In this paper, the acoustic impedance of a liner is educed by a novel semi-analytical inverse technique. The liner sample is placed flush with the solid walls in a rectangular duct with grazing flow. The technique uses complex acoustic pressure measured at four positions at the wall of the duct, upstream and downstream of the lined section, and educes the impedance with a mode-matching method. Previous studies neglected grazing flow nonuniformity and the pressure discontinuity that appears at the liner-wall boundary caused by the discontinuity of the acoustic particle velocity into the wall. In the present paper, the mode-matching formulation is rederived in terms of pressure instead of velocity potential which is found to be more numerically stable. Moreover, the proposed methodology is validated with benchmark data from an experiment performed by NASA. First, the ability of the code to reproduce the pressure field for a given impedance is tested. Second, the ability to educe the correct impedance for a given pressure distribution is tested. The results of the mode-matching code are in very good agreement with the experimental data. The effect of shear flow is investigated and it can be concluded that the assumption of uniform flow is appropriate for the chosen liner, duct size, and frequency range of interest. Nomenclature A = admittance of the liner, 1=rayl a = half the width of the duct, m a q , a q = amplitudes of the incident and reflected qth mode in the inlet hard duct, Pa b q , b q = amplitudes of the incident and reflected qth mode in the lined duct, Pa c q , c q = amplitudes of the incident and reflected qth mode in the outlet hard duct, Pa j = complex unity k = normalized wave number !=c a k q m , k q n = normalized transversal wave number in the x and y directions, respectively k q z = normalized axial wave number of the qth mode L = normalized length of the lined duct M = flow Mach number p = temporal Fourier transform of the pressure, Pa q = mode number u, w = unsteady velocity field in the x and y directions, respectively, m=s x, y, z = normalized Cartesian coordinates x : x=a; z coordinate in the direction of the duct axis q = two-dimensional mode shape of the qth mode after separating the z dependence

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In this work, the objective was to investigate the influence of Active Flow Control on the improvement of a DU96-W-180 airfoil aerodynamic performance. A numerical simulation was done for incompressible unsteady low Reynolds Number flow at high angle of attack. The innovative approach was the use of an "Active Slat" where the periodic blowing effect was achieved by periodically opening and closing the slat passage. The major benefit of this concept is being flexible to a desired operating condition. A new OpenFOAM ® solver was developed from the existing pisoFoam solver to simulate the active slat flow control technique. To get the best aerodynamic performance, the active slat should operate at the domain dominant frequencies. A Fast Fourier Transform (FFT) was performed to achieve the optimum slat excitation frequency. These frequencies will help in controlling the inherent instabilities in the boundary layer and thus improving the aerodynamic performance. Finally, active flow control simulations were applied using different excitation. Using the optimum FFT excitation frequency ( 0.68 f = in the wake region) yields the best aerodynamic improvement of all tested frequencies. Improvements in lift coefficient were achieved up to 8%. Hence, the slatted airfoil is superior to the conventional clean configuration airfoil.

This article presents analytical and computational fluiddynamics (CFD) solutions of the unsteady flow resulting from a horizontal circular disk moving downward at a constant velocity toward a horizontal floor seeded with spherical micro-particles, and the effect of this flow on particle detachment and levitation. The selected configuration is a simplification of numerous practical applications in which particle resuspension is important, for example a foot or an object impacting a dusty floor, or a squeeze film thrust bearing with particle contamination. The resulting radial and axial velocity field, coupled with a particle detachment model and the particle equations of motion were employed to compute particle trajectories in the gap. The CFD solutions were utilized to describe the high-speed radial wall jet and the vortices developing outside the disk and to explain their role in particle levitation and entrainment. It is shown that as the gap narrows the resulting radial velocity close to the disk perimeter is high enough to detach and levitate μm-size particles, and that the vortices shed by the descending disk and its high-velocity radial wall jet create an upward convective motion that contributes to particle resuspension from the floor and entrainment in any far-field flow that might be present around the descending disk.

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