"Keywords:
Direct numerical simulation, Bluff body flows, Turbulent flows, Instability, Normal flat plate, Kelvin-Helmholtz instability, Time-series analysis, Proper orthogonal decomposition
Abstract:
The bluff-body wake dynamics play a significant role in various engineering domains such as heat exchangers, vehicle aerodynamics, flow dynamics around natural and human-made structures, and even explosion dynamics. Relative motion between an object and a fluid at a sufficient Reynolds number leads to the separation of the flow from the object, forming a large wake behind it. In the past, numerous studies have been performed for a circular cylinder wake flow, but not much attention was given for a simple flow configuration like normal flat plate. The studies performed for the normal flat plate body were primarily experimental and at a higher Reynolds number range. Very limited data available for normal flat plate configuration at lower Reynolds numbers. Considering the limited availability of high-fidelity data for a thin, normal flat plate body, fully resolved three-dimensional Direct numerical simulations (DNS) were performed at six different Reynolds numbers 400, 500, 625, 750, 950, and 1200. At Reynolds number 400, the low-drag and high-drag regimes were observed due to low-frequency modulation of the near wake. The low-drag regime was characterized by less coherent von-Karman (V-K) vortices, while highly coherent V-K vortices were shed in the high-drag regime. Distinct flow structures were observed in the two drag regimes. Data-driven techniques were implemented to obtain further knowledge about these flow structures. Additionally, statistical analysis was performed to understand the variation of fluctuation kinetic energy and Reynolds stresses. To explore the effect of Reynolds number change on statistical quantities, data at Reynolds numbers 400 and 1200 were compared.
In all bluff body flows, hydrodynamic instability plays a vital role in the evolution of wake. One such important instability, which is extensively studied for circular cylinder wake flow, is Kelvin-Helmholtz (K-H) instability. Beyond a critical Reynolds number, K-H instability renders the near wake turbulent due to shear layer breakdown and subsequent K-H vortices shedding. For a normal flat plate body, the critical Reynolds number/range for K-H instability is still undefined. Thus, the later part of the study focuses on identifying the critical Reynolds number/range for normal flat plate configuration. Intermittency, a peculiar feature of K-H instability, is studied, and its challenges in identifying critical Reynolds number is discussed. To mitigate these challenges, a Continuous Wavelet Transform (CWT) based time-series analysis algorithm was developed. Further, the relation between low frequency modulation and K-H instability appearance is also explored."