An energy-efficient pathway to turbulent drag reduction.

Simulations and experiments at low Reynolds numbers have suggested that skin-friction drag generated by turbulent fluid flow over a surface can be decreased by oscillatory motion in the surface, with the amount of drag reduction predicted to decline with increasing Reynolds number. Here, we report direct measurements of substantial drag reduction achieved by using spanwise surface oscillations at high friction Reynolds numbers ([Formula: see text]) up to 12,800. The drag reduction occurs via two distinct physical pathways. The first pathway, as studied previously, involves actuating the surface at frequencies comparable to those of the small-scale eddies that dominate turbulence near the surface. We show that this strategy leads to drag reduction levels up to 25% at [Formula: see text] = 6,000, but with a power cost that exceeds any drag-reduction savings. The second pathway is new, and it involves actuation at frequencies comparable to those of the large-scale eddies farther from the surface. This alternate pathway produces drag reduction of 13% at [Formula: see text] = 12,800. It requires significantly less power and the drag reduction grows with Reynolds number, thereby opening up potential new avenues for reducing fuel consumption by transport vehicles and increasing power generation by wind turbines.

Are surgical masks manufactured from sterilisation wrap safe?

BACKGROUND: Due to regional shortages some health services have proposed using surgical masks manufactured from sterilisation wrap. However, there has been little assessment of the safety of this practice. Therefore, we developed our own prototypes and evaluated whether they met regulatory standards. METHODS: Surgical mask prototypes were manufactured from two thickness grades of commercial sterilisation wrap. Safety was assessed in the context of regulatory standards. As it was not previously reported, we developed and performed differential pressure and synthetic blood penetration resistance experiments in accordance with official methodology. RESULTS: Bacterial filtration efficiency was comparable between sterilisation wrap and commercial surgical masks. Both prototypes met regulatory standards for synthetic blood resistance, whilst only our thinner mask fulfilled acceptable differential pressure ('breathability') thresholds. CONCLUSION: Acceptable barrier and breathability properties can be achieved with surgical masks produced from sterilisation wrap. Therefore, this may be a reasonable method to supplement stock if required. Unless there are shortages mandating alternatives, health-care workers should always use approved personal protective equipment.