Pressure-sensitive Paint Measurements Research Articles

Characterization of the Endwall Flow in a Low-Pressure Turbine Cascade Perturbed by Periodically Incoming Wakes, Part 2: Unsteady Blade Surface Measurements Using Pressure-Sensitive Paint

Unsteady pressure-sensitive paint (i-PSP) measurements were performed at a sampling rate of 30 kHz to investigate the near-endwall blade suction surface flow inside a low-pressure turbine cascade operating at engine-relevant high-speed and low-Re conditions. The investigation focuses on the interaction of periodically incoming bar wakes at 500 Hz with the secondary flow and the blade suction surface. The results build on extensive PIV measurements presented in the first part of this two-part publication, which captured the ’negative-jet-effect’ of the wakes throughout the blade passage. The surface pressure distributions are combined with CFD to analyze the flow topology, such as the passage vortex separation line. By analyzing data from phase-locked PIV and PSP measurements, a wake-induced moving pressure gradient negative in space and positive in time is found, which is intensified in the secondary flow region by 33% with respect to midspan. Furthermore, two methods of frequency-filtering based on FFT and SPOD are compared and utilized to associate a pressure fluctuation peak around 678 Hz with separation bubble oscillation.

A deep-learning image registration method for pressure-sensitive paint measurements

Image registration is an essential step in the data processing of pressure-sensitive paint (PSP) measurements. As PSP technology is gradually expanded to increasingly harsh environments, it faces challenges such as severe image misalignment due to model deformations/motions, which pose difficulties to traditional feature-based registration algorithms. To improve registration accuracy and efficiency, we propose an end-to-end image registration method based on deep learning. Initially, a PSP dataset based on PSP images is constructed through data augmentation. Three types of residual network and three training strategies are then adopted to prepare the deep-learning model for automatic image registration. The optimal combination of the residual network and training strategy is selected for validation using fabricated PSP images and outperforms two traditional algorithms (i.e., the Sift and Watershed methods). Finally, the performance of the deep-learning method is compared with that of traditional algorithms adopting a new metric of the overlapping rate for assessing the registration accuracy on experimental PSP images. The results show that the deep learning method outperforms the traditional algorithms in terms of registration accuracy and robustness.

Fast pressure-sensitive paint measurements of dynamic stall on a pitching airfoil via intensity- and lifetime-based methods

Fast pressure-sensitive paint measurements of dynamic stall on a pitching airfoil via intensity- and lifetime-based methods

Flow mechanism study and geometrical parameters analysis of fluidic oscillators based on pressure-sensitive paint measurements and modal analysis

Flow mechanism study and geometrical parameters analysis of fluidic oscillators based on pressure-sensitive paint measurements and modal analysis

Temperature Dependency Model in Pressure Measurement for the Motion-Capturing Pressure-Sensitive Paint Method.

Pressure-sensitive paint (PSP) has received significant attention for capturing surface pressure in recent years. One major source of uncertainty in PSP measurements, temperature dependency, stems from the fundamental photophysical process that allows PSP to extract pressure information. The motion-capturing PSP method, which involves two luminophores, is introduced as a method to reduce the measurement uncertainty due to temperature dependency. A theoretical model for the pressure uncertainty due to temperature dependency is proposed and demonstrated using a static pressure measurement with an applied temperature gradient. The experimental validation of the proposed model shows that the motion-capturing PSP method reduces the temperature dependency by 37.7% compared to the conventional PSP method. The proposed model also proves that a PSP with zero temperature dependency is theoretically possible.

Low-frequency large-scale unsteadiness analysis of lateral jet interactions on a slender cylinder in hypersonic flows using fast-responding pressure-sensitive paint

Lateral jet interactions, commonly encountered in high-speed vehicles, are typical complex three-dimensional (3D) shock wave/boundary layer interactions. The spatiotemporal features of surface pressure are crucial for understanding such complex interactions. In this study, fast-responding pressure-sensitive paint measurements are implemented at a sampling rate of 3000 Hz and a spatial resolution of 0.35 mm/pixel to investigate global pressure fields in lateral jet interactions on a slender cylinder under hypersonic flow conditions. Tests are conducted at three angles of attack (0°, 5°, and 10°) and two sideslip angles (0° and 2.5°) at Mach 6 and a Reynolds number of 1.35 × 107/m. The mean pressure results indicate the presence of a 3D high-pressure region beneath the bow shock feet, the area and pressure level of which depend on the cylinder attitude. Moreover, significant low-frequency broadband unsteadiness associated with the large-scale motion of bow shock is found in the high-pressure region. Statistical, coherence, and modal analyses are performed to clarify the characteristics of the large-scale unsteadiness. The results indicate that the dominant pressure mode is induced by streamwise oscillation of the bow shock, which exhibits two subregions with opposite pressure-fluctuation variations. Furthermore, the effects of the cylinder attitude on the unsteadiness extent and intensity are discussed.

Fast-Responding Pressure-Sensitive Paint Measurements of the IC3X at Mach 7.2

Global surface pressure measurements of a 5.7% scale AFRL Initial Concept 3.X vehicle (IC3X) were obtained using a fast-responding ruthenium-based pressure-sensitive paint (PSP) at the UTSA Mach 7 Ludwieg Tube Wind Tunnel at two different angles of attack, 0° and 2.5°. Static calibration of the paint was performed over a range of 0.386 kPa to 82.7 kPa to relate luminescent intensity to pressure. Details on the facility, paint preparation, application, calibration, and image processing techniques are provided in the manuscript. The results from statistical, spectral, and proper orthogonal decomposition (POD) analyses are presented to characterize the pressure field observed on the model. The experimental results qualitatively follow the expected trends and correspond to the occurrence of shock waves and expansion fans, which were visualized via Schlieren imaging. The theoretical pressure range obtained from conical shock analysis for 0° agrees with the experimentally derived pressure range for the model, and the outliers are attributed to errors in image registration. This study presents preliminary pressure measurements that pave the way for obtaining time-resolved global PSP measurements to train and validate aerothermodynamic machine learning models.

Lifetime based pressure and temperature sensitive paint measurement on a civil aircraft wing under conditions near the onset of shock-induced separation

Lifetime-based pressure- and temperature-sensitive paint (PSP and TSP) measurements were conducted in a large-scale industrial transonic wind tunnel to obtain high-quality pressure data for validation of computational fluid dynamics (CFD) simulations. A wind tunnel test was performed using the NASA common research model in the JAXA 2 m × 2 m transonic wind tunnel. The freestream Mach number was varied in the range of 0.70–0.89, and aerodynamic forces and moments were acquired in order to obtain the pitch break condition related to the onset of shock-induced separation. Polymer-based PSP was coated on a main wing of the model, and TSP was used in tandem with PSP to correct the temperature dependence of PSP. The two-gate lifetime-based method was applied to obtain the PSP and TSP emissions. An a-priori/in-situ hybrid calibration was conducted to convert ratio-of-ratios to pressure, and measured pressure distributions were mapped onto a three-dimensional (3D) model grid. The root-mean-square error for the pressure measurements was evaluated by pressure tap data and was approximated to be 0.8 kPa for all Mach numbers tested. The obtained pressure distributions exhibited a significantly high signal-to-noise ratio and were used for comparison with CFD results on a 3D grid. The high-spatial-resolution PSP measurements helped to accurately localize the differences from the CFD simulation results and showed that the prediction of the shock location along the main wing is still a relevant challenge.

Film Cooling Effectiveness on Pressure Surface and Suction Surface of Turbine Guide Vane With Diffusion Slot Holes

Abstract This paper investigated the film-cooling effectiveness of diffusion slot holes in a turbine nozzle guide vane. The pressure-sensitive paint measurement technique was employed to obtain the film-cooling effectiveness at a density ratio of DR = 1.5. The mainstream Reynolds number based on the axial chord length and the exit velocity was 6 × 105. The mainstream turbulence intensity was approximately 3.7%. Three diffusion slot hole geometries with cross-sectional aspect ratios (ASs) of 2.3, 3.4, and 4.9 were tested and compared with a typical fan-shaped hole. The experiments were performed at three typical hole row locations on the pressure surface (PS) and suction surface (SS). The average blowing ratios varied from M = 0.5 to 2.5. The results showed that throughout the blowing ratio range, on the PS, a substantially higher film-cooling effectiveness than the fan-shaped hole is always obtained from the diffusion slot hole with a large aspect ratio (AS = 4.9); on the SS, the diffusion slot hole with a small AS (AS = 2.3). The influence of hole row positioning is inconsistent for diffusion slot holes with different ASs. The diffusion slot hole is less affected by the PS when the AS is moderate and less affected by the SS when the AS is large. The film-cooling effectiveness of the diffusion slot holes is basically the lowest where the PS has a maximum concave curvature and the highest where the SS has a large favorable pressure gradient.

Experimental characterization of shock-separation interaction over wavy-shaped geometries through feature analysis

A canonical wavy surface exposed to a Mach 2 flow is investigated through high-frequency Pressure Sensitive Paint, Kulite measurements, and shadowgraph imaging. The wavy surface features a compression and expansion region, two shock-boundary layer interactions, and two shock-separation regions. The unsteady characteristics of the wall pressure and shock angles are presented, demonstrating an increase in the amplitude of the instabilities when traveling through the shock systems. The pressure sensitive paint measurements confirm a two-dimensional flow pattern with small transversal unsteadiness. Higher-Order Dynamic Mode Decomposition and Spectral Proper Orthogonal Decomposition are implemented to dissect the different flow features, revealing several dominant low-frequency and medium-frequency phenomena. The separation region appears at frequencies with Strouhal numbers between 0.01 and 0.2, confirmed by the frequency content in the local pressure measurement using Kulites and the pressure sensitive paint.

An accumulated imaging method with phase-locking for rotor pressure-sensitive paint measurements

An accumulated imaging method with phase-locking for rotor pressure-sensitive paint measurements

Mach and Reynolds number effects on transonic buffet on the XRF-1 transport aircraft wing at flight Reynolds number

This work provides an overview of aerodynamic data acquired in the European Transonic Windtunnel using an XRF-1 transport aircraft configuration both at cruise conditions and at the edges of the flight envelope. The goals and design of the wind tunnel test were described, highlighting the use of the cryogenic wind tunnel’s capability to isolate the effects of M_{infty }, {text{Re}}_{infty } and the dynamic pressure q/E. The resulting dataset includes an aerodynamic baseline characterization of the full span model with vertical and horizontal tailplanes and without engine nacelles. The effects of different inflow conditions were studied using data from continuous polars, evaluating the changes in aeroelastic deformation which are proportional to q/E and the influence of M_{infty } and {text{Re}}_{infty } on the shock position. Off-design data was analyzed at the lowest and highest measured Mach numbers of 0.84 and 0.90, respectively. Wing lower surface flow and underside shock motion was analyzed at negative angles of attack using {{c}_{text{p}}} distribution and unsteady pressure transducer fluctuation data, identifying significant upstream displacement of the shock close to the leading edge. Wing upper-side flow and the shock motion near buffet onset and beyond was analyzed using unsteady pressure data from point transducers and unsteady pressure-sensitive paint (PSP) measurements. Buffet occurs at lower angles of attack at high Mach number, and without clearly defined lift break. Spectral contents at the acquired data points in the buffet range suggest broadband fluctuations at Strouhal numbers between 0.2 and 0.6, which is consistent with recent literature. The spanwise shock propagation velocities were determined independently via analysis of unsteady PSP and pressure transducers to be in the range between u_{text{s}} / u_{infty } = 0.24 and 0.32, which is similarly in line with published datasets using other swept wing aircraft models.

Prediction of wing buffet pressure loads using a convolutional and recurrent neural network framework

AbstractIn the present study, a hybrid deep learning reduced-order model (ROM) is applied for the prediction of wing buffet pressure distributions on a civil aircraft configuration. The hybrid model is compound of a convolutional variational neural network autoencoder (CNN-VAR-AE) and a long short-term memory (LSTM) neural network. The CNN-VAR-AE is used for the reduction of the high-dimensional flow field data, whereas the LSTM is applied to predict the temporal evolution of the pressure distributions. For training the neural network, experimental buffet data obtained by unsteady pressure sensitive paint measurement (iPSP), is applied. As a test case, the Airbus XRF-1 configuration is selected, considering two different experimental setups. The first setup is defined by a wind tunnel model with a clean wing, whereas the second setup includes an ultra high bypass ratio engine nacelle on each wing. Both configurations have been tested in the European Transonic Windtunnel, considering several transonic buffet conditions. Finalizing the training of the hybrid neural networks, the trained models are applied for the prediction of buffet flow conditions which are not included in the training data set. A comparison of the experimental results and the pressure distributions predicted by the hybrid ROMs indicate a precise prediction performance. Considering both aircraft configurations, the main buffet flow features are captured by the hybrid ROMs.

Uncertainty and validation of unsteady pressure-sensitive paint measurements of acoustic fields under aero engine-like conditions

Fast response pressure-sensitive paint (PSP) allows optical measurements of pressure fluctuations on a surface with high spatial and temporal resolution. This technique is evaluated for aeroacoustic measurements inside an aeroacoustic wind tunnel (AWT). The AWT is a test rig especially designed for investigating the excitation and propagation of sound under conditions typical for turbomachinery. The aim of this work is to compare the results of sound pressure measurements of tonal sound fields in a circular duct conducted with PSP and microphone arrays in order to assess the applicability of PSP in turbomachinery acoustics applications. A data analysis process is presented, which projects the camera image of the PSP data onto a given surface. To analyze the spatial pressure fluctuations, the PSP data are transformed in the frequency domain using pixel-wise fast Fourier transform. Measurements with a mean Mach number up to 0.109 and 5 kHz excitation frequency are conducted. An acoustic mode generator is used to excite the sound field with specific circumferential mode order. The pressure fluctuations obtained with the PSP measurement visualize the measured acoustic field well and allow early interpretation. The pressures of PSP and microphones are in good agreement; for example, the maximum detected deviation in pressure at 2700 Hz is 30 Pa. A preview on using radial mode analysis to decompose the acoustic field, measured by PSP, into acoustic modes is provided. The results are confirmed by a decomposition using conventional arrays of flush-mounted microphones.Graphical

Experimental and numerical investigations on the unsteady flow and film cooling characteristics of the trailing edge cutback

The film cooling characteristics of the trailing edge cutback slot of a high pressure turbine were experimentally and numerically investigated. The unsteady flows have an important effect on the mixing process of the mainstream and coolant flow, resulting in deteriorated film performance. To illustrate the interaction between the unsteady flows and coolant injection, eight blowing ratios ranging from 0.3 to 1.2 were considered for the baseline cases. Pressure sensitive paint measurements were conducted to acquire the adiabatic film cooling effectiveness distribution. The counter-intuitive decrease of the film cooling effectiveness was confirmed and further revealed. Particle image velocimetry showed that the unsteady flow patterns downstream of the trailing edge slot varied with increasing blowing ratio. Furthermore, the high-fidelity RANS/LES hybrid method was employed to resolve the detailed flow fields. The numerical method was carefully validated, and it successfully predicted the unsteady flows and mixing process. The spanwise vorticity showed that the two branches of the shedding vortices alternatively become strong. At the blowing ratio of 0.75, the strength of the two branches was comparative, yielding the worst cooling performance. Additionally, the density ratio and lip thickness to slot height ratio were investigated. Thus, the density ratio affects the coolant velocity ratio, causing the forward movement of the inflection point of the film cooling effectiveness map. The decrease of the lip thickness to slot height ratio leads to the reduction of the strength of the shedding vortices and therefore increases the film cooling effectiveness.

Pressure-sensitive paint measurements on the cavity with passive control devices under transonic flow

Abstract In this study, the pressure-sensitive paint (PSP) technique, specifically the mesoporous-particle-based PSP, was employed to compare rectangular cavities with varying length-to-depth ratios (L/D) and different trailing edge shapes under transonic conditions. By utilizing PSP, comprehensive and quantitative pressure data were obtained, enabling the simultaneous observation of surface flow field distribution. The results obtained using PSP were found to be consistent with those obtained from conventional pressure sensors. The study revealed that the pressure distribution within the cavities changed with increasing L/D, and cavities with different trailing edge shapes demonstrated a reduction in pressure at the bottom region. Furthermore, the comparison of results obtained through the oil flow method corroborated the PSP findings, indicating that a beveled or sawtooth-shaped trailing edge of the cavity induced air flow deflection, effectively disrupting the upstream shear flow structure and altering the pressure distribution at the cavityʼs bottom.

Time-resolved pressure-sensitive paint measurements for cryogenic wind tunnel tests

A first time-resolved pressure-sensitive paint (PSP) test campaign at the European Transonic Wind Tunnel (ETW) was conducted within the research initiative “Unsteady flow and interaction phenomena at High Speed Stall conditions”. One of the objectives of this wind tunnel campaign was to resolve time-series of surface pressure distributions caused by complex 3-D buffet phenomena on a full-span airplane model XRF-1 transport aircraft configuration. At higher angle-of-attack and high Mach number, pressure fluctuations with a frequency of several hundred Hertz are expected to occur on the main wing and the horizontal tail plane (HTP) caused by the buffet effect. To capture the expected buffet phenomena by PSP, the German Aerospace Center developed a time-resolved PSP measurement and data acquisition system as well as a post-processing method for measurements in ETW. The measurements were conducted on the main wing and HTP simultaneously, with a camera frame rate of up to 2 kHz. The transonic buffet phenomena were observed at the flight relevant Reynolds number Re = 12.9 × 106 and Re = 25.0 × 106. The time-varying surface pressure distribution on the model was successfully captured by PSP. The time-series and spectra of both PSP and pressure transducer data match very well.

Experimental assessment of wing lower surface buffet effects induced by the installation of a UHBR nacelle

The installation of large bypass ratio engines on classical under wing configurations may lead to shock/boundary layer interaction on the wing lower surface, limiting the flight envelope in a similar way to classical buffet occurring on the wing suction side at high incidences in transonic flight. In this study, buffet effects on the lower surface of the wing induced by the installation of a Ultra-High-Bypass-Ratio through flow nacelle are assessed by means of wind tunnel testing. Unsteady pressure sensitive paint measurements were used to assess the pressure field on the wing with high temporal and spatial resolution. Strong unsteady shock motion associated with shock induced separation was found on the wing lower surface for various combinations of Mach number, Reynolds number and angle of attack. The wing lower surface buffet effects are found to increase with reducing angle of attack and are present over a wide range of Reynolds numbers. Preliminary spectral analysis suggests an upper limit for the buffet frequency at a Strouhal number of about 0.4.

Time-series image denoising of pressure-sensitive paint data by projected multivariate singular spectrum analysis

Time-series data, such as unsteady pressure-sensitive paint (PSP) measurement data, may contain a significant amount of random noise. Thus, in this study, we investigated a noise-reduction method that combines multivariate singular spectrum analysis (MSSA) with low-dimensional data representation. MSSA is a state-space reconstruction technique that utilizes time-delay embedding, and the low-dimensional representation is achieved by projecting data onto the singular value decomposition (SVD) basis. The noise-reduction performance of the proposed method for unsteady PSP data, i.e., the projected MSSA, is compared with that of the truncated SVD method, one of the most employed noise-reduction methods. The result shows that the projected MSSA exhibits better performance in reducing random noise than the truncated SVD method. Additionally, in contrast to that of the truncated SVD method, the performance of the projected MSSA is less sensitive to the truncation rank. The projected MSSA achieves denoising effectively by extracting smooth trajectories in a state space from noisy input data. Expectedly, the projected MSSA will be effective for reducing random noise in not only PSP measurement data, but also various high-dimensional time-series data.Graphic abstract

A dual-camera single-shot lifetime method with fully controllable exposure gates for pressure-sensitive paint measurements on fast-rotating surfaces

This paper proposes a dual-camera single-shot lifetime method for pressure-sensitive paint (PSP) measurements on fast-rotating surfaces. Two cameras with fully controllable exposure gates are used for image acquisition to reduce the motion blur due to the open-ended exposure of the interline transfer charge-coupled device camera which was typically used for the single-shot lifetime method. The images can thus be processed without deblurring for a wider rotating speed range than is possible with existing techniques. More importantly, both frames have equal exposure times and therefore equal blur lengths. This enables precise pixel-to-pixel image matching and minimizes image registration errors. In addition, the signal strengths of the frames can be balanced, enabling shot-noise-induced errors to be reduced by independent adjustment of the aperture ratio. A pressure uncertainty estimation model is established to evaluate the measurement errors, which helps determine the system parameters. The dual-camera PSP system is calibrated and then validated through the surface pressure measurement of a fast-rotating disk. It is found that the exposure time t e has little effect on the PSP sensitivity, whereas the delay time t d affects both the pressure sensitivity and the measurement error. Both t e and t d are set to 5 μs in the rotating disk experiment. Images with a blur length of 1.4 pixels are captured and processed without deblurring to yield reasonable surface pressure results. The pressure error is decreased from 1.05 to 0.65 kPa by increasing the aperture ratio F from 2 to 4.