Differential luminescent imaging method for pressure field measurement

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.

Discussion on measurement mechanism of pressure-sensitive paints

While pressure sensitive paint (PSP) technique has been widely used to measure adiabatic film cooling effectiveness distributions on the surfaces of interest based on a mass transfer analog to traditional thermal-based measurements, very little can be found in literature to provide a comprehensive analysis on the uncertainty levels of the measured film cooling effectiveness distributions derived from PSP measurements. In the present study, a detailed analysis is performed to evaluate the effects of various associated uncertainties in the PSP measurements on the measured film cooling effectiveness distributions over the surfaces of interest. The experimental study is conducted in a low-speed wind tunnel under an isothermal condition. While airflow is used to represent the “hot” mainstream flow, an oxygen-free gas, i.e., carbon dioxide (CO2) gas with a density ratio of DR = 1.5 for the present study, is supplied to simulate the “coolant” stream for the PSP measurements to map the adiabatic film cooling effectiveness distribution over a flat test plate with an array of five cylindrical coolant holes at a span-wise spacing of three diameters center-to-center. A comprehensive analysis was carried out with focus on the measurement uncertainty and process uncertainty for the PSP measurements to determine the film cooling effectiveness distributions over the surface of interest. The final analysis indicates that the total uncertainty in the adiabatic film cooling effectiveness measurements by using the PSP technique depends strongly on the local behavior of the mixing process between the mainstream and coolant flows. The measurement uncertainty is estimated as high as 5% at the near field behind the coolant holes. In the far field away from the coolant holes, the total measurement uncertainty is found to be more uniform throughout the measurement domain and generally lower than those in the near field at about 3%.

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.

Surface pressure measurements were taken on a hemisphere-on-cylinder turret in a wind tunnel using pressure sensitive paint and fast response pressure transducers. Four different turret protrusion distances were tested to study the characteristics of the unsteady pressure field on the backside and wake of the turret. Proper orthogonal decomposition was used to identify the dominant spatial surface pressure modes acting on the turret in this parametric study. It was found that the further the turret protruded into the freestream flow, the more the surface pressure field became dominated by spanwise antisymmetric surface pressure distributions resulting from anti-symmetrical vortex shedding at a normalized frequency of approximately StD=0.2. For the case of the partial hemisphere, this anti-symmetrical vortex shedding was essentially absent, insinuating that at some protrusion distance, the surface pressure environment on the turret fundamentally changes. The normalized net force rms was calculated on the turret for each configuration. It was found that the greater the turret protrusion, the greater the net force acting in the spanwise direction.

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.

The objective of this study is to examine the effect of the geometrical modification of the land on the overall film cooling effectiveness on the cutback region of a turbine blade model. A room-temperature experiment was conducted, in which nitrogen serves as the cooling stream, and the mainstream flow is air. The adiabatic film cooling effectiveness was mapped employing the pressure-sensitive paint (PSP) technique. Data was acquired at five different blowing ratios (from 0.45 to 1.65) for both the baseline and the modified model. Detailed film cooling effectiveness from PSP measurements in correlation with the flow map in streamwise and spanwise planes from particle image velocimetry (PIV) measurements was performed, characterizing the effect of rounding the edges of the lands. The results show that the rounded edges enable the coolant flow to reach the top surface of the land area more readily, especially at low blowing ratios. Superior coolant coverage on the land surface observed in the PSP measurements are well correlated with the PIV measurements. At the high blowing ratio of 1.65, the round edge of the lands helps regulate the mixing between the coolant and mainstream flows, therefore the film cooling effectiveness in the slot region is also improved.

The measurement of surface pressure in wind tunnel or in flight tests gives very important data for aerodynamics analysis. In fact, the pressure measurement gives valuable information about air flow phenomena such as shock, flow separation and boundary layer transition. In this paper a real-time image processing system for pressure sensitive paint (PSP) measurement and calibration has been designed using a calibrated pressure sensor. In the implementation of the system, TCL paper is used to luminaphore inside a windowed enclosure and a factory-calibrated pressure sensor for validation of measurement. The pressure measurement of PSP is based on the images acquired from a CCD camera sent to the MyRIO processor module in which the real-time image processing has been applied. The results are compared with the calibrated sensor from which the A and B coefficients of the Estern Volmer Equation have been calculated. During the several tests done for the pressure measurement, the accuracy of the PSP measurement has been calculated in this paper. Both normal and real-time image processing analysis have been compared as well.

Unsteady Pressure-Sensitive Paint (PSP) measurement of a rocket faring model was conducted at the JAXA 2m×2m Transonic Wind Tunnel as a part of the campaign to investigate the steady and unsteady flow field and to obtain the CFD validation data. Unsteady PSP measurement results of M=0.8 and =0 and +4° were described in this paper. Anodized-aluminum PSP (AA-PSP) was coated on the model surface, and it was illuminated by a high-power blue laser diode. AA-PSP luminescence was measured by a high-speed camera, whose frame rate was 5k frame-per-sec. The high-speed camera images were reduced to the time-series Cp distribution images using in-situ method involving pressure transducer data. Several kinds of data analysis were applied to the time-series Cp images and discussed about steady pressure distribution, unsteady pressure variation, and the results of the spectral analysis of the flow field around a rocket faring configuration.

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.

In image-based measurements, quantitative image data must be mapped to three-dimensional object space. Analytical photogrammetric methods, which may be used to accomplish this task, are discussed from the viewpoint of experimental fluid dynamicists. The Direct Linear Transformation (DLT) for camera calibration, used in pressure sensitive paint, is summarized. An optimization method for camera calibration is developed that can be used to determine the camera calibration parameters, including those describing lens distortion, from a single image. Combined with the DLT method, this method allows a rapid and comprehensive in-situ camera calibration and therefore is particularly useful for quantitative flow visualization and other measurements such as model attitude and deformation in production wind tunnels. The paper also includes a brief description of typical photogrammetric applications to temperature- and pressure-sensitive paint measurements and model deformation measurements in wind tunnels.

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.

Experimental and computational studies are performed to study pressure and temperature distributions and flow patterns on impingement target surface subject to a single impinging air jet from a plenum. The experiments cover a range of jet-to- target plate distance, Z/D, from 1.5 to 12 for Reynolds number range from 5000 to 60000. The main objective is to investigate the optimal jet-to-target distance (Z/D) for stagnation point heat transfer and location of second peak of local heat transfer at small Z/D value of 1.5. Pressure and temperature sensitive paints measurements techniques are implemented to obtain the distribution of pressure and temperature on target surface. Flow visualization test has also been performed using surface oil and smoke technique to obtain the streamline distribution over the impinged surface and to qualitative study jet characteristics. The optimal (Z/D) is found to be 4.8 and second peak location for Z/D of 1.5 is at radial location (r/D) of 1.8. Comparison of average Nu with correlation from open literature, shows agreement to within experimental uncertainty for Z/D=5, while for Z/D=1.5 a 23% difference is found. Experimental results are compared to computational (CFD) prediction using Realizable κ-ε turbulence model.

A new Pressure-Sensitive Paint (PSP) measurement technique to acquire unsteady pressure fields is established. It is a FFT based frequency analysis using a large number of high-speed camera images, which measure variation of PSP luminescence. The measurement system and the data processing concept of this method are described. Then, a low-speed wind tunnel test was conducted to confirm its performance to apply it to actual flow fields. An unsteady flow field around a two-dimensional cylinder induced by Karman vortex was measured. Its unsteady pressure fields, amplitude and phase information, were visualized by this method. Its distribution had a good agreement with that measured by pressure transducers.

The Second International Vortex Flow Experiment provided a variety of experimental data for a 65° delta wing with sharp and blunt/rounded leading edges. Flow measurements including forces and moments, surface pressures, Pressure Sensitive Paint measurements, and off-surface flow variables from Particle Image Velocimetry were made available for comparisons with computational simulations. A number of test cases were chosen for simulation by seven numerical groups, and a summary of their results is presented here. The ability of computational fluid dynamics to predict such flow features as the dual primary vortex system found on the blunt leading edge configuration and a shock/vortex interaction for the sharp leading edge are assessed. While computational simulation has made great strides in recent years, there are still areas where further improvement can be made, including in turbulence modeling, transition modeling, and the ability to accurately compute unsteady flows.