Times Of Velocity Research Articles

Live-bed local scour around vertical-wall abutments

Local scour experiments were performed with four different uniform cohesionless sediment diameters d50 = 0.26, 0.42, 1.06, and 1.92 mm and five vertical-wall abutments with projected lengths l = 0.04, 0.06, 0.08, 0.10, and 0.12 m perpendicular to the flow. These tests were conducted at different flow velocity (V) varying from 0.71 to 5 times of the critical velocity (Vc). The plotting of scour depths dse vs. V/Vc shows that the scour depths increase up to the threshold value of flow (V/Vc = 1) and after that it reduces and again increases to attain the live-bed maximum in the flow range V/Vc = 3–5. It is observed that the scour depth increases with the increase of sediment sizes and abutment lengths. Further, the scour depth decreases with the increases of nonuniformity under all flow conditions. A design equation is proposed based on the present experimental data and existing live-bed scour data. The measured values of scour depth are compared with the calculated values using proposed equation and also with three different live-bed local scour equations for the conditions of the tests.

Fluid aspects of electron streaming instability in electron-ion plasmas

Electrons streaming in a background electron and ion plasma may lead to the formation of electrostatic solitary wave (ESW) and hole structure which have been observed in various space plasma environments. Past studies on the formation of ESW are mostly based on the particle simulations due to the necessity of incorporating particle's trapping effects. In this study, the fluid aspects and thermodynamics of streaming instabilities in electron-ion plasmas including bi-streaming and bump-on-tail instabilities are addressed based on the comparison between fluid theory and the results from particle-in-cell simulations. The energy closure adopted in the fluid model is the polytropic law of d(pρ−γ)/dt=0 with γ being a free parameter. Two unstable modes are identified for the bump-on-tail instability and the growth rates as well as the dispersion relation of the streaming instabilities derived from the linear theory are found to be in good agreement with the particle simulations for both bi-streaming and bump-on-tail instabilities. At the nonlinear saturation, 70% of the electrons are trapped inside the potential well for the drift velocity being 20 times of the thermal velocity and the pρ−γ value is significantly increased. Effects of ion to electron mass ratio on the linear fluid theory and nonlinear simulations are also examined.

Onset of cellular instability in adiabatic H2/O2/N2 premixed flames anchored to a flat-flame heat-flux burner

The onset of cellular instability in adiabatic H2/O2/N2 premixed flames anchored to a heat-flux burner is investigated numerically. Both hydrodynamic instability and diffusional-thermal instability are shown to play an important role in the onset of cellular flames. The burner can effectively suppress cellular instability when the flames are close to the burner, otherwise the burner can suppress the instabilities only at large wavenumbers. Because of differential diffusion, local extinction can occur in lean H2/O2/N2 flames. When the flames develop to take on cellular shapes, the surface length, the overall heat release rate and the mean burning velocity are all increased. For near stoichiometric fuel-rich flames the mean burning velocity can increase by as much as 20%–30%. For lean flames with an equivalence ratio of 0.56, the mean burning velocity can be 2–3 times of the burning velocity of the corresponding planar flame.

Parameter Sensitivity Analysis on Pressure Drop of Gas-solid Flow for Absorber Sphere Pneumatic Conveying

The pressure drop of gas-solid flow for absorber sphere pneumatic conveying is important for the design and operation of the gas circulator. The simplified one-dimension model is introduced to predict the pressure drop of gas- solid flow for absorber sphere pneumatic conveying. The pressure drop of gas-solid flow in the vertical pipe is the main component part of the total gas-solid flow for absorber sphere conveying. The pressure drop of gas-solid flow for absorber sphere conveying in the vertical pipe with the inside diameter 47mm and conveying height 23 m is predicted by the simplified model. The superficial gas velocity considered is in the range of 1.5 to 3.0 times of the terminal velocity of a single particle. The solid mass flow rate considered is in the range of 0.2 to 1.2kg/s. Particle velocity, solid friction factor and solid mass flow rate are important parameters to predict pressure drop of vertical pipe. The typical empirical correlations of particle velocity and solid friction factor reported in the literature are adopted to predict the pressure drop of vertical pipe. The superficial gas velocity for absorber sphere conveying is suggested to be two times of the terminal velocity of a single particle. For the suggested superficial gas velocity, the predicted pressure drop of vertical pipe by using empirical correlations is in the range of 15 to 20kPa and 31 to 50kPa for solid mass flow rate at 0.2kg/s and 1.2kg/s, respectively. For the suggested superficial gas velocity, the relatively conservative predicted pressure drop of vertical pipe is in the range of 20 to 78kPa for solid mass flow rate in the range of 0.2 to 1.2kg/s. Further work is needed to verify the pressure drop model and to modify the key parameters for absorber sphere conveying in helium.

Modelling and analysis of a novel wind turbine structure

This study introduces a novel wind turbine structure for an urban environment. A computational modelling has been conducted to investigate the effect of the new structure on the flow behaviour of entrance wind through the structure and the feasibility of the new wind turbine working at different wind speeds in an urban area. The wind flow behaviour through a chamber of the wind turbine structure has resulted in an increase of 1.3 times of the wind velocity at the outlet of the wind turbine. This is equivalent to 2.5 times increase of wind energy. The wind tunnel tests were carried out to validate the simulation results. There is a good correlation between the experimental and computational results. It is evident that the presented computational method can predict and evaluate the performance of this new type of shroud structure in an urban environment.

Flow Field Around Single and Tandem Piers

The present study provides a comparison between the flow pattern around two circular piers in tandem and a single pier set up on a moderately rough flat bed in a laboratory flume. Velocities are measured by an Acoustic Doppler Velocimeter (ADV). The contours of the time-averaged velocity components, Reynolds shear stresses, turbulence intensities and turbulence kinetic energy at different planes are presented. Streamlines and vectors are used to study the flow features. The analysis of power spectra around the piers is also presented. The results show that the presence of downstream pier changes the flow structure to a great extent, particularly in the near-wake region. Within the gap between the two piers, a stronger and substantial upflow is shaped. However, a weaker transverse-deflection is formed in comparison with that in the single pier. Near the bed, the velocity of flow approaching the downstream pier decreases to 0.2–0.3 times of the approach mean velocity (U 0) due to the sheltering effect of the upstream pier. In the wake of downstream pier, the flow structure is completely different from the one in the wake of single pier. In comparison with the single pier, the values of turbulence kinetic energy and turbulence intensities show a considerable decrease around the tandem piers. In tandem piers, the high values of turbulence characteristics are found near the downstream pier. There is a recirculation zones just upstream of the sheltered pier close to the bed and another behind that pier near the free surface. The results show a decrease in the strength of vortical structure in the wake of tandem piers in comparison with single pier. It is shown that the formation of flow with different Reynolds number along the flow depth due to the effect of bed roughness, as well as pier spacing, can influence the type of flow regime of tandem case. In addition to enhancing the flow structure indulgence, the present detailed measurements can also be used for verification of numerical models.

Hydrodynamics of a fluidized bed co-combustor for tobacco waste and coal

The fluidization characteristics of binary mixtures containing tobacco stem (TS) and cation exchange resin (a substitute for coal) were studied in a rectangular bed with the cross-section area of 0.3×0.025m2. The presence of herbaceous biomass particles and their unique properties such as low density and high aspect ratio resulted in different fluidization behaviors. Three fluidization velocities, i.e. initial, minimum and full fluidization velocities, were observed as the TS mass fraction increased from 7% to 20%, and four hydrodynamic stages were experienced, including the static, segregation, transition and mixing stages, with increasing operational gas velocities. The results suggest that the operational gas velocity should be in the range of 2.0–5.0 times of the minimum fluidization velocity of the binary mixtures, and less than 7% TS mass fraction should be used in an existing bubbling fluidized bed. Higher TS fraction inclusion requires the introduction of central jet gas to improve the mixing effect.

Synthesis of time-optimal control of inverted rod

The problem of controlling a system consisting of a heavy rod fastened at its lower end on a hinge to a movable support is studied. In the linear statement and under the assumption that the support moves without inertia at a bounded velocity, a time-optimal motion of the rod from an arbitrary initial state with a nonzero velocity to a vertical unstable equilibrium position with the zero velocity of the rod at this position is found assuming that the point of support must travel a prescribed distance. The domain where a solution exists is found. Formulas for determining the switch times of the support velocity depending on the boundary conditions are obtained. A numerical example illustrating the procedure of choosing the control depending on the initial position of the rod in the absence of disturbances is considered.

Effect of Venturi System on Acceleration of Low-speed Water Flow at the Venturi Throat Installed at the Inlet of Hydro Turbine

For a hydro turbine electricity generation system in river or bay, a venturi system could be applied to accelerate flow speed at the inlet of the turbine system in a flow field. In this study, a steady flow simulation was conducted to understand the effect of venturi system on the acceleration of current speed at the inlet of a hydro turbine system. According to the continuity equation, the flow speed is inversely proportional to the cross-section area in a conduit flow; however, it would be different in an open region because the venturi system would be an obstruction in the flow region. As the throat area is 1/5 of the inlet area of the venturi, the flow velocity is accelerated up to 2.1 times of the inlet velocity. It is understood that the venturi system placed in an open flow region gives resistance to the upcoming flow and disperses the flow energy around the venturi system. The result of the study should be very important information for an optimum design of a hydro turbine electricity generation system.

Heat transfer characteristics in a horizontal swirling fluidized bed

An innovative horizontal swirling fluidized bed (HSFB) with a rectangular baffle in the center of an air distributor and three layers of horizontal secondary air nozzles located at each corner of fluidized bed was developed. Experiments on heat transfer characteristics were conducted in a cold HSFB test model. Heat transfer coefficients between immersed tubes and bed materials in the HSBF were measured with the help of a fast response heat transfer probe. The influences of fluidization velocity, particle size of bed materials, measurement height, probe orientation, and secondary air injection, etc. on heat transfer coefficients were intensively investigated. Test results indicated that heat transfer coefficients increase with fluidization velocity, and reach their maximum values at 1.5–3 times of the minimum fluidization velocity. Heat transfer coefficients are variated along the circumference of the probe, and heat transfer coefficients on the leeward side of the probe are larger than that on the windward side of the probe. Heat transfer coefficients decrease with increasing of measurement height; heat transfer coefficients of the longitudinal probe are larger than that of the transverse probe. The proper secondary air injection and particle size of bed materials can generate a preferred hydrodynamics in the dense zone and enhance heat transfer in a HSFB.

Effect of AC electric fields on the stabilization of premixed bunsen flames

The stabilization characteristics of laminar premixed bunsen flames have been investigated experimentally for stoichiometric methane–air mixture by applying AC voltage to the nozzle with the single-electrode configuration. The detachment velocity either at blowoff or partial-detachment has been measured by varying the applied voltage and frequency of AC. The result showed that the detachment velocity increased with the applied AC electric fields, such that the flame could be nozzle-attached even over five times of the blowoff velocity without having electric fields. There existed four distinct regimes depending on applied AC voltage and frequency. In the low voltage regime, the threshold condition of AC electric fields was identified, below which the effect of electric fields on the detachment velocity is minimal. In the moderate voltage regime, the flame base oscillated with the frequency synchronized to AC frequency and the detachment velocity increased linearly with the applied AC voltage and nonlinearly with the frequency. In the high voltage regime, two different sub-regimes depending on AC frequency were observed. For relatively low frequency, the flame base oscillated with the applied AC frequency together with the half frequency and the variation of the detachment velocity was insensitive to the applied voltage. For relatively high frequency, the stabilization of the flame was significantly affected by the generation of streamers and the detachment velocity decreased with the applied voltage.

切欠きオリフィス衝突噴流の流動・伝熱特性(熱工学,内燃機関,動力など)

Jet flow from an orifice nozzle is applicable to mixing and entraining of ambient fluid effectively because of a large shearing stress layer with a large velocity gradient at the jet edge due to the vena contracta effect, but the sudden contraction at the nozzle exit accounts for a large flow resistance. The centerline velocity increases to the downstream and reaches to the maximum of 1.2 times of the nozzle exit maximum velocity at about x/d_0=2 (d_0: nozzle exit diameter), this is also effective to enhance the heat transfer performance of impinging jet. In order to reduce the flow resistance remaining a vena contract effect, use of a cone orifice nozzle has been considered. In this study, the effects of use of a notch for the orifice nozzle on the flow characteristics are examined experimentally. Hot wire measurements were conducted to demonstrate the spreading or mixing performance of the notched orifice nozzle having a reduced flow resistance of the nozzle and an increasing turbulent intensity more than that for a conventional orifice nozzle. Moreover, heat transfer performance of the impinging jet from an orifice nozzle is made clear and the improvement or enhancement by a notched orifice nozzle and a tapered notched nozzle will be shown by experimental research works.

Experimental study on average movement characteristics of bed-load particles

The three-dimensional movements of bed-load particles in an open channel with different grain diameters and different specific gravities under the conditions of different hydraulic gradient are measured by means of particle tracking velocimetry (PTV) technique. The average movement characteristics of the particles are analyzed by statistics. The results show that the longitudinal average velocities of the particles neither agree with the law suggested by Einstein nor obey the law assumed by Bagnold. Einstein’s law is that the particle velocity is equal to 11.6 times of shear velocity, and the Bagnold’s law assumes the velocity equal to the difference between flow velocity at corresponding water depth and settling velocity of particle. Two formulas for determining the velocity of particle in the forms of expression given by Einstein and Bagnold are obtained according to the statistical results of experimental data, respectively.

Non-steady electro-osmotic flow of a micropolar fluid in a microchannel

We formulated the initial-boundary-value problem of non-steady electro-osmotic flow of a micropolar fluid in a rectangular microchannel of height much larger than the Debye length and length much larger the height. Solving the governing differential equations numerically when a spatially uniform electric field is applied as an impulse of finite magnitude, we found that the effect is instantaneous on the flow, just as for simple Newtonian fluids. The decay times of the fluid velocity and the microrotation, however, are smaller in micropolar fluids than in simple Newtonian fluids. The maximum magnitude of microrotation decreases as the micropolarity increases. The effect of microrotation on the stress tensor is more dominant than that of the fluid speed, and a threshold effect with respect to the magnitude of the zeta potential is evident in the spatial profile of the couple stress tensor. We expect similar trends even when the applied electric field varies over some finite interval of time.

Floating internals in fast bed of cohesive particles

Four kinds of spherical materials with diameter between 0.3 mm to 0.6 mm were employed as floating internal in fast bed of cohesive particles to reduce agglomerate size and narrow the size distribution. The fast bed of cohesive particles with floating internals preserves basic fluidizing characteristics of fast fluidization. However, the size of agglomerates in fast beds of all four types of cohesive particles employed in experiments was notablely reduced. The effect of floating internals could be obviously observed only when the fluidizing gas velocity is higher than about 3–4 times of incipient fluidizing velocity of floating internals. This phenomenon may indicate an “active velocity” for floating internals to take effect. Under specific operating conditions and a matching between cohesive powder and floating internal, there will be a maximal agglomerate size that floating internals can break. This suggests a proper startup procedure for fast bed with floating internals to prevent the formation of oversize agglomerates. Methods for estimating the gas velocity at which floating internal take effect and the terminal velocity of floating internals and agglomerates were developed to give operating range of fluidizing gas velocity of fast bed with floating internals. A model for calculating maximal agglomerate size that floating internal can break was also developed, and an analysis on the influence of fast bed's operating conditions on the maximal size was made based on the model.

Mechanical and EMG responses of the vastus lateralis and changes in biochemical variables to isokinetic exercise in endurance and power athletes

Twelve endurance athletes and six power athletes performed fatiguing isokinetic knee flexions/extensions. Isokinetic torque was recorded during the exercise. Isometric torque, cortisol and lactate responses, electromyographic (EMG) mean power frequency, average rectified value, and conduction velocity were analysed before and after the isokinetic exercise to determine correlations between electrophysiological variables and mechanical performances and/or blood concentrations of biomarkers in the two groups of athletes. The EMG variables were estimated from signals recorded from the vastus lateralis in both voluntary and electrically elicited isometric contractions. Power athletes recorded higher values than endurance athletes for the following variables: pre-exercise isometric maximal voluntary contraction (MVC), isokinetic MVC, rate of mechanical fatigue during isokinetic contractions, pre – post exercise variations and recovery times of conduction velocity and mean power frequency, and lactate concentrations. Moreover, conduction velocity overshooting was observed in endurance athletes during the recovery phase after exercise. The correlation analyses showed that the higher the rate of mechanical fatigue, the higher the lactate production and the reduction in conduction velocity due to the exercise.

Regulating characteristics of loop seal in a 65 t/h oil shale-fired circulating fluidized bed boiler

In this paper, an industrial cold experiment was conducted for studying regulating characteristics of a loop seal in a 65 t/h oil shale-fired circulating fluidized bed (CFB) boiler. The starting characteristic of the loop seal, and the effect of the supplying air and the fluidizing air in the loop seal were investigated. Compared with other regulating modes, the combined regulating mode of keeping the fluidizing air rate constant and regulating the supplying air rate can make the loop seal obtain better regulating quality, and offer more reliable guarantee for steady operation of the CFB boiler as well. In order to prevent circulating material depositing and slagging at the bottom of the loop seal, it was suggested that the fluidizing air rate and the supplying air rate is 2–3 and 1.2–1.5 times of the minimum fluidizing velocity of circulating material, respectively. These experimental results may be used as a reference for regulating the loop seal of the 65 t/h CFB boiler in hot condition and designing a new loop seal.

A Superheated-Steam Fluidized-Bed Dryer for Parboiled Rice: Testing of a Pilot-Scale and Mathematical Model Development

This article describes the testing of a pilot-scale superheated-steam fluidized-bed dryer for parboiled rice along with development of a mathematical model for predicting the changes in temperature of steam and moisture content of parboiled rice during drying. Based on the obtained results, it was found that the superficial velocity of steam from 1.3 to 1.5 times of the minimum fluidization velocity had no significant effect on the drying rates of rice. The energy consumption for reducing the moisture content of paddy from 0.43 to 0.22 kg/kg dry basis was approximately 7.2 MJ/kg water evaporated. Drying temperature caused the appreciable change of parboiled rice qualities as characterized by water adsorption, whiteness and pasting viscosities, white belly, and hardness. Soaking paddy at a temperature of 70°C for 7–8 h before drying was sufficiently enough for producing parboiled rice, with no white belly. The gelatinization of starch during drying resulted in higher head rice yield of the product as compared to that of raw paddy.

A210 垂直浮力噴流の組織構造の可視化と同時速度計測

In order to understand the turbulent structure of the buoyant plume, the simultaneous flow visualization and the velocity measurements by PIV are carried out in vertical and horizontal planes of the plumes. The flow visualization result shows the periodic formation of mushroom type vortices in the plume, which is induced by the ejection of the hot fluid from the nozzle exit by the influence of buoyancy. It is found from the PIV measurement that the ejection velocity reaches more than 3 times of the plume velocity at the nozzle exit and entrains the cold fluid into the plume, which contributes the large growth rate of the plume development.

二次元噴流の渦構造と移流速度評価に関する実験的研究(G05-11 渦・噴流,G05 流体工学部門一般講演)

To investigate the coherent structure in a turbulent plane jet. it is important to estimate the convective velocity of the coherent structure. In this study, the simultaneous measurement at 2 points by the X-type hot wire probe has been performed, and the new method was suggested to estimate the convective velocity more accurately by using the power spectrum and the spatial correlation of the lateral velocity fluctuation v. It is found that the convective velocity for the turbulent plane jet of this study is 0.676 times of the mean velocity of the jet centerline.