The growth of computer information capacity has profoundly changed the conduct of research in fluid mechanics. Properly executed, a direct numerical simulation can be a valid and rich source of ‘data’ for the study of certain aspects of turbulence. Multi-point experiments contain less information than direct numerical simulations, but they are ‘real’, and they are still far richer in information than classical single-point experiments. To make significant advances in understanding complex flows, especially turbulence, new practices must be used that exploit the richness of these databases. These include stricter adherence to the principal of using concepts rooted in theoretical formulations to define the quantities to be measured, quantitative concepts based on new methods of kinematic analysis for three-dimensional visualization, more extensive use of statistical analysis based on multi-point correlations and related methods, and tightly coupled combined experimental/computer simulation studies. Lastly, dissemination of the information requires new approaches.
Paper briefly describes applications of shock wave phenomena to interdisciplinary research, which are in progress in the Interdisciplinary Shock Wave Research Center. This project is funded by the grant-in-aid science research under the COE program entitled the investigation of shock wave phenomena in complex media and its interdisciplinary application. Its goal is to extend results of this basic research not only to interdisciplinary research programs and also to establish a new paradigm of shock wave research.
The aspects of verification and validation of computational fluid dynamics must always be addressed with a emphasis on the quantification of the uncertanties due to the model assumptions (either physical or geometrical), and to the numerical and experimental approximations. The credibility of computational fluid dynamics can only be established by a rigorous process of verification and validation. Verification is the process of determining how accurate a computational simulation represents a given conceptual model. Validation establishes how accurate a simulation (i. e. the conceptual model) represents the phenomenon to be investigated. In the present paper we discuss a roadmap toward the development of a verification and validation procedure through the use of a flow library on the Web. In particular, we describe the experience of the web-based FLOWnet database and we analyze some representative test cases installed in the data base.
The paper presents the recent developments in Hierarchical Parallel Evolutionary Algorithms to speed up optimisation of aerodynamic shapes. One is the implementation of different models in different layers of a Parallel Genetic Algorithm. The other is Asynchronous Hierarchical Evolution Strategy. These methods are employed to reconstruct a one-dimensional transonic nozzle and a two-dimensional aerofoil shape. Considerable speed up is achieved as a result.
In this study, numerical simulation was performed with the SIMPLER method on the amplification of Tollmien-Schlichting (T-S) wave in the boundary layer over a flat plate and its suppression with a wall motion actuator. The T-S wave was induced by the interaction between sound wave introduced by a speaker and a protuberance on the surface of the plate, and amplified in the unstable region of the boundary layer. The vertical vibration of a wall motion actuator was used to control the T-S wave. The influence of the amplitude and phase of actuator vibration on the control effectiveness was investigated in the simulation. The results show that the T-S wave can be effectively suppressed by the motion of the actuator surface if the phase and amplitude of the actuator are appropriately adjusted.
The surfactant used in this study is well known as an additive for drag reduction in straight (non-swirling) pipe flow. This paper deals with swirling flow characteristics of a surfactant solution. We investigated the effects of surfactant concentration and Reynolds number on flow characteristics. It is shown that the drag reducing effect is less than in straight pipe flow, and that the surfactant solution must be highly concentrated with great elasticity in order to produce a drag reduction. As the results of velocity measurement by LDV and wall pressure measurement in the axial direction, it is expected that the swirl intensity of the drag reducing swirling flow with a surfactant decays more quickly as the flow progresses downstream. And we propose that this mechanism helps create the drag reduction in swirling flow. For the velocity profiles of drag reducing swirling flow, only the forced-vortex type is observed in this experiment. This suggests that the velocity profiles of drag reducing swirling flow change more quickly to a forced-vortex type from a Rankin's combined vortex type. It is also reported that turbulence intensity of drag reducing swirling flow is smaller than turbulence intensity of a solvent.
The cause of 350 Hz large-amplitude rotor vibration of the H-2A rocket's LE-7A fuel turbopump was investigated by wideband oscillating-pressure measurement. Measurement was successfully conducted by quartz-type pressure sensors in full-load liquid-hydrogen tests. The phase difference of pressure oscillations between two pressure measurement ports under conditions of pump cavitating operation revealed a phenomenon similar to rotating stall in turbomachinery which had not been previously observed. The rotating speed of a cell was 350 Hz, about a half the rotor speed. This phenomenon is different from rotating cavitation in which the cavitation pattern rotates around the periphery of an impeller faster than shaft rotating speed. Based on these findings, it was concluded that this phenomenon cased the large-amplitude 350 Hz vibration of the LE-7A fuel turbopump. Results of FFT analysis of measured oscillating pressure are presented.
Unsteady stress-of 4-bladed inducer blades was measured at the blade root near the leading edge with and without inlet flow distortion. The following results were obtained: (1) Without the inlet flow distortion, the mean stress of the blade with shorter cavity is smaller than that with longer cavity under the alternate blade cavitation at intermediate cavitation number. At lower cavitation number, the stress fluctuation occurs caused by the cavitation instability such as rotating cavitation/cavitation surge, which is one of the major causes of stress fluctuations in inducers. (2) The magnitude of the blade stress fluctuation due to the interaction with the inlet flow distortion basically decreases as we decrease cavitation number. With inlet flow distortion, the occurrence ranges of alternate blade cavitation and cavitation instabilities shift toward lower cavitation number. Under the alternate blade cavitation, the fluctuation of longer cavity causes the stress fluctuation of the adjacent blade with shorter cavity. The stress fluctuations caused by the rotating cavitation and acvitation surge are nearly the same level as those in case without inlet flow distortion.
Magnetorheological (MR) fluids are suspensions of magnetizable micrometer particles in a liquid. The rheological properties of the fluids can be changed rapidly, reversibly and repeatedly from that of a liquid to that of a solid when an external magnetic field is applied. This tunability of the fluids comes from the microscopic structure of the particles. Different microscopic structures are observed, quantified and correlated with rheological properties. The yield stress and viscosity of MR fluids depends on the particle size and volume fraction, sample cell geometry, magnetic field strength and application rate. The application of MR fluids in cancer therapy is discussed. In-vitro experiments show the blood flow can be blocked leading to a tumor by injecting a dilute MR fluid in a blood vessel and placing a magnet on top of the tumor. Without blood supply, tumor necrosis occurs shortly.
Transition phenomena for the natural convection of a magnetic fluid in a square cavity was studied numerically for an externally imposed magnetic field. In the present study, particular attention was focused to verify heat transfer characteristics for non-gravity situation when the magnetic field was imposed. By applying magnetic fields the convection state many be largely affected improving heat transfer characteristics followed by various flow transitions in the non-gravity simulation. It was revealed that magnetic field could enhance the convection and consequently improves heat transfer characteristics, indicating that the magnetic field would be used as a substitute for the gravity in the zero-gravity situation of space.
The effect of ambient air temperature on the minimum fluidization velocity Umf of glass beads is investigated by Distinct Element Method (DEM). Under different ambient air temperatures, the pressure drop curves are calculated by DEM to determine Umf. The mean kinetic energy E of particles is defined to judge whether the particles move or not. The fluidization characteristics of fluidized bed are investigated in details. The calculated Umf is compared with empirical data. The different behavior of Geldart B and D particles against temperature is reproduced numerically. Umf decreases with increasing ambient temperature for Geldart B particles, while it increases for Geldart D particles.
Great efforts have been made to attain a good understanding of boiling heat transfer, but we have a poor understanding for the boiling structures in high heat-flux boiling. For example, there are still opposing opinions for the mechanism of the critical heat flux in saturated boiling and there are no rigorous models for that in subcooled boiling. In the present paper, the nucleation site density (NSD) was measured using high-speed video pictures and an image processing technique. The video pictures were taken from below the boiling surface made of single crystal sapphire with a total reflection technique. Based on the experimental results of NSD, a simplified model predicting the contact-line-length density (CLLD) at CHF is proposed.
The propagation of sound waves in a finite vortex is investigated by numerically solving the linearized Euler equations in two dimensions. Two types of vortices are considered: homentropic, when the entropy is uniformly distributed in space, and non-homentropic, or entropy stratified, when it is distributed non-uniformly. In the latter case, the results reveal instability of the perturbation field, which is caused by entropy-rotational waves excited in the vortex region by incident sound waves.
An implicit multiblock Navier-Stokes solver, which contains the LU-SGS subiteration method and the HLLEW scheme, has been developed for numerical simulations on complex are realistic aerodynamic configurations. Two-level halo cells are used to communicate data between abutting blocks. The transfinite interpolation (TFI) and the elliptic method with boundary control are employed to generate the initial multiblock grid and to smooth the grid distribution in each block. A comparison is first done for the AGARD supercritical LANN-wing with single- and multi-block grids. Then the present method is applied to a NASA transport wing/fuselage configuration and the NAL supersonic transport (SST) model. The choice of multiblock grid topology from the view of aeroelastic calculation and the comparison of Euler and Navier-Stokes solutions are investigated.
Early stages of transition to turbulence of compressible shear layers were investigated in the viewpoint of a linear stability analysis. The primary stage of the transition is by which instability of a laminar flow (primary instability), in which disturbances develop into spanwise vortices and form a secondary flow. The secondary stage of the transition is by which instability of the secondary flow (secondary instability), in which disturbances develop into streamwise vortices. In the present study, the primary and secondary instability problems were formulated as eigenvalue problems and solved numerically using spectral methods. At first, the primary instability problems were solved. From the eigenvectors of the primary instability, we obtain density, velocity, temperature and pressure fields of the secondary flow. Using these quantities, the instability of the secondary flow was calculated. For the secondary instability, it is shown that (1) growth rates of disturbances are larger than those in the primary instability, (2) growth rates of disturbances have a peak when the ratio of wavelength of the primary and secondary instability is approximately 0.8 and (3) growth rates of disturbances are less affected by flow compressibility than those in the primary instability.
An implicit time-marching method based on the LU-SGS scheme developed for self-field magneto-plasma dynamic (MPD) fully-ionized viscous flows is extended to the method for self-field MPD viscous flows considering a finite rate of ionization. The axisymmetric compressible Navier-Stokes equations with Lorentz force and Joule heating, the equation of magnetic induction induced from Maxwell's equations with Ohm's law, and the continuity equation of electron are simultaneously solved using the present time-marching method. Partially ionized flows in an experimental MPD thruster are simulated and compared with the experiments. Also the effect of flow conditions such as the inlet temperature, the total current, and the rate of ionization to the flow field is numerically investigated.
In order to investigate the spray combustion mechanism, a new methodology (Fine Wire Sustaining method) was established. Fine wires of 14µm in diameter were used to sustain the droplets. Any arrangement of the droplets could be performed with this method. In this study, 33 fuel droplets arranged in symmetrically were subjected to the quiescent high temperature air in an electric furnace. The temperature of the environment air was about 1000K. Fuel was n-eicosane and the mean droplet diameter was 0.58mm. The standard deviation of the droplet diameter was 0.02mm. A high-speed video camera of 250ftp was provided to observe the auto-ignition and flames of fuel droplet clouds. The experiments were done at atmospheric pressure using the JAMIC drop shaft that provides 10 seconds of effective period of time for the micro-gravity. As the results, the time histories of the diameter of the particle flames had maximum and that of the diameter of the group flame had the minimum.
In the present work, the effect of probability density function (PDF) selection and intermittency on the result of the numerical simulation are examined by the simulation of a turbulent methane-air jet diffusion flame, where air is preheated to 1073K. As to the PDFs, beta-function and clipped Gaussian are considered. It is found that intermittency has a large impact on the numerical result, while the choice of PDF has a minor influence. When intermittency is considered, the flame height becomes larger and maximum temperature decreases. Comparison of radial profiles with experimental data shows that it is necessary to consider intermittency also for high temperature air diffusion flames.
We investigate a solid-on-solid model of epitaxial crystal growth in 1+1 dimensions. The emerging distribution of terrance sizes and the resulting currents on surfaces with a given inclination angle are calculated analytically. Furthermore, we evaluate the stable magic slope which is selected due to the compensation of the competing effects. Results of Monte Carlo Simulations are in very good agreement with the theoretical predictions.
A two-dimensional fluid/Monte Carlo simulation model was developed to study plasma “molding" over surface topography. The plasma sheath evolution and potential distribution over the surface were predicted with a self consistent fluid simulation. The trajectories of ions and energetic neutrals were then followed with a Monte Carlo simulation. In this paper, energy and angular distributions of ions and energetic neutrals bombarding an otherwise planar target with a step are reported. As one approaches the step, the ion flux decreases and the ion impact angle increases drastically. For a time invariant sheath (DC case), the ion energy distributions (IED) remain relatively unaffected. When the plasma sheath oscillates at radio frequencies, the IED narrows, while the ion angular distribution becomes broader as one approaches the step. The energetic neutral flux is found to be significant near the vertical step wall. The simulation results are in good agreement with experimental data.
A simple transient network model is introduced to describe creation and annihilation of junctions in the networks of associating polymers. Stationary non-linear viscosity is calculated by the theory and by Monte Carlo simulation to study shear thickening. The dynamic mechanical moduli are calculated as functions of the frequency and the chain disengagement rate. From the peak of the loss modulus, the lifetime τx of the junction is estimated, and from the high frequency plateau of the storage modulus, the number of elastically effective chains in the network is found. Transient phenomena such as stress relaxation and stress overshoot are also theoretically studied. Results are compared with the recent experimental reports on the rheological study of hydrophobically modified water-soluble polymeters.
The evolution of interfacial patterns during growth processes is shortly summarized. The analogy between hydrodynamic dewetting patterns and diffusion controlled growth patterns is demonstrated. A phase-field method for the treatment of hydrodynamic flow with free interfaces is presented and illustrated with some examples.
Turbine manufacturers have been concerned about efficient utilization of limited energy resources and prevention of environmental pollution. For steam turbine power plants, a higher efficiency gain is necessary to reduce the fuel consumption rate. Blade configurations have been studied for reductions of profile loss and endwall loss that lead to decreased steam turbine internal efficiency, by applying recent aerodynamic technologies based on advanced numerical analysis methods. This paper discusses increase of pitch-chord ratio by 14% (reduction of rotor blade numbers by 14%) and increased blade aerodynamic loading without deterioration of performance. A new rotor cascade is found which improves blade performance, especially at the root section where the reduction in the energy loss coefficient is about 40%. This rotor blade also provides lower manufacturing cost.
Our previous study confirmed that ellipsoidal differential equations can be used to interpolate particle tracking velocimetry (PTV) measurement results. In the preset study, we deal with extensions of the equations; to time serial processing and to the detection of mismatched vectors. The performance of these algorithms is evaluated by using the flow around a rectangular cylinder and a vortex flow. Furthermore, the actual advantages of utilization of these methods are discussed by applying them to the experimental result of sudden vortex generation due to the collision of a circular cylinder with a flat wall.
It is well known that the boundary layer separation due to the adverse pressure gradient of the pseudo-shock affects the dynamic feature of the pseudo-shock such as the shape of the first shock, the pressure recovery and the self-induced oscillation. The suppression of the separation is effective to control the interaction between the shock wave and the boundary layer. In this research, the boundary layer control with the slot injection is studied experimentally and numerically. The experiment is carried out in a blow-down supersonic wind tunnel at free stream Mach number of 2.0. The numerical simulation is made with the two-dimensional compressible Navier-Stokes code. In solving the equations, the second-order accurate Harten-Yee's upwind TVD scheme and k-ε turbulence model modified for the compressible flow are used. The results show that the separation is appreciably suppressed and the shape of the first shock is changed, resulting the wall pressure fluctuation to be greatly reduced. Furthermore the total pressure loss is decreased with the low slot height.
The numerical simulations for the model coating flow of concentrated solutions of hydroxypropylcellulose (HPC) are carried out using the Doi model for rod-like polymers. The high degree of molecular orientation is induced by an elongational flow at the die lip and the molecules align parallel to the free surface. This molecular orientation near the free surface in coating flow is discussed in connection with the surface alignment of liquid crystals (LCs) on the HPC films. In the experiment on surface alignment of 5CB on the HPC film formed by coating, parallel surface alignment of 5CB is obtained in the conditions of large coating speed, thin coating thickness and high concentration of HPC. This tendency is well explained from the simulation results. From both the results of the numerical simulation and the experiment, it is confirmed that the surface alignment of LCs on the HPC films formed by coating results from the interaction of LC molecules with HPC molecules oriented parallel to the free surface.
The paper presents a new dynamic SGS model of two-way coupling for large eddy simulation of particle-laden turbulent flow based on Yuu's model (1997). The advantage of this new model is that coupling of fluid-particles SGS components is taken into account and at the same time the coefficient of proposed SGS model can be optimized by Germano's (1991) dynamic procedure. To investigate the capability of this model, numerical simulations of particle-laden turbulent flow at Re=644 in a vertical channel were performed using this new dynamic SGS model and using Van Driest wall function. By comparing the calculation results with that of using single-phase SGS models, in which the coupling of SGS components was not considered, the present dynamic SGS model for two-way coupling was verified. In addition, the roles of particles GS and SGS components played in the turbulence modulation of fluid flow were clarified for particle-laden channel turbulent flows.
Fast summation algorithms for the particle simulation are presented. The diffusion velocity, which appears in the vortex methods, is treated as an example of rapidly-decaying potential and the convection velocity as far-field potential. Computational count can be reduced from O(N2) to O(N), where N is the particle number, for the diffusion velocity and to O(N logN) for the convection velocity by the use of blocks with various widths which are arranged in a computational domain based on simple principles. Our method is applied to heat-vortex interaction and it is shown that the theoretical efficiency is achieved, and that our code is sufficiently parallelised.
This study deals with the sorption characteristics of a honeycomb-type sorption element composed of a new organic sorbent that was composed of the cross-linked polymer of sodium acrylate. Transient experiments in which moist air was passed into the honeycomb-type sorption element were conducted under various conditions of air velocity, temperature, relative humidity and honeycomb length. As a result, the effective mass transfer coefficient of the organic sorbent adsorbing the water vapor was non-dimensionalized as a function of Reynolds number, modified Stefan number and non-dimensional honeycomb length.
Pressure waves produced by explosions of combustible mixtures formed by the accidental spillage of fuel gases have attracted considerable attention from the safety point of view. In order to assess damage caused by such explosion hazards, real-scale experiments are necessary. However, it is often difficult to perform such experiments because of space limitations and experimental safety. Small-scale experiments are preferred if an appropriate scaling law can be verified. The objectives of this study are to elucidate the decay processes of planar blast waves initiated by deflagration and detonation waves in a tube and to assess the applicability of the energy-based scaling law to these phenomena.
Rotational temperature of NO molecule in methane/air premixed flame was estimated by a spectral matching method. A tunable narrow band ArF excimer laser was used to excite the D2Σ+←X2Π(0, 1) band system of NO. Laser beam was introduced in a flame, and the laser-induced fluorescence was resolved into a spectrum by using a spectrograph. On this spectrum, ε and δ bands of upper vibrational level of v´=0 were analyzed. In order to use a spectral matching method, profiles of ε and δ band spectra were calculated theoretically in detail with reliable molecular constants and exact equations, and they were modulated by an experimental slit function. Since the profile of band spectrum was determined as a function of a rotational temperature, a rotational temperature could be estimated using the temperature where the profile of every band spectrum obtained theoretically is fitted to that of experimentally obtained. Applying a spectral matching method on the ε(0, 3), ε(0, 4) and δ(0, 2) band of NO, it was obtained that the rotational temperature is about 1000K. The obtained rotational temperature is almost agreed with the thermocouple temperature.
The EGI system has a severe problem in that heavy HC emission is generated due to large fuel droplet and fuel film attachment. Therefore, for the purpose of reducing HC emission, various atomization techniques are being developed. One of those techniques is the air shrouded injector, which has a better atomization ability and demands less power loss than other atomizers. In this study, the atomization and fuel film characteristics of an air shrouded injector were analyzed by using a particle size measurement system and a fuel film measurement device. From these experiments, design factors were established to develop air shrouded injectors.