Vijayakumar Rajagopalan

Proper functioning of certain critical systems like cooling of the main machinery and diesel alternator at various operating speed regimes on high-speed vessels, largely depends on the availability/flow characteristics of seawater at the sea chest (used as a coolant). The general practice is to provide sea tubes/sea chests in the underwater hull for intake of seawater. However, in high-speed planning vessels, the appropriate positioning of the sea chest along with the sea tube with a positive pressure gradient at the mouth of the sea tube becomes the vital parameter that influences the effective intake of seawater. In a high-speed planning vessel, which is designed to operate at various planning modes,(the majority of the hull is hydrodynamically lifted from the sea) the pressure developed at the entry of the sea tube may not be adequate for drawing seawater inside the hull. Hence this paper presents numerical analysis using RANSE solver on sea chest performance for a planning vessel at various speeds and suggest modification for improvement, such as altering the position of sea tube, dimension of sea chest, and also retrofitting a scoop element for improving the flow characteristics. The results are analyzed for the pressure developed in each of the above cases for favorable effect on the seawater intake system.

An early estimation of the ship–helicopter dynamic interface flow environment is one of the most challenging and precarious tasks in any naval organization across the globe. The First-Of-Class Flying Trials (FOCFT) is one of the most common methods to evaluate the complexities associated with the ship–helicopter dynamic interface. These trials are very expensive and highly demanding, with added limitation that these can be conducted only after post-construction of the ship and restricting the scope of any further design modifications. This study presents an investigation to gain understanding of the aerodynamic impact of helicopter on shipboard flight deck flow physics under a ship–helicopter dynamic interface flow environment. The prime goal of this work is to investigate the influence of helicopter downwash aerodynamics over the ship flight deck using the simplified dynamic interface (SDI) model. A parametric analysis has been conducted for three identified rotor configurations at different crossflow conditions. The paper reports the influence of rotor ground effect over the flight deck region in terms of the downwash airflow characteristics, variation of rotor plane velocity gradients and the airwake existing over the flight deck region. Finally, an attempt has been made to grade the effect of ground on a ship–helicopter dynamic interface for safeguarding the helicopter operations.

The combined operation of a ship and a helicopter is ubiquitous in every naval organization. The operation of a ship in combination with the landing and take-off operations of a helicopter results in a very complex flow phenomenon. This is due to the presence of ship airwakes, strong velocity gradients and widely varying turbulence length scales. An accurate assessment of the resultant flow phenomena is an outstanding challenge for naval architects as well as researchers. A conceptual method to gain insight into the combined ship-helo flow phenomena over a helodeck is presented. The prime objective of this work is to develop an economical design tool employing both experimental as well as computational techniques to simulate the ship-helicopter coupled environment regime at early design stages reasonably well so as to ease the burden of expensive and risky sea trials. For this purpose, a simplified dynamic interface (SDI) model is proposed to investigate the coupled effects of ship airwake and helicopter rotor downwash. The paper reports a parametric analysis to study the coupled ship-helo airwake behaviour and its impact on helicopter fuselage over the ship helodeck for different ship speed regimes by the proposed SDI model. Further, an attempt has been made to set up preliminary design criteria to grade the ship-helicopter interface for ensuring minimum standards of safe helo operations. Finally, we discuss the impact of the coupled flow dynamics in terms of induced fuselage drag, cross-flow characteristics, rotor plane wake and velocity gradients existing over the helodeck region and evaluate the proposed design criteria.

Underwater Gliders are unique buoyancy propelled oceanographic profiling vehicles. Their speed and endurance in longitudinal motion are affected by the symmetry, sweep, dihedral angle and span of the control surfaces. In the lowvelocity regime, these parameters can be varied to examine the flow around the glider. They also affect the lift-to-drag ratio (L/D) essential for the manoeuvring path in longitudinal and transverse motions. In this paper, the sweep angle of the main wing of a blended wing autonomous underwater glider configuration is varied as 10., 15., 30., 45. and 60. and the resulting hull forms are numerically simulated in the commercial software, STARCCM+. The main wing is a tapered NACA0018 section (taken as per the general arrangement requirement) with 1.5m chord at the root and 0.1m at the tip. The numerical model is validated using the CFD results of NACA0012 airfoil from Sun.C et al., 2015 [1]. The hydrodynamic forces are obtained by varying the angle of attack (a) of the body from -15. to 15., for flow velocity of 0.4m/s. The hydrodynamic coefficients (lift-todrag ratios) and flow physics around the wing are analyzed to arrive at an optimum Lift-to-drag ratio for increased endurance.

Helicopters have become an integral part as they provide extended capabilities in areas where land-based operations are not feasible. In order to facilitate helo operations, frontline warships are provided with a helo deck and a helo hangar. Hangar shapes which were traditionally designed with stowage volume and associated aviation systems availability are being additionally optimised for stealth aspects. However, the flow considerations on helo-deck are verified by Experimental Test Pilots (ETPs) performing high risk trials in later stages of warship construction. The non-availability of a design tool at initial design stage of a warship which can effectively optimize the dynamic flow conditions on the helo deck poses a significant challenge to the designer as well as the operator. Across the globe, combinations of warship forms and helicopter types are increasing and the task of qualifying multiple helo-decks is challenging for all Navies. This paper examines the effects of helicopter downwash interacting with ship airwake for a Simplified Frigate Ship (SFS2). The flow visualization studies, with simplified rotor downwash (SRD) and airwake measurements carried out in wind tunnel at IIT Delhi are presented. These studies give a valuable insight into the interaction of varying helicopter operations at different warship speed regimes. Further, they provide critical data for numerical model validation and parametric studies at initial design stages.

The hydrodynamic performance of high-speed displacement vessels can be enhanced by installing basic stern fittings such as stern wedges, stern flaps, interceptors, and hull vanes. The hydrodynamic performance of a high-speed displacement vessel with hull vane is analysed and compared to the bare hull condition in this study. The hull vane is a hydrofoil wing attached transversely to the bottom of the transom that focuses largely on minimising the wave-making resistance of the vessel by altering the flow at the stern. It increases flow on its upper surface, which interferes favourably with the stern wave system, hence lowering wave-making resistance. This research investigates the effects of installing the hull vane on a high-speed displacement vessel at various locations in the X and Z directions from the transom edge. The tests were carried out on model ship with and without a hull vane to verify the accuracy of the computational findings. The experiments were carried out in the towing tank facility available at Indian Institute of Technology Madras (IIT Madras), Department of Ocean Engineering (DOE). Computational fluid dynamics (CFD) simulations were carried out for model vessel with and without hull vane for a range of Froude numbers from 0.17 to 0.42 using RANSE-based commercial software. The research examines the resistance, trim, pressure distribution, and stern flow development of the vessel at various speeds and in calm water conditions. The results indicate that installing a stern hull vane reduces resistance and trim of the vessel. The effect of a hull vane is enhanced when it is positioned away from the stern of the vessel.

The shipping business expects to develop energy-saving and drag-reducing techniques addressing the cost of shipping and environmental problems. It has been reported that for slow-moving vessels, frictional resistance accounts for up to 80% of the total resistance, needing urgent attention to reduce the same. To reduce frictional resistance, the air has been used as lubricant, which is injected below the moving body known as Bubble Drag Reduction or the Air Lubrication System. In this article, results obtained from experimental investigations into drag reduction of a 1:23 scaled model of an 8000-ton deadweight bulk carrier by injecting air bubbles below it are presented. Investigations were carried out for a speed range of 6–10 knots, and for each speed, the effect of six injection flow rates of .5–3.0 CFM were investigated. To investigate the effect of different sizes of injection holes, two types of injector units have been used: one with injection holes of 1 mm diameter and the other with injection holes of 2 mm diameter. The study carried out has many practical implications because it is easier to create bigger size holes which will reduce the power required to inject air, thereby increasing the efficiency of the entire technique.

Composite materials are widely popular and outperform metallic counterparts with superior properties. The current study aims to predict a B series composite propeller's hydro-structural performance in self-propulsion mode. Numerical simulations were carried out in full-scale conditions using RANS equation-based CFD and FEM based software to predict the hydrodynamic and structural performance. Using the FSI coupling algorithm, a connection was established between two solvers to estimate the hydro-structural performance. Two geometrically ideal propellers were chosen for a candidate hull form with Steel and Carbon fibre material properties. Initially, the fluid load was calculated using CFD and transferred to the structural solver, and deformation and stresses were computed. Later, the structural deformation details were transferred to the fluid solver within the same step. The above process was repeated until convergence was achieved. Hydrodynamic parameters such as resistance, trim, and sinkage of the candidate hull were established numerically and validated with towing tank experiments. The open water curves tip deflection and von-mises stress of the same propeller were already established using the FSI algorithm [1]. The results obtained from structural solvers such as deflection and stress were compared with open water results and identified the deviation of bend-twist characteristics under wake conditions.

This paper discusses the effect of modifying the hangar shape on the dynamic interaction of ship airwake and rotor downwash of the helicopter using computational fluid dynamics (CFD). A traditional 1:100 scaled simplified frigate ship (SFS-2) is modified to obtain trapezoidal and rectangular configurations of the hangar. The coupled interaction of ship airwake formed behind the ship subjected to free stream velocity of 6 m/s with the downwash generated by helicopter rotor hovering at 5000 rpm is simulated using STARCCM + CFD solver. The Reynolds-averaged Navier–Stokes equation is solved with conventional k-ω two-equation turbulence model to simulate the flow. The helicopter rotor thrust coefficient on hovering plane and landing plane is calculated for all the three SFS-2 configurations. The resultant velocity flow field of the ship airwake and rotor downwash is surveyed to compare the turbulence intensities at four equidistant lateral planes along the flight deck for a zero wind-over-deck (WOD) angle. The modified hangar configurations are shown to improve the coupled flow aft of the hangar.

Research on the air flow disturbances in the aircraft carrier environment has gained prominence in recent times. However, there is presently no representative carrier model analogous to the Simplified Frigate Shape (SFS) which is generic naval frigate for air flow investigation. In the present study, a Generic Aircraft Carrier (GAC) model is proposed, as a simplified, benchmark model for aerodynamic research. With the motivation to provide validation data for future CFD studies, baseline experimental data is generated in the wind tunnel, in terms of pressure distribution over the deck, for two variants, namely, a complete flat deck configuration with no island and secondly, with the island in the baseline position of the GAC. Effect of the island in modifying the flow is discussed by a comparison between the two variants. Particle Image Velocimetry (PIV) is employed to record velocity and turbulence levels in the GAC environment, highlighting regions of velocity deficits, and unsteady flow which may hinder the landing procedure of an approaching pilot. Comprehensive database of experimental data is presented as baseline data for future work and for validation of numerical models. Traditional tuft and smoke visualization studies are also conducted to provide corroboratory qualitative insights.