Corridors of barchan dunes: stability and size selection

Barchans are crescentic dunes propagating on a solid ground. They form dune fields in the shape of elongated corridors in which the size and spacing between dunes are rather well selected. We show that even very realistic models for solitary dunes do not reproduce these corridors. Instead, two instabilities take place. First, barchans receive a sand flux at their back proportional to their width while the sand escapes only from their horns. Large dunes proportionally capture more than they loose sand, while the situation is reversed for small ones: therefore, solitary dunes cannot remain in a steady state. Second, the propagation speed of dunes decreases with the size of the dune: this leads -- through the collision process -- to a coarsening of barchan fields. We show that these phenomena are not specific to the model, but result from general and robust mechanisms. The length scales needed for these instabilities to develop are derived and discussed. They turn out to be much smaller than the dune field length. As a conclusion, there should exist further - yet unknown - mechanisms regulating and selecting the size of dunes.Comment: 13 pages, 13 figures. New version resubmitted to Phys. Rev. E. Pictures of better quality available on reques

Minimal model for aeolian sand dunes

We present a minimal model for the formation and migration of aeolian sand dunes. It combines a perturbative description of the turbulent wind velocity field above the dune with a continuum saltation model that allows for saturation transients in the sand flux. The latter are shown to provide the characteristic length scale. The model can explain the origin of important features of dunes, such as the formation of a slip face, the broken scale invariance, and the existence of a minimum dune size. It also predicts the longitudinal shape and aspect ratio of dunes and heaps, their migration velocity and shape relaxation dynamics. Although the minimal model employs non-local expressions for the wind shear stress as well as for the sand flux, it is simple enough to serve as a very efficient tool for analytical and numerical investigations and to open up the way to simulations of large scale desert topographies.Comment: 19 pages, 22 figure

Experiments on the automatic control of boundary-layer transition

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semiconductor

versus direct parallel substructuring methods i

Automatic control of laminar boundary-layer transition

Interest has recently been renewed in the use of distributed suction for the production of laminar flow over substantial areas of the surface of aircraft wings and engine nacelles. Suction may be most efficiently applied by using a number of independently controllable panels through which fluid is withdrawn. The need to determine the distribution of suction flow rates that results in a given streamwise location of boundary-layer transition with minimum power consumed in providing suction gives rise to a nonlinearly constrained optimization problem. A gradient descent algorithm is shown to be successful in both experimental and numerical studies in determining the optimal suction distribution. We present results of experiments performed on a flat plate in a wind tunnel with two suction panels and an optional pressure gradient. We show that the system successfully maintains the laminar-turbulent transition at a given point and minimizes the amount of suction power required to do so.</p

Optimum drag balance for boundary-layer suction

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