• Influence of Yawed Wind Flow on the Blade Forces/Bending Moments and Blade Elastic Torsion for an Axial-Flow Wind Turbine

      Ahmadi, Mohammad H. B.; Yang, Zhiyin; University of Derby (American Society of Mechanical Engineers, 2021-09-16)
      Effects of yawed incoming flow on wind turbine blades forces and root bending moments (RBMs) are not fully understood. To advance our current understanding, numerical studies of a small-scale three-bladed horizontal axis wind turbine at TSR = 6.7 with yaw angles of zero and 45° have been carried out to examine the variations of blade and rotor loading due to the yawed incoming flow. An approach combining Large Eddy Simulation (LES) with Actuator Line Modelling (ALM) has been employed in the present study. The predicted phase-averaged 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 yawed 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.
    • Large-eddy simulation of transitional flows using a co-located grid

      Langari, Mostafa; Yang, Zhiyin; Page, Gary J.; University of Sussex, Brighton, UK; University of Derby, UK (Taylor and Francis, 2013-07-12)
      A large-eddy simulation (LES) of a transitional separated flow over a plate with a semi-circular leading at low (<0.2%) and high (5.6%) free-stream turbulence (FST) has been performed, using a co-located grid with the Rhie–Chow pressure smoothing. A numerical trip is used to produce a high FST level and a dynamic subgrid-scale model is also employed in the current study. The entire transition process leading to breakdown to turbulence has been shown clearly by the flow visualisations using instantaneous spanwise vorticities, and the differences between the low- and high-FST cases are clearly visible. Coherent structures are also visualised using isosurfaces of the Q-criterion, and for the high-FST case, the spanwise-oriented quasi-two-dimensional rolls, which are clearly present in the low-FST case, are not visible anymore. Detailed quantitative comparisons between the present LES results and experimental data and the previous LES results at low FST using a staggered grid have been done and a good agreement has been obtained, indicating that the current LES using a co-located grid with pressure smoothing can also predict transitional flows accurately.
    • Large-eddy simulation: a glance at the past, a gaze at the present, a glimpse at the future

      Yang, Zhiyin; University of Derby, UK (2014-09-15)
      Large-Eddy Simulation (LES) was originally proposed for simulating atmospheric flows in the 1960s and has become one of the most promising and successful methodology for simulating turbulent flows with the improvement of computational power. It is now feasible to simulate complex engineering flows using LES. However, apart from the computing power, significant challenges still remain for LES to reach a level of maturity that brings this approach to the mainstream of engineering and industrial computations. This paper will describe briefly LES formalism first, present a quick glance at its history, review its current state focusing mainly on its applications in transitional flows and gas turbine combustor flows, discuss some major modelling and numerical challenges/issues that we are facing now and in the near future, finish with the concluding remarks.
    • Numerical study of the coupling between the instantaneous blade loading/power of an axial wind turbine and upstream turbulence at high Reynolds numbers

      Ahmadi, Mohammad H.B.; Yang, Zhiyin; University of Derby (Elsevier BV, 2020-07-06)
      Little is known about how the range of scales in the approaching turbulent flow can interact dynamically with wind turbines and influence its ability to produce power. Here, a numerical study of a horizontal-axis wind turbine at different Reynolds numbers (corresponding to different tip speed ratios) has been conducted to investigate the instantaneous turbine response to upstream turbulence. A computational approach, combining large eddy simulation with actuator line modelling, is adopted. Comparison between Power Spectral Density (PSD) of the turbine thrust/power and PSD of the velocity at the rotor plane and one rotor diameter upstream of it confirms that there is a coupling between the instantaneous turbine thrust/power and the upstream turbulence (one diameter upstream of the turbine) for frequencies below a critical frequency. Furthermore, it has been shown for the first time, that PSD of the turbine thrust/power and the velocity PSD at the rotor plane are very similar, indicating that the instantaneous turbine thrust/power and the velocity at the rotor plane are coupled for all frequencies. This means that the PSD of velocity at the rotor plane or shortly behind it can provide interesting information for the instantaneous turbine loads that are very important for the turbine operational life.
    • On wind turbine power fluctuations induced by large-scale motions

      Ahmadi, Mohammad; Yang, Zhiyin; University of Derby (Elsevier, 2021-04-21)
      Our current understanding on the dynamic interaction between large-scale motions in the approaching turbulent flow and wind turbine power is very limited. To address this, numerical studies of a small-scale three-bladed horizontal axis wind turbine with cylinders placed in front of it to produce energetic coherent structures of varying scale relative to the turbine size have been carried out to examine the temporary variations of the turbine power. The predicted spectra reveal a strong interaction between large-scale turbulent motions generated by cylinders and the instantaneous turbine power. More specifically, it shows how the large dominant turbulent scales of incoming flow affect the spectral characteristics of turbine power, i.e, determining the level and trend of the turbine power spectrum. Comparisons reveal that there are two critical frequencies recognisable in the turbine power spectrum: the first one, close to the turbine rotational frequency, above which the coupling of upstream flow and turbine power disappears; the second one, identified for the first time and related to the dominant large-scale motions which dictate the level and trend of the turbine power spectrum. This study also shows that the strong scale-to-scale interaction between the upstream flow and turbine power reported previously does not appear at high Reynolds numbers.