Upstream Wall Research Articles

WITHDRAWN: Utilization of filter for seepage phenomenon reduction under hydraulic structures

Abstract Water movement under hydraulic structures through a permeable layer of soil is considered as one of the main causes of failure of hydraulic structures. Therefore, this study aims to minimize the influence of water movement under hydraulic structures by using a filter. The case was simulated using the finite element analysis software ANSYS where many valuables were studied such as head of water, depth of upstream cut off wall, filter depth, and filter location. Such variables usually influence the uplift pressure, exit gradient and seepage discharge, which control the performance of the hydraulic structure. The results have shown that the position of filter at downstream leads to increases the seepage discharge and exit gradient, while at upstream, it reduces the uplift pressure significantly.

Impact of a wall downstream of an air curtain nozzle on air curtain separation efficiency

Air curtains (ACs) employ plane turbulent jets to separate two environments in terms of heat and mass transfer, while still allowing unrestricted access through the opening between these environments. Most previous studies focused on ACs discharged from nozzles located just above the opening. However, in some cases ACs have to be installed close to the ceiling at a substantial distance from the top of the opening. The AC blown downwards along the vertical wall then first resembles a wall jet and after reaching the top of the opening starts resembling a free jet. The present study analyzes the behavior and performance of an AC with upstream wall above the opening. 2D steady RANS CFD simulations are performed based on grid-sensitivity analyses and validation with experimental data for a wall jet and a free jet. The total opening height is 4 m and vertical walls of 0.5 m, 1 m and 2 m, partly closing this opening, are considered. AC performance is evaluated both with the separation efficiency η (based on infiltration) and the adapted separation efficiency η* (based on infiltration and exfiltration). It is shown that the presence of the wall reduces jet decay. The longer the wall, the larger the jet momentum over the opening height. This reduces infiltration and increases η, but it increases exfiltration and therefore decreases η*. In practice, the jet discharge velocity (jet momentum) will have to be adjusted to keep high η*.

The Effects of Upstream Wall Roughness on the Spatio-Temporal Characteristics of Flow Separations Induced by a Forward-Facing Step

Abstract Separating and reattaching turbulent flows induced by a forward-facing step submerged in thick oncoming turbulent boundary layers (TBL) developed over smooth and rough upstream walls were investigated using time-resolved particle image velocimetry. The examined upstream walls resulted in smooth, transitionally rough, and fully rough wall conditions. The upstream boundary layer thicknesses were 4.3 and 6.7 times the step height in the smooth and rough wall cases, respectively. The Reynolds number based on the step height and freestream velocity was 7800. The effects of upstream wall roughness on the mean flow characteristics, Reynolds stresses defined in both Cartesian and curvilinear coordinate systems, as well as the unsteadiness of the turbulent separation bubbles were critically examined. The results show that upstream wall roughness increases the boundary layer thickness and turbulence intensity and consequently, promotes early mean flow reattachment over the step. Distinct regions of significantly elevated vertical Reynolds normal stress and Reynolds shear stress were observed upstream of the step in the fully rough wall case compared to the smooth wall case. Proper orthogonal decomposition (POD) and the reverse flow area over the step were employed to investigate the unsteadiness of the separation bubbles. The first POD mode coefficient and the reverse flow area over the step were strongly correlated and exhibited the same dominant frequency.

MYB Transcription Factors and Its Regulation in Secondary Cell Wall Formation and Lignin Biosynthesis during Xylem Development.

The secondary wall is the main part of wood and is composed of cellulose, xylan, lignin, and small amounts of structural proteins and enzymes. Lignin molecules can interact directly or indirectly with cellulose, xylan and other polysaccharide molecules in the cell wall, increasing the mechanical strength and hydrophobicity of plant cells and tissues and facilitating the long-distance transportation of water in plants. MYBs (v-myb avian myeloblastosis viral oncogene homolog) belong to one of the largest superfamilies of transcription factors, the members of which regulate secondary cell-wall formation by promoting/inhibiting the biosynthesis of lignin, cellulose, and xylan. Among them, MYB46 and MYB83, which comprise the second layer of the main switch of secondary cell-wall biosynthesis, coordinate upstream and downstream secondary wall synthesis-related transcription factors. In addition, MYB transcription factors other than MYB46/83, as well as noncoding RNAs, hormones, and other factors, interact with one another to regulate the biosynthesis of the secondary wall. Here, we discuss the biosynthesis of secondary wall, classification and functions of MYB transcription factors and their regulation of lignin polymerization and secondary cell-wall formation during wood formation.

Lean Operation of a Pulse Detonation Combustor by Fuel Stratification

Abstract Pressure gain combustion is a promising concept to substantially increase the thermal efficiency of gas turbines. One possible implementation that has been frequently investigated is pulse detonation combustors (PDCs), as they permit stable and reliable operation. At the same time, the need for part-load operation and low NOx emissions requires combustion concepts in the lean regime. However, realizing lean combustion is still very challenging in PDCs since the deflagration to detonation transition (DDT) is very sensitive to the reactant composition. This work investigates an approach to realize lean combustion in PDC by applying fuel stratification experimentally. The scope is to find the necessary increase of fuel concentration inside the predetonation chamber to provide reliable DDT with respect to the overall equivalence ratio. Emission measurements in the exhaust of the PDC allow for a quantification of the NOx emissions as a function of the injected fuel profile. A valveless PDC test rig is used, which contains a shock focusing geometry for detonation initiation and is ignited by a spark plug close to the upstream end wall. The subsequent expansion of the burned gas and interaction of the flame front with turbulence leads to the formation of a leading shock inside the predetonation chamber, which is then focused inside a converging-diverging geometry. The successful initiation of a detonation wave by shock focusing is very sensitive to the pressure ratio across the leading shock, which can be influenced by initial pressure, reactant composition and flow velocity. Results reveal that fuel stratification allows for reliable detonation initiation at a global equivalence ratio of φglob=0.65, whereas repeatable successful operation with nonstratified fuel injection is limited to φglob≥0.85.

Experimental and numerical simulation of free surface flow over an obstacle on a sloped channel

In the present study, both experimental and numerical were conducted on a free surface flow over an obstacle. Numerical simulations were performed using the Renormalization Group (RNG-k-ɛ) based Reynolds-Averaged Navier–Stokes (RANS) turbulence model coupled with the Volume OF Fluid (VOF) method in FLUENT Software to investigate the effect of the channel slope on the flow pattern upstream, above and downstream the obstacle. Respectively, 5%, 7%, 8%, 10%, 20% and 50% channel slopes were considered. Numerical simulation has showed a good agreement compared against experimental results. Effect of the slope on the flow is observed particularly upstream of the obstacle where the flow takes the vertical direction after hitting the upstream wall. The more the slope becomes steeper, the higher the level of the water is. Recirculation zones in the case of a horizontal channel are elongated downstream the weir, whereas in the case of a sloped channel, they are localized just at the foot of the downstream wall.

Experimental study on rectangular cut-throated flume: Effects of flume walls slopes and channel longitudinal slope

This research conducted to study the flow through a rectangular cut-throated flume (RCTF). The flume is simply formed by placing two vertical triangular prisms (two vertical folded plates) on either side of a rectangular open channel. Both channel and flume cross-sections are rectangular. The investigated flume is inexpensive, easy to install and does not require high maintenance. A wide experimental investigation, carried out under free outflow conditions and under upstream subcritical flow regime to investigate the effects of the channel longitudinal slope, the flume throat width, and slopes of upstream and downstream flume walls on the stage-discharge relationship. The stage-discharge relationships were deduced by applying the dimensional analysis and were calibrated using the data of this study. The proposed stage-discharge equation for horizontal channel has an average absolute relative error of 2.97% with the relative errors restricted in the range of ±10%, and 80% of the data points are in the error range of ±5%. The proposed stage-discharge equation for sloping channel has an average absolute relative error of 3.97% with the relative errors restricted in the range of ±10%, and 66% of the data points are in the error range of ±5%. The measurements indicate that slopes of upstream and downstream walls affect the stage-discharge relationship of the CTFs only in sloping channels and flow discharge is not influenced by the flume walls slopes in a horizontal channel. The proposed unified stage-discharge equation for both horizontal and sloping channels has an average absolute relative error of 3.38% with the relative errors restricted in the range of ±10%, and 74% of the data points are in the error range of ±5%. The proposed stage-discharge model demonstrates favorable accurate and convenient estimation of discharge for flows through the CTFs.

Experimental study on two oil supplies control schemes of scramjet combustor

The two oil supplies control scheme test was carried out using the combustor model of scramjet as the test object, and the feasibility of the closed-loop fuel control using wall pressure as the controlled parameter was studied on the combustion test equipment of Northwestern Polytechnical University. In the closed-loop control test, the wall pressure of a certain point or two points at the suitable position was selected as the controlled parameters of kerosene control, and it was used as the feedback signal to adjust the kerosene flow in the two oil supplies test to achieve the expected wall pressure. The test results showed that: (1) the two-loops closed-loop fuel control test using the wall pressure of combustor as the controlled parameters achieved the expected purpose, and the scheme was feasible; (2) comparing the time when the pressure distribution of combustor tended to be stable, the time of two-loops closed-loop control was the shortest, the single-loop closed-loop control test time was slightly longer, and the two-loops open-loop control test took the longest time; (3) the pressure distribution on the upstream and downstream walls of the combustor were controlled separately in the two-loops closed-loop control with two fuel supplies test.

Analysis of surrounding rock anchoring effect under the condition of squeezed mountain

In the process of orogenesis, the geological conditions of vertical strata were formed due to the intense compressive tectonic movement, and lithologic stratification occurred in the rock mass of the plant along the horizontal plane and vertically upstream and downstream. The main powerhouse of a power station straddles two types of rock formations along the axis. Considering the lithological differences between the two ends, traditional support methods will inevitably fail to meet the requirements of safety and economy. In this paper, under the condition of systematic bolt support for the whole plant, prestressed anchor cables are added to the upstream and downstream side walls of the plant in the type III surrounding rock area to increase the stability, and the anchoring effect is analyzed and compared with the calculation of the burrow. The results show that the deformation around the cave in the surrounding rock areas of type II and III under unsupported conditions differs by 31.48%, and the depth of the plastic zone in the surrounding rock area of type III exceeds the anchoring depth of the bolt; After partition support, The horizontal displacements of typical cross-sections in the two types of surrounding rock areas have decreased by 8.66% and 17.11% respectively, the stress distribution is more uniform, the distribution range of the plastic zone in the downstream wall of the plant is significantly reduced, and the stability of the surrounding rock is guaranteed.

Microfluidic assay of antiplatelet agents for inhibition of shear-induced platelet adhesion and activation.

We have developed a microfluidic system to perfuse whole blood through a flow channel with an upstream stenotic region and a downstream protein capture region. This flow-based system was used to assay how effectively antiplatelet agents suppress shear-induced platelet adhesion and activation downstream of the stenotic region. Microcontact printing was used to covalently attach one of three platelet binding proteins [fibrinogen, collagen, or von Willebrand factor (vWf)] to the surface of the downstream capture region. Whole blood with an antiplatelet agent was transiently exposed to an upstream high wall shear rate (either 4860 s-1 or 11 560 s-1), and subsequently flowed over the downstream capture region where the platelet adhesion was measured. Several antiplatelet agents (acetylsalicylic acid, tirofiban, eptifibatide, anti-vWf, and anti-GPIbα) were evaluated for their efficacy in attenuating downstream adhesion. Following antibody blocking of vWf or GPIbα, downstream platelet activation was also assessed in perfused blood by flow cytometry using two activation markers (active GPIIb/IIIa and P-selectin). Acetylsalicylic acid demonstrated its inability to diminish shear-induced platelet adhesion to all three binding proteins. GPIIb/IIIa inhibitors (tirofiban and eptifibatide) significantly reduced platelet adhesion to fibrinogen. Antibody blocking of vWf or GPIbα effectively diminished platelet adhesion to all three capture proteins as well as platelet activation in perfused blood, indicating an essential role of vWf-GPIbα interaction in mediating shear-induced platelet aggregation.

Execution of Safe Blasting Under Adverse Conditions of a Powerhouse Complex: A Revisit to Sardar Sarovar Project, India

When the excavation of the underground powerhouse of the Sardar Sarovar Project, India was nearly complete, cracks were observed on the upstream and downstream walls of the powerhouse, and the installed instrumentation readings sounded an alert for the instability of the powerhouse cavern that could possibly derail the project, further excavation in the powerhouse cavern was halted. After completing stabilisation measures, the remaining underground excavations by drill and blast method were to be completed. This paper revisits case studies of controlled blasting for the remaining excavations, namely a construction ramp, turbine pits, draft tube tunnels connecting the powerhouse, and the concrete plugs erected at the exit ends of the draft tube tunnels. To ensure overall stability around the excavations, blast vibration was controlled by planning the excavations in proper sequences. The damage outside the planned line of excavations was controlled by adopting modified line drilling/smooth blasting techniques. The details of the sequence of excavations, drilling and blasting parameters, compiled from previous publications, are presented in this paper. This paper also describes the reasons why concrete plugs were erected in the draft tube tunnels, the details of the concrete plugs, the optimised drilling and blasting procedure for safe removal of the plugs, and the method adopted to quantify the damage.

Numerical modeling of fractional viscoelastic non-Newtonian fluids over a backward facing step – Buoyancy driven flow and heat transfer

The unsteady flow and natural convection heat transfer of fractional Maxwell viscoelastic fluid over sudden expansion geometry such as vertical backward facing step is studied. The time–space fractional derivatives are calculated based on Caputo fractional derivative. The nonlinear governing equations are solved using finite difference method mixed with a L-1 algorithm. The step, upstream and downstream walls were set at a constant temperature. The range of step length is 0≤S≤0.5 and dimensionless temperature is 0.25≤θw≤1. The effects of velocity and temperature fractional derivatives (αand β) and various physical parameters, such as step length, dimensionless temperature, on Nusselt number, friction coefficient, profile of velocity and temperature, are investigated numerically. The results reveal that the fractional derivative parameters, the variation of wall temperature and step length have significant effects on the rate of heat transfer, the friction coefficient, the velocity and temperature of backward facing step. The average of Nusselt number variation increases with increasing of β and decreases of α parameters. The results show that the velocity and thermal boundary layer of the non-Newtonian fluid (ordinary Maxwell) is higher than Newtonian fluid. Also, the Nusselt number for the ordinary Maxwell fluid flow is greater than the Newtonian fluid flow. With the increase of the step length of backward facing step, the average Nusselt number, friction coefficient, velocity boundary layer, and temperature boundary layer increases. The obtained results can improve significantly the performance of the systems design that operate with the non-Newtonian fluids.

Simulation of Convective Heat Transfer in 3D Forward Facing Step Using Various Turbulence Models

In this work, a modified solver from the OpenFOAM 4.1 software was used to study the fluid flow and heat transfer characteristics over a forward facing step (FFS) considering various turbulence models, viz., k-e , k-ω , k-ω SST and v 2 -f . Numerical computations were performed using a newly developed transient solver, pisoTempFoam. Modeling and meshing of the geometry and setting of the boundary conditions were done with OpenFOAM. The bottom (upstream, step and downstream) walls were heated at a constant temperature of 350K, while the fluid inlet temperature was 298K. The simulation results were compared with those available in the literature. Variation of skin friction coefficient ( C f ), coefficient of pressure ( C p ), and Nusselt number (Nu) for different Reynolds numbers (Re), contraction ratios (CR) and different fluids are presented. This article also presents information about recirculation bubbles in the upstream and downstream regions of the FFS. The results show that the combined effect of turbulence models and parameters, such as CR, Re and Pr, change the flow and heat transfer characteristics of the FFS. The present CFD simulation plays a pivotal role in the analysis of flow over airfoils at a large angle of attack in heat exchangers and pipes whose area suddenly changes.

Flow around a scoured bridge pier: a stereoscopic PIV analysis

We performed stereoscopic particle image velocimetry of the turbulent flow inside a scour hole around a cylinder in a sandy bed. At two planes, symmetry plane and 45^circ with respect to the approach flow, the flow and its turbulence structure were investigated. We used two Reynolds numbers (20, 000 and 39, 000) based on the cylinder diameter and the depth-averaged velocity in the symmetry plane. The flow is characterized by a strong down-flow in front of the cylinder, a large horseshoe vortex inside the scour, and an upstream directed wall jet underneath. The values of vorticity in the horseshoe vortex and of the velocity in the wall jet are larger than in a comparable configuration on a flat bed. Enhanced levels of turbulent kinetic energy are found around the horseshoe vortex and in the shear layer detaching from the rim. The orientation of the main axis of the velocity fluctuations changes when the flow enters the scour hole: from about wall-parallel in the detaching shear layer to vertical at the horseshoe vortex. The production of turbulent kinetic energy shows a maximum upstream of the horseshoe vortex centre with considerable production in the shear layer and in the wall jet underneath the horseshoe vortex. Furthermore, strong wall-parallel velocity fluctuations are visible in this region, and bimodal velocity distributions are found, but not anywhere else. The time-averaged wall-shear stresses are largest under the horseshoe vortex and most likely larger than in a corresponding flat-bed configuration.Graphic abstract

Numerical investigation on film cooling characteristics of slot-sectional diffusion holes combined with an internal cross-flow channel

Film cooling characteristics of several slot-sectional diffusion hole geometries were investigated in a perpendicular cross-flow channel using numerical methods. A traditional circle-sectional fan-shaped hole was compared as a baseline. Four slot-sectional hole geometries included two fusiform diffusion holes (upstream or downstream wall outer convex) and two rectangular diffusion holes (cross-sectional aspect ratio AS = 3.5 or 5.0). The cross-flow channel had an 8D × 8D square cross-section and divided into smooth, 45° rib and 135° rib channels. Compared with plenum-fed, the film cooling effectiveness profiles of the slot-sectional diffusion holes were all transformed to asymmetric bi-peak patterns under the influences of cross-flow. Compared with the fan-shaped hole, a considerable advantage in film cooling effectiveness was found, especially at moderate and high blowing ratios. The largest increase in spatially averaged effectiveness was 160%, 112% and 50% respectively in the smooth, 45° rib and 135° rib channel. The rectangular hole with a large cross-sectional aspect ratio always yielded the highest cooling effectiveness in the different cross-flow channels. ln the ribbed channels, the performance of the post rib hole was significantly higher than that of the pre rib hole. The discharge coefficients of the slot-sectional diffusion holes were slightly higher than the fan-shaped hole in the same cross-flow channel.

Effects of elapsed time on downstream platelet adhesion following transient exposure to elevated upstream shear forces

Transient exposure to elevated shear forces is known to prime platelets for enhanced downstream adhesion, but how far downstream these priming effects persist is not known. In the present study, the platelet capture regions, prepared by immobilizing fibrinogen, collagen, or von Willebrand factor, were placed at three different distances from the upstream stenotic region to vary the elapsed time of circulating platelets downstream. Platelet adhesion increased with the increase of upstream wall shear rates from 1620 s−1 to 11,560 s−1 for all three downstream proteins, but only the adhesion to fibrinogen increased significantly with the distance between the upstream stenotic region and the downstream capture region. In contrast, platelet adhesion to downstream collagen remained essentially independent on the distance and the adhesion to von Willebrand factor marginally increased with the distance after transient platelet exposure to upstream wall shear rates of 2145 s−1 and 11,560 s−1. The results implied that the activation of fibrinogen receptor GPIIb/IIIa by transient exposure to high upstream wall shear rates progresses in a time-dependent manner during the downstream flow of platelets. The highly elevated upstream wall shear rate of 11,560 s−1 altered the morphology of many platelets adhered to downstream fibrinogen from their native ellipsoidal to spread circular form. The platelet shape analysis showed that longer periods of post-stenotic flow increased the surface coverage fraction of ellipsoidal platelet population and decreased the surface coverage fraction of fully spread platelets on fibrinogen for both transiently elevated upstream wall shear rates.

Investigation of the effect of the different configurations of double-cutoff walls beneath hydraulic structures on uplift forces and exit hydraulic gradients

In the design of hydraulic structures, cutoff walls are needed to reduce uplift force (U) and the exit hydraulic gradient (GR). Usually two cutoff walls are required; an upstream cutoff wall is used to reduce the resultant of uplift force and a downstream cutoff wall is used to reduce the exit hydraulic gradient. This study numerically investigates double-cutoff walls beneath hydraulic structures with variations in their location and depth. Governing equations with specified boundary conditions are solved using the finite element method (FEM). Results show that if the downstream cutoff wall is deeper than the upstream cutoff wall, the resultant of uplift force will increase (compared to the no-cutoff wall case). With a constant hydraulic structure width (B), decreasing the distance between the two cutoffs (L), results in a reduction of uplift force. For L/B = 0.7, the change in the slope of the uplift force is large. Increases in the permeable depth (D) and reductions of B will lower the uplift forces. Increases in the downstream cutoff depth (d2) and L result in a reduction in the exit hydraulic gradient (GR). When the two cutoffs are located at the end of the hydraulic structure, GR is lower than when the downstream cutoff wall is located at a more downstream location. Comparisons between the available analytical solutions and computational results for two equal cutoffs show that the FEM can predict U and GR with a maximum of 5% error.

Decapod crustacean larval community structure of the submarine canyon off Blanes (NW Mediterranean Sea)

Decapod crustacean fisheries play a crucial role on the northwestern Mediterranean coast due to their high commercial value. Although knowledge of larval ecology and recruitment dynamics of these species is essential to establish a sustainable fisheries management, they are still poorly known. In this paper, we describe the composition, abundance and distribution of decapod crustacean larvae in the submarine canyon off Blanes (northwestern Mediterranean Sea) during summer thermal stratification conditions. Samples were collected in September 2011 with a multi-net system and a 60-cm bongo net at 22 stations with bottom depths of between 100 and 1800 m. A total of 635 larvae from 60 genera were identified. The most relevant taxa were Aristeus antennatus (7.93 individuals/1000 m3), the family Sergestidae (7.24) and Alpheus glaber (6.78). These three taxa were dominant ( > 20% of total decapod larvae) at more than half of the stations. Decapod larval communities were found to be richer and more diverse at the canyon head, a finding which could be explained by the higher retention rates when compared with the upstream and downstream walls and the canyon axis.

Electrokinetic‐vortex formation near a two‐part cylinder with same‐sign zeta potentials in a straight microchannel

Vortex formation near a two-part cylinder with zeta potentials of different values but the same sign under an external DC electric field is numerically investigated in this paper. The cylinder, inserted in a straight microchannel filled with an aqueous solution, is composed of an upstream part and a downstream part. When a DC electric field is applied in the channel, under certain conditions, the vortex will form near the cylinder due to the different velocities of electroosmotic flow generated on the cylinder surface. The numerical results reveal that the larger the velocity difference of electroosmotic flow generated on the two-part cylinder and the smaller the channel width, the more conducive to vortex formation in the channel. In addition, if the zeta potential ratios of cylinder downstream part to upstream part and channel wall to cylinder upstream part are unchanged, the DC electric field strength and the zeta potential value do not affect the pattern of vortices formed in the channel. This study provides a way for vortex formation in microchannels and has the potential application in microfluidic devices.

Experimental investigation of inclining the upstream wall of a scramjet cavity

The cavity flameholder has been widely used in scramjets to improve mixing and combustion rates. While many have investigated modifications to the cavity downstream-wall, few have investigated modifying the upstream-wall - a rear-facing step. The rapid expansion produces a low pressure region and base drag force, while only the larger of the induced twin vortices contributes substantially to mixing. In this paper, the upstream-wall of a scramjet combustor cavity was inclined, aiming to reduce cavity base drag and remove superfluous cavity vortex structures. Upstream-wall angles of 90°, 45° and 22.5° were examined, at cavity length-to-depth (L/D) ratios from 4 to 7. Cavity downstream-wall angles of 90° and 22.5° were examined for each configuration. Reflected shock tunnel experiments were conducted at Mach 7 enthalpy, scramjet combustor conditions. Experimental Schlieren imaging showed that reducing the upstream-wall angle did not significantly affect shear layer separation for L/D ratios of 4 and 5. At L/D ratios 6 and 7, however, reducing the upstream-wall angle saw the shear layer penetrating deeper into the cavity. Reynolds-averaged Navier-Stokes computations examined the internal flow structure and it was shown that reducing the upstream-wall angle to 45° retained the dominant mixing vortex in the cavity, with base drag reduced by 21% compared to the standard 90° upstream-wall case.