Experimental Investigation of Wall-Normal Pressure Gradient over the Bottom of a Turbulent Open-Channel Flow

This paper presents the results of an experimental investigation of a wall-normal pressure gradient in a turbulent open-channel flow with a smooth bottom (wall). Neutrally buoyant particles 1 mm in size were used in the experiments. Particle trajectories were photographed in the side view, and the particle accelerations in the vertical direction were determined from the particle trajectory data. The acceleration data were used to determine the wall-normal pressure gradient, implementing the vertical component of the equation of motion of the particle. The conditionally averaged wall-normal pressure gradient, upward directed or downward directed, is plotted as a function of depth. It was found that the latter was radically different from the wall-normal mean-pressure gradient, obtained from the Reynolds equation. The conditionally averaged pressure gradient, upward or downward, reaches very large values near the wall in the range O(5)<y+<O(100), a range which practically coincides with the near-wall region where fluid ejections originate in the turbulent bursting process. It was further found that the instantaneous wall-normal pressure gradient can attain values as much as a factor of 2 of their mean values. The latter two results potentially explain the observed vigorous, impulsive suction of the low-speed fluid during the ejection events of the bursting process near the wall.