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 G16: Flow Control: Active IIIControl
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Chair: Sunil Manohar, Singapore University of Technology and Design Room: 603 |
Monday, November 20, 2017 10:35AM - 10:48AM |
G16.00001: Drag control of wall-bounded turbulent flows. Xi Chen, Jie Yao, Fazle Hussain Using direct numerical simulations of turbulent channel flow, we present a new method for skin friction reduction, enabling large-scale flow forcing without requiring instantaneous flow information. We show that the lack of drag reduction at high Re (Re$_{\mathrm{\tau }}$ \begin{figure}[htbp] \centerline{\includegraphics[width=0.25in,height=0.20in]{300720171.eps}} \label{fig1} \end{figure} $=\quad 550)$ recently reported by Canton \textit{et al. }[J. Canton \textit{et al.}, PRF (2016)] is remedied by a proper choice of the large-scale control flow, i.e. via near-wall spanwise opposed wall-jet forcing (SOWF), each wall-jet covering multiple streaks. The control method is characterized by three parameters, namely, the wall-jet amplitude A$^{\mathrm{+}}$, the spanwise wall-jet spacing $\Lambda^{\mathrm{+}}$, and the wall-jet height y$^{\mathrm{+}}_{\mathrm{c}}$ ($+$ indicates viscous scaling). We show as an example that with a choice of A$^{\mathrm{+}}\approx $.015, $\Lambda ^{\mathrm{+}}\approx $1200 and y$^{\mathrm{+}}_{\mathrm{c}} \quad =$30 (these three parameters values were found to produce maximum drag reduction for Re$_{\mathrm{\tau \thinspace }}=$ 180), the flow control definitely suppresses the wall shear stress at a series of Reynolds numbers, namely, 19{\%}, 14{\%}, and 12{\%} drag reductions at Re$_{\mathrm{\tau }}=$ 180, 395, and 550, respectively. Vortex structures ($\lambda_{\mathrm{2}})$ and flow statistics (Reynolds shear stress, rms of vorticities, kinetic energy budget, etc.) are further examined to explain the mechanism of drag reduction and increase. [Preview Abstract] |
Monday, November 20, 2017 10:48AM - 11:01AM |
G16.00002: Reducing turbulent boundary layer drag by a sustainable thin-air film Cong Wang, David Jeon, Morteza Gharib Reduction of hydrodynamic frictional drag through introduction of air bubbles or films at the wall regions has been tried by several groups in the past. The main challenge for these approaches has been to sustain the air bubble or film under high turbulent velocity fluctuations. We will report a novel technique that allows maintaining stable oscillating air films over solid surface in order to obtain large drag reduction effect. Based on our DPIV results, we will present a potential mechanism for the Reynolds stress suppression in the near wall region. [Preview Abstract] |
Monday, November 20, 2017 11:01AM - 11:14AM |
G16.00003: In-plane travelling waves for turbulent skin friction drag reduction. James Bird, Matthew Santer, Jonathan Morrison The control of turbulent boundary layers via spanwise wall forcing has long been the subject of investigation, where large reductions in skin friction drag have been observed in experimental and numerical studies. When the wall forcing takes the form of streamwise travelling waves of spanwise velocity, the same drag reductions occur, but also with the potential for net power saving, when the waveform has certain dimensionless qualities. The production of waveforms of this nature experimentally, and their influence on the flow, is the subject of the work presented. A flat surface, $3$ m in length, was developed comprising of a compliant substructure, based on the Kagome lattice geometry, supporting a membrane skin. The substructure was designed such that when it was actuated, it produced in-plane waveforms of variable length and speed. Various waves were generated under a turbulent boundary layer, with $Re_{\tau}=1125$, and, for certain forcing parameters, a large drag reduction of $20\%$ was measured, in line with, and following the same trends as, existing numerical studies. [Preview Abstract] |
Monday, November 20, 2017 11:14AM - 11:27AM |
G16.00004: Resolvent analysis of suboptimal control for turbulent skin friction drag reduction Satoshi Nakashima, Koji Fukagata, Mitul Luhar We study the drag reduction mechanisms of suboptimal control (Lee et al. 1998) via the resolvent formulation developed by McKeon and Sharma (2010). Under this formulation, the nonlinear term in the Navier-Stokes equations is regarded as a forcing which acts upon the linear dynamics to output a velocity response across Fourier space. This analysis enables targeted analyses of the effects of the control on modes resembling dynamically important coherent structures such as the near-wall (NW) cycle. Suboptimal control generates blowing and suction at the wall that is proportional to the streamwise (Case ST) or spanwise (Case SP) wall shear-stress, with the magnitude of blowing and suction being a design parameter. Both Case ST and SP can suppress resolvent modes resembling the NW cycle. However, for Case ST, the analysis reveals that the control leads to substantial increase in amplification for structures that are long in the spanwise direction. High actuation of such energetic spanwise structures was confirmed by conducting limited direct numerical simulations. In addition to the study of modes resembling the NW cycle, we will discuss modes of varying propagating speed and wavelength to provide insight into the effects of suboptimal control across spectral space. [Preview Abstract] |
Monday, November 20, 2017 11:27AM - 11:40AM |
G16.00005: Resolvent-based feedback control for turbulent friction drag reduction Aika Kawagoe, Satoshi Nakashima, Mitul Luhar, Koji Fukagata Suboptimal control (Lee et al. 1998) for turbulent friction drag reduction has been studied extensively. Nakashima et al. (accepted) extended resolvent analysis (McKeon and Sharma 2010) to suboptimal control, and for the control where the streamwise wall shear stress is used as an input (Case ST), they revealed the control effect across spectral space is mixed: there are regions of drag increase as well as reduction. This suggests that control performance may be improved if the control is applied for selective wavelengths, or if a new law is designed to suppress the spectral region leading to drag increase. In the present study, we first assess the effect of suboptimal control for selective wavelengths via DNS. The friction Reynolds number is set at 180. For Case ST, resolvent analysis predicts drag reduction at long streamwise wavelengths. DNS with control applied only for this spectral region, however, did not result in drag reduction. Then, we seek an effective control law using resolvent analysis and propose a new law. DNS results for this law are consistent with predictions from resolvent analysis, and about 10{\%} drag reduction is attained. Further, we discuss how this law reduces the drag from a dynamical and theoretical point of view. [Preview Abstract] |
Monday, November 20, 2017 11:40AM - 11:53AM |
G16.00006: “Large”- vs Small-scale friction control in turbulent channel flow Jacopo Canton, Ramis Örlü, Cheng Chin, Philipp Schlatter We reconsider the “large-scale” control scheme proposed by Hussain and co-workers (Phys. Fluids 10, 1049--1051 1998 and Phys. Rev. Fluids, 2, 62601 2017), using new direct numerical simulations (DNS). The DNS are performed in a turbulent channel at friction Reynolds number $Re_\tau$ of up to 550 in order to eliminate low-Reynolds-number effects. The purpose of the present contribution is to re-assess this control method in the light of more modern developments in the field, in particular also related to the discovery of (very) large-scale motions. The goals of the paper are as follows: First, we want to better characterise the physics of the control, and assess what external contribution (vortices, forcing, wall motion) are actually needed. Then, we investigate the optimal parameters and, finally, determine which aspects of this control technique actually scale in outer units and can therefore be of use in practical applications. In addition to discussing the mentioned drag-reduction effects, the present contribution will also address the potential effect of the naturally occurring large-scale motions on frictional drag, and give indications on the physical processes for potential drag reduction possible at all Reynolds numbers. [Preview Abstract] |
Monday, November 20, 2017 11:53AM - 12:06PM |
G16.00007: Impact of Drag Reduction Control on Energy Box of a Fully Developed Turbulent Channel Flow Yosuke Hasegawa, Davide Gatti, Andrea Cimarelli, Bettina Frohnapfel, Maurizio Quadrio We introduce the Constant Power Input (CPI) concept to clarify how a drag reduction control affects energy budget of a fully developed turbulent channel flows. The entire kinetic energy is decomposed into the mean and fluctuating components, and the total dissipation is accordingly divided into the dissipation of the mean filed and the turbulent dissipation. The CPI condition is essential in the present study, since it strictly restricts the amount of power applied to the flow system. This allows us to identify how each flow control strategy changes the energy flows between each component and the viscous dissipation. Ultimately, if we succeed in suppressing all turbulence, the turbulent dissipation should vanish and the power applied to the flow system should be dissipated only by the dissipation of the mean velocity, which should have a parabolic profile. Our fundamental question in the present study is whether there exists unique relationship between the changes in the turbulent dissipation and the resultant drag reduction effect. In order to provide the definite answer to this question, we introduce triple decomposition of the velocity field, and validate our approach by considering two different flow control strategies. [Preview Abstract] |
Monday, November 20, 2017 12:06PM - 12:19PM |
G16.00008: Turbulent channel flows over complex walls Marco Edoardo Rosti, Luca Brandt We perform numerical simulations of turbulent channel flows over porous walls and deformable hyper-elastic walls. The flow over porous walls is simulated using volume-averaged Navier–Stokes equations within the porous layers, while the multiphase flow over deformable walls is solved with a one-continuum formulation which allows the use of a fully Eulerian formulation. New insights on the effect of these complex walls on the turbulent flows in terms of friction, statistics and flow structures are discussed using a number of post-processing techniques. The turbulent flow in the channel is affected by the porous and moving walls in a similar manner even at low values of porosity and elasticity due to the non-zero fluctuations of vertical velocity at the interface that influence the flow dynamics. The near-wall streaks and the associated quasi-streamwise vortices are strongly reduced near porous and deformable isotropic wall while the flow becomes more correlated in the spanwise direction. On the contrary, an opposite behavior is noticed in the case of anisotropic porous layers, with an increase of streamwise correlation due to a strengthening of the low- and high-speed streaks. [Preview Abstract] |
Monday, November 20, 2017 12:19PM - 12:32PM |
G16.00009: Trajectory of a synthetic jet issuing into a high Reynolds number turbulent boundary layer Tim Berk, Rio Baidya, Charitha de Silva, Ivan Marusic, Nicholas Hutchins, Bharathram Ganapathisubramani Synthetic jets are zero-net-mass-flux actuators that can be used in a range of flow control applications. For several pulsed/synthetic jet in cross-flow applications the variation of the jet trajectory in the mean flow with jet and boundary layer parameters is important. This trajectory will provide an indication of the penetration depth of the pulsed/synthetic jet into a boundary layer. Trajectories of a synthetic jet in a turbulent boundary layer are measured for a range of actuation parameters in both low- and high Reynolds numbers (up to $Re_\tau = 13000$). The important parameters influencing the trajectory are determined from these measurements. The Reynolds number of the boundary layer is shown to only have a small effect on the trajectory. In fact, the critical parameters are found to be the Strouhal number of the jet based on jet dimensions as well as the velocity ratio of the jet (defined as a ratio between peak jet velocity and the freestream velocity). An expression for the trajectory of the synthetic (or pulsed) jet is derived from the data, which (in the limit) is consistent with known expressions for the trajectory of a steady jet in a cross-flow. [Preview Abstract] |
Monday, November 20, 2017 12:32PM - 12:45PM |
G16.00010: Control of wake vortex street behind a square cylinder using surface travelling waves Sunil Manohar Dash, Michael S Triantafyllou, Pablo Valdivia y Alvarado A novel travelling wave (TW) flow separation control strategy is developed to suppress the adverse downstream wake effects on a square cylinder of side, L, at low Reynolds number (Re$=$100). Our 2D numerical simulations suggest that when the downstream cylinder surface carries appropriately designed TWs, in the presence of an incoming flow of velocity U, a series of small scale vortices are formed in the trough regions of TWs. These small vortices inhibit momentum transfer between the thin fluid layer adjacent to the wall and the freestream. Consequently, the von-Karman vortex street behind the cylinder is suppressed and more than 70{\%} reduction in drag force and complete elimination of fluctuating lift force is observed. The optimum TW control mechanism is determined by conducting a series of numerical simulations with various wave speeds (c) and wave amplitudes (A) for a fixed wave number (N$=$L/$\lambda =$4, where $\lambda $ is the wavelength). Total suppression of the von-Karman vortex street is achieved when c/U is greater than 5, whereas only limited suppression of wake effects is seen at lesser c/U. The effect of wave amplitude is insignificant in the range of A/L$=$0.02 to 0.03. Energy efficiency to generate TW is also investigated in this study. [Preview Abstract] |
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