Flow reattachment on the roof of a 6 m cube

Five ultrasonic anemometers are used to measure flow velocities above the roof of a 6 m cube and at a reference point upstream. Various analysis techniques are applied to the data in order to illustrate the differences between: the mean reattachment position when the mean wind is normal to the windward face (0°), which is at x/h≈0.6; the median instantaneous reattachment position at x/h=0.66 and the reattachment position that would occur if the wind direction was held at a constant 0°, which is at x/h=0.75 and is also the position of zero conditionally averaged u velocity at instants when the v velocity is zero. It is also shown that the flow is highly unsteady and that the reattachment length varies from negligible separation to no reattachment, which occurs for 20% of the time. Some of these variations are related to fluctuations in the onset wind speed and direction, but they are also influenced by the dynamic response of the separation vortex system. The formation and shedding of vortices means that certain frequencies in the turbulence spectrum, around 1 Hz, are slightly amplified, whereas frequencies above 10 Hz are filtered out as a result of the inertia of the vortex system. The effects of reattachment length on the pressure distribution are briefly considered but it is shown that these do not account for the differences between the Silsoe field data and typical wind-tunnel results. It is suggested that the pressure differences may be related to Reynolds number, but it appears that this is not associated with changes in reattachment length.     

On the wake with and without vortex shedding suppression behind a two-dimensional square cylinder in proximity to a plane wall

A comparative study of the wake characteristics behind a two-dimensional square cylinder in proximity to a plane wall was made on two systems, i.e. G/D=0.25 and 0.5, which corresponds to the wakes with and without suppression of the vortex shedding, respectively. Here, G is the gap distance and D is the width of the square cylinder. Synchronized measurements of wall-pressure fluctuations and velocity fluctuations were made using a microphone array and a split-fiber film, respectively. For the system G/D=0.5, a regular Karman-like vortex shedding at fD/U₀=0.13 was found by inspection of the streamwise velocity spectra. When the gap width is reduced to G/D=0.25, a low-speed separation bubble was formed in the region 4.25<x/D<9.25, while the intermittent existence of large-scale vortices at the reduced frequency fD/U₀=0.116 was identified in the upper shear layer. The convective vortex dynamics were analyzed in terms of the cross-correlation of the wall-pressure fluctuation, coherence and cross-correlation of wall-pressure fluctuations with velocity fluctuations at various positions located at the lower edge of the cylinder (y/D=G/D) and the outer edge of the upper shear layer. The convection velocities of the Karman-like vortices in the system G/D=0.5 was determined to be U_c/U₀=0.64. However, in the system G/D=0.25 the convection velocities before and behind x/D=3.25 were determined to be U_c/U₀=0.32 and 0.64, respectively.     

An examination of the effect of boundary layer thickness on vortex shedding from a square cylinder near a wall

A computational study has been undertaken to examine the effect of boundary layer thickness δ/D on vortex shedding from a square cylinder in proximity to a solid wall. The computations were carried out in a second-moment turbulence modeling framework using a finite-volume technique. The computed results show that, in general, thickening of the wall boundary layer causes wake periodicity to persist for increasingly smaller cylinder-to-wall gap widths, S/D. The nature of the periodic motion changes as S/D approaches the critical value for complete suppression of vortex shedding. Similar to experimental observations, the location at which coupled shear layer motion is first observed shifts downstream of the base region. This shift is characterized by a rise in the shedding frequency and a drop in the time-averaged drag and lift on the cylinder. In addition, the pressure distribution along the lower wall is seen to change significantly due to the reduced size of the recirculation region in the cylinder wake.     

Full scale consolidation test on ultra-soft soil improved by prefabricated vertical drains in MAE MOH mine,Thailand

The reclamation of slurry pond with ultra-soft soil deposit using the prefabricated vertical drains (PVDs) with preloading technique in the Mae Moh mine, Lampang, Thailand is a challenging work and is illustrated in this paper. Geotextile reinforcement was used to strengthen bearing capacity of the soil foundation prior to the installation of sand platform. The delay of excess pore water pressure dissipation at the early loading stage occurred despite the occurrence of large settlements was a distinct behavior of the ultra-soft soil. Within the delayed time, the calculated average degree of consolidation based on measured settlements, U_s increased while the calculated average degree of consolidation based on measured excess pore water pressures, U_e and the undrained shear strength, S_u remained unchanged. Beyond the delayed time, both U_e and S_u increased significantly with time and when U_s > 90%, the difference between U_e and U_s was observed to be small. It was suggested to use U_s for approximation of S_u when U_s > 90% based on the SHANSEP's method. The successful installation and application of PVD to improve ultra-soft soil in this research is applicable for the ground improvement of similar problematic ultra-soft soils in international land reclamation projects.     

Non-linear effects on solute transfer between flowing water and a sediment bed

A previously developed model of periodic pore water flow in space and time, and associated solute transport in a stream bed of fine sand is extended to coarse sand and fine gravel. The pore water flow immediately below the sediment/water interface becomes intermittently a non-Darcy flow. The periodic pressure and velocity fluctuations considered are induced by near-bed coherent turbulent motions in the stream flow; they penetrate from the sediment/water interface into the sediment pore system and are described by a wave number (χ) and a period (T) that are given as functions of the shear velocity (U_∗) between the flowing water and the sediment bed. The stream bed has a flat surface without bed forms. The flow field in the sediment pore system is described by the continuity equation and a resistance law that includes both viscous (Darcy) and non-linear (inertial) effects. Simulation results show that non-linear (inertial) effects near the sediment/water interface increase flow resistance and reduce mean flow velocities. Compared to pure Darcy flow, non-linear (inertial) effects reduce solute exchange rates between overlying water and the sediment bed but only by a moderate amount (less than 50%). Turbulent coherent flow structures in the stream flow enhance solute transfer in the pore system of a stream bed compared to pure molecular diffusion, but by much less than standing surface waves or bed forms.     

Investigation of vortex-induced vibration of a suspension bridge with two separated steel box girders based on field measurements

This paper investigates the vortex-induced vibration of a twin steel box girder suspension bridge with a centre span of 1650 m based on field measurements. Two ultrasonic anemometers, two tri-axial accelerometers and 52 wind pressure sensors are installed at the quarter span section. The other 20 pressure sensors are installed in another 5 sections, and each section has 4 pressure sensors. Four vortex-induced oscillation events are measured. The analytical results indicate that the vortex-induced vibration more likely occurs in a low wind speed range of 6-10 m/s, with the wind direction nearly perpendicular to the bridge line, and low turbulence intensity. The mean pressure distribution on the surface of the bridge deck is obtained and the characteristics of fluctuant pressures are analysed by proper orthogonal decomposition (POD) method. Moreover, the spatial-temporal evolution of flow around the bridge deck is investigated. The results indicate that in the beginning stage of vortex-induced resonance, the regular vortex shedding phenomena occur only in the gap of the deck and at tailing region of downstream deck; however, when in the lock-in stage, the vortex shedding is strengthened due to the dramatic vibration, and the regular vortex shedding phenomena extend to the entire lower surface of downstream deck and the tail of upstream deck, the vortex shedding regions in the gap and lower surface link together. In the lock-in range, the span-wise correlation of the wind pressure is analysed, and the correlations of wind pressure along the bridge line are very high and do not decrease with the increase in distance.     

Shear strength response of reinforced pond ash

Reinforced pond ash is a composite material, which can be used as an alternative construction material in the field of geotechnical engineering. To study the shear strength response of reinforced pond ash, a series of unconsolidated undrained (UU) triaxial test has been conducted on both unreinforced and reinforced pond ash. In the present investigation the effects of confining pressure (σ₃), number of geotextile layers (N), and types of geotextiles on shear strength response of pond ash are studied. The results demonstrate that normal stress at failure (σ_1f) increases with increase in confining pressure. The rate of increase of normal stress at failure (σ_1f) is maximum for three layers of reinforcement, while the corresponding percentage increase in σ_1f is around (103%), when the number of geotextile layers increases from two layers to three layers of reinforcement. With increase in confining pressure the increment in normal stress at failure, Δσ increases and attains a peak value at a certain confining pressure (threshold value) after that Δσ becomes more or less constant. The threshold value of confining pressure depends on N, dry unit weight (γ_d) of pond ash, type of geotextile, and also type of pond ash.     

Effective stress law for the permeability of a limestone

The effective stress law for the permeability of a limestone is studied experimentally by performing constant-head permeability tests in a triaxial cell with different conditions of confining pressure σ and pore pressure p_f. Test results show that a pore pressure increase and a confining pressure decrease both result in an increase of the permeability, and that the effect of the pore pressure change on the variation of the permeability is more important than the effect of a change of the confining pressure. A power law is proposed for the variation of the permeability with the effective stress (σ′=σ–n_kp_f). The permeability-effective stress coefficient n_k increases linearly with the differential pressure and is greater than unity as soon as the differential pressure exceeds few bars. The test results are well reproduced using the proposed permeability-effective stress law. A conceptual pore-shell model based on a detailed observation of the microstructure of the studied limestone is proposed. This model is able to explain the experimental observations on the effect of the total stress and of the pore pressure on the permeability of the limestone. Effective stress coefficients for the stress-dependent permeability which are greater than one are obtained. It is shown that the controlling factor is the ratio of the different bulk moduli of the various constituents of the rock. This ratio is studied experimentally by performing microhardness tests.     

A computational model to calculate the flow-induced pressure fluctuations on buildings

A computational model to predict the flow-induced pressure fluctuation on bluff bodies is presented. Unlike direct and large-eddy simulation, the present model employs a stochastic model to generate plausible velocity fluctuations (synthetic turbulence) that satisfy the mean turbulent quantities such as turbulent kinetic energy (k) and dissipation energy rate (ε). This model has three main components: (1) prediction of mean flow quantities by solving the 3D Navier-Stokes equations using the standard k-ε model with Kato and Launder modifications; (2) generating a synthetic turbulent velocity field using a stochastic model and finally (3) solving the Poisson equation that governs the pressure fluctuations field. Flow around the low-rise building at Texas Tech was analyzed using the developed model. Two different wind angles of attack are considered for the analysis. Results obtained using the developed model are compared with wind tunnel and field measurements. The computed rms values for pressure fluctuations show good agreement with the experimental results.     

Interference effects on wind loading of a row of closely spaced tall buildings

Interference effects on a row of square-plan tall buildings arranged in close proximity are investigated with wind tunnel experiments. Wind forces and moments on each building in the row are measured with the base balance under different wind incidence angles and different separation distances between buildings. As a result of sheltering, inner buildings inside the row are found to experience much reduced wind load components acting along direction of the row (x) at most wind angles, as compared to the isolated building situation. However, these load components may exhibit phenomena of upwind-acting force and even negative drag force. Increase in x-direction wind loads is observed on the upwind edge building when wind blows at an oblique angle to the row. Other interference effects on y-direction wind loads and torsion are described. Pressure measurements on building walls and numerical computation of wind flow are carried out at some flow cases to explore the interference mechanisms. At wind angle around 30° to the row, wind is visualized to flow through the narrow building gaps at high speeds, resulting in highly negative pressure on associated building walls. This negative pressure and the single-wake behavior of flow over the row of buildings provide explanations for the observed interference effects. Interference on fluctuating wind loads is also investigated. Across-wind load fluctuations are much smaller than the isolated building case with the disappearance of vortex shedding peak in the load spectra. Buildings in a row thus do not exhibit resonant across-wind response at reduced velocities around 10 as an isolated square-plan tall building.     

Drag reduction on a circular cylinder using dual detached splitter plates

Drag reduction on a circular cylinder using dual detached splitter plates is numerically studied. Two splitter plates with the same length as the cylinder diameter (d) are placed along the horizontal centerline; one is upstream of the cylinder and the other is in the near-wake region, respectively. Their positions are described by the gap ratios G₁/d, G₂/d, where G₁ represents the gap between the cylinder stagnation point and the rear edge of the upstream splitter plate, and G₂ denotes the gap between the cylinder base point and the leading edge of the downstream splitter plate. The drag varies with the two gap ratios; it has the minimum value at a certain set of gap ratios for each Reynolds number. The upstream splitter plate reduces the stagnation pressure by friction, while the downstream one increases the base pressure by suppressing vortex shedding. This combined effect causes a significant drag reduction on the cylinder. In particular, the drag sharply increases past the optimum G₂/d; this is related to the restarted vortex shedding in the wake region.     

Poroelastic contribution to the reservoir stress path

Pore pressure/stress coupling is the change in the smaller horizontal stress σ_h associated with changes in pore pressure P, and has been measured in numerous reservoirs worldwide. These measurements suggest that the change in minimum horizontal stress Δσ_h is on average ca. 64% of the change in the reservoir pore pressure ΔP, but can be as low as 34% and as high as 118%. Conventionally it is assumed that the total vertical stress σ_v, given by the overburden, is not affected by changes in pore pressure, in contrast to the horizontal stresses σ_H and σ_h. We investigate analytically and numerically the spatio-temporal pore pressure and stress evolution in poroelastic media for continuous fluid injection at a point source, and calculate from the numerical modelling results the ratio Δσ/ΔP. Analytically, we show that the measured average of Δσ_h/ΔP can mathematically be deduced from the long-term limit of the spatio-temporal evolution of pore pressure and horizontal stress caused by fluid injection at a point source. We compare our numerical results to the analytical solution for continuous point injection into homogeneous poroelastic media as well as to Δσ_h/ΔP values measured in the field, and show that all stress components change with a variation in P. We use the concept of poroelasticity to explain the observed coupling between pore pressure and stress in reservoirs, and we consider different measurement locations and measurement times as one possible reason for the measured variation in Δσ_h/ΔP in different oil fields worldwide.     

Effect of shear displacement on the hydraulic conductivity of a fracture

The effect of shear displacement inclined relative to macroscopic water flow on the hydraulic conductivity of a rock fracture was estimated, using synthetic fractures that reproduce a tensile fracture in granite. The results showed that the hydraulic aperture normalized by the mean aperture increased with the angle between the directions of shear displacement and macroscopic water flow, according to a sinusoidal function of twice the angle. Formulae were established to estimate the hydraulic aperture of the fracture as a function of the mean aperture, the standard deviation of the initial aperture, the shear displacement, and the angle between the shear displacement and macroscopic water flow, based on results obtained in both this work and previous work, but neglecting scale effects. By assuming the mechanical properties of the fracture based on experimental results for granite, but neglecting scale effects, the hydraulic conductivity of the fracture with an arbitrary direction under a given state of stress (σ₁=29 MPa, σ₂=25 MPa and σ₃=13.5 MPa) was estimated for macroscopic water flow in the directions of both σ₁ and σ₂. When the contour map of the transmissivity of the fracture is plotted on a stereonet of the normal direction of the fracture in the principal axes of stress, there is a ridge (line of the local maximum) of transmissivity in the circumferential direction, and the inclination angle of the ridge from the σ₃-axis decreases with shear displacement, since shear dilation increases with both a decrease in normal stress and an increase in shear displacement. Furthermore, for the condition of stress given in this study, the transmissivity for macroscopic water flow in the direction of σ₁ is maximum for a fracture with a normal direction within the σ₂–σ₃ plane, while that in the direction of σ₂ is maximum for a fracture with a normal direction within the σ₁–σ₃ plane.     

Aerodynamic forces on structures of circular cross-section. Part 2. The influence of turbulence and three-dimensional effects

The review discusses the influence of turbulence and certain three-dimensional effects on the mean drag coefficient, the Strouhal Number and the root-mean-square lift coefficient. The data show that the nature of the effects of turbulence depends on the ratio of the scale of turbulence and the diameter of the cylinder. The effects of large scale turbulence are adequately predicted by quasi-steady theory. Small scale turbulence is better able to penetrate the shear layers around the cylinders and thus influence the mean and fluctuating pressure distributions, and also to cause premature transitions from one Reynolds number regime to another. Small scale turbulence can cause dramatic increases in fluctuating lift. The strength of pressure fluctuations due to vortex shedding is shown to be closely related to angle of separation.The data on the spanwise correlation of fluctuating lift display considerable scatter. While the data suggest that this parameter is sensitive to the effects of turbulence, aspect ratio etc. it is difficult to quantify the influences because of the limited data, particularly for high Reynolds numbers. The same is true of the data on the effects of finite aspect ratio. The latter subject is discussed in detail.     

Wind-tunnel modelling of the Silsoe Cube

1:40 scale wind-tunnel modelling of the Silsoe 6 m Cube at the University of Auckland is reported. In such situations, it is very difficult to model the full turbulence spectra, and so only the high-frequency end of each spectrum was matched. It is this small-scale turbulence that can directly interact with the local flow field and modify flow behaviour. This is illustrated by studying data from tests conducted in a range of European wind tunnels. It is recommended that spectral comparisons should be carried out by using turbulence-independent normalising parameter, such as plotting fS(f)/U² against reduced frequency f=nz/U. Using parameters such as the variance and integral length scale can easily mask major differences. It is noted that it is the size of the tunnel that limits the low-frequency end of the spectra, and so the longitudinal and transverse turbulence intensities were lower than in full scale. In spite of this similar pressure distributions are obtained. Some differences are observed and these are partially attributed to the reduced standard deviation of wind directions, which affects both the observed mean and peak pressures by reducing the band of wind directions occurring during a run centred on a particular mean direction. The reduced turbulence intensities also affect the peak-to-mean dynamic pressure ratio. However, since the missing turbulence is at low frequencies, the peak pressures appear to reduce in proportion. By expressing the peak pressure coefficient as the ratio of the extreme surface pressures to the peak dynamic pressure observed during the run, reasonable agreement is obtained. It is argued that this peak–peak ratio is also less sensitive to measurement system characteristics or analysis method, provided the measurement and analysis of the reference dynamic pressure is comparable with that used for the surface pressures.     

Experimental study of the interaction between a pair of circular cylinders normal to a uniform shear flow

The flow about two cylinders of diameter D, displaced in a plane normal to a uniformly sheared free stream with a centreline separation H, has been investigated experimentally. The freestream shear parameter (D/U_c)dU/dy was 1.48 × 10^?2, and the Reynolds number based on the freestream central velocity was 4.3 × 10⁴. Three distinct flow regions were observed: for 1.1<H/D<1.8 the flow through the gap between the cylinders is biased and unstable; for 1.8?H/D<2.0 the flow through the gap is biased and stable; and for 2?H/D the gap flow is not biased. The present paper describes the effects of mutual interference on the pressure distributions, the vortex-shedding frequencies and the interaction of the vortex streets for these three regions.     

Gust alleviation using rapidly deployed trailing-edge flaps

This paper presents the results of a numerical and experimental investigation into the use of a small, rapidly actuated, actively controlled trailing-edge flap (4% chord) to alleviate the unsteady loading experienced by wind turbine blades due to atmospheric turbulence and the atmospheric boundary layer.The computational investigation demonstrated that the rejection of realistic flow disturbances should be feasible with the use of load measurements on the blade and the feedback control of a small flap. The experimental prototype subsequently successfully rejected intentionally introduced flow disturbances from the vortex street of a square block located upstream of a sting-mounted, strain-gauged wing fitted with flap. This application of control provided a very significant reduction in the unsteady loading experienced (∼81% of C_LRMS).The findings show the potential of this method of load control for the rejection of unsteady aerodynamic loading by the sole use of load measurements from the wing for simple feedback control (PID/LQG).     

Numerical investigation on the three-dimensional unsteady flow past a 5:1 rectangular cylinder

This paper deals with the three-dimensional simulation of the unsteady flow around a stationary 5:1 rectangular cylinder at zero-degree angle of attack, low Mach number (M_∞=0.1) and high Reynolds number (Re=26,400, based on the plate thickness). Detached-Eddy Simulation (DES) was adopted as strategy of turbulence modeling. Results obtained with a hybrid mesh show satisfactory agreement when validated against experimental data and other computational results from the literature. Particular attention is devoted to the effects of the spanwise extension of the computational domain. Results show that the common choice of a spanwise period equal to the chord of the cylinder might not be enough to allow the natural loss of correlation of the pressure fluctuations and the free development of large-scale turbulent structures. The key role played by the amount of numerical dissipation, which is introduced by the second-order central difference scheme used to discretize the inviscid fluxes in the governing equations, is highlighted. The promising results obtained with DES for this benchmark test case suggest that this hybrid method is well suited for complex problems of high-Reynolds number bluff body aerodynamics and fluid-structure coupling.     

Critical ventilation velocity for multi-source tunnel fires

Ventilation is an effective method for controlling smoke during a fire. The “critical ventilation velocity” u_cr is defined as the minimum velocity at which smoke is prevented from spreading under longitudinal ventilation flow in tunnel fire situations. All previous studies on this topic have simulated fire scenarios in which only one fire source exists. This study conducted small-scale experiments and numerical simulations to investigate u_cr for cases in which two tunnel fires occur simultaneously. The tunnel was 4 m long, 0.6 m wide and 0.6 m tall. Three cases of two variously separated fires were experimentally explored and six cases were examined numerically. Both the experimental and simulation results indicated that for two identical fires, u_cr declines with separation. When the two fire sources are separate completely, u_cr can be determined by considering only a single fire. When the larger fire is upstream of the smaller downstream fire, u_cr also decreases with the separation. When two such fires sources are completely separate, u_cr can be evaluated by considering only the larger fire. The concurrent ventilation flow and flow of downstream smoke from the larger fire are strong enough to suppress the smoke flow from the smaller fire. However, when the smaller fire is upstream of the larger fire, the decrease in u_cr becomes insignificant as distance increases and the flow at u_cr must overcome the flow from both fires.     

考虑特征湍流影响的体育场悬挑屋盖脉动风压谱模型

基于风洞试验对体育场悬挑屋盖的脉动风压谱进行系统研究,旨在得到适用于此类结构的脉动风压谱模型,为风振响应分析提供必要的信息。通过对屋盖表面脉动风压进行谱分析,可知在屋盖前缘处的风压谱与来流风速谱较接近,但屋盖后缘处则差异很大,表现出明显的漩涡脱落特征。因此脉动风压自谱采用来流谱与漩涡脱落谱相结合的形式来描述,并通过权数因子体现屋盖表面不同位置处流场作用的特点。对于脉动风压互谱则用指数衰减函数来表示,并确定了适用于悬挑屋盖的衰减系数。为验证所提出风压谱模型的有效性及特征湍流对风致效应的影响,对系列悬挑屋盖结构进行风振响应分析,风荷载时程分别采用风洞试验测得的风压时程、基于建议风压谱模型模拟生成的风压时程、按拟定常假设生成的风压时程。基于建议模型得到的响应结果与试验结果基本一致,基于拟定常假设的风振响应极值偏小10%～15%,均方根值偏小30%～40%,脉动风压谱建模中不可忽略特征湍流的影响。