Numerical simulations reproducing a wind tunnel experiment on two in-line wind turbines have been performed. The flow features and the array performances have been evaluated in different inflow conditions. Following the experimental setup, different inlet conditions are obtained by simulating two grids upstream of the array. The increased turbulence intensity due to the grids improves the wake recovery and the efficiency of the second turbine. However, the inlet grid induces off-design operation on the first turbine, decreasing the efficiency and increasing fatigue loads. Typical grid flow patterns are observed past the rotor of the first turbine, up to the near wake. Further downstream, the signature of the grid on the flow is quite limited. An assessment of numerical modeling aspects (subgrid scale tensor and rotor parameterization) has been performed by comparison with the experimental measurements.

Control algorithms seeking to maximize wind plant power production may not require that all turbines communicate with each other for the purpose of coordinating an optimal control solution. In practice, an efficient and robust control solution may result by coordinating only turbines that are aerodynamically coupled through wake effects. The implementation of such control strategy would require information of which clusters of turbines are coupled in this way. As the wind changes direction, the clusters of coupled turbines may vary continuously within the array. Hence, in practical applications, the identification of these clusters has to be performed in real time in order to efficiently apply a coordinated control approach. Results from large eddy simulations of the flow over a wind farm array of 4 × 4 turbines are used to mimic Supervisory Control And Data Acquisition (SCADA) data needed for the cluster identification method and to evaluate the effectiveness of the yaw control applied to the identified clusters. Results show that our proposed method is effective in identifying turbine clusters, and that their optimization leads to a significant gain over the baseline. When the proposed method does not find clusters, the yaw optimization is ineffective in increasing the power of the array of turbines. This study provides a model-free method to select the turbines that should communicate with another to increase power production in real time. In addition, the analysis of the flow field provides general insights on the effect of the local induction, as well as of the wind farm blockage, on yaw optimization strategies.

Heat transport over super-hydrophobic (SHS) and liquid-infused surfaces (LIS) is studied using direct numerical simulations of a turbulent channel flow. The lower wall of the channel consists of either longitudinal or transversal bars with a secondary fluid locked in the cavities between the surface elements. We consider two viscosity ratios between the fluids mimicking SHS and LIS. The thermal diffusivity is varied to assess its effect on thermal performance. We investigate the dependence of heat transfer on the elements pitch-to-height ratio and on the interface dynamics. The interface deformation (dependent upon the surface tension between the two fluids) is fully coupled to the fluid governing equations with the level-set method. Present simulations are consistent with published results about the drag-reducing potential of SHS and LIS in turbulent flow compared to the smooth wall. In the limiting case of infinite surface tension (the interface remains flat and slippery), heat transfer efficiency (heat transfer to drag ratio) can be enhanced compared to a smooth wall. Although the total heat flux is marginally reduced, SHS and LIS with longitudinal ridges achieve a comparatively larger drag reduction, which increases the efficiency. A model is derived from the energy equation to correlate the heat transfer performance with the thermal slip length ( and a smaller heat transfer efficiency than the smooth wall. In the case of finite surface tension, the dynamics of the interface generates a turbulent flux which improves the thermal performance, but tends to decrease the amount of drag reduction. Liquid-infused surfaces are more robust than SHS to the deformation of the interface thanks to the larger viscosity of the secondary fluid and maintain about the same drag reduction as in the infinite surface tension case. For SHS longitudinal bars, simultaneous increase in heat transfer and reduction in drag are observed, leading to an apparent breakdown of the Reynolds analogy. Also in the case of finite surface tension, heat transfer efficiency scales with the relative magnitude of the thermal and streamwise slip lengths. The results suggest a potential expansion of the control space for engineers. Depending on the application, one can reach an optimal combination of heat transfer and drag by tuning the shape of the substrate and the viscosity and diffusivity ratios.

Liquid-infused surfaces can reduce friction drag in both laminar and turbulent flows. However, the heat transfer properties of such multi-phase surfaces have still not been investigated to a large extent. We use numerical simulations to study conjugate heat transfer of liquid-filled grooves. It is shown that heat transfer can increase for both laminar and turbulent liquid flows due to recirculation in the surface texture. Laminar flow simulations show that for the increase to be substantial, the thermal conductivity of the solid must be similar to the thermal conductivity of the fluids, and the recirculation in the grooves must be sufficiently strong (Péclet number larger than 1). The ratio of the surface cavity to the system height is an upper limit of the direct contribution from the recirculation. While this ratio can be significant for laminar flows in microchannels, it is limited for turbulent flows, where the system scale (e.g. channel height) usually is much larger than the texture height. However, heat transfer enhancement of the order of $10\,\%$ is observed (with a net drag reduction) in a turbulent channel flow at a friction Reynolds number ${{Re}}_\tau \approx 180$ . It is shown that the turbulent convection in the bulk can be enhanced indirectly from the recirculation in the grooves.