Kartik V. Bulusu

A synergistic approach to the design, fabrication and evaluation of 3D printed micro and nano featured scaffolds for vascularized bone tissue repair.

3D bioprinting has begun to show great promise in advancing the development of functional tissue/organ replacements. However, to realize the true potential of 3D bioprinted tissues for clinical use requires the fabrication of an interconnected and effective vascular network. Solving this challenge is critical, as human tissue relies on an adequate network of blood vessels to transport oxygen, nutrients, other chemicals, biological factors and waste, in and out of the tissue. Here, we have successfully designed and printed a series of novel 3D bone scaffolds with both bone formation supporting structures and highly interconnected 3D microvascular mimicking channels, for efficient and enhanced osteogenic bone regeneration as well as vascular cell growth. Using a chemical functionalization process, we have conjugated our samples with nano hydroxyapatite (nHA), for the creation of novel micro and nano featured devices for vascularized bone growth. We evaluated our scaffolds with mechanical testing, hydrodynamic measurements and in vitro human mesenchymal stem cell (hMSC) adhesion (4 h), proliferation (1, 3 and 5 d) and osteogenic differentiation (1, 2 and 3 weeks). These tests confirmed bone-like physical properties and vascular-like flow profiles, as well as demonstrated enhanced hMSC adhesion, proliferation and osteogenic differentiation. Additional in vitro experiments with human umbilical vein endothelial cells also demonstrated improved vascular cell growth, migration and organization on micro-nano featured scaffolds.

Non-Newtonian perspectives on pulsatile blood-analog flows in a 180° curved artery model

Complex, unsteady fluid flow phenomena in the arteries arise due to the pulsations of the heart that intermittently pumps the blood to the extremities of the body. The many different flow waveform variations observed throughout the arterial network are a result of this process and a function of the vessel properties. Large scale secondary flow structures are generated throughout the aortic arch and larger branches of the arteries. An experimental 180° curved artery test section with physiological inflow conditions was used to validate the computational methods implemented in this study. Good agreement of the secondary flow structures is obtained between experimental and numerical studies of a Newtonian blood-analog fluid under steady-state and pulsatile, carotid artery flow rate waveforms. Multiple vortical structures, some of opposite rotational sense to Dean vortices, similar to Lyne-type vortices, were observed to form during the systolic portion of the pulse. Computational tools were used to assess the effect of blood-analog fluid rheology (i.e., Newtonian versus non-Newtonian). It is demonstrated that non-Newtonian, blood-analog fluid rheology results in shear layer instabilities that alter the formation of vortical structures during the systolic deceleration and onwards during diastole. Additional vortices not observed in the Newtonian cases appear at the inside and outside of the bend at various times during the pulsation. The influence of blood-analog shear-thinning viscosity decreases mean pressure losses in contrast to the Newtonian blood analog fluid.

Shannon Entropy-Based Wavelet Transform Method for Autonomous Coherent Structure Identification in Fluid Flow Field Data

The coherent secondary flow structures (i.e., swirling motions) in a curved artery model possess a variety of spatio-temporal morphologies and can be encoded over an infinitely-wide range of wavelet scales. Wavelet analysis was applied to the following vorticity fields: (i) a numerically-generated system of Oseen-type vortices for which the theoretical solution is known, used for bench marking and evaluation of the technique; and (ii) experimental two-dimensional, particle image velocimetry data. The mother wavelet, a two-dimensional Ricker wavelet, can be dilated to infinitely large or infinitesimally small scales. We approached the problem of coherent structure detection by means of continuous wavelet transform (CWT) and decomposition (or Shannon) entropy. The main conclusion of this study is that the encoding of coherent secondary flow structures can be achieved by an optimal number of binary digits (or bits) corresponding to an optimal wavelet scale. The optimal wavelet-scale search was driven by a decomposition entropy-based algorithmic approach and led to a threshold-free coherent structure detection method. The method presented in this paper was successfully utilized in the detection of secondary flow structures in three clinically-relevant blood flow scenarios involving the curved artery model under a carotid artery-inspired, pulsatile inflow condition. These scenarios were: (i) a clean curved artery; (ii) stent-implanted curved artery; and (iii) an idealized Type IV stent fracture within the curved artery.

Evaluation of Efficiency in Compressible Flow Ejectors

A steady-flow ejectors are fluid dynamic flow induction devices that directly transfer energy and momentum from a high energy primary fluid to a low energy secondary fluid through the work provided by entrainment and turbulent mixing. A variety of applications from refrigeration systems that are thermally energized and capable of using environmentally friendly refrigerants such as water and air, to desalination of water are currently being explored with this technology. The potential benefit of using environmentally friendly refrigerants makes it extensively useful for commercial air conditioning and refrigeration, particularly in applications where a source of waste heat is readily available. While steady-flow ejectors operate on entrainment and turbulent mixing between the primary (driving) flow, and the secondary (driven) flow, eliminating mechanically moving parts, the turbulent entrainment mechanism itself, is inherently dissipative of energy and little can be done to improve it. The process of mixing, which is irreversible is not accounted for explicitly in the existing definitions of ejector efficiency. Moreover, efficiency of ejectors is based on the concepts of a indirect flow induction viz., turbine-compressor analogy and compressor efficiency, and entrainment ratio. An experimental steady flow ejector with compressed air as the motive fluid and ambient air as suction fluid was fabricated and tested in this study. Comparisons of efficiency are made with the assumption of adiabatic and complete mixing of primary and secondary fluids before fluid discharge from the ejector. An important goal in our research is the definition of a proper measure of ejector efficiency which is appropriate for non-dissipative, non-steady ejectors. Such a definition would enable a more systematic methodology for evaluating ejector performance. There are three proposed methods that will be presented and compared.Copyright

The Influence of Shear Layer Turbulence on Stationary Pseudoblades in Supersonic Pressure Exchange Inducing Flow Fields

The pressure exchange process can be initiated by nonsteady pressure forces that arise due to moving fluid dynamic interfaces in the laboratory frame of reference. The fluid interfaces are flow features of “pseudoblades” that can be generated by an expanding supersonic primary flow, impinging on freely spinning cone-vane type of rotors. These pseudoblades are fluidic vanes that interface with an entrained, compressible secondary fluid and can mimic the action of impellers as in conventional turbomachinery. The overarching goal of this research is the development of a novel fluid impeller-based ejector. The authors’ motivation towards this study was in understanding the boundary conditions leading to spatial deterioration of pseudoblades. Flow around stationary, axisymmetrically aligned rotors (the ramp vane and double cone type), held in a primary supersonic flow field (Mach 1.44 jet), were investigated by laser Doppler velocimetry (LDV) measurements of shear layer turbulence intensity (TI) under alternative seeding of primary and entrained secondary flows. Rotors were tested and compared for shear layer TI distribution-based boundary conditions, anticipated pseudoblade conditions and an “effective persistence length of stationary pseudoblades.” The results suggest that the double cone rotor is most conducive for pseudoblade stability. The TI distribution-based boundary conditions for this rotor indicate that the effective pseudoblade persistence length approximately equals the exit diameter of the supersonic nozzle.

Supersonic Flow Visualization Over Patented Rotors for a Novel Crypto-Steady Pressure Exchange Ejector Using Schlieren Photography

The process of pressure exchange occurs where flows exchange mechanical energy through work of mutually exerted pressure forces at their interfaces. A novel ejector based on the concept of supersonic crypto-steady pressure exchange rather than the more energy dissipative turbulent entrainment phenomenon is being developed. To better understand the flow structures in context of the novel ejector, schlieren photography is being used as a flow visualizaton tool. The crypto-steady mode of pressure exchange can be achieved with rotors that enable the creating of psuedoblades and entrainment gullies by a primary supersonic fluid. The primary fluid can perform work on an entrained subsonic secondary fluid in a non-steady mode of fluid-fluid interaction. The primary fluid jets emanating from the rotor form a helical pattern whereby the secondary fluid becomes entrapped in the interstices of the helices. In the non-rotating case of the rotors, the voids between psuedoblades that create helical structure end up, providing entrainment gullies for the secondary fluid. In the rotating case, work is done by the expanding primary fluid on the secondary fluid by the pressure forces acting across the helical boundary between the two fluids. Crypto-steady pressure-exchange has the potential of providing society with a highly efficient means of compressing a low energy fluid through direct contact with a relatively high energy fluid, thereby circumventing the complexity and the energy dissipation associated with intervening machinery inherent in conventional compressors and turbulent mixing in ejectors. Global entropy can be calculated using steady inviscid two dimensional equations. The paper will report progress made by using schlieren photography on three patented rotors.Copyright

Self-Entrainment Characteristics and Performance of a Compact Air Ejector

A one dimensional method of analysis and experimentally determined entrainment and compression ratios are presented with considerations made for constant pressure and constant area mixing. A set of three nozzles, one converging and two converging diverging, were used to study the isentropic characteristics of the ejector performance. Ejector efficiencies are calculated using the turbomachinery analogue of compressors where adiabatic and complete mixing of primary and secondary fluids is assumed before discharge. Efficiencies are characterized against non-dimensional parameters chosen in context. A generalized algorithm and corresponding MATLAB™ based computer program was developed for performance analysis. While exploring the possibility of a refrigeration system for automotive applications where the size of the ejector could play an important role, a compact experimental ejector was designed and tested.Copyright