Author(s): Jan Peter Balmes

Linked Author(s): Jan Peter Balmes

Keywords: Large wood; Drag Force; Drag coefficient; Horizontal cylinder

Abstract: As a key habitat feature of creeks and rivers, large woody structures (LWS) are applied more often in ecological motivated river restoration projects. In those cases, engineers must consider the tradeoff between flood safety especially in urban creeks and rivers, and ecological goals. Regarding flood safety, the stability of LWS are depending on characteristics including relative size to the flow cross-section, the ratio between length and diameter, shape, orientation, flow magnitude, and the overall hydraulic conditions. All these parameters must be considered to avoid the unintended mobilization of engineered LWS during floods. Major concepts for the proof of stability considering the lift and drag forces in given situations. The latter must be calculated to perform a proof of stability including all measures needed for the dimensioning of suitable fixations. Different approaches are available, which are often only applicable by simplifying the LWS into an idealized 1D/2D cylinder case. Neither alignment nor rotation in 3D space is considered. The general drag force equation uses an empirical drag coefficient that does not consider blockage ratio or orientation. This usually results in under- or overestimation of drag forces. These uncertainties in the associated calculation of drag forces and the choice of drag force coefficients are the focus of the present work. Established blockage corrections are applied, and a new term for the orientation correction supplements the drag force equation. The combination of the two corrections shows good agreement among the calculated, measured, and simulated flow forces. Experiments were performed in a 10 m long and 80 cm wide glass flume designed with a rough bed to ensure a fully turbulent velocity profile in the investigation area. Smooth PVC cylinders were used to represent simplified large wood. The cylinders have a fixed diameter, lengths between 20 and 40 % of the channel width, and were installed with rotation angles between 15 and 90 degrees with respect to the flow. Various subcritical conditions with different flow depths and flow velocities were investigated. A dynamic load cell was used to measure the drag forces in the flow direction, which was connected to the cylinder with a vertical round rod. CFD simulations were added to the study to extend the flume studies and the parameter space in further investigations. The experimental data were used to evaluate the accuracy of the modeling approach. In the current phase, the focus is likewise on the integral calculation of forces exerted on the cylinder. For the numerical simulations, the open-source software OpenFOAM was used. A three-dimensional, incompressible, and steady-state solver with a high Re k-omega SST turbulence model was selected. This configuration provides an appropriate prediction of flow separation and has applicable behavior in adverse pressure gradients.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221506

Year: 2022