Investigating the effects of a non - uniform magnetic field on heat and flow characteristics of a ferrofluid

A numerical study is performed to investigate the effect of a non - uniform magnetic field from a current carrying wire on the ferrofluid flow. The analysis is carried out for a semi circular annulus with three different locations of wire relative to it, by solving coupled set of flow field equations, energy equations and the Maxwell’s magnetostatics equations. Results from the present study offers better insight about the ferrofluid behaviour and heat transfer mechanism. It also explains the dependency of flow distribution on the location of the electric wire and the magnitude of current flowing through it.

Quantification of noncircular stent expansion after TAVR into a pathological annulus and its impact on paravalvular leakage

Patients undergoing transcatheter aortic valve replacement (TAVR) may suffer severe clinical complications, caused by paravalvular leakage (PVL) which is defined as leakage between TAVR and aortic annulus. PVL is often facilitated by a severely calcified annulus. This limits the expansion of a self-expandable TAVR stent. To assess TAVR performance in terms of leakage, measurement of regurgitation fraction in a pathophysiological annulus is recommended according to ISO 5840. For this purpose, a configuration of a circular annulus with a calcification nodule has been proposed in the recently published ISO 5840. The impact of the proposed pathophysiological annulus model on the expansion of self-expandable TAVR stents and on the regurgitation fraction was investigated in this study. For this purpose, two commercially available selfexpandable TAVRs (Evolut R and Portico) were implanted in a calcified annulus model. Circular expansion of the TAVR stents was investigated based on μCT scans of the implanted TAVR. The calcification-induced area in which retrograde flow can occur during diastole was detected. These results were then compared with the experimentally determined regurgitation fraction obtained from pulse duplicator tests. The results of the μCT scans showed a continuous leakage area in the region of the annulus for the Evolut R compared to a locally larger leakage area of the Portico, which, however, reattaches to the annulus in the distal inflow region. The hydrodynamic measurements confirmed a smaller leakage in the pathological annulus for the Portico. In summary, it can be assumed that a continuous leakage area in the TAVR stent inflow region encourages the PVL of TAVR.

Cementing Irregular Horizontal Wellbores

In this work, we study the effect of borehole irregularities during primary cementing of a horizontal section of well. We use a simplified 2D gap-averaged model to compute the displacement of a drilling mud by a spacer within an elliptical annulus that represents an oval irregularity. We also present a series of 3D numerical simulations using a Volume of Fluid method to capture the interface between the fluids. The 3D model allows us to study the effects of more local irregularities such as wall roughness that can be imported from a caliper log. The dynamics of the displacement of two fluids in a horizontal uniform circular annulus is governed by buoyancy, eccentricity and the rheology of the fluids. A positive density difference combined with a slow mean pumping speed promotes slumping of the second fluid towards the bottom of the annulus. Nevertheless, high eccentricity values (e = 1-standoff) are common due to the weight of the casing pulling downwards, opposing the buoyancy force. Finally, the rheology of the fluids is relevant to determine the presence of un-displaced layers of mud, e.g. at the walls. The same competition described above holds true in the elliptical annulus. Results from the 2D gap-averaged model suggest that the elliptical shape incorporates an additional way of altering the velocity field around it. The effect is more evident when orienting the largest radius of the elliptical annulus at different angles. Results from 3D simulations show that the interface follows irregularities and the local roughness can improve the displacement by inducing secondary flows. However, enlargements result in poor displacement.

Dispersion as a waste-clearance mechanism in flow through penetrating perivascular spaces in the brain

Accumulation of metabolic wastes in the brain is correlated with several neurodegenerative disorders, including Alzheimer’s disease. Waste transport and clearance occur via dispersion, the combined effect of diffusion and advection by flow of fluid. We examine the relative contributions of diffusion and advection in the perivascular spaces (PVSs) that surround penetrating cortical blood vessels and are filled with cerebrospinal fluid (CSF). To do so, we adapt prior analytic predictions of dispersion to the context of PVSs. We also perform advection-diffusion simulations in PVS-like geometries with parameters relevant to transport of amyloid-$$\beta$$ β (associated with Alzheimer’s) in a variety of flows, motivated by in vivo measurements. Specifically, we examine solute transport in steady and unsteady Poiseuille flows in an open (not porous) concentric circular annulus. We find that a purely oscillatory flow enhances dispersion only weakly and does not produce significant transport, whereas a steady flow component, even if slow, clears waste more effectively.

Circumferential defect detection using ultrasonic guided waves

Purpose Quantitatively detecting surface defects in a circular annulus with high levels of accuracy and efficiency has been paid more attention by researchers. The purpose of this study is to investigate the theoretical dispersion equations for circumferential guided waves and then develop an efficient technique for accurate reconstruction of defects in pipes. Design/methodology/approach The methodology applied to determine defects in pipelines includes four steps. First, the theoretical work is carried out by developing the appropriate dispersion equations for circumferential guided waves in a pipe. In this phase, formulations of strain-displacement relations are derived in a general equidistant surface coordinate. Following that, a semi-analytical finite element method (SAFEM) is applied to solve the dispersion equations. Then, the scattered fields in a circular annulus are calculated using the developed hybrid finite element method and simulation results are in accord with the law of conservation of energy. Finally, the quantitative detection of Fourier transform (QDFT) approach is further enhanced to efficiently reconstruct the defects in the circular annuli, which have been widely used for engineering applications. Findings Results obtained from four numerical examples of flaw detection problems demonstrate the correctness of the developed QDFT approach in terms of accuracy and efficiency. Reconstruction of circumferential surface defects using the extended QDFT method can be performed without involving the analytical formulations. Therefore, the streamlined process of inspecting surface defects is well established and this leads to the reduced time in practical engineering tests. Originality/value In this paper, the general dispersion equations for circumferential ultrasonic guided waves have been derived using an equidistant surface coordinate and solved by the SAFEM technique to discover the relationship between wavenumber of a wave and its frequency. To reconstruct defects with high levels of accuracy and efficiency, the QDFT approach has been further enhanced to inspect defects in the annular structure.