Impingement of offset jets on rigid and movable beds.

The present thesis describes an experimental and theoretical investigation of the flow field and scour action of an offset jet. The hydraulic characteristics of the offset jet and the resulting scour developed in the bed were investigated in the laboratory. Tests were carried out using a fixed bed and a single offset ratio (height of jet above bed / jet thickness). Three flow rates were used. Velocity measurements in two directions, especially in the recirculating zone, were of help in understanding the flow field and in providing the necessary data for comparison with the theoretical results. The development of scour, on a uniform sand, was monitored at set time intervals, in most cases until the asymptotic state was reached. A new effective and simple method for measuring the scour profile, while the experiment was running, was devised. The experiments were conducted using four different offset ratios and several flow-rates. Results showed dependency of the scour characteristics on Froude number, time and especially the offset ratio. The findings of each experiment were combined dimensionlessly to produce relationships which describe the development of scour characteristics for the tested range of parameters. Scour profiles were found to be similar for a given offset ratio, but differed from one offset ratio to another. The second part of the work was concerned with developing a general integral method capable of the prediction of velocity fields of different flow situations, including those of offset jet impinging on rigidand eroded beds. The combination of strip integral method in a curvilinear system with the k-E and algebraic stress turbulence models provided such a method. Application of this method to a variety of selected test cases revealed the ability of the model to capture the main features of the flow within the considered range of interest. The algebraic stress model was found to give better results in curved and wall effected flows

Experimental Investigation of Saltation Characteristics of Particles near Channel Bed

This paper studies the mechanical characteristics of saltation of bed-load particles in turbulent flows. Experiments have been carried out by means of high speed photography to obtain saltation characteristics such as length, height, velocity, impacting and rebounding angles for different hydraulic conditions, particle size, density and bed roughness by analyzing and processing pictures taken at 250 frames per second. The measured values of the parameters showed that increases in turbulence parameters of the flow increase saltation length, height, and velocity but decrease the impacting and rebounding angles. It was also found that saltation length and height tended to increase as the average size of the bed material (d50) increased. To solve numerical trajectory equations, the initial velocity component of the particle must be known. The initial longitudinal particle velocity was found to range from 3u* to 8u* and its vertical velocity ranged from 1.5u* to 3.5u*, which were different from the values reported in the literature. The longitudinal and vertical components of the restitution coefficient were also measured and it was seen that the Shields parameter (t*) had no meaningful effect on these components, remaining almost constant with increasing