Fabrication and Characterization of a Large-array Hot-film Sensor for Detection of Separated Flow

Hot-film anemometers relate heat transfer on a small resistive element to local velocity and shear stress in a flow. Large-array surface-mounted hot-films expand velocity and stress measurements to an entire surface, enabling measurement of unsteady flow phenomena including separation. The development of these sensor arrays requires alternative methods of fabrication in addition to analysis of sensor frequency response and methods of data validation. Various applications exist where information on the boundary layer state is desired, such as a wind turbine blade, but due to the physically large scales a standard hot-film sensor remains impractical. This work centers on fabrication and testing of a large-scale, surface-mount, hot-film sensor array. The objective of the array is to provide information on the flow condition, specifically whether the flow remains attached or separated, under unsteady inflow conditions. A constant temperature anemometer (CTA) is used to resolve the flow condition under unsteady inflow, including detection of flow separation or unsteadiness due to turbulence. The sensor array response is verified using fluorescent oil film for the static case and particle image velocimetry for the dynamic velocity tests. The CTA demonstrates sensitivity to separated, attached, and turbulent boundary layer states through variance integration in the frequency domain. A simple classification system based on the measured variance is provided for flow state identification at the low Reynolds number (Re) which were tested.