Effectiveness of Fractal Square Grids to Enhance Turbulence: Proper Orthogonal Decomposition Analysis

Fractal square grids have been used as a passive flow control device to enhance fluid mixing in many engineering applications. While producing a desirable high turbulence intensity peak downstream, the fractal square grid also generates a pressure loss that needs to be considered in the design. It is not well understood how the multiple fractal scales affect turbulence enhancement and the pressure loss. This work is to measure the effectiveness of a fractal square grid by comparing the level of enhancement of turbulence and the pressure drop across it in well-controlled water-tunnel experiments. A set of fractal square grids with increasing number of fractal scales (N = 1, 2, 3, and 4) and the same largest scale are employed. Turbulent velocity fields downstream of each fractal square grid are measured by planar Particle Image Velocimetry (PIV) at a Reynolds number of 3400, and the static pressure drop measured by a differential pressure transducer. Proper Orthogonal Decomposition (POD) analysis on the velocity field is used to identify the coherent structures associated with different fractal length scales. This work can potentially benefit a wide variety of applications where energy efficient mixing or convective heat transfer is a crucial process.