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Wake interactions play a crucial role in modern aerodynamics and hydrodynamics. Thisdissertation focuses on the wake interactions that occur between two oscillatory propulsors as well as between a thrust producing jet wake and a drag producing separated wake. First, experiments are presented on the interaction between a pair of in-line, sinusoidally pitching hydrofoils with fixed Strouhal number of St = 0.25 and Reynolds number of Re = 10, 000. The spatial locations of the hydrofoils relative to one another in the streamwise and spanwise directions is varied. Streamwise spacings range from 0.625 chords of spacing to 1.125 chords of spacing. Spanwise spacings ranged from fully aligned (no spacing) up to 0.5 spans of spacing. The phase delay, or offset, between the oscillations of the hydrofoils is also varied ranging from 0� to 360�. The hydrodynamic forces acting on both hydrofoils is recorded to investigate changes in time-averaged thrust, lift, spanwise force, power consumption, and propulsive efficiency. Additionally, a novel method to mitigate confounding factors from connecting rod drag is employed using streamlined fairings. The effects of spanwise spacing on the interaction of the hydrofoils were found to depend on the spanwise contraction of the leader hydrofoil�s wake as it advected downstream before impinging on the span of the follower hydrofoil. The rate of spanwise contraction of the leader�s wake is combined with force measurements to define small and large spanwise spacings. Small spanwise spacings are those which do not reduce the portion of the follower hydrofoil�s span where the incoming wake shed by the leader is directly impinging. Small spanwise spacings were found to only minimally change the thrust, power, and efficiency of the follower hydrofoil. Large spanwise spacings are those which do reduce the portion of the follower hydrofoil�s span directly interacting with the wake of the leader hydrofoil, linearly weakening the wake interaction between the hydrofoils. This results in the hydrodynamic performance of the follower hydrofoil being driven towards its isolated value. The delineation between small and large spanwise spacings was also dependent on the streamwise spacing between the hydrofoils. Smaller streamwise spacings are found to narrow the range of spanwise spacings that would be considered small. Moreover, negligible spanwise forces are measured for all of the spanwise spacings tested, which shows that independently swimming foils would be neutrally stable to spanwise perturbations. In addition the performance of the follower hydrofoil showed a strong dependence on its streamwise positioning relative to the leader hydrofoil. Near streamwise spacing (0.625 to 0.875 chords) and far streamwise spacing (0.875 to 1.125 chords) regions were identified, with the delineation between the two occurring at 0.875 chords of streamwise spacing. Distinct trends in the performance of the follower hydrofoil were identified in each streamwise region. In the far region moving the hydrofoils closer together in the streamwise direction resulted in an increase in the maximum time-averaged power consumption of the follower hydrofoil while this trend reversed in the near region. Using these experimental results, an empirical model for the average power consumption of the follower hydrofoil is developed. Separately, an experimental study on the interaction of a small Uncrewed Aerial System (sUAS) with a fast-moving ground vehicle in a wind tunnel setting is presented. In contrast to the first study, which considered the interaction of two thrust producing wakes of equal magnitude, the goal of this study is to investigate the interactions of thrust and drag producing wakes of different sizes relative to one another. The study records the aerodynamic forces experienced by the sUAS and the resulting flow fields for a variety of relative spacings between the models. The flow around each of the models is characterized using both time averaged and high speed particle image velocimetry (PIV). In order to better understand the wakes of the models, efforts are made to bound the wakes using readily observable flow features in the time average PIV data. The extents of the separated wake structure of the ground vehicle was found to be easily bounded by contours of Utotal = 0.95U?. The jet wake of the sUAS was found to be bound by contours of URMS = 0.10U?. Interaction with the wake of the RGV was found to cause significant changes to the aerodynamic performance characteristics of the sUAS. In particular, close proximity to the RGV was found to increase the unsteady lift and thrust forces acting on the sUAS as well as the time average thrust force. Analysis of the interaction flow fields at a variety of relative positions between the models revealed several locations where the interaction flow field appeared to be a simple summation of the isolated flow fields. From this information an additive wake hypothesis was formulated and investigated through the further investigation of the isolated flow fields as well as through an energy spectrum analysis of the wake flow fields. This study categorizes the observed wake interactions as: (1) Interactions where the sUAS did not modify the wake of the ground vehicle model; (2) Interactions where the sUAS altered the RGV wake, the sUAS jet wake was visible in the interaction case, did not modify the upstream flow, and some similarities were observed between the interaction case and a combination of isolated flow fields; (3) Interactions where the two wakes interacted in a complex manner, which occurred at locations where the sUAS was positioned near the heavily decelerated regions of flow in the wake of the ground vehicle, modified the upstream wake of the ground vehicle, or was close enough for ground effect. This third interaction region demonstrated that an sUAS, which is considerably smaller than a commercial ground vehicle, has the capability to significantly alter the flow field around a ground vehicle and thus significantly impact the type of wake interaction that it may encounter while operating near the ground vehicle. These studies emphasize the importance of relative positioning between objects in flow and highlight the effects of relative spacing on the types of wake interactions that will take place in a variety of flow conditions.