Michael Todorov

Analysis of torsional oscillation of the drive train in horizontal-axis wind turbine

The unsteadiness of rotor inflow caused by the atmosphere creates continuous variations of blade loads and rotor torque. Usually these loads are amplified by rotor shaft and gearbox elasticity and inertia. The authors propose here a dynamic multi-body model where the wind turbine includes a rotor, a drive train and an electrical generator. The drive train has a three stage gearbox which contains two high-speed parallel gear stages and a low-speed planetary gear stage. The model consists of 10 bodies and has 8 degrees of freedom taking into account the stiffness of the engaged tooth pairs. In this model the aerodynamic torque is applied as an external load. The calculation permits to obtain natural frequencies, mode shapes, time series of torsional oscillations and amplitude-frequency characteristics for an industrial wind turbine. The results show that transient loads in the gearbox are very high and need special attention.

Aerodynamic characteristics of airfoil with single plain flap for light airplane wing

A numerical study was performed on a NACA 23012 airfoil with a single plain flap to examine the aerodynamic coefficients at Reynolds number of 3×106, and to help for identifying the forces acting on a light airplane wing. Besides, in the paper the flow fields around the airfoil with single plain flap were shown. All calculations were made using a CFD code. For a turbulent model the Spalart-Allmaras method was chosen. Conclusions were made about the aerodynamic efficiency of the proposed configuration wing-single plain flap.

Numerical model for rapid computatuon of the flow-field of a rotor in hover

In the present article, the authors are performing a comparison study between two vortex models for the rapid numerical computation of the static thrust of a helicopter rotor in hover. The aim of this study is to access the rapidity and accuracy of a vortex model, comprised of a series of vortex rings and a single semi-infinite vortex cylinder, compared to a helical wake model with straight-line segmentation. Studies are performed for the optimal number of vortex elements, as well as for their optimal positioning, in order to provide both fast computation and good agreement with the experimental data obtained from a wind tunnel test of a model rotor. The studied vortex model is shown to significantly reduce the amount of computational time required to evaluate the performance of the model rotor, when compared to the helical wake model, while offering a similar degree of accuracy.

NUMERICAL STUDY ON THE INFLUENCE OF WAKE CONTRACTION ON THE COMPUTATIONAL ACCURACY AND RAPIDITY OF A MODEL OF A ROTOR IN HOVER

In the present article, the authors are performing a comparison study between two vortex models for the numerical computation of the static thrust of a model helicopter rotor in hover. The aim of this study is to access the rapidity and accuracy of the vortex models, comprised of a series of vortex rings and a single semi-infinite vortex cylinder. The first model has a cylindrical arrangement of the vortex elements, while the second model has a contracting arrangement of its vortex elements. Studies are performed for the optimal positioning and spacing of the vortex elements, in order to provide both rapid computation and good agreement with the experimental data, obtained from a wind tunnel test of the model rotor. The results of the study show a slight increase in the required computational time for the case of the contracting wake, while offering a significantly better accuracy than the cylindrical wake. Another interesting result from the numerical study is that in the case of the contracting wake model there is a reduction of the spacing (compression) between the vortex rings in the near wake, which results in the reduction of the overall length of the vortex system trailing downstream of the rotor.

INFLUENCE OF THE NUMBER OF VORTEX ELEMENTS ON THE STABILITY AND ACCURACY OF THE NUMERICAL SIMULATION FOR A ROTOR IN HOVER

Numerical simulations based on vortex models are very sensible from both the number of vortex elements and the initial conditions. The aim of this study is to evaluate the influence of the number of the vortex elements, modelling the wake, on the stability and accuracy of the numerical solution for the performance of a rotor in hover. For each computational case the number of the vortex elements varies from the number of blades per rotor and from the number of the emitted vortices per blade, which are taken into account. It was observed that larger series of vortex rings provide more accurate results, while a quicker convergence of the numerical solution is achieved with a smaller number of vortex elements. The use of time-stepping predictor-corrector scheme of second order, contributed for an increased stability of the simulation of the convection and propagation of the free-wake in the computational domain. The numerical results are compared against previously obtained experimental data for a model rotor.