Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session L15: Flow Control: Vortices and Turbulence |
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Chair: Raghuraman Govardhan, Indian Institute of Science Room: 203 |
Monday, November 23, 2015 4:05PM - 4:18PM |
L15.00001: Vorticity dynamics in the interaction of a single bubble with a vortex ring Raghuraman Govardhan, Narsing Jha Bubbly turbulent flows occur in a number of engineering applications, such as in drag reduction using bubbles. In the present work, we study an idealization of this problem, namely, the interaction of a single bubble with a single vortex ring. The vortex ring is generated in water using a piston-cylinder arrangement, and an air bubble in injected close to it. The changes in vorticity during the interaction are measured using time-resolved PIV, while the bubble dynamics are visualized using high speed imaging. Interactions are studied over a large range of Weber numbers, which is defined using the vortex ring strength. The results show that the interactions can significantly affect the vortex ring, including reduction in its convection speed, and fragmentation of its core with a resultant large decrease in its enstrophy. We present vorticity fields during the interaction over a range of Weber numbers to help understand the physics of the interaction. The present results for the bubble ring interactions show many phenomena also seen in bubbly turbulent flows such as reduction in enstrophy, which suggests that results from the present study may help to better understand interaction of bubbles with vortical structures in turbulent flows. [Preview Abstract] |
Monday, November 23, 2015 4:18PM - 4:31PM |
L15.00002: PIV investigation of the intake flow in a parallel valves diesel engine cylinder P. Henrik Alfredsson, Jean Rabault, Julie A. Vernet, Bj\"orn Lindgren The flow of air (gas) inside the cylinder of internal combustion engines prior to compression may have a large influence on the combustion process. The structure of the in-cylinder flow, which can be swirl or tumble dominated, is to a large extent controlled by the design of the intake ports. In this study the admission flow generated by a parallel valves diesel engine cylinder head was investigated in a steady flow test bench through planar and stereo PIV measurements in both the swirl and tumble planes. By combining several sets of measurements a full three-dimensional, three-component reconstruction of the mean flow field was made. The flow out of the valves has a radial jet character, making the air hit the cylinder wall before flowing down along the cylinder wall. This leads to the formation of a recirculation bubble in the tumble plane. In the swirl plane complex jet dominated structures are found just below the valves giving rise to a counter-rotating vortex pair, where the strongest vortex becomes predominant giving rise to a single coherent swirling structure away from the cylinder head. Variations of the location and strength of the swirling structure may give rise to cycle-to-cycle variations and its stability was analysed by tracking the vortex centre. [Preview Abstract] |
Monday, November 23, 2015 4:31PM - 4:44PM |
L15.00003: Pressure fluctuations and time scales in turbulent channel flow Kamthon Septham, Jonathan Morrison, Sourabh Diwan Pressure fluctuations in turbulent channel flow subjected to globally stabilising linear feedback control are investigated at $Re_{\tau} = 400$. The passivity-based control is adopted and explained by the conservative characteristics of the nonlinear terms contributing to the Reynolds-Orr equation (Sharma \textit{et al. Phys. Fluids} 2011). The linear control operates via $\textit{v}U'$; the maximum forcing is located at $y^{+} \approx 20$, corresponding to the location of the maximum in the mean-square pressure gradient. The responses of the rapid (linear) and slow (nonlinear) pressure fluctuations to the linear control are investigated using the Green's function representations. It demonstrates that the linear control operates via the linear source terms of the Poisson equation for pressure fluctuations. Landahl's timescales of the minimal flow unit (MFU) in turbulent channel flow are examined at $y^+=20$. It shows that the timescales of MFU agree well with the theoretical values proposed by Landahl (1993). Therefore, the effectiveness of the linear control to attenuate wall turbulence is explained by Landahl's theory for timescales, in that the control proceeds via the shear interaction timescale which is significantly shorter than both the nonlinear and viscous timescales. [Preview Abstract] |
Monday, November 23, 2015 4:44PM - 4:57PM |
L15.00004: The effect of butterfly-scale inspired patterning on leading-edge vortex growth Jacob Wilroy, Amy Lang Leading edge vortices (LEVs) are important for generating thrust and lift in flapping flight, and the surface patterning (scales) on butterfly wings is hypothesized to play a role in the vortex formation of the LEV. To simplify this complex flow problem, an experiment was designed to focus on the alteration of 2-D vortex development with a variation in surface patterning. Specifically, the secondary vorticity generated by the LEV interacting at the patterned surface was studied and the subsequent affect on the growth rate of the circulation in the LEV. For this experiment we used butterfly inspired grooves attached to a flat plate and compared the vortex formation to a smooth plate case as the plate moved vertically. The plate is impulsively started in quiescent water and flow fields at Re $=$ 1500, 3000, and 6000 are examined using Digital Particle Image Velocimetry (DPIV). The vortex formation time is 3.0 and is based on the flat plate travel length and chord length. [Preview Abstract] |
Monday, November 23, 2015 4:57PM - 5:10PM |
L15.00005: Flow around new wind fence with multi-scale fractal structure in an atmospheric boundary layer Sarah McClure, Sang-Joon Lee, Wei Zhang Understanding and controlling atmospheric boundary-layer flows with engineered structures, such as porous wind fences or windbreaks, has been of great interest to the fluid mechanics and wind engineering community. Previous studies found that the regular mono-scale grid fence of 50{\%} porosity and a bottom gap of 10{\%} of the fence height are considered to be optimal over a flat surface. Significant differences in turbulent flow structure have recently been noted behind multi-scale fractal wind fences, even with the same porosity. In this study, wind-tunnel tests on the turbulent flow and the turbulence kinetic energy transport of 1D and 2D multi-scale fractal fences under atmospheric boundary-layer were conducted. Velocity fields around the fractal fences were systematically measured using Particle Image Velocimetry to uncover effects of key parameters on turbulent flows around the fences at a Reynolds number of approximately 3.6x10$^{4}$ based on the free-stream speed and fence height. The turbulent flow structures induced by specific 1D/2D multi-scale fractal wind fences were compared to those of a conventional grid fence. The present results would contribute to the design of new-generation wind fences to reduce snow/sand deposition on critical infrastructure such as roads and bridges. [Preview Abstract] |
Monday, November 23, 2015 5:10PM - 5:23PM |
L15.00006: Energetically efficient Proportional-Integral control of flow past a circular cylinder Pramode Kesavadas, Vijay Anand, B.S.V. Patnaik, A.J. Shaiju In this numerical study, we present an energetically efficient Proportional (P) and Integral (I) control strategy for the cessation of vortex shedding behind a circular cylinder. Reflectionally symmetric controllers are designed such that, they are located on a small sector of the cylinder over which, tangential sliding mode control is imparted. Energetically efficient optimal parameters for the P, I and PI controls have been numerically assessed. An estimation of the time-averaged kinetic energy of different flow regimes using Proper Orthogonal Decomposition (POD) is also carried out. These values are obtained with and without the optimal controllers. The Navier-Stokes equations along with an evolution equation for the PI controller, is numerically solved using finite volume method. The optimization procedure is formulated as a standard Linear Quadratic (LQ) problem and the time-averaged kinetic energy is obtained by summation of POD eigenvalues. The energetic efficiency for the, I controller was observed to be superior compared to the other two classes of controllers. By performing detailed fluid flow simulations, it was observed that, the system is energetically efficient, even when the twin eddies are still persisting behind the circular cylinder. [Preview Abstract] |
Monday, November 23, 2015 5:23PM - 5:36PM |
L15.00007: Identification of secondary instabilities in the near wake of a blunt trailing edge profiled body Ross Cruikshank, Wenyi Zhao, Philippe Lavoie Aerodynamic research into blunt trailing edge (BTE) airfoils is driven by their structural and aerodynamic advantages over sharp trailing edge airfoils. However, the wake of BTE airfoils is dominated by a vortex street, which causes increased drag. One method to reduce the spanwise coherence of the vortex street is to generate streamwise vorticity in the wake. Recent evidence suggests that the efficiency of this control method can be improved by forcing at the same wavelength as a secondary instability (SI) of the vortex street, present at Reynolds numbers (based on airfoil thickness, $d$) above 470. The objective of the present study was to investigate the variation of the SI wavelength at $2000< Re_d <35,000$, and to examine the effect of forcing on the wake topology. The velocity field in the wake of a BTE profiled model was measured using particle image velocimetry, and proper orthogonal decomposition was applied as a filter for measurement noise. It was found that, for a laminar boundary layer, the SI wavelength decreased as $Re_d$ increased. Following boundary layer transition to turbulence, the SI wavelength was insensitive to $Re_d$. This study will also examine the effect of forcing at different wavelengths on the dominant spanwise wavelength of the wake velocity field. [Preview Abstract] |
Monday, November 23, 2015 5:36PM - 5:49PM |
L15.00008: Feedback Control of Bistability in the Turbulent Wake of an Ahmed Body Rowan Brackston, Andrew Wynn, Juan Marcos Garcia de la Cruz, Georgios Rigas, Jonathan Morrison Three-dimensional bluff body wakes have seen considerable interest in recent years, not least because of their relevance to road vehicles. A key feature of these wakes is spatial symmetry breaking, reminiscent of the large scale structures observed during the laminar and transitional regimes. For the flat backed Ahmed body, this feature manifests itself as a bistability of the wake in which the flow switches randomly between two asymmetric states. This feature is associated with instantaneous lateral forces on the body as well as increased pressure drag. Starting from the modelling approach of Rigas \textit {et al.} (J. Fluid Mech. \textbf{778}, R2, 2015)we identify a linearised model for this mode of the flow, obtaining parameters via a system identification. The identified model is then used to design a linear feedback controller with the aim of restoring the flow to the unstable, symmetric state. The controller is implemented experimentally at Re $\sim 3 \times 10^5$ and is found to both suppress the bistability of the flow and reduce the drag on the body. Furthermore, the control system is found to have a positive energy balance, providing a key demonstration of efficient feedback control applied to a 3D bluff body at Reynolds numbers representative of road vehicle wakes. [Preview Abstract] |
Monday, November 23, 2015 5:49PM - 6:02PM |
L15.00009: A statistical approach characterizing the effectiveness of flow control on a dynamically pitching airfoil Keith Taylor, Michael Amitay The presence of dynamic stall on wind turbines complicates the goal of energy production, as variations in input loading runs counter to the end goal of producing continuous level power output from a wind turbine. While dynamic stall has been extensively studied experimentally and computationally, the control of dynamic stall through active flow control is still a nascent field of research. In order to understand the flow field around a dynamically pitching finite span airfoil, a new method of characterizing the effectiveness of flow control in a statistical sense is presented. This method leverages the gamma one criterion on Particle Image Velocimetry images to identify the vortices shed, then statistically describes how the distribution of the circulation strength of identified vortices changes during dynamic stall. This is in contrast to previous work, which only addressed the phase averaged flow field, which does not fully illustrate how the flow field varies loop by loop, as there is significant variation between phase averaged flow fields and instantaneous flow fields measured. The purpose of this work is to present a new method of characterizing the effectiveness of flow control under dynamic conditions, without the need to capture PIV at high frequencies. [Preview Abstract] |
Monday, November 23, 2015 6:02PM - 6:15PM |
L15.00010: Characterization of base pressure fluctuations in a blunt trailing edge wake with three-dimensional forcing Heather Clark, Philippe Lavoie The wakes of many nominally two-dimensional bluff bodies exhibit multiple intrinsic three-dimensional instabilities whose spatiotemporal structure and growth rate depend on geometry and Reynolds number. Here, these features are investigated experimentally for a blunt trailing edge profiled body using simultaneous measurements of velocity and fluctuating surface pressure on the model rear face near separation. Passive three-dimensional forcing of the wake is implemented with an array of vortex generators that are distributed according to the characteristic spanwise wavelength of the dominant secondary instability. For a Reynolds number of 8000 based on model thickness, the control strategy is found to increase the base pressure coefficient by 26\% while globally reducing the amplitude of base pressure fluctuations, relative to the unforced flow. Additionally, amplitude modulation of the pressure signals that is observed in the natural wake decreases in strength with distributed forcing as a result of the modified three-dimensional flow structure. The spanwise distribution of pressure will be further examined for the baseline and controlled flows via temporal spectral analysis and spatial modal decomposition. [Preview Abstract] |
Monday, November 23, 2015 6:15PM - 6:28PM |
L15.00011: On vortex pairing in several free shear layer containing high Reynolds number flows Mo Samimy, Michael Crawley There are several free shear flows with-well known Kevin Helmholtz instability, which contain an additional instability mechanism. For example, a jet has shear layer and jet column instabilities, a stalled airfoil has shear layer and wake instabilities, and a cavity flow has a shear layer instability and Rossiter modes. The shear layer's most amplified frequency is normally several times larger than that of the other instability. Typically, the structures associated with the lower frequency instability are observed in the experiments. There is not much information in the literature, especially in high Reynolds number flows, on whether these structures are generated directly or by multiple merging of smaller structures generated by the shear layer instability. Single or multiple merging has been shown in the literature in only low Reynolds number flows (e.g. in jets). Our recent experimental results in high Reynolds number flows excited by plasma actuators seem to show the occurrence of multiple merging events before the observation of lower frequency large-scale coherent structures. The experimental PIV images obtained in jets using reconstructed flow and in stalled airfoils obtained using phase averaging. [Preview Abstract] |
Monday, November 23, 2015 6:28PM - 6:41PM |
L15.00012: Interaction of Suction and Pulsed Blowing with a Laminar Boundary Layer Avraham Seifert, Liad Marom The presentation will describe a fundamental study of active flow control (AFC) using the steady suction and oscillatory blowing actuator (SaOB), identifying its effects on a laminar boundary layer. Recent experiments showed this effective and efficient actuator as a drag reduction device .......[e.g., Wilson et al, AIAA J, 2013]. However, improved fundamental understanding of the boundary layer (BL) interaction with suction and oscillatory blowing and the combination of these two effects in close proximity is desired. The current experiment, performed in a laminar flow, will result in improved efficiency of the actuator and will enable development of a reliable predictive capability of this flow control method. The interaction with a laminar BL is crucial for the project due to the lack of interaction with the random turbulence, the thicker BL and lower skin-friction that enables greater effect of the controlled BL. Furthermore, fundamental interaction principles could be easier to identify and understand in laminar flows, where critical trends will not be masked by turbulence, and the averaging process will better represent the time dependent flow. The results demonstrate that while the oscillatory blowing is robust and has a strong effect on the flow evolution, the steady suction introduced upstream has a crucial role in the efficient operation of the AFC system. [Preview Abstract] |
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