Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session G31: NLD Model Reduction and Transition I 
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Chair: Aditya Nair, university of nevada,reno Room: 156 
Sunday, November 19, 2023 3:00PM  3:13PM 
G31.00001: Leveraging Tangent Manifolds for Efficient TimeDependent Basis Reduced Order Models Mohammad Hossein Naderi, Hessam Babaee In this study, a novel technique is proposed for timedependent basis reduced order modeling (TDBROM) of problems involving coherent structures that evolve over time. The method leverages the spatial and temporal locality of these coherent structures by utilizing a matrix differential equation to derive efficient TDBROMs. The approach begins by locally approximating the fullmodel solution in time using a reduced space based on tangent manifolds. This locally reduced tangent manifold is then updated at each time step with basis updates obtained from querying the full model at a small number of selected spatial coordinates. The TDB approach using tangent manifolds has demonstrated its effectiveness on various benchmark problems, achieving significant runtime speedups compared to full models and traditional ROMs. The use of tangent manifolds allows the reduced basis to accurately capture the local dynamical behavior of the coherent structures. Further work will explore the sensitivity of the approach to different manifold learning techniques for capturing the intrinsic local geometry. 
Sunday, November 19, 2023 3:13PM  3:26PM Author not Attending 
G31.00002: Abstract Withdrawn

Sunday, November 19, 2023 3:26PM  3:39PM 
G31.00003: Actuator selection based on singular vector method in linearized GinzburgLandau model Masahito Watanabe, Yasuo Sasaki, Keigo Yamada, Takayuki Nagata, Taku Nonomura In this research based on singular vector method we propose a numerical algorithm to place multiple actuators in a linear system so that the norm of a physical quantity is amplified efficiently. Further, we verify the validity of the proposed method by numerically analyzing the impulse response in a linearized GinzburgLandau model, which is known as a simple model of flow fields. Let us briefly introduce the proposed method. We consider a linear model, where the initial physical quantity on some selected points can be set to some values, while that on the other points are set to zero. From the physical point of view, such model corresponds to a model, where impulse input is added from actuators at some selected points. Then, we obtain the state transition matrix associated with the initial and terminal state, and a singular vector decomposition is applied to the matrix to extract the dominant modes. Here, the right singular vectors indicate the sensitivity of the terminal state to the initial state. It is shown that the norm of the terminal state may be maximized when the determinant of the matrices associated with the right singular vectors for the selected points is maximized. Therefore, in this study we select points for actuators by greedy method so that the determinant of such matrices become large as possible, where greedy method is a numerical algorithm that selects each element one by one to obtain the quasi‐optimum solution of combinational problems. 
Sunday, November 19, 2023 3:39PM  3:52PM 
G31.00004: Minimal seeds for transition to turbulence in the Stokes boundary layer Tom S Eaves The Stokes boundary layer is the oscillatory flow above a plate, with oscillations driven either by (1) transverse sinusoidal motion of the plate or (2) a sinusoidal applied pressure gradient. Beyond a critical Reynolds number of 2511, the laminar solution of the Stokes boundary layer is susceptible to linear instability (the eigenvalues of the linear problems in cases 1 and 2 are identical, Blennerhassett & Basson, 2002). However, this instability is subcritical given that turbulence is observed for Reynolds numbers above approximately 700 (Ozdemir et al., 2014) despite the flow being linearly stable in this subcritical range of Reynolds numbers. In order to examine potential mechanisms which may cause transition to turbulence from the laminar flow in this subcritical range, Biau (2016) computed linear optimal perturbations in the Stokes boundary layer using linear transient nonmodal analysis, noting the importance of the Orr mechanism. However, it is wellestablished (see Kerswell, 2018, for a review) that linear transient growth results offer limited insight into the inherently finiteamplitude disturbances which are most likely to trigger transition to turbulence. Instead, the `minimal seed' for turbulence, the smallest amplitude perturbation that causes transition, provides such a nonlinear description. This talk will describe minimal seeds and their dynamics in the Stokes boundary layer, how they differ between cases (1) and (2), and their dependence on Reynolds number. 
Sunday, November 19, 2023 3:52PM  4:05PM 
G31.00005: PODbased analysis of the onset of turbulence in pipe flow Alex Yakhot, Basheer A Khan The Openpipeflow NavierStokes solver (openpipeflow.org) was used to perform direct numerical simulations of a laminarturbulent transitional flow in a pipe for Re=1,920. The onset of turbulence is manifested by a localized turbulent zone called "puff". Using the proper orthogonal decomposition (POD), we modify the various components of the velocity fluctuations to disrupt the selfsustaining mechanism of turbulence. Velocity components have been expanded into POD modes and several modes have been removed. The simulation resumes with the field reconstructed from the truncated POD modes. We have found that the turbulence was maintained even by the first two POD azimuth modes, which accounted for 17% of the overall azimuth energy. The complete elimination of the radial component did not prevent the onset of turbulence. We found that the removal of all longitudinal fluctuations, that is, the elimination of inflection points in the longitudinal velocity profile, but retaining only the first two azimuthal POD modes, also did not prevent the onset of turbulence. We believe that PODbased manipulation of velocity components can be valuable for understanding and managing laminarturbulent transitional flow. 
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