Towed acoustic arrays and hull mounted arrays are used both in military and in civil systems. For example, the seismic industry is an extensive user of acoustic arrays.
Various noise sources will limit the performance of both hull mounted and towed acoustic arrays. One such performance degrading mechanism is caused by the turbulent boundary layer surrounding the arrays when they are moved through water.
The fluctuating pressure field of the turbulent boundary layer will potentially be responded to by pressure fluctuations within the array. This might directly cause a noisy pressure field recorded by the hydrophones. In addition, the fluctuating pressure field will cause mechanical vibrations of the entire system with a wider impact than just the pressure fields' impact at the hydrophones.
In addition to noise sources caused by turbulence (noise from the sea is not treated here), there may be vibrations propagated in the hull or through the towing system, noise from the towing ship, noise from depressors etc.
Hydrodynamics, hydroacoustics and structure mechanics can be used to explain and describe the physical processes of the noise mentioned above that will degrade the performance, but also the propagation of the acoustical signals that the systems are designed to detect.
The physical description and modelling of acoustic arrays involve solution of a multi-physics problem where fluid turbulence, acoustics and structural vibrations are more or less coupled. The modelling is done by solving the Navier-Stokes equations and the elasticity equations numerically and then couple the solutions of both systems.
This coupling can be one- or two-way. Two-way coupling is a big challenge. Modelling of turbulent boundary layers with sufficient precision for these problems is also a challenge. Therefore we run physical experiments involving towed and hull mounted acoustic arrays. This is done to validate the computer codes used for modelling and also to validate our view of which physical processes are involved and which do dominate these systems.