The present study aims investigating experimentally wing/blade geometries in which the leading edge is modified by the presence of artificial bumps, following examples in nature (“biomimetics”). Specifically, the tubercles observed in humpback whales are considered with a special focus on easy manufacturing and performance improvements, trying to overcome the observed lift coefficient reduction before stall in comparison with a standard wing. To this end, different tubercle geometries are tested, by measuring overall forces acting on the wings and by deriving detailed velocity fields using particle image velocimetry. Measurements indicate performance improvements for all trailing edge tubercle geometries here tested. In addition, the detailed analysis of mechanisms underlying the improvement of performances suggests that a triangular shape of the leading edge combines the advantages of easy manufacturing and improvements of pre-stall behaviour. So far, a simple mathematical model, describing tubercles as delta wings, is presented and verified by experimental data. The objective of the present work is focusing on the basic fluid-mechanics phenomena involved, to show that beneficial effects of tubercles are present even when tubercle details are simplified, in order to couple performance improvement and ease of assembly.
Graphical Abstract
Contra-rotating propellers represent a non-conventional approach for marine propulsion whose main advantage lies in the increase of propulsive efficiency. This is achieved by recovering part of the energy loss due to the rotational flow generated by a single propeller by means of a contra-rotating downstream propeller. The hydrodynamics of such configuration is quite complex due to the interaction between upstream and downstream propellers and a deep understanding of their features is critical to driving the design phase. In this work a methodology based on planar (2D-2C) Particle Velocimetry is presented to investigate on the flow in the wake of two contra-rotating propellers. An ad-hoc mixed hardware and software phase-locking technique is developed in order to analyze the contribution of each propeller to the overall hydrodynamics of the system.
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