Abstract
In order to study the energy loss of bi-directional hydraulic machinery under cavitation conditions, this paper uses high-speed photography combined with six-axis force and torque sensors to collect cavitating flow images and lift signals of S-shaped hydrofoils simultaneously in a cavitation tunnel. The experimental results show that the stall angle of attack of the S-shaped hydrofoil is at ±12° and that the lift characteristics are almost symmetrical about +1°. Choosingα= +6° andα= −4° with almost equal average lift for comparison, it was found that both cavitation inception and cloud cavitation inception were earlier atα= −4° than atα= +6°, and that the cavitation length atα= −4° grew significantly faster than atα= +6°. Whenα= +6°, the cavity around the S-shaped hydrofoil undergoes a typical cavitation stage as the cavitation number decreases: from incipient cavitation to sheet cavitation to cloud cavitation. However, whenα= −4°, as the cavitation number decreases, the cavitation phase goes through a developmental process from incipient cavitation to sheet cavitation to cloud cavitation to sheet cavitation to cloud cavitation, mainly because the shape of the S-shaped hydrofoil at the negative angle of attack affects the flow of the cavity tails, which is not sufficient to form re-entrant jets that cuts off the sheet cavitation. The formation mechanism of cloud cavitation at the two different angles of attack (α = +6°、−4°) is the same, both being due to the movement of the re-entrant jet leading to the unstable shedding of sheet cavity. The fast Fourier analysis reveals that the fluctuations of the lift signals under cloud cavitation are significantly higher than those under non-cavitation, and the main frequencies of the lift signals under cloud cavitation were all twice the frequency of the cloud cavitation shedding.
Highlights
The instability and shedding of sheet cavitation will cause noise and performance decline of hydraulic machinery, and severe shedding will cause vibration and material damage (Brennen, 1995; Franc and Michel, 2005; Luo et al, 2016)
The lift and drag characteristics of S-shaped hydrofoil at different attack angles are recorded by cavitation tunnel, and the cavitation images at different cavitation numbers are collected by high speed photography
(2) The cavitation map of S-shaped hydrofoil shows that regardless of positive or negative angle of attack, cloud cavitation always occurs when the maximum cavity length develops to about 0.25 l, The development of cavitation at the positive angle of attack is similar to that of the conventional hydrofoil, but there is an unconventional phenomenon at negative angle of attack α∈(−6°, 0°): During the process of cavitation number decreases, the cavitation will change from cloud cavitation to sheet cavitation
Summary
The instability and shedding of sheet cavitation will cause noise and performance decline of hydraulic machinery, and severe shedding will cause vibration and material damage (Brennen, 1995; Franc and Michel, 2005; Luo et al, 2016). Xiao et al (Xiao and Tan, 2020) adopted the design method of controllable velocity method to suppress the pressure pulsation in the impeller under different IGVFs, provides a new view for the design of hydraulic machinery under the condition of two-phase flow. These studies provide a reasonable theoretical and experimental basis for understanding the complex dynamic flow in cavitation flow. At the same time, according to the lift-drag signal data, the correlation between lift-drag and cloud cavitation was analyzed, and the main frequency that caused the change of lift-drag around the S-shaped hydrofoil was revealed
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