Unsteady high-lift mechanisms from heaving flat plate simulations

Abstract

Flapping animal flight is often modeled as a combined pitching and heaving motion in order to investigate the unsteady flow structures and resulting forces that could augment the animal's lift and propulsive capabilities. This work isolates the heaving motion of flapping flight in order to numerically investigate the flow physics at a Reynolds number of 40,000, a regime typical for large birds and bats and challenging to simulate due to the added complexity of laminar to turbulent transition in which boundary layer separation and reattachment are traditionally more difficult to predict. Periodic heaving of a thin flat plate at fixed angles of attacks of 1, 5, 9, 13, and 18 degrees are simulated using a large-eddy simulation (LES). The heaving motion significantly increases the average lift compared with the steady flow, and also surpasses the quasi-steady predictions due to the formation of a leading edge vortex (LEV) that persists well into the static stall region. The progression of the high-lift mechanisms throughout the heaving cycle is presented over the range of angles of attack. Lift enhancement compared with the equivalent steady state flow was found to be up to 17% greater, and up to 24% greater than that predicted by a quasi-steady analysis. For the range of kinematics explored it is found that maximum lift enhancement occurs at an angle of attack of 13 degrees, with a maximum lift coefficient of 2.1, a mean lift coefficient of 1.04.

Reducing flow separation of an inclined plate via travelling waves

“…Until an angle of attack of , the results follow exactly the experimental results, but there are small discrepancies with the experimental results for higher angles of attack ( ), which can be related to the effect of Reynolds number, as the Reynolds number in the experiment of Pelletier & Mueller (2000) is 80 000. Nevertheless, the agreement of our LES with the experimental results is similar to that of Franck & Breuer (2017). Comparing the lift coefficient for of the 2-D plate and the 3-D flat inclined plate 2 reveals that the of a 3-D plate is approximately 39 % smaller than of 2-D plate due to the generation of spanwise flow, which also reduces flow separation, i.e.…”

“…Until an angle of attack of , the results follow exactly the experimental results, but there are small discrepancies with the experimental results for higher angles of attack ( ), which can be related to the effect of Reynolds number, as the Reynolds number in the experiment of Pelletier & Mueller (2000) is 80 000. Nevertheless, the agreement of our LES with the experimental results is similar to that of Franck & Breuer (2017).…”

“…In addition, the lift coefficient for an inviscid thin plate is plotted, where lift coefficient is (with in degrees). The results of this study agree well with the experimental data of Pelletier & Mueller (2000) and the LES results of Franck & Breuer (2017). Until an angle of attack of , the results follow exactly the experimental results, but there are small discrepancies with the experimental results for higher angles of attack ( ), which can be related to the effect of Reynolds number, as the Reynolds number in the experiment of Pelletier & Mueller (2000) is 80 000.…”

“…In order to further validate our simulations, a flat inclined 2-D plate is simulated at angles of attack ranging from to with , which is the same as in the LES of Franck & Breuer (2017). The grid dimension and resolution in and directions are the same as grid 2 (table 1), but there are 21 nodes with a constant spacing of along the spanwise direction.…”

Large Eddy simulation of a heaving wing on the Cusp of transition to turbulence

“…The work of [25] on two and three-dimensional pitching and plunging flat plates at Reynolds numbers O (10 4 ) and reduced frequency k = 0.2, considering shallow stall and a deep stall motion, showed massive leading-edge separation at the sharp leading edge of the flat plate. Similar observations were also made by [26] on a heaving flat plate.…”