Aerodynamic characterization of a soft kite by in situ flow measurement

Abstract

Abstract. Wind tunnel testing of large deformable soft kites for wind energy conversion is practically not feasible. Computational simulation of the coupled fluid-structure interaction problem is scientifically challenging and of limited practical use for aerodynamic characterization. In this paper we present a novel experimental method for aerodynamic characterization of flexible membrane kites by in situ measurement of the relative flow, while performing complex flight maneuvers. We find that the measured aerodynamic coefficients agree well with the values that are currently used for flight simulation of soft kites. For 5 flight operation in crosswind maneuvers where the traction force is kept constant, the angle of attack and the apparent flow velocity exhibit opposite trends. For entire pumping cycles, the measurements show considerable variations of the aerodynamic coefficients, while the angle of attack of the kite varies in fact only in a narrow range. This finding questions the commonly used representation of aerodynamic coefficients as sole functions of the angle of attack and stresses the importance of aeroelastic deformation for this type of wing. Considering the effect of the power setting (identical to the trim) solely as a rigid-body pitch 10 rotation does not adequately describe the aero-structural behavior of the kite. We show that the aerodynamic coefficients vary as functions of the power setting (trim) of the kite, the steering commands and flight direction.

Identification of kite aerodynamic characteristics using the estimation before modeling technique

“…These difficulties are intrinsic to our low cost experimental setup, which involves relatively short tether lengths and, due to limitations related to the load cells and the manual control system of the kite, it should operate with low wind speeds. Other experimental setups do not exhibit these issues and low AoA (around 10°) were measured by performing figure‐eight trajectories in cross‐wind conditions with a LEI kite 22 . Since the pilot could not perform figure‐eight maneuvers, our kite flew in post‐stall conditions most of the time and, around 1 min after takeoff, the kite crashed and the multi‐hole Pitot tube was damaged.…”

“…Experimental studies on the aerodynamics of AWE systems were also carried out. Besides wind tunnel experiments, 13,19 flight test setups have been developed to determine the aerodynamic characteristics of AWE systems 20‐23 . In situ flow measurements of a soft kite in complex flight maneuvers were obtained 18,22 .…”

“…Besides wind tunnel experiments, 13,19 flight test setups have been developed to determine the aerodynamic characteristics of AWE systems 20‐23 . In situ flow measurements of a soft kite in complex flight maneuvers were obtained 18,22 . The Pitot tube was installed on a rigid frame attached to the two power lines, instead of the kite, and the inflow angles were measured with vanes.…”