J.C.S. Lai

Flapping Wing Aerodynamics: Progress and Challenges

It is the objective of this paper to review recent developments in the understanding and prediction of flapping-wing aerodynamics. To this end, several flapping-wing configurations are considered. First, the problem of single flapping wings is treated with special emphasis on the dependence of thrust, lift, and propulsive efficiency on flapping mode, amplitude, frequency, and wing shape. Second, the problem of hovering flight is studied for single flapping wings. Third, the aerodynamic phenomena and benefits produced by the flapping-wing interactions on tandem wings or biplane configurations are discussed. Such interactions occur on dragonflies or on a recently developed micro air vehicle. The currently available two- and three-dimensional inviscid and viscous flapping-wing flow solutions are presented. It is shown that the results are strongly dependent on flapping frequency, amplitude, and Reynolds number. These findings are substantiated by comparison with the available experimental data.

Jet characteristics of a plunging airfoil

Water-tunnel tests of a NACA 0012 airfoil that was oscillated sinusoidally in plunge are described. The flowered downstream of the airfoil was explored by dye flow visualization and single-component laser Doppler velocimetry (LDV) measurements for a range of freestream speeds, frequencies, and amplitudes of oscillation. The dye visualizations show that the vortex patterns generated by the plunging airfoil change from drag-producing wake flows to thrust-producing jet flows as soon as the ratio of maximum plunge velocity to freestream speed, i.e., the nondimensional plunge velocity, exceeds approximately 0.4. The LDV measurements show that the nondimensional plunge velocity is the appropriate parameter to collapse the maximum streamwise velocity data covering a nondimensional plunge velocity range from 0.18 to 9.3

Oscillation frequency and amplitude effects on the wake of a plunging airfoil

The flow over a NACA 0012 airfoil, oscillated sinusoidally in plunge, is simulated numerically using a compressible two-dimensional Navier-Stokes solver at a Reynolds number of 2 ×10 4 . The wake of the airfoil is visualized using a numerical particle tracing method. Close agreement is obtained between numerically simulated wake structures and experimental wake visualizations in the literature, when the flow is assumed to be fully laminar. The wake structures, and the lift and thrust of the airfoil, are shown to be strongly dependent on both the Strouhal number and the reduced frequency k of the plunge oscillation at this Reynolds number. Leading-edge separation appears to dominate the generation of aerodynamic forces for reduced frequencies below approximately k = 4 but becomes secondary for higher frequencies. Wake structures appear to be controlled primarily by trailing-edge effects at all frequencies tested up to k=20. Aerodynamic force results obtained at this Reynolds number show marked differences from those predicted by potential flow analyses at low plunge frequency and high amplitude but are similar at high frequency and low amplitude, consistent with the effect of leading-edge separation.

Brake squeal: a literature review

Abstract Brake squeal, which usually falls in the frequency range between 1 and 16 kHz, has been one of the most difficult concerns associated with automotive brake systems since their inception. It causes customer dissatisfaction and increases warranty costs. Although substantial research has been conducted into predicting and eliminating brake squeal since the 1930s, it is still rather difficult to predict its occurrence. In this paper, the characteristics and current difficulties encountered in tackling brake squeal are first described. A review of the analytical, experimental and numerical methods used for the investigation of brake squeal is then given. Some of the challenges facing brake squeal research are outlined.

Mechanisms Influencing the Efficiency of Oscillating Airfoil Propulsion

A NACA0012 airfoil undergoing pitching and plunging motion at Re = 20,000-40,000 was simulated using a two-dimensional Navier-Stokes flow solver. Results were compared with experimental measurements in the literature and those from an inviscid analytical method and an unsteady panel method code. Although the peak in propulsive efficiency with Strouhal number demonstrated in the experimental results was predicted by the inviscid methods, it was found to be significantly modified by leading-edge vortex shedding and viscous drag at low Strouhal numbers. The occurrence and influence of vortex shedding is controlled by both the motion of the airfoil (amplitudes and phases of plunging and pitching) and the flapping frequency, which limits the time available for vortex formation and convection over the airfoil surface. Thus, Strouhal number alone is insufficient to characterize the efficiency of flapping-foil propulsion.

Comparison of flow characteristics in the near field of two parallel plane jets and an offset plane jet

Two parallel plane air jets and offset air jets have several common features such as the existence of a subatmospheric pressure region and the formation of a flow recirculation zone adjacent to the nozzle plate. In particular, the symmetry plane that exists between two parallel plane jets may appear to affect the flow field in much the same way as a solid wall does in a reattaching offset jet. It is obvious, however, that there are significant differences far downstream from the nozzles because the two parallel plane jets will combine to form a single free jet while the offset jet will develop into a wall jet. Differences in the near field between these two jet configurations with a separation ratio of 2.125 are examined here under identical initial flow exit conditions through laser Doppler anemometer measurements of the mean velocity components, turbulence intensities, and Reynolds shear stress. The results indicate that the wall exerts significant retarding and turbulence suppression effects on the off...

Numerical Analysis of an Oscillating-Wing Wind and Hydropower Generator

The extraction of energy from wind or water streams is generally accomplished by means of rotary systems. However, it is recognized and it has been demonstrated that oscillating wings can also be used for this purpose. A newly developed oscillating-wing wind and hydropower generator is described. Its potential for the generation of electric power from tidal flows and high-altitude jet streams is studied using two-dimensional Navier―Stokes simulations at Re = 20, 000. Results for a single NACA 0014 wing power generator undergoing nonsinusoidal pitch― plunge motion indicate around 17 % increase in power generated and around 15 % increase in efficiency over that for sinusoidal motion. Two airfoils operating in tandem, undergoing both sinusoidal and nonsinusoidal motions, are also studied. It is found that for sinusoidal motion both averaged power output and efficiency per foil are reduced by around 20% for tandem configurations compared with a single foil in sinusoidal motion, and similar performance reductions are experienced for nonsinusoidal motions.

Prediction of natural frequencies of finite length circular cylindrical shells

Abstract Analysis of the vibration characteristics of finite circular cylindrical shells is more complex than for beams and plates. This is because the coupling of vibration of shells between the three directions can no longer be neglected. A literature review of the subject reveals that in traditional analysis, assumptions are made to simplify the equations of motion so that they can be solved directly with the appropriate boundary conditions. Consequently, the results obtained could be in error under certain conditions. Based on the Loves equations and an infinite length model, a novel wave approach is introduced to predict the natural frequencies of finite length circular cylindrical shells with different boundary conditions without simplifying the equations of motion. Results obtained compare favourably with those obtained using the finite element method.

Vortex Lock-In Phenomenon in the Wake of a Plunging Airfoil

The flow over a NACA0012 airfoil, oscillated sinusoidally in plunge, is simulated numerically using a two-dimensional Navier-Stokes solver at a Reynolds number of 20,000. The wake of the airfoil is visualized using a numerical particle tracing method for high reduced frequencies (1.0 <k< <10.0) and small nondimensional amplitudes (h < 0.1). Anomalous vortex shedding modes (involving multiple vortices shed per half-cycle of airfoil motion) observed experimentally in the literature are reproduced numerically and are shown to be the result of interaction between the plunging frequency and a natural bluff-body shedding frequency. This results in a vortex lock-in phenomenon analogous to that seen for oscillating cylinders. However, the lock-in boundary is not symmetric about the natural shedding frequency, due to the sharp trailing edge forcing the flow to separate at the trailing edge on the windward side of the airfoil for the majority of the plunge cycle at higher frequencies and amplitudes.

Numerical Simulation of Fully Passive Flapping Foil Power Generation

A fully passive flapping foil turbine was simulated using a two-dimensional Navier–Stokes solver with two-way fluid-structure interaction at a Reynolds number based on freestream flow Re=1100 and 1.1×106 with a NACA 0012 foil. Both pitch angle and angle-of-attack control methodologies were investigated. Efficiencies of up to 30% based on the Betz criterion were found using pitch control, which is commensurate with values reported in the literature for prescribed motion studies. Nonsinusoidal foil pitching motions were found to be superior to sinusoidal motions. Efficiencies exceeding 41% were found using angle-of-attack control, and nonsinusoidal angle-of-attack profiles were found to be superior. The key to improving the efficiency of energy extraction from the flow is to control the timing of the formation and location of the leading-edge vortex at crucial times during the flapping cycle and the interaction of the vortex with the trailing edge. Simulations using Reynolds-averaged Navier–Stokes turbulenc...