Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session R20: Flow Instability: Control and Vortex Dominated |
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Chair: Aarthi Sekaran, SUNY Polytechnic Institute Room: 146C |
Monday, November 20, 2023 1:50PM - 2:03PM |
R20.00001: Wavelet based reduced-order-modelling of intermittent combustor flows Saarthak Gupta, Anagha Madhusudanan, Santosh Hemchandra We study the reduced-order-modelling of intermittent flow events encountered in real engineering applications. Characterizing such intermittent events using Fourier-based methods, such as, for instance, spectral-proper orthogonal decomposition (SPOD), would require a large set of modes. Representing such flow features using a smaller subset of modes would require a basis that can optimally capture events localised in time, such as a wavelet basis. We therefore use wavelet-based decompositions to model the flow. Two separate methods are presented: (1) POD on wavelet-filtered data and (2) POD on the coefficients obtained from a wavelet-transform. We apply these decompositions on a turbulent swirl nozzle combustor (Re~20,000) flow field which shows intermittent precessing vortex core (PVC) oscillations. The time-resolved data for the analysis is obtained from large eddy simulation (LES). The low-rank models obtained from this analysis can potentially explain the mechanisms that cause such intermittent behaviour in the flow. This could in turn impact future efforts towards building control strategies for such flows. |
Monday, November 20, 2023 2:03PM - 2:16PM |
R20.00002: Identifying wavemakers of self-excited flow oscillations using information theory and complex network analysis Santosh Hemchandra, Vivek Thazhathattil, Saarthak Gupta Flow regions sustaining large scale self-excited coherent oscillations are called wavemakers. Identifying these regions can aid informed engineering design decisions aimed at suppressing these oscillations when they are not beneficial. Structural sensitivity maps determined from linear stability analysis (LSA) is a physics-based approach for identifying the wavemaker. Geometrically complex flows in engineering applications whose time-averaged flows may not have symmetries, can yield large and computationally challenging LSA eigenvalue problems. We present an alternative data-driven approach using complex network analysis (CNA) to determine the wavemaker of flows with large scale oscillations, using only time-series data determined from large eddy simulation (LES) or experimental measurements. A network representation of the dataset is constructed using measurement points in space as nodes. Two ways of determining node connectivity: using correlation and mutual information between velocity fluctuations at the nodes are evaluated. Nodes with high weighted closeness centrality in the network are mapped back into physical space to identify the wavemaker. Using LES data from a turbulent swirl combustor (Re~20,000) with a coherent precessing vortex core (PVC) oscillation, the CNA predicts the wavemaker region in good agreement with structural sensitivity analysis. Disrupting the flow field in the wavemaker region suppresses the PVC oscillations, thereby, validating the CNA approach. |
Monday, November 20, 2023 2:16PM - 2:29PM |
R20.00003: Scalar mixing efficiency in pulsatile channel flow Yijie Li, Jacob Page, Colm-cille P Caulfield Pulsatile channel flow exhibits destabilising effects at certain frequencies^{ }with potentially significant non-modal transient growths. We explore the mixing efficiency of oscillatory channel flows at Reynolds number Re=7500 as they act on various distributions of passive scalars. The flows are controlled by a pulsating pressure gradient (defined with mass flow rate Q(t) = Q(1 + Q̃sin(Ωt)) and Womersley number Wo = h(Ω/ν)^(1/2)). Distinctive subharmonic eigenvalue orbits<span style="font-size:10.8333px"> (that form due to the non-normality of the Orr-Sommerfeld operator) can be tracked and identified. The eigenfunctions associated with the unstable eigenvalues can then be used as initial perturbations of Direct Numerical Simulations. We report that although instabilities and non-normal growths can be observed for a range of Womersley numbers, the most significant mixing events still occur within the ‘ballistic’ regime with relatively low pulsation frequency and large amplitudes. For larger Womersley numbers, although non-normal growths are also observed, the mixing efficiency is much lower due to the high pulsation frequency reducing the thickness of the oscillatory boundary layers. Therefore, we conjecture that it is more appropriate to employ the variational 'direct-adjoint-looping' method^{ }to locate the optimal pulsation scheme via the minimization of scalar variance and/or mix-norms instead of maximizing perturbation energy growth. |
Monday, November 20, 2023 2:29PM - 2:42PM |
R20.00004: Exploiting instability mechanisms for the efficient data-driven forecasting of extreme events in airfoil flow Benedikt Barthel, Themistoklis Sapsis For certain Reynolds numbers, airfoil flow is susceptible to sporadic high amplitude fluctuations in the aerodynamic forces. These extreme excursions may be seen as prototypical of the kind of unsteady and intermittent dynamics relevant to the flow around airfoils and wings in a variety of real-world applications. Through a wavelet and spectral analysis of the surface pressure and vorticity fields we find that these extreme events arise due to the instability of a second slower frequency component distinct from the vortex shedding mode. During these events, this extreme event frequency draws energy from the energetically dominant vortex shedding flow and undergoes an abrupt transfer of energy from small to large scales. We exploit this phenomenon for the data-driven forecasting of extreme events from sparse measurements of the surface pressure through a preprocessing algorithm which extracts this extreme event frequency content from the measured data. Using our preprocessing algorithm, we are able to accurately forecast extreme events using a simple feed-forward network architecture – a significant reduction in computational complexity as compared to the recursive architectures more commonly used for such tasks. |
Monday, November 20, 2023 2:42PM - 2:55PM |
R20.00005: Computational analysis of the formation of Görtler vortices in the flow past a three-element airfoil Hussein Kokash, G. Gilou Agbaglah A high-order spectral element method is used to study the flow characteristics of a 30P30N three-element high lift wing at low Reynolds numbers. The primary focus is on investigating the flow structures within the slat cove and the formation of Görtler vortices on the upper surface of the main airfoil. By evaluating the spanwise velocity component within the slat cove, a distinct sinusoidal pattern with a fixed wavelength corresponding to that of the Görtler vortices is observed. The amplitude of the spanwise velocity exhibits an exponential growth before reaching a saturation state. Proper Orthogonal Decomposition (POD) analysis of the flow structures in the slat cove shows that the flow is initially characterized by shear layers, which subsequently evolve into spanwise perturbations. Especially, Görtler vortices are shown to arise from the separation flow of the trailing edge of the slat. |
Monday, November 20, 2023 2:55PM - 3:08PM |
R20.00006: Visualizing flow instabilities and control mechanisms in 3D cylindrical cavities with a top bounding wall Aarthi Sekaran Cavity flows studied over the past few decades have led to an increased understanding of flow physics and instability modes. Most studies focus on two-dimensional cavities with a limited number on three-dimensional (cuboidal) and a few on cylindrical cavities, using either numerical or experimental techniques. There is, however, a significant variation in the flow with the presence of a top bounding wall, a configuration that occurs most commonly in turbomachinery hole-pattern seals. Recent (detached eddy) simulations by the author revealed 2D and 3D instabilities in such settings akin to shear-layer and wake-mode instabilities in traditional subsonic open cavity flows. The instabilities show modified dynamics accounting for the top bounding wall but similar Rossiter modes as open cavity flows. Mode-switching from the shear-layer mode to the wake mode is also documented and leads to large-scale variations in the drag characteristics of the overall flow. The present study complements the numerical work via smoke flow visualization enabling matched comparisons to the simulations. The experimental setup is updated with two (individual) passive flow control mechanisms designed to prevent instability mode switching. The effect of the flow control devices is qualitatively analyzed via the smoke flow visualizations by altering the operating conditions and the resulting dynamics are used to set up future numerical simulations for parametric studies. |
Monday, November 20, 2023 3:08PM - 3:21PM |
R20.00007: Abstract Withdrawn |
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