arXiv subjects

David Greenblatt

Publications and source records attributed to David Greenblatt.

4 recordsLinked to original sources

arXiv2026

A Universal Vortex-Formation Law for Insect Hovering

Flapping insect wings generate leading-edge vortices (LEVs) that produce much of the lift required for hovering, yet the frequency governing their formation has not been connected to classical vortex shedding. Roshko scaling for bluff-body wakes, together with the Sigurdson extension to wall-bounded separated flows, identifies a universal vortex-formation Strouhal-number range of approximately 0.15 to 0.17. Here we show that this same range governs LEV formation on both stationary wings and hovering insect wings. We first develop an approximate scaling for stationary flat-plate wings in which discrete LEVs are generated by periodic leading-edge perturbations, and show that maximum lift consistently occurs within the universal Strouhal range. In hovering insects, one LEV forms during each translational half-stroke, allowing the same framework to be applied to published flight data. Despite large differences in morphology, kinematics and Reynolds number, the insect data recover the same Strouhal range. These results reveal a common vortex-formation timescale linking classical separated flows, artificially generated LEVs and the natural LEVs that sustain insect hovering.

physics.flu-dyn↗

arXiv2026

Nanosecond DBD-Induced Shock and Thermal Perturbations on Blunt Bodies in Hypersonic Flow

Nanosecond-pulsed dielectric barrier discharge (DBD) plasma actuators were investigated on a generic blunt body in a Mach 6 Ludwieg tube to characterize the pressure and thermal perturbations relevant to hypersonic boundary-layer transition control. Complementary quiescent experiments were also conducted over ambient pressures representative of those predicted in the model nose region to isolate the influence of local thermodynamic conditions on actuator operation. Pulse-energy measurements and schlieren imaging showed that decreasing pressure reduced the deposited electrical energy per pulse, weakened the actuator-generated shock, and increased the spatial extent of the residual heated region owing to energy deposition over a larger plasma volume. Under Mach 6 Ludwieg-tube conditions, the actuator-generated shock interacted with and reflected from the detached bow shock, temporarily increasing the bow-shock stand-off distance by approximately 11%, while the residual heated region was advected downstream along the body. The schlieren images further permitted the evolution of the thermal disturbance to be distinguished from that of the actuator-generated shock. The results demonstrate two distinct perturbation mechanisms -- a short-duration compression wave and a longer-lived thermal disturbance -- whose relative importance is governed by the local thermodynamic conditions and which may independently promote hypersonic boundary-layer transition.

physics.flu-dyn↗

arXiv2025

Frequency Scaling Laws for Flat Plate Wing Active Separation Control

Dimensionless frequency scaling laws for active separation control on flat-plate wings, using dielectric barrier discharge plasma actuators, were examined on the basis of maximum increases to lift coefficient, and compared with hovering insect wing-flapping frequencies. Data for a range of angles of attack ($24^\circ$ to $32^\circ$), Reynolds numbers (3,000 to 20,000) and semispan wing aspect ratios (0.75 to $\infty$), collapsed best when scaled with the streamwise-directed height of the chord length. The ``forcing Strouhal number'' that produced the largest lift coefficient increments, equal to $0.26 \pm 0.04$, was linked to the conventional bluff-body Strouhal number by recognizing that drag and lift on flat plate wings are directly proportional at a fixed angle of attack. Flowfield measurements, to determine the time-averaged separation bubble height and local velocity at separation, showed that the universal Strouhal number of approximately $0.16 \pm 0.01$ --developed for bluff-body and separation bubble vortex shedding--can be further generalized to active separation control, when the natural shedding frequency is substituted by the forcing frequency. Insect wing flapping frequencies in hover were examined on the basis of Strouhal number scaling and corresponded reasonably well to the optimum forcing Strouhal number range, although angle of attack estimates were a source of uncertainty. Furthermore, universal Strouhal number scaling can only be validated with accurate separation bubble height and separation velocity measurements.

physics.flu-dyn↗

arXiv2022

Airfoil Boundary Layer Bubble Separation and Transition in a Surging Stream

The effect of high-amplitude harmonic surging on airfoil laminar separation bubbles was investigated theoretically, and experimentally in a dedicated surging-flow wind tunnel. A generalized pressure coefficient was developed that accounts for local static pressure variations due to surging. This generalization, never previously implemented, facilitated direct comparisons between surging and quasi-steady pressure coefficients, and thus unsteady effects could be distinguished from Reynolds number effects. A momentum-integral boundary layer analysis was implemented to determine movement of the bubble separation point, and movement of the transition point was extracted from experimental surface pressure coefficients. The most significant finding was that bubble bursting occurs, counterintuitively, during early imposition of the favorable temporal pressure gradient. This is because the favorable pressure gradient rapidly drives the bubble aft, rendering it unable to reattach. Bursting resulted in large lift and form-drag coefficient oscillations, and failure to implement the generalized pressure coefficient definition resulted in temporal form-drag errors of up to 400 counts.

physics.flu-dyn↗