Shintaro Yamashita

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.

An Experiment on a Taylor Vortex Flow in a Gap with a Small Aspect Ratio : Bifurcation of Flows in a Symmetric System

SummaryBoth Taylor vortex flows in a symmetric or asymmetric system exhibit various patterns (cell modes). They can be classified by the process of cell formation, the number of cells and the direction of flow for the cell, into primary modes or secondary modes, and normal modes or anomalous modes. Following the previous report in which flows in a symmetric end condition were classified, in the present work, for flows in an asymmetric end condition, the Reynolds number at which a secondary mode bifurcates into another mode is experimentally investigated, and the bifurcation of the Taylor vortex flows in an asymmetric system when the Reynolds number is gradually decreased is presented in a bifurcation diagram.

An experimental study on the two-dimensional opposed wall jet in a turbulent boundary layer: Change in the flow pattern with velocity ratio

Results of the measurement of flow properties in a two-dimensional turbulent wall jet which is injected into the turbulent boundary layer in the direction opposite to that of the boundary layer flow are presented by varying the ratio of the jet issuing velocity to the mainstream velocity λ. This flow includes the flow separation and the recirculating flow, and so it requires the magnitude and direction of instantaneous velocity be measured. In the present work, a tandem hot-wire probe is manufactured and its characteristics are examined experimentally. With the use of this probe the change in the penetration length of the jet with the velocity ratio is investigated. It is clarified that two regimes of flow patterns exist: in the low velocity ratio the penetration length of the jet changes little with λ, and in the high velocity ratio it goes far from the nozzle with increasing λ. Streamlines, turbulence intensity contours and static pressure contours are presented in the two typical velocity ratios corresponding to each of two flow patterns, and they are compared.

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.

Experiments on Three-Dimensional Mixing of a Wall Jet with a Turbulent Boundary Layer

Measurements are presented of the mean flow and turbulence properties of threedimensional wall jets injected form a circular nozzle located adjacent to a wall into the turbulent boundary layers, with and without or adverse pressure gradient and a quiescent fluid. The maximum velocities of the wall jets in the boundary layers decrease more slowly toward downstream than in the quiescent fluid, and also these inverse values grow not linearly with the axial distance. The half-widths in the normal and spanwise directions to the wall, Ly and Lz, are considerably small in the wall jets injected into the boundary layers in comparison with those into the quiescent fluid. The variations of the maximum turbulent intensities are discussed in connection with those of twodimensional wall jets, with and without wall flow. The patterns of the isopleths of the mean velocity and turbulent intensity are exhibited.

Experiments on the two-dimensional impinging jets on a wedge. Mean flow properties.

An experimental study has been performed on the mean flow properties of the two-dimensional impinging jets on a wedge whose included angle is 90°. The variations in static pressure along the streamwise and transverse directions are large near the vertex of the wedge, and should not be neglected in calculations for these flows. The non-dimensional velocity profiles in the fully-developed-flow region far from the vertex do not coincide with the similarity profile of the two-dimensional wall-jet. Also the trends in the decay of the maximum velocities and in the growth of the half width of the present wall-jets are different to those of the two-dimensional wall jets. The local skin friction coefficients are estimated and compared with the empirical formula proposed by several authors. The logarithmic velocity distribution is examined by using the estimated values of the skin friction coefficients.

Measurements of the Vorticity Behind a Circular Cylinder in a Uniform Stream

The present study is designed to experimentally investigate the flow field past a circular cylinder in a uniform stream. The circular cylinder was set across an open test section of water channel. Experiments were conducted with Reynolds number based on a cylinder diameter of 120 to 800, and the flow field was visualized by the hydrogen-bubble method and visualization of the flow velocity pattern in the lateral direction was made by using an ultrasonic velocity profiler (UVP). Fluctuating velocity fields, v(y, t), were measured. Changes in the deflection amount and width of the separated shear layer are indicated from intensity distributions of velocity fluctuation. According to the analysis based on the POD, it is clarified that the present flow fields may be represented by the reduced order model of the vortex structure. Integral of v(y, t) with respect to y is related with the averaged vorticity, and its time history indicates a stability of vortex shedding mode.© 2011 ASME

Flow Visualization of a Three-Dimensional Wall Jet using the Tuft-Grid Method

This study is to clarify the flow structure of a three-dimensional wall jet discharged on a flat surface from a round nozzle. Instantaneous flow pattern parallel to the wall was visualized by depth-tuft method, and the feature of the large-scale structures existed in the present wall jet was examined. It was revealed that a streak flow pattern corresponding to a ridge pattern of tufts was dominant structure, and that the streaks were generated quasiperiodically. The flow following capability of the tufts was calibrated using a sinusoidally oscillating wind-tunnel.

Experimental Study on the Flow behind a Swept Backward-Facing Step (1st Report, Mean Flow Field in a Whole Region)

In the present study, the flow over a swept backward-facing step, at a Reynolds number (based on the step height and the free stream velocity) of 8 000 for the swept angle of 30 degrees, was investigated experimentally. Measurements were made for the mean flow field such as the surface flow direction, forward flow fraction, and mean velocity components. The flow field was examined in the regions of separation, reattachment, and following recovery to the two-dimensional flow. Lateral flows induced by the swept step significantly contribute to the variation of mean velocity profiles in the separated region. Reattachment distance of the flow over the swept step becomes about 10% shorter than that of the two-dimensional one. The mean velocity profiles collapse reasonably well on the Johnstons triangular model in the recovery region, and the profiles near the wall also collapse on the three-dimensional logarithmic law proposed by Johnston.

Experiments on the Flow Structure in a Cavity using UVP and PIV

Experiments on the Flow Structure in a Cavity using UVP and PIV Yoshihiro INOUE, Shintaro (AMASHITA, Takashi HARA and Chiharu FUKUSHIMA ABSTRACT This research has been performed to clarify the oscillation of a separating shear layer over a cavity formed peripherally on a cylinder in a uniform axial stream. Experiments have been carried out for the cavity of a width-todepth ratio of 4 in a water channel. Measurements have been made along the shear layer using UVP monitor. The spatio-temporal behaviors of the separated shear layer are examined with POD and orthonormal wavelet ransform. Maps of the mean velocity vector and vorticity are shown from the PIV measurement.