Correlations between art and CFD through colour and shape

Victorian stained glass artists were among the first to link vibrant colour with strong abstract geometry. This link was further exploited by artists of the early 20th century. Here may be mentioned the works of designers and artists like Mackintosh and of well-known Bauhaus group members like Klee and Kandinsky. Computational fluid dynamics (CFD) and art, particularly abstract art, are undoubtedly intrinsically linked not only by colour, but also by shape as often both contain regular geometries like rectangles and triangles. The use of colour has a multitude of functions in both the pre- and post-processing stages in CFD. These are discussed with an emphasis on the representation of CFD results. Moreover, modelling of fluid dynamics should be seen as but one example of numerical modelling in general. It may be that such common features between these very different metiers are the reason why the numerical modellers amongst us seem to have a natural liking for colourful abstract art. This paper investigates the correlations between art and CFD and is written from the view points of both the professional engineer and the hobby artist.     

Diffusion within a near-critical nanopore fluid

Results are presented for grand canonical Monte Carlo (GCMC) and both equilibrium and non-equilibrium molecular dynamics simulations (EMD and NEMD) conducted over a range of densities and temperatures that span the two-phase coexistence and supercritical regions for a pure fluid adsorbed within a model crystalline nanopore. The GCMC simulations provided the low temperature coexistence points for the open pore fluid and were used to locate the capillary critical temperature for the system. The equilibrium configurational states obtained from these simulations were then used as input data for the EMD simulations in which the self-diffusion coefficients were computed using the Einstein equation. NEMD colour diffusion simulations were also conducted to validate the use of a system averaged Einstein analysis for this inhomogeneous fluid. In all cases excellent agreement was observed between the equilibrium (linear response theory) predictions for the diffusivities and non-equilibrium colour diffusivities. The simulation results are also compared with a recently published quasi-hydrodynamic theory of Pozhar and Gubbins (Pozhar, L. A., and Gubbins, K. E., 1993, J. Chem. Phys., 99, 8970; 1997, Phys. Rev. E, 56, 5367.). The model fluid and the nature of the fluid wall interactions employed conform to the decomposition of the particle–particle interaction potential explicitly used by Pozhar and Gubbins. The local self-diffusivity was calculated from the local fluid–fluid and fluid wall hard core collision frequencies. While this theory provides reasonable results at moderate pore fluid densities, poor agreement is observed in the low density limit.     

Four dimensional bolus tagging imaging of pulsatile flow

Inversion bolus tagging MR methods were used to provide a graphic depiction of the axial velocity in three spatial dimensions for pulsatile flow through complex geometries. Visualization of the flow field was readily apparent, and a train of tagged boli were depicted providing an immediate overview of the displacement of flowing fluid over the entire pulsatile cycle. Tagging efficiency obtained using adiabatic inversion pulses was improved compared to that with a windowed sinc pulse. Results from phantom experiments on steady flow were correlated with computational fluid dynamic (CFD) simulations. The use of 3D methods reduced spatial partial volume effects, and the displacement of boli in a steady flow experiment correlated well with CFD simulations. The use of adiabatic inversion pulses resulted in sharp edged inversion regions with good retention of longitudinal magnetization. However in order to keep the pulse duration short, of the order of 2-5 ms, a rather large RF amplitude had to be used. The inversion bolus tagging method is useful in visualizing the flow field in multiple levels for pulsatile fluid flowing through complex geometries, and may be useful in fluid dynamic applications.     

Multiple pure tone noise prediction

This paper presents a fully numerical method for predicting multiple pure tones, also known as “Buzzsaw” noise. It consists of three steps that account for noise source generation, nonlinear acoustic propagation with hard as well as lined walls inside the nacelle, and linear acoustic propagation outside the engine. Noise generation is modeled by steady, part-annulus computational fluid dynamics (CFD) simulations. A linear superposition algorithm is used to construct full-annulus shock/pressure pattern just upstream of the fan from part-annulus CFD results. Nonlinear wave propagation is carried out inside the duct using a pseudo-two-dimensional solution of Burgers? equation. Scattering from nacelle lip as well as radiation to farfield is performed using the commercial solver ACTRAN/TM. The proposed prediction process is verified by comparing against full-annulus CFD simulations as well as against static engine test data for a typical high bypass ratio aircraft engine with hardwall as well as lined inlets. Comparisons are drawn against nacelle unsteady pressure transducer measurements at two axial locations as well as against near- and far-field microphone array measurements outside the duct.     

Visualization of respiratory flows from 3D reconstructed alveolar airspaces using X-ray tomographic microscopy

Abstract  

A deeper knowledge of the three-dimensional (3D) structure of the pulmonary acinus has direct applications in studies on acinar fluid dynamics and aerosol kinematics. To date, however, acinar flow simulations have been often based on geometrical models inspired by morphometrical studies; limitations in the spatial resolution of lung imaging techniques have prevented the simulation of acinar flows using 3D reconstructions of such small structures. In the present study, we use high-resolution, synchrotron radiation-based X-ray tomographic microscopy (SRXTM) images of the pulmonary acinus of a mouse to reconstruct 3D alveolar airspaces and conduct computational fluid dynamic (CFD) simulations mimicking rhythmic breathing motion. Respiratory airflows and Lagrangian (massless) particle tracking are visualized in two examples of acinar geometries with varying size and complexity, representative of terminal sacculi including their alveoli. The present CFD simulations open the path towards future acinar flow and aerosol deposition studies in complete and anatomically realistic multi-generation acinar trees using reconstructed 3D SRXTM geometries.     

Adaptation of mesoscale turbulence parameterisation schemes as RANS closures for ABL simulations

The present study presents different k-ε turbulence closures for atmospheric boundary layer flows using computational fluid dynamics (CFD) simulations that are consistent with inflow conditions from numerical weather prediction (NWP) simulations. Eight different mesoscale turbulence parameterisation schemes of the Weather Research and Forecasting (WRF) model are covered. To ensure consistency between the NWP and CFD simulations, different closure coefficients of the k ? ε turbulence model for each NWP scheme are proposed. This is achieved by combining production–dissipation closure coefficient relationships based on the Monin–Obukhov similarity theory and the formulation based on the Coriolis parameter proposed by Detering and Etling. The proposed methodology has been implemented in the open source CFD toolbox OpenFOAM and is demonstrated at near-neutral stability conditions for the classical Askervein Hill case.     

基于面元法的水平轴风力机数值模拟

使用基于速度面元法的势流数值模拟方法,以NREL PhaseⅥ为例进行了叶片气动载荷和风轮近尾流场的数值模拟。将势流数值模拟、叶素动量理论和计算流体力学CFD方法的计算结果与实验数据进行了对比分析。结果表明使用速度面元法计算风轮绕流场具有较高的计算精度和求解效率,为大规模风力机群的流场计算和出力预报提供支撑。     

Novel multi-tubular fixed-bed reactors’ shell structural analysis based on numerical simulation method

In the present contribution, a numerical study of fluid flow and heat transfer performance in a pilot-scale multi-tubular fixed-bed reactor with a novel configuration for propylene-to-acrolein oxidation reaction is presented using a three-dimensional computational fluid dynamics method (CFD) to ensure the uniformity condition using molten salt as a heat carrier medium on shell side. The effects of multiscale structural parameters including the number of baffles, baffles cut, central nontube region and the number of flow channels on pressure drop and heat transfer are considered. The simulations suggest that heat transfer coefficient per pressure drop is reduced with increasing number of baffles. By the single factor sensitivity analysis it was shown that the central region is the key factor in the structural design of a multi-tubular fixed-bed reactor.     

Numerical simulation and flow visualization using soap film of the self-organized vortex structure in the wake of an array of cylinders

Abstract  

With the aid of computational fluid dynamics (CFD) and simple flow visualization technique using flowing soap-film, we present here the wake structures behind an array of cylinders for Reynolds numbers corresponding to both laminar and turbulent flow regimes. The image results illustrate interesting vortex interactions past these equally spaced cylinders; for low Reynolds number flow, well-organized wake pattern persists and manifests unsteadily to different symmetry states. An increase of free stream flow velocity causes the wake transition, resulting in the formation of asymmetric flow wake with chaotic mixing at the far wake. Observations from both the numerical simulations and soap-film are in good agreement at least qualitatively.     

Steering in computational science: Mesoscale modelling and simulation

This paper outlines the benefits of computational steering for high performance computing applications. Lattice-Boltzmann mesoscale fluid simulations of binary and ternary amphiphilic fluids in two and three dimensions are used to illustrate the substantial improvements which computational steering offers in terms of resource efficiency and time to discover new physics. We discuss details of our current steering implementations and describe their future outlook with the advent of computational grids.     

Structure of a tethered polymer under flow using molecular dynamics and hybrid molecular-continuum simulations

We analyse the structure of a single polymer tethered to a solid surface undergoing a Couette flow. We study the problem using molecular dynamics (MD) and hybrid MD-continuum simulations, wherein the polymer and the surrounding solvent are treated via standard MD, and the solvent flow farther away from the polymer is solved by continuum fluid dynamics (CFD). The polymer represents a freely jointed chain (FJC) and is modelled by Lennard-Jones (LJ) beads interacting through the FENE potential. The solvent (modelled as a LJ fluid) and a weakly attractive wall are treated at the molecular level. At large shear rates the polymer becomes more elongated than predicted by existing theoretical scaling laws. Also, along the normal-to-wall direction the structure observed for the FJC is, surprisingly, very similar to that predicted for a semiflexible chain. Comparison with previous Brownian dynamics simulations (which exclude both solvent and wall potential) indicates that these effects are due to the polymer–solvent and polymer–wall interactions. The hybrid simulations are in perfect agreement with the MD simulations, showing no trace of finite size effects. Importantly, the extra cost required to couple the MD and CFD domains is negligible.     

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为进一步优化实验包层模块的热工水力性能,分别研究了实验包层模块的两种不同冷却方案在正常和极端两种不同工况下的温度分布。用流体数值分析软件FLUENT模拟了实验包层模块关键部件的实际温度分布状况。     

Modified split-potential model for modeling the effect of DBD plasma actuators in high altitude flow control

Surface DBD plasma actuators are novel means of actively controlling flow. They have shown promising ability in reducing drag, postponing transition from laminar to turbulent flow, suppression of separation, noise reduction and enhancement of mixing in different applications. The CFD simulation of the effect of plasma actuator in such kind of applications could provide more information, and insight, for optimization and design of close looped flow control systems. However, the fluid models for simulating the formation of the plasma and its effect are computationally expensive such that, although they provide more detailed information about the physics related to the formation plasma, they are still not viable to be used in large scale CFD simulations. In this paper, we present the modified version of a simpler model that predicts the thrust generated by the plasma actuator with acceptable accuracy and can be easily incorporated in CFD calculations. This model is also free of empirical fitting parameters, being based on pure flow physics scaling.     

Simulations of multiphase reactive flows in fluidized beds using in situ adaptive tabulation

A novel algorithm, in situ adaptive tabulation (ISAT), has been implemented into a multiphase computational fluid dynamics (CFD) code to treat complex chemistry calculations. In this work, isothermal silane pyrolysis in a fluidized bed reactor is presented to test the feasibility and explore the capabilities of ISAT with a finite-volume two-fluid code (MFIX). Based on the results of simulations, an error tolerance of 10^?5 is found to be satisfactory for maintaining the accuracy for the examples investigated. Due to the rapidly changing time step used in the CFD code, the performance enhancement was found initially to be minimal. However, the performance is significantly improved when ISAT is called using a fixed time step.     

Gas puff nozzle characterization using interferometric methods andnumerical simulation

One of the source terms of Z-pinch experiments is the gas puff density profile. In order to characterize the gas puff, we have used two interferometrical methods and performed some numerical simulations. The merits of both optical techniques are presented in terms of sensitivity, accuracy, and full time recording. Hence, one technique has been chosen to characterize the gas puff. The computation fluid dynamics (CFD) code (ARES) has been used to simulate the gas flow with the aim of testing its performances. Comparing experimental and numerical data shows off the taking into account of gas viscosity in computations. Given these consistent results, the nozzle geometries can be optimized in order to obtain specific Z-pinch gas puffs and check the computation with the interferometric method. Results obtained with a cylindrical nozzle are presented herein     

CFD-RANS prediction of individual exposure from continuous release of hazardous airborne materials in complex urban environments

One of the key issues of recent research on the dispersion inside complex urban environments is the ability to predict individual exposure (maximum dosages) of an airborne material which is released continuously from a point source. The present work addresses the question whether the computational fluid dynamics (CFD)–Reynolds-averaged Navier–Stokes (RANS) methodology can be used to predict individual exposure for various exposure times. This is feasible by providing the two RANS concentration moments (mean and variance) and a turbulent time scale to a deterministic model. The whole effort is focused on the prediction of individual exposure inside a complex real urban area. The capabilities of the proposed methodology are validated against wind-tunnel data (CUTE experiment). The present simulations were performed ‘blindly’, i.e. the modeller had limited information for the inlet boundary conditions and the results were kept unknown until the end of the COST Action ES1006. Thus, a high uncertainty of the results was expected. The general performance of the methodology due to this ‘blind’ strategy is good. The validation metrics fulfil the acceptance criteria. The effect of the grid and the turbulence model on the model performance is examined.     

A new approach to design a control system for a FGR furnace using the combination of the CFD and linear system identification techniques

A new approach to design control systems for an industrial furnace with flue gas recirculation (FGR) is presented. To facilitate the control system design, a linear dynamic model is needed for the furnace. Full-scale computational fluid dynamics (CFD) simulations are used to generate the required small signal input and output data sets. Subsequently, a least squares based system identification technique is used to obtained the linear dynamic models. After model validation, feedback controller is designed based on these linear dynamic models. Finally, the performance of the designed closed-loop control system is also evaluated using both linear dynamic model and full-scale nonlinear CFD model. The comparison shows that the control system designed using the proposed approach can minimize the deviation of nitric oxides (NO) emission from the design point by minimize the dynamic NO formation, hence to prevent any excessive NO formation in the combustion process when the system subjects to disturbances.     

Noninvasive quantification of fluid mechanical energy losses in the total cavopulmonary connection with magnetic resonance phase velocity mapping

A major determinant of the success of surgical vascular modifications, such as the total cavopulmonary connection (TCPC), is the energetic efficiency that is assessed by calculating the mechanical energy loss of blood flow through the new connection. Currently, however, to determine the energy loss, invasive pressure measurements are necessary. Therefore, this study evaluated the feasibility of the viscous dissipation (VD) method, which has the potential to provide the energy loss without the need for invasive pressure measurements. Two experimental phantoms, a U-shaped tube and a glass TCPC, were scanned in a magnetic resonance (MR) imaging scanner and the images were used to construct computational models of both geometries. MR phase velocity mapping (PVM) acquisitions of all three spatial components of the fluid velocity were made in both phantoms and the VD was calculated. VD results from MR PVM experiments were compared with VD results from computational fluid dynamics (CFD) simulations on the image-based computational models. The results showed an overall agreement between MR PVM and CFD. There was a similar ascending tendency in the VD values as the image spatial resolution increased. The most accurate computations of the energy loss were achieved for a CFD grid density that was too high for MR to achieve under current MR system capabilities (in-plane pixel size of less than 0.4 mm). Nevertheless, the agreement between the MR PVM and the CFD VD results under the same resolution settings suggests that the VD method implemented with a clinical imaging modality such as MR has good potential to quantify the energy loss in vascular geometries such as the TCPC.     

On the use of slow manifolds in molecular and geophysical fluid dynamics

Constraints typically arise from the elimination of high frequency oscillations in mechanical systems. Examples are provided by bond constraints in molecular simulations and incompressibility constraints in fluid dynamics. A key issue is the accuracy of constrained dynamics with regard to the full dynamics. In this review we focus on the smooth solution components and discuss the concept of slow manifold and soft constraints in molecular and geophysical fluid dynamics. While the formal mathematical derivation of constraints is the same for both molecular and fluid dynamics, the predominant numerical techniques for dealing with constraints are different in the two fields. Semi-implicit time- stepping methods are often used in geophysical fluid dynamics while explicitly enforced constraints are more common in molecular dynamics.     

Parametric studies on hydrocarbon fireball using large eddy simulations

Occurrences of fireball close to plant buildings due to the release of flammable hydrocarbon fuel caused by the formation of fuel vapour cloud poses severe safety concerns. On the availability of potential ignition source, the induced fireball would cause the damage to the structures of nuclear power plant by direct contact, radiation and/or convection of hot combustion products through the opening of air intakes and ducts. In the present paper, the accidental/ experimental observations and theoretical studies of fireball are summarised. Computational fluid dynamics (CFD) analyses have been carried out to study the behaviour of fireball using OpenFOAM CFD software. The parametric studies are conducted by varying the mass of fuel, inlet velocity and inlet diameter. The new correlations for fireball diameter and duration have been proposed based on the parametric studies using CFD simulations. The fireball with a larger amount of fuel releases the heat slower and for a longer duration. The high heat released rate (HRR) is observed in case of a larger inlet diameter used for the same mass. The incident radiation from the fireball is calculated at different locations to assess thermal hazard. Analysis performed show that various parameters like fireball diameter, duration and the radiative flux falling at different locations can be predicted well using CFD code.