Study of the thermomolecular pressure difference phenomenon in thermal creep flows through microchannels of triangular and trapezoidal cross sections

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.A detailed study of pressure and temperature driven flows through long channels of triangular and trapezoidal cross sections is carried out. The solution is based on the linearized Shakhov model subject to Maxwell boundary conditions and it is valid in the whole range of the Knudsen number. In addition to the dimensionless flow rates, a methodology is presented to estimate the pressure distribution along the channel, as well as the coefficient of the thermomolecular pressure difference

Wall-pressure spectra models for supersonic and hypersonic turbulent boundary layers

This paper presents an investigation of pressure fluctuations spectra beneath supersonic and hypersonic turbulent boundary layers at zero pressure gradient. High-order implicit Large Eddy Simulations have been used to provide numerical data for assessing existing models and further improving their qualitative accuracy in predicting wall-pressure spectra. Several different models have been investigated and it is shown that existing models fail to capture the correct behaviour of pressure fluctuations in supersonic and hypersonic boundary layers across a broad range of frequencies. The models have been modified by introducing compressibility corrections. The modified models are validated against implicit Large Eddy Simulations, Direct Numerical Simulations, and experimental data. The qualitative accuracy of the models is discussed and the most promising model is identified

Acoustic loading beneath hypersonic transitional and turbulent boundary layers

This paper concerns a study of pressure fluctuations beneath hypersonic transitional and turbulent boundary layers and associated acoustic loading on a flat surface. We have employed high-order implicit large eddy simulations in conjunction with the atmospheric (von Kármán) multimode energy spectrum as initial condition, and conducted simulations at Mach 4, 6 and 8 and for different inflow turbulence intensities. The spectral analysis of the pressure fluctuations shows consistent results with the available theoretical, experimental and numerical data for fully turbulent boundary layers. In the transition region the spectrum roll-off diverges from the existing scaling predictions for incompressible, as well as fully-turbulent compressible flows. This study shows that the spectrum in the transition region is governed by different scaling laws. The Mach number has a direct impact on the spectrum for both transitional and fully turbulent flows, especially in the high-frequency region of the spectrum. Increasing the inlet turbulence intensity leads to higher amplitude pressure fluctuations in the mid-to-high-frequency region, faster transition to turbulence, and higher acoustic loading on the solid surface

Computational aeroacoustics beneath high speed transitional and turbulent boundary layers

This paper concerns a study of pressure fluctuations beneath hypersonic shock- wave turbulent boundary layer interactions and the associated acoustic loading on a compression/expansion ramp. We have employed high-order implicit large eddy simulations and conducted simulations at Mach 7.2. The spectral analysis of the pressure fluctuations at various locations of the compression/expansion ramp are compared with the spectra calculated beneath a hypersonic transitional boundary layer. Similarities and differences between the two hypersonic boundary layers, in the context of acoustic loading, are drawn. Extremely high values of pressure fluctuations are recorded after the shock re-attachement where the maximum pressure gradients are also observed, indicating that acoustic loading is correlated with areas of high pressure gradients. Finally, we show the impact of the boundary layer state (attached flow, turbulence bursts, recirculations, shock oscillations, shock re-attachment and expansion fans) on the frequency spectrum of the pressure fluctuations

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

Electrowetting controls the deposit patterns of evaporated salt water nanodroplets

So-called “coffee-ring” stains are the deposits remaining after complete evaporation of droplets containing non-volatile solutes. In this paper we use Molecular Dynamics to simulate the evaporation of salt water nanodroplets in the presence of an applied electric field. We demonstrate, for the first time, that electrowetted nanodroplets can produce various deposit patterns, which vary substantially from the original ring-like deposit that occurs when there is no electric field. If a direct current (DC) electric field with strength greater than 0.03 V/Å is imposed parallel to the surface, after the water evaporates the salt crystals form a deposit on the substrate in a ribbon pattern along the field direction. However, when an alternating current (AC) electric field is applied the salt deposit patterns can be either ring-like or clump, depending on the strength and frequency of the applied AC field. We find that an AC field of high strength and low frequency facilitates the regulation of the deposit patterns: the threshold electric field strength for the transition from ring-like to clump is approximately 0.006 V/Å. These findings have potential application in fabricating nanostructures and surface coatings with desired patterns

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 frac- ture 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

Hybrid molecular-continuum simulations of water flow through carbon nanotube membranes of realistic thickness

We present new hybrid molecular-continuum simulations of water flow through filtration membranes. The membranes consist of aligned carbon nanotubes (CNTs) of high aspect ratio, where the tube diameters are ~1–2 nm and the tube lengths (i.e. the membrane thicknesses) are 2–6 orders of magnitude larger than this. The flow in the CNTs is subcontinuum, meaning standard continuum fluid equations cannot adequately model the flow; also, full molecular dynamics (MD) simulations are too computationally expensive for modelling these membrane thicknesses. However, various degrees of scale separation in both time and space in this problem can be exploited by a multiscale method: we use the serial-network internal-flow multiscale method (SeN-IMM). Our results from this hybrid method compare very well with full MD simulations of flow cases up to a membrane thickness of 150 nm, beyond which any full MD simulation is computationally intractable. We proceed to use the SeN-IMM to predict the flow in membranes of thicknesses 150 nm–2 μm, and compare these results with both a modified Hagen–Poiseuille flow equation and experimental results for the same membrane configuration. We also find good agreement between experimental and our numerical results for a 1-mm-thick membrane made of CNTs with diameters around 1.1 nm. In this case, the hybrid simulation is orders of magnitude quicker than a full MD simulation would be

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