Metamaterials for Hypersonic Flow Control: Experimental Tests on Novel Ultrasonically Absorptive Coatings

Since the drag and heat transfer rates in hypersonic turbulent boundary layer can be several times higher than those of a laminar one, different strategies to delay the transition from laminar to turbulent boundary layer are currently investigated. One possibility to delay the transition is by using porous surfaces called ultrasonically absorptive coating (UAC). This type of metamaterial scatters the ultrasonic waves propagating within the laminar boundary layer that promote the transition. The acoustic scattering problem affects the design of UAC for hypersonic laminar flow control. The present research is focused on the experimental studies to custom-make and test a novel UAC concept. A comparison between a conventional and a novel UAC has been performed to investigate the effectiveness of the innovative concept herein proposed. The conventional UAC, constructed according to the theoretical prescription, is based on cylindrical cavities, whereas the new strategy consists in a flat plate where the top surface is characterized by a regular distributed pyramidal pattern. The main idea is to promote the scattering of the incident ultrasonic wave to inhibit the boundary layer transition precursors. The proposed surface machining also allows to reduce the overall processing time, which is a primary requirement for the engineering of this type of technology. For the sake of completeness, another UAC geometry that can be considered a middle ground between the classical and the novel one is investigated, tested and discussed. To the best of our knowledge the control approach proposed in the present manuscript is a no man's land in the field of hypersonic fluid mechanics.