The butt dissimilar joints of Al-Mg-Si Alloy A6063 and carbon steel S45C by means of friction stir welding (FSW) were prepared for investigating variation of hardness and amount of deformation during cyclic loading of the joints. And variation of hardness and amount of deformation were also measured on heat treated specimen. Hardness distribution were measured by using Vickers hardness testing machine before applying cyclic load and when the cyclic loading was interrupted at discretional number of cycles, and amount of deformation were measured by measuring the distance between each indentation when the cyclic loading was interrupted. From the experimental results, it was found that the significant deformation and hardness recovering were occurred at early stage of the fatigue life in both case of non-heat treated specimen and heat treated specimen. After the early stage of fatigue life, variation of hardness and amount of deformation were not so significant. The maximum amount of deformation was indicated at where the hardness was minimum, and fatigue fracture occurred at that region, except for the case that the fracture occurred at welding interface.

In order to realize a new structure and function, direct joining method of dissimilar materials has been studied. According to previous results, friction stir welding (FSW) is possible to make dissimilar materials joint such as steel and aluminum alloy, copper alloy and aluminum alloy, etc.. It is expected to add various functions like composite materials by joining materials with different properties. The present research work aims to develop a thin dissimilar materials joint plate produced with rolling process followed by FSW between aluminum alloy and various metallic materials. In this report, effect of rolling process on strength characteristics under static and cyclic loading condition in a dissimilar materials FSW joint between copper and aluminum alloy is shown.

It has been reported that the joint strength of resin/metal dissimilar materials joint changed depending on the environmental condition. Moreover, the mechanical properties of the resin are possible to change due to heating during the joining process or environmental effect in the service condition. The strength of the dissimilar materials joint can be affected by the change in interfacial strength itself and change in the mechanical property of resin too. In this study, the effect of heat treatment on the mechanical property of resin material and the effect of the change in the mechanical property of resin on the interfacial strength of the dissimilar materials joint were investigated. Tensile strengths of resin materials were affected by heat treatment. In case of PA66 (Polyamide), the tensile behavior changed significantly from ductile to brittle with an increase of heat treatment time. According to FT-IR result, the chemical bond might be changed before and after heat treatment. The indentation test was conducted to evaluate the peeling force of the interface in dissimilar materials adhesive joint between PA66 and A5052. The maximum load corresponding to delamination decreased with an increase in heat treatment time of resin.

In order to improve corrosion resistance in Friction Stir Spot Welding (FSSW) dissimilar materials joint, inserting a resin film between metallic material sheets was proposed. In the present study, A1100 and Zn plated steel was welded by FSSW with inserting a PET film. In case of A1100 / Zn plated steel FSSW joint welded without PET film, tensile shear strength of 26.6 MPa was obtained. Moreover, according to immersion test inside salt water for 1100 / Zn plated steel FSSW joint welded without PET film, corrosion significantly occurred. FSSW was carried out for welding between A1100 and Zn plated steel inserted PET film with different thickness. Zn was detected above and below the PET film in the joint inserted a thick PET film. In case of a joint welded with thinner PET film, damage of the film induced during FSSW process was observed. In a joint welded with a PET film with the middle thickness, flowing of molten Zn to above and to below the PET film and damage in the film were not found. According to observation of delaminated joint, it was found that the PET film was bonded to A 1100 and to Zn plated steel by FSSW. Improving of corrosion resistance could be expected in A1100 / Zn plated steel FSSW joint by inserting a PET film with appropriate thickness.

In a gas turbine combustor operated at high temperature, cracks are generated from cooling holes in the combustor due to thermal stress and combustion oscillation. Since the combustor consists of thin-walled and cylindrical structure, it is difficult to repair the combustor by conventional overlay welding. Therefore, a new method for repairing the combustor with cracks is needed. We proposed a laser repairing technology for thin-walled component with crack. This technology is based on melting and bonding of crack by a pulsed laser. However, influence of laser conditions and thermal properties of material on the molten geometry has not been well understood yet. In this study, in order to clarify the relationship between laser condition and molten geometry, four kinds of metallic materials (SUS304, SUS430, SS400 and A5052) were irradiated by a fiber laser. The influence of laser power, scan speed and thermal properties of the materials on width and depth as the molten geometry was investigated. As a result, it was confirmed that the width and depth strongly depend on laser power, scan speed, and material. Based on those results, a new parameter called “laser operating parameter (LOP)” was proposed for estimating the molten geometry.

Thermal Barrier coatings (TBCs) are used to protect the hot sections of gas turbine engines and airplane engines, and help maintain lower temperature of the underlying superalloy substrate. The TBC system comprises a substrate, bond coat, and TBC top coat. Temperature-dependent Young's modulus of TBC is an essential mechanical property as they allow the evaluation of parameters of material mechanics. In order to evaluate temperature-dependent Young's modulus of TBC, we used the thermal stress model of three-layered material and the deflection during a thermal cycling. First, we verified experimentally the effectiveness of the model. Further, we successfully evaluated the temperature-dependent Young's modulus up to 700°C for TBC specimens with various preheat treatment.

In general, tapping screws are used for tightening of CFRP member. However since tapping screws are tightened while forming internal threads, residual stress is generated at the thread roots of the internal threads. Hence it is considered that the tightening strength decreases. In our previous study, we found that the tightening strength of the CFRP member tightened with metric threads was higher than that of the tapping screw. In this study, we investigated the effects of lubricant on tightening characteristics of CFRP members due to metric threads. As a result of this study, it was found that lubricants are counterproductive in the tightening of CFRP by the metric threads.

In a recent automotive industry, the weight reduction of vehicle body is required for intense fuel efficiency regulation. Multi-material structures such as steel with aluminum or CFRP has been adapted nowadays. Adhesive bondings are attracted for joining different materials, but the reliability of adhesive joints under various loading conditions are important issues. Therefore, fracture mechanics evaluations of adhesive joints in mixed mode loading are required, but it has not been established yet. In this study, a method for obtaining J-integral values from path integrals using the Digital Image Correlation(DIC) as the fracture mechanics evaluation method of the adhesive bonding material is studied. At first, we adapted the developed method to aluminum alloy A5052 and confirmed that J-integral values from DIC is agree with that of FEM analyses in mixed mode tensile tests. Then the developed method was applied to adhesive joints and comparison between experiment and numerical results were discussed.

In the present study, an adhesive contact analysis is conducted considering van der Waals force between a rigid indenter and a substrate with wavy surfaces. The wavy surface is expressed by a product of cosine functions with respect to x₁ and x₂ and an amplitude A. The rigid indenter approaches to the wavy surface every step of 0.08nm until the tip of the indenter reaches to -3.0nm. After that, the indenter is pulled up every step of 0.08nm. During the separation process of indenter, the maximum adhesion force is observed. In the present study, the amplitude of surface is varied from 0.0 to 2.0nm. The maximum adhesion force is obtained in the amplitude of surface of 0.08nm. It is supposed that the increase of adhesion force in the separation process of the indenter is attributed to the adhesion area in the concave part of the wavy surface.

In recent years, to achieve reducing the fuel consumption, transportation equipment for automobiles has been applying to lightweight materials such as high strength steels. At the same time, establishment of a welding technique to a wide range of materials is required, and this study focused on adhesive bonding. Adhesive bonding enables multi-material structure by dissimilar welding, high strength for joining thin plate is expected by surface joining. However, there are few researches on fatigue strength for adhesive bonding joints using automotive adhesives. This study investigated the influence of heating time on the fatigue strength of joints bonded with automotive steel sheets SPCC by one-pack thermosetting epoxy resin to create a bonded joint with higher strength and accuracy. The static and fatigue strength of the joint improves as increasing the heating time. In particular, to improve the fatigue strength of adhesive bonded joints, it is necessary to improve the interface strength between the adherend and the adhesive. In other words, it was found that an increase in the cohesive failure rate affects improvement in fatigue strength.

In the automotive industry, it is required to improve fuel consumption such as lightweight body in order to solve environment issues, the increasing use of high-strength steel is expected. However, high-strength steel spot welds have problems of reducing the fatigue strength, moreover, for the conventional shear-type joints, which subject to mode I loading may cause the welded part inclined. Therefore, evaluation of fatigue strength by considering purely shearing loading may lead to inaccuracy prediction. Thus, this study investigated the effect of loading mode on the fatigue characteristics of spot welded joint by restraining the jigs for suppressing the rotational deformation of the joint. As a result, there is no difference between the fatigue strength of low- and high-strength steel in the conventional shear-type joints. However, in the shear-type under mode II loading only using restrained jigs, the fatigue strength of the high-strength steel tended to improve because of the difference in the fatigue crack propagation path.

Bio Diesel Fuel (BDF) contains a lot of oxygen, but the ignitability is inferior because the viscosity is high compared with gas oil. However, previous research has found that while gas oil and BDF have different fuel properties, use of these under operating conditions with diesel engines eliminates the drastic differences in thermal efficiency and exhaust gas characteristics. Therefore, the purpose of this study is to grasp the effects of oxygen in fuel in the combustion process of diesel engines. To this end, engine performance testing using mixed fuels of BDF and gas oil with different oxygen content rates was performed. The results clarified that using fuel with a moderate mix of BDF and light oil improves the exhaust gas characteristics of the diesel engine compared to using BDF alone.

In order to utilize palm acid oil (PAO) for combustion fuel, improvement on the low temperature fluidity of PAO-diesel mixed fuel is demonstrated using polyglycerol fatty acid ester (PFAE). PFAE has an effect on reducing the PAO deposition at low temperature definitely in exchange of viscosity increase. One of the PFAEs: THL-17 puts the aggregation of PAO particles off in the beginning of the cooling in addition to keeping the uniform distribution of the PAO particles. Combustion experiment using an industrial diffusion burner fueled by the PAO-diesel mixed fuel with PFAE is carried out to evaluate the combustion and emissions characteristics. The result shows that the furnace outlet temperature of THL-17:2wt.% mixed fuel almost keeps as high as the temperature of diesel fuel. On the other hand, the THL-17:2wt.% mixed fuel decreases NOx by maximum 30 % in comparison with the diesel fuel, showing a possibility that NOx is reduced by PAO.

The self-circulating thermosyphon (SCTS) for transporting solar energy was developed. This device is driven by buoyancy of vapor generated in the heater. Thus, SCTS can operate without an external power supply, and as simple structure. The solar radiation varies time to time. Therefore, the transient behaviors against varying thermal energy are important subjects. We examined about the dynamic characteristics for fluctuations of thermal energy in this study. To examine the dynamic characteristics of SCTS, the thermal energy at the heater was changed in time. When the thermal energy increased, the fluctuations of flow velocity occurred. And, even when the thermal energy decreased, fluctuations occurred. The frequencies of these fluctuations were almost the same.

In order to expand the introduction amount of renewable energy, it is necessary to solve various problems such as suppression of output fluctuation, cost of power supply compensator for reducing output fluctuation, and lack of transmission capacity. On the other hand, it is known that output fluctuation of renewable energy is leveled by interconnecting renewable energy dispersedly arranged in a wide area. Therefore, it is possible to reduce the cost of the system by optimally distributing and link the renewable energy to a wide area. Therefore, in this study, we developed computer algorithms to optimize the location and introduction amount of renewable energy that will conduct wide area interconnections based on actual transmission network equipment. The target of the analysis was the Hokkaido area in Japan with extensive land and abundant natural energy. Using the proposed algorithm, we evaluate the relationship between economical renewable energy location and capacity, renewable energy supply rate and grid capacity. As a result, it was possible to realize an economical power system with a high percentage power supply ratio of renewable energy.

Darrieus type turbines with guide vanes for the Oscillating water column were simulated by LES analysis. The equations are discretized by Finite volume method for space and Fractional step method for time. In this study, turbine has symmetric two guide vanes near the rotor. Main parameters of guide vane are camber, setting angle and position. Four blades of rotor are NACA0018 and chord length is 17.2% of diameter. The following are conclusions of analysis. Suitable guide vanes increase efficiency under the oscillating flow. However, it can't increase maximum efficiency under the steady flow. Large gap of vane and rotor can't increase efficiency.

Magnus wind turbine is wind turbine that uses Magnus effect to generate electricity. Magnus wind turbines are unique wind turbine systems that rotate with cylinders instead of using the more common propeller-type blades. In a previous study, it was confirmed that the vortex behavior produced by the interaction between the freestream and the fin was significantly related to lift generation. However, many parameters such as the fin shape and the number of fins have room for improvement. In this study, in order to investigate relationship between fluid force and number of fins, the rotation cylinders with two straight fins and three straight fins are compared. As a result, the lift force of the rotating cylinder with three straight fins increased under the low peripheral speed ratio. However, the drag force increased, too. So, the lift drag ratio of the rotating cylinder with two straight fins and that with three fins ware almost the same for a part.

A gain-scheduling approach for model predictive control of a floating offshore wind turbine-generator system is developed to improve the reduction effect of both the power output fluctuation and the platform pitching motion. In our previous model predictive control, the internal model to predict the control behavior was obtained by linearizing the dynamic response of the floating wind turbine-generator system at a fixed operating wind speed. To deal with variations in the dynamic characteristics to the inflow wind speed mainly caused by the blade pitch manipulation, the coefficient matrixes in the internal model are varied according to the spatial mean of the inflow wind speed to the wind turbine. An aero-elastic-hydro-control coupled simulation reveals that employing the gain-scheduled model predictive control improves the reduction of the damage equivalent load at the tower base as well as the controllability of both the power output and the platform pitch in comparison to the model predictive control using the fixed internal model and this improvement is prominent at the wind speed lower than 18 m/s.

A hybrid turbine that is being evaluated for performance deterioration has the possibility to improve its performance by engineering examination of the flow field. Since the inner turbine has a large influence on the flow field of the entire turbine, it is necessary to pay attention to the influence on the flow field conforming to the condition for generating the torque, not the characteristic curve of the individual turbine of the turbine to be configured. For more efficient torque generation, it is necessary to effectively convert lift to torque. Here, a characteristic inner turbine which is not the Savonius type is adopted. The radial velocity component of the flow velocity field, which has an important role in the performance improvement mechanism, was investigated by visualization experiments in the flow region on the side of the turbine, and showed the existence of a flow which is a factor to improve the torque.

This study aims to investigate flow field for a drag type vertical-Axis wind turbine (VAWT) consisting of three quarter circular-arc blades attached to a cylindrical core. In this study, four values of the attachment angle of the circular-arc blades are selected as α=5o, 20o, 45o and 75o. The flow visualization around the rotor can be carried out experimentally with the particle image velcimetry (PIV) . The pressure distributions on the surface of the rotor were successfully obtained from the mesured velocity fields.

For investigating the effects of horizontal arms (cross sections: airfoil, rectangle, circle) on a straight blade vertical axis wind turbine, surface shear stress distributions and integrated friction drag were analyzed based on the CFD (Computational Fluid Dynamics) results. Since the flow separated from the corners of the leading edge side of a rectangular arm and made a counter-flow region, the wall shear stress worked as a propulsion force for the rectangular arm. The integrated friction drags were one tenth as small as the integrated pressure-based drags that were reported before. It was found that the tendency of power coefficient Cp of the wind turbine (the Cp of rectangular arm rotor < the Cp of cylindrical arm rotor) cannot be explained by the friction drag.

The control of the wind turbine rotor aims to increase the turbine output and to reduce the fluctuations of the output and the structural load. The change in the operating condition, such as the blade pitch control, causes the aerodynamic transient of the rotor, which affects the fluctuations of the output and/or the structural load. The yaw misalignment of the rotor axis to the inflow wind direction often occurs in the natural wind condition. When the wind flows diagonally to the rotor, the angle of attack of the blade relative to inflow wind speed changes during a rotation of the blade, which occurs the fluctuation of the torque and the structural load. The present study examines the aerodynamic transient characteristics of the rotor due to the blade pitch control, for the horizontal axis wind turbines for values of the tip speed ratio and the yaw misalignment.

For the further introduction of wind power generation, to improve the reliability of wind turbines is required. When the wind turbines are operated above rated wind velocity, the output power is kept constant, however, the rotor thrust may fluctuate due to the wind velocity fluctuations. The load fluctuations of rotor cause many kinds of failures in the drivetrains. The failures in drivetrain need the long repair period. Suspended operation makes the loss of power production. The reliability of drivetrain is most important issue for the wind power cost. Therefore, the suppression of load fluctuation in the rotor thrust leads to improve the reliability of wind turbines. In this paper, the possibility of rotor thrust control supported by inflow measurement using ultrasonic anemometer was evaluated for suppressing the fluctuation of rotor thrust. The feedforward control for the pitch system is demonstrated with a 30kW test wind turbine. For the feedforward control, the blade pitch angle is set to keep the rotor thrust as constant level according to the inflow wind velocity. The target pitch angle is decided by referring to the database calculated by the result of the preliminary measurement. The time series of pitch angle is fitted the timing of inflow wind arrival to the rotor surface. The pitch angle can control the rotor torque and thrust. The effect of rotor thrust fluctuation suppression by feedforward control with pitch angle operation was compared and evaluated with rotor thrust fluctuation by normal control.

Coriolis force, centrifugal force, pressure gradient force in flow direction, etc. act on the flow inside the radial flow impeller such as a turbomachine. These forces also influence the boundary layer developed on the impeller wall surface, forming a very complicated internal flow. In order to improve the performance of fluid machinery and the theoretical design of impeller shape, it is indispensable to elucidate the flow phenomena inside the impeller. In this study, the final goal is to clarify the flow phenomena in the impeller by using a rectangular straight channel with low aspect ratio (1:7) modeling the impeller of actual machine. Therefore, the time mean and fluctuating velocities and the Reynolds stresses are measured by using a hot-wire anemometer with various type slanted hot-wire probes. As the result, the uniformity of the relative velocity vector to the direction of the rotation axis, known as Taylor-Proudman theorem, was observed, and the structural change in the turbulent boundary layer was clarified.

Turbulence structures in recovery region of downstream flow over a forward-facing step are studied in Poiseuille and Couette flows. Large scale turbulence vortices are intermittently released from reattachment point of separation. Spectral analysis is carried out for velocity sequence obtained with hot wire velocimetry, and then pre-multiplied spectra is estimated in the downstream recovery region. Large scale turbulence vortices in the Couette flow are convected upward to the opposite wall, while the vortices in the Poiseuille flow are weakened due to the counter-rotating vortices developed near the opposite wall.

A unique micro sensor for flow-direction and velocity measurement has been developed. The sensor is composed of a hot-wire and a group of cold-wires. The normal hot-wire placed upstream is used not only to measure an instantaneous velocity magnitude but to operate as a heat source. On the other hand, the group of cold-wires placed downstream is used to detect a thermal wake formed behind the upstream hot-wire. By applying the Gaussian interpolation technique to the cold-wire outputs to find the position of a maximum temperature rise, we can also measure an instantaneous vector of fluid velocity. As reported previously, by using this sensor, fluctuating two velocity components are successfully measured in a turbulent wake flow behind a cylinder. In this study, we assess the performance of this sensor to reveal whether this technique can also be applied successfully to a turbulent boundary layer flow. It is found that if we apply the response compensation to the cold-wire outputs, the measurement result of the wall-normal velocity component is quite reasonable as well as that of streamwise velocity component.

A unique measurement technique for visualizing the spatial velocity distribution of a non-uniform flow field has been developed. By this technique which we call a “fluid velocity-field scanner (FVS),” two-dimensional velocity profiles can be obtained by tracking the path of a rod-type velocity probe with a high-speed CMOS camera, and superimposing the measurement results on the image of a measured object photographed by the same camera. The rod-type velocity probe consists of ten hot-wires operated by constant-temperature anemometer. Moving- and ensemble-average techniques are applied to the instantaneous velocity signals of the ten hot-wires to obtain the representative values on the virtual grid points for drawing a contour map of mean-velocity profile, which can be regarded as the two-dimensional time-mean velocity distribution measured by a normal hot-wire probe. Furthermore, by applying the similar moving- and ensemble-averaging to the square of the residual velocity fluctuation between the instantaneous signal and the ensemble-averaged one, turbulent intensity can also be estimated. It is demonstrated that the non-uniform distributions of mean velocity and turbulent intensity in a turbulent air jet flow can be visualized readily and quantitatively.

We performed direct numerical simulations of turbulent boundary layers along rough walls and investigated the characteristics of turbulence structures from a viewpoint of turbulence statistics. The results showed that, in the case that the height of roughness is larger than the buffer layer, the mean distance between the structures and the wall surface was increased. In addition, the mean spanwise spacing of the structures was also increased. In this study, we introduced a new spatial scaling method which was defined as an integral formulation of viscous length depending on the distance from the wall surface. Consequently, it was found that we could apply the scaling method to the present results and then observe the spatial features of turbulence structures on a rough wall surface universally comparing with those on a smooth wall surface.

The concept of the analytical wall function (AWF) for RANS is extended to define the SGS eddy viscosity inside the wall-cells (which are the wall adjacent numerical grid cells). With such an eddy-viscosity in the relatively coarse wall-cells, this SGS-AWF integrates the thin-layer approximated momentum equation that includes the temporal, convection, viscous and pressure gradient terms, to give the wall boundary condition for the momentum equation. The model coefficient of the SGS-AWF depends dynamically on the ratio of the SGS and GS time-scales. Coupled with the Smagorinsky and Abe’s one equation SGS models, the SGS-AWF is confirmed to perform better than the traditional wall function in a rib-mounted channel flows.

The direct numerical simulation (DNS) of the fully developed turbulent channel flows with random and local injection and suction at the bottom wall is performed in this work. The spots of the blowing and suction occupy 5% of the bottom wall and the blowing and suction velocity is changed by five patterns. The mean velocity is not affected by the random blowing and suction, while the Reynolds normal stress and fluctuating vorticity intensities become large in the viscous sublayer. From the viewpoint of the probability density function (PDF) of the wall-normal velocity, the peculiar distribution with the three peaks transitions into PDF without blowing and suction in the viscous sublayer. The instantaneous vortex field is influenced in the immediate vicinity of the wall.

We conduct a direct numerical simulation of a high-Reynolds-number turbulent boundary and identify the hierarchy of vortices by applying a Gaussian filter to the simulated velocity fields. We quantitatively show how the hierarchy of vortices is generated by evaluating the contribution of the scale-dependent strain-rate to the scale-dependent enstrophy production rate. Largest-scale vortices, that is, eddies with the size in the order of the distance from the wall are stretched and amplified predominantly by the mean flow. In contrast, small-scale vortices, that is, eddies sufficiently smaller than the height are generated mainly by the vortices twice as large as themselves. The generation mechanism of small-scale vortices is similar to the one in fully developed turbulence in a periodic cube, and it also explains the disappearance of hairpin vortices observed in the turbulent boundary layer as the Reynolds number increases.

We have investigated invariant solutions (time-independent solutions) to the Boussinesq equations which are the simplified model for Rayleigh–Bénard convection. In this work, steady and unsteady flows are obtained for three-dimensional domains between horizontal parallel plates with no-slip boundary conditions, which are held at constant temperature difference. It is well known that Rayleigh–Bénard convection is characterized by the Rayleigh number Ra and the Prandtl number Pr. For Pr=1, an unstable three-dimensional steady solution, which bifurcates from a thermal conductive solution at Ra~10³, is obtained up to Ra~10⁷ by using Newton-Krylov iteration. At high Ra, the three-dimensional solution consists of large-scale convection cells and small-scale vortex structures. The multi-scale invariant solution exhibits a scaling of the Nusselt number Nu with Ra, Nu~Ra^0.31, observed turbulent Rayleigh–Bénard convection, and represents well the spatial structure and statistics in a turbulent state.

We propose that the Stokes-number dependence of inertial-particle clustering in turbulence is explained in terms of the structures extracted from the coarse-grained fluid acceleration. To verify this proposal, we conduct direct numerical simulations of small heavy particles subjected to the linear Stokes drag in homogeneous isotropic turbulence. We investigate the surfaces, in three-dimensional space, satisfying the condition for the fluid acceleration which was introduced by Goto and Vassilicos [“Sweep-stick mechanism of heavy particle clustering in fluid turbulence”, Physical Review Letters, Vol. 100 (2008), 054503] and we apply the condition to the coarse-grained fluid acceleration. Our DNS results show that the surfaces thus extracted from the fluid acceleration coarse-grained at different lengths well correlate with the clusters of particles at different Stokes numbers.

To determine the ignition criteria and effects of turbulence on the localized ignition delay time, direct numerical simulations (DNS) of forced ignition of premixed mixtures have been performed for Toluene Reference Fuel (TRF)-air mixture at a high exhaust gas recirculation (EGR) rate and low equivalence ratio conditions at high pressure. A high-temperature kernel is imposed as an initial ignition kernel, and one-dimensional DNS have been preliminary performed to determine the ignition criteria of the laminar condition. It is found that ignition criteria of the laminar case are determined in terms of the ignition source energy and the heat conductivity from the ignition kernel regardless of fuel. Subsequently, two-dimensional DNS have been performed with an initial kernel size and temperature which yields successful ignition derived from the one-dimensional laminar cases, to clarify the influence of the turbulent flow on the ignition delay time. The two-dimensional DNS results are investigated to clarify the influence of the local and global strain rates on ignition delay time and the mechanism by which the turbulence influences the ignition kernel establishment. In the case of TRF-air mixture, when energy input is suitable, the relationship between local strain rate and ignition delay time holds regardless of fuel. However, the local strain rate alone dose not explain the deviated ignition delay time.

We investigated experimentally a turbulent diffusion process of ethylene emitted from a point source in a flat-plate turbulent boundary layer using a simultaneous measurement system with a fast-response flame ionization detector (FFID) and an X-type hot-wire anemometer. Instantaneous waveforms of streamwise and wall-normal velocity and concentration fluctuations are measured quantitatively. It is found that the streamwise velocity and concentration fluctuations correlate well with each other, and the high-frequency wall-normal velocity fluctuations produce the turbulent mass flux intermittently. Moreover, we estimated the turbulent Schmidt number to be about 0.8, which shows overall similarity between the velocity and concentration fields.

From the viewpoint of energy conservation the heat transfer enhancement of heat exchanger is also desired. The conventional circular double tube type heat exchanger, CP-tube, is one of the compact one with a high heat transfer performance. The authors discussed the heat transfer performance of double tube type heat exchanger (pipe outer diameter: 22.0 [mm], pipe length: 2.4 [m]) with petal shaped special inner tube having a large wet perimeter. The 6 petal shaped inner tube, 6P-tube, 5P-tube, 5P’-tube with a shallow petal and CP-tube were used, and it was made clear that 5P-tube has an excellent heat transfer performance. In this study, in order to improve and enhance the heat transfer performance of 5P-tube more the decrease of a large pressure loss of outer tube will be discussed using a triple tube, 5P-tri-tube, and will be shown the excellent heat transfer performance of it. The hot water of 60 [°C] flowed in the inner tube and cold water of 20 [°C] flowed in middle and outer tubes as countercurrent flow. As a result, it was shown that for example, at a low Reynolds number of Re_h =4,000 the 5P-tri-tube have an excellent heat transfer efficiency η of 2.84 times of 5P-tube (where, η of 5P-tube is 2.1 times of CP-tube).

A pulsating turbulent channel flow is studied by direct numerical simulation (DNS) to clarify the effect of pulsation frequency on turbulence statistics. The flow pulsation was forced to a turbulent channel flow by sinusoidal variation of pressure gradient. Four pulsation frequencies were tested at Womersley number, Wo = 19.8, 28.0, 39.6 and 56.0 for time-mean friction Reynolds number, Re_τs = 300. As a result, profiles of phase-averaged streamwise velocity and temperature change sinusoidally at each phase especially in the log layer and the variation amplitude increases as Womersly number increases. Phase-averaged friction Reynolds number and Nusselt number also change during one cycle with different amplitude and phase lag relative to the change of bulk Reynolds number, depending on Womersley number. Consequently, it is found that the time-local dissimilarity between momentum and heat transfer occurs due to the flow pulsation.

The purpose of this study is to find what kind of vortices should be introduced in order to enhance dissimilar heat transfer and to interpret its physical mechanism. We carry out a direct numerical simulation on heat and momentum transfer in plane Couette flow. Dimensionless boundary conditions for the streamwise velocity and the temperature are similar. The Prandtl number is set to unity. As the first step, an axisymmetric decaying vortex is directly introduced into laminar Couette flow, whose initial circumferential velocity, position and radius are changed to investigate their effects on heat and momentum transfer. The parameters of this system are the Reynolds number, the circumferential velocity, the position, the radius and the inclination angle from the spanwise to the streamwise direction of the vortex. Consequently, for dissimilar heat transfer enhancement, if a strong vortex can be generated, a spanwise-oriented vortex whose spanwise vorticity has the opposite sign to the background-shear vorticity, namely a spanwise-oriented anti-cyclonic vortex, should be introduced, otherwise, a spanwise-oriented cyclonic vortex should be done. The reason why the cyclonic vortex (or the anti-cyclonic vortex) realizes dissimilar heat transfer enhancement is that the streamwise pressure gradient and the streaklines due to the vortex intensifies the difference between the temperature and the streamwise velocity in the upper and lower (or downstream and upstream) regions of the vortex, which makes the Reynolds stress dominate over the turbulent heat flux.

Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan The objectives of this study are to observe and investigate characteristics of turbulent heat transfer phenomena in various combined-convection turbulent boundary layers with adverse pressure gradient (APG) on the suddenly changing wall thermal condition, via direct numerical simulation (DNS). In the carried out DNS, since an isothermal wall from inlet to middle of the computational domain followed by the adiabatic wall conditions are applied, the effect of buoyancy changes due to variation of wall thermal conditions. It is found from DNS results that local wall friction coefficients vary from suddenly changing wall thermal condition point due to effects of both buoyancy and variation of wall thermal condition. On the other hand, variations of wall temperature are almost same with/without APG in cases of neutral and unstable conditions, whereas different variations are clearly observed in the case of stable condition. Also, it is observed that the relationship between a wall-normal pressure gradient and a buoyancy relate to these peculiar phenomena. The present DNS reveals the characteristics and structures in various combined-convection turbulent boundary layers with APG on the suddenly changing wall thermal condition.

The compressible turbulent flows through a square duct constituted by three isothermal walls and an adiabatic wall at Re=1000 are investigated using the direct numerical simulation. The flow pattern and number of the secondary circulation drastically changes as the Mach number (M) increases, This behavior is similar to the previous low-Reynolds-number result. In the bulk-energy budget, the work between the internal and turbulent energy becomes large and the viscous heat generation is suppressed at the large Mach number case. The turbulence structures are strongly influenced by adiabatic wall at large M.

It is known that a lubricant sheared at a nanometer sized gaps has unique mechanical properties (viscoelasticity) different from those in the bulk state. On the other hand, polymer additives are used for lubricating oils to control their temperature characteristics. The influence of the added polymer on the viscoelasticity of lubricating oil in the nanogaps has not been clarified. In this study, we measured the viscoelasticity of the lubricant with the polyisobutylene as a polymer additive in the nanogaps by our original measuring method.

This paper aims at making in a theoretical way an analysis on incompressible flows with low Reynolds number in a microscopic and tapered circular-tube, an axisymmetric tube with its diameter contracting linearly along the axis. Based on the boundary-layer approximation, the momentum equation is derived from Navier-Stokes equation in cylindrical coordinate system, from which the second-order nonlinear ordinary differential equation for the pressure is obtained using the velocity profile of a fourth-order polynomial function with Pohlhausen’s Λ. Runge-Kutta method for numerical solution of this equation is used to investigate velocity profiles at various positions in the tube. The velocity profiles are greatly dependent on Reynolds number R_e, i.e., the larger R_e is, the larger the velocity gradient is. The profiles for R_e=1 in the tube with a small taper angle are similar to the Hagen-Poiseuille profile. Our model can successfully predict the development of the flow in the tube.