Upstream Velocity Research Articles

Parameter identification of two coupled oscillator model for pure intrinsic thermoacoustic instability

Thermo-acoustic instabilities are a well-known problem in combustions systems. In general, the type of these modes can be divided in two categories: (i) modes with an acoustic origin and (ii) modes with an intrinsic thermo-acoustic (ITA) origin. It is an challenging task to observe the ITA modes experimentally. Therefore, a setup with a burner in an acoustic embedding with close to anechoic up- and downstream terminations is required. In previous research we introduced such a setup and conducted an experimental bifurcation analysis on the dominant pure ITA mode. In this bifurcation analysis, we observed that for increasing upstream velocity the flames loose stability through a supercritical Hopf bifurcation and subsequently exhibit limit cycle, quasi-periodic and period-2 limit cycle oscillations. The quasi-periodic oscillations were characterized by low frequency amplitude and frequency modulation. In this research we present a phenomenological model consisting of two coupled oscillators that is able to reproduce all the different experimentally observed regimes. This model consists of a nonlinear Van der Pol oscillator and a linear mass-spring-damper oscillator, which are nonlinearly coupled to each other. Furthermore, we executed a parameter identification of the model and concluded that the model is able to describe the experimental data quantitatively well.

The response of a conical flame to a dual-frequency excitation

This work investigates the response of a conical premixed flame to a dual-frequency excitation, based on the integrated CH* signal collected from a photomultiplier tube (PMT), the upstream velocity disturbance measured by a hotwire, and the chemiluminescence signal captured by high-speed imaging. The results show that, in addition to the excitation frequencies, a notable flame response can also be observed at the difference frequency, where the corresponding velocity fluctuation is relatively small. This result means that, at the difference frequency, the velocity fluctuation contributes little to the flame response. Such interacted response generally occurs at intermediate excitation frequencies but tends to disappear as either excitation frequency is below the cut-off frequency. And it increases linearly with the excitation amplitude, with nearly zero dependence on the phase difference. Furthermore, the flame front is extracted based on the chemiluminescence images to analyze the flame area fluctuation. The resultant phase response implies that the fluctuation of the difference frequency propagates downstream convectively, similar to that of the excitation frequencies. Interestingly, the flame area fluctuation at the difference frequency shows significant response and a low-pass characteristic, whereas the CH* fluctuation approaches zero at those low frequencies.

Biomimetic crossflow filtration with wave minimal surface geometry for particulate biochar water treatment

Wave minimal surfaces (WMSs) are mathematically defined structures that are commonly observed in nature. Their unique properties have allowed researchers to harness their potential for engineering applications. Since WMSs can be represented by mathematical equations, the geometry can be parametrized and studied using computational fluid dynamics (CFD) for particle separation. Low energy particle separation in water treatment can yield low-carbon footprint technology approaches such as biochar water treatment where removal and recovery of adsorbed N and P on biochar can address water pollution, climate change and food security. The objective of this work was to demonstrate the capability of WMS as a crossflow filtration system to remove particulates in water. For this purpose we used CFD to optimize WMS geometry and studied the performance of the 3D-Printed (3DP) optimized WMS using experimental fluid dynamics (EFD) in a water tunnel. CFD studies quantified planar vorticity, fluid filtrate flux, and particle behavior of WMS. For inflow velocities of 0.2–0.4 m/s, CFD results showed that a reverse wave filter design with convex shape leading-edge, angle of incidence of 90o, and maximum width of n = 1.0 captured 15–25% of upstream velocity at the filter port. CFD analysis showed more than 95% separation efficiency at velocities and pressures of 0.2–0.32 m/s and 5–35 kPa, respectively. Particle Image Velocimetry (PIV) was used for EFD fluid flow measurements with an optimized wave minimal surface (OMWS). Comparison of OMWS CFD and PIV velocity fields showed good agreement with a root-mean-square error of less than 10%. Particle size analysis showed that the 3DP OMWS could filter particle sizes ranging from 1–30 μm with at least 50% particle count reduction in the filtrate. Thus, we successfully demonstrated a novel framework for analyzing a crossflow water filtration system from conceptual design to initial benchtop experiments using iterative CFD, 3DP, and EFD.

Experimental and multiphase modeling of small vertical-axis hydrokinetic turbine with free-surface variations

Vertical-axis hydrokinetic turbines are promising option to harness the low velocity currents. However, limited investigations have been carried out considering the interactions between the turbine rotor and the channel section, including the free-surface. Thus, a vertical-axis turbine model has been designed and manufactured, to be tested in an open channel. Also, a three-dimensional multiphase simulation has been carried out, using the volume of fluid (VOF) model, to capture the air–water interface and to investigate the free-surface variations effects on the turbine output. Experimentally, the turbine model has been characterized under different flow conditions and free-surface levels. The peak power coefficients are found to increase with the upstream velocity. This effect is directly linked to the blockage ratio and the Froude number. A reasonably good match has been found between the experimental and the numerical results. The VOF model is able to simulate the free-surface longitudinal variations, and the effect of the turbine blockage of the channel. The velocity field and the pressure coefficient distribution, around the turbine rotor, have been studied and correlated with the free-surface variations from upstream to downstream of the turbine. Finally, the vortex street generated by the blade–plate interaction has been analyzed.

VALIDATION OF A STOCHASTIC BREAKUP MODEL FOR TURBULENT JETS IN HIGH-SPEED CROSSFLOW: ASSESSMENT OF TURBULENT INTERACTIONS AND SENSITIVITY TO BOUNDARY CONDITIONS

Improving the mixing of fuel and air by injecting a turbulent liquid fuel jet into a high-speed cross-flowing gas can reduce the emissions of gas turbine applications. To facilitate and hasten the development of such low-emissions technologies, accurate predictions of the spray characteristics are needed. The objective of the present study is to validate the predictive capabilities of a stochastic breakup model for turbulent transverse jets over a wide range of representative pressures and atomization characteristics. The effect of turbulence modeling is also assessed to provide accurate and computationally less expensive Eulerian-Lagrangian transient approaches. To do so, the predictions made with the large eddy simulation (LES) approach for different subgrid-scale (SGS) models and with the synthetic eddy method (SEM) are compared to the ones made using the unsteady Reynolds-averaged Navier-Stokes (RANS) approach with and without a turbulent dispersion model. The sensitivity of the numerical methodology to the upstream velocity profile, pressure, and momentum flux ratio were also assessed. Properly accounting for the upstream gas velocity profile was found to be critical to ensure accurate predictions of the spray characteristics. The unsteady RANS (URANS) turbulent approach coupled with the turbulent dispersion model showed good agreement with experimental data, but the LES approach tends to overpredict the spray penetration and underpredict the Sauter mean diameter (SMD). This could be due to the lower turbulent interactions it predicts, which may lead to lower momentum transfer between the phases.

Study on the flow mixing of hot and cold fluids in the T-junction under rolling motion condition

The present study numerically investigates the flow mixing characteristics in the T-junction with momentum ratio (MR) of 0.95 under rolling motion condition. The effects of different inflow rates of 0.47–1.9 kg·s−1 and the synergistic effects between velocity and temperature are discussed. The results show that the rolling motion significantly changes the upstream instantaneous velocity field, especially that near the wall, which induces the variations of the instantaneous MR and the mixing length of the temperature field. The rolling motion increases the range of the velocity fluctuation while decreasing that of the temperature fluctuation. Under rolling motion condition, increasing the inflow rates decreases the range of the velocity fluctuations and inhibits the variation of MR. This weakens the flow mixing of main and branch fluids thereby increasing the range of the temperature fluctuations. In addition, it is found that the synergistic effects between velocity and temperature occur at the junction of the main and branch fluids, where the power spectrum densities (PSDs) of y velocity component fluctuation and temperature fluctuation exhibit the same frequency peaks of 0.2 Hz, 5.8 Hz, 14.7 Hz, and 31.2 Hz. By comparing the PSDs along with the y-axis, it is revealed that the high temperature fluctuation area of y/D = 0–0.33 has a faster energy cascade with a larger slope of −5/3 in the low-frequency area (0.1–10 Hz) and −13/2 slope in the high-frequency area (>10 Hz).

Architectural Knitted Windbreaks for Improved Wind Comfort in the City: A Wind Tunnel Study of Custom-Designed Porous Textile Screens

There is a need to shield from the wind to improve pedestrian comfort in urban environments. Perforated windbreaks, such as fences, vegetation or textile nets, have proven to be an efficient solution, whereas knitted textiles have not yet been explored. The purpose of this study was to evaluate the capacity of knitted textile windbreaks to reduce wind velocities, to inform further research and promote wider architectural applications. Five custom-knitted textile prototypes, representing fragments of textile windbreaks, were tested in a wind tunnel and compared against a perforated and a nonperforated solid board. Forces on the models, as well as upstream and downstream velocities, were measured. The results indicate that the optimal optical porosity of knitted windbreaks should be around 10%, which differs from the porosity for perforated windbreaks recommended by prior studies. Moreover, it was observed that a textile windbreak knitted using the drop-stitch technique efficiently reduces the wind, while not generating a large drag force. Furthermore, the drag coefficient for the knitted windbreak is reduced with increased windspeed. With this, the presented study demonstrates that knitted structures exposed to wind influence have the functional potential of becoming efficient windbreaks, thus improving wind comfort and aesthetic user experience in the urban space.

A study on the effect of driver section length on the flow field inside a two-dimensional closed-ended viscous shock tube: A numerical investigation

The interaction of a reflected shock wave with the boundary layer inside a closed-ended shock tube is studied by many researchers using different numerical schemes. However, the interaction of a reflected shock wave having a decaying pressure profile ahead of the reflected shock is lacking in the literature. It is essential to quantify the deviation in flow characteristics experienced in tests performed inside the shock tube compared to field tests while studying blast wave attenuation and mitigation. In the present study, the effect of transient upstream pressure on the shock wave reflecting from a solid wall and the associated flow separation inside the boundary layer is examined in detail through numerical simulations using the viscous shock tube problem for Re = 1000. Simulations were performed using an in-house 13th-order high-resolution hybrid solver with 50 million grid points. The transient pressure profile behind the incident shock wave is achieved by varying the non-dimensional driver section length from 0.5 to 0.02 while keeping the overall shock tube length as 1. The flow field is scrutinized through numerical schlieren and vorticity plots. It is observed that the decaying pressure profiles behind the incident shock wave have an immense impact on the dynamics of shockwave-boundary layer interaction and the vortices emanating from the shear layer due to Kelvin–Helmholtz Instability. The secondary vortices inside the boundary layer, vortices at the shear layer, and compression waves in the separated boundary layer region reduce with a decrease in driver section length. The triple point height increases as driver section length decreases due to the weakening of upstream velocity. The Kelvin–Helmholtz vortices originating from the triple point moves back into the separated boundary layer rather than interacting with the end wall. It suggests that the decaying pressure behind the incident shock wave minimizes the secondary effects on the solid wall during shock wave impingement.

On wind turbine loading induced by non-uniform approaching flow at high Reynolds numbers

Influences of non-uniform incoming flow on the wind turbines blades forces and root bending moments (RBMs) are not fully understood. To advance our current understanding, numerical studies of a three-bladed horizontal axis wind turbine with cylinders placed in front of it to produce non-uniform flow approaching the turbine with different non-uniformity levels have been carried out to examine the variations of blade and rotor loading due to the non-uniform incoming flow. The phase-averaged predicted blade forces reveal that the blade tangential force, in-plane RBM, and power coefficient are much more sensitive to the upstream streamwise velocity variations and are much more strongly affected than the blade axial force, out-of-plane RBM, and thrust coefficient. It also shows that for non-uniform incoming flows the blade axial force to the blade tangential force ratio fluctuates significantly during one rotor revolution, resulting in large variations of the blade elastic torsion and that the total blade force (magnitude and direction) undergoes a non-linear change in the circumferential and radial directions, which will likely lead to the reduction in the turbine operational life significantly, especially for long lightweight blades of large size wind turbines. This study also shows different behaviors of the blade forces along the blade span under non-uniform upstream flows in terms of the amplitudes and standard deviations of their oscillations. For the blade tangential force, λ and σ increase monotonously along the blade span up to near the blade tip, whereas those of the blade axial force increase up to approximately 0.6 blade span and show an opposite trend behind that.

Scaling of reconnection parameters in magnetic island coalescence: Role of in-plane shear flow

A 2D incompressible viscoresistive-MHD model [Mahapatra et al., Phys. Plasmas 28, 072103 (2021)] is used to study the scaling of reconnection parameters in the magnetic island coalescence problem under two interesting scenarios. First, the effect of changing island half-width at a fixed system size is investigated. As the island half-width increases, the total magnetic flux content of the islands increases, resulting in an increase in upstream magnetic field, upstream velocity field, and unnormalized reconnection rate. However, the downstream magnetic field, current sheet length and normalized reconnection rate (normalized to the upstream magnetic field and upstream Alfvénic velocity) remain independent of it. Interestingly, the reconnection rate is found to be different from the upstream to downstream velocity ratio as well as from the aspect ratio of the current sheet, as opposed to the findings of the Sweet–Parker model. Second, the in-plane shear flow effects are studied, keeping the island width and system size fixed. Here, thickness and length of the current sheet, the upstream magnetic and velocity field components, reconnection rate and time, current sheet inclination angle with shear flow length scale, and amplitude are calculated. Interestingly, the inclination angle of the current sheet and the diffusion region are found to be different, and the differences are more in stronger shear flows. These results are significantly different from the Harris sheet setup with shear flow.

Lab-scale measurements of wind farm blockage effects

The inflow an individual wind turbine experiences in a farm layout is strongly dependent on flow interaction effects with the turbines surrounding it. In addition to the well-known wake flows, lateral and upstream flow interaction effects, commonly described as wind farm blockage effects, have also recently gained some attention.In this work, flow measurements in the upstream induction zone of neighboring turbines are presented for different arrangements of three to seven porous actuator discs. It is shown how the upstream induction zones of individual discs merge into a single zone when decreasing the lateral spacing between three discs. As a result, the central disc experiences a higher thrust force than its neighboring discs. For farm arrangements in two rows, an aligned disc setup is observed to result in a larger upstream velocity decrease than offset arrangements.

Effect of Airflow Non-Uniformities on the Thermal Performance of Water–Air Heat Exchangers—Experimental Study and Analysis

The thermal performance of fin-and-tube heat exchangers (HX) is a crucial aspect in a multitude of applications and fields; several design and operational parameters influence this performance. This study focuses on the issue of flow maldistribution and its effect on the HX thermal performance. For this purpose, an experimental setup is designed and implemented to emulate the conditions under which an automotive heat exchanger operates in regard to the non-uniform upstream airflow velocity distribution over the HX surface. The setup allows obtaining various configurations of airflow velocity non-uniformity of some desired mean velocity and standard deviation. The experimental results reveal that a higher degree of non-uniformity (higher standard deviation of the velocity distribution) causes an increased deterioration of the HX thermal performance. For example, at a water flowrate of 200 L/h and a mean airflow velocity of 2 m/s, increasing the standard deviation from 0 to 2 m/s (i.e., moving from the lowest to highest degrees of non-uniformity) causes a total deterioration of 27% in the performance (3.78 to 2.75 kW, respectively), which can also be observed in the increased level of outlet water temperature (53.8 to 58.2 °C, respectively). The obtained results confirm the numerical results reported in the literature.

Determination of the Response Amplitude Operator of a tidal turbine as a spectral transfer function

A transfer function determination method is proposed in this study to predict the unsteady fluctuations of the performance of a tidal turbine model. This method is derived from the Response Amplitude Operator (RAO) applied in the offshore industry to predict linear wave-induced loads on large structures. It is based on a spectral approach and requires the acquisition of a turbine parameter (e.g. torque, thrust, power or root-blade force) in synchronization with an upstream flow velocity measurement. On the frequency range where the causality between these two signals is proven, the transfer function is established using the ratio between the cross-spectral density and the spectral density of the incoming velocity.The linearity is verified using the coherence function, which shows validity for the turbine power in the lowest frequencies only. This transfer function is then used to reconstruct the power fluctuations which is compared to the recorded one for a particular flow condition with bathymetry generated turbulence. The result shows the dependence on the accurate location of the velocity measurement point used for the reconstruction. This point must exactly correspond to the expected turbine location, i.e. where the turbine response needs to be processed. Bearing in mind its limits, the method can be used to predict the loadings of extreme events on the turbine structure and the performance variations corresponding to the unsteady characteristics of a turbulent flow, for a better grid integration.

Time-resolved velocity and ion sound speed measurements from simultaneous bow shock imaging and inductive probe measurements.

We present a technique to measure the time-resolved velocity and ion sound speed in magnetized, supersonic high-energy-density plasmas. We place an inductive ("b-dot") probe in a supersonic pulsed-power-driven plasma flow and measure the magnetic field advected by the plasma. As the magnetic Reynolds number is large (RM > 10), the plasma flow advects a magnetic field proportional to the current at the load. This enables us to estimate the flow velocity as a function of time from the delay between the current at the load and the signal at the probe. The supersonic flow also generates a hydrodynamic bow shock around the probe, the structure of which depends on the upstream sonic Mach number. By imaging the shock around the probe with a Mach-Zehnder interferometer, we determine the upstream Mach number from the shock Mach angle, which we then use to determine the ion sound speed from the known upstream velocity. We use the sound speed to infer the value of Z̄Te, where Z̄ is the average ionization and Te is the electron temperature. We use this diagnostic to measure the time-resolved velocity and sound speed of a supersonic (MS ∼ 8), super-Alfvénic (MA ∼ 2) aluminum plasma generated during the ablation stage of an exploding wire array on the Magpie generator (1.4 MA, 250ns). The velocity and Z̄Te measurements agree well with the optical Thompson scattering measurements reported in the literature and with 3D resistive magnetohydrodynamic simulations in GORGON.

Reconstruction of flow around a high-rise building from wake measurements using Machine Learning techniques

This paper investigates the unsteady flow around a high-rise building using OpenFoam. A Vortex Method is developed to generate upstream unsteady fluctuations that are validated considering the numerical simulation of a neutral atmospheric boundary layer around a high-rise building. The Vortex Method parameters are tuned to produce an upstream velocity profile whose turbulence characteristics are comparable to experimental data. Comparison between mean and fluctuating velocity profiles of the numerical data measured in the building’s wake with experimental data shows satisfactory results.The resulting database is used to reconstruct the flow from limited velocity measurements inside the wake and static wall pressure measurements on the building surface. Machine learning techniques such as linear regressions (Ridge, Lasso, ElastikNet, MultiTaskLasso regression) and Artificial Neural Network are tested and compared. The flow reconstruction using velocity measurements inside the wake leads to a better result compared to the flow reconstruction from the wall pressure measurements. At the same time, it was noticed that the Artificial Neural Network regression does not lead to more satisfactory results compared to linear regression techniques.

A framework for exploring environmental risk of the longest inter-basin water diversion project under the influence of multiple factors: A case study in China

Multi-factor risk assessment is an important prerequisite for water quality protection and the safe operation of mega hydro-projects. As the largest long-distance inter-basin water diversion project in the world, the Middle Route of the South-to-North Water Diversion Project of China (MRSNWDPC) has been in operation for 8 years and has benefited 79 million people along the canal. However, concerns have been raised in recent years about the potential negative effects of abnormal algal proliferation in the MRSNWDPC. It is very important for the safety of water supply to carry out relevant risk analysis and formulate regulatory management. In order to quantitatively evaluate the risk of algal proliferation in the MRSNWDPC under the influence of multiple factors, a multivariate risk assessment method based on Vine Copula theory and Monte Carlo simulation was proposed. Five key factors (water temperature, flow velocity, flow rate, algal cell density, and dissolved oxygen) were used and multiple dependency models in each section of the MRSNWDPC from January 2016 to January 2019 were established to study the risk of algal proliferation under multiple scenarios. The results demonstrate that water temperature can be used as an appropriate early-warning indicator of algal proliferation. The early-warning interval (unit: °C) of water temperature in the upper, middle, and lower reaches are 26–29°C, 23–26°C, and 21–23°C, respectively. Unlike bivariate analysis, the multiple dependency model describes the relationship between variables more accurately and enriches the scenarios of multiple conditional probabilities. When the water temperature fluctuates in the early-warning interval, regulating the upstream, midstream, and downstream flow velocity to be higher than 0.6 m/s, 0.5 m/s, and 0.6 m/s, respectively, can effectively reduce the risk of algal proliferation. This research not only provides a reference for the ecological control of algae in the MRSNWDPC and similar mega hydro-projects but also enriches the application of the Vine Copula theory coupled with the random sampling method for multi-variable risk analysis.

LES-based prediction of technically premixed flame dynamics and comparison with perfectly premixed mode

The present study combines Large Eddy Simulation (LES) with System Identification (SI) to determine the Flame Transfer Functions (FTFs) of technically premixed flames that respond to fluctuations of upstream velocity as well as equivalence ratio. Two variants to obtain the corresponding FTFs from numerically determined time series data are reported and compared with the experimental results. The experiment does not measure heat release rate directly but instead the CH* chemiluminescence. This is insufficient for FTF identification of technically premixed flames but can be used for the validation of the simulation. We implemented a CH* post-processor in the simulation and validated with the experiment. After validation, the simulation is used to identify the contributions of velocity and equivalence ratio to the FTF of technically premixed flame dynamics. We propose and compare two approaches for the identification of FTFs. The direct approach via multiple-input single-output system identification requires one simulation with simultaneous excitation of fuel and air inlets and carefully chosen input signals. The second approach reconstructs the FTF decomposition from two separate simulations, one perfectly premixed and one technically premixed, with reduced requirements on signal quality. We compare both approaches and discuss the FTFs of perfectly and technically premixed flames. Overall, the LES/SI approach proved to be flexible and reliable for technically premixed flames.

Flow Dynamics Between Two Car Models: Influence Of The Intervehicle Distance

Air pollution is a crucial issue since it is one of the major causes for worldwide deaths. Car drivers and passengers are daily exposed to high level of pollutant concentrations. The particles emitted from the tailpipe can disperse in the wake of the vehicle and then infiltrate the following car c abin. It has been demonstrated that the dynamics of such particles are highly affected by the properties of the flow downstream the emitting vehicle. In the present paper, the flow characterization in the wake of a simplified car model followed by a second vehicle is performed. Reduced-scaled Ahmed bodies are used. The upstream velocity is set to U∞ = 14.3 m/s, corresponding to a Reynolds number based on the simplified model’s height ( hc = 54 mm) of Re = 49500. Two leading vehicles with two different rear slant angles (φ = 0°, 25°) are considered. Two sets of experiments are carried out, one in the wind tunnel at ESTACA and the other in the wind tunnel at CORIA’lab. In the first measurement campaign, 2D LDV is used and six inter-vehicle distances ranging from d = 0.93 hc to 5.56 hc are considered and presented. The ITTT (Interarrival Time and Transit Time) method is applied to LDV data in order to ensure reliable results. Mean and turbulent properties of the wake between the two vehicles are described. The role of the rear slant angle is highlighted. A critical distance above which the followed vehicle has an insignificant influence on the flow topology downstream of the leading vehicle is identified. For further details, and taking into consideration the first results (from LDV measurements), Stereo-PIV measurements are used to get access to the three components of the velocity. Due to time constraint, only three inter-vehicle distances are discussed in this paper (0.93 hc, 1.85 hc and 3.70 hc). The SPIV results validate those obtained from LDV measurements and provide complementary information. The sensitivities of the bi-stability of the transverse velocity to the short inter-vehicle distance (0.93 hc) and to the slant rear angle are investigated. SPIV measurements are then correlated with LDA measurements taking into account this behavior. These measurements are completed by further measurements of concentrations of the exhausted particles from the tailpipe.

Including realistic upper atmospheres in a wind-farm gravity-wave model

Abstract. Recent research suggests that atmospheric gravity waves can affect offshore wind-farm performance. A fast wind-farm boundary layer model has been proposed to simulate the effects of these gravity waves on wind-farm operation by Allaerts and Meyers (2019). The current work extends the applicability of that model to free atmospheres in which wind and stability vary with altitude. We validate the model using reference cases from literature on mountain waves. Analysis of a reference flow shows that internal gravity-wave resonance caused by the atmospheric non-uniformity can prohibit perturbations in the atmospheric boundary layer (ABL) at the wavelengths where it occurs. To determine the overall impact of the vertical variations in the atmospheric conditions on wind-farm operation, we consider 1 year of operation of the Belgian–Dutch wind-farm cluster with the extended model. We find that this impact on individual flow cases is often of the same order of magnitude as the total flow perturbation. In 16.6 % of the analyzed flows, the relative difference in upstream velocity reduction between uniform and non-uniform free atmospheres is more than 30 %. However, this impact is small when averaged over all cases. This suggests that variations in the atmospheric conditions should be taken into account when simulating wind-farm operation in specific atmospheric conditions.

On loading fluctuations induced by energetic large-scale motions for horizontal axis wind turbines

The interaction of horizontal axis wind turbines (HAWTs) and large-scale energetic flows is not fully understood. The unsteady flow approaching turbines could result in highly unsteady blade forces and root bending moments (RBMs) which would have a strong influence on the turbine's operational life span, especially for large-scale wind turbines. To provide a better understanding of this interaction, numerical simulations of a three-bladed HAWT with a cylinder located at its upstream to produce energetic motions of varying scale relative to the turbine size have been carried out to examine the temporary variations of the turbine root bending moments. The predicted spectra reveal that the energetic large-scale motions produced by the cylinder strongly interact with the instantaneous blade forces and, thus, root bending moments. It specifically shows how dominant large-scale motions approaching the turbine significantly influence the spectral characteristics of flapwise and edgewise RBMs in terms of level and trend. The comparison shows that below about half of the blade passage frequency, RBMs spectra correlate well with those of upstream velocity, whereas above this frequency, RBMs spectra are mainly influenced by the turbine rotational frequency. This study also shows that the strong scale-to-scale interaction between the upstream flow and turbine loading reported previously does not appear at high Reynolds numbers in the present study.