Yoshiyuki Tsuji

Pressure fluctuation in high-Reynolds-number turbulent boundary layer: results from experiments and DNS

We have developed a small pressure probe and measured both static pressure and wall pressure simultaneously in turbulent boundary layers up to Reynolds numbers based on the momentum thickness . The measurements were performed at large experimental facilities in Sweden, Australia, and Japan. We find that the measured pressure data are contaminated by the artificial background noise induced by test section and are also affected by the flow boundary conditions. By analyzing data from different wind tunnels acquired at the same Reynolds number, we evaluate the effect of background noises and boundary conditions on the pressure statistics. We also compare the experimental results with results of direct numerical simulations and discuss differences in boundary conditions between real and simulated wind tunnels.

Instantaneous measurement of velocity fields in developed thermal turbulence in mercury.

Using ultrasonic velocimetry we measured the vertical profile of the velocity fluctuation in high-Rayleigh-number thermal convection in a cell with aspect ratio of 0.5, filled with a low-Prandtl-number fluid, mercury. The intriguing fluctuating dynamics of the mean flow and universal nature of the kinetic energy cascade are elucidated utilizing spectral decomposition and reconstruction. The scaling properties of the structure functions and the energy spectrum are directly calculated without the use of Taylors frozen-flow hypothesis. Despite the complex nature of the mean flow, it is found that the energy cascade process exhibits universal laws in thermal turbulence.

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.

Friction factor and mean velocity profile for pipe flow at high Reynolds numbers

The friction factor for a fully developed pipe flow is examined at high Reynolds numbers up to ReD = 1.8 × 107 with high accuracy using the high Reynolds number actual flow facility “Hi-Reff” at AIST, NMIJ. The precise measurement of the friction factor is achieved by the highly accurate measurement of the flow rate, and the measurement uncertainty is estimated to be approximately 0.9% with a coverage factor of k = 2. The result examined here is obviously different from the Prandtl equation and the experimental results from the superpipe at Princeton University. The deviation of the present result from the Prandtl equation in the lower Reynolds number region is approximately 2.5% and −3% at the higher Reynolds number. For ReD 2.0 × 105, and it increases with the Reynolds number and reaches −6% at ReD = 1.0 × 107. The Karman constant estimated by the measured fric...

Universality of probability density distributions in the overlap region in high Reynolds number turbulent boundary layers

The probability density functions (PDFs) of the streamwise mean velocity in high Reynolds number turbulent boundary layers have been studied. The hypothesis of self-similar, normalized with the root mean square velocity, PDFs has been tested using the KTH database of high Reynolds number zero pressure-gradient turbulent boundary layer flow. The self-similarity was tested using the Kullback–Leibler divergence measure and it was found that the region of self-similar PDFs extends from about 160 viscous wall units to about 0.3 boundary layer thicknesses (in δ95). This region is somewhat larger than the universal overlap region. The PDF shape in the universal overlap region is close to Gaussian allowing for a Gram–Charlier expansion approximation of the measured PDFs. A remarkable collapse was found for 57 normalized PDF distributions for different positions within the universal overlap region and Reynolds numbers based on the momentum-loss thickness between 4300 and 12u200a600, strongly indicating a high degree o...

Large-scale anisotropy effect on small-scale statistics over rough wall turbulent boundary layers

According to the local isotropy hypothesis presented by Kolmogorov, small-scale velocity fluctuations should be universal in any kind of turbulent flow when the Reynolds number is sufficiently large. This is one of the key assumptions in turbulence phenomena. At this stage, the question is not whether this assumption is correct or not, but rather how the local isotropy works as a good approximation depending on the nature of the large-scale anisotropy. In this paper, we report on how the large-scale anisotropy penetrates the small scales. Based on the experiments performed in the strong mean shear flow on the rough-wall boundary layer, we consider how the local isotropy is restored. The anisotropic parameter S* is defined as a ratio of the time scale caused by the mean velocity gradient and the Kolmogorov time scale. It is found that the local isotropy is achieved in the dissipation range even in S*≃0.1. On the other hand, there is no clear evidence of isotropy in the inertial range. Due to the strong mea...

Intermittency effect on energy spectrum in high-Reynolds number turbulence

The energy spectrum in fully developed turbulence shows the power-law relation, E(k)∝k−(5/3+μ). The deviation from −5/3 is due to the effect of intermittency. Analyzing data in high-Reynolds number turbulence [Rλ≃O(104)], it was found that there are two different power-law regions in E(k). One locates close to the spectral bump and the other is in the lower wave number range. Intermittency corrections are μ=0.075 and μ=0.025, respectively. This result is compared with recent direct numerical simulations (Rλ≃1200) [Phys. Fluids 15, L21 (2003)] in which μ is not negligible, but is evaluated to be 0.1. We also comment on the Kolmogorov constant and the scaling exponent of velocity structure function.

High-Reynolds-number experiments: the challenge of understanding universality in turbulence

At the 40th anniversary meeting of the Japan Society of Fluid Mechanics, the author presented the results obtained in three high-Reynolds-number experiments. The results dealt with issues such as the turbulence energy spectrum, the mean velocity profile in the boundary layer and the skin friction coefficient of a flat plate. This publication presents a summary of the first topic, and makes a case for the necessity of high-Reynolds-number experiments by attempting to answer the question Why do we need high-Reynolds-number experiments?

INTERMITTENCY FEATURE OF SHEAR STRESS FLUCTUATION IN HIGH-REYNOLDS-NUMBER TURBULENCE

Instantaneous shear stress fluctuations are considered for high-Reynolds-number (up to Rλ≈104) turbulent flow field using the concept of maximum norm. The maximum norm is defined as the largest change within a box of a given size. We have applied this technique to a variety of flow fields and a wide range of Reynolds numbers. Results indicated that the maximum norm has a power law behavior with the size of the box. This novel approach is applied to the dissipation field and the scaling behavior is found to agree very well with that obtained using conventional methods. Thus, we obtain the intermittency exponents for both shear stress and the dissipation rate. We find that the shear stress fluctuation is less intermittent than the dissipation field. Implications of these results are discussed briefly.

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...