VOLUME 3, JULY 2010

Shahril Rizal Hassan
Research Officer, Postgraduate Student (MSc.)
B.Eng in Mechatronic Engineering (USM)
Project Title: Design and Development of an Autonomous Surface Vehicle.

COMPUTATIONAL FLUID DYNAMIC (CFD) FOR USVs HULL ANALYSIS

Computational fluid dynamics (CFD) is one of the branches of fluid mechanics that uses numerical methods and algorithms to solve and analyze problems that involve fluid flows. Computers are used to perform the millions of calculations required to simulate the interaction of liquids and gases with surfaces defined by boundary conditions. Even with high-speed supercomputers only approximate solutions can be achieved in many cases. Ongoing research, however, may yield software that improves the accuracy and speed of complex simulation scenarios such as transonic or turbulent flows. Initial validation of such software is often performed using a wind tunnel with the final validation coming in flight test [1].

Computational Fluid Dynamic (CFD) has played an important role as a tool to help the engineer and designer to achieve a better understanding of physic aspects of fluid flow and to visualizing the flow in the area that difficult to adopt using other measuring technique [2]. The design or selection of USVs hull is differ from any other large engineering structure because it must be design to move effectively through the water with minimal of external resistance. The hull form that optimum in calm water is not necessarily perform well in rough sea water condition. Therefore, the CFD analysis is needed to analyze the performance of USVs hull in calm and rough condition.

The objective of this article is to give an overview of modeling technique for Unmanned Surface vessel (USVs) hull hydrodynamics and their interaction with wave while in maneuvering.

The important challenge for numerical method in general is to gain the confident in the simulation results, which required verification and validation studies [3]. The 3D sketch of USVs hull used in simulation is show in figure 1. For this article the flow of water is study in positive x-direction while moving forward and positive y-direction when the water is toward the starboard side of the hull is neglect.

From the CFD analysis on the hull above, the drag force on the hull is measuring to know the minimum force need to maneuver the USVs. The drag force for USVs hull can be obtaining using:

Where the drag force is measured in N (Newton) and is the drag coefficient measured in N/m. Table 1 shows the drag force result obtain from CDF analysis.

Table 1: Drag force analysis

Drag force calculation using computer fluid dynamics were performed based on a CAD model for vessel hull. The hull is sketch using SOLIDWORK software and exported to GAMBIT that used for mesh generation and to prescribe boundary condition. Finally the FLUENT software is used for perform the analysis.

Based on table 1, the value of drag force is presented at different wave velocity. The drag force on hull is increased when velocity increased. Based on the result, the minimum thrust produced by propeller need to maneuvering the USVs can be determined for different wave velocity. Therefore, the power consuming by the USVs can be minimize and user can obtain how longer operation time available for vehicle at different condition. Besides that, result from this analysis can give advantage on designing the control algorithm for USVs and also useful in process to developed the System Identification Process.

This article only addresses some of the most recent results on numerical fluid dynamic modeling obtained by using CFD.

Reference:

[1]http://en.wikipedia.org/wiki/Computational_fluid_dynamics

[2] C. L. R. Siqueira, N. Spogis, R. Damian, M. Reis “OVERVIEW OF CFD MODELLING FOR SHIP HULL INTERACTIONS WITH WAVES AND PROPELLERS” in 3th International workshop on Applied offshore hydrodynamics.

[3] M. Salas, R. Luco, P. K Sahoo, N. Browne and M. López “Experimental and CFD Resistance Calculation of a Small Fast Catamaran” in Proc. of High Performance Yacht Design Conference

VOLUME 3, MAY 2010

Muzammer Zakaria

Research Officer, Postgraduate Student (MSc.)
B.Eng in Mechatronic Engineering (USM)

New Sensor Detects Direction of Sound Underwater

Using optical fibers, researchers at the Georgia Institute of Technology have created a sensor that detects the direction from which a sound is coming under water—an important improvement over current technology. Credit: Georgia Institute of Technology.

A new sensor that measures the motion created by sound waves under water could allow the U.S. Navy to develop compact arrays to detect the presence of enemy submarines. These new arrays would detect quiet underwater targets, while also providing unambiguous directional information.

Using optical fibers, researchers at the Georgia Institute of Technology have found a way to create a sensor that detects the direction from which a sound is coming under water. This directional component is an important improvement over the current technology, researchers said.

“Detecting quiet sounds under water can be very difficult,” said Francois Guillot, a research engineer in Georgia Tech’s George W. Woodruff School of Mechanical Engineering. “But our sensor detects small sounds over the noise of the ocean and also provides directional information.”

The sensor uses a mechanism inspired by how fish hear under water. Inside a fish’s ear, there are thousands of tiny hairs that move when a sound wave passes through the fish. These hairs then communicate with nerves allowing fish to hear under water. Because fish excel at detecting sound so they don’t get eaten, the Georgia Tech researchers chose the fish hearing system as their model, they said.

Guillot described the novel underwater sensor late last fall at the 4th joint meeting of the Acoustical Society of America and the Acoustical Society of Japan in Honolulu, Hawaii. His presentation was part of a session titled “Underwater Acoustics: Array Processing, Sensors, and Technology.”

In the field of underwater acoustics, there is always a need to develop more sophisticated sensors, researchers said. The Navy currently tows long lines of hydrophones to listen to sound under water—much like a microphone listens to sound in the air. A hydrophone measures the pressure change associated with the propagation of a sound wave. It converts acoustic energy into electrical energy and is used in passive underwater systems to listen only. One hydrophone identifies a sound nearby, and multiple hydrophones can help tell the direction from which it’s coming. But directional ambiguity exists. A line array of hydrophones cannot tell if the sound is coming from the left or right.

Guillot and collaborators David Trivett, a principal research scientist, and Peter Rogers, a professor—both in the School of Mechanical Engineering—have developed a more compact, more sensitive sound detector that can provide unambiguous directional information. In addition, the sensor can be modified to measure the water deformation, known as shear, associated with a sound wave— a quantity typically difficult to measure because it requires very sensitive instruments. This new sensor shows promise that it can be successfully modified to detect this acoustic shear, which will enhance the directional information, the researchers said.

The sensor is designed with two small plates attached by a hinge. One plate is held rigidly, and the other plate—made of a composite material with the same density as water—is free to move. The freely moving plate shifts in the sound field and follows the motion of water. A light signal sent through an optical fiber glued to both plates is modified by the motion of the freely moving plate. Analyzing the light signal with a photodetector provides information relative to the sound waves.

The sensor developed at Georgia Tech offers advantages over existing systems, researchers said. Guillot hopes the new sensor changes the way the Navy detects sound under water.“If the Navy tows an array of hydrophones thousands of feet long, it makes it difficult to maneuver the ship,” Rogers said. “Since we can cut that length by a factor of more than five, it will cost less money to operate and be easier to handle.”

The current prototype sensor has been tested in the School of Mechanical Engineering’s large underwater acoustic tank facility to observe the behavior of the sensor under water. The facility includes a rectangular concrete water tank 25 feet deep, 25 feet wide and 34 feet long; it contains about 160,000 gallons of water. The researchers hope to field test the prototype system soon to see if it outperforms current technology.

Source: Georgia Institute of Technology

VOLUME 3, JUNE 2010

Zulkifli Zainal Abidin
Postgraduate Student(Phd)
B.Eng Computer and Information Engineering (International Islamic University Malaysia)
Project title: A Novel Fly Optimization Algorithm: Animal-inspired Metaheuristic

DROSOBOTS: Swarm of mini ASVs with Animal-Inspired Algorithm for Marine Applications

The study of animal behavior in their natural habitat has led to Animal-Inspired Metaheuristic Algorithms which may be adopted to be used in the development of Autonomous Surface Vehicles (ASVs). In this study, a new concept of an ASV is proposed, mimicking the predictable biological balancing nature of the Drosophila or the fruit fly.

Due to the factors of complexity in instrument deployment, expensive tools for marine applications, time and resources consuming for data collections, swarming concept is proposed by using mini ASVs namely Drosobots. The factors influencing the conceptual model, the choice of shape, the parameters influencing the control in the design, the practicalities encountered in navigational issues and mechanisms of communication amongst a group of these ASVs based upon the Drosophila’s optimal swarming movements are also investigated.

Even though the accuracy of the GPS is about 3 meters, compass sensor ±0.5°, and depth transducer ±15cm only, the trial results show that there is potential for the ASVs to be further developed for real applications such as the multi-agents usage for sea-bed mapping, mine-sweeping, environmental and oceanographic measurements, port security, oil spill tracking and search and rescue mission.

Click image to watch the slide.