Takashi Atobe

Numerical Simulation around Airfoil with Natural Transition in High Reynolds Numbers

A new approach for the analysis of subsonic flow is proposed and the capability of capturing the detail flow properties is investigated. Especially, the natural transition phenomenon is focused on. The flows around two-dimensional aerofoil of the NACA0012 under relatively high Reynolds number conditions (Re ≃ 10 6 ) are analyzed. The development of so called T-S wave and the laminar-turbulent transition are clearly captured. Locations of transition points are compared with the experiments and agreements are excellent. The detail comparison with the linear stability analysis indicates that the most unstable disturbance in the linear unstable region is captured quantitatively by taking care of grid resolution in the chord direction. The disturbance distribution inside the boundary layer in the transitional region is examined and the phase jump is also captured.

Lift Enhancement of a Pitching Airfoil in Dynamic Stall by DBD Plasma Actuators

A dielectric barrier discharge (DBD) plasma actuator was applied to control leading-edge flow separation on periodically oscillated NACA 0012 airfoil. The effectiveness of flow control by the plasma actuator was investigated through pressure measurements on the surface of the airfoil. All of the cases exhibited a higher cycle-integrated lift and an improvement in the lift cycle hysteresis. The lift enhancement by the plasma actuator was sensitive to frequency of unsteady actuator operation. The maximum peak of efficiency of lift enhancement was observed around F=0.5. The time-resolved PIV measurement was also conducted to understand the flow control mechanism by the plasma actuator. The clear vortices appeared at the leading-edge at high angle of attack, and moved along the airfoil surface toward trailing edge. These vortices bring entrainment of main flow and the lift enhancement of the oscillating airfoil can be achieved.

Numerical Simulation Around Airfoil with High Resolution in High Reynolds Numbers

A new approach for the analysis of subsonic flow is proposed and the capability of capturing the detail flow properties is validated both qualitatively and quantitatively. Especially, the natural transition phenomenon is focused on. The flows around twodimensional aerofoil of the NACA0012 under relatively high Reynolds number conditions ( Re ≃ 10 ) are analyzed. The development of so called T-S wave and the laminar-turbulent transition are clearly captured. Locations of transition points are compared with the experiments and agreements are excellent. The same peak spectrum as in the experiment is also obtained. The detail comparison with the linear stability analysis indicates that the most unstable disturbance in the linear unstable region is still slightly underestimated. The acoustic field around the airfoil is also captured and some discussions are given.

Pressure Gradient Effects on Supersonic Transition over Axisymmetric Bodies at Incidence

Boundary-layer transition on axisymmetric bodies at a nonzero angle of attack in Mach 2 supersonic flow was investigated using experimental measurements and linear stability analysis. Transition over four axisymmetric bodies (namely, the Sears–Haack body, the semi-Sears–Haack body, the straight cone, and the flared cone) with different axial pressure gradients was measured in two different facilities with different unit Reynolds numbers. The semi-Sears–Haack body and flared cone were designed specifically to achieve a broader range of axial pressure distributions. Measurements revealed a dramatic effect of body shape on transition behavior near the leeward plane of symmetry. For a body shape with an adverse pressure gradient (that is, a flared cone), the measured transition patterns show an earlier transition location along the leeward symmetry plane in comparison with the neighboring azimuthal locations. For a nearly zero pressure gradient (that is, the straight cone), such leeward-first transition is ob...

Numerical Study on Transition of a Channel Flow with Longitudinal Wall-oscillation

Laminar-turbulent transition of a channel flow with longitudinal walloscillation is numerically investigated. In a model flow, two walls are oscillated in phase and Reynolds number based on a half width of the walls and the global maximum of the mean flow of non-oscillating channel flow is fixed to 10000. Direct numerical simulation shows that, due to the wall-oscillation, in some case the transition to turbulence is accelerated in comparison with the non-oscillating channel flow, and in other case is not. It is revealed from visualization of the vorticity that the streaks existing near the walls are divided in the accelerated case. It is found that the transition is suppressed in a certain area of the parameter space. In this case, it seems that the wall oscillation prevents the growth of T-S wave in the channel flow. Furthermore, the phenomena have correlation with the stability of Stokes layer flow.

Numerical Investigation of Feedback Mechanism around Two Dimensional Airfoil

A new numerical simulation approach is applied to the flow around 2-D aerofoil with tonal noise generation. Two flow conditions are considered. The first case is the flow around the NACA0015 airfoil with the transition process on the suction side. The second case is the flow around the NACA0012 airfoil with the Reynolds number of about 8,000. The acoustic fields clearly show the radiation of sound waves from just after the trailing edge region. Numerical simulations also capture a separated region at the trailing edge for both cases. For the high Reynolds number case, the linear stability analysis shows the possibility of the existence of T-S waves and the most unstable frequency up to the separation almost coincides with the peak frequency. This indicates that the generation of T-S waves on the pressure side is strongly relevant to the determination mechanism of the selection of peak frequency of the tonal edge noise and that the self-excited feedback mechanism exists between just after the stagnation point at the leading edge and the trailing edge region. For the low Reynolds number case, very small sound wave is captured and the frequency is the same as that in the wake region. No development of unstable waves inside the boundary layer is observed by the stability analysis and it is still unclear whether the flow structure before the separation region around trailing edge has some effects for the determination mechanism of the selection of peak frequency of the tonal edge noise.

Stability Analysis of Boundary-Layer Transition Using Accurate Velocity Profiles Obtained by an Advanced LES

Laminar-turbulent transition of the boundary layer is discussed from a view point of accuracy of the computational fluid dynamics. When highly accurate velocity profiles are used for stability analysis, curve of N factor of the e N method becomes apart from the curve of conventional laminar-flow computation. Furthermore, N value does not reach sufficient large value even in the situation that laminar-turbulent transition may occur. With the excellent computation, usefulness of the accurate velocity profiles for the transition issue is sought.