Increasing Blockage Ratio Research Articles

Study on the propagation characteristics of gas explosions disturbed by crushed rock in tunnels through gas-containing stratum

Gas explosions pose a serious risk to tunnel construction when through a gas-containing stratum. The crushed rock accumulated in tunnel could have a significant impact on explosion. To understand the mechanism of crushed rock influence on gas explosion, methane/air explosion experiments under the crushed rock with different blockage ratios and void ratios were carried out using an independently designed experimental system. The results show that the crushed rock can significantly affect the explosion intensity, and the various blockage ratios show a higher degree of influence than the void ratio. The crushed rock has the combined effect of accelerating and quenching on the flame, and there has an optimum blockage that dramatically accelerates the flame propagation. The needle-shaped, divergent and reversed flames appeared with high blockage ratio and void ratio conditions. The velocity ratio (ratio of the flame front velocities upstream and downstream of the crushed rock) has an exponential function relationship with the blockage ratio. Under certain void ratio conditions, the maximum explosion overpressure shows a parabolic trend with increasing blockage ratio. This study is helpful to guide the understanding of the explosion characteristics under practical tunnel conditions and the change law of flame structure with crushed rock obstruction.

Effects of Blockage Ratio on the Spatiotemporal Dynamics of Turbulent Flow Separation Around a Square Cylinder at Moderate Reynolds Numbers

Abstract The effects of blockage ratio (BR) on turbulent flows around square cylinders at moderate Reynolds numbers are investigated using a time-resolved particle image velocimetry (TR-PIV). The blockage ratios range from 2.5% to 15%, and the Reynolds numbers based on the freestream velocity and cylinder thickness are 3000, 7500, and 15,000. The flow dynamics are examined in terms of the mean flow, Reynolds stresses, frequency spectra, reverse flow area, and proper orthogonal decomposition (POD). The results show that the wake characteristics are nearly independent of the Reynolds number and blockage ratio. Spectral analyses of the velocity fluctuations demonstrate that the von Kármán (VK) shedding frequency is independent of the Reynolds number and blockage ratio, however, the Kelvin–Helmholtz (KH) frequencies increase with increasing Reynolds number and blockage ratio. The probability density function of the reverse flow area shows unimodal and bimodal distributions for the lower (BR ≤ 5%) and higher (BR ≥ 10%) blockage ratios, respectively, and the mean reverse flow area and its standard deviation decrease with increasing blockage ratio. The results also show that the contributions from the first POD mode pair to the total energy increase with blockage ratio but independent of the Reynolds number. The POD mode coefficients show significant cycle-to-cycle variation at lower blockage ratios, suggesting that the energetic structures are comparatively less organized at lower blockage ratios. The spectra of the velocity fluctuations, reverse flow area, and POD mode coefficients all show dominant peaks at the fundamental shedding frequency.

Influences of longitudinal ventilation and two same of obstacles placed upstream and downstream of the fire source on gas temperature distribution in a tunnel

In case of tunnel fire, vehicles can be blocked both upstream and downstream of the fire source. This paper presents a numerical study on influences of longitudinal ventilation and two same of obstacles placed upstream and downstream of the fire source on gas temperature distribution in a tunnel. The heat release rate, ventilation velocity and blockage ratio are varied. Results show that for a given ventilation velocity, as the blockage ratio increases, the inclination of fire plume gradually changes from the downstream to the upstream and the downstream smoke spread distance below the ceiling decreases. The Coanda effect (which refers to the adhesion of fluid on a convex surface) and the velocity vector field are introduced to explain the attachment of fire plume to the upstream obstacle. As the blockage ratio increases, the gas temperatures in the upstream of the fire source increase, while the gas temperatures in the downstream of the fire source decrease. The maximum temperature tends to decrease with increasing ventilation velocity. The effect of blockage ratio on maximum temperature is obvious for small ventilation velocities, while it becomes insignificant for large velocities. Then the maximum temperature model concerning the block effect and ventilation velocity are proposed.

Blockage slope and ratio effects on box culvert hydraulics

Culvert blockage at the inlet can dramatically alter the local hydraulic characteristics that often increase backwater rise, change the flow patterns, and thereby increase the risk of upstream flooding. The impact of culvert blockage is not usually included in the initial hydraulic design, although it can cause significant losses once the culvert becomes blocked. Culvert hydraulics including backwater rise with respect to blockage slope and ratio has been neglected in the studies conducted so far. In this study, we conducted a series of narrow flume tests to investigate the effects of four bottom blockage slope angles and five blockage ratios on the hydraulic characteristics of box culverts, including backwater rise, and upstream mean velocity at various Froude numbers. The results showed that Froude number, blockage ratio, and slope angle were regarded as the controlling variables for culvert backwater rise. Backwater rise increased dramatically with increasing blockage ratio and Froude number. Compared to blockage ratio B = 0.20, ΔH/hu increased by 2, 5, 9, and 13 times for B = 0.40, 0.60, 0.80, and 1, respectively. However, when the blockage slope angle decreased, the backwater rise reduced and upstream mean velocity increased in compared with vertical blockage. The relative backwater rise reduced from 0.32 to 0.29, when blockage slope angle decreased from 90° to 18.44° at B = 1 and Fo = 0.10. In addition, the length and maximum depth of water surface depression inside the culvert itself were larger with increasing blockage ratio and Froude number. For Fo = 0.10, d/ho = 0.13 and L/ho = 0.94 for B = 0.20 compared to d/ho = 0.32 and L/ho = 4.34 for B = 1. The results were also combined to develop an equation to predict backwater rise in the range 0.10 ≤ Fo ≤ 0.25, 0 ≤ B ≤ 1, and 18.44° ≤ α ≤ 90°. To enhance the accuracy, it is recommended to apply the equation for ΔH/hu ˃ 0.10. It is useful and practical in applications to add the probability of backwater rise or losses owing to culvert blockage within accounting of losses during the hydraulic design of culverts, especially in channels exposed to debris.

Numerical study on hydrogen heterogeneous reaction characteristic in a micro catalytic combustor with blunt body

A numerical investigation has been conducted to explore the effect of an inserted blunt body on the heterogeneous reaction (HTR) characteristics in a micro-catalytic combustor operating with premixed hydrogen/air. The two-dimensional numerical model incorporates a comprehensive heterogeneous reaction mechanism of hydrogen employed by experimental verification. The influences of blockage ratio at various inlet velocities have been investigated to obtain optimized design parameters of the combustor. The findings of this research highlight analysis of the positive influence of an inserted blunt body within the combustor, including enhancement of chemical reaction rate and hydrogen conversion ratio. Numerical results shows that the gaseous mixture between the blunt body and catalytic surface noticed a considerable velocity increase, leading to a greater adsorption–desorption of bulk species on the catalytic surface. At the same inlet velocity, the hydrogen conversion ratio gradually increases with higher blockage ratios. When the blockage ratio is less than or equal to 0.6, the average inner wall temperature is positively affected by the increased blockage ratio. However, the effect becomes negative at higher blockage ratios (0.8). The Pr (Prandtl number) number near the gas–solid interface adjacent to the blunt body initially decreases and then increases, while Nu/Nu0 (Nusselt number) and Pe (Peclet number) exhibit an initial increase followed by a decrease. The presence of the blunt body leads to a reduction in the cross-sectional area of the channel, causing an acceleration in fluid velocity near the blunt body and exerting an impact on the boundary layer near the gas–solid interface. This indicates that the addition of the blunt body reduces the boundary layer thickness and enhances the mass and heat transfer characteristics in the region. The study reveal that as the blockage ratio increases, the overall field synergy of the combustor initially increases and then decreases. The peak field synergy is attained at an optimal blocking ratio of 0.6. The findings of this study have important implications for the design of micro-scale combustors.

Study on aerodynamic characteristics and parameters of high-speed elevator car-counterweight intersection

When the high-speed elevator is running, the air in the hoistway is rapidly compressed and released to form the piston effect. Especially when the car and counterweight intersect, the piston effect makes the aerodynamic force of the car and counterweight change sharply, affecting the stability of elevator operation. In this study, a three-dimensional car counterweight intersection model is constructed, and a multi-region dynamic layered method is proposed to calculate the aerodynamic flow field in the whole moving process, the changes of aerodynamic drag force and aerodynamic lateral force of the car and the counterweight under different length ratio, blockage ratio and spacing are analyzed and simulated, and the influence of structural changes in the hoistway on the aerodynamic flow field is studied. Furthermore, the accuracy of the numerical results is verified by the real elevator test. Finally, the results show that when the car intersects the counterweight in the hoistway, the interaction between the car and the counterweight is enhanced, and the drag force coefficient and lateral force coefficient of the car suddenly change, and return to the normal state after the intersection, with the decrease of length ratio and spacing and the increase of blockage ratio, the sudden change degree of drag force also increases, and the sudden change of lateral force of the car is mainly affected by the car counterweight spacing, and increases with the decrease of spacing.

Inertial migration of rigid red blood cell particles in Poiseuille flow

Inertial migration of single and multiple red blood cell (RBC) particles in a tube Poiseuille flow is studied using the lattice Boltzmann method coupled with the Galilean invariant momentum exchange method. Different blockage ratios, Reynolds numbers, and solid volume fractions are considered. In the single particle case, the RBC particle migrating in tumbling and rolling modes is called the tumbling particle and rolling particle, respectively. Simulation results show that the tumbling particle has a higher equilibrium position and longer rotation period than the rolling particle. For both the tumbling and rolling particles, when the blockage ratio increases, the equilibrium position moves closer to the tube centerline, and the rotation period becomes longer. The higher Reynolds number strongly affects the angular velocity of the RBC particle, leading the RBC particle to rotate faster; however, the equilibrium position is insensitive to the Reynolds number based on the limited observation. In the multiparticle case, the second and third inner particle annuluses unexpectedly form inside the Segré–Silberberg annulus as the solid volume fraction increases. Furthermore, smaller particles form the perfect Segré–Silberberg annulus at a lower solid volume fraction than larger particles. The third inner annulus of smaller particles is located further from the tube centerline than that of larger particles. This work is beneficial for understanding the motion of RBC particles in Poiseuille flow. It can positively contribute to the design of microfluidic devices for focusing, separating, and transporting red blood cells.

Three-dimensional numerical investigation on flow behaviors around a diversion dike

Three-dimensional numerical simulations of a flow past a non-submerged diversion dike are performed for various blockage ratios (1/6 ≤ β ≤ 1/3) and Froude numbers (0.152 ≤ Fr ≤ 0.253). The instantaneous and time-averaged flow characteristics, the evolution process of mixing layers, and the bed shear stress of the dike are analyzed. The blockage ratio β plays an important role in the momentum redistribution and turbulent kinetic energy of the diversion dike. As the blockage ratio increases, the flow velocity gradient and the vortex shedding frequency in the mixing layer significantly increase. The Froude number Fr has little effect on the flow field. A large recirculation zone is observed in the wake of the dike, and the recirculation length is more depended on the length of the dike rather than the Froude number. The bed shear stress around the dike increases significantly with the increase in the blockage ratio.

Harvesting airflow energy from circular cylinder wake via a thin polyvinylidene fluoride film

To improve the characteristics of airflow energy harvesting, this paper presents a new piezoelectric wind energy harvester (PWEH), in which a polyvinylidene fluoride (PVDF) film was directly located in the wake of the circular cylinder. Four important parameters were independently varied for detailed experimental investigations: wind speed, spacing ratio for the distance between the center of the circular cylinder and the tip of the PVDF film to the circular cylinder diameter, diameter of the circular cylinder, and load resistance. The results indicated that the designed PWEH performed optimally with the occurrence of resonance, maintaining a relatively high output when significantly exceeding the resonance wind speed. The PVDF film inhibited the vortex shedding of the circular cylinder, which weakened with increasing spacing ratio. The optimum value of the spacing ratio should be approximately 2.5, which yields an effective coupling between the vortex and structure. The diameter of the circular cylinder played an important role in determining the output power. The match between the load resistance and PWEH also contributed to the high-level performance. Moreover, with an increase in the diameter of the circular cylinder, the increasing blockage ratio unexpectedly enhanced the vortex shedding frequency and decreased the theoretical resonance wind speed, which provided a new orientation for improving the performance of such PWEHs.

Numerical Investigation on The Effect of Grille Blockage Ratio on Air Flow Characteristics of Air Vents

Vents with louvers are an important component in the indoor heating, ventilation, and air conditioning (HVAC) system in providing a degree of freedom to the occupants to direct the air flow and prevent foreign objects from entering or exiting the air ducts. As a result, whether designing air ventilation or any duct design that involves louvers, the influence of louver design cannot be ignored. For vent design, aside from louver angle, blockage ratio is an important factor in air distribution due to its effect on pressure drop and flow distribution to indoor space. The blockage ratio is often described as the ratio of the projected area of the structure in flow direction to the cross sectional area of the domain around the structure. The purpose of this study is to investigate the effect of varying blockage ratios on the air flow characteristics of air vents including velocity and pressure drop. The chosen air vent model for this study is the Proton Wira air vent. Results are obtained using computational fluid dynamics (CFD) analysis and by utilizing the free and easily available open source software via Code-Saturne. The parameters were set according to available literature. The study finds that the pressure drop increases with the increasing blockage ratio. The velocity drops almost 7% for blockage ratio greater than 3 and the pressure drop increases more than 4%.

Flame acceleration and transition to detonation in non-uniform hydrogen-air mixtures in an obstructed channel with different obstacle arrangements

Numerical simulations were performed to study flame acceleration (FA) and deflagration-to-detonation transition (DDT) in non-uniform hydrogen-air mixtures in an obstructed channel. The fully compressible reactive Navier-Stokes equations coupled to a calibrated chemical-diffusive model were solved using a high-order numerical method on a dynamically adapting mesh. The simulations are in satisfactory agreement with previous experiments. For the cases with average H2 concentration 30 vol%, the presence of composition gradient weakens FA and causes delayed DDT due to lower total heat release since the unburned material is either fuel-rich or fuel-lean, although the composition gradient leads to more unburned pockets. The effects of obstacle arrangement and blockage ratio were explored in the mixtures with a linear gradient at 50 vol% average H2 concentration. The results show that FA and DDT can be promoted by increasing blockage ratio from 0.3 to 0.6 or placing obstacles along the sidewalls with lower H2 concentration because the formation and flame properties of unburned pockets are enhanced. An analytical model for predicting FA in fuel-rich non-uniform mixtures is suggested and validated against the numerical simulations. The model shows that flame acceleration rate mainly depends on blockage ratio and the thermal expansion ratio in unburned pockets.

Study on the Influence of Vent Shape and Blockage Ratio on the Premixed Gas Explosion in the Chamber with a Small Aspect Ratio.

Given the current situation of deflagration caused by gas leakage for domestic use and the study of the single shape of the vent, the differential change law of vent parameters on the small aspect ratio of the chamber explosion flame evolution and explosion overpressure and other effects were experimentally investigated. Based on the theory of geometric similarity, the experimental platform of a small length–diameter ratio explosion chamber was designed and built, and the premixed gas explosion experiment was carried out by changing the shape (square and rectangle) and blockage ratio (0.1, 0.3, 0.5, 0.7, and 0.9). The explosion flame structure, flame front position, flame propagation speed, explosion pressure waveform, overpressure peak value, and so on were tested and analyzed. The results showed that the blockage ratio had the most significant effect on flame propagation and explosion flame evolution. With the increase of blockage ratio, the stretching degree of the flame became more and more obvious, the flame front became sharper and sharper, and the sharp flame changed from the lower part to the upper part. The position of the flame front increased rapidly, and the steepness and peak value of explosion overpressure became larger. The oscillation of the flame propagation velocity was more violent after the turn, and the speed of propagation to the outside of the chamber gradually accelerated. In the same blockage ratio, there was a difference in the time point at which the flame propagation velocity turned under the rectangular and square shape of the vent. In blockage ratios of 0.1, 0.3, 0.5, and 0.9, the peak overpressure reduction under the shape of the blast square relative to the rectangle was 41.3, 47.9, 1.03, and 27.6%. This indicated that the explosion relief effect of the square was better than that of a rectangle. The research results can provide a reference and basis for the reasonable deployment of explosion venting and explosion decompression work.

Modeling spherical turbines for in-pipe energy conversion

In-pipe hydrokinetic systems employing spherical turbines that operate in confined flows with high blockage ratio are a relatively new technology. The number of modeling and computational studies analyzing the performance of spherical turbines operating in confined area is quite limited. The main objective of this study is to model a novel in-pipe cross-flow turbine and perform numerical analyses. Accordingly, a five bladed spherical rotor was designed and numerical simulations were conducted for various inlet velocities (0.8–6.5 m/s), tip speed ratios (0.6–5.2) and blockage conditions (0.59–0.83). The relationships between geometric, dynamic parameters and power performance were discussed. The findings showed that increasing blockage ratio significantly improved the power performance by shifting the CP-λ curve to the right-upward direction. The output power was maximized by a factor of 2.5 where the pipe diameter is reduced from 1.3 to 1.1 times the turbine diameter. A linearly increasing relationship was achieved between power coefficient and blockage where the angle was proportional with λ. Optimum λ was obtained around 4.2 at high blockage where it was mostly between 1.5 and 2 in free flow turbines. This research is expected to contribute to spherical turbine studies for optimal power generation and energy efficiency in water transmission pipelines.

Effects of obstacle layout and blockage ratio on flame acceleration and DDT in hydrogen-air mixture in a channel with an array of obstacles

Numerical simulations were performed to study flame acceleration and deflagration-to-detonation transition (DDT) in hydrogen-air mixture in a channel with a two-dimensional array of cylindrical obstacles. A high-order numerical algorithm with adaptive mesh refinement was applied to solve reactive Navier-Stokes equations. The effect of obstacle layout was examined by considering three layouts at a fixed blockage ratio of 0.5: staggered, inline-concentrated, and inline-scattered. Three blockage ratios, 0.33, 0.5, and 0.67, were used for the case of staggered obstacles to explore the influence of blockage ratio. The results show that both obstacle layout and blockage ratio have significant effects on flame acceleration and DDT occurrence, although the basic mechanism of detonation initiation is consistent for all the cases involving shock focusing. In the staggered case, the head-on collisions of flame and pressure waves with obstacles greatly promote the growth of flame surface area and thus lead to the fastest flame acceleration and shortest detonation onset time. In the inline-concentrated case, flame propagates slower than that in the staggered case due to smaller flame surface area. However, compared to the zigzag path in the staggered case, the straight passages parallel to flame propagation direction in the inline-concentrated case are more conducive to producing strong shock focusing and thus result in the shortest DDT distance. In the inline-scattered case, the straight passage along the centerline of channel facilitates the early acceleration of flame, but it has the slowest flame propagation in lateral directions and thereby the longest DDT time and distance. For the staggered obstacles at different blockage ratios, flame acceleration rate increases with increasing blockage ratio. The occurrence of DDT is hindered by the most congested obstacles, because the shock focusing is insufficiently strong to initiate detonation after passing through the excessively narrow gaps.

Numerical simulations of gas explosion using Porosity Distributed Resistance approach Part −1: Validation against small-scale experiments

A new model, PDRFOAM, has been developed for the prediction of gas explosions in congested plant. It uses the PDR approach to model the effect of small-scale obstacles, e.g., pipes and vessels on flame propagation and on explosion overpressure. While the effects of large obstacles i.e., that is the large scales are explicitly resolved. The equations for mass, momentum, enthalpy and Favre-averaged regress variable are solved. In addition, porosity modified standard k-ε turbulence model and the transport equations for the flame wrinkling parameter are solved. The model PDRFOAM, is built as a new application in OpenFOAM, suite of models. OpenFOAM is an open-source CFD package of routines for solution of systems of partial differential equations.In addition, to the PDRFOAM model, the CADPDR program was developed which generates various fields (volume blockage, area blockages, surface area, sub-grid drag and turbulence generation parameters) needed by PDRFOAM. CADPDR needs as input obstacle files that list coordinates and dimension of the obstacles e.g. pipes and vessels. The PDRFOAM code solves porosity-modified momentum and continuity equations with sub-grid source terms. The combustion model in PDRFOAM is based on flame area transport. The turbulent burning velocity correlation used is based on Markstein and Karlowitz number. Flame area generation due to the folding of the flame around obstacles is explicitly modelled.This paper presents the formulation of PDRFOAM and validation of the PDRFOAM code against three series of small- and medium-scale experiments i.e., ERGOS, MERGE and Buxton S-Series experiments. A total of more than 150 experiments were used for validation which includes variation in blockage ratio, grid pitch, size of the congested region, equivalence ratio, confinement, partial fill, obstacle diameter, different obstacle shapes and three different fuels. The model is compared against flame position, flame speed as a function of time and maximum overpressure obtained from experiments. Simulations predict the experimental trends of increase in overpressure with increase in blockage ratio, laminar burning velocity, partial fill of the gas cloud, size of the congested region, confinement, grid pitch and obstacle diameter. It also predicts the trends of increase in overpressure with increase in equivalence ratio, variation of maximum overpressure with ignition location, change in obstacle shape and obstacle configuration. The maximum overpressure predicted by simulations (see Fig. 1) is in general within the uncertainty of a factor of two.

Effect of the blockage ratio on the smoke extraction efficiency in tunnel fires with natural ventilation

Natural ventilation by shafts is widely applied in urban shallow tunnels due to its simple structure, steady smoke extraction efficiency and environment-friendly means. In order to investigate the influence of vehicle blockage on the smoke extraction efficiency of the shaft in tunnel fires, a series of n-heptane fire experiments were conducted in a 1:15 small-scale tunnel model with seven blockage ratios (0, 0.16, 0.24, 0.32, 0.40, 0.48, 0.56). The smoke extraction efficiency is calculated by the measured smoke temperature and CO volume fraction in the shaft and in the tunnel near the exhaust vent. Results show that the smoke extraction efficiency of the tunnel with shafts decreases firstly, then increases and drops lastly with the increasing blockage ratio. The smoke temperature in the shaft and at the downstream tunnel change similarly, which indicates that the vehicle blockage can affect the thermal energy discharged by a shaft. The main influence factors are the blocking effect, the necking effect caused by the blockage and the stack effect of the shaft, resulting in the variability of the smoke extraction efficiency. The smoke extraction efficiency reaches the maximum around the blockage 0.48 in the present tunnel, which can be referred to design the tunnel cross section in engineering applications.

Experimental investigation of turbulent flow under different Reynolds numbers and blockage ratios in a heated rectangular channel

Particle Image Velocimetry (PIV) technology is used in current paper to investigated turbulent flow of central blocked accidents in a heated rectangular channel. Proper Orthogonal Decomposition (POD) method and Power Spectral Density (PSD) method are also used to expose the turbulent characteristics in the wake. The velocity field at different Reynolds numbers and blockage ratios is analyzed. During the blockage experiments, the movement of eddies are captured in detail in the instantaneous flow field. The vortex gradually decreases with the increase of Reynolds number, but gradually increases with the increase of blockage ratio. The dominant vortex structures in the first two POD modes gradually become sharper at higher blockage ratio. Subsequently, the peak frequencies of the instantaneous flow field are the same as those of POD modes at various blockage ratios.

Experimental study on the lock-in of an oscillating circular cylinder confined in a plane channel

The results of an experimental study concerning wall boundary effects on the lock-in behavior of the shedding vortex of a circular cylinder, which is forced to oscillate transversely in a uniform flow at moderate Reynolds number Re = 2000,are presented in this paper. A height-adjustable flow channel is designed to study the boundary effect. Two sets of experiments are organized to study the lock-in behavior by changing the blockage ratio and oscillation amplitude separately. When the blockage ratio exceeds a critical value, the boundary effects are observed and the Strouhal number of the stationary cylinder increases non-linearly. As the blockage ratio increases, both the lower and upper limits of the lock-in region move to the lower value of excitation frequency, and the extent of the lock-in region decreases first and then increases. The boundary changes the effects of oscillation amplitude on lock-in behavior. When free from the boundary effect, as the oscillation amplitude increases, a V-shaped boundary of the lock-in region is observed, and the extent of the lock-in region increases monotonically. In comparison, under the influence of the boundary, the extent of the lock-in region first increases with the increase of the oscillation amplitude, and then decreases with the increase of the oscillation amplitude. Hysteresis loops are first observed near the lower limits of the lock-in region for some ranges of blockage ratio and oscillation amplitude in a forced-vibration system, and the extent of the hysteresis loop is affected by the two parameters.

Some new characteristics of the confined flow over circular cylinders at low reynolds numbers

This study summarises some new characteristics of the fluid flow over a confined circular cylinder at low Reynolds numbers. Results from both two- and three-dimensional direct numerical simulations are presented at blockage ratio between 0.1 and 0.9 and Reynolds number between 120 and 500. Floquet stability analysis of selected cases will also be presented. From the two-dimensional simulations, it is found that the fluctuating lift forces decreases with blockage ratio and becomes zero (where the flow is steady) at blockage ratio of approximately 0.7–0.8. Upon further increasing the blockage ratio to 0.9, the simulations show a dramatic increase in the fluctuating lift forces, nearly an order of magnitude greater than previously reported for an unconfined cylinder flow. It is also found that for blockage ratio of 0.5, there is a long term two-dimensional instability that becomes more prominent with increasing Reynolds number. This instability has a time scale of approximately 105 time units (D/Umax) at Reynolds number of 500. In addition, the transition between two- and three-dimensional flow at blockage ratios up to 0.5 is investigated. It is shown that the transition Reynolds number decreases with increasing blockage ratio. At high blockage ratio of 0.5, as we increase the Reynolds number, the transition to three-dimensional flow is shown to go from unsteady two-dimensional to steady three-dimensional before transitioning to unsteady three-dimensional flow.

Flood attenuation hydraulics of channel-spanning leaky barriers

Natural flood management aims to enhance natural processes to build resilience into flood risk management alongside hard engineering methods of flood defence, using ‘soft engineering’ methods such as leaky barriers. This study addresses the research gaps pertaining to the backwater effects of different leaky barrier designs and the physical characteristics that determine the extent of flood attenuation. Porous and non-porous leaky barrier designs, which varied by longitudinal length, blockage ratio and log arrangement, were tested in a laboratory flume with a compound channel cross-section. Flow area afflux (defined as the upstream increase in flow area caused by the leaky barrier compared to the uniform flow condition without the barrier) and headloss were used to quantify the backwater effects of the leaky barrier under 80 and 100% bankfull discharges. For inbank flows, leaky barrier longitudinal length and cross-sectional blockage ratio governed head loss and drag coefficients, which were higher for non-porous than for porous leaky barriers. The cross-sectional blockage ratio was the primary factor increasing area afflux, indicating that leaky barrier designs which maximise channel obstruction will result in higher flood attenuation. Longitudinal length had a limited effect on stage and area afflux, unless it was accompanied by an increase in blockage ratio, especially for the non-porous structures. The use of uniformly distributed logs resulted in equal or higher area afflux than the more physically complex barriers that used varied log orientations. The non-porous structures resulted in at least twice the area afflux compared to their porous counterparts, indicating that over time, accumulation of organic matter and sediments, which render the barriers more watertight, will enhance backwater effects, flood storage and downstream attenuation.