Characterizing Fundamental Resonance Peaks on Flat-Lying Sediments Using Multiple Spectral Ratio Methods: An Example from the Atlantic Coastal Plain, Eastern United States

ABSTRACT Damaging ground motions from the 2011 Mw 5.8 Virginia earthquake were likely increased due to site amplification from the unconsolidated sediments of the Atlantic Coastal Plain (ACP), highlighting the need to understand site response on these widespread strata along the coastal regions of the eastern United States. The horizontal-to-vertical spectral ratio (HVSR) method, using either earthquake signals or ambient noise as input, offers an appealing method for measuring site response on laterally extensive sediments, because it requires a single seismometer rather than requiring a nearby bedrock site to compute a horizontal sediment-to-bedrock spectral ratio (SBSR). Although previous studies show mixed results when comparing the two methods, the majority of these studies investigated site responses in confined sedimentary basins that can generate substantial 3D effects or have relatively small reflection coefficients at their base. In contrast, the flat-lying ACP strata and the underlying bedrock reflector should cause 1D resonance effects to dominate site response, with amplification of the fundamental resonance peaks controlled by the strong impedance contrast between the base of the sediments and the underlying bedrock. We compare site-response estimates on the ACP strata derived using the HVSR and SBSR methods from teleseismic signals recorded by regional arrays and observe a close match in the frequencies of the fundamental resonance peak (f0) determined by both methods. We find that correcting the HVSR amplitude using source term information from a bedrock site and multiplying the peak by a factor of 1.2 results in amplitude peaks that, on average, match SBSR results within a factor of 2. We therefore conclude that the HVSR method may successfully estimate regional linear weak-motion site-response amplifications from the ACP, or similar geologic environments, when appropriate region-specific corrections to the amplitude ratios are used.

Direct numerical simulations of hypersonic boundary-layer transition for a flared cone: fundamental breakdown

Direct numerical simulations (DNS) were carried out to investigate the laminar–turbulent transition for a flared cone at Mach 6 at zero angle of attack. The cone geometry of the flared cone experiments in the Boeing/AFOSR Mach 6 Quiet Tunnel (BAM6QT) at Purdue University was used for the simulations. In the linear regime, the largest integrated spatial growth rates ($N$-factors) for the primary instability were obtained for a frequency of approximately $f=300~\text{kHz}$. Low grid-resolution simulations were carried out in order to identify the azimuthal wavenumber that led to the strongest growth rates with respect to the secondary instability for a fundamental and subharmonic resonance scenario. It was found that for the BAM6QT conditions the fundamental resonance is much stronger compared to the subharmonic resonance. Subsequently, for the case which led to the strongest fundamental resonance onset, detailed investigations were carried out using high-resolution DNS. The simulation results exhibit streamwise streaks of very high skin friction and of high heat transfer at the cone surface. Streamwise ‘hot’ streaks on the flared cone surface were also observed in the experiments carried out at the BAM6QT facility using temperature sensitive paint. The presented findings provide strong evidence that the fundamental breakdown is a dominant and viable path to transition for the BAM6QT conditions.

High Intensity Focused Ultrasound Transducer Using Inversion Layer Technique for Ultrasound Therapy

Recently, high intensity focused ultrasound (HIFU) has been used for non-invasive surgery of prostate, uterus, and brain. However, a HIFU therapy is suffered from relatively long surgical time mainly due to small focal area per each sonication. In order to solve this problem, a HIFU therapy using multi-frequency was suggested by several researchers, and they demonstrated that this technique can increase the area of the coagulated lesion due to enhanced cavitation effect compared to single-frequency HIFU [1–3]. To generate multi-frequency especially dual-frequency, dual-element and dual-layer HIFU transducers have been developed and provided an expanded lesion size [1–3]. In this study, we present an alternative technique of making dual-frequency HIFU transducer using inversion layer technique. Generally, a single layer piezoelectric element can excite the strong fundamental resonance (f0) and the weak odd-order harmonic resonance (3f0) [4]. In the inversion layer technique, on the other hand, a piezoelectric component consisting of two piezo-ceramic plates bonded together with opposite poling directions and different thicknesses can produce the relatively strong even-order harmonic (2f0) in addition to the fundamental resonance [5]. Additionally, only a pair of electrode at the outside of the each piezo-ceramic plate is required to stimulate dual-frequency ultrasound while two pairs of electrodes are typically required for conventional dual-element and dual-layer transducers [2,3]. A specially designed prototype HIFU transducer was built, and we verified that the dual-frequency ultrasound was successfully generated through electrical impedance and pulse-echo response measurements.