Active Control of Normal Particle Velocity at a Boundary between Two Media

Algorithm for Active Suppression of Radiation and Acoustical Scattering Fields by Some Physical Bodies in Liquids

“…The wave, which was radiated by a Huygens' source, does not depend on impedance in back-space and can be simply added to any wave passing without any interaction with the source. In [36] we can see the same effect of the one side radiation without mechanical support for the monochromatic excitation (on some frequency ω) of two piezoelectric layers (with the phase difference Δφ = ωh c /c c of excitation). A very small phase difference (necessary for one side radiation) is defined by stationary device tuning.…”

“…The new approach allows quick suppression of scattering fields with minimum information about the boundary problem. The one-sided direction of the radiation of Huygens' sources allows us to form the distribution of oscillatory velocities (or displacements), which we need, on the outside surface of an active coating, without inducing additional oscillations of the protected body [25,36,37]. At this point the analogy between Malyuzhinets's method and our approach is over.…”

“…This represents two-layer plane segments [31][32][33][34][35] of a piezocomposite material. Longitudinal impedance and speed of propagation of longitudinal sound are the same for an active coating and the external fluid respectively [33,36,38]. Such active pistons almost fully cover the protected surface.…”

“…Time interval of averaging satisfies the condition It was shown [15], that in a one-dimensional problem (at L V →∞) such a double layer of piezoelectric material (or piston) with the above mentioned magnitude-temporal relation (Figure 7с) between inducing electric pulses (Figure 7a,b) creates a Huygens' source (or Huygens' piston). This double layer provides a bipolar pulse of media particles displacement of duration T B = 3τ V + τ c. These pulses are moving through space (like wavelets [36], Figure 8) with speed c w . A wavelet (we will also call it a "basic pulse") is only radiated forwards (outside the surface S V ) and must always contain an interval of zero magnitude and of duration τ V , between two pulses of mutually opposite polarity and same duration τ V .…”

“…The first (left part) interval of the next basic pulse is covered by the third (right part) interval of the previous basic pulse. The absence of the source (one-dimensional [36]) and wave field in back-space means that we can generate forward radiation waves without any mechanical support of both piezoelectric layers, due to the special construction of the Huygens' source. Besides one-side radiation this Huygens' source is an acoustically transparent source.…”