Vijayakumar Rajagopalan

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.

In recent years, underwater technology is being developed for coastal survey and strategic applications. Autonomous Underwater Vehicles (AUVs) are a classic example of such technology. The Unmanned Underwater Gliders, unique among them, use a buoyancy propulsion mechanism to execute said operations with minimal or no acoustic noise without the need to disrupt the aquatic life. To increase the speed in the horizontal motion of such gliders, a new type of hull-form is being developed on the basis of the flying wing design called the Blended-winged Unmanned Underwater Gliders (BWUUGs). The increased lift-to-drag ratio due to the high lift generated by imitating the hull of the glider with a wing, results in lowering the glide angle, thereby increasing maneuvering efficiency. This paper gives the CFD results obtained from various hull-forms leading to the evolution of BWUUGs focusing on the wings of the glider by comparing the lift-todrag ratios of classical hull-forms with that of the BWUUGs. The simulations are performed in a commercial CFD software – STARCCM+ with SST k-ω turbulence model validated using results from Nakamura et al., (2013).

The primary objective of this project is to investigate if the designed hull form when realistic wave conditions are taken into account should be more slender than the current blunt bows. The added resistance is also highly dependent upon environmental forces like wave conditions the vessel experiences. Also, ship operators are mostly interested in fuel savings with minimum modifications in hull form; thus, a study has been made on one operation regime considering the wave data for all the design variations. MOERI KVLCC2 has been considered for this study http://www.simman2008.dk/KVLCC/KVLCC2/tanker2.html, as it is available in public domain and widely used for CFD calculations in industry. Hull forms have been transformed using FFD transformation in NAPA looped with optimisation and CFD tools in NAPA. Systematic algorithm was used to do optimisation. Six designs have been investigated changing the water lines and entrance angles resulted from blunt to sharp. In this thesis, KVLCC2_0 is the original design of MOERI tanker with no flare. KVLCC2_-1.5, KVLCC2_-1 and KVLCC2_-0.5 are blunt designs than the other design variations used. KVLCC2_0.5, KVLCC2_1 and KVLCC2_2 are more slender ships by moving the volume from the shoulder to the bulb area. KVLCC2_-1.5 is more blunt ship and has some restrictions in calculating the full-scale resistance after CFD study, so this design variation is neglected in the reports. However, to show the design variations, it has been shown at some places. KVLCC2_0 has been elongated by 8m and can be seen in KVLCC2_2 design. One route has been chosen to represent the actual operational areas of a similar vessel. The route selected is from Arabian Gulf (AG) to Japan. Resistance of calm water has been calculated and verified with experimental data. The wave resistance was calculated numerically using NAPA. KVLCC2_-1 has the greatest calm water resistance compared to rest of the designs. Designs KVLCC2_0, KVLCC2_0.5 and KVLCC2_1 have very similar calm water resistance but slightly lower than KVLCC2_-0.5 and KVLCC2_2. The added resistance was calculated by NAPA seakeeping subsystem. Sharper bow designs have lower resistance in the regime considered as expected. Fuel consumption calculations were done by including operational profile of the voyage, viscous resistance, added resistance, and propulsion characteristics in NAPA. The results show that KVLCC2_0.5 and KVLCC2_1 have good fuel efficiency of 11.8 and 12.6%, respectively. From the results, it is obvious that a sharper bow will have better advantage over a blunter bow when actual operational conditions are considered.

The reduction of ship’s resistance is one of the most effective way to reduce emissions, operating costs and to improve EEDI. It is reported that, for slow moving vessels, the frictional drag accounts for as much as 80% of the total drag, thus there is a strong demand for the reduction in the frictional drag. The use of air as a lubricant, known as Micro Bubble Drag Reduction, to reduce that frictional drag is an active research topic. The main focus of authors is to present the current scenario of research carried out worldwide along with numerical simulation of air injection in a rectangular channel. Latest developments in this field suggests that, there is a potential reduction of 80% & 30% reduction in frictional drag in case of flat plates and ships respectively. Review suggests that, MBDR depends on Gas or Air Diffusion which depends on, Bubble size distributions and coalescence and surface tension of liquid, which in turn depends on salinity of water, void fraction, location of injection points, depth of water in which bubbles are injected. Authors are of opinion that, Microbubbles affect the performance of Propeller, which in turn decides net savings in power considering power required to inject Microbubbles. Moreover, 3D numerical investigations into frictional drag reduction by microbubbles were carried out in Star CCM+ on a channel for different flow velocities, different void fraction and for different cross sections of flow at the injection point. This study is the first of its kind in which, variation of coefficient of friction both in longitudinal as well as spanwise direction were studied along with actual localised variation of void fraction at these points. From the study, it is concluded that, since it is a channel flow and as the flow is restricted in confined region, effect of air injection is limited to smaller area in spanwise direction as bubbles were not escaping in spanwise direction.

Flow structures and air wake studies are crucial features which need thorough understanding in the initial stages of design, to enhance the primary role of aircraft carriers- to launch and recover aircraft, safely and swiftly. Hardly any information is available in the open literature which could be used to establish the important flow features of the carrier environment. Any attempt to establish such a benchmark requires to be generic, repeatable and scalable. The benchmark data would be useful for validation of numerical models. The relative ease of setting up CFD simulations compared to physical experiments can consequently be exploited to undertake numerous parametric analyses to evaluate design alternatives. This paper presents experimental studies carried out in wind-tunnel at IIT Delhi to measure the airflow over a generic aircraft carrier model and establish baseline data of the pressure distribution over the flight deck. Several pressure-taps embedded over the deck give the pressure at various locations which are extracted for data analysis. Numerical simulations have been validated against the experimental data and a suitable turbulence model has been identified for the problem. These studies offer valuable insight into the flow conditions over the flight deck and provide a logical start point to extend the work to qualify multiple geometries and locations of the island for optimizing airflow. The explication of flow physics thus obtained is a prerequisite to understanding the influence of these structures in the critical ‘burble’ zone of aircraft landing approach.

Autonomous underwater gliders are a class of AUVs that execute motions using change in buoyancy in conjunction with wings. Lack of conventional propeller restricts the speed of the vehicle. Speed of vehicle depends on operation of the buoyancy engine. Gliders follow a saw-tooth profile path and spiral path for undertaking motions in 2D and 3D planes. Traditionally, “legacy gliders” have been using roll and pitch correction mechanisms for conducting the 3D motion. This paper attempts to characterize the mass definitions of one laboratory-based AUG being developed at IIT Madras and predicts the path the glider will execute with participation of roll and pitch correction mechanisms that are being developed. The laboratory-scale gliding fish (small-scale glider) consists of a rudder and a roll control mechanism. The roll control mechanisms consist of movable mass, rotating and traversing about the principal axis of the glider. The effect of rudder and roll control mechanism on the path traversed by glider in 3D steady state is studied individually and in combination by solving the equations of motion using FSOLVE algorithm of MATLAB.

Numerical investigation of the effects of installing a rotating cylinder at the hangar top edge of a frigate on the dynamic interaction between the airwake generated by a Simplified-Frigate-Ship (SFS) during its propulsion and downwash formed by a 3-bladed helicopter rotor trying to perform landing/take-off operations on the flight deck of SFS is undertaken. The SFS hangar is attached with this rotating cylinder to suffice as an active flow device and the flow field so created by this dynamic interaction is analyzed and compared. The modified frigate is modeled using a scale ratio of 1:100. Measurements are taken in terms of rotor thrust coefficient, recirculation length and turbulence intensity at identified locations. STAR CCM+ code that uses FVM (Finite Volume Method) solver to solve the RANS (Reynolds-Averaged-Navier-Stokes) equation along with two-equation - turbulence model as a CFD tool are used for carrying out this numerical analysis. Firstly, the airflow analysis is carried out for SFS-2 in isolation in order to establish a baseline understanding of flow followed by airflow analysis for modified SFS-2, the one equipped with a rotating cylinder. Further, simulations for studying the dynamic interface of ship airwake and the rotor downwash are carried out which involve both the helicopter rotor with ROBIN fuselage and SFS-2 in the first case while the helicopter rotor with ROBIN fuselage and SFS-2 modified with rotating cylinder in the second case. These simulations were varied based upon three distinct cylinder diameter to hangar height ratios for each one of the two rotor hovering-planes positioned parallel to eachother. The inference obtained from this study is that the hangar with a rotating cylinder gives better flow field in terms of recirculation length and thrust co-efficient with zero WOD (wind over deck angle).

The increase in fuel costs and looming restrictions on carbon dioxide emissions are driving the shipowner into reducing the ship’s resistance and required installed power. It was earlier reported that, merchant vessels operating at lower speeds, the frictional drag accounts of almost 70–80% of the total drag; thus, there is a strong demand for the reduction in the fluid frictional drag, especially in the marine transportation business. The use of air as a lubricant, by injecting below the plate or the body, which is famously known as microbubble drag reduction (MBDR) in order to reduce that frictional drag is an active research topic. Latest developments in this field suggests that there is a potential reduction of 80% in frictional drag in case of flat plates and about 30% reduction in case of ships, which encourages researchers to investigate further. In this study, 3D numerical investigations into frictional drag reduction by microbubbles were carried out in Star CCM+ on a channel for different flow velocities, different void fractions and different cross sections of flow at the injection point. This study is the first of its kind in which variation of coefficient of friction both in longitudinal and transverse directions was studied along with actual localized variation of void fraction at these points. The numerical framework consists of the Reynolds-averaged Navier–Stokes (RANS) equations and the standard k−ε turbulence model with standard wall function treatment, which is validated in both conditions of with and without microbubbles with the existing experimental data. The design exploration study was carried out for various flow speeds, injector flow rates, cross sections of the channel/heights of channels and of course void fractions. Coefficient of friction and void fraction values are measured at 12 longitudinal positions, and at each longitudinal position, 11 in number transverse and 10 in number depthwise positions were studied. In all, for one simulation, data at more than 1000 positions were collected. More than 60 simulations were carried out to understand the effect. From the study, it is concluded that since it is a channel flow and as the flow is restricted in confined region, effect of air injection is limited to smaller area in transverse direction as bubbles were not escaping in transverse direction.

Underwater Gliders are a class of AUVs that move without a conventional propeller and are known to behave like Dubin's car for point to point traverse (Dubins, 1957). The turning motion or the spiral maneuver hence becomes an important motion trajectory that has to be studied for efficient maneuvering and control of the vehicle. Control of the glider to follow a given path has been extensively studied from control strategies. However, the effect of hull form on maneuvering characteristics has not been researched extensively. This paper reviews the published literature for maneuverability of underwater gliders from the viewpoint of Naval Architecture and hydrodynamics of hull form. The dynamics equation formulation for a glider, hull forms of underwater gliders in use, hydrodynamics coefficients and methods of their estimation are discussed. Further, analysis of spiral maneuver is undertaken for the characteristic variables of the maneuver, namely velocity, turning control inputs and radius of the spiral trajectory. Methods of numerical prediction of a glider executing spiral are discussed. Scope for further research based on the review is proposed.

The impact of increased Underwater Radiated Noise (URN) over the past two decades on marine mammals has resulted in the pressing requirement to reduce it. Shipping contributes immensely to the URN. Propeller noise is a major source of URN. The reduction in Propeller noise can hence significantly help in the reduction of URN. With the sole objective of improving the hydrodynamic performance of propellers ways to prevent cavitation are being developed. However, the reduction of non cavitating noise produced by the propeller would still remain a challenge. The change in the propeller geometry can modify the acoustic characteristics. In this present study, effect of modifying the tip of DTMB4119 propeller on the acoustic and hydrodynamic characteristics is presented. The change in the flow pattern at the tip due to introduction of tip rake is also discussed. The SPL has been calculated by using the two-step Ffowcs William and Hawkings (FW-H) equations from the pressure distribution at various points around the propeller. SPL at various points in the downstream and propeller disk plane are numerically predicted and discussed.