Vishwanath Nagarajan

Uncertainty assessment for ship maneuvering mathematical model

An 8 degrees of freedom (DoF) ship maneuvering motion model for a twin-propeller twin-rudder high-speed hull form is developed from captive model experiment data available from literatures. The degrees of freedom considered are surge, sway, yaw, roll, rudder rate and propeller rate. Besides maneuvering motion model, variation of port/starboard propeller thrust and torque and port/starboard rudder normal force and rudder torque are also included in the model. An uncertainty analysis computation for the mathematical model is carried out. Uncertainties in the experimental data and the polynomial curve fitting during modeling are included in the computation. It is shown that the mathematical model uncertainty is higher than the experimental uncertainty. Uncertainty is propagated to full-scale zigzag maneuver using the conventional Monte Carlo simulation (MCS) method. The uncertainty analysis results will be useful for further improvement of mathematical model, validation of CFD simulation results of appended hull maneuvering tests, etc. The authors have also shown the utility of asymmetric operations of the twin-propeller and twin-rudder by carrying out full-scale simulation a zigzag maneuver and showing the variation of the rudder normal force and torque.

A Stochastic Response Surface Approach for Uncertainty Propagation in Ship Maneuvering

The uncertainty of various coefficients of 8 degrees of freedom (DoF) twin-propeller twin-rudder (TPTR) maneuvering mathematical model has been determined. A stochastic response surface method (SRSM) is developed for propagating these uncertainties to the full scale simulations of ship maneuvers. The proposed SRSM uses Hermite polynomial chaos (PC) expansion of standard random variables (SRVs) for analysis. Here SRVs are Gaussian variables. The SRSM approximates all model inputs and outputs as a function of SRVs and develops an approximate surrogate model for a specific output. The SRSM treats any deterministic model as a “black-box”. SRSM requires less computational time as compared to standard Monte Carlo Simulation (MCS) method. Full scale simulations of a TPTR model installed with gas turbine propulsion at a cruising speed of 30 knots have been carried out. There are 60 uncertain inputs and 14 uncertain outputs present in the model. A linear sensitivity study has been carried out to select a set of most sensitive inputs for each one of the different outputs. Only the sensitive inputs are considered for computing the output distribution. Asymmetric behavior and uncertainty in the characteristics of twin-propeller twin-rudder system are significant. Uncertainty for overshoot angle, advance, tactical diameter, propeller revolution and thrust, engine torque, rudder normal force and torque are presented.

Observation on Forces and Motions of a Mariculture Cage From Model and Prototype Experiments

In India, several experimental coastal aquaculture cages have been installed on sites along the Indian coast. Many of them have been commercially successful, while some have faced technical difficulties. The construction of the Indian coastal aquaculture cage is based on ad hoc designs adopted from other countries. The performance of the coastal aquaculture cage shows that there is a need for development of a cage design with a location-specific mooring system to withstand local wave conditions for a longer period in different coastal zones of the country. This requires that designs of the cages suiting Indian conditions be made based on sound engineering principles. To design the cage, the current- and wave-induced tensions in the mooring chain and net twine and the cage motions need to be estimated. In this study, a model cage was fabricated and tested in a towing tank under different waves and towing conditions. The tension on the mooring chain was measured during model experiments along with the towing speed and wave parameter. A prototype coastal aquaculture cage with single point mooring near the coast of Visakhapatnam in South East India was identified for measurement of forces and motions. On the prototype cage, measurements of cage motions, tensions on mooring chain, and net twine and cage orientation were carried out. Characteristics of the cage response, observed in model and prototype experiments, are presented in this paper. The model cage is not a geometrically scaled down version of the prototype cage. The diameter/depth ratios of the model and the prototype are different. The mooring pattern and net twine of the model cage is the same as that of the prototype. The data presented in this paper could be used as a reference for validating the numerical model for simulating the cage forces and motions under wave and current loads.

Analysis of Sloshing in Tanks Using Image Processing

Sloshing is referred to as the violent movement of liquid in a partially filled tank that undergoes dynamic motion. There are several examples of such types of motions. In ships, sloshing motion occur in oil tankers, liquefied natural gas carriers and large fuel oil tanks. In case of rockets, it happens in the liquid hydrogen (LOH) and liquid oxygen (LOX) containing external fuel tanks. The sloshing motion is mainly due to large dimensions of tanks with smooth plane surfaces in contact with the liquid. The tank layout fails to damp the sloshing motion of the liquid. The sloshing motion becomes more violent when the parent vehicle’s motion contains energy in the vicinity of the natural frequencies for liquid motion inside the tank. Determination of these frequencies is critical to determine the nature of fluid motion inside the tank and thereby predict impact load on the structure holding the liquid. The determination of hydrodynamic pressure on the tank walls due to liquid sloshing motion finds application in the design and construction of liquid tanks in rockets and ships. In some cases, determination of liquid motion inside the tank is also critical as it can get coupled with the parent vehicle’s motion dynamics. This paper deals with extraction of data from video recording of liquid sloshing motion inside a rectangular tank. Image processing techniques are used for this purpose. The important fluid dynamics properties which can be determined by image processing are discussed in the paper. The analysis presented is mainly for 2D motions.