Ikuo Nakamura

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.

Probability density function in the log-law region of low Reynolds number turbulent boundary layer

The logarithmic velocity region is considered in zero-pressure-gradient turbulent boundary layers. We propose a new definition of the log-law region using the probability profiles of streamwise velocity fluctuation. The log-law profile is extracted readily from the experimental data as well as from the probability density function (pdf) equation. The measure called Kullback Leibler divergence is applied for distinguishing the probability profiles. If the logarithmic profile, U+=A⋅logu200ay++B, is a good representation of experimental data, our results show that A is independent of the Reynolds number while B depends on it. The ratio of boundary layer thickness δ to the upper end of log-law extent, δL, is not constant but approaches the value δL/δ=0.2 as the Reynolds number increases.

Simultaneous measurements of concentration and velocity in a CO2 jet issuing into a grid turbulence by two-sensor hot-wire probe

A new simple method has been developed to measure the concentration and velocity simultaneously for the gas mixture (CO2–air) flow by two hot-wire probes. By using a calibration map, the values of the instantaneous concentration and velocity can be determined from the voltages of two hot-wire probes with different overheat ratios (OHR) by digital data processing. This method does not require subtle adjustment of the OHR to achieve insensitivity to the CO2 concentration, as in the past work by Chassaing (Melange turbulent de gaz inertes dans un jet tube libre, These Doc. Sciences 42, Inst. Nat. Poly. de Toulouse, 1979). The measurements have been performed in a CO2 jet in grid turbulence. The experimental results for the centerline and radial distributions of the mean and r.m.s. values of mass fraction concentration and velocity, and axial scalar flux are consistent with previous data.

Pressure regulator valve by Bondgraph

Abstract The Bondgraph represents mechanical, hydraulic and electric components with their power flow in an unified manner and is used here for a three port regulator valve for pressure. It consists of a valve spool and a valve body or casing, and an electric solenoid. The valve assembly has three ports and two variable metering orifices, represented by modulated R-elements. They are the combination of a proportional and a quadratic characteristic, each by separate R-elements. One of them is in conductance causality, the other one in resistance causality and they are on a 1-junction. This gives an algebraic loop that can be broken by a first order element. Even with a closed port, there is some passage, or leakage in the valve due to clearance. The solenoid is non-linear. The whole is represented by a Bondgraph and simulated by the TUTSIM program, where real parameters are used. This leads the valve dynamics much quicker than solving differential equations

The effect of a circular cylinder on the diffusion of matter by a plume

Experimental results are given for the mean and fluctuation concentration in a plume from a point source in grid-generated turbulence, which has developed so as to have a width comparable to the cylinder diameter, is disturbed by a cylinder with its axis intersecting the plume axis perpendicularly. It is shown that the disturbance effsct of the cylinder appears in the upstream region of about 1.5 diameter from the stagnation point. As the stagnation point is approached, the increased travel time and the enhanced molecular diffusion cause a noticeable amplification of the dissipation of concentration variance. Also, the characteristics of some conditional statistics of the plume entrained in the wake are examined in order to investigate the intermittent structure of the plume meandering produced by the Karman vortex street.

The fractal aspect of an isovelocity set and its relationship to bursting phenomena in the turbulent boundary layer

In this paper, the fractal property of an isovelocity set which is defined by the instantaneous streamwise velocity signal obtained in a turbulent boundary layer is reported. In the present study, the aim is to provide further evidence for the fractal property of the set and to investigate which physical and dynamical quantities significantly affect the fractal property of an isovelocity set. The instantaneous Reynolds stress is found to be a key factor of this fractal property. Using the variable interval time average (VITA) method and the quadrant analysis technique, the bursting events in the turbulent boundary layer are conditionally sampled and investigated in relation to the isovelocity set. The quadrant analysis technique is used to demonstrate that the fractal property appears only in the bursting period and that there is a close connection between the Reynolds stress producing structure and the fractality. Also considerations are given on the bursting distribution itself by means of fractals. In ...

Existence of extra vortex and twin vortex of anomalous mode in Taylor vortex flow with a small aspect ratio

SummaryThis experimental work on Taylor vortex flow in a gap with a small aspect ratio is concerned with two extra vortices and a twin vortex system, each of which depends on an anomalous cell of the anomalous mode. Extra vortices are smaller than other vortices such as defined cells. At any Reynolds number and aspect ratio extra vortices can be found at the corner of the end plate and inner rotating cylinder and at the corner of the end plate and outer stationary cylinder. For a one-cell flow (anomalous one-cell mode) in a symmetric system, an outer extra vortex develops and grows to the same size as the main cell, only in an aspect ratio of less than one. A twin vortex is observed to form when two vortices are aligned in the direction of the radius. There are three flow fields on the end plate; two are extra vortex flows and the other is the main cell flow. The flow direction of the anomalous cell is from the inner cylinder to the outer one, at the end plate opposite of the flow direction of the normal cell.

Is intermittent motion of outer flow in the turbulent boundary layer deterministic chaos

Intermittent phenomena observed in the outer regions of the turbulent boundary layer are studied. A new idea is proposed in which the dynamics generating the intermittent phenomena can be described by a one‐dimensional map; Xi+1=(1+e)Xi+uXzi, for 0≤Xi≤Xc 1, u>0). The binary sequence {si} is constructed from carefully processed data of instantaneous streamwise velocities, and also from the map. The encoding is made in such a way that the turbulent state is referred to as si=1 and the nonturbulent state as si=0 at a discretized time i. Both sequences are found to have the common essential properties of intermittent chaos; the probability P(l) of finding nonturbulent states with length l exhibits the power law with an exponent r for l<lc, lc being a cutoff. From the exponent r, one can assign z almost equal to 4, while the control parameter e is closely related to the distance from the wall. The scaling function of P(l) agrees very well with the one predicted from the map. The cutoff length lc i...

Fractal analysis of a circulating flow field with two different velocity laws

Abstract The turbulent shear flow around a rotating cylinder in a quiescent flow is a simple case of a rotating turbulent flow field, where centrifugal force works. Two different power-law mean-velocity distributions exist in this flow field. One is U∝1/r and the other is U∝1/r2, where r is a distance from the surface of a cylinder. The behavior of chaos and fractal properties for this complex flow field are investigated. The former concerns the dynamic property in fluid flow and the latter is useful to characterize complex flow patterns or the distribution of turbulence quantities. From the instantaneous velocity signal, we defined the iso-velocity set, and its fractal property was investigated both in the U∝1/r and U∝1/r2 region. The instantaneous Reynolds stress is found to be a key factor in this fractal property as conceived for the flat plate boundary layer. The intermittency chaos was applied to investigate the turbulent and non-turbulent distribution in the outer region. A simple one-dimensional model could be useful to identify turbulent and non-turbulent distributions even in this complex flow.