INSEAN_CN20130 pages

FLUID-STRUCTURE INTERACTION

INSEAN, Rome, Italy

Sonar Dome Vibration Analysis
Scientists at INSEAN, The Italian Ship Model Basin, have developed a way
to calculate the effects of turbulent boundary layer flow on a bulbous bow housing
sonar system with an easy-to-use simulation model to reduce self-noise.
BY JENNIFER HAND
bulbous bow, the protruding bulb
that sits below the waterline at
the front of a vessel, has become a
standard design feature in large ships. It
significantly reduces wave resistance when
traveling long distances at a speed near
to maximum capacity, resulting in more
stability and fuel efficiency.
In recent years there has been a move
towards utilizing the bulbous bow to
house different types of sonar system but
there are questions about the extent to
which structural vibration interferes with
the functioning of the transducer array
inside such a sonar dome. In particular,
the pressure fluctuation that arises from
the turbulent boundary layer (TBL), a
thin layer of fluid where gradient of
velocity and the resulting shear stress
magnitudes are much higher than in the
laminar case, is thought to be one of the
major causes of self-noise for on-board
sensors. A team from INSEAN therefore
set out to investigate.

A

Testing a Scale Model
Francesca Magionesi, a researcher at
INSEAN, explains: “Most existing research
on wall pressure fluctuation has been
done using simple geometries and ideal
flow conditions that do not take into
account the free flow of water over a
complex curvature. So our first aim was
to understand the frequency spectra of
wall pressure fluctuations induced by
the turbulent boundary layer acting on a
bulbous bow.”
This was done by building a largescale model bulbous bow that was
rigid and conducting an experiment to
characterize wall pressure fluctuations
within one of the two towing tanks
at INSEAN. “This was tricky not only
because of the effects of structural
curvature and fluid loads but also
because we had to capture the higher
frequency component of pressure
fluctuations using miniaturized pressure

42

COMSOL NEWS / 2013

FIGURE 1: Experimental campaign on a 1:8 scaled model of the bulbous bow. A section of the
bulbous bow has been substituted with a transparent material on which accelerometers have
been placed (bottom middle). The simulation shows the system’s structural response (bottom right).
transducers,” reports Magionesi. Despite
this, the team managed to measure
wall pressure fluctuations at different
locations along the bulbous bow and
model these loads in terms of auto and
cross spectral densities.
In a second experiment the
researchers substituted a section of
the bulbous bow with an elastic linear
material (a transparent polymethyl
methacrylate, PMMA, thermoplastic)
and obtained its structural response
(see Figure 1).

The team then turned to simulation
to replicate the results of the physical
testing.
“To evaluate the dynamical response
of an elastic structure to the turbulent
boundary layer load it would normally
be necessary to use direct numerical
simulation of the fluid flow coupled with
a structural code,” explains Magionesi.
“But the high Reynolds number typical
of a real naval application makes this
approach impossible because of the
tremendous computational time and