A transient approach flow acting on a square cylinder - flow pattern and horizontal forces

Nils Goseberg, Gian Bremm, Torsten Schlurmann, Ioan Nistor

Friday 3 july 2015

11:15 - 11:30h at Amazon (level 1)

Themes: (T) Water engineering, (ST) River and coastal engineering

Parallel session: 15E. Engineering - River

Man-induced hazards such as dam or dike breaches as well as natural ones such as tsunamis have often generated unprecedented consequences for humans and the infrastructure. In the context of the extreme flooding generated by such disastrous phenomena, it is of major importance to quantify the time-history of forces which occur during the interaction of the rapidly transient flow with various structures and the characteristics of the induced flow patterns. An idealized infrastructure by means of a free-surface-piercing structure is exposed to flow conditions resembling onland tsunami-induced flow. The flow is physically modeled in a tsunami flume through long leading depression waves which run up and down a 1:40 smooth and impermeable sloping beach. The flow in the flume was generated by a volume-driven wave maker and horizontal forces in the x- and y-directions were measured. The area around the structure was monitored with two CCD cameras and were analyzed by Particle Image Velocimetry (PIV). To determine the stream-wise total drag force exerted on the structure and to compare it with the total measured base force, preparatory steady-state experiments were accomplished. These tests allowed determining the drag force coefficient of the square cylinder at various water depths. Estimated drag force coefficients were used to calculate the drag force share of the total forces on the cylinder during the transient flow experiments. The flow acting on the structure resulted in distinct flow pattern which was characteristic for this type of flow-structure interaction. Besides shock wave propagating upstream, a standing or partially-standing wave was observed in front of the structure together with a wake formation downstream, while a transient von Kármán vortex street developed during the deceleration phase of the flow motion and during draw-down. Force measurements indicated a sudden increase in the stream-wise total force starting with the arrival of the flow front during initial run-up. Lateral velocities showed significant oscillations in correlation with the transient von Kármán vortex street development. A comparison of the total measured base force with the analytically-calculated share of the drag force revealed that forces were clearly drag-dominated. Inertial forces were only of minor importance for the configurations presented and discussed in this laboratory study.