Effect Of Pressure Gradient Research Articles

Examination of Couette Flow with a Pressure Gradient and Heat Conduction Using Molecular Dynamics Simulation

As computer capabilities improve, Molecular Dynamics simulations are becoming more important for solving various flow problems. In this study, Couette and Poiseuille flows at different wall temperatures were investigated using a hard-sphere Molecular Dynamics simulation approach. Although a low spacing ratio was used in the simulations, the results are valid for rarefied gas flows when proper scaling based on the Knudsen number was used because only binary collisions with a hard-sphere model were considered. The main focus of this study was the examination of the effects of various wall speeds, pressure gradients, and wall temperatures. A pressure gradient was generated by developing a modified selective periodicity condition in the flow direction. With the combined effect of the pressure gradient and the wall velocities, subsonic, transonic, and supersonic speeds in nanochannels were examined. With the combination of different parameters, 1260 simulation cases were conducted. The results showed that there are temperature and velocity slips that are dependent on not only the temperature and velocity values but also on the magnitudes of a pressure gradient. The pressure gradient also caused nonlinearities in temperature and velocity profiles.

Micro‐mesoscopic analysis of 3D void formation and evolution in CFRP composite using high‐resolution x‐ray microtomography

AbstractVoid defect is prevalent during the composite manufacturing process and may reduce the mechanical properties of composite structures, while limited information about three‐dimensional void distribution and evolution can be found due to the difficulties in void observation. In this work, void defects at different positions were detected and analyzed using x‐ray microtomography (XRM), and the random forest algorithm was implemented for segmenting components of CFRP samples. Results showed a trend of gradual rising porosity in the axial direction, with an increase of ~250% of porosity at the outlet of resin injection compared to the one at the inlet during the composite manufacturing process. The voids mainly featured with microscale between carbon fibers, with less distribution between fiber bundles and resin‐rich zones. Void defects achieved aggregation at the middle position of manufactured laminate, which is probably related to pressure interception. In the axial direction, the proportion of large‐diameter voids gradually increased, and a large volume of connected voids appeared near the outlet. Due to the shear deformation of resin and fiber and the pressure gradient effect, most of the voids showed stripe shape. The spatial orientation of voids was predominantly along the direction of resin flow and fluctuated due to fiber curl.Highlights 3D void characteristics in CFRP were studied using the XRM technique. Void formation and the effects of fibers on void morphology were analyzed. Dynamic void transport mechanisms during CFRP manufacturing were studied. Location and morphology of voids are helpful for CFRP damage modeling. The effective partitioning of CFRP was achieved using a random forest algorithm.

Effect of Displacement Pressure Gradient on Oil–Water Relative Permeability: Experiment, Correction Method, and Numerical Simulation

Relative permeability is a fundamental parameter affecting reservoir development performance analysis. During the development of oil and gas fields, the displacement pressure gradient changes with time and space. This paper studies the effect of displacement pressure gradient on relative permeability. The oil–water relative permeability curves of a Bohai Oilfield under different displacement pressure gradients are obtained through experimental analysis. Based on the experimental data, a correction model of the permeability curve is established by regression of the Willhite model parameters. The correction model is introduced into the black oil numerical simulation, and the production performance and remaining oil are compared and analyzed. The results show that the displacement pressure gradient can have an obvious impact on the relative permeability curve. As the displacement pressure gradient increases, the two-phase span of the relative permeability curve increases, the oil displacement efficiency increases, and the water relative permeability increases. The relative permeability curves under different displacement pressure gradients can be accurately characterized by the Willhite model. The consideration of the displacement pressure gradient has an obvious impact on numerical simulation results. The conventional method of using a fixed relative permeability curve cannot truly reflect the production performance and the remaining oil distribution. This paper proposes a set of realization methods including obtaining laws from experiments, utilizing the empirical model to correct, and simulating to characterize reservoir changes.

Analysis of stall characteristics of the cuspidal waverider at subsonic conditions

The stall characteristics of the cuspidal waverider under subsonic conditions were studied at M = 0.4 and H = 0 km incoming flow. The stall characteristics were compared with those of delta wing models with the same leading-edge swept angle and the same aspect ratio. It can be found that the flow field changes of the cuspidal waverider are similar to those of Model 1 and Model 2. The critical angle of attack of the cuspidal waverider is more influenced by the aspect ratio, but less related to the leading-edge swept angle. The lift change of the cuspidal waverider is closely related to the vortex structure on the leeward side. As the angle of attack increases, the vortex structure becomes thicker and stronger, and moves toward the middle of the model under the action of the overflowing high pressure gas on the lower surface. Under the same angle of attack, the vortex structure under the effect of the adverse pressure gradient, the intensity along the flow direction decreases while the vortex continues to move away from the wall. Besides, the possible splitting of large vortex into smaller ones will be another result of the effect of the inverse pressure gradient. The smaller the overall aspect ratio of the model, the greater actual swept angle and the greater effective swept angel of the vortex structure, which allows the vortex to exist at higher angles of attack. When the vortex structure is broken to form a larger recirculation area in the leeward area, the negative pressure at the leeward surface decreases, the lift coefficient decreases, and a stall occurs.

Simulations of scalar transport in oscillatory flow over a wavy wall

A three-dimensional model that couples Reynolds–averaged Navier–Stokes equations with an advection–diffusion solver for flow and concentration fields over a fixed, non-permeable boundary is presented. The model simulates oscillatory flow over a wavy wall with a constant scalar source to assess the effects of a boundary-induced pressure gradient on passive scalar mixing. Results were compared to experimental data collected in an oscillating tray facility. Hydrodynamic quantities are in favourable agreement with observations. The model captured vortex dynamics correctly, and turbulence statistics were modelled reasonably well. Comparisons of time-resolved and bulk scalar quantities for crest and trough sources were in favourable agreement with observations. Results indicate that the model can be utilized to study processes with highly intermittent phenomena, such as the transport of wastes and dissolved nutrients in the coastal bottom.

Wind turbine wake superposition under pressure gradient

We investigate the effect of pressure gradient on the cumulative wake of multiple turbines in wind tunnel experiments spanning across a range of adverse pressure gradient (APG), zero pressure gradient (ZPG), and favorable pressure gradient (FPG). Compared to the upstream-most turbine, the in-wake turbines exhibit lower (higher) wake velocity in APG (FPG) than in the ZPG. The maximum velocity deficit shows a lesser difference for the in-wake turbine between different cases compared to the upstream-most one. This is linked to the effect of the wake of the upstream turbine. Conversely, the wake width varies more for the in-wake turbines. A new analytical approach to model the cumulative wake velocity deficit is proposed. This approach extends the application of the analytical pressure gradient model to multiple turbine wakes. Specifically, the new approach explicitly accounts for the effect of the pressure gradient induced by the wake of the upstream turbine on the wake of the downstream one. The new method is compared to the linear summation approach and experimental data. It agrees well with the experiments and outperforms the linear summation approach.

Examining laminar, non-stationary viscoelastic fluid flow between two parallel planes

The generalized Maxwell model is used to handle problems involving the unsteady flow of a viscoelastic fluid in a flat channel under the effect of a constant pressure gradient. Formulas for fluid flow, velocity distribution, and other hydrodynamic characteristics were found. Transient processes during unsteady flow of a viscoelastic fluid in a flat channel are investigated based on the discovered formulas. The analysis’s conclusions demonstrated that, at small Debord number values, the procedures of changing a viscoelastic fluid’s properties from an unstable to a stationary state essentially don’t differ from those of a Newtonian fluid. Exceeding the Debord number relatively unity, it has been established that the process of transition of a viscoelastic fluid from an unsteady state to a stationary state is of a wave nature, in contrast to the transition process of a Newtonian fluid, and the transition time is several times longer than that of a Newtonian fluid. It was also discovered that perturbed processes can arise during the transition. These disturbances occurring in unsteady flows of a viscoelastic fluid can be stabilized by mixing the Newtonian fluid within it. The implementation of this property is important in preventing technical failures or malfunctions.

Electrokinetic energy conversion of nanofluids in porous microtubes with Green’s function

Abstract Micro-devices fabrication has led to extensive scientific research on microfluidics and microelectromechanical systems. These devices are used for a wide range of technological applications, but research on microfluidic devices for nanofluids is relatively scarce. In response to this problem, the electrokinetic energy conversion (EKEC) efficiency of nanofluids is provided under the coupling effect of pressure gradient and magnetic field through porous microtubes using the Debye–Hückel linearization and the Green’s function method. The results show that the periodic excitation of the square waveform is more effective in increasing the EKEC efficiency. In addition, compared with previous studies, the average velocity is in good agreement with the cosine waveform at R = 0.2. It is worth noting that compared to cosine waves, the average velocity reaches 47% in triangular waves and 85% in square waves.

Effect of sinusoidal pressure gradient on MHD flow of viscoelastic fluid in a channel

Unsteady Hydro Magnetic oscillatory flow of incompressible viscoelastic fluid in a channel packed with porous material is investigated under the influence of oscillatory pressure gradient. Moreover, the applied magnetic field of moderate strength interacts with electrically conducting fluid, to generate a resistive volumetric body force. In addition to it, viscoelastic fluidity, characterizing the non-Newtonian behavior in conjunction with permeability of the medium as well as sinusoidal sheared flow due to imposed favorable pressure gradient, contribute to the novelty of the study. The previous work was confined to constant motion of the lower plate but the present work includes the sinusoidal motion of the lower plate in addition to the constant motion. The importance lies with the fact that the coupling effect of sinusoidal fluctuation and viscoelastic fluidity go hard in industrial applications, particularly in process industry. The governing equations characterizing the flow are solved analytically. The striking results of this study are: the formation of inverted boundary layer in case of oscillatory lower plate with moderately escalated time span but such instability leading to flow reversal, does not appear in case of constant motion of the lower plate. Hence, it is recommended that for stability of flow and to avoid flow reversal, higher frequency of oscillation as well as larger time span is not desirable. The elastico-viscosity property reduces the volume flow rate which may be useful to control the efflux and differentiates the state of motion (resulting low skin friction) and state of rest (resulting high skin friction) in respect shearing effects at the bounding surfaces.

Effects of Pressure Gradient on Fluid Flow and Energy Distribution in a Bending Square Channel

A precise knowledge on fluid flow and energy distribution in a bending channel is important for the thermal management of various engineering problems. The literature currently lacks a comprehensive understanding on how fluid characteristics and heat transfer in a bending channel are affected by pressure gradients, as well as when and how fluids transit into a turbulent state under continuous pressure gradient. The present study aims to explore the bifurcation structure of the steady solutions, linear stability and velocity distribution of the solutions and the transitional behavior from stead-state to other flow states (i.e., periodic or chaotic) for an extensive domain of the Dean number 0 < Dn ≤ 5000. The geometry of the problem is that the outer and bottom walls are heated while the inner and top are at room temperature. A proper grid analysis and validation are performed as well. The study successfully analyzed four branches of steady solutions where stability exists only in the first branch up to Dn = 2593.3709 and 2- to 7-vortex solutions are observed in the secondary flow. The flow transitions in the unsteady solutions are analyzed exquisitely by performing time-advancement of the solutions followed by inquisition of the phase space of time-dependent solutions. Results show that, if Dn > 2593.3709, the steady-state flow becomes periodic followed by a chaotic solution and 2- to 5-vortex solutions are observed for the unsteady flow. It is noticed that, if Dn > 0, the value of the Nusselt number (Nu) was initially higher on the cooling wall compared to the heated wall and at Dn = 102.67, Nu is found to be equal on both sidewalls and then it started increasing rapidly on the heated walls. The present study figured out how centrifugal force impacts fluid flow for larger pressure gradient, which assists fluid mixing and consequently enhances heat transfer in the fluid.

Dramatic mitigation of capacity decay and volume variation in vanadium redox flow batteries through modified preparation of electrolytes

Electrolyte imbalance caused by the undesired vanadium-ions cross-over and water transport through the membrane is one of the main critical issues of vanadium redox flow batteries, leading to battery capacity loss and electrolytes volume variation. In this work, the evolution of discharged capacity and electrolyte volume variation were firstly investigated adopting commercial electrolyte for hundreds of charge-discharge cycles in vanadium redox flow batteries employing different membranes, varying thickness and equivalent weight. Subsequently, with the support of a 1D physics-based model, the origin of the main phenomena regulating capacity decay and volume variation has been identified and different modifications in the preparation of electrolytes have been proposed. Electrolytes characterized by an equal proton concentration between the two tanks at the beginning of cycling operation turned out to limit capacity decay, while increasing electrolyte proton concentration was effective also in the mitigation of volume variation. The most promising electrolyte preparation combined the effect of high proton concentration and null osmotic pressure gradient between the two tanks: compared to commercial electrolyte this preparation reduced the capacity decay from 47.7% to 20.9%, increased the coulombic efficiency from 96.2% to 98.9% and the energy one from 79.9% to 83.4%, and also implied a negligible volume variation during cycles. The effectiveness of this electrolyte preparation has been verified with different membranes, increasing the range of validity of the results, that could be thus applied in a real system regardless of the adopted membrane.

Modeling density-driven flow and solute transport in heterogeneous reservoirs with micro/macro fractures

In this paper, a numerical model is presented to simulate the density-driven flow in heterogeneous porous media with micro- and macro-fractures. The micro-fractures (like fissures) with relatively small fracture lengths compared to the reservoir are modeled using an equivalent continuum model. The macro-fractures (like faults) with large fracture lengths are modeled using the extended finite element method (XFEM). Governing equations are based on the mass conservation equation for both the matrix and fracture domains together with the proper constitutive laws for the fluid velocity, dispersive velocity of solute, and equation of state for density of fluid phase. Several parameter studies are performed to investigate the effects of fracture characteristics on the solute spread in the medium. It is demonstrated that in the absence of macro-fractures, the influence of micro-fractures’ aperture on the solute spread is fully substantial and predictable. However, in the presence of macro-fractures, there is a region where the macro-fracture performs like a vacuum diminishing the influence of nearby micro-fractures. Finally, a real hydrological problem of the Gödöllő Hills reservoir located in Hungary with five different layers and eight faults, which is full of saline water is numerically modeled to investigate the effect of pressure gradient, anisotropy, and density difference on the solute transport. It is illustrated that in the top layers of the Gödöllő Hills reservoir, the gravitational force and vertical permeability are mostly accountable for the solute flow across the faults, whereas in the bottom layers, the horizontal pressure gradient is primarily responsible for the solute transport.

Simulations of the Characteristics of the Entropy Mode in Dipole-Magnetic-Confined Plasmas.

Plasmas confined in a dipole magnetic field widely exist in both space and laboratories, and this kind of plasma draws much attention from researchers both in plasma physics and in space science. In this paper, the characteristics of the collisionless electrostatic instability of the entropy mode in a dipole-magnetic-confined plasma are simulated with the linear gyrokinetic model. It is found that the entropy mode can be generated in dipole-magnetic-confined plasmas, and there are two typical stages of the entropy mode, with another transitional stage at different values of η. The main instability changes from the ion diamagnetic drift to the electronic diamagnetic drift as η becomes larger. In addition, the MHD mode predicts that the most stable point is at η~2/3 when k⟂ρi << 1. However, we find that η and k⟂ρi are coupled with each other, and the most stable point of the mode moves gradually to η~1 as k⟂ρi increases. There is a peak value for the entropy mode growth rate around k⟂ρi~1.0, and more complicated modes are induced so that the dispersion relation has been changed when the driving force of the plasma pressure gradient effect is obvious. For example, the characteristics of the interchange-like modes gradually emerge when the driving effect of the plasma pressure becomes stronger. Further investigations should be taken to reveal the characteristics of the entropy mode in magnetospheric plasmas.

Incipient Motion of Bed Material in a Channel with Varying Width and Vegetated Channel Walls

This experimental study aims to investigate the characteristics of turbulent flow in channels with vegetated banks and varying channel width under the condition of the incipient motion of bed material. The natural reeds were used as emergent vegetation on the sidewalls of a laboratory flume. In total, nine experimental runs have been conducted with different experimental setups by using three different particle sizes of bed material and three different channel bed slopes. An Acoustic Doppler velocimetry (ADV) was used to acquire velocity components in three directions. The results of this study indicate that the streamwise velocities have the maximum and minimum values at the cross sections with the narrowest and widest width, respectively. When the aspect ratio is less than 5, the maximum velocity occurs below the water surface, due to presence of the secondary currents. It is found that, at all measurement points, the distribution of the Reynolds shear stress has a Z-shaped profile owing to presence of vegetation on the channel sidewalls. By extrapolating the profiles for flow velocity and Reynolds shear stress towards the surface of the channel bed, the near-bed incipient velocities and the corresponding shear stresses for the incipient motion have been determined. By increasing the channel bed slope, the estimated near-bed parameters for all particle sizes decreased, indicating the dominance of the gravity effect over the pressure gradient effect. It was also observed that the Shields method was invalid for assessing the incipient motion of bed material in the presence of vegetation on the sidewalls of a channel that has a varying width.

Physics of generalized couette flow of immiscible fluids in anisotropic porous medium

This study presents an investigation on the generalized Couette flow of two immiscible Newtonian fluids in an anisotropic porous medium. The flow occurs between two parallel plates, driven by the combined effects of an axial pressure gradient and the movement of the upper plate. The region between the plates is filled with an anisotropic porous medium characterized by directional permeability. An exact solution is derived for the proposed problem by considering appropriate boundary and interface conditions. The slip condition is employed at the lower plate of the channel which has a significant role in the flow mechanics of fluids flowing through porous medium. The effects of directional permeability on the flow of immiscible fluids are explored, and specific cases are presented. This study reveals a significant influence of directional permeability on the flow velocity of both fluids. Furthermore, quantitative conclusions are drawn regarding the shear stress at the lower and upper wall of the channel, with implications for the anisotropic nature of blood arteries. The findings contribute to a deeper understanding of fluid flow in anisotropic porous media and have potential applications in various engineering domains.

Aeroacoustics of finite wall-mounted square cylinders in pressure gradient flows

Flow-induced noise produced by finite wall-mounted cylinders (FWMCs) is a major noise source for aircraft landing gear, rail pantographs and submarine appendages. These applications often encounter flows with various pressure gradients, however, there is little information in the literature on the effects of pressure gradients. Our recent work has demonstrated that the presence of a pressure gradient can significantly affect the near-wake flow structures of a square FWMC with a low aspect ratio of 2.4, thereby suppressing/enhancing the vortex-shedding tones. The current study extends this work to square FWMCs with varying aspect ratios, focusing on the role that aspect ratio plays in the noise generation of square FWMCs in pressure gradient flows. Experiments were undertaken using the open-jet pressure-gradient test rig in the UNSW anechoic wind tunnel, where the square FWMC model was immersed in flows with favourable-, near-zero-, and adverse-pressure gradients at a width-based Reynolds number of 28800. The square FWMC model was installed on a traversing system to realize the change in cylinder aspect ratio. Inside the test model, a series of channels and surface pressure taps were created to measure the unsteady surface pressure using a remote microphone technique. Far-field noise and unsteady surface pressure signals were simultaneously acquired to characterize the combined effects of aspect ratio and pressure gradient on the far-field noise production and reveal the noise generation mechanisms.

Single focused-beam acoustical tweezers: Trapping cells in 3-D

Single focused beams have shown great potential in acoustofluidics and medical ultrasound because of their large pressure gradient and acoustic-thermal effect, such as acoustical tweezers and high-intensity focused ultrasound (HIFU) technique. However, the three-dimensional trapping of typical cells is still challenging since cells in water have a positive acoustic contrast factor and will be repulsive from the focus [Gong and Baudoin, Phys. Rev. Applied 18, 044033 (2022)]. In this work, to reverse this positive acoustic contrast factor into negative, we propose to use a cell-friendly medium (made of iodixanol with water) [Augustsson et al., Nat. Commun. 7, 11556 (2016)]. This enables a single focused beam to selectively trap and control cells in 3-D, and more importantly, keeps the manipulated cells with good viability. This work will extend the applications of acoustical tweezers, which may be beneficial to single cell analysis, cellular phenotyping, precise assembly of different cells in tissue engineering, and controlled drug delivery.

Effect of Borehole Radial Pore Pressure Gradient on the Initiation and Propagation of Sandstone Hydraulic Fractures

Effect of Borehole Radial Pore Pressure Gradient on the Initiation and Propagation of Sandstone Hydraulic Fractures

Micro-structured fluid within a channel under static and oscillatory pressure gradients: A novel Darcy-Forchheimer flow investigation

The pertinence and importance of Darcy-Forchheimer flows can be found in large spectrum of engineering, biological and technological mechanisms like solar absorption and filtration, drainage problems, solar cells, ground stream hydrology, solar panels, irrigation and so forth. The major concern of this study is to interpret that how much a Darcy-Forchheimer flow of a micro-structured fluid (e.g., micropolar fluid) within a channel is influenced by the combined effects of static and oscillatory pressure gradients. Numerical treatment is carried out via three steps explicit Runge-Kutta method. Impacts of the preeminent parameters on the flow, mass and heat transport features of the problem are over looked by means of graphical and tabular representations. The numerical consequences evidently point out that the influence of oscillatory component is to elevate the fluid velocity whereas static component of pressure gradient tends to decelerate the velocity inside the channel. It is also determined here that Darcy parameter sufficiently reduces the angular velocity in the flow regime.

Effects of Variable Pressure Outlets for Array Jet Impingement Cooling With a Bidirectional Exit Air Scheme

Abstract Array jet impingement in conjunction with other cooling methods such as effusion cooling is used in gas turbine combustion zones to provide optimized cooling in the form of double wall cooling around a combustion chamber. Utilizing a transient liquid crystal (TLC) technique an experimental investigation into the effects of pressure gradients and single versus multiple exits for array jet impingement crossflow is evaluated in the form of a detailed heat transfer analysis. In this study, four pressure gradients to bias mass flow ratios as (1:1), (2:1), (3:1), and (1:0), two jet array configurations either inline or staggered with jet to jet spacings (x/D = y/D) of 1.4, 1.9, and 2.2, three jet to target distances (z/D) ranging from 2 to 4, and three Reynolds number from 5000 to 15,000 are considered. In total, a test matrix of 72 different performance conditions was evaluated. Results are presented as local and area averaged Nusselt number plots along with local heat transfer coefficient contours. Overall, Nusselt number decreases with increased (z/D) and increased pressure gradient bias toward a single exit from (1:1) to (1:0). There is also slightly better performance from inline jet array configurations compared to staggered configurations.