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Smart Vortex Leveraging Tabs

    Submarine sailplanes and rudders generate concentrated vortices quite similar to the wake system produced by aircraft (Figure 1). The importance of identifying effective methods to mitigate adverse vortex wake effects on submarines has motivated Navy interest in wake deintensification in recent years. Not only are the noise and vibration due to wake ingestion in propulsors a concern, but the wake signature left by submarines in both cruise and maneuver is a significant issue in non-acoustic antisubmarine warfare. Thus, achieving rapid dissipation of the organized vortex wake is an important feature of preserving the stealthiness of the current submarine fleet.


Figure 1: Schematic of wake vortex systems trailing from typical submarines.


    DARPA-sponsored work in the late 1990s at CDI saw the development and testing of a novel method for mitigating adverse vortex wake effects using control surfaces actuated via Shape Memory Alloy (SMA) smart materials technology. This project originated with an analysis effort that identified a method for introducing small, secondary vortices of periodic, time-varying strength to promote the deintensification of the primary vortex wake systems of submarines and aircraft. Computational analyses of wake breakup using this "vortex leveraging" strategy were undertaken, and showed up to an order of magnitude increase in the dissipation rate of concentrated vortex wakes (Figure 2).


Figure 2: Computational modeling of the use of Vortex Leveraging Tabs (VLTs) to effect deintensification of submarine control plane wakes.


    The first phase of this effort addressed the preliminary design of actuation mechanisms for the deflectable surfaces that provided the time-varying wake perturbations required to implement vortex leveraging, using smart materials (SMA) actuators. These active surfaces, or Smart Vortex Leveraging Tabs (SVLTs), exploit the high-force, high-deflection capabilities of SMA-driven devices, which have proven to be well suited for the low frequency actuation requirements of the vortex leveraging concept. Work in the second phase was divided into three major elements: extended computational analysis of the vortex leveraging concept; design and testing of prototype SVLT hardware; and scale model experimental demonstrations of vortex leveraging in a tow tank environment.

    The experimental portion of the effort built on prior experience with SMA-driven devices and culminated in benchtop and water tunnel demonstrations of a prototype SVLT design that met the performance specifications for this device in terms of amplitude and frequency response (Figures 3 and 4). Further important validation of the vortex leveraging concept was provided by a third element, two tow tank experiments that both provided flow visualization of predicted wake on wake interactions and measured velocity data indicating substantial mitigation of sailplane wake effects.



Figure 3: Overview of SMA devices and their application in VLT design.



Figure 4: Prototype SVLT demonstrator in the Carderock water tunnel (left); time history of lift on the demonstrator at 10 kts. tunnel speed.


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