Pressure sensitive paint measurements on a hemispherical turret

Discussion on measurement mechanism of pressure-sensitive paints

Wind tunnel experiments were conducted to measure the unsteady surface pressure field of a hemisphere-on-cylinder turret in subsonic flow. These measurements were obtained using pressure transducers coupled with fast response pressure sensitive paint. The surface pressure field data resulting from Mach 0.5 flow (ReD ≈ 2 × 10⁶ ) over three different turret protrusion distances were analyzed. Previously, dominant surface pressure modes on the turret were found using proper orthogonal decomposition. The results of which showed that greater turret protrusion into the freestream flow increased the prevalence of spanwise anti-symmetric surface pressure field fluctuations. These anti-symmetric pressure fluctuations are caused by anti-symmetrical vortex shedding. However, when a partially submerged hemispherical turret geometry is used, it was shown that this anti-symmetric mode was of much lower relative energy. This suggests that there is a transition in flow field phenomena as protrusion is changed from partially submerged to a full hemisphere configuration. Further investigation into this so-called “mode switching” is the emphasis of the work presented here. This research heavily relied on modal analysis to identify correlations between turret and wake surface pressure fields. The fluctuations in the surface pressure field around the partial hemisphere were found to be mostly dominated by the wake with little influence from fluidic structures on the turret itself. For the hemisphere and hemisphere-on-cylinder configurations, both symmetric and anti-symmetric unsteady separation grew to be the largest influence and was coupled with the wake fluctuations.

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.

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.

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.

The Ohio State 6” × 22” Unsteady Transonic Wind Tunnel was used to study different methods of experimentally capturing the time-varying location of a shock wave on a NACA 0012 airfoil. The unsteady wind tunnel enables a freestream Mach number oscillation of 0.51 ± 0.1 for a Reynolds number range of 17 – 43 million per meter. Fast-response pressuresensitive paint (PSP) and particle image velocimetry (PIV) were evaluated for their ability to capture time-accurate location of the oscillating shock wave. Unsteady surface pressure and temperature measurements were made using a fast-response bi-luminophore pressuresensitive paint with a single-shot intensity-based technique. This method allows for high spatial resolution surface pressure and temperature maps. Phase-locked planar particle image velocimetry data were also acquired in this experiment, demonstrating an ability to resolve a moving shock. These advanced diagnostic tools enable detailed understanding of the impact of time-varying compressibility both on and off the airfoil surface. The information obtained with each of these techniques is compared in terms of spatial resolution and accuracy with data acquired via surface pressure taps. A NACA 0012 airfoil was tested at 9, 10 and 11 degrees angle of attack and at low reduced frequencies.

The Andrew Fejer Unsteady Wind Tunnel was modified to add a suction duct on top of the test section to generate a vertical velocity component (cross flow). The suction produced on the top was distributed by the individually controlled louvers at the inlet of the suction duct so that the velocity components in the test section vary both temporally and spatially. Steady and traveling-wave transverse flows were generated in the test section. We present theoretical models for the unsteady flow field generated by this configuration and validate these models with experimental measurements. The results indicated that the transverse gusts generated in the test section were essentially irrotational, and a potential flow model makes relatively accurate predictions of the flow field. The suction-driven approach greatly reduced the turbulence level in the flow field relative to jet-driven cross flow approaches, and demonstrated high levels of repeatability. Traveling waves in 1−cos form were generated and showed evident influence on the flow angle of attack. This paper presents a general method to extract a surface pressure field from a skin friction field in complex flows as an inverse problem, focusing on its application to global luminescent oil-film (GLOF) skin friction measurements. The main technical aspects of this method are described, including the basic equation relating surface pressure to skin friction, variational method, numerical algorithm, and approximate method with the constant boundary enstrophy flux (BEF). The proposed method is evaluated through simulations in the Falkner–Skan flow and the flow over a 70°-delta wing to investigate the effects of the Lagrange multiplier, downsampling rate, noise level, and the value of a constant source term in the approximate method. Further, the approximate method is applied to the skin friction fields obtained by GLOF measurements in the flow over a 65°-delta wing and the square junction flow to obtain the normalized surface pressure fields. The proposed method provides a useful tool to obtain the high-resolution fields of both surface pressure and skin friction by GLOF measurements in complex flows (particularly at low speeds)

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.

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 focus of this paper is on investigating the noise produced by an airfoil at high angles of attack over a range of Reynolds number Re≈2×10⁵–4×10⁵. The objective is not modeling this source of noise but rather understanding the mechanisms of generation for surface pressure fluctuations, due to a separated boundary layer, that are then scattered by the trailing edge. To this aim, we use simultaneous noise and surface pressure measurement in addition to velocimetric measurements by means of hot wire anemometry and time-resolved particle image velocimetry. Three possible mechanisms for the so-called “separation-stall noise” have been identified in addition to a clear link between far-field noise, surface pressure, and velocity fields in the noise generation.

Pressure sensitive paint (PSP) is a novel measurement system developed to obtain surface pressure distribution on wind-tunnel testing models. The objective of the present work is to examine the feasibility of this novel surface pressure mapping technique for truly unsteady supersonic flow testing. Using a fast-responding porous PSP and a fast-framing complementary metal oxide semiconductor (CMOS) camera, we have measured unsteady flows induced in a two-dimensional Laval nozzle of a blow-down supersonic wind tunnel. The PSP used in this study was composed of a dye (bathophenanthroline ruthenium(II) complex, Ru(Ph 2-phen) and a silica-gel thin layer chromatography (TLC) plate, on which the dyes were absorbed. The PSP was illuminated by light-emitting-diode (LED) lamps. The results obtained by the present measurement system shows the starting process of supersonic wind tunnel at 5kHz frame rate.

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.

Wind flow over the low-rise building models with gabled roofs having different pitch angles

To clarify the unsteady pressure distributions on the rotor blades of an axial fan, a pressure-sensitive paint (PSP) technique was used. To capture the image of the rotating fan as a static image, an optical derotator method with a dove prism was adopted. It was confirmed by preliminary experiments with a resonator and a speaker that the pressure fluctuations with 347 Hz can be measured by the present PSP. The measured mean pressure distributions were compared with the predicted results based on large-eddy simulations. The measured instantaneous surface pressure is instrumental to identify acoustic source of fan noise in the design stage.