A miniature light detection and ranging (LIDAR) velocimetry sensor has been developed to analyze high-velocity and boundary layer flows in real-world conditions. Using Rayleigh scattering, as opposed to the more common particle scattering, the patent-pending sensors provide multiple flow parameters without the need for particle-seeded flows. The compact fiber-optic sensor design can be embedded directly in a test surface, and allows for a variety of near-surface measurement formats enabling real-time, three-component flow velocity mapping, composition, gas density, and temperature data. The versatility of the Micro-LIDAR sensor platform offers broad utility in advanced aerodynamic and fluid dynamic applications requiring boundary-layer, unseeded flow measurements.

This sensor was developed to enable high-speed (supersonic and hypersonic) boundary layer flow analyses in wind tunnels and flight vehicles without the need for particle-seeded flow. The velocity of a gaseous flow can be optically measured by sending laser light into the gas flow, and then measuring the scattered light signal that is returned from matter within the flow. Scattering can arise from either gas molecules within the flow itself (Rayleigh scattering) or from particles within the flow (Mie scattering). Mie scattering provides a much larger return signal than Rayleigh scattering, and is the basis of all commercial laser Doppler and particle imaging velocimetry systems, which require particle seeding of the flow, and are time-of-flight techniques.

The Micro-LIDAR sensor combines high laser intensities over a small flow volume near a surface, and good signal collection efficiencies to enable Rayleigh scattering measurements, which eliminates the need for particle-seeded flows. In addition to unseeded flow velocimetry, the sensor format can be reconfigured as spectroscopy, induced fluorescence, and differential measurement devices to provide flow composition, gas density, temperature, and pressure data. The multiparameter sensor format provides for an unobtrusive in-flight method to determine laminar versus turbulent flow in the boundary layer. When combined with surface actuators in a closed-loop control system, the sensor creates the potential for active boundary layer control.

This work was done by Paul Danehy and Michael Walsh of Langley Research Center. LAR-16538-1

NASA Tech Briefs Magazine

This article first appeared in the September, 2015 issue of NASA Tech Briefs Magazine.

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