Analysis of the suction performance, cavitation development, and internal flow fields of the impeller of a flat-helical inducer with two blades were performed. As the suction pressure is reduced, the balanced cavity development on both blades is destroyed. Alternate blade cavitation, in which cavity development evolves on one blade while it weakens on the other, can occur. When it does, the theoretical and actual pump heads can decrease quickly. Investigations were conducted to determine how this phenomenon develops.

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Fundamental investigation of non-evaporating diesel spray disintegration was carried out using single and double nano-spark back-light photography of diesel sprays injected into a constant volume chamber and relatively clear single and double exposure images of droplets were obtained. Droplet size, shape and velocity as well as the two-dimensional velocity vectors of the droplets have been simultaneously measured by analyzing the spray photographs, using an image processing system and developed analysis method. It was observed that even at the early period of injection, some droplets have a velocity much lower than the penetration velocity of the spray tip. The velocity of droplets around the spray tip is much hogher than the velocity of droplets near the nozzle tip. At the early period of injection, droplets flew away from the spray flow around the nozzle tip. However later at the same position, droplets were entrained into the spray flow. No droplet entrains into the spray around the spray tip at the early period of injection.

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Exhaust emissions(HC, CO, NO_x, PM, and PAH(polycyclic aromatic hydrocarbons in PM))from a commercial DI diesel engine were measured for petroleum-derived fuels with a wide range of fuel properties. Fuel properties such as cetane number, aromatic content and type, and distillation temperature were adjusted to be independent from each other. The test results showed that total aromatic content was the controlling factor for NO_x emission, and PM emission mainly depended on polycyclic aromatic content in the fuel. The increases in PM emission with increase in aromatics was mainly caused by the increase in SOF emission. Both of NO_x and PM emissions were seldom affected by cetane number and 90% distillation temperature. Therefore, chemical characteristics of fuel are the controlling factor for NO_x and PM emission, not the physical properties. The amount of sulfate in PM proportionally increased with increase in sulfur content of fuel. PAH emission was evaluated by analyzing SOF using a high performance liquid chromatography with a chemiluminesence detector. Also, the PAH highly depended on aromatic content, and the emission characteristics caused by aromatics was quite similar to PM emission.

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A computational study is performed on a chemically reacting laminar boundary layer over a porous horizontal wall with a diffusion flame. The generation of counter-rotating vortices along the flame sheet is observed. Our attention is focused on i)local acceleration, which induces an anomaly in the velocity distributions across the flat plate boundary layer, and ii)the origin of hydrodynamic instability, which may disturb and wrinkle the flame downstream. Results show that when reaction exothermicity is taken into account in the numerical simulation, both the aerodynamic structures and the mass transfer of the chemical species change significantly, especially in the region near the front edge of the burner. Counter-rotating vortices are generated along the flame sheet and are expected to induce hydrodynamic instability. A high pressure zone in front of the leading flame edge and local acceleration are numerically simulated in agreement with experimental observation. Buoyancy plays a negligible role in the behavior of flow and combustion in regions upstream of the burner.

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By using an opposed-jets flow composed of an upper single jet and a lower coaxial jet, a planar diffusion flame located near the stagnation plane and a trumpet-shaped diffusion flame stabilized along the interface of coflowing jets were experimentally developed to simulate and study diffusion burning structures modified by interactions of oxidizer eddies within fuel zone in turbulent combustible flows. It was shown that in comparison with the trumpet-shaped flame, the planar flame is not only iasier to be stabilized in the given flow field, but also easier to generate soot layer;and that the propane flame has better flame stabilization and is relatively sooty than the methane flame. Interaction and transition of two diffusion flames further indicated that the development of a diffusion flame is strongly influenced by its own oxygen supply, and may be sensitive to oxygen supply of the other stronger flame;and that soot formation strongly depends on oxygen and fuel supplies, and fuel region being closed by the flame or not. In this study, detailed comparisons on individual diffusion flames and discussions on interaction and transition of two diffusion flames therefore enrich our understandings on interactions of diffusion flamelets in the turbulent flow.

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An experimental study was conducted to investigate the effects of acoustic waves on the development of the turbulent statistics of a plane mixing layer which were generated downstream from a splitter plate towed in a straight long tunnel. Acoustic forcing was applied to excite the shear layer and induce wave resonance in the free shear layer. The mixing layer large-scale structures / turbulent statistics in the transitional regimes at Re_m≈100 based on the momentum thickness were investigated by means of the visualization / flying hot-wire technique. We found that turbulent quantities of the spanwise velocity component were homogenized and reduced because of the subharmonic resonance. These results are close to the observations of simulated mixing layers at Re_m=112.

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Theoretical study of three-dimensional boundary layer transition on a sphere rotating in still fluid has been carried out by a linear stability analysis. It is shown that the neutral stability curve shifts toward the lower Reynolds number and the lower spiral vortex number, either closer to the equator or with the decrease in the vortex angle. The critical Reynolds numbers obtained in the analysis are close to but smaller than the experimental results. The rotational speed ratio between the spiral vortices and the surface agrees with the experiments. It is also shown that the critical Reynolds number becomes insensitive to the vortex angle near the equator. On the other hand, the critical wave number of spiral vortices becomes larger with the increase in the vortex angle and decreases near the equator. Computation including the streamline-curvature effects in general gives the critical reynolds numbers closer to the experiments.

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Experimental and computational study on the linear instability region downstream of a two-dimensional roughness element(a square rod)placed in a laminar boundary layer on a flat plate is described. The numerical solutions of the two-dimensional N-S equations were obtained by the MAC(marker-and-cell)method. Hot wire measurements were conducted in the boundary layer on a flat plate in a low-turbulence wind tunnel. A sinusoidal fluctuation, which grows exponentially, is observed behind the roughness element. The characteristics of sinusoidal fluctuation(linear region)depend on κ / δ^*_κ, where κ is the height of the roughness element and δ^*_κ is the displacement thickness at the roughness element. At κ / δ^*_κ=1.0, the predominant frequency of naturally excited sinusoidal fluctuation lies in the unstable zone, which is predicted using the theoretical calculation based on the Orr-Sommerfeld equation applied for Lin's profile. The response characteristics of the linear instability region to artificial external excitation were investigated in detail using sound as an exciting agent. Also, flow fields with forced time-dependent disturbances are studied by the numerical method. The effects of the artificial external excitation become maximum when the frequency of disturbance coincides with the natural frequency. The numerical results for the linear instability region(frequency, phase velocity and amplification rates)agree with the experimental measurements

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A method is proposed for analyzing the swimming motions of bacteria. It is assumed that a bacterium consists of a spherical cell body and several rotating flagella. The forces and torques exerted on each flagellum are evaluated in a coordinate system fixed to the flagellum, using the resistive force theory. They are transformed into a coordinate system fixed to the cell body. Then, the velocity and angular velocity of the cell body can be determined because the net force and torque on the bacterium are zero. This method is applicable to the motion of bacteria with rotating flagella in any form of curve attached at any point on the cell body. A numerical example shows that the trajectory of bacteria with a flagellum slanting from the radial direction of the cell body follows the pattern of a double helix. Another example shows that a doubly flagellated bacterium swims faster than a singly flagellated bacterium.

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In this study, we consider the dynamic characteristics of a rectangular parallele-piped panel / tank having infinitely rigid side walls and bottom plate, but a thin elastic plate as a cover. All 6 plates of the parallelepiped panel / tank constitute a box-like cavity. The bottom and the cover have dimensions of L₁ by L₂. The dimension L₃ represents the depth of the panel cavity, which is filled with inviscid acoustical or compressible liquid. The elastic cover plate and the liquid in the cavity constitute a hydroelastic structural system(HES)that vibrates under external perturbations. The adiabatic(isoentropic)dynamic interaction between the elastic plate and the acoustical liquid of the cavity under outside perturbation has already been considered, but some details regarding the correct treatment of the problem were omitted. In the present investigation a general solution is formulated and implemented. As a result, corrections of the dispersion curves for the first four natural frequencies of the HES are obtained for a large variation of the generalized parameter of the acoustical stiffness of the system, and these corrections are evaluated quantitatively. It is observed that for high acoustical stiffnesses, the dispersion curves possess asymptotes. A multitude of diagrams of the first four modes for a large variety of parameters are computed and some of them are shown. The general idea used in the work of Pretlove and Craggs(1970)was evaluated as the correct one.

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Friction drag reduction of pipe flow using a flexible tube was experimentally investigated and compared with a rigid tube. It was found that the coefficient of friction of the flexible tube was lower than that of the rigid tube. Furthermore, when the thickness of the flexible tube was decreased and the Reynolds number was increased, the rate of drag reduction increased, except when using a very thin flexible tube. To clarify these phenomena more precisely, the fluctuating velocity on the center axis of the tube, the fluctuating pressure on the inner wall surface of the tube and the fluctuating displacement on the outer wall surface of the tube were measured. From the power spectrum distribution and the overall band energy obtained by analyzing these waves, it was clarified that there exists an interrelation between the turbulent energy and the drag reduction and a negative correlation between the fluctuating pressure and the fluctuating displacement.

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Problems of high technological interest increasingly cut across traditional disciplinary boundaries. An integrated multidisciplinary approach not only enables greater reach in attacking such problems, but can also offer opportunities for achieving goals beyond those defined by conventional design constraints. In this lecture we illustrate such a process. The example used, which arises in the context of the development of "smart jet engines", is the dynamic control of aerodynamic instabilities in gas turbine compression systems. The lecture shows:1)the progression of technology and basic understanding of phenomena from the initial "back of the envelope calculation" stage through increasingly complex models, experiments, and physical situations, and 2)the interaction between disciplines which was necessary for program success.

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The understanding of bluff body flows and their prediction are important in many areas of industrial development. The complex nature of these flows provides a challenge to both experimental and computational fluid dynamicists. Some of the more recent advances are reviewed, including the understanding of the near wake region and the instabilities that develop there. The role of vortex dislocations is discussed and it is shown how encouraging their formation can reduce drag. In addition to describing the improved insight that can be provided by computational fluid dynamics, the likely impact of new, non-intrusive measurement techniques is also assessed.

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The interaction between grid-generated turbulent flow and a 2-dimensional circular cylinder is investigated experimentally. Particularly, the effect of the velocity fluctuations induced by the cylinder wake on the upstream turbulent field is quantitatively estimated and the spatial region where the wake vortices affect the upstream turbulence is determined. The fundamental characteristics of the grid turbulence and the pressure distributions on the cylinder surface are first investigated, then measurements of the turbulent velocity tangential to the mean stream line are carefully made by a single hot wire probe mainly in the upstream region of the cylinders. The effect of the wake-induced fluctuation appears in the spatial region upstream at a distance of more than 5 times the redius of the circular cylinder. This effect is largest near the separation point and is smaller on the stagnation line as compared with the effect in the other region.

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Measurement of the local skin friction coefficient and re-examination of the Reynolds-number-effect on the mean flow quantities were made in a zero-pressure gradient, smooth wall, turbulent boundary layer. In order to do this fairly, the two-dimensionality of the flow field was carefully adjusted. And, the wall shear stress τ_ω was determined by direct measurement using a floating element device. The Reynolds number based on the momentum thickness R_θ=U₁θ / ν ranges from 840 to 6 220. The present experimental results of c_f propose a new empirical formula which well represents the present experimental data and confirms that the Karman constant χ=0.41, which implies a slope of the logarithmic linear layer, is independent of R_θ down to R_θ=860, whereas the additive constant varies with R_θ. The magnitude of the wake parameter takes an asymptotic value of 0.62 if the Reynolds number is sufficiently high. The effect of R_θ on all the turbulent intensities is evident even within the viscous sublayer. The constant stress layer does exist in the Reynolds shear stress profiles despite that the constant value varies with R_θ.

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Physical and numerical experiments and computations reported in the literature conclude that the kinetic energy of free-survace bounded turbulent flows appears to increase as the free surface is approached from below. There is in the previous literature no satisfactory explanation of the mechanism underlying this feature. In order to provide insight revealing the mechanism, an analytic investigation of an oscillating laminar flow bounded on the lower side by a horizontally moving no-slip wall and on the upper side by a shear-free constant-pressure boundary is presented herein. Although the motion is periodic, rather than turbulent, essential features including momentum diffusion are present and hence the flow is a valid candidate for indicating the mechanism. A mathematical solution for the velocity and the kinetic energy is analyzed to determine the physical mechanism leading to the increase in kinetic energy. The finding is that the relaxation of the shear stress at the free surface produces enhanced viscous acceleration of the fluid in a thin layer adjacent to the free surface. The enhanced acceleration produces a nearly constant kinetic energy in the layer. It is concluded that the effect of the shear-free constant-pressure boundary of a fluctuating flow is to produce a nearly uniform flow in a thin layer adjacent to the surface, and that the effects of the free surface penetrate far into the bulk fluid with exponentially decreasing significance.

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Coherent structure modeling in the outer and wall regions of a turbulent boundary layer were proposed, which yielded results in agreement with experimental observations in several important aspects. The so obtained results were then used in the calculation of the transport of passive quantities, specifically, the transport of heat in the turbulent boundary layer of a heated plate.

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A technique has been developed to measure three components of mean velocity, six components of the Reynolds stress, ten components of the triple correlation, and fifteen components of the quadruple correlation of fluctuating velocity in steady turbulent flow using Hot-Wire Anemometry(HWA). The hot-wire signal was acquired by rotating a single inclined hot-wire probe toward a number of spatial orientations at the same location in a turbulent flow-field. Velocity information has been computed by decomposing the response equation for the effective velocity of the hot-wire probe.

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The cross-correlation tracking technique is widely used to analyze image data, in particle image velocimetry(PIV). The technique assumes that the fluid motion, within small regions of the flow field, is parallel over short time intervals. However, actual flow fields may have some distorted motion, such as rotation, shear and expansion. Therefore, if the distortion of the flow field is not negligible, the fluid motion cannot be tracked well using the cross-correlation technique. The author proposed a new particle tracking technique, based on the particle cluster matching using linear affine transformation. The algorithm can be applied to flow fields which exhibit characteristics such as rotation, shear and expansion. The algorithm is based on pattern matching of particle clusters between the first and second image. The deformation of the cluster pattern is expressed by the linear affine transformation. The parameter of the transformation can be determined using the least squares technique from the particle positions. The residue of the particle deformation is taken as the pattern matching parameter. The smallest residue pattern in the second image is the most probable pattern match to the correspondent original pattern in the first image. Therefore by finding the best matches, particle movements can be tracked between the two images. The effectiveness of the proposed technique was verified with synthetic data of three-dimensional flow. It demonstrated a high degree of accuracy for the three-dimensional calculation.

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A gas-droplet mixture flow has been investigated by a light extinction and scattering method in an expansion-shock tube. A mist flow was produced by an adiabatic expansion of a mixture of water vapor and N₂ gas. The expanded mixture is at 190 K and 13 kPa. A shock wave propagates in the mist. The propagation in the gas-droplet mixture was observed by an optical nethod. The fundamental setup of optical observation is a kind of spatial filtering method. A knife edge was replaced by the spatial filter, which was designed to pass a transmitted light and cut scattering light. The recorded image was converted to a digital one by the film scanner with a high resolution. The variation of the droplet radius was obtained from the recorded photograph using a suitable procedure. On the other hand, the ratio of the number density of the mist across the shock wave was obtained from the image of the photograph experimentally obtained. The computed result was compared with the experimental one. The experimental results show a good agreement with the computed one.

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In order to improve the reliability of megawatt wind turbines, the passive teetered brake & damper mechanism is applied. Its two unique effects, as its name implies, are braking and damping. The passive brake & damper mechanism is useful for variable speed control of the large wind turbine. It is comprised of teetering and feathering mechanisms. When the wind speed exceeds the rated wind speed, the blade is passively teetered in a downwind direction and, at the same time, a feathering mechanism, linked to toe teetering mechanism through a connecting rod is activated. In this study, two kinds of blades, a twisted tapered blade and a non-twisted rectangular one, are used with this passive mechanism. Testing of the model horizontal axis wind turbine in a wind tunnel showed that the passive mechanism can suppress the over-rotational speed of the rotor. The velocity distribution around the rotating blade is measured by a two-dimensional laser Doppler velocity meter. It is found that the passive teetered brake & damper mechanism can suppress the over-rotational speed of the rotor, and the braking effect is caused by the reduction of the angle of inflow for the blade.

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The objective of this paper is to show numerically the aerodynamic effectiveness of winglets on the performance of a horizontal axis wind turbine(HAWT). The winglets used in this study are an inclined extension of the rotor blade. For the numerical analysis, a vortex lattice method with a free wake model was used, as the method can be applied to arbitrary blade shapes and the model needs no parameter based on empirical data about the wake geometry. Calculations were performed on the flow field of the rotor wake and the rotor performance, and results were compared between rotors with and without winglets. To examine the effects on the strength of blades, the flatwise bending moment was also calculated. A small installation angle of winglets causes a large increase in the power coefficient but a small increase in the flatwise bending moment compared with the blades with radially extended winglets.

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The Wells trubine is one element in a chain of devices by which te oceans' wave power resource can be tapped. This paper reports a study of the performance and aerodynamics of a Wells turbine using a three-dimensional, flow solver for the Reynolds-averaged Navier-Stokes equations. Calculations have been performed for a monoplane device comprised of NACA 0015 blades under the conditions : Reynolds number 8×10⁵, tip Mach number 0.4, hub-to-tip ratio 0.6 and tip clearance 2%. To study the effect of solidity calculations were performed for different numbers of blades. It is shown that the predicted effect of solidity on the turbine pressure drop, torque and efficiency agrees qualitatively and quantitatively with experimental data. The discrepancies can be partly explained by geometric differences between the experimental and numerical turbines. It is also shown that increasing turbine solidity increases the resistance to stall by strengthening the blade-to-blade interactions in the hub region.

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The leak flow through clearances between the fixed and orbiting scrolls of a dry scroll vacuum pump is a dominant factor that determines the pumping performance. Because the leak flow sometimes ranges from viscous to slip to free molecule flow along a clearance, the equation that describes the flow through the three flow regimes is developed by a weighted linear combination of the two equations for free molecule and slip flows. The pressure distribution in gas pockets is another important factor in pumping performance. The validity of this theory is examined by experiments.

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Transitional two-dimensional boundary-layer flows are considered. The alge-braic eddy-viscosity model for a simulation of nonturbulent(laminar)boundary layers is proposed. It takes into account, in a simple form, an intensity and a length scale of the free stream turbulence(FST)and a value of the favorable pressure gradient. Transition is modeled by employing the intermittency factor. The Baldwin-Lomax model is used to calculate turbulent portions of the boundary layers. The boundary layer(BL)and heat transfer measurements for flows over a flat plate were employed to verify the proposed "nonturbulence" model and for an evaluation of the numerical code abilities to predict heat transfer for transitional flows. The code was applied for the calculation of heat transfer coefficients of two different gas turbine vanes, and the results were compared with available experimental data at different free-stream turbulence and Reynolds number conditions.

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A comparison between two numerical flow solution methods, one based on atructured grids and the other based on unstructured grids, is made by using both methods to solve for steady and unsteady flowfields in turbomachinery blade rows. Numerical results from the two different methods are compared to each other as well as to available experimental data. Both methods solve the three-dimensional Reynolds-averaged Navier-Stokes equations using the finite volume technique, and can be run in steady-state or time-accurate unsteady modes. The structured solver uses a pressure-based method applied to hexahedral control volumes and a standard two-equation turbulence model. The unstructured solver uses a density-based method applied to tetrahedral control volumes and the one-equation Spalart-Allmaras turbulence model. The objective of the comparison is to assess the current capabilities of the unstructured flow solver relative to those of a well established structured flow solver, and to establish areas for further development to improve accuracy and economy.

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The rotordynamic forces generated by the fluid flow through the impeller leakage path of a centrifugal pump are now well established. The present paper examines the effects of modifying the leakage path geometry by changing the front shroud, from a conical shape to a more typical curved design, and the effects of low pressure seal design on these forces. It is found that only the cross-coupled stiffness is affected by the change of pathgeometry. Changing the low pressure seal from an axial to a radial clearance does, however, significantly affect the rotordynamic forces. A bulk flow numerical model is found to predict the same general result for the low pressure seal tests. The model agrees with the general trends with increasing leakage flow coefficient exhibited by the data, but appears to underpredict the magnitude of the normal force.

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Studies of the forces acting upon and the flow patterns around bluff bodies submerged in oscillatory flow are fundamental to fluid engineering structures. In this work, measurements were made of both in-line and transverse forces of flat plates at various angles of attack in a wide range of the Keulegan-Carpenter number(KC). The force signals were spectrally analyzed to determine dominant frequencies, and the flow visualization technique was also employed to understand the correspondence between flow patterns and force coefficients. In this paper we discussed how the angle of attack influences the flow and aerodynamic characteristic of a flat plate. It was found that, for the present bluff bodies, several classes of flow patterns appear which depend strongly on KC numbers. Furthermore, the correlations between KC numbers and flow patterns were revealed. From the predominant frequency components of the fluctuation transverse forces, it was found that Strouhal numbers, which are reduced at maximum velocity, agree well with the results in the case of steady flow.

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A new approach to simulate flow-induced vibrations in piping systems and circular cylindrical structures is introduced. In the developed method, three-dimensional unsteady flow is solved by a direct simulation method. Based on the fluid dynamic calculation, fluid forces on structure surfaces(piping inner wall or cylinder surface)are estimated by integrating fluid pressure and shear stress. The calculated fluid forces are translated to finite element nodes in structure surfaces and treated as external forces acting on these surfaces. Then dynamic response of the structures is calculated by using modal analysis or the Newmark-β method. For a full coupling analysis, the moved surfaces become the new coordinate boundaries at the next time step. As an application of the developed method, flow-induced vibrations in an elbow and an array of tubes were evaluated. Turbulent intensity agreed well with experimental data in the unsteady flow analysis and vibration of the elbow was successfully simulated in the dynamic response analysis. For a 3×3 square array, the occurrence of fluid elastic instability could be simulated.

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The effort to solve the fundamental problem of convective flows and heat transfer in a vertical channel is continued in this work, considering a new control parameter for the thermal flow regime. A numerical investigation of the developing heat convection in the channel is presented, while considering the effects of a vertical forced flow on the Nusselt number at the cold wall for a wide range of related parameters. By the analysis of these results, it is possible to predict heat transfer behavior especially in cases when the Nusselt number at the cold wall becomes practically zero. The main results obtained from this study are the critical Reynolds number variation obtained for different geometrical aspect ratios(A=H / L=2;4;8;16)and the natural convection parameter, Ra=10⁴-10⁷. Based on the scaling analysis it is determined that the critical Reynolds number is independent of Ra and scaled as R^HT_ecr∝A², where A is the aspect ratio.

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To develop a vortex-generator-jet(VGJ)method for separation control an experimental study was carried out using a 14 deg conical diffuser with and without VGJs. Static pressure recovery coefficient and a coefficient for loss of total pressure were both used as criteria to evaluate the effectiveness of several VGJ arrangements where two geometric parameters, jet-hole-size and the number-of-jets, were varied. It was found that VR(ratio of jet speed to freestream velocity)rather than Qj / Qm(ratio of jet flow-rate to total flow-rate)is more useful as a key parameter to understanding the control of the separation in a diffuser because the effects of change of geometrical parameters tend to be reduced. The total pressure losses were reduced by increasing VR up to values near 1.5 to 2.0 where losses reached a minimum. Due to the increasing losses associated with the production of the jets, total pressure losses increase with further increase of VR beyond 2.0.

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