Temperature (T) is probably the most fundamental parameter in all kinds of science. Respective sensors are widely used in daily life. Besides conventional thermometers, optical sensors are considered to be attractive alternatives for sensing and on-line monitoring of T. This Review article focuses on all kinds of luminescent probes and sensors for measurement of T, and summarizes the recent progress in their design and application formats. The introduction covers the importance of optical probes for T, the origin of their T-dependent spectra, and the various detection modes. This is followed by a survey on (a) molecular probes, (b) nanomaterials, and (c) bulk materials for sensing T. This section will be completed by a discussion of (d) polymeric matrices for immobilizing T-sensitive probes and (e) an overview of the various application formats of T-sensors. The review ends with a discussion on the prospects, challenges, and new directions in the design of optical T-sensitive probes and sensors.

Luminescent probes and sensors for temperature.

Simple aerodynamic configurations under even modest conditions can exhibit complex flows with a wide range of temporal and spatial features. It has become common practice in the analysis of these flows to look for and extract physically important features, or modes, as a first step in the analysis. This step typically starts with a modal decomposition of an experimental or numerical dataset of the flowfield, or of an operator relevant to the system. We describe herein some of the dominant techniques for accomplishing these modal decompositions and analyses that have seen a surge of activity in recent decades [1–8]. For a nonexpert, keeping track of recent developments can be daunting, and the intent of this document is to provide an introduction to modal analysis that is accessible to the larger fluid dynamics community. In particular, we present a brief overview of several of the well-established techniques and clearly lay the framework of these methods using familiar linear algebra. The modal analysis techniques covered in this paper include the proper orthogonal decomposition (POD), balanced proper orthogonal decomposition (balanced POD), dynamic mode decomposition (DMD), Koopman analysis, global linear stability analysis, and resolvent analysis.

https://arc.aiaa.org/doi/pdf/10.2514/1.J056060

Modal Analysis of Fluid Flows: An Overview

Actuators are transducers that convert an electrical signal to a desired physical quantity. Active flow control actuators modify a flow by providing an electronically controllable disturbance. The field of active flow control has witnessed explosive growth in the variety of actuators, which is a testament to both the importance and challenges associated with actuator design. This review provides a framework for the discussion of actuator specifications, characteristics, selection, design, and classification for aeronautical applications. Actuator fundamentals are discussed, and various popular actuator types used in low-to-moderate speed flows are then described, including fluidic, moving object/surface, and plasma actuators. We attempt to highlight the strengths and inevitable drawbacks of each and highlight potential future research directions.

Actuators for Active Flow Control

This paper presents the results of a parametric experimental investigation aimed at optimizing the body force produced by single dielectric barrier discharge plasma actuators used for aerodynamic flow control. A primary goal of the study is the improvement of actuator authority for flow control applications at higher Reynolds number than previously possible. The study examines the effects of dielectric material and thickness, applied voltage amplitude and frequency, voltage waveform, exposed electrode geometry, covered electrode width, and multiple actuator arrays. The metric used to evaluate the performance of the actuator in each case is the measured actuator-induced thrust which is proportional to the total body force. It is demonstrated that actuators constructed with thick dielectric material of low dielectric constant produce a body force that is an order of magnitude larger than that obtained by the Kapton-based actuators used in many previous plasma flow control studies. These actuators allow operation at much higher applied voltages without the formation of discrete streamers which lead to body force saturation.

Optimization of Dielectric Barrier Discharge Plasma Actuators for Active Aerodynamic Flow Control

The current paper describes the development of pressure-sensitive paint (PSP) technology as an advanced measurement technique for unsteady flow fields and short-duration wind tunnels. Newly developed paint formulations have step response times approaching 1 μs, making them suitable for a wide range of unsteady testing. Developments in binder technology are discussed, which have resulted in new binder formulations such as anodized aluminium, thin-layer chromatography plate, polymer/ceramic, and poly(TMSP) PSP. The current paper also details modeling work done to describe the gas diffusion properties within the paint binder and understand the limitations of the paint response characteristics. Various dynamic calibration techniques for PSP are discussed, along with summaries of typical response times. A review of unsteady and high-speed PSP applications is presented, including experiments with shock tubes, hypersonic tunnels, unsteady delta wing aerodynamics, fluidic oscillator flows, Hartmann tube o...

https://journals.sagepub.com/doi/pdf/10.1243/09544100JAERO243

A review of pressure-sensitive paint for high-speed and unsteady aerodynamics

The development and capabilities of fast-responding pressure-sensitive paint (fast PSP) are reviewed within the context of recent applications to aerodynamic and acoustic investigations. PSP is an optical technique for determining surface pressure distributions by measuring changes in the intensity of emitted light, whereas fast PSP is an extension applicable to unsteady flows and acoustics. Most fast PSP formulations are based on the development of porous binders that allow for rapid oxygen diffusion and interaction with the chemical sensor. This article reviews the development of porous binders, the selection of luminophore molecules suitable for unsteady testing, dynamic calibrations of PSP, data-acquisition methods, and noteworthy applications for flow and acoustic diagnostics. Calibrations of the dynamic response of fast PSP show a flat frequency response to at least 6 kHz, with some paint formulations exceeding a response of 1 MHz. Various applications of fast PSP are discussed that highlight the capabilities of the technique, and concluding remarks highlight the need for the future development of fast PSP.

Fast Pressure-Sensitive Paint for Flow and Acoustic Diagnostics

The microfluidic oscillator is a new microscale actuator developed for flow control applications. These patented devices can produce a 325-ytm-wide oscillating gas jet at high frequencies (over 22 kHz) and very low flow rates (∼1 1/ min or ∼1 g/ min). Furthermore, microfluidic oscillators have no moving parts; the jet oscillations depend solely on the internal fluid dynamics. In this work, the flowfield of a microfluidic oscillator is characterized using pressure transducers, water visualization, and pressure-sensitive paint. The acoustic field and frequency spectrum were characterized for the oscillator at several flow rates. The results indicate that the external flowfield of the microfluidic oscillator is marked by two distinct operating regimes, separated by a transitional increase in turbulent noise. This work also demonstrates a significant advance in pressure-sensitive paint technology. New instrumentation was developed to resolve small-scale, time-resolved measurements of a high-frequency micro flowfield. A macro imaging system was used to provide a spatial resolution of approximately 3 mm per pixel and time-resolved, full-unsteady pressure measurements at oscillation frequencies up to 21 kHz.

Characterization of the Microfluidic Oscillator

*† ‡ § This paper details the principles behind the time-averaged force production by a dielectric-barrier discharge plasma actuator A theoretical derivation shows that the force produced is due to the acceleration of ions through the applied electric field, and subsequent collisions with neutral particles This work shows that the force production is independent of the density of the neutral particles, but is governed by ion density, volume of the plasma, and the applied electric field Force production of the plasma actuator was experimentally measured in a large vacuum chamber used to control air pressure Power dissipation, applied voltage, and plasma intensity were also measured in these experiments These results indicate a linear relationship between force production and air pressure, with the force going to zero at vacuum conditions Dependencies of electric field strength and number of ions in the plasma as a function of pressure are also measured These two nonlinear relationships are determined to be the only factors affecting the amount of force produced by the plasma actuator

Force Production Mechanisms of a Dielectric-Barrier Discharge Plasma Actuator

This review provides a detailed discussion of the historical development of fluidic oscillators and their application to flow control. Fluidic oscillators were initially developed in the 1960’s for a variety of applications, and have seen resurgent interest for their suitability for modern flow control applications. The devices produce an oscillating jet of fluid over a wide fan angle and have no moving parts, making them an attractive actuator concept. This review aims to highlight the most important historical papers of relevance to modern fluidic oscillator development. The reviewed works will extend from the early 1960’s to the most recent investigations, with a focus on the fundamental operating mechanisms of fluidic oscillators. The authors present this review as a short synopsis of fluidic oscillators for flow control, while a more comprehensive review will be submitted for archival publication in the near future.

James W. Gregory

Papers

A review of pressure-sensitive paint for high-speed and unsteady aerodynamics

Fast Pressure-Sensitive Paint for Flow and Acoustic Diagnostics

Characterization of the Microfluidic Oscillator

Force Production Mechanisms of a Dielectric-Barrier Discharge Plasma Actuator

A Review of Fluidic Oscillator Development