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The detailed mechanism of supersonic mixing enhancement by a novel device is investigated computationally based on the Navier-Stokes equations. The device is a wall-mounted three-dimensional cavity, several parts of the rear face of which are cut out, and the lower wall of the cut-out part has a slope directed toward the injector from which the injectant is discharged. The flow structures and behaviors observed in the computational results are confirmed in the experimental results. Experimental information about the high-frequency (several tens of kHz) pressure oscillation in the cut-out part is acquired in this study using a pressure-sensitive paint (PSP) responding to rapid change in pressure. The computational results reveal that a hairpin-like vortex is formed inside the cavity. The vortex works on the shear layer across the cavity to deflect it upward. The deflected shear layer shields the injectant from the primary flow, which causes the injectant to penetrate deep into the primary flow. The computational results also clarify that the expansion and compression waves produced in the rectangular part of the cavity are delivered effectively to the injector along the slope of the cut-out part, which is confirmed in the PSP measurement results. The delivered waves actuate the jet issuing from the injector to fluctuate up and down periodically with a large amplitude.

We propose a novel method for solving optimal sensor placement problem for high-dimensional system using an annealing machine. The sensor points are calculated as a maximum clique problem of the graph, the edge weight of which is determined by the proper orthogonal decomposition (POD) mode obtained from data based on the fact that a high-dimensional system usually has a low-dimensional representation. Since the maximum clique problem is equivalent to the independent set problem of the complement graph, the independent set problem is solved using Fujitsu Digital Annealer. As a demonstration of the proposed method, the pressure distribution induced by the K\'arm\'an vortex street behind a square cylinder is reconstructed based on the pressure data at the calculated sensor points. The pressure distribution is measured by pressure-sensitive paint (PSP) technique, which is an optical flow diagnose method. The root mean square errors (RMSEs) between the pressure measured by pressure transducer and the reconstructed pressures (calculated from the proposed method and an existing greedy method) at the same place are compared. As the result, the similar RMSE is achieved by the proposed method using approximately 1/5 number of sensor points obtained by the existing method. This method is of great importance as a novel approach for optimal sensor placement problem and a new engineering application of an annealing machine.

Polymer solutions under shear flow are often observed in manufacturing processes. Classically, polymer behavior is represented by Kuhn’s bead-spring model, in which only the elongation of polymer chains is assumed. In recent years, the compression of polymer chains under shear flow has been reported. In this study, we investigated the behavior of polymer chains dissolved in various concentrations under shear flow. We measured the time variation of the fluorescence intensity emitted from a FRET (fluorescence resonance energy transfer) polymer, which enabled us to study the change in the distance between both ends of a polymer chain. The polymer chains appeared to stretch and compress depending on the concentration of the polymer solution. The results showed that the deformation of polymer chains was different from the classical theory. The FRET measurement is a promising diagnostic method for understanding the dynamics of polymer chains.

Pressure-Sensitive Paint (PSP) is a powerful measurement technique to obtain pressure distribution on a model of interest by measuring the emission intensity of the PSP coating with a camera. Since a PSP coating is prepared by applying a solution containing an organic solvent, generally, by sprayer, the properties such as the pressure- and the temperature-sensitivity depends on the skill of the person applying it. This fabrication process is one of the barriers to use of the PSP technique because of the legal restrictions on the use of organic solvents. Thus, a sticker-like PSP coating is useful because it does not require the use of organic solvent and the applying skill. In this study, we have fabricated freestanding Pressure-Sensitive Nano-Sheet (PSNS) by a sacrificial layer process using a spin-coating method. We employed Pt(II) meso-tetra(pentafluorophenyl)porphine (PtTFPP) as a pressure-sensitive dye and poly(1-trimethylsilyl-propyne) (PTMSP) and poly(L-lactic acid) (PLLA) as a polymer binder; thus, the PSNS samples based on PTMSP and PLLA were prepared. The pressure- and the temperature-sensitivity, the lifetime of the luminescence, and the quantum yield of the fabricated PSNS have been investigated. The pressure-sensitivity of PTMSP-based PSNS is higher than that of PLLA-based PSNS. Conversely, the quantum yield of PLLA-based PSNS is higher than that of PTMSP-based PSNS.

Polymer/ceramic pressure sensitive paint (PC-PSP) is an optical fast-responding pressure sensor. PC-PSP consists of a polymer and particles as the material for a porous binder layer, and a pressure-sensitive luminophore. The solution of the polymer and particles, and then the solution of the luminophore are separately applied to a model surface. The response time of PSP is mainly determined by the thickness and oxygen diffusivity of the binder layer. However, we found that PC-PSP showed the significantly different response time depending on the solvents for the luminophore despite the same total thickness of the binder layer. Therefore, we investigated the effect of solvents in dissolving luminophore on the PC-PSP characteristics including response time, pressure sensitivity, and luminescence lifetime. PC-PSP’s prepared with solvents for luminophore that poorly dissolve the polymer in the binder layer achieved a faster response time of those prepared with solvents that can dissolve the polymer. Furthermore, it was also found that the solvents caused the change in the response time, luminescence intensity and luminescence lifetime of PC-PSP. We have concluded that the different solubility of the solvents may provide the different state of porous structure and the local distribution of luminophore in the binder layer, and it causes the variation of the dynamic and static characteristics of PC-PSP.

The conductive heat transfer through a rarefied binary gas mixture of helium and argon is experimentally investigated as a function of a molar fraction. The heat flux is measured from the free-molecular to near free-molecular flow regimes, and it is analyzed both in dimensional and dimensionless forms to clarify the effect from the gas-surface interaction. Then, the thermal accommodation coefficient is obtained by assuming a gas mixture as a single “virtual” species gas. The measured thermal accommodation coefficients for single component gases of helium and argon show good agreement with our previous results and data in literature. The thermal accommodation coefficient for a binary gas mixture is compared with the theoretical value derived in the free-molecular flow regime as a superposition of an independent heat transfer by each gas component. The measured results are well explained by the theory. However, there is a little discrepancy between them, suggesting variation in the thermal accommodation coefficient with a molar fraction in flow regimes other than the free-molecular flow regime., ファイル公開：2021-02-01

We propose a noise reduction method for unsteady pressure-sensitive paint (PSP) data based on modal expansion, the coefficients of which are determined from time-series data at optimally placed points. In this study, the proper orthogonal decomposition (POD) mode calculated from the time-series PSP data is used as a modal basis. Based on the POD modes, the points that effectively represent the features of the pressure distribution are optimally placed by the sensor optimization technique. Then, the time-dependent coefficient vector of the POD modes is determined by minimizing the difference between the time-series pressure data and the reconstructed pressure at the optimal points. Here, the coefficient vector is assumed to be a sparse vector. The advantage of the proposed method is a self-contained method, while existing methods use other data such as pressure tap data for the reduction of the noise. As a demonstration, we applied the proposed method to the PSP data measuring the K\'arm\'an vortex street behind a square cylinder. The reconstructed pressure data is agreed very well with the pressures independently measured by pressure transducers., Comment: This article has been submitted to Physics of Fluids

We propose a pressure-sensitive paint (PSP) method under ambient light based on structured light illumination. In the PSP method, pressure is obtained by measuring the variation of the emission light intensity from a PSP coating when it is illuminated by a light source. Since the extraction of the emission light from ambient light is difficult in the conventional PSP method, the existence of ambient light induces a noticeable measurement error. While spatially uniform light is used as illumination light in the conventional PSP method, structured light, the spatial intensity of which is actively controlled, is used in the proposed method. By measuring the PSP emission by varying the spatial pattern of the structured light illumination, one can eliminate ambient light, which is not synchronized with the structured light and can measure pressure without the effect of ambient light. In this study, we demonstrate the pressure measurement under ambient light using structured light with a sinusoidal wave generated by a digital mirror device. The measured pressure agrees well with the pressure measured by a pressure transducer connected to a pressure tap. The proposed method is promising under ambient light conditions.

In the high Knudsen number flows, like micro gaseous flows and flows in vacuum, the gas-surface interaction plays an important role. Among many gas species, water is a common and useful fluid in wide area of engineering applications, and it is one of the main residual gas components in the high vacuum environment. However, experimental measurements on the gas-surface interaction of water molecules are still limited. In this study, we focus on the accommodation coefficient, which represents the mean behavior of gas molecules on the scattering process from a surface. We measure the thermal accommodation coefficient (TAC) and the tangential momentum accommodation coefficient (TMAC) of water molecules. TAC is measured from the heat flux between two surfaces with different temperatures in vacuum. TMAC is derived through the viscous slip coefficient from the measurement of the mass flow rate through a microtube. To investigate the characteristics of water molecules clearly, the measured accommodation coefficients are compared with those of noble gases, helium, neon, and argon, measured with the same apparatus and procedures. For the accommodation coefficients of noble gases, TAC increases with increasing the molecular weight, while TMAC decreases, which are in good agreement with the previous studies. The TAC and TMAC of water are slightly larger than those estimated from the corresponding molecular weight of noble gases.

We propose a novel approach to analyze random walks in heterogeneous medium using a hybrid machine-learning method based on a gamma mixture and a hidden Markov model. A gamma mixture and a hidden Markov model respectively provide the number and the most probable sequence of diffusive states from the time series position data of particles/molecules obtained by single-particle/molecule tracking (SPT/SMT) method. We evaluate the performance of our proposed method for numerically generated trajectories. It is shown that our proposed method can correctly extract the number of diffusive states when each trajectory is long enough to be frame averaged. We also indicate that our method can provide an indicator whether the assumption of a medium consisting of discrete diffusive states is appropriate or not based on the available amount of trajectory data. Then, we demonstrate an application of our method to the analysis of experimentally obtained SPT data.

We have proposed an instant method for painting of pressure- and temperature-sensitive paints (PSP and TSP) using a commercial inkjet printer. This method enables us to finely and separately paint PSP and TSP to prevent them from being mixed and the interaction between PSP and TSP dyes. Moreover, the pressure-sensitivity of inkjet-printed PSP was similar to those of conventional PSPs. We consider this method is promising to measure pressure distribution with temperature variation, because the printing pattern of PSP and TSP is easily designed by an illustration software to adjust pressure distribution of interest. The pressure distribution induced by jet impingement was measured by the grid pattern of PSP and TSP as a demonstration. The pattern of PSP and TSP was well-defined; thus, the emissions from PSP and TSP can be easily separated and analyzed even when the emissions were captured by a CCD camera at the same time. This is also an advantage of our method. The pressure distribution agreed well with the pressures measured by pressure taps.

Single particle/molecule tracking is widely used for the evaluation of diffusion coefficients using diverse tracking algorithms. Many of them require the subtle decision of displacement threshold parameter to enable the appropriate tracking, though it depends on both of the diffusion coefficient itself and the measurement system. A simple and highly transferable technique is proposed to overcome this difficulty based on the evaluation of diffusion coefficient from the peak position of the distribution of squared displacements in the logarithmic scale. In combination with its linear dependence on time, this protocol is remarkably robust against the too large value of the displacement threshold including that covers the whole image. Furthermore, the proposed technique is compatible with many of the existing algorithms by construction.

Industrial applications of cellulose nanofibres (CNFs) include the additive to a wide range of materials from display and cosmetic to pigment inks. Many of these expected applications are based on the mixture of colloidal particles and CNFs in aqueous dispersion. In particular, the mixed dispersion is typically processed in such a way that the droplet or shallow wide volume of dispersion is dried on the substrate or container. In this work, the basic physical properties that these diverse kinds of potential applications share in common are focused on. The colloidal films were fabricated consisting of polystyrene particles and CNFs under the condition of various concentration combinations through the drying of aqueous dispersion in polystyrene well plates. It has been found that the films fabricated from higher concentrations of CNF tend to peel-off spontaneously in the drying process. The basic mechanism of phenomenon is attributed to the contraction of CNFs during the drying by hydrogen bonding between the filaments.

Graphical Abstract, We report that the addition of a small amount of cellulose nanofibers (CNFs) into an aqueous dispersion of colloidal particles suppresses the coffee-ring effect when the dispersion dries on a solid substrate, as revealed by the computational analysis of experimental time-series images and by particle image velocimetry. The addition of CNFs is much more effective than the increase of colloidal particle concentration at the same weight percentage; it is also more environment friendly than the use of typical molecular surfactants. This finding is promising for the fabrication of metamaterials from colloidal dispersions and for ink printing in electronics, where CNFs can also serve as a substrate for flexible devices.

Diatoms in seawater, Chaetoceros gracilis and Chaetoceros calcitrans, were successfully deactivated by plasma generated in the seawater flow. Not only in situ formed oxidants such as hydrogen peroxide and free residual chlorine, but also the direct effects of the plasma such as UV illumination and collisions with high-energy particles seemed to contribute to the deactivation of the algae.

This study experimentally investigates the mixing of a two-layer density-stratified fluid of water (upper layer) and aqueous sodium chloride (NaCl) solution (lower layer) induced by the interaction between a vortex ring and the density interface. The vortex ring, which consists of water, is launched from an orifice in the upper layer toward the density interface, after which its motion, along with the behavior of the lower fluid, is visualized through a planar laser-induced fluorescence method. The Atwood number that expresses the nondimensional density jump across the density interface is set at 0.0055, and the Reynolds number Re of the vortex ring is varied from 2050 to 3070. The visualization experiment clarifies that the vortex ring penetrating the density interface is bounced while collapsing in the lower fluid. Furthermore, it elucidates that the bounced upper fluid entrains the lower fluid into the upper layer by inducing a second vortex ring consisting of the lower fluid. Thus, this study reveals the effect of Re on the mixing of the upper and lower fluid induced by the launched vortex ring.