Chee Lum

Hydroxyl tagging velocimetry in a supersonic flow over a cavity

Hydroxyl tagging velocimetry (HTV) measurements of velocity were made in a Mach 2 (M 2) flow with a wall cavity. In the HTV method, ArF excimer laser (193 nm) beams pass through a humid gas and dissociate H2O into H + OH to form a tagging grid of OH molecules. In this study, a 7 x 7 grid of hydroxyl (OH) molecules is tracked by planar laser-induced fluorescence. The grid motion over a fixed time delay yields about 50 velocity vectors of the two-dimensional flow in the plane of the laser sheets. Velocity precision is limited by the error in finding the crossing location of the OH lines written by the excimer tag laser. With a signal-to-noise ratio of about 10 for the OH lines, the determination of the crossing location is expected to be accurate within +/- 0.1 pixels. Velocity precision within the freestream, where the turbulence is low, is consistent with this error. Instantaneous, single-shot measurements of two-dimensional flow patterns were made in the nonreacting M 2 flow with a wall cavity under low- and high-pressure conditions. The single-shot profiles were analyzed to yield mean and rms velocity profiles in the M 2 nonreacting flow.

Physical factors that trigger cholesterol crystallization leading to plaque rupture

BACKGROUND Triggers of plaque rupture have been elusive. Recently it was demonstrated that cholesterol expands when transforming from a liquid to a crystal state, disrupting overlying plaque. This study examined the effect of physical conditions including saturation, temperature, hydration, pH on cholesterol crystallization. METHODS Graduated cylinders were filled with varying amount of cholesterol powder (1, 2 and 3g) and dissolved in corn oil at 37 degrees C. Change in volume expansion (DeltaVE) and time to crystallization were measured for each saturation. The same was repeated while varying temperature (22-44 degrees C); hydration (1-3ml H(2)O); pH (5-8.4) and combination of saturation and temperature. Scanning electron microscopy was performed to evaluate crystal morphology and X-ray diffractometry to assess molecular structure of cholesterol. RESULTS Increasing saturation raised both DeltaVE (3g: 0.53+/-0.1ml vs. 1g: 0.14+/-0.02ml and 2g: 0.3+/-0.1ml; p<0.0001; p<0.01) and rate of change over 3min (3g: 60% vs. 1g: 14%). Crystal morphology was the same seen with crystals perforating human plaques. Temperature drop increased DeltaVE (44 degrees C: 0.05+/-0.01ml vs. 22 degrees C: 0.5+/-0.07ml; p<0.0001) and initiated earlier crystallization. Hydration resulted in greater DeltaVE (3ml: 0.7+/-0.07 vs. 0ml: 0.1+/-0.05; p<0.001) with corresponding changes in cholesterol molecular structure. Rising pH was associated with increased DeltaVE (1.3+/-0.03ml vs. 0.1+/-0.02ml; p<0.001). Combined increase in saturation and temperature had greater DeltaVE than expected from the sum of each alone. CONCLUSIONS Physical factors influenced both volume and rate of cholesterol crystallization. This suggests that local factors may play an important role in triggering plaque rupture. Combination of several factors may even be a more powerful trigger for acute cardiovascular events.

Hydroxyl Tagging Velocimetry in a Mach 2 Flow with a Wall Cavity

Abstract : Hydroxyl tagging velocimetry (HTV) measurements of velocity were made in a Mach 2 flow with a wall cavity. In the HTV method, ArF excimer laser (193 nm) beams pass through a humid gas and dissociate H2O into H + OH to form a tagging grid of OH molecules. In this study, a 7x7 grid of hydroxyl (OH) molecules is tracked by planar laser-induced fluorescence. The grid motion over a fixed time delay yields about 50 velocity vectors of the two-dimensional flow. Instantaneous, single-shot measurements of two-dimensional flow patterns were made in the non-reacting Mach 2 flow with a wall cavity under low and high pressure conditions. The single-shot profiles were analyzed to yield mean and rms velocity profiles in the Mach 2 non-reacting flow.

The Impact of Stent Strut Porosity on Reducing Flow in Cerebral Aneurysms

Flow diversion techniques are increasingly used to treat cerebral aneurysms. The optimal stent porosity to achieve aneurysm obliteration would allow clinicians to treat aneurysms more effectively. We sought to determine the optimal porosity threshold in an in vitro flow model that would lead to stagnation of flow in an aneurysm.