Sandia Wake Imaging System (SWIS)
The Sandia Wake Imaging System (SWIS) has undergone development to improve the precision of velocity measurements within the inflow and wake of wind turbines through DOE funding. SWIS development began with a goal to capture instantaneous coherent turbulent structures within the tip-vortex region (Figure 12) of the near wake for the purpose of validating high-fidelity models. SWIS operates using the Doppler global velocimetry (DGV) method, also referred to as planar Doppler velocimetry, because of its ability to scale to large fields of view while capturing instantaneous coherent structures, or velocity images, of the flow field. Doppler global velocimetry scales to larger fields of view because individual particles are not resolved, though particulate seeding does improve the SNR ratio of the measurement (McKenzie 1996; Meyers et al. 2001; Herges et al. 2015, 2016; Tropea and Yarin 2007).
Other velocity measurement technologies, such as lidar, radar, and PIV, were considered. Lidar and radar instrumentation are limited in their capability to instantaneously acquire coherent turbulent structures at short spatial and temporal scales (Sjöholm et al. 2013a; Hirth et al. 2015), whereas PIV has extreme challenges with scaling to large fields of view because individual particles need to be resolved by the imaging device (Tropea and Yarin 2007; Bosbach et al. 2009; Adrian and Westerweel 2011; Pol and Balakumar 2013; Scarano et al. 2015).
DGV measures the velocity by capturing the Doppler shift frequency of light scattered from aerosol particles within a flow field illuminated by a narrow line width laser sheet. The scattered light is collected through a molecular iodine absorption filter contained within an optical cell (glass cylinder with optical windows) to convert the Doppler shift frequency into an intensity variation measureable by cameras to produce a velocity image (Elliott and Beutner 1999; Mosedale et al. 2000). The Doppler shift of the seeded and naturally occurring particles is calculated at each pixel of the imaging device using the Doppler shift equation, the measured intensity and camera observation unit vector, the iodine transmission profile, and the incident light-sheet-laser frequency and unit vector (Mosedale et al. 2000). The measured velocity component is the result of the system layout, wherein the bisector angle between the observation and incident laser light unit vector defines the direction of the velocity sensitivity, thus, multiple velocity components can be measured with additional observation vectors (Elliott and Beutner 1999). An example of the velocity sensitivity component relative to the laser sheet and observation vector is shown in Figure 13 for a possible SWIS configuration to measure one velocity component of the wind turbine tip vortex region at the SWiFT site. The SWIS acquires velocity images with a spatial resolution of 256-by-256 pixels at a rate of 15 Hz.
The SWIS system has undergone development from an initial laboratory viewing area of 15 cm by 15 cm, to a system characterization experiment with a 2-m-by-2-m viewing area (Herges et al. 2015) and finally a field demonstration viewing area of 3.5 m by 3.5 m centered at a height of 9.8 m from the ground to demonstrate safe operation of the laser and aerosol system outdoors (Herges et al. 2016). During this process, a SWIS modeling tool was created and calibrated to facilitate further system development, optimization, and experimental planning at the SWiFT facility (Herges et al. 2016). The modeling tool calculates measurement quality based on a specific experimental configuration and flow field of interest. As an example, Figure 12 and Figure 13 show the representative flow field from a simulation of the wind turbine tip vortex and the system layout, respectively, whereas Figure 14 (left) provides the velocity image captured with the current SWIS configuration, and Figure 14 (right) displays the velocity image acquired with a proposed upgraded SWIS system.
The upgraded system improvements include a new pulse-burst laser, modified iodine cell, and an adjusted aerosol generation system with a larger coverage area. The SWIS has a TRL of 6.