As skin-friction drag in wall turbulence has a considerable environmental impact, a feedback control technique is applied to provide the drag reduction effect. Specifically, opposition control, a feedback control technique, is used, which involves applying blowing and suction from the wall to cancel out turbulent vortical structures in the region near the wall. Opposition control typically involves the use of detected velocity immediately as the control input. However, in the present study, the near-wall velocity is predicted by an orbit-instability-based forecasting method based on deterministic chaos in low-Reynolds-number flow. We performed direct numerical simulations of turbulent channel flows to investigate the drag reduction performance. The maximum drag reduction rate was found to be comparable to that of the original opposition control. Although the drag reduction rate decreased slightly with increasing prediction time in the orbit-instability-based forecasting method, a positive drag reduction rate was achieved even when the prediction time was more than 100 times the time resolution of direct numerical simulation.

The forces acting on a delta wing are predominantly influenced by local flow structures, specifically leading-edge vortices (LEVs). This study applies colour-coded particle image velocimetry (PIV) to quantify wake dynamics and lift generation for a delta wing at varying angles of attack in wind tunnel experiments. Key contributions include an application of the novel colour-PIV technique through comparison with theoretical models and previous experimental data, and an in-depth analysis of vortex dynamics. Results reveal that LEVs exhibit maximum stability and strength at an angle of attack of 30°, corresponding to peak lift generation. These findings provide insights into the aerodynamic characteristics of delta wings and demonstrate the advantages of colour-PIV in experimental fluid mechanics.

Single string cavitation (SC) and a swirling flow around it may occur in a fuel injector of diesel engines at the moment of low needle lift, if the tip of the needle valve is close to the nozzle entrance. The swirling flow in a nozzle of the injector induces a hollow-cone spray with a very large spray angle and enhances spray atomization, which reduces the emission of particulate matters. However, the characteristics of the complicated gas-liquid two-phase swirling flow in a nozzle and its effect on spray angle have not been clarified yet. In this study, a simple one-dimensional model is developed to simulate the swirling flow, based on the mass and momentum conservation equations for the annular swirling liquid film around a single SC inside a nozzle and the hollow-cone liquid film in stagnant air. In order to obtain the boundary condition at the entrance and exit of a nozzle and to verify the validity of the swirling film flow model, we conduct a high-speed visualization of a single SC in a transparent mini-sac nozzle by visible light with refractive index matching for diesel as well as a high-speed X-ray phase contrast imaging of a hollow-cone spray to measure spray angle and the thickness of the hollow-cone liquid film along the axis. Then, we carry out a number of one-dimensional simulations to investigate the effects of the axial and azimuthal velocity components of the swirling liquid film flow in the nozzle on spray cone angle. As a result, we obtain the following conclusions. (1) The proposed one-dimensional model on the swirling film flow with single SC can quantitatively predict the motion of the swirling film flow within a nozzle and the hollow-cone spray. (2) We derived a correlation on spray angle based on the axial and azimuthal velocity components in the nozzles, and verified its validity.

Single string cavitation (SC) and a swirling flow occur in a nozzle of diesel fuel injector under low needle lift conditions, when the distance Z_N between needle tip and nozzle entrance is short. The swirling flow induces a hollow-cone spray and enhances the atomization of fuel spray, by which the emission of particulate matter is reduced. Previous studies have shown that spray angle α can be predicted by the ratio of the azimuthal velocity v_θ to the axial velocity v_z in the nozzle. In order to evaluate v_θ and v_z, we have to predict the circulation Γ_en in a nozzle and the diameter D_sc of single SC. However, empirical correlations for Γ_en and D_sc have not been established yet. In this study, five transparent single-hole nozzles with different sac inlet widths and needle lifts were designed to change and measure the circulation Γ_sac at the sac upstream of the nozzle by particle image velocimetry (PIV). We estimate Γ_en using one-dimensional simulation model and measure D_sc by image analysis. As a result, we obtained the following conclusions. (1) Circulations Γ_sac in the sacs with various needle lifts and inflow velocities are proportional to the local inlet velocity v_in at the sac inlet. (2) The diameter D_sc of single string cavitation can be simply evaluated using Γ_sac in the sac. (3) The circulation Γ_en at the entrance of the nozzle can be evaluated by the correlation based on W_sac, Z_N, D and v_N. (4) By using the correlations on Γ_en and D_sc, we can evaluate v_θ, v_z, and the resulting spray angle α.