Richard M. Kelso

An experimental study of round jets in cross-flow

The structure of round jets in cross-flow was studied using flow visualization techniques and flying-hot-wire measurements. The study was restricted to jet to freestream velocity ratios ranging from 2.0 to 6.0 and Reynolds numbers based on the jet diameter and free-stream velocity in the range of 440 to 6200. Flow visualization studies, together with time-averaged flying-hot-wire measurements in both vertical and horizontal sectional planes, have allowed the mean topological features of the jet in cross-flow to be identified using critical point theory. These features include the horseshoe (or necklace) vortex system originating just upstream of the jet, a separation region inside the pipe upstream of the pipe exit, the roll-up of the jet shear layer which initiates the counter-rotating vortex pair and the separation of the flat-wall boundary layer leading to the formation of the wake vortex system beneath the downstream side of the jet. The topology of the vortex ring roll-up of the jet shear layer was studied in detail using phase-averaged flying-hot-wire measurements of the velocity field when the roll-up was forced. From these data it is possible to examine the evolution of the shear layer topology. These results are supported by the flow visualization studies which also aid in their interpretation. The study also shows that, for velocity ratios ranging from 4.0 to 6.0, the unsteady upright vortices in the wake may form by different mechanisms, depending on the Reynolds number. It is found that at high Reynolds numbers, the upright vortex orientation in the wake may change intermittently from one configuration of vortex street to another. Three mechanisms are proposed to explain these observations.

Horseshoe vortex systems resulting from the interaction between a laminar boundary layer and a transverse jet

The horseshoe vortex system resulting from the interaction between a laminar boundary layer and a round transverse jet was studied over a range of Reynolds numbers and velocity ratios using hydrogen bubble wire visualization in a water channel. The study shows that the horseshoe vortex system can be steady, oscillating, or coalescing, depending on the flow conditions. Topological concepts are used to interpret the observed flow patterns and compare these patterns with those observed and computed upstream of wall‐mounted circular cylinders. The Strouhal numbers of the observed oscillating and coalescing systems agree reasonably well with those appearing in the literature for wall‐mounted circular cylinders. The relationship between the unsteady horseshoe vortex motions and the unsteady vortex motions in the wake is studied for a velocity ratio of 4. Here it is shown that the oscillating regime occurs at the same frequency as the wake and the coalescing regime occurs at approximately double the frequency of...

Performance Variations of Leading-Edge Tubercles for Distinct Airfoil Profiles

An experimental investigation has been undertaken to determine the influence of sinusoidal leading-edge protrusions on the performance of two NACA airfoils with different aerodynamic characteristics. Force measurements on full-span airfoils with various combinations of tubercle amplitude and wavelength reveal that when compared to the unmodified equivalent, tubercles are more beneficial for the NACA 65-021 airfoil than the NACA 0021 airfoil. It was also found that for both airfoil profiles, reducing the tubercle amplitude leads to a higher maximum lift coefficient and larger stall angle. In the poststall regime, however, the performance with largeramplitude tubercles is more favorable. Reducing the wavelength leads to improvements in all aspects of lift performance, including maximum lift coefficient, stall angle, and poststall characteristics. Nevertheless, there is a certain point at which further reduction in wavelength has a negative impact on performance. The results also suggest that tubercles act in a manner similar to conventional vortex generators.

Cardiac Flow Analysis Applied to Phase Contrast Magnetic Resonance Imaging of the Heart

Phase contrast magnetic resonance imaging is performed to produce flow fields of blood in the heart. The aim of this study is to demonstrate the state of change in swirling blood flow within cardiac chambers and to quantify it for clinical analysis. Velocity fields based on the projection of the three dimensional blood flow onto multiple planes are scanned. The flow patterns can be illustrated using streamlines and vector plots to show the blood dynamical behavior at every cardiac phase. Large-scale vortices can be observed in the heart chambers, and we have developed a technique for characterizing their locations and strength. From our results, we are able to acquire an indication of the changes in blood swirls over one cardiac cycle by using temporal vorticity fields of the cardiac flow. This can improve our understanding of blood dynamics within the heart that may have implications in blood circulation efficiency. The results presented in this paper can establish a set of reference data to compare with unusual flow patterns due to cardiac abnormalities. The calibration of other flow-imaging modalities can also be achieved using this well-established velocity-encoding standard.

N-Acetylcysteine, a Novel Treatment for Helicobacter pylori Infection

N-Acetylcysteine (NAC), being both a mucolytic agent and a thiol-containing antioxidant, may affect the establishment and maintenance of H. pylori infection within the gastric mucus layer and mucosa. Agar and broth dilution susceptibility tests determined the MIC of H. pylori strain SSI to NAC. H. pylori load in SSI strain-infected C57BL mice was determined as colony forming units per gram of gastric tissue. Gastritis assessment was scored and gastric surface hydrophobicity was determined by contact angle measurement. MICs of NAC were 5 to 10 and 10 to 15 mg/ml using the agar dilution and broth dilution methods, respectively. NAC (120 mg per day for 14 days) reduced the H. pylori load in mice by almost 1 log compared with sham treatment. Pretreatment with NAC (40 mg/day) also significantly reduced the H. pylori load but did not prevent H. pylori colonization. Both H. pylori infection and NAC reduced the surface hydrophobicity of murine gastric mucosa. No significant differences were observed in the gastritis scores of H. felis- or H. pylori-infected mice receiving either NAC or sham treatments. This study demonstrates that NAC inhibits the growth of H. pylori in both agar and broth susceptibility tests and in H. pylori-infected mice. NAC did not alter the severity of H. pylori- or H. felis-induced gastritis.

Phase-averaged velocity in a fluidic precessing jet nozzle and in its near external field

Abstract Phase averaged laser-Doppler measurements of the axial velocity components made within and in the near exit field of a precessing-jet nozzle have verified a number of flow features reported in the research literature. The nozzle is a short cylindrical tube with an axisymmetric inlet at one end, and with a centrebody and a small exit lip at the other end. The diameter ratio of the abrupt expansion at the inlet is 1:5. The measurements of the internal flow field reveal a radially deflected internal jet which reattaches asymmetrically and precesses around the wall of the chamber. The phase-averaged flow inside the chamber can be divided into regions of forward flow and regions of reverse flow. The distribution of these regions inside the chamber implies the presence of large-scale recirculation. Representative reverse mean flow speeds of recirculation are about 30% of the forward flow speed. Measurements inside the chamber suggest that the effect of reversed flow on the velocity decay of the inlet-jet flow is similar to that of an ambient counter flow. Measurements in the external jet suggest that the initial entrainment rate of the external precessing jet is between six and seven times that of an equivalent free turbulent jet. The phase-averaged deflection angle of the present emerging jet is 50° but this decreases to about 30° within 0.4 chamber diameters of the exit plane.

Cardiac flow component analysis

In a chamber of the heart, large-scale vortices are shown to exist as the result of the dynamic blood flow and unique morphological changes of the chamber wall. As the cardiovascular flow varies over a cardiac cycle, there is a need for a robust quantification method to analyze its vorticity and circulation. We attempt to measure vortex characteristics by means of two-dimensional vorticity maps and vortex circulation. First, we develop vortex component analysis by segmenting the vortices using an data clustering algorithm before histograms of their vorticity distribution are generated. The next stage is to generate the statistics of the vorticity maps for each phase of the cardiac cycle to allow analysis of the flow. This is followed by evaluating the circulation of each segmented vortex. The proposed approach is dedicated to examining vortices within the human heart chamber. The vorticity field can indicate the strength and number of large-scale vortices in the chamber. We provide the results of the flow analysis after vorticity map segmentation and the statistical properties that characterize the vorticity components. The success of the cardiac measurement and analysis is illustrated by a case study of the right atrium. Our investigation shows that it is possible to utilize a data clustering algorithm to segment vortices after vorticity mapping, and that the vorticity and circulation analysis of a chamber vorticity can provide new insights into the blood flow within the cardiovascular structure.

Three-dimensional structure of a low-Reynolds-number turbulent boundary layer

A low-Reynolds-number zero-pressure-gradient incompressible turbulent boundary layer was investigated using a volumetric imaging technique. The Reynolds number based on momentum thickness was 700. The flow was tagged with a passive scalar from two spanwise dye slots to distinguish between fluid motions originating in the inner and outer portions of the boundary layer. The resulting volumetric scalar field was interrogated using a laser sheet scanner developed for this study. Twoand three-dimensional time-dependent visualizations of a 50 volume time series are presented (equivalent to 17δ in length). In the outer portion of the boundary layer, scalar structures were observed to lie along lines in the (x, z)-plane, inclined to the streamwise (x-)direction in the range ±50 ◦ . The ejection of brightly dyed fluid packets from the near-wall region was observed to be spatially organized, and related to the passage of the large-scale scalar structures.

The formation mechanism and impact of streamwise vortices on NACA 0021 airfoil's performance with undulating leading edge modification

Wings with tubercles have been shown to display advantageous loading behavior at high attack angles compared to their unmodified counterparts. In an earlier study by the authors, it was shown that an undulating leading-edge configuration, including but not limited to a tubercled model, induces a cyclic variation in circulation along the span that gives rise to the formation of counter-rotating streamwise vortices. While the aerodynamic benefits of full-span tubercled wings have been associated with the presence of such vortices, their formation mechanism and influence on wing performance are still in question. In the present work, experimental and numerical tests were conducted to further investigate the effect of tubercles on the flow structure over full-span modified wings based on the NACA 0021 profile, in the transitional flow regime. It is found that a skew-induced mechanism accounts for the formation of streamwise vortices whose development is accompanied by flow separation in delta-shaped regions n...

MEDICAL IMAGING AND PROCESSING METHODS FOR CARDIAC FLOW RECONSTRUCTION

Intra-cardiac blood flow imaging and visualization is challenging due to the processes involved in generating velocity fields of flow within specific chambers of interest. Visual analysis of cardiac flow or wall deformation is crucial for an accurate examination of the heart. Cardiac chamber boundary encapsulation is one of the key implementations for region definition. To provide intelligible results describing flow within the human heart, cardiac chamber segmentation is a pre-requisite so that fluid motion information can be presented within a region of interest defined by the chamber boundary. A technique that is used to establish contouring along the cardiac wall is described mathematically. This article also sets the practical foundation for flow vector synthesis and visualization in the cardiac discipline. We have outlined conceptual development and the construction of flow field based on a three-dimensional Cartesian grid that can give a greater insight into the blood dynamics within the heart. We developed a framework that is able to present both anatomical as well as flow information by overlaying velocity fields over medical images and displaying them in cine-mode. By addressing most of the methods involved from the programming perspective, procedural execution and memory efficiency have been considered. Our implemented system can be used to examine abnormal blood motion behaviour or discover flow phenomena in normal or defective hearts.