Onset of turbulence in a regular porous medium: An experimental study

This paper presents the results from an experimental study on turbulent flows through a regular porous matrix of spheres packed in a cubic arrangement. The special pore geometry made it possible to measure the velocity within individual pores, using the ultrasonic velocity profiler (UVP). The UVP recorded the time series of the average cross-sectional velocity at a series of points along the pore. The measurements were performed for various hydraulic gradients resulting in a pore Reynolds number between 70 and 430. Three different regimes were detected: unsteady laminar, transition to turbulence, and turbulent. An additional measurement was performed using particle image velocimetry (PIV), which recorded two velocity components within a window which covered only the widest pore section. The PIV test had a Reynolds number of 370, which corresponds to transitional flow just before the onset of turbulence. The PIV result revealed the spatial distribution of mean velocities and turbulence intensities, as well as the shapes of coherent flow structures during significant turbulent events.