We describe the experimental generation of isolated vortical gusts and the interaction between these gusts and a downstream airfoil at a Reynolds number of 20,000. A standard method of generating a vortical gust has been to rapidly pitch an airfoil. A different approach is presented here: heaving a plate across a tunnel and changing direction rapidly to release a vortex. This method is motivated by the desire to limit a test article's exposure to the wake of the gust generator by moving it to the side of the tunnel. Two suites of experiments were performed to characterize the performance of the gust generators and to measure the forces on and flow around the downstream airfoil. The novel mechanism allowed for measurement of the resumption of vortex shedding from the downstream airfoil, which was impossible with the pitching generator. Nomenclature C L = lift coefficient, lift/(dynamic pressure × wing area) c a = airfoil chord length, m c p = plate chord length, m h = plate heave distance, m Re c = Reynolds number with respect to airfoil's chord length S = heaving speed ratio, V heave ∕U T = heaving time ratio, t heave ∕t cp t = time, s t a = normalized plate acceleration time, t accel ∕t cp t accel = plate acceleration time, s t c = chordwise convective time across airfoil, s t cp = chordwise convective time across plate, s t heave = heaving time, s t p = normalized airfoil pitching time, t pitch ∕t c t pitch = generator pitching time, s U = freestream speed, m∕s u = velocity in x direction, m∕s V heave = plate heaving speed, m∕s v = velocity in y direction, m∕s x = streamwise coordinate, m x 0 = streamwise position computed from unwrapping process, m y = cross-stream coordinate with respect to tunnel midline, m y 0 = initial plate distance from midline, m y peak = cross-stream position of maximum displacement of heaving plate, m y plate = cross-stream position of heaving plate, m y upstream = cross-stream position of pitching gust generator, m z = spanwise coordinate, m α = angle of attack of test article, rad (unless otherwise noted) α 1 = initial angle of pitching gust generator, rad (unless otherwise noted) α 2 = final angle of pitching gust generator, rad (unless otherwise noted) α eff = effective angle of attack of heaving gust generator, rad (unless otherwise noted) α upstream = angle of attack of pitching gust generator, rad (unless otherwise noted) Γ v = circulation of shed vortex, m 2 ∕s Γ v;est;heave = estimated circulation of shed vortex from heaving generator, m 2 ∕s Γ v;est;pitch = estimated circulation of shed vortex from pitching generator, m 2 ∕s Γ 2 = vortex identification criterion Δx = streamwise distance between generator and test article, m τ a = normalized time, t∕t c ω z = vorticity measured in x-y plane, s −1

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This work was supported by the "RevealFlight" Concerted Research Action (ARC) of the Federation Wallonie-Bruxelles Groups of aircraft have the potential to save significant amounts of energy by flying in formations; all but the leading aircraft can benefit from the upwash of the wakes of preceding aircraft. A potential obstacle as the number of aircraft in such a formation increases is that disturbances at one aircraft, for example caused by turbulence or wake meandering, can propagate and grow as each following aircraft tries to track the optimal energy-saving position relative to the one in front. This phenomenon, known as string instability, has not yet been adequately examined in the context of aircraft formations. We discuss some trade-offs involved in designing string stable controllers whose objective is to minimize energy, and present a control design method to achieve both string stability and energy efficiency of an aircraft formation. In simulations of a 10-aircraft linear formation in the presence of 2% turbulence intensity, our controller achieves string stability while reduced energy consumption by an average of 13% with respect to solo flight.

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