The Doppler effect refers to the change in frequency of a wave source as a consequence of the relative motion between the source and an observer. Veselago theoretically predicted that materials with negative refractions can induce inverse Doppler effects. With the development of metamaterials, inverse Doppler effects have been extensively investigated. However, the ideal material parameters prescribed by these metamaterial design approaches are complex and also challenging to obtain experimentally. Here, we demonstrated a method of designing and experimentally characterising arbitrary broadband acoustic metamaterials. These omni-directional, double-negative, acoustic metamaterials are constructed with ‘flute-like’ acoustic meta-cluster sets with seven double meta-molecules; these metamaterials also overcome the limitations of broadband negative bulk modulus and mass density to provide a region of negative refraction and inverse Doppler effects. It was also shown that inverse Doppler effects can be detected in a flute, which has been popular for thousands of years in Asia and Europe.

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The design and manufacture of a second-generation adaptive damper composed of an electrorheological (ER) fluid, and piezoelectric ceramics is presented in this paper. Compared with the first-generation dampers, there are three aspects of modification adopted in the design of this. These include (a) the number, mode of arrangement, and electrocircuit connecting style of piezoceramics, (b) load applying mechanism, and (c) holechannel in the concentric cylinder electrode. The experiment conducted shows that the amplitude of frequency response function decreases drastically in the frequency range of 220-250 Hz under an exciting acceleration amplitude of 3.5 g (g the gravitational acceleration) and the resonance frequency moves from 198 to 208 Hz. The new damper excels the old one in antivibration performance. In the on-off state model, numerical simulations clearly show the vibration suppression effect of the damper. The design method of combining the ER fluid and the piezoelectric ceramic as a self-coupled system can be implemented extensively in other control systems, such as acoustic insulation and control.

Dynamic soaring improves the endurance of Unmanned Aerial Vehicles (UAVs) by obtaining energy from the horizontal wind shear gradient. The use of dynamic soaring in small solar UAVs can mitigate the trade-off between energy capacity and battery weight to achieve continuous all-day flight. The goal of this study is to determine the optimal energy acquisition methods for small solar UAVs using dynamic soaring and to decrease the battery weight to achieve all-day flight. A dynamic soaring UAV model that considers the influence of the wind shear gradient and a solar power energy model are established. The conditions to obtain a closed-loop energy system during daytime and nighttime flights are discussed, and the minimum mass of the energy system required for these conditions is determined. Simulations of single-cycle circular flights and a 72-h continuous flight of a small solar UAV are performed. The analyses and simulation results show that: (1) the combination of dynamic soaring and solar technology significantly reduces the energy consumption and reduces the required battery weight, (2) the flight speed and flight attitude angles have significant effects on the optimal total energy acquisition and (3) wind fields with a large horizontal gradient and strong solar illumination provide energy and load advantages.