Daniel Feszty

Driving Mechanisms of High-Speed Unsteady Spiked Body Flows, Part 2: Oscillation Mode

The driving mechanism of the unsteady e ow mode pulsation arising over axisymmetric spiked bodies has been analyzed by using computational e uid dynamics as a tool. Laminar, axisymmetric e ow at Mach 2.21 and Reynolds number (based on the blunt-body diameter) of 0.12 £106 was simulated by a spatially and temporally second-order-accurate e nite volume method. The model geometry was a forward facing cylinder of diameter D equipped with a spike of length L/D=1.00. After reviewing previous pulsation hypotheses, the numerical results were analyzed in detail. A new driving mechanism was proposed, its main features being the creation of a vortical region in the vicinity of the foreshock-aftershock intersection causing mass ine ux into the dead-air region, the existence of supersonic e ow within the dead-air region, the liftoff of the shear layer from the spike tip, and the collision of the recirculated and penetrating e ows within the expanded separated region.

Alleviation of Airfoil Dynamic Stall Moments via Trailing-Edge-Flap Flow Control

Trailing-edge-flap flow control for the mitigation of large negative pitching moments and negative aerodynamic damping caused by helicopter rotor blade dynamic stall was studied by means of computational fluid dynamics. A discrete vortex method was used for the simulations. The model geometry was a NACA 0012 airfoil oscillating in an α(t) = 15 deg + 10 deg sin(ωt) motion at the reduced frequency of k = 0.173. The freestream flow conditions were of M = 0.117 and Re = 1.463296 x 10 6 . The flap actuation was a brief pulse signal of a sinusoidal shape

Application of Multi-Input Volterra Theory to Nonlinear Multi-Degree-of-Freedom Aerodynamic Systems

This paper presents a reduced-order-modeling approach for nonlinear, multi-degree-of-freedom aerodynamic systems using multi-input Volterra theory. The method is applied to a two-dimensional, 2 degree-of-freedom transonic airfoil undergoing simultaneous forced pitch and heave harmonic oscillations. The so-called Volterra cross kernels are identified and shown to successfully model the aerodynamic nonlinearities associated with the simultaneous pitch and heave motions. The improvements in accuracy over previous approaches that effectively ignored the cross kernels by using superposition are demonstrated.

Reduced Order Modeling of Nonlinear Transonic Aerodynamics Using a Pruned Volterra Series

The following paper presents a reduced-order-modeling approach for nonlinear aerodynamic systems utilizing a pruned Volterra series. The method is applied to a two-dimensional transonic airfoil undergoing forced pitch oscillations. Pruned Volterra series reduced-order-models up to fourth-order are identified and compared against computational fluid dynamics models. Very favorable accuracies are attained over a wide range of Mach number, reduced frequency and oscillation amplitude. The computational resources associated with the pruned Volterra series are demonstrated to be several ordersof-magnitude lower compared to the standard Volterra series.

Smart Spring Control of Vibration on Helicopter Rotor Blades

c1, c2 = Smart Spring viscous damping coefficients associated with primary and secondary load paths F = external (input) force applied to the Smart Spring k = effective dynamic stiffness of the Smart Spring k1, k2 = Smart Spring constants associated with primary and secondary load paths m = effective inertia of the Smart Spring m1, m2 = mass of external and internal sleeves in the Smart Spring N = contact force applied by piezoelectric stack T = Smart Spring period of actuation t = time x = displacement (output) yielded by the Smart Spring y = displacement associated with the Smart Spring secondary load path = dynamic stiffness complex coefficients = dry friction coefficient = Smart Spring control frequency, 2 =T ! = Smart Spring frequency of excitation

Review of Active Rotor Control Research in Canada

The current status of Canadian research on rotor-based actively controlled technologies for helicopters is reviewed in this paper. First, worldwide research in this field is overviewed to put Canadian research into context. Then, the unique hybrid control concept of Carleton University is described, along with its key element, the “stiffness control” concept. Next, the smart hybrid active rotor control system (SHARCS) project`s history and organization is presented, which aims to demonstrate the hybrid control concept in a wind tunnel test campaign. To support the activities of SHARCS, unique computational tools, novel experimental facilities and new know-how had to be developed in Canada, among them the state-of-the-art Carleton Whirl Tower facility or the ability to design and manufacture aeroelastically scaled helicopter rotors for wind tunnel testing. In the second half of the paper, details are provided on the current status of development on the three subsystems of SHARCS, i.e. that of the actively controlled tip, the actively controlled flap and the unique stiffness-control device, the active pitch link.

Numerical simulation of the hysteresis phenomenon in high-speed spiked body flows

The hysteresis phenomenon occurring in unsteady 00~s over an axisymmetric spiked cylinder at Mach 2.21, Re=O.l2million was numerically simulated using a 2nd order time-accurate and 2nd order space discretised finite volume method. The continuous inward/outward motion of the spike was modelled by employing a deforming mesh technique. Two spike speeds were considered and the hysteresis phenomenon was predicted qualitatively in both cases. The lower boundary of the hysteresis range was found to be in excellent agreement with the experiment at both spike speeds. The upper boundary of the phenomenon was overpredicted by the numerical method. A detailed description of the transition processes is given, in which a new mechanism supporting the pulsation mode at extremely large spike lengths is revealed.

ALLEVIATION OF ROTOR BLADE DYNAMIC STALL VIA TRAILING EDGE FLAP FLOW CONTROL

A trailing-edge flap flow control for the mitigation of large negative pitching moments and the associated negative damping during helicopter rotor blade dynamic stall was studied by means of CFD. A discrete vortex method was used for the simulations. The model geometry was a NACA 0012 airfoil, oscillating in a fi(t) = 15o + 10osin(!t) motion at the reduced frequency of k=0.173. The freestream flow conditions were M=0.117 and Re=1,463,296. The flap actuation was a brief pulse signal and it was shown that for optimum results upward flap deflection for the duration of about the 1/3 of the oscillation time period should be employed. The pulse signal should start in the 3rd quarter of the azimuth. Detailed analysis of the flowfield showed that the trailing-edge vortex (TEV), induced by the downstream convecting dynamic stall vortex (DSV), is responsible for the occurence of large negative pitching moments and negative damping. The flap flow control technique proved to be successful in mitigating these eects by displacing the TEV to a higher location where the DSV could only push it o the trailing edge, thus eliminating its eect. The method was shown to be promising for other cases from the helicopter flight envelope as well.

Whirl-tower Open-loop Experiments and Simulations with an Adaptive Pitch Link Device for Helicopter Rotor Vibration Control

In the present work are presented both experimental and numerical simulation results obtained with an open-loop control law applied to an individual blade control system that incorporates a mechanism for blade active impedance adaptation at the root, the active pitch link or “smart spring”. It is demonstrated that the active pitch link provides parametric excitation of the blade in the rotating frame and, with it, alters the vibration spectra of the vibration loads both in the blade structure and transmitted to the nonrotating frame by the hub. The experimental results were obtained in whirl tower tests where blade periodic excitation was provided by a fan located at the base of the rotor and generated a transversal flow at a range of blade azimuth angles.

Identification of the Smart Spring properties from FRFs measurements

The objective of this paper is the dynamic identi cation of a reduced-scale helicopter blade system that incorporates an active pitch link or smart spring for vibration control. The identi cation of the Smart Spring parameters, in terms of the masses and sti nesses associated to its components, is carried out in the frequency domain using a developed sensitivity-based updating method. This method, called Predictor-Corrector, iteratively minimizes a residual vector of correlation functions, de ned on the Frequency Response Functions (FRFs), in order to obtain the unknown values of the parameters that well represent the dynamic behavior of the smart spring. In the paper the accuracy of the solution provided by the developed technique is assessed through several numerical analyses. For this purpose, a lumped parameter numerical model of the Smart Spring was developed and the e ects of various mass and sti ness distribution scenarios on the modal properties of the system are presented. Due to the nonlinear dynamic behavior of the smart spring system, a linear approximation of the system around a prescribed operative working condition is considered. Finally, the developed approach is applied for the identi cation of the dynamic parameters of a real smart spring system. It is shown that acceptable values of the equivalent lumped parameters were achieved also considering experimental data such as those recorded during a test campaign carried out at the Smart Rotor Laboratory of the Carleton University, thus validating the identi cation approach.