Experiments preformed in the JHU refractive index matched facility examine flow phenomena developing in the rotor passage of an axial compressor at the onset of stall. High-speed imaging of cavitation performed at low pressures qualitatively visualizes vortical structures. Stereoscopic particle image velocimetry (SPIV) measurements provide detailed snapshots and ensemble statistics of the flow in a series of meridional planes. At prestall condition, the tip leakage vortex (TLV) breaks up into widely distributed intermittent vortical structures shortly after rollup. The most prominent instability involves periodic formation of large-scale backflow vortices (BFVs) that extend diagonally upstream, from the suction side (SS) of one blade at midchord to the pressure side (PS) near the leading edge of the next blade. The 3D vorticity distributions obtained from data recorded in closely spaced planes show that the BFVs originate form at the transition between the high circumferential velocity region below the TLV center and the main passage flow radially inward from it. When the BFVs penetrate to the next passage across the tip gap or by circumventing the leading edge, they trigger a similar phenomenon there, sustaining the process. Further reduction in flow rate into the stall range increases the number and size of the backflow vortices, and they regularly propagate upstream of the leading edge of the next blade, where they increase the incidence angle in the tip corner. As this process proliferates circumferentially, the BFVs rotate with the blades, indicating that there is very little through flow across the tip region.

The flows in the tip regions of two rotors with blades of similar geometry but different tip clearance are studied experimentally to determine the effect of gap on the flow structure at different operating conditions. The experiments have been performed in the JHU optically index-matched facility, where the refractive index of the fluid is matched with that of the acrylic rotor blades and casing, facilitating unobstructed Stereo Particle Image Velocimetry (SPIV) measurements. The blade geometries are based on the first one and a half stages of the Low Speed Axial Compressor (LSAC) facility at NASA Glenn. The tip gap sizes are 0.49% and 2.3% of the blade chordlength, and measurements are performed for two flow rates, the lower of which is just above stall conditions. The presence and trajectories of the tip leakage vortex (TLV) and secondary structures are visualized by recording high speed movies of cavitation at lower pressures. The results consist of performance curves, distributions of velocity, circumferential vorticity and turbulent kinetic energy, as well as the strength and trajectory of vortices. Increasing the tip gap reduces the static-to-static pressure coefficient for all flow conditions. For the higher flow rate, a wider tip gap has several effects: (i) It delays the rollup of the TLV and its detachment from the suction side (SS) corner of the blade, presumably due to the larger distance from the endwall casing and the ‘image vortex’. (ii) It alters the blade loading and reduces the circulation shed from the blade. (iii) It delays the onset of TLV bursting in the aft part of the rotor passage. (iv) For both gaps, the endwall boundary layer separates at the point where the leakage flow meets the opposite-direction main passage flow. For the wide gap, the separated layer with opposite sign vorticity remains above the TLV; while for the narrow gap, the TLV entrains this layer around itself. And (v) consistent with the major differences in flow structure, the spatial distributions and magnitudes of all the turbulence intensity are also very different. Trends and flow structure are quite different at pre-stall conditions. Most notably, TLV rollup is still delayed for the wide gap, but vortex bursting and associated arrival of multiple secondary structures to the pressure side (PS) of the next blade occur earlier. Consequently, the turbulence level on both sides of the blade tip is substantially higher, and remnants of the previous TLV are ingested into the next tip gap.

Vascular atresia are often treated via transcatheter recanalization or surgical vascular anastomosis due to congenital malformations or coronary occlusions. The cellular response to vascular anastomosis or recanalization is, however, largely unknown and current techniques rely on restoration rather than optimization of flow into the atretic arteries. An improved understanding of cellular response post anastomosis may result in reduced restenosis. Here, an in vitro platform is used to model anastomosis in pulmonary arteries (PAs) and for procedural planning to reduce vascular restenosis. Bifurcated PAs are bioprinted within 3D hydrogel constructs to simulate a reestablished intervascular connection. The PA models are seeded with human endothelial cells and perfused at physiological flow rate to form endothelium. Particle image velocimetry and computational fluid dynamics modeling show close agreement in quantifying flow velocity and wall shear stress within the bioprinted arteries. These data are used to identify regions with greatest levels of shear stress alterations, prone to stenosis. Vascular geometry and flow hemodynamics significantly affect endothelial cell viability, proliferation, alignment, microcapillary formation, and metabolic bioprofiles. These integrated in vitro-in silico methods establish a unique platform to study complex cardiovascular diseases and can lead to direct clinical improvements in surgical planning for diseases of disturbed flow.

Continuing preliminary data submitted last year, this paper focuses on effect of operation point on the structure of a tip leakage vortex (TLV) in compressor-like settings. Experiments are being performed at the Johns Hopkins University refractive index-matched facility. The transparent acrylic blades of the one and a half stage compressor have the same geometry, but lower aspect ratio as the inlet guide vanes and the first stage of the Low Speed Axial Compressor facility at NASA Glenn. The refractive index of the liquid, an aqueous NaI solution is matched with that of the blades and transparent casing, facilitating unobstructed stereo-PIV measurements. As the flow rate is reduced close to stall conditions, the leakage flow is confined to rotor chordwise sections further towards the leading edge, and the TLV rollup occurs further upstream, and more radially inward. However, as the leakage flow stops in the aft part of the passage, the near-stall TLV migrates faster to the PS side of the next blade. Instantaneous realizations demonstrate that the TLV consists of multiple interlaced vortices and never rolls up into a single structure, but when phased-averaged, it appears as single structure. The circumferential velocity peak is located radially inward of the mean vorticity center. Turbulent kinetic energy (TKE) is high in the TLV center, in the shear layer connecting the suction side (SS) corner to the TLV feeding vorticity into it, as well as in the region of flow separation on the endwall casing where the leakage flow meets the passage flow. The normal and shear Reynolds stress demonstrate high inhomogeneity and anisotropy, with the streamwise velocity fluctuations being the largest contributor to TKE. The dominant inplane contributors to TKE production rate involve contraction in the region of endwall casing separation and near the SS tip corner, and shear production in the shear layer. Fragmentation and rapid growth of the TLV occurs at mid passage, moving upstream with decreasing flow rate.