Flexible rocket nozzles
In this project, the behavior of flexible rocket nozzles during start up is investigated. Previous investigations have shown that by allowing the nozzles to become more flexible, the aeroacoustic loads experienced by the base of the rocket can be heavily influenced. A goal of this project is to find out whether or not aeroelastic effects can be manipulated in such a way that the resulting structure is lighter than a stiff version and to gain general insight into the characterization of nozzle behavior during start up.
For this purpose, a variety of rubber and metallic scaled parabolic nozzles are tested on a cold gas nozzle test stand. A focus is hereby placed on the use of optical measurements, which allow unaltered characterization of the nozzle’s vibratory behavior.
Research personnel: Marc Eitner
An extremely flexible rotor blade
In this work, we carry out the design, analysis, and testing of an unconventional rotor featuring extremely flexible, retractable blades. These rotor blades are composed of a flexible matrix composite material; they are so flexible that they can be rolled up and stowed in the rotor hub. The motivation for this study is to equip the next generation of unmanned rotary-wing vehicles with morphing rotors that can change their diameter in flight, based on mission requirements. Due to their negligible structural stiffness, the static and dynamic behavior of these blades is dominated by centrifugal effects. Passive stabilization of the flexible blades is achieved by centrifugal stiffening in conjunction with an appropriate spanwise and chordwise mass distribution. In particular, such blades are susceptible to large bending and twist deformations.
There have been several publications over the past few years related to this project. Our current focus is to measure three-dimensional rotor blade deformation over the entire rotor disk and identify modal parameters in the rotating frame, including natural frequencies and mode shapes. Analyses that are being investigated are the natural excitation technique (NExT), the eigensystem realization algorithm (ERA) and a variety of Independent Component Analysis techniques. This work was presented at 6th Asian-Australian Rotorcraft Forum, Kanazawa, Japan.
Research personnel: Daiju Uehara
Unsteady flow in rotating ducts – Circulation control rotors
Circulation control can be used on rotor blades to increase the stall margin by alleviating retreating blade stall. The Sikorsky X-Wing experimental helicopter implemented circulation control on a stopped rotor by steady blowing through the rotor blades. Unsteady blowing at 1/rev can be used to target retreating blade stall alleviation. In this case, the flow through the rotor blade is unsteady and compressible, and is strongly affected by the centrifugal force field as well as time-varying aerodynamic boundary conditions.
In this project, a numerical model of the unsteady flow in a rotating duct is developed. The model solves the quasi one-dimensional Euler equations using Advective Upstream Splitting Methd (AUSM) to calculate convective fluxes. Losses are incorporated using a friction factor as well as loss coefficients for the inlets and outlets. The effect of duct sweep and flow control valves anywhere along the duct can be captured. The boundary conditions are derived using Riemann invariants and stagnation conditions. The model is validated by measurements on a 4.25ft long, 2 inch diameter circular cross-section tube rotating at a tip speed of up to 600ft/s. The flow through the tube can be modulated by a valve at the root of the tube that can change its opening at 1/rev. Quantities measured include wall static pressure (kulite transducers) and mass flow rate of air (hot wire sensor).
A Railgun based Plasma Actuator (RailPAc)
The primary goal of this project is to investigate the ability of a magnetohydrodynamic plasma actuator to alleviate dynamic stall on an airfoil. This research is motivated by the potential of extending the stall boundary of a rotor in edgewise flight. In particular, we hope to demonstrate that the use of a surface mounted plasma actuator can effectively reduce flow separation on the retreating blade of a helicopter.
To gain insight into the RailPAc actuation mechanism, various experimental methods are employed; Arc morphology study using high speed imaging, Inducued velocity measurement with particle image and schlieren velocimetry technique.
Research personnel: Young-Joon Choi
A flexible flapping wing
The successful development of bio-inspired flapping wing micro aerial vehicles (MAVs) requires precise control and sensing of the forces generated by the flapping wing. These forces are generated by a coupled fluid-structure interaction which includes unsteady aerodynamics and large structural deflections. The objective of the current research is to develop a reduce-order methodology technique which may be used to sense flapping forces from wing deflections in real-time. The wing deflections are reconstructed in real time through a reduced-order modal shape sensing approach which utilize an array of piezoelectric strain sensors. This array of piezoelectric sensors is designed such that the sensor outputs are linearly relates to the modal coordinates of the wing. The real-time measurement of the wing deflection enables the use of a reduced-order methodology to reconstruct the flapping wing loads at all times. Some of the experimental methods used in this project include high-speed digital image correlation, particle image velocimetry, and experimental modal analysis.
Research personnel: Jason Tran