Experimental investigation of current forces on a square caisson with small aspect ratios during the sinking process

ABSTRACT

The study of flow around a square caisson considering the combined effects of free surface, free end and bottom boundary is limited. Experiments were performed in this study to investigate the characteristics of current forces on a square caisson model considering the combined effects of free surface, free end and bottom boundary. Results show that the drag coefficient and the root mean square coefficient of transverse force generally increase with aspect ratio (AR), and significantly increase at the instant when the free end disappears. The bottom boundary decreases the values of root mean square coefficients of drag force and vertical force. The drag coefficient decreases with incident angle ($\alpha$) when $0^{\circ }\le \alpha \le {15}^{\circ }$, but increases with $\alpha$ when ${15}^{\circ }\le \alpha \le {45}^{\circ }$. The trim and squat phenomenon caused by shallow water effect when the caisson is nearly touching the riverbed (seabed) during the sinking process should receive extra attention in practice.

Keywords:

• Bottom boundary effect
• current force
• free surface
• free end
• physical experiment
• square caisson

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notation

$A$	=	projected area (m2)
AR	=	aspect ratio (–)
$C_d$	=	drag coefficient (–)
$\overline{C_d}$	=	averaged coefficient (–)
$C_{drms}$	=	root mean square coefficient of drag force (–)
$\overline{C_{drms}}$	=	averaged root mean square coefficient of drag force (–)
$C_{lrms}$	=	root mean square coefficient of transverse force (–)
$\overline{C_{lrms}}$	=	averaged root mean square coefficient of transverse force (–)
$C_{zrms}$	=	root mean square coefficient of vertical force (–)
$\overline{C_{zrms}}$	=	averaged root mean square coefficient of vertical force (–)
D	=	side length of caisson (m)
e	=	gap between caisson bottom surface and flume bottom surface (m)
$F_x\left(t\right)$	=	drag force (N)
$F_y\left(t\right)$	=	transverse force (N)
$F_z\left(t\right)$	=	vertical force (N)
$\overline{F_x}$	=	mean value of drag force (N)
$\overline{F_y}$	=	mean value of transverse force (N)
$\overline{F_z}$	=	mean value of vertical force (N)
$\mathsf{F}$	=	Froude number (–)
H	=	height of caisson (m)
U	=	flow velocity (m s−1)
${\mathsf{R}}_{\mathsf{e}}$	=	Reynolds number (–)
$\alpha$	=	incident angle (°)
${\lambda }_L$	=	scale ratio of length (–)
${\lambda }_u$	=	scale ratio of velocity (–)
$\rho$	=	density of water density of water (kg m−3)

Additional information

Funding

This work is supported by the National Key Research and Development Program of China [grant no. 2021YFC3100700], the Fundamental Research Funds for the Central Universities [grant no. 2682022ZTPY019], and Scientific Research Service of Lingang Yangtze River Bridge on Zigong Yibin Line of New South Sichuan Intercity Railway [grant no. SRIG2019FW0001].