Vortical Gusts: Experimental Generation and Interaction with Wing

Abstract

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

Analysis of vortical gust impact on airfoils at low Reynolds number

“…For MAVs, the small size of the vehicle results in a localized gust, where the characteristic length of the velocity perturbation is of the same order of magnitude (or even smaller) than the chord of the airfoil. Most experimental and numerical realization of these types of gusts are based on either airfoils/wings passing over a vertical jet (i.e, localized transverse gusts as in 24,6,15,9), or airfoils/wings encountering a localized vortex in the free-stream (i.e., vortical gusts as in 12,4,30,1,22,21,5,7). The results of the localized transverse gusts are very similar to the plunge maneouvers discussed earlier when the jet width is sufficiently large [15,9].…”

Generation of periodic gusts with a pitching and plunging airfoil

“…In order to minimize vorticity effects in the flow downstream of the gust generator, the wake of the generator itself should be minimized along with the persistence of dynamic effects. To this end, Hufstedler and McKeon (2019) developed a vortical gust generator using an impulsively plunging plate. The plunging motions of the plate meant that the wake vorticity from the plate had a limited effect on the gust region, since the wake vorticity would only cross through the measurement domain twice per cycle.…”

“…Larose, 1999). The vortices shed by an impulsively plunging plate can also be used to represent gust disturbances (Hufstedler and McKeon, 2019).…”

representative driven system to interrogate passive dynamics of an airfoil in the wake of a cylinder

“…Lefebvre and Jones (2019) also demonstrated mean thrust production, even for a stationary airfoil, for most of the cases considered in the study when the geometric angle of attack remained moderate. To connect instantaneous dynamics with the passage of particular structures in the flow, Hufstedler and McKeon (2019) collected time-resolved PIV as well as force measurements of a stationary airfoil encountering an isolated vortical gust. Such studies of stationary airfoils provide a basis to interpret results for more complex studies that allow for either actuated or passive airfoil motion.…”