Study on the Vortex in a Pump Sump and Its Influence on the Pump Unit

The vortex in a pump sump is a negative problem for the pump unit, which can lead to the decline of pump performance. Focusing on the internal pressure characteristics of the floor-attached vortex (FAV) and its influence on the pump unit, the FAV was analyzed adopting the previously verified numerical simulation method and experiment. The results show that the pressure in the vortex core gradually decreases with time, drops to a negative pressure at the development stage, and then reaches the lowest pressure during the continuance stage. When the negative pressure of the vortex tube is around the vaporization pressure of the continuance stage, it can cause a local cavitation at the impeller inlet. The evolution of the FAV is accompanied by a change of pressure gradient in the vortex core which is discussed in detail. This research provides theoretical guidance for a better understanding of the vortex characteristics and the optimal design for the pump.

Numerical Analysis of Leading-Edge Vortex Effect on Tidal Current Energy Extraction Performance for Chord-Wise Deformable Oscillating Hydrofoil

To improve the energy extraction performance of the oscillating hydrofoil, the lift force that acts on the oscillating hydrofoil is analyzed. The pressure difference between the oscillating hydrofoil‘s opposing surfaces is dominant to generate the lift force. Forming and shedding of the leading-edge vortex from the hydrofoil surface determines the pressure difference between the opposing surfaces of the oscillating hydrofoil. In this paper, the hydrofoil with different chord flexibility coefficients and maximum offset at the trailing edge are analyzed to obtain the power coefficient, lift coefficient, and moment coefficient of the oscillating hydrofoil. The influence mechanism of chord-wise deformation of the oscillating hydrofoil on the energy extraction performance is explored. According to the Kutta–Joukowsky condition and the Stokes’ theorem, the relationship between the attached vortex on the hydrofoil and the surface pressure of the hydrofoil, the surface pressure difference of the hydrofoil, and the lift force that acts on the hydrofoil are investigated. By quantifying the vortex intensity, the ascending-shedding process of the attached vortex on the hydrofoil is characterized. Finally, the complete influence chain among the chord-wise flexure, the attached vortex on the hydrofoil, and the energy extraction performance of the oscillating hydrofoil is established.

Experimental investigation of pressure pulsation induced by the floor-attached vortex in an axial flow pump

The pressure pulsation test in an axial flow pump with and without the floor-attached vortex was performed. Pressure sensors were mounted on the impeller inlet section and impeller outlet section and guide vane section outlet of the axial flow pump. The investigations showed that the pressure pulsation in the axial flow pump was mainly affected by the impeller rotation. The time-domain characteristic curves of the pressure pulsation at the impeller inlet and outlet changed the most at different periods when the floor-attached vortex appeared in the pump sump. There was no significant difference between the time-domain characteristic curves of the pressure pulsation with and without the floor-attached vortex at the guide vane outlet. The pressure pulsation induced by the floor-attached vortex was a low-frequency pulsation of 2.12 Hz, which fluctuates periodically with time in the form of a trigonometric function. The pressure pulsation amplitudes with the floor-attached vortex were larger than those without the floor-attached vortex. The floor-attached vortex mainly affected the pressure pulsation in the impeller and had less influence on the pressure pulsation at the guide vane outlet due to the rectifying effect of the guide vane.

Visualization and analysis of flow structures in an open cavity

A numerical study is performed on the supersonic flow over an open cavity at Mach number of 1.5. A newly developed visualization method is employed to visualize the complicated flow structures, which provide an insight into major flow physics. Four types of shock/compressive waves which existed in experimental schlieren are observed in numerical visualization results. Furthermore, other flow structures such as multi-scale vortices are also obtained in the numerical results. And a new type of shocklet which is beneath large vortices is found. The shocklet beneath the vortex originates from leading edge, then, is strengthened by successive interactions between feedback compressive waves and its attached vortex. Finally, it collides against the trailing surface and generates a large number of feedback compressive waves and intensive pressure fluctuations. It is suggested that the shocklets beneath vortex play an important role of cavity self-sustained oscillation.

Flow Structure on a Retreating Rotor Blade at High Advance Ratios

This paper provides unambiguous velocity-field proof that a strong edge vortex occurs on a rotating blade in reverse flow. The rotor blades of a helicopter encounter reverse flow during flight at high advance ratio. Reverse flow is a limiter in rotorcraft design with excursions in pitch link loads and bending moments. Stereo particle-image velocimetry is used on the flow under the retreating blade of a two-bladed teetering rotor system in a low speed wind tunnel. The results are correlated with earlier aerodynamic loads and flow visualization data using the same rotor blade planform placed in a yawed, fixed-wing position. Results obtained with the blade held fixed at several rotor azimuths and angles of attack are used to ascertain the rotation effects on the flowfield by comparison with rotating blade results. Initial results suggest that radial velocity due to rotation hinders separation and delays the formation of an attached vortex, compared to the static case. The circulation of the reverse flow attached vortex is of the same order of magnitude as the bound circulation of the airfoil section, proving that the vortex contributes significantly to the lift force in addition to the pitching moment.

Rotational lift: something different or more of the same?

SUMMARYThis paper addresses the question, do the rotational forces in the hovering fruit fly Drosophila melanogaster reflect something different (the Magnus effect) or more of the same (circulatory-and-attached-vortex force)?The results of an unsteady blade-element model using empirically derived force coefficients from translating (root-oscillating) wings are compared with recent results derived from both the measured forces on a dynamically scaled Drosophila wing and the computational fluid dynamic (CFD)-modeled forces on a virtual Drosophila wing. The behavior of the forces in all three models during wing rotation supports the hypothesis that rotational lift is not a novel aerodynamic mechanism but is caused by the same fluid-dynamic mechanism that occurs during wing translation. A comparison of the unsteady model with a quasi-steady model that employs empirically derived rotational coefficients further supports the hypothesis that rotational forces are more of the same. Finally, the overall similarity of the results between the unsteady model, the physical wing model and the CFD model suggests that the unsteady model can be used to explore the performance consequences of kinematic variation and to investigate locomotor control in freely moving animals.