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A lowspeed wind tunnel is developed for fluid dynamics research at Reykjavik University. The tunnel is designed for conducting research on the flow past micro air vehicles, as well as fundamental research on turbulence. High flow quality is elemental for both research projects. The tunnel is of open suction type and is composed of a square inlet with a honeycomb and turbulence screens, settling chamber, contraction, experimental section housing, diffuser, and axial fan. Here, we describe the details of the design optimization procedure of the contraction, which is a key to getting a high quality flow in the experimental section. A high fidelity computational fluid dynamic (CFD) flow solver is used to capture the nonlinear flow physics. Due to the high computational cost of the CFD simulations, surrogate based optimization (SBO) is used to accelerate the design process. The SBO approach replaces direct optimization of the high fidelity (accurate but computationally expensive) model by iterative optimization of a properly corrected low fidelity model obtained from low fidelity CFD simulations. The optimum contraction design is verified using high fidelity CFD simulation, as well as by experimental measurements.

The role of turbulence in droplet growth in clouds is controversial, in part because of the difficulty of studying underlying processes in the cloud environment, and in part because of the difficulty of achieving real cloud conditions in controlled laboratory or computational studies. This paper is a synthesis of research on turbulence effects on cloud droplets that includes field and laboratory studies. Results from cloud measurements show that the turbulence exhibits similar internal intermittency to that observed in the laboratory, and in direct numerical simulations. We explore the consequences of this by relating measurements of droplet accelerations in the laboratory, to conditions observed in the clouds. We show that there is a strong likelihood of droplet accelerations in clouds exceeding the acceleration due to gravity. We discuss these observations in terms of the dynamics of droplets, including velocity statistics and clustering, and their influence on droplet growth.