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 M15: Flow Control: General |
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Chair: Avraham Seifert, Tel Aviv University Room: Georgia World Congress Center B302 |
Tuesday, November 20, 2018 8:00AM - 8:13AM |
M15.00001: Feedback flow control for transient energy growth minimization using convex-optimization techniques Aniketh Kalur, Maziar Sam Hemati The sub-critical transition of channel flow from a laminar to turbulent regime results from large transient energy growth (TEG) of disturbances. TEG can be minimized using previously formulated convex-optimization-based feedback control laws, requiring the solution to a standard linear matrix inequality (LMI) problem; however, methods for solving the associated LMI problem suffer from the curse of dimensionality, making controller synthesis intractable for high-dimensional fluid flows. In this talk, we develop reduced-order models to facilitate LMI-based controller synthesis in the context of a linearized channel flow with wall-normal blowing and suction actuators. The talk will focus on lessons learned, addressing fundamental modeling and design challenges. Performance and robustness trade-offs are examined and compared with other common optimal flow control strategies. |
Tuesday, November 20, 2018 8:13AM - 8:26AM |
M15.00002: Optimization of mixing in stirred, binary fluids Maximilian Eggl, Peter J Schmid Mixing is a process that is present in a wide-range of industrial applications, and optimization of mixing under time and energy constraints is a research area of great interest. We employ a computational framework based on nonlinear direct-adjoint looping for the optimization of mixing processes in a binary fluid system. By optimizing the stirring protocol of two cylindrical stirrers on circular paths, we seek to improve the mix-norm of the binary fluid system and ultimately generate a more homogeneous mixture. A Fourier-based pseudo-spectral numerical approach coupled to a Brinkman penalization of the immersed bodies is used to solve the direct and adjoint problem. The extracted gradient information is then processed in an optimization algorithm to gradually improve the stirring strategy. This optimization is attempted while observing a prescribed finite time horizon and an upper bound on the expended control energy.
Several cases of mixing enhancement, covering different time horizons and energy budgets, will be presented to demonstrate the effectiveness, efficiency and flexibility of the computational direct-adjoint approach. In all cases, significant improvements in mixing can be observed. |
Tuesday, November 20, 2018 8:26AM - 8:39AM |
M15.00003: Nonlinear optimal control of buoyancy-driven flow using direct-adjoint-looping method for various ventilation strategies Saleh Nabi, Piyush Grover, Colm-cille P. Caulfield We consider the problem of optimally controlling turbulent buoyancy-driven flows in the built environment in the presence of a known time-varying disturbance. Using unsteady Reynolds-averaged equations (URANS), a Direct-Adjoint-Looping (DAL) implementation of the nonlinear optimal control problem yields time-varying values of temperature and velocity of the inlet flow that lead to `optimal' time-averaged temperature relative to appropriate objective functionals in a region of interest. As a test case, we consider a time-varying heat-source in a displacement ventilation scenario, where maintaining a certain temperature profile in the region is of significant practical value. Our results can inform optimal sensor and/or actuator placement tasks, as well as provide a way to obtain open loop control that results in robustness to unknown disturbances. |
Tuesday, November 20, 2018 8:39AM - 8:52AM |
M15.00004: Quasi-periodic behavior of closed-loop control in convective flows Vladimir Parezanovic, Andreas Spohn, Laurent Cordier Closed-loop control of convective flows, arranged in a feedback configuration (sensors downstream of the actuators), can usually be made to establish a quasi-periodic behavior when the actuation creates a perturbation of an order one magnitude. Many studies have shown a tendency of the controller to establish this quasi-periodicity at or near the local natural frequency of the flow (at sensor location). In this presentation, we will discuss whether this is by design or by chance. We will show how convective times and amplification properties of shear layers (as an example of convective flows) can modify this quasi-periodic cycle of the feedback controller, sometimes to values very far from the natural local frequency. This can be either an advantage or a difficulty, if the objective of control depends on a precise frequency selection (as is usually the case). We will also propose an analogy between the feedback control of a free mixing layer and a mixing layer flowing over a passive cavity of given properties. |
Tuesday, November 20, 2018 8:52AM - 9:05AM |
M15.00005: Multi-objective Shape Optimization of Unsteady Heat-convection Fields Eiji Katamine, Naoya Okada This paper presents a numerical solution to multi-objective shape optimization problem of unsteady heat-convection fields. |
Tuesday, November 20, 2018 9:05AM - 9:18AM |
M15.00006: Static output feedback strategies for suppressing transient energy growth in channel flow Huaijin Yao, Maziar Sam Hemati Transient energy growth (TEG) of disturbances can lead a laminar flow to become turbulent. Full-state feedback control can be used to suppress TEG and has been shown to delay transition in simulations. However, full-state feedback is not always possible in practical applications; instead, sensor-based output feedback is required. In this study, we consider a linearized channel flow with wall-normal blowing/suction actuation and wall shear-stress sensors. We find that widely used observer-based feedback controllers will lead to severely degraded worst-case TEG performance, even relative to the uncontrolled flow. An optimal static output feedback (SOF) approach is proposed, which is shown to reduce TEG in the case of purely spanwise disturbances. We further show that by introducing a small number of additional sensors, the optimal SOF approach can recover full-state feedback performance and reduce TEG for all disturbances. |
Tuesday, November 20, 2018 9:18AM - 9:31AM |
M15.00007: Large eddy simulation of a cross-flow fan and its control using sinusoidal protrusions Beomjun Kye, Wonhyuck Yoon, Keuntae Park, Sehyeong Oh, Haecheon Choi, Myungsung Lee, Joo-Han Kim The flow characteristics in a cross-flow fan is investigated using large eddy simulation with an immersed boundary method. The fan operates at the Reynolds number of Re = 5,400 based on the blade chord length and the blade tip velocity at its outer radius. Due to the rotation of cross-flow fan, an eccentric vortex locates at the lower right corner near a stabilizer, and the main flow passes by this vortex. A recirculation region at the corner of the inflow region and flow separation on blade suction surfaces are also captured in the simulation. To analyze the effects of these flow structures on the fan performance, the variation of the torque coefficient on each blade along the azimuthal direction is examined. Most of the efficiency comes from the flow in the lower left and upper right corner of the fan, but the recirculation zone narrows the inflow region, resulting in a considerable loss. The flow on the upper part also separates, providing lower efficiency and pulsations. With the protrusion installed on the leading edge of each blade, the recirculation region and the flow separation in the upper part is significantly reduced, increasing the fan efficiency. |
Tuesday, November 20, 2018 9:31AM - 9:44AM |
M15.00008: Testing Nikola Tesla’s fluidic diode Quynh Nguyen, Leif Ristroph A basic component of flow control circuits is the check valve or diode, which passes current one way while impeding flow in the reverse direction. The inventor Nikola Tesla proposed an ingenious fluidic diode with no moving parts that relies only on an intricate (and static) internal geometry. We experimentally test this device for both steady (DC) and unsteady (AC) flows, and we show how it may be used in applications such as a pump or AC-to-DC converter. While these results clearly demonstrate the utility of the device, they also raise questions about Tesla’s proposed mechanism and his claims of its effectiveness. |
Tuesday, November 20, 2018 9:44AM - 9:57AM |
M15.00009: Sensitivity Analysis of Thermoacoustic Instability with Adjoint Helmholtz Solvers Matthew Juniper Thermoacoustic oscillations often occur late in the gas turbine design process. They can usually be eliminated by making a small change to the system. The challenge is to identify the optimal change systematically, cheaply, and accurately. Often there is one linearly unstable natural oscillation (eigenmode) and many possible changes to the system. This means that linear adjoint methods are ideal for identifying the optimal change. This paper applies linear adjoint methods to a thermoacoustic Helmholtz solver to evaluate, in a single calculation, the sensitivity of an eigenmode to all possible changes. These sensitivities are calculated with finite difference and finite element methods, in the weak form and the strong form, with the discrete adjoint and the continuous adjoint, and with two solution methods. This reveals that: the discrete adjoint of finite difference should be avoided; the discrete adjoint of finite element is simple and robust; if the strong form equations must be used, e.g. in order to use an existing direct solver, then the continuous adjoint is best. Finally, physical interpretation of these results shows that the well-known Rayleigh criterion should be written in terms of the adjoint pressure, not the direct pressure. |
Tuesday, November 20, 2018 9:57AM - 10:10AM |
M15.00010: Flow control to disturb the training of storms for the prevention of flood and landslide risks Daisuke Hiruma, Ryo Onishi, Keiko Takahashi, Koji Fukagata Localized torrential downpours over a period of several hours triggered by linear rainbands or so-called storm training often exceed the capacity of the infrastructure even in developed countries, causing severe damage. Even though it will not be feasible to remove those rainband systems entirely, it would be possible to diffuse the centralization of the precipitation zone to limit the maximum precipitation accumulation within the manageable amount. |
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