Session G13: Boundary Layers: General II

Reduced-order modelling of winglet wind turbine wake characteristics.

Power production of a horizontal-axis wind turbine (HAWT) is affected by unsteady turbulent structures which span large range of length scales. To curb this challenge winglets are attached to the tip of horizontal axis wind turbines to optimize performance by reducing the wing tip vortices which causes extra drag and lift. In this work, we used Proper Orthogonal Decomposition (POD) method as a reduced order model in analyzing the wake characteristics of a horizontal-axis wind turbine (HAWT) with winglets. The method is used to identify energic turbulent modes for near, mid, and far stream wake structures of the optimized three bladed winglet configuration. Measured data in this work was taken from a wind tunnel experimental facility using a prototype scaled model with neutral inlet boundary layer. A high-resolution Particle Image Velocimetry (PIV) was used to capture and characterize the wake vortex structures. The accuracy of the POD reconstruction technique will be validated qualitatively with the statistical-averaged PIV measurement results. The reduced order model provides knowledge of the underlying physics and how the changes in the wake flow affects power and thrust coefficient measurements.

 

 

    

Characterizing yawed wind turbine wakes under stable stratification and wind veer

The wind veer near the surface of an Atmospheric Boundary Layer (ABL) shears the wake behind a wind turbine in the lateral direction. The sheared wake of upstream turbines may affect the power output of downstream turbines, thereby affecting overall wind farm performance. The veer shear rate depends strongly on the stability of the ABL; therefore, it is crucial to understand how this dependence affects the flow mechanisms associated with wind turbine performance. To this end, we perform Large Eddy Simulations (LES) of a yawed wind turbine in a stably stratified ABL and study the influence of stratification on its wake evolution. Results confirm that strong stability leads to winds with stronger veer, causing significant lateral wake deformation. We then analytically predict wind veer shear rate under stable atmospheric conditions by determining the spanwise wake deformation from the veer model and using it to augment the Gaussian wake model (see Abkar et al., Energies, 2018). Model results are compared to LES.   

Turbulent flows over porous/rough walls: Response of velocity to stress on surfaces

Porous/rough walls with finite grain sizes and depths affect the overlying turbulence via multiple mechanisms including slip, transpiration, roughness, and bottom reflection. Understanding these mechanisms is a fundamental prerequisite for a higher-level theory on the behavior changes of turbulence itself. In this study, we investigate the scale-local response of velocity to stress on porous/rough wall surfaces. First, the superficial velocity is correlated with the stress based on solutions to the linearized Navier-Stokes (LNS) equations, yielding a response matrix as a function of exciting wavevectors and frequencies. This matrix reveals a substrate's inherent attributes, particularly the bottom pressure reflection that principally distinguishes rough walls from deep porous walls. Second, we examine the superficial velocity-stress correlations for the data of direct numerical simulations that fully resolve turbulence over porous/rough walls. Strong correlations are observed for most scales and are highly consistent with the LNS results. This indicates that the LNS framework captures the superficial responding mechanisms, particularly the phase delay of transpiration responding to pressure, a crucial effect that cannot be captured by conventional Stokes-flow-based approaches.   

Winglet size effect on the wake dynamics of model wind turbines

A laboratory investigation was conducted to evaluate the wake dynamics of model wind turbines with downwind-facing winglets of different sizes operating in a turbulent boundary layer (TBL). Particular focus is placed on exploring the like effect of the turbines under low-level jets (LLJs) with velocity peaks coincident with the top tip and hub heights. A standard rotor without winglets was included as a base case; the winglets spanned 0.096, 0.107, and 0.118 times the rotor radius. Results show that a larger winglet span enhances the energy fluxes in the far wake. Wake energy entrainment showed minor dependence on the winglet size for the turbine operating under LLJs. The work offers insight for strategic wind farm planning considering the modulation of TBLs and LLJs on turbines with winglets.

    

Finite retention in non-wetting lubricant-infused surfaces

Surfaces patterned with long parallel grooves infused with a lubricating fluid (LIS) can be used for drag reduction, anti-fouling, and heat transfer enhancement in marine applications. Design criteria have been established to mitigate failure modes mainly related to lubricant drainage when the surfaces are exposed to shear flows. A critical requirement to assure their robustness is that the lubricant has to wet the surface when surrounded by the working liquid. Technical constraints related to the experimental apparatus or to the choice of materials can be an obstacle to meet such condition. In this study a non-wetting LIS, where the canonical condition is violated, is manufactured and tested in a new turbulent water channel facility. We demonstrate that, despite the energetic unfavourable condition, a finite length of lubricant is retained in a stable manner even when subjected to a turbulent flow. In comparison, when the configuration of a wetting lubricant is applied on the same LIS geometry and fluids, the lubricant drainage happens much earlier if retention barriers are not used. We formulate an analytical model which shows, in agreement with the experimental results, that the contact line hysteresis has a critical role in the lubricant retention mechanism.