The control effect of micro-vortex generators (VGs) on the instability of attached cavitation was investigated in a series of experiments. The micro-VGs, located at the leading edge of a NACA0015 hydrofoil, were used to alter the near-wall flow and control the attached cavitation dynamics. The effect of the nondimensional height of micro-VGs on the nondimensional cavity length was quantitatively evaluated by regression equations through response surface methodology. The micro-VGs increased the nondimensional cavity length. The counter-rotating streamwise vortices induced by micro-VGs had a rectifying effect on the near-wall flow and withstood the flow disturbance in the spanwise direction. Additionally, the micro-VGs partially suppressed Rayleigh–Taylor instability and Kelvin–Helmholtz instability arising from reverse flow underneath the cavity. Under a partial cavity oscillation (PCO) condition, the growth of sheet cavitation was highly two-dimensional in the spanwise direction, and the cloud cavity shedding had a strict periodicity with a smaller Strouhal number (St) than for the smooth hydrofoil. The shedding cloud cavity was captured in a single spanwise vortex core, which was advected toward the trailing edge of the hydrofoil. The transition from PCO to transitional cavity oscillation (TCO) occurred when the cavity length was larger than 0.8 of chord length. Under the TCO condition, the concave cavity closure line of sheet cavitation on the hydrofoil showed perfect symmetry and the St was nearly constant. As a result of our investigation, the micro-VGs have high potential to manipulate and control the attached cavitation dynamics.

In this study, we investigate the effects of micro vortex generators (VGs) installed close to the leading edge of a quasi-two-dimensional NACA0015 hydrofoil under cavitating and non-cavitating conditions. Our aim is to improve physical insight into interaction mechanisms of the boundary layer with the formation and stability of partial cavities. Under non-cavitating conditions, the proposed micro VGs effectively suppress laminar separation. However, under cavitating conditions, even very small micro VGs within the boundary layer promote the formation of counter-rotating cavitating vortices. In comparison with the smooth hydrofoil surface (without micro VGs), the cavitation onset is shifted toward the leading edge. Additionally, classical “fingering structures” and Tollmien–Schlichting waves are no longer present. Since the onset of the cavity does no longer appear at (or close to) the laminar separation line, a novel onset mechanism is observed experimentally. The mechanism consists of stable vortex cavitation, followed by vortex break-down into bubbly structures that are finally accumulated into an attached cavity region. By reduction in the height of the micro VGs, a delayed vortex break-down is found, leading to an increase in the length of the cavitating vortex pattern. This allows for enhanced control on the cavity dynamics, especially with respect to the penetration depth of the re-entrant jet. As a result of our investigation, we conclude that well suited micro VGs show a high potential to manipulate and control cavity dynamics.

This letter is devoted to the layout reconstruction via the ultra-wideband (UWB) through-wall imaging radar under one single observation and simultaneously takes account of the real-time human indication. In the proposed framework, layout reconstruction is taken as the preprocessing, where a coherent processing interval consisting of several successive received echoes in the initial stage is first employed to construct a range-Doppler (RD) spectrum. Then, in the RD spectrum, a series of selected discrete Doppler frequency signals is used to form Doppler back projection (BP) images. Finally, in the Doppler BP image stack, we design a 3-D constant false alarm rate detector to extract the building layout. Once completed, the achieved layout as auxiliary information is fused with the simultaneous human indication. Through-wall experiments show that the proposed method can effectively extract the covered layout of multiple walls under one single view and accordingly provide strong support for the concealed human sensing.

To investigate the stream-wise distribution of skin-friction drag reduction on a flat plate with bubble injection, both experiments and simulations of bubble drag reduction (BDR) have been conducted in this paper. Drag reductions at various flow speeds and air injection rates have been tested in cavitation tunnel experiments. Visualization of bubble flow pattern is implemented synchronously. The computational fluid dynamics (CFD) method, in the framework of Eulerian-Eulerian two fluid modeling, coupled with population balance model (PBM) is used to simulate the bubbly flow along the flat plate. A wide range of bubble sizes considering bubble breakup and coalescence is modeled based on experimental bubble distribution images. Drag and lift forces are fully modeled based on applicable closure models. Both predicted drag reductions and bubble distributions are in reasonable concordance with experimental results. Stream-wise distribution of BDR is revealed based on CFD-PBM numerical results. In particular, four distinct regions with different BDR characteristics are first identified and discussed in this study. Thresholds between regions are extracted and discussed. And it is highly necessary to fully understand the stream-wise distribution of BDR in order to establish a universal scaling law. Moreover, mechanism of stream-wise distribution of BDR is analysed based on the near-wall flow parameters. The local drag reduction is a direct result of near-wall max void fraction. And the near-wall velocity gradient modified by the presence of bubbles is considered as another important factor for bubble drag reduction.

In this study, a double volute centrifugal pump with relative low efficiency and high vibration is redesigned to improve the efficiency and reduce the unsteady radial forces with the aid of unsteady computational fluid dynamics (CFD) analysis. The concept of entropy generation rate is applied to evaluate the magnitude and distribution of the loss generation in pumps and it is proved to be a useful technique for loss identification and subsequent redesign process. The local Euler head distribution (LEHD) can represent the energy growth from the blade leading edge (LE) to its trailing edge (TE) on constant span stream surface in a viscous flow field, and the LEHD is proposed to evaluate the flow field on constant span stream surfaces from hub to shroud. To investigate the unsteady internal flow of the centrifugal pump, the unsteady Reynolds-Averaged Navier–Stokes equations (URANS) are solved with realizable k–ε turbulence model using the CFD code FLUENT. The impeller is redesigned with the same outlet diameter as the baseline pump. A two-step-form LEHD is recommended to suppress flow separation and secondary flow encountered in the baseline impeller in order to improve the efficiency. The splitter blades are added to improve the hydraulic performance and to reduce unsteady radial forces. The original double volute is substituted by a newly designed single volute one. The hydraulic efficiency of the centrifugal pump based on redesigned impeller with splitter blades and newly designed single volute is about 89.2%, a 3.2% higher than the baseline pump. The pressure fluctuation in the volute is significantly reduced, and the mean and maximum values of unsteady radial force are only 30% and 26.5% of the values for the baseline pump.