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.

@en

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. @en

The aerodynamic characteristics of a modern road cycling wheel in crosswind are studied through force measurements and 3D velocimetry in TU Delft’s Open Jet Facility. The performance of the 62 mm deep rim is evaluated for two tire profiles, and yaw angles up to 20◦ . 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 trend, also referred to as the sail-effect, is limited by the stall angle of the tire-rim profile. The stall angle is found to be dependent on the tire surface texture and varies between 14◦ and 20◦.@en

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.].

@en

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.

@en

The flow field around a full-scale swimmer’s hand model with varying thumb positions is investigated by robotic volumetric PIV. The experiment is conducted in the Open Jet Facility wind tunnel at TU Delft at 15m/s. Quantitative flow field information is constructed with 3D-PTV in a 120 liter volume, encompassing the full hand and arm. The effect of spatial resolution on the time-averaged flow field is investigated. A large-scale recirculating wake behind the hand is accurately identified at a linear bin size of 20mm whereas the accelerated flow between individual fingers can only be resolved at bin sizes below 10mm where 5mm results in a statistically unconverged velocity field. The influence of the thumb is limited to one side of the hand where its presence results in a larger stagnated region in front and larger wake behind the hand, depending on the thumb position. Closing the thumb strengthens the recirculation but results in a smaller velocity deficit downstream, suggesting a smaller propulsive force generation which is considered disadvantageous in competitive swimming. @en

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. @en

This study describes the working principles of the coaxial volumetric velocimeter (CVV) for wind tunnel measurements. The measurement system is derived from the concept of tomographic PIV in combination with recent developments of Lagrangian particle tracking. The main characteristic of the CVV is its small tomographic aperture and the coaxial arrangement between the illumination and imaging directions. The system consists of a multi-camera arrangement subtending only few degrees solid angle and a long focal depth. Contrary to established PIV practice, laser illumination is provided along the same direction as that of the camera views, reducing the optical access requirements to a single viewing direction. The laser light is expanded to illuminate the full field of view of the cameras. Such illumination and imaging conditions along a deep measurement volume dictate the use of tracer particles with a large scattering area. In the present work, helium-filled soap bubbles are used. The fundamental principles of the CVV in terms of dynamic velocity and spatial range are discussed. Maximum particle image density is shown to limit tracer particle seeding concentration and instantaneous spatial resolution. Time-averaged flow fields can be obtained at high spatial resolution by ensemble averaging. The use of the CVV for time-averaged measurements is demonstrated in two wind tunnel experiments. After comparing the CVV measurements with the potential flow in front of a sphere, the near-surface flow around a complex wind tunnel model of a cyclist is measured. The measurements yield the volumetric time-averaged velocity and vorticity field. The measurements of the streamlines in proximity of the surface give an indication of the skin-friction lines pattern, which is of use in the interpretation of the surface flow topology.

@en

A novel approach to the measurement of large-scale complex aerodynamic flows is presented, based on the combination of coaxial volumetric velocimetry and robotics. Volumetric flow field measurements are obtained to determine the time-averaged properties of the velocity field developing around a three-dimensional full-scale reproduction of a professional cyclist. The working principles of robotic volumetric PIV are discussed on the basis of its main components: helium-filled soap bubbles as tracers; the compact coaxial volumetric velocimeter; a collaborative 6 degrees of freedom robot arm; particle image analysis based on Shake-the-Box algorithm and ensemble statistics to yield data on a Cartesian mesh in the physical domain. The spatial range covered by the robotic velocimeter and its aerodynamic invasiveness are characterised. The system has the potential to perform volumetric measurements in a domain of several cubic metres. The application to the very complex geometry of a full-scale cyclist in time-trial position is performed in a large aerodynamic wind tunnel at a flow speed of 14 m/s. The flow velocity in the near field of the cyclist body is gathered through 450 independent views encompassing a measurement volume of approximately 2 m³. The measurements include hidden regions between the arms and the legs, otherwise very difficult to access by conventional planar or tomographic PIV. The time-averaged velocity field depicts the main flow topology in terms of stagnation points and lines, separation and reattachment lines, trailing vortices and free shear layers. The wall boundary layers developing on the body surface hide below the level resolvable by the present measurements.

@en

Some recent trends in the development of PIV techniques are leading to the realization of velocity measurement at large scale. Moreover, methods for evaluating the fluid flow pressure from PIV data have been successfully demonstrated at small scale and for simple geometries. Methods for quantitative visualization of surface flow properties such as skin-friction and pressure are comparatively less developed, despite the great interest for the aerodynamic insight they provide for the study of the flow around 3D complex objects. The accuracy of the pressure evaluation at the surface is inquired in this work by dedicated measurements over a sphere at Red = 8 × 104. The measurements made with robotic PIV are compared to direct surface pressure measurements. The solution of the Reynolds averaged momentum equations is performed with the Poisson problem formulation and the results are compared with potential flow theory and some simplified models. The accuracy of stagnation pressure depends upon the measurement resolution. Instead, the suction peak region is generally underestimated. The pressure in the separated region is retrieved with sufficient accuracy. The surface flow properties visualization of a time-trialing full-scale cyclist replica at 14 m/s (Re = 5.5×105) is performed as a demonstration of the above method to a complex 3D problem at large scale. The data obtained with the robotic PIV technique is analyzed in close proximity of the solid surface and the skin friction lines of the time averaged velocity field are inspected. The topological analysis of the skin friction lines yields clearly the evolution of the flow around the body segments returning the details of the separated flow regions and local recirculation zones. The analysis of the surface pressure distribution yields results in accordance with the skin-friction lines, the outer flow velocity field as well as the vortex topology.@en