A Lagrangian criterion of unsteady flow separation for two-dimensional periodic flows

The present paper proposes a Lagrangian criterion of unsteady flow separation for two-dimensional periodic flows based on the principle of weighted averaging zero skin-friction given by Haller （HALLER, G. Exact theory of unsteady separation for two-dimensional flows. Journal of Fluid Mechanics, 512, 257-311 （2004））. By analyzing the distribution of the finite-time Lyapunov exponent （FTLE） along the no-slip wall, it can be found that the periodic separation takes place at the point of the zero FTLE. This new criterion is verified with an analytical solution of the separation bubble and a numerical simulation of lid-driven cavity flows.

Numerical Analysis of the Effects of Periodic Gust Flow on the Wake Structure of Ventilated Supercavities

A computational model is established to investigate the effects of a periodic gust flow on the wake structure of ventilated supercavities.The effectiveness of the computational model is validated by comparing with available experimental data.Benefited from this numerical model,the vertical velocity characteristics in the entire flow field can be easily monitored and analyzed under the action of a gust generator;further,the unsteady evolution of the flow parameters of the closed region of the supercavity can be captured in any location.To avoid the adverse effects of mounting struts in the experiments and to obtain more realistic results,the wake structure of a ventilated supercavity without mounting struts is investigated.Unsteady changes in the wake morphology and vorticity distribution pattern of the ventilated supercavity are determined.The results demonstrate that the periodic swing of the gust generator can generate a gust flow and,therefore,generate a periodic variation of the ventilated cavitation numberσ.At the peakσ,a re-entrant jet closure appears in the wake of the ventilated supercavity.At the valleyσ,a twin-vortex closure appears in the wake of the ventilated supercavity.For the forward facing model,the twin vortex appears as a pair of centrally rolled-up vortices,due to the closure of vortex is affected by the structure.For the backward facing model,however,the twin vortex appears alternately as a pair of centrally rolled-up vortices and a pair of centrally rolled-down vortices,against the periodic gust flow.

Thrombelastography and color Doppler flow imaging in the perioperative period after closed lower limb fracture

Objective To observe the changes of thrombelastography (TEG) and color Doppler flow imaging (CDFI) in the perioperative period after closed lower limb fracture.Methods Fasting venous blood samples in the morning from 11 healthy adults were used

Assessing reservoir connectivity with pressure/rate surveillance data

Estimating reservoir connectivity is critical for assessing infill-drilling prospects and initiating fluid injection in enhanced oil recovery operations. Several methods have appeared in the literature over decades to meet these business needs, given that all tools, including seismic imaging, have limitations. Besides imaging, geochemical fingerprinting constitutes a powerful tool to gauge the compartmentalization question. However, real-time pressure/rate surveillance data allows assessing interwell connectivity vis-à-vis the overall drainage volume.This study presents a simplified approach to using the reciprocal-productivity index (RPI) vs. the total-material-balance time (tTMB) plot. This tool exhibits the same slope for those wells in the same compartment beyond the start of the boundary-dominated flow (BDF) period. The wells showing different slopes imply that they are in separate drainage volumes. The early-time transient period remains muted to minimize confusion on this Cartesian plot. We validated the proposed tool's efficacy with 2D and 3D models with increased degrees of reservoir complexity, followed by the verification phase with four field examples. The use of other analytical tools complemented our findings.

3D numerical study on flow structure and heat transfer in a circular tube with V-baffles

A 3D numerical investigation has been carried out to examine periodic laminar flow and heat transfer characteristics in a circular tube with 45°V-baffles with isothermal wall.The computations are based on the finite volume method(FVM),and the SIMPLE algorithm has been implemented.The fluid flow and heat transfer characteristics are presented for Reynolds numbers ranging from 100 to 2000.To generate main longitudinal vortex flows through the tested section,V-baffles with an attack angle of 45°are mounted in tandem and in-line arrangement on the opposite positions of the circular tube.Effects of tube blockage ratio,flow direction on heat transfer and pressure drop in the tube are studied.It is apparent that a pair of longitudinal twisted vortices(P-vortex)created by a V-baffle can induce impingement on a wall of the inter-baffle cavity and lead a drastic increase in heat transfer rate at tube wall.In addition,the larger blockage ratio results in the higher Nusselt number and friction factor values.The computational results show that the optimum thermal enhancement factor is around 3.20 at baffle height of B=0.20 and B=0.25 times of the tube diameter for the V-upstream and V-downstream,respectively.

Formation of Singularities in One—Dimensional Hydromagnetic Flow

Two results on the formation of singularities in solutions to the system of one-dimensional hydromagnetic dynamics are presented.In particular,it is shown that shocks form from a smooth spatial periodic flow in a finite time if the initial amounts of entropy and the “magnetic field” in each period are smaller than those of sound waves.A quantitative estimate of blow-up time is also given.

Exact solution of electroosmotic flow in generalized Burgers fluid

An exact solution is developed for the time periodic electroosmotic flow of a non-Newtonian fluid between the micro-parallel plates.The constitutive equations of a generalized Burgers fluid are used in the mathematical formulation.The resulting problem is solved by a Fourier transform technique.Graphs are plotted and discussed for various emerging parameters of interest.

3D modeling for effect of tool eccentricity on coupled thermal and material flow characteristics during friction stir welding

A novel three-dimensional numerical model is proposed to investigate the effect of tool eccentricity on the coupled thermal and material flow characteristics in friction stir welding(FSW) process.An asymmetrical boundary condition at the tool-workpiece interface,and the dynamic mesh technique are both employed for the consideration of the tool eccentricity during tool rotating.It is found that tool eccentricity induces the periodical variation of the heat densities both at the tool-workpiece interface and inside the shear layer,but the fluctuation amplitudes of the heat density variations are limited.However,it is demonstrated that tool eccentricity results in significant variation of the material flow behavior in one tool rotating period.Moreover,the material velocity variation at the retreating side is particularly important for the formation of the periodic characteristics in FSW.The modeling result is found to be in good agreement with the experimental one.

Laminar Periodic Flow and Heat Transfer in a Rectangular Channel with Tri-angular Wavy Baffles

This paper presents a numerical analysis of laminar periodic flow and heat transfer in a rectangular constant temperature-surfaced channel with triangular wavy baffles (TWBs).The TWBs were mounted on the opposite walls of the rectangular channel with inline arrangements.The TWBs are placed on the upper and lower walls with attack angle 45?.The numerical is performed with three dif-ferent baffle height ratios (BR=b/H=0.05 0.3) at constant pitch ratio (PR) of 1.0 for the range 100 ≤ Re ≤ 1000.The computational results are shown in the topology of flow and heat transfer.It is found that the heat transfer in the channel with the TWB is more effective than that without baffle.The in-crease in the blockage ratio,BR leads to a considerable increase in the Nusselt number and friction factor.The results indicate that at low BR,a fluid flow is significantly disturbed resulting in inefficient heat transfer.As BR increases,both heat transfer rate in terms of Nusselt number and pressure drop in terms of friction factor increase.Over the range examined,the maximum Nu/Nu0 of 7.3 and f/f0 of 126 are both found with the use of the baffles with BR=0.30 at Re=1000.In addition,the flow structure and temperature field in the channel with TWBs are also reported.

Five-equation and robust three-equation methods for solution verification of large eddy simulation

This study evaluates the recently developed general framework for solution verification methods for large eddy simulation（LES） using implicitly filtered LES of periodic channel flows at friction Reynolds number of 395 on eight systematically refined grids.The seven-equation method shows that the coupling error based on Hypothesis I is much smaller as compared with the numerical and modeling errors and therefore can be neglected. The authors recommend five-equation method based on Hypothesis II, which shows a monotonic convergence behavior of the predicted numerical benchmark（S_C）, and provides realistic error estimates without the need of fixing the orders of accuracy for either numerical or modeling errors. Based on the results from seven-equation and five-equation methods, less expensive three and four-equation methods for practical LES applications were derived. It was found that the new three-equation method is robust as it can be applied to any convergence types and reasonably predict the error trends. It was also observed that the numerical and modeling errors usually have opposite signs, which suggests error cancellation play an essential role in LES. When Reynolds averaged Navier-Stokes（RANS） based error estimation method is applied, it shows significant error in the prediction of S_C on coarse meshes. However, it predicts reasonable S_C when the grids resolve at least 80% of the total turbulent kinetic energy.

Assessment of the cytotoxic and mutagenic potential of the Jialu River and adjacent groundwater using human-hamster hybrid cells

The Jialu River in China has been seriously polluted by the direct discharge of industrial and domestic wastewater. The predominant contaminants of the Jialu River and its adjacent groundwater were recently investigated. However, the potential genotoxic impact of polluted water on human health remains to be clarified. Here, we used human–hamster hybrid（AL） cells, which are sensitive for detecting environmental mutagens. We found that the cytotoxicity and mutagenicity of the groundwater in the Jialu River basin were influenced by the infiltration of the Jialu River. Hydrological periods significantly affected the cytotoxicity, but not the mutagenic potential, of surface and groundwater. Further, the mutagenic potential of groundwater samples located 〈 1 km from the Jialu River（S（M-2） water samples） was detected earlier than that of groundwater samples located approximately 20 km from the Jialu River（SN water samples）. Because of high cytotoxicity, the mutagenic potential of water samples from the Jialu River（S（M-1） water samples） was not significantly enhanced compared with that of untreated controls. To further assess the mutagenic dispersion potential, an artificial neural network model was adopted. The results showed that the highest mutagenic potential of groundwater was observed approximately 10 km from the Jialu River. Although further investigation of mutagenic spatial dispersion is required, our data are significant for advancing our understanding of the origin, dispersion,and biological effects of water samples from polluted areas.

Hydrocarbon migration in fracture-cave systems of carbonate reservoirs under tectonic stresses:A modeling study

The Tahe oilfield,located in the southwest of the Akekule nosing structure,northern Tarim basin,was the most prolific oilfield targeting at the Ordovician carbonate reservoirs in China.The reservoir space was dominant with fracture-cave systems commonly induced by tectonics and karstification.Although hydrocarbon production had proceeded for two decades in the Tahe oilfiled,the control of oil and gas accumulations was still doubtful.In this work,the periodic fluid flow induced by cyclic tectonic stresses was proposed as the mechanism of hydrocarbon migration in the fracture-cave systems of carbonate reservoirs.The fracture networks formed conduits for fluid flow,and the fluid pressure in caves transmitted from stress field provided the driving force.The constitutive equations were established among stresses,fracture densities and flow velocities.Four quasi-3D geological models were constructed to simulate the flow velocities on the Ordovician surface of Akekule nosing structure in the critical tectonic stages.The simulated results supplied indicative information on oil and gas migration and accumulation in the tectonic stages.Combining with the oil and gas charge history,a conceptual model was built to reveal the multi-stage oil and gas charge and accumulation in the Ordovician of Akekule nosing structure.

Hydrocarbon migration in fracture-cave systems of carbonate reservoirs under tectonic stresses: A mechanism study

The fracture-cave systems of carbonate reservoirs have almost stored 30% of recoverable oil and gasaround the world. However, it is still doubtful about the mechanism of hydrocarbon migration in thefracture-cave systems. In this work, deducing from the Eshelby’ solution, we derived the equation tocalculate the bulk strain of elliptic cylinder caves applied by stresses. Calculated results indicate that thebulk strain of caves negatively increases with the radius ratio of the elliptic cave axials under fixedstresses. In the case of the effective horizontal stress increasing from 30 MPa to 80 MPa, the bulk straindifference of a cave could be up to 0.5%. It may result in 0.4% of the total cave volume of fluid transportingthrough the fracture-cave systems within such a stress cycle. Since the tectonic stresses transform in acyclic way, the volumetric ratio of new-to-old fluid in a cave would increase with the number of stresscycles. As a result, we proposed that the periodic fluid flow induced by cyclic tectonic stresses could be animportant mechanism for hydrocarbon migration in the fracture-cave systems of carbonate reservoirs.

Turbulent heat transfer and pressure loss in a square channel with discrete broken V-rib turbulators

Turbulent periodic flow, heat transfer, friction loss and thermal enhancement characteristics in a three-dimensional horizo- ntal square channel with broken V-ribs （B-VR） are numerically investigated. The computations are based on the finite volume method, and the SIMPLE algorithm with QUICK scheme is implemented. The B-VR were installed on both sides of a plate which was diagonally placed in a square channel to produce longitudinal vortex flows through the tested section. Effects of different open comer ratios （ d/H = 0, 0.01, 0.02, 0.03, 0.04 and 0.05） on heat transfer and pressure loss in the channel and the results of the B-VR are studied. The pitch ratio （PR = p/H） and blockage ratio （BR = p/H） of B-VRs were fixed at 1.0 and 0.15, respectively. As compared with the channel without V-rib, the one with B-VRs possessed considerably higher heat transfer and friction loss. It is observed that apart from the rise of Reynolds number, the reduction of the open comer ratios leads to an increase in the Nusselt number and friction factor due to the weaker turbulence and lower resistance to the flow. According to the computational results for B-VRs, the optimum thermal enhancement is fotmd at d/H = 0.