We studied numerically the intrinsic instability of high-temperature premixed flames, where the burned-gas temperature was constant, under the conditions of constant density and constant pressure in the unburned gas. A sinusoidal disturbance with sufficiently small amplitude was superimposed on a planar flame to obtain the relation between the growth rate and wave number, i.e. the dispersion relation. As the unburned-gas temperature became higher, the growth rate increased and the unstable range widened, which was due to the increase of the burning velocity of a planar flame. In sufficiently small wave-number range, the obtained numerical results were consistent with the theoretical solutions. When the growth rate and wave number were normalized, the same dispersion relations were found under the conditions both of constant density and constant pressure in the unburned gas. The normalized growth rate decreased with an increase of the unburned-gas temperature, and the normalized unstable range narrowed. This was because that the thermal-expansion effects became weaker owing to the decrease of the difference in temperature between the burned and unburned gases. To clarify the characteristics of cellular flames induced by intrinsic instability, we superimposed a disturbance with the critical wavelength. The superimposed disturbance evolved, and a cellular-shaped flame front formed. The behavior of cellular flames became milder as the unburned-gas temperature became higher, even though the growth rate increased. The burning velocity of a cellular flame normalized by that of a planar flame decreased, which was due to the weakness of the thermal-expansion effects and diffusive-thermal effects. Moreover, the burning velocity of a cellular flame increased monotonously as the length of computational domain became larger, and the dependence of burning velocity on domain length became weaker with an increase of the unburned-gas temperature.
Turbulent plane jets are prototypical free shear flows on which fundamental research can expand the overall understanding of turbulent flows. In this study, flow characteristics of the turbulent plane jets are studied by means of direct numerical simulation (DNS) based on the finite difference method. The effects of the initial conditions at the jet exit (i.e. Reynolds number and velocity profile) on the spatial development of velocity and scalar fields are mainly investigated. Meanwhile the instantaneous coherent structures, based on the Q-criterion and the local minimum of pressure, are presented. The results suggest that the plane jet flow with the higher Re (=Ubd/ν : Ub is the bulk mean velocity at the jet exit, d is the slot width, and ν is the kinematic viscosity) and the parabola velocity profile at the jet exit will correspond to the shorter potential core, furthermore, the lower Re and the parabola velocity profile will strengthen the decay of the mean field in the process of flow transition to full turbulence. The flow self-similarity will be achieved at shorter streamwise distance from the jet exit for the higher Re and the parabola velocity profile at the jet exit. Moreover, the dependency of coherent structures on the initial Reynolds number is remarkable, especially in the flow development region, but the change of initial velocity profile at the jet exit hardly acts on the scale of coherent structures. It is also observed that the scalar field is more sensitive to changes of initial conditions than the velocity field.
Immersed boundary-lattice Boltzmann methods (IB-LBM) with the single-relaxation time (SRT) cause non-physical distortion in fluid velocity when the Reynolds number is low, i.e. the relaxation time τ is high, and IB-LBM requires high spatial resolution to stably simulate high Reynolds number flows. An immersed boundary-finite difference lattice Boltzmann method (IB-FDLBM) using two-relaxation times (TRT) is therefore proposed in this study to simulate low and high Reynolds number flows stably and accurately. Benchmark problems such as circular Couette flows, flows past a circular cylinder and a sphere at various Reynolds numbers are carried out for validation. The main conclusions obtained are as follows: (1) TRT reduces numerical errors causing non-physical distortion in the fluid velocity at low Reynolds numbers, and accurate predictions are obtained when the parameter Λ, which is a function of the two relaxation times, is low, (2) for stable simulation the parameter Λ should be decreased as the Reynolds number increases, (3) implementation of TRT and the implicit direct forcing method into IB-FDLBM can solve two problems in simulation of low Reynolds number flows, i.e. non-physical velocity distortion and non-physical penetration of flow into the solid body, and (4) IB-FDLBM with TRT gives good predictions of the drag coefficients of a circular cylinder and a sphere in uniform flows for a wide range of the Reynolds number, Re, i.e., 0.1 ≤ Re ≤ 1x104.
The vibration and noise caused by pressure pulsation, referred to as fluid-borne vibration or fluid-borne noise, are some of the most detrimental problems in hydraulic systems. A Helmholtz silencer with multiple degrees of freedom was proposed to attenuate several harmonic frequencies generated in the hydraulic systems. The silencer consists of a cylindrical vessel with several chokes inside the vessel. The final goal of this research project is the development of a multiple-degree-of-freedom type of Helmholtz silencer that can be applied to hydraulic pump systems operated at different speeds. The aim of this report is to establish the design criteria of the silencer specifications. In particular, the effects of the diameter and length of the choke and the cylindrical volume on attenuation performance were analytically and experimentally investigated.
The target of this research is to develop a micro electric power generator for a low cost small river monitoring device. Firstly, the power generation by vortex induced vibrations (VIVs), its performance coefficient and the optimum condition of the generator were estimated from energy balance analysis based on the assumption that VIVs can be regarded as a resonance oscillation of a linear system. Secondly, water tunnel experiments were carried out and it was confirmed that the trailing vortex induced vibration (TVIV) occurs on a cruciform circular-cylinder/strip-plate system over a velocity range about 15 times wider than that of Kármán vortex induced vibration (KVIV). Finally, power generation experiments were carried out utilizing TVIV. The generator circuit consists of coils mounted on the circular cylinder vibrated by TVIV and magnets fixed on rigid supports. The generator is shown to extract energy from the water flow in the same way as a viscous damper over the expected velocity range. Although the performance coefficient of TVIV electricity generation is lower than that of KVIV, it is more appropriate for natural rivers of which flow velocity changes greatly.
We conducted an experiment to investigate the vortex-induced vibration (VIV) of an elastically supported rigid cylinder in fluid flow in order to develop a renewable energy system that utilizes the VIV. We constructed an experimental setup involving rotational machinery that can operate even under weak flow conditions, such as those in the deep sea. The displacement of the vibration and the lift force acting on the cylinder were measured and processed using Fourier and Hilbert transforms. The measured VIV exhibited modulations of amplitude and frequency, and a few response branches. Relationships between those and the efficiency of energy conversion from fluid flow into the VIV were analyzed. The modulations were observed only in the initial and upper branches. Our analysis found that the role for exciting the VIV periodically exchanges between locked-in and unlocked-in components of the lift force, thus causing the modulations. The maximum efficiency, about 10% in the upper branch, is insufficient and remains an issue. In addition, the present study has confirmed the predominance of the nonlinear damping force acting on the VIV, particularly in the upper branch. These experimental results will contribute to the next design of the energy system.
The saturation of gas trapped in porous rocks by capillarity depends on many factors. Herein, we focused on the effect of gas saturation at flow reversal on capillary trapping saturation. To investigate gas trapping in various sandstone cores, experiments were carried out under supercritical conditions. Residual gas saturation increased with increasing initial gas saturation. The local residual gas saturation fluctuated with heterogeneity due to the sedimentary structure. To evaluate the effect of the initial gas saturation on the residual gas saturation at the pore scale, experiments were also carried out under room temperature. For a fixed capillary injection flow rate, the initial gas saturation depended on the pore size distribution and heterogeneity due to the sedimentary layers. For vertical Berea sandstone cores, the capillary entrance pressure, associated with the layered structure, caused injected gas to enter in the porous layer of the core. However, for horizontal cores, injected gas flowed through a few layers with high permeability. On the other hand, for Kimachi sandstone cores, injected gas only entered the large pores, whereas for Tako sandstone cores, it entered both large and small pores. Therefore, high initial gas saturation can be achieved.
This report aims to clarify the relationship between the preferred frequency and jet diffusion. The disturbance frequency used to control mixing, based on the preferred frequency of a flow in the initial area of the jet column mode, is often selected to enhance the diffusion of the jet flow. In the diffusive mixing enhancement of gaseous fuel and air, the properties of the jet flow and the surrounding gas differ. Thus, for the jet flow, air, CO2, and He were used. Three types of nozzle, convergent, orifice and pipe were used. First, the laser light sheet technique was used to visualize the cross section of the jet flow. The distance from the nozzle to the position at which the vortex rings form increases with the density ratio. The fluctuation velocity of the jet flow was measured with a hot-wire anemometer and PIV to investigate the preferred frequency and determine the Strouhal number. The Strouhal number was found not to depend on the Reynolds number and remain at an almost constant value. These findings elucidate the effects of the jet density and exit velocity distribution on the relationship between the preferred frequency and jet diffusion.
Wind tunnel experiments are carried out to investigate the interference effect of a downstream strip-plate of width w = d on the crossflow vibration of a square cylinder of side length d = 26 mm. While the vibration induced by the Karman vortex is insufficiently suppressed by plates with crossflow height less than ld = 4d, the galloping is suppressed even by a square plate, i.e. ld = d, at gap ratios 1 < s/d < 2.0. When a plate of ld ≤ 2d is located at a gap ratio s/d < 1-1.4, a large vibration occurs at U > 5 m/s with maximum amplitude around s/d = 0.3. This amplitude increases divergently with increasing flow velocity. Measurement of the lift force on the square cylinder and the velocity in the near wake for the corresponding fixed system indicates that the vibration is not caused by periodic flow changes. The quasi-static hypothesis predicts that the vibration is caused by fluid-elastic instability but not by a periodic vortex shedding. Hence, the name ‘Wake Body Interference Fluid Elastic Vibration (WBIFEV)’ is proposed for this vibration.
In the present study, we consider the air entrainment into a suction pipe which is vertically inserted down into a suction sump across a mean free-water surface. This configuration is often referred to as the “vertical wet-pit pump”, and has many practical advantages in construction, maintenance and operation. In particular, we focus our concern upon the critical submergence depth Sc, which is one of the prime and conventional indicators for the air-entrainment occurrence. By a systematic approach, we experimentally investigate the influences of kinetic and geometric parameters upon Sc. As the kinetic parameters, we consider the Reynolds number Re and the Weber number We, in addition to the Froude number Fr, on such a basis as Fr is not much larger than unity in many actual cases. As the geometric parameters, we consider back clearance X, sump breadth B and bottom clearance Z. Here, all parameters are non-dimensionalised by the outside diameter D and the intake velocity Vi of the suction pipe. As a result, we reveal the effects of such six parameters upon Sc. The We effect, namely, the surface-tension effect can be ignored at We > 12. And, the Re effect, namely, the viscous effect becomes negligibly small at Re > 3×104. Under such conditions for We and Re, we could consider only the Fr effect, namely, the gravitational effect. Concerning the X/D and B/D effects, Sc/D attains the maximum at a certain X/D or B/D. On the other hand, the Z/D effect is monotonic, and becomes small at Z/D > 2.5. Some aspects of these geometric effects can be evaluated by a local-Froude-number effect on the basis of the global relation between Sc/D and Fr. And, the other aspects is necessarily considered to be related with the flow structure in the suction sump.
The present aim is to reveal the flow past a pipe which is immersed parallel to the mainstream at high Reynolds numbers. In a wind tunnel, we carry out (1) base-pressure measurements, (2) velocity-fluctuation measurements using a hot-wire anemometer and (3) flow visualisations by a smoke-wire method with PIV analyses, where we take consecutive picutures using a high-speed camcorder to obtain quantitative flow-field information such as velocity vector and vorticity. The tested parameter ranges are as follows: Re = 2.0×103 - 1.3×104, d/t = 4.0 - 10.0 and l/t = 1.0 - 10.0, where Re, d, t and l are the Reynolds number, mean diameter, thickness and length of the pipe, respectively. As a result, the Re effects are negligible. The base-suction coefficient -Cpb monotonically decreases with decreasing d/t, or with increasing l/t. We propose a unified formula to predict -Cpb, which are consistent with both a two-dimensional prism and a rod for l/t < 4.0 in addition to a ring. In contrast, the Strouhal number St almost coincides with that for a two-dimensional prism at any l/t, if we can detected any dominant frequencies. In addition, we conduct flow visualisations, and reveal the effects upon axisymmetry of wake. Finally, we classify the flow into three modes based on both periodicity and axisymmetry. Such a modal classification reveals that the enhancement of flow's irregularity corresponds to the decrease of -Cpb.
The single dielectric barrier discharge plasma actuators (PAs) are known to be effective for flow control process. This study investigated the winglet-type thin and small PAs, which were suspended in the quiescent air flow. We conducted the PIV measurements and flow computations for the PA induced flow. The induced flow was discussed on three regions: a radial flow toward the covered electrode, a wall shear flow downstream from the covered electrode, and a jet-like flow downstream from the winglet. When the electrode was located at the leading edge of the winglet PA, the streamwise flow was effectively enhanced on the covered electrode. On the other hand, when the electrode was located at the trailing edge, the friction loss was minimized for the near-wall high shear flow over the winglet, and thus the highest momentum integral was obtained for the downstream jet-like flow. From these results, the most effective flow actuation was suggested to be achieved by the short winglet that had the exposed electrode at a leading edge and the covered electrode at a trailing edge.
Investigations of the definition and randomness of turbulence were reviewed. The Kolmogorov complexity measure of randomness was then introduced. Numerical and graphic data in the mixing layer formed downstream of a two-dimensional nozzle exit were compressed with the aid of a compression program. Approximated Kolmogorov complexity, AK, and normalized compression distance, NCD, were obtained. The AK indicated the regularity of the laminar flow and the randomness of the turbulent flow quantitatively. The NCD of the numerical value varied with data length. Between the same data, it approached zero, yet, between different data, it approached unity as the data length increased. The NCD of the numerical value in the natural transition process in the mixing layer increased monotonically downstream. Thus, the NCD appears to be the measure of the transition process. In the natural transition process in the mixing layer, the AK of the numerical value and the NCD of the graphic data did not change monotonously in the downstream direction. They therefore contain some uncertainty for measurement of the transition process.
This experimental research concerns a simple air-bubblejet from a bottom nozzle in water. We try to conduct the measurements of the flow by applying a three-dimensional particle tracking velocimetry (3D-PTV) technique, in order to specify both bubble (air) and liquid (water) velocities. As tracer particles, we regard bubbles themselves in bubble-velocity measurements, and polyethylene particles suspended in water in liquid-velocity measurements. Then, we visualise the three-dimensional motions of the bubbles and the liquid. As we record stereo images using a pair of high-speed video cameras, we can get temporally-consecutive spatial information of both the bubbles' and liquid's motions. As a result, we quantitatively reveal the three-dimensional and instantaneous behaviour of the unsteady bubble-jet flow. And, using the obtained three-dimensional information, we show the fundamental flow structure of the bubble jet from a statistical view point.
The laminar-turbulent transition of a boundary layer induced by a jet injection in the inlet region of a circular pipe was experimentally investigated. The jet was periodically injected radially from a small hole in the inlet region into the pipe flow. Axial velocity was measured by a hot-wire anemometer. The turbulence induced by the jet within the boundary layer developed into turbulent patches which then grew in the axial, circumferential and radial directions downstream. The shape of the patches shown by the intermittency factor in the diametrical plane was similar to the turbulent spot in the flat plate boundary layer at first, then became similar to the turbulent slug in the pipe flow developed downstream. The turbulent patches protruded from the boundary layer after they grew and reached the circumferential opposite side, although they stayed within the boundary layer as long as the shape was turbulent spot-like in the diametrical plane. The propagation velocity at the leading edge became faster than the cross-sectional velocity, though it turned slower at the trailing edge. Therefore, the growth rate of its axial length varied downstream. The growth rate of the patches' circumferential length was smaller than that in the turbulent spot under zero pressure gradient and was almost the same as the spot under favorable pressure gradient.
In a previous paper, the effect of angle and length of the inlet guide vane on the performance of a cross-flow fan was examined. By installing a guide vane having one sheet in the tongue division side in the suction region, the performance of the cross flow fan resulted in higher pressure and higher efficiency than the case without the guide vane. In this study, a guide vane having several sheets is installed as a more convenient method in the suction region of the cross-flow fan. First, the reason of the generation of inlet pre-whirl flow of the cross-flow fan is investigated. Next, we show that the pre-whirl is controlled by the inlet guide vane. As a result, the following facts became clear. (1) The pre-whirl is generated by drawing in the inlet flow for the eccentric vortex of the interior rotor, and the inlet velocity therefore becomes non-uniform in the circumferential direction. (2) The cross flow fan becomes a high-efficient and high-pressure fan by controlling the pre-whirl using the inlet guide vane.
A high-speed train entering a tunnel generates a compression wave that propagates through the tunnel toward its exit. When the compression wave reaches the tunnel exit, a pressure pulse causing environmental problems (the micro-pressure wave) is radiated from the exit portal. The micro-pressure wave magnitude is approximately proportional to the maximum pressure gradient of the tunnel compression wave arriving at the exit portal. In a long Shinkansen tunnel with concrete slab tracks, the compression wavefront steepens due to the nonlinear effect during its propagation because all surfaces of the tunnel wall are smooth. Therefore, it is necessary to investigate the compression wave distortion in a tunnel and to clarify the characteristics of the compression wave propagation for estimating and reducing the magnitude of the micro-pressure wave. In this paper, we introduce a new simple equation governing distortion of the tunnel compression wave propagating through a Shinkansen tunnel with concrete slab-tracks. We propose a new simple scheme for numerical calculations. A space evolution type equation with one variable is derived from the three conservation equations (mass, momentum, and energy including the wall friction and heat transfer terms) of the 1D CFD by assuming small disturbances excited by the compression wave. The numerical calculation scheme based on the simple equation reduces the computing time remarkably because its CFL condition is relaxed. The calculation results obtained using the proposed scheme agree well with those by a conventional scheme based on the 1D CFD. The accuracy of the simple equation is verified.