Daisuke Hitomi

Numerical Analysis of Asymmetric Vortex Breakdown in a Cylindrical Container Flow with a Rotating Disk

Numerical analysis has been performed for three-dimensional and time-depending vortex breakdown in an open and confined cylindrical container flow generated by a rotating disk. Vortex breakdown in swirling flows has been the sudject of much attention since it was first recognized in the tip of delta winged aircraft. Under certain conditions vortex flows undergo sudden structural changes near their rotation axis, called vortex breakdown, which are characterized by the existence of free stagnation point upstream of a region with reversed axial flow. Recently, experimental results suggest that the asymmetry of the vortex breakdown is found to be related to the existence of asymmetric flow separation on the container wall. Adding to this, it is important to reproduce such asymmetry vortex breakdown by numerical method because there is no report to predict an asymmetric vortex breakdown numerically. In the numerical calculation, boundary fitted coordinate system has been used for this flow in order to make the cause of asymmetry vortex clear. Calculated results of angular moment and flow vectors in circular cross section are compared with the experimental data in order to examine the validity of the presented numerical method. As a result of this analysis, it has been found out that the present method can reproduce vortex breakdown reasonably for an open and confined cylindrical container. Although the present method could not predict the separated flow near the wall which has been reported experimentally as the cause of asymmetry vortex generation, we have showed numerically that asymmetry vortex is also able to be produced by shifting rotating axis from center within the gap between rotating disk and cylindrical wall of experimental apparatus.

Numerical Analysis of Turbulent Heat Transfer in a Square Duct with Different Rib Shapes

Numerical analysis has been performed for three-dimensional developed turbulent flow in a square duct with rib-roughened walls. Special attention pays for the prediction of turbulent heat transfer with roughened wall. Roughened wall is composed of small ribs which are located periodically on bottom wall of square duct. In order to clarify the influence of rib cross sectional shape on flow and temperature fields, three kinds of ribs, that are square, triangular and elliptical cross section, is examined from the point of heat transfer. In numerical calculation, algebraic Reynolds stress model is selected for flow field in order to predict anisotropic turbulent flow precisely and zero equation model assuming constant Prandtl number is applied for temperature field to make clear whether such simple model is able to evaluate Nusselt number correctly. Periodic boundary condition has been used for this flow to save computational time. Calculated results of temperature field are compared with the experimental data in order to examine the validity of the presented numerical method. As a result of this calculation, it has been found out that the present method could predict temperature contour lines and Nusselt number qualitatively although agreement is certainly not perfect in all detail. Adding to this, turbulent structure is affected by rib cross section shape, although rib height is small compared with side length of square. Especially, elliptical cross section promotes the production of vertical fluctuating velocity and shear stress which leads to generate the secondary flow of the second kind more actively than the other rib shapes. At the same time, calculated results suggest that heat transfer is enhanced by the elliptical cross section.

Camera Calibration Considering Scheimpflug Condition

Presently, a new calibration scheme for the camera under the Scheimpflug condition is proposed, which represents an exact projection from the object through the lens to the inclined image sensors and also handles the distortion by the lens theoretically. By this scheme, a highly precise three-dimensional calibration can be carried out and thus the accurate three-dimensional projection, which is inevitable for the development of three-dimensional three-component 3D3C PIV, can be calculated. By taking the images of the precisely manufactured target which traverses perpendicularly with high accuracy, the detailed tests for the accuracy of the present scheme, compared to those of conventional calibration schemes, have been carried out. The results show that the present scheme gives a better projection than the normal projection scheme and much better results than the conventional two-dimensional calibration schemes. The three-dimensional polynomial fit gives a fairly good result only for the case of small depth of field.

Assessment of Volume Traking Algorithms in a Three-Diemensional Flow Field Dominated by Surface Tension.

In this study, numerical analysis has been performed to clarify the assessment of volume tracking algorithms in a three-dimensional flow field dominated by surface tension. The FLAIR method has been extended to three-dimensional problems in this study. The distinct feature of FLAIR developed in two-dimension is that the slope of line segment is estimated based only on two neighboring volume fractions. This feature is also adopted in the three-dimensional FLAIR method proposed in this study, even if the three-dimensional slope of interface is neglected. This three-dimensional FLAIR is applied to a non-straining flow field and a surface tension dominated flow field. The results are compared with those of the donor-acceptor method, the SURFER method and the CIP method with digitization. Consequently, it has been found that the precision of translation of interface is much more improved by the use of the CIP method with digitization and the three-dimensional FLAIR method than that of the other methods. However, the CIP method with digitization will produce an uneven interface.

Numerical Analysis of Three-Dimensional Turbulent Flow in a 180.DEG.-Bent Tube Using by an Algebraic Reynolds Stress Model.

A numerical analysis has been performed for three dimensional developing turbulent flow in a 180° bent tube with straight inlet and outlet sections using by an algebraic Reynolds stress model. To our knowledge, only very few numerical investigations exist in which the detailed comparison between calculated results and experimental data contained Reynolds stresses. In numerical analysis, an algebraic Reynolds stress model in conjunction with a boundary-fitted coordinate system is applied to a 180° bent tube in order to solve the anisotropic turbulent structure precisely. As a result of this analysis, it is found that the calculated results show a comparatively good agreement with the experimental data of the time averaged velocity and the secondary vectors in the bent tube and the straight outlet sections. For example, the location of the maximum streamwise velocity, which appears near the top or bottom wall in the bent tube, is predicted correctly by the present method. As for the comparison of Reynolds stresses, the present model has been found to simulate many characteristic features of streamwise normal stress and shear stresses in the bent tube satisfactorily, but has a tendency to underpredict its value. Judging from the comparison between the calculated and the experimental results, the algebraic Reynolds stress model is applicable to the developing turbulent flow in a bent tube which is known as a flow with a strong convective effect.