Force and Moment Measurements with Pressure-Sensitive Paint

This paper will present details of a Pressure Sensitive Paint (PSP) system for measuring global surface pressures on the tips of rotorcraft blades in simulated forward flight at the 14- x 22-Foot Subsonic Tunnel at the NASA Langley Research Center. The system was designed to use a pulsed laser as an excitation source and PSP data was collected using the lifetime-based approach. With the higher intensity of the laser, this allowed PSP images to be acquired during a single laser pulse, resulting in the collection of crisp images that can be used to determine blade pressure at a specific instant in time. This is extremely important in rotorcraft applications as the blades experience dramatically different flow fields depending on their position in the rotor disk. Testing of the system was performed using the U.S. Army General Rotor Model System equipped with four identical blades. Two of the blades were instrumented with pressure transducers to allow for comparison of the results obtained from the PSP. Preliminary results show that the PSP agrees both qualitatively and quantitatively with both the expected results as well as with the pressure taps. Several areas of improvement have been indentified and are currently being developed.

The capability of a lifetime-based Pressure-Sensitive Paint (PSP) method for application in cryogenic wind tunnels was investigated. The relevant experiments were conducted in the Pilot-European Transonic Windtunnel (PETW). The 2D airfoil model (CAST10-II) was painted with PtTFPP/PTMSP paint and tested in a nitrogen atmosphere at 150 K. PSP and pressure tap data were acquired at Mach 0.78 at a total pressure of 160 kPa. The oxygen concentration of the flow was set to 1000 ppm by continuously supplying dry air. As a result, the lifetime-based PSP method successfully visualized the pressure distribution on the airfoil model under cryogenic conditions. The quantitative agreement of PSP and pressure tap data was obtained and the difference in both data came out to be 0.05 in the pressure coefficient Cp. The accuracy of PSP data is comparable to results achievable at ambient conditions. Therefore, it may be concluded that the lifetime-based method was successfully applied under cryogenic conditions. In addition to the lifetime-based method, the intensity-based method was also conducted as a comparison. Furthermore the applicability of the lifetimebased method to tests with model deformation was evaluated.

This paper presents the application of pressure-sensitive paint (PSP) measurement technology to a large-scale commercial turbine-engine test stand. In this work, the test article is the engine-inlet bell mouth. A sol-gel-based PSP is applied to the inlet and illuminated using the blue (460-nm) output of eleven LED arrays. PSP data are acquired using a scientific-grade CCD camera. The application of PSP measurements in the engine-test-stand environment requires test instrumentation to be fixed within a housing located upstream of the test article. Challenges associated with performing PSP in this hostile environment are discussed, with focus on the strategies implemented to recover surface-pressure distributions on the engine-inlet bell mouth.

Image measurement using pressure-sensitive paint (PSP) is an effective tool for analyzing the unsteady pressure field on the surface of a body in a low-speed air flow, which is associated with wind noise. In this study, the surface pressure fluctuation due to the tonal trailing edge (TE) noise for a two-dimensional NACA 0012 airfoil was quantitatively detected using a porous anodized aluminum PSP (AA-PSP). The emission from the PSP upon illumination by a blue laser diode was captured using a 12-bit high-speed complementary metal-oxide-semiconductor (CMOS) camera. The intensities of the captured images were converted to pressures using a standard intensity-based method. Three image-processing methods based on the fast Fourier transform (FFT) were tested to determine their efficiency in improving the signal-to-noise ratio (SNR) of the unsteady PSP data. In addition to two fundamental FFT techniques (the full data and ensemble averaging FFTs), a technique using the coherent output power (COP), which involves the cross correlation between the PSP data and the signal measured using a pointwise sound-level meter, was tested. Preliminary tests indicated that random photon shot noise dominates the intensity fluctuations in the captured PSP emissions above 200 Hz. Pressure fluctuations associated with the TE noise, whose dominant frequency is approximately 940 Hz, were successfully measured by analyzing 40,960 sequential PSP images recorded at 10 kfps. Quantitative validation using the power spectrum indicates that the COP technique is the most effective method of identification of the pressure fluctuation directly related to TE noise. It is possible to distinguish power differences with a resolution of 10 Pa $$^2$$ (4 Pa in amplitude) when the COP was employed without use of another wind-off data. This resolution cannot be achieved by the ensemble averaging FFT because of an insufficient elimination of the background noise.

Discussion on measurement mechanism of pressure-sensitive paints

We propose a noise reduction method for unsteady pressure-sensitive paint (PSP) data based on modal expansion, the coefficients of which are determined from time-series data at optimally placed points. In this study, the proper orthogonal decomposition (POD) mode calculated from the time-series PSP data is used as a modal basis. Based on the POD modes, the points that effectively represent the features of the pressure distribution are optimally placed by the sensor optimization technique. Then, the time-dependent coefficient vector of the POD modes is determined by minimizing the difference between the time-series pressure data and the reconstructed pressure at the optimal points. Here, the coefficient vector is assumed to be a sparse vector. The advantage of the proposed method is a self-contained method, while existing methods use other data, such as pressure tap data for the reduction of the noise. As a demonstration, we applied the proposed method to the PSP data measuring the Kármán vortex street behind a square cylinder. The reconstructed pressure data agreed very well with the pressures independently measured by pressure transducers. This modal-based approach will be applicable not only to PSP data but other types of experimental data.

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.

There appears fluttering phenomena in a hard disk drive system with high-speed disks rotating inside a closed space, leading to degrade of reading and writing performance. The precise pressure distribution on the disk may improve the performance, but there has been no report because it is very hard to measure the surface pressure using conventional techniques, such as pressure taps. While pressure sensitive paint (PSP) seems to be suitable for the pressure measurement on the disk, we have to compensate its highly temperature-sensitive characteristics of PSP, because the temperature distribution on the disk is not assumed to be uniform. We employed PySO3H based PSP, which has small temperature sensitivity, and have obtained the pressure distribution on the disk rotated at various speeds (10000–20000 rpm) successfully. The result showed that the pressure is higher at the disk outside than at the center, and forms a concentric circle distribution. Moreover, we found that the pressure difference between the inner and outer region of the disk increases as a square of disk rotation speed.

In the present study, new performance evaluation parameters for pressure-sensitive paint (PSP) were proposed, and the effects of ambient pressure on the characteristics of the PSP were evaluated. The proposed parameters allow us to determine the optimal pressure range on the basis of the following quantities: (1) the Stern–Volmer coefficient, (2) the normalized pressure sensitivity to intensity changes due to flow-induced pressure fluctuations, (3) the change in the intensity of PSP emissions in response to the given change in pressure, and (4) the signal-to-noise ratio of the change in the PSP emission intensity due to flow-induced pressure fluctuations. The characteristics of several types of PSP were evaluated using the proposed parameters. It was demonstrated that the proposed parameters enable a comparison of the effects of ambient pressure on the characteristics of PSP, and the optimal pressure range for aerodynamic measurements could be successfully identified for the different PSP.

The videogrammetric model deformation (VMD) measurement technique is developed, which is integrated with fast-responding pressure sensitive paint (PSP) to measure simultaneously unsteady aeroelastic deformation and pressure field of a wing in transonic and supersonic wind tunnels. The measurement system and data processing are described. The method of generating a deformed wing surface based on VMD measurements for PSP data mapping on the surface is developed and evaluated through simulation. Simultaneous VMD and PSP measurements on an oscillating airfoil section have been conducted in a transonic wind tunnel, and VMD data are obtained from the registration targets in PSP images. Furthermore, dynamic aeroelastic deformation of a thin flexible rectangular wing is measured by using a single-camera VMD system in a low-speed wind tunnel, and the vibration modes are extracted at different angles of attack and freestream velocities.

<div class="htmlview paragraph">Engine manufacturers and product designers are currently under significant pressure to reduce fuel consumption, emissions, and noise from two-stroke engines. Improved engine breathing goes a long way toward overcoming these challenges by providing better charge delivery, complete exhaust gas scavenging, and minimal fuel short-circuiting.</div> <div class="htmlview paragraph">Reed valve motion and the resulting gas flow are highly coupled phenomena. Understanding and controlling these dynamics and their interactions is critical to effective intake breathing. A comprehensive effort to model reed valves at a level sufficient for engine design using engine simulations is still incomplete. This work is focused on the assessment, development and validation of a reed valve model suitable for engine simulations.</div> <div class="htmlview paragraph">In this work, a representative two-stroke engine reed valve is modeled in a detailed fluid/structures, multi-physics CFD code which fully resolves both the flow-field and the details of the deflecting reed plate.</div> <div class="htmlview paragraph">This same reed valve geometry is modeled in a 1D engine simulations code. In lieu of detailed experimental measurements, the CFD simulation results are used to provide the flow and lift data for various differential pressures. These are input into the 1D model as flow coefficients vs. lift.</div> <div class="htmlview paragraph">A comparison of results, between the 1D engine simulation and the original fluid/structure CFD, shows excellent agreement for the maximum lift, the lift vs. time profile, and for the overall flow predictions.</div> <div class="htmlview paragraph">In the 1D simulation code, the standard component model representation of a reed valve is as a spring-mass-damper physical model. It was found that this model, while not an exact representation of the bending reed, did provide adequate agreement.</div> <div class="htmlview paragraph">Additionally, the predicted lifts were compared with uniformly loaded deflecting beam theory assuming the up-stream and down-steam pressures are uniformly applied on either side of the reed valve. It was found that the while this assumption is an adequate first level approximation for low lift conditions and suitable for preliminary design analysis, the CFD results indicate that it does not hold for high lift conditions.</div> <div class="htmlview paragraph">In this work, a method was demonstrated which allows design engineers to investigate reed valve design options from the detailed CFD perspective and from the 1D engine simulations perspective, and thus to develop a good reed valve model for analysis prior to testing. This allows the engineer to screen potential designs with simulation before the more expensive task of building and testing prototypes. The validated reed valve model can be incorporated directly into a full engine simulation with confidence.</div>

Imaging of pressure-sensitive paint (PSP) for pressure measurement on moving surfaces is problematic due to the movement of the object within the finite exposure time of the imager, resulting in the blurring of the blade edges. The blurring problem is particularly challenging when high-sensitivity PSP with a long lifetime is used, where the long luminescence time constant of exponential light decay following a burst of excitation light energy results in blurred images. One method to ameliorate this effect is image deconvolution using a point spread function (PSF) based on an estimation of the luminescent time constant. Prior implementations of image deconvolution for PSP deblurring have relied upon a spatially invariant time constant in order to reduce computational time. However, the use of an assumed value of time constant leads to errors in the point spread function, particularly when strong pressure gradients (which cause strong spatial gradients in the decay time constant) are involved. This work introduces an iterative method of image deconvolution, where a spatially variant PSF is used. The point-by-point PSF values are found in an iterative manner, since the time constant depends on the local pressure value, which can only be found from the reduced PSP data. The scheme estimates a super-resolved spatially varying blur kernel with sub-pixel resolution without filtering the blurred image, and then restores the image using classical iterative regularization tools. A kernel-free forward model has been used to generate test images with known pressure surface maps and a varying amount of noise to evaluate the applicability of this scheme in different experimental conditions. A spinning disk setup with a grazing nitrogen jet for producing strong pressure gradients has also been used to evaluate the scheme on a real-world problem. Results including the convergence history and the effect of a regularization-iteration count are shown, along with a comparison with the previous PSP deblurring method.

Pressure-Sensitive Paint (PSP) technique has been applied to measure pressure distribution on airfoil models at low Reynolds number (Re = 4.1x10 4.3x10) in the Mars Wind Tunnel (MWT) at Tohoku University. In this study, the capability of PSP for pressure measurement was evaluated with a particular emphasis on its applications to thin airfoil models. The obtained results show that PSP can clearly visualize pressure distributions on the airfoils, allowing us to analyze a behavior of the separated shear layer on the airfoil including laminar-to-turbulent transition and reattachment. A comparison between the PSP and balance measurements indicates that the evolution of a laminar separation bubble over the airfoil surface has a strong effect on the lift and drag characteristics. It has been also verified by this experiment that, using the proposed method for temperature correction, pressure distribution can be obtained quantitatively even on thin airfoils.

Ejector side-wall pressure distribution is a key indicator of supersonic jet-mixer ‐ejector performance. When documenting pressurepatterns on an ejectorwall using pressure-sensitivepaint (PSP), onehasto considertemperaturevariationscaused by the supersonicjet e owwithin theejector becausethesecan causesignie cantlocalerrors in the PSP results. If the temperature sensitivity of PSP is not corrected for in complex internal supersonic e ows, large localized errors could contaminate the results. In the present work, temperature-sensitive paint maps the temperature distribution on the ejector wall and corrects PSP results point-by-point for temperature sensitivity. The experiments were conducted on multijet supersonic mixer ‐ejector cone gurations with straight, convergent (6-deg), and divergent (6-deg) side walls. A comparison of corrected and uncorrected PSP readings shows that at Mj = 1:55, the error with respect to true data from static pressure ports can be reduced from 4.98 to 2.84% for the case of a simple ejector with parallel walls. For the complex 6-deg convergent ejector at Mj =1:39, the error reduces by almost an order of magnitude (from 20.83 to 2.66% ). Ourresults indicatethat the use of this correction technique can signie cantly reduce PSP errors in complex internal supersonic e ow situations.

Vortex interactions and breakdown play a critical role in determining the static and dynamic stability of aircraft, particularly at high angles of attack. This study investigates the relationship between vortex interactions and aircraft stability characteristics, focusing on how chord ratio, angle of attack, and sideslip angle influence stability metrics. Utilizing force and moment measurements along with pressure-sensitive paint (PSP) for surface pressure distributions, the results show the effects of varying double-delta wing chord ratios at a target Mach number of 0.15. The stability metrics derived from force and moment measurements demonstrate directional stability improvements with increasing sideslip angle due to larger vortex-induced differential drag induced by advanced windward wing vortex breakdown. This asymmetric vortex breakdown was confirmed through PSP-measured surface pressure distributions. The study metrics revealed trend reversals in lateral stability due to the varying state of vortex systems on different models, and higher reduced frequencies, pitch rates, and chord ratios correlated with greater dynamic instability and sensitivity. This is attributed to the more rapid and extensive vortex breakdown associated with these higher parameters. These findings provide deeper insights into the flow physics governing high-alpha maneuvers, offering a foundation for future research, enhancing the design and performance of next-generation aircraft.