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The NASA single-aisle turboelectric aircraft with an aft boundary layer propulsor (STARC-ABL) concept utilizes a novel electrically driven aft fan that ingests the fuselage boundary-layer for increased propulsive efficiency. In this paper we examine how aerodynamic shaping of the fuselage diffuser and nacelle inlet can reduce the flow distortion at the aft fan. Adjoint-based aerodynamic shape optimization with the ARP1420 distortion metric objective is used to automatically determine the optimal shapes for minimal fan-face distortion. Single and multipoint optimizations are carried out for simplified body-duct and wing-body-duct configurations. These two configurations highlight the importance of including the wing downwash effects when designing the propulsor. The optimizations showed the body-duct configuration can obtain cruise distortion values of approximately 1% while the wing-body-duct configuration can obtain distortion values of just over 2%.

Flows over airfoils and blades in rotating machinery for unmanned and microaerial vehicles, wind turbines, and propellers consist of different flow regimes. A laminar boundary layer near the leading edge is often followed by a laminar separation bubble with a shear layer on top of it that experiences transition to turbulence. The separated turbulent flow then reattaches and evolves downstream from a nonequilibrium turbulent boundary layer to an equilibrium one. Typical Reynolds-averaged Navier–Stokes (RANS) turbulence modeling methods were shown to be inadequate for such laminar separation bubble flows (Spalart and Strelets, 2000, “Mechanisms of Transition and Heat Transfer in a Separation Bubble,” J. Fluid Mech., 403, pp. 329–349). Direct numerical simulation (DNS) is the most reliable but is also the most computationally expensive alternative. This work assesses the capability of large eddy simulations (LES) to reduce the resolution requirements for such flows. Flow over a flat plate with suitable velocity boundary conditions away from the plate to produce a separation bubble is considered. Benchmark DNS data for this configuration are generated with the resolution of 59 × 106 mesh points; also used is a different DNS database with 15 × 106 points (Spalart and Strelets, 2000, “Mechanisms of Transition and Heat Transfer in a Separation Bubble,” J. Fluid Mech., 403, pp. 329–349). Results confirm that accurate LES are possible using O(1%) of the DNS resolution.

Fluid systems are most efficient for fully attached flows, and designers therefore seek to avoid flow separation. Active flow control can help achieve this goal, and closed-loop control offers improved performance at off-design conditions. However, this requires feedback of accurate state estimates to the controller in real time. This motivates a physics-based, state-estimation technique that economically extracts key dynamical features of the flow. This work aims to extract dynamical characteristics of a laminar separation bubble on a flat plate at a chord Reynolds number of 10 5 using a linear array of unsteady surface pressure measurements. First, Dynamic Mode Decomposition (DMD) is employed on high-dimensional time-resolved PIV velocity and corresponding estimated pressure fields to identify the dynamically relevant spatial structure and temporal characteristics of the separated flow. Then, results are presented of various open-loop control cases using pulse-modulation of a zero-net mass-flux actuator slot located just upstream of separation. Real-time estimates of the dynamical characteristics are provided by performing online DMD on measurements from a linear array of 13 unsteady surface pressure transducers. The results show that this method provides reliable estimates of the modal characteristics of the separated flow subject to forcing at a rate much faster than the characteristic time scales of the flow. Therefore, online DMD applied to the surface pressure measurements provides a time-varying linear estimate of the evolution of the controlled flow, thereby enabling closed-loop control.

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