Abstract
We explore the stability of wakes arising from 2D flow actuators based on linear momentum actuator disc theory. We use stability and sensitivity analysis (using adjoints) to show that the wake stability is controlled by the Reynolds number and the thrust force (or flow resistance) applied through the turbine. First, we report that decreasing the thrust force has a comparable stabilising effect to a decrease in Reynolds numbers (based on the turbine diameter). Second, a discrete sensitivity analysis identifies two regions for suitable placement of flow control forcing, one close to the turbines and one far downstream. Third, we show that adding a localised control force, in the regions identified by the sensitivity analysis, stabilises the wake. Particularly, locating the control forcing close to the turbines results in an enhanced stabilisation such that the wake remains steady for significantly higher Reynolds numbers or turbine thrusts. The analysis of the controlled flow fields confirms that modifying the velocity gradient close to the turbine is more efficient to stabilise the wake than controlling the wake far downstream. The analysis is performed for the first flow bifurcation (at low Reynolds numbers) which serves as a foundation of the stabilization technique but the control strategy is tested at higher Reynolds numbers in the final section of the paper, showing enhanced stability for a turbulent flow case.
Highlights
The prediction and control of unsteady wakes is of capital importance in engineering applications since unsteady wakes and associated fluctuating forces are often correlated to undesired phenomena, such as unpredictable unsteady forces, reduced fatigue life or noise generation.Some of these effects can undermine the structural integrity and reduce the operating life of the device and its prediction and control is paramount
We model turbines using a localised flow resistance and passive flow control to enhance the stability of the wake arising from the turbines and based on linear momentum actuator disc theory (LMADT) by Rankine [14] and Froude [15]
The analysis is performed for the first flow bifurcation which serves as a foundation of the stabilization technique but the control strategy is tested at higher Reynolds numbers in the final section of the paper, showing enhanced stability for a turbulent flow case
Summary
The prediction and control of unsteady wakes is of capital importance in engineering applications since unsteady wakes and associated fluctuating forces are often correlated to undesired phenomena, such as unpredictable unsteady forces (e.g., dynamic stall), reduced fatigue life or noise generation. Some of these effects can undermine the structural integrity and reduce the operating life of the device and its prediction and control is paramount. The control of the wake unsteadiness may enhance energy production, alleviate structural damage and extend the turbine life
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