# Research

## Turbomachinery

### Non-synchronous vibration (NSV) study of a NACA0012

This project seeks to bring some light into the physical flow mechanisms behind non-synchronous vibrations (NSV). A better understanding of the parameters that drive NSV is needed to establish guidelines to design NSV-free engines.

### Forced response analysis of multi-row compressors

Duke and Purdue Universities have joined forces to propose a comprehensive experimental and computational program for GUIde 6. The proposed research directly addresses many of the “Grand Challenge” topics, including higher order modes and low order excitation. Through a collaboration with the High-speed compressor lab at Purdue university, a combined experimental/computational effort will produce the much-needed information through a comprehensive set of experimental data and comparisons with advanced computational results.

## Aeroelastic Systems

### Flapping Flag Flutter

The interaction between a cantilevered elastic plate and a uniform axial flow is a canonical fluid-structure interaction problem. If the flow is aligned normal to the clamped edge then the system is referred to as flag-like. It is well-known that this system exhibits a flutter instability when the flow velocity is increased above a critical velocity.

### Plate and Membrane Flutter

The aeroelastic stability of rectangular plates are well-documented in literature for certain sets of boundary conditions. Specifically, wing flutter, panel flutter, and divergence of a plate that is clamped on all sides are well-understood. However, the on-going push for lighter structures and novel designs have led to a need to understand the aeroelastic behavior of elastic plates for other boundary conditions.

### Piezoelectric Systems

The purpose of this project intended to answer the question: how to maximize the piezoelectric power extraction of an aeroelastic system? This involved an experimental investigation of a simple rectangular cantilever plate, which experiences non-linear aeroelastic limit cycle oscillations (LCO).

## Rotary Wing Aircraft

### Rotary Wing Aerodynamics

For this project, we are researching the optimal aerodynamic rotor design for conventional and compound helicopters. This includes determining the blade twist, chord, and root inputs that produce the most efficient flight.

### Rotor Vibration Reduction Using Multi-Element Multi-Path Design

Rotor blade vibration reduction has become an important consideration in aerospace engineering, turbomachinery, automobile engineering, etc. In the design of rotorcraft and wind turbines there is always a need to reduce the rotor blade vibration to maintain the stability and operational life of the whole structure. The approach to be explored in this research to solve vibration problem is the multi-element multi-path (MEMP) structures that inherently suppress vibration. The following figures show the evolution of this concept on helicopter rotor blades.

## Acoustics

### Bliss Aerodynamics and Acoustics Lab

In acoustics, the group is led by Dr. Bliss whoose primary effort is in the application of ANM to structural acoustics, particularly to acoustic scattering from submerged elastic bodies with structural discontinuities. He has also developed a method called Alternate Resonance Tuning (ART) to prevent low frequency sound transmission into flexible wall enclosures, with applications to aircraft interior noise. He also conducts research on mathematical homogenization applied to structural acoustic systems, and on general boundary conditions for bulk-reacting sound absorbing surfaces.

## Reduced Order Modeling in Turbulence and Aerodynamics

### Model Order Reduction for High Reynolds Number Flows

Turbulence is a phenomenon characterized by chaotic, multi-scale dynamics, both, in space and time. At high Reynolds numbers, the dynamics of turbulence exhibit an energy cascade: large scale eddies are broken down into smaller and smaller eddies until the scales are fine enough so that viscous forces can dissipate their energy. We generalize the POD-based Galerkin model order reduction approach by incorporating Navier-Stokes equation constraints.

### POD Decomposition for Vortex Induced Vibrations Around a Cylinder

For a certain range of Reynolds numbers, flow passing by a cylinder will lead to a fluid dynamic instability, often referred to as von Karman vortices. These alternating, shedding vortices cause unsteady forces on the surface of the cylinder, which tend to make the cylinder oscillate; a phenomenon called 'vortex-induced vibrations'.

### Extrapolative Potential of Turbulent ROMs

Variations of the proper orthogonal decomposition present the potential for a linear modal basis optimized for dynamically significant characteristics of multiscale, nonlinear fluid flows.

## Nonlinear Aeroelasticity and Dynamics

### Nonlinear Asymptotic Modal Analysis

Quickly and accurately describing the behavior of complicated and intricate dynamical systems opens the door to revolutionary design possibilities, including sensing technology in extreme environments, reliable failure predictions, and electromagnetic field control of plasmas. Time-marching simulations and Classical Modal Analysis (CMA) are often too computationally demanding to offer solutions to practical problems, but methods such as Statistical Energy Analysis (SEA) and Asymptotic Modal Analysis (AMA) can offer the quick insight necessary to appropriately develop design criteria.

### System Identification of Nonlinear Aeroelastic Systems

We explore a novel, sparse representation of the Volterra series whose identification costs are significantly lower than the identification costs of the full Volterra series. We demonstrate that sparse Volterra reduced-order models are capable of efficiently modeling aerodynamically induced limit cycle oscillations of the prototypical NACA 0012 benchmark model.

### Nonlinear Dynamics of the Lid-Driven Cavity

Bifurcations observed in the lid-driven cavity as the Reynolds number was increased were tracked and explained in both the phase space and the frequency domain.