Flow past bluff bodies on a mobile bed: blockage effects on local scour and coherent structures at equilibrium

Bridges that traverse streams or rivers are susceptible to degradation or collapse due to riverbed erosion (i.e. “scour”) around bridge-supporting piers. Erosion is induced by rapidly moving currents that accelerate as they pass the piers. It is usually studied in small-scale experiments in laboratory flumes with a layer of sand placed on the flume bottom. However, wide piers are difficult to study in this configuration because they partially block flow through the flume, which is not representative of piers in natural streams. As a result, erosion around a wide pier in a narrow laboratory flume differs from erosion observed around piers in rivers. This is problematic because these results are used to improve the design of bridges over streams and rivers. This project will quantify how flow blockage affects scour around piers in laboratory experiments, develop a methodology to correct for these effects, and incorporate this correction to obtain more accurate predictions of scour for full-scale piers. The results will enhance the resiliency and safety of current U.S. bridges and the future manufacturing of other similar structures. The overall objective of this project is to gain new insights into the effect of flow blockage on the physics of local scour due to large-scale turbulent flows around bridge piers, using circular cylinders placed on an erodible bed. The research will evaluate the influence of blockage ratio (the ratio between pier width and flume width) on the flow field around bluff bodies in mobile beds. Experiments and numerical simulations will be conducted for select bluff body geometries and bed coarseness values to identify the physics-based parameters related to channel blockage. Time-resolved particle image velocimetry measurements will be acquired to capture the size, circulation, and location of horseshoe vortices, the width, circulation, and length of wake vortices, and the magnitude of the “contracted jet region” in the streamwise-vertical plane around the cylinders. Numerical simulations will elucidate additional three-dimensional characteristics of the flow. The effect of blockage ratio will be used to identify the dimensionless parameters that are linked to blockage effects. These results will be used to establish a framework of conditions to mitigate blockage effects in future experiments and to develop correction factors. This approach can be applied to future testing other bluff bodies in a flume test involving an erodible bed (e.g. aquatic vegetation patches and marine infrastructure components). The results will also be used to develop an improved bridge scour design methodology and introduce STEM students to bridge design at the undergraduate level. The project will train STEM students in numerical modeling and experimental testing and improve the resiliency of the bridge infrastructure for public safety. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria. NSF Award ID: 2614004 | Program: 01002627DB NSF RESEARCH & RELATED ACTIVIT | Principal Investigator: Priscilla Williams | Institution: University of Iowa, IOWA CITY, IA | Award Amount: $508,573 View on NSF Award Search: https://www.nsf.gov/awardsearch/show-award/?AWD_ID=2614004 View on Research.gov: https://www.research.gov/awardapi-service/v1/awards/2614004.html