The emergence of distributed electric propulsion (DEP) concepts for aircraft systems has enabled new capabilities in the overall efficiency, capabilities, and robustness of future air vehicles. Distributed electric propulsion systems feature the novel approach of utilizing electrically-driven propulsors which are only connected electrically to energy sources or power-generating devices. As a result, propulsors can be placed, sized, and operated with greater flexibility to leverage the synergistic benefits of aero-propulsive coupling and provide improved performance over more traditional designs. A number of conventional aircraft concepts that utilize distributed electric propulsion have been developed, along with various short and vertical takeoff and landing platforms. Careful integration of electrically-driven propulsors for boundary-layer ingestion can allow for improved propulsive efficiency and wake-filling benefits. The placement and configuration of propulsors can also be used to mitigate the trailing vortex system of a lifting surface or leverage increases in dynamic pressure across blown surfaces for increased lift performance. Additionally, the thrust stream of distributed electric propulsors can be utilized to enable new capabilities in vehicle control, including reducing requirements for traditional control surfaces and increasing tolerance of the vehicle control system to engine-out or propulsor-out scenarios. If one or more turboelectric generators and multiple electric fans are used, the increased effective bypass ratio of the whole propulsion system can also enable lower community noise during takeoff and landing segments of flight and higher propulsive efficiency at all conditions. Furthermore, the small propulsors of a DEP system can be installed to leverage an acoustic shielding effect by the airframe, which can further reduce noise signatures. The rapid growth in flight-weight electrical systems and power architectures has provided new enabling technologies for future DEP concepts, which provide flexible operational capabilities far beyond those of current systems. While a number of integration challenges exist, DEP is a disruptive concept that can lead to unprecedented improvements in future aircraft designs.

A series of wind tunnel experiments were conducted on an NACA 0012 airfoil undergoing a linear pitch ramp maneuver at a fixed dimensionless pitch rate of Ω + = 0.05 and across three transitional Reynolds numbers, Re c = 0.2 × 10 6 , Re c = 0.5 × 10 6 , and Re c = 1.0 × 10 6. The primary objectives of these experiments were to perform a detailed analysis of the flow evolution, with particular emphasis on the underlying physical mechanisms, and to extract the dominant scales associated with the flow perturbations, for a canonical dynamic stall process. A series of unsteady surface pressure measurements, with a high sampling frequency, were acquired in order to investigate the time-dependent behavior of the flow in the immediate vicinity of the airfoil. These surface pressure measurements were used to identify the region of boundary layer transition during the initial stages of the dynamic stall process. A spatially-contracting laminar separation bubble was also identified near the airfoil leading edge from the characteristic pressure plateau in the surface pressure distribution. The dominant frequencies associated with the laminar separation bubble were extracted using a continuous wavelet transform technique. These frequencies were observed to span a wide range of chordbased Strouhal numbers between St = 50 and St = 105, at Re c = 0.5 × 10 6. The off-body flow evolution was inferred and described using a combination of surface pressure measurements and time-resolved particle image velocimetry. For Re c = 0.2 × 10 6 and Re c = 0.5 × 10 6 , the dynamic stall vortex was observed to emerge from a collective interaction of the discrete vortices that were ejected from the leading edge of the airfoil. At Re c = 1.0 × 10 6 , however, the near-wall vortices were observed to amalgamate into two regions, forming a distinct primary and a secondary coherent structure. After formation, these two structures were observed to interact with each other, following a co-rotating vortex merging process and resulting in the emergence of a single, coherent dynamic stall vortex. The process of emergence of the dynamic stall vortex at Re c = 1.0 × 10 6 , observed from the present experiments, is therefore quite distinct from the classical understanding of the dynamic stall vortex formation, which was observed at the lower Reynolds numbers. The time-dependent spectra of the velocity field were calculated using a combination of empirical mode decomposition and Hilbert transformation. From the velocity spectra, the fluctuations in the flow were observed to attain an amplified state during the initial ejection of vorticity from the leading-edge region of

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