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【書報討論】107/10/18 Prof. Michael A Leschziner, Aeronautics Department, Imperial College London, UK



演講者:Prof. Michael A Leschziner, Aeronautics Department, Imperial College London, UK 講題:The effect of large-scale oblique undulations on a solid wall on the turbulent drag

摘要:The presentation considers the flow configuration shown in the figure below.

Channel-flow geometries with wavy walls

Extensive DNS studies for this geometry have been undertaken in an effort to answer the question of whether the introduction of wall waviness can yield friction-drag reduction. The wavy walls, skewed with respect to the mean flow direction, are introduced as a means of emulating a Spatial Stokes Layer (SSL) induced by in-plane wall motion. The transverse shear strain above the wavy wall is shown to be similar to that of a SSL, thereby affecting the turbulent flow, and leading to a reduction in the turbulent skin-friction drag. However, some important differences with respect to the SSL case are brought to light too. In particular, the phase variations of the turbulent properties are enhanced and, unlike in the SSL case, there is a region of increased turbulence production over a portion of the wall, above the leeward side of the wave, thus giving rise to a local increase in dissipation. The pressure- and friction-drag levels are carefully quantified for various flow configurations, exhibiting a combined maximum overall-drag reduction of about 0.5%. The friction-drag reduction is shown to behave approximately quadratically for small wave slopes and then linearly for higher slopes, whilst the pressure-drag penalty increases quadratically. The transverse shear-strain layer is shown to be approximately Reynolds-number independent when the wave geometry is scaled in wall units.