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Abstract Large‐eddy simulation (LES) has been adopted to study wind turbine wakes because it can capture fine‐scale details of turbulent wind flows and interactions with wind turbines. Here, we use the LES version of the Weather Research and Forecasting (WRF) model with an actuator disk model to gain insights on several wake effects that have been traditionally difficult to measure. The first finding is that the wake has a “dual nature,” meaning that the wind speed deficit behaves differently from the added turbulent kinetic energy (TKE) and the two are not co‐located in space. For example, the wind speed deficit peaks at hub height and reaches the ground within 8D (D is the rotor diameter), but added TKE peaks near the rotor tip and generally remains aloft. Second, temperature changes near the ground are driven by the added TKE in the rotor area and by atmospheric stability. The combination of these two factors determines the sign and intensity of the vertical heat flux divergence below the rotor, with convergence and warming associated with stable conditions and weak divergence and modest cooling with unstable conditions. Third, wakes do not expand indefinitely, as suggested by similarity theory applied to the wind speed deficit, but eventually stop expanding and actually contract, at different rates depending on atmospheric stability. The implication of these findings is that, in order to study wakes, it is not sufficient to focus on wind speed deficit alone, because TKE is also important and yet behaves differently from the wind speed deficit.
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Baroclinicity adds another layer of complexity to the much-studied barotropic atmospheric boundary layers (ABLs) by modulating the pressure gradient in height. Despite the prevalence of baroclinicity in real-world ABLs, our knowledge of the interacting effects of baroclinicity and atmospheric stability is limited. In this talk, we aim to address this knowledge gap by systematically varying baroclinicity and stability using the large-eddy simulation (LES) technique. We will present how baroclinicity alters the friction velocity, Obukhov length, shear production, ABL height, and low-level jet (LLJ) in diabatic baroclinic atmospheric flows. It will be shown that while baroclinicity significantly impacts unstable, neutral, and weakly stable ABLs, its effects reduce with increased stratification in the ABL. In strongly stratified ABLs, the LLJ height, friction velocity, and Obukhov length converge to a constant asymptote independent of the baroclinicity regime. We will demonstrate that this behavior is attributed to the strong turbulence destruction in very stable ABLs that decouples the surface from higher elevations where baroclinicity is more important. Finally, two rescaling methods in the inner and outer layers of stable baroclinic ABLs will be presented to non-dimensionalize and collapse the wind profiles in baroclinic environments. The developed reduced model for different baroclinic wind profiles will be shown against the LES results. The findings of this research elucidate the underlying physics of baroclinic diabatic ABLs and are useful for characterizing the wind profiles in weather/climate models, field measurements, and various industrial applications.more » « less
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Abstract The extremum seeking control (ESC) algorithm has been proposed to determine operating parameters that maximize power production below rated wind speeds (region II). This is usually done by measuring the turbine's power signal to determine optimal values for parameters of the control law or actuator settings. This paper shows that the standard ESC with power feedback is quite sensitive to variations in mean wind speed, with long convergence time at low wind speeds and aggressive transient response, possibly unstable, at high wind speeds. The paper also evaluates the performance, as measured by the dynamic and steady state response, of the ESC with feedback of the logarithm of the power signal (LP‐ESC). Large eddy simulations (LES) demonstrate that the LP‐ESC, calibrated at a given wind speed, exhibits consistent robust performance at all wind speeds in a typical region II. The LP‐ESC is able to achieve the optimal set‐point within a prescribed settling time, despite variations in the mean wind speed, turbulence, and shear. The LES have been conducted using realistic wind input profiles with shear and turbulence. The ESC and LP‐ESC are implemented in the LES without assuming the availability of analytical gradients.
