Data-driven spatiotemporal analysis of cloud cavitation by means of spectral proper orthogonal decomposition

IF 2.3 3区 工程技术 Q2 ENGINEERING, MECHANICAL
Grigorios Hatzissawidis, Moritz Sieber, Kilian Oberleithner, Peter F. Pelz
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引用次数: 0

Abstract

The global dynamics of cloud cavitation are not always obvious; cloud cavitation may exhibit chaotic, multimodal and intermittent behaviour, where dominant flow structures are hidden to the naked eye. To address this, spectral proper orthogonal decomposition (SPOD) is applied, a method that can continuously transition between proper orthogonal decomposition (POD) and discrete Fourier transformation (DFT)/dynamic mode decomposition (DMD). This provides the opportunity to break down the complex dynamics of interacting and transient processes into interpretable modal bases. Experiments were conducted in a high-speed cavitation tunnel using a two-dimensional NACA 0015 hydrofoil at a fixed Reynolds number of \(8 \times 10^5\) and an incidence of \(12^\circ\) for varying cavitation numbers. The cavitation was recorded using a synchronised dual-camera set-up with simultaneously captured pressure signals. Shockwave-driven and re-entrant flow-driven cloud shedding is identified, as well as the transition regime in between, exhibiting more complex behaviour. The transition from shockwave-driven to re-entrant flow-driven cloud cavitation is smooth, with shockwaves becoming more dominant as the cavitation number decreases. SPOD modes allow for a frequency and amplitude variation, which successfully decomposes the data into the dominant modes, whereas classical modal decomposition methods such as POD and DMD do not provide interpretable decompositions. SPOD grants access to a transient analysis of the data via the SPOD time coefficients. We validate the SPOD results using space–time plots and power spectral density (PSD) of the pressure signals, being in good agreement with the SPOD spatial modes and time coefficients. The complex time coefficients give access to instantaneous mode frequencies and allow calculating a standard deviation of the frequency modulation of the modes. The findings provide a deep insight into the spatial and temporal behaviour of cloud cavitation and support the understanding of its physics.

基于光谱固有正交分解的数据驱动云空化时空分析
云空化的全局动力学并不总是很明显;云空化可能表现出混乱、多模态和间歇性的行为,其中主要的流动结构是肉眼看不到的。为了解决这个问题,应用了光谱固有正交分解(SPOD)方法,这种方法可以在固有正交分解(POD)和离散傅立叶变换(DFT)/动态模态分解(DMD)之间连续转换。这提供了将相互作用和瞬态过程的复杂动力学分解为可解释的模态基础的机会。实验采用二维NACA 0015水翼,在固定雷诺数\(8 \times 10^5\)和不同空化数发生率\(12^\circ\)的条件下,在高速空化隧道中进行。空化现象的记录采用同步双摄像头设置,同时捕获压力信号。确定了冲击波驱动和重入流驱动的云脱落,以及两者之间的过渡状态,表现出更复杂的行为。由激波驱动向再入流驱动的云空化过渡是平稳的,随着空化数的减少,激波的作用更加明显。SPOD模态允许频率和幅度变化,从而成功地将数据分解为主导模态,而POD和DMD等经典模态分解方法无法提供可解释的分解。SPOD允许通过SPOD时间系数访问数据的暂态分析。利用压力信号的空时图和功率谱密度(PSD)验证了spd结果,与spd的空间模态和时间系数吻合较好。复时间系数允许访问瞬时模式频率,并允许计算模式的频率调制的标准偏差。这些发现为云空化的时空行为提供了深刻的见解,并支持对其物理学的理解。
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来源期刊
Experiments in Fluids
Experiments in Fluids 工程技术-工程:机械
CiteScore
5.10
自引率
12.50%
发文量
157
审稿时长
3.8 months
期刊介绍: Experiments in Fluids examines the advancement, extension, and improvement of new techniques of flow measurement. The journal also publishes contributions that employ existing experimental techniques to gain an understanding of the underlying flow physics in the areas of turbulence, aerodynamics, hydrodynamics, convective heat transfer, combustion, turbomachinery, multi-phase flows, and chemical, biological and geological flows. In addition, readers will find papers that report on investigations combining experimental and analytical/numerical approaches.
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