James W. Gregory

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

Abstract 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 oscillations, acoustics, and turbomachinery. Flowfields with fundamental frequencies as high as 21 kHz have been successfully measured with porous PSP formulations.

Force Production Mechanisms of a Dielectric-Barrier Discharge Plasma Actuator

*† ‡ § 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.

Characterization of the Microfluidic Oscillator

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.

Porous pressure-sensitive paint for characterizing unsteady flowfields

The fast response time characteristics of porous pressure-sensitive paint (porous PSP) are applied to unsteady flowfield measurements. The unsteady flowfield of a fluidic oscillator is investigated by using anodized aluminum (AA), thin-layer chromatography (TLC), and polymer/ceramic (PC) as porous supporting matrices. The frequency response of these PSPs is measured using a shock tube, showing responses of 12.2, 11.4, and 3.95 kHz for AA-PSP, TLC-PSP, and PC-PSP, respectively. Flow oscillations of various phases are captured with these porous PSPs in the fluidic oscillator tests, with AA-PSP giving the sharpest images

Pressure‐sensitive paint as a distributed optical microphone array

Pressure-sensitive paint is presented and evaluated in this article as a quantitative technique for measurement of acoustic pressure fluctuations. This work is the culmination of advances in paint technology which enable unsteady measurements of fluctuations over 10 kHz at pressure levels as low as 125 dB. Pressure-sensitive paint may be thought of as a nano-scale array of optical microphones with a spatial resolution limited primarily by the resolution of the imaging device. Thus, pressure-sensitive paint is a powerful tool for making high-amplitude sound pressure measurements. In this work, the paint was used to record ensemble-averaged, time-resolved, quantitative measurements of two-dimensional mode shapes in an acoustic resonance cavity. A wall-mounted speaker generated nonlinear, standing acoustic waves in a rigid enclosure measuring 216 mm wide, 169 mm high, and 102 mm deep. The paint recorded the acoustic surface pressures of the (1,1,0) mode shape at approximately 1.3 kHz and a sound pressure level of 145.4 dB. Results from the paint are compared with data from a Kulite pressure transducer, and with linear acoustic theory. The paint may be used as a diagnostic technique for ultrasonic tests where high spatial resolution is essential, or in nonlinear acoustic applications such as shock tubes.

POROUS PRESSURE-SENSITIVE PAINT FOR MEASUREMENT OF UNSTEADY PRESSURES IN TURBOMACHINERY

This work details the development and application of porous pressure-sensitive paint (PSP) for measuring unsteady surface pressures in turbomachinery. The advantages of porous PSP over conventional methods include global pressure measurement, fast response time, and instrumentation of thin turbomachinery components that are otherwise difficult or impossible to instrument. In this paper, the development, calibration, and application of porous PSP to turbomachinery are discussed. Porous PSP formulations were developed to ensure fast response times and good adhesion characteristics to rotating parts. Experimental methods such as phase-locking and image registration were developed to acquire quality data. Dynamic response calibrations of PSP with a fluidic oscillator were performed, indicating that porous PSP formulations have a response time of up to 40 kHz. For a turbomachinery application, the inlet wall and impeller blades of a turbocharger compressor were painted with fast-responding polymer/ceramic PSP. The turbocharger was operated at 100,000 rpm, corresponding to a blade-passage frequency (BPF) of 10 kHz. Even at this high speed and BPF, porous PSP was able to resolve the unsteady wall pressure distributions about the blade. In addition, a flow blockage was created to induce an inlet flow distortion on the compressor. Porous PSP was able to resolve the 1.67 kHz unsteady blade loading. Potential applications of porous PSP to unsteady turbomachinery testing include evaluations of rotor/stator interaction, flutter, inlet flow distortion, rotating stall, and surge.

Effect of Quenching Kinetics on Unsteady Response of Pressure-Sensitive Paint

Pressure-sensitive paints (PSP) have recently been extended to high-frequency flowfields. Paint formulations have been used effectively to characterize pressure fluctuations on the order of 100 kHz. As the limits of PSP are extended, various experimental results indicate that the unsteady response characteristics are nonlinear. A thorough understanding of the photophysical mechanisms in paint response is needed. Gas transport properties, coupled with the nonlinear nature of the Stern‐Volmer relationship have an effect on the paint response. This work discusses the full implications of a diffusion-based model for the unsteady response of pressure-sensitive paint. Based on this model, it is shown that the indicated pressure response of PSP is faster for a decrease in pressure, and slower for a pressure increase. Effects of other factors, such as pressure-jump magnitude, pressure-jump range, and Stern‐Volmer nonlinearity, are evaluated. Furthermore, a fluidic oscillator is used to demonstrate experimentally the quenching kinetics of two types of PSP—polymer/ceramic and fast FIB. Results from the oscillator operated with argon, nitrogen, and oxygen gases at 1.59 kHz demonstrate behavior that agrees with the diffusion model. The polymer/ceramic PSP exhibited no delay between different test gases, indicating a flat frequency response of at least 1.59 kHz. Fast FIB, on the other hand, demonstrated a significant delay in rise time between the nitrogen and oxygen cases. Both the diffusion model and the experimental results demonstrate that the different responses to nitrogen and oxygen only become critical when the period of the flowfield oscillations is shorter than the response time of the paint formulation.

Microchannel Pressure Measurements Using Molecular Sensors

Fluid mechanics on the microscale is an important subject for researchers who are interested in studying microdevices since physical phenomena change from macroscale to microscale. Channel flow is a fundamental topic for fluid mechanics. By using a molecular sensor known as pressure-sensitive paint (PSP), detailed pressure data can be obtained inside the microchannel and at the channel entrance. The achievable spatial resolution of the acquired pressure map can be as high as 5 mum. PSP measurements are obtained for various pressure ratios from 1.76 to 20, with Knudsen number (K n) varying from 0.003 to 0.4. Compressibility and rarefaction effects can be seen in the pressure data inside the microchannel and at the channel entrance.

Single-shot, lifetime-based pressure-sensitive paint for rotating blades

A single-shot, lifetime-based pressure-sensitive paint (PSP) technique is proposed as a pressure sensor for applications requiring high pressure sensitivity on a moving model such as a rotor blade. The method is based on a single pulse of high-energy excitation light and a double-frame exposure on an interline transfer charge-coupled device camera for recording luminescent lifetime. Small pressures can be measured on surfaces that are moving in an aperiodic manner (which precludes phase averaging). Measurements in environments having overall surface pressure gradients as small as 1 kPa show that the technique is capable of accurately resolving small pressure fluctuations. The pressure sensitivity to the oxygen concentration of some commonly available PSP formulations has been investigated with respect to capabilities and limitations of the paints for this single-shot lifetime application. A system with ruthenium-based pressure-sensitive paint, 532 nm wavelength laser and a CCD camera is demonstrated on a 0.126 m diameter propeller rotating at 70 Hz. Pressure data are acquired within a single pulse of excitation light energy, with no image averaging required.

Motion-deblurred, fast-response pressure-sensitive paint on a rotor in forward flight

A pressure-sensitive paint (PSP) system capable of measuring the global, unsteady pressure distribution on a rotating surface without resorting to phase averaging is applied to a two-bladed model propeller in edgewise freestream flow. A gated lifetime-based technique captures the paint luminescence after a single pulse of high-energy laser excitation, yielding a signal-to-noise ratio sufficient to avoid image averaging. The selection of a porous polymer/ceramic matrix base with platinum tetra(pentafluorophenyl) porphyrin (PtTFPP) as the luminophore afforded high frequency response and pressure sensitivity, but the long lifetime of PtTFPP caused blurring in the long-exposure image of the rotating blade. An approach to deblurring based on the lifetime of the paint and surface motion is described and validated by results obtained from a disc of 17.8 cm diameter spinning at 70 Hz. An infrared camera recorded wind-on and -off temperature maps to provide a temperature correction for the PSP. The single-shot PSP technique with motion deblurring and temperature correction is then applied to a vertically mounted model propeller with a 25.4 cm diameter and 10.2 cm pitch. Surface pressure maps for the advancing and retreating blades are presented for a spin rate of 70 Hz and advance ratio of 0.3. The higher suction peak and other features on the advancing blade due to its larger effective velocity are detected by the paint system, while the retreating blade shows a qualitatively different distribution.