There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

A review of shape memory alloy research, applications and opportunities

To improve the performance of particle image velocimetry in measuring instantaneous velocity fields, direct cross-correlation of image fields can be used in place of auto-correlation methods of interrogation of double- or multiple-exposure recordings. With improved speed of photographic recording and increased resolution of video array detectors, cross-correlation methods of interrogation of successive single-exposure frames can be used to measure the separation of pairs of particle images between successive frames. By knowing the extent of image shifting used in a multiple-exposure and by a priori knowledge of the mean flow-field, the cross-correlation of different sized interrogation spots with known separation can be optimized in terms of spatial resolution, detection rate, accuracy and reliability.

Theory of cross-correlation analysis of PIV images : Image analysis as measuring technique in flows

Aircraft wings are a compromise that allows the aircraft to fly at a range of flight conditions, but the performance at each condition is sub-optimal. The ability of a wing surface to change its geometry during flight has interested researchers and designers over the years as this reduces the design compromises required. Morphing is the short form for metamorphose; however, there is neither an exact definition nor an agreement between the researchers about the type or the extent of the geometrical changes necessary to qualify an aircraft for the title ‘shape morphing.’ Geometrical parameters that can be affected by morphing solutions can be categorized into: planform alteration (span, sweep, and chord), out-of-plane transformation (twist, dihedral/gull, and span-wise bending), and airfoil adjustment (camber and thickness). Changing the wing shape or geometry is not new. Historically, morphing solutions always led to penalties in terms of cost, complexity, or weight, although in certain circumstances, thes...

A Review of Morphing Aircraft

Originally introduced in the fluid mechanics community, dynamic mode decomposition (DMD) has emerged as a powerful tool for analyzing the dynamics of nonlinear systems. However, existing DMD theory deals primarily with sequential time series for which the measurement dimension is much larger than the number of measurements taken. We present a theoretical framework in which we define DMD as the eigendecomposition of an approximating linear operator. This generalizes DMD to a larger class of datasets, including nonsequential time series. We demonstrate the utility of this approach by presenting novel sampling strategies that increase computational efficiency and mitigate the effects of noise, respectively. We also introduce the concept of linear consistency, which helps explain the potential pitfalls of applying DMD to rank-deficient datasets, illustrating with examples. Such computations are not considered in the existing literature, but can be understood using our more general framework. In addition, we show that our theory strengthens the connections between DMD and Koopman operator theory. It also establishes connections between DMD and other techniques, including the eigensystem realization algorithm (ERA), a system identification method, and linear inverse modeling (LIM), a method from climate science. We show that under certain conditions, DMD is equivalent to LIM.

On Dynamic Mode Decomposition: Theory and Applications

The development of a biomimetic active hydrofoil that utilizes shape memory alloy (SMA) actuator technology is presented. This work is the first stage prototype of a vehicle that will consist of many actuated body segments. The current work describes the design, modeling and testing of a single-segment demonstration SMA actuated hydrofoil. The SMA actuation elements are two sets of thin wires on either side of an elastomeric component that joins together the leading and trailing edges of the hydrofoil. Controlled heating and cooling of the two wire sets generates bi-directional bending of the elastomer, which in turn deflects the trailing edge of the hydrofoil. In this paper the design of the hydrofoil and the experimental tests preformed thereon are explained. A detailed account of SMA actuator preparation (training) and material characterization is given. Finite-element method (FEM) modeling of hydrofoil response to electrical heating of the SMA actuators is carried out using a thermomechanical constitutive model for the SMA with input from the material characterization. The modeling predictions are finally compared with experimental measurements of the trailing edge deflection and the SMA actuator temperature.

https://iopscience.iop.org/article/10.1088/0964-1726/9/5/312/pdf

Development of a shape memory alloy actuated biomimetic vehicle

We present the application of the new synthetic jet actuator (SJA) to flow separation control over a NACA 0015 wing. The actuator is compact enough to fit in the interior of the wing that has a chord of 0.375 m. The wing was tested in the Texas A&M University Aerospace Engineering 3 ftX4 ft wind tunnel. An experimental investigation into the effects of the synthetic jet actuator on the performance of the wing is described. Emphasis is placed on the capabilities of the actuator to control the separation of the flow over the wing at high angles of attack. The results include force balance measurements, on surface and off surface flow visualization, surface pressure measurements, and wake surveys. All of the reported tests were performed at a free-stream velocity of 35 m/s, corresponding to a Reynolds number of 8.96×10 5 . The angle of attack was varied from -2.0 deg to 29.0 deg.

A New Class of Synthetic Jet Actuators—Part II: Application to Flow Separation Control

The development and testing of a biomimetic active hydrofoil that utilizes Shape-Memory-Alloy (SMA) actuator technology is presented. This work is the second stage in the development of a vehicle that has a skeletal structure similar to that of aquatic animals and SMA actuators for muscles. The current work describes the development and testing of a six-segment demonstration vehicle and the control schemes used. Each SMA actuation element consists of a thin wire that joins together two adjacent vertebrae segments of the hydrofoil skeleton and induces relative movement of one with respect to the other. Controlled heating and cooling of the wire sets generates bi-directional rotation of the vertebrae, which in turn causes a change in the shape of the hydrofoil. Each SMA wire is embedded in an elastic water channel that facilitatesfast active SMA cooling via forced water circulation. This hydrofoil was able to deform to several shapes mimicking aquatic animal swimming, with controlled oscillation frequencies...

Development of a Shape-Memory-Alloy Actuated Biomimetic Hydrofoil

Although strong potential of synthetic jets as flow separation control actuators has been demonstrated in the existing literature, there is a large gap between the synthetic jet actuators (SJA) used in laboratory demonstrations and the SJAs needed in realistic fullscale applications, in terms of compactness, weight, efficiency, control authority and power density. In most cases, the SJAs used in demonstrations are either too large or too weak for realistic applications. In this work, we present the development of compact, high-power synthetic jet actuators for realistic flow separation control applications and demonstrate the developed SJA technology in representative, flow separation control problems, including control of steady separation/stall. The developed actuators are compact enough to fit in the interior of a 14.75" chord, NACA0015 wing, have maximum power of 2.0 HP and can produce (for the tested conditions) exit velocities as high as 80 m/sec. Flow separation control was demonstrated over a 14.75" chord, NACA 0015 wing at angles of attack and free stream velocities as high as 25 degrees and 45 m/s, respectively and pressure data was acquired over the wing for a range of conditions. INTRODUCTION The separation of the boundary layer is associated with large energy losses, and in most applications adversely affects the aerodynamic loads in the form of lift loss and drag increase. Therefore, there is a strong incentive to delay or manipulate the occurrence of flow separation. For example, if the separation of the boundary layer formed over a bluff body is delayed, the pressure drag is greatly reduced; also separation delay will permit the operation of an airfoil/wing at higher angles of attack. Me Cormick (2000) showed that delay or elimination of separation * Research Assistant, Aerospace Engineering Department, Student Member AIAA. § Associate Professor, Aerospace Engineering Department, Member AIAA Copyright © 2000 by J. L. Gilarranz and O. K. Rediniotis. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. can increase the pressure recovery in a diffiiser. Hence, separation control is of great importance to most of the systems involving fluid flow, such as air, land or underwater vehicles, turbomachines, diffusers, etc. Many researchers have developed and tested methods of separation control in a variety of applications. Gad-el-Hak and Bushnell (1991) provide a comprehensive review on the research in the area of separation control previous to the year 1991. Typically the separation control techniques may be grouped in two categories: passive and active techniques. Most of the techniques, developed for passive separation control, may be found in the review by Gad-el-Hak and Bushnell (1991). Some of the parameters affecting the selection of a separation control technique include, but are not limited to: weight of system, power consumption (active type), power density, parasitic drag of device, size, reliability, cost and efficiency. Active separation control methods have included the application of: • steady boundary layer suction to remove the low momentum fluid. • wall heat transfer to control and modify the viscosity of the fluid. • moving walls in order to use the no-slip condition at the surface to energize the fluid close to the wall. • momentum addition to the boundary layer by steady blowing. • oscillatory blowing and suction. In the recent years the development of the socalled "synthetic jet" or "zero mass flux" devices and their potential for flow control has received a great amount of attention from the fluid dynamics community. This type of systems mostly involves small-scale, low-energy, typically high-frequency actuators, whose operation is based on the concentrated input of energy at high receptivity regions of the flowfield. They take advantage of the physical flow evolution processes to amplify the applied disturbance, which stands apart from the traditional brut force 1 American Institute of Aeronautics and Astronautics c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

Compact, High-Power Synthetic Jet Actuators for Flow Separation Control

6Wood, D. H., “A Three-Dimensional Analysis of Stall-Delay on Horizontal-Axis Wind Turbine,” Journal of Wind Engineering and Industrial Aerodynamics, Vol. 37, 1991, pp. 1–14. 7Cebecci, T., An Engineering Approach to the Calculation of Aerodynamic Flows, Springer, Berlin, 1999, pp. 51–62. 8Banks, W. H. H., and Gaad, G. E., “Delaying Effect of Rotation on Laminar Separation,” AIAA Journal, Vol. 1, No. 4, 1963, pp. 941, 942.

Othon K. Rediniotis

Papers

Development of a shape memory alloy actuated biomimetic vehicle

A New Class of Synthetic Jet Actuators—Part II: Application to Flow Separation Control

Development of a Shape-Memory-Alloy Actuated Biomimetic Hydrofoil

Compact, High-Power Synthetic Jet Actuators for Flow Separation Control

Microstructured Hydrophobic Skin for Hydrodynamic Drag Reduction