The Burble Effect: Superstructure and Flight Deck Effects on Carrier Air Wake2010 BRADLEY E. CHERRY & MATTHEW M. CONSTANTINO“The purpose of the present work was to qualitatively and quantitatively model the air wake created by an aircraft carrier flight deck and superstructure in order to understand how it affects aircraft on approach and landing. The “burble effect” is the name given by navy pilots to the velocity deficit and downwash field immediately aft of an aircraft carrier. This turbulent region of air has adverse effects on landing aircraft and can cause pilots to bolter, missing the arresting wires and requiring another landing attempt. The experimental approach involved using a five-hole Pitot probe rake system for mapping of the air wake of a 6 ft aircraft carrier model. Wind tunnel tests were performed at Reynolds numbers of 11,000,000 and wake maps of 288 point measurements each were generated for configurations with and without the superstructure. The experiment showed that the superstructure created a region of extremely low flow velocity and downward flow angle. While the hull/deck configuration produced velocity deficits as low as 17%, the presence of the superstructure increased this to almost 30%. Furthermore, the addition of the superstructure decreased the downwash angle at the deck edge from -5 to -8 deg. Under these conditions, an aircraft on approach will experience an increased descent rate as it passes through the burble region. The presence of strong downwash compounds this adverse effect, making it more severe closer to the flight deck. Results also show that the superstructure geometry affects the severity of the burble effect.
IntroductionHE superstructure and deck/hull features of an aircraft carrier are known to generate turbulent airflow behind the carrier. This region of turbulent air has become known as “the burble,” and it is often encountered by pilots immediately before landing. The burble causes planes to drop slightly, thus requiring a small amount of power to be temporarily added to maintain the glide slope though this region. The region is characterized by a downwards suction in the airflow and it is created by “the interference of the structure of the ship with the relative wind and its influence is felt mainly in the last half mile of the approach to the ship”. Additionally, sea state, crosswinds, and features such as raised jet blast deflectors and flight deck traffic contribute to the burble....
...The air wakes of ships which deploy aircraft have been the subject of numerous research programs, with the central focus to quantify the flowfield along the aircraft approach paths. Previous research has shown that subtle changes and protuberances in hull/deck/superstructure can have significant changes in the external flow characteristics and that these flow features can be severe. This has been shown for DDG, LHA, and CV ship classes. For the aircraft carrier (CV), the flow pattern is primarily characterized by the formation of free vortices. One of these can appear along the leeward side of the hull while others form at sharp edges of the deck-edge and superstructure. Cut-outs in the deck or sharp corners on the superstructure in general trap standing vortices which stabilize the wake structure in general and create a flowfield that is largely independent of the Reynolds number. The fact that the overhang of the deck is a major cause for flow disturbance was experimentally validated by Lehman on a Forrestal-class carrier. Durand and Wasicko also studied the effects of upwash and downwash on the glide path control of the approaching aircraft. More recently, the burble was modeled in CFD by Polsky and Naylor. They found that the geometry of the stern had a significant impact on the airwake qualities along the aircraft approach path. Other researchers have concentrated on the dynamic flowfield effects on the aircraft/ship coupling....
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A second feature that was investigated was deck fillets. Removable fillets were built to install in the two notches in the back of the deck. Increasing the deck space can be useful for taxiing aircraft and parking them on deck, therefore the effects of how rounding out the corners of the aircraft carrier’s deck would influence the burble was investigated. Figure 5 shows the two fillets and it is noted the fillet on the starboard side is the small fillet....
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C. Flight Deck Geometry Effects for Ford ClassFurther analysis was conducted in order to study the possible effects of the flight deck geometry on the burble. These tests were conducted for the configurations listed below in Table 4. It was found that the flight deck geometry mainly affected the port side of the flow field behind the carrier and was independent of the type of superstructure installed. Therefore, this section seeks to characterize the deck/hull vortex for the Ford class aircraft carrier...
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D. Discussion/ConclusionIt is important to note that the study conducted on the effect of the fillets on the deck/hull vortex was conducted at a length of two feet behind the carrier. As previously stated, this length corresponds to distance of 380 feet aft of a real, full-scale carrier. Even at this distance, the vortex and burble appeared relatively intact. Due to equipment and wind tunnel mounting restraints, it was not possible to analyze the deck/hull vortex and burble effect immediately over and aft of the flight deck. The deck/hull vortex appears to be “sucked” upwards through the “notch” in the back of the carrier flight deck and subsequently rolls downstream behind the carrier. The addition of the large fillet drastically appears to reduce the intensity of the deck/hull vortex, even at a full-scale distance of 380 feet. Therefore, it would appear to be beneficial to fill in the “notch” in the back, port corner of the flight deck on aircraft carriers. Not only would this fillet reduce the deck/hull vortex and prevent it from rushing up and over the landing/approach area, but it would also provide room for aircraft and/or equipment storage.
The burble effect is caused by multiple factors such as - velocity deficits, upwash and downwash, and vortices that are generated from the superstructure and the deck/hull. All of these various aspects of the burble combine to produce the increased sink rate on approach that has been described by many carrier pilots. Additionally, an aircraft carrier’s superstructure and flight deck geometry both contribute to the burble effect. Since Nimitz class carrier construction has ended and Ford class carrier construction has just begun, the potential exists for future carriers to be designed such that the burble effect becomes reduced.”
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