Upstream Flow Velocity Research Articles

Effect of Nonlinear Magnetic Forces on Transverse Galloping Dynamics of Square Cylinders

Under the influence of cross-fluid flow, a cylinder of a square cross-section may gallop. Galloping is a self-excited vibration mode that can be utilized for low-power harvesting applications. The harvested power depends on several factors, including upstream flow velocity and system dynamics. This study explores the potential of magnetically-induced nonlinear stiffness to improve the power output of galloping-based energy harvesters. In this experimental study, the vibration response of a square rod with a mass ratio of 10 is investigated at a Reynolds number of 200. The vibration behavior of two identical coaxial square rods with magnetic monopoles at opposite ends is analyzed. Results reveal that the magnets’ configuration and strength significantly affect vibration amplitude and the critical flow velocity necessary for the onset of galloping.

Research on the Optimal Spacing of Multiple Roof Smoke Blocking Structures in a Long Corridor

In a long and narrow corridor, the installation of roof smoke blocking structures is a measure to slow down the spread of fire smoke. When employing multiple smoke blocking structures, the spacing between these structures is a critical parameter that needs to be considered for optimal effectiveness. This paper analyzes the smoke blocking performance of double structures at different spacing and measures the smoke flow velocity both upstream and downstream of the double structures. According to the analysis of the smoke velocity vector obtained from numerical simulation, the smoke can be divided into three zones based on the flow state of the smoke after passing through the front smoke screen structure, namely the vortex zone, surge wave zone, and steady flow zone. When the rear smoke screen is located in the surge zone, the smoke blocking effect is optimal. Analysis of the morphology of the smoke layer indicates that the length of the vortex region is directly proportional to the upstream smoke flow velocity. The numerical and experimental results both indicate that an excessively large or small spacing between the structures fails to achieve optimal smoke control effectiveness. When the spacing is within an optimal range, the smoke velocity is the lowest. Finally, using a real architectural corridor as a case background, this paper presents a design example of roof smoke blocking structures. In order to arrange as many smoke blocking structures as possible, an appropriate spacing between the structures should be slightly larger than the vortex region. The smoke control effectiveness of multiple roof structures was validated through numerical simulation. As a result, the time required for smoke to pass through the corridor increases by 110 s.

Influence of magnetically-induced nonlinear added stiffness on the lift galloping of square cylinders at low Reynolds number

A square cylinder may gallop if subjected to fluid flow, experiencing a self-excited vibration mode that can harvest energy for low-power applications. The harvested power is typically low and depends on the upstream flow velocity and system dynamic parameters. In this study, the influence of nonlinear stiffness induced by two repulsive magnetic poles on the galloping response of square rods is investigated at a mass ratio of 10 and a Reynolds number of 200. The vibration response of two identical coaxial square rods with magnetic poles attached to their opposite ends is analyzed. Two simplified configurations are selected to model the magnetic forces: a pair of identical monopoles and a pair of identical dipoles. Results show that the magnet configuration and strength significantly affect the vibration amplitude and the onset flow velocity. A weak magnetic force can enhance vibration and reduce onset flow velocity, while a stronger pair of dipoles may push the bistable system to a state of low-amplitude vibration with a mean displacement. The influence of magnets in this configuration has potential as a control technique for energy harvesting applications.

Experimental and numerical investigation of a disc-attached cylinder near a wall

An experimental and numerical study is carried out to investigate the flow up- and downstream of a disc-attached cylinder near a wall with a Reynolds number of Re=U0d/ν=5.4⋅104 (U0 is the mean upstream flow velocity, d is the diameter of the cylinder, and ν is the kinematic viscosity). Laser Doppler Velocimetry measurements were conducted to obtain statistical flow parameters such as the mean velocity and turbulent kinetic energy. The experimental results show that the mean flow can be characterized as two-dimensional at a transverse distance of more than d away from the disc. Furthermore, it is shown that the effect of the disc and the cylinder fades away downstream. Analysis of a power spectrum of the vertical velocity fluctuations shows that vortex shedding is disturbed downstream of the disc. The numerical simulation is performed using a Reynolds-Averaged Navier–Stokes model. The turbulent kinetic energy and mean velocities are well-predicted by the numerical model. An increase in the peak amplitudes of the power spectra obtained from the vertical velocity fluctuations is found at a transverse distance of 3.75d from the centerline of the disc for both the experimental and numerical study, caused by vortices being transported by a transverse flow related to the disc.

Power-generation enhancements and upstream flow properties of turbines in unsteady inflow conditions

Energy-harvesting systems in complex flow environments, such as floating offshore wind turbines, tidal turbines and ground-fixed turbines in axial gusts, encounter unsteady streamwise flow conditions that affect their power generation and structural loads. In some cases, enhancements in time-averaged power generation above the steady-flow operating point are observed. To characterize these dynamics, a nonlinear dynamical model for the rotation rate and power extraction of a periodically surging turbine is derived and connected to two potential-flow representations of the induction zone upstream of the turbine. The model predictions for the time-averaged power extraction of the turbine and the upstream flow velocity and pressure are compared against data from experiments conducted with a surging-turbine apparatus in an open-circuit wind tunnel at a diameter-based Reynolds number $Re_D = 6.3\times 10^5$ and surge-velocity amplitudes up to 24 % of the wind speed. The combined modelling approach captures trends in both the time-averaged power extraction and the fluctuations in upstream flow quantities, while relying only on data from steady-flow measurements. The sensitivity of the observed increases in time-averaged power to steady-flow turbine characteristics is established, thus clarifying the conditions under which these enhancements are possible. Finally, the influence of unsteady fluid mechanics on time-averaged power extraction is explored analytically. The theoretical framework and experimental validation provide a cohesive modelling approach that can drive the design, control and optimization of turbines in unsteady flow conditions, as well as inform the development of novel energy-harvesting systems that can leverage unsteady flows for large increases in power-generation capacities.

A Study on the Measurement Characteristics of the Spring-Plate Flow Measurement Device

In order to improve the effective utilization of agricultural irrigation water and to reasonably allocate water resources in irrigation areas, it is necessary to use open channel flow measurement devices that are accurate and easy to carry. In this study, a spring-plate flow measurement device with different plate widths was designed. Through a combination of theoretical analysis and numerical simulation, the measurement characteristics of the device in specific channels under conditions of 20–105 m3/h flow were studied, the relationship between the flow rate and the force acting on the plate surface was fitted, and the hydraulic characteristics of water flow during its use, such as pressure, velocity distribution, and head loss, were analyzed. The results show that in the process of using the spring-plate flow measurement device, the force on the plate surface increases with the increase in the flow rate, and the force on the plate surface is related to the flow rate in the channel by a power of 5/6. The width of the measurement plate impacts the accuracy of flow measurement, and the smaller the plate width, the larger the error in flow measurement. The distribution of pressure on the measurement plate is similar to that of static pressure, and the pressure increases with the increase in the width of the measurement plate. The upstream flow velocity of the device is small, and the water level increases due to obstruction of the measurement plate. When it connects to the downstream water surface, the water level rapidly decreases, and the flow velocity increases. In using the spring-plate flow measurement device to measure flow, head loss will be produced, and the magnitude of this loss increases with the increase in the width of the measurement plate. The research results provide a theoretical basis for the application of spring plate flow-measuring devices in irrigation areas.

Determination of the Response Amplitude Operator of a tidal turbine as a spectral transfer function

A transfer function determination method is proposed in this study to predict the unsteady fluctuations of the performance of a tidal turbine model. This method is derived from the Response Amplitude Operator (RAO) applied in the offshore industry to predict linear wave-induced loads on large structures. It is based on a spectral approach and requires the acquisition of a turbine parameter (e.g. torque, thrust, power or root-blade force) in synchronization with an upstream flow velocity measurement. On the frequency range where the causality between these two signals is proven, the transfer function is established using the ratio between the cross-spectral density and the spectral density of the incoming velocity.The linearity is verified using the coherence function, which shows validity for the turbine power in the lowest frequencies only. This transfer function is then used to reconstruct the power fluctuations which is compared to the recorded one for a particular flow condition with bathymetry generated turbulence. The result shows the dependence on the accurate location of the velocity measurement point used for the reconstruction. This point must exactly correspond to the expected turbine location, i.e. where the turbine response needs to be processed. Bearing in mind its limits, the method can be used to predict the loadings of extreme events on the turbine structure and the performance variations corresponding to the unsteady characteristics of a turbulent flow, for a better grid integration.

A framework for exploring environmental risk of the longest inter-basin water diversion project under the influence of multiple factors: A case study in China

Multi-factor risk assessment is an important prerequisite for water quality protection and the safe operation of mega hydro-projects. As the largest long-distance inter-basin water diversion project in the world, the Middle Route of the South-to-North Water Diversion Project of China (MRSNWDPC) has been in operation for 8 years and has benefited 79 million people along the canal. However, concerns have been raised in recent years about the potential negative effects of abnormal algal proliferation in the MRSNWDPC. It is very important for the safety of water supply to carry out relevant risk analysis and formulate regulatory management. In order to quantitatively evaluate the risk of algal proliferation in the MRSNWDPC under the influence of multiple factors, a multivariate risk assessment method based on Vine Copula theory and Monte Carlo simulation was proposed. Five key factors (water temperature, flow velocity, flow rate, algal cell density, and dissolved oxygen) were used and multiple dependency models in each section of the MRSNWDPC from January 2016 to January 2019 were established to study the risk of algal proliferation under multiple scenarios. The results demonstrate that water temperature can be used as an appropriate early-warning indicator of algal proliferation. The early-warning interval (unit: °C) of water temperature in the upper, middle, and lower reaches are 26–29°C, 23–26°C, and 21–23°C, respectively. Unlike bivariate analysis, the multiple dependency model describes the relationship between variables more accurately and enriches the scenarios of multiple conditional probabilities. When the water temperature fluctuates in the early-warning interval, regulating the upstream, midstream, and downstream flow velocity to be higher than 0.6 m/s, 0.5 m/s, and 0.6 m/s, respectively, can effectively reduce the risk of algal proliferation. This research not only provides a reference for the ecological control of algae in the MRSNWDPC and similar mega hydro-projects but also enriches the application of the Vine Copula theory coupled with the random sampling method for multi-variable risk analysis.

Convective Velocity Perturbations and Excess Gain in Flame Response as a Result of Flame-Flow Feedback

Convective velocity perturbations (CVPs) are known to play an important role in the response of flames to acoustic perturbations and in thermoacoustic combustion instabilities. In order to elucidate the flow-physical origin of CVPs, the present study models the response of laminar premixed slit flames to low amplitude perturbations of the upstream flow velocity with a reduced order flow decomposition approach: A linearized G-equation represents the shape and heat release rate of the perturbed flame, while the velocity perturbation field is decomposed into irrotational and solenoidal contributions. The former are determined with a conformal mapping from geometry and boundary conditions, whereas the latter are governed by flame front curvature and flow expansion across the flame, which generates baroclinic vorticity. High-resolution CFD analysis provides values of model parameters and confirms the plausibility of model results. This flow decomposition approach makes it possible to explicitly evaluate and analyze the respective contributions of irrotational and solenoidal flows to the flame response, and conversely the effect of flame perturbations on the flow. The use of the popular ad hoc hypothesis of convected velocity perturbation is avoided. It is found that convected velocity perturbations do not result from immediate acoustic-to-hydrodynamic mode conversion, but are generated by flame-flow feedback. In this sense, models for flame dynamics that rely on ad-hoc models for CVPs do not respect causality. Furthermore, analysis of the flame impulse response reveals that for the configuration investigated, flame-flow feedback is also responsible for “excess gain” of the flame response, that is, the magnitude of the flame frequency response above unity.

Application of the proximal isovelocity surface area method for estimation of the effective orifice area in aortic stenosis

Although the echocardiographic effective orifice area (EOA) calculated using the continuity equation is widely used for the assessment of severity in aortic stenosis (AS), the existence of high flow velocity at the left ventricular outflow tract (LVOT) potentially causes its overestimation. The proximal isovelocity surface area (PISA) method could be an alternative tool for the estimation of EOA that limits the influence of upstream flow velocity. EOA was calculated using the continuity equation (EOACont) and PISA method (EOAPISA), respectively, in 114 patients with at least moderate AS. The geometric orifice area (GOA) was also measured using the planimetry method in 51 patients who also underwent three-dimensional transesophageal echocardiography. Patients were divided into two groups according to the median LVOT flow velocity. EOAPISA could be obtained in 108 of the 114 patients (95%). Although there was a strong correlation between EOACont and EOAPISA (r = 0.78, P < 0.001), EOACont was statistically significantly larger than EOAPISA (0.86 ± 0.33 vs 0.75 ± 0.29 cm2, P < 0.001). Both EOACont and EOAPISA similarly correlated with GOA (r = 0.70, P < 0.001 and r = 0.77, P < 0.001, respectively). However, a fixed bias, which is hydrodynamically supposed to exist between EOA and GOA, was not observed between EOACont and GOA. In contrast, there was a negative fixed bias between EOAPISA and GOA with smaller EOAPISA than GOA. The difference between EOACont and GOA was significantly greater with a larger EOACont relative to GOA in patients with high LVOT flow velocity than in those without (0.16 ± 0.25 vs - 0.07 ± 0.10 cm2, P < 0.001). In contrast, the difference between EOAPISA and GOA was consistent regardless of the LVOT flow velocity (- 0.07 ± 0.12 vs - 0.07 ± 0.15 cm2, P = 0.936). The PISA method was applied to estimate EOA in patients with AS. EOAPISA could be an alternative parameter for AS severity grading in patients with high LVOT flow velocity in whom EOACont would potentially overestimate the orifice area.

Thermalized collisional pre-sheath detected in dense plasma with coherent and incoherent Thomson scattering

In the direct vicinity of plasma-facing surfaces, the incident plasma particles interact with surface-recombined neutrals. Remarkably high near-surface plasma pressure losses were observed in the high-flux linear plasma generator Magnum-PSI. Combining the incoherent and coherent Thomson scattering diagnostics, we directly measured particle, momentum and energy fluxes down to 3 mm from the plasma target surface. At the surface, the particle and total heat flux were also measured, using respectively an in-target Langmuir probe and thermographic methods. The near-surface momentum and energy losses scale with density, and amount to at least 50% and 20%, respectively, at n e = 8 × 1020 m−3. These losses are attributed to the efficient exchange of charge, momentum and energy between incident plasma and surface-recombined neutrals. In low-temperature plasmas with sufficient density, incident particles go through several cycles of interaction and surface deposition before leaving the plasma, thereby providing an effective alternative dissipation channel to the incident plasma. Parallel plasma parameter profiles exhibit a transition with increasing plasma density. In low-density conditions, the plasma temperature is constant and near-surface ion acceleration is observed, attributed to the ambipolar electric field. Conversely, deceleration and plasma cooling are observed in dense conditions. These results are explained by the combined effect of ion–neutral friction and electron–ion thermal equilibration in the so-called thermalized collisional pre-sheath. The energy available for ambipolar acceleration is thus reduced, as well as the upstream flow velocity. In the ITER divertor, enhanced near-surface p–n interaction is expected as well, given the overlap in plasma conditions. Including these effects in finite-element scrape-off layer models requires a near-surface resolution smaller than the neutral mean free path. This amounts to 1 mm in Magnum-PSI, and possibly an order of magnitude smaller in ITER.

KAJIAN NUMERIK FENOMENA UNDULAR TIDAL BORES DALAM MEMPENGARUHI PROSES EROSI PADA DAERAH ALIRAN SUNGAI (Numerical study of undular tidal bores phenomenon in influencing erosion processes of watersheds)

Riverbank erosion is one indication of watershed damage. One of the causes is the phenomenon of tidal bores waves that occur in a river channel.The strength of tidal bores wave's can be measured based on its shear force parameter and dissipation energy. Wave shear force and dissipation energy are the parameters that play a role in a riverbank erosion process. Both of them are characterized by the Froude number (Fr) which is a function of the upstream river flow velocity (V0), the tidal bores flow velocity from the estuary (Vb), the river depth (h1), and the gravity acceleration (g). A numerical study of the phenomenon of undular tidal bores has been carried out in this article. Five undular bores simulations have been built using the open-source Computational Fluid Dynamics (CFD) software, OpenFOAM. This study aims to analyze the effect of the Froude number variations (Fr) on the magnitude of the wave shear coefficient (ϵ) and dissipation energy ( ) on undular bores cases. Five simulations of undular bores have been generated based on five Froude's numbers, Fr = 1.0, 1.1, 1.2, 1.3, and 1.4. The validation has been performed by comparing the experimental and numerical results from the scientific literature. The analysis results show that the increase in Fr has a significant effect on the increase in the ϵ and .These results indicate that the Froude number variations have influenced the wave shear coefficient and dissipation energy on the undular bores cases. Increasing the Fr values have triggered an increase in the value of ϵ linearly and exponentially. Thus, the erosion that occurs on the riverbank in the undular tidal bores phenomenon could be determined based on Froude's number.

Flow Behavior Behind A Freely Suspended Cylinder in the Wake of A Stationary Cylinder

The experiment of flow past a freely suspended circular cylinder in the wake of an upstream stationary cylinder was carried out in a re-circulating water channel using the Particle Image Velocimetry (PIV) technique. The upstream cylinder was fixed, while the downstream cylinder was suspended from a platform and allowed to move freely in the horizontal plane. The centre-to-centre spacing ratio between two tandem cylinders was initially kept at a constant value of 3.0. The instantaneous flow field and the orbital trajectories were analyzed to reveal the effect of the presence of the upstream cylinder and flow velocity on the dynamic response of the downstream suspended cylinder. The results showed that the upstream stationary cylinder has significant effect on modifying the flow patterns behind two tandem cylinders. Different trajectories of the downstream suspended cylinder with variation of flow velocity U were observed, such as: (1) depicting a figure-8 type motion at U = 0.27 m/s; (2) undergoing intermittent oscillations as it travels downstream at 0.3 m/s ⩽ U ⩽ 0.37 m/s; (3) successive moving downstream, no obvious streamwise oscillation observed at U = 0.43 m/s.

Water Energy Resource Innovation on the Cavitation Characteristics

Abstract The main purpose of this study is to numerically correlate the amount of generated vapor over a hydrofoil to the lift and drag coefficients acting on it. Cavitation characteristics were investigated of a hydrofoil in the cavitating, sub-cavitating, and non-cavitating flows for different angles of attacks (AoA) with the high upstream flow velocity. The hydrofoil was tested in a square water tunnel with water entering the tunnel at various velocities for each AoA ranges from 9.1 m/s to 12.2 m/s. It was found that lift and drag forces acting on the hydrofoil follow the trend of the experimental data quite closely. While the cavitation can be identified by a unique number (averaged vapor volume fraction), the work done created an inverse correlation between this number and the cavitation number at the same angle of attack. The lift force declines with the increase in the vapor content on the hydrofoil surface, meanwhile the drag force peaks at certain vapor volume fraction, and then, a huge reduction occurs with the considerable decrease in the corresponding cavitation number. A fourth-order correlation generated between the lift to drag (L/D) and the cavitation number (σ). It was found the lift-to-drag ratio decreases by the formation of the cavitation over the hydrofoil, thus causing a drop in the efficiency of the turbomachines.

Исследование влияния структуры потока на точность измерения расхода перед гидрометрическим сооружением в открытом русле

The reliability of water flow measurement in irrigational canals depends on the measurement method and design features of the flow-measuring structure and the upstream flow velocity profile. The flow velocity profile is a function of the channel geometry and wall roughness. The article presents the study results of the influence of the upstream flow velocity profile on the discharge measurement accuracy. For this, the physical and numerical modeling of two structures was carried out: a critical depth flume and a hydrometric overfall in a rectangular channel. According to the data of numerical simulation of the critical depth flume with a uniform and parabolic (1/7) velocity profile in the upstream channel, the values of water discharge differ very little from the experimental values in the laboratory model with a similar geometry (δ &lt; 2 %). In contrast to&#x0D; the critical depth flume, a change in the velocity profile only due to an increase in the height of the bottom roughness by 3 mm causes a decrease of the overfall discharge coefficient by 4…5 %. According to the results of the numerical and physical modeling, it was found that an increase of backwater by hydrometric structure reduces the influence of the upstream flow velocity profile and increases the reliability of water flow measurements.

High-velocity air-water flows downstream of sluice gates including selection of optimum phase-detection probe

High-velocity air-water flows occur commonly in hydraulic structures including low-level outlets. Novel experiments of air-water flow patterns and properties were conducted along a tunnel chute downstream of a high-head sluice gate for a wide range of flow conditions including Froude numbers between 10 and 45, Reynolds numbers up to 2.4 × 106 and flow velocities up to 22 m/s, limiting potential scale effects at close to prototype scale. The study documents the evolution of distribution shapes and characteristic values for a range of air-water flow properties along the tunnel chute. The results show an increase in flow aeration and bubble break-up processes with increasing upstream head and flow velocity. The measurement of air-water flow properties was conducted with a side-by-side double-tip conductivity probe, amongst others, which in a comprehensive comparative analysis of four types of phase-detection intrusive probe designs was identified as the most suitable one in high-velocity air-water flows as studied herein. The performance of the conductivity probe for the present flow conditions suggests that the probe may be a promising instrumentation for future prototype measurements of high-velocity air-water flows.

Effects of vegetation buffer strips on concentrated flow hydraulics and gully bed erosion based on in situ scouring experiments

AbstractVegetation on gully beds could intercept runoff, reduce incision of gully bed, and trap sediment from upstream area. The aim of this study is to evaluate the influences of vegetation buffer strips on concentrated flow hydraulics and gully bed erosion downstream of gully headcuts. Five gully head plots containing different buffer strip widths with similar vegetation density were constructed for in situ scouring experiments. Each plot received 11 tests with 2 discharges (i.e., 7 tests simulating 83.3 L min−1 and 4 tests simulating 166.7 L min−1). Buffer strips clearly decrease flow hydraulics (about 56–70%) and increase flow resistance (1.2 to 1.5 times) according to the selected hydraulic parameters. After 11 tests, total net erosion volume (NEV) and average sediment concentration from each gully bed showed an exponential decreasing relationship with increasing widths of buffer strips. All the selected hydraulic parameters showed a strong correlation with total NEV from the bare soil sections (0.43 &lt; R2 &lt; .61 with significance level reaching .01), whereas the relationship was very weak in the buffer strips. The gross deposition volume in buffer strips showed a linear correlation with upstream NEV (NEVu) and flow velocity (R2 = .91, p &lt; .001). Split flow effects expressed by braid index increased with increasing widths of buffer strips and scouring times. Both bare and vegetated sections suffered from erosion clearly, but the average NEV from buffer strips was about 30.2% of bare soil sections. This indicated the buffer strip effects on reducing erosion were more significant than trapping sediments were in this study.

Phenol Biodegradation by Pseudomonas putida in an Airlift Reactor: Assessment of Kinetic, Hydrodynamic, and Mass Transfer Parameters

An airlift biofilm reactor was employed to study phenol biodegradation by Pseudomonas putida. Hydrodynamic tests were also conducted in a conventional column to facilitate the comparison of the dynamic behavior in different types of columns. The three-phase airlift column offered better aeration than the conventional column as liquid and solid circulation in the downcomer favored bubble breakup, increasing oxygen dissolved in the liquid phase and favoring the phenol biodegradation process. Kinetic parameters of phenol biodegradation by P. putida were obtained in an agitated batch reactor, with the initial phenol concentration varying from 10 to 750 mg/L. Experimental data were fitted using different microbial growth models found in literature. The Yano and Koga model, which considers the formation of multiple inactive enzyme–substrate complexes, fitted well with our experimental data, with a correlation coefficient, R 2 = 0.952. An internal loop airlift bioreactor was used for aerobic phenol biodegradation in which polystyrene particles were utilized to support biomass immobilization. Several tests were performed by varying the influent phenol concentration, hydraulic retention time, upstream flow, and superficial air velocity. It was concluded that until an influent phenol concentration of approximately 300 mg/L, phenol acted as the limiting substrate. For higher phenol concentrations, oxygen became the limiting substrate. An increase in the oxygen concentration resulted in the complete consumption of phenol under high phenol concentration of 500 mg/L.

Higher-order modulation theory for resonant flow over topography

The flow of a fluid over isolated topography in the long wavelength, weakly nonlinear limit is considered. The upstream flow velocity is assumed to be close to a linear long wave velocity of the unforced flow so that the flow is near resonant. Higher order nonlinear, dispersive and nonlinear-dispersive terms beyond the Korteweg-de Vries approximation are included so that the flow is governed by a forced extended Korteweg-de Vries equation. Modulation theory solutions for the undular bores generated upstream and downstream of the forcing are found and used to study the influence of the higher-order terms on the resonant flow, which increases for steeper waves. These modulation theory solutions are compared with numerical solutions of the forced extended Korteweg-de Vries equation for the case of surface water waves. Good comparison is obtained between theoretical and numerical solutions, for properties such as the upstream and downstream solitary wave amplitudes and the widths of the bores.

Bubble size and flow characteristics of bubbly flow downstream of a ventilated cylinder

Bubbly flows downstream of a ventilated cylinder in a water tunnel are experimentally studied. Emphasis is placed upon the relationship between bubble property and carrier flow parameters. Under no-ventilation condition, the pure-water wake flow is measured with particle image velocimetry technique. Bubbles are generated with ventilation and the bubbly flow is visualized using shadow image velocimetry. The separation and statistical treatment of bubbles in the captured images are accomplished with an in-house code. The influence of upstream flow velocity and air flow rate is examined. Sauter mean diameter of the bubbles and bubble velocity distribution are obtained. Instantaneous bubbly flow pattern is in accordance with the carrier flow characteristics. Across the high-vorticity region, bubbles experience a remarkable bubble size variation, large bubbles are annihilated. As for cross-sectional bubble size distribution, the tendency obtained with image processing agrees with the result obtained with the formula associating turbulent kinetic energy dissipation with bubble size. As upstream velocity increases, the percentage of small bubbles increases. Both bubble volume fraction and the most predominant bubble size increase with air flow rate. The percentage of large bubbles varies slightly with air flow rate.