J. Michael R. Graham

Assessment of Wake-Tail Interference Effects on the Dynamics of Flexible Aircraft

This work investigates the effect of aerodynamic interference in the coupled nonlinear aeroelasticity and flight mechanics of flexible lightweight aircraft at low speeds. For that purpose, a geometrically exact composite-beam formulation is used to model the vehicle flexible-body dynamics by means of an intuitive and easily linearizable representation based on the displacement and Cartesian rotation vectors. The aerodynamics are modeled using the unsteady vortex-lattice method, which captures the instantaneous shape of the lifting surfaces and the free inviscid wake, including large deformations and interference effects. This results in a framework for simulation of high aspect ratio planes that provides a medium-fidelity representation of flexible-aircraft dynamics with a modest computational cost. Previous independent studies on the structural-dynamics and aerodynamics modules are complemented here with the integrated simulation methodology, including vehicle trim, and linear and nonlinear time-domain solutions. A numerical investigation is next presented on a simple wing-fuselage-tail configuration, assessing the interference effects between wing wake and horizontal tail, and the downwash due to the proximity of the wake is shown to play a significant role in the longitudinal dynamics of the vehicle. Finally, a brief discussion of direct wake-tail encounters is included to show the limitations of the approach.

Fluid Dynamic Loading on Curved Riser Pipes

In order to gain a preliminary understanding of the fluid dynamics developed past a curved riser pipe, a numerical investigation into the flow past curved cylinders at a Reynolds number of 100 has been performed. To approximate the flow conditions on curved riser pipes, different velocity profiles and flow directions were applied and the corresponding results compared. In addition, the fluid dynamic loading and the wake structures for curved cylinder flows were investigated. The fully three-dimensional simulations were computed with a spectral/hp element method. The computational results were compared with experiments undertaken in the towing tank facility of the Department of Aeronautics of Imperial College.

Aeroelastic Control of Long-Span Suspension Bridges

The modeling, control, and dynamic stabilization of long-span suspension bridges are considered. By employing leading- and trailing-edge flaps in combination, we show that the critical wind speeds for flutter and torsional divergence can be increased si g nificantly. The relatively less well known aerodynamic properties of leading-edge flaps will be studied in detail prior to their utilization in aeroelastic stability and control system design studies. The optimal approximation of the classical Theodorsen circulation function will be studied as part of the bridge section model building exercise. While a wide variety of control systems is possible, we focus on compensation schemes that can be implemented using passive mechanical components such as springs, dampers, gearboxes, and levers. A single-loop control system that controls the leading- and trailing-edge flaps by sensing the main deck pitch angle is investigated. The key finding is that the critical wind speeds for flutter and torsional divergence of the sectional model of the bridge can be greatly increased, with good robustness characteristics, through passive feedback control. Static winglets are shown to be relatively ineffective.

An Experimental and Numerical Assessment of Airfoil Polars for Use in Darrieus Wind Turbines—Part I: Flow Curvature Effects

A better comprehension of the aerodynamic behavior of rotating airfoils in Darrieus Vertical-axis wind turbines (VAWTs) is crucial both for the further development of these machines and for improvement of conventional design tools based on zero or one-dimensional models (e.g. BEM models).When smaller rotors are designed with high chord-to-radius (c/R) ratios so as not to limit the blade Reynolds number, the performance of turbine blades has been suggested to be heavily impacted by a virtual camber effect imparted on the blades by the curvilinear flow they experience.To assess the impact of this virtual camber effect on blade and turbine performance, a standard NACA0018 airfoil and a NACA0018 conformally transformed such that the airfoil’s chord line follows the arc of a circle, where the ratio of the airfoil’s chord to the circle’s radius is 0.25 were considered. For both airfoils, wind tunnel tests were carried out to assess their aerodynamic lift and drag coefficients for Reynolds numbers of interest for Darrieus VAWTs.Unsteady CFD calculations have been then carried out to obtain curvilinear flow performance data for the same airfoils mounted on a Darrieus rotor with a c/R of 0.25. The blade incidence and lift and drag forces were extracted from the CFD output using a novel incidence angle deduction technique.According to virtual camber theory, the transformed airfoil in this curvilinear flow should be equivalent to the NACA0018 in rectilinear flow, while the NACA0018 should be equivalent to the inverted transformed airfoil in rectilinear flow.Comparisons were made between these airfoil pairings using the CFD output and the rectilinear performance data obtained from the wind tunnel tests and XFoil output in the form of pressure distributions and lift and drag polars.Blade torque coefficients and turbine power coefficient are also presented for the CFD VAWT using both blade profiles.Copyright

CFD Simulations of the Vortex-Induced Vibrations of Model Riser Pipes

The paper reports results from two strip theory CFD investigations of the Vortex-Induced Vibrations of model riser pipes. The first investigation is concerned with the vibrations of a vertical riser pipe that was subjected to a stepped current profile. An axial spatial resolution study was conducted to determine the number of simulation planes required to achieve tolerably converged numerical solutions. It was found that six to seven simulation planes are required per half-wavelength of pipe vibration in order to obtain convergence. The second investigation is concerned with the simultaneous in-plane and out-of-plane vibrations of a model Steel Catenary Riser that was subjected to a uniform current profile. The pipe’s simulated vibrations were found to agree very well with those determined experimentally. This result was achieved despite the questionable usage of simulation planes at high angles to the flow direction.© 2005 ASME

An Experimental and Numerical Assessment of Airfoil Polars for Use in Darrieus Wind Turbines—Part II: Post-stall Data Extrapolation Methods

Accurate post-stall airfoil data extending to a full range of incidences between −180° to +180° is important to the analysis of Darrieus vertical-axis wind turbines (VAWTs) since the blades experience a wide range of angles of attack, particularly at the low tip-speed ratios encountered during startup.Due to the scarcity of existing data extending much past stall, and the difficulties associated with obtaining post-stall data by experimental or numerical means, wide use is made of simple models of post-stall lift and drag coefficients in wind turbine modeling (through, for example, BEM codes). Most of these models assume post-stall performance to be virtually independent of profile shape.In this study, wind tunnel tests were carried out on a standard NACA0018 airfoil and a NACA 0018 conformally transformed to mimic the “virtual camber” effect imparted on a blade in a VAWT with a chord-to-radius ratio c/R of 0.25.Unsteady CFD results were taken for the same airfoils both at stationary angles of attack and at angles of attack resulting from a slow VAWT-like motion in an oncoming flow, the latter to better replicate the transient conditions experienced by VAWT blades.Excellent agreement was obtained between the wind tunnel tests and the CFD computations for both the symmetrical and cambered airfoils. Results for both airfoils also compare favorably to earlier studies of similar profiles. Finally, the suitability of different models for post-stall airfoil performance extrapolation, including those of Viterna-Corrigan, Montgomerie and Kirke, was analyzed and discussed.Copyright

Efficient aeroservoelastic modeling and control using trailing-edge flaps of wind turbines

This paper presents a computationally efficient aeroservoelastic modeling approach for dynamic load alleviation in large wind turbines with trailing-edge aerodynamic control surfaces. The aeroelastic model is expressed directly in a state-space formulation and trailing-edge flaps are modeled directly in the unsteady aerodynamics. The linear model of a single rotating blade is used to design a Linear-Quadratic-Gaussian regulator for minimizing the root-bending moments, which is shown to provide load reductions of about 20% in closed-loop on the full wind turbine non-linear aeroelastic model.

Flutter control of long-span suspension bridges

The dynamic stabilization of a sectional model of a long-span suspension bridge is considered. Feedback control is achieved using leading- and trailing-edge flaps as actuators. While a wide variety of control systems is possible, we focus on compensation schemes that can be implemented using passive mechanical components such as springs, dampers, and a rack and pinion mechanism. A single-loop control system is investigated that controls the flaps by sensing the main deck heave velocity. A symmetrical control scheme is used on both flaps to make the feedback system insensitive to the wind direction. The key finding is that the critical wind speed for the flutter instability of the sectional model of the bridge can be greatly increased, with good robustness characteristics, through passive feedback control.

Aeroelastic Modelling of Long-Span Suspension Bridges

Abstract The 2D aerodynamic modelling of long-span suspension bridges is considered. We use thin airfoil theory from the aircraft industry and a sectional bridge model with an integrated controllable trailing-edge flap. The relatively less well known aerodynamic properties of leading-edge flap will be studied in detail. The optimal approximation of the classical Theodorsen circulation function will be studied as part of the bridge sectional model building exercise, which can therefore be re-casted in a form suitable for control systems analysis and design. The critical wind speeds for flutter and torsional divergence are predicted precisely. Static winglets are shown to be relatively ineffective to mitigate torsional divergence.

Experimental flutter and buffet suppression of a sectional suspended-bridge

This paper conducts flutter and buffet suppression of a section of a long-span bridge deck mounted elastically across a wind tunnel working section. The incident stream turbulence for buffet tests is generated by a standard biplanar grid. The bridge deck response in its two major degrees of freedom, heave and pitch. A mechanical flutter control system is developed which senses the vertical velocity of flap pivots and adjusts the leading and trailing-edge flap angles accordingly. The wind tunnel experiments show that the mechanical flutter control system can not only increase the critical flutter speed of the bridge deck but also can suppress the buffeting greatly.