Active flow control (AFC) was applied to a tandem configuration of two SD7003 airfoils. The tandem configuration consisted of an upstream airfoil (forefoil) with a pitching motion at a fixed frequency and a downstream airfoil (hindfoil) that was not moving. Synthetic jet actuators (SJAs) were applied on both airfoils to control the flow fields at the low Reynolds number of 30,000. The flow physics inherently involved three different frequencies: frequency of the pitching forefoil and two actuation frequencies of the two of SJAs. In this study we kept all three frequencies fixed at 5 Hz. However, we allowed for phase differences between them. An optimization study was conducted in order to improve total aerodynamic performance defined as the combined total time-averaged value of lift-to-drag ratio of both airfoils (L/D)_tot. Injection angle of the two SJAs, phase differences between each SJA frequency and frequency of the pitching motion in addition to vertical spacing between the airfoils were considered as design variables of the optimization study. Optimization algorithm was coupled with a machine learning method to reduce computational cost. We found that lift coefficients were enhanced and drag coefficients were reduced for the optimum controlled case in comparison with the uncontrolled case which led to an aerodynamic performance improvement of 304%. However, drag force was the dominant parameter in determining final performance value. For all design variables, drag force determined the final optimum values.

Ocean Alkalinity Enhancement (OAE) is a technique proposed to address ocean acidification and global warming. This study examines rainbowing (i.e. water jets pumped into the air from ships) for the slaked lime slurry distribution. The fluid dynamic behaviour—both the in-air trajectory and subsurface—was studied using the Smoothed Particle Hydrodynamics method. Various outflow velocities were simulated for calm seas; different dilution factors and environmental slaked lime concentration spikes have been found. Linking slurry concentration to pH, discharge conditions that avoid damage to marine biota have been identified. A preliminary cost analysis compares rainbowing to ship-wake discharge. Rainbowing requires high power consumption to project large volumes of water far from ships. Very high dilution is needed, mainly because of the artificial requirement not to momentarily exceed 24-hour environmental pH limits; the effect of ocean turbulence on instantaneous pH peaks is negligible and is not considered in the analysis. Emissions due to energy consumption almost equalize carbon dioxide removed during discharge, making the method inefficient (only 176 kg of CO₂ removed per ton of slaked lime spread). Previous ship-wake discharge analysis shows greater OAE performance and lower cost (5.5 € versus 388 € per ton of CO₂ removed). In conclusion—based on the conservative environmental limits imposed, and the specific modeling undertaken—rainbowing is not demonstrated as an efficient method for OAE.

A previous paper by Cimarelli et al. [“General formalism for a reduced description and modelling of momentum and energy transfer in turbulence,” J. Fluid Mech. 866, 865–896 (2019)] has shown that every decomposition of turbulent stresses is naturally approximated by a general form of tensorial eddy viscosity based on velocity increments. The generality of the formalism is such that it can also be used to give a reduced description of subgrid scalar fluxes. In the same work, this peculiar property of turbulent stresses and fluxes has been dynamically exploited to produce tensorial eddy viscosity models based on the second-order inertial properties of the grid element. The basic idea is that the anisotropic structure of the computational element directly impacts, although implicitly, the large resolved and small unresolved scale decomposition. In the present work, this new class of turbulence models is extended to compressible turbulence. A posteriori analysis of flow solutions in a compressible turbulent channel shows very promising results. The quality of the modeling approach is further assessed by addressing complex flow geometries, where the use of unstructured grids is demanded as in real world problems. Also in this case, a posteriori analysis of flow solutions in a periodic hill turbulent flow shows very good behavior. Overall, the generality of the formalism is found to allow for an accurate description of subgrid quantities in compressible conditions and in complex flows, independent of the discretization technique. Hence, we believe that the present class of turbulence closures is very promising for the applications typical of industry and geophysics.