Oceanography | Vol. 38, No. 3
82
MECHANICAL DESIGN: OCEAN DRIFTER DYNAMICS
Using the curriculum, students learn that drifter motion is
shaped by water movement, wind drag, wave action, and buoy-
ancy. To track surface currents accurately, they explore how min-
imizing wind drag improves performance. Instead of relying on
mass or inertia, students design their drifters to be highly sensi-
tive to water flow.
Specifically, the motion of a surface drifter subject to oceanic
and atmospheric forcing can be described by Newton’s second law:
m dv
—
dt = Fdrag + Fwind + Fbuoyancy + Fgravity + FCoriolis + Fpressure,
(1)
where v is the drifter velocity and m is its mass. For small, low-
inertia drifters, Coriolis and pressure gradient forces may be negli-
gible over short timescales (Niiler et al., 1987; Furnans et al., 2008).
BUOYANCY AND GRAVITY
To maintain surface tracking, the drifter is designed to be slightly
positively buoyant (ρwV > m; Bjørnestad et al., 2021):
Fbuoyancy = ρwgV,
(2)
Fgravity = mg,
(3)
where ρw is the density of seawater, V is the displaced volume, and
g is gravitational acceleration.
HYDRODYNAMIC AND WIND DRAG
The primary forces affecting horizontal motion are drag from both
water and air:
Fdrag, water = ½Cd,wρwAw(v – uw)2,
(4)
Fdrag, wind = ½Cd,aρaAa(ua – v)2,
(5)
where uw and ua are the water and wind velocities, Cd is the drag
coefficient, A is the effective cross-sectional area, and subscripts w
and a refer to water and air, respectively.
MINIMIZING WIND SLIPPAGE
Niiler et al. (1987) showed that to keep wind-induced slip under
1 cm s–1 when the wind speed is 10 m s–1, the drag area ratio (R)
should be greater than 40.
R = Aw
—
Aa
,
(6)
≈ 6.6 · 10–4 ·
Fwind
|ua|
Fwater
|uw|
(7)
A higher R ensures the water drag force dominates over the drag
from wind forces. The area ratio of the drifter provided as an exam-
ple to the students is approximately 40.
INERTIAL RESPONSE TIME
The drifter’s responsiveness to transient currents is governed by a
time constant:
τ =
meff
ρwCd,w Aw(vsteady state − uw)
(8)
where meff includes added mass from the surrounding fluid, an
important concept in hydrodynamics (Stelson and Mavis, 1957).
Increasing the water drag area lowers the τ values, yielding better
TABLE 1. List of provided materials.
PART NAME
PURPOSE
COST
USD
COMMERCIAL GPS SYSTEM
LandAirSea GPS tracker
Commercial cellular GPS tracker
$28
Cellular antenna
Replacement antenna for Cellular GPS tracker
$8
SHARED DRIFTER MATERIALS
Cotton canvas fabric
Underwater drogue
$2
3 mm thick jute rope
Frame construction and parts attachment
$2
Cork
Surface flotation
$4
20 mm dia by 1.6 m wooden dowel
Frame construction
$5
4 x 8 mm dia by 0.5 m wooden dowel
Frame construction
$3
Iron fishing weights
Counterbalance
$1
Cylindrical bamboo container
Electronic stack container
$8
Shellac
Waterproofing coating
$2
Coconut wax
Waterproof potting material
$2
CUSTOM GPS SYSTEM (OPTIONAL)
Lilygo T-sim7000g
Custom GPS cellular development board
$35
18650 Li-ion battery 1300 mAh
Custom GPS unit battery
$4
BMP390 breakout board
Barometric pressure and altimeter
$13
NTC 3950
Waterproof temperature sensor
$2
LTE-M SIM card
Cellular service provider
$20