Modeling of Air Flow Over a Generic Aircraft Carrier With and Without Island Structure

Abstract

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.

Effect of modifications to island shape and geometrical configuration on the external aerodynamics of a generic aircraft carrier

“…The greater the shift to port, the greater is the TKEAverage, irrespective of configuration. This is because, as the glideslope path shifts to port, the island’s contribution to the turbulence along the glide path becomes less, and the more significant contribution is because of the turbulence generated due to the forward port notch and to a lesser extent by the aft port notch 29. The effect of the port notches increases as the glideslope path shifts further to the port.…”

“…A detailed literature survey focusing on the external aerodynamics of an aircraft carrier has been carried out in one of our earlier publications. 5 The earliest studies to map the airwake around aircraft carriers were conducted experimentally in a wind tunnel using pressure probes to measure the steady-state velocity field and hot wire anemometry to measure the turbulence. The results were compared with full-scale trials on board the existing aircraft carriers using instrumented aircraft.…”

“…Both the figures have an inset of the GAC/BGAC model, with the red line indicating the centerline for which the results have been plotted.

Figure 6.Comparison of Cp along center line—SST k-ω model versus Expt Data (BGAC). 5

Figure 7.Comparison of Cp along center line—SST k-ω model versus Expt Data (GAC). 5

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“…5 The aim of the exercise was to mathematically model the turbulence to indicate the increased pilot workload and burble effect in terms of the airflow parameters such as the velocities (u,w) and pressure (p) and its derived quantities such as TI and turbulence kinetic energy (TKE) based on studies undertaken by Rudowsky et al, 26 who showed that any decrease in forward velocity component (u) would be considered unfavorable for pilot workload and safety, as it directly reduces the lift force and causes a sinking effect and Vivaldi 27 who analyzed the effect of turbulence and crosswinds on the mental workload of pilots. Three mathematical constructs, namely, TKEAverage, bold-italicuAv_Variation, and bold-italicwAv_Variation as defined by equations (6) to (8) were developed 5 . w Av-Variation is the average downwashTogether, TKE average , u Av-Variation , and w Av-Variation would give a quantitative estimate of the burble produced by different parametric changes and a potential quantitative indication of the pilot workload.…”

“…Validation with in-house experiments. A detailed description of the experiments undertaken on GAC and BGAC models has been extensively covered in our earlier publications 1,5 and also by Vignesh, 22 and hence only the gist will be given here. The mapping of the pressure field over the flight deck of the GAC with and without the island structure was carried out by installing 105 pressure taps at various locations distributed over the entire flight deck so as to capture the distribution of pressure over the complete flight deck.…”