Zhigang Zuo

Numerical investigations of hydrodynamic performance of hydrofoils with leading-edge protuberances

Leading-edge protuberances on airfoils or hydrofoils have been considered as a viable passive method for flow separation control recently. In this article, the hydrodynamic performance of a NACA 634-021 (baseline) foil and two modified foils with leading-edge protuberances was numerically investigated using the Spalart–Allmaras turbulence model. It was found that modified foils performed worse than the baseline foil at pre-stall angles, while the lift coefficients at high angles of attack of the modified foils were increased. Both the deterioration of pre-stall and the improvement of post-stall performance were enhanced with larger amplitude of protuberance. Near-wall flow visualizations showed that the leading-edge protuberances worked in pairs at high angles of attack, producing different forms of streamwise vortices. An attached flow along some valley sections was observed, leading to a higher local lift coefficient at post-stall angles. The leading-edge protuberances were considered as sharing a similar mechanism as delta wings, increasing nonlinear lift at large angles of attack. The specific stall characteristics of this leading-edge modification could provide some guidelines for the design of some special hydrofoils or airfoils.

Numerical analyses of pressure fluctuations induced by interblade vortices in a model Francis turbine

Interblade vortices can greatly influence the stable operations of Francis turbines. As visible interblade vortices are essentially cavitating flows, i.e., the ones to cause interblade vortex cavitations, an unsteady simulation with a method using the RNG k-ε turbulence model and the Zwart-Gerber-Belamri (ZGB) cavitation model is carried out to predict the pressure fluctuations induced. Modifications of the turbulence viscosity are made to improve the resolutions. The interblade vortices of two different appearances are observed from the numerical results, namely, the columnar and streamwise vortices, as is consistent with the experimental results. The pressure fluctuations of different frequencies are found to be induced by the interblade vortices on incipient and developed interblade vortex lines, respectively, on the Hill diagram of the model runner’s parameters. From the centrifugal Rayleigh instability criterion, it follows that the columnar interblade vortices are stable and the streamwise interblade vortices are unstable in the model Francis turbine.

Periodic and aperiodic flow patterns around an airfoil with leading-edge protuberances

Recently leading-edge protuberances have attracted great attention as a passive method for separation control. In this paper, the effect of multiple leading-edge protuberances on the performance of a two-dimensional airfoil is investigated through experimental measurement of aerodynamic forces, surface tuft visualization, and numerical simulation. In contrast to the sharp stall of the baseline airfoil with large hysteresis effect during AOA (angle of attack) increasing and decreasing, the stall process of the modified airfoil with leading-edge protuberances is gentle and stable. Flow visualization revealed that the flow past each protuberance is periodic and symmetric at small AOAs. Streamwise vortices are generated on the shoulders of the protuberance, leading to a larger separation around the valley sections and a longer attachment along the peak sections. When some critical AOA is exceeded, aperiodic and asymmetric flow patterns occur on the protuberances at different spanwise positions, with leading-e...

Effect of a Single Leading-Edge Protuberance on NACA 634-021 Airfoil Performance

April 10-15, 2016 Abstract Leading-edge protuberances on airfoils or wings have been considered as a viable passive control method for flow separation. In this paper, the aerodynamic performance of a modified airfoil with a single leading-edge protuberances was investigated and compared with the baseline NACA 634-021. Spalart-Allmaras turbulence model was applied for numerical simulation and smoke flow was used for experimental visualization. Compared to the sharp decline of baseline lift coefficient, the stall angle of the modified foil is advanced and the decline of lift coefficient becomes mild. The post-stall performance of the modified airfoil is also improved. The attached flows along the peak of the protuberance help improving the total performance of the airfoil. Asymmetrical flows along the spanwise direction are observed on the modified airfoil, which may be responsible for the performance decline at pre-stall attack angles. The formation mechanism and suppression method of the symmetry breaking phenomenon should be investigated more deeply in the future to guide the practical application of the passive control method. .

Numerical simulation of the two-phase flows in a hydraulic coupling by solving VOF model

The flow in a partially filled hydraulic coupling is essentially a gas-liquid two-phase flow, in which the distribution of two phases has significant influence on its characteristics. The interfaces between the air and the liquid, and the circulating flows inside the hydraulic coupling can be simulated by solving the VOF two-phase model. In this paper, PISO algorithm and RNG k–e turbulence model were employed to simulate the phase distribution and the flow field in a hydraulic coupling with 80% liquid fill. The results indicate that the flow forms a circulating movement on the torus section with decreasing speed ratio. In the pump impeller, the air phase mostly accumulates on the suction side of the blades, while liquid on the pressure side; in turbine runner, air locates in the middle of the flow passage. Flow separations appear near the blades and the enclosing boundaries of the hydraulic coupling.

Evaluation of shaft forces in a vertical canned motor through local hydraulic loss analysis

In order to improve the stability and the lifespan of rotating components in a canned motor of a centrifugal pump, a local hydraulic loss analysis has been performed based on detailed flow field analysis by Reynolds-averaged Navier–Stokes–based re-normalization group k-ε turbulence model simulation. An energy balance equation was applied to analyze the local hydraulic loss distribution. The stator flow channel in the motor was modeled as a porous medium, by utilizing the Ergun equation. Due to the complex structure and the velocity gradient in the complicated flow field, a strong circulation flow exists in the thrust bearing outer ring. Moreover, circumferentially asymmetric Taylor vortices in the vicinity of the hydrodynamic bearing and unsymmetrical Taylor vortices in the transitional region after the thrust bearing were predicted. Dean vortices in the distributary pipes are also captured. These complex vortices are accompanied with high dissipations. A strong increase in hydraulic losses are observed in the corresponding canned motor parts (x/xmax = (0.03, 0.23), (0.87, 1)). Moreover, a strong increase in the radial force and the axial force are predicted in the relative parts of canned motor (x/xmax = (0.84, 0.97), x/xmax = (0.24, 0.17), (0.84, 0.97), respectively).

Two-Step Stall Characteristic of an Airfoil with a Single Leading-Edge Protuberance

An experimental investigation has been conducted to examine the influence of a single leading-edge protuberance on the performance of a baseline NACA 634–021 airfoil. Static aerodynamic forces of t...