Effect Of Slip Length Research Articles

Effect of slip length on flow dynamics and heat transport in two-dimensional Rayleigh–Bénard convection

We report a direct numerical simulation (DNS) study of the heat transport and temperature profiles of the plume ejecting and impacting regions in the two-dimensional turbulent Rayleigh–Bénard (RB) convection with slippery plates and horizontally periodic boundary conditions. The numerical study is conducted in the parameter range of Rayleigh number from to and the slip length b from 0 (NS) to ∞ (FS) for the top and bottom plates. Two distinct flow patterns can be seen depending on b, namely convection roll state and zonal flow, which affect the Nusselt number and the Reynolds number . We show that the zonal flow occurs when the normalised slip length , where is the thermal boundary layer thickness for the no-slip (NS) plates. and increase with increasing , and can reach the optimum before the generation of the zonal flow. It is observed that with the effective scaling exponent for the convection roll state, and for the zonal flow. Furthermore, for the convection roll state, the power-law scaling of the local heat flux is in the plume ejecting region, while in the plume impacting region, for varying slip length . The DNS data with different slippery plates for both plume ejecting and impacting regions agree well with the predicted temperature profiles by Huang et al. (J Fluid Mech. 2022;943:A2).

Droplet formation under wall slip in a microfluidic T-junction

Although no slip is a universally accepted condition for wall-bounded flows, velocity slip occurs due to lubricating action by stable air pockets sandwiched between the fluid and solid surface asperities when a flow takes place along a superhydrophobic surface. The effects of velocity slip on single-phase flow systems have been reported by several researchers. However, its impact on the two-phase flow system has been less investigated. The present work investigates the effect of velocity slip at the wall during droplet formation in a microfluidic T-junction. The presence of velocity slip exhibited by the dispersed phase liquid on the channel wall has been experimentally measured with micro-PIV, and numerical simulations have been performed for investigating the effects of continuous phase fluid slip length on the two-phase flow system. We have also developed an analytical model to establish the effect of wall slip on the droplet length for various properties. All the results (numerical, experimental, and analytical) produce a satisfactory agreement. The effect of slip length on the shear stress at a lower flow rate is more pronounced than at a higher flow rate ratio. An increase in the slip of the continuous phase liquid enhances the vorticity inside the droplet, which resists the droplet detachment, increasing the droplet length.

The Effects of Asymmetric Slip Flow Between Parallel Plates of a Microchannel Under Uniform Heat Flux

Abstract Microchannels have many applications in the field of modern technologies. Today, it is necessary to increase the efficiency of these systems due to the increasing rate of thermal loads. Recent researches have shown that liquid flows in the microchannels depend on their size and surface properties. Consequently, there is no symmetric flow condition in a symmetric geometric channel necessarily. In this study, the effect of slip length on the amount of asymmetric heat transfer in a microchannel is investigated numerically. By calculating the parameters such as Nusselt number and local pressure drop coefficient, it is observed that the asymmetric slip flow affects the flow profile and could lead to a decrease or increase in the heat transfer in the microchannel. According to the results, asymmetric slip flow can lead to a 20–40% reduction or increase in the Nusselt number. The most important point in the design of micro-scale asymmetric cooling systems is the increase of slip length at high temperatures.

Effect of the slip length on the flow over a hydrophobic circular cylinder

The laminar flow (Reynolds number ranging from 60 to 180) past a hydrophobic circular cylinder is conducted by the time-dependent simulation with focus on the effect of dimensionless slip length, LS (ranging from 0.01 to 0.25), on total, viscous and pressure drag coefficients. It is found that the viscous drag plays a major role in drag reduction, for low Reynolds (Re < 100) and small slip length (LS < 0.02). When the Reynolds number and slip length are large enough, the decrease of total drag mainly depends on the pressure drag. The pressure drag coefficient increases with the Reynolds number when LS ranges from 0 to 0.02. For LS of 0.05, however, it has a different trend. Due to small proportion of viscous drag to total drag coefficient, the effect of viscous drag on the total drag is less important. The adverse pressure gradient appears on the circular cylinder and increases with the slip length. The pressure distribution is mainly affected by the vorticity gradient, fluid convection and the stability. Meanwhile, as Reynolds number equals to 100, the separation point moves downstream. This results in the increase of vortex shedding frequency and the decrease of vortex intensity at LS ranging from 0 to 0.11. As slip length increases to 0.25, the flow is stable. These results can provide useful information on the drag reduction control of the cylinder surface.

Computational fluid dynamics modeling of fluid flow and heat transfer in the central pore of carbon nanopipes

Carbon nanopipes, which can be employed in micro- and nano-fluidic devices, have some extraordinary mechanical, optical, flow and electrical properties. In addition, these devices can manipulate very small amounts of liquid. One of the main aspects of these pipes is an unusual enhancement in the flow rate, and it should be explored whether this enhancement can be justified by continuum fluid mechanics or not. Computational fluid dynamics were employed for exploring this phenomenon. To explore this unusual flow enhancement, different parameters, such as temperature, inlet pressure, tube diameter, tube length and slip length were used. With varying parameters, the slip length can explain this phenomenon. The heat transfer was also numerically investigated and an improvement in the heat transfer coefficient and the convective heat flux magnitude were observed for the carbon nanopipes. Then, based on the numerical investigation, correlations were developed for the friction factor and the Nusselt number, and these correlations were verified by the reported experimental data for nanopipes. Moreover, the effects of this slip length on the velocity profile was investigated and the observed shape of the velocity profile was also completely changed from convex to plug-like by importing the effect of slip length on the walls of the tubes.

Effect of Slip Length on Entropy Generation of Fully Developed Forced Convection in a Micro-Channel for a Power Law Fluid

In this paper, the effect of slip length on entropy generation of fluid flow in a microchannel is analyzed. The flow between the parallel plates is considered fully developed both hydrodynamically and thermally. Uniform heat flux boundary condition is assumed. The fluid is supposed to be non Newtonian, following a power law model. To model the slip, Navier's linear slip law has been used. The spatial distribution of fluid velocity, non dimensional temperature, non dimensional entropy generation and Bejan number has been investigated for different slip length ratios. For dilatants fluids, it is found that that the effect of slip length is significant and cannot be neglected.

The effect of slip length on vortex rebound from a rigid boundary

The problem of a dipole incident normally on a rigid boundary, for moderate to large Reynolds numbers, has recently been treated numerically using a volume penalisation method by Nguyen van yen, Farge, and Schneider [Phys. Rev. Lett. 106, 184502 (2011)]. Their results indicate that energy dissipating structures persist in the inviscid limit. They found that the use of penalisation methods intrinsically introduces some slip at the boundary wall, where the slip approaches zero as the Reynolds number goes to infinity, so reducing to the no-slip case in this limit. We study the same problem, for both no-slip and partial slip cases, using compact differences on a Chebyshev grid in the direction normal to the wall and Fourier methods in the direction along the wall. We find that for the no-slip case there is no indication of the persistence of energy dissipating structures in the limit as viscosity approaches zero and that this also holds for any fixed slip length. However, when the slip length is taken to vary inversely with Reynolds number then the results of Nguyen van yen et al. are regained. It therefore appears that the prediction that energy dissipating structures persist in the inviscid limit follows from the two limits of wall slip length going to zero, and viscosity going to zero, not being treated independently in their use of the volume penalisation method.

The reduction of viscous extrusion stresses and extrudate swell computation using slippery exit surfaces

Wall slip has been pointed out as a way to reduce cracks and swelling on polymer extrudates. To investigate this phenomenon, the capillary extrusion of a purely viscous generalized Newtonian fluid is computed with the finite element code POLYFLOW with a realistic slip law. This fits with the friction curve data for a polydimethylsiloxane (PDMS) in a steel die. It is based on molecular dynamics theory and contains a critical stress below which there is qualitative adhesion at the wall. The existence of a slip boundary condition is shown to modify the morphology of the velocity field which tends toward a plug flow. It thus largely reduces the fully developed stress level and the exit stress concentration. Localized slip at the die exit appears even for fully developed flow under shear rate values which are lower than the critical shear rate corresponding to the occurrence of slip at the wall in a Poiseuille flow. This is explained by exit stress concentration. The effect of slip length is then examined: it is found that exit stress and extrudate swell can be largely reduced by using a very short slip length only. This result shows the interest of optimising slippery surfaces in the design of extrusion processes.