Real-time implementation of fast checkerboard demodulation background-oriented schlieren

The background-oriented schlieren (BOS) technique is a widely used diagnostic for visualizing density gradients in high-speed aerospace flows. For BOS applications requiring quantitative density gradient measurements, careful system calibration is necessary to link measured background displacements to line-of-sight integrated density gradients via the optical geometry and the Gladstone–Dale relation. However, the objective of the present work is the real-time, qualitative visualization of flow density gradients for monitoring steady-state setpoint operations in aerospace test facilities. Conventional BOS implementations employ a camera to image a speckle background, where density gradients within the flow field distort the recorded pattern. These distortions are routinely resolved using cross-correlation algorithms adapted from particle image velocimetry (PIV). In these setups, the density gradients themselves are not directly measured; rather, the system estimates pixel displacements that are proportional to these density gradients. While such PIV-based processing of BOS image data is highly robust, its spatial resolution remains limited, and its computational cost is high. In contrast to these conventional approaches, the fast checkerboard demodulation (FCD) method uses a checkerboard background pattern to isolate distinct frequency bands, enabling an alternative Fourier-based processing approach. This work introduces a true real-time implementation of the FCD approach for background-oriented schlieren (RT-FCD-BOS), in which images are acquired by a camera, processed, and the resulting displacement field is displayed instantaneously. Unlike earlier real-time BOS systems that relied on graphics processing unit (GPU) acceleration, RT-FCD-BOS delivers pixel-level displacement estimates, which are proportional to the density gradients, by utilizing multicore central processing units (CPUs). This approach simplifies hardware requirements while maintaining high spatial resolution, providing a practical and efficient tool for qualitative real-time flow monitoring in aerospace ground test facilities. A video supplement to this paper demonstrates the RT-FCD-BOS system in active operation with a plug nozzle test article within the NASA Glenn Research Center’s Nozzle Acoustic Test Rig (NATR).