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Accelerator-driven neutron sources have been developed worldwide for boron neutron capture therapy (BNCT) instead of nuclear reactors. Nagoya University is currently developing a compact DC accelerator driven BNCT system using a sealed lithium (Li) target. The target receives a high heat load by a proton beam, and the heat then requires efficient removal. This study developed a high-efficiency heat removal technique that uses V-shaped staggered ribs on the heat removal side of the flow channels. We performed experiments using an electron beam to confirm the technique performance. The results showed double the heat removal performance compared to the smooth surface channel. The value expected to sufficiently remove 6.6 MW/m2 of heat load provided by the proton beam at the Nagoya University neutron source for BNCT.

We investigate the turbulent structures associated with positive and negative high-amplitude wall pressure peaks in fully developed turbulent channel flows. The analysis is performed by conditional sampling of a channel flow dataset computed in direct numerical simulations. The Reynolds number was varied from 180 to 4000 based on the channel half-depth h and the friction velocity u τ . Positive and negative wall pressure events were associated with small-scale vortex structures identified by Q-criterion. When scaled in wall units, the overall size of the structure was independent of Reynolds number. Positive and negative wall pressure events were also associated with a large-scale sweep motion (of ) from the outer layer, which constitutes a shear layer in the near-wall region. In a statistical analysis of the instantaneous pressure field, a sequence of negative and positive pressure peaks was found to predominate at the wall. The relative contributions from the large- and small-scale structures were qualitatively assessed by investigating the rapid and slow pressure terms from Poisson's equation. Both terms contributed nearly equally to the positive wall pressure peaks, but the slow term dominated the negative pressure peaks (with a relative contribution of approximately 60%).

Experimental investigations were carried out to establish a design method for a thermoacoustic cooling system. When a straight acoustic tube is used, the acoustic power is supplied from the tube end. The ceramic stack, which consists of many narrow channels, is inserted into the tube. By changing the insertion position and the stack channel radius, a temperature decrease at the stack end is observed as the energy conversion parameter. A nondimensional parameter, ωτ, considering the relaxation time τ is determined from resonance frequency and stack channel radius. The temperature decrease is examined using several values of ωτ. Results show that the insertion position for which the maximum temperature decrease is obtained depends on ωτ. Therefore, the best stack insertion position can be designed according to the given frequency and stack channel radius. For that reason, ωτ is an important index for thermoacoustic cooling systems.

The probability density function (pdf) of a streamwise velocity component is studied in zero-pressure gradient boundary layers. From analyzing the data up to Rθ13,000, it is found that pdfs have self-similar profiles ranging from y+180 to 0.049Δ+, where Δ is Rotta–Clauser boundary layer thickness. Pdf profiles asymptote to the universal shape very close to the Gaussian, but are positively skewed at the core region, indicating smaller values in the tail parts. Based on this experimental fact, the mean velocity profile is reconsidered from the standpoint of pdf equation. The log-law profile is expected as the mean velocity distribution. The Karman constant is evaluated to be 0.38, and the log-region starts at ys+ = 180 and ends at yr+ = 0.021Δ++96.5 for Rθ > 4000. The end point locates in 0.15δ+ ≤ yr+ ≤ 0.2δ+. The relation to the new scaling law derived from Lie-group theory is also discussed.

Helium (He) plasma irradiation to tungsten (W) leads to morphology changes in nanometer scale by the formation and growth of He bubbles. Initially pinholes and protrusions are formed on the surface followed by the formation of nanostructures. In this study, based on experimental observation, the growth process of the fiberform nanostructures are revisited and the swelling process of the structure is discussed. The novel nanostructures are analyzed from the viewpoint of fractality. It is found that the number of the initially formed pinholes and its sizes have a fractal relation, indicating that the size and number of bubbles formed near the surface have fractality. The fractal dimension is estimated from the brightness variation of a transmission electron microscope (TEM) micrograph and gas adsorption property. Moreover, it is revealed from TEM image analysis that the nanostructure has multifractal feature, probably because of the fractality identified between the number and the size of bubbles near the surface.

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?'

Experimental investigations have been carried out to improve the energy conversion efficiency of the loop tube: a thermoacoustic cooling system. Generally, increasing the proportion of thermal boundary layers in a stack improves the energy conversion efficiency of a thermoacoustic cooling system. Decreasing the channel radius of the stack is necessary to increase the proportion of the thermal boundary layer in the cross section of the stack. It is difficult for a ceramic stack to reduce the channel radius of the stack because of the present manufacturing techniques. Thus, we use a lamination mesh formed by piled up stainless mesh to do so. Nevertheless, the lamination mesh cooling property is unclear, as it is related to ωτ, which is a parameter for determining heat exchange. In this expression, ω is the angular frequency of the sound wave and τ is the time required for thermal equilibrium in the cross section of the stack channel. Using the acoustic tube, experiments were carried out by changing the mesh number of the lamination mesh and the insertion position to assess the cooling characteristics of the lamination mesh. Experimental results reveal a tendency resembling that shown by a ceramic stack. Moreover, the ωτ and channel radius of the lamination mesh were estimated using the result in our previous work. Results show that this design method can be adapted to the use of the lamination mesh and ceramic stack.

For the first time, fluctuation characteristics at the high-field side (HFS) and low-field side (LFS) scrape-off-layers (SOLs) in L-mode plasmas of the JT-60U tokamak are compared in detail; the plasma transport is studied using reciprocating Langmuir probes. A statistical analysis based on probability density functions (PDFs) is employed to describe intermittent (nondiffusive) transport in SOL plasma fluctuations. It is found that positive bursty events associated with blobby plasma transport appear frequently at the LFS midplane. Further, the PDF at the LFS midplane is strongly positively skewed, while the PDF at the HFS SOL is close to a Gaussian distribution. The conventional conditional averaging method is improved by using the variable-interval time-averaging method along with it, which enables the precise reconstruction of the burst profile exhibiting a rapid increase and a slow decay. The radial evolution of plasma blobs is analysed by using an autocorrelation function, the short-time Fourier transform and wavelet analysis.

The total skin friction on a flat plate is directly measured by using a towing tank up to Reynolds number ReL 107 (or Rθ 104). Plates of 3.3 and 4.3 m in length are towed in still water, balancing the vertical weight by small flotation devices, and their drag force is measured by a highly sensitive load cell. We have developed a new technique to correct wave-making resistance, pressure resistance and drag on a turbulence simulator. When the measured total drag is converted into local drag, it is found that the local frictional resistance is about 6% smaller than that given by the Karman–Schoenherr formula. But it is consistent with the values obtained by the floating element technique, oil film interferometry and asymptotic evaluations.