Wave forces on a vertical cylinder
0
l
l
FT   dFD   dFl
Objectives
To understand interactions between waves
and a vertical cylinder. To learn how to measure
and calculate wave forces.
Description
Using a load cell, students will learn how to
measure wave forces acting on a cylinder.
Different sizes of cylinders and various wave
parameters will be used to examine different
flow regimes.
where
uu
dz ,
2
D 2 du
dFI  C M 
dz
4 dt
in which D is the diameter of the cylinder, l
the submerged length of the cylinder, C M the
inertia or the mass coefficient, and C D the drag
dFD  CD D
coefficient.
Procedures
8 cm
0
14 cm
For small amplitude wave the free surface
profile at a fixed x, say x=0, can be expressed
as
  A cost
The corresponding horizontal velocity and
acceleration can be written as
8.1 cm
8.5 cm
Figure 1. Location of the cylinders relative to the
bottom of the wave tank.
water depth should be approximately 22 cm.
stroke = 1 inch (in all speeds)
1) Calibrate one wave gage
2) For each cylinder, run speed = 3, 4, 5, 6 and
7
3) Remember to measure the stationary freesurface and force-free output voltages.
Theory
Based on the Morrison’s formula, the wave
force acting on the cylinder can be expressed as
cosh k ( z  h)
cos t
sinh kh
du
cosh k ( z  h)
  A 2
sin t
dt
sinh kh
u  A
in which z   h is the bottom of the wave
tank.
Report
The following minimum information should be
included:
1) A graph showing wave forces, in both
longitudinal and lateral directions as
function of wave period and cylinder
diameter.
2) Compare experimental data with theory.
3) Discussion on the choice of mass and drag
coefficients.