For pressure-driven turbulent channel flows undergoing spanwise system rotation, it has been observed that the absolute vorticity, i.e., the sum of the averaged spanwise flow vorticity and system rotation, tends to zero in the central region of the channel. This observation has so far eluded a convincing theoretical explanation, despite experimental and numerical evidence reported in the literature. Here we show experimentally that three-dimensional laminar structures in plane Couette flow, which appear under anticyclonic system rotation, give the same effect, namely, that the absolute vorticity tends to zero if the rotation rate is high enough. It is shown that this is equivalent to a local Richardson number of approximately zero, which would indicate a stable condition. We also offer an explanation based on Kelvin's circulation theorem to demonstrate that the absolute vorticity should remain constant and approximately equal to zero in the central region of the channel when going from the nonrotating fully turbulent state to any state with sufficiently high rotation.

In an effort to characterize noise induced by separated turbulent boundary layers, surface pressure fluctuations on a DU96-W-180 airfoil were measured using miniature pressure sensors. Because of limitation in amplifier channels and available sensors, a rearrangeable configuration of sensors was applied. Chordwise distributions of the surface pressure were obtained for aerodynamic angles of attack of −0.8 • ≤ α ≤ 10.3 • and at three Reynolds numbers (0.8, 1.0, and 1.2) ×10 6 . The boundary layer profile at 1%c behind the trailing edge was measured using constant temperature anemometry. The boundary layer thickness compares well with that simulated using XFOIL for α ≤ 7.8 • . Within the limits of the simulation, other relevant boundary layer properties from XFOIL were used to calculate the surface pressure spectrum predicted from published empirical models for zero and non-zero pressure gradient turbulent boundary layers. Finally, a modified Blake-TNO semiempirical model was used to predict the surface pressure spectrum near the trailing edge for separated flow. The modification is introduced to the so called 'moving axis spectrum' and the chord-normal correlation length scale. It is found that in the low frequency range, the modified semi-empirical model fits well with the measured surface pressure spectrum of a separated turbulent boundary layer. Nomenclaturea Speed of sound, m·s −1 c Chord length, m c l Coefficient of lift, -c 1 , c 2 Modification parameters to Φ m , -k e Eddy containing wavenumber, 0.7468/L y , m −1 k x Chordwise wavenumber, 2π/L x , rad·m −1 k z Spanwise wavenumber, 2π/L z , rad·m −1 L Wetted span, m mix Mixing length, 0.085δ tanh(κy/0.085δ), m L x Chordwise correlation length scale, m L y Chord-normal correlation length scale, m L z Spanwise correlation length scale, m R Radial distance from a sound source, m Re Reynolds number, -R T Ratio of the outer and inner boundary layer time scales, u τ δ/ν C f /2 U (y) Time averaged velocity component, m·s −1 U ∞ Freestream velocity, m·s −1 U c (y) Convective velocity, m·s −1 U e Turbulent boundary layer edge velocity, m·s −1 u, v, w Instantaneous velocity components in the order x, y, z, m·s −1

The introduction of quiet short take-off and landing for civil aircraft operations in close proximity to the population poses important technological challenges. One critical aspect is the realization of extreme lift augmentation at low acoustic emissions. The aircraft concept selected to achieve this goal is a high-lift system equipped with an active flow-control non-slotted flap and a droop nose. For this specific configuration, trailing edge noise becomes a dominant noise source. Porous materials as a passive means for trailing-edge noise reduction are selected and characterized. Results of extensive experimental investigations in the acoustic wind tunnel Braunschweig are presented and discussed to point out the potential and limitations of the selected porous devices. Practical issues related to material manufacturing and integration into the wind tunnel model are addressed. The noise reduction potential of passive porous trailing-edge devices is found to strongly depend on both these aspects. Issues related to the characterization of the porous materials properties are described. Although porous materials are found to be successful at reducing trailing-edge noise emissions, the results indicate that there is still a need for more generic investigations to further clarify the parametric dependencies between noise reduction and material properties.

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