A common, worldwide atmospheric phenomenon known as a low-level jet (LLJ) offers benefits to wind energy production. Despite the fact that this effect may be used to increase the capacity factor of wind farms, the interaction between LLJs and wind power plants is not entirely known. By producing a synthetic LLJ velocity profile under well-controlled laboratory conditions, we show that this phenomenon enhances energy entrainment in the wake of wind turbines. The mechanism is associated with the increased velocity shear around the wind farm canopy height, which leads to higher turbulent mixing and, consequently, more energy flux for inner turbines within wind farms. The new insight from this work offers an opportunity to strategically develop, configure, and operate wind farms by taking into account the particular modulation of LLJs.

Flow separation and vortex shedding are some of the most common phenomena experienced by bluff bodies under relative motion with the surrounding medium. They often result in a recirculation bubble in regions with adverse pressure gradient, which typically reduces efficiency in vehicles and increases loading on structures. Here, the ability of an engineered coating to manipulate the large-scale recirculation region was tested in a separated flow at moderate momentum thickness Reynolds number, $R{e}_{\theta }=\mathrm{1,200}$. We show that the coating, composed of uniformly distributed cylindrical pillars with diverging tips, successfully reduces the size of, and shifts downstream, the separation bubble. Despite the so-called roughness parameter, $k^{+}\approx 1$, falling within the hydrodynamic smooth regime, the coating is able to modulate the large-scale recirculating motion. Remarkably, this modulation does not induce noticeable changes in the near-wall turbulence levels. Supported with experimental data and theoretical arguments based on the averaged equations of motion, we suggest that the inherent mechanism responsible for the bubble modulation is essentially unsteady suction and blowing controlled by the increasing cross-section of the tips. The coating can be easily fabricated and installed and works under dry and wet conditions, increasing its potential impact on a diverse range of applications. Significance Flow separation on moving bodies has a negative effect on energy efficiency. Reducing recirculating regions is key in the design of energy-efficient systems. Efficient design decreases fuel consumption and pollutant emissions, including the systems’ carbon footprint. The engineered bio-inspired coating presented here aims to contribute in that direction. The relative ease of manufacturing and installation and its cost effectiveness, as well as its functionality under both wet and dry conditions, make it a versatile solution of potentially high impact in a broad range of applications, including transportation, wind power, and underwater vehicles.

A zero-pressure-gradient turbulent boundary layer flowing over a transitionally rough surface (24-grit sandpaper) with $k^{+}\approx 11$ and a momentum-thickness Reynolds number of approximately 2400 is studied using direct numerical simulation (DNS). Heat transfer between the isothermal rough surface and the turbulent flow with molecular Prandtl number $Pr=0.71$ is simulated. The dynamic multiscale approach developed by Arayaet al.(J. Fluid Mech., vol. 670, 2011, pp. 581–605) is employed to prescribe realistic time-dependent thermal inflow boundary conditions. In general, the rough surface reduces mean and fluctuating temperature profiles with respect to the smooth surface flow when normalized by Wang & Castillo (J. Turbul., vol. 4, 2003, 006) inner/outer scaling. It is shown that the Reynolds analogy does not hold for $y^{+}<9$ . In this region the value of the turbulent Prandtl number departs substantially from unity. Above this region the Reynolds analogy is only approximately valid, with the turbulent Prandtl number decreasing from 1 to 0.7 across the boundary layer for rough and smooth walls. In comparison with the smooth-wall case, the turbulent transport of heat per unit mass, $\overline{v^{\prime }v^{\prime }{\it\theta}^{\prime }}$ , towards the wall is enhanced in the buffer layer, but the transport of $\overline{v^{\prime }v^{\prime }{\it\theta}^{\prime }}$ away from the wall is reduced in the outer layer for the rough case; similar behaviour is found for the vertical transport of turbulent momentum per unit mass, $\overline{v^{\prime }u^{\prime }v^{\prime }}$ . Above the roughness sublayer (3 $k$ –5 $k$ ) it is found that most of the temperature field statistics, including higher-order moments and conditional averages, are highly similar for the smooth and rough surface flow, showing that the Townsend’s Reynolds number similarity hypothesis applies for the thermal field as well as the velocity field for the Reynolds number and $k^{+}$ considered in this study.

The flow over a mushroom-shaped microscale coating was experimentally inspected over a diverging channel that followed the pressure side of a wind turbine blade (S835). High-resolution particle image velocimetry was used to obtain in-plane velocity measurements in a refractive-index-matching flume at Reynolds number Re_θ ≈ 1200 based on the momentum thickness. The results show that the evolution of the boundary layer thickness, displacement thickness, and shape factor change with the coating, contrary to the expected behavior of an adverse pressure gradient boundary layer over a canonical rough surface. Comparison of the flow with that over a smooth wall revealed that the turbulence production exhibited similar levels in both cases, suggesting that the coating does not behave like a typical rough wall, which increases the Reynolds stresses. Proper orthogonal decomposition was used to decompose the velocity field to investigate the possible structural changes introduced by the wall region. It suggests that large-scale motions in the wall region lead to high-momentum flow over the coated case compared to the smooth counterpart. This unique behavior of this surface coating can be useful in wind-turbine applications, with great potential to increase the power production.

An experimental investigation was carried out to explore the interaction and wake statistics of model wind turbines operating individually and in pairs over two-dimensional hills with varying heights. The hills shared a sinusoidal shape and extended L/D=20 in the streamwise direction, where D represents the diameter of the turbine rotor. The peak heights of the hills were H/D=0, 0.5, 1, and 1.5. The first turbine was located at the beginning of the hill development, and the second turbine was positioned halfway between the first and the hill's peak, downwind. The flow in the intermediate wake regions was characterized using particle image velocimetry, focusing on the recovery mechanisms of streamwise momentum on the windward side of the hills, ranging from gentle to steep-up slopes. The results indicate that the advection terms play a more significant role than turbulence in the wake recovery mechanism with steeper hill slopes. Associated reduced turbulence levels are attributed to flow acceleration, which led to a higher power availability at the top of the hills.

We experimentally explored the effect of a series of low-level-jet (LLJ) velocity profiles on the energy entrainment of a single and a pair of counter-rotating Giromill–Darrieus vertical-axis wind turbines (VAWTs) models under two types of rotations. Planar particle image velocimetry (PIV) was used to obtain the mean flow and turbulence statistics in the wake of the model wind turbines. Incoming LLJ profiles had peaks coincident with the turbines midspan and top tip; complementary flow characterization with a canonic turbulent boundary layer is also included for comparison. Results show that the positive shear region of the LLJ velocity profiles increased the wake asymmetry due to higher vertical velocity gradients. The positive shear of the LLJ contributed to the enhancement of energy entrainment in the wake compared with a standard turbulent boundary layer profile by increasing the mean kinetic energy advection into the wake in the single and a pair of counter-rotating VAWTs. Comparatively, high vertical velocity gradients may be responsible for this phenomenon; it promoted stronger cross-flow and counter-rotating vortices in the wake.