| 30 Oct 2025
Flow Field Analysis of a Leading-Edge Inflatable Kite Rigid Scale Model Using Stereoscopic Particle Image Velocimetry
Abstract. Leading-edge inflatable (LEI) kites are characterized by a pronounced downward curvature of the wing and flow recirculation zones on the pressure side. This study presents novel stereoscopic particle image velocimetry (PIV) measurements of a 1:6.5 rigid scale model of the TU Delft V3 LEI kite. The flow-field measurements were conducted in the Open Jet Facility of Delft University of Technology for two angles of attack and seven chordwise measurement planes positioned between mid-span and tip, and were compared with results from Reynolds-averaged Navier–Stokes (RANS) simulations. The double-curved anhedral wing geometry presented several challenges, such as surface reflections that required careful data processing and the use of a lateral velocity filter. The circulation distribution was analyzed, using both elliptical and rectangular boundary curves, showing good agreement in trends between the vortex-step method (VSM), RANS, and PIV data. The lift and drag coefficients of each chordwise measurement plane were estimated using the Kutta–Joukowski theorem, surface pressure integration of RANS CFD data, and Noca's method – an inherently three-dimensional reformulation of the momentum conservation equations expressed solely as surface integrals over the control-volume boundary – applied here in two dimensions. While the mid-span to tip variation of lift coefficients was in accordance with the anhedral shape and tip-vortex effects, the drag measurements and predictions deviated from the expected behavior by exhibiting negative values. Especially near the tip region, significant discrepancies were observed, attributed to increased measurement uncertainty. The surface pressure integration revealed discrepancies at the strut junction, likely due to local three-dimensional strut-induced flow effects and increased airfoil thickness. This study provides comprehensive validation data for CFD simulations of LEI kites while highlighting the challenges in PIV measurements of double-curved anhedral wings and characterizing local aerodynamic phenomena.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Wind Energy Science.
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The manuscript presents a high-quality experimental study on a 1:6.5 scale LEI kite using stereoscopic PIV, providing a benchmark dataset for validating CFD and Vortex-Step Method models. The work is scientifically significant, as it elucidates local 3D aerodynamic phenomena, including strut-induced flow effects, and directly confirms stall onset at high angles of attack. Methodology and analysis are rigorous, and results are clearly presented. Minor technical revisions are suggested to improve clarity, including sentence refinements, explicit description of pseudo-2D assumptions in force calculations, and clarification of some PIV limitations. Overall, the manuscript represents a valuable contribution to the field of kite aerodynamics and is recommended for acceptance after technical corrections.
Technical correction :
Split long sentences in Discussion and Conclusion for readability for example : “Characterizing the aerodynamics of LEI kites with numerical prediction and experimental measurement poses several challenges, owing to the highly flexible nature, pronounced anhedral and sweep, and unconventional airfoil geometries.”; “The combination of increased downward, sideways, and upstream flow near the strut suggests the presence of a tilted or angled vortex structure.” ; “Measurement quality could be further improved by employing a narrower laser light sheet to concentrate laser power, potentially reducing reflection intensity.”