FORMATION OF WAVES ON A HORIZONTAL ERODIBLE BED OF PARTICLES

The mechanisms responsible for the initial growth of sand waves on the surface of a settled layer of particles are studied experimentally an d theoretically. Experiments employ water-glycerin solutions of 1-14 c P and glass spheres (rho(s) = 2.4 g/cm(3)) that are either 100 or 300 mu m in diameter. The particle Reynolds number and Shields parameter a re of order one and the flow Reynolds number is of order 1000 to 10,00 0. Experimentally obtained regime maps of sand wave behavior and data on the wavelengths of the sand waves that first appear on the surface of the settled bed are presented. Turbulence in the clear liquid is no t necessary for formation of waves and there is no dramatic change in behavior as the flowrate is increased across the turbulent transition. The initial wavelength varies as the Froude number to the first power . Because a flowing suspension phase is observed before waves form, li near stability analysis of the clear-layer-suspension-layer cocurrent two-phase flow is presented. The suspension phase is modeled as a cont inuum that has an either constant or exponentially increasing viscosit y. Neither of the models correctly predicts the wavelength for the fir st observed waves, their growth rate or their speed. However, the init ial wavelength is found to agree well with the trajectory length for a saltating particle obtained from a model for forces on individual par ticles.