Junichi Akatsuka

Flow Visualization by a Simplified BOS Technique

This paper proposes a simplified background oriented schlieren (S-BOS) technique for flow visualization. Our new method does not require a cross-correlation algorithm, which results in a more straightforward and automated technique. The difficulty in optimizing parameters and removing incorrect vectors, which is typical in the conventional BOS technique, is eliminated. It allows us to output images easily in real time. In the proposed BOS method, the image displacement associated with density gradient is algebraically calculated from the image intensities using a periodic background pattern. Because the proposed optical setup is simpler than the conventional schlieren technique, it could be used in various situations including field tests. A wind tunnel test was conducted using the proposed and conventional methods in a 1 m × 1 m supersonic wind tunnel and the images displayed the value of the new, simplified technique. Nomenclature A = resolution [mm] c = circle of confusion [mm] d = image dimension [mm] f = focal length [mm] f# = aperture value G = sensor size of 1 pixel [m] I = intensity K = Gladstone-Dale constant M = Mach number m = magnification factor n = refractive index P0 = tunnel stagnation pressure [Pa] uf044X = displacement in the image along the x-direction [pixel] uf044Y = displacement in the image along the y-direction [pixel] Zbo = distance between background and object [mm] Zoc = distance between object and camera lens [mm] Zbc = distance between background and camera lens [mm] Zs = depth of view [mm] uf061 = angle of view [rad] uf065 = deflection angle [rad] uf06c = wavelength of light [m] uf071uf020i = incident angle [rad] uf071uf020r = refractive angle [rad]

Noise Reduction Concept Using Variable Exhaust Nozzle for Supersonic Aircraft

This paper proposes a new variable nozzle for reducing jet mixing noise in supersonic aircraft. The variation of the nozzle is used to achieve a geometry that enhances mixing in the mixing layer without the use of additional devices. Validation tests were conducted using scale models and the thrust loss was evaluated by numerical analysis. It was found that the proposed concept produced an acoustic benefit of 1–2 dB in the overall sound pressure level and a 2.5%–3% thrust loss at takeoff. No cruise thrust penalty is expected considering the clean convergent-divergent configuration of the nozzle under cruise conditions. It was also observed that the characteristics of the nozzle were similar to those of a chevron nozzle, based on which a similar noise reduction performance is expected.

The Effect of Diffuser Geometry on the Starting Pressure Ratio of a Supersonic Wind Tunnel

To reduce the starting and stopping loads at intermittent blow-down supersonic wind tunnels, diffuser performance is important to keep the starting pressure ratio as low as possible. A parametric study on diffuser performance for the JAXA 1m x 1m supersonic wind tunnel (JSWT) was conducted by numerical simulation. It was found that it is difficult to estimate performance by using only quasi-1D non-viscous theory because of shock waveboundary layer interactions. To improve the diffuser performance, a modified diffuser configuration was designed with a viewpoint of shock train phenomena. The modified JSWT diffuser is more slender than the original and its higher performance was confirmed by numerical simulation. This design concept was also validated experimentally with a 10% scale model tunnel at a Mach number of 3.0.