Oscillatory Flow Conditions Research Articles

Computational Model for Early-Stage Aortic Valve Calcification Shows Hemodynamic Biomarkers.

Heart disease is a leading cause of mortality, with calcific aortic valve disease (CAVD) being the most prevalent subset. Being able to predict this disease in its early stages is important for monitoring patients before they need aortic valve replacement surgery. Thus, this study explored hydrodynamic, mechanical, and hemodynamic differences in healthy and very mildly calcified porcine small intestinal submucosa (PSIS) bioscaffold valves to determine any notable parameters between groups that could, possibly, be used for disease tracking purposes. Three valve groups were tested: raw PSIS as a control and two calcified groups that were seeded with human valvular interstitial and endothelial cells (VICs/VECs) and cultivated in calcifying media. These two calcified groups were cultured in either static or bioreactor-induced oscillatory flow conditions. Hydrodynamic assessments showed metrics were below thresholds associated for even mild calcification. Young's modulus, however, was significantly higher in calcified valves when compared to raw PSIS, indicating the morphological changes to the tissue structure. Fluid-structure interaction (FSI) simulations agreed well with hydrodynamic results and, most notably, showed a significant increase in time-averaged wall shear stress (TAWSS) between raw and calcified groups. We conclude that tracking hemodynamics may be a viable biomarker for early-stage CAVD tracking.

The effect of operating conditions on heat transfer for parallel plate heat exchangers of thermoacoustic standing wave systems

Thermoacoustic systems represent a promising technology for clean cooling and/or power generation. However, understanding of the behavior of oscillatory flow inside the system under high drive ratio operating conditions, at elevated mean pressures, range of frequencies, and high flow amplitudes, remains limited and poses challenges for designing efficient systems with minimal losses. This paper presents a numerical investigation into the flow and heat transfer characteristics across a coupled cold and hot parallel-plate heat exchanger operating under standing-wave type oscillatory flow conditions. The models were developed for various conditions, spanning frequencies from 13 Hz to 100 Hz and mean pressures between 1 and 20 bar, using both nitrogen and helium as working fluids. The study reveals significant phenomena such as heat accumulation and annular effects in both temperature and velocity/vorticity fields. These findings were discussed in the context of their impact on the heat transfer performance of the heat exchanger. Specifically, the emergence of vortices was examined in relation to mean pressure, alongside jet-like velocity profiles and natural convection effects. Furthermore, the paper includes a comparative analysis between the heat transfer model employed in this study and previous works, aiming to validate the findings. This research enhances the understanding of fluid dynamics and heat transfer phenomena within heat exchangers of thermoacoustic devices, thereby contributing to advancements in future design improvements.

Eco-Designing Cosmetic Products while Preserving the Sensorial-Application Properties: An Instrumental Approach toward Sustainable Formulations

Driven by growing environmental concerns and regulations, cosmetic companies are seeking reliable methods to promptly assess the possibility of replacing high-impact ingredients with sustainable alternatives. In this work, we exploited rheological and texture analyses to evaluate the possibility of using natural and biodegradable raw materials for reformulating three commercial oil-in-water skin care emulsions from an eco-design perspective. Synthetic texturizers, like nylon-12 and PMMA, were replaced with starch, maltodextrin, and silica, while acrylic rheological modifiers were substituted with polysaccharide associations of sclerotium gum, xanthan gum, diutan gum, and carrageenan. Plant-based emollients and a biodegradable elastomer were used as alternatives to silicone oils. The flow and viscoelastic properties of the samples were characterized using rheological tests under continuous and oscillatory flow conditions. The immersion/de-immersion texture analysis allowed us to measure the mechanical properties of firmness, adhesiveness, and stringiness. A double-blind sensory test assessed the products’ application and sensory characteristics. The results revealed that rheology and texture analysis are complementary and correlated techniques, useful for predicting cosmetics’ sensory characteristics. While perfect replication of the original formulas might not be achievable, this protocol can aid formulators in selecting new eco-friendly ingredients ensuring the products’ desired application and sensory properties without compromising consumer experience.

Abstract TP181: Identification Of Hub Genes In Endothelial Cells Under Oscillatory Flow

Objective: Abnormal blood flow can influence gene expression of endothelial cells. Atherosclerotic plaques are more likely to form at the arterial bifurcation or the downstream of stenosis with low or oscillatory wall shear stress (WSS). However, the mechanism remains elusive. This study aims to explore the hub genes and the transcription factors (TF) regulating endothelial gene expression under oscillatory flow through bioinformatics. Methods: Microarray expression profile (GSE16706) was obtained from the Gene Expression Omnibus database to explore differentially expressed genes (DEGs) of human umbilical vein endothelial cells (HUVECs) under oscillatory flow (time-average 1 dyne/cm2; range: -11 - +11 dynes/cm2; 1 Hz). Enrichment analysis was conducted to infer the function of DEGs. Protein-protein interaction network was also plotted to further explore hub genes. TF for hub genes were predicted through NetworkAnalyst. Results: 303 DEGs, with 219 up-regulated genes and 84 down-regulated genes, were identified by comparing gene expression of HUVECs under oscillatory flow or static condition. 4 hub genes (CCNA2, CDC45, BUB1B, KIF20A) related to cell cycle were verified in GSE83476. ELF1 which could regulate all these 4 hub genes was selected as a key TF. Conclusion: We found 4 hub genes and 1 TF associated with oscillatory flow treated HUVECs, which might also be involved in oscillatory flow induced atherosclerosis.

A Dimensional optimization study on cyclone performance under the oscillating boundary condition

In this study, the effect of changing the diameter of the vortex finder, cone and hydraulic inlet diameter of the cyclone under oscillating flow conditions is compared to under uniform flow. In using cyclones as an inlet air filter for the internal combustion engine, the cyclone flow cannot be considered ideally uniform, and flow oscillations must be applied to the boundaries. Also, optimizing the dimensions of the cyclone can provide the necessary space for other uses in the engine space. Numerical solution validation has been performed and seven cyclone models have been created. Then, the effect of geometry variation on the flow field is investigated by examining the frequency and power of active vortices in the vortex finder, cylinder and cone zones. Also, the cyclone performance parameters, including Euler number and efficiency, have been investigated for different models, and correlations have been proposed to estimate them under non-uniform flow. Multi-objective optimization is performed using the genetic algorithm for introducing the most optimal cyclone geometry. Under non-uniform flow conditions, increasing the vortex finder and cone diameter by 50% and decreasing the hydraulic inlet diameter by 40% reduce the Euler's number by 52 to 62%.

Stability and Application Properties of Surfactant-Free Cosmetic Emulsions: An Instrumental Approach to Evaluate Their Potential

Technological innovation in the cosmetic field must necessarily consider not only the safety and efficacy requirements but also the growing attention to environmental issues and the need for sensory pleasure during application to satisfy the consumers’ expectations. This work aimed to formulate an oil-in-water fluid emulsion stabilized by the combination of organic and inorganic powders, namely Zea mays starch and zinc oxide associated at different ratios in presence of a polysaccharidic rheological modifier. After verifying the physical stability of the prototypes with a mechanical stress test in a centrifuge, the rheological analyses were conducted in continuous and oscillatory flow conditions, and the immersion/de-immersion test, conducted using a texture analyzer, allowed identifying the system with the viscoelastic characteristics and the parameters of textures, such as firmness, consistency, and adhesiveness, more suitable for the formulation of skin-care products with low viscosity and high spreadability. Finally, we studied the compatibility of the powders with sunscreen filters and pigments, to optimize the systems and assess their potential use in finished products.

A coupled flow and beam model for fluid–slender body interaction

In this study, a coupled numerical model for simulation of 3D flow interacting with a slender body is proposed. A discrete-forcing immersed boundary method is implemented to couple the fluid motion and structure deformation. To model the dynamic response of a slender structure, an efficient nonlinear beam model is developed. A new surface reconstruction method is proposed to accurately represent the occupation of the moving structure with different cross-sections in the fixed Cartesian grids of the fluid domain. A quadratic interpolation based on the least-squares method is proposed to realize the local fluid velocity near the relocated solid surface. To validate the model, several fluid–structure interaction problems including a flexible plate attached to a circular cylinder in a flow, a flexible flag in a flow, etc. are simulated. The numerical results are in good agreement with reference results from either experiments or numerical modelling. The model is further employed to investigate the vortex-induced vibration of a flexible riser under unidirectional and oscillatory flow conditions as well as impulse force analysis for a swinging carangiform swimmer in different swing frequencies, the latter has a varying cross-sectional shape along the body’s central axis.

Morphodynamics of vortex ripple creation under constant and changing oscillatory flow conditions

The morphodynamics of orbital vortex ripples due to oscillatory flow and induced sediment transport was studied numerically. The methodology was based on large-eddy simulations of the full coupling of turbulent oscillatory fluid flow, bed and suspended sediment transport, and bed evolution. The Immersed Boundary method was implemented for the imposition of fluid and sediment boundary conditions on the mobile bed surface. Results are presented for ripple creation starting from flat or rippled beds, as well as results of ripples adapting to changing flow (wave) direction. The computed equilibrium ripple dimensions are in agreement with existing empirical predictions; however, the computed ripple profile is not as steep in the vicinity of the ripple crest as the analytical one in Sleath (1984). The time needed for the bed geometry to reach equilibrium, in cases where the initial bed is almost flat or it is formed by ripples that shorten towards equilibrium, is longer than in cases where the initial bed is formed by ripples that widen towards equilibrium. Moreover, the ripple crests are reoriented when the flow changes direction by 45°; the new equilibrium ripple crestline is perpendicular to the flow direction. Finally, the numerical results are compared with corresponding results of flow above fixed ripples. The bed load flux does not present substantial differences between the fixed and mobile bed cases. On the contrary, a strong increase of the suspended load flux is observed for the mobile bed case in comparison to the fixed bed case. • LES Coupling between oscillatory flow, sediment transport, and bed morphodynamics. • Same flow forcing results into same bed equilibrium regardless of initial bed form. • Evolution of ripples longer than equilibrium ones is slower than for shorter ones. • Ripples are reoriented perpendicular to the flow direction. • Suspended load over mobile rippled bed is larger than over fixed ripples.

Experimental Evaluation of Two-Phase Heat Transfer Coefficient Under Oscillatory Flow Conditions and Correlation Development

Abstract Natural circulation boiling water reactors (NCBWRs) are prone to flow instabilities. Due to instabilities, heat transfer deteriorates. However, the extent to which the heat transfer coefficient (HTC) deteriorates is not well established. In this work, experiments are conducted under controlled flow oscillations to evaluate the impact of the time period, mean mass flux, and amplitude ratio on HTC. Existing correlations for HTC are compared with experimental results and are found to be not satisfactory. To improve the predictions of HTC, correction factors (CFs) are proposed for existing correlations. Further, an improved HTC correlation has been developed, incorporating new nondimensional numbers.

Heat Transfer for the Oscillatory Flow of Thermoacoustics across an in-Line Tube-Banks Heat-Exchanger

Heat exchangers are an important component for many energy devices including those that operate under oscillatory flow conditions. Unfortunately, the oscillatory flow is less understood leading to difficulties in predicting the performance of energy systems. This paper reports an investigation based on heated in-line tube-banks that are placed inside an oscillatory flow condition with thermoacoustic settings at a resonance frequency of 14.2 Hz. The rig contains air at atmospheric pressure that travels as a standing-wave inside a quarter wavelength resonator. Changes in velocity and temperature in oscillatory flow with drive ratios between 0.45% and 3.20% are discussed. Discussions are supported by results from validated two-dimensional models solved using a two-equation turbulence model with boundary conditions that matched the experimental conditions. Changes in oscillatory flow dynamics and temperature are presented. The heat transfer performance was presented by the Colburn-j factor but with a comparison of results between two different definitions of temperature drop. This study found that the heat transfer performance might be underpredicted if it was calculated using the conventional definition of temperature difference. This finding should be considered and explored further in any applications related to heat transfer for oscillatory flow conditions especially those related to the thermoacoustic environment.

A novel model for macroscopic simulation of oscillating heat and fluid flow in porous media

In thermoacoustics, stacks and regenerators are porous media where energy conversion takes place. Modelling full thermoacoustic devices with a CFD approach, in order to capture some nonlinearities, can be extremely expensive from a computational perspective compared to a standard linear approach used in the frequency domain. At the same time, macroscopic models for porous media developed for steady-state flows cannot be directly applied in oscillating flow conditions. Moreover, macroscopic models in the available literature for oscillating flows are inaccurate at high frequencies or require a closure coefficient to be determined numerically (with Direct Numerical Simulations) or experimentally. In this article, a time domain macroscopic model for heat and fluid flow is proposed based on the concepts of complex Darcy and Nusselt numbers in the linear regime. Such coefficients, introduced in the past to describe the oscillatory phenomena, have been used for the first time to build a CFD macroscopic model in terms of their real and imaginary parts. For two different porous media, a parallel plate and a transversal pin array, the developed macroscopic model is verified with the microscopic solution. Furthermore, for a transversal pin array stack, the proposed model is validated against experimental data from the available literature, showing a very good agreement. The findings of this paper can help to strongly reduce the computational costs of oscillatory flow simulations without prior direct numerical simulations of the porous core.

Computational Fluid Dynamics Investigation of Flow in Scour Protection Around a Mono-Pile With the Volume-Averaged k-ω Model

Abstract The study numerically investigates the flow behavior around a mono-pile with scour protection under steady and oscillatory flow conditions. A hydrodynamic model based on volume-averaged Reynolds-averaged Navier–Stokes (VARANS) equations with the volume-averaged k-ω turbulence closure is developed and implemented in openfoam. Three porosity transition types, i.e., constant, linear and parabolic, near the interface between stone cover and free flow are first evaluated in two-dimensional models. The simulated results, i.e., flow velocities, turbulence levels and bed shear stresses, are compared with previous experiments under steady and oscillatory flow conditions. The parabolic transition shows the best agreement with the measurements and is therefore used in the developed model. Under steady current, a three-dimensional model is validated against experimental measurements including flow features both inside and outside of the scour protection around a mono-pile, and it exhibits relatively good performance. Further, the volume-averaged k-ω model shows better agreement to experiments in porous medium compared to results from k-ω shear stress transport and volume-averaged k-ɛ models. The model is applied to investigate the flow patterns under the oscillatory flow condition. The results show that a horseshoe vortex is formed, and it penetrates the entire scour protection, which generates high flow velocities and bed shear stresses; erosion is most likely to occur at the area in the presence of vortex, which poses a threat to the pile stability. The simulations demonstrate the ability of the developed model to evaluate the flow behavior in scour protection.

Design Optimization of Tubular Heat Exchangers for a Free-Piston Stirling Engine Based on Improved Quasi-Steady Flow Thermodynamic Model Predictions

This paper presents the design optimization of a heat exchanger for a free-piston Stirling engine (FPSE) through an improved quasi-steady flow (iQSF) model and a central composite design. To optimize the tubular hot heat exchanger (HHX) design, a design set of central composite designs for the design factors of the HHX was constructed and the brake power and efficiency were predicted through the iQSF model. The iQSF model is improved because it adds various heat and power losses based on the QSF model and applies a heat transfer model that simulates the oscillating flow condition of an actual Stirling engine. Based on experimental results from the RE-1000, an FPSE developed by Sunpower, the iQSF model significantly improves the prediction error of the indicated power from 66.9 to 24.9% compared to the existing QSF model. For design optimization of the HHX, the inner diameter and the number of tubes with the highest brake power and efficiency were determined using a regression model, and the tube length was determined using the iQSF model. Finally, the brake output and efficiency of FPSE with the optimized HHX were predicted to be 7.4 kW and 36.4%, respectively, through the iQSF analysis results.

The effect of varying degrees of stenosis on transition to turbulence in oscillatory flows

Many complications in physiology are associated with a deviation in flow in arteries due to a stenosis. The presence of stenosis may transition the flow to weak turbulence. The degree of stenosis as well as its configuration whether symmetric or non-symmetric to the parent artery influences whether the flow would stay laminar or transition to turbulence. Plenty of research efforts focus on investigating the role of varying degrees of stenosis in the onset of turbulence under steady and pulsatile flow conditions. None of the studies, however, have focused on investigating this under oscillatory flow conditions as flow reversal is a major occurrence in a number of physiologic flows, and is of particular relevance in cerebrospinal fluid flow research. Following up on the previous work in which a 75% stenosis was studied, this contribution is a detailed investigation of the role of degrees of stenosis on transition in an oscillatory flow. A cylindrical pipe with 25%, 50% and 60% reductions in area in axisymmetric and eccentric configurations is studied for transition with 3 different pulsation frequencies of a purely oscillatory flow. Cycle averaged Reynolds numbers between 1800 and 2100 in steps of 100 are studied for each configuration resulting in 72 simulations each conducted on 76,800 CPU cores of a modern supercomputer. It is found that a higher degree of stenosis and eccentricity causes earlier transition to turbulence in oscillatory flow. The results further demonstrate that a higher frequency of oscillation results in larger hydrodynamic instability in the flow, which is more prominent in smaller degrees of stenosis.

Loss of intermediate‐flow states only evident when considering sub‐daily flow metrics in a major tributary of the Limpopo basin

AbstractAs the impacts of the anthropocene intensifies, there is an increasing need to understand how these changes affect both daily and sub‐daily stream flow variability, timing and flow quantities, as these are some of the most influential drivers of spatial and temporal dynamics of stream biota. In this paper, long‐term changes in flow patterns of a strategic water source area (Luvuvhu catchment) in an arid region of southern Africa were quantified, focusing on the relation between daily and sub‐daily flow and its potential impact on fish biota of the catchment. Long‐term temporal trends in stream flow were modelled using generalized least squares (GLS), while sub‐daily and daily mean flow of the same stations were compared using a suite of metrics. Periods of similar stream flow patterns were identified using K‐means cluster analysis. A spreadsheet rule‐based model was developed linking fish communities to streamflow patterns, providing a predictive framework for fish assemblage responses to stream flow classes. Long‐term reduction in flow in the Luvuvhu catchment has a strong seasonal component, with significant decreases during the wet season, not linked to long‐term rainfall patterns. The flow regime of the Luvuvhu river system has become more variable over time. Several sub‐daily flow metrics were positively related to daily flow metrics. Oscillating flow conditions and the loss of intermediate‐flow states may permanently exclude certain fish flow guilds. However, temporal partitioning is only evident when sub‐daily metrics are considered, highlighting their importance for assessing ecological resilience.

Experimental investigation of dryout phenomena under oscillatory flow conditions

In water-cooled nuclear reactors, the maximum power which can be extracted from the core is limited by critical heat flux (CHF). CHF in the high-quality region is known as dryout. In advanced nuclear reactors, the coolant flow occurs solely by virtue of natural circulation; however, instabilities may occur during off-normal operations. This may lead to premature dryout due to lower coolant flow rates seen by the heater during such oscillations. This paper describes the experimental investigation on the effect of flow oscillations on the CHF with the time period of 120 s, which is observed typically in the large-scale natural circulation system. Based on observations made with respect to temperature transient, the continuous dryout is preceded by the transient dryout for higher flow oscillations. But as flow fluctuation decreases, the transient dryout phenomenon is found to disappear. The applicability of the look-up table to predict CHF under oscillatory flow conditions using suitable correction factors (CFs) for premature dryout has been evaluated. CFs for the CHF under oscillations suggested by previous authors have been compared. The maximum possible degradation in CHF value suggested by previous authors has been found to agree with the present experimental data. Percentage fluctuation in heat transfer coefficient (HTC) at fully developed annular flow conditions has been evaluated, and it is found that fluctuation in HTC is in phase with the fluctuation in flow.

Numerical study on a thermoacoustic refrigerator with continuous and staggered arrangements

Thermoacoustic devices require heat exchangers with oscillating flow, but there is currently no viable design approach for them. A heat exchanger with a staggered structure can efficiently improve the velocity disturbance and promote heat transfer in steady flow. The flow and heat transfer characteristics of a standing-wave thermoacoustic refrigerator and an ambient heat exchanger with staggered parallel plates under the oscillating flow condition are investigated in this study, primarily focusing on the geometric influences and differences between staggered and non-staggered (continuous) arrangements. The CFD simulation is a mainstream tool for the numerical simulation of complex thermoacoustic phe?nomena. The flow field around the stack and heat exchanger plate is simulated by introducing the dynamic mesh boundary conditions. Through numerical simulation, the flow field characteristics of non-linear vortices generation around the heat exchanger are presented. By changing the staggered column number in the ambient heat exchanger, it is observed that the larger the column number of staggered parallel plates, the more significant the refrigeration effect through the thermoacoustic effect.

Study of the wave period effect on the time-averaged suspended sediment concentration distribution under the oscillatory sheet flow condition

ABSTRACT Existing sheet flow experimental data show the measured time-averaged suspended sediment concentration (TSSC) may increase, decrease, or even remain constant if the wave period decreases. With respect to the available sheet flow experimental data, it is confirmed that the TSSC of relatively fine sand undergoes three stages, i.e. first decreases, then increases, and finally decreases again in the case that the wave period decreases from 12 s to 2 s, whereas for the relatively coarse sand, its vertical profile remains almost unchanged. The criterion for identifying the relatively fine or coarse sand was proposed in terms of the nondimensional ratio between the settling velocity and the root-mean-square flow velocity. If this ratio is less than 0.04–0.043, it is deemed as relatively fine sand. Otherwise, it is classified as relatively coarse sand. Applying the classical gradient diffusion model, sediment diffusivity is confirmed to be insensitive to the wave period. Keeping other experimental conditions uniform, TSSCs with different wave periods are proportional to each other and only affected by the reference concentration. Subsequently, a simple quantitative expression considering the wave period effect on the TSSC under the oscillatory sheet flow regime was proposed and verified with the experimental data.

Effect of oscillatory flow conditions on crystalliser fouling investigated through non-invasive imaging

Conditions that lead to undesired fouling events in oscillatory flow crystallisers are investigated. The moving fluid oscillatory baffled crystalliser is used to mimic the operating conditions of continuous oscillatory baffled crystallisers, while reducing materials and energy consumption. A non-invasive imaging method is deployed to determine fouling induction times as a function of oscillatory flow conditions and supersaturation in crystallisation of glycine and L-glutamic acid from aqueous solutions. Heterogeneous nucleation kinetics are extracted from the distribution of fouling induction times, showing that higher fouling nucleation rates are observed when the frequency and amplitude of the oscillatory flow are increased. Higher oscillatory Reynolds numbers result in increased fluid shear in the crystalliser, promoting heterogeneous nucleation at the glass-solution interface and leading to subsequent fouling. It is therefore essential to consider the dependence of fouling kinetics on operating conditions to enable rational design of continuous crystallisers that ensure smooth and robust operation.

Numerical Investigation of Scour Beneath Pipelines Subjected to an Oscillatory Flow Condition

The present study carries out two-dimensional numerical simulations to investigate scour beneath a single pipeline and piggyback pipelines subjected to an oscillatory flow condition at a Keulegan–Carpenter (KC) number of 11 using SedFoam (an open-source, multi-dimensional Eulerian two-phase solver for sediment transport based on OpenFOAM). The turbulence flow is resolved using the two-phase modified k−ω 2006 model. The particle stresses due to the binary collisions and enduring contacts among the sediments are modeled using the rheology model of granular flow. The present numerical model is validated for the scour beneath a single pipeline, and the simulated sediment profiles are compared with published experimental data and numerical simulation results. The scour process beneath three different piggyback pipelines under the same flow condition are also considered, and the scour development and surrounding flow patterns are discussed in detail. Typical steady-streaming structures around the pipeline due to the oscillatory flow condition are captured. The scour depth during the initial development of the scour process for the piggyback pipeline with the small pipeline placed above the large one is the largest among all the investigated configurations. The phase-averaged flow fields show that the flow patterns are influenced by the additional small pipeline.