Abstract
Fluctuating pressure is an important feature of the bottom of a stilling basin with step-down floor. To analyze the frequency domain characteristics and energy distribution of this fluctuating pressure, the Hilbert–Huang transform (HHT) method is used. First, empirical mode decomposition is performed on the pressure fluctuation signal to obtain a number of intrinsic mode functions (IMFs), and then the Hilbert transformation is performed on each IMF to obtain the Hilbert spectrum and marginal spectrum for characterizing the pressure fluctuation signal. The results show that the fluctuating pressure signal of the stilling basin with step-down floor has obvious characteristics of low frequency and large amplitude. The dominant frequencies of the head and tail of the stilling basin are very prominent, and most of the energy is concentrated below 5.0 Hz; with the increase in the relative position of the measuring point, the energy distribution in stilling basin with step-down floor changes from high-frequency component to low-frequency component. The fluctuating pressure signal of the stilling basin with step-down floor has random amplitude modulation and frequency modulation. The marginal spectrum obtained by the HHT method can obtain the local characteristics of the signal more accurately and is more suitable for processing nonlinear and nonstationary signals.
Highlights
In recent years, with the continuous increase in the number of high dam water conservancy projects, research on energy dissipation and antiscouring technology under high water head and high flow conditions has become increasingly important [1]
According to the basic theory of turbulence, the high-frequency band mainly exists in the fluid vortex with small scale, and the low-frequency band mainly exists in the fluid vortex with large scale. e energy of the fluctuating pressure signal changes from high-frequency dominant to low-frequency dominant, which means that the small-scale vortex in the stilling basin with step-down floor changes into large-scale vortex [23]. e Hilbert–Huang transform directly reveals the inherent law that the energy distribution of intrinsic mode functions (IMFs) component of empirical mode decomposition (EMD) decomposition of the pressure pulsation signal changes with the relative position of the measuring point [24]
The Hilbert–Huang transform (HHT) method was used to analyze the pulsating pressure signal, and the pulsating pressure signal on the stilling basin with step-down floor was studied from a different perspective from the traditional perspective, and the following conclusions were obtained: (a) e pulsating pressure signal of the bottom of the stilling basin with step-down floor has obvious characteristics of low frequency and large amplitude; near the jet impact point, the signal energy is mainly concentrated in the low-frequency region; that is, the energy is mainly concentrated in the large-scale fluid vortex; the dominant frequency at the head and tail of the stilling basin is very prominent, and most of the energy is concentrated below 1.0 Hz
Summary
With the continuous increase in the number of high dam water conservancy projects, research on energy dissipation and antiscouring technology under high water head and high flow conditions has become increasingly important [1]. To this end, researchers have proposed a series of new energy dissipation structures to meet the energy dissipation requirements under such extreme conditions. Stilling basin with step-down floor is a representative new energy dissipation structure [2]. E stilling basin with step-down floor has a sill at the head of the conventional stilling basin to improve the hydraulic properties of the stilling basin floor. Because of its many advantages, an increasing number of high dam projects have adopted this form of energy dissipation
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