Team members: Ryan Wallace, Beverley McKeon
There is potential benefit to an active manipulation of the flow around wind turbine blades tuned to the angular phase of rotation. We examine the influence of a mechanical proxy for a controllable, time-dependent surface roughness profile on the flow over representative VAWT blades, under different angles of attack and rotational speeds. In particular, the sensitivity of the time-averaged drag coefficient to flow separation characteristics is investigated. Ultimately, we will characterize the potential of this actuation method to optimize the efficiency of the full wind turbine.
The lifetime performance of VAWT rotor blades can be improved through the use of architected, nanostructured materials. For example, octet micro-truss structures have been shown to have exceptional strengths with very low densities. These structures combined with nanometer-sized features allow for the creation of bulk structures with constant specific strengths over a range of structure density and flow strengths. Applied to VAWT rotor blades, this approach results in significantly lower structure weight, as the specific strength of the rotor can be reduced until other design parameters begin to dominate strength requirements (e.g. impact resistance, survivability in high winds, etc.) . The tailorability of these truss structures also allows the blades to be functionally graded to minimize the effects of the centrifugal forces. The blades are engineered such that regions experiencing the highest stresses, e.g. the rotor-tower connections, are the densest and thus strongest pieces of the blade, whereas the remaining portions of the blade have very low density thus mitigating the impact of the centrifugal forces.
In this project, we consider changes to the nanostructure and surface morphology of wind turbine blades to maximize aerodynamic performance. Recent research has shown that nanostructured materials have significantly different properties than their bulk counterparts. They are often stronger, more corrosion/wear resistant, and present a tunable platform for controlling surface wetting properties. One key roadblock preventing these nanostructured materials from leading to breakthroughs in damage tolerance in wind turbine blades is the difficulty of replicating these nanostructures and their enhanced functionality in relevant metals, polymers, and composites. This project develops techniques to design optimal nano-topologies using standard semiconductor processing techniques, with the aim of eliciting, and subsequently tuning, the air flow properties over the blade. In parallel, we investigate surface morphological features capable of favorably influencing boundary layer characteristics. Turbine efficiency can be optimized by carefully controlling the characteristic length scales and shape of the surface in order to manipulate the drag profile.
As a fish swims, it sheds vortices from its tail into its wake. It has been previously proposed that schooling fish take advantage of these shed vortices, thereby minimizing the energy required for locomotion. In this project, conceptual analogy is made between fish-generated vortices and the wind-generated rotation of vertical-axis wind turbines. In both cases, simplified fluid dynamic models predict that the performance of the group (i.e. fish school or wind farm) can be enhanced by constructive aerodynamic interference between adjacent group members. This approach is predicted to achieve significant increases in wind farm power density (i.e. power output per area of land) compared to existing wind farms. Ongoing field research has confirmed the initial predictions of the theoretical models, and the empirical data is being used to refine those models.
Investigation of Mechanical Properties, Deformation and Failure Mechanisms in Composite Wind Turbine Blades
For wind turbine blades, fatigue is a prominent cause of stiffness degradation and ultimately failure. We are conducting a comprehensive suite of monotonic and cyclical nano-mechanical tests on specific locations in the composite blade. Our in-situ nano-mechanical instrument, SEMentor, is instrumental in performing this research as it enables mechanical testing and deformation observation of small-scale (i.e. comparable to the microstructural critical dimensions) samples from highly-strained locations. This is particularly relevant in studying fracture toughness and failure mechanisms through fiber pull-out tests on individual fibers and uniaxial deformation tests on extracted composite specimens to ascertain strength, shear properties, and fracture toughness as a function of fiber density, distribution, and size. Such information assists in highlighting the failure mechanisms - for example, allowing to distinguish failures due to localized shear deformation from ones due to excessive localized tensile deformation.
A principal cause of the relatively low power density of modern wind farms is the deleterious effect of vortex shedding from the individual wind turbines, which degrades the quality of airflow past turbines further downwind in an array. This project utilizes computational fluid dynamics simulations to accurately model the wake dynamics of vertical axis wind turbines with realistic input wind parameters based on field measurements. The discovery of VAWT operating parameters and array configurations that reduce vortex shedding in the wake enables the turbines to be placed in closer proximity, thereby increasing the net power output per unit area of the VAWT arrays and potentially increasing the individual turbine efficiency.
Previous research has shown vertical-axis turbines can be effective at extracting energy from wind with variable speed and direction. Strategic placement of counter-rotating vertical-axis turbines can further enhance energy extraction. This project applies these principles for the purpose of energy extraction from marine environments. Especially in near-shore regions, these flows are characterized by temporal and spatial variability over a broad spectrum of scales, and therefore are not compatible with most existing ocean energy technologies. The performance of vertical-axis water turbines is studied in the 40-meter water tunnel facility at Caltech, where flows of varying spatial and temporal frequencies are simulated. Turbine configurations that prove most effective are selected for further refinement and testing in the field, and the discovered design principles are used to inform wind energy research as well.
To analyze aerodynamic interactions between vertical-axis wind turbines in detail, it is essential to be able to observe their flow fields. Quantitative in situ measurements pose a challenge because of the large spatial dimensions, high flow velocities and the remote locations of the VAWTs. This project implements Particle Image Velocimetry (PIV) in horizontal cross-sections of the VAWTs as well as in the regions between neighboring turbines in the wind farm. Novel methods for flow seeding and illumination are being developed, as well as the incorporation of high-speed, high-resolution cameras and optical sectioning techniques. PIV yields instantaneous, two-dimensional, two-component velocity fields together with the out of plane component of vorticity, and is therefore a considerable advantage over the single-point techniques that are currently available for field measurements of wind turbines.
The Caltech Field Lab is used to demonstrate and test new wind energy technologies at full-scale and under natural wind conditions. To support research at the site, this project involves a comprehensive characterization of wind resources at the site. Sonic anemometers and LIDAR are deployed at multiple sites around the facility in order to measure time-dependent, three-component wind profiles from 1 to 100 m above the ground. Measurements are conducted both upwind and in the wake of the VAWT array to determine parameters including the turbulence fluctuations and the effective roughness length and zero-plane displacement of the wind farm.
Professor Dabiri focuses on mechanics and dynamics of biological propulsion, and fluid dynamic energy conversion.