Browsing by Author "Iowa State University. Department of Aerospace Engineering. Advanced Flow Diagnostics and Experimental Aerodynamics Laboratory"

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    Analysis of Turbine Wake Characteristics using Proper Orthogonal Decomposition and Triple Decomposition Methods
    Premaratne, Pavithra; Tian, Wei; Hu, Hui (Virginia Tech, 2015-06)
    In the present study, we report the progress made in our efforts to examine the wake flow characteristics behind a commonly-used three-bladed horizontal-axis wind turbine. A series of experiments were performed in a large-scale wind tunnel with a scaled wind turbine model placed in a typical Atmospheric Boundary Layer (ABL) wind under neutral stability conditions. In addition to measuring dynamic wind loads acting on the model turbine by using a force- moment sensor, a high-resolution digital particle image velocimetry (PIV) system was used to achieve detailed flow field measurements to quantify the characteristics of the turbulent vortex flow behind the turbine model. Besides conducting “free-run” PIV measurements to determine the ensemble-averaged statistics of the flow quantities such as mean velocity, Reynolds stress, and turbulence kinetic energy (TKE) distributions in the wake flow, “phase-locked” PIV measurements were also performed to elucidate further details about evolution of the unsteady wake vortex structures in relation to the position of the rotating turbine blades. The detailed flow field measurements were used to validate the analytical models for the velocity deficit prediction in turbine wakes. Proper Orthogonal Decomposition (POD) method was employed in the present study for the data reduction of the PIV measurement results to identify the high energy modes that dominate the turbulent kinetic energy distributions in the turbine wakes. Triple Decomposition (TD) approach was also used to analyze the phase-locked PIV measurement results to elucidate the underling physics related to the intensive turbulent mixing process in the wake flow, which would promote the vertical transport of kinetic energy to entrain more high-speed airflow from above to re-charge the wake flow behind the wind turbine model.
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    An Experimental Investigation on Surface Water Transport and Ice Accreting Process Pertinent to Wind Turbine Icing Phenomena
    Hu, Hui (Virginia Tech, 2015-06)
    Wind turbine icing represents the most significant threat to the integrity of wind turbines in cold weather. Ice accretion on turbine blades would decrease power production of the wind turbines significantly. Ice accretion and irregular shedding during wind turbine operation would lead to load imbalances as well as excessive turbine vibration, often causing the wind turbine to shut off. Icing issues can also directly impact personnel safety due to falling and projected large ice chunks. It should be noted that the icing hazard is often most severe in the locations which are best suited for wind turbine sites, such as northern latitudes, off-shore wind farms and high altitudes (i.e. mountains). Wind turbines in these regions are more prone to water contamination and icing in cold weather. Advancing the technology for safe and efficient wind turbine operation in atmospheric icing conditions requires the development of innovative, effective anti-/de-icing strategies tailored for wind turbine icing mitigation and protection. Doing so requires a keen understanding of the underlying physics of complicated thermal flow phenomena pertinent to wind turbine icing phenomena, both for the icing itself as well as for the water runback along contaminated surfaces of wind turbine blades. In the present study, a series of experimental investigations were conducted to characterize the transient behavior of wind-driven water film/rivulet flows over a NACA 0012 airfoil model and the dynamic ice accreting process over the airfoil model in order to elucidate the underlying physics of the important microphysical processes pertinent to wind turbine icing phenomena. The experimental study was conducted in an icing research tunnel available at Aerospace Engineering Department of Iowa State University. A suite of advanced flow diagnostic techniques, such as molecular tagging velocimetry and thermometry (MTV), digital image projection (DIP), and infrared (IR) imaging thermometry techniques, were developed and applied to achieve quantitative measurements of the film thickness distributions of the surface water film/rivulet flows and the temperature distributions of the water/ice mixture flows over the airfoil model surface at different test conditions. The new findings derived from the present icing physics study would lead to a better understanding of the important micro-physical processes, which could be used to improve current icing accretion models for more accurate prediction of ice formation and ice accretion on wind turbine blades as well as development of effective anti-/de-icing strategies tailored for safer and more efficient operation of wind turbines in cold weather.