Image registration for pressure-sensitive paint applications

Pressure-sensitive paint (PSP) data are generally derived by ratioing two images of a PSP-coated wind tunnel model. One image is taken at a known wind-off reference pressure while the other is taken at the test condition, in order to factor out the effects of nonuniform illumination and paint thickness. However, airloads on the model cause it to move during testing with respect to both the camera and the illumination source used to excite the P & P. Thus, the raw PSP images must be aligned, or registered, by means of a spatial transform. Once a proper ratioed image is obtained, image coordinates must be related to model coordinates so that pressure data can be obtained at the desired points on the model. This article compares several transforms used for image registration, and demonstrates the use of the projective equation of photogrammetry to relate model to image coordinates. An application in a small-scale wind tunnel test is shown, demonstrating mapping of pressure data to the three-dimensional model surface.Han Park was responsible for the original data reduction software, and much of the look and feel of the final product. Joel Mendoza provided the XB-70A model used in this study.     

Accuracy of correlation-based image registration for pressure-sensitive paint

The accuracy of correlation-based image registration (CBIR) in the analysis of pressure-sensitive paints (PSP) was investigated. CBIR has been developed to perform accurate image registration without the need for control points, even for model motions containing nonlinear local deformations. In the present study, the influence of displacement errors and their sensitivity on the accuracy of pressure measurement was examined by uncertainty analysis. The error sources in image registration were classified and several factors affecting the accuracy of image registration were examined. The performances of image registration were evaluated under several artificial model motions. Local intensity variations due to speckles, which enhance the image correlation in CBIR, may act as a source of image noise. The local pressure sensitivity in the presence and absence of speckles was investigated through pixel-by-pixel calibration. A spatial filtering was employed to reduce the local intensity variations. It was found that application of a median filter decreased the fluctuations in the local pressure sensitivity and significantly reduced the sensitivity of the intensity error to misregistration.     

Investigation of three-dimensional dynamic stall on an airfoil using fast-response pressure-sensitive paint

Dynamic stall on a pitching OA209 airfoil in a wind tunnel is investigated at Mach 0.3 and 0.5 using high-speed pressure-sensitive paint (PSP) and pressure measurements. At Mach 0.3, the dynamic stall vortex was observed to propagate faster at the airfoil midline than at the wind-tunnel wall, resulting in a “bowed” vortex shape. At Mach 0.5, shock-induced stall was observed, with initial separation under the shock foot and subsequent expansion of the separated region upstream, downstream and along the breadth of the airfoil. No dynamic stall vortex could be observed at Mach 0.5. The investigation of flow control by blowing showed the potential advantages of PSP over pressure transducers for a complex three-dimensional flow.     

Application of fast-responding pressure-sensitive paint to a hemispherical dome in unsteady transonic flow

The current work focuses on the development and application of fast-responding polymer/ceramic pressure-sensitive paint (PSP) as an advanced surface pressure measurement technique for unsteady flow fields in large-scale wind tunnels. To demonstrate the unsteady PSP technique, the unsteady surface pressure distribution over a hemispherical dome placed in the United States Air Force Research Laboratory’s Trisonic Gasdynamics Facility (TGF) was studied by phase-locking to the characteristic frequency in the flow caused by an unsteady separated shear layer shed from the dome. The wind tunnel was operated at stagnation pressures of 23.92 and 71.84 kPa, with the test section flow at Mach 0.6. Under the two operating conditions, the predominant shear layer frequency was measured to be 272 and 400 Hz, respectively. The quasi-periodic shear layer frequency enabled a phase-averaged method to be employed for capturing the unsteady shock motion on the hemisphere. Unsteady pressure data resulting from this technique are shown to correlate well with measurements acquired by conventional measurement techniques. Measurement uncertainty in the phase-averaging technique will be discussed. To address measurement uncertainties from temperature sensitivity and model movement, a new implementation of an AC-coupled data representation is offered.     

Application of novel pressure-sensitive paint formulations for the surface flow mapping of high-speed jets

The principle of pressure-sensitive paints (PSPs) is based upon excitation of the luminophore molecules at a certain wavelength and the emission of this absorbed energy at a higher wavelength. By isolating these two wavelengths we insure that the results obtained are not affected by any background radiation. Various international research groups, such as: the Central Aero-Hydrodynamic Institute (Russia), the University of Washington, NASA Ames, Boeing and McDonnell Douglas (USA), have developed their PSP formulations and some are commercially available.Two paints, which have been developed in-house at the Aero-Physics Laboratory (APL) at the University of Manchester, are studied here. One formulation uses hydrochloric acid (PSP1–HCl) and the other acetone as the solvent (PSP2–Ace). The current study employs the well known schlieren photography technique together with the relatively new PSP method, with comparison to discrete measurements, to examine the flow through a two-dimensional air-ejector system and examines the efficacy of the PSP formulations in providing an accurate global pressure field of the aforementioned setup. Detailed analysis of the errors and drawbacks involved in PSP measurements along with possible solutions to overcome them are also presented. Fully expanded jet Mach numbers in the range of 0.52 ? M_j ? 1.36 were examined.     

Characterization of pressure dynamics in an axisymmetric separating/reattaching flow using fast-responding pressure-sensitive paint

This collaborative work discusses the results of time-resolved pressure-sensitive paint measurements performed on a model of a generic spacecraft under sub- and transonic test conditions. It is shown that optical pressure measurements using an active layer from platinum?Cporphyrin complexes (PtTFPP) in combination with a polymer-ceramic base layer are able to measure dynamic flow phenomena in the trisonic wind tunnel facility up to sampling rates of 2?kHz. Low amplitude fluctuations in the order of 0.1?kPa were determined by means of this measurement technique. The buffet dynamics, as well as the spatial extent of the recirculation area in the near-wake, compare well with numerical predictions and PIV measurements. Furthermore, characteristic coherent pressure modes on the base were resolved, which were predicted by large-eddy simulations.     

Pressure-sensitive paint measurements in a shock tube

 Surface pressures were measured in the short-duration, transient flow environment of a small-scale, low pressure-ratio shock tube using thin-film pressure-sensitive paint (PSP). Issues regarding coating formulation, measurement uncertainty, optical system design, and temperature and illumination compensation are discussed. The pressure measurements were acquired during steady flow conditions following the passage of normal shocks and expansion regions along a flat sidewall and a wedge sidewall. The PSP characteristic response time was 3 to 6 ms. Overall pressure uncertainty for the shock tube measurements ranged up to 5% over one atmosphere and compared well with theoretical estimates of uncertainty. Received: 20 April 1998 / Accepted: 9 September 1998     

Dynamic surface pressure measurements on a square cylinder with pressure sensitive paint

The dynamic and static surface pressure on a square cylinder during vortex shedding was measured with pressure sensitive paints (PSPs) at three angles of incidence and a Reynolds number of 8.9×10⁴. Oscillations in the phosphorescence intensity of the PSP that occurred at the vortex shedding frequency were observed. From these phosphorescent oscillations, the time-dependent changes in pressure distribution were calculated. This work extends PSP’s useful range to dynamic systems where oscillating pressure changes are on the order of 230 Pa and occur at frequencies in the range of 95–125 Hz.     

Application of pressure-sensitive paint for determination of the pressure field and calculation of the forces and moments of models in a wind tunnel

The visualization and measurements of aerodynamic effects on a 3D aircraft model were conducted using an optical pressure measurement system, based on the pressure-sensitive paint (PSP) technique. PSP technology provides a good understanding of the flow around the wind tunnel model. The PSP technique can be used to carry out absolute pressure measurements on a surface of the model and to determine additional aerodynamic data using scientific-grade cameras and image processing techniques. Surface pressures from the top, bottom, left, and right viewing directions were obtained using the DLR-PSP system on the entire surface, which can be observed by eight CCD cameras. Finally, the measured pressures can be integrated to calculate the forces and moments of the complete model, or parts thereof.     

Thermal-transient effects on pressure measurements

A set of experiments were performed on a flush-mounted strain-gage pressure transducer to determine the nature of erroneous responses induced by two types of thermal transients. The thermal transients studied were a constant step increase in transducer-diaphragm surface temperature, and a constant step increase in diaphragm-surface heat flux. The responses of the transducer to changes in ambient temperature and to temperature gradients in the transducer housing at steady state were investigated also. Analytic models for the transducer were solved under the experimental conditions and compared with the experimental results for the first two transients. These models predicted the response of the transducer quite well for about the first 25 percent of the transient period. Beyond this short time, the transducer behavior was drastically different from the models.     

Extension and characterization of pressure-sensitive molecular film

Pressure-sensitive paint (PSP) has the potential as a diagnostic tool for pressure measurement in high Knudsen number regime because it works as a so-called “molecular sensor”. However, there are few reports concerning application of PSP to micro-devices, because conventional PSPs are too thick owing to polymer binders. In our previous work, we adopted the Langmuir–Blodgett (LB) technique to fabricate the pressure-sensitive molecular film (PSMF) using Pd(II) Mesoporphyrin IX (PdMP), which has pressure sensitivity only in the low pressure range (below 130 Pa). In this study, aiming for pressure measurement under an atmospheric pressure condition, we have constructed four samples of PSMFs composed of Pt(II) Mesoporphyrin IX (PtMP), Pt(II) Mesoporphyrin IX dimethylester (PtMPDME), Pt(II) Protoporphyrin IX (PtPP) and Cu(II) Mesoporphyrin IX dimethylester (CuMPDME) as luminescent molecules. The pressure sensitivity of those PSMFs was measured, and it was clarified that the pressure sensitivity of PSMF-PtMP is the highest among the four samples. Moreover, the temperature dependency of PSMF-PtMP was investigated, and we found that the temperature dependency of PSMF is dominated not by the oxygen diffusion in the layer, but by non-radiative deactivation process of excited luminescent molecules.     

Instrument effects on stress jump measurements

Stress jumps occur in fluids that possess a viscous contribution to their total stress. On cessation of shear, such a fluid will show an instantaneous loss of stress, while some of the stress will remain and subsequently decay in some manner, either exponential or otherwise. The results in this paper show that filtering, present in most rheometers, will eliminate the stress jump and not allow its measurement. The torque head inertia for a spring-type torque measuring system is found to greatly influence the measurements and a stress jump is not seen. In fact, the material's true stress relaxation behaviour is far from that measured. The force rebalance torque measuring system is found to accurately measure stress jumps.     

Base strain effects on force measurements

Force transducers directly interact with their environment. In this study, a force transducer is attached to the midpoint of a free-free beam and is used to measure the force on a rigid mass that vibrates with the beam. The ratio of force to mass times acceleration is measured for several different masses over a frequency range that includes the first four odd natural frequencies of the beam. Then the force transducer is mounted so that the beam's strain is isolated from the transducer. The tests are repeated. The results dramatically illustrate the effects of base strain on the force measurements. A simple theoretical model is developed that explains the vertical axis shift in the calibration curve. Paper was presented at SEM 1994 Spring Conference and Exhibits.     

Quantitation of pressure-sensitive film using digital image scanning

An experimental technique is presented for measurement of contact stress distribution using Fuji Pressensor film. The development of packets of Pressensor disks for use in small areas or surfaces of complex curvature is explained. A digital-image-scanning procedure has been developed to substantially increase the spatial resolution with which stress distributions can be constructed from stained Fuji Pressensor film. The technique is developed for small, discrete disks as well as for larger, continuous sheets of Pressensor. As an illustration, sealed packets of small Pressensor disks are used to measure the stress distribution across a cadaveric juvenile femoral head under load.     

Image-averaging in the analysis of data from pressure-sensitive film

Experimental Techniques - Image addition is a useful tool for determining interface pressures in situations demanding greater spatial and pressure resolution than those resolutions possible from...     

The spatial and pressure resolution of fuji pressure-sensitive film

Due to its ease of application, Fuji prescale pressure-sensitive film is currently one of the more popular methods, within the biomechanics community, for assessing contact areas and pressures within articulating joints—in addition to its use in industry. This material produces a stain on the application of pressure due to the rupture of microscopic bubbles releasing a liquid which, in turn, causes patches of color to be formed; a greater pressure produces a darker stain. These stains are often converted into digital images and manipulated to produce false-color pressuremaps, an approach which is beyond the simple methods of analysis suggested by the manufacturer. Due to the granular nature of Fuji film stains, the two user-defined variables which will determine the accuracy of any pressure-map are: (a) the size of the sample-area used to capture data from the original stain during the digitization process and (b) the number of pressure-intervals identified on each map; the chosen values should match the spatial and pressure resolutions of the film. Despite the importance of these factors, the literature presents a bewildering array of values, particularly for the number of pressure-intervals, with no validation of those chosen; consequently, little guidance is provided for other potential users of Fuji film. This paper discusses the relationship between sample-area and pressure-interval and introduces a method for examining their effect on the resulting pressure-maps. The results obtained using Super Low grade Fuji film suggest that the authors of some previously reported methods may have been overambitious in their choice of sample-area and pressure-intervals. Finally, a series of suggested values of sample-area size and pressure-intervals are provided.