Turbulence and Pressure Fluctuations Characteristics of Downstream of Weir Using Particle Image Velocimetry and Bubble Image Velocimetry

There are a lot of river-crossing hydraulic structures such as weirs in Korea. The turbulence and pressure fluctuations are very complex around the hydraulic structure and these can trigger local scour and slab failure in an apron which is one of the main factors associated with structural instability. Understanding the flow and turbulence around hydraulic structures is essential for safe design and river training works. This study analyzed the flow fields using Particle Image Velocimetry (PIV) to find the flow and turbulence characteristics downstream of a weir. However, the hydraulic jump area created at the downstream of weirs is highly aerated containing not only a large number of bubbles but also very large sizes of bubbles. The highly aerated bubbly flow caused the traditional PIV technique to fail due to the uncontrollable scattering of laser light. A modified PIV method, called bubble image velocimetry (BIV), was introduced by directly using bubbles as the tracer and measuring the bubble velocity by correlating the `texture` of the bubble images. Mean velocities, turbulence, and pressure fluctuation quantities are obtained by averaging instantaneous velocities acquired using PIV and BIV. Various flow conditions are considered by controlling the tailwater depth (h). As the tailwater depth is fairly low, the hydraulic jump occurs with an oscillating jet which produces a large wave of non-periodicity, and high turbulence fluctuations are observed around the jet flow. The high flow occurs along an apron and the stagnation zone of recirculating flow is observed outside the submerged jet.