Search

The design of rigid vortex generators (RVG) influences the thermal performance of various technologies. We employed Discrete Adjoint-Based Optimization to show the optimal development of vortex generators. Under turbulent flow conditions, different bi-objective functions on the RVG design were examined. Specifically, we aimed at an optimal RVG shape that minimizes the pressure drop and maximizes the local heat transfer in a rectangular channel. We show that an optimal design of an RVG can be obtained using computational fluid dynamics in conjunction with the Pareto Front at a computational cost of the order ~ O (10 - 1) . We obtained three essential vortex generator shapes based on the RVG morphing technique. Compared to the baseline geometry of a delta winglet pair DWP, the first morphed design reduced the pressure drop by 39 % , however, at the expense of a 21 % reduction in the Nusselt number. The second vortex generator design enhanced the heat transfer by 18 % , however, at the cost of a significant increase in pressure drop of about 40 % . The final morphed design achieved the highest thermal performance factor of 1.28, representing a heat transfer enhancement of 6 % with a moderate increase in pressure drop of about 13 % compared to DWP vortex generators. Furthermore, we investigated the effect of introducing different size holes on the mass reduction of vortex generators and their thermal performances. The mass of vortex generators can be reduced by 9 % and with an increase of 7 % in thermal performance factor concerning the DWP baseline. The findings of this study will lead to highly efficient lightweight heat exchangers. [ABSTRACT FROM AUTHOR]

According to WHO, by 2050, at least one person out of two will suffer from an allergy disorder resulting from the accelerating air pollution associated with toxic gas emissions and climate change. Airborne pollen, and associated allergies, are major public health topics during the pollination season, and their effects are further strengthened due to climate change. Therefore, assessing the airborne pollen allergy risk is essential for improving public health. This study presents a new computational fluid dynamics methodology for risk assessment of local airborne pollen transport in an urban environment. Specifically, we investigate the local airborne pollen transport from trees on a university campus in the north of France. We produce risk assessment maps for pollen allergy for five consecutive days during the pollination season. The proposed methodology could be extended to larger built-up areas for different weather conditions. The risk assessment maps may also be integrated with smart devices, thus leading to decision-aid tools to better guide and protect the public against airborne pollen allergy. [ABSTRACT FROM AUTHOR]

This paper presents an overview of past and current research in computational modelling of micro- and nanofluidic systems with particular focus on recent advances in multiscale modelling. Different mesoscale and hybrid molecular-continuum methods are presented. The contributions of these methods to a broad range of applications, as well as the physical and computational modelling challenges associated with the development of these methods, are also discussed. [ABSTRACT FROM AUTHOR]

This paper concerns the behaviour of a copper-argon nanofluid confined in a nanochannel. Using molecular dynamics simulations, it is shown that in narrower channels, the thermal conductivity increases by approximately 20 % compared to macroscopic cases. The results suggest that the structured liquid layers surrounding the solid particles occupy a greater percentage of the system in narrower channels, thus enhancing the thermal conductivity of the nanofluid. [ABSTRACT FROM AUTHOR]

Application of ultrasound to biological tissues has been identified as a promising cancer treatment technique relying on temporal enhancement of biological membrane permeability via shock wave impact. In the present study, the effects of ultrasonic waves on a 1,2-dipalmitoyl-sn-phosphatidylcholine biological membrane are examined through molecular dynamics simulations. Molecular dynamics methods traditionally employ periodic boundary conditions which, however, restrict the total simulation time to the time required for the shock wave crossing the domain, thus limiting the evaluation of the effects of shock waves on the diffusion properties of the membrane. A novel method that allows capturing both the initial shock wave transit as well as the subsequent longer-timescale diffusion phenomena has been successfully developed, validated and verified via convergence studies. Numerical simulations have been carried out with ultrasonic impulses varying from 0.0 to 0.6 mPa s leading to the conclusion that for impulses ≥0.45 mPa s, no self-recovery of the bilayer is observed and, hence, ultrasound could be applied to the destruction of localized tumor cells. However, for impulses ≤0.3 mPa s, an increase in the transversal diffusivity of the lipids, indicating a consequent enhancement of drug absorption across the membrane, is initially observed followed by a progressive recovery of the initial values, thereby suggesting the advantageous effects of ultrasound on enhancing the chemotherapy efficiency. [ABSTRACT FROM AUTHOR]

This paper presents an artificial neural network-based multiscale method for coupling continuum and molecular simulations. Molecular dynamics modelling is employed as a local 'high resolution' refinement of computational data required by the continuum computational fluid dynamics solver. The coupling between atomistic and continuum simulations is obtained by an artificial neural network (ANN) methodology. The ANN aims to optimise the transfer of information through minimisation of (1) the computational cost by avoiding repetitive atomistic simulations of nearly identical states, and (2) the fluctuation strength of the atomistic outputs that are fed back to the continuum solver. Results are presented for prototype flows such as the isothermal Couette flow with slip boundary conditions and the slip Couette flow with heat transfer. [ABSTRACT FROM AUTHOR]

The axisymmetric concave body is a typical configuration about which shock/shock interactions appear. Various shapes of axisymmetric concave bodies are used in a variety of applications in aeronautics, for example, axisymmetric jet inlets with conical centerbody, ballistic missiles drag reduction by spike, plasma or hot gas injection, parachutes for pilot-ejection capsules. However, it is well known that two distinct modes of instability appear around a concave body in the high-speed flow regime for a certain range of geometric parameters. These instabilities can cause undesirable effects such as severe vibration of the structure, heating and pressure loads. According to the experimental evidence, the unsteady flow is characterised by periodic radial inflation and collapse of the conical separation bubble formed around the forebody (pulsation). Various explanations have been given for the driving mechanism of the instabilities. In the present, merging of the leading explanations is done, and basic rules for the passive suppression of the instabilities are applied, in order to enforce their proposed driving. In addition, the effect of the flow initialisation method on the flow structure predicted by numerical simulations is examined. For certain configurations, bifurcation of the time-dependent flow has been found. This behaviour is explained with recourse to the phenomenon of hysteresis, which is an inherent feature of the examined flows. [ABSTRACT FROM AUTHOR]

Collisions of Lennard-Jones nanoparticles (NPs) may be used to study the generic collision behavior of NPs. We study the collision dynamics of amorphous NPs for oblique collisions using molecular dynamics simulation as a function of collision velocity and impact parameter. In order to allow for NP bouncing, the attraction between atoms originating from differing NPs is reduced. For near-central collisions, a finite region of velocities – a 'bouncing window' – exists where the 2 NPs bounce from each other. At smaller velocities, energy dissipation and – at larger velocities – also NP deformation do not allow the NPs to surpass the attractive forces such that they stick to each other. Oblique collisions of non-rotating NPs convert angular momentum into NP spin. For low velocities, the NP spin is well described by assuming the NPs to come momentarily to a complete stop at the contact point ('grip'), such that orbital and spin angular momentum share the pre-collision angular momentum in a ratio of 5:2. The normal coefficient of restitution increases with impact parameter for small velocities, but changes sign for larger velocities where the 2 NPs do not repel but their motion direction persists. The tangential coefficient of restitution is fixed in the 'grip' regime to a value of 5/7, but increases towards 1 for high-velocity collisions at not too small impact parameters, where the 2 NPs slide along each other. [ABSTRACT FROM AUTHOR]