Development and Testing of Deployable "Pop Up Vortex Generators" (PUVGs) Using SMA Actuation

    Fixed-vane vortex generators (VGs) have been in existence for over 50 years and are still among the most effective available flow control devices. However, it is generally acknowledged that once such fixed VGs have been configured to improve performance in one flight regime, they often penalize the performance in other operational conditions. Recent NASA-sponsored work at CDI included an effort to improve upon the fixed VG by making it deployable "on demand" and thereby eliminating any penalty when the generators are not needed (Figure 1). The technology enabling this improvement is the use of Shape Memory Alloy (SMA) based actuators to deploy Pop Up Vortex Generator (PUVG) devices. Because of the favorable force/stroke characteristics of SMA actuators, it is possible to design PUVGs so that they retract flush to the wing surface and so have negligible aerodynamic penalty when not deployed (Figure 2). In addition, novel self-locking actuation devices developed at CDI enable the maintenance of deployed PUVGs with no power expenditure.

Figure 1. Array of Pop-Up Vortex Generators (PUVGs) deployed (left) and stowed (right). Note that VGs can be individually deployed and stowed.

Figure 2: Top, retracted (left) and deployed (right) PUVGs actuated with NiTi SMA wire. Below, closeups showing minimal, near-flush height of retracted PUVG, and the large displacement of the deployed surface.

    A near full scale test of PUVG performance was conducted in the Glenn L. Martin Wind Tunnel (GLMWT) at the University of Maryland. The test wing was mounted vertically in the section, as shown in Figure 3; the wing was 24 in. in chord and 92 in. in span, was untwisted, and used an NACA 23015 airfoil section. An array of PUVGs was attached to the wing along the full span, with a device width to lateral spacing ratio of 1:3 at the 30% chord (maximum thickness) location. Five rows of tufts were attached to the wing at the 50-90% chord locations, at intervals of 10% of chord.

Figure 3: Overall view of vertically mounted wing model.

    Figure 4 shows a comparison of the flow over the upper (suction) surface of the wing at an angles of attack of 15 and 16 deg. and a chord Re of 1x106 (the nominal stall angle for the foil is 15 deg.) for wing segments where the PUVGs are retracted and deployed. The ability of the PUVGs to maintain attached flow in this case is clearly indicated.

Figure 4: Separated flow behavior and angles of attack near stall; split picture shows the effect of PUVG deployment (retracted above, deployed below)

    The same favorable effect is reflected in measurements of wing performance, as indicated in the enhancement of maximum lift coefficient evident in Figure 5. Full documentation of these results is available in the CDI project report to NASA, which will form part of an upcoming AIAA paper.

Figure 5: Effect of PUVG deployment on maximum CL (note: current retracted PUVGs impose a 0.6 count - 5% - penalty on profile drag; this number is susceptible of considerable improvement).

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