Assessment of a common nonlinear eddy-viscosity turbulence model in capturing laminarization in mixed convection flows

Laminarization is an important topic in heat transfer and turbulence modeling. Recent studies have demonstrated that several well-known turbulence models failed to provide accurate prediction when applied to mixed convection flows with significant re-laminarization effects. One of those models, a well-validated cubic nonlinear eddy-viscosity model, was observed to miss this feature entirely. This paper studies the reasons behind this failure by providing a detailed comparison with the baseline Launder–Sharma model. The difference is attributed to the method of near-wall damping. A range of tests have been conducted and two noteworthy findings are reported for the case of flow re-laminarization

Left Ventricular Assist Devices: Impact of Flow Ratios on the Localisation of Cardiovascular Diseases Using Computational Fluid Dynamics

AbstractThe use of Left Ventricular Assistive Devices (LVADs) is increasing for people with heart failure. The present computational fluid dynamics study provides insight into the significance of the flow ratio between the cannula and the aortic root on the prediction of lesion localisation in a typical LVAD configuration, which in turn affects the design and manufacture of these devices. Three cases were studied with varying percentages of flow through the two inlets; the cannula inlet was assumed steady-state, whilst the aortic root inlet had a scaled pulsatile profile, representative of the extent of heart failure. Results suggest that as the flow exiting the heart decreases in velocity, if not orientated properly, the jet exiting the cannula graft can ‘obstruct’ flow to the innominate, common carotid and left subclavian arteries. Therefore, patients with less severe heart failure will generally experience relatively good perfusion of these arteries. However, for the more severe cases of heart failure, the orientation of the cannula graft should be such that adequate perfusion to the aforementioned arteries is maintained

Experimental Techniques against RANS Method in a Fully Developed Turbulent Pipe Flow:Evolution of Experimental and Computational Methods for the Study of Turbulence

Fully developed turbulent flow in a pipe was studied by considering experimental and computational methods. The aim of this work was to build on the legacy of the University of Manchester, which is widely regarded as the birthplace of turbulence due to the pioneering work of the prominent academic Professor Osborne Reynolds (1842–1912), by capturing the evolution of fluid turbulence analysis tools over the last 100 years. A classical experimental apparatus was used to measure the mean velocity field and wall shear stress through four historical techniques: static pressure drop; mean square signals measured from a hot-wire; Preston tube; and Clauser plot. Computational Fluid Dynamics (CFD) was used to simulate the pipe flow, utilizing the Reynolds-averaged Navier–Stokes (RANS) method with different two-equation turbulence models. The performance of each approach was assessed to compare the experimental and computational methods. This comparison revealed that the numerical results produced a close agreement with the experiments. The finding shows that, in some cases, CFD simulations could be used as alternative or complementary methods to experimental techniques for analyzing fully developed turbulent pipe flow

Flow Control Techniques for Enhancing the Bio-Recognition Performance of Microfluidic-Integrated Biosensors

Biosensors are favored devices for the fast and cost-effective detection of biological species without the need for laboratories. Microfluidic integration with biosensors has advanced their capabilities in selectivity, sensitivity, controllability, and conducting multiple binding assays simultaneously. Despite all the improvements, their design and fabrication are still challenging and time-consuming. The current study aims to enhance microfluidic-integrated biosensors’ performance. Three different functional designs are presented with both active (with the help of electroosmotic flow) and passive (geometry optimization) methods. For validation and further studies, these solutions are applied to an experimental setup for DNA hybridization. The numerical results for the original case have been validated with the experimental data from previous literature. Convection, diffusion, migration, and hybridization of DNA strands during the hybridization process have been simulated with finite element method (FEM) in 3D. Based on the results, increasing the velocity on top of the functionalized surface, by reducing the thickness of the microchamber in that area, would increase the speed of surface coverage by up to 62%. An active flow control with the help of electric field would increase this speed by 32%. In addition, other essential parameters in the fabrication of the microchamber, such as changes in pressure and bulk concentration, have been studied. The suggested designs are simple, applicable and cost-effective, and would not add extra challenges to the fabrication process. Overall, the effect of the geometry of the microchamber on the time and effectiveness of biosensors is inevitable. More studies on the geometry optimization of the microchamber and position of the electrodes using machine learning methods would be beneficial in future works

Pore-scale Conjugate Heat Transfer Analysis of Turbulent Flow over Stochastic Open-cell Metal Foams

Fundamental understanding of turbulent flow and heat transfer in composite porous-fluid systems, which consists of a fluid-saturated stochastic open-cell metal foam and a flow passing over it, is crucial for fostering technological development in numerous applications such as transpiration cooling in aerospace, packed-bed thermal storage and thermal management of electronic devices. In this work, conjugate heat transfer simulations were adopted to explore the turbulent flow and heat transfer features in a composite porous-fluid system at the pore-scale. Simulations were performed to account for the influence of the blockage ratios (i.e., BR = 0.5, 0.8 and 1.0) on pressure drop and heat transfer rate by introducing a new concept called penetration cooling length. Furthermore, the effect of Reynolds numbers (i.e., Re = 1800, 3600 and 7200) at different blockage ratios was investigated in terms of pressure drop, fluid and solid temperatures, interstitial heat transfer coefficient, and flow leakage. Results indicate that for a fixed blockage ratio, as the Reynolds number increases by a factor of 3.0, there is a 14.9-fold increase in the pressure drop and a 2.9-fold increase in the interstitial heat transfer coefficient. Additionally, for a fixed Reynolds number, when the blockage ratio increases by a factor of 2.0, there is a 6.8-fold increase in the pressure drop and a 1.8-fold increase in the interstitial heat transfer coefficient. Flow visualisation indicated that the penetration cooling length is influenced by flow leakage from the porous-fluid interface. A correlation of IHTC is proposed based Reynolds number, blockage ratio and development length of the metal foam. Results show at small blockage ratios and low Reynolds numbers, a significant portion of the flow from the porous region leaves it to the clear region on top of the porous block. While, at high Reynolds numbers and large blockage ratios, the flow leakage is reduced. Additionally, for a low blockage ratio (BR<0.5), the amount of flow leakage depends on the Reynolds number, while it is independent of the Reynolds number for BR>0.8

Impact of heart failure severity on ventricular assist device haemodynamics: a computational study

From Springer Nature via Jisc Publications RouterHistory: received 2020-04-17, registration 2020-08-24, accepted 2020-08-24, pub-electronic 2020-08-29, online 2020-08-29, pub-print 2020-12Publication status: PublishedFunder: University of ManchesterAbstract: Purpose: This computational fluid dynamics study investigates the necessity of incorporating heart failure severity in the preoperative planning of left ventricular assist device (LVAD) configurations, as it is often omitted from studies on LVAD performance. Methods: A parametric study was conducted examining a common range of LVAD to aortic root flow ratios (LVAD/AR-FR). A normal aortic root waveform was scaled by 5–30% in increments of 5% to represent the common range of flow pumped by the left ventricle for different levels of heart failure. A constant flow rate from the cannula compensated for the severity of heart failure in order to maintain normal total aortic flow rate. Results: The results show that LVAD/AR-FR can have a significant but irregular impact on the perfusion and shear stress-related haemodynamic parameters of the subclavian and carotid arteries. Furthermore, it is found that a larger portion of the flow is directed towards the thoracic aorta at the expense of the carotid and subclavian arteries, regardless of LVAD/AR-FR. Conclusion: The irregular behaviour found in the subclavian and carotid arteries highlights the necessity of including the LVAD/AR-FR in the preoperative planning of an LVAD configuration, in order to accurately improve the effects on the cardiovascular system post implantation

Thermal-hydraulic analysis of gas-cooled reactor core flows

In this thesis a numerical study has been undertaken to investigate turbulent flow and heat transfer in a number of flow problems, representing the gas-cooled reactor core flows. The first part of the research consisted of a meticulous assessment of various advanced RANS models of fluid turbulence against experimental and numerical data for buoyancy-modified mixed convection flows, such flows being representative of low-flow-rate flows in the cores of nuclear reactors, both presently-operating Advanced Gas-cooled Reactors (AGRs) and proposed ‘Generation IV’ designs. For this part of the project, an in-house code (‘CONVERT’), a commercial CFD package (‘STAR-CD’) and an industrial code (‘Code_Saturne’) were used to generate results. Wide variations in turbulence model performance were identified. Comparison with the DNS data showed that the Launder-Sharma model best captures the phenomenon of heat transfer impairment that occurs in the ascending flow case; v^2-f formulations also performed well. The k-omega-SST model was found to be in the poorest agreement with the data. Cross-code comparison was also carried out and satisfactory agreement was found between the results.The research described above concerned flow in smooth passages; a second distinct contribution made in this thesis concerned the thermal-hydraulic performance of rib-roughened surfaces, these being representative of the fuel elements employed in the UK fleet of AGRs. All computations in this part of the study were undertaken using STAR-CD. This part of the research took four continuous and four discrete design factors into consideration including the effects of rib profile, rib height-to-channel height ratio, rib width-to-height ratio, rib pitch-to-height ratio, and Reynolds number. For each design factor, the optimum configuration was identified using the ‘efficiency index’. Through comparison with experimental data, the performance of different RANS turbulence models was also assessed. Of the four models, the v^2-f was found to be in the best agreement with the experimental data as, to a somewhat lesser degree were the results of the k-omega-SST model. The k-epsilon and Suga models, however, performed poorly. Structured and unstructured meshes were also compared, where some discrepancies were found, especially in the heat transfer results. The final stage of the study involved a simulation of a simplified 3-dimensional representation of an AGR fuel element using a 30 degree sector configuration. The v^2-f model was employed and comparison was made against the results of a 2D rib-roughened channel in order to assess the validity and relevance of the precursor 2D simulations of rib-roughened channels. It was shown that although a 2D approach is extremely useful and economical for ‘parametric studies’, it does not provide an accurate representation of a 3D fuel element configuration, especially for the velocity and pressure coefficient distributions, where large discrepancies were found between the results of the 2D channel and azimuthal planes of the 3D configuration.EThOS - Electronic Theses Online ServiceEngineering and Physical Sciences Research CouncilGBUnited Kingdo