Flat Hydrofoil Research Articles

Investigation on the vortex formation mechanism of flat hydrofoil under stall condition considering cavitation and non-cavitation states

Stalls will affect the efficiency of hydraulic machinery, resulting in vibration and noise. When stall occurs, it is often accompanied by cavitation. To study the flow mechanism under stall conditions, the influence of cavitation on the flow field is investigated using a symmetrical flat hydrofoil (SFH) as the research object. The effect of cavitation state on hydrofoils under stall is investigated by a combination of experiments and numerical simulations. It was found that the lift coefficient of the SFH starts to drop after about 12° and enters a stall condition. The area and range of cavitation variation also increase with increasing angle of attack (AoA). A quasi-periodic character is observed in the change of the cavitation area under stall conditions. With a 16° angle of attack, cavitation leads to increased lift amplitude and magnitude, and changes the structure and evolution of the stall vortex. The generation of the oblique vortex structure may be related to the collision of the re-entrant jet with the main flow. The difference in leading-edge vortex leads to the difference in vortex structure in the flow field between the cavitation and non-cavitation states. The study of flow field shows that non-cavitating states have more influence on rear flow field, while cavitating states have more influence on hydrofoil performance itself. This study provides insight into the vortex generation mechanism of stall conditions, laying a foundation for further research into the stall field under hydraulic machinery.

Evaluation of the spatiotemporal unsteady characteristics of the tip leakage vortex based on a direct numerical simulation database

The spatiotemporal evolution of the tip clearance vortical structures behind a flat hydrofoil immersed in a turbulent boundary layer over a flat plate was investigated by means of direct numerical simulation, with focus on the unsteady behaviors of the tip-leakage vortex (TLV) and their effects on the flow blockage. It is found that the TLV evolutionary processes can be characterized by three typical phases: the formation phase, the vortex wandering phase, and the vortex splitting and breakdown phase. In the second phase, the TLV is subject to the wall-normal low-frequency wandering motion, which is excited by the induced vortex. The abrupt increase in vortex wandering intensity near the trailing edge of the hydrofoil can be attributed to the frequent occurrence of vortex splitting and breakdown events in the third phase. The time-averaged vortex intensity of the TLV increases gradually in the first and second phases. On the other hand, instantaneous vortex intensity shows an initially decreasing and then increasing trend, as a result of the breakdown of the TLV and the formation of the secondary TLV, respectively. In addition, the investigation of flow blockage caused by the TLV indicates that along the streamwise direction, the time-averaged blockage area and blockage coefficient both follow an exponential distribution. The present results provide a qualitative and quantitative characterization for the spatiotemporal evolution of the TLV, which is critical for improving the efficiency loss and mechanical vibration caused by the unsteady behaviors of the TLV.

Numerical study on cavitation over flat hydrofoils with arc obstacles

Cavitation is a complex and unsteady phenomenon. When the cavity generated at the leading edge extends to the trailing edge, a cloud/supercavitation phenomenon appears. This type of cavitation moving downstream causes the flow to become more unstable. In this paper, the influence of different-sized arc obstacles on the cavitation evolution process is studied. The simulation accuracy is verified by comparing the numerical calculation results with the experimental results. The large edgy simulation results can effectively reproduce the unsteady evolution process of cavity formation, development, and shedding on flat hydrofoils with different structures. Results show that the obstacles of different structures affect the frequency of cavitation shedding and the distribution of air content on the flat hydrofoils. The arc obstacles on the flat hydrofoil can stabilize the cavity at the leading edge and reduce the size of the shedding cavity, thereby inhibiting the evolution of cavitation.

Experimental and numerical studies on the cavitation over flat hydrofoils with and without obstacle

To control the shedding of cavitation, an obstacle is placed on the surface of a flat hydrofoil. Both experimental and numerical studies are carried out. Images of cavitation evolution are recorded by a high-speed camera. 3-D simulations are performed to investigate the cavitating flows around the hydrofoil. The results show that the re-entrant jet plays an important role during the process of cavitation shedding. A kind of U-type shedding is identified during the evolution of the cloud cavitation. The length of the cavity is apparently reduced due to the placement of the obstacle. It is interesting to find that the cavitation shedding changes from the large-scale mode to a small-scale mode, as an obstacle is placed on the hydrofoil surface. As we can observe from both experimental and numerical results, the small-scale cavitation shedding dominates the cavitating flow dynamics, we thereby conclude that the placement of an obstacle is favorable for the inhibition of cavitation shedding.

Inhibition of cloud cavitation on a flat hydrofoil through the placement of an obstacle

Unsteady cavitation is an important topic due to its potential to cause huge damage to the hydraulic machinery. To control the shedding of cloud cavitation, the cavitation over a flat hydrofoil with an obstacle is investigated experimentally and numerically. A series of experiments around the flat hydrofoil without/with obstacle are carried out to study the evolution of cavitation. Periodic re-entrant jet and large shedding of cloud cavitation are observed in the case without obstacle, while the shedding of cloud cavitation in the case with obstacle is much weaker. Numerical simulations of the 2D unsteady cavitating flows around the hydrofoil are also performed. The transient and averaged fields of numerical simulations are presented and compared with the experimental data. The results show that in cases without obstacle, the averaged cavity length becomes longer with the decrease of the cavitation number. While in cases with obstacle, there is a range of cavitation number, in which the averaged cavity length almost keeps constant. The existence of obstacle changes the strength and direction of the transient re-entrant jet as well as the pressure distribution at the tail part of the cavity, leading to the weaker shedding of the cloud cavitation.

A Parallel Algorithm of Non-Linear Fluid-Solid Coupling Problem for Hydrofoil

A parallel algorithm of the non-linear hydrofoil fluid-solid coupling problem is discussed. Based on hydrodynamics and solid mechanics, a variational formula is derived on the mapping plane with a fixed boundary between solid and fluid, and calculated using both parallel finite element method and the finite difference method. The calculation steps of an example are given, and the numerical results of flat hydrofoil are presented.

The Wall Effect for Unsteady, Choked Supercavitating Flow

A linear theory for the two-dimensional supercavitating flow past a flat hydrofoil heaving and pitching with small amplitudes in a choked tunnel is developed using the acceleration potential. It is supposed that the streamlines of the cavity always spring from the leading and trailing edges of the hydrofoil and that the hydrofoil is inclined at a small angle to the oncoming flow. Also the pressure in the cavity is assumed to be constant. Force and moment coefficients are calculated as functions of reduced frequency and the ratio of tunnel height to chord length. The pressure disturbances caused by the unsteady motion in a tunnel do not die out far upstream; these also depend on the chord-tunnel height ratio and reduced frequency.