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Grid fins are unconventional control surfaces consisting of an outer frame supporting an inner grid of intersecting planar surfaces. Although afflicted with higher drag, these have been credited for their enhanced lifting characteristics at high angles of attack and high Mach numbers, alongside reduced hinge moments accounting for the recent upsurge in their usage on numerous aerospace applications. Present investigations carry out elaborate flow field visualization and characterization underlining the rudimentary physics through a sequence of subsonic numerical simulations performed at different angles of attack and different gap (between the members) to chord ratios on a simplified grid fin variant called cascade fin. The study makes use of a new nondimensionalization technique called cumulative nondimensionalization to decipher the effect of cascading on individual members of the fin. Hence, after a comprehensive examination of the aerodynamic coefficients, pressure coefficient distribution, pressure gradient, velocity gradient, boundary layer velocity profile, and flow field visualization, the study elucidates physics associated with hastened stall angle, augmented lift-drag, and bounded efficiency accretion for gap increment.

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In this paper, superhydrophobicity of multiscale hierarchical structures and wettability transition phenomenon of a water droplet on the superhydrophobic surface at different substrate temperature are extensively studied. Spin coating and spray coating methods have been regulated to fabricate EPF (Expanded polystyrene foam)/candle soot and EPF/camphor soot particle composites films. Maximum water contact angle of 165° and roll off angle of 2° were obtained with spray coated EPF/camphor soot composites which demonstrates their superhydrophobicity. By the measurement of water contact angle at different substrate temperature, we have demonstrated the influence of heat energy and pressure developed by micro water droplets during spreading phase. The reduction in water repellence observed on the protrusion induced hierarchical nanostructure is explained by the balancing pressure induced by a droplet of volume 4 µL and work of adhesion. The droplet contact area of hierarchical surfaces was profoundly influenced by change in viscosity of water droplets with temperature. The best performing spray coated EPF/camphor soot particle composite film impressively maintained superhydrophobicity with critical pressure of 200 Pa.The resulting insights can be used to tailor wettability of composite films, control liquid flow patterns for engineering applications such as micro-fluidics and bio-sensing devices.

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