The increasing importance of drag reduction in commercial vehicles, traditionally motivated by rising energy costs, and more recently accelerated through policymaking in a bid to reduce emissions, has resulted in the development of increasingly sophisticated add-on aerodynamic devices for the semi-trailer. Of these devices, the greatest uptake amongst trucking fleets has been for devices fitted to the underbody, primarily on account of allowing free movement of freight into and out of the trailer, hence incentivizing a better understanding of the flowfield in this region. The current research addresses the aerodynamics of trailer wheels in tandem, which form a key component of the semi-trailer underbody, by investigating the effects of wheel rotation, trailer side-skirts, and wheel cavity covers on the flowfield and drag. The configurations tested include a short side-skirt terminating before the front edge of the leading wheel, a long side-skirt covering both the wheels, wheel covers without openings (solid disks), and wheel covers with openings varying in coverage area and radial position.

Stereoscopic (2D-3C) PIV is applied to examine the flow topology in the near wake and on selected planes besides the wheels. The measured velocity data in the wake is further used to derive the pressure field (by solving the Poisson equation for pressure), which together are then used to calculate the drag using a control volume approach. The uncertainty in the derived pressure field and drag values is determined using linear uncertainty propagation.

The largest drag reduction from the baseline case, i.e. without a side-skirt or wheel covers, is seen for the long side-skirt, followed by the short side-skirt with wheel covers, short side-skirt only, and wheel covers only, in that order. The effectiveness of both the long side-skirt and wheel covers is seen to increase with wheel rotation, with an appreciable reduction in drag with the wheel covers fitted only seen for rotating wheels. Investigation of the flowfield in the wake shows significant differences between the stationary and rotating wheel, particularly within the region of the projected wheel profile, and indicates to an earlier separation of flow along the upper surface of the rotating wheel. The effect of the side-skirt on the velocity in the wake is seen primarily in a lower streamwise velocity deficit, narrower wake, and higher horizontal symmetry of the wake for the skirted configuration. Wheel covers show a comparatively limited effect on the velocity field in the wake, showing a marginally wider wake and a slightly higher velocity deficit for the uncovered wheel. On planes beside the wheels, the side-skirt and wheel covers show a greater influence than wheel rotation, considered to be a consequence of a static floor combined with a gap between the wheel and the ground. The non-skirted configurations here exhibit a larger region of separated flow and greater velocity deficit whereas the uncovered wheels show an outflow at the bottom of the wheel cavity and an inflow at the top. Finally, the wheel covers with openings show a behavior between that of a covered and an uncovered wheel, with the inflow/outflow depending significantly on the coverage percentage and radial position.

The available energy density supplied by conventional light sources prohibits the use of time-resolved volumetric Particle Image Velocimetry (PIV) with submillimetre helium-filled soap bubbles (HFSB) on a cubic metre scale. In connection with attempts to expand the maximum attainable measurement domain of PIV, a research project is carried out into the upscale of the seeding particles, capable of scattering more light. The production flexibility and scalability of HFSB is investigated by designing and improving the conventional HFSB nozzle generator used by Delft University of Technology. Experiments are performed on various nozzle designs, focusing on the average bubble diameter. Through geometrical scaling and further developments, the bubble diameter is scaled from 0.5 mm, produced by conventional nozzles, to 2.5 mm. The obtained upscale in bubble diameter corresponds to a 3-fold increase along all dimensions compared to the current operational domain for HFSB experiments. Furthermore, a novel physics-based model is proposed relating particle size to the nozzle orifice and flow rates of air and helium and is benchmarked with the acquired experimental data.

Experimental fluid dynamics and computational fluid dynamics have traditionally been treated as disparate fields of study. However, each field has its own unique set of advantages and disadvantages. Data assimilation is a field that can be used to leverage some of the advantages each field offers to help compensate mutual
weaknesses. In this thesis, a state observer based data assimilation method is used to assimilate 3-D experimental data obtained in a wind tunnel experiment onto a steady RANS simulation. The experimental data is considered as the ground truth and is used to condition the RANS simulation. An understanding of the working of the method along with a study on the effect of different parameters of the state observer method are gathered by first applying it on the 1-D viscous Burgers equation and a 2-D CFD simulation. For the 3-D case, experimental data is obtained by performing a wind tunnel experiment using robotic PIV to map the time-averaged velocity field around a bluff body following which the data is assimilated onto a steady RANS simulation of the same body. Application of this method helps to recreate topological features and velocity
fields of the flow with better accuracy than a baseline CFD simulation. Finally, the effects of the different parameters on the success of the method along with recommendations for improving the method are provided.

In experimental aerodynamics, pressure is an important parameter which provides insight into the various features of the flow around the test object. By obtaining the surface pressure information, various inferences can be drawn such as flow separation, local flow velocity etc. Traditionally, the most common method of obtaining pressure has been through direct measurements by incorporating the test model with pressure taps, which makes the model expensive and complicated. However, often there arises a requirement to obtain qualitative information and visualise the aerodynamic features during the test. Particle Image Velocimetry (PIV) solves provides this qualitative information as well as quantitative information in the form of a flow field. Pressure can be further computed using this velocity field. This technique holds a number of advantages over traditional wind tunnel measurements, especially with reduction in resources for manufacturing, ease of handling and non-intrusiveness. However, the conventional PIV technique came with the disadvantage of only having 2D2C information. Other techniques like stereo PIV and tomographic PIV were developed to tackle these limitations and the most recent development was that of Coaxial Volumetric Velocimetry (CVV). Along with the advent of Helium Filled Soap Bubbles (HFSB), this novel technique allows for large scale 3D3C measurements. This research is conducted to assess the feasibility of this promising technique to obtain surface pressure.
The flow field around an inverted wing in ground effect is investigated using CVV by robotic manipulation. Using HFSB as tracer particles, the experiment was carried out in the W-Tunnel of TU Delft. The wing was tested at two angles of attack, α = 5° and α = 8° and three ground clearance configurations
(h/c= 1, 0.6 and 0.3 at α = 5°) at airspeed of 10 m/s. The wing is equipped with a total of 30 surface pressure taps along the chord-wise direction, both on the pressure and suction side. The velocity field is obtained using Lagrangian particle tracking algorithm, Shake-the-Box and time-averaged velocity field is obtained by ensemble averaging of the obtained particle tracks. Static pressure is calculated using Poisson’s equation using the information obtained from velocity field. The pressure thus obtained is then compared with the readings obtained from pressure taps.
Such an assessment not only shows the feasibility of the technique but also highlights the shortcomings and potential improvements. To the best of the author’s knowledge through a literature study, this particular technique has not been used to obtain surface pressure information. Most of the work published in the field of CVV is to study flow features, mostly in the wake. Hence, it is expected that this research and the recommendations help to further development of such a technique in the future.