The effects of stabilizing and destabilizing longitudinal curvature on the structure of turbulent, two-stream mixing layers

The construction and development of the multi-component traversing system and associated control hardware and software are presented. A hydrogen bubble/laser sheet flow visualization technique was developed to visually study the characteristics of the mixing layers. With this technique large-scale rollers arising from the Taylor-Gortler instability and its interaction with the primary Kelvin-Helmholtz structures can be studied

The influence of physiological flow development on popular wall shear stress metrics in an idealized curved artery

We numerically investigate the influence of flow development on secondary flow patterns and subsequent wall shear stress distributions in a curved artery model, and we compute vascular metrics commonly used to assess variations in blood flow characteristics as it applies to arterial disease. We model a human artery with a simple, rigid 180-degree curved tube with circular cross-section and constant curvature, neglecting effects of taper, torsion and elasticity. High-fidelity numerical results are computed from an in-house discontinuous spectral element flow solver. The flow rate used in this study is physiological. We perform this study using a Newtonian blood-analog fluid subjected to a pulsatile flow with two inflow conditions. The first flow condition is fully developed while the second condition is undeveloped (i.e. uniform). We observe and discuss differences in secondary flow patterns that emerge over the rapid acceleration and deceleration phases of the physiological waveform, and we directly connect the variation in intensity of these secondary flow patterns along the curvature to differences in the wall shear stress metrics for each entrance condition. Results indicate that decreased axial velocities under an undeveloped condition produce less intense secondary flow that, in turn, reduces both the oscillatory and multidirectional nature of the wall shear stress vector, and we link this effect to abnormalities in computed stress metrics. These results suggest potentially lower prevalence of disease in curvatures where entrance flow is rather undeveloped-a physiologically relevant result to further understand the influence of blood flow development on disease.Comment: 10 pages, 10 figures. arXiv admin note: text overlap with arXiv:2108.0237

Cyber-fluid dynamics

Issued as final reportNational Science Foundation (U.S.

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

Self-Oscillating Vocal Fold Model Mechanics Associated with Aging

From infancy to old age, humans depend on their ability to voice speech in order to effectively communicate their needs. Voiced speech is produced when a critical lung pressure is achieved, forcing air through two bands of tissue stretched across the airway, known as the vocal folds (VFs). The aerodynamic forces then impart energy to the tissues of the VFs and induce self-sustained oscillations. Growths on the VF surfaces, such as nodules and polyps, can result from the repeated and prolonged collision between the tissues of opposing VFs, and can be devastating to one’s daily life. Approximately 30% of people will suffer from a voice disorder at some point in their lives with this probability doubling for those who rely heavily on their voice for work, such as teachers and singers. Further, by 2060, there is expected to be 92 million adults over the age of 64. As this large segment of the population grows, the incidence of voice disorders is expected to be between 12-35%. The objective of this research is to study and improve synthetic VF models to replicate physiological VF motion and characteristic parameters of human speech and relate that to aging. This study includes clinical aerodynamic measures from healthy young men as well as healthy aged men to determine whether the various VF models may be differentially valuable for study of non-normative populations. Synthetic, self-oscillating, and geometrically idealized multi-layer VF models are fabricated from Smooth-On EcoFlex silicone to exhibit material properties representative of the different layers of human VFs. Two-layer VF models are fabricated with cover layers of varying moduli of elasticities to mimic loss of muscle tone associated with aging. The VF models are evaluated experimentally in a vocal tract simulator to replicate physiological conditions. The fabricated silicone synthetic VF models possessed modulus of elasticity values within the range of physiological values, and exhibited good repeatability in our experiments. Further, the two-layer pathological VF models exhibited mean speed quotient values within the range of healthy aged voice physiological values, indicating that silicone is an acceptable material for modeling aging VFs. This study integrates speech science with engineering and flow physics and extends the use of synthetic VF models by assessing their ability to replicated behaviors observed in human subject data to advance a means of investigating voice production as well as the vocal changes associated with aging

In Vitro Hydrodynamic Investigation of Polyvinyl Alcohol Scaffolds for Drug Delivery

In the field of tissue engineering, rapidly-prototyped polymer scaffolds are being researched for allografting and in vivo, ultra-fast drug release applications [1]. Degradation of these scaffolds must match patient recovery time. General degradation of polymers occurs in two stages: polymeric matrix swelling and structural failure [2]. Accordingly, the objective of this study is to monitor the degradation, diffusion, and transport of polyvinyl alcohol (PVA) scaffolds in an alkaline medium. The diffusion coefficient of diluted PVA was measured using a Polson cell apparatus and UV spectrophotometer [3]. A Beer’s Law calibration curve was found using known concentrations of dilute PVA and their absorption at 275 nm wavelength light. The base concentrations were then inserted into the bottom section of the Polson Cell while deionized (DI) water was inserted into the top as the solvent. The Polson Cell-sections were aligned for certain time durations and then offset. The absorption at 275 nm wavelength light of the solution from the top section was found. The diffused concentrations were determined using the absorbance values and the Beer’s Law calibration curve. Using these diffused concentrations, the diffusion coefficient for a base concentration of 50 mg/mL was found to be 0.6096 × 10-5 cm^2/s (± 4.3e-06 at ~ 25 °C), analytically. The dynamic degradation was studied in a lab-scale, curved artery-based flow loop system with steady and unsteady flow conditions for PVA scaffold geometries of 20%, 40%, and 60% infill. The unsteady flow rate was modeled with a carotid artery-based pulsatile flow rate waveform. Two microelectromechanical systems-based (MEMS) were used to measure the pressure differential across the scaffold in the flow loop and the degradation of the PVA-scaffolds was monitored. All experiments were performed at room temperature (~ 25 °C ± 1 °C) with deionized (DI) water as the working fluid and pre-wetted scaffolds to ensure homogeneity. The results have tremendous potential to impact our understanding of drug-release and transport in clinically-relevant scenarios. References [1] D. W. Hutmacher, and S. Cool, Concepts of scaffold-based tissue engineering – the rationale to use solid free-form fabrication techniques. J. Cell. Mol. Med. Vol 11.4, pp 654-669, 2007. [2] Y. Fu, and W. J. Kao, Drug release kinetics and transport mechanisms of nondegradable and degradable polymeric delivery systems. Expert Opin Drug Deliv. Vol 7.4, pp 429-444, 2010. [3] A. Polson, A new method for measuring diffusion constants of biologically active substances. Nature Vol 154 pp 823, Dec. 1944

3D printing of novel osteochondral scaffolds with graded microstructure

Osteochondral tissue has a complex graded structure where biological, physiological, and mechanical properties vary significantly over the full thickness spanning from the subchondral bone region beneath the joint surface to the hyaline cartilage region at the joint surface. This presents a significant challenge for tissue-engineered structures addressing osteochondral defects. Fused deposition modeling (FDM) 3D bioprinters present a unique solution to this problem. The objective of this study is to use FDM-based 3D bioprinting and nanocrystalline hydroxyapatite for improved bone marrow human mesenchymal stem cell (hMSC) adhesion, growth, and osteochondral differentiation. FDM printing parameters can be tuned through computer aided design and computer numerical control software to manipulate scaffold geometries in ways that are beneficial to mechanical performance without hindering cellular behavior. Additionally, the ability to fine-tune 3D printed scaffolds increases further through our investment casting procedure which facilitates the inclusion of nanoparticles with biochemical factors to further elicit desired hMSC differentiation. For this study, FDM was used to print investment-casting molds innovatively designed with varied pore distribution over the full thickness of the scaffold. The mechanical and biological impacts of the varied pore distributions were compared and evaluated to determine the benefits of this physical manipulation. The results indicate that both mechanical properties and cell performance improve in the graded pore structures when compared to homogeneously distributed porous and non-porous structures. Differentiation results indicated successful osteogenic and chondrogenic manipulation in engineered scaffolds. © 2016 IOP Publishing Ltd