, military, and industrial applications. In particular, AUVs are becoming a significant option for undersea search and survey. Bottom following, tight turning radius, stability, and elimination of tow cables make AUVs appealing in this role. Recent commercial success has proven that AUVs can be competitive survey platforms. While AUVs that perform surveys have become more common and capable, there are few designed for close inspection tasks. These missions call for an AUV that can move slowly, on the order of 10-15 cm/sec, maneuver equally well in all three dimensions, and maintain a very stable orientation over the seafloor, usually for imaging and production of photo mosaics. The range and endurance requirements of an inspection class AUV are expected to be small compared to an AUV designed to survey large areas. The MIT AUV Lab is actively investigating the design of a new type of underwater vehicle, known as an Inspection Class AUV, for missions such as marine archaeology and fisheries habitat studies. Two new designs have been created, both of which are radical evolutions of existing Odyssey class vehicles. One design takes the proven subcomponents of the Odyssey II, namely the pressure spheres, sensors and actuators, and transfers them to an entirely new mechanical framework. The other design capitalizes on the modular nature of the Odyssey III vehicles and adds new modules that allow an Odyssey III to perform inspection tasks. This paper discusses the details of these new designs, their anticipated performance specifications and lessons learned from deployment of conventional Odyssey vehicles for inspections tasks.

Mathematical formulation and demonstration of a dynamic system-level ship thermal management tool

Abstract This paper presents the mathematical formulation and unique capability of a system-level ship thermal management tool, vemESRDC, developed to provide quick ship thermal responses in early design stages. The physical model combines principles of classical thermodynamics and heat transfer, along with appropriate empirical correlations to simplify the model and expedite the computations. As a result, the tool is capable of simulating dynamic thermal response of an entire ship, characterized by intricate thermal interactions within a complex ship structure, within an acceptable time frame. In this work, vemESRDC is demonstrated through three case studies in which transient thermal responses of an all-electric ship to different ship operation modes, weather conditions, and partial loss of cooling are investigated. The analysis examines particularly the following: (1) the required cooling capacities to maintain each ship component within its design limit; (2) equipment temperature variations with respect to partial cooling loss in battle mode; and (3) the assets of installing seawater heat exchangers to pre-cool deionized freshwater before chillers. For the notional all-electric ship conceived and assessed in this work, the results verify the capability of vemESRDC to capture dynamic thermal interactions between shipboard equipment and their respective surroundings and cooling systems, e.g., the tool provides practical insights into pulse load cooling strategy, and different solutions are obtained for distinct weather conditions. In addition to the case studies performed in this work, vemESRDC can be employed to conduct diverse studies based on which concrete ship thermal management strategies can be formulated in early design stages.

Modular IPS machinery arrangement in early-stage naval ship design

Electrical power demands for naval surface combatants are projected to rise with the development of increasingly complex and power intensive combat systems. This trend coincides with the need to achieve maximum fuel efficiency at both high and low hull speeds. A proposed solution to meet current and future energy needs of conventionally powered naval surface combatants is through the use of an Integrated Power System (IPS), which is seen as the next evolution in naval ship design. In an effort to enhance the relationship between new-concept designs and historically-based ship design processes, this paper focuses on a novel approach of incorporating IPS at the earliest stage of the design process as part of assessing system-level tradeoffs early within the ship design process. This paper describes a methodology for the systematic design and arrangement of an IPS machinery plant to meet a desired power generation level. In conjunction with the methodology development, a hierarchical process and design tool were produced to assist in rapid development and evaluation of various IPS arrangements. The result of this process, through several case studies, provides insight into equipment selection philosophy, the initial sizing of the ships machinery box, and the initial definition of electrical zones.

Graph theory applications in FOCUS-compliant ship designs

Recent developments in the storage of system data in the Navys data repository, LEAPS, using the FOCUS product meta-model have opened the doors to graph-theory applications in the design of Navy ship systems in the early stages of design. In this paper, we demonstrate the ability to extract graphs from ship data and present pertinent applications of such graphs including a vulnerability metric for early-stage design, an equipment-sizing algorithm for automated system design, and a network design process that includes vulnerability assessment with preliminary ship arrangements.

Fisheries habitat survey with a small low cost AUV

Summary form only given. The AUV Xanthos was deployed in spring 2003 to demonstrate how the capabilities of autonomous underwater vehicles may be applied to the preservation and study of fisheries resources. Xanthos carries a low-light camera, LED illumination, and side scan sonar. The vehicles small size and robustness permit the use of a vessel of opportunity (a fishing trawler) to examine fisheries habitat. Surveys were conducted in protected zones and heavily fished areas, and the results compared.

A Wearable Vehicle Interface for augmented reality in operating AUVs

Running an operation with an autonomous underwater vehicle (AUV) presently requires that the pilots situate themselves, primarily in a sedentary position for extended periods of time, in front of a computer that is in communication with the vehicle. Such an operations workstation is likely to be a networked PC that cannot be exposed to the elements, but has access to all of the software necessary for data reduction and visualization. At the MIT AUV Lab we have extensive experience with this mode of operation. In order to overcome the deficiencies in the current paradigm, we have developed a lightweight computing system that is worn by members of the AUV deployment team to investigate the human factors issues associated with immersing a user in a computing environment that is being used to control an underwater robot. The wearable vehicle interface (WeVI) is intended for use in the marine environment and fits over a US Coast Guard approved personal flotation device. The WeVI provides for connectivity to the AUV as well as other WeVI suits via Radio Frequency and 802.11 b wireless standards. The operators point-of-view and GPS location may be shared among WeVI suits to promote situational awareness for the entire operations team. Mission control (planning, scripting, and launching), and visualizing datasets is performed through the graphical user interface that is displayed in a users head-mounted display. Research into particular attributes of the augmented reality provided by such a suit is ongoing and presented in this paper.