Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session Q16: Flow Control: Actuators & General ApplicationsControl
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Chair: Robert J. Martinuzzi, University of Calgary Room: 603 |
Tuesday, November 21, 2017 12:50PM - 1:03PM |
Q16.00001: Internal Fluid Dynamics and Frequency Scaling of Sweeping Jet Fluidic Oscillators Jung Hee Seo, Erik Salazar, Rajat Mittal Sweeping jet fluidic oscillators (SJFOs) are devices that produce a spatially oscillating jet solely based on intrinsic flow instability mechanisms without any moving parts. Recently, SJFOs have emerged as effective actuators for flow control, but the internal fluid dynamics of the device that drives the oscillatory flow mechanism is not yet fully understood. In the current study, the internal fluid dynamics of the fluidic oscillator with feedback channels has been investigated by employing incompressible flow simulations. The study is focused on the oscillation mechanisms and scaling laws that underpin the jet oscillation. Based on the simulation results, simple phenomenological models that connect the jet deflection to the feedback flow are developed. Several geometric modifications are considered in order to explore the characteristic length scales and phase relationships associated with the jet oscillation and to assess the proposed phenomenological model. A scaling law for the jet oscillation frequency is proposed based on the detailed analysis. [Preview Abstract] |
Tuesday, November 21, 2017 1:03PM - 1:16PM |
Q16.00002: Modifying the inlet characteristics of a Turbulent Coanda Wall Jet Anshuman Pandey, James W. Gregory The Coanda effect has been ingeniously used over the past century for augmentation of lift. More recently, NOTAR helicopters have employed the use of the Coanda effect for producing the stabilizing side force with quieter and safer configurations. A manifestation of the Coanda effect in its simplest form is a wall jet issuing tangentially to a cylinder that tends to stay attached to the cylinder over turning angles as large as 180 degrees. An experimental study on such a configuration has been performed in this work to understand the effect of inlet characteristics on the evolution of the wall jet. In previous studies, it has been found that the radial momentum influx is maximum near the inlet and it provides the necessary centrifugal force for the jet to stay attached. So it can be hypothesized that a protrusion of the upper wall of the nozzle that issues the jet would lead to an earlier separation and decreased efficiency. On the other hand, the predisposition of the jet to stay attached to the flat surface of the protrusion could create a separation bubble between the jet and the curved wall and lead to increased suction. These competing phenomena have been explored using Particle Image Velocimetry performed in the streamwise direction at mid-span location of a 3 ft long cylinder. The effect of varying the length of the protrusion for different combinations of initial jet width and flow velocity has been studied to understand how changing the inlet affects the efficiency of the wall jet. [Preview Abstract] |
Tuesday, November 21, 2017 1:16PM - 1:29PM |
Q16.00003: Turbulence Modulation of a Weakly Compressible Wall-Jet Cristale Garnica, Bertrand Rollin Wall-jets are flows of paramount importance in modern engineering, where applications in thermal protection, combustion, flow control and noise generation are numerous. It can be seen as being composed of two canonical flows: a boundary layer and a free mixing flow. In this paper, the focus is turned to the modulation of turbulence in weakly compressible isothermal wall-jets, when subject to changes in the jet-inlet conditions. Direct Numerical Simulations (DNS) of wall-jets are carried out using PyFR$^{\mathrm{\thinspace [1]}}$, a Python based computational fluid dynamics framework. Analysis of mean profiles and turbulence quantities response to carefully designed excitation profiles will be presented, as well as changes in coherent structures of the turbulent flow. Finally, of particular interest is the relation between the Kelvin-Helmholtz instability and the modulation of turbulence in both the outer and the inner-layer.\\ \\$[1]$Witherden, F. D., et al., PyFR: \textit{An open source framework for solving advection-diffusion type problems on streaming architectures using the flux reconstruction approach.} Computer Physics Communications, 185(11), 3028-3040, 2014. [Preview Abstract] |
Tuesday, November 21, 2017 1:29PM - 1:42PM |
Q16.00004: Open-loop control for complete relaminarisation of turbulent pipe flow Jakob Kühnen, Davide Scarselli, Björn Hof We show that a simple and predetermined perturbation is able to annihilate turbulent pipe flow by disrupting the sustaining mechanism of turbulence at the wall, causing complete relaminarisation further downstream. The annihilation of turbulence is effected by a steady, open-loop manipulation of the streamwise velocity component only. We control and perturb the flow such that the streamwise velocity profile becomes more flat, i.e. we increase the streamwise velocity close to the wall and decrease the velocity in the center of the pipe. We will present several different control schemes from laboratory experiments which achieve the required perturbation of the flow for total relaminarisation up to Reynolds numbers of 40 000. We will present measurements with high-speed particle image velocimetry, measurements of the pressure drop and videos of the development of the flow during relaminarisation. Transient growth, a linear amplification mechanism measuring the efficiency of eddies in redistributing shear that quantifies the maximum perturbation energy amplification achievable over a finite time in a linearized framework, is shown to set a clear-cut threshold below which turbulence is impeded in its formation and thus permanently annihilated. [Preview Abstract] |
Tuesday, November 21, 2017 1:42PM - 1:55PM |
Q16.00005: Experimental control of a fluidic pinball using genetic programming Cedric Raibaudo, Peng Zhong, Bernd R. Noack, Robert J. Martinuzzi The wake stabilization of a triangular cluster of three rotating cylinders was investigated in the present study. Experiments were performed at Reynolds number $Re \sim$ 6000, and compared with URANS-2D simulations at same flow conditions. 2D2C PIV measurements and constant temperature anemometry were used to characterize the flow without and with actuation. Open-loop actuation was first considered for the identification of particular control strategies. Machine learning control was also implemented for the experimental study. Linear genetic programming has been used for the optimization of open-loop parameters and closed-loop controllers. Considering a cost function $\mathcal{J}$ based on the fluctuations of the velocity measured by the hot-wire sensor, significant performances were achieved using the machine learning approach. [Preview Abstract] |
Tuesday, November 21, 2017 1:55PM - 2:08PM |
Q16.00006: Trade-offs for feedback control of the linearized Ginzburg-Landau system Simon Illingworth, Stephan Oehler We consider feedback control of the linearized Ginzburg-Landau system. The particular focus is on any trade-offs present in the single-input single-output control problem. The work is in three parts. First, we consider the estimation problem in which a single sensor is used to estimate the entire flow field (without any control). By considering the optimal sensor placement with varying system stability, a fundamental trade-off for the estimation problem is made clear. Second, we consider the full-information control problem in which the entire flow field is known, but only a single actuator is available for control. We show that a similar trade-off exists when placing the single actuator. Third, we consider the overall feedback control problem in which only a single sensor is available for measurement; and only a single actuator is available for control. By varying the system stability, a similar fundamental trade-off is made clear. Implications for effective feedback control with a single sensor and a single actuator are discussed. [Preview Abstract] |
Tuesday, November 21, 2017 2:08PM - 2:21PM |
Q16.00007: Disturbance Source Separation in Shear Flows Using Blind Source Separation Methods Igal Gluzman, Jacob Cohen, Yaakov Oshman A novel approach is presented for identifying disturbance sources in wall-bounded shear flows. The method can prove useful for active control of boundary layer transition from laminar to turbulent flow. The underlying idea is to consider the flow state, as measured in sensors, to be a mixture of sources, and to use Blind Source Separation (BSS) techniques to recover the separate sources and their unknown mixing process. We present a BSS method based on the Degenerate Unmixing Estimation Technique. This method can be used to identify any (a priori unknown) number of sources by using the data acquired by only two sensors. The power of the new method is demonstrated via numerical and experimental proofs of concept. Wind tunnel experiments involving boundary layer flow over a flat plate were carried out, in which two hot-wire anemometers were used to separate disturbances generated by disturbance generators such as a single dielectric barrier discharge plasma actuator and a loudspeaker. [Preview Abstract] |
Tuesday, November 21, 2017 2:21PM - 2:34PM |
Q16.00008: Observer-based feedback control of transient energy growth: Revisiting the separation principle Huaijin Yao, Maziar Hemati Sub-critical transition to turbulence is often attributed to a linear mechanism for transient energy growth. Numerous strategies have been formulated to suppress transient energy growth via feedback control. Usually, in practice, not all states needed for feedback control can be measured directly, so a state observer is used to estimate these states from available sensor measurements. Subsequently, an observer-based feedback law is typically synthesized by invoking the separation principle, which is used to justify independent designs of the controller and estimator. In this talk, we show that controller synthesis based on the separation principle can lead to degraded performance for transient energy growth suppression. Using a number of illustrative examples, we show that observer-based control strategies can be greatly improved by abandoning the separation principle. [Preview Abstract] |
Tuesday, November 21, 2017 2:34PM - 2:47PM |
Q16.00009: An Explorative Study to Use DBD Plasma Generation for Aircraft Icing Mitigation Hui Hu, Wenwu Zhou, Yang Liu, Cem Kolbakir An explorative investigation was performed to demonstrate the feasibility of utilizing thermal effect induced by Dielectric-Barrier-Discharge (DBD) plasma generation for aircraft icing mitigation. The experimental study was performed in an Icing Research Tunnel available at Iowa State University (i.e., ISU-IRT). A NACA0012 airfoil/wing model embedded with DBD plasma actuators was installed in ISU-IRT under typical glaze icing conditions pertinent to aircraft inflight icing phenomena. While a high-speed imaging system was used to record the dynamic ice accretion process over the airfoil surface for the test cases with and without switching on the DBD plasma actuators, an infrared (IR) thermal imaging system was utilized to map the corresponding temperature distributions to quantify the unsteady heat transfer and phase changing process over the airfoil surface. The thermal effect induced by DBD plasma generation was demonstrated to be able to keep the airfoil surface staying free of ice during the entire ice accretion experiment. The measured quantitative surface temperature distributions were correlated with the acquired images of the dynamic ice accretion and water runback processes to elucidate the underlying physics. [Preview Abstract] |
Tuesday, November 21, 2017 2:47PM - 3:00PM |
Q16.00010: Lift enhancement by thermal forcing at the trailing edge Sumit Vashishtha, MF Baig, Nadeem Hasan Using numerical simulations of Viscous Compressible flow around a canonical case of 2-D flat plate airfoil, it is shown how a simple yet unexplored active flow control method - Thermal Forcing at the trailing edge - can enhance the lift to drag ratio by as much as $60\%$ at low Reynolds numbers and low angles of attack. Here thermal forcing at the trailing edge implies heating a small portion - $5\%$ of the chord length - at the lower surface of the trailing edge. Moreover, such active control when applied at locations upstream of the trailing edge is ineffective in enhancing the lift to drag ratio and most of the energy supplied is simply drained without influencing the aerodynamic performance. Application of the control on the upper surface in fact deteriorates the lift to drag ratio. A Vorticity Dynamics analysis reveals that, for the case of thermal forcing at the trailing edge, the viscous term in vorticity evolution equation for viscous compressible flows acts as a strong source of negative vorticity near the trailing edge. The negative vorticity thus generated induces an additional clockwise circulation around the airfoil and thereby enhances the lift. [Preview Abstract] |
Tuesday, November 21, 2017 3:00PM - 3:13PM |
Q16.00011: Time-resolved, three-dimensional imaging of the initial region of a jet discharged from a round nozzle with a tab Akinori Muramatsu Enhancement or suppression of mixing between a jet fluid and the ambient fluid can be realized by operating the flow structure, namely the vortical structure, in the initial region of the jet. Changing the exit form of a jet nozzle is a method for managing the vortical structure in the jet. When the projections (tabs) are installed on the nozzle exit, it is known that the vortical structure of the jet changes by the numbers, the shapes, and the arrangement methods of the tabs. In this study, a triangular tab was attached on the exit of a round nozzle. Air jet was formed at a Reynolds number of 2,000. The Reynolds number is based on the nozzle diameter and the jet velocity. The changed vortical structure in the initial region by the triangular tab was visualized using a time-resolved, three-dimensional imaging. The three-dimensional visualization of the jet was performed using a scanning, planar laser Mie scattering and a high-speed digital video camera. A three-dimensional image for the jet was constructed by volume-rendering method from a large number of cross-sectional images. A pair of wakes are formed just behind the nozzle exit by the tab, and is a pair of streamwise vortexes. The streamwise vortexes merge with vortex rings. [Preview Abstract] |
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