Pulse wave imaging (PWI) is an ultrasound imaging modality that estimates the wall stiffness of an imaged arterial segment by tracking the pulse wave propagation. The aim of the present study is to ...integrate PWI with vector flow imaging, enabling simultaneous and co-localized mapping of vessel wall mechanical properties and 2-D flow patterns. Two vector flow imaging techniques were implemented using the PWI acquisition sequence: 1) multiangle vector Doppler and 2) a cross-correlation-based vector flow imaging (CC VFI) method. The two vector flow imaging techniques were evaluated in vitro using a vessel phantom with an embedded plaque, along with spatially registered fluid structure interaction (FSI) simulations with the same geometry and inlet flow as the phantom setup. The flow magnitude and vector direction obtained through simulations and phantom experiments were compared in a prestenotic and stenotic segment of the phantom and at five different time frames. In most comparisons, CC VFI provided significantly lower bias or precision than the vector Doppler method (<inline-formula> <tex-math notation="LaTeX">{p} < {0.05} </tex-math></inline-formula>) indicating better performance. In addition, the proposed technique was applied to the carotid arteries of nonatherosclerotic subjects of different ages to investigate the relationship between PWI-derived compliance of the arterial wall and flow velocity in vivo . Spearman's rank-order test revealed positive correlation between compliance and peak flow velocity magnitude (<inline-formula> <tex-math notation="LaTeX">{r}_{s} = {0.90} </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">{p} < {0.001} </tex-math></inline-formula>), while significantly lower compliance (<inline-formula> <tex-math notation="LaTeX">{p} < {0.01} </tex-math></inline-formula>) and lower peak flow velocity magnitude (<inline-formula> <tex-math notation="LaTeX">{p} < {0.001} </tex-math></inline-formula>) were determined in older (54-73 y.o.) compared with young (24-32 y.o.) subjects. Finally, initial feasibility was shown in an atherosclerotic common carotid artery in vivo . The proposed imaging modality successfully provided information on blood flow patterns and arterial wall stiffness and is expected to provide additional insight in studying carotid artery biomechanics, as well as aid in carotid artery disease diagnosis and monitoring.
In order to investigate the effect of impeller tip clearance on the internal flow fields and the hydraulic losses in mixed-flow pump, the entropy production method with computational fluid dynamics ...(CFD) is employed to analyze the energy losses in a low specified number mixed-flow pump with guide vane. The results show that the size of tip clearance is closely related to the external characteristic performance of mixed-flow pump, and the effect of tip clearance on the flow fields of mixed-flow pump is obvious at design flow rate condition. When the tip clearance raises from 0.2 mm to 1.1 mm, the head drop loss coefficient increases 1.62 times in the impeller. As the tip clearance augments from 0.2 mm to the 1.1 mm, the total entropy production in impeller increases by 142%. Whereas, the total entropy production in guide vane descends by 21.8% slightly. It indicates that the increase of tip leakage flow (TLF) may increase the energy losses in impeller but the hydraulic losses in guide vane is suppressed to some extent as a result of an existence of TLF. Therefore, for the sake of improving the energy performance of mixed-flow pump, it is necessary to take the scale of blade tip clearance into account and consider optimizing the hydraulic design structure of guide vanes comprehensively to match the tip clearance.
•Energy losses induced by tip leakage flow are analyzed by entropy production method.•The effect of tip clearance size on head loss of pump is analyzed in detail.•Entropy production in impeller is positively correlated with tip clearance.•Entropy production in guide vane is negatively correlated with tip clearance.•Tip clearance size and structure of guide vane are essential to optimize the pump.
With increasing coverage, density, and accuracy of the global inland water altimetry record, remote sensing observations of water surface elevation (WSE) and water surface slope (WSS) are becoming ...available for the world's rivers. In steady, uniform flows, WSS is invariable, while there is a unique one‐to‐one relationship between WSE and discharge, the rating curve. While the assumptions of steady uniform flow are appropriate for many rivers, they are violated upstream of river confluences. We present a simple analytical hydraulic model of river confluences using the theory of steady, gradually varied flow. We apply the model to four river confluences in the Mississippi‐Missouri river system. We determine the spatial extent of the backwater‐affected zones and map WSE‐discharge and WSS‐discharge relationships. We show that coincident measurements of WSE and WSS from new satellite altimetry missions effectively constrain discharge estimates from space in the backwater‐affected zones upstream of river confluences.
Plain Language Summary
New satellite altimetry missions monitor water levels in global rivers at unprecedented spatio‐temporal resolution and accuracy. One principal objective of this monitoring effort is to map river flow in space and time, understand the impacts of climatic change on river flows, and provide river flow estimates for practical water resources applications. In many locations and situations, there is a one‐to‐one relationship between river flow and river water level. This so‐called rating curve can be used to directly translate water level observations into river flow estimates. However, the concept of the rating curve breaks down whenever the flow in the river is significantly different from steady uniform flow, which is the flow occurring in a long and uniform river reach with constant boundary inflow. Upstream of river confluences, conditions are often non‐uniform, because high flow in one tributary can coincide with low flow in the other, leading to significant backwater effects in both tributaries. This study models water level and water surface slope dynamics upstream of river junctions and highlights the value that data sets from new satellite missions provide for understanding the hydraulics around river confluences and for estimating flow upstream of river confluences from space.
Key Points
The classic stage‐discharge rating curve concept fails upstream of river confluences because of backwater effects
Hydraulic modeling confirms that unique stage‐slope‐discharge relationships exist upstream of river confluences
New satellite altimetry observations of river stage and slope reveal stage‐slope‐discharge relationships for selected river confluences
•Oil-water vertical flow pattern maps under high temperature/pressure were constructed.•Effects of temperature and pressure on flow patterns transition were investigated.•Effect of properties of oil ...and water has been considered on the flow patterns at high temperature/pressure.•Holdups and frictional pressure gradients were investigated.
We conducted experimental measurements to examine the upward oil-water flow patterns in a vertical pipe (0.02 m I.D.) under high temperatures up to 130 °C and high pressures up to 20.51 MPa. The viscosity of the oil sample was 85.113 mPa·s at 40 °C. All the experiments were conducted with an in-house-built high pressure/temperature flow apparatus. This apparatus is equipped with a view window that allows us to visually observe the upward flow patterns of oil-water two-phase flow in a 0.02 m I.D. stainless steel pipe. The effects of pressure, temperature, input water fraction (IWF) and mixture flow rate on the flow patterns and friction pressure gradients were systematically investigated. We plot the observed flow patterns on a flow pattern map in which input water fraction was used as the y-axis and the mixture flow velocity was used as the x-axis. Based on the measurement results, we observed nine flow patterns including four water-in-oil flow, transition flow and four oil-in-water flow patterns. With an increase in temperature at a given input water fraction, slug flow, plug flow with large water drops tended to transform into bubbly flow and dispersed flow with smaller water drops, water-in-oil very dense bubbly flow (VDB W/O) was observed at high temperature due to the significantly increasing of in-situ water holdups. The transition flow and the boundaries in the flow pattern maps tend to occur at lower input water fraction values. The effect of pressure on the flow patterns was found to be opposite to that of temperature. The possible causes leading to the changes in the flow patterns subjected to pressure/temperature variations were provided in terms of changes in density ratio, interfacial tension, and viscosity ratio between oil and water. The phase inversion points at different temperatures and pressures reflected from friction pressure gradients were found to be in agreement with the results of flow patterns. This work contributes to a better understanding of oil-water two-phase flow behavior under high temperature/pressure.
The internal flow in a vertical axial-flow pump is a complex unsteady three-dimensional viscous flow. An unstable flow often produces complex flow phenomena such as flow separation, vortices, and ...secondary reflux, which reduces the operating efficiency of the pump and can endanger safety and stability. In this paper, computational fluid dynamics is used to calculate the flow characteristics in an axial-flow pump using the shear stress transport and curvature correction (SST-CC) model for turbulence modified to account for the rotational curvature. Furthermore, the dependability of the numerical results was confirmed by a test with an actual model of a pump. The transient deviation angle at the impeller inlet of the pump, the stream field attributes in various spanwise parts of the impeller and guide vane, and the velocity distributions at the impeller inlet and outlet were analyzed. The omega method was utilized to recognize the vortex structure inside the guide vane. Moreover, the development of the transient vortex structure inside the guide vane was studied. As the flow rate increased, the scale and turbulent kinetic energy of the vortex structure gradually decreased. The time-domain graph for the impeller inlet is clearly periodic, with three peaks and three troughs in an impeller rotational period. The dominant frequency in the spectrum at each monitoring point was basically the blade frequency, and the secondary dominant frequency was twice the blade frequency.
Common modal decomposition techniques for flow-field analysis, data-driven modelling and flow control, such as proper orthogonal decomposition and dynamic mode decomposition, are usually performed in ...an Eulerian (fixed) frame of reference with snapshots from measurements or evolution equations. The Eulerian description poses some difficulties, however, when the domain or the mesh deforms with time as, for example, in fluid–structure interactions. For such cases, we first formulate a Lagrangian modal analysis (LMA) ansatz by a posteriori transforming the Eulerian flow fields into Lagrangian flow maps through an orientation and measure-preserving domain diffeomorphism. The development is then verified for Lagrangian variants of proper orthogonal decomposition and dynamic mode decomposition using direct numerical simulations of two canonical flow configurations at Mach 0.5, i.e. the lid-driven cavity and flow past a cylinder, representing internal and external flows, respectively, at pre- and post-bifurcation Reynolds numbers. The LMA is demonstrated for several situations encompassing unsteady flow without and with boundary and mesh deformation as well as non-uniform base flows that are steady in Eulerian but not in Lagrangian frames. We show that application of LMA to steady non-uniform base flow yields insights into flow stability and post-bifurcation dynamics. LMA naturally leads to Lagrangian coherent flow structures and connections with finite-time Lyapunov exponents. We examine the mathematical link between finite-time Lyapunov exponents and LMA by considering a double-gyre flow pattern. Dynamically important flow features in the Lagrangian sense are recovered by performing LMA with forward and backward (adjoint) time procedures.
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•Performed complete hydrate slurry flow characteristics in fully visual flow loop.•Providing the alarm points for hydrate agglomeration, deposition and bedding.•Analyzing the water ...conversion rate and the hydrate effective volume fraction.•Finding the hydrate accumulation position in the slope pipes.
Gas hydrates pose impeded flow risks and serious safety hazards in oil and gas transportation pipelines. This makes it imperative to look for appropriate hydrate control strategies. In this study, the hydrate slurry flow characteristics were investigated under multiphase flow conditions using a high-pressure fully visual flow loop. At different liquid loadings and mixture velocities, pressure drop variations were monitored from the initial hydrate formation to deposition and bedding, and the flow patterns were observed throughout the experiments. It was found that the hydrate slurry flow may involve four stages of hydrate formation, agglomeration, deposition and bedding prior to pipeline blockage. The effective volume and water conversion fractions were also obtained at different stages. The results showed that the lower liquid loading causes quicker and more severe hydrate blockage problem mainly due to more mass transfer of gas into water hence higher water conversion rate. Moreover, lower mixture velocity resulted in a higher hydrate bedding tendency. The results also confirmed further formation and deposition of gas hydrates due to a drastic temperature drop immediately after the liquid flow stopped in the system. In addition, the flow characteristics in sloped pipe sections (both upslope and downslope) were studied to investigate the effect of driving forces on the hydrate deposition. Slug flow was observed in the upslope while stratified flow in the downslope. Furthermore, hydrate accumulation tends to occur in the transition position between the horizontal pipes and the sloped pipes due to the effect of gravity.
To understand the influence of shear on the hydraulic properties of rock fractures, shear-flow tests were carried out on rock fractures with different surface roughnesses. Each rough-walled fracture ...was replicated in four specimens, which were sheared at different displacements under normal stresses that varied from 0.5 to 2.0 MPa. At each shear displacement, a series of hydraulic tests with different hydraulic gradients were performed, and the nonlinear flow regimes of the fluid within the fractures were investigated. The results show that Forchheimer’s law can well describe the nonlinear relationship between the flow rate and the hydraulic gradient in rough-walled fractures. Both the linear coefficient and nonlinear coefficient decrease during shearing but increase as the normal stress increases. The critical hydraulic gradient increases with an increase in the shear displacement and normal stress. With an increase in the joint roughness coefficient, the critical hydraulic gradient decreases. The normalized transmissivity exhibits a strong correlation with the Reynolds number. As the shear displacement increases, the fitted curves of the normalized transmissivity versus the Reynolds number shift upward but the curves shift downward with an increase in normal stress. Additionally, the Forchheimer coefficient decreases with an increase in the shear displacement but increases with an increase in the applied normal stress. Visualization tests show that the number of flow paths is large when the shear displacement is small due to various distributions of the contact areas and that the flow of dyed water over the entire fracture decreases. As the shear displacement increases, the flow resistance decreases due to the shear dilation-induced increase in the aperture, and the advantage channel flow is distinct in the fracture. The contact ratio rapidly decreases as the shear displacement increases from 1 to 3 mm and then slightly varies with a continuously increasing maximum shear displacement of 9 mm.
•Flow patterns and plug hydrodynamics of liquid–liquid flow are investigated.•A criterion for plug tail cap turns from a curved surface to flat is developed.•The Capillary number has a significant ...effect on the plug length.•Prediction correlation on plug length is developed in the liquid–liquid system.
The flow patterns and the plug hydrodynamics of liquid–liquid two-phase flow in a small circular channel were experimentally investigated. Five distinct flow patterns, including droplet flow, slug flow, throat-annular flow, annular flow and stratified flow were observed in the silicone oil–water system. The flow pattern map was present systematically based on Re, We, water volume fraction and mixture velocity to evaluate competitive relations of internal forces in each flow regime. Three parameters were defined to characterize the plug shape, and the change law of the plug shape was synthesized based on this. A criterion for plug tail cap turns from a curved surface to flat was developed. Capillary number in the liquid–liquid system has been proved to affect the plug length prediction significantly. A new plug length prediction correlation was summarized through the two linear fits of the plug length under different flow ratios. The water-isooctanol and water-dioctyl phthalate systems were selected to correct the plug length prediction correlation. The predicting results agree well with the literature data, and the maximum error is about 19.7%.
•Construction of unified gas-kinetic wave-particle (UGKWP) method for rarefied flow.•The multiscale solution is captured through the coupled wave and particle evolution.•In the hypersonic rarefied ...flow, UGKWP becomes a particle method.•In the continuum flow regime, UGKWP recovers the gas-kinetic scheme.•UGKWP achieves high efficiency in the corresponding flow regime.
The unified gas-kinetic scheme (UGKS) provides a framework for simulating multiscale transport with the updates of both gas distribution function and macroscopic flow variables on the cell size and time step scales. The multiscale dynamics in UGKS is achieved through the coupled particle transport and collision in the particle evolution process within a time step. In this paper, under the UGKS framework, we propose an efficient multiscale unified gas-kinetic wave-particle (UGKWP) method. The gas dynamics in UGKWP method is described by the individual particle movement coupled with the evolution of the probability density function (PDF). During a time step, the trajectories of simulation particles are tracked until collision happens, and the post-collision particles are evolved collectively through the evolution of the corresponding distribution function. The evolution of simulation particles and distribution function is guided by the evolution of macroscopic variables, and guarantees the conservation of the scheme in the final wave-particle formulation. A new concept of multiscale multi-efficiency preserving (MMP) method is introduced. Multiscale preserving means UGKWP method preserves the flow regime from collisionless regime to hydrodynamic regime without requiring the cell size and time step to be less than the mean free path and collision time. Multi-efficiency preserving means the computational cost of the scheme including the computational time and memory cost is on the same level as the most efficient method in the corresponding regime, such as the particle methods in the rarefied regime and hydrodynamic solvers in continuum regime. The UGKWP method is shown to satisfy the MMP requirement. The UGKWP method is specially efficient for hypersonic flow simulation in all regimes in comparison with the discrete ordinate methods, and presents a much lower stochastic noise in the continuum flow regime in comparison with the particle-based Monte Carlo methods. Numerical tests for flows over a wide range of Mach and Knudsen numbers are presented. The examples include the hypersonic flow passing a circular cylinder at Mach numbers 20 and 30 and Knudsen numbers 1 and 10−4, low speed lid-driven cavity flow, laminar boundary layer, shock structure, and shock tube problems. These results validate the accuracy, efficiency, and multiscale and multi-efficiency property of UGKWP method.
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