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This paper presents the results of investigations into the flow around a rectangular cylinder with a chord-to-depth ratio equal to 4. The studies are performed through wind tunnel dynamic pressure measurements along a cross-section combined with Unsteady Reynolds-Averaged Navier-Stokes (urans) and Delayed-Detached Eddy Simulation (ddes). These experimental and numerical studies are complementary and combining them allows a better understanding of the unsteady dynamics of the flow. The comparison of experimental and numerical data is performed using statistics and Dynamic Mode Decomposition. It is shown that the rectangular cylinder involves complex separation-reattachment phenomena that are highly sensitive to the Reynolds number. In particular, the mean lift slope c lα increases rapidly with the Reynolds number in the range 7.8×10 3 ≤ Re ≤ 1.9×10 4 due to the modification of the mean vortex strength, thickness and distance from the surface. Additionally, it is shown that both urans and ddes simulations fail to accurately predict the flow at all the different incidence angles considered. The urans approach is able to qualitatively estimate the spatio-temporal variations of vortices for incidences below the stall angle α = 4 •. Nonetheless, urans does not predict stall, while ddes correctly identifies the stall angle observed experimentally.

Using flight measurements conducted between altitudes of 700 m and 3000 m, this work characterizes atmospheric turbulence and investigates the effects of an increase in turbulence level on the laminar-turbulent transition taking place on the pressure side of a laminar airfoil. Flight conditions ranging from calm to moderately turbulent and natural transition driven by Tollmien-Schlichting waves are considered. The inflow conditions are first characterized and reported using single and two-point statistics. Moreover, it is shown how characteristic parameters can be estimated from the turbulence intensity. Then, the sensitivity of the transition location to an increase of turbulence level is investigated. Flight results show a low sensitivity of the transition location to an increase of turbulence level, when the latter is not associated with significant variations of pressure gradient. Similar investigations are also conducted in a wind tunnel where the turbulence level is increased using an active grid and a significant change of the transition location is observed with increasing turbulence level. The differences in the response of the transition to freestream turbulence level in flight and in the wind tunnel is postulated to be attributable to differences in the probability density distributions of the inflow velocity fluctuations.

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Wind tunnel investigations of how Natural Laminar Flow (NLF) airfoils respond to atmospheric turbulence require the generation of turbulence, whose relevant characteristics resemble those in the atmosphere. The lower, convective part of the atmospheric boundary layer is characterized by low to medium levels of turbulence. The current study focuses on the small scales of this turbulence. Detailed hot-wire measurements have been performed to characterize the properties of the turbulence generated by grids mounted in the settling chamber of the Laminar Wind Tunnel (LWT). In the test section, the very low base turbulence level of Tuu ≅ 0.02% (10 ≤ f ≤ 5000 Hz) is incrementally increased by the grids up to Tuu ≅ 0.5%. The turbulence spectrum in the u-direction shows the typical suppression of larger scales due to the contraction between grids and test section. Still, the generated turbulence provides a good mapping of the spectrum measured in flight for most of the frequency range 500 ≤ f ≤ 3000 Hz, where Tollmien-Schlichting (TS)-amplification occurs for typical NLF airfoils. The spectra in v and w-direction exhibit distinct inertial subranges with slopes being less steep compared to the − 5/3 slope of the Kolmogorov spectrum. The normalized spectra in u-direction collapse together well for all grids, whereas in v- and w-directions the inertial- and dissipative subranges are more clearly distinguished for the coarser grids. It is demonstrated that the dissipation rate ε is a suitable parameter for comparing the wind tunnel turbulence with the atmospheric turbulence in the frequency range of interest. By employing the grids, turbulence in the range 4.4 × 10–7 ≤ ε ≤ 0.40 m2/s3 at free-stream velocity U∞ = 40 m/s can be generated in the LWT, which covers representative dissipation rates of free flight NLF applications. In the x-direction, the spectra of the v and w-components develop progressively more pronounced inertial- and dissipative subranges, and the energy below f ≈ 400 Hz decreases. In contrast, the spectral energy of the u-component increases across the whole frequency range, when moving downstream. This behavior can be explained by the combination of energy transport along the Kolmogorov cascade and the incipient return to an isotropic state. Graphic Abstract