Experimental and Analytical Study of under Water Pressure Wave Induced by the Implosion of a Bubble Generated by Focused Laser.

In various domains of material processing, such as surface cleaning and surface treatment, cavitation phenomenon may become an alternative to traditional methods if this phenomenon is well understood. Due to experimental and mathematical difficulties in theoretical models, it is still a challenge to accurately measure the physical mechanism of the fluid/structure interactions. In this study, we verified the feasibility of using polyvinylidene fluoride (PVDF) sensors to quantitatively measure the under-water pressure wave generated by the collapse of a single cavitation bubble. The electrical signal obtained by PVDF can be converted into pressure information only by using the sensor material parameters provided by the supplier. During the conversion process, only the capacitance of the acquisition chain needs to be additionally measured. At the same time, a high-speed video recording system was used to visualize the evolution of the cavitation bubble. The Gilmore analytical model and an associated wave propagation model were used to simulate the pressure peak of the first collapse of the cavitation bubble. This theoretical pressure was compared with the experimental results. The result showed that, for bubbles with a normalized standoff distance γ larger than 5, the PVDF sensor had the ability to quantitatively measure the pressure wave generated by a single cavitation bubble.

Computational fluid dynamics simulation to compare large volume irrigation and continuous spraying during nasal irrigation.

BACKGROUND: Nasal irrigation is now widely recognized as a treatment for chronic rhinosinusitis and during the postoperative period. However, there are no guidelines for performing irrigation. This study used computational fluid dynamics (CFD) simulation objective physical parameters to optimize and increase the efficiency of nasal irrigation and to compare large-volume, manual, and gravity pressure irrigation vs small-volume continuous spraying. METHODS: A 3-dimensional (3D) sinonasal model was constructed from a healthy adult high-resolution computed tomography (CT) scan. The 3D nasal model was constructed using a tetrahedral and hex-dominant mesh grid with TGRID™ 16 (ANSYS Inc., Villeurbanne, France) software. A structured hex mesh was created inside the domain using the Hexcore meshing method. The final mesh had a total of 9.6 × 106 cells with an average size of 0.29 mm3 , or an average volume of 2.42 × 10-2 mm3 . Navier-Stokes equations were resolved with the standard k - ε model. RESULTS: Large-volume irrigation (15 mL/s) covered all zones (136 to 310 cm2 ) rapidly with strong shear stress and prolonged contact time (310 mPa 3.26 seconds for gravity mode and 280 mPa 3.35 seconds for manual pressure mode). Continuous spraying (3 mL/second) covered all areas (76 to 310 cm2 ) but with far less volume, more slowly, with low shear stress (50 mPa), and with shorter contact time (1.84 seconds). The surface wetted by time in contact was 135.4, 113.9, and 46.6 cm2 for gravity, manual pressure mode, and continuous spraying, respectively. CONCLUSION: CFD simulation visualizes the circulation of water during nasal irrigation and makes it possible to determine objective parameters to decide which mode of irrigation may be used.

Numerical simulation of two consecutive nasal respiratory cycles: toward a better understanding of nasal physiology.

BACKGROUND: Computational fluid dynamic (CFD) simulations have greatly improved the understanding of nasal physiology. We postulate that simulating the entire and repeated respiratory nasal cycles, within the whole sinonasal cavities, is mandatory to gather more accurate observations and better understand airflow patterns. METHODS: A 3-dimensional (3D) sinonasal model was constructed from a healthy adult computed tomography (CT) scan which discretized in 6.6 million cells (mean volume, 0.008 mm3 ). CFD simulations were performed with ANSYS©FluentTMv16.0.0 software with transient and turbulent airflow (k-ω model). Two respiratory cycles (8 seconds) were simulated to assess pressure, velocity, wall shear stress, and particle residence time. RESULTS: The pressure gradients within the sinus cavities varied according to their place of connection to the main passage. Alternations in pressure gradients induced a slight pumping phenomenon close to the ostia but no movement of air was observed within the sinus cavities. Strong movements were observed within the inferior meatus during expiration contrary to the inspiration, as in the olfactory cleft at the same time. Particle residence time was longer during expiration than inspiration due to nasal valve resistance, as if the expiratory phase was preparing the next inspiratory phase. Throughout expiration, some particles remained in contact with the lower turbinates. The posterior part of the olfactory cleft was gradually filled with particles that did not leave the nose at the next respiratory cycle. This pattern increased as the respiratory cycle was repeated. CONCLUSION: CFD is more efficient and reliable when the entire respiratory cycle is simulated and repeated to avoid losing information.

Data acquisition analysis for femoral morphometric X-ray absorptiometry.

Bone mineral density (BMD) is a contributing factor of hip fracture risk. Other factors, such as lifestyle, the propensity for falls, and femoral geometry may influence the risk of hip fracture. The DMS Lexxos densitometer, a dual-energy X-ray densitometer can produce either a single-energy X-ray or a BMD image. The purpose of this study was to evaluate which of these 2 images enables the best detection to make femoral morphometry measurements. Spatial resolution, contrast, and noise were evaluated separately. A contrast-detail phantom was also used for the purpose of overall visual analysis. The spatial resolution was the same in the 2 images. The contrast was better with the BMD image, but the noise was higher. Using the contrast-detail phantom, the single-energy X-ray image allowed globally a better detection of the objects, but results were in the same range with high contrast values. Hip volunteers' morphometric measurements and the Singh Index were evaluated 3 times for each image by 3 observers, and the intra-, inter-, and global reproducibility was computed. The reproducibility of the measurements seems to be better with the single-energy X-ray image but results were not statistically significantly different. These results suggest that even if the image-quality indices were different, the single-energy X-ray image and BMD image are closely useful for clinical morphometric femoral evaluation.