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.

The most expansive active grid parametric study to date is conducted in order to ascertain the relative importance of the various grid parameters. It is identified that the three most important parameters are the Rossby number, Ro, the grid Reynolds number, Re M , and the wing geometry. For Ro > 50, an asymptotic state in turbulence intensity is reached where increasing Ro further does not change the turbulence intensity while other parameters continue to vary. Three wing geometries are used: solid square wings, solid circular wings, and square wings with holes. It is shown that the wings with the greatest blockage produce the highest turbulence intensities and Re λ , but that parameters such as the Kolmogorov, Taylor and integral scales are not significantly influenced by wing geometry. Finally, it is demonstrated that for several different sets of initial conditions that produce the same Re λ , the spectra are collapsed everywhere but at the largest scales. This result suggests that regardless of the very different origins of the turbulence, the shape of the spectra at high wavenumbers is dependent only on Re λ when normalized by Kolmogorov variables, hence demonstrating a degree of independence from the initial conditions.

A gliding bird's ability to stabilize its flight path is as critical as its ability to produce sufficient lift. In flight, birds often morph the shape of their wings, but the consequences of avian wing morphing on flight stability are not well understood. Here, we investigate how morphing the gull elbow joint in gliding flight affects their static pitch stability. First, we combined observations of freely gliding gulls and measurements from gull wing cadavers to identify the wing configurations used during gliding flight. These measurements revealed that, as wind speed and gusts increased, gulls flexed their elbows to adopt wing shapes characterized by increased spanwise camber. To determine the static pitch stability characteristics of these wing shapes, we prepared gull wings over the anatomical elbow range and measured the developed pitching moments in a wind tunnel. Wings prepared with extended elbow angles had low spanwise camber and high passive stability, meaning that mild perturbations could be negated without active control. Wings with flexed elbow angles had increased spanwise camber and reduced static pitch stability. Collectively, these results demonstrate that gliding gulls can transition across a broad range of static pitch stability characteristics using the motion of a single joint angle.

We focus on an estimate of the decay exponent ͑m͒ in the initial period of decay of homogeneous isotropic turbulence at low Taylor microscale Reynolds number R ͑Ӎ20-50͒. Lattice Boltzmann simulations in a periodic box of 256 3 points are performed and compared with measurements in grid turbulence at similar R . Good agreement is found between measured and calculated energy spectra. The exponent m is estimated in three different ways: from the decay of the turbulent kinetic energy, the decay of the mean energy dissipation rate, and the rate of growth of the Taylor microscale. Although all estimates are close, as prescribed by theory, that from the Taylor microscale has the largest variability. It is then suggested that the virtual origin for the decay rate be determined from the Taylor microscale, but the actual value of m be estimated from the decay rate of the kinetic energy. The dependence of m on R ͑0͒ ͑the value of R at the beginning of the simulation͒ is also analyzed, using the present data as well as data from the literature. The results confirmed that m approaches 1, as R ͑0͒ increases.