Suction Distribution Research Articles

Effects of aggregate size on water retention capacity and microstructure of lime-treated silty soil

Lime treatment is a common technique of improving the workability and geotechnical properties of soils. In this study, the aggregate size effects on the water retention capacity and microstructure of lime-treated soil were investigated. Two soil powders with different maximum aggregate sizes (Dmax = 0·4 and 5 mm) were prepared and stabilised by 2% lime (by weight of dry soil). Soil samples were prepared by compaction at dry side of optimum water content (w = 17%) with a dry density of 1·65 Mg/m3. Suction and pore size distribution were determined after different curing periods. The results obtained show that: (a) the treated soil with smaller Dmax presents relatively smaller modal sizes and lower frequency of macropores (10–330 μm); (b) lime addition effectively improves the soil water retention capacity and decreases both the modal sizes of macro- and micropores gradually over time. Moreover, a higher air entry value and larger water retention capacity were also observed for a smaller Dmax value, in agreement with the pore size distributions.

DLR natural and hybrid transonic laminar wing design incorporating new methodologies

AbstractIn the present work natural laminar flow (NLF) and hybrid laminar flow (HLF) wing designs are presented which were obtained by combining new methodologies with experience and knowledge obtained with traditional laminar wing design methods. The NLF wing design is performed for wing-body configurations with backward swept wing (BSW) and forward swept wing (FSW). Initial aerofoil sections were obtained by using a new sectional conical wing method which allows the design of transonic wing sections, taking into account the effects of sweep and taper for the computational cost of a 2D method. Except for flow regions with strong 3D influence, wings constructed with these aerofoils showed an acceptably large region with laminar boundary layer and small shocks at design and specified off-design conditions. For regions close to the body and the tip a 3D inverse design method was further required. For the BSW case, due to cross flow a premature transition occurred. Therefore, a HLF panel was required to obtain a larger laminar region. A suction distribution was obtained using the suction distribution module of the automated target pressure generator (ATPG). This generator optimises the pressure distributions in terms of minimising drag while keeping certain boundary conditions constant, e.g. lift and momentum. Using the ATPG, the laminar extent of the BSW NLF design could be further improved for the inboard wing. With the new methodologies design work was reduced. They lead to a design with reserves that allow for acceptable off-design performance qualities by keeping the wing laminar over a wide range of flight conditions.

불포화토 사면 안정해석에서 선행강우의 영향에 관한 연구

불포화토 사면 거동은 불포화토의 수리학적 특성 및 역학적 특성, 지반 투수계수, 지반 내 지층 구조, 강우조건 즉 강우강도, 강우패턴, 강우 지속시간, 선행강우 등의 영향을 받는다. 이와 같은 여러 요소 중 산사태는 사면 파괴 당시 강우조건보다 그 이전에 발생했던 선행강우의 영향을 크게 받는 것으로 알려져 있어 선행강우의 영향은 사면 안정성 해석에서 중요한 요소로 작용할 것으로 예상된다. 따라서 강우에 의한 산사태 발생을 예측하고 대책을 마련하기 위하여 선행강우에 따른 사면 내 모관흡수력 분포를 반드시 고려하여야 하며 이후에 발생한 강우특성을 반영하여 사면 안정성 검토를 수행하여야 할 것으로 판단된다. 본 연구의 경우 실트질 흙 함량 구성을 5%, 10%, 20%로 달리한 3가지 시료를 조성한 후 각각의 시료에 대하여 흙-함수특성곡선 시험을 수행하여 관련 매개변수들을 분석하였다. 한 달 동안의 선행강우 적용과 집중호우에 해당하는 12시간 동안의 강우가 지속적으로 내릴 때 불포화토 내 모관흡수력 분포 및 응력, 강도와 같은 역학적 특성 변화 등을 분석하고 사면안정 수치해석을 수행한 후 선행강우와 흙-함수특성곡선이 사면의 안정에 미치는 영향을 비교, 분석하였다. The behavior of the unsaturated soil slope can be influenced by the various factors such as the hydraulic characteristics, the mechanical characteristics, the coefficient of conductivity, the stratifications, the rainfall conditions i.e. the rainfall intensity, the rainfall pattern, the duration time of the rainfall and the antecedent rainfall etc. It is known that the slope failure is influenced greatly by the antecedent rainfall rather than the rainfall condition at the failure time, so the antecedent rainfall is supposed to be a very important factor in slope stability analysis among these factors. To predict and to prevent the slope failure by the rainfall, the distribution of the matric suction by the antecedent rainfall must be considered first of all and the slope stability analysis should be carried out by considering the successive rainfall characteristics. In this research, 3 samples with different quantity (5%, 10%, 20%) of silts were prepared and the SWCC (Soil-water characteristic curve) tests were carried out and the associated parameters were analyzed. After analyzing the distribution of the matric suction and the change of the mechanical characteristics such as the stress and the strength when applying the antecedent rainfall for one month and the successive intensive rainfall for 12 hours, the slope stability analyses were carried out numerically. And the influence of the antecedent rainfall for one month and the SWCC on the stability of a slope were compared and analyzed.

Field study on influence of root characteristics on soil suction distribution in slopes vegetated with Cynodon dactylon and Schefflera heptaphylla

AbstractVegetation evapotranspiration (ET) induced soil water suction reduces hydraulic conductivity and increases shear strength of slopes. Several field studies have been conducted to investigate suction distribution in vegetated slopes. However, these studies were conducted on natural slopes, which are prone to heterogeneity in vegetation and soil conditions. Moreover, studies quantifying the effect of different vegetation species, root characteristics (root depth and root area index) and transpiration reduction function (Trf) on suction in slopes under natural variation are rare. This study investigated the suction distribution and root characteristics in recompacted slopes vegetated with two different species, i.e. Cynodon dactylon (Bermuda grass) and Schefflera heptaphylla (ivy tree). Bare slope served as a control. Suction distributions during different seasons and rainfall events were monitored. It is found that during the dry season, slope vegetated with young Schefflera heptaphylla seedlings have substantially higher suction within the root zone compared with bare slope and slope vegetated with Cynodon dactylon. This is because Schefflera heptaphylla has a higher root biomass, Trf and ET than Cynodon dactylon. It was also found that suctions within root zones of vegetated slopes and bare slope were completely destroyed under rainfall events corresponding to 2 years and 20 years return period. Copyright © 2015 John Wiley & Sons, Ltd.

Investigation of soil density effect on suction induced due to root water uptake by Schefflera heptaphylla

AbstractAn increase in soil water suction due to root water uptake is significant for stability of slopes and landfill covers as it increases shear strength of soil by 50%. Coupled effects of soil density changes in soil hydraulic properties [i.e., soil water characteristic curve (SWCC) and hydraulic conductivity] and root characteristics (root depth, vertical root distribution) found in past experimental studies may affect soil water suction, which are not fully understood. The objective of this study is to numerically investigate these coupled effects of soil density by systematically comparing suction profiles under three different scenarios: (1) changes in only soil hydraulic properties, (2) changes in only root characteristics, and (3) changes in both soil properties and root characteristics. Silty sand with two different degrees of compaction (DoC) (i.e., 82% and 89%), which are typically found on man‐made slopes in subtropical regions, is considered. Schefflera heptaphylla is selected as vegetation species, which is commonly used for ecological restoration for man‐made slopes. A laboratory experiment was conducted to quantify suction distribution in silty sand vegetated with Schefflera heptaphylla at DoC of 89%. Vertical root distribution was measured using image analysis, while an axi‐symmetric finite element analysis was conducted to calibrate transpiration rate from measured suction. Root characteristics corresponding to DoC of 82% were then deduced using observations from literature. Richards equation coupled with sink term was numerically solved using Vadose/W to simulate water flow and root uptake on a flat ground (5 m depth, 10 m wide). Soil hydraulic parameters from a completely decomposed granite soil for two different DoCs (82% and 89%) were considered. Simulations assume constant transpiration rate for a period of 10 d. With increase in soil density from DoC of 82% to 89%, computed suction increases by 110% for scenario 1 (only effect on soil hydraulic properties), whereas it decreases by at least 21% for scenarios 2 (only root characteristics) and 3 (coupled effect). Here, a higher unsaturated hydraulic conductivity and a lower actual transpiration rate balance effects of a lower desorption rate of SWCC.

DYNAMIC RESPONSE OF PENGKENG LANDSLIDE TO RAINFALL AND ITS MONITORING AND EARLY WARNING

This paper takes Pengkeng landslide as a case study on the basis of the theory of unsaturated soil mechanics. The finite element method is used to calculate and analyze the slope seepage, deformation properties and stability under rainfall infiltration. The calculation results are utilized to study dynamic response mechanism of landslide due to rainfall and the relationship between safety factor and displacement under different real working conditions. Results show that the loose structure of the slip mass has provided a convenient condition for rainwater infiltrated. Thus it changes the spatial distribution of matrix suction and weakens the rock-soil mass parameters. The ground water level is risen under sustained rainfall. Deformation instability occurs in the zone of slope toe, then pulls the slope rear edge and causes the tensile fissures, which forms partial landslide in unsaturated area during rainfall period. In the initial stage of the heavy rainfall, the safety factor of the slope drops rapidly, which easily causes the landslide. The safety coefficient reduces further due to the continued infiltration of rainwater. Landslide occurrence after rainfall should be paid more attention. Safety factor is affected and vibrated with water in different parts of the slope. Finally, the concept of risk factor is proposed to reflect the harmful degree of slope under groundwater level and rainfall condition variation. The relationship between the risk factor and incremental displacement is established to describe the changes of surface displacement of slope measure point at different time periods. The above research and method will be useful and meaningful as reference for the management and early warning of the similar projects.

Influence of transport Layer on Transient Suction Distribution in a Two-Layered Slope

Residual soil slope failure due to rainfall infiltration is one of geotechnical hazards receiving much attention in many tropical climate countries. The infiltrating water eliminates matric suction in the residual soil slope and results in slope failure. A capillary barrier is used to prevent excessive rainfall infiltration and preserve matric suction in the residual soil slope and hence prevent rainfall-induced slope failure. A numerical study to examine the performance of a transport layer in a two-layered slope using capillary barrier principle was presented in this paper. Material properties of tropical residual soils consisting of Grade V (silty gravel) and Grade VI (sandy silt) were used and modelled a two-layered slope. These material properties were obtained from representative soil sample of Balai Cerapan slope in Universiti Teknologi Malaysia, Johor Bahru campus. A granite chips (Gravel) was also incorporated to act as a transport layer in the numerical model. The simulated slope model was then subjected to three different rainfall intensities of 9 mm/h (rainfall 1), 22 mm/h (rainfall 2) and 36 mm/h (Rainfall 3) representing short, medium and high intensity rainfalls, respectively. A total of six numerical schemes were performed by restricting the thickness of the transport layer to 0.1 m. However, to assess the effect of the transport layer thickness on suction distribution; the thickness was increased to 0.2 m. The results of the study show that inclusion of gravelly transport layer enables the top layer of fine sandy silt residual soil to retain the infiltrating water as a result of capillary break developed at the interface and also divert it above the interface towards the direction of the toe of the slope. Similarly the transport layer is found to be effective in preventing water breakthrough occurrence into the underlying coarser soil layer of the two-layered slope, especially when the thickness of the transport layer is optimum.

Analysis of physical testing of rainfall-induced soil slope failures

Rainfall is a significant factor that triggers slope failures around the world. This paper reports a series of physical tests, which were conducted to simulate rain-induced slope failures. The experiments dealt with two scenarios including (1) rainwater infiltration into the slope and (2) slope failures induced by artificial rainfall with different initial conditions. Slope deformation and slope failures were observed and possible mechanisms were interpreted based on the experimental results. The results confirm the hypothesis that pore-water pressure and water content in a loose soil slope change rapidly and that water infiltration into cracks in the slope has a great impact on landslide development. The observed slope failures can be divided into three types: overall sliding failure, partial sliding failure and flow slide. The effect of slope gradient, rainfall intensity and distribution of initial suction on the slope deformation and failure process are also summarized for possible applications under the similar conditions.

Precipitation Influence on the Distribution of Pore Pressure and Suction on a Coastal Hillside

The frequent interruption of roads in some freeways in Brazil due to slope failure has caused economic losses and potential harm to users. The paper presents data from a monitoring system installed at Serra do Mar in Santa Catarina State, Brazil. The slope monitored is called Morro do Boi and is located on BR-101 south, near the municipality of Itapema. It was stabilised using anchor and flexible metal mesh. Hence, all the pore water pressure measured is due to environmental changes. After approximately sixteen months of readings, it was possible to observe that the suction variation in the unsaturated layers presents a time delay in relation to the rainfall observed in the area. The ground water table presented a variation of about 1.5 m. The analyses of the data allowed establishing a trigger accumulated rainfall that reduces the negative pore water pressure to values below 10 kPa. So far, no significant positive pore water pressure has been observed above the ground water table.

Effect of Surface Flux Boundary Conditions on Transient Suction Distribution in Homogeneous Slope

Rainfall-induced slope failure is a common natural disaster that occurs frequently in many parts of the world. Many studies have been conducted on the effect of advancement of wetting front on reduction or elimination of the additional shear strength provided by matric suction in unsaturated soil. However, the inclusion of boundary flux condition such as evaporation in predicting soil suction for rainfall-induced slope failure analyses is still limited. Therefore, this study investigate the combined effect of two boundary flux (infiltration and evaporation) on transient suction distribution and hence the factor of safety (FOS) of a modelled slope which consist of slope height and horizontal distance of 5 and 47 m respectively with a slope angle of 21°. One year rainfall and evaporation data of a site in Johor Bahru, Malaysia were collected for the study and two months were selected to represent the wet and dry seasons based on the two predominant wind systems which brings rainfall in Malaysia (Northeast and Southwest monsoons). Soil samples were collected from the site for laboratory testing and the relevant soil properties required in the analyses were obtained. Initial soil condition was determined from Soil Water Characteristics Curve (SWCC) of the soil sample from the study area and these initial conditions were simulated at the beginning of each analysis to enable realistic condition of the soil to be obtained. The seepage analyses were conducted with Seep/w and the pore water pressure determined from the seepage analyses were used as input parameters in the slope stability analyses carried out with slope/w. The results of the analyses show that considering flux boundary condition has a great influence in maintaining negative pore water pressure in unsaturated soil especially during wet period when there is more water infiltrating the soil. Moreover, inclusion of evaporation in seepage analyses give more realistic pore water pressure and hence better prediction of FOS of the slope. Keywords: Evaporation, Flux Boundary, Rainfall, Slope Stability, Suction Distribution

Effect of surface flux boundary conditions on transient suction distribution in homogeneous slope

Rainfall-induced slope failure is a common natural disaster that occurs frequently in many parts of the world. Many studies have been conducted on the effect of advancement of wetting front on reduction or elimination of the additional shear strength provided by matric suction in unsaturated soil. However, the inclusion of boundary flux condition such as evaporation in predicting soil suction for rainfall-induced slope failure analyses is still limited. Therefore, this study investigate the combined effect of two boundary flux (infiltration and evaporation) on transient suction distribution and hence the factor of safety (FOS) of a modelled slope which consist of slope height and horizontal distance of 5 and 47 m respectively with a slope angle of 21°. One year rainfall and evaporation data of a site in Johor Bahru, Malaysia were collected for the study and two months were selected to represent the wet and dry seasons based on the two predominant wind systems which brings rainfall in Malaysia (Northeast and Southwest monsoons). Soil samples were collected from the site for laboratory testing and the relevant soil properties required in the analyses were obtained. Initial soil condition was determined from Soil Water Characteristics Curve (SWCC) of the soil sample from the study area and these initial conditions were simulated at the beginning of each analysis to enable realistic condition of the soil to be obtained. The seepage analyses were conducted with Seep/w and the pore water pressure determined from the seepage analyses were used as input parameters in the slope stability analyses carried out with slope/w. The results of the analyses show that considering flux boundary condition has a great influence in maintaining negative pore water pressure in unsaturated soil especially during wet period when there is more water infiltrating the soil. Moreover, inclusion of evaporation in seepage analyses give more realistic pore water pressure and hence better prediction of FOS of the slope.

Site monitoring of suction and temporary pore water pressure in an ancient landslide in the Three Gorges reservoir area, China

Rainfall is the main factor influencing the stability of landslides, but the precise mechanisms and processes are still poorly known. Unsaturated soil mechanics can be used to study how suction and temporary pore water pressure affect landslide stability. During rainfall, the groundwater level in the landslide will change, and the suction and temporary pore water pressure will also change with time and rainfall. Construction of a field observation well is an effective method of studying changes in suction. An observation well was constructed in an ancient landslide in the Three Gorges reservoir area in China. The well has a depth of 20 m, a diameter of 2 m, and is the largest observation well for monitoring of suction and temporary water pressure in the world. Suction, temporary water pressure, and rainfall were measured between October 2002 and December 2004; this is the longest observation period for this type of study. Analysis of the results shows that the distribution of suction is determined by the depth, soil materials, gradation, and structure of the landslide. The changes in suction are related to the intensity and duration time of rainfall, the season, and the climate. If the suction does not change promptly after rain, there is a time difference between the timing of rainfall and of suction change. The suction varied between 0 and 15 kPa. These conclusions form a basis for further scientific studies on how rainfall affects landslides.

Local wind pressures acting on walls of low-rise buildings and comparisons to the Japanese and US wind loading provisions

This paper examines peak external wind pressures acting on walls of low-rise buildings using various parameters. Results indicate that positive pressure distribution is relatively uniform regardless of parameters considered and its magnitude decreases as averaging area size becomes larger. Large suction pressure distribution occurs at lateral edges of walls and its magnitude does not average out with increasing averaging area size as rapidly as for positive pressure. Based on these observations, we find that zoning of wall area for design purposes is only necessary for wall suction pressures. We further find that it is more suitable to base zoning on building height to some extent. When the present results are compared with the design values in the AIJ Recommendations for Loads and Buildings in Japan, the design values significantly underestimate positive wall pressures for small averaging areas, but the degree of underestimation diminishes as area becomes larger. Similarly, design values underestimate negative wall pressures up to an area of 10m2, but begin to overestimate past that point. A similar observation was made for a comparison with ASCE7-10; however, the degree of overestimation of negative code values at larger areas was smaller than the one observed in the comparison with the AIJ Recommendations.

Numerical Studies on a Rotor with Distributed Suction for Noise Reduction

Minimizing the flow-induced noise is an important issue in the design of modern onshore wind turbines. There is a number of proven passive means to reduce the aeroacoustic noise, such as the implementation of serrations, porous trailing edges or the aeroacoustic airfoil design. The noise emission can be further reduced by active flow control techniques. In the present study the impact of distributed boundary layer suction on the noise emission of an airfoil and a complete rotor is investigated. Aerodynamic and aeroacoustic wind tunnel tests were performed for the NACA 64-418 airfoil and supplemented by numerical calculations. The aeroacoustic analyses have been conducted by means of the institute's Rnoise prediction scheme. The 2D studies have shown that noise reductions of 5 dB can be achieved by suction at moderate mass flow rates. To study the impact of three-dimensional effects numerical investigations have been conducted on the example of the generic NREL 5MW rotor with suction applied in the outer part of the blade. The predictions for the complete rotor provided smaller benefits compared to those for the isolated airfoil, mainly because the examined suction configurations were not optimized with respect to the extent of the suction patch and suction distribution.

Numerical studies on the effect of temperature on the unsaturated hydraulic response of geotextiles

A series of coupled thermo-hydraulic simulations were performed on a soil–geotextile column to understand the effect of temperature on suction distribution throughout the soil column and on the hydraulic performance of the geotextile as a drainage/capillary barrier layer. Two different constant temperatures of 0°C and 38°C and a temperature gradient of 4°C along the column were modeled. Changing the temperature from 0°C to 38°C did not have a significant effect on the suction head distribution in the soil–geotextile column. The temperature gradient resulted in appreciable thermal vapor flow and changes in suction head and hydraulic conductivity of the geotextile. During drainage, the temperature gradient and lower temperature at the top of the column increased suction in the geotextile and its ability to function as a capillary barrier. During capillary rise, the temperature gradient and lower temperature at the top of the column decreased the suction in the geotextile and its ability to function as a capillary barrier. Changing the direction of the thermal gradient reversed the water vapor flow direction and its effect on the suction in the geotextile. A temperature gradient did not have a noticeable effect on the suction head of the geotextile when positive pore pressure was developed in the geotextile and adjacent soil during drainage.

Effects of soil density on grass-induced suction distributions in compacted soil subjected to rainfall

Grass is recognized as being beneficial in reducing rainfall infiltration in some kinds of surface cover systems such as landfill cover, because rainwater discharges as surface runoff due to reduced water permeability caused by evapotranspiration-induced soil suction as well as foliage interception. However, the distributions of grass-induced suction in various compacted soils during rainfall are rarely reported. Moreover, it is not straightforward to determine an optimum soil dry density for minimizing rainfall infiltration and at the same time encouraging plant growth. This is because there are conflicting requirements for vegetated cover systems, i.e., compacted soil should not be too dense as to impede root growth, while on the other hand to minimize infiltration. This study thus aims to investigate, quantify, and compare grass-induced suction distributions in silty sand compacted at different densities when subjected to artificial rainfall in the laboratory. A grass species, Cynodon dactylon, which is common in many parts of Asia, was selected for testing. Compacted soil with and without a growing grass patch was tested at three relative compactions (RCs) of 70%, 80%, and 95%, in six test boxes. Test results reveal that at an RC of 95%, suction (40 kPa) retained in vegetated soil after rainfall is 100% higher than that (20 kPa) in bare soil. Among the vegetated soil compacted at the three RCs, suction retained was the highest at an RC of 95% (40 kPa), whereas suction decreased to 0 kPa at an RC of 70% after rainfall. As the average depth of grass roots decreased by 36% due to an increase in RC from 70% to 95%, the depth of influence of suction for vegetated soil at an RC of 95% reduced to less than half of root depth, which was the shallowest among the three compacted soil specimens.

Prediction of Seepage and Slope Stability in a Flume Test and an Experimental Field Case

As a participant to round robin test, Middle East Technical University (METU) team performed infiltration and slope stability analyses to simulate a laboratory flume infiltration test and a field experiment, and calculated, for both cases, changes in the suction distribution and in slope stability due to rainfall and evaporation. The overall methodology, the details of the numerical model, calibrations to assess accurate soil properties and estimation procedure for both cases are explained and presented in this study. Use of separate wetting and drying soil hydraulic properties are found to be necessary for proper infiltration simulations. For field cases, it is concluded that evaporation and its duration also plays a critical role and must be considered in numerical analyses.

Finite Element Solution of Unsteady Mixed Convection Flow of Micropolar Fluid over a Porous Shrinking Sheet

The objective of this investigation is to analyze the effect of unsteadiness on the mixed convection boundary layer flow of micropolar fluid over a permeable shrinking sheet in the presence of viscous dissipation. At the sheet a variable distribution of suction is assumed. The unsteadiness in the flow and temperature fields is caused by the time dependence of the shrinking velocity and surface temperature. With the aid of similarity transformations, the governing partial differential equations are transformed into a set of nonlinear ordinary differential equations, which are solved numerically, using variational finite element method. The influence of important physical parameters, namely, suction parameter, unsteadiness parameter, buoyancy parameter and Eckert number on the velocity, microrotation, and temperature functions is investigated and analyzed with the help of their graphical representations. Additionally skin friction and the rate of heat transfer have also been computed. Under special conditions, an exact solution for the flow velocity is compared with the numerical results obtained by finite element method. An excellent agreement is observed for the two sets of solutions. Furthermore, to verify the convergence of numerical results, calculations are conducted with increasing number of elements.

Three-dimensional aspects of cylinder drag reduction by suction and oscillatory blowing

An efficient and robust active flow control (AFC) system was applied to a nominally 2D circular cylinder, an archetype bluff-body configurations, with the purpose of drag reduction and wake stabilization. This is a fundamental cornerstone of a larger research program aimed at similar objectives but focusing on more realistic engineering applications such as heavy vehicles, rotorcraft fuselage and buildings subjected to cross-winds.The current study is focused on drag reduction by separation delay and manipulation of the natural vortex shedding regime, affected by the Suction and Oscillatory Blowing (SaOB) actuator, as part of the development of a new active flow control device for heavy vehicles aerodynamic drag reduction and fuel savings.The experiments were carried out in a low-speed, low-turbulence wind tunnel at cylinder diameter Reynolds numbers between 50,000 and 250,000, with smooth and rough surface conditions. With the rough surface, the Reynolds number became a weak parameter, between Re=100,000 and 200,000. Several key aspects of the AFC system operation and its interaction with the external-flow were tested: the number of actuators placed along the span of the model, which directly affect the energetic efficiency of the system and the influence of different actuation parameters such as the excitation magnitude, its frequency, suction distribution and phase relations between adjacent actuators. The measurements included steady and unsteady pressures on the model and in its wake. Optimal operational conditions were identified, leading to significant drag reduction (up to 60%), complete suppression of vortex shedding and increased overall system efficiency, by about 15%. The three-dimensional span-wise structure of the flow was examined and successfully utilized for enhanced energetic efficiency and significant drag reduction. The preferred wave-length was one to two cylinder diameters, in agreement with known low Reynolds number instabilities.

MULTIPLE LOADING EFFECTS ON WIND-INDUCED STATIC PERFORMANCE OF SUPER-LARGE COOLING TOWERS

Currently, four grouped 177 m super-large cooling towers, i.e. column-supported hyperboloidal shells, are to be constructed in a typical electric power plant in Southeast China. To this end, simultaneous pressure measurements on 1:200 rigid tower models are carried out in an atmospheric boundary layer (ABL) wind tunnel, aimed at accurately obtaining the external/internal cladding wind loads on these shells. The wind-induced static behavior of the cooling towers is analyzed by applying the wind loads acquired via the pressure model tests, using both linear elastic and nonlinear elastic finite element (FE) analyses. The corresponding responses (structural deformation, internal force and local buckling state) are compared with those obtained by the traditional design approach, focused on the effects of internal suction and external pressure distribution. Besides, the tower group interference effects are studied by comparing the results computed of a freestanding tower, with those of the tower groups during two different construction stages. The main findings about the loading effects on the static performance of the super-large cooling towers are helpful for improving the current Chinese Codes that govern the design of super-large cooling towers.