Kanako Yasue

Towards EFD/CFD Integration: Development of DAHWIN - Digital/Analog-Hybrid Wind Tunnel

The development of ‘Digital/Analog-Hybrid WINd tunnel (DAHWIN),’ which is an innovative system integrating CFD (Computational Fluid Dynamics) with EFD (Experimental Fluid Dynamics), is presented. The aim of DAHWIN is to improve efficiency, accuracy, and reliability of aerodynamic characteristics evaluation in aerospace vehicle developments through mutual support between EFD and CFD. DAHWIN is constructed as a system seamlessly connecting two large facilities, JAXA 2m x 2m Transonic Wind Tunnel for EFD and JAXA Supercomputer System for CFD. The function of this system consists of optimization of test planning, an accurate correction of the wind tunnel wall and support interaction effects, quasi-simultaneous monitoring of EFD data in comparison with corresponding CFD data, the most probable aerodynamic characteristics estimation based on both EFD and CFD data, and so forth. Each function is described with some examples of applications. Key technical challenges in the system development, such as an automatic grid generator, high-speed CFD solver, and a high-speed data processing technique for image measurement data, are also addressed. Some preliminary applications of DAHWIN to practical wind tunnel tests, such as civil transport tests, showed the usefulness and reliability of DAHWIN in terms of increasing efficiency and accuracy of both EFD and CFD.

Three-Dimensional Simulation of Bow-Shock Instability Using Discontinuous Galerkin Method

Many experiments and numerical simulations for a bow shock that forms over a blunt body have been conducted. In general, the bow shock formed in a uniform flow is stable, and a steady bow shock can be easily obtained. However, instability of the bow shock was observed in front of nearly flat bodies in a difluorodichloromethane atmosphere, using a ballistic range 30 years ago [1]. Baryshnikov et al. classified the features of this bow-shock instability into three types: small deformation (Fig. 1(a)), large deformation (Figs. 1(b) and (c)), and complete disruption of shock wave (Fig. 1(d)). From experiments under various conditions, it was concluded that bow-shock instability occurs depending on not only the Mach number and atmospheric pressure, but also the roundness of the edge and the curvature of the body surface. They suggested two candidates for the main mechanism of this phenomenon. One is dynamical nonequilibrium behind the shock wave due to a low specific heat ratio γ of the difluorodichloromethane; the other is chemical nonequilibrium with a quick increase in temperature at the shock front. Since direct experimental analysis of these mechanisms is difficult, numerical analysis using a sophisticated computational fluid dynamics (CFD) technique is expected to identify the mechanism that has not yet been revealed.

Bow-shock instability induced by Helmholtz resonator-like feedback in slipstream

Bow-shock instability has been experimentally observed in a low-γ flow. To clarify its mechanism, a parametric study was conducted with three-dimensional numerical simulations for specific heat ratio γ and Mach number M. A critical boundary of the instability was found in the γ-M parametric space. The bow shock tends to be unstable with low γ and high M, and the experimental demonstration was designed based on this result. The experiments were conducted with the ballistic range of the single-stage powder gun mode using HFC-134a of γ = 1.12 at Mach 9.6. Because the deformation of the shock front was observed in a shadowgraph image, the numerical prediction was validated to some extent. The theoretical estimation of vortex formation in a curved shock wave indicates that the generated vorticity is proportional to the density ratio across the shock front and that the critical density ratio can be predicted as ∼10. A strong slipstream from the surface edge generates noticeable acoustic waves because it can be deviated by the upstream flow. The acoustic waves emitted by synchronizing the vortex formation can propagate upstream and may trigger bow-shock instability. This effect should be emphasized in terms of unstable shock formation around an edged flat body.