Turbulent Flow Conditions Research Articles

Spectral analysis of the transition to turbulence downstream a delta winglet pair vortex generator in an airflow channel

The purpose of this experimental study is to provide a detailed analysis of the transition to turbulence in an airflow channel perturbed by a delta winglet pair vortex generator. Measurements of instantaneous velocity are carried out by a laser Doppler anemometry technique. Experiments are conducted for Reynolds numbers (based on the hydraulic diameter) ranging from Re = 400 to Re=12 000 and at several axial position downstream of the vortex generator. The flow behavior is characterized in each regime by means of a frequency analysis. We identified the inlet conditions corresponding to laminar weakly unsteady, transitional, and fully turbulent flow regimes. Furthermore, we have constructed a correlation between the Strouhal number and the Reynolds number that characterizes the vortex instability mechanism. Finally, under turbulent flow conditions, the dissipation rate was estimated, which showed an increase followed by an exponential decrease with the distance downstream of the vortex generator.

Аеродинамічний аналіз впливу повітряних потоків від вантажівки на рух мотоцикла

The article examines the problem of ensuring the safety of motorcycle movement in the conditions of a turbulent air trail created by a large truck. The development of computer technology, software and methods of computational fluid dynamics (CFD) allows conducting theoretical studies of various aspects of the movement of motor vehicles, which are difficult to reproduce experimentally and which at the same time can have a significant impact on accidents on public roads. A motorcycle is one of the most dangerous vehicles in terms of traffic accidents with human casualties. The analysis of accidents involving motorcycles showed that most of the time their causes are insufficient skill and inability to drive a motorcycle, as well as insufficient attention. At the same time, when driving a motorcycle, movement in conditions of turbulent air flows with a sharp change in their speed and direction is a particular danger. Such conditions arise when overtaking large trucks moving at high speed. An aerodynamic model of the movement of a truck and a motorcycle was built for their different relative positions. From the position of the motorcycle in the turbulent wake behind the truck and after passing the compressed air zone in front of the truck. A study of turbulence models was conducted to assess the accuracy of the most common Reynolds-averaged Navier-Stokes models such as k–ε and k–ω. Zones in which the largest changes in air pressure occur during motorcycle movement have been established, and the magnitude of these changes has been determined. Accordingly, recommendations are provided regarding the characteristics of motorcycle movement when overtaking a large-sized truck and the safe trajectory in which such a maneuver is the safest for drivers with insufficient experience in driving a motorcycle.

Thermal-fluctuation effects on small-scale statistics in turbulent gas flow

Kolmogorov's theory of turbulence assumes that the small-scale turbulent structures in the energy cascade are universal and are determined by the energy dissipation rate and the kinematic viscosity alone. However, thermal fluctuations, absent from the continuum description, terminate the energy cascade near the Kolmogorov length scale. Here, we propose a simple superposition model to account for the effects of thermal fluctuations on small-scale turbulence statistics. For compressible Taylor–Green vortex flow, we demonstrate that the superposition model in conjunction with data from direct numerical simulation of the Navier–Stokes equations yields spectra and structure functions that agree with the corresponding quantities computed from the direct simulation Monte Carlo method of molecular gas dynamics, verifying the importance of thermal fluctuations in the dissipation range.

Statistics and spectral analysis of turbulent duct flows with flexible and rigid polymer solutions

Statistics and spectral analysis of turbulent duct flows with flexible and rigid polymer solutions

Dynamic stall of pitching tubercled wings in vortical wake flowfield

In consideration of the turbulent inflow condition of engineering applications, the flow mechanisms of dynamic stall of a tubercled airfoil have been comprehensively analyzed with an upstream cylinder. Numerical simulation of the flowfield of a tubercled wing with NACA0021 (National Advisory Committee for Aeronautics) airfoil has been conducted with the large eddy simulation model. Then, flow mechanisms have been analyzed based on the aerodynamic performances and flow structure descriptions. Meanwhile, proper orthogonal decomposition (POD) analysis has been carried out at both trough and peak sections to reveal the flow dynamics. It turns out that the dynamic stall process vanishes, and performances would be obviously impacted by the incoming cylinder wake in the case of αA=5° due to the enforced resistance of adverse pressure gradient. Furthermore, the first leading POD mode corresponds to the pitching movement at both trough and peak sections, while the high-order modes represent the influence of cylinder wake. Eventually, the influence of pitching amplitude has also been discussed in the case of αA=15°. Different from the case of αA=5°, dynamic stall phenomenon emerges, and the influence of wake impingement could be barely detected from the mode information except for the third mode at the trough section. The detachment of the dynamic stall vortex takes place corresponding to the dynamic stall onset, which is driven by the streamwise pressure gradient near the trough leading-edge.

Synthesis, thermophysical characterization and thermal performance analysis of novel Cu-MXene hybrid nanofluids for efficient coolant applications.

Hybrid nanofluids are considered as an alternative for conventional heat transfer fluids and mono nanofluids due to its remarkable enhancement in thermo-physical properties. However, there are some limitations in achieving the better thermo-physical properties due to the stability of nanoparticles in different base fluids at higher concentration. This work aims at synthesizing, thermo-physical characterization and thermal performance estimation of stable Cu-MXene based hybrid nanofluids using various base fluids at very low volume concentration of Cu and MXene nanostructures. Two step method is employed to prepare Cu-MXene hybrid nanofluids by dispersing the low volume concentration of as prepared Cu and MXene nanostructures (ranging from 0.01-0.05 vol%) containing SDS surfactant in various base fluids such as water, methanol, castor oil and silicon oil. Synthesized mono and hybrid nanofluids shows excellent stability against aggregation up to 7 days as evidenced from higher zeta potential values. Wettability studies conducted using contact angle measurement suggests that the castor oil, methanol and silicon oil based hybrid nanofluids exhibits hydrophilic behavior (showing contact angle less than 90°). Hybrid nanofluids display excellent enhancement in thermal conductivity at very low concentration of nanostructures (more than 70% for methanol based Cu-MXene hybrid nanofluid). Viscosity of the silicone oil based hybrid nanofluids show a remarkable enhancement followed by water, methanol and castor oil based hybrid nanofluids. Thermal conductivity and viscosity of hybrid nanofluids are effectively validated with existing theoretical models. Moreover, specific heat and pumping power of the hybrid nanofluids with respect to volume concentration of nanostructures are determined using the existing theoretical equations. Thermal performance of hybrid nanofluids was successfully estimated using Figure of Merit (FOM) analysis and suggested the better heat transfer fluid for improving the heat transfer performance under laminar and turbulent flow conditions for efficient cooling applications.

Comparison of methods for measuring and investigating water permeability of woven filter media

This paper presents, discusses, and compares three methods for measuring the water permeability of metal woven meshes. Two pieces of equipment were designed for this purpose. The first uses declining pressure – falling head method (FHM). The other uses constant pressure – the constant head method (CHM). The results obtained with these two empirical methods were compared with the fluid flow simulation through the studied samples. The results showed that each studied technique provided valuable and unique information for the water permeability investigation. All three techniques were in good agreement when they all worked under turbulent flow conditions. These conditions were observed for the samples whose pore size was above 30 µm. FHM worked only under turbulent flow. Therefore, it was recommended to work with high permeable samples. The samples whose pore size was below 30 µm required laminar flow. CHM could provide these conditions. This method was also the fastest with the lowest standard deviation value. It is recommended as the basis for forming a standard for measuring the water permeability of woven filter media. Data AvailabilityThe datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Thickener as an automatic control object

Introduction. Grain motion in the process of pulp sedimentation in a radial thickener was studied to use the typical links of the automatic control system. Research objective is to explore the possibility of describing the sedimentation process by means of typical links of the automatic control theory based on the physics of grain motion in a radial thickener and the equation of grain motion under laminar and turbulent flow conditions. Methods of research. Stokes’ law that determines the drag force resisting a solid sphere when it slowly moves forward in an unbounded viscous fluid (laminar motion) and the Newton– Rittinger law for the turbulent motion of grain formed the basis of the mathematical model, making it possible to develop a program that chooses a particular equation to calculate the grain sedimentation time in accordance with the grain size. The equations helped to obtain the sedimentation time and hence the time taken to reach the thickener’s bottom, which, in their turn, together with the fractional composition, determine the performance of the thickener. Transition curves were thereby obtained for various granulometric composition, percentage of ore in the pulp, thickener height, ore and pulp density. In the future, this made it possible to approximate the process of sediment accumulation by the equations of one or two lag blocks of the automatic control system. Research results. A computer program has been used that describes the process of sediment accumulation in a thickener, considering the granulometric composition as well as pulp and ore grain density. The program showed that the process of sediment accumulation in a thickener can be described with fair accuracy by a first-order lag block for laminar or turbulent grain motion. For a mixture of coarse and fine fractions, ore sedimentation can be approximated by two parallel lag blocks. Conclusions. Sedimentation process approximation by lag blocks, obtained with fair accuracy, can be used when analyzing, designing and adjusting sedimentation automatic control systems.

Intensification of thermal efficiency of a cross-flow heat exchanger under turbulent flow conditions using CuFe2O4/water nanofluid

The use of nanofluids has opened new intellectual horizons in improving the performance of heat exchangers. Therefore, in this work an experimental study was conducted for investigating the effect of CuFe2O4/water nanofluid on the heat transfer rate enhancement in a cross-flow type heat exchanger under turbulent flow conditions. Heat transfer rate were examined at different volume percentages of CuFe2O4 nanoparticle (0≤ φ ≤ 0.9% vol.) with a nominal diameter of 28 nm at three inlet fluid temperatures of 72, 80 and 88 °C, water pump rotational speeds of 2000 rpm and 3000 rpm, and cross-flow fan speeds of 1500, 2000 and 2500 rpm. According to the results, the heat transfer rate in the case of using nanofluids with a volume percentage of 0.1% was significantly higher than that of distilled water in all test conditions. The highest relative increase in the universal heat transfer coefficient (U) equal to 92.3%, compared to the use of water (φ = 0) was obtained at 0.1% vol% of nanoparticles, the lowest temperature of fluid entering the heat exchanger (72 °C), medium pump speed (2000 rpm) and low fan speed (1500 rpm). The maximum Performance Evaluation Criterion (PEC) improvement of 44.46% also occurred at this working condition.

CFD analysis of heat transfer enhancement in a concentric tube counter flow heat exchanger using nanofluids (SiO2/H2O, Al2O3/H2O, CNTs/H2O) and twisted tape turbulators

A concentric tube, counter flow heat exchanger with nanofluids under turbulent flow conditions has been examined using a computational fluid dynamics model. The proposed model consists of Twisted Tape Turbulators (TTT) on the annular side and inside the tube along with different nanofluids. Twisted Tape Turbulators produce a swirl motion within the fluids to increase heat transfer. Al2O3/H2O, SiO2/ H2O, and CNTS/ H2O nanofluids (with varying volume concentrations of 1 %, 2 %, and 3 %) with the turbulent flow (Reynold number Re = 2000 to 10000) were considered to determine three major parameters i.e., heat transfer rate (Q), overall heat transfer coefficient (U) and Nusselt number (Nu). This study resulted in 6.03 %, 16.74 %, and 6.74 % enhancement of overall heat transfer coefficient (U) with a 3 % volume concentration of CNTS/H2O nanofluid, Al2O3/H2O nanofluid, and SiO2/H2O nanofluid, respectively, when equated to without twisted tape tabulators. This analysis shows the maximum value of Nusselt number as 143,136.68 and 140.12 for CNTS/H2O, Al2O3/H2O, and SiO2/H2O, respectively, for the twisted tape tubular arrangement. It is concluded that when using twisted tape turbulator, the heat transfer rate (Q), overall heat transfer coefficient (U), and Nusselt Number (Nu) increase with increasing Reynolds number (turbulent flow conditions) and volume concentrations of nanofluid.Novelty statement: Twisted Tape Turbulators (TTT) arrangement with different concentrations and types of nanofluids combined are infrequent in the study of concentric tube heat exchanger with turbulent flow to obtain enhanced heat transfer.

Numerical study on heat transfer and flow characteristics of coaxial geothermal heat exchangers with helical finned inner tubes

In recent years, geothermal energy as well has become the first choice for building heating. In order to improve the heat transfer performance of the coaxial GHE, the flow characteristics and heat transfer characteristics of the coaxial GHE when the inner tube of the coaxial GHE is assembled with HF of different width ratio (a), thickness ratio (b) and pitch ratio (S) are numerically studied in this paper. Water is chosen as the working medium, and the Reynolds number range is 10000–60000. HF have a significant impact on heat exchanger performance by changing the fluid flow structure and improving the efficacy of heat transfer between the fluid and the wall. Numerical results show that as the Reynolds number increases, the Nusselt number (Nu) of coaxial GHE with different a, b, and S increases, while the friction factor(f) and thermal enhancement factor (TEF) decreases. The Nu and f of the coaxial GHE with HF of different a, b, and S on the inner tube are higher than those of the coaxial GHE with smooth inner tube. The change in S of HF has a greater effect on Nu and f than the changes in a and b of HF have on Nu and f. To discover the enhanced heat transfer mechanism of the coaxial GHE with HF assembled on the inner tube, flow lines, velocity, turbulence, turbulence, and temperature distribution are provided. According to the analysis, the blocking of the HF causes the fluid to appear as a spiral rotating flow structure, which improves fluid mixing and heat exchange between the fluid and the wall. Increases in both a and b of HF can improve the heat transfer strength of the fluid to the wall, while decreases in S of HF can hasten fluid mixing. Finally, two correlations are established based on the numerical simulation data to predict Nu and f as functions of design parameters in turbulent flow states.

Comparison of large eddy simulations against measurements from the Lillgrund offshore wind farm

Abstract. Numerical simulation tools such as large eddy simulations (LESs) have been extensively used in recent years to simulate and analyze turbine–wake interactions within large wind farms. However, to ensure the reliability of the performance and accuracy of such numerical solvers, validation against field measurements is essential. To this end, a measurement campaign is carried out at the Lillgrund offshore wind farm to gather data for the validation of an in-house LES solver. Flow field data are collected from the farm using three long-range WindScanners, along with turbine performance and load measurements from individual turbines. Turbulent inflow conditions are reconstructed from an existing precursor database using a scaling-and-shifting approach in an optimization framework, proposed so that the generated inflow statistics match the measurements. Thus, five different simulation cases are setup, corresponding to five different inflow conditions at the Lillgrund wind farm. Operation of the 48 Siemens 2.3 MW turbines from the Lillgrund wind farm is parameterized in the flow domain using an aeroelastic actuator sector model (AASM). Time-series turbine performance metrics from the simulated cases are compared against field measurements to evaluate the accuracy of the optimization framework, turbine model, and flow solver. In general, results from the numerical solver exhibited a good comparison in terms of the trends in power production, turbine loading, and wake recovery. For four out of the five simulated cases, the total wind farm power error was found to be below 5 %. However, when comparing individual turbine power production, statistical significant errors were observed for 16 % to 84 % of the turbines across the simulated cases, with larger errors being associated with wind directions resulting in configurations with aligned turbines. While the compared flapwise loads in general show a reasonable agreement, errors greater than 100 % were also present in some cases. Larger errors in the wake recovery in the far wake region behind the lidar installed turbines were also observed. An analysis of the observed errors reveals the need for an improved controller implementation, improvement in representing meso-scale effects, and possibly a finer simulation grid for capturing the smaller scales of wake turbulence.

Toward control co-design of utility-scale wind turbines: Collective vs. individual blade pitch control

A large-eddy simulation framework has been coupled with controller modules to systematically investigate the impacts of collective (CPC) and individual (IPC) pitch control strategies on utility-scale wind turbine energy production and fatigue loads. Wind turbine components were parameterized using an actuator surface model to simulate the rotor blades and the turbine nacelle. The baseline CPC and IPC algorithms, consisting of single-input single-output proportional–integral controllers and two integral controllers, respectively, were incorporated into the numerical framework. A series of simulations were carried out to investigate the relative performance of the two controllers under various turbulent inflow conditions, spanning hub-height velocities of 7 to 14 m/s. The numerical simulation results of this study showed that, in comparison to the CPC, the IPC controller could successfully reduce the damage equivalent loads of utility-scale turbines at regions 2 and 3 of turbine operation by about 3% and 40%, respectively, without any penalty on the power production of the turbine. It was also shown that, despite its minor impact on the turbulence kinetic energy of the wake, the IPC controller did not influence the recovery of the turbine wake.

The effects of calcium carbonate precipitation on high-velocity flow behavior in 3D printed fracture models

A large amount of water is produced in oil and geothermal fields. Produced water is used in water injection for oil displacement and pressure maintenance in oil fields. However, there is a possibility of forming different precipitations when the injected water and formation water combine, which can cause serious problems in the reservoirs. In the world, more than 60% of oil and more than 40% of gas reservoirs are fractured carbonate rocks, and the mineral precipitation observed in these formations is mostly CaCO3 due to temperature and incompatibility between the injected and formation water. There are few techniques available for CaCO3 scaling removal due to their relative hardness and low solubility. In most cases, the scaled-up wells are caused by the formation of carbonate. In the well tests, it is seen that the flow from the matrix to the fracture is quite low, and the flow in the fracture dominates the whole flow. Therefore, it is essential to properly understand the kinetics of CaCO3 scale formation and its detrimental effect on fracture.It is known from studies that kinetic models can give better results in determining precipitation behavior. However, there are not enough experimental studies on the effects of flow on mineral precipitation in fractured media. Therefore, the kinetic and thermodynamic models are used in this study to reliably predict the amount of mineral precipitation. This work investigates the behavior of precipitation in different temperature conditions, laminar or turbulent flow conditions, and fracture properties like high roughness, length, and aperture. The amounts of precipitation in fracture under different conditions that have not been determined in the literature are given. After flow experiments, the amount of CaCO3 precipitation and pressure differences were measured. When the temperature is 80 °C, the flow rate is 4.16 × 10−5 m³/s, and the roughness is high, precipitation is 47.5% higher than when the temperature is 50 °C. Repeating the same fracture characteristics for different flow conditions in fracture flow tests is essential. By using 3D fracture models, the same fracture properties were obtained under different flow conditions.In addition to precipitation amounts, this article presents decreasing crack permeability due to precipitation. Precipitation of minerals can drastically influence the reservoir's permeability and ability to transmit mass and energy. With the pressure differences measured in the experiments, permeability decreases due to precipitation were calculated using Poiseuille's law (cubic law) for laminar flow and Forchheimer's equation for turbulent flow. After 3.5 days of flow, the study's permeability at high roughness and high flow decreased by 23% at 50 °C and 44% at 80 °C. This study's findings can help better understand that the permeability reduction of fracture is greater in turbulent flow conditions compared to laminar flow. In addition, temperature and high roughness are very effective parameters in precipitation and permeability change. At high temperatures and high roughness, more minerals precipitate, and permeability values decrease considerably. In the later injection periods, the decrease in permeability caused by the precipitation can make the fracture impermeable and increase the pressure losses during the injection.

Research on the flow characteristics of the bulk carrier wake field based on particle image velocimetry

The wake field has three-dimensional flow separation characteristics, affecting various ship performances. The research on the characteristics of the wake field can provide theoretical guidance for the formation of new ship design methods and provide an understanding of flow mechanisms for improving ship performance. To explore the flow mechanism of the wake field, particle image velocimetry was used to carry out the detailed flow measurement of bulk carriers. First, the experimental uncertainty and convergence are analyzed. Then, the spatial distribution characteristics of the time-averaged field, instantaneous field, and turbulent flow statistics are discussed in detail, and a criterion for discriminating turbulent anisotropy is proposed. The results show that the vortex structure significantly affects flow characteristics, and the axial velocity contours present a U-form distribution with prominent “hook-like” features. Compared with the time-averaged velocity field, the instantaneous velocity field is chaotic and has multiple additional vortex structures, and the velocity contours and streamlines have prominent non-smooth characteristics. The wake field has an apparent double vortex structure, and the aggregation of many small vortices forms the bilge vortex. The instantaneous rotation characteristics of vortices in the wake field are highly time-dependent and fluctuate with time. The turbulent kinetic energy, the root mean square of fluctuation velocity, and the Reynolds stress have a U-form distribution. The U-form region is concentrated in the area with a large gradient. The wake distribution is in a state of turbulent anisotropy, and the kinetic energy change layer and low kinetic energy region have a low turbulent anisotropy.

Modal analysis of propeller wake dynamics under different inflow conditions

The evolution mechanisms from near to far field of propeller wakes under different inflow conditions were analyzed. Modal decomposition was performed on the propeller wakes calculated by previous numerical simulations [Wang et al., “Propeller wake instabilities under turbulent-inflow conditions,” Phys. Fluids 34, 085108 (2022)]. In particular, the Sparsity-Promoting Dynamic Mode Decomposition (SPDMD) was employed to identify the flow modes that play a dominant role in the inception mechanisms of the propeller wake destabilization. The results showed that the turbulent inflow contributes to the destabilization process of the tip vortex system under the low-turbulence inflow condition, and as a perturbation, it accelerates the interference between adjacent tip-vortex structures, intensifies the instability motion of the tip vortex, and then accelerates the breakdown of the tip vortex system. SPDMD succeeds in selecting a subset of dynamic mode decomposition modes that have the most substantial influence on the quality of approximation of the snapshot sequence. The present study further extends knowledge of propeller wake instability inception mechanisms under different inflow conditions, which can support the development and improvement of next-generation propellers.

Space-filling single square and square fractal grids induced turbulence: Reynolds stress model parameters-optimization

The employment of grid-induced turbulent flow structures as a passive means of augmenting solid-fluid heat transfer is receiving considerable attention, especially in HVAC applications. However, there presently exists a gap in simulation approaches, where industry-accessible CFD packages are unable to accurately express crucial second-order turbulent flow statistics generated by space-filling single square grids (SSGs) and square fractal grids (SFGs). The present study reports a successful application of a revised Reynolds Stress Model (RSM), which accurately replicates the streamwise distributions of centerline mean flow velocity and turbulence intensity in the lee of one SSG (operating under three different flow Reynolds number ReDh) and five geometrically different SFG test cases after undergoing Nelder-Mead downhill simplex optimization of key RSM kernel parameters. The optimized RSM presents a disagreement of, at worst, 4.30% and 9.98%, respectively against experimental hot-wire anemometry measurements of first-order and second-order statistics, and is the first known instance of the RSM being validated for turbulence intensity predictions of SSG- and SFG-induced turbulence. Examination of RSM parameters reveals that the pre-factors for the rates of turbulence dissipation production and destruction (C1,ε and C2,ε) hold greatest effect on simulation accuracy, with additional optimization of the turbulent viscosity pre-factor (Cμ) required for SFG cases. This is attributed to the effect of enhanced turbulent transport due to the cascading and multiscale nature of SFG turbulence, which insofar could not be replicated by Reynolds-Averaged Navier-Stokes (RANS) models. The values of optimized C1,ε, C2,ε, and Cμ range between 1.057 to 1.697, 2.226 to 2.556, and 0.17 to 2.27, respectively. This leads to the largest deviation of −26.6%, 33.1%, and 200% for C1,ε, C2,ε, and Cμ, respectively, when compared to their corresponding default values. With regards to the sensitivity of this parameter set on grid design, it is shown that the grid's fractal iteration number N and thickness ratio tr have greatest influence on the variation of C1,ε, C2,ε, and Cμ, while the effect of ReDh is insignificant. Overall, this study presents an alternative approach to capture the anisotropic and inhomogeneous nature of SSG- and SFG-induced turbulence for industrial heat-transfer applications via an accessible RANS package, which was previously constrained to expensive DNS and LES studies.

Numerical and Physical Modeling of Ponte Liscione (Guardialfiera, Molise) Dam Spillways and Stilling Basin

Issues such as the design or reauditing of dams due to the occurrence of extreme events caused by climatic change are mandatory to address to ensure the safety of territories. These topics may be tackled numerically with Computational Fluid Dynamics and experimentally with physical models. This paper describes the 1:60 Froude-scaled numerical model of the Liscione (Guardialfiera, Molise, Italy) dam spillway and the downstream stilling basin. The k-ω SST turbulence model was chosen to close the Reynolds-averaged Navier–Stokes equations (RANS) implemented in the commercial software Ansys Fluent ®. The computation domain was discretized using a grid with hexagonal meshes. Experimental data for model validation were gathered from the 1:60 scale physical model of the Liscione dam spillways and the downstream riverbed of the Biferno river built at the Laboratory of Hydraulic and Maritime Constructions of the Sapienza University of Rome. The model was scaled according to the Froude number and fully developed turbulent flow conditions were reproduced at the model scale (Re > 10,000). From the analysis of the results of both the physical and the numerical models, it is clear that the stilling basin is undersized and therefore insufficient to manage the energy content of the fluid output to the river, with a significant impact on the erodible downstream river bottom in terms of scour depths. Furthermore, the numerical model showed that a less vigorous jet-like flow is obtained by removing one of the sills the dam is supplied with.

Numerical investigations of coupled aeroelastic performance of wind turbines by elastic actuator line model

With the economic considerations of wind power generation, blades of wind turbines are gradually becoming longer and more flexible. Therefore, it is necessary to research the influence of coupling aerodynamic and elastic structure of blades of wind turbines. Moreover, in a wind farm, the turbulent wind is generated by the irregular movement of the atmosphere, which makes the elastic deformations of blades and the complex wake interference effect between wind turbines. Therefore, the coupled aeroelastic performance of the blade is applied to analyze the interaction of the wake flow field of two tandem wind turbines based on the elastic actuator line model and the Euler-Bernoulli beam theory. Firstly, the correctness of the coupled aeroelastic model is illustrated by comparing the aeroelastic response with OpenFAST software. Secondly, the influence of aeroelastic performances is researched on the structural deformations of blades. It is found that the aerodynamic power and thrust decrease after considering the deformations of blades. Furthermore, the interference effect on the thrust is more evident than aerodynamic power. Thirdly, the far wake region is affected apparently when the deformations of blades are taken into consideration, which causes the tip vortex interference phenomenon to delay. Finally, compared with the uniform inflow condition, the displacement and speed of the deformations of the blades are both increased under the turbulent inflow condition. Meanwhile, the disturbance of the wake vortex in the far wake region by the turbulent wind is stronger. Besides, by comparing the differences between the four vortex identification methods, it is significant that the ΩR method can better capture the vortex system behind the rotor plane without manual intervention threshold selection. The vortex structure also represents the vorticity of the wake vortex evolution.

Approach towards local isotropy in statistically stationary turbulent shear flows

We analyse the approach towards local isotropy in statistically stationary turbulent shear flows using the transport equations for the fourth-order moments of the velocity derivative. It is found that terms of these equations representing the large-scale contribution associated with the uniform mean velocity gradient gradually decrease as the Taylor microscale Reynolds number $Re_\lambda$ increases, and finally disappear when $Re_\lambda$ is sufficiently large. This gradual weakening of the large-scale effect is accompanied by a gradual approach towards local isotropy of the small-scale motion. The rate at which local isotropy is approached depends on the weakening of the large-scale forcing, which is controlled by the magnitude of the non-dimensional velocity shear parameter $S^*$ ( $\equiv \overline {u_1^2}({{\partial {{\bar U}_1}}}/{{\partial {x_2}}})/{\bar {\varepsilon }_{iso}}$ , where $\bar {\varepsilon }_{iso}$ is the isotropic mean turbulent energy dissipation rate, $\overline {u_1^2}$ is the streamwise velocity variance, and ${\partial {{\bar U}_1}/\partial {x_2}}$ is the uniform mean velocity gradient in the transverse direction). In particular, we show that the approach towards local isotropy can be recast in the form $C\, Re_\lambda ^{-1}$ , where $C$ is the product of $S^*$ and a ratio of transverse-to-streamwise velocity derivative variances. This is consistent with the behaviour of the normalized third-order moments of transverse velocity derivatives. With the further use of the transport equations for the eighth- and twelfth-order velocity derivative moments, it is found that the even moments of transverse velocity derivatives can significantly affect the rate at which local isotropy is approached, especially for higher orders.