Konstantinos Ritos

Barriers to superfast water transport in carbon nanotube membranes

Carbon nanotube (CNT) membranes hold the promise of extraordinary fast water transport for applications such as energy efficient filtration and molecular level drug delivery. However, experiments and computations have reported flow rate enhancements over continuum hydrodynamics that contradict each other by orders of magnitude. We perform large scale molecular dynamics simulations emulating for the first time the micrometer thick CNTs membranes used in experiments. We find transport enhancement rates that are length dependent due to entrance and exit losses but asymptote to 2 orders of magnitude over the continuum predictions. These rates are far below those reported experimentally. The results suggest that the reported superfast water transport rates cannot be attributed to interactions of water with pristine CNTs alone.

Flow enhancement in nanotubes of different materials and lengths

The high water flow rates observed in carbon nanotubes (CNTs) have previously been attributed to the unfavorable energetic interaction between the liquid and the graphitic walls of the CNTs. This paper reports molecular dynamics simulations of water flow in carbon, boron nitride, and silicon carbide nanotubes that show the effect of the solid-liquid interactions on the fluid flow. Alongside an analytical model, these results show that the flow enhancement depends on the tubes geometric characteristics and the solid-liquid interactions.

Dynamics of nanoscale droplets on moving surfaces

We use molecular dynamics (MD) simulations to investigate the dynamic wetting of nanoscale water droplets on moving surfaces. The density and hydrogen bonding profiles along the direction normal to the surface are reported, and the width of the water depletion layer is evaluated first for droplets on three different static surfaces: silicon, graphite, and a fictitious superhydrophobic surface. The advancing and receding contact angles, and contact angle hysteresis, are then measured as a function of capillary number on smooth moving silicon and graphite surfaces. Our results for the silicon surface show that molecular displacements at the contact line are influenced greatly by interactions with the solid surface and partly by viscous dissipation effects induced through the movement of the surface. For the graphite surface, however, both the advancing and receding contact angles values are close to the static contact angle value and are independent of the capillary number; i.e., viscous dissipation effects are negligible. This finding is in contrast with the wetting dynamics of macroscale water droplets, which show significant dependence on the capillary number.

Electric fields can control the transport of water in carbon nanotubes

The properties of water confined inside nanotubes are of considerable scientific and technological interest. We use molecular dynamics to investigate the structure and average orientation of water flowing within a carbon nanotube. We find that water exhibits biaxial paranematic liquid crystal ordering both within the nanotube and close to its ends. This preferred molecular ordering is enhanced when an axial electric field is applied, affecting the water flow rate through the nanotube. A spatially patterned electric field can minimize nanotube entrance effects and significantly increase the flow rate.

Pressure- and temperature-driven flow through triangular and trapezoidal microchannels

A detailed study of pressure- and temperature-driven flows through long channels of triangular and trapezoidal cross sections is carried out. Due to the imposed pressure and temperature gradients there is a combined gas flow consisting of a thermal creep flow from the cold toward the hot reservoir and a Poiseuille flow from the high- toward the low-pressure reservoir. The formulation is based on the linearized Shakhov model subject to Maxwell boundary conditions, and it is solved numerically using a finite-difference scheme in the physical space and the discrete velocity method in the molecular velocity space. The results are valid in the whole range of the Knudsen number. In addition to the dimensionless flow rates, a methodology is presented to estimate for a certain set of input data the mass flow rates and the pressure distribution along the channel. Finally, special attention is given to the case of zero net mass flow and to the computation of the coefficient of the thermomolecular pressure difference.

Mechanical properties of pristine and nanoporous graphene

We present molecular dynamics simulations of monolayer graphene under uniaxial tensile loading. The Morse, bending angle, torsion and Lennard-Jones potential functions are adopted within the mdFOAM library in the OpenFOAM software, to describe the molecular interactions in graphene. A well-validated graphene model using these set of potentials is not yet available. In this work, we investigate the accuracy of the mechanical properties of graphene when derived using these simpler potentials, compared to the more commonly used complex potentials such as the Tersoff-Brenner and AIREBO potentials. The computational speed up of our approach, which scales O(1.5N), where N is the number of carbon atoms, enabled us to vary a larger number of system parameters, including graphene sheet orientation, size, temperature and concentration of nanopores. The resultant effect on the elastic modulus, fracture stress and fracture strain is investigated. Our simulations show that graphene is anisotropic, and its mechanical properties are dependant on the sheet size. An increase in system temperature results in a significant reduction in the fracture stress and strain. Simulations of nanoporous graphene were created by distributing vacancy defects, both randomly and uniformly, across the lattice. We find that the fracture stress decreases substantially with increasing defect density. The elastic modulus was found to be constant up to around 5% vacancy defects, and decreases for higher defect densities.

Implicit large eddy simulation of acoustic loading in supersonic turbulent boundary layers

This paper investigates the accuracy of implicit large eddy simulation in the prediction of acoustic phenomena associated with pressure fluctuations within a supersonic turbulent boundary layer. We assess the accuracy of implicit large eddy simulation against direct numerical simulation and experiments for attached turbulent supersonic flow with zero-pressure gradient, and further analyze and discuss the effects of turbulent boundary layer pressure fluctuations on acoustic loading both at the high and low frequency regimes. The results of high-order variants of the simulations show good agreement with theoretical models, experiments, as well as previously published direct numerical simulations.

Physical insight into a Mach 7.2 compression corner flow

High-order implicit Large Eddy Simulations were conducted to study shock-boundary layer interaction around a 33° compression corner at Mach 7.2 and Reynolds number of Reθ = 3,500 based on the momentum thickness. A grid-convergence study was performed to reduce the computational uncertainty and the results were compared with experiments and theoretical predictions. Furthermore, the turbulent flow properties were analysed with respect to the Reynolds normal stress, skewness and flatness, and conclusions were drawn regarding the shock boundary layer interaction behavior.

Thermoacoustic effects in high-speed compressible transitional and turbulent boundary layers

A numerical investigation of the thermal and acoustic effects in high-speed compressible flows is presented. Two case studies are considered: i) transition to turbulence in supersonic flows over a flat plate, and ii) supersonic shock wave turbulent boundary layer interaction (SWTBLI) over a compression ramp. Implicit Large Eddy Simulations (iLES) are performed using the second and fifth order Monotone-Upstream Central Scheme for Conservation Laws (MUSCL) and the ninth order Weighted Essentially Non-Oscillatory (WENO) schemes. The aim of this study is twofold: i) to examine the acoustic and thermal effects associated with transitional and turbulent boundary layers, particularly in the near wall region; ii) to investigate the effects of numerical accuracy on acoustic and thermal loading. The results are compared with theoretical models, Direct Numerical Simulations (DNS) and experiments.

Microflow leakage through the clearance of a metal-metal seal

The motivation behind this study is to simulate high pressure gas flow through the clearance between a valve seat and disc when in a closed position using a representative model. This leakage phenomenon is common in metal-to-metal seal pressure relief valves. As a pressure relief valve reaches the set pressure, it is known for the leakage to increase. The representative model that we studied is of an ideal-gas flow through a 2D micro-channel in the slip flow regime. We used a laminar continuum flow solver which solved the mass, momentum and energy equations. In addition, we applied low pressure slip boundary conditions at the wall boundaries which considered Maxwells model for slip. The channel height was varied from 1μm to 5μm while the length remained at 1.25 mm, this means the length to height ratio varied from 1250 to 250. Inlet pressure was varied from a low pressure (0.05 MPa) to a high pressure (18.6 MPa), while the outlet remained constant at atmospheric. The calculated mass flow rate is compared to an analytical solution giving very good agreement for low pressure ratios and high length to height ratios.