On synthetic jet actuation for aerodynamic load control

Abstract

SummaryA major goal for the wind energy industry is the reduction of the cost of energy. This drives the design towards increasingly larger wind turbines. The technology of smart rotor control is expected to allow wind turbines to increase even further in size. Smart rotor control consists of a sequence of local load control mechanisms, distributed along the span of a wind turbine blade, which are operated individually based on local sensory information. This technology has the potential to reduce fatigue inducing variations in blade loads.Fatigue loads are induced by cyclic effects, such as wind shear, gravity and yaw misalignment, or by stochastic effects, such as turbulence in the upstream flow. Alleviation of the fatigue loads reduces the structural requirements of multiple components of a wind turbine, which allows for relatively lighter structures and less maintenance. It is expected that this will decrease the cost of energy, due to a combination of increased energy production and relatively lower capital and maintenance costs of a large but lighter wind turbine equipped with smart rotor control.The aerodynamic effect needed for smart rotor control is 'local pitch control', which aims for changes in the local aerodynamic characteristics, mainly the lift coefficient (c l ), over the range of angles of attack (α) in the linear c l (α)-regime. This aerodynamic effect can also be employed for other turbomachinery and in wing aerodynamics in the field of aeronautics. Potential options for local pitch control are trailing edge flaps, micro-tabs (small deployable Gurney flaps) and blade morphing. Active fluidic control close to the trailing edge by means of jets is an alternative option.In the present research, synthetic jets have been investigated as a potential option for local pitch control. Synthetic jets are generated by repeated ingestion and subsequent ejection of air, into and out of a cavity below the surface of the blade, respectively, through holes or slits in the surface of this blade. This oscillatory ejection/ingestion is caused by a vibrating wall inside the cavity, such as a piston or a piezoceramic composite diaphragm. The technology of synthetic jets can be used for boundary layer separation control, as well as pitch control.In the present thesis, a multi-purpose computational method has been developed for the simulation of unsteady compressible viscous flows. This method is, in principle, able to address the relevant characteristic flow effects associated with synthetic jet actuation. The computational method solves the unsteady Reynolds-averaged Navier-Stokes (URANS) equations for unsteady compressible viscous flow, together with the equation(s) of a linear eddy-viscosity turbulence model. The equations are discretized on unstructured computational grids, employing the Finite Volume method on cell-centered control volumes. The discretization is nominally of second order accuracy, both in space and time.iii An exception is the discretization of the convective flux in the equation(s) of the tur...

O a method for simulation-based wind turbine blade design

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“…In regions where the pressure shows smooth behaviour, the second-order dissipation term is small and the fourth-order term is non-zero, resulting in a second-order spatial accuracy. Typical values for the dimensionless constants [20,199] are…”

“…To determine suitable boundary conditions for situations like these, the properties of the underlying mathematical model are considered. Due to the hyperbolic nature of the Euler equations, the number of boundary conditions imposed must equal the number of characteristics that enter the computational domain [199,202]. For subsonic outflow, the only characteristic that enters the domain, corresponds to the acoustic wave with eigenvalue (u · n − c) -where n is the unit normal vector pointing into the domain.…”