Vijayakumar Rajagopalan

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.

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.

An early assessment of the ship airwake characteristics is one of the most challenging tasks associated with the designing of vessels. The design of warship superstructures has traditionally followed the basic polyhedron shape (box type structures) to achieve the desired stealth capability. However, presence of such a box shape bluff superstructure generates massively separated airwake over the ship helodeck region. This airwake results into complex flow phenomena which carry strong velocity gradients in space and time, along with widely varying turbulence length scales. Under such conditions, the launch and recovery of a shipboard helicopter operations are very hazardous. Thus, an accurate assessment of the resultant ship airwake flow phenomena at early design stages is desirable. We present a comparative time-accurate assessment study in order to gain a better understanding of the capability of the Unsteady Reynolds-Averaged Navier-Stokes (URANS), the Scale-Adaptive Simulation (SAS) and the Detached Eddy Simulation (DES) turbulence models in predicting turbulent ship airwake characteristics. Detailed comparisons are conducted with respect to the in-house experimental data. Results show that the DES and SAS produce nearly similar trends of the mean flow properties when compared to the experimental results. However, comparisons of velocity spectra indicate that SAS can resolve the dominant large-scale turbulent flow structures with less computational burden. Further, this study also attempts to compare the variation of mean flow quantities with steady RANS approach in order to quantify the percentage variation between the predictions of the steady and unsteady turbulence modelling approach.

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.

An early assessment of the ship airwakes flow characteristic is one of the most challenging tasks associated with the designing of vessels. The presence of ship airwake creates very complex flow phenomena due to the presence of strong velocity gradients in space and time and widely varying high levels of recirculation and turbulence. Under such condition, the landing and take‐off operation of a helicopter over the ship helodeck is very complex and accurate prediction represents a computational challenge. We present timeaccurate scale‐adaptive simulation (SAS) of turbulent flow around a simple frigate ship to gain insight into the flow phenomena over the helodeck. Numerical analysis is carried out after several grids and time‐steps refinement to ensure the spatial and temporal accuracy of the numerical data. The instantaneous iso‐surface of eddy flow structures and vorticity have been analysed across the vertical and longitudinal plane. Results show good agreement with experimental data. Comparisons of mean quantities and velocity spectra show good agreement, indicating that SAS can resolve the large‐scale turbulent structures which can adversely impact ship‐helo combined operations. Overall, the SAS approach is shown to capture the unsteady flow features of massively separated ship airwake characteristics with reasonable accuracy.

The study of external aerodynamics of an aircraft carrier is of utmost importance in ensuring the safety of aircraft and pilots during take-off and recovery. The velocity deficit in the forward direction and the downwash together combine to give a sinking effect to the aircraft, along its glideslope path and is known as the ‘burble’ in naval aviation parlance. This phenomenon is primarily responsible for the potential increase in pilot workload on approach to the aircraft carrier. There is little literature in the open domain regarding ways and means to alleviate the burble effect. Unlike in the case of the automobile industry, which has the generic ‘Ahmed body’ and for the frigates/destroyers, for which there is the Simplified Frigate Ship (SFS), on which experiments and validation through CFD could be carried out, by researchers from all over the world, there is no generic Aircraft Carrier model for carrying out experiments and validation of CFD codes. The aim of this study is to define the Generic Aircraft Carrier Model (GAC), as developed at IIT Delhi, and to carry out numerical studies on the GAC and a variant of GAC without the island, BGAC (Baseline GAC), to assess the contribution of the island to the burble behind an Aircraft Carrier. This study gives a quantitative estimation of the effect and contribution of individual components of an Aircraft Carrier (like flight deck, island, etc.) to the burble behind the carrier, and would give a Naval Ship Designer an understanding of the effect of the geometrical configuration of the flight deck and the island on generation of the burble behind the carrier, which could aid the designer in potentially reducing the pilot workload.