Pressure-Sensitive Paint Application to an Oscillating Shock Wave in a Transonic Flow M.-C. Merienne, * P. Molton, † R. Bur, ‡ and Y. Le Sant § ONERA–The French Aerospace Lab, 92190 Meudon, France DOI: 10.2514/1.J053744 This paper presents an application of the pressure-sensitive paint technique to investigate two-dimensional unsteady flow in a transonic channel. This work is a contribution to the study of the transonic interaction between an oscillating shock wave and a separated boundary layer in a channel flow. The shock-wave oscillation is forced by the periodic variation of the section of a second throat by means of a rotating elliptical shaft located in its section. The channel’s lower wall is equipped with a contour profile, or bump, allowing for flow separation. To achieve a reduced response time for surface pressure measurements, we use anodized-aluminum coating as pressure-sensitive paint instead of usual paint. An aluminum insert including the bump was manufactured and coated with anodized- aluminum pressure-sensitive paint. Images were acquired by using a high-speed camera, and pressure-sensitive paint results were compared with pressure tap and Kulite sensor measurements implemented in the insert. Spectral analysis was carried out to assess the ability of anodized-aluminum pressure-sensitive paint for understanding unsteady aspects of such a complex channel flow. Nomenclature A i , B ij = temperature-dependent calibration coefficients I = intensity of light emitted by pressure-sensitive paint at some pressure I Ref = intensity of light emitted by pressure-sensitive paint at reference pressure p = surface pressure, Pa p Ref = reference surface pressure, Pa p 0 = initial pressure level before a step change, Pa p 1 = final pressure level after a step change, Pa S T = temperature sensitivity of the pressure-sensitive paint, %∕K S p = pressure sensitivity of the pressure-sensitive paint, %∕kPa S pT = pressure sensitivity due to temperature, Pa∕K T = temperature, K t 99 = pressure settle time, ms τ = response time, ms I. Introduction P RESSURE-SENSITIVE paint (PSP) is widely used in aerody- namic testing, as this technique introduces an innovative concept of instrumentation allowing pressure measurements at any point on a model surface provided that this point is viewed by optical instru- mentation. PSP is now a well-established surface pressure measure- ment method for a large domain of velocities ranging from low-speed to supersonic steady-state flows [1,2]. Application to unsteady pressure measurements is a difficult challenge for the PSP technique, as it requires a fast-responding coating. For the unsteady PSP, highly gas diffusive materials are needed as a binder, since the PSP response time depends on the gas diffusivity through the paint thickness. Unsteady applications concern different types of research in fluid mechanics. Rotating machinery generates very short periodical phe- nomena, whereas turbulent flows induce random phenomena. Un- steady flows are present in a large domain of applications like reduction of noise engines, control of airfoil buffeting or vortex breakdown, and reduction of automobile drag and noise. Usually, unsteady pressure measurements are obtained with discrete sensors, which complicates the model design and increases the manufacturing time. In addition, fast pressure sensors are very expensive, so their number is usually limited. Thus, the development of an optical method able to measure the unsteady pressure on the whole model surface would be of great interest. Most PSPs have a response time of 1 s, which is incompatible with unsteady measurements. The composition of a PSP consists of a porous binder mixed with luminescent molecules. The pressure sensitivity being the result of the oxygen-quenching process of the luminescence, the paint binder must be porous in order to allow energy transfer with oxygen molecules. The paint porosity deter- mines the PSP response time. For unsteady measurements, it can be necessary to reduce the response time below 1 ms, which represents several orders of magnitude below the actual response time. Thus, the PSP coating has to be designed in such a way that the luminescent sensor is directly in contact with oxygen. Different materials have been tested in order to increase the paint porosity: for example, by using a high-porosity binder [3,4]. Other solutions consist of increasing the porosity of the binder by adding hard ceramic particles in the polymer [5,6]. Another solution is to remove the binder and to fix the luminophore directly on the model surface. Gregory [7] and Asai et al. [8] developed a fast-responding PSP based on porous anodized aluminum with the dye adsorbed on the surface. This type of coating leads to a response time on the order of a few tens of microseconds. An extensive overview of the state of the art on unsteady PSP development was summarized by Gregory et al. [9]. A previous investigation using anodized-aluminum PSP (AA- PSP) was performed at ONERA–The French Aerospace Lab in 2007 to study the flowfield inside a nozzle during the starting process and to define the pressure gradient in the outlet region during the steady- state phase. The inner surface of the nozzle was prepared with AA- PSP coating, and images were acquired up to 5000 Hz [10]. During this test campaign, it was demonstrated that the use of a 12 bit depth complementarity metal-oxide-semiconductor (CMOS) high-speed camera is suitable for PSP investigation. In the present work, PSP is used to investigate a quasi two- dimensional unsteady transonic flow in a channel with a strong Presented as Paper 2010-4921 at the 27th AIAA Aerodynamic Measurement Technology and Ground Testing Conference, Chicago, IL, 28 June–1 July 2010; received 7 July 2014; revision received 18 May 2015; accepted for publication 11 June 2015; published online 7 September 2015. Copyright © 2015 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 1533-385X/15 and $10.00 in correspondence with the CCC. *Research Scientist, Fundamental and Experimental Aerodynamics Department, 8 rue des Vertugadins; marie-claire.merienne@onera.fr. † Research Scientist, Fundamental and Experimental Aerodynamics Department, 8 rue des Vertugadins; pascal.molton@onera.fr. ‡ Research Scientist, Fundamental and Experimental Aerodynamics Department, 8 rue des Vertugadins; reynald.bur@onera.fr. § Research Scientist, Fundamental and Experimental Aerodynamics Department, 8 rue des Vertugadins; yves.le_sant@onera.fr. 3208 AIAA JOURNAL Vol. 53, No. 11, November 2015 Downloaded by UNIVERSITA DEGLI STUDI DI MILANO on January 6, 2016 | http://arc.aiaa.org | DOI: 10.2514/1.J053744