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Carotid artery diseases, such as atherosclerosis, are a major cause of death in the United States. Wall shear stresses are known to prompt plaque formation, but there is limited understanding of the complex flow structures underlying these stresses and how they differ in a pre-disposed high-risk patient cohort. A ‘healthy’ and a novel ‘pre-disposed’ carotid artery bifurcation model was determined based on patient-averaged clinical data, where the ‘pre-disposed’ model represents a pathological anatomy. Computational fluid dynamic simulations were performed using a physiological flow based on healthy human subjects. A main hairpin vortical structure in the internal carotid artery sinus was observed, which locally increased instantaneous wall shear stress. In the pre-disposed geometry, this vortical structure starts at an earlier instance in the cardiac flow cycle and persists over a much shorter period, where the second half of the cardiac cycle is dominated by perturbed secondary flow structures and vortices. This coincides with weaker favorable axial pressure gradient peaks over the sinus for the ‘pre-disposed’ geometry. The findings reveal a strong correlation between vortical structures and wall shear stress and imply that an intact internal carotid artery sinus hairpin vortical structure has a physiologically beneficial role by increasing local wall shear stresses. The deterioration of this beneficial vortical structure is expected to play a significant role in atherosclerotic plaque formation.

In contact mode scanning probe microscopy (SPM), the microcantilever probe's dynamics are governed by the (short-range) surface interaction forces, where the tip is "always-in-contact" with the sample. In intermittent contact modes such as "tapping" or bimodal SPM, on the other hand, these are governed by the frequency of the microcantilever's own external excitation. However, when contact mode is employed with high scan speeds (for "video-rate" SPM), we see intermittent transitions—within a single oscillation cycle—between the "always-in-contact" regime and another which is dominated by the microcantilever's inertia. We find—through experiments and physical modeling—that the fast in-plane motion of the sample relative to the probe results in a high surface excitation frequency v / λ (and its harmonics), which excite the microcantilever's out-of-plane eigenmodes and cause it to "break free" of the surface and "overshoot" and "parachute." The impacts of the tip that consequently occur upon the sample inject energy over a wide frequency band into the higher eigenmodes, especially when operating in a low dissipation ambient environment. The microcantilever, then, exhibits phenomena such as eigenmode switching, sidebands, and fractional and combination resonances; such behavior is not seen in, say, tapping mode SPM, since, there, energy is injected at an externally-determined temporal rate. This article investigates the transition from the dynamics of the microcantilever at low speeds to that exhibited at high speeds. The model for dynamic contact loss is validated against the experiments and can be used to propose mitigation of such dynamics in order to achieve high-resolution imaging. [ABSTRACT FROM AUTHOR]

We experimentally study flow-induced vibration (FIV) of a thin, cantilevered, flexible plate attached on the lee side of a circular cylinder in a free stream airflow. The Reynolds number (ReD) based on the diameter of the cylinder was varied in the range of (4 × 103, 5 × 10 4 ). The plates of different lengths and the same span were tested with varying airflow velocities (U) in a wind tunnel. The reduced velocity (UR) based on the length and the second natural frequency of the plate was in the range of (1, 11). We describe the plate dynamics showing displacement, frequency, phase plane, amplitude spectral density (ASD), oscillation envelope, modal energy contribution, and flow structures. Based on the motion of the plate, three FIV regimes are observed, namely, initial excitation, transition, and lock-in. During lock-in, the plate attains self-sustained limit cycle oscillation. We observe the presence of small-amplitude higher harmonics in the ASD plot. The second mode is dominant around the onset of lock-in, while the first mode is dominant later in the lock-in regime. The dimensionless displacement increases with an increase in mass ratio (Ms) in the lock-in regime. With a decrease in Ms, the slope of the dimensionless frequency with UR increases. To explain measurements, we develop a wake oscillator model (WOM) in which the plate and fluctuating lift force are modeled as an Euler–Bernoulli cantilever beam and van der Pol oscillator, respectively. The coupled WOM qualitatively and quantitatively predict the measured amplitude and frequency response, respectively, in the lock-in regime. [ABSTRACT FROM AUTHOR]