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You are here : eLibrary : IAHR World Congress Proceedings : 36th Congress - The Hague (2015) ALL CONTENT : Water engineering : A transient approach flow acting on a square cylinder - flow pattern and horizontal forces
A transient approach flow acting on a square cylinder - flow pattern and horizontal forces
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. To investigate this topic,
an idealized infrastructure by means of a free-standing, 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 by a 2-axis force transducer. The
area around the structure was monitored with two CCD cameras. Recorded images were analyzed by Particle Image
Velocimetry (PIV) and were also used to reconstruct the time series of the water levels on the cylinder surface. 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 Krmn 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 Krmn 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.
File Size : 4,611,803 bytes
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Chapter : IAHR World Congress Proceedings
Category : 36th Congress - The Hague (2015) ALL CONTENT
Article : Water engineering
Date Published : 20/08/2015
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