Keisuke Asai

Application of pressure-sensitive paints to low-pressure range

The pressure-sensitive paint (PSP) technique has potential as a powerful diagnostic tool for measurements in the high Knudsen number regime because it is based on luminescence of molecules. Three types of PSP [two composed of organic dye and polymer (luminophore/binder), platinum octaethylporphyrin (PtOEP)/silicone polymer (GP197) and platinumtetrakis (pentafluorophenyl) porphyrin (PtTFPP)/poly[1-(trimethylsilyl)-propyne] [poly(TMSP)], and the other ruthenium II tris (4,7-diphenyl-1, 10-phenanthrolin chloride (Bath-Ru) adsorbed on anodized aluminum] are applied to the rarefied gas flow mainly lower than 150 Pa (about 1 torr) to examine fundamental properties, such as pressure/temperature sensitivity, time response of luminescence, and so on

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.

Computational and Experimental Analysis of a High-Performance Airfoil Under Low-Reynolds-Number Flow Condition

A high-performance Ishii airfoil was analyzed using both a wind-tunnel and large-eddy simulations at a low-Reynolds-number condition (Re=23,000). The design guidelines for an airfoil shape with a high lift-to-drag ratio under the aforementioned condition are described by analyses of flowfields and aerodynamic characteristics of the Ishii airfoil. Compared with conventional airfoils, such as the NACA 0012 and NACA 0002, the shape characteristic effects of the Ishii airfoil on its flowfield and aerodynamic characteristics are discussed. The shape on the suction side of the Ishii airfoil can cause delays in the flow separation at low angle of attacks. The separated flow reattaches, and a separation bubble forms even when trailing-edge separation changes to leading-edge separation. The separation bubble contributes to an increase in lift coefficient. In addition, the Ishii airfoil can gain a high positive pressure on the pressure side as compared with the other two symmetric airfoils due to the camber near th...

Mechanisms of surface pressure distribution within a laminar separation bubble at different Reynolds numbers

Mechanisms behind the pressure distribution and skin friction within a laminar separation bubble (LSB) are investigated by large-eddy simulations around a 5% thickness blunt flat plate at the chord length based Reynolds number 5.0 × 103, 6.1 × 103, 1.1 × 104, and 2.0 × 104. The characteristics inside the LSB change with the Reynolds number; a steady laminar separation bubble (LSB_S) at the Reynolds number 5.0 × 103 and 6.1 × 103, and a steady-fluctuating laminar separation bubble (LSB_SF) at the Reynolds number 1.1 × 104, and 2.0 × 104. Different characteristics of pressure and skin friction distributions are observed by increasing the Reynolds number, such that a gradual monotonous pressure recovery in the LSB_S and a plateau pressure distribution followed by a rapid pressure recovery region in the LSB_SF. The reasons behind the different characteristics of pressure distributions at different Reynolds numbers are discussed by deriving the Reynolds averaged pressure gradient equation. It is confirmed that the viscous stress distributions near the surface play an important role in determining the formation of different pressure distributions. Depending on the Reynolds numbers, the viscous stress distributions near the surface are affected by the development of a separated laminar shear layer or the Reynolds shear stress. In addition, we show that the same analyses can be applied to the flows around a NACA0012 airfoil.

Development of low density wind tunnel to simulate atmospheric flight on Mars

A new type of wind tunnel that can simulate airfoil flow in the Mars atmosphere has been designed and constructed at Tohoku University. This wind tunnel is driven by ejector and designed to simulate low-density Martian atmosphere consisting of CO2. The preliminary performance tests were conducted using air to investigate the operating envelop of the tunnel. It was demonstrated that this wind tunnel could cover the Reynolds number range from 10 4 to 10 5 and the Mach number range up to 0.71, allowing us to simulate low Reynolds number and high subsonic flow on Mars airplanes. Calibration tests were also conducted to investigate the flow characteristics in the test section. It was confirmed that the flow was kept uniform in the test section even when the total pressure was 1 kPa. There were significant effects of the boundary layer development on static pressure gradient along the test section. It was found that these effects of boundary layers could be corrected by inclining both the upper and lower walls of the test section.

Wind Tunnel Testing on Start/Unstart Characteristics of Finite Supersonic Biplane Wing

This study describes the start/unstart characteristics of a finite and rectangular supersonic biplane wing. Two wing models were tested in wind tunnels with aspect ratios of 0.75 (model A) and 2.5 (model B). The models were composed of a Busemann biplane section. The tests were carried out using supersonic and transonic wind tunnels over a Mach number range of with angles of attack of 0°, 2°, and 4°. The Schlieren system was used to observe the flow characteristics around the models. The experimental results showed that these models had start/unstart characteristics that differed from those of the Busemann biplane (two dimensional) owing to three-dimensional effects. Models A and B started at lower Mach numbers than the Busemann biplane. The characteristics also varied with aspect ratio: model A () started at a lower Mach number than model B () owing to the lower aspect ratio. Model B was located in the double solution domain for the start/unstart characteristics at , and model B was in either the start or unstart state at . Once the state was determined, either state was stable.

Feasibility of skin-friction diagnostics based on surface pressure gradient field

An intrinsic relation is given between the skin-friction vector and the surface pressure gradient through the boundary enstrophy flux (BEF), and it is used to study the possibility to extract some skin-friction structures from a surface pressure field. This attempt contains two related parts. In the first part, when the BEF field is given, a projected skin-friction field in the image plane can be sought from a surface pressure image based on a variational solution of an optical-flow-like equation. This approach is validated in several classical flows. The second part deals with a practical problem in which the BEF field is not known a priori. In this case, a so-called auxiliary skin-friction field is determined from a surface pressure image alone by using the same variational approach. The auxiliary skin-friction field has the magnitude proportional to the skin-friction magnitude and the direction of the negative surface pressure gradient. The physical meaning of the auxiliary skin-friction field and its applicability to global skin-friction diagnostics are discussed.

Effects of mach number and specific heat ratio on low-reynolds-number airfoil flows

The effects of Reynolds number, Mach number, and gas species (air and CO2) on aerodynamic characteristics of a thin flat plate and a NACA 0012-34 airfoil were investigated under low-Reynolds-number (Re=0.43×104 to 4.1×104) and high-subsonic-flow (M=0.1 to 0.6) conditions. In addition to lift and drag measurements by a two-component balance system, the pressure-sensitive paint technique was applied to measure pressure profiles on the model surface. For the flat plate, the Reynolds number moderately affects the lift and drag characteristics because of a simple behavior of the leading-edge separation bubble; the length of the separation bubble increases as the angle of attack increases. By contrast, the Mach number and specific heat ratio contribute little to the aerodynamic performance. For the NACA 0012-34 airfoil, the lift curves are highly dependent on the Reynolds number because of the formation, shift, and burst of the separation bubble, whereas the compressibility affects only the stall characteristic...

Development of Ultrafast-Response Anodized-Aluminum Pressure-Sensitive Paints

In order to apply Pressure-Sensitive Paint (PSP) to unsteady shock-wave phenomena, Anodized-Aluminum PSP (AA-PSP) with ultrafast response was fabricated and its response time to a step pressure change was evaluated by using a shock tube. Phosphoric acid was used as electrolyte in anodization and the anodic alumina with pore diameter as large as 160 nm was successfully fabricated. The improved AA-PSP achieved a response with the time constant of 0.35 uf06ds. This is the fastest PSP ever reported. We applied this AA-PSP to interactions of a moving shock wave with a circular cylinder. The results show that the improved AA-PSP can visualize the shock reflections and the shock diffractions with the ever-highest spatial and temporal resolution. Nomenclature Ai = Stern-Volmer coefficients d = pore diameter, nm Deff = effective diffusion coefficient, m 2 s -1

Ultrafast-Response Anodized-Aluminum Pressure-Sensitive Paints for Unsteady Flow Measurement

To apply pressure-sensitive paint (PSP) to unsteady shock-wave phenomena such as shock–obstacle interactions in shock-tube experiments, anodized-aluminum PSP (AA-PSP) with ultrafast response was fabricated and its response time to a step pressure change was evaluated by using a shock tube. Phosphoric acid was used as electrolyte in the anodization process, and the anodic alumina with pore diameter as large as 160xa0nm was successfully fabricated. This improved AA-PSP achieved a response with the time constant of 0.35u2009u2009μs. This AA-PSP was applied to interactions of a moving shock wave with a circular cylinder. The results show that the improved AA-PSP can visualize the shock reflections and the shock diffractions with the ever-highest spatial and temporal resolution.