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The aerodynamic characteristics of a modern road cycling wheel in cross wind are studied through force- and planar PIV measurements in the TU Delft Open Jet Facility. The performance of the 62 mm deep rim is evaluated for three tire profiles, and yaw angles up to 24°. All measurements are executed at 12.5 m/s (45 km/h) freestream- and wheel-rotational velocity. The wheel's rim-tire section in crosswind is found to behave similar to an airfoil at incidence, ultimately resulting in a reduction of the wheel's aerodynamic resistance with increasing yaw angle magnitude. This phenomenon, also referred to as the sail-effect, is limited by the stall angle of the tire-rim profile. The stall angle is found to depend critically on the tire's surface structure. Larger stall angles, resulting in lower resistance, are obtained if the tire profile triggers laminar-to-turbulent boundary layer transition., Flow Physics and Technology, Aerodynamics

Particle image velocimetry (PIV) is the state of the art for quantitative, full-field, 3D flow diagnostics. Despite the maturity of the technique, two bottlenecks are identified which are addressed in this thesis: the achievable measurement volume size, and the optical access to geometrically complex objects. Both aspects are well illustrated when considering the human body in sports action. Characterising the aerodynamic flow topology around an athlete demands measurement volumes on the cubic-meter scale, whereas the simultaneous illumination and imaging of the flow near the athlete’s body is challenged by the geometric complexity of the human body and the sports equipment. Focusing on sport performance, especially in timed disciplines, it is recognized that due to the shape of the human body, the aerodynamic resistance is often dominated by pressure drag. Therefore, a third element addressed in this thesis is the PIV-based pressure evaluation in the flow and on an object surface. To overcome the identified measurement constraints, a PIV system for the 3D diagnostics of large-scale and low-speed flows has been developed, synthesizing advancements in PIV imaging and illumination hardware, automation technology, tracer particle generation, and particle tracking algorithms. The so called robotic volumetric PIV concept is proposed in Part I of this thesis, along with dedicated data analysis methods to retrieve the shape of the test object, the total pressure in the fluid flow, and the aerodynamic pressure on the object surface. Part II features applications of the proposed tools in the context of sport aerodynamics, with specific examples in cycling and swimming., Aerodynamics

The present work reviews the recent developments in the domain of particle image velocimetry (PIV) with attention to its use for large-scale problems of interest for industrial aerodynamics. The article introduces the fundamental principles of flow seeding for large-scale experiments based on the helium-filled soap bubble (HFSB) technique. The measurement of flow tracers in a three-dimensional domain is presented, which is based on Lagrangian tracking methods. Furthermore, the concept of co-axial volumetric velocimetry is introduced and its combination with robotic arm manipulation is shown. The work presents two main examples of applications to aerodynamic problems: The first is the flow field around a propeller aircraft at wind tunnel free-stream velocity up to 50 m s-1, which to date is considered the boundary of large-scale PIV application with HFSB. The second example is the complex three-dimensional flow field around a cyclist at full scale. The work summarizes developments obtained in the last 3 years of research, demonstrating that the domain of PIV techniques is still undergoing vivid developments., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Aerodynamics, Wind Energy & Propulsion, Aerodynamics

Abstract: A method to identify the surface of solid models immersed in fluid flows is devised that examines the spatial distribution of flow tracers. The fluid–solid interface is associated with the distance from the center of a circle to the centroid of the tracers ensemble captured within it. The theoretical foundation of the method is presented for 2D planar interfaces in the limit of a continuous tracer distribution. The discrete regime is analyzed, yielding the uncertainty of this estimator. Also the errors resulting from curved interfaces are discussed. The method's working principle is illustrated using synthetic data of a 2D cambered airfoil, showing that one of the limitations is the treatment of an object thinner than the search circle diameter. The method is readily adapted to 3D and applied to the 3D PTV data of the flow around a juncture. The surface is reconstructed within the expected uncertainty, and specific limitations, such as the smoothing of sharp edges is observed. Graphic abstract: [Figure not available: see fulltext.]., Aerodynamics, Aerodynamics, Wind Energy & Propulsion

An experimental approach for the measurement of the time-average fluid flow pressure over the surface of generic three-dimensional objects is presented. The method is based on robotic volumetric PTV measurements followed by the integration of the pressure gradient. The domain for pressure evaluation is subdivided in two parts: in the irrotational region the static pressure is obtained following Bernoulli relation; in the turbulent wake and close to the object the pressure gradient is integrated. An approach based on the total pressure distribution is proposed to estimate the boundary between these two regions. The method is first assessed with experiments around a sphere equipped with pressure taps. A criterion for minimum spatial resolution is formulated in terms of maximum ratio between bin size and local radius of curvature of the object. An experimental database from a three-dimensional problem of higher geometrical complexity is considered: the time-averaged flow field around a full-scale cyclist. The surface pressure distribution is discussed in connection to the topological features of near-surface streamlines and streamwise vortices., Aerodynamics, Aerodynamics, Wind Energy & Propulsion

A multi-model method for static pressure evaluations based on 3D PTV data is presented. The volumetric measurement domain is partitioned into an irrotational part where pressure is evaluated in a point-wise manner through Bernoulli’s equation, and a rotational domain where the pressure gradient is spatially integrated. In a third step, the field pressure is mapped onto the solid surface of the object. The method is first assessed on the flow around a 15 cm diameter sphere at 10 m/s. A good agreement is observed for the surface pressure prior to flow separation. Discrepancies in the order of 0.1 to 0.2 Cp are found towards the separated wake region. There is little dependency on spatial resolution, as long as the ensemble averaging cell remains below 50% of the local radius of curvature of the object. The potential of the method to address generic three-dimensional problems of higher geometrical complexity is demonstrated for the time-averaged flow field around a full-scale cyclist, within a 2 m3 measurement domain. The proposed combination of the multi-model pressure evaluation with robotic PIV enables surface pressure measurements in the low-speed flow regime, for unprecedented aerodynamic analysis otherwise possible only by massive instrumentation of the test model with surface pressure taps., Aerodynamics, Aerodynamics, Wind Energy & Propulsion