We experimentally investigate drop impact dynamics onto different superhydrophobic surfaces, consisting of regular polymeric micropatterns and rough carbon nanofibers, with similar static contact angles. The main control parameters are the Weber number We and the roughness of the surface. At small We, i.e., small impact velocity, the impact evolutions are similar for both types of substrates, exhibiting Fakir state, complete bouncing, partial rebouncing, trapping of an air bubble, jetting, and sticky vibrating water balls. At large We, splashing impacts emerge forming several satellite droplets, which are more pronounced for the multiscale rough carbon nanofiber jungles. The results imply that the multiscale surface roughness at nanoscale plays a minor role in the impact events for small We less than or approximately equal 120 but an important one for large We greater than or approximately equal 120. Finally, we find the effect of ambient air pressure to be negligible in the explored parameter regime We less than or approximately equal 150.

Drop Impact upon Micro- and Nanostructured Superhydrophobic Surfaces

We fabricated biomimetic hierarchical superhydrophobic surfaces using the Tobacco mosaic virus and investigated the role of each length scale during droplet impact by decomposing the micro and nanoscale components. We found that 10 μl water droplets rebounded at impact velocities greater than 4.3 m/s on the hierarchical surfaces, outperforming the nanostructured surfaces, which underwent an observable wetting transition at an impact velocity of 2.7 m/s. This finding demonstrates that each length scale plays a distinct, but complementary, role in maximizing water repellency during droplet impact and, thus, provides insight into the evolutionary development of highly water-repellant hierarchical plant leaves.

Biotemplated hierarchical surfaces and the role of dual length scales on the repellency of impacting droplets

Tuned bistable nonlinear energy sink for simultaneously improved vibration suppression and energy harvesting

Exploiting bi-stable magneto-piezoelastic absorber for simultaneous energy harvesting and vibration mitigation

Unmanned Aerial Vehicles have undergone rapid developments in recent decades. This has made them very popular for various military and civilian applications allowing us to reach places that were previously hard to reach in addition to saving time and lives. A highly desirable direction when developing unmanned aerial vehicles is towards achieving fully autonomous missions and performing their dedicated tasks with minimum human interaction. Thus, this paper provides a survey of some of the recent developments in the field of unmanned aerial vehicles related to safe autonomous navigation, which is a very critical component in the whole system. A great part of this paper focus on advanced methods capable of producing three-dimensional avoidance maneuvers and safe trajectories. Research challenges related to unmanned aerial vehicle development are also highlighted.

Towards Fully Autonomous UAVs: A Survey.

Micro aerial vehicles using flapping wings are under investigation, as an alternative to fixed-wing and rotary-wing micro aerial vehicles. Such flapping-wing vehicles promise key potential advantag...

Recent progress in flapping wings for micro aerial vehicle applications

Fluid-structure interaction of a fully passive flapping foil for flow energy extraction

Recent studies on understanding of natural flyers have encouraged researchers in development of micro aerial vehicles mimicking birds and insects such as hummingbirds, dragonflies, bats and many mo...

Recent advancements in flapping mechanism and wing design of micro aerial vehicles

Energy extraction through flapping foils is a new concept in the domain of renewable energy, especially when the system is fully driven by incoming free-stream flow, a phenomenon known as flow-induced vibration. To investigate this concept, a water tunnel test-rig was designed and fabricated, where a flat plate foil made from plexiglass performs two-degrees of freedom pitch and plunge motion under the influence of incoming water flow. For this study a power-takeoff system was not introduced, hence energy harvesting performance was evaluated through real-time force and motion measurements with the help of sensors. The energy harvester performed self-sustained flapping motions when the free-stream velocity reached a threshold value, known as the cut-off velocity, which for this test-rig is 0.40 m/s (without sensors) and 0.50 m/s (with sensors). To support these self-sustained flapping motions, inertial mass blocks were placed to provide the necessary inertia especially when the flat plate foil performed the pitching or stroke reversal action. Different inertial mass units (mib = 0.45, 0.90 & 1.35 kg/block) were tested to analyze their effect on the flat plate foil kinematics and its energy harvesting performance. Other parameters such as pitching amplitude (θo = 30, 43 & 60) and free-stream velocity (U∞ = 0.57 m/s, 0.65 m/s and 0.78 m/s) were varied at fixed pivot location (xp = 0.65 chords (c)) to augment the varying inertial mass unit study. In the first section at fixed mib of 0.45 kg/block and xp = 0.65c from leading edge, energy harvesting performance (C̅p & η) was observed to increase with increase in pitching amplitude, while it degraded as the free-stream velocity increased. Best energy harvesting performance of η = 52.5% and C̅p = 1.124 was achieved with mib = 0.45 kg/block, θo = 60 and U∞ = 0.57 m/s. Varying mib also had a considerable effect on the energy harvesting performance of the test-rig, where the mib = 0.90 kg/block case showed a 36.5% and 21.13% decline in performance compared to the mib = 0.45 and 1.35 kg/block cases, respectively at θo = 60 and U∞ = 0.57 m/s. This shows that the energy harvester is sensitive to changes in inertial loads, affecting the forcemotion synchronization which eventually affects its performance.

https://jafmonline.net/JournalArchive/download?file_ID=49812&issue_ID=1002

Muhammad Nafees Mumtaz Qadri

Papers

Recent progress in flapping wings for micro aerial vehicle applications

Fluid-structure interaction of a fully passive flapping foil for flow energy extraction

Recent advancements in flapping mechanism and wing design of micro aerial vehicles

An Experimental Investigation of a Passively Flapping Foil in Energy Harvesting Mode

Flow-energy harvesting using a fully passive flapping foil: A guideline on design and operation