・About Fluid Dynamics Group
The fluid dynamics group is conducting the wind tunnel experiments of unique conditions and discussing the fluid dynamics in such conditions. The Mars Wind Tunnel and the Magnetic Suspension and Balance System are used for this purpose. 

・Mars Wind Tunnel
Currently, Mars airplanes are being studied as a new Mars exploration method. The atmospheric environment of Mars is very different from that of the earth, and the atmospheric density is approximately 1/100 of that of the earth. As a result, the aerodynamic force obtained by the wing is reduced, and since the flight is performed at a low Reynolds number, the performance of the wing is significantly reduced. To obtain the lift required for flight under such conditions, a high-speed flight is necessary so that the Mach number during flight increases. In addition, about 95% of the Martian atmosphere is composed of carbon dioxide, which is very different from the atmospheric composition of the earth. With a common wind tunnel, it was difficult to produce a low Reynolds number and a high Mach number conditions. Therefore, our laboratory designed and developed a Martian atmospheric wind tunnel that can simulate compressible low-Reynolds number flow. The wind tunnel installed in the vacuum chamber can produce the compressible low-Reynolds-number flows, and the working gas can be replaced with any composition. Currently, we are studying the flow around a basic shape with a low Reynolds number, thin airfoil, and rotor blade using this wind tunnel.

1. Visualization and Measurement of Flow around Basic Shape
A blunt body such as a circular cylinder or a square cylinder is considered as a simplified shape or element shape of various objects such as buildings and bridges, cables, cars, etc., and the flow field has been investigated in the wide range of Reynolds and Mach numbers. However, compressible low-Reynolds-number flow is difficult to reproduce, and the current knowledge of the flow field is poor. Therefore, returning to the basic shape, we are working to clarify the flow around the basic shape in a compressible low-Reynolds-number environment that has not been elucidated so far. The figure below shows PSD and phase difference of pressure fluctuation on cylinder surface and schlieren visualization of the wake. In a common wind tunnel test, Mach number 0.5 is a high Reynolds number flow, so the Reynolds number dependence cannot be seen. By using the Mars wind tunnel, on the other hand, Reynolds number dependencies on the position of wake vortices and the phase difference in the cylinder span direction were observed.

Schlieren visualization of flow over a circular cylinder at Mach number 0.5: (a) Re = 1000; (b) Re = 2000; (c) Re = 3000; (d) Re = 4000.

PSD and phase difference of pressure fluctuation: (a) Re = 2000; (b) Re = 3000; (c) Re = 4000; (d) Re = 5000.

2. Visualization and Measurement of flow around thin Airfoil
In low-Reynolds-number flow, it is considered that wing performance is dominated by fluid phenomena such as laminar separation, turbulent transition, and reattachment. For incompressible low-Reynolds-number flow, various studies have been conducted to improve the wing performance at low-Reynolds-number flow by using a wing shape that mimics the wings of organisms such as insects and birds (biomimetics). However, it is unclear whether performance can also be improved in compressible low-Reynolds-number flows because the flight of organisms is in the low-Mach-number region. In this research, we are investigating those phenomena through visualization of flowfield and evaluation of wing performance of lift under compressible conditions, and are conducting research toward the realization of Mars aircraft. The figure below shows the surface pressure coefficient distribution of a wing with a serrated leading edge. The low-pressure region due to the vertical vortex generated in the serrated portion is observed, and it can be seen that the low-pressure region expands as the Mach number increases.

Pressure distribution on a serrated wing with an attack angle of 6 degrees: a) Re = 11,000, M = 0.46; b) Re = 13,000, M = 0.64.

3. Measurement of Pressure Distribution on Rotor Blade
It is known that in a low Reynolds number flow, a strong leading-edge vortex is formed on the rotor blade surface and the lift is increased. Therefore, it is considered that rotor aircraft are more suitable for flight than fixed-wing aircraft in the Martian atmosphere. In our team, we are measuring the pressure distribution with a high spatial resolution by applying PSP and elucidating the mechanism of lift increase due to the separation bubble at the leading edge. The left figure shows the rotor blade model illuminated with excitation light, and the right figure shows the pressure distribution obtained by PSP measurement, which shows a low-pressure region at the leading edge (bottom of the right figure). In addition, since PSP measurement in low-pressure environments has various difficulties, we are conducting researches to improve PSP measurements in low-pressure environments, such as evaluation of optimal experimental conditions and PSP measurement with variable oxygen concentrations.

Rotor model installed in the chamber of Mars wind tunnel (left) and Pressure distribution on the blade surface measured by PSP

・MSBS
MSBS is a system to levitate and support a wind tunnel model using the magnetic force generated by electromagnets placed around the flow channel. It can be used to solve the support interference problem that cannot be avoided in normal wind tunnel tests. This device can control the posture of the model with multiple degrees of freedom by changing the coil current in dynamic. In addition, the air force acting on the model can be estimated from the coil current value. In addition to static support, it is also possible to move the model as shown in the movie below. We are using two MSBS with different sizes, one of which is 0.3-m MSBS, which can be attached to the Tohoku-University basic aerodynamic research wind tunnel (T-BART). The other one is 1.0-m MSBS, which is owned by the Institute of Fluid Science, Tohoku University. This is the world’s largest MSBS and can be installed in a low-turbulence heat transfer wind tunnel.

1. Study on flow around a freestream-aligned circular cylinder (0.3-m MSBS)
　The figure below shows a PIV measurement of a statically supported cylinder using 0.3-m MSBS, which enables us to measure the flow field without support interference and enables us to acquire highly accurate data even in problem settings that are easily affected by mechanical support devices. By measuring the velocity field in synchronization with the aerodynamic force, the relationship between the wake and the aerodynamics can be discussed. In addition, we have developed a wireless pressure measurement device that can be built into the model, which enables us to measure the pressure on the surface of the model synchronously.

Statically supported cylinder using 0.3-m MSBS

2. Study on dynamic wind tunnel testing of flow around a freestream-alined circular cylinder (1.0-m MSBS)
　MSBS can be used for dynamic support as well as static support. The figure below shows the visualization of the wake in static (left side) and dynamic (right side) wind tunnel tests of a cylinder using 1.0-m MSBS. It is shown that the wake width increases when the excitation is matched to the fluctuating frequency of the wake, and it is clarified that the aerodynamic force acting on the cylinder is amplified. We can see that there is a difference in the vortex structure in the wake.

3. Improvement of model position detection method toward wind tunnel testing of a non-axisymmetric model using MSBS (1.0-m MSBS)

The MSBS operated at Tohoku University has been basically applied only to axisymmetric objects due to the characteristics of the model position detection method. However, for practical applications, such as aircraft models, it is necessary to support non-axisymmetric models. The figure below shows a mock-up of a spaceplane supported by 1.0-m MSBS. The model position detection method has been improved for wind tunnel testing of non-axisymmetrical objects and has been applied to tunnel testing of several kinds of model geometries.

Spaceplane model levitated by 1-m MSBS