Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session X29: Flow Instability: General |
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Chair: Vassilis Theofilis, Technion - Israel Institute of Technology Room: 255 A |
Tuesday, November 26, 2024 8:00AM - 8:13AM |
X29.00001: Hydrodynamic instability of a flow through a rotating channel filled with anisotropic porous material Mrityunjoy Saha, Sukhendu Ghosh Due to its relevance in numerous industrial and geophysical applications, there is an increasing need to understand the dynamics and stability of flow through porous media. In recent times, sufficient effort has been put into gaining pure hydrodynamic instability covering a wide range of permeabilities, effective viscosity, fluid viscosity, and flow speeds for different types of porous-fluid systems. The current work investigates the linear stability of a revolving flow in a spanwise rotating channel loaded with an anisotropic porous substance. The extended Darcy-Brinkman model is considered to depict such physical systems in which the Coriolis force terms are incorporated into the momentum equations. The ratio of vertical permeability to horizontal permeability is taken into analysis to describe the anisotropic flow phenomena. Applying normal mode analysis, we get a fourth-order coupled Orr-Sommerfeld-Squire eigenvalue problem that encapsulates the linear instability of the perturbed flow, which is solved numerically by the Chebyshev collocation method. The main goal of this study is to investigate the effect of Coriolis force and anisotropy of the porous layer in the marginal stability boundaries and estimate the critical parameters that destabilize the flow. |
Tuesday, November 26, 2024 8:13AM - 8:26AM |
X29.00002: High-order meshless Solver for Compressible Fluid Flow and Global Stability Analysis Using Polyharmonic Spline Radial Basis Functions Akash Unnikrishnan, Vinod Narayanan, Surya P VANKA This study presents a high-order meshless global stability analysis of compressible flow within a Taylor Couette apparatus using Polyharmonic Spline Radial Basis Functions (PHS-RBF) appended with polynomials as interpolation functions. Analyzing the flow within the Taylor Couette apparatus is fundamental in fluid dynamics, providing critical insights into flow instabilities and transition phenomena that impact the understanding of rotating fluid systems and their practical applications. Our methodology involves a two-step computational approach. First, a meshless solver is developed for solving compressible fluid flow equations. The base flow profiles are computed from this solver. This base flow solution is then utilized as input for a developed stability code that performs the global stability analysis. The PHS-RBF method offers several advantages, including high accuracy and flexibility in handling complex geometries without a structured grid, making it particularly suited for this type of analysis. This approach also ensures that the spatial discretization of the perturbed and linearized Navier-Stokes equations is highly accurate and stable. The stability analysis results reveal the critical Reynolds and Mach numbers at which instabilities arise, offering deeper insights into the transition mechanisms in compressible flow within the Taylor Couette apparatus. This work demonstrates the effectiveness of the meshless method for high-order stability analysis and opens up new possibilities for studying flow instabilities in complex fluid systems without the limitations imposed by traditional mesh-based methods. The insights gained from this research can be directly applied to improving the design and analysis of rotating fluid systems, enhancing their efficiency and stability in various engineering applications. |
Tuesday, November 26, 2024 8:26AM - 8:39AM |
X29.00003: Effect of outflow boundary conditions in global stability analysis of bluff body wakes Guangyao Cui, Amit Sigawi, Michael Karp Global linear stability problems are solved using matrix-forming or time-stepping. While the former approach avoids the need to have a linearized direct numerical solver, it is limited due to the need to store the matrix in the memory, which might prevent its application to problems with many degrees of freedom. This study investigates the effect of outflow boundary conditions (BCs) in the matrix-forming approach, with emphasis on reduction of the computational domain size as much as possible. Incompressible bluff body wakes, including cylinders and airfoils, are examined, in particular when the global modes are spatially amplified downstream. It is shown that below a certain Reynolds number, when the global mode is stable, it is spatially amplified downstream even when the wake is virtually inexistent. The ability of various outflow BCs from the literature to adequately resolve such eigenmodes is discussed. The BCs include Dirichlet, Neumann, and Robin, where the latter is based on a Gaster-type transformation and incorporates predictions from local stability analysis of the outflow streamwise velocity profile. It is shown that appropriate BCs facilitate the convergence of the global mode, even when the computational domain size is reduced, thus appreciably improving the efficiency of the global stability analysis. Our findings pave the way towards application of the matrix-forming approach in more complex stability problems, such as Floquet analysis and compressible flows. |
Tuesday, November 26, 2024 8:39AM - 8:52AM |
X29.00004: Tri-Global stability analysis of reacting, swirling flows Parth Patki, Benjamin L Emerson, Vishal Srinivas Acharya, Timothy C Lieuwen Swirling jets are used as canonical flow fields in combustion systems with the well-established advantage of a swirl-stabilized flame. Modeling unsteady vortical hydrodynamic structures is considered a serious challenge for swirling flows in literature as the structures significantly perturb the flame. Hydrodynamic stability analysis has been a prominent, efficient, yet developing technique for low order modeling of coherent structures for the past few decades. The objective of this study is to develop a reduced order model by means of a Tri-Global hydrodynamic stability framework to identify self-excited natural hydrodynamic modes of a LES mean flow based on a commercial nozzle. Tri-Global linear stability analysis is employed by exploiting the sparsity patterns of linearized Navier Stokes equations through a high accuracy, centered finite difference scheme. Upon discretizing the linearized governing equations about a 7-point stencil, a generalized-eigenvalue problem is formulated leading to a sparse stiffness and mass coefficient matrices for a structured 20x20x20 cubical grid. The GEVP is solved using a shift-and-invert technique to detect global unstable modes with high growth rates of the mean-flow in a low-frequency search region. A helical mode decomposition is carried out to extract the azimuthally periodic helical modes commonly associated with swirling jets. Lastly, a comparison between Bi-Global and Tri-Global stability analyses is shown to understand effects of varying modal characteristics from two to three dimensions. |
Tuesday, November 26, 2024 8:52AM - 9:05AM |
X29.00005: ABSTRACT WITHDRAWN
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Tuesday, November 26, 2024 9:05AM - 9:18AM |
X29.00006: Global stability analysis of thermal plasma jets in inductively coupled plasma facilities Prathamesh Sirmalla, Alberto Padovan, Alessandro Munafo, Daniel J Bodony, Marco Panesi Understanding the hydrodynamic behavior of thermal plasma jets is important in several applications including plasma spray coating, detecting the composition of a chemical solution, and testing thermal protection systems at high temperatures. Of particular interest to us is the plasma jet issuing from an inductively coupled plasma (ICP) torch at the Plasmatron X ICP facility at the University of Illinois Urbana-Champaign. Several experimental campaigns have shown that the hydrodynamics of the jet depend heavily on the operating chamber pressure and input power and, in this work, we describe this behavior from the standpoint of global linear stability. We linearize our finite volume solver about an axisymmetric steady base flow obtained via direct numerical simulation, and we compute the modal and non-modal behavior of the resulting linear operator. Preliminary results suggest that there exist critical pressure and power combinations where the flow transitions from a linearly stable steady solution to an unsteady oscillatory one through a baroclinic torque instability. |
Tuesday, November 26, 2024 9:18AM - 9:31AM |
X29.00007: Global stability analysis of a double Taylor-Couette system using a higher-order meshless method Akash Unnikrishnan, Vinod Narayanan, Surya P VANKA Meshless methods have recently gained popularity for numerically solving partial differential equations, such as those governing fluid flows, heat transfer, and species transport. They have also been employed for the linear stability analysis of fluid flows (Physics of Fluids, 36 (6): 064103 (2024), and Journal of Computational Physics, 474, 111756 (2023)). A numerical solver is developed using a meshless framework using polyharmonic spline–radial basis functions (PHS-RBF) with appended polynomials to efficiently compute the hydrodynamic stability of fluid flows in complex geometries. The key advantage of this method is that linear stability analysis can be performed of flows within complex geometries without the need for coordinate transformations, which result in complex Navier–Stokes operators involving transformation matrices. Instead, this approach uses a Cartesian system, resulting in the same Navier–Stokes operator regardless of the complexity of the geometry. This leads to significant computational efficiency for solving the eigenvalue problem. |
Tuesday, November 26, 2024 9:31AM - 9:44AM |
X29.00008: Abstract Withdrawn
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Tuesday, November 26, 2024 9:44AM - 9:57AM |
X29.00009: Abstract Withdrawn |
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