Ningqun Guo

The interaction of Lamb waves with delaminations in composite laminates

The S0 Lamb mode can propagate over distances of the order of 1 m in composite laminates and so has the potential to be used in long‐range nondestructive inspection. This paper discusses the interaction of the S0 Lamb mode with delaminations. The dispersion curves and the corresponding stress and displacement mode shapes of the lower order Lamb modes are obtained analytically and the interaction of the S0 mode with delaminations at different interfaces in a composite laminate is then studied both by finite element analysis and by experiment. It is shown that the amplitude of the reflection of the S0 mode from a delamination is strongly dependent on the position of the delamination through the thickness of the laminate and that the delamination locations corresponding to the maximum and minimum reflectivity correspond to the locations of maximum and minimum shear stress across the interface in the S0 mode.

Rayleigh wave interaction with surface-breaking cracks

This paper investigates Rayleigh wave interaction with simulated, surface breaking cracks using a finite element method, in which the scattered wave modes giving rise to the in-plane and out-of-plane displacements are presented. By looking at the contribution from all of the transmitted, reflected, and mode-converted signals at the crack, the magnitude of signal enhancement in the near field and the mechanism by which this occurs can be fully explained. Furthermore, oscillations in the Rayleigh wave reflection and transmission coefficients with crack depth in the far field can be explained by means of multiple reflected and transmitted wave modes at the crack, whose relative amplitudes are dependent on the crack depth. Results agree with previously published experimental measurements.

Beam pattern calculation for optimization of broadband array transducers

Broadband array transducers are widely used in medical imaging systems, and their beam patterns determine the imaging quality. The characteristics of the beam patterns such as mainlobe beamwidth and sidelobe levels are the major criteria for designing and optimizing sparse array systems. The spatial impulse response approach can be used to calculate the beam patterns rigorously in both near field and far field. However, it is time consuming due to the high sampling frequency required, and thus not suitable for array optimization. On the other hand, assuming the array elements to be omnidirectional point sources the field calculation can be much simplified and fast but at the expense of poor accuracy. This paper presents a modified model that has the accuracy comparable to that from the impulse response model while keeping the efficiency close to that of the simple point source model. The developed algorithm is particularly suitable for stochastic methods for two-dimensional (2D) sparse arrays design and optimization such as genetic algorithms (GA), in which the iterative beam pattern calculation dominates the optimization program running time.

Viscous Dissipation Effect on Entropy Generation of Nanofluid Flow in Microchannels

An analytical study of the viscous dissipation effect on entropy generation of forced convection of water-alumina nanofluid in a circular microchannel subjected to exponential wall heat flux is reported. Closed form solutions of the temperature distributions in the streamwise direction for the models with and without viscous dissipation term in the energy equation are obtained. The two models are compared by analyzing their relative deviations in entropy generation for different Reynolds number and nanoparticle volume fraction. The incorporation of viscous dissipation prominently affects the temperature distribution and consequently the entropy generation. The increase in the entropy generation is mainly attributable to the increase in the fluid friction irreversibility. The addition of nanoparticle increases the effective thermal conductivity and viscosity of nanofluid which induces escalation in the heat transfer and fluid friction irreversibilities, respectively. By taking the viscous dissipation effect into account, the exergetic effectiveness for forced convection of nanofluid in microchannels attenuate with increasing nanoparticle volume fraction. From the aspect of the second law of thermodynamics, the widespread conjecture that nanofluids possess advantage over pure fluid associated with higher overall effectiveness is invalidated.© 2013 ASME