Statistical moments and power spectra of velocity fluctuations and their temporal derivative were measured behind four square-mesh grids at very small bulk Reynolds numbers. Three distinct states of such flows were identified: (1) in cases with insufficiently small Reynolds number, the flow was characterized as conventional grid turbulence, in which the turbulent kinetic energy was scaled with the mean velocity; (2) when the Reynolds number dropped below a threshold, which was specific for each grid, the flow was characterized as approximately steady; and (3) for Reynolds numbers that were between the two previous bounds (“intermediate state”), the flow structure exhibited peculiarities, the details of which depended on the grid geometry and the Reynolds number. The Reynolds stress anisotropy increased drastically in the intermediate state. Moreover, power spectra in the intermediate state differed strongly, when normalized by Kolmogorov scales based on the measured dissipation rate, but nearly collapsed in their large wave number ranges, when normalized by a dissipation rate that was fitted to a universal normalized spectrum. The flow state could not be characterized accurately by the Reynolds number based on mesh size, but the change of state from the conventional to the intermediate one occurred at ${\mathrm{Re}}_{\lambda }\approx 10$ for all grids. Finally, measurements quantified the dependence of the skewness and flatness factors of the streamwise velocity derivative, the ratio of production, and dissipation of vorticity fluctuations and the dissipation parameter upon ${\mathrm{Re}}_{\lambda }$.

• Received 6 November 2023
• Accepted 2 February 2024

DOI:https://doi.org/10.1103/PhysRevFluids.9.024607