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This study experimentally determines high-frequency aeroacoustic noises of the high-speed dual-impinging jet, focusing on the generation and evolution mechanism between impingement tones modes and far-field acoustic spectra. The variables of the high-speed impinging jet are Mach number (Ma = 0.9 and 1.1), fixed diameter (D) of nozzle, nozzle spacing (3.5D), and impingement distance (4D). A novel fast-responding pressure-sensitive paint (fast-PSP) with a significantly extended frequency response ability was designed to develop an accurate phase-resolving propagation process of aeroacoustic noises and handle high impingement momentum challenges posed by the high-speed impinging jet. The PSP raw data were enhanced by calibration, image restoration, and proper orthogonal decomposition filtering. Two distinct far-field spectral characteristics were identified based on synchronized acoustic measurements. The existence of the stagnation region affected by the fountain effect in the dual-impinging jet was determined using spatial–temporal cross-correlation analysis. Subsequently, the concurrent axisymmetric dual annulus mode and its coupling behavior at Ma = 0.9 and the non-axisymmetric helical mode and its periodic fragmentation-reconstruction patterns at Ma = 1.1 were identified by spectral proper orthogonal decomposition. Finally, the spatial–temporal evolution of the phase-locked first-order mode was extracted, and the transient variations of coherent structures and their mechanisms for discrete tone noise generation were quantitatively investigated. The expansion and coupling of the dual annulus modes promoted the dominance of single-tone peaks across the entire acoustic spectrum. The helical mode, exhibiting both rotational and expansion behaviors, enhanced the coherent vorticity at the periphery of the coherent structures, resulting in intense impingement tones sound sources. [ABSTRACT FROM AUTHOR]

Fast pressure-sensitive paint (PSP) was applied to an inlet/isolator designed using the Osculating Internal Waverider Inlet with Parallel Streamlines (OIWPS) method. The dorsal isolator surface pressure was measured using anodized-aluminum PSP through transparent cast acrylic that makes up the ventral portion of the isolator. Temperature-sensitive paint was utilized to correct for the PSP's temperature sensitivity. The model was tested under Mach 5.7 flow at Re = 8.5 × 10 6 /m and 10.2 × 10 6 /m in the AFOSR–Notre Dame Large Mach-6 Quiet Tunnel (ANDLM6QT) under conventional noise conditions. Flow phenomena, such as shocks originating in the inlet and flow separation at the throat, were visualized with high spatial resolution. The dynamics measured by the PSP and pressure transducers matched well where the spectral signal-to-noise ratio was above unity. Power spectral densities showed significant frequency content at ≈ 1 kHz in the shock-wave/boundary-layer interaction (SWBLI) regions. Coherence analysis showed a linear relationship between the unsteady pressures at locations underneath different SWBLI in the isolator, with the exception of the Busemann throat shock. Temporal correlation of shock positions indicated that disturbances propagated downstream at 114% of the core-flow velocity; however, improved calculations of the core-flow velocity are needed to refine this assessment. The surface pressure fields at Re = 8.5 × 10 6 /m and 10.2 × 10 6 /m were quantitatively very similar, and the results in the ANDLM6QT were qualitatively similar to previous studies in the Boeing/AFOSR Mach-6 Quiet Tunnel under noisy flow. [ABSTRACT FROM AUTHOR]

Pressure-sensitive paint (PSP) was applied to the surface of a NACA0012 airfoil to investigate pressure fluctuations associated with trailing edge (TE) noise under low-velocity flow conditions. The primary focus is to assess the feasibility of employing laser pulses exposed at the airfoil surface to mitigate TE noise. However, the weak pressure fluctuations accompanying TE noise pose a challenge, as they are overshadowed by image sensor noise in high-speed cameras capturing PSP emission changes. To address this issue, a novel time-resolved phase-locking technique was introduced, utilizing the signal from a semiconductor pressure transducer at the trailing edge as a phase-lock trigger source. By repetitively conducting phase-locked measurements (1150 times), time series ensemble averaged data based on PSP emission images were obtained, enabling the capture of these subtle pressure fluctuations. Quantitatively, fluctuations with a dominant frequency of 679 Hz and an amplitude of 50 Pa are resolved within an accuracy of about 15 Pa, achieved at a recording rate of 19.2 kHz. Both the suppression and subsequent redevelopment of the pressure field with the TE noise offer valuable insights into the dynamics of TE noise and open avenues for targeted noise reduction strategies in aerodynamic applications. [ABSTRACT FROM AUTHOR]

Highly maneuverable aircraft configurations can be subjected to tail buffeting phenomena at moderate subsonic speeds and medium to high angles of attack. Detailed measurement of the aerodynamic surface pressure fluctuations and the corresponding vibrations is of great importance to investigate these buffeting effects. Fast-response pressure sensitive paint (iPSP) was developed at the German Aerospace Center (DLR) to measure unsteady pressure fluctuation effects in a setup with high-speed cameras. The setup allows a high spatio-temporal resolution of the unsteady surface pressures. In the present paper the iPSP method is applied to a mixed-material, flexible aircraft wind tunnel model with double-delta wing and horizontal tail plane in these flow conditions. The low-speed buffeting effects on the configuration are measured using a synchronized system of five connected high-speed cameras to simultaneously record the unsteady surface pressures with iPSP and the dynamic three-dimensional structural deformation. The present application case is particularly challenging due to low levels of pressure fluctuation in the low-speed regime, multi-frequency aeroelastic phenomena on the model, a long distance between the cameras and the model, as well as the combination of synchronized measurement of unsteady surface pressure and structural deformation. Advanced data analysis methods are applied in a post-processing tool chain to cope with these challenges. The focus of the present paper is the setup of the synchronized measurement chain as well as the data analysis tools. First results are evaluated with respect to iPSP unsteady surface pressures. The root-mean square surface pressures show good agreement with high-fidelity CFD results for the flow conditions of interest. Particular focus of the analysis is on the lessons learned regarding the UV illumination setup, the camera arrangement as well as post-processing tools. [ABSTRACT FROM AUTHOR]

The evolution of pressure-sensitive paint (PSP)-based surface pressure measurement technology for rotating machinery is significant for flow mechanism analysis and blade design. Considering the challenges of reducing motion blur and improving signal-to-noise ratio (SNR) in the application of PSP technology for the rotating machinery, an accumulated imaging method with phase-locking based on the periodic characteristics of the rotor flow field has been proposed in this study, which is capable of capturing the phase-averaging pressure on the rotor blade surface. The motion blur is reduced by pulse acquisition of PSP luminescent, and the SNR is improved by accumulating luminescent signals of the same phase multiple times, which can address the challenge. Single PSP luminescence acquisition time and accumulation number are the two most crucial parameters for measurement accuracy, and the constraints of the parameters are discussed in detail according to the principle of PSP. The PSP measurement system for rotating machinery based on the proposed method has been established, and the method is successfully applied to the rotating blade surface pressure measurement of a transonic axial fan. Besides, the proposed method can also be applied to the temperature measurement of the rotating blades based on temperature-sensitive paint. The results offer a technical foundation for the pressure measurement of rotating machinery. [ABSTRACT FROM AUTHOR]

Pressure-sensitive paint (PSP) measurement for turbine blades and vanes is often limited by the optical window and occlusion of the complex blade structure. Endoscopic PSP imaging systems have been designed for the pressure measurement of integrated inter-turbine duct (IITD) guide vanes. Notably, the objective lens of the endoscope leads to severe and dynamic image distortion. The classical direct linear transformation (DLT) algorithm does not consider nonlinear distortion, which reduces the accuracy of three-dimensional (3D) pressure field reconstruction. Therefore, in this study, a dynamic distortion correction endoscopic (DDCE) PSP technique is proposed. Real-time endoscopic images are registered to the post-calibration images through affine transform. Next, the distortion was compensated using the post-calibrated distortion and intrinsic matrix, and the relationship between the pixels on the distorted two-dimensional (2D) PSP images and 3D points on the vanes can be reliably acquired. Reprojection errors are used to evaluate the performance of the proposed technique. The maximum and average errors decrease from 30 to 4 pixels and from 4.8 pixels to 0.8 pixels, respectively. Subsequently, the DDCE PSP technique is applied for the pressure measurement on turbine guide vanes with outlet Mach number ranging from 0.65 to 0.95. Endoscopic PSP images of a structural vane and two aero-vanes are recorded, and the 2D pressure fields of the three vanes are defined through in situ calibration of the PSP. Finally, the 3D pressure fields are reconstructed by mapping the 2D pressure field to the 3D point cloud using the DDCE PSP technique. [ABSTRACT FROM AUTHOR]

A pressure distribution inside a two-dimensional rectangular cavity with a ratio L / D = 5.0 of the cavity length L to the depth D over a supersonic flow was obtained using an improved fast anodized-aluminum pressure-sensitive paint (AA-PSP). The freestream Mach number was M ∞ = 1.9 and the frequencies of the dominant cavity modal phenomena, called "Rossiter" modes, were on the order of 1–10 kHz. The fast AA-PSP using a free-based porphyrin luminophore and a high-frequency-repetition double-pulsed laser were applied, and instantaneous pressure fields were captured. This measurement system has a 60 kHz sampling rate and is suitable for the purpose of a direct measurement of the Rossiter mode phenomena. The result of the power spectral density (PSD) of the PSP measurement showed that this measurement system is capable of observing phenomena of approximately 18 kHz. Time-resolved pressure transducer measurement was also employed, and PSD obtained by the PSP measurement was validated. The frequency spectra and the amplitudes of PSD obtained by the PSP measurement were in good agreement with those obtained by the pressure transducer data. A spectral proper orthogonal decomposition (SPOD) analysis was also conducted, and the symmetric and asymmetric spatial modes were extracted at each peak frequency. In this analysis, the seventh Rossiter mode phenomena of approximately 21 kHz were visualized. The peak frequencies of symmetric modes were in good agreement with those of the Rossiter mode, and lower than that of asymmetric modes. The energy of the symmetric modes at each peak frequency was several times higher than that of the asymmetric mode. The phase of the SPOD mode revealed the propagation direction of the pressure waves. It showed the existence of the upstream propagation waves and standing waves inside the cavity. [ABSTRACT FROM AUTHOR]

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 ∞ , Re ∞ 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 ∞ and Re ∞ 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 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 s / u ∞ = 0.24 and 0.32, which is similarly in line with published datasets using other swept wing aircraft models. [ABSTRACT FROM AUTHOR]

Unsteady surface pressure measurements have been carried out on a flat-plate test article in the Air Force Research Laboratory's Mach-6 Ludwieg Tube at a negative angle of attack using PCB pressure sensors. The acoustic waves of the second-mode boundary-layer instability mechanism were successfully measured as indicated by the fluctuating surface pressure spectra. The effectiveness of a novel porous material (Silicon-Carbide (SiC) foam) at absorbing these fluctuations was investigated by comparing porous-wall and impermeable-wall spectra. The impermeable-wall spectra indicate second-mode peaks in amplitude, which agree with companion computations, while the equivalent porous material showed no signs of the instability in the spectra therefore exhibiting a strong dampening ability. However, the porous-wall spectra demonstrate similar breakdown to turbulence to the impermeable material at higher length Reynolds numbers, which might be attributed to surface-roughness effects. Acoustic-absorption bench tests also indicated high levels of ultrasonic absorption for the SiC foam—the acoustic absorption coefficient was consistently higher than 0.90 for second-mode relevant frequencies. A band-limited amplitude integration of the experimental PCB spectra indicate lower amplitude growth in the porous material compared to the impermeable material; however, beyond a length Reynolds number of ≈ 4 · 10 6 , the band-limited amplitude of the porous material approach a similar value to the impermeable one. Linear-stability (LST) computations confirmed the experimentally observed frequencies only when supported by base-flow calculations capturing the leading-edge shock, which entails sudden thickening of the boundary layer at the tip not captured by the Blasius solution. Computed N factors for the porous material were approximately 30% of the associated impermeable values where the material inserts were located, downstream of which, they approached the same value. This study indicates that SiC foam has a strong ability to absorb acoustic waves characteristic of the hypersonic second-mode boundary-layer instability mechanism, and is therefore a candidate material for passive boundary-layer control. [ABSTRACT FROM AUTHOR]

Fluidic thrust vectoring (FTV) offers a novel approach to aerodynamic control, circumventing some of the issues associated with mechanical systems. One method is shock vector control which involves injecting a fluid into the exhaust nozzle of an engine to redirect the gases and thus, produce a control force. An experimental model which incorporated FTV was designed and tested at Mach 6 in the Oxford high density tunnel (HDT). The model was a simplified two-dimensional scramjet geometry with two different configurations to compare an internal and external exhaust nozzle. The FTV injection system consisted of a slot at the rear edge of the exhaust nozzle fed from an internal plenum. In the experimental campaign, a range of gas injection pressures and free stream stagnation pressures were tested to assess the effectiveness of both configurations. Two new measurement methods were successfully implemented in the HDT: pressure sensitive paint and a 6-axis load cell. The FTV system has been shown to be effective with observable increases in lift and pitching moment. A linear relation between the injection pressure ratio and the control forces could be observed for both configurations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Combined measurement of shape and aerodynamic force is effective for understanding the mechanism of transonic wing flutter. To explore one method, a random-dot pressure-sensitive paint (PSP) was applied to the time-resolved global measurement of the deformation and pressure field of a transonic flow over an airfoil. The random-dot PSP has a patterned texture with luminescence intensity, which is made by intentional photodegradation of a PSP dye. These dots allow simultaneous measurement of pressure and the surface profile of the airfoil using stereo-digital image correlation. The feasibility of the random-dot PSP was tested in a transonic wind tunnel using a three-dimensional backswept wing model, which started to flutter at Mach 0.89 with a dominant frequency of 100 Hz. The unsteady surface profile and pressure distribution of the airfoil were measured using 12-bit high-speed cameras as the deformation amplitude increased. The experimental results indicate that the deformation and surface pressure distribution during flutter were successfully measured over time. The accuracy of the measured deformations and pressures was evaluated by comparison with several point data obtained with a laser displacement meter and semiconductor pressure transducers. The accuracy at each time-resolved image obtained at a recording rate of 6.25 kHz was within 1 mm for displacement and about 1 kPa for pressure. [ABSTRACT FROM AUTHOR]

A novel measurement method is developed for a simultaneous measurement of pressure and temperature on an airfoil by sensitive paints. The proposed method requires two sets of measurements: in the first set, the temperature distribution is measured on the entire surface of the airfoil by temperature-sensitive paint (TSP). This temperature field is further utilized to evaluate sparse sensor locations based on the sensor selection methods. For the second set of measurements, TSP was sprayed on sparse points and pressure-sensitive paint (PSP) on the remaining airfoil surface. A full temperature field can be reconstructed using temperature data measured at those sparse locations. The temperature-induced error due to temperature sensitivity of PSP is corrected, and a time-averaged pressure field is compared with the pressure tap data. The proposed method is demonstrated on a flow over a NACA 0015 airfoil. Time-averaged and spanwise averaged pressure agrees very well with pressure sensor data measured simultaneously with PSP giving further confidence in our measurement. The present results also show that the Bayesian estimation and a corresponding sensor selection method overperform the linear least squares estimation and a corresponding sensor selection method, and the Bayesian estimation framework is recommended for the practical sparse sensor for temperature reconstruction. [ABSTRACT FROM AUTHOR]

Shock wave oscillations and surface flow fields on unswept-wing and 10 deg swept-wing models were experimentally investigated in a two-dimensional transonic wind tunnel facility to clarify the effects of wing sweep on the shock buffet phenomena. High-speed shadowgraph imaging, oil flow, and unsteady pressure-sensitive paint (PSP) techniques were used to visualize the shock oscillation over the models, surface stream lines, and the surface pressure distributions, respectively. Shock waves oscillated periodically on both models under specific flow conditions, but the oscillation frequency for the 10-deg swept wing was 1.4 times higher than that for the unswept wing. A spectral analysis and a mode decomposition analysis of PSP data for the 10 deg swept-wing model showed that higher-frequency shock oscillation developed mainly on the outboard wing, where a complicated 3D surface flow structure was formed. We confirmed that the wing sweep angle is an important factor affecting characteristic Strouhal number of shock buffet according to the present data and the results of previous studies. [ABSTRACT FROM AUTHOR]

The mixing between the coolant and the boundary-layer gas downstream of an injector—for transpiration/film cooling—has been extensively studied for turbulent flows; however, only a handful of studies concerning laminar mixing exist, particularly in hypersonic flows. In this paper, the concentration of the coolant gas at the wall and the heat flux reduction downstream of a transpiring injector in a hypersonic laminar flow are experimentally measured and examined. Experiments are performed in the Oxford High Density Tunnel at Mach 7. A flat-plate model is coated with pressure-sensitive paint (PSP) to spatially resolve the film and obtain a film effectiveness based on coolant concentration. Thin-film arrays are installed to measure the heat flux reduction. Six different cases are studied featuring nitrogen and helium as the coolant gas, where the blowing ratio is varied from 0.0406% to 0.295 % . The unit Reynolds number of the flow is 12.9 × 10 6 m - 1 . A coolant concentration of up to 95 % is achieved immediately (2 mm) downstream of the injector. The film concentration drops in a monotonic fashion farther downstream; however, a constant film coverage of 5–20 mm immediately downstream of the injector is observed in cases with a higher blowing ratio. A film coverage above 15% over three injector lengths is present even for the lowest blowing ratio. Heat flux reduction is achieved in all cases. The concentration effectiveness obtained from PSP is compared with the thermal film effectiveness calculated from the heat flux reduction. The latter is found to be higher than the former for all data points. Finally, a collapse of the thermal effectiveness is achieved and a modified analytical correlation is proposed. [ABSTRACT FROM AUTHOR]

Fast-responding pressure-sensitive paint (fast PSP) is a valuable tool for high spatial and temporal resolution full-field pressure measurement. However, when fast PSP is used for quantitative measurement of pressure fluctuation, there remain challenges including signal attenuation and delay and noise contamination. This paper first discusses the effect of PSP system's response and noise contamination on pressure fluctuation measurement and data analysis (spectral analysis and dynamic modal analysis). Then, the ability of fast PSP to extract high-frequency dynamic features from an experimental system comprising an inclined subsonic jet impinging on a plate is characterized. In this system, the jet velocity was Mach number 0.78–0.84, the feature frequency of pressure fluctuation was as high as 2500 Hz, while the 3 dB attenuation of the PSP used in this work occurred at approximately 1300 Hz. Proper orthogonal decomposition is used to realize noise reduction with no loss in spatial resolution. This noise reduction and concomitant spectral correction substantially improve the accuracy of results up to a frequency of 2500 Hz. Spatiotemporal correlation, band-limited spectra, and spectral proper orthogonal decomposition (SPOD) are then used to analyze the pressure fluctuations due to jet impingement. The SPOD results reveal the kilohertz dynamic features caused by vortex motions on the plate. [ABSTRACT FROM AUTHOR]

This work applied fast mesoporous-particle-based pressure-sensitive paint (PSP) to obtain the time-resolved flow dynamics inside fluidic oscillators with jet speed up to 0.7 Mach and oscillation frequencies higher than 1 kHz. The frequency of the oscillator decreases as the feedback channel length increases. The frequency characteristics can be divided into a linear growth stage and a slow change stage. The external velocity characteristic of the oscillator also presented a double-peak phenomenon. Pressure wave reflection theory was used to explain the double-peak phenomenon of velocity, which was verified according to the PSP dynamic pressure field. In addition, the proper orthogonal decomposition method was applied to the PSP snapshots to further explain the dominating pressure propagation and small-scale pressure changes. [ABSTRACT FROM AUTHOR]

The study presents an optimized pressure-sensitive paint (PSP) measurement system that was applied to investigate unsteady surface pressures on recently developed double-swept rotor blades in the rotor test facility at the German Aerospace Center (DLR) in Göttingen. The measurement system featured an improved version of a double-shutter camera that was designed to reduce image blur in PSP measurements on fast rotating blades. It also comprised DLR's PSP sensor, developed to capture transient flow phenomena (iPSP). Unsteady surface pressures were acquired across the outer 65% of the rotor blade with iPSP and at several radial blade sections by fast-response pressure transducers at blade-tip Mach and Reynolds numbers of M tip = 0.282 - 0.285 and Re tip = 5.84 - 5.95 × 10 5 . The unique experimental setup allowed for scanning surface pressures across the entire pitch cycle at a phase resolution of 0.225 deg azimuth for different collective and cyclic-pitch settings. Experimental results of both investigated cyclic-pitch settings are compared in detail to a delayed detached eddy simulation using the flow solver FLOWer and to flow visualizations from unsteady Reynolds-averaged Navier–Stokes (URANS) computations with DLR's TAU code. The findings reveal a detailed and yet unseen insight into the pressure footprint of double-swept rotor blades undergoing dynamic stall and allow for deducing "stall maps", where confined areas of stalled flow on the blade are identifiable as a function of the pitch phase. [ABSTRACT FROM AUTHOR]

This paper presents the performance of pressure-sensitive paint (PSP) for the direct measurement of species concentration on a porous surface with mass injection. It is used to measure the ability of an injected gas to reduce the mass transfer of freestream species to the surface. A porous alumina sample was sprayed with a PSP luminophore solution. The sample was installed into a flat plate model and exposed to hypersonic cross-flows in the Oxford High-Density Tunnel. Tests were conducted with no coolant injection, air injection, and nitrogen injection at increasing blowing ratios. Oxygen partial pressure maps on the transpiration-cooled surface were obtained for several conditions at unit Reynolds numbers between 2.58 - 5.0 × 10 7 / m and blowing ratios between 0.016 - 0.078 % . The oxygen pressure decreases as the unit Reynolds number decreases and the blowing ratio increases. [ABSTRACT FROM AUTHOR]

Flow over thin elastic panels that undergo small amplitude vibrations has a common occurrence in supersonic and hypersonic platforms. The resulting fluid–structure interactions (FSI) are shown to excite the panel oscillation over different elastic modes. The consequent pressure field over the panel poses a stiff measurement challenge because of the fundamental limitations with using pressure transducers. Fast response pressure-sensitive paints make a viable alternative to unravel the pressure dynamics over the panel surface. However, the dominant contribution from the camera noise and the finite number of images acquired introduce broadband fluctuations in the pressure fluctuation power spectral density (PSD) over the panel surface that mask the elastic mode contributions to the PSD in weakly coupled FSI. In this work, an approach is introduced to unearth the elastic mode contributions in the PSD of the panel surface pressure. This approach involves averaging multiple power spectra over a representative region to subdue the broadband fluctuations while retaining the elastic mode contributions. The number of samples to be averaged is determined from the theoretical scaling of the r.m.s. of the broadband spectral fluctuations with averaging. This approach is demonstrated in an impinging shock boundary layer interaction (SBLI) unit generated over an elastic panel at a Mach number of 2.5. The PSD within the incoming boundary layer revealed an elastic mode contribution in the (2,1) mode with spectral sample averaging that was otherwise submerged within the broadband noise. Similarly, the contributions from the different elastic modes were revealed within the intermittent region and the relaxing boundary layer with spectral sample averaging. Whereas the Welch's algorithm to compute the PSD retained the noise floor, the use of cross-spectral density significantly mitigated the noise floor and provided a more accurate determination of the PSD. [ABSTRACT FROM AUTHOR]

Global surface pressure measurements have been carried out on a 7° half-angle circular cone/flare model at nominally zero angle of attack using pressure-sensitive paint (PSP). These experiments were conducted to illustrate the PSP technique's usefulness and effectiveness at measuring the unsteady structures inherent to hypersonic shock-wave/boundary-layer interactions (SWBLI) on a global scale. Mean and fluctuating surface pressure was measured with a temperature-corrected, high-frequency-response ( ≈ 10 kHz) anodized-aluminum pressure-sensitive paint (AA-PSP). This AA-PSP was made in-house to provide the high frequency response required. Methodologies for tracking the boundary-layer separation and reattachment shock feet in both time-averaged and instantaneous senses are provided and discussed. Excellent agreement is observed between the different metrics. In addition, spectral analyses were conducted on a global scale providing insights into the unsteady dynamics of the shock feet and structures under the separated shear layer. These spectral analyses identified a smooth, low-frequency bandwidth centered at ≈ 500 Hz, which is characteristic of the shock-foot oscillations. These experimental findings validate the usefulness of AA-PSP to provide global physical insights of unsteady SWBLI surface behavior in the hypersonic flow regime. Similar methodologies can be incorporated in future experiments to investigate complex and novel SWBLI. [ABSTRACT FROM AUTHOR]

Pressure-sensitive paint (PSP) has become an important technique for global pressure measurement on rotating surfaces such as helicopter rotor blades. Due to the temperature sensitivity of PSP, it is essential to apply accurate temperature correction method in PSP measurements. In previous studies, one-dimensional (1D) correction methods were generally adopted which only considered the radial temperature gradient on the blade. However, in the current study, it was found that the systematic error of this 1D assumption was non-negligible in large-scale experiments due to the growing chordwise temperature gradient. Accordingly, a two-dimensional (2D) temperature correction method considering both radial and chordwise temperature gradients was developed to improve the accuracy of PSP. In this method, a 2D temperature map was firstly generated based on the TSP results. Calibrations were then conducted under a series of temperature and pressure values to generate a calibration surface. According to the calibration results, the pressure and temperature fields on the rotor blade were solved directly. The proposed 2D method was validated through PSP measurements on a 2 m diameter BO-105 helicopter rotor in hover. Compared with 1D temperature correction, the results from the current method showed clearly improved agreement with the CFD results and reduced overall measurement error. [ABSTRACT FROM AUTHOR]

This review summarizes state-of-the-art knowledge on fast-responding pressure-sensitive paint (fast PSP), which has evolved into a powerful experimental tool for studying complex flow problems. As the formulation of porous paint with kilohertz response is now well established, full-field pressure measurements with high spatial and temporal resolution have been achieved on both stationary and moving targets. Recent studies have significantly advanced every aspect of this technology, including paint development, theoretical modeling, system integration and data processing. Novel paint formulations with superior sensing properties and additional functions for multi-physical measurements are being continually developed. The dynamic response mechanism is better understood through analysis and modeling considering the processes of photoluminescence, gas diffusion and light transmission. More importantly, applications of fast PSP are being expanded from regular wind-tunnel tests to more challenging conditions featuring hypervelocity, fast rotation and high temperature. Interdisciplinary research has played a key role in these development processes, and will remain vital for future breakthroughs in PSP technology. [ABSTRACT FROM AUTHOR]

Recognizing the need for global surface measurement techniques to characterize the time-varying, three-dimensional loading encountered on rotating wind turbine blades, fast-responding pressure-sensitive paint (PSP) has been evaluated for resolving unsteady aerodynamic effects in incompressible flow. Results of a study aimed at demonstrating the laser-based, single-shot PSP technique on a low Reynolds number wind turbine airfoil in static and dynamic stall are reported. PSP was applied to the suction side of a Delft DU97-W-300 airfoil (maximum thickness-to-chord ratio of 30 %) at a chord Reynolds number of 225,000 in the University of Wyoming open-return wind tunnel. Static and dynamic stall behaviors are presented using instantaneous and phase-averaged global pressure maps. In particular, a three-dimensional pressure topology driven by a stall cell pattern is detected near the maximum lift condition on the steady airfoil. Trends in the PSP-measured pressure topology on the steady airfoil were confirmed using surface oil visualization. The dynamic stall case was characterized by a sinusoidal pitching motion with mean angle of 15.7°, amplitude of 11.2°, and reduced frequency of 0.106 based on semichord. PSP images were acquired at selected phase positions, capturing the breakdown of nominally two-dimensional flow near lift stall, development of post-stall suction near the trailing edge, and a highly three-dimensional topology as the flow reattaches. Structural patterns in the surface pressure topologies are considered from the analysis of the individual PSP snapshots, enabled by a laser-based excitation system that achieves sufficient signal-to-noise ratio in the single-shot images. The PSP results are found to be in general agreement with observations about the steady and unsteady stall characteristics expected for the airfoil. [ABSTRACT FROM AUTHOR]

Fast pressure-sensitive paint (PSP) is very useful in flow diagnostics due to its fast response and high spatial resolution, but its applications in low-speed flows are usually challenging due to limitations of paint's pressure sensitivity and the capability of high-speed imagers. The poor signal-to-noise ratio in low-speed cases makes it very difficult to extract useful information from the PSP data. In this study, unsteady PSP measurements were made on a flat plate behind a cylinder in a low-speed wind tunnel (flow speed from 10 to 17 m/s). Pressure fluctuations (Δ P) on the plate caused by vortex-plate interaction were recorded continuously by fast PSP (using a high-speed camera) and a microphone array. Power spectrum of pressure fluctuations and phase-averaged Δ P obtained from PSP and microphone were compared, showing good agreement in general. Proper orthogonal decomposition (POD) was used to reduce noise in PSP data and extract the dominant pressure features. The PSP results reconstructed from selected POD modes were then compared to the pressure data obtained simultaneously with microphone sensors. Based on the comparison of both instantaneous Δ P and root-mean-square of Δ P, it was confirmed that POD analysis could effectively remove noise while preserving the instantaneous pressure information with good fidelity, especially for flows with strong periodicity. This technique extends the application range of fast PSP and can be a powerful tool for fundamental fluid mechanics research at low speed. [ABSTRACT FROM AUTHOR]

Traditional pressure-sensitive paint (PSP) systems can provide data with high spatial resolution; however, the bandwidth is limited to a few Hz by the response time of the paint. Fast-responding paints have demonstrated response times of up to 100 kHz. Ultra-bright LEDs and fast-framing cameras combined with a porous polymer PSP can be used to produce a system capable of both high spatial resolution and high temporal bandwidth. Measurements of mean and unsteady pressure have been acquired on an experimental setup composed of a Mach-2 channel flow with transverse jet injection. The unsteady pressure data clearly resolve structures not present in the mean pressure data, including multiple lambda shocks upstream of a strong bow shock, high-frequency perturbations in the location of these shocks, and significant deformations of the bow shock structure. Time series of data can be extracted at each pixel, and the spectral content and phase relationship of the flow can be presented as maps of pressure fluctuations at specific frequencies or as correlation coefficients between a control point and the remaining flow. This type of map can be created using arrays of fast pressure transducers; here, we present data representing an array of over 26,000 fast pressure transducers. [ABSTRACT FROM AUTHOR]

Unsteady pressure-sensitive paint (PSP) measurements were acquired on an articulated model helicopter rotor of 0.26 m diameter in edgewise flow to simulate forward flight conditions. The rotor was operated at advance ratios (free stream velocity normalized by hover tip speed) of 0.15 and 0.30 at a cycle-averaged tip chord Reynolds number of 1.1 × 10, with collective and longitudinal cyclic pitch inputs of 10° and 2.5°, respectively. A single-shot data acquisition technique allowed a camera to record the paint luminescence after a single pulse of high-energy laser excitation, yielding sufficient signal-to-noise ratio to avoid image averaging. Platinum tetra(pentafluorophenyl) porphyrin (PtTFPP) in a porous polymer/ceramic binder served as the PSP. To address errors caused by image blurring and temperature sensitivity, a previously reported motion deblurring algorithm was implemented and the temperature correction was made using temperature-sensitive paint measurements on a second rotor blade. Instantaneous, unsteady surface pressure maps at a rotation rate of 82 Hz captured different aerodynamic responses between the two sides of the rotor disk and were compared to the nominally steady hover case. Cycle-to-cycle variations in tip unsteadiness on the retreating blade were also observed, causing oblique pressure features which may be linked to three-dimensional stall. [ABSTRACT FROM AUTHOR]

Spatially temporally resolved unsteady pressure fields on a surface of a hemisphere-on-cylinder turret with either a flat or a conformal window with realistic features such as gaps and 'smile' cutouts were characterized using fast-response pressure-sensitive paint at M = 0.33 for several window viewing angles. Various statistical properties of pressure fields were computed, and geometry effects on the unsteady pressure fields were analyzed and discussed. Proper orthogonal decomposition was also used to extract dominant pressure modes and corresponding temporal coefficients and to analyze and compare instantaneous pressure structures for different turret geometric features and the window viewing angles. An unsteady separation off the turret and a recirculation region downstream of the turret were identified as dominant sources of the unsteady pressure. It was found that while all geometric features affected the unsteady pressure field, the 'smiles,' positioned spanwise-symmetrically on both sides of the turret, were the leading cause of these changes, followed by the looking forward flat window. The gaps, the side- and the back-looking flat window introduced only small local changes. [ABSTRACT FROM AUTHOR]

We proposed applying the cross-spectrum analysis to the unsteady pressure-sensitive paint (PSP) images to eliminate the electronic shot noise component from the pressure fluctuation spectrum. The data of surface pressure fluctuation behind a square cylinder which measured by means of the polymer/ceramic PSP with a frequency response of approximately 5 kHz which is sufficiently high for studying the Kármán vortex shedding was used for the validation of this analysis method. The cross spectrum is compared with the auto-spectrum of PSP images and that of the Kulite pressure transducer that corresponds to the reference signal. The cross spectrum can drastically reduce the electronic shot noise component in the pressure fluctuation spectrum. The second peak of vortex shedding which is not observed in the result of auto-spectrum can be observed thanks to the noise reduction. The convergence of calculation is faster as the size of the spatial area for calculating cross spectrum increases. [ABSTRACT FROM AUTHOR]

Pressure-sensitive-paint (PSP) measurement was conducted for unsteady phenomena at various frequencies up to the order of kHz in low-speed flow to evaluate measurement accuracy of PSP. Pressure fluctuations on the floor surface induced by the Karman vortex were measured by PSP and unsteady pressure transducer. The dominant frequency of the pressure fluctuations is varied from 0.15 to 1.7 kHz by changing the size of the square cylinder. While regions with large pressure fluctuations could be visualized by calculating root mean square of pressure fluctuations from PSP images, the values significantly differed from those measured by pressure transducer. By applying Fast Fourier Transform (FFT), the power spectral density (PSD) at peak frequencies could be obtained within an error of 20%. Singular-value decomposition (SVD) yields a remarkable improvement in signal-to-noise ratio. However, amplitude of pressure fluctuations is changed depending on the way how to select modes. Three mode-selection methods for SVD filtering/reconstruction analysis are proposed in this study which show good improvement compared with convection method and are proved capable of extracting characteristic behaviors of the flow phenomena even below the noise floor. [ABSTRACT FROM AUTHOR]