The effects of initial conditions on grid turbulence are investigated for low to moderate Reynolds numbers. Four grid geometries are used to yield variations in initial conditions and a secondary contraction is introduced to improve the isotropy of the turbulence. The hot-wire measurements, believed to be the most detailed to date for this flow, indicate that initial conditions have a persistent impact on the large-scale organization of the flow over the length of the tunnel. The power-law coefficients, determined via an improved method, also depend on the initial conditions. For example, the power-law exponent m is affected by the various levels of large-scale organization and anisotropy generated by the different grids and the shape of the energy spectrum at low wavenumbers. However, the results show that these effects are primarily related to deviations between the turbulence produced in the wind tunnel and true decaying homogenous isotropic turbulence (HIT). Indeed, when isotropy is improved and the intensity of the large-scale periodicity, which is primarily associated with round-rod grids, is decreased, the importance of initial conditions on both the character of the turbulence and m is diminished. However, even in the case where the turbulence is nearly perfectly isotropic, m is not equal to −1, nor does it show an asymptotic trend in x towards this value, as suggested by recent analysis. Furthermore, the evolution of the second-and third-order velocity structure functions satisfies equilibrium similarity only approximately.

A novel square-fractal-element grid was designed in order to increase the downstream measurement range of fractal grid experiments relative to the largest element of the grid. The grid consists of a series of square-fractal-elements mounted to a background mesh with spacing L 0 = 100 mm. Measurements were performed in the region 3.5 x/L 0 48.5, which represents a significant extension to the x/L 0 < 20 of previously reported square fractal grid measurements. For the region x/L 0 24 it was found that a powerlaw decay region following q 2 ∼ (x − x 0 ) m exists with decay exponents of m = −1.39 and −1.37 at Re L0 = 57, 000 and 65, 000, respectively. This agrees with decay values previously measured for regular grids (−1 m −1.4). The turbulence in the near-grid region, x/L 0 < 20, is shown to be inhomogeneous and anisotropic, in apparent contrast with previous fractal grid measurements. Nonetheless, power-law fits to the decay of turbulent kinetic energy in this region result in m = −2.79, similar to m ≈ −2.5 recently reported by Valente & Vassilicos (2011) for space-filling square fractals. It was also found that C is approximately constant for x/L 0 25, while it grows rapidly for x/L 0 < 20. These results reconcile previous fractal-generated turbulence measurements with classical grid turbulence measurements.

Recent technological advancements have made the use of particle image velocimetry (PIV) more widespread for studying turbulent flows over a wide range of scales. Although PIV does not threaten to make obsolete more mature techniques, such as hot-wire anemometry (HWA), it is justifiably becoming an increasingly important tool for turbulence research. This paper assesses the ability of PIV to resolve all relevant scales in a classical turbulent flow, namely grid turbulence, via a comparison with theoretical predictions as well as HWA measurements. Particular attention is given to the statistical convergence of mean turbulent quantities and the spatial resolution of PIV. An analytical method is developed to quantify and correct for the effect of the finite spatial resolution of PIV measurements. While the present uncorrected PIV results largely underestimate the mean turbulent kinetic energy and energy dissipation rate, the corrected measurements agree to a close approximation with the HWA data. The transport equation for the second-order structure function in grid turbulence is used to establish the range of scales affected by the limited resolution. The results show that PIV, due to the geometry of its sensing domain, must meet slightly more stringent requirements in terms of resolution, compared with HWA, in order to provide reliable measurements in turbulence

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This paper revisits values of the normalized energy dissipation rate (Cϵ) in different flows (two-dimensional wakes, grid turbulence, and homogeneous shear flow). Previously published as well as new data are considered over a relatively wide range of the Taylor-microscale Reynolds number Rλ. Cϵ exhibits wide scatter (in the range 0.5–2.5 for Rλ&gt;50) although, for a given flow and initial conditions, it is independent of Rλ when the latter is sufficiently large. An alternative definition [B. R. Pearson, P.-Å. Krogstad, and W. van de Water, “Measurements of the turbulent energy dissipation rate,” Phys. Fluids 14, 1288 (2002)] of Cϵ has been checked in the same flows but has failed to yield a universal value for the coefficient.