Orientation distribution of fibers and rheological property in fiber suspensions flowing in a turbulent boundary layer

A model relating the translational and rotational transport of orientation distribution function （ODF） of fibers to the gradient of mean ODF and the dispersion coefficients is proposed to derive the mean equation for the ODE Then the ODF of fibers is predicted by numerically solving the mean equation for the ODF together with the equations of turbulent boundary layer flow. Finally the shear stress and first normal stress difference of fiber suspensions are obtained. The results, some of which agree with the available relevant experimental data, show that the most fibers tend to orient to the flow direction. The fiber aspect ratio and Reynolds number have significant and negligible effects on the orientation dis- tribution of fibers, respectively. The additional normal stress due to the presence of fibers is anisotropic. The shear stress of fiber suspension is larger than that of Newtonian solvent, and the first normal stress difference is much less than the shear stress. Both the additional shear stress and the first normal stress difference increase with increasing the fiber concentration and decreasing fiber aspect ratio.     

利用风廓线雷达谱宽反演晴空湍流耗散率

 从晴天风廓线雷达的回波信号机制出发,研究了利用风廓线雷达谱宽反演晴空湍流耗散率的方法,即从雷达测量的多普勒速度谱宽中提取出湍流因素引起的谱宽,进而由湍流谱宽和湍流动能耗散率的关系计算出湍流动能耗散率。通过对合肥地区风廓线雷达谱宽资料的分析,得到合肥上空300 m高度上湍流动能耗散率的大小分布在10-4~10^-1 m²/s³之间,与理论相符,说明利用雷达谱宽反演湍流耗散率的方法可行。     

Kolmogorov scaling of turbulent flow in the vicinity of the wall

How to scale even the simplest of turbulent flows continues to be a cause for considerable controversy. In the present research, a data base compiling results from channel flow direct numerical simulations and turbulent boundary layer experiments is employed to investigate the properties of shear and normal Reynolds stresses very close to the wall. Two types of scaling based on Kolmogorov length and velocity scales are analyzed. It is shown that it is highly likely that large length scales of the order of the channel half-width or the boundary layer thickness play an important role even in the innermost regions of wall-bounded turbulent flows, which hints at the persistence of Reynolds number effects in even high Reynolds number flows.     

Three-dimensional direct simulations and structure of expanding turbulent methane flames

Direct numerical simulations (DNS) are ideally suited to investigate in detail turbulent reacting flows in simple geometries. For an increasing number of applications, detailed models must be employed to describe the chemical processes with sufficient accuracy. Despite the huge cost of such simulations, recent progress has allowed the direct numerical simulation of turbulent premixed flames while employing complete reaction schemes. We briefly describe our own developments in this field and use the resulting DNS code to investigate more extensively the structure of premixed methane flames expanding in a three-dimensional turbulent velocity field, initially homogeneous and isotropic. This situation typifies, for example, the initial flame development after spark ignition in a gas turbine or an internal combustion engine. First investigation steps have been carried out at low turbulence levels on this same configuration in the past Symposium, and we build on top of these former results. Here, a considerably higher Reynolds number is considered, the simulation has been repeated twice in to limit the possibility of spurious, very specific results, and several complementary post-processing steps are carried out. Characteristic features concerning the observed combustion regime are presented. We then investigate in a quantitative manner the evolution of flame surface area, global stretch-rate, flame front curvature, flame thickness, and correlation between thickness and curvature. The possibility of obtaining reliable information on flame front curvature from two-dimensional slices is checked by comparison with the exact procedure.     

Experimental and computational study of the developed flow field in a flat plate integrated collector storage (ICS) solar device with recirculation

The flow structure in a flat plate integrated collector storage device, with recirculation of the storage water, is studied experimentally and theoretically. To facilitate flow visualization, an experimental device was constructed by transparent material (Plexiglas). Flow velocities and fluctuations are measured, using a LDV system. A three-dimensional CFD-model was developed using the FLUENT code. The standard k–ω model is selected as the most appropriate. The model is validated, with good agreement, against experimental measurements. Furthermore, copper tubes, in the form of embedded heat exchanger, are placed inside the device and another similar 3D model was developed. The model was used to examine the behavior of the system, when the service water enters the heat exchanger, thus being indirectly heated by the stored hot water. It is shown that the outlet temperature of the service water is enough higher, when recirculation occurs.     

The stabilization mechanism and structure of turbulent hydrocarbon lifted flames

The structure and stabilization mechanism of turbulent lifted non-premixed hydrocarbon flames have been investigated using combined laser imaging techniques. The techniques include Rayleigh scattering, laser induced predissociation fluorescence of OH, LIF of PAH, LIF of CH₂O, and planar imaging velocimetry. The geometrical structure of multi-reaction zones and flow field at the stabilization region have been simultaneously measured in 16 hydrocarbon flames. The data reveal the existence of triple flame structure at the stabilization region of turbulent lifted flames. Increasing the jet velocity leads to an increase of the lift-off height and to a broadening of the lift-off region. Further analysis of the stabilization criterion at the lift-off height based on the premixed nature of triple-flame propagation and flow field data has been presented and discussed.     

Turbulent flow in a tube containing an offset rod

A numerical finite volume prediction method for arbitrary-shaped passages has been applied to the case of fully developed axial turbulent flow past a rod eccentrically placed in a circular tube. The numerical method was based on an orthogonal curvilinear mesh and employed an algebraic stress transport model to calculate the full three-dimensional velocity field directly from the governing partial differential equations. This study is one of a series of applications of this prediction method to a range of different non-circular passages that have been made in order to establish the capabilities and usefulness of this type of procedure. The present eccentric rod case was the subject of a comprehensive experimental investigation by Kacker¹ which has enabled a detailed comparison to be made between the present predictions and the measurements. This comparison included local distributions of axial velocity, wall shear stress and secondary velocities; and although found to be satisfactory overall, some differences in detail revealed possible shortcomings in the measurement of secondary flow. This, together with other previously reported cases, indicates, that, although the present method cannot be expected to replace experiment in providing turbulent passage flow data, it has an important role to play in interpreting and supplementing experiments.