Modeling and measurement of optical turbulence by tomographic imaging of a heated air flow

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Abstract
An eight-view tomographic system based on one-dimensional Hartmann sensors is currently in use to image heated air flows. The system produces two-dimensional maps of refractive index at rates of several kilohertz. The high rate and good resolution enable comparison of measured results to expected results from fluid flow models. However, the novel nature of the application complicates validation of the system's performance. This paper describes computer simulations and experimental results quantifying the performance of the tomographic system at resolving spatial and temporal structures within the flow. The computer simulations model the physics and noise sources inherent in the wavefront sensing and tomographic imaging systems. An error budget for the system and an effective resolution metric allow quantitative comparison of performance against a given model of the flow. However, the accuracy of the simulation's predictions depends upon the accuracy of the disturbance model. By combining computer models of fluid flow, time averaged measurements such as temperature across the flow, and intrusive measurements such as smoke visualizations, we refine our flow model and improve the simulation. Experimental results show good agreement with this model, and the model allows us to discriminate and remove reconstruction artifacts from the imaged flow.

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