Surfaces that induce slip at the wall have been shown to reduce skin friction drag in the turbulent regime. While superhydrophobic and liquid-infused surfaces are capable of drag reduction in turbulent hydrodynamic flows, neither would be feasible in aerodynamic applications due to the added roughness and the minuscule slip lengths created by the unfavourable viscosity ratio between the liquid and air. This thesis explores the novel idea of liquid-infused superhydrophobic surfaces (LISHySs). These surfaces use liquid droplets held in superhydrophobic features to induce slip at the air-liquid interface while reducing resistance to the motion of the liquid within the cavity.

Since no framework existed on the drag-reducing mechanism of such a surface, this was devised and the parameters of importance were identified. These were seen to be the area of the air-liquid interface, the exposed and submerged areas of the droplets in the cavities, and the degree of superhydrophobicity of the surface. Two types of LISHySs were consequently designed - one with spherical droplets and another with spanwise cylindrical droplets. These were produced by 3D printing surfaces with cavities and applying a superhydrophobic coating, into which water was infused.

Direct force measurements on the surface showed an increase in the drag coefficient (between 10% and 17%) with respect to a smooth reference surface. While PIV for the flow over this surface showed an increase in mean streamwise velocity and a decrease in Reynolds stress in the overlap layer, the high degree of reflection produced by the surface meant that meaningful near-wall data could not be obtained. Observations of the air-liquid interface demonstrated that the hypothesised steady rolling motion of liquid droplets occurred at low freestream velocities, but this was observed to become more chaotic at higher velocities. Pressure drag due to droplets acting as roughness elements, among other reasons, further contributed to the total drag produced.

While the results showed that the current implementation of LISHySs was unsuccessful, the lessons from this first attempt point to the directions in which future studies could be made. Cylindrical droplets oriented in the streamwise direction offer high potential due to the creation of streamwise slip. Advancements in the fields of material science and hydrophobicity would allow for surfaces with finer cavities that exhibit higher superhydrophobicity to be produced. Measures such as these would pave the way for the passive reduction of turbulent drag through the novel concept of LISHySs.

This research studies the fluid-structure interaction (FSI) of compliant surfaces in air flows, with an objective of finding possible turbulent viscous drag reduction. A compliant surface is a thin layer of viscoelastic material drawing inspiration from dolphin epidermis, which was thought to have drag-reducing capabilities by Gray (1936). Research into the drag reduction capabilities of compliant surfaces has been long going for more than five decades, starting with Kramer (1960), yet no firm conclusion has been reached. In addition, most of the experimental research has been focused on water flows, with air flows regarded as incapable of inertially forcing a compliant surface to deform. This research attempts to disprove this assumption by applying the proper inertial scaling to the FSI between the compliant surface and air flows. Compliant surfaces are characterised by the stiffness and thickness, which are presumed to be of first-order influence on the FSI. A parameter sweep of these compliant surface properties was conducted by drag delta measurements and flow visualisation by planar particle image velocimetry (PIV) in the M-Tunnel at the Low Speed Laboratory (LSL) Drag delta results confirm the possibility of turbulent viscous drag reduction by compliant surfaces, with a measured drag delta of -3.43 %. This result is further supported by the decrease in 1D turbulence intensity at the test plate's trailing edge from hot-wire measurements, and a smaller decay of the shape factor H from PIV. Quadrant analysis of the PIV data found evidence of a reduction in combined Q2 and Q4 events, further supporting the drag delta measured. Correlation between the compliant surface's viscoelastic properties and the drag delta found a negative correlation between the magnitude of complex shear modulus and a positive correlation between the loss tangent and drag reduction. The high loss tangent for the drag-reducing compliant surface and its subtle positive correlation with the drag reduction indicates that the viscoelasticity might have a greater influence on the FSI than expected.