Youssef Bichiou

Unsteady aeroelastic behaviors of rigid airfoils with preset angles of attack

The effects of varying the angle of attack on the flutter speed and limit cycle oscillations of an aeroelastic system are investigated. This system consists of a plunging and pitching rigid airfoil supported by linear springs. The unsteady vortex lattice method is used to model the unsteady flow. The objective is to determine how the flutter boundary is affected by changing the angle of attack. To solve simultaneously and interactively the governing equations, an iterative scheme based on Hamming’s fourth order predictor–corrector model is employed. Several numerical simulations are conducted for various angles of attack to determine their effects on the dynamic behavior of the aeroelastic system and particularly on the dynamic stability or flutter speed and the nonlinear response of the system. The results show that the flutter speed increases as the angle of attack is increased. It is also determined that increasing the preset angle of attack results in a decrease in the dynamic amplitudes of the nonlinear response. In other words, increasing the angle of attack offers a way to control the system in terms of delaying flutter and reducing the limit-cycle oscillations amplitudes.

Nonlinear aeroelastic characterization of wind turbine blades

A nonlinear aeroelastic characterization of wind turbine blades is performed. A two-dimensional aerodynamic model based on the quasi-steady approximation is coupled with a plunging and pitching blade section. The governing nondimensional equations are derived. The normal form of the Hopf bifurcation is derived and used to characterize the behavior of the system. Using linear analysis, it is demonstrated that, as the blade radius and/or operating rotational speed are increased, wind turbine blades become more susceptible to flutter at freestream velocities that are close to the cut-out speed. The nonlinear analysis, based on the normal form of the Hopf bifurcation, shows that, depending on the nonlinear structural parameters and initial conditions, subcritical instability may take place which means that high limit-cycle oscillation amplitudes may take place at freestream velocities that are lower than the linear flutter speed.

Investigation on the Effectiveness of a Nonlinear Energy Sink on an Aeroelastic System

The effectiveness of a passive control system, namely, the Nonlinear Energy Sink (NES) in the control of the response of a nonlinear aeroelastic system is investigated. The system consists of a rigid airfoil elastically mounted on linear and nonlinear springs. The structure equations are derived using Lagrange’s equations. The quasi steady aerodynamics are used to model the aerodynamic loads. The NES having linear damping is attached to the aeroelastic system. The parameters of the passive controller are varied in order to investigate the efficiency in supressing undesirable aeroelastic behavior. The results suggest that the nonlinearity of the NES influences the nonlinear dynamic behavior of the aeroelastic system and may yield undesirable responses. Nomenclature h Plunge motion θ Pitch motion y2 Nes motion α0 Preset angle of attack αeff Effective angle of attack m1 Mass of the airfoil m2 Mass of the nes e Position of the center of gravity relatively to the elastic axis d Position of the nes relatively to the elastic axis Icg Mass moment of inertia of the airfoil relatively to the center of gravity kh0 Airfoil linear plunging stiffness kh1 Airfoil quadratic plunging stiffness kh2 Airfoil cubic plunging stiffness Ch Airfoil plunging motion viscous damping coefficient kθ0 Airfoil linear pitching stiffness kθ1 Airfoil quadratic pitching stiffness kθ2 Airfoil cubic pitching stiffness Cθ Airfoil pitching motion viscous damping coefficient kn0 Nes linear plunging stiffness kn1 Nes quadratic plunging stiffness kn2 Nes cubic plunging stiffness Cy2 Nes plunging motion viscous damping coefficient L Aerodynamic Lift Force M Aerodynamic Moment b Semi-chord Clα Lift coefficient ∗Department of Engineering Science and Mechanics, MC 0219, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA. Email:mhajj@vt.edu, Phone number:+15402314190 Fax:+15402314574 1 of 13 American Institute of Aeronautics and Astronautics Cmα Moment coefficient cs Nonlinear aerodynamic coefficient U Freestream velocity ρ Air density

Quantifying grade effects on vehicle fuel consumption for use in sustainable highway design

ABSTRACT The paper develops an analytical formulation that quantifies the effect of vertical grade on vehicle fuel consumption and then illustrates the use of the developed procedure in the identification of the fuel efficient freeway layouts. Specifically, the Virginia Tech Comprehensive Power-based Fuel consumption Model (VT-CPFM) was used to develop the formulation and then applied using 2015s 10 most-sold vehicles in the U.S. and Europe to quantify the vertical grade effect on vehicle fuel consumption rates. An increase in fuel consumption of approximately 140% was found when the roadway grade increased from 0.5% to 6%. The proposed selection procedure uses Geographical Information System (GIS) applications in the design phase to evaluate possible freeway layouts. A multi-criteria analysis is performed to rank the feasible alternatives. The yearly fuel consumed by cars traveling on each feasible layout is then predicted and the alternatives are sorted in ascendingly. If the alternative selected by the multi-criteria analysis gives the least yearly fuel consumption, then that alternative should be constructed. If not, then the alternative that results in the least yearly fuel consumption should be re-evaluated with respect to the one selected by the multi-criteria analysis tool. The proposed procedure is validated using a real case study involving the construction of a new freeway in Cameroon. As much as a 12% difference in fuel consumption was found between the alternative with the least estimated yearly fuel consumption and that selected based on a multiple-criteria decision analysis.

Unsteady Aeroelastic Response of Rigid Airfoils with Nonzero Angles of Attack

The effects of varying the angle of attack on the flutter speed and limit cycle oscillations of an aeroelastic system are investigated. This system consists of a rigid airfoil supported by linear springs that undergoes plunging and pitching motions. The Unsteady Vortex Lattice Method (UVLM) is used to model the aerodynamic loads. To solve simultaneously and interactively the governing equations, an iterative scheme based on Hamming’s fourth order predictor-corrector model is employed. The effects of the angle of attack on the dynamic response including the flutter speed and ensuing limit cycle oscillations are investigated. The results show that the flutter speed increases as the angle of attack is increased. It is also determined that increasing the preset angle of attack results in a decrease of the amplitudes of the limit cycle oscillations.