Value Of Pressure Coefficient Research Articles

Aerodynamic Performance and Numerical Validation Study of a Scaled-Down and Full-Scale Wind Turbine Models

Understanding the aerodynamic performance of scaled-down models is vital for providing crucial insights into wind energy optimization. In this study, the aerodynamic performance of a scaled-down model (12%) was investigated. This validates the findings of the unsteady aerodynamic experiment (UAE) test sequence H. UAE tests provide information on the configuration and conditions of wind tunnel testing to measure the pressure coefficient distribution on the blade surface and the aerodynamic performance of the wind turbine. The computational simulations used shear stress transport and kinetic energy (SST K-Omega) and transitional shear stress transport (SST) turbulence models, with wind speeds ranging from 5 m/s to 25 m/s for the National Renewable Energy Laboratory (NREL) Phase VI and 4 m/s to 14 m/s for the 12% scaled-down model. The aerodynamic performance of both cases was assessed at representative wind speeds of 7 m/s for low, 10 m/s for medium, and 20 m/s for high flow speeds for NREL Phase VI and 7 m/s for low, 9 m/s medium, and 12 m/s for the scaled-down model. The results of the SST K-Omega and transitional SST models were aligned with experimental test measurement data at low wind speeds. However, the SST K-Omega torque values exhibited a slight deviation. The transitional SST and SST K-Omega models yielded aerodynamic properties that were comparable to those of the 12% scaled-down model. The torque values obtained from the simulation for the full-scale NREL Phase VI and the scaled-down model were 1686.5 Nm and 0.8349 Nm, respectively. Both turbulence models reliably predicted torque and pressure coefficient values that were consistent with the experimental data, considering specific flow regimes. The pressure coefficient was maximum at the leading edge of the wind turbine blade on the windward side and minimum on the leeward side. For the 12% scaled-down model, the flow simulation results bordering the low-pressure region of the blade varied slightly.

Estimating pore pressure in tight sandstone gas reservoirs: A comprehensive approach integrating rock physics models and deep neural networks

Pore pressure serves as an important driving power for subsurface fluid migration and therefore has a significant impact on gas accumulation and enrichment in tight sandstone reservoirs. Tight gas is typically produced in overpressure regions, where pressure coefficients are notably elevated. Thus, it is crucial to establish an effective methodology for precise pore pressure estimation. This study introduces an approach to improve pore pressure prediction by incorporating rock physical modeling and deep neural networks (DNNs) into the classical Eaton method. Compared to conventional techniques relying on empirical correlations between pressure coefficients and elastic properties, the proposed method considers the influence of porosity, fluids, and lithology, which could enhance reliability in pore pressure prediction. Meanwhile, a prediction model is developed using logging data and DNNs to estimate mineralogical volumetric fractions based on elastic properties. This prediction model allows improved estimation of rock matrix elastic properties using seismic-inverted data, which is crucial for estimating normal compaction velocity to extend pore pressure prediction from individual boreholes to the whole study area. Real data applications demonstrate that the predicted pressure coefficients derived from seismic data using the method presented in this paper align well with the gas enrichment estimated in previous studies for the tight sandstone reservoirs. Furthermore, regions with high values of pressure coefficients correspond to high gas content. These findings validate the effectiveness of the proposed methodology, which can provide valuable insights for identifying potential tight sandstone reservoirs.

Wind-induced interference effect of complex building clusters on long-span roof structures

The presence of complex buildings introduces interference effects and complicates the wind field around the long-span roof structure. The complexity of the problem makes it challenging to predict wind load of long-span roof structures, resulting in a scarcity of design data available to engineers and designers. Thus, the influence of interference effect on mean and peak pressure coefficients of a long-span tri-centered cylindrical roof structure was investigated through wind tunnel tests conducted in an atmospheric boundary layer wind tunnel. The study focused on the interference effects caused by two cooling towers, chimney, and some low-rise buildings, considering all wind directions. The results show that the interference effects are the result of a combination of amplification effects from the adjacent cooling towers and chimney and shielding effects from the surrounding low-rise buildings. Due to the shielding effect of low-rise buildings, the wind suction on the roof is mitigated compared to that of a single building, while at the end of the roof, wind suction is amplified by cooling tower effects. The interference effect of the building clusters, however, amplifies the fluctuating wind pressure coefficient and the peak pressure coefficient on the roof. To accurately estimate the wind load of building components and envelopes, furthermore, this study investigates the influence mechanism of interference effect on non-Gaussian characteristics of wind pressure combining with fluctuating wind pressure spectrum, spatial correlation of fluctuating wind pressure, and probability distribution characteristics of standardized wind pressure coefficient. The spatial correlation of wind pressure in the roof interference area exhibits a strong association, and the probability density distribution characteristics of the standardized wind pressure coefficient significantly deviate from the Gaussian curve, with a peak factor exceeding 3.5. Thus, it can be concluded that the wind pressure within this region demonstrates significant non-Gaussian characteristics. Hence, the zone values of peak pressure coefficients are determined using the peak factor approach to inform the wind resistance design of the envelope structure.

Angle-of-attack characteristics of opposing jet for improving drag and heat reduction

While the opposing jet technique has the potential to achieve efficient drag and heat reduction, it can be severely affected by the incoming angle of attack. To analyze the angle-of-attack characteristics of opposing jet for improving drag and heat reduction, a three-dimensional blunt model was studied under various jet stagnation pressure ratios and angles of attack using the verified numerical method. The results showed that the enhanced reattachment shock on the windward side resulted in a higher pressure and temperature rise, which led to the deterioration of drag and heat reduction. Under the influence of the incoming angle of attack, the recirculation vortex transformed into a longitudinal vortex, resulting in a slanted U-shaped distribution of the surface pressure coefficient and Stanton number. Increasing the jet stagnation pressure ratio widened the coverage of the recirculation vortex on both the windward and leeward sides, which brought an improvement in drag and heat reduction. The interaction between the incoming angle of attack and the opposing jet caused a double-peak distribution of Stanton number due to the recirculation vortex reattachment and the compression of the incoming flow. The inclined opposing jet could reduce the peak values of pressure coefficient and Stanton number when subjected to the incoming flow with an angle of attack by spreading the recirculation vortex along the windward side. There should exist an optimal inclination angle that can effectively reduce the peak caused by the compression of the incoming flow without generating an excessive peak due to the recirculation vortex reattachment.

The Effect of Adding a Square Disturbance on the Flow Characteristics across Circular Cylinders Arranged in Tandem

Circular cylinder is one form of bluff body that is often used in engineering and industrial applications. The addition of a square placed in front of the circular cylinder aims to accelerate the flow from laminar to turbulent so that flow separation can be delayed more slowly so that the resulting pressure drag is smaller. The research method used was experimental using two circular cylinders arranged in tandem with a diameter (D) of 25 mm and variations in the distance between cylinders (L/D) 2;2,5;3;3,5;4 and two square-shaped square bodies (SDB) in front of each cylinder with dimensions of 4 mm. Position of SDB angle is 30o and the gap distance is 0.4 mm. The Reynolds number used is 2.3x104. This study aims to obtain the distribution of pressure coefficient (CP), drag pressure coefficient (CDP) and velocity profile behind the test specimen. The results showed that the addition of two square-shaped square bodies on each cylinder can reduce the drag force on the cylinder which causes a difference in values, where the largest pressure coefficient value occurs at a distance of L/D 4 with a value of -1.073 and the lowest value of drag pressure coefficient (CDP) at a distance of L/D 2 on the upstream cylinder with a value of 0.0786. While the downstream cylinder is located at a distance of L/D 3 with a value of -0.079 and the lowest speed value is located in the variation of L/D 4 with a value of 9.52 m/s.

Aerodynamic analysis of building with zigzag-patterned façade: Insights from large eddy simulation

High-rise buildings have diverse shapes and configurations to harmonize with the architectural, engineering, environmental, economic, and functional considerations, and aspirations. However, the architectural shape and configuration plays a vital role in determining the aerodynamic characteristics of the surrounding flow field and wind forces acting on the building. This paper numerically investigates the aerodynamic characteristics of the zigzag-patterned building by adopting the three-dimensional large eddy simulation (LES). The flow around a square cylinder in high Reynolds number is used to valid the numerical method. The results show that the zigzag pattern has a little effect on the mean streamwise velocity but significantly influences the fluctuation of streamwise and vertical velocities in the wake region. It is observed that the zigzag pattern on the leeward wall results in a narrower wake region, while on the windward wall, leads to a thinner and longer shear layers and shorter recirculation region. As for the pedestrian level wind, the flow velocity within the separation area is noticeably reduced when incorporating a zigzag pattern on the windward wall. Moreover, the fluctuating pressure coefficient values at the sidewalls of a building with a zigzag windward wall is significantly smaller than that of a smooth configuration building due to the façade pattern's impact in diminishing the curvature of the nearby separation at the leading edge.

Investigation on natural to ventilated cavitation considering the air-vapor interactions by Merging theory with insight on air jet location/rate effect

Cavitation is of significant practical importance since unstable flow characteristics can have noticeable consequences on objects nearby. An essential approach for controlling the cavitation flow field's instability is air injection. This work aims to conduct a numerical and experimental investigation on the natural to ventilated cavitation around a Clark Y hydrofoil. Having three phases: water, vapor, and air, the cavitation model is adjusted based on the Merging theory to consider the impact of dissolved air on cavitation. Furthermore, the Density Corrected-based Method (DCM) is used to alter the turbulence model. The experimental tests are carried out in the water tunnel, which can maintain the constant water flow rate (Qwater=490 m3⁄h) and regulate the pressure level (105–180 kPa). The Clark Y hydrofoil is fixed at an angle of attack of 8° and includes injection and pressure taps. Two holes, called Tap1-injection and Tap5-injection, are alternatively used for air injection purposes.Two cavitation numbers (σ=1.1, 1.6) and three air injection rates (Q = 0, 0.5, 1 l/min) are considered current case studies. The results demonstrate the meaningful impact of location and rate of aeration on the dynamic/average characteristics of cavitation. Increasing the air injection rate results in an increase in the pressure coefficient values, a decrease in the shedding frequency, and an elongation of the cavity with an M-shaped structure. Also, during a cycle of cavity development, air injection may take place in the reversed jet, perpendicular jet, or direct jet configurations. In addition, a close agreement between numerical and experimental results is recorded.

NUMERICAL MODELLING OF WIND LOADS ON WINDOWS. VALIDATION FOR A HIGH-RISE SQUARE PLAN BUILDING

This paper presents the results of numerical and experimental modelling of mean and peak pressure coefficients for window assemblies on a rectangular building. The applicability of the SST and GEKO k-ω turbulence models implemented in ANSYS Fluent is investigated. A simple steady-state method for determining the peak wind pressures, which utilizes the turbulence kinetic energy calculation, is shown to be sufficient for practical purposes. A comparative analysis with the values of pressure coefficients given in the Russian building code is carried out. The obtained results can be used in the design of window assemblies of typical buildings.

Force distribution on polymeric geostrip reinforcements in MSE walls under seismic excitation

In this experimental study, the values and distributions of dynamic tensile force along the polymeric geostrip reinforcements were investigated under stepped increasing sinusoidal excitations with a series of shaking table tests on 1500 mm high or 1/3 scale physical mechanically stabilized earth wall (MSE) model. The effects of the peak ground acceleration ([Formula: see text]), slope angle of the cohesionless backfill material ([Formula: see text]), and stiffness of the polymeric geostrip reinforcement were assessed on both the potential failure surface geometry in the backfill soil and dynamic tensile forces along the lengths of polymeric geostrips. Three potential failure surfaces from the results of shaking table tests in the backfill cohesionless soil were detected as for states of quasi-elastic, plastic, and failure. However, neither the current design codes for MSE walls nor the theoretical recommendations truly reflect potential failure surface geometry in the backfill soil. Additionally, the values and distribution geometry of maximum incremental dynamic tensile forces ([Formula: see text]) on the reinforcements along the height of MSE wall recommended by design codes are not compatible with test results; however, the normalized location of total maximum incremental dynamic tensile force ([Formula: see text]) acclaimed by design codes is consistent with the test results. The traditional pseudo-static limit equilibrium methods result in over conservative horizontal dynamic earth pressure coefficient ([Formula: see text]) values generally as compared with the test results. The slope angle of the backfill material and stiffness of reinforcement have a negligible effect on the potential failure surface geometry; however, incremental dynamic tensile forces, total maximum incremental tensile forces, and horizontal dynamic earth pressure coefficient values increase with an increase in the stiffness of reinforcement and inclination angle of the backfill and peak ground acceleration value.

CO2 fluxes at the water-atmosphere interface in fluvial environments: an overview of studies in Brazilian rivers

Recent advances in Brazilian scientific production on CO2 fluxes at the water-atmosphere interface in rivers were reviewed, including estimates of CO2 partial pressure and fluxes. A total of 17 studies were reviewed. We compiled information regarding the location studied, the methodology used by each author, and the values of CO2 partial pressure (pCO2), CO2 fluxes (FCO2), and gas exchange coefficient (k) found in each region. The results were spatialized and synthesized. The important role of fluxes in CO2 degassing and their influence on climate change, as well as the global lack of data on these environments, were the main motivations for this study. The information was scarce, and most studies are focused on the Amazon Basin. However, high-resolution mapping of CO2 fluxes, at the scale of micro basins and streams, proved to be scarce. We emphasize, therefore, the importance of further studies in the country including other hydrographic regions, and in high resolution. These studies would add to our knowledge of how natural and anthropic processes influence CO2 flux, in addition to providing better estimates in tropical river systems.
 Keywords: climate change, CO2 emission, CO2 partial pressure.

Mechanism analysis on wake-induced vibration of parallel hangers near a long-span suspension bridge tower

To reproduce the vibration phenomenon and explore the mechanism of parallel hangers near a suspension bridge tower based on the practical engineering, a pair of hangers in the wake of the bridge tower were taken as the research object. The influences of different wind yaw angles on the flow patterns of the bridge tower and aerodynamic characteristics of parallel hangers were investigated using the computational fluid dynamics (CFD) method. Then, based on the amplitude spectrum results, the relationship between the aerodynamic interference intensities of the tower wake and the displacements of parallel hangers were quantitatively described. Moreover, the coherence between the bridge tower and the parallel hangers were studied. Finally, based on the surface pressure distributions, energy input analysis, and wind speed characteristics, the vibration mechanism of parallel hangers near the bridge tower were explored. The results show that when the wind yaw angle is in the range of 0° ∼ −30°, the tower wake and the vortex shedding from the hangers jointly affect the aerodynamic characteristics of the hangers, which leads to the vibration displacements and movement tracks are observed as a sawtooth shape with multiple frequencies and synchronous skewed ellipses, respectively. The aerodynamic interference intensities of the bridge tower are positively correlated with the displacements of the hangers, and the peak amplitude at the wind yaw angle of −30° with strong aerodynamic interference is 35 mm, which is about 9 times that at the wind yaw angles of ±60° with weak aerodynamic interference. Moreover, at the wind yaw angles with strong aerodynamic interference, the mean wind pressure coefficients of the parallel hangers tend to be negative and their root mean square (RMS) values of wind pressure coefficient exhibit an overall upward trend compared with those at wind yaw angles with weak aerodynamic interference. Based on the analysis of energy input and wind speed characteristics, it is found that the low-frequency vortex shedding from the tower wake is the internal factor that continuously maintain the large vibrations of the parallel hangers.

An Experimental Study on Estimation of the Lateral Earth Pressure Coefficient (K) from Shaft Friction Resistance of Model Piles under Axial Load

Estimating a pile shaft’s frictional capacity is challenging and has been a controversial subject among researchers. In this study, the shaft friction resistance of non-displacement (pre-installed) model piles under axial load was investigated. Four different model piles were used, including steel, timber, and two composite piles (FRP and PVC filled with concrete). The angle of interface friction (δ) between test sand, and pile materials was determined using an interface shear test (IST) at four relative densities. Axial pile load experiments were implemented in a soil tank and piles were embedded into loose to very dense sand. Model pile load tests were performed in such a way that there was no end (point) bearing capacity (only friction was generated), and lateral friction resistance between the pile material and the soil along the pile shaft formed the complete bearing capacity of the model pile. According to experimental results, it was observed that, with increasing sand relative density and surface roughness of the pile material, the shaft friction resistance of the model pile increases. A back-calculation analysis was also performed to find the values of lateral earth pressure coefficient (K) using Burland’s (1973) equation with the help of measured shaft friction capacity of the model pile load test. By performing multivariate regression analysis, an equation was obtained between the back-calculated lateral earth pressure coefficient (K) and other parameters. The obtained equation was used to calculate the K values given in other studies in the literature. It was determined that the obtained equation was in good agreement with the data in other studies. This equation can be beneficial in practice and can be advantageous for further study in the future.

An investigation of forces on a representative surface in a pulp flow through rotating and non-rotating grooves

Softwood pulp flow in rotating and non-rotating grooves is numerically simulated in the present study to investigate the fluid flow and the forces acting on a representative surface mounted in the groove. The viscosity of softwood pulp with various consistencies is available from the measurements reported in the literature providing the opportunity to examine the effects of fiber consistency on the velocity and pressure distribution within the groove. The simulations are carried out in OpenFOAM for different values of gap thickness, angular velocity and radial positions from which the pressure coefficient and shear forces values are obtained. It is found that the shear forces within the gap increase linearly with the angular velocity for all fiber consistencies investigated and in both grooves. Also, this behavior can be successfully predicted by modeling the gap flow as a Couette flow in a two-dimensional channel. Meanwhile, a more detailed analysis of the flow kinetic energy close to the stagnation point using Bernoulli’s principle is carried out to provide a better understanding of the pressure coefficient variation with angular velocity in the non-rotating groove. A comparison of pressure coefficients obtained numerically with those calculated by considering the compression effects revealed that the comparison effects are dominating in the pulp flow within the groove.

Wind effect on scallop domes with negative amplitude and prominence using Experimental and Numerical Study

This study investigates the wind effect on hexa-sectored scallop domes using Computational Fluid Dynamics (CFD) and wind tunnel tests. Similar to the shape of a seashell, the scallop dome is one of the most common domes used to cover large spans. The scallop dome has an additional curvature equal to its sectors compared to a base dome (spherical). Hence, it has better structural efficiency compared to a spherical dome under the same condition. The curvatures on the scallop dome create alternate “ridges” and “grooves” on it. According to the computations of the article, the grooves created on this dome in the sector, as well as their position angle to the wind direction significantly affect pressure coefficient value. Due to these differences, wind pressure on scallop domes significantly differs from wind pressure on spherical domes. The results indicate that the ridges cause negative wind pressure coefficients whose magnitude reaches a maximum of −2.0 for an angle of alignment of 30°. The analyses have been conducted through Ansys-Fluent. This study presents the equation of wind pressure coefficient in the most critical of dome groove positions. The arch created between the grooves of a scallop dome is another effective parameter on maximum negative pressure. In the case study scallop dome, if the wind effect angle is considered α = 15, the maximum deformation in the structure will be created, which is 20% higher than that of α = 0.

Effect of cavitation evolution on power characteristics of tidal current turbine

With the development of large-scale tidal current turbines and the increase in tidal current velocity, the possibility of cavitation increases. Furthermore, unsteady cavitation is a complicated multiphase flow that causes power degradation of tidal current turbine blade. There has been no comprehensive investigation of it so far. In this study, the blade captured power is obtained at different cavitation numbers using the Schnerr–Sauer cavitation model. The numerical uncertainty for the mesh and the time step is calculated by the grid convergence index method. It has been shown that, when the cavitation number is 5 and 2, cavitation has no effect on the blade power. With the decrease in the cavitation number, the rise in cavitation intensity occurs when the vapor distribution area stretches from the blade tip to the blade root and from the leading edge to the trailing edge, respectively. With a fall in cavitation number to 1.3, the vapor volume fraction rises, and the viscosity of the mixed phase reduces, resulting in a reduction in viscous power. When the cavitation number is 0.8, there exists a larger region in which an absolute value of minimum pressure coefficient is less than the cavitation number, a smaller blade load is present, and the pressure difference power is substantially decreased. Because of the huge inverse pressure gradient created by cavitation, the negative pressure difference power is generated, resulting in a decline of the blade power coefficient to 14%, when the cavitation number is 0.5.

Numerical study of unsteady flow past compound cylinders

The flow over compound cylinders is simulated using computational fluid dynamic (CFD). Three contacting cylinders of different diameters, arranged in tandem with respect to the oncoming flow were examined at Reynolds number ReD = 333 based on the diameter of the middle cylinder, D. The proposed configuration simulates the wind flow over the satellite launch vehicles during the period of launch. Computations were performed for diameter-to-diameter ratio, d / D, of 0.8 and 0.6. The numerical results revealed the vortex formation and vortex shedding mechanism in the wake downstream of the cylinders. For d / D = 0.6, the near wake is wider than that of d / D = 0.8. It is also found that the base pressure coefficient values for d / D = 0.6 are lower than those for d / D = 0.8. These findings imply that the acting drag force on the cylinders for d / D = 0.6 is larger than that for d / D = 0.8. Concluding that, the base pressure coefficient depends on the diameter ratio d / D whereas, the computed vortex-shedding Strouhal number value was found to be 0.25 and is independent on the d / D ratio. Bangladesh J. Sci. Ind. Res. 57(4), 215-220, 2022

An exact solution of active earth pressures based on a statically admissible stress field

Static lateral earth pressure problems are a central and long-standing topic in geotechnical engineering, but their exact solutions are lacking. This study is dedicated to providing an exact solution for active earth pressure exerted against rigid retaining walls with various wall geometries, backfill geometries, and wall roughness. A numerical model based on the rigid plastic analysis, namely a fully plastic solution, has been successfully developed to search out statically admissible stress fields in the backfills. In particular, Sokolovski’s theory coupled with the Mohr-Coulomb criterion has been adopted to enable the statically admissible stress fields to be developed. Robust numerical frameworks based on the fifth-order adaptive Runge-Kutta method, and a shooting technique has demonstrated efficacy in coping with the stress singularity that appeared in the analyzed domain, thus the admissible stress fields have been successfully figured out. The accuracy of the proposed solution is satisfactorily verified and validated by comparing the obtained results with exact values and those of experimental results available in the literature. The admissible stress fields explored in the backfills of rigid walls elucidate the load transfer mechanism of cohesionless materials in the backfills; the potential effects of various parameters e.g., wall geometries, backfill geometries, and wall roughness on the active earth pressure coefficients have been disclosed thereof. In essence, this study provided a conservative answer for the static active earth pressure problems whereby the exact values of active earth pressure coefficients are tabulated and provided for engineering practice.

Wind Pressure Distribution on the Façade of Stand-Alone Atypically Shaped High-Rise Building Determined by CFD Simulation and Wind Tunnel Tests

The investigation of wind pressure distribution on a façade of an atypically shaped 162 m tall building is discussed in this paper. The horizontal cross-section was changed with the height of the structure (the square in the bottom part and the polygon in the top). The surface of the structure was smooth. A structural system was created using a combination of the tube structural system and exoskeleton structure. The building was stand-alone, located in urban terrain. In this case, the information in standards were not sufficient for its design. Therefore, other available tools had to be used for the determination of required input parameters (mean external pressure coefficients). At first, wind tunnel tests (WT) were performed on a reduced-scale model (1:300). Then, the obtained results were compared with data from a computational fluid dynamics (CFD) simulation. The accuracy of the simulation was evaluated by the method of three metrics. Short descriptions of the reduced-scale model, boundary layer wind tunnel, used measuring devices, and the methodology of tests are mentioned. The aim of this research was to identify the influence of the shape modification on the values of mean external pressure coefficients (in the comparison with the original shape, which was the cuboid). In the case of the cuboid, good agreement between the values determined by the CFD and the values from Eurocode was achieved. Larger discrepancies occurred on the roof. The modification of the total shape of the structure from the cuboid to atypical structure had the positive effect on the mean values of external pressure coefficients cpe. These values were smaller (at some levels significantly). Mainly, this effect was noticeable on the leeward side. For the wind directions 0° and 180°, the changes of the values were relatively large. For the other two wind directions (45° and 67.5°), the values on the windward sides were similar. The large advantage of this atypical structure is that the negative pressures on side walls and leeward side are smaller in the comparison with the cuboid. This is very useful for the fixing of façade components, where the values of negative pressures are larger than the positive pressures on the cladding in the larger heights.

Performance characteristics of an axial‐flow gas induction impeller

AbstractPower number, pressure coefficient, and induced gas flow rate of a novel axial‐flow gas induction impeller with large openings for gas flow are reported. The effects of rotational speed, impeller diameter, submergence, and pumping mode are considered in turbulent operation. The pressure coefficient is measured using the reduction in pressure at speeds below the minimum induction speed as well as from the minimum induction speed. The minimum induction speed approach consistently yields lower pressure coefficient values, indicating the existence of impeller exit losses that are required to force gas from the centre of the impeller into the liquid. The induced gas flow rate is modelled using two approaches. The first approach relates the gas flow rate to the available pressure difference, and in this case, the relation is found to be strongly affected by impeller diameter. The second approach is a literature model that likens the induction process to water jet injection. While this approach appears promising, flow measurement device pressure losses make it difficult to definitively evaluate this method.

Numerical investigation of flow around two cylinders in tandem above a scoured bed

Flow mechanisms around two cylinders in tandem arrangement above a scoured bed have been investigated using the three-dimensional unsteady Navier–Stokes equations with the Spalart–Allmaras improved delayed detached-eddy simulation model. A turbulent inlet boundary layer generation method is adopted to obtain more realistic inlet boundary conditions. First, uniform flow over a single cylinder at Re = 3900 and flow over a single cylinder above scoured beds at Re = 6000 were simulated to validate the numerical model, boundary layer generation method, and mesh density effect. Second, two cylinders in the tandem arrangement above scoured beds with six different pitch ratios L/D are investigated numerically in terms of instantaneous vortex characteristics, the hydrodynamic force, and time-averaged flow fields. The simulation results in scoured beds are compared with simulations under the near flat wall and wall-free conditions. The major findings can be summarized as follows. (1) When L/D≤2.0, the wake of two tandem cylinders is dominated by the intermittent shedding, and the downstream sand dune in the scoured bed hinders the Kármán vortex formation at the rear of the downstream cylinder. Lift force fluctuations of the two cylinders have small amplitudes, and their spectra show a multi-peak distribution and no dominant peak frequency in the spectrum of the upstream cylinder. A squarish cavity-like recirculation zone is formed between two cylinders at L/D=2.0. (2) When L/D≥3.0, the periodic vortex shedding is evident in the wake of the upstream cylinder, and the small sand berm between two cylinders has an impact on the bottom shear layer of the upstream cylinder. The downstream cylinder is periodically impacted by the vortices shed from the upstream cylinder. Lift force spectra of the upstream and downstream cylinders have the same peak frequency. (3) Due to the influence of scoured bed and the inlet boundary layer, the time-averaged lift coefficient of the upstream cylinder remains negative when L/D≥1.5, and the critical spacing for drag inversion is relatively smaller compared with under wall-free conditions. The negative pressure coefficient values of the upstream cylinder are smaller than the values in near flat wall and wall-free conditions.