Nonlinear control of boundary impedance in an acoustic waveguide.

Wavetubes are employed for measurements of acoustic properties in various fluids. The ability to manipulate and control the frequency-dependent boundary impedance of the tube improves the estimation accuracy. Passive solutions, which use composite materials to change the boundary impedance, enable one to realize a finite combination of boundary impedances. In this paper, the tube boundary impedance is tuned at will by using two loudspeakers. The suggested method operates in the presence of dispersion by estimating, in real-time, a parametric reduced-order model using a multichannel least mean square algorithm. The identified model is fed to a nonlinear, adaptive control algorithm to realize modal traveling wave ratio (TWR) control. It has been noted that the TWR is smooth and parabolic across closed regions in the parameter space, thus assuring the convergence of the nonlinear control. Several methods to estimate the TWR gradient are considered and compared based on an analytical model of a rigid impedance tube. An experimental case study utilizing an air-filled impedance tube with two loudspeakers is presented. The results demonstrate the ability to control the dynamics of the principal acoustic mode at will, thus enabling one to set the desired tube's boundary impedance.

Waveguide dispersion curves identification at low-frequency using two actuators and phase perturbations.

Dispersion curves of fluid-filled elastic-tubes are used for non-destructive measurement of material acoustic properties. The underlying physics leads to a singular numerical procedure when several modes or long-wavelength scenarios take part in the tube dynamics. The literature describes several methods to identify dispersion curves that require a large ratio of samples per length. Described is a method to enrich the amount of available information of an otherwise ill-posed problem, by multiple boundary phase perturbations at each excitation frequency. The method uses two actuators, one at either end of the waveguide to produce different relative phases, followed by a nonlinear model fitting procedure. Presented are a model-based derivation and experimental verification of the proposed approach on an air-filled elastic-tube. The latter shows the capability of the method to recover the dispersion curves even for very weak structural-acoustic coupling and at low frequencies. The portrayed scheme can be applied on various waveguides by using two actuators and only a single sensor, and hence makes dispersion curve estimation realistic in formerly inaccessible cases.

On the sensing and tuning of progressive structural vibration waves.

Progressive flexural waves can be generated only in finite structures by fine tuning the excitation and the boundary conditions. The tuning process eliminates the reflected waves arising from discontinuities and edge effects. This work presents and expands two new methods for the identification and tuning of traveling waves. One is a parametric method based on fitting an ellipse to the complex spatial amplitude distribution. The other is a nonparametric method based on the Hilbert transform providing a space-localized estimate. With these methods, an optimization-based tuning of transverse flexural waves in a one-dimensional structure, a vibrating beam, is developed. Existing methods are designed for a single frequency and are based on either combining two vibration modes or mechanical impedance matching. Such methods are limited to a designated excitation frequency determined by a specific configuration of the system. With the proposed methods, structural progressive waves can be generated for a wide range of frequencies under the same given system configuration and can be tuned in real time to accommodate changes in boundary conditions. An analytical study on the nature of the optimal excitation conditions has been carried out, revealing singular configurations. The experimental verification of the sensing and tuning methods is demonstrated on a dedicated laboratory prototype. The proposed methods are not confined to mechanical waves and present a comprehensive approach applicable for other physical wave phenomena.

Noncontacting lateral transportation using gas squeeze film generated by flexural traveling waves--numerical analysis.

This paper presents the theory describing the dynamical behavior of a noncontacting lateral transportation of planer objects by means of a gas squeeze film created by traveling flexural waves of a driving surface. An oscillating motion in the normal direction between two surfaces can generate a gas film with an average pressure higher than the surrounding. This load-carrying phenomenon arises from the fact that a viscous flow cannot be instantaneously squeezed; therefore, fast vibrations give rise to a cushioning effect. Equilibrium is established through a balance between viscous flow forces and compressibility forces. When the oscillatory motion between two surfaces creates traveling waves, lateral viscous forces are generated in addition to the normal levitation forces. These forces are produced as a result of nonuniform pressure gradients in the lateral direction between the surfaces. The combination of normal and lateral forces could be used for transporting objects without any direct contact with the driving surface. The numerical algorithm in this work couples the squeeze film phenomenon, which is represented by means of finite difference equations, to model a variant of the Reynolds equation, together with the equations describing the dynamics of the floating object. Numerical simulations are presented and investigated to highlight noteworthy topics.