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Tidal currents are renewable and predictable energy sources that could prove fundamental to decrease dependency from fossil fuels. Tidal currents, however, are highly unsteady and non uniform, resulting in undesirable load fluctuations on the blades and the drive train of turbines. A passive morphing blade concept capable to reduce the load fluctuations without affecting the mean loads has recently been formulated and demonstrated with numerical simulations (Pisetta et al., 2022). In this paper, we present the first demonstration of this morphing blade concept, through experimental tests on a 1.2 m diameter turbine. We show that fluctuations in the root-bending moment, thrust and torque are consistently reduced over a broad range of tip-speed ratios. This work also highlights some critical design aspects of morphing blades. For instance, it is showed that the friction resistance can substantially decrease the effectiveness of the system and thus must be minimised by design. Overall this paper demonstrates for the first time the effectiveness of morphing blades for tidal turbines, paving the way to the future development of this technology., 28 pages, 12 figures (11 in main body, 1 in appendix). Dataset available at https://doi.org/10.7488/ds/3483

The SARS-CoV-2 virus is primarily transmitted through virus-laden fluid particles ejected from the mouth of infected people. Face covers can mitigate the risk of virus transmission but their outward effectiveness is not fully ascertained. Objective: by using a background oriented schlieren technique, we aim to investigate the air flow ejected by a person while quietly and heavily breathing, while coughing, and with different face covers. Results: we found that all face covers without an outlet valve reduce the front flow through by at least 63% and perhaps as high as 86% if the unfiltered cough jet distance was resolved to the anticipated maximum distance of 2-3 m. However, surgical and handmade masks, and face shields, generate significant leakage jets that may present major hazards. Conclusions: the effectiveness of the masks should mostly be considered based on the generation of secondary jets rather than on the ability to mitigate the front throughflow.

Tidal energy has the potential to significantly contribute to energy security by providing predictable renewable energy. New technology is needed to decrease the levelised cost of energy and to make this energy sector competitive in the energy market. A key area where technology can contribute to decrease costs is mitigating the hydrodynamic load fluctuations, and thus increasing the fatigue life of the turbine. Here, we formulate a passive morphing blade concept that aims to mitigate the unsteady thrust without affecting the mean torque and thus the harvested power.We show that a blade with a trailing edge that deflects perfectly elastically can suppress virtually all fluctuations without varying the mean loads. The effect of the hydrodynamic and blade’s inertia, the material damping, and the radial shear stress, decrease the performances.Using a low-order model of the blade, we show that when a gust occurs, the angle of attack experienced by a rigid blade increases, whilst that experienced by a well-designed morphing blade decreases. This counter-intuitive mechanism is what makes morphing blades highly effective. While blades that could pas- sively twist have previously been developed, this theoretical study suggests that chordwise flexibility is a suitable alternative that should be further explored. Keywords: unsteady load mitigation, passive load control, pitch control, morphing, fluid-structure interaction, tidal turbine

Tidal turbines experience large load fluctuations due to the unsteady environment and the shear in the tidal flow. Mitigating these fluctuations without affecting the mean load would result in lower capital and operational costs. In this paper we discuss how this could be achieved through blades that passively and elastically adapt their camber and angle of attack to counteract unsteady flow conditions. Firstly, we discuss the underlying principles of unsteady thrust mitigation. We show that complete cancellation of the thrust fluctuations would be possible if every blade section could pitch passively and independently of neighbouring sections. Secondly, we provide proof of principle for two practical implementations through physical experiments and computational fluid dynamics simulations. We consider a blade that is rigid near the leading edge and flexible near the trailing edge. We show that the unsteady load mitigation is proportional to the ratio between the length of the flexible and rigid parts of the blade. For example, for a blade section where the flexibility is concentrated in a hinge at 3/4 of the chord, the amplitude of the fluctuations is 3/4 of the original amplitude. Secondly, we consider a solid, rigid blade with a passive pitch mechanism. We show that, for a 1 MW turbine operating in shear flow, more than 80% of the unsteady loading is mitigated. These results demonstrate the potential effectiveness of morphing blades for mitigating thrust fluctuations on tidal turbines.

Respiratory droplets are the primary transmission route for SARS-CoV-2, a principle which drives social distancing guidelines. Evidence suggests that virus transmission can be reduced by face coverings, but robust evidence for how mask usage might affect safe distancing parameters is lacking. Accordingly, we set out to quantify the effects of face coverings on respiratory tract droplet deposition. We tested an anatomically realistic manikin head which ejected fluorescent droplets of water and human volunteers, in speaking and coughing conditions without a face covering, or with a surgical mask or a single-layer cotton face covering. We quantified the number of droplets in flight using laser sheet illumination and UV-light for those that had landed at table height at up to 2 m. For human volunteers, expiratory droplets were caught on a microscope slide 5 cm from the mouth. Whether manikin or human, wearing a face covering decreased the number of projected droplets by less than 1000-fold. We estimated that a person standing 2 m from someone coughing without a mask is exposed to over 10 000 times more respiratory droplets than from someone standing 0.5 m away wearing a basic single-layer mask. Our results indicate that face coverings show consistent efficacy at blocking respiratory droplets and thus provide an opportunity to moderate social distancing policies. However, the methodologies we employed mostly detect larger (non-aerosol) sized droplets. If the aerosol transmission is later determined to be a significant driver of infection, then our findings may overestimate the effectiveness of face coverings.

Fins, wings, blades and sails can generate lift and drag in both attached and separated flow conditions. However, the common understanding of the lift generation mechanism holds only for attached flow conditions. In fact, when massive flow separation occurs, the underlying assumptions of thin airfoil theory and lifting line theory are violated and the concept of bound circulation cannot be applied. Therefore, there is a need to develop an intuitive understanding of the force generation mechanism that does not rely on these assumptions. This paper aims to address this issue by proposing a paradigm based on established concepts in theoretical fluid mechanics, and impulse theory in particular. The force generation can be intuitively associated with the vorticity field, which can be gathered with computational fluid dynamics or particle image velocimetry. This paradigm reconciles key known results about wing aerodynamics, and provides designers of lifting surfaces a measurable objective to optimise the shape in separated flow conditions. It will hopefully underpin both a deeper understanding of how lift and drag are generated, and the development of low order models in different fields of application.

The leading-edge separation bubble (LESB) is a flow feature occurring on the suction side of thin foils as a result of separation at the sharp leading-edge followed by reattachment downstream along the chord. For a flat plate at zero angle of attack, the reattachment length of the bubble depends on the plate thickness t. At a non-zero incidence, instead, the underlying scale governing bubble length is not clear. To investigate, we undertake a critical review of experimental and theoretical studies, and we develop an analytical formulation to predict the reattachment length of plates both at zero and at small incidences. We focus on conditions where the bubble is turbulent, i.e. when transition occurs at a negligible distance from the point of separation. This occurs at thickness- and chord-based Reynolds numbers Ret=10,000, Rec=100,000. At angle of attack α = 0, we find that the reattachment length is xR ≈ 4.8t when the chord-to- thickness ratio is c/t > 12. At α > 0, we find that xR/c = πσα^2, where σ ≈ 7.9 is the inverse of the growth rate of a turbulent free shear layer. These results allow estimating xR on the thin wings of, for example, aerial vehicles and yacht sails.