Laminar wake suppression of airfoil by rotating rod at low Reynolds number

The objective of this research is to investigate the efficacy of Magnus effect on a circulation control airfoil (CCA). To achieve this, the Reynolds-averaged Navier–Stokes method is employed to numerically simulate the flow around a three-dimensional CCA configuration. The numerical verification demonstrates that the k–ω shear stress transport turbulence model provides better characterization of flow circulation and separation in the trailing edge area of the airfoil while ensuring the independence of the grid. The rotation speed is set at 15%, 30%, and 45% of the free-stream velocity, respectively, encompassing both clockwise and counterclockwise directions. The analysis focused on the characteristics of lift-drag forces, velocity circulation, and flow separation. The findings reveal that as the rotational speed increases in a clockwise direction, there is a progressive enhancement in aerodynamic performance of the airfoil. Both the flow separation point and stagnation point exhibit a backward shift in their respective positions. However, counterclockwise rotation produces an opposing effect. The amplitudes of fluctuating aerodynamic force coefficients are suppressed completely whether in clockwise or in counterclockwise rotation, resulting in a stable flow pattern consisting of a pair of vortices near the trailing-edge area. While the qualitative application of Bernoulli's principle explains the correlation between velocity circulation and surface pressure, there is a 12% margin of error in the quantitative analysis. Furthermore, evaluating the efficiency of circulation control while taking into account the input energy reveals that rotating the cylinder at 45% of free-stream velocity leads to a 23.6% increase in the lift-to-drag ratio.

Novel design of a circulation control airfoil with cylinder rotation

Zhang,

Zhao

2023

Wake suppression of a cylinder immersed in turbulence using rotating rods

Song,

Bao,

Zhang

et al. 2024

In this study, we conducted three-dimensional direct numerical simulations to investigate the control performance of the free-stream turbulence past a circular cylinder with two small rotating rods at Re = 3900. In the case A and case B, two control rods with different rotating ratio α are positioned symmetrically at the circumferential angle θ=±45° and θ=±120° from the forward stagnation point of the main cylinder, respectively. Here, α is the ratio of the tangential velocity of the control rod surface ur to the inflow velocity U∞. Compared to the bare cylinder immersed in turbulence, both installation positions of the rotating rods at α = 2 result in a significant suppression on the wake flow. Specifically, the time-averaged drag coefficient Cd¯ was reduced by 25%, and the root mean square value of the lift coefficient measured to reduce by up to 30%. Statistical analysis is then performed, in terms of the Reynolds stresses, mean field, and the turbulent wake visualization to show variations in the flow dynamics. As expected, the front-mounted control rods inject kinetic energy into the boundary layer, effectively suppressing the turbulence fluctuations on both sides of the main cylinder. The work done by viscous forces around the rotating control rods and the suppression of turbulent fluctuations contribute to the pressure recovery observed in the rear-mounted control rod case. Proper orthogonal decomposition method is further employed to analyze the key features of the controlled wake of two cases with α = 2.

Ground-induced suppression of chaos in the self-excited flow behind a plunging airfoil

Chung,

Guan,

et al. 2024

We numerically investigate the forced synchronization of the self-excited flow behind a plunging airfoil in ground effect at a Reynolds number of Re = 1000. On varying the plunging amplitude and frequency, we find a rich array of nonlinear dynamics, such as a period-1 limit cycle due to natural vortex shedding as well as two-frequency quasiperiodicity on a torus attractor (T2). For certain non-resonant plunging frequencies without a ground surface, we find that low-dimensional chaos emerges via the Ruelle–Takens–Newhouse route. However, we find that the chaos can be suppressed by introducing a ground surface, inducing a direct transition from T2 quasiperiodicity to 1:1 phase locking as the plunging amplitude rises over the boundaries of the Arnold tongue. Apart from suppressing chaos, the ground surface also causes the lift and drag coefficients to become less sensitive to the plunging motion itself. Knowledge of the critical plunging conditions required for forced synchronization and chaos could be useful in various engineering applications, such as the design of pico air vehicles.