Comparison of the homogeneous turbulence created by a turbulence box to that of an active grid.

For the investigation of homogeneous turbulence, both turbulence boxes and active grids are widely used. In this study, we compare the design, operation, and performance of these two systems. Our turbulence box uses a unique actuation mechanism in which 64 actuators are individually controlled, enabling spatial variation in control parameters including power and frequency, similar to the capability of active grids. The synthetic jet turbulence box, featuring 64 speakers within a 24"×24"×24" cubical chamber, and the active grid, with a 12"×12" cross-sectional area and a 6×6 mesh operated by 12 stepper motors, were both controlled via Python code. The turbulence box operation involved adjusting power, frequency, and on/off timing, while the active grid's operation included modifications in shaft speed, acceleration/deceleration, angular displacement, flipping, and winglet position relative to the flow direction. Performance evaluation was based on turbulent energy, isotropy, integral length scale, integral time scale, Reynolds number based on Taylor microscale, and Kolmogorov scales. Additionally, the effect of mesh size on turbulence and shear turbulence was studied, comparing both generators. The research analyzed the variation of turbulence parameters under similar operational modes, including speed/power, winglet opening and closing frequency versus speaker frequency, periodic sinusoidal velocity variations, velocity steps and single gusts, spatial operation of the shaft versus the speakers' rows/columns operation, and shear generation. This study provides a comprehensive comparison and insight into the operational and performance characteristics of the synthetic jet turbulence box and the active grid with a wind tunnel.