The generation of large-scale magnetic field due to small-scale motion of a conducting fluid, which is called the dynamo action, is important in understanding magnetic fields in geo-/astro-physical objects and in controlled fusion devices. A large eddy simulation of magnetohydrodynamic (MHD) turbulent channel flow is carried out and turbulent statistics are obtained to investigate the dynamo effect. It is shown that the flow rate is greater than that in the non-MHD case due to the decrease in the wall-normal velocity fluctuation. A streamwise mean magnetic field is generated due to the effect of the turbulent electromotive force. It is suggested that the cross-helicity dynamo effect contributes to the turbulent electromotive force. [This abstract is not included in the PDF]
It has been recognized that the turbulent cross helicity (correlation between the velocity and magnetic-field fluctuations) can play an important role in several magnetohydrodynamic (MHD) plasma phenomena such as the global magnetic-field generation, the turbulence suppression, etc. Despite its relevance to the cross-helicity evolution, little attention has been given to the dissipation rate of the turbulent cross helicity. In this paper, we consider the dissipation rate of the turbulent cross helicity and propose an algebraic model and an evolution equation of the cross-helicity dissipation rate (eW equation). We apply the model to the solar-wind turbulence, where several observations have been made on the turbulent cross helicity, and validate the model of cross-helicity dissipation. It is shown that, as far as the solar-wind application is concerned, the simplest possible algebraic model is useful enough to elucidate the spatial evolution of the solar-wind turbulence. [This abstract is not included in the PDF]
Large-eddy simulation (LES) of turbulent channel flow using the transport equation of sub-grid scale (SGS) kinetic energy ksgs is carried out. In this study, we aim to establish a one-equation model which doesn’t have length scale in SGS viscosity and each term in the transport equation of ksgs. Modeling of SGS dissipation term is paid much attention to and it is finally modeled by first derivative of ksgs. As a result, its performance is almost equivalent to the standard Smagorinsky model. [This abstract is not included in the PDF]
In this paper, as a basic investigation, solutions via LES and standard k-epsilon model were compared regarding the flow field around a simplified exterior vehicle body (ASMO) with flat underbody. An applicability of LES was shown for aerodynamic developments of the vehicle body. A practical evaluation method for the standard Smagorinsky model based LES solutions for practical problems, where direct validation via experiments is not available, was proposed. By using the method proposed, LES solutions calculated for the flow fields around the complex exterior vehicle body (V50) with semi-complex underbody configuration were evaluated. [This abstract is not included in the PDF]
We investigate the mechanism of self-sustained oscillations over a rectangular cavity with a length-to-depth ratio of 2:1 by directly solving the compressible Navier-Stokes equations. The boundary layer over the cavity is turbulent and the freestream Mach numbers are M =0.4 and 0.7. The results clarify that the self-sustained oscillations occur in the shear layer of the cavity and two-dimensional large-scale vortices are formed in the shear layer of the cavity. When these vortices collide with the downstream edge, expansion waves are radiated. The propagation of the acoustic waves at the upstream edge produces disturbances in the shear layer. The disturbances are developed by the Kelvin-Helmholtz instability and finally the two-dimensional large-scale vortices are formed. As a result, a feedback loop is formed and the self-sustained oscillations occur. [This abstract is not included in the PDF]
A steady state airflow distribution is calculated using CFD and concentration response factors from each cell and inlet to each observation point are obtained through concentration calculations on the flow field in a room. Assuming cells and inlets as contaminant emission points, the concentration equations of convolution of emission rate and response factors at observation points are set up based on the response factor method. Simultaneous equations of contaminant emission rates are formulated by applying a weighted residual method to the equations to minimize residuals between measured and calculated concentrations. Identification by solving the simultaneous equations is the inversion analysis of the convolution of concentration. Here we present the calculation method as well as examples of the identification of the source.
The main subject of this paper is the development of a method for extracting heat transport phenomena in rooms from limited CFD analyses and integrating the data into a flow network model analysis. The calculation method is based on the fact that we can regard static flow field as a linear system, and the transient heat response on static flow field can be calculated through computation of convolution by using response factors. In this paper, we present the calculation steps: method obtaining heat response factor on flow filed; and method calculating transient heat response using those response factors.
The present research introduces a technique to determine pollution source locations in urban environments -when the pollutant concentration field is known- through the use of reversed time marching method (RTMM). The method depends primarily on the solution of the scalar transport equation with time integration in the negative direction. This leads to reversing the velocity field and also the diffusion term. The study demonstrates how to use the inverse CFD model with the reversed time marching method to identify pollution sources in urban environments. In order to examine the accuracy of RTMM in identifying pollution sources in urban areas, two examples were given. In the first one, a simple laminar flow was considered and a pollutant source was emitted for variable wind conditions. In the second example, the wind flow around a single building was investigated for two different source locations. In all cases, steady state numerical simulations were carried out at first in order to estimate the wind flow fields. With the steady-state airflow patterns, direct CFD modeling (forward-time simulation) was used to calculate pollutant concentration distributions for step-function sources. In the last stage, the scalar transport equation was solved again but with the reversed flow field and the negative diffusion term. By using peak concentration, one could identify the pollution source location. Results of the study demonstrated that the RTMM can identify pollution sources locations in urban areas with a satisfied accuracy. However, more efforts are needed in order to decrease the wide spread of the concentration fields around the source location and facilitates the source location identification. [This abstract is not included in the PDF) KEYWORDS: Reverse simulation; Reversed time marching method; Outdoor environments; CFD
For analyzing the complex flow field such as urban space, Finite Volume Method (FVM) with the unstructured grid system is often used because of its’ high adaptability to complex geometry. In this study, the numerical accuracy of FVM with the unstructured grid system in respect of some mesh geometries is estimated and compared with that of Finite Differential Method (FDM). Especially, the validity of Muzaferijal and Gosman’s Defer Correction Formula in the unstructured FVM is evaluated. In this paper, the truncation errors when FVM with the unstructured grid system in respect of some mesh geometries is applied, is clarified and notation when Muzaferijal and Gosman’s Defer Correction Formula is used, is shown.
The numerical simulation was executed for Caspian Sea, which has a peculiar brackish water region ecosystem, using a three-dimensional hydrostatic model called MEC (Marine Environmental Committee). The numerical simulation could capture qualitatively the seasonal variation of the circulation and the stratification structure, however the several cyclonic gyres, the absolute values of water temperature, and the thickness of the mixing layer were not reproduced well. As future studies, the boundary conditions, especially the solar radiation conditions, should be improved and the effects of tides are considered in order to reproduce the detail structure of water current and stratification in the Caspian Sea. [This abstract is not included in the PDF]
Increasing awareness of environmental needs is driving changes in the way the concrete industry operates. Demand for cement and concrete products is increasing in response to economic development, and yet the industry must somehow reduce its environmental impact while meeting supply needs. Research into sustainable development in the concrete industry has, until now, been fairly limited. Some areas have potential for promoting sustainable ideas, such as the utilization of waste material from other industries as cementitious replacement products and the recycling of waste products both within and from beyond the concrete industry as filler material in new concrete construction. This paper reviews the challenges faced by the concrete industry and the research work which has been performed to meet this challenge. [This abstract is not included in the PDF)
In this paper, the authors propose the personal mobility vehicle (PMV), which is friendly for the human and the environment. The PMV consists of the bicycle mode that is used at high speed and parallel two-wheel mode that is used at low speed. The steer-by-wire and the drive-by-wire system are used in order to increase its stability. The parallel two-wheel mode is stabilized by the controller using the stabilization method of an inverted pendulum. In addition to the conventional step type vehicle that is fully operated by the electric power, the hybrid type that uses the human power for its running is investigated. [This abstract is not included in the PDF]