Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session F15: Flow Control: Coherent Structures and Reduced Order Modeling 
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Chair: Redha Wahidi, University of Texas of the Permian Basin Room: Georgia World Congress Center B302 
Monday, November 19, 2018 8:00AM  8:13AM 
F15.00001: Relation of Largescale motions with inlet blowing perturbations in turbulent wallbounded flows Venkatesh Pulletikurthi, Suranga I Dharmarathne, Fazle Hussain, Luciano Castillo Direct numerical simulations (DNS) were performed in a turbulent channel flow to demonstrate the role of upstream perturbations in controlling heat transfer downstream. Snapshot POD and 3D adaptive Gaussian filtering were used to separate energetic and spatially largescale motions. These scales carry most of the Reynolds stresses and turbulent kinetic energy. It is shown that the blowing perturbations enhance wall heat transfer at 3D to 5D (where D is jet diameter) downstream of the jets; this is a direct consequence of the proliferation of coherent vortical structures. The largescale motions which form horseshoe vortices create strong sweep events downstream of perturbations; thus, enhancing heat transfer by moving hot fluid from the wall to the outer flow. 
Monday, November 19, 2018 8:13AM  8:26AM 
F15.00002: Energy amplification and coherent structure evolution due localized forcing in flat plate boundary layer flow Igal Gluzman, Dennice F Gayme Inputoutput analysis of the linearized Navier Stokes equations is used to evaluate the effect of localized forcing for laminar and turbulent flat plate boundary layer base flows. First, a stochastic forcing is applied at different heights above the plate to determine the wallnormal location that leads to the largest steadystate variance (output energy). Next, the time evolution of streamwise coherent structures due to impulse forcing at two wallnormal locations is studied. Forcing at the location associated with maximum energy amplification is compared with forcing at the wall, where inputs are typically applied in experiments. The inclination angles of the coherent structures as well as the wallnormal positions associated with maximum energy are compared for the laminar and turbulent base flows. The results show that the wallnormal location of the forcing matters more for turbulent base flows than for laminar ones. This study may provide insights for determining actuator placement in flow control applications. 
Monday, November 19, 2018 8:26AM  8:39AM 
F15.00003: Modulation of largescale motions due to blowing and suction Suranga Dharmarathne, Venkatesh Pulletikurthi, Murat Tutkun, Luciano Castillo Our previous studies revealed that largescale energy containing structures in turbulent channel flows can be modulated by using blowing perturbations to yield improved heat and mass transfer. In the present study, we investigate the impact of suction perturbations on largescale motions in a turbulent channel flow at a moderate Reynolds number. The flow is decomposed into its constituent scales by using proper orthogonal decomposition. Anisotropic factor for each mode is computed and used to separate the flow into two groups: largescale motions and smallscale motions. The effects of both blowing and suction on Reynolds stresses and turbulent heat fluxes are demonstrated. 
Monday, November 19, 2018 8:39AM  8:52AM 
F15.00004: Targeted disruption of vortical structures Saikishan Suryanarayanan, David B. Goldstein, Robert A. Handler Coherent structures act as engines of momentum transport and contribute to high wall shear stress and drag. Polymer molecules disrupt such vortical structures and hence lead to drag reduction. While moderately useful for internal flows, the large quantities of polymer required for external flows impose serious cost and pollution constraints. The majority of previous studies have considered the polymer to be uniformly present in the flow, or involved bleeding the polymer into the entire nearwall turbulent boundary layer. On the other hand, we examine the effect of the hypothetical ability to place the polymer only within or immediately adjacent to specific drag producing vortical structures. Direct numerical simulations of a hairpin vortex in a channel flow are performed and a scalar field is introduced in its neighborhood. On entering the vortical structure, the scalar field is observed to remain within the structure for the duration of the simulation. While this is a consequence of Kelvin’s theorem, we demonstrate its applicability to the near wall viscous regime. Our simulations also show that if the scalar field is associated with a FENEP fluid concentration, the hairpin is disrupted suggesting the viability of this targeted control. 
Monday, November 19, 2018 8:52AM  9:05AM 
F15.00005: Koopman mode representations of vortex interactions and instabilities Samaneh Sadri, Hassan Arbabi, Igor Mezic, Paolo Luzzatto Fegiz Coherent vortices play an important role in many systems like turbomachinery and aircraft. The dynamics of vortices affect the performance and safety of the system and neighboring ones. While stability and control of these vortices has been the subject of a great deal of work, there are still several unsolved issues. Here we propose to address these challenges using dynamical systems theory, including recent advances on Koopman mode analysis. First, a meaningful and practical definition of stability is needed. In many cases, where the overall flow is robust, linear stability analysis yields unstable eigenvalues associated with an extremely small amount of vorticity. Second, the presence of realworld imperfections may be sufficient to drive drastically different behavior from canonical predictions. Note that decompositions based on energycontaining modes (such as POD) may also fail in this case, as lowenergy vortices can drive drastic changes in the dynamics. By contrast, a Koopman mode approach will be better suited to represent these flows. In this talk, we apply KMD to vortical flows, starting with vortex merger and proceeding to more complex examples. In all cases, KMD robustly recovers unstable eigenvalues. 
Monday, November 19, 2018 9:05AM  9:18AM 
F15.00006: On the Robustness of PODGalerkin ROMs with Symmetrizable Governing Equations Elnaz Rezaian, Mingjun Wei Evolution of ReducedOrder Models (ROMs) to successfully respond to a variety of flow conditions is crucial to their adjustment to flow control demands. This study is focused on the influence of inner product definition on the robustness of PODGalerkin models. The quality of Symmetry inner product in improving stability of compressible flow ROMs has been demonstrated in previous studies. The current work shows that, when unsteady discontinuities in supersonic flows resolved by original highfidelity simulation are not resolved in a lowerorder space over a limited number of leading POD modes through Galerkin ROMs, using the Symmetry inner product notably restricts the amount by which the ROM deviates from its ideal stable and accurate state. The L2 ROM on the other hand, easily leaps toward extremely unstable conditions in a loworder space. Meanwhile, when the originally unstable linear and nonlinear ROMs based on L2 and Symmetry inner products are stabilized by an Eigenvalue Reassignment method empowered by Particle Swarm Intelligence, the Symmetry ROMs have shown to be more robust against suboptimal control laws. 
Monday, November 19, 2018 9:18AM  9:31AM 
F15.00007: Robust reconstruction of flow fields from limited measurements Jared Callaham, Kazuki Maeda, Steven L Brunton In many applications it is important to estimate the structure of a flow field from limited and possibly corrupt measurements. Many current methods in flow estimation use least squares regression to reconstruct the flow field, which searches for the minimumenergy solution that is consistent with the measured data. However, this approach is known to be prone to overfitting and is sensitive to noise. To address these challenges we instead seek a sparse representation of the data in a library of examples rather than a minimumenergy solution. Sparse representation has been widely used in image recognition and reconstruction examples, and is wellsuited to structured data with limited measurements, corruption, and outliers. We demonstrate sparse representation for flow reconstruction using various fluid data sets, including vortex shedding past a cylinder at low Reynolds number, a mixing layer, and two geophysical flows. In particular we find considerable improvements in overall estimation accuracy and robustness compared with least squares methods such as gappy POD. Sparse representation is a promising framework for extracting useful information from complex flow fields with realistic measurements. 
Monday, November 19, 2018 9:31AM  9:44AM 
F15.00008: Abstract Withdrawn

Monday, November 19, 2018 9:44AM  9:57AM 
F15.00009: Effectiveness of Fractal Square Grids to Enhance Turbulence: Proper Orthogonal Decomposition Analysis Alexis Kathleen Omilion, Jodi C Turk, Wei Zhang Fractal square grids have been used as a passive flow control device to enhance fluid mixing in many engineering applications. While producing a desirable high turbulence intensity peak downstream, the fractal square grid also generates a pressure loss that needs to be considered in the design. It is not well understood how the multiple fractal scales affect turbulence enhancement and the pressure loss. This work is to measure the effectiveness of a fractal square grid by comparing the level of enhancement of turbulence and the pressure drop across it in wellcontrolled watertunnel experiments. A set of fractal square grids with increasing number of fractal scales (N = 1, 2, 3, and 4) and the same largest scale are employed. Turbulent velocity fields downstream of each fractal square grid are measured by planar Particle Image Velocimetry (PIV) at a Reynolds number of 3400, and the static pressure drop measured by a differential pressure transducer. Proper Orthogonal Decomposition (POD) analysis on the velocity field is used to identify the coherent structures associated with different fractal length scales. This work can potentially benefit a wide variety of applications where energy efficient mixing or convective heat transfer is a crucial process. 
Monday, November 19, 2018 9:57AM  10:10AM 
F15.00010: Influence of orifice aspect ratio on synthetic jet trajectory in crossflow Girish K. Jankee, Bharathram Ganapathisubramani The periodic ingestion and ejection of fluid through an orifice yield vortex rings forming an unsteady jet. The ability to impart momentum with a zero net mass flux makes synthetic jet actuators coveted components in flow control applications, most of which include interaction of the jet with an incoming boundary layer. Knowledge of the trajectory of the jet becomes important under such circumstances as it allows targeted use of the actuator. Trajectory of the synthetic jet relies on critical parameters such as the Strouhal number, the velocity ratio and the orifice geometry. In this investigation, a synthetic jet is issued from rectangular orifices with aspect ratio 3, 6 and 12, for different operating frequencies and blowing ratios. We will attempt to establish the scaling characteristics of the jet trajectory that accounts for aspect ratio, momentum ratio and Strouhal number of the jet. 
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