Daisuke Yorita

Transition Detection on Rotating Propeller Blades by means of Temperature-Sensitive Paint

The laminar-to-turbulent boundary-layer transition on a high-speed rotating propeller was measured using Temperature-Sensitive Paint (TSP). The TSP formulation, the experimental set-up, the image acquisition, and processing procedure had been evaluated in a pretest. High resolution propeller images were obtained even at rotation speeds of 9600 rpm. Obtained temperature images clearly show the temperature steps due to the laminar-to-turbulent boundary layer transition. Based on the pretest results, a wind tunnel experiment was conducted in the low-speed wind-tunnel BLSWT of AIRBUS in Bremen at propeller rotation speeds up to 14400 rpm. The TSP results of the wind tunnel test visualized the development of laminar and turbulent regions on the propeller blade depending on propeller speed.

Unsteady PSP Measurement in Low-Speed Flow - Overview of Recent Advancement at Tohoku University

Recent activities on PSP applications to unsteady low-speed flow at Tohoku University are reported. One significant achievement is the determination of the frequency response of PSP using an acoustic resonance tube. This apparatus can produce sinusoidal pressure variations in the frequency range from 0.1 to 10 kHz. The amplitude and the phase delay characteristics of Polymer-Ceramic PSP were measured and it was found that PC-PSP had the time response of at least 3.8 kHz. For unsteady low-speed measurements, two new types of image processing techniques have been developed; one is the modified phase-lock method and the other the conditional image sampling method. The frequency-domain technique based on pixel-by-pixel FFT analysis has also been implemented. These techniques were applied to measure unsteady pressure fields around a 3D square cylinder caused by Karman vortex shedding. It has been demonstrated that pressure fluctuation images with the peak frequency of 150 Hz and the pressure amplitude of several hundred Pa can be measured using PSP.

Enhancing the signal-to-noise ratio of pressure sensitive paint data by singular value decomposition

When using pressure sensitive paint under unsteady conditions in low-speed applications, the signal-to-noise ratio is usually low and may hinder the proper evaluation of the acquired data. Here, we ...

Unsteady PSP Technique for Measuring Naturally-Disturbed Periodic Phenomena

To understand unsteady flow phenomena, it is important to measure time-resolved pressure variations with high spatial resolution. Pressure Sensitive Paint (PSP) has a capability of providing high-resolution pressure images, but the Signal-to-Noise Ratio (SNR) is not high enough to analyze unsteady flow structures in low-speed flow. The phase-lock method is often applied to PSP measurement of periodic flow phenomena, but it is difficult to apply this technique to unsteady flow containing natural disturbances in amplitude and phase. To solve this problem, we propose a new type of phase-lock method in which images of unsteady pressure field are phase-locked using a trigger signal processed with signal conditioning techniques such as band-pass filtering and signal differentiation. The effectiveness of this method was verified using simulated signal and applied to low-speed wind tunnel testing of a 3-D square cylinder model. As a result, we could obtain time-resolved unsteady pressure field on the side of the square cylinder due to periodic Karman vortex shedding. The fluctuating pressures were found to be high at the lower part of the side wall, that is in good agreement with the pressure transducers data.

Unsteady pressure-sensitive paint measurement based on the heterodyne method using low frame rate camera.

The pressure-sensitive paint technique based on the heterodyne method was proposed for the precise pressure measurement of unsteady flow fields. This measurement is realized by detecting the beat signal that results from interference between a modulating illumination light source and a pressure fluctuation. The beat signal is captured by a camera with a considerably lower frame rate than the frequency of the pressure fluctuation. By carefully adjusting the frequency of the light and the camera frame rate, the signal at the frequency of interest is detected, while the noise signals at other frequencies are eliminated. To demonstrate the proposed method, we measured the pressure fluctuations in a resonance tube at the fundamental, second, and third harmonics. The pressure fluctuation distributions were successfully obtained and were consistent with measurements from a pressure transducer. The proposed method is a useful technique for measuring unsteady phenomena.

Simulation of the trajectory of a punted rugby ball taking into account the asymmetrical pressure distribution caused by the seams

This paper describes the effect of the seams of a rugby ball on the side force and the flight trajectory of the punted kick. Measurement of the aerodynamic force on a non-spinning rugby ball reveals that the side force coefficient depends on the position of the seam as well as the angle of attack. It was found from pressure-sensitive paint measurements that the seam of the ball is the trigger for initiating low pressure when the seam is situated around 60° from the stagnation point. The flight trajectory of the fluctuating ball can be obtained by numerically integrating the six degree-of-freedom non-linear equations of motion. It was shown that a slower spinning ball fluctuates from side to side during flight because of the asymmetrical pressure distribution on the sides of the ball.Graphical Abstract

Application of Lifetime-based Pressure-Sensitive Paint Technique to Cryogenic Wind Tunnel Tests

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.

Investigation of the Instabilities of Supersonic Impinging Jets Using Unsteady Pressure Sensitive Paint

At given nozzle to plate spacings, the flow field of high speed impinging jets is known to be characterized by a resonance phenomenon. Large coherent structures that convect downstream and impinge on the surface create strong acoustic waves that interact with the inherently unstable shear layer at the nozzle exit. This feedback mechanism, driven by the coherent structures in the jet shear layer, can either be axisymmetric or helical in nature. Fast response pressure sensitive paint is applied to the impingement surface to map the unsteady pressure distribution associated with these resonant modes. Phase-conditioned results at several kHz are obtained using a flush mounted unsteady pressure transducer on the impingement plate as a reference signal. Tests are conducted at nozzle to plate spacings of x/Dj = 4 and 4.5. The resulting phase-conditioned pressure distribution reveals dramatically di↵erent flow fields at the corresponding impingement heights. The existence of a purely axisymmetric mode is identified at x/Dj = 4.5 characterized by concentric rings of higher/lower pressure that propagate radially with increasing phase. Two simultaneous modes are observed at x/Dj = 4. One being a dominant symmetric mode and the second a sub-dominant helical mode exhibiting a unique ‘yin-yang’ pressure distribution. Phaseconditioned Schlieren images are also given to visualize the flow structures associated with each mode. Results at other impingement heights not shown here are also discussed in connection with the existence of axisymmetric and helical modes.

Application of Carbon Nanotubes and Temperature Sensitive Paint for the Detection of Boundary Layer Transition under Cryogenic Conditions (Invited)

For aerodynamic profile tests on aircraft models, transition detection is of great interest. Under ambient flow conditions the infrared technique (IR) is a well-established image-based method for this purpose. In high Reynolds number tests which are conducted at cryogenic temperatures the IR technique is of only limited suitability. In contrast, the image-based Temperature-Sensitive Paint (TSP) technique is well-suited for these conditions. Boundary layer transition detection by means of TSP generally requires an artificial temperature difference between model surface and flow. For wind tunnels operated under cryogenic conditions this temperature difference can be generated by changing the liquid nitrogen injection rate of the working fluid causing a rapid change of the flow temperature. The drawback of this procedure is that du ring the change of the flow temperature neither the Reynolds nor the Mach number can be kept constant. To overcome this problem, we have recently published an alternative approach where Carbon Nanotubes (CNT) are used to electrically heat the model surface and thus generate a well-defined temperature difference to visualize laminar-turbulent transition. The combination of CNT and TSP, which we call cntTSP, delivered excellent results in different wind tunnel tests from ambient down to 150 K. At lower temperatures the previously used CNT layer failed to work, since the nanotubes were embedded into an acrylic binder material which is known to be unsuitable for cryogenic temperatures. In this paper we describe a new sensor development based on the idea that all paint components are mixed (CNT) or dissolved (TSP) in a polyurethane binder material, which has demonstrated its suitability for cryogenic testing in previous tests. After pre-testlng CNT and TSP in the laboratory, a cryogenic wind tunnel experiment was conducted in the pilot facility of the European Transonic Windtunnel (PETW) with the aim to visualize the laminar-turbulent transition on a twodimensional model for Reynolds number Re c S 10xl06• A comparison between the results of the standard temperature-step method of the working fluid and of the new approach based on the model surface heating by means of CNT will be presented and discussed.