Toward a wall-stress model for LES over rough boundaries ; contributions to experimental and simulation technology

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To contribute to research on simulation of wall turbulence, the present research project has developed relevant measurement technology, and has proposed a hybrid LES-RANS wall-stress modeling technique and a new fictitious-domain method for simulating turbulent flow with highly concentrated dispersed particles.Concerning measurement technique, we have developed Stereo PIV (Particle Image Velocimetry) as applied to an open water-channel. First, we pointed out some simple but new aspects of camera calibration. Next, we proposed a new image-processing Technique to directly measure the rate of shear at a wall, both as seen by a single camera, and as reconstructed in 3D from two camera views. This "PIV/Interface Gradiometry" technique was successfully applied to experimental images from a sinusoidal wall. Such basic contributions will facilitate application of (stereo) PIV to regions near possibly complex boundaries.To allow numerical simulation of solid-liquid two-phase flow near a sediment bed, we extended Kajishima's fictitious-domain technique to handle cases of high particle concentration. Tests on "closed" and "open" rotating drums, partially filled with glass beads, gave good agreements on bed angles and other important parameters. Following these tests, we considered a biperiodic "minimal" turbulent channel containing 1500 spherical beads. Parameters were adjusted to yield particle Reynolds number around 15, and nondimensional shear stress (Shields parameter) in the range 0.06 1.1. Over this range, results for sediment flux agreed to within 20% with the prediction of the Meyer-Peter Mueller equation.To contribute to the unresolved theoretical issues related to LES over a boundary of complex shape, the PI has also derived an equation for the pressure drag on a "patch" of wall in terms of the advection of vorticity nearby.