Near-wall velocity measurement has been performed at y⁺=2.6 in a fully developed turbulent square-duct flow in order to evaluate fluctuating wall skin friction. Turbulence structures, which induce high wall shear rates, have been identified using simultaneous flow visualization. It is shown that quasi-streamwise vortical structures play a dominant role in high-friction generation, and that they can be classified into the 'cyclonic' or the 'anticyclonic' vortex, which is tilted in the spanwise direction with its vorticity parallel or antiparallel, respectively, to the mean shear vorticity. The generation of high wall shear rates is often seen on the sweep sides of the cyclonic and anticyclonic streamwise vortices. In addition to the experiments, the long-wavelength instability of a spanwise vortex filament in a background shear flow has been investigated using the Biot-Savart law with a suitable cutoff in order to demonstrate the generation mechanism of streamwise vortices. It is found that the unstable modes lead to the generation of streamwise vorticity, and that the most unstable mode has a spanwise wavelength of the order of 100 wall units in the near-wall region.

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A quasi-three dimensional analytical model is proposed to simulate swirling flow with spiral vortex core in a pipe. The vortex core is expressed as a helical vortex filament, and the boundary condition of zero normal velocity at the pipe wall is specified by distributing image helical vortex filaments on a cylindrical surface which is located outside the pipe. Thus, the velocity in a pipe is given by the superposition of velocity induced by helical vortex filaments and that of an exisymmetric base flow. The proposed model is applied to the swirling flow in the draft tube of a hydraulic turbine for predicting the flow pattern at the inlet section and the rotating frequency of the vortex core. The calculated results are compared with experimental ones, and reasonable agreement is obtained.

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Three-dimensional steady and unsteady flows through the bifurcation of the porcine renal artery were analyzed numerically using the finite-element method. As the bifurcation geometry plays a primary role in determining the hemodynamical variables, the present bifurcation geometry was precisely developed in accordance with the measured data. In the present calculations, the arterial wall was assumed to be rigid and the non-Newtonian effect of blood was not taken into consideration. The calculated results showed that the present difurcation geometry had an effect of suppressing the flow separation. In addition, it was shown that the present bifurcation geometry flattened the spatial distributions of wall shear stress, which has important implications for the concept of Wall Shear Stress Gradient.

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We discuss the characteristics of a flapper-nozzle system for water hydraulic servovalves. High-pressure water at the supply port is used for the first time as the working fluid for the hydrostatic bearings supporting the spool. Spool valve adhesion induced by poor lubrication with water is thus avoided. The fluid is then led to the ends of the spool and is used as the working fluid of the flapper-nozzle system. In this flapper-nozzle system the circumferential clearance of the spool becomes a laminar restriction that replaces the fixed orifice used in conventional servovalves. The linearity in the pressure-displacement relationship of the new flapper-nozzle system is better than that of conventional fixed orifice systems.

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In this paper, flow force due to nozzle flow and its influence on the flapper-nozzle system for water hydraulic servovalves are discussed. The flow force acting on the flapper is estimated using the momentum theory, and expressed as a function of the nozzle pressure and flapper-nozzle gap. Analysis shows that the double nozzle flapper gives a quasi-linear characteristic to the relationship among the force on the flapper, the flapper gap and the spool velocity. The numerical result based on the theory is used to estimate the accuracy of the classical linear approximation. The moment due to the flow force resists input moment acting on the armature. Consequently the flow force reduces the magnitude of the change of the nozzle backpressures that drive the spool of the water hydraulic valve. Experimental verification of the theory is also presented.

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An electrohydraulic servovalve that uses fresh water as a pressure medium(water hydraulic servovalve)is developed. A conventional electrohydraulic servovalve cannot be operated with such a hostile fluid. In this paper, we explain the structure and functions of components, and the relationship between them, as well as the developmental procedure of the water hydraulic servovalve. The basic idea for the servovalve is to support the spool of the valve with hydrostatic bearings and to lead the water from the bearings to the flapper-nozzle system. The hydrostatic bearings constitute a laminar restriction that takes the place of the turbulent restriction in the conventional flapper-nozzle system. We also explain the design procedure of the hydrostatic bearing in the valve and the relationship between electromagnetic elements and fluidic elements. The water hydraulic servovalves fabricated show good characteristics of smooth motion, endurance and controllability, as well as good dynamic characteristics.

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We propose a new method for measurement of temperature in high temperature air using LIF(laser-induced fluorescence)of O₂ molecules excited by a broad band ArF excimer laser. In this method, temperature can be measured by using temperature dependence of ratios between fluorescence intensities integrated over two spectral regions including spectral peaks. The theoretical broadband spectra of O₂ molecules are calculated and compared with the experimental ones of static hot air(300K-1100K)inside an electric furnace. Temperatures measured by this method agree with those measured by a thermocouple within about ± 100K at a temperature below 1000K.

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In this paper we describe a detailed experimental investigation of turbulent heat transfer in radially rotating circular smooth-walled and square rib-roughened ducts with focuses on the variable rotational effects along the leading and trailing surfaces of these two test ducts due to differences in the shape of the cross section and the surface conditions of duct walls. The experimental data reconfirmed the presence of strong notation-induced secondary flows for both the test sections with an attendant relative increase in local heat transfer on the trailing surface. On the leading surfaces of both test sections, heat transfer was significantly less than that of the pure forced convection level. In qualitative terms, even with the agitated flow field caused by ribs inside the square ribbed duct, considerable rotational effects within the ribbed duct occurred in a similar manner to that found inside the circular smooth-walled duct. However, with equivalent rotating conditions, the heat transfer impediment on the leading surface was more severe for the ribbed duct than for the smooth-walled duct and the heat transfer enhancement on the trailing surface of the ribbed duct was significantly alleviated. A low degree of peripheral heat transfer variation was found in the ribbed test duct. Since the difference between smooth-walled or rib-roughened ducts is considerable, the selection of either circular smooth-walled or square ribroughened coolant dhannel for gas turbine rotor blades must be performed after considering the variable rotational effects in order to achieve the optimum design of an internal cooling system.

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Hypersonic projectiles were fired into CO-O₂ and CO-O₂-H₂ gas mixtures. For observation of combustion phenomena around the projectiles, we employed a high-speed framing schlieren technique. When we used CO-O₂ mixtures as combustible gases, no shock-induced combustion was observed. However, addition of a small amount of H₂ into the CO-O₂ mixtures enabled combustion. We experimentally investigated the dependence of combustion phenomena on the molar fraction of H₂ in the CO-O₂-H₂ gas mixtures. The high-speed framing schlieren technique made it possible to observe and discuss unsteady shock-induced combustion phenomena.

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Flame spread phenomena in a suspended fuel droplet array were experimentally investigated for n-hexadecane in a high-pressure ambience. Seven droplets of the same size were arranged at equal horizontal spacings. Flame spread rates were measured based on OH emission histories detected by a high-speed video camera with an image intensifier for droplet diameters of 0.50, 0.75, and 1.0 mm at ambient pressures from 0.1 to 2.0 MPa. Results show that, as droplet spacing becomes smaller, flame spread rate increases and attains a maximum value at a specific spacing. A further decrease in droplet spacing causes the spread rate to decrease due to the large latent heat of vaporization. Experiments were also conducted in a microgravity field to determine if these characteristics of flame spread are affected by natural convection.

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The characteristics of NO_x emission in the high-temperature air combustion technology were investigated numerically. Two kinds of methods were used. Detailed chemistry and transport properties were used for a two-dimensional laminar diffusion flame, and a simplified reaction scheme was used for calculating a turbulent flame in a furnace. Results show that the thermal mechanism dominates NO_x emission in high-temperature air combustion. If only the preheated air temperature is increased, NO_x emission will rise significantly. However, the combination of air preheating and flue gas recirculation not only improves the combustion efficiency, but also suppresses NO_x emission in the combustion process.

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Measurements of the instantaneous in-cylinder flow fields at the end intake stroke at bottom dead center(BDC)were carried out in an FloDyne^TM water analog engine simulation rig using 3-D particle tracking velocimetry(3-D PTV). The measurements were performed in a 4-valve per cylinder, typical pent-roof type combustion chamber with both intake valves activated generating mainly tumble vortex structures. The experiments were repeated for two different operating points simulating an idle condition(600 PRM)and a low load condition(1200 RPM). These two operating points represent almost a factor of two in effective Reynolds(Re)numbers. For each case, 100 cycles of data were acquired. Efforts were made to optimize the particle seeding density(and the resulting number of 3-D velocity vectors)to yield 500 to 600 instantaneous vectors at each cycle. The raw data coosisted of sets(between 50 000 and 60 000 velocity vectors each)of 3-D instantaneous(but phase conditioned at BDC)velocity vectors, randomly(but relatively uniformly)distributed over the entire cylinder volume. Sophisticated statistical tools were applied to these data sets to evaluate the ensemble averaged mean flow field and total fluctuation fields in the root mean square(r.m.s.)sense. In addition, "conventional" integral measures such as tumble, cross-tumble and swirl ratios were computed from the data by integrating the angular momentum components over the cylinder volume. The results indicate that the Re number effects are relatively small between the two conditions investigated, with a tendency of the smaller scale structures at BDC for the higher Re number case. This paper illustrates the tremendous potential of the water analog engine simulation used in conjunction with 3-D PTV as a rapid engine flow field evaluation tool. This approach allows to conveniently obtain 3-D maps of the mean and total r.m.s.fluctuation levels of each velocity components as well as integral parameters such as tumble and ratios(under realistic unsteady conditions). Furthermore, this method also allows various configurations or operating points to be conveniently compared with respect to many of their flow characteristics.

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Various types of heat engines using shape memory alloys are proposed and studied. From the viewpoints of thermal efficiency and operational control of an engine, an efficient heat exchanger is required for heating and cooling shape memory alloys. Furthermore, a heat engine operating in a small range of strain is required because the fatigue life of shape memory alloys is strongly dependent on the strain of shape memory alloys. In this paper, a novel operation system comprising of a reciprocating-type heat engine using shape memory alloys was proposed. This proposed engine could be operated continuously even within a small strain range. The shape memory alloys used were straight wires of a TiNiCu alloy which were heated and cooled by forced convection. The working characteristics of the engine were investigated by experiment. Experiments were conducted at various heating temperatures and under various strains. Working characteristics such as the amount of work are discussed.

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Occasionally the continuum equations of fluid mechanics must be supplemented by information about processes occurring at microscopic scales. Examples include the formulation of boundary conditions, the resolution of singular solutions of the governing equations, and the determination of constitutive relations. In such situations, molecular dynamics calculations in which an entire flow is simulated at the atomic scale can provide the missing information. We review the numerical procedure and then discuss several recent applications of this technique. We consider the validity of lubrication theory at very short distances, the smoothing of corner singularities for both Newtonian and non-Newtonian fluids, and some molecular aspects of wetting phenomena, particularly contact angle hysteresis.

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The flow past bluff bodies, which occurs in many engineering situations, is very complex, involving often unsteady behaviour and dominant large-scale structures, and it is therefore not very amenable to simulation by statistical turbulence models. The large-eddy simulation technique is more suitable for these flows, and recent work in the area of large-eddy simulations of bluff body flows is summarised, with emphasis on work by the author's research group as well as on experiences gained from two LES workshops. Results are presented and compared for the vortex-shedding flow past square and circular cylinders and for the flow around a surface-mounted cube. The performance, the cost and the potential of the LES method for simulating bluff body flows, also vis-a-vis statistical turbulence models, is assessed.

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The present paper describes the numerical as well as the experimental study on the axisymmetric supersonic jet associated with a sootblower nozzle for boiler cleaning. The navier-Stokes equations are solved numerically, and the experimental work is also performed with the low-density wind tunnel. Calculations and experiments both cover the region from the nozzle exit to the downstream distance of 20 times the nozzle exit diameter. The effect of the nozzle divergence angle is discussed by comparing the experiments with calculations, and it is pointed out that, even when the expansion ratio across the nozzle is correct, the regular shock-cell structure appears in the jet for the nozzles with finite divergence angles. Also, the present numerical results on the impact pressures along the jet center-line agree favorably well with those reported by the other researchers.

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For blunt-nosed, ramped flat plates at angles of attack of 0°, 20°, 30° and 35°, the heat transfer and the flow field around the compression ramp have been investigated experimentally based on infrared thermography measurement and Schlieren photograph observation in the NAL 1.27m and ONERA S4MA hypersonic wind tunnel at M_∞=10.0 and Re_∞=1.5-2.3×10⁶ / m. We also compared CFD simulations using NAL and ONERA N.S.codes for the examination and comparison of the flow field and the magnitude of the heat transfer. The results showed an increase in heat transfer on the flat plate that was upstream from the corner and the reduction of heat transfer on the ramp when the leading-edge is blunt. An analytical study to obtain a single correlation for the measured heat transfer along the plates showed that ratio x / r_n of x distance from the leading-edge and the nose radius is a dominant similarity parameter for heat transfer distribution on the ramp downstream of the reattachment as well as on the flat plate part upstream from the ramp corner.

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Nonequilibrium radiation behind strong shock waves in low-density air is studied in terms of a computer-aided, high-speed photography using a gated I.I.CCD camera system for shock waves generated by a free-piston, double-diaphragm shock tube. The range of shock velocity is from 9 km / s to 12 km / s at the initial pressure 13.3 Pa. Numerical simulation for radiation profiles is also performed using a three-temperature model. It is found from a two-dimensional, time-frozen photography for the total radiation profile that the shock velocity for the transition criteria from 1-peak profile to 2-peaks profile is about 11 km / s, and that there are some discrepancies between observed and calculated profiles. Some remarkable phenomena are observed from the time-resolved spectroscopy: e.g., the longer radiation duration of the N⁺₂(1-)(1, 0)line and the temporal oscillations of the intensities of spectral lines of the N⁺₂(1-)band system. A comparison between observed and calculated streak images of the time-resolved spectroscopy is also presented.

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An overview is given regarding experimental activities in the field of hypersonic flow. After a brief introduction into the history of hypersonic ground based facilities modern high-enthalpy wind tunnels are described including their capabilities and flow diagnostic techniques. After the HERMES technology program came to an end in Europe, an ESA technology program, called Manned-Space-Transportation Program(MSTP)has been initiated among several institutions in Europe to preserve the knowledge in hypersonics. Besides of the ESA activities 3 Sonderforschungsbereiche(centers of excellence)exist at German universities. These centers are related to hypersonic flow research;they are supported by the Deutsche Forschungsgemein-schaft DFG.

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This paper describes results of holographic interferometric observation of the evolution of shock waves and the primary vortex loop discharged from the open-end of a 40 mm × 40 mm square tube. Experiments were conducted by using the square tube connected to a 60 mm × 1500 mm diaphragmless shock tube. Holographic interferometric flow visualization viewed from the axial direction was carried out using a diffusively-reflected beam as the object beam. In order to interpret the observed three-dimensional shock-wave motion, numerical simulations were also carried out by solving the Euler equations with the dispersion-controlled scheme. The experimental interferogram was compared with numerical visualization results. The three-dimensional transition of the shock wave configuration from a planar to a spherical shape and the development of the primary vortex loop from a square shaped to the three-dimensional structure were clearly observed.

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Considered here is combustion of an explosion-driven turbulent jet in a thin cylindrical enclosure. The process is initiated in a small pre-chamber, filled with acetylene mixed with 10% air, and provided with an explosive igniter charge. The prechamber is connected to the main enclosure, filled with air at NTP, by sealed orifice. Upon ignition of the explosive charge, the acetylene is thermally decomposed, generating a detonation in the pre-chamber. The high pressure thus engendered breaks the seal and creates a blast wave that injects the fuel jet into the main enclosure which undergoes then an exothermic process of combustion with air. The mixing layer is repeatedly compressed and expanded due to blast wave reverberations within the enclosure. Experimental results, in the form of cinematic shadow photographs and transient pressure records, are modeled by a two-dimensional CFD analysis of combustion dominated by turbulent mixing. The results reveal the dynamic features of turbulent combustion when the exothermicity is controlled by fluid-mechanic transport in a turbulent field, rather than by the effects of the exothermic reaction and diffusion, that is usually the case.

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A 1 D Particle-in-Cell(PIC-MCC)simulation has been used to study the effect of temporal modulation(pulsing)of an rf(13.56MHz)source voltage on the evolution of an argon plasma. Plasma breakdown occurs during the first on-time of the pulse and analytic expressions for the density evolution during and after breakdown are presented for the single pulse case. The effect of multiple pulsing on the plasma development is then examined and compared with the steady-state evolution. Experimental measurements of pulsing in a parallel plate rf system are also presented, which have a high qualitative agreement with the simulation results.

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A self-consistent two-dimensional three moment radio-frequency glow discharge model has been developed for methane feed gas using a fluid model to study the charged species dynamics and its effects on deposition in a polyatomic gas discharge. Swarm data as a function of electron energy are provided as input to the model. The discharge is electropositive since positive ion density is more than negative ion density. The importance of two-dimensional modeling has been emphasized by comparing the plasma variables with those of a one-dimensional model. The model predicts both the radial and axial variations of species densities, electron energy and electric field. The off-axis maxima in species densities, typical for two-dimensional discharge, can be observed. The asymmetry of the discharge generates a self dc bias that has been predicted by a trial and error method. The model also calculates the radial variations of species fluxes to the cathode that are important to predict the film deposition.

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This paper is concerned with nonequilibrium condensation from a vapor to liquid phase on the plane endwall of a shock tube behind a reflected shock wave. The growth of a liquid film on the endwall is measured by an optical interferometer using a laser beam. Condensation coefficient of methanol vapor is determined in a wide range of thermodynamic nonequilibrium conditions of the vapor at the vapor-liquid interface. It is found that condensation coefficient is drastically influenced by the thermodynamic nonequilibrium conditions, it increases with the increase of interfacial vapor temperature and decreases with the increase of number density.

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The purpose of the present paper is to examine numerically the theoretical results given by the previous work, in which an integral equation of Fredholm type with respect to vorticity is derived from the Navier-Stokes equations. From this integral equation, we derive the first and second order approximation and carry out the numerical calculation for the Oseen type vorticity distribution to confirm the validity of the core spreading method in this paper. Numerical results show that the classical core spreading method, the first order approximation, is valid within some finite time and the difference from the second order approximation is very small. This result implies that Greengard's notice that the core spreading method converges to a system of equations different from the Navier-Stokes equations is not surely correct in sense of engineering applications of the core spreading method.

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A direct numerical simulation(DNS)of turbulent flow in a wavy channel was carried out using the consistent finite-difference method. The boundary wave in the mainstream direction causes periodic pressure gradient, successive acceleration and deceleration, and flow separation. Tuning the amplitude and period of the wave allows continuous observation of turbulence structure from a simple flow in a plane channel to a complex one. The drag imbalance on flat and wavy wall sides is mainly caused by the pressure drag, while the friction drags are almost even between the two walls. The characteristic of the flow field can be explained by a simple linearized analysis. Good agreement between linearized and DNS results validates the thin-layer approximation. A discrepancy is observed in the limited region where intermittent and local separation takes place even though the mean flow does not separate from the wall.

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The recent work on large eddy simulation(LES)of turbulent flows at the Center for Turbulence Research is reviewed. This includes progress on issues surrounding the governing equations and filtering, subgrid scale and wall layer modeling, and spatial discretization. Recent results from LES of separated flows, and two promising applications of LES to flows encountered in combustors are presented.

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The important role of nuclei on cavitation inception and cavitation development is now well known. Several recent studies have allowed a precise correlation between cavitation inception for different types of cavitation on marine propellers and nuclei distributions(20th ITTC report). In a standard cavitation facility the nuclei distribution will depend on the hydrodynamic operating conditions, and can change during whole cavitation tests. Then, to be able to transpose the model results to full-scale, it is necessary to determine precisely the nuclei distribution corresponding to each cavitation operating point. An illustration is given by analyzing the influence of the operating conditions on a complete cavitation inception <<bucket>>. Moreover, as nuclei govern the whole cavitation process, nuclei measurements are also conducted at sea. The main purpose of all these studies is the accurate prediction of full-scale cavitation behavior from precise transposition laws. This is possible only when nuclei distributions are determined.

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Simple but reliable correlations for a drag coefficient, C_D, of single bubbles under a wide range of fluid properties, bubble diameter and acceleration of gravity were developed based on a balance of forces acting on a bubble in a stagnant liquid and available empirical correlations of terminal rising velocities of single bubbles. The proposed C_D consists of three equations, each of which corresponds to pure, slightly contaminated and contaminated systems. The effect of a frictional pressure gradient due to a liquid flow is also taken into account by introducing a concept of an effective body acceleration. Terminal rising velocities of single bubbles were calculated using the proposed C_D, and compared with measured data under the condition of 10^-2<Eo<10³, 10^-14<M<10⁷ and 10^-3<Re<10⁵ where Eo, M and Re are Eotvos, Morton and bubble Reynolds numbers, respectively. As a result, it was confirmed that the proposed C_D gives better predictions than available drag coefficient models.

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A new method is developed to solve the three-dimensional hydrodynamic field generated by multiple bubbles, which are arbitrarily configured at low Reynolds number, taking the effects of both bubble interactions and the slight deformation of the bubble surface into consideration, for the case of ∈=We / Re<<1.The numerical results for the case of two equi-radial bubbles confirm the validity of this method with the restriction that the gap between the two bubbles is to be not less than half of the radius. It is shown that the hydrodynamic force acting as drag is smaller in the case of a bubble moving away from another stationary bubble than in the case of one bubble approaching another. This difference is explained as the nonlinear effect originating from the bubble surface deformation.

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Experiments have been conducted for turbulent liquid plane Couette flow with an immiscible droplet in order to understand interfacial deformation and the modification of the structure of shear-dominant turbulence due to immiscible droplets in turbulent dispersed liquid-liquid two-phase flows. Flow visualisation, an image-capturing method and image processing have been used for the measurement of the fluctuating velocity field. Mean velocity, turbulence intensities, turbulent kinetic energy and mean vorticity have been estimated from the velocity field. The present work clarifies that the turbulence energy and the vorticity increase at the confluence of the axial flow and the flow along the interface in the direction perpendicular to the moving wall, and that they decrease in the stagnation region of the main flow near the interface.

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In this paper, the instability and transition of a radial liquid-film flow which is regarded as a kind of boundary layer is described. This flow has been generated by a water discharge to the atmosphere from the thin cylindrical gap between a pipe end and a disk surface. The spatial linear stability of the flow, including the effect of the radial spreading of vortices, is investigated. The numerical results show that the effect of the radial spreading of vortices increases the instability of the flow. The frequency of the most amplifying disturbance at each radial position, which is estimated from the evolution of the disturbance of the fixed frequency as it moves downstream, is in good agreement with that detected through wall pressure measurement. The correlation between the amplification of the linear disturbance with the transition point obtained by flow observation shows that the transition to turbulence occurs when the total amplification of disturbance is about e⁹.

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The present study describes the control characteristics of the gas temperature and velocity fields and also plasma parameters of RF induction argon nonequilibrium plasma by injecting a premixed cold hilium gas axially at atmospheric pressure. The flow and the gas temperature fields and the plasma parameters in the RF induction mixed gas plasma are obtained by solving the axisymmetric turbulent 2D MHD equations taking account of the variable transport properties of gas mixtures and using two-temperature model coupled with 2D Maxwell's equations. The mixing rate of a secondary injected gas is also evaluated using the species conservation equation. It is examined how the thermofluid and diffusion characteristics, the plasma parameters and the nonequilibrium degree in the RF induction plasma are influenced by the positions and the inlet mass fractions of injected helium gas. Finally, the calculated gas temperature taking account of the nonequilibrium effect shows good agreement with the previously obtained experimental data.

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This paper presents the stability and dynamic characteristics of an axisymmetrically oscillating magnetic liquid column subjected to a uniform magnetic field in the presence of gravity. Initially, the axisymmetric mode response of a magnetic liquid column is analyzed using potential theory based on a magnetic field perturbation method. Then, an experiment is described in which a liquid column is formed between Helmholtz coils and vibrated vertically to produce axisymmetric oscillations by an electodynamic exciter. The surface frequency response of the column is measured optically using a high-speed video camera, a strobovision-snalyzer and a stroboscope. The results show that the magnetic field exerts a stabilizing effect on the column and the resonance frequency increases for increased liquid column aspect ratio(diameter / height). Both the experimental interfacial response and resonance frequency show good agreement with the theoretical prediction.

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