This thesis investigates the aerodynamics of two finite wall-mounted cylinders in tandem, focusing on drag reduction as a function of governing parameters.
For the experimental wind tunnel campaign, two measurement techniques were employed: balance measurements and stereoscopic particle image velocimetry. While balance measurements exhibited good repeatability, drag values obtained with PIV saw high uncertainty and only limited conclusions could be made from it.

Cylinders were mounted to the floor of a closed wind tunnel test section. The trailing cylinder was rigidly attached to the balance underneath, while the leading cylinder could move upstream to the de- sired distance. Stereoscopic PIV images of the wake at various distances upstream and downstream were taken through the transparent sides of the wind tunnel. These images, in combination with the control volume approach, were used to determine the drag of a trailing cylinder.

Coefficients of drag, obtained with a balance for isolated cylinders of various aspect ratios, were in line with similar results from the literature, albeit on the higher side. For cylinders in tandem of the same aspect ratio, AR, as the distance between them increased, the CD of a trailing cylinder converged to that of an isolated cylinder. Comparing tandem configurations with different AR and at the same nondi- mensionalized in-between distance, trailing cylinders with larger AR experienced larger drag reduction.

Introducing cylinder diameter ratio as an additional degree of freedom showed that smaller diameter trailing cylinders experienced greater drag reduction at close distances. However, at a certain distance further downstream, this trend reversed.

The drag reduction values obtained with PIV confirmed the findings from balance measurements. How- ever, due to the limited set of usable data, further work would need to be carried out to gain more confidence in the method.

In cycling, the presence of drag reduction from drafting, when multiple cyclists ride in tandem, is a considerable area of performance enhancement for a cycling team and the customisation of this tandem group has the potential of maximising the drag reduction for the entire group or a specific cyclist. The flow interactions between two tandem bluff bodies is complex and incorporates several fluctuating variations in flow phenomena which have considerable effects on the drag forces experienced. An experimental campaign for tandem finite cylinders and cyclists of differing sizes and in different spacing configurations is undertaken, including a drag force measurement and PIV flow visualisation campaign. It is observed that differing magnitudes of drag reduction occur for tandem cylinders and cyclists, which are however, caused by similar variations in flow structures. The size and spacing ratios are found to have notable impact on drag reduction for tandem objects.

The design of current skating suits is based on the assumption that the flow across the skater body parts is highly similar to cylinder flow. The latter features drag crisis behaviour, resulting in significant drag reduction at the critical Reynolds number. However, whether the aforementioned assumption is valid and whether a drag crisis along the different body parts occurs, so far remains unknown.

The goal of this study is to investigate Reynolds number effects along the leg of a skater mannequin. To do so, potential drag crisis behaviour is studied via robotic Particle Image Velocimetry and Infrared Thermography for speeds ranging between 5 m/s and 25 m/s. The boundary layer state and the critical velocity distribution along the leg, based on the wake width variation, are evaluated for the bare mannequin and the mannequin wearing a skating suit optimized for ∼ 15 m/s.

Results reveal drag crisis behaviour along the knee, lower and upper leg. Furthermore, the flow topology is not only governed by the leg geometry, but also by streamwise vortices. These streamwise vortices cause an increase of the wake width below the calf and a reduced velocity deficit behind the upper leg. Most significant differences in wake width between the bare and the dressed leg are observed at 17.5 m/s. The latter observation is also supported by the Infrared Thermography results.

It can be concluded that the flow across the leg partly differs from cylinder flow, mainly because of streamwise vortices that locally affect the drag crisis behaviour.

The AerGo is a recreational ultralight water-aircraft, designed to be transportable by bike and operated without a license. This report outlines its concept, characteristics, and feasibility.

The AerGo features a biwing design with swept wings, closed by vertical plates at the tips. It has a constant wing chord of 0.79 m and a wingspan of 12 m. The aircraft is powered by two six-bladed propellers mounted on the top wing and driven by electric engines. The lower wing is attached to a buoyant hull, stabilized by side floats. A paddle is used for taxiing and docking on water, as the aircraft is designed exclusively for water take-off and landing. The pilot sits inside the hull and controls pitch by shifting weight on a swing. Roll control is achieved through wing warping, and yaw by rudders. The total empty mass is 44.7 kg. A multipurpose trailer system facilitates storage, transport, and deployment. The AerGo is designed for single-person assembly and operation and is safe to fly for over 270 days per year.

A market analysis of ultralight aircraft reveals a European fleet of over 25,000, with 500 in the Netherlands. While most ultralights are custom-built, successful models like the Woopyfly and Lazair have seen large-scale production. The AerGo’s mobility, ease of use, and hydrodynamic capabilities position it competitively. A conservative estimate projects annual sales of 20 units, with 25% for the Dutch market and 75% for Europe. Expansion to North America is a future opportunity.

The design is driven by weight and energy efficiency, with batteries forming a significant part of the empty weight. The primary energy requirement comes from cruise flight, optimized through airfoil selection. The NACA 6415 airfoil, chosen for its high lift coefficient at zero angle of attack, minimizes drag and maximizes cruise efficiency. The hydrodynamic model, based on DSDS data, estimates a take-off speed of 9.9 kg and a 170 m take-off distance. The computed climb rate is 1.43 m/s, reaching a cruise altitude of 150 m in 105 seconds.

The engine and battery design prioritize take-off power while ensuring a minimum flight time of 60 minutes. With a cruise speed of 15 m/s, the AerGo has a range of 40 km. The pilot’s longitudinal position, optimized at 1.6 m, ensures stability. The upper wing has a positive stagger of 0.45 m forward, creating a seesaw effect that smooths weight-shift control. Roll is controlled by wing warping, and yaw by rudders placed at the wing tips. A 15° wing sweep enhances rudder effectiveness, allowing safe operation on a single engine.

The aircraft’s lightweight structure consists of a skin-on-frame design, with a nylon-covered carbon fiber wing weighing 11.3 kg and a dacron-covered carbon fiber hull at 4 kg. Noise reduction is achieved with six-blade propellers, keeping levels below 40 dB at 100 m distance. Approximately 74% of the aircraft is recyclable, with a carbon footprint of 800 kg CO2 per unit.

At an estimated price of €20,000 per unit and annual sales of 20, the break-even point is 9.5 years, with a projected return on investment of €700,000 after 12 years. Further analysis is required on cost budgeting, structural impact resistance, and user assembly instructions to ensure feasibility.

Aerodynamics investigations in speed sports generally consist of measurements of aerodynamic loads through wind tunnel experiments. Lately, a new technique has emerged, known as the Ring of Fire. It is the only drag measurement technique capable of retrieving flow field data on-site, by using Large-Scale PIV. In this Ring-of-Fire project a Low-Speed PIV system is implemented in an indoor facility and used to test an amateur cyclist in both pedalling and non-pedalling conditions. Twenty cyclist’s passages can be executed in thirty minutes and provide a mean drag area with an uncertainty equal to 2%. Drag differences equal to 6% are measured between two postures. Limitations arise due to the lack of repeatability of seeding density and cyclist’s position. With the contribution of this research, the Ring of Fire becomes a more mature technique to investigate speed sports aerodynamics, with future applications to curvilinear trajectories and multiple athletes.

Laser diagnostic techniques such as Particle Image Velocimetry and Particle Tracking Velocimetry to measure the flow of a fluid around an object have been in prevalence for a few decades. Typically, the fluid, whose motion is of interest, is seeded with micron scale particles and illuminated with a laser. Due to scattering inefficiencies of these micron scale particles the maximum measurement volumes were about a few litres. Development of the neutrally buoyant sub-millimetre scale Helium-Filled Soap Bubbles allows for much larger scattering cross-sections thus enabling large-scale measurements in air (Caridi et.al. 2015).

Large scale Particle Tracking Velocimetry has been used to study the flow in the wake of a cyclist at typical time-trial speeds. A novel PTV algorithm known as Shake-The-Box (STB) (Schanz et.al. 2013) has been used to obtain the particle tracks from the images acquired in a thin volume in the cyclist’s wake using the wake-scanning method. The Lagrangian particle tracking technique was found to be more accurate than the Time Resolved Tomographic PIV through another experiment. Particularly for large field of views involving scanning at different positions, it produces whole-field results free from the boundary effects that arise between different positions of the scans. This yields accurate velocity components and their gradients that are crucial for pressure reconstruction.

Flow-field results from 4D-PTV show that the wake structure is similar to those found in the literature. Two large regions of momentum deficit are present, one behind the thighs and one near the lower legs and the wheel axis. Furthermore, they reveal the presence of some vortices which were not reported previously. Based on the observed signs of the vortices the points of origination of these vortices are identified based on separation mechanisms. There is a good agreement in the flow-field results with the literature. Pressure fields reconstructed from the velocity flow-field show low pressure pockets in the regions of the vortex cores and separated flow over the lower back.

Wake flow-field is utilised to compute the drag using the control volume approach (van Oudheusden 2007) and compared with the drag forces obtained from an external balance measurement to obtain the accuracy of large scale 4D-PTV. In total, five measurements at different velocities in the range of 13m/s to 15m/s are analysed. The accuracy of the drag estimates from 4D-PTV is obtained within 5%. Out of the three terms of the drag force, the momentum term contributes approximately 95% and the contribution from the pressure term is less than 0.3%. Rest is contributed by the Reynolds stresses in streamwise direction. Almost all the variation in the drag force comes from the momentum term.

A novel measurement system, the Ring of Fire, is deployed which enables the aerodynamic drag estimation of transiting cyclists. The system relies upon the use of large-scale stereoscopic PIV and the conservation of momentum within a control volume in a frame of reference moving with the athlete. The rider cycles at a velocity of approximately 8 m/s, corresponding to a torso based Reynolds number of 3.17 × 10^5. The measurements upstream and approximately 1000 × 1700 m^2. The non-dimensional, phase-locked, time-averaged streamwise velocity fields compare well to literature and the time-averaged drag area shows a rather constant value along the wake with an uncertainty of 5%. A comparison with wind tunnel force balance measurements shows significant discrepancies, which may be partly attributed to the bike supports and stationary floor in the wind tunnel measurements. The 25 % drag difference measured between a rider in upright and time-trial position, instead, matches literature well.