Principal Investigator: Charng Ning Chen (Georgia Institute of Technology)
Sponsor: GWRI
Start Date: 1970-11-01; Completion Date: 1970-11-01;
Keywords:
Description:
The mechanics of the process by which a sediment particle is removed from a stream bed was investigated. An idealized model, consisting of a one-inch diameter sphere protruding through a flat plate, a sphere-supporting base, and two equal height sphere-restraining pins aligned perpendicular to the flow direction, was used to simulate the condition of a cohesionless particle lying on a stream bed. By means of this model, the statistical variables of the angle of repose, the protrusion condition, and the approach velocity distribution became controllable at deterministic values.
In spite of the replacement of most of the statistical variables by the deterministic variables, initial motion of the particle is a fluctuating phenomenon which must be described in probabilistic terms because the fluid-dynamic force is fluctuating in nature. The transition from a stationary state to the removal of the spherical particle was found to be gradual rather than instantaneous. The transition is characterized by the random rocking motion of the sphere. The initial stage of the transition is defined as the condition at which the cumulative per cent time of contact between the sphere and the base is 95 per cent. The final stage is defined as the condition at which the sphere would be rolled over definitely. The condition of initial stage is established by equating the static weight-restoring moment to a moment level which exceeds the fluctuating fluid-driving moment 95 percent of the time. The final stage is established by equating an additional impulsive moment to the difference of the driving moment and the restoring moment as used in establishing the initial stage. The flow condition associated with the transition is expressed in terms of the mean velocity at the height of the protruding sphere. The effect of nonuniform velocity distribution is accounted for by the use of a momentum correction coefficient.
The fluid-dynamic moments and forces were determined experimentally in air flow with both uniform and non-uniform approach velocity profiles. Since the transitional stage is associated with the balance of moments, a method has been developed using the weight-restoring moment at the transitional stage as a gauge to measure the unknown fluid-driving moment. The corresponding fluid-driving force pattern was then determined through a set of three algebraic equations defining the equilibrium of moments due to the driving force and the restoring force. Experimental results indicated that the ratio of the coefficient of lift to drag decreased from approximately 1.6 to 0.4 as the ratio of the protrusion height to the sphere diameter increased from 25 per cent to 100 per cent. Also, the resultant fluid driving force could be considered as passing through the centroid of the sphere for protrusion height equal to or less than 75 per cent of the sphere diameter.