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Direct Wall Shear Stress Measurement for Rotor Blades



OBJECTIVE: Directly measure mean and fluctuating shear stress on a rotor blade.

DESCRIPTION: Aerodynamic loads on rotor blades are driven in large part by the dynamics of the boundary layer. Each point of the blade undergoes large variations in aerodynamic regimes throughout its operation; including tangential speed variations along the span, variation of both mean and fluctuating angles of attack as a result of the setting of collective and cyclic controls, as well as variation in the magnitude of the oncoming flow speed throughout the rotation in forward flight. All of these factors influence the behavior of the boundary layer and ultimately lead to the overall aerodynamic performance of a vertical lift vehicle platform.Numerical calculation of these loads from high-fidelity computation fluid dynamics models is possible, but validation of sufficiently complex models is difficult without the ability to directly measure surface pressure and shear stress at various locations on the rotor blade system. Direct point-wise sensing of these quantities would permit model validation, as well as insight into the boundary layer physics. Many boundary layer models are developed from investigations that do not include the full complexity of the actual flows (i.e. 2D vs 3D, swept wing vs rotation, Mach and Reynolds number mismatches, etc.) and thus suffer from empiricism and questionable applicability to the vehicle system. Capturing the behavior of the boundary layer subject to all the relevant physical mechanisms has potential to significantly advance fundamental understanding of the unsteady boundary layer physics, which in turn will permit more advanced vehicle/rotor system designs.Historically, hot-film anemometry and oil-film interferometry have been used as wall-shear stress measurements, but suffer from directionality, bandwidth, and the need to infer wall-shear stress behavior rather than sense it directly. A sensor capable of conducting these measurements will need to meet several challenges associated with operation in this domain: the sensor must 1) be able to be installed in rotor blades with realistic geometries, to include thin/narrow airfoils, 2) operate reliably while undergoing dynamic motion (e.g. pitch, rotation), 3) have sufficient bandwidth, dynamic range, directional sensitivity, and spatial resolution to capture relevant boundary layer physics (both mean and fluctuating quantities), and 4) provide a means for accurate readout during rotational operation of the rotor blade system. Current MEMS-based or photonics-based sensing modalities, while capable of direct wall-shear stress measurement in a steady environment, need additional development to address all of the above-mentioned challenges.

PHASE I: Perform an analysis of the required sensor performance metrics for implementation on a current full-scale vertical lift vehicle platform. The analysis should consider the challenges listed in the description, considering the boundary layer physics (both mean and fluctuating quantities) on a rotor blade for a full-scale vertical-lift vehicle, the effect of dynamic motion (e.g. pitch and rotation), methodologies for data readout from the rotating environment, and form factors capable of being integrated on realistic geometries without necessitating compromise of the rotor blade structure.Provide a conceptual design of a wall shear stress sensor that addresses the operational environment; including form factor, acceleration compensation, readout connectivity, and overall integration with the rotor blade system.Phase I will conclude with a viable sensor design for development in Phase II.

PHASE II: Develop a working shear stress sensor prototype that meets the identified requirements and demonstrate operation in a relevant environment. This phase should demonstrate and characterize all aspects of the measurement system, to include: 1) sensing element, 2) transducer, 3) measurement signal routing, and 4) all necessary electronics for useful signal output, such that the sensor can be directly utilized in conjunction with typical COTS data acquisition systems.

PHASE III: Refine prototype designed in Phase II for technology transfer for commercial and military applications, to include university laboratories, DoD laboratories and research centers, NASA vertical- lift research efforts and helicopter and wind-turbine manufacturers. Successful implementation of this measurement technology will enable future design and performance analysis of vertical lift systems capable of increased performance (range, endurance, efficiency, safety, etc.).

KEYWORDS: wall shear stress, rotor blade, vertical lift, boundary layer physics


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