•Machine learning models are compared for simulation and forecasting of streamflows.•Support Vector Regression, Artificial Neural Networks and Random Forest are used.•A base flow separation method is ...coupled with machine learning models.•North Fork, Chehalis, Carson and Sacramento rivers in the United States are analysed.•Base flow separation method improved streamflow simulation to a certain degree.
Efficient simulation of rainfall-runoff relationships is one of the most complex problems owing to the high number of interrelated hydrological processes. It is well-known that machine learning models could fail in simulating streamflows from only meteorological variables in the absence of antecedent streamflow values. The main reason for this could be low and lagged relationships between streamflow and meteorological variables. To overcome this inefficiency, for the first time, we developed a simulation framework by coupling a base flow separation method to three machine learning methods. It was demonstrated that separating streamflow into different components such as base flow and surface flow can be useful for improving simulation and forecasting capabilities of machine learning models. We simulated streamflow in four rivers in the United States with Support Vector Regression (SVR), Artificial Neural Networks (ANNs) and Random Forest (RF) as a function of precipitation (P), temperature (T) and potential evapotranspiration (PET). We concluded that the base flow separation method improved the simulation performances of the machine learning models to a certain degree. Apart from the simulation scheme, we also employed a forecasting scheme by using the antecedent observed discharge values in addition to P, T, and PET. We discussed performances of models in simulation and forecasting of streamflow regarding model types, input structures and catchment dynamics in detail.
Objective of this study is to elucidate relationship with aerodynamic properties and vortex shedding from suction surface and wake of wind turbine blade at low Reynolds numbers. Force measurement of ...NACA 4412 airfoil was conducted at various angles of attack and Reynolds numbers. Furthermore, smoke-wire experiment was performed to clearly visualize flow patterns such as flow separation or laminar separation bubble (LSB) over the airfoil. Also, velocity measurements at near wake region were done to highlight coactions among LSB and trailing edge vortices. Hot-film experiment was performed to obtain more information with regards to progress and formation of LSB. Experiments indicated that location and formation of LSB were affected by variety of both Reynolds number and angle of attack. Besides, mean and rms velocities showed that flow characteristics near wake were influenced by LSB and trailing-edge separation. At lower angles of attack, short bubble occurred, and frequency of vortex shedding due to short bubble were high whereas shedding frequency of long bubble at moderate angles of attack were low. The bubbles can cause vibration and noise at the wind turbine blade, hence they should be better known and eliminated, which aerodynamic performance and energy efficiency of turbine can be increased.
•Vortex shedding from suction surface and wake of aerofoil at low Reynolds numbers.•Strouhal number of vortices due to bubble and trailing edge varies with incidence.•Separation bubble physically changes as Reynolds number and angle of attack vary.•Strouhal number of vortex shedding due to bubble changes at low angles of attack.
•Adaptive flap for use by a high-solidity vertical axis wind turbine is developed.•Performance of the adaptive flap is studied by considering different flap lengths and locations.•Optimal parameters ...of the adaptive flap have been predicted.•Advantage and weakness of the adaptive flap are discussed.
Adaptive flap as a new flow control technique with adaptability to the changing flow separation has recently attracted much attention. This study uses the adaptive flap to mitigate the flow separation of a vertical axis wind turbine with a high solidity of 0.75 and investigate its performance and flow control mechanism by considering different flap lengths and locations. The fluid flow is simulated using computational fluid dynamics with the shear-stress transport k-ω model, and the flap motion is calculated based on the fluid–solid interaction methodology. The results show that the flap can be adaptively raised by the backflow caused by flow separation and used to block the backflow. The blocking of the backflow alleviates the flow separation problem and increases the blades’ aerodynamic torque. However, the long flap causes a negative effect due to its inability to retract timely when the flow tends to the attached state at high tip speed ratio scenario. It is observed that the short flap can avoid this problem when it is located far from the blade leading edge. Also, the short flap located closer to the blade leading edge performs better at low tip speed ratios, even though the performance is observed to be weakened by the trailing edge vortices suppressing the flap from deployment. This study provides a technical approach and theoretical basis for better alleviating the flow separation problems in vertical axis wind turbine.
•Application of improved k-ω-γ model of inflatable decelerator is conducted.•Angle of attacks effects on performances of k-ω-γ models are analyzed.•Crossflow instability, separation instability and ...first mode dominate the transition.
The scalloping surface of Hypersonic Inflatable Aerodynamic Decelerator (HIAD) promotes flow transition. Precise prediction of flow transition and surface aeroheating over the HIAD is imperative to the design of thermal protection system. In this paper, the improved k-ω-γ model considering the separation instability is first applied to the deformed HIAD aeroshell under various angles of attack to assess and verify its performance on such complex configuration, where multiple transition mechanisms coexist and interact. The results of the original k-ω-γ model are also provided for comparison and the prediction mechanisms of the improved model are further dissected. The results reveal that the flow separation and crossflow caused by the scalloping surface are highly sensitive to the angle of attack (α). With the α increasing, an opposite tendency between them can be found, that is, larger α causes stronger and earlier crossflow, but much later flow separation, which leads to the alternation of dominant instabilities for transition in turn. At the relatively high angle of attack, the crossflow becomes critical, at a small angle of attack, however, the separation instability is dominant. The first-mode disturbance can also accelerate the flow instability, while the influence of second-mode disturbance can be neglected due to relatively low post-shock Mach numbers. As a result, the original k-ω-γ model, not attempting to incorporate separated flow transition, predicts an opposite tendency to the experiments. While the transition onset, extent and shape simulated by the improved k-ω-γ model compare quite well with the experiments.
As a core component in high-pressure hydrogen storage systems for hydrogen fuel cell vehicles, the spring-loaded hydrogen decompression valve (SHDV) directly affects the performance of the hydrogen ...fuel cells. In this study, the transient flow characteristics of fluid– structure interaction in SHDV are investigated using the dynamic mesh method and real gas model. The complete closure of the valve spool is realized by the porous medium model. Adverse pressure gradients, flow separation, and unsteady vortex evolution in the decompression valve are numerically calculated with large eddy simulation, and the reliability of the calculation is verified by the experiments and theoretical model. Results show that the characteristic curve of the SHDV startup process can be divided into three phases: rapid response phase, closure phase, and equilibrium phase. The change in spool angle slightly affects the rapid response phase but substantially affects the fluctuations of pressure and flow during the closure and equilibrium phases. In the equilibrium phase, typical adverse pressure gradients and flow separation occur downstream of the spool, gradually propagating into the control chamber and causing pressure instability. Backflow under the large-angle spool is primarily caused by adverse pressure gradient and wall shear stresses, whereas backflow under the small-angle spool is mainly formed by the obstruction of the upper wall at the control chamber. By adding a convex cylinder structure to the spool, adverse pressure gradients and flow separation downstream of the large-angle spool can be effectively suppressed.
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•The transient flow characteristics of fluid-structure interaction are investigated.•The complete closure of the spool is realized by the porous medium model.•The adverse pressure gradient and flow separation are investigated.•The evolution mechanisms of unstable vortices are clarified.•Optimization of unsteady flow downstream of the spool by convex cylinder.
The thermo-hydraulic characteristics of a solar air heater with quarter-circular ribs on the absorber plate are studied numerically. A comparison of the three different arrangements of the ...quarter-circular ribs is performed. The ribs are aligned periodically in a transverse manner to the flow. The numerical simulation of the mass, momentum, and energy conservation equations are carried out for a two-dimensional computational domain using the finite volume multi-grid solver of ANSYS-Fluent-18. A constant and uniform heat flux of 1000 W/m2 is supplied to the aluminum collector plate. Many pertinent input parameters, viz. flow parameters like Reynolds number and roughness parameters like relative roughness pitch and relative roughness height are varied. Reynolds number is varied from 3800 to 18000, whereas the relative roughness pitch is varied from 7.14 to 17.86, and the value of relative roughness height is kept fixed, i.e., 0.042. The impact of relative roughness pitch and Reynolds number on the thermo-hydraulic performance of the ribbed solar air heater is analyzed here. The optimized rib geometry in the investigated range of parameters is obtained. Pumping power calculations are done. The contours of temperature, pressure, and velocity are illustrated pictorially to understand the flow physics with clarity. It is found that the solar air heater with quarter-circular ribs having a relative roughness pitch of 7.14 delivers a maximal thermal enhancement ratio (representing the overall energy performance) of 1.88 in the studied parametric range. The Nusselt number and friction factor correlations are proposed by performing a non-linear regression analysis.
Large-eddy simulation is performed to study the turbulence statistics and flow structures of the water past a rotating axial-flow pump under different flow-rate working conditions. A novel ...sharp-interface level-set based immersed boundary method is applied to capture the complex geometry of the pump. An unstructured triangular mesh is used to discretize the complex surface geometry of the pump, and a ray-tracing method is employed to classify the computational domain into fluid and solid regions. Turbulence statistics, including the mean velocity, turbulent kinetic energy (TKE), turbulence production, and turbulence dissipation, are analyzed under five different flow-rate working conditions around the designed condition. The results show that unsteady wake, tip leakage flow, and flow separation are accompanied by a high TKE magnitude. For the high turbulence intensity under off-designed working conditions, the tip leakage flow plays a leading role at low flow-rates, and flow separation dominates at high flow-rates.
•LES study of detailed flow structures around a rotating axial-flow pump.•A novel immersed boundary method is applied to capture the complex pump geometry.•The additional efficiency loss is found to be correlated to the large TKE magnitude.•The TKE transport is analyzed to study the mechanism underlying the efficiency loss.
Abstract
Stall will deteriorate flight safety and cause serious accidents for civil aircraft. RANS-LES hybrid approaches have become a compromise choice due to insufficient ability of RANS for large ...flow separation. In present work, Zonal Detached-eddy-simulation coupled with high-order spatial scheme were employed to investigated grid density effect in the numerical simulations. The calculations show that the grid density would affect the numerical simulation of the small-scale flow structure in the wake of region of the main wing, and further affect the disturbance of main wing to the horizontal tail. It is necessary to properly densify the grid downstream of the main wing in order to accurately predict the aircraft stall.
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