Strength Of Pressure Gradient Research Articles

Nanoscopic insights of saline water in carbon nanotube appended filters using molecular dynamics simulations.

Nanotube appended membranes are shown to be very promising due to their ultrafast water transport and very high salt rejection ability. Using classical molecular dynamics, the present study reports the nanoscopic assessment of various molecular events for nanotube-based desalination, which might be useful for nanoscale devices during process operation at the macroscopic scale. The characteristics of water and ion flow are discussed with varied strength of pressure gradient and salt concentration for different scales of confinement. The results revealed that the membranes comprising nanotubes of 1.0-1.1 nm diameter can be optimized for efficient water desalination with more than >95% salt rejection. Furthermore, the anomalies in water flux through nanotubes are linked with the hydration characteristics of ions inside CNTs. The results show the maximum hydration of confined ions inside the nanotubes, which indicated the minimum permeability of water due to freezing effects. Furthermore, the MD results revealed that akin to bulk phases, the mass transport through nanotubes can be linked with the component diffusivity in the medium. It has been demonstrated that not only the diffusivities of water and ions, but even the gradient of water to ion diffusivity might be utilized to predict and explore the experimental observations, which might be helpful in optimizing the operational regime in nanotube-based filtrations. Moreover, the thermodynamic characteristics of the flow are discussed in terms of the entropy of water and ions using the robust two-phase thermodynamic (2PT) method. The results reflect that the entropy of water is linked to the distortion of the hydrogen bond network inside the nanotube confinement, at the nanotube-water interface and at the bulk solution, whereas the entropy of ions seems to be majorly dominated by their oscillation. Also, the interconnection of hydration structure, mass flux and the diffusivity of water and ions along with their thermodynamic origin are discussed.

Adverse-Pressure-Gradient Effects on Turbulent Boundary Layers: Statistics and Flow-Field Organization

This manuscripts presents a study on adverse-pressure-gradient turbulent boundary layers under different Reynolds-number and pressure-gradient conditions. In this work we performed Particle Image Velocimetry (PIV) measurements supplemented with Large-Eddy Simulations in order to have a dataset covering a range of displacement-thickness-based Reynolds-number 2300 <Re_{delta ^{*}}< 34000 and values of the Clauser pressure-gradient parameter β up to 2.4. The spatial resolution limits of PIV for the estimation of turbulence statistics have been overcome via ensemble-based approaches. A comparison between ensemble-correlation and ensemble Particle Tracking Velocimetry was carried out to assess the uncertainty of the two methods. The effects of β, Re and of the pressure-gradient history on turbulence statistics were assessed. A modal analysis via Proper Orthogonal Decomposition was carried out on the flow fields and showed that about 20% of the energy contribution corresponds to the first mode, while 40% of the turbulent kinetic energy corresponds to the first four modes with no appreciable dependence on β and Re within the investigated range. The topology of the spatial modes shows a dependence on the Reynolds number and on the pressure-gradient strength, in line with the results obtained from the analysis of the turbulence statistics. The contribution of the modes to the Reynolds stresses and the turbulence production was assessed using a truncated low-order reconstruction with progressively larger number of modes. It is shown that the outer peaks in the Reynolds-stress profiles are mostly due to large-scale structures in the outer part of the boundary layer.

Large-scale motions in turbulent boundary layers subjected to adverse pressure gradients

It is known that large-scale streamwise velocity-fluctuating structures ($u^{\prime }$) are frequently observed in the log region of a zero pressure gradient turbulent boundary layer, and that these motions significantly influence near-wall small-scale $u^{\prime }$-structures by modulating the amplitude (Hutchins &amp; Marusic, J. Fluid Mech., vol. 579, 2007, pp. 1–28; Mathis et al., J. Fluid Mech., vol. 628, 2009, pp. 311–337). In the present study, we provide evidence that the spatial organization of large-scale structures in the log region is significantly influenced by the strength of adverse pressure gradients in turbulent boundary layers based on a direct numerical simulation dataset. For a mild adverse pressure gradient boundary layer flow, groups of hairpin vortices are coherently aligned in the streamwise direction to form hairpin vortex packets, and streamwise merging events of the induced large-scale $u^{\prime }$-structures create a larger streamwise length scale of structures than that for a zero pressure gradient boundary layer flow. As the pressure gradient strength increases further, however, the formation of hairpin packets is continuously suppressed, and large-scale motions are consequently not concatenated to create a longer motion, resulting in a significant reduction of the streamwise coherence of large-scale structures in the log layer. Although energy spectrum maps for $u^{\prime }$-structures show that the large-scale energy is continuously intensified above the log layer with an increase in the pressure gradient, amplitude modulation of the near-wall small-scale motions is dominantly induced by log region large-scale structures for adverse pressure gradient flows. Conditional averaged flow fields with large-scale Q2 and Q4 events indicate that large-scale counter-rotating roll modes play an important role in organizing the flows under the pressure gradients, and the large-scale roll modes associated with Q4 events are more enhanced in the outer layer than those associated with Q2 events, reducing the streamwise coherence of the vortices in a packet.

Mass transport analysis of non-Newtonian fluids under combined electroosmotically and pressure driven flow in rectangular microreactors

Hydrodynamically fully developed flow of power-law fluids under combined action of electroosmotic and pressure gradient forces in rectangular microreactors is analyzed considering heterogeneous catalytic reactions. The Poisson-Boltzmann, Cauchy momentum, and concentration equations are considered in two dimensions and after being dimensionless are numerically solved applying a finite difference algorithm. Variation of axial concentration gradient, and axial and horizontal mass diffusions are taken into account as well. To accomplish a more general analysis, the velocity distribution is obtained by solving continuity and Cauchy momentum equations and is not considered as an average axial velocity for solving the concentration equation. Profiles of mean concentration of reactants are analyzed as functions of operating variables. Results reveal that mean concentration increases with increasing Péclet number and dimensionless Debye-Hückel parameter. It is also a decreasing function of Damkohler number. Presence of pressure gradient will affect the decreasing trend of mean concentration through the channel. Shear-thinning fluids, due to their nature, are most strongly influenced by pressure gradient and Newtonian and shear-thickening fluids are less affected, respectively. Favorable pressure gradient will decelerate mean concentration rate of decrease and adverse pressure gradient acts the opposite. Moreover, a larger aspect ratio leads to slower descending rate of mean concentration in purely electroosmotic and pressure-assisted flows while in a pressure-opposed flow, the behavior might change depending on the strength of pressure gradient and the value of channel aspect ratio.

Investigation of impinging single and twin circular synthetic jets flow field

Synthetic jets are largely used in the electronic cooling field; indeed their heat transfer performances have been widely investigated. The heat transfer performances have been enhanced through the design of innovative synthetic jet devices, as the twin synthetic jets device. Obviously the heat transfer performances of the classic and innovative synthetic jet devices are strictly related to their impinging flow field. Therefore the behavior of impinging single and twin circular synthetic jets in phase opposition is experimentally investigated by using Particle Image Velocimetry (PIV) at Reynolds and Strouhal numbers equal to 5100 and 0.024, respectively. Several nozzle-to-plate distances (H), ranging between 2 and 10 nozzle diameters (D), have been investigated. The time-averaged behavior of the velocity components has been reported and discussed. Their distributions, near the impinging plate, have been described. For the single jet, at short nozzle-to-plate distances (H/D<4) the axial velocity profile near the impinging plate shows a double peak with a minimum on the jet axis. Instead, at high nozzle-to-plate distance (H/D>6), the axial velocity profile is bell-shaped. This is ascribed to the adverse pressure gradient strength and the potential core-like region extension. External oscillations are observed in all the flow field quantities near the impinging plate at 2 diameters from the stagnation point due to a secondary counter rotating vortex ring generation. The presence of such a counter rotating vortex ring decreases as the nozzle-to-plate distance increases. Comparing the two synthetic jet configurations, higher axial velocity and turbulence level but lower axial phase-correlated organized contribution to velocity have been found for the twin case because of the jets interaction. The evolution of the flow field for both configurations has been explained through phase-averaged measurements. High turbulence is observed along the shear layer emanated by the nozzle edge and in the vortex ring core. During the suction phase the saddle point shows a different behavior in the two configurations. In the single case, the saddle point reaches the impinging plate causing injection of air from the plate into the device. Differently the twin configuration generates two saddle points which do not reach the impinging plate because of the presence of the other impinging synthetic jet.

The log behaviour of the Reynolds shear stress in accelerating turbulent boundary layers

Direct numerical simulation of highly accelerated turbulent boundary layers (TBLs) reveals that the Reynolds shear stress,$\overline{u^{\prime }v^{\prime }}^{+}$, monotonically decreases downstream and exhibits a logarithmic behaviour (e.g. $-\overline{u^{\prime }v^{\prime }}^{+}=-(1/A_{uv})\ln y^{+}+B_{uv}$) in the mesolayer region (e.g. $50\leqslant y^{+}\leqslant 170$). The thickness of the log layer of$\overline{u^{\prime }v^{\prime }}^{+}$increases with the streamwise distance and with the pressure gradient strength, extending over a large portion of the TBL thickness (up to 55 %). Simulations reveal that$V^{+}\,\partial U^{+}/\partial y^{+}\sim 1/y^{+}\sim \partial \overline{u^{\prime }v^{\prime }}^{+}/\partial y^{+}$, resulting in a logarithmic$\overline{u^{\prime }v^{\prime }}^{+}$profile. Also,$V^{+}\sim -y^{+}$is no longer negligible as in zero-pressure-gradient (ZPG) flows. Other experimental/numerical data at similar favourable-pressure-gradient (FPG) strengths also show the presence of a log region in$\overline{u^{\prime }v^{\prime }}^{+}$. This log region in$\overline{u^{\prime }v^{\prime }}^{+}$is larger in sink flows than in other spatially developing FPG flows. The latter flows exhibit the presence of a small power-law region in$\overline{u^{\prime }v^{\prime }}^{+}$, which is non-existent in sink flows.

0135 振動するはく離点を持つ平板上のはく離流れの制御(OS1-7 噴流,後流および剥離流れの流動解析と応用,オーガナイズドセッション)

The flow separation on a flat plate under adverse pressure gradient was investigated by flow visualization. The strength of pressure gradient was adjusted by changing the angle of the ceiling plate. The results showed that the location of the separation bubble moved upstream by the increase in the adverse pressure gradient.

Transpired Turbulent Boundary Layers Subject to Forced Convection and External Pressure Gradients

The problem of forced convection transpired turbulent boundary layers with external pressure gradient has been studied by using different scalings proposed by various researchers. Three major results were obtained: First, for adverse pressure gradient boundary layers with suction, the mean deficit profiles collapse with the free stream velocity, U∞, but into different curves depending on the strength of the blowing parameter and the upstream conditions. Second, the dependencies on the blowing parameter, the Reynolds number, and the strength of pressure gradient are removed from the outer flow when the mean deficit profiles are normalized by the Zagarola/Smits [Zagarola, M. V., and Smits, A. J., 1998, “Mean-Flow Scaling of Turbulent Pipe Flow,” J. Fluid Mech., 373, 33–79] scaling, U∞δ*/δ. Third, the temperature profiles collapse into a single curve using the new inner and outer scalings proposed by Wang and Castillo [Wang, X., and Castillo, L., 2003, “Asymptotic Solutions in Forced Convection Turbulent Boundary Layers,” J. Turbulence, 4(006)], which produce the true asymptotic profiles even at finite Pe´clet number.

Numerical simulation of the aeroacoustic noise in the separated laminar boundary layer

The unsteady flow characteristics and the related noise of separated incompressible laminar boundary layer flows (Reδ*=614, 868, and 1,063) are numerically investigated. The characteristic lines of the wall pressure are examined to identify the primary noise source, related with the unsteady motion of the vortex at the reattachment point of the separation bubble. The generation and propagation of the vortex-induced noise in the separated laminar boundary layer are computed by the method of Computational Aero-Acoustics (CAA), and the effects of Reynolds number, Mach number and adverse pressure gradient strength are examined.

The Dynamics of Northwest Summer Winds over the Santa Barbara Channel

A mesoscale model is used to examine the dynamics of northwest flow over the Santa Barbara Channel region. Three cases are considered, each characterized by typical summertime synoptic conditions, but with differences in pressure gradient strength and marine boundary layer depth (MBL). The first case examines a relatively deep MBL and strong pressure gradient. Case 2 is characterized by a more shallow MBL and weaker pressure gradient, and case 3 represents a transition from a deep MBL to shallow conditions. In all cases, simulated surface winds show reasonable agreement with observations over most of the model domain, with the exception of regions near abrupt terrain changes. Results from the model indicate that the flow with a deep MBL (∼400 m) and strong pressure gradient (case 1) is supercritical, causing regions of acceleration and expansion in the lee of Point Conception. When the MBL is shallow (∼150 m) (case 2), a transcritical flow scenario exists with subcritical flow upstream from Point Conception and a supercritical flow region over the Santa Barbara Channel and downstream from the Channel Islands. Flow over the channel is strongly affected by diurnal heating in shallow MBL cases, reversing direction in step with a land breeze circulation induced by nighttime cooling. The land breeze forces an internal wave disturbance that propagates westward across the channel, eliminating the supercritical flow region in the lee of Point Conception. Conditions with a deep MBL (∼400 m) produce less variability in the surface winds, except for the region sheltered by the Santa Ynez Mountains. An expansion fan is still evident in this case, but it is produced by the interaction of the flow with higher terrain north and east of the channel. The low hills on Point Conception and the Channel Islands do not have a large blocking effect on the surface flow when the MBL is deep. Analysis of the momentum budget supports the conclusion that the boundary layer behaves like a transcritical hydraulic flow when the MBL is shallow. Except for the open ocean region, the Coriolis term is minor in comparison with the pressure and advection terms. Diurnal heating effects are evident in the nearshore pressure term, which varies from offshore during the late evening to onshore in the afternoon. These effects are most significant when the MBL is shallow and can augment the hydraulically forced pressure pattern, causing a stronger expansion fan in the late afternoon and a collapse of the expansion fan during the early morning.

Mean-flow measurements in a supersonic three-dimensional turbulent boundary layer

Measurements are presented of the mean flow in a supersonic turbulent boundary layer subjected to a constant weak transverse pressure gradient. The temperature and longitudinal velocity and the axial shear stress were only slightly affected by the three-dimensionality but a clearly defined crossflow component appeared, in the manner suggested by theory and confirmed in earlier experiments. The flow deflection angle and transverse-velocity component achieved a self-preserving form so long as the transverse-pressure gradient remained constant, and both achieved a maximum at the sublayer edge; the deflection angle seemed to decrease again between the latter and the surface. An empirical relation was found between the pressure-gradient strength and the maximum in the crossflow, and the dependence of the latter on distance from the surface was used to test analytical crossflow predictions. The data are in general agreement with Van Den Berg's law of the wall. The data also support the so-called parabolic law following a relaxation distance, especially if inviscid gradients are accounted for, and if the normal coordinate is contracted by the compressibility transformation.