A. Dropkin

Bio: A. Dropkin is an academic researcher from Naval Undersea Warfare Center. The author has contributed to research in topics: Lift-to-drag ratio & Airfoil. The author has an hindex of 1, co-authored 1 publications receiving 53 citations.

An experimental study on flow separation control of hydrofoils with leading-edge tubercles at low Reynolds number

Abstract: Hydrodynamic characteristics of hydrofoils with leading-edge tubercles were experimentally investigated in a water tunnel at a Reynolds number of Re=1.4×104. Particle image velocimetry measurements and particle-streak visualizations reveal that the tubercles improve flow separation behaviour. In particular, hydrofoils with larger wave amplitudes and smaller wavelengths tend to perform significantly better in flow separation control. Cross-stream flow measurements indicate that streamwise counter-rotating vortex pairs are generated over the tubercles and mitigate flow separation. Analysis confirms that the tubercles function as vortex generators, due to their comparable heights relative to the boundary layer thickness. The vortex pairs meander and interact with adjacent flows, causing the flow separation behaviour to be occasionally unstable, thus leading to variable flow separation region sizes. This suggests that measures may have to be taken to ensure the stability of the counter-rotating vortex pairs for more persistent and predictable improvements.

Experimental study of flow separation control on a low-Re airfoil using leading-edge protuberance method

Abstract: An experimental study of flow separation control on a low-Re c airfoil was presently investigated using a newly developed leading-edge protuberance method, motivated by the improvement in the hydrodynamics of the giant humpback whale through its pectoral flippers. Deploying this method, the control effectiveness of the airfoil aerodynamics was fully evaluated using a three-component force balance, leading to an effectively impaired stall phenomenon and great improvement in the performances within the wide post-stall angle range (22°–80°). To understand the flow physics behind, the vorticity field, velocity field and boundary layer flow field over the airfoil suction side were examined using a particle image velocimetry and an oil-flow surface visualization system. It was found that the leading-edge protuberance method, more like low-profile vortex generator, effectively modified the flow pattern of the airfoil boundary layer through the chordwise and spanwise evolutions of the interacting streamwise vortices generated by protuberances, where the separation of the turbulent boundary layer dominated within the stall region and the rather strong attachment of the laminar boundary layer still existed within the post-stall region. The characteristics to manipulate the flow separation mode of the original airfoil indicated the possibility to further optimize the control performance by reasonably designing the layout of the protuberances.

Aerodynamic Characteristics of Finite Span Wings with Leading-Edge Protuberances

Abstract: A series of water-tunnel experiments were conducted to determine the effect of sinusoidal leading-edge protuberances on the aerodynamic characteristics of finite span wings. The models consisted of seven rectangular planform wings, two swept-leading-edge wings, and two wings with a planform resembling humpback-whale flippers. All models had an underlying NACA 634-021 profile with protuberance amplitudes of 0.025–0.12 times the chord length. The models were examined at Reynolds numbers up to 4.5×105 and angles of attack up to 30 deg. The lift and drag coefficients were nearly independent of Reynolds numbers above 3.6×105. Specific rectangular-planform models had appreciably greater lift coefficients over a limited angle-of-attack range when compared to the baseline model. However, with the exception of the planform that resembled the humpback-whale flipper, the lift-to-drag ratio of all leading-edge modified models was comparable to or less than the equivalent baseline model. The flipper model had a slight...

Mimicking the humpback whale: An aerodynamic perspective

Abstract: This comprehensive review aims to provide a critical overview of the work on tubercles in the past decade. The humpback whale is of interest to aerodynamic/hydrodynamic researchers, as it performs manoeuvres that baffle the imagination. Researchers have attributed these capabilities to the presence of lumps, known as tubercles, on the leading edge of the flipper. Tubercles generate a unique flow control mechanism, offering the humpback exceptional manoeuverability. Experimental and numerical studies have shown that the flow pattern over the tubercle wing is quite different from conventional wings. Research on the Tubercle Leading Edge (TLE) concept has helped to clarify aerodynamic issues such as flow separation, tonal noise and dynamic stall. TLE shows increased lift by delaying and restricting spanwise separation. A summary of studies on different airfoils and reported improvement in performance is outlined. The major contributions and limitations of previous work are also reported.

A large-eddy simulation on a deep-stalled aerofoil with a wavy leading edge

Abstract: A numerical investigation on the stalled flow characteristics of a NACA0021 aerofoil with a sinusoidal wavy leading edge (WLE) at chord-based Reynolds number Re∞= 1.2×105 and angle of attack α =20◦ is presented in this paper. It is observed that laminar separation bubbles (LSBs) form at the trough areas of the WLE in a collocated fashion rather than uniformly/periodically distributed over the span. It is found that the distribution of LSBs and their influence on the aerodynamic forces is strongly dependent on the spanwise domain size of the simulation, i.e. the wavenumber of the WLE used. The creation of a pair of counter-rotating streamwise vortices from the WLE and their evolution as an interface/buffer between the LSBs and the adjacent fully separated shear layers are discussed in detail. The current simulation results confirm that an increased lift and a decreased drag are achieved by using the WLEs compared to the straight leading edge (SLE) case, as observed in previous experiments. Additionally, the WLE cases exhibit a significantly reduced level of unsteady fluctuations in aerodynamic forces at the frequency of periodic vortex shedding. The beneficial aerodynamic characteristics of the WLE cases are attributed to the following three major events observed in the current simulations: (i) the appearance of a large low-pressure zone near the leading edge created by the LSBs; (ii) the reattachment of flow behind the LSBs resulting in a decreased volume of the rear wake; and, (iii) the deterioration of von-Karman (periodic) vortex shedding due to the breakdown of spanwise coherent structures.