Experimental hydrodynamics

Future surface combatant designs are focused on multirole mission capabilities and unconventional efficient propulsion systems. Concepts in naval design and related systems introduced by several nations in the past two decades shows great amount of advancements from the traditional platform operation criteria such as speed, shock & noise reduction, infrared signature to futuristic demand criteria like flexible mission capability, alternative propulsion architecture, higher degree of stealth, longer duration of loitering capability with smaller crew and cleaner emissions. Few of such platforms are already in operation and many more are under design/construction phase by the world navies. Mission effectiveness of the platform largely depends on two major entities i.e. hull form and propulsor configuration. It has been reviewed that most of advanced marine vehicles rely on unconventional propulsion such as waterjets & podded propulsors for effective platform performance. Studies have been carried out at Naval Science & Technological Laboratory (NSTL), India on a trimaran with waterjet propulsion for hydrodynamic performance evaluation at concept level. The present paper highlights the hydrodynamic studies on trimaran with waterjets carried out at NSTL.

Advances in the field of hydrodynamics have brought out a number of varieties of high speed vessels including advanced hybrid crafts. Yet there is a thirst worldwide in ship building industry/Research & Development organizations to innovate and redefine the hydrodynamic technology for intensifying their performance. There has been a tremendous response for competent hydrodynamic performance technology especially for high speed planing hull forms leading to an innovative idea christened as " Interceptors " which claims to be an effective solution to enhance vessels performance in calm as well as rough waters. The basic Interceptor system offers the high-speed craft to optimise the ship speed and the ride comfort by control of running trim and list. The fundamental working principle of an interceptor is to generate an increase in pressure on the hull bottom plating directly ahead of the transom by intercepting the flow with a blade. Interceptors require less power than any other vertical or horizontal trim tab/ wedge / flap devices. Interceptor concept also offers excellent directional stability when aligned with other thrust producing devices such as waterjets and can operate as a motion damper. The present paper throws light on the performance of interceptors as its application to planing hull form.

Hydrodynamic model studies were carried out at Naval Science & Technological Laboratory (NSTL), Vishakhapatnam, India to estimate the resistance contribution of appendages. This paper discusses categorization of ship hull appendages from a hydrodynamic viewpoint. An attempt is made to qualitatively assess the resistance contribution of each of these categories of appendages to the total ship resistance. A relative appraisal of the resistance contribution of appendages on a naval surface combatant, a submarine, a tanker, a container, a dredger and a single screw ship is made to emphasize the large appendage drag on naval ships. Results obtained from hydrodynamic model experiments conducted at High Speed Towing Tank (HSTT) facility at NSTL on a fast displacement hull form used for naval applications (FDHFNA) are discussed at length as a case study. A criterion for rating appendages is proposed. ISSN: 2278 0211 (Online)

The mechanism of wake-induced vibrations (WIV) of a pair of cylinders in a tandem arrangement is investigated by experiments. A typical WIV response is characterised by a build up of amplitude persisting to high reduced velocities; this is different from a typical vortex-induced vibrations (VIV) response that occurs in a limited resonance range.
We suggest that WIV of the downstream cylinder is excited by the unsteady vortex-structure interactions between the body and the upstream wake. Coherent vortices interfering with the second cylinder induce fluctuations in the fluid force that are not synchronised with the motion. A favourable phase lag between the displacement and the fluid force guarantees that a positive energy transfer from the flow to the structure sustains the oscillations. If the unsteady vortices are removed from the wake of the upstream body then WIV will not be excited.
We introduce the concept of wake stiffness, a fluid dynamic effect able to sustain a body without any structural stiffness into oscillatory motion. The role played by the unsteady wake is central to this idea and contributes to the understanding of WIV as a wake-excited and wake-sustained mechanism. A simple analytical model predicts the frequency of response rather well, but fails to model the displacement because important nonlinear effects are not taken into account. We conclude that while unsteady vortex-structure interactions provide the energy input to sustain WIV, it is the wake stiffness effect that defines the character of the response.
Suppression of cross-flow and in-line VIV, with resulting drag coefficients less than that for a static plain cylinder, is achieved using two-dimensional control plates free to rotate around the body. Devices based on parallel plates show great potential to suppress VIV and WIV of offshore structures with considerable reduction in drag.

Waterjet propulsors are successfully used in high speed vessels (HSV). It is efficient both in deep and shallow waters. Recent advances in waterjet propulsion technology have placed them ahead of conventional propulsion systems in both performance and reliability, mainly when used in HSVs. Waterjet propulsion exhibits excellent propulsive performance and maneuverability within a specified speed range which are favorable for high speed applications and also for vessel maneuvers in restricted waters. The waterjet propulsor manufacturers provide only the performance data of waterjet system at a specified operational condition, without the vessel influence effects. But, the overall scenario of the waterjet system interaction effects on the performance of the vessel and that of the hull on the propulsors are neither known nor provided. Several experimental and numerical studies are being carried out worldwide to understand and evaluate the performance of hull -waterjet system as a whole. This paper tries to bring out the interaction between hull and waterjet system numerically using CFD methods based on the RANSE solvers and also the same experimentally based on model tests carried out as part of the present research work. Use of these hydrodynamic interaction parameters go as an input into the design of a more efficient waterjet system for a specific vessel.

Ship wave pattern resistance evaluation has been an important event in the optimization process of variety of hull forms ranging from small crafts to fast displacement hull forms including multihull vessels. Several experimental methods for wave pattern analysis like Hogben Matrix Element Method (MEM), Longitudinal & transverse wave cuts involving Landweber Fourier Transform (LFT) and Matrix solution method, theoretical methods like Michell's thin ship theory, Rankine source-sink and Kochin-Fourier method exist today with their merits and limitations. Ship wave pattern resistance is mostly a derived quantity from experiments either as a subset of residuary resistance or when total viscous component is known. In general applying Froude's hypothesis, the residuary resistance coefficient is not subjected to scaling effects from model to prototype and evaluation of total viscous component for surface ships by model experiments is seldom done. Optimization of hull forms through wave pattern studies is a powerful tool and in-house potential flow solvers/experimental evaluation techniques for free surface flows have been developed in many hydrodynamic test facilities. The present paper focuses on ship wave pattern evaluation using Michell's theory (using Kochin wave amplitude function) and its comparison with Shipflow ® & Longitudinal wave cut method (using LFT) on bench mark test model R/V Athena. Matlab ® code development and ship wave pattern resistance evaluation sensitivity for the method chosen is also a part of present discussion.

Trimaran is the most promising hull form for naval vessels because of its excellent performance characteristics notably reduced wave making resistance, larger deck area, good sea keeping and ability to perform optimally in a range of speeds. In its most generic form it consists of a slender centre hull and two side hulls. High slenderness ratio, differential breadth to draft ratios, stagger/separation of the hull forms gives an edge over equivalent monohulls/catamarans in minimising wave making resistance. However by means of advancements to the existing design which can include bulb & wave piercing forms, stern wedge/flap/interceptors, asymmetry in side hull configuration , lifting bodies etc. The present paper investigates the qualitative performance of trimaran hull form modified to various configurations using Shipflow ®. ISSN: 2278 0211 (Online)

Hydrocephalus is a pathophysiology due to the excess of cerebrospinal fluid in the brain ventricles and it can be caused by congenital defects, brain abnormalities, tumors, inflammations, infections, intracranial hemorrhage and others. Hydrocephalus can be followed by significant rise of intraventricular pressure due to the excess of production of cerebrospinal fluid over the absorption, resulting in a weakening of intellectual functions, serious neurological damage (decreased movement, sensation and functions), critical physical disabilities and even death. A procedure for treatment involves the placement of a ventricular catheter into the cerebral ventricles to divert/drain the cerebrospinal fluid flow to a bag outside of the patient body -provisory treatment known as external ventricular drainage (EVD). Another option is the permanent treatment, internal ventricular drainage (IVD), promoting the cerebrospinal fluid drainage to other body cavity, being more commonly the abdominal cavity. In both cases, EVD and IVD, it is necessary to use of some type of neurological valve in order to control the flow of cerebrospinal fluid. In the present work is proposed an experimental procedure to test the hydrodynamic behavior of a complete drainage system, or parts of them, in order to verify its performance when subjected to pressure gradients found in the human body. Results show that the method is well adapted to quantify the pressure drop in neurological systems.

In the modern world waterjet propulsors have rapidly gained acceptance as the leading means of propulsion for all types of conventional high speed vehicles like ferries, workboats, patrol crafts, yachts and notably to some unconventional hull forms like Pentamarans, Trimarans and Swaths. Recent advances in waterjet propulsion technology have placed them ahead of conventional propulsion systems in both performance and reliability. Waterjet propulsors manufacturers provide only the performance data of waterjet system and the overall scenario of  hull - waterjet system performance in terms interaction may be well away and makes it inconvenient to accept the waterjet performance data alone. Model tests / results in Towing tanks for hull - waterjet system as a whole are very limited and a costly affair to carry out subjected to sufficient and necessary instrumentation. This  paper  brings out a methodology of  testing of hull – waterjet system in wind tunnel and thus transferring useful inputs for hull – waterjet system performance evaluation.

Hydrocephalus is a pathophysiology due to the excess of cerebrospinal fluid in the brain ventricles and it can be caused by congenital defects, brain abnormalities, tumors, inflammations, infections, intracranial hemorrhage and others. Hydrocephalus can be followed by significant rise of intraventricular pressure due to the excess of production of cerebrospinalfluid over the absorption, resulting in a weakening of intellectual functions, serious neurological damage (decreased movement, sensation and functions), critical physical disabilities and even death. A procedure for treatment involves the placement of a ventricular catheter into the cerebral ventricles to divert/drain the cerebrospinal fluid flow to a bag outside of the patient body – provisory treatment known as external ventricular drainage (EVD). Another option is the permanent treatment, internal ventricular drainage (IVD), promoting the cerebrospinal fluid drainage to other body cavity, being more commonly the abdominal ...