Flow Velocity Fluctuations Research Articles

Unsteady behavior of surge and rotating stall in an axial flow compressor

Unsteady behaviors as well as unsteady cascade flow fields of a single-stage axial flow compressor were experimentally investigated by detail measurements of unsteady performance characteristics and casing wall pressure and internal flow velocity fluctuations. The main feature of the test compressor is a capacity tank connected directly to the compressor outlet in series through slits and a concentric duplex pipe, and also jet nozzles in order to inject compressed air toward the rotor tip region. Research attention is focused on the post-stall characteristics of surge and rotating stall which occur simultaneously. When the compressor was connected to the capacity tank, surge was generated with rotating stall in accordance with the capacitance increment of whole compressor system. The surge behavior changed irregularly with throttling valve installed behind the compressor, and several types of surge cycles were observed. In addition, the surge cycle changed by jet injection to the rotor tip region. The results suggested that the blockages of the cascade flow which were generated by a stall cell play an important role in deciding the surge behaviors.

Hummocky cross-stratification-like structures and combined-flow ripples in the Punta Negra Formation (Lower-Middle Devonian, Argentine Precordillera): A turbiditic deep-water or storm-dominated prodelta inner-shelf system?

Turbidity-current and storm-induced deposits may exhibit similarities, in particularly when the latter is laid down by a combination of oscillatory and unidirectional flows. Recent progress in facies analysis helps to discriminate the sedimentary effects of oscillatory from unidirectional components of the flow. On the basis of detailed analysis of sedimentary facies, strata geometry, and palaeocurrent data, the present study reinterprets the Punta Negra Formation (PNF) (Lower-Middle Devonian, Argentine Precordillera), previously considered as a depositional system of deep-water, as a storm-dominated prodeltaic shelf depositional system.In the sandstone beds of the PNF, planar, low-angle and undulating laminations with weakly asymmetric hummocky and swaley bedforms, combined-flow ripples, accretionary hummocky cross-stratification-like (HCS-like), and anisotropic HCS-like suggest the action of oscillatory currents combined with unidirectional currents in forming the deposits. Different hypotheses on the origin of the oscillatory currents have been examined. The most convincing interpretation is that the oscillatory component of the velocity is attributed to storm-induced waves. The palaeocurrent data indicate offshore current directions, suggesting that the unidirectional flow was a gravity-induced bottom current. Inverse grading at the base and overlying normally graded divisions of the sandstone beds testify to waxing–waning behaviour of the depositional flows; interbedding of sedimentary structures (undulating laminations, low-angle and parallel laminations, and combined-flow ripples) in the lower and intermediate divisions of the beds indicate fluctuations of flow velocity. This organisation of the sedimentary structures permits association of the unidirectional component with hyperpycnal bottom currents. The terrestrial origin of the hyperpycnal flows is suggested by the abundance of terrestrial plant remains, the mineralogical and textural immaturity of the sandstone composition, and the relative scarcity of bioturbation, which was likely controlled by fresh-water input and a high rate of sedimentation. Storm-influenced, hyperpycnal flows generated subaqueous channelised forms at the mouth of the river deltas, which later filled with sand. At the distal end of the channelised forms, lobe-shaped sandstone beds were deposited, evolving distally into thin sandstone beds alternating with sandy mudstone.

Transient growth of flow disturbances in triggering a Rijke tube combustion instability

Combustion instabilities in a Rijke tube could be triggered by the transient growth of flow disturbances, which is associated with its non-normality. In this work, a Rijke tube with three different temperature configurations resulting from a laminar premixed flame are considered to investigate its non-normality and the resulting transient growth of flow disturbances in triggering combustion instabilities. For this, a general thermoacoustic model of a Rijke tube is developed. Unsteady heat release from the flame is assumed to be caused by its surface variations, which results from the fluctuations of the oncoming flow velocity. Coupling the flame model with a Galerkin series expansion of the acoustic waves present enables the time evolution of flow disturbances to be calculated, thus providing a platform on which to gain insights on the Rijke tube stability behaviors. Both eigenmodes orthogonality analysis and transient growth analysis of flow disturbances are performed by linearizing the flame model and recasting it into the classical time-lag N-τ formulation. It is shown from both analyses that Rijke tube is a non-normal thermoacoustic system and its non-normality depends strongly on the temperature configurations and the flame position. Furthermore, the most ‘dangerous’ position at which the flame is more susceptible to combustion instabilities are predicted by real-time calculating the maximum transient growth rate of acoustical energy.

A coupled neutronics/thermal–hydraulics tool for calculating fluctuations in Pressurized Water Reactors

This paper describes a tool for estimating fluctuations in neutron flux, fuel temperature, moderator density and flow velocity in Pressurized Water Reactors by coupling a dynamic thermal–hydraulic module and a dynamic neutron kinetic module. The code calculates the static solution first, giving the profile of the static fuel temperature, moderator density, velocity and neutron flux. The fluctuations (called noise in this work) are the differences between the actual time-dependent values and the corresponding mean values. The fluctuations are in general induced by perturbations in the thermal–hydraulic parameters, e.g. moderator temperature or density, at the inlet of the core. There is also a possibility to directly define the perturbations in the macroscopic cross-sections and to supply them to the neutronic part of the model. The model was validated against two separate calculations using two different commercial tools.

Correlation between Aerodynamic Noise and Velocity Fluctuation of Tip Leakage Flow of Axial Flow Fan

In the present paper the attention is focused on correlation between fan noise and velocity fluctuations of tip leakage vortex around rotor blade of a low pressure axial flow fan at the maximum pressure operating point. We measured time fluctuating velocity near the rotor tip around the rotor blades by using a hot-wire sensor from a relative flame of refer- ence fixed to the rotor blades. As the results, it is clear that the velocity fluctuation due to tip leakage vortex has weak periodicity and the hump portion appeared in its spectrum. If the flow rate was lower than the design condition, the tip leakage flow became to attach to the following blade and the sound pressure level at frequency of velocity fluctuation of this flow was increased. The correlation measurements between the velocity fluctuation of tip leakage flow and the aerodynamic noise were made using a rotating hot-wire sensor near the rotor tip in the rotating frame. The correlation between the velocity fluctuation due to tip leakage flow and acoustic pressure were increased due to generation of weak acoustic resonance at the maximum pressure operating point.

Correlation between Aerodynamic Noise and Velocity Fluctuation of Tip Leakage Flow of Axial Flow Fan

In the present paper the attention is focused on correlation between fan noise and velocity fluctuations of tip leakage vortex around rotor blade of a low pressure axial flow fan at the maximum pressure operating point. We measured time fluctuating velocity near the rotor tip around the rotor blades by using a hot-wire sensor from a relative flame of refer- ence fixed to the rotor blades. As the results, it is clear that the velocity fluctuation due to tip leakage vortex has weak periodicity and the hump portion appeared in its spectrum. If the flow rate was lower than the design condition, the tip leakage flow became to attach to the following blade and the sound pressure level at frequency of velocity fluctuation of this flow was increased. The correlation measurements between the velocity fluctuation of tip leakage flow and the aerodynamic noise were made using a rotating hot-wire sensor near the rotor tip in the rotating frame. The correlation between the velocity fluctuation due to tip leakage flow and acoustic pressure were increased due to generation of weak acoustic resonance at the maximum pressure operating point.

Impact upon Navigation Conditions of River Reach between the Two Dams by Peak Shaving at Three Gorges Hydropower Station

In order to give full play to peak shaving capacity of Three Gorges Hydropower Station and reduce impact upon shipping by peak shaving operation at Three Gorges Hydropower Station (TGHY), the paper carried out a preliminary research with a 1:110 physical model on mode of daily regulation during low-flow season and mode of peak shaving during flood season of the power station. The results of the study demonstrated that impact upon navigation due to unsteady flow generated in water course between the above-mentioned two dams by daily regulation at TGHS during low-flow season is limited because regulating flow quantity and regulating capacity is relatively small. But during flood season, both peak shaving amplitude and flow quantity fluctuation amplitude of TGHS are large. As a result, flow velocity, gradient ratio and water level fluctuation along river reach between the two dams increase dramatically during peak shaving process of the power station as compared with those during low-flow season. When average daily flow quantity and peaking capacity reaches or exceeds 20000m3/s and 8 million KW respectively, flow velocity and gradient ratio at key shoals of water course between the two dams exceed the allowable values for a 10,000-ton fleet. Therefore, sail of up-bound vessel becomes very difficult.

Study on the analogy between velocity and temperature fluctuations in the turbulent rotating channel flows

Turbulent rotating channel flow is of great interest in engineering practice, such as the cooling of the blades in turbo machinery. This paper investigates the turbulence characteristics of temperature fluctuations in rotating turbulent channel flow by direct numerical simulation. It is known that the analogy exists between temperature and velocity fluctuations in non-rotating channel flows. In this paper the analogous property is investigated between temperature and streamwise velocity fluctuations in rotating turbulent channel flows and it has been found that the analogy exists at pressures side in near wall region but the analogy is violated at the suction wall. The influence of the large scale flow structure in rotating channel, known as the Taylor-Görtler vortex, on the characteristics of temperature fluctuations are revealed.

Nonlinear Self-Excited Combustion Oscillations of a Premixed Laminar Flame

Self-excited combustion oscillations are caused by a coupling between acoustic waves and unsteady heat release. A premixed laminar flame in a Rijke tube, anchored to a metal gauze, is considered in this work. The flame response to flow disturbances is investigated by developing a nonlinear kinematic model based on the classical-equation, with the assumption of a time-invariant laminar flame speed. Unsteady heat release from the flame is assumed to be caused by its surface variations, which results from the fluctuations of the oncoming flow velocity. The flame is acoustically compact, and its presence causes the mean temperature undergoing a jump, whose effect on the dynamics of the thermo-acoustic system is discussed. Coupling the flame model with a Galerkin series expansion of the acoustic waves present enables the time evolution of the flow disturbances to be calculated. It was found that the model can predict the mode shape and the frequencies of the excited combustion oscillations very well. Moreover, the fundamental mode is found to be the easiest one to be triggered among all acoustic modes. To gain insight about the mode selection and triggering, further numerical investigation is conducted by linearizing the flame model and recasting into the popular formulation.

Wire-rod type electrohydrodynamic gas pumps with and without insulation cover over corona wire

The effect of an insulation cover on a corona wire for a wire-rod type electrohydrodynamic (EHD) gas pump was experimentally investigated and one-dimensional cross sectional averaged analyses were performed. The insulation cover over the corona wire causes a fluctuation of the discharge current, resulting in fluctuations of the EHD gas flow. The insulation cover also changes the EHD force distribution, which is the product of an electric field and ion charge density. The EHD force concentrated at the center of the flow channel, and was directed toward the rod electrode in a straight line. Thus the rod electrode supposed to become a significant disturbance to the flow. The flow velocity fluctuations and the modified distribution of the EHD force with the insulation cover enlarged the flow head losses, and eventually resulted in smaller flow velocity than that without the insulation cover over the corona wire.

Derivation and validation of a binary multi-fluid Eulerian model for fluidized beds

The paper presents a multi-fluid Eulerian model derived from binary kinetic theory of granular flows, free path theory and an empirical friction theory. The effects of the inter- and inner-particle collisions, particle translational motions and particle–particle friction are included. As the effects due to fluiddynamic particle velocity differences and particle–particle friction are considered, some unconventional terms are produced compared with the previous models. Model validation using the data from Mathiesen et al. (2000) shows that the coupling terms give a stronger and more realistic particle–particle coupling because the effects due to the fluiddynamic velocity differences are considered. The model gives reasonable predictions of the particle volume fraction, particle velocities and velocity fluctuations. The model analysis reveals that the basic particle velocity fluctuations constitute 2 terms: the velocity fluctuations of the discrete particles, and the velocity fluctuations of the continuous fluid flow. Furthermore, the simulation results show that the velocity fluctuations of the continuous fluid flow are dominant in a binary riser flow.

Analysis of pressure fluctuations in fluidized beds. I. Similarities with turbulent flow

Experimental data for pressure fluctuation time series in a fluidized bed are analyzed by several distinct, but complementary methods. First, we compute a multiscale probability density function (PDF) for the successive increments of the data. The results demonstrate the evolution of the PDF from the short to long time scales, and indicate striking similarity to the Castaing equation that has been proposed for modeling velocity fluctuations in turbulent flows. Next, to further check the results we compute the structure function of the successive increments of the data. We find that the fluctuations exhibit multifractal behavior, which is also prevalent in turbulence. The multifractality implies that the pressure fluctuates differently over distinct time scales. To understand the origin of the multifractality, we use a powerful method of analysis, namely, the multifractal detrended fluctuation analysis (MF-DFA) in order to analyze the data. The results confirm the multifractal property of the data. To better understand the similarities between the pressure fluctuations in fluidized beds and velocity fluctuations in turbulent flow, and whether the multifractality is due to extended correlations in the data or because the PDF of the successive increments is broad, we also construct the shuffled and surrogate series for the data and analyze them by the MF-DFA method. Comparison of results for the original data with the shuffled and surrogate time series indicates that the correlations in the pressure are responsible for the multifractality of the data, rather than the broadness of the PDF.

Bridging the gap between turbulence and larger scales of flow motions in rivers

ABSTRACTAlthough flow turbulence in rivers is of critical importance to earth scientists, ecologists and engineers, its relations with larger flow scales are not well understood, thus leaving a fundamental gap in our knowledge. From an analysis of a long time series of the streamwise and vertical flow velocity fluctuations measured in a gravel‐bed river, we show that the signature of the fundamental turbulent flow structures (e.g. ejections and sweeps) is embedded within increasingly larger flow scales in a self‐similar manner. The imbrication of turbulent structures into large flow pulsations of flow acceleration and deceleration covers more than two‐orders of magnitude from a few seconds to nearly 10 minutes. This property is explained by the clustering of turbulent events creating an emergent pattern at larger scales. The size of the larger flow pulsations scales with the spacing of the pools and riffles in the river. This implies a mutual adjustment between turbulence generation mechanisms and long pulsations of flow acceleration and deceleration controlled by the bed morphology. These results bridge a gap in our understanding of flows in rivers and offer a new perspective on the interactions between the turbulent flow with larger scales of flow motion that are critical for sediment transport, habitat selection and fish behaviour. Copyright © 2011 John Wiley & Sons, Ltd.

Crystallization and intermittent dynamics in constricted microfluidic flows of dense suspensions

A strong coupling between shear, confinement and microstructure governs the flow of dense colloidal suspensions through narrow channels. In this paper we explore the additional complexity that arises when the channel is partially blocked by a constriction in the centre of the channel. The reduction of the particle flux due to the constriction induces a densification upstream from the constriction, resulting in the formation of extended crystalline domains. A dynamic equilibrium establishes, which is characterized by large fluctuations in the instantaneous flow velocity and the local density. In some cases, the individual events leading to this intermittent behaviour are clearly visible as transient and localized jamming and unjamming events.

Active Control of Separated Flows behind a Backward-Facing Step with Synthetic Jet (2nd Report, Study on Details of the Flow Structures and Consideration of Mechanism of Flow Modification from a Viewpoint of Coherent Structure)

Flow fields with coherent turbulent structures behind a backward-facing step have been investigated with Laser Doppler Velocimetry to clarify a mechanism of flow control with synthetic jets. Synthetic jets with a frequency which is observed in the velocity fluctuation of the backward-facing step flow without the jet have been fed perpendicularly to the main flow from the vicinity of the step edge. Periodic component of velocity fluctuation with the same frequency of the jet is extracted with procedure of phase locked averaging. Vortices shed from the step edge have an organized structure which is synchronized with the jet and these vortices are advected immediately above the separated stream line at a speed of around 45% of the free stream velocity. Amplitude of non-periodic velocity fluctuation are larger than those of periodic fluctuations due to the vortices. The kinetic energy of the non-periodic velocity fluctuations comes from the main flow and the periodic velocity fluctuations are just the beginning of such taking in. The reattachment position closely correlates with total kinetic energy of the periodic and non-periodic velocity fluctuations. This implies the flow reattachment is promoted by the mixing of the fluids in the main stream and the reversed flow region.

D113 上部加熱型熱サイフォンに関する研究(OS2 再生可能エネルギー(太陽・地熱・温度差))

The thermosyphon can operate without an external power supply, and have simple structures. Therefore, thermosyphon can be expected to reduce energy consumption and consequently reduce CO_2 emissions. Ito et al. developed a device that the reservoir and the condenser are separate. Using this device, it was possible to drastically reduce the time before circulation commences. In this study, we examined unsteady characteristics of Ito's thermosyphon system. When the supplied power increased, the start-up time decreased. After the Working fluid has begun to flow, the fluctuation of flow velocity occurred. After several minutes, the flow in a top heat s thermosyphon became steady state.

Design, modeling and characterization of microfluidic architectures for high flow rate, small footprint microfluidic systems

We propose a strategy for optimizing distribution of flow in a microfluidic chamber for microreactor, lateral flow assay and immunocapture applications. It is aimed at maximizing flow throughput, while keeping footprint, cell thickness, and shear stress in the distribution channels at a minimum, and offering a uniform flow field along the whole analysis chamber. In order to minimize footprint, the traditional tree-like or "rhombus" design, in which distribution microchannels undergo a series of splittings into two subchannels with equal lengths and widths, was replaced by a design in which subchannel lengths are unequal, and widths are analytically adapted within the Hele-Shaw approximation, in order to keep the flow resistance uniform along all flow paths. The design was validated by hydrodynamic flow simulation using COMSOL finite element software. Simulations show that, if the channel is too narrow, the Hele-Shaw approximation loses accuracy, and the flow velocity in the chamber can fluctuate by up to 20%. We thus used COMSOL simulation to fine-tune the channel parameters, and obtained a fluctuation of flow velocity across the whole chamber below 10%. The design was then implemented into a PDMS device, and flow profiles were measured experimentally using particle tracking. Finally, we show that this system can be applied to cell sorting in self-assembling magnetic arrays, increasing flow throughput by a factor 100 as compared to earlier reported designs.

J091043 角柱から発生する広帯域騒音の数値解析([J09104]流体関連の騒音と振動(4))

In this paper, it is investigated that computational grid resolution influences on the LES results in order to predict the broadband noise generated from a square cylinder with high accuracy by decoupled method. Large eddy simulation with dynamic Smagorinsky model is performed on the condition of Reynolds number of 40,000 based on a one side of the cylinder and uniform flow velocity. Analysis of high resolution with grid space to about 1/3 of the thickness of the boundary layer at the leading edge of the cylinder, gives predictions with high accuracy in the frequency range from low frequency to the Strouhal number of 20 in the wake and to the Strouhal number of 10 near the leading edge on the flow velocity fluctuation spectrum. It is clear that improvement of the grid resolution near the corners of the upstream effects on the development of boundary layer of the leading edge and vorticity of the wake.

Permeability of soft porous media under one-dimensional compaction

A novel experimental approach to measure permeability of porous material samples under variable longitudinal compaction has been developed. The material has a non-linear structural behavior and exhibits a small hysteresis during mechanical loading and unloading experiments. The new permeameter includes a piston moving inside a Plexiglas cylinder with controllable speed and a test section where the porous material sample is placed under compaction by two grids with adjustable positions. Time-dependent pressure was recorded at four different locations along the sample together with the velocity of the piston. Experiments with two different sample lengths have been performed at three different Reynolds numbers based on the apparatus diameter. The results show that pressure gradient and permeability data do not depend on initial uncompacted sample length. All experiments included measurements at various compaction ratios of the material followed by measurements during relaxation/expansion of the material. No hysteresis was observed in the pressure gradient and permeability data during compaction and expansion of the material for a wide range of compaction ratio. The effects of small velocity fluctuations due to variable friction of the moving piston with cylinder’s wall were also considered. These velocity fluctuations cause pressure fluctuations within the sample which are high close to the inlet part of the material sample and are reduced almost completely towards its outlet. However these pressure fluctuations when scaled with the corresponding mean pressure retained their time-dependent amplitude and phase unchanged along the material. These relative pressure fluctuations cancelled out the flow velocity fluctuations resulting insignificant fluctuations in permeability. It was found that permeability, which is a material property, is drastically reduced with increased compaction ratio of the material while its solid fraction changes substantially but its porosity remains practically unchanged. A comparison with the Cármán–Kozeny expression for random porous media was also examined. Cármán–Kozeny expression predicts qualitatively the reduction of permeability with compaction. However, the predicted values of permeability are very sensitive to the initial value of porosity.

Effects of hot-wire length on the measurement of turbulent spectra in anisotropic flows

Fluid velocity measurements obtained using hot-wire anemometry are subject to spatial filtering due to the finite length of the heated sensing element. This effect can lead to a negative bias error in the magnitude of the velocity fluctuations in turbulent flows when the smallest length scales of the flow are comparable to the wire length. Here an analytical model is used to provide a quantitative description of these errors in anisotropic flows. The model is then applied to a high Reynolds number pipe flow in which two hot-wire probes of different lengths were used to measure velocity spectra at three locations in the boundary layer. These data were used to estimate the anisotropy of the turbulence as a function of frequency (or streamwise wavenumber) and wall-normal distance.