Masashi Ichimiya

Properties of a relaminarizing turbulent boundary layer under a favorable pressure gradient

Abstract Behavior of the turbulent boundary layer on a flat plate in a relaminarization process with flow acceleration due to a convergence of the sectional area is experimentally investigated. Statistical properties are measured and the turbulent structure is examined in detail. The bursting phenomena are also investigated with the VITA technique. The ensemble averages of the streamwise fluctuating velocities are plotted, and the characteristics in each process are investigated. In the relaminarization process, low-amplitude fluctuating portions appear intermittently in the velocity signal. In the retransition process an irregular velocity fluctuation appears intermittently; then the fluctuation grows and gradually becomes a turbulent condition. In the relaminarization process, vorticity increases in a large eddy and decreases in a small one. The small eddies become large, and the contribution from the high wave number decreases. In the ensemble averages of the fluctuating velocity, it is considered that the relaminarization changes the ejection and sweep, though it does not particularly attenuate the bursting in the inner layer. This phenomenon is qualitatively explained with the aid of the mean velocity profiles and mixing length concept.

The Effects of Splitter Plates on Turbulent Boundary Layer on a Long Flat Plate Near the Trailing Edge

An experimental investigation has been made on a turbulent boundary layer near the trailing edge on a long flat plate. The flow was controlled by an additional splitter plate fitted to the trailing edge along the wake centerline. The length of the splitter plate, I, was varied from a half, to five times the trailing edge thickness, h. Measurements of base pressure behind the trailing edge and of mean velocity and pressure distribution in the turbulent boundary layer on the flat plate were made under the freestream zero-pressure gradient. The absolute value of the base pressure coefficient of the long flat plate was considerably smaller than that of the short flat plate without the splitter plate. A significant increase in the base pressure coefficient was achieved with the splitter plate (l/h≥1), fitted to the long flat plate. Within an inner layer in the turbulent boundary layer near the trailing edge, the mean velocity increased more than that in the upstream position in the case without the splitter plate. With the splitter plate, however, the base pressure rise made the mean velocity distribution more closely approach that of a fully developed turbulent boundary layer.

Structure of a Turbulence Wedge Developed from a Single Roughness Element on a Flat Plate

A turbulence wedge which develops downstream from a single roughness element placed in a laminar boundary layer on a flat plate is experimentally investigated. Mean and fluctuating velocities and the intermittency factor were measured. Contours of the wedge and its development in the normal and spanwise directions were clarified by drawing intermittency contour lines. The mean properties inside of the wedge just behind the roughness did not reach fully developed turbulence, and they gradually developed in the downstream direction. The contour of the wedge is not formed by straight lines very near or far from the roughness, and the extent of the wedge in which the lines are straight is limited. Just behind the roughness, distributions of the mean and fluctuating velocities are complicatedly distorted, so the existence of several vortices is presumed. The intermittency contour Lines in the section normal to the flow are also presented.

Properties of a Relaminarizing Turbulent Boundary Layer under a Favorable Pressure Gradient.(Analysis of Bursting Structure with VITA Technique).

The bursting phenomena of the relaminarizing turbulent boundary layer on a flat plate are experimentally investigated using the VITA technique. First, an attempt is made to obtain a normalized bursting frequency universally applicable to the development process of the turbulent boundary layer, the relaminarization process and the retransition process. The ensemble averages of the streamwise fluctuating velocities are drawn and the characteristics of each process are investigated. No time scales can render the non-dimensional bursting frequencies universally constant. The tendency of the distribution of the bursting frequency in the relaminarization process is different from that in the development and retransition process of the turbulent boundary layer. This presumably supports the previous visualization result, according to which the relaminarization process begins from the outer region. In the ensemble averages of the fluctating velocity, it is considered that the relaminarization changes the ejection and sweep, though it does not particularly attenuate the bursting in the inner layer. This phenomenon is qualitatively explained with the aid of the mean velocity profiles and the mixing length concept.

The Effects of Splitter Plates on Turbulent Boundary Layer Developing on a Flat Plate near the Trailing Edge

An experimental investigation has been made for a turbulent boundary layer near the trailing edge on a long flat plate. The flow was controlled by an additional splitter plate fitted to the trailing edge along the wake center line. The length of the splitter plate, l, was varied from a half, to five times the trailing edge thickness, h. Measurements of base pressure behind the trailing edge and of mean velocity and pressure distribution in the turbulent boundary layer on the flat plate were made under the freestream zero-pressure gradient. The absolute value of base pressure coefficient of the long flat plate was considerably smaller than that of the short flat plate (bluff body) without splitter plate (l/h=0). A significant increase in the normalized base pressure coefficient (about 50% as same as short one) was achieved with the splitter plate (l/h≥1) fitted to the long flat plate. Within an inner layer in the turbulent boundary layer near the trailing edge, mean velocity increased than that in the upstream position, especially in the case without splitter plate. With splitter plate, however, the base pressure rise made mean velocity distribution come closer to that of fully developed turbulent boundary layer. In the case of l/h≥1, the affected range of acceleration decreased from 10 h upstream of the trailing edge without splitter plate to 5 h upstream.

Unsteady Fluid Forces on a Blade in a Cross-Flow Turbine.

The internal flow in a cross-flow turbine is nonuniform because the water passes through only part of the runner. Therefore, the unsteady fluid forces act on a blade through rotation. The experimental and theoretical studies for determination of fluid forces on the blade in a cross-flow turbine are conducted. In the experiment, the tangential and radial forces are measured on a test blade using the strain gauges and slip rings. On the other hand, in the theoretical study, they are calculated numerically using the unsteady momentum theory. The calculated results are compared with experimental data and good agreement between them is demonstrated. Furthermore, the maximum forces are found to occur immediately before the blade leaves the nozzle exit in both the experimental and theoretical results.

Effects of Single Protrusion on the Boundary Layer around a Cylinder Rotating in Axial Flow ( Change in Flow Properties in the Turbulence Wedge with the Speed Ratio )

Results of the measurement of flow properties in the boundary layer on a cylinder rotating in an axial flow in the presence of a single protrusion are presented. A wedge-shaped region of turbulent flow extends downstream with streamwise vortices on either side. Mean and fluctuating velocities are measured at various rotation speeds of the cylinder. The mean velocity profiles become asymmetric with respect to the relative streamline of the main flow passing the protrusion point as the rotation speed becomes high; this asymmetry is presumably due to the Coriolis effect. Representations of the mean velocity distribution on the basis of relative streamline coordinates and polar plots confirm the existence of a pair of streamwise vortices. The profiles of the fluctuating velocities within the turbulent-wedge region suggest the existence of many small streamwise vortices. The effect of the Coriolis force on the streamwise vortices is examined by using the vorticity equation expressed in a rotating coordinate system.