Features of the flow velocity and pressure gradient of an undular bore on a horizontal bed

2020 ◽  
Vol 32 (4) ◽  
pp. 043603 ◽  
Author(s):  
Rajkumar Venkatesh Raikar ◽  
James Yang
Keyword(s):  
Pressure Gradient ◽  
Flow Velocity ◽  
Undular Bore

The diastolic flow velocity-pressure gradient relation and dpv50 to assess the hemodynamic significance of coronary stenoses

2006 ◽  
Vol 291 (6) ◽  
pp. H2630-H2635 ◽  
Author(s):  
Koen M. J. Marques ◽  
Machiel J. van Eenige ◽  
Hugo J. Spruijt ◽  
Nico Westerhof ◽  
Jos Twisk ◽  
...  
Keyword(s):  
Pressure Gradient ◽  
Flow Velocity ◽  
Stress Testing ◽  
Coronary Vessels ◽  
Velocity Data ◽  
Severe Stenosis ◽  
Coronary Pressure ◽  
Coronary Stenoses ◽  
Sensitivity Specificity ◽  
Velocity Pressure

To evaluate the hemodynamic impact of coronary stenoses, the fractional (FFR) or coronary flow velocity reserve (CFVR) usually is measured. The combined measurement of instantaneous flow velocity and pressure gradient (v-dp relation) is rarely used in humans. We derived from the v-dp relation a new index, dpv50 (pressure gradient at flow velocity of 50 cm/s), and compared the diagnostic performance of dpv50, CFVR, and FFR. Before coronary angiography was performed, patients underwent noninvasive stress testing. In all coronary vessels with an intermediate or severe stenosis, the flow velocity, aortic, and distal coronary pressure were measured simultaneously with a Doppler and pressure guidewire after induction of hyperemia. After regression analysis of all middiastolic flow velocity and pressure gradient data, the dpv50 was calculated. With the use of the results of noninvasive stress testing, the dpv50 cutoff value was established at 22.4 mmHg. In 77 patients, 124 coronary vessels with a mean 39% (SD 19) diameter stenosis were analyzed. In 43 stenoses, ischemia was detected. We found a sensitivity, specificity, and accuracy of 56%, 86%, and 76% for CFVR; 77%, 99%, and 91% for FFR; and 95%, 95%, and 95% for dpv50. To establish that dpv50 is not dependent on maximal hyperemia, dpv50 was recalculated after omission of the highest quartile of flow velocity data, showing a difference of 3%. We found that dpv50 provided the highest sensitivity and accuracy compared with FFR and CFVR in the assessment of coronary stenoses. In contrast to CFVR and FFR, assessment of dpv50 is not dependent on maximal hyperemia.

Flow Properties of Fluids Confined in Parallel-Plate Nanochannels

2012 ◽  
Vol 229-231 ◽  
pp. 22-25
Author(s):  
Zhong Qiang Zhang ◽  
Guang Gui Cheng ◽  
Jian Ning Ding ◽  
Zhi Yong Ling
Keyword(s):  
Pressure Gradient ◽  
Flow Velocity ◽  
Parallel Plate ◽  
Stokes Equations ◽  
Channel Width ◽  
Mean Flow ◽  
The Mean ◽  
Dynamics Simulations ◽  
Mean Flow Velocity ◽  
Pressure Driven

Molecular dynamics simulations are carried out to explore the fluid flows in parallel-plate nanochannels. A “channel moving” pressure-driven model is utilized to study the planar Poiseuille flows. Considering the slip boundary conditions, relationships among the pressure gradient, mean flow velocity and the channel width are investigated to couple the atomistic regime to continuum. The results show that the mean flow velocity almost linearly increases with the increase of the pressure gradient. The slope of the linear relationship between the pressure gradient and the mean flow velocity is nonlinearly decreased with increasing the channel width. The results indicate that the approximate accuracy is reduced with decreasing the channel width while the pressure-driven flows confined in nanochannels are approximately described by the Navier-Stokes equations.

A FRACTAL MODEL FOR LOW-VELOCITY NON-DARCY FLOW IN TIGHT OIL RESERVOIRS CONSIDERING BOUNDARY-LAYER EFFECT

Fractals ◽  
2018 ◽  
Vol 26 (05) ◽  
pp. 1850077 ◽  
Author(s):  
FUYONG WANG ◽  
ZHICHAO LIU ◽  
JIANCHAO CAI ◽  
JIAN GAO
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Flow Velocity ◽  
Oil Reservoirs ◽  
Darcy Flow ◽  
Tight Oil ◽  
Fluid Viscosity ◽  
Low Velocity ◽  
Layer Effect ◽  
Tight Oil Reservoirs

Flow in nanoscale pore-throats of tight oil reservoirs is strongly affected by boundary-layers, and exhibits low-velocity non-Darcy flow phenomena. The relationship between flow velocity and pressure gradient is highly nonlinear and difficult to be modeled mathematically. This paper proposed a low-velocity non-Darcy flow model which can account for boundary-layer effect in tight oil reservoirs. First, a modified Hagen–Poiseuille equation coupled with boundary-layer effect in a single capillary tube was derived. Then, assuming pores in tight formations following fractal distribution, an analytical expression of nonlinear correlation between flow velocity and pressure gradient in fractal porous media was developed. Finally, the proposed model was validated with experiment data, and parameters influencing low-velocity non-Darcy flow were quantitatively evaluated. The research results show that the decreasing boundary-layer thickness with the increase pressure gradient is the main reason of low-velocity non-Darcy flow in tight oil reservoirs. Our model can effectively describe the nonlinear relationship between flow velocity and pressure gradient. The relationship between threshold pressure gradient (TPG) and pseudo threshold pressure gradient (PTPG) can also be predicted using our model. Fluid viscosity has great impact on nonlinear flow behavior, and with fluid viscosity increasing TPG and PTPG increase significantly. TPG is the function of fluid type, fluid viscosity and maximum pore diameter, and decreases exponentially with the increasing maximum pore size.

scholarly journals Hydrodynamic Features of an Undular Bore Traveling on a 1:20 Sloping Beach

Water ◽  
2019 ◽  
Vol 11 (8) ◽  
pp. 1556 ◽  
Author(s):  
Chang Lin ◽  
Wei-Ying Wong ◽  
Ming-Jer Kao ◽  
James Yang ◽  
Rajkumar V. Raikar ◽  
...  
Keyword(s):  
Free Surface ◽  
Pressure Gradient ◽  
Peak Level ◽  
Horizontal Velocity ◽  
Surface Elevation ◽  
Free Surface Elevation ◽  
Undular Bore ◽  
Run Up ◽  
The Moment ◽  
Sloping Beach

The hydrodynamic characteristics, including local and convective accelerations as well as pressure gradient in the horizontal direction, of the external stream of an undular bore propagating on a 1:20 sloping beach are experimentally studied. A bore with the water depth ratio of 1.70 was generated downstream of a suddenly lifted gate. A high-speed particle image velocimetry was employed to measure the velocity fields during the run-up and run-down motions. The time series of free surface elevation and velocity field/profile of the generated bore, comprising a pure bore accompanied by a series of dispersive leading waves, are first demonstrated. Based on the fast Fourier transform (FFT) and inverse FFT (IFFT) techniques, the free surface elevation of leading waves and the counterpart of pure bore are acquired separately at a specified measuring section (SMS), together with the uniform horizontal velocity of the pure bore. The effect of leading-wave-induced velocity shift on the velocity profiles of the generated bore are then evaluated at the SMS. To understand the calculation procedure of accelerations and pressure gradient, three tabulated forms are provided as illustrative examples. Accordingly, the relationships among the partially depth-averaged values of the non-dimensional local acceleration, convective acceleration, total acceleration and pressure gradient of the generated/pure bore acquired at the SMS versus the non-dimensional time are elucidated. The trends in the non-dimensional accelerations and pressure gradient of the external stream of generated bore are compared with those of the pure bore. During the run-up motion from the instant of arrival of the bore front to the moment of the peak level at the SMS, continuous decrease in the onshore uniform horizontal velocity, and successive deceleration of the pure bore in the onshore direction are evidenced, exhibiting the pure bore under the adverse pressure gradient with decreasing magnitude. However, the pure bore once ridden by the leading waves is decelerated/accelerated spatially and accelerated/decelerated temporally in the onshore direction during the rising/descending free surface of each leading wave. This fact highlights the effect of pre-passing/post-passing of the leading wave crest on the velocity distribution of generated bore. It is also found that, although the leading waves have minor contribution on the power spectrum of the free surface elevation as compared with that of the pure bore, the leading waves do play an important role on the magnitudes of both accelerations and pressure gradient. The largest magnitude of the acceleration contributed by the leading waves is around 26 times the counterpart contributed by the pure bore. Further, during the run-down motion right after the moment for the peak level of the bore, a linear increase in the magnitude of the offshore uniform horizontal velocity and a constant local acceleration with increasing time are both identified. The partially depth-averaged value of the non-dimensional pressure gradient is equal to a small negative constant (−0.0115) in the offshore direction, indicating that the bore is subject to a constant favorable pressure gradient.

Separations and secondary structures due to unsteady flow in a curved pipe

2017 ◽  
Vol 815 ◽  
pp. 26-59 ◽  
Author(s):  
C. Vamsi Krishna ◽  
Namrata Gundiah ◽  
Jaywant H. Arakeri
Keyword(s):  
Reynolds Number ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Pressure Gradient ◽  
Flow Velocity ◽  
Secondary Flow ◽  
Flow Separation ◽  
Unsteady Flows ◽  
Stress Components ◽  
Wall Shear

Unsteady flows in highly curved geometries are of interest in many engineering applications and also in physiological flows. In this study, we use flow visualization and computational fluid dynamics to study unsteady flows in a highly curved tube ($\unicode[STIX]{x1D6FD}=0.3$) with square cross-section; here, $\unicode[STIX]{x1D6FD}$ is the ratio of the half edge length to the radius of curvature of the tube. To explore the combined effects of curvature and pulsatility, we use a single flow pulse of duration $T$ and peak area averaged axial velocity $U_{p(max)}$, which are independently varied to investigate a range of Dean and Womersley numbers. This range includes cases corresponding to flows in the ascending aorta. We observe radially inward moving secondary flows which have the structure of wall jets on the straight walls; their subsequent collision on the inner wall leads to a re-entrant radially outward moving jet. The wall jet arises due to an imbalance between the centrifugal force and the radial pressure gradient. During the deceleration phase, the low-axial-momentum fluid accumulated in the jet reverses direction and leads to flow separation near the inner wall. We use boundary layer equations to derive scales, which have not been reported earlier, for the secondary flow velocities, the wall shear stress components and the distance ($\hat{P}$) traversed by the secondary flow structures in the transverse plane. We show that $\hat{P}$ predicts the movement of vortical structures until collision. In the limit $\unicode[STIX]{x1D6FD}\rightarrow 0$, the Reynolds number based on this secondary flow velocity scale asymptotes to the secondary streaming Reynolds number proposed by Lyne (J. Fluid Mech., vol. 45 (01), 1971, pp. 13–31) in loosely curved pipes. The magnitude of the secondary flow velocity is high and ${\sim}40\,\%$ of $U_{p(max)}$ for physiological flow conditions. We show that the flow separation on the inner wall has origins in the secondary flow, which was reported in a few earlier studies, and is not due to the axial pressure gradient in the tube as proposed earlier. The wall shear stress components, hypothesized to be important in arterial mechanobiology, may be estimated using our scaling relations for geometries with different curvatures and varying pulsatilities.

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