Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session Q26: Flow Control: Turbulence |
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Chair: Wei Zhang, Cleveland State University Room: 608 |
Tuesday, November 26, 2019 7:45AM - 7:58AM |
Q26.00001: Turbulent Wake Induced by a Seal-Whisker-Inspired Power Turbine Blade Wei Zhang, Robert Ahlman, Curtis Flack, Vikram Shyam Power turbines operate over a large range of flow incidence and at relatively low Reynolds numbers (Re). For a fixed-wing aircraft at cruising altitude, it is challenging for current power turbine blades to maintain desirable aerodynamic performance as the Re number has dropped to substantially lower than that at sea level. Therefore, it is imperative to improve the aerodynamics of turbine blades in low Re regimes. However, the performance of state-of-the-art turbine blades for aero-propulsive systems has plateaued. Inspired by the exceptional hydrodynamics of harbor seal whiskers, this study applies the key features of the three-dimensional undulating morphology of seal whiskers to the turbine blade leading edge. Turbulent wake flows generated by a seal-whisker-inspired variable speed power turbine (VSPT) blade are quantified and compared against a baseline untreated VSPT blade in a water tunnel using Particle Image Velocimetry (PIV). Focus is on the wake velocities and turbulence statistics at a range of angles of attack (AOA $=$ -- 10 to 10 degrees). Results of the seal-whisker-inspired blade will be used not only to improve the design of power turbine blades, but also to inform a wide variety of bio-inspired aerodynamic applications. [Preview Abstract] |
Tuesday, November 26, 2019 7:58AM - 8:11AM |
Q26.00002: Resolvent-informed design of anisotropic permeable substrates for turbulence control Andrew Chavarin, Garazi Gomez-de-Segura, Ricardo Garcia-Mayoral, Mitul Luhar We utilize an extended version of the resolvent formulation to design anisotropic permeable substrates for passive turbulence control. The resolvent formulation interprets the Fourier transformed Navier-Stokes equations as a forcing-response system: the linear terms map the action of the nonlinear terms to a velocity and pressure response. A gain-based decomposition of the forcing-response transfer function (the resolvent operator) identifies flow features (resolvent modes) that reproduce important structural and statistical features of wall-bounded turbulent flows. One particular resolvent mode serves as a useful surrogate for the dynamically important near-wall cycle. The effect of permeable substrates is introduced in this framework using the Volume-Averaged Navier-Stokes equations. Substrates with high streamwise permeability and low wall-normal permeability are found to suppress the near-wall resolvent mode, which is consistent with conditions in which drag reduction has been observed in recent numerical simulations. Performance deteriorates when high-gain spanwise constant rollers resembling Kelvin-Helmholtz vortices emerge over the porous medium. A parametric study is used to identify permeability combinations that have drag reduction potential. [Preview Abstract] |
Tuesday, November 26, 2019 8:11AM - 8:24AM |
Q26.00003: Wall-Jet Turbulence and Mixing Control by Way of a Pulsed Inlet Velocity Cristale Garnica, Bertrand Rollin Attaining skin friction reduction and increasing flow mixing is of utmost importance to enable breakthroughs in fuel efficiency, heat transfer, as well as drag and noise reduction. The turbulent plane wall-jet constitutes a typical flow configuration where turbulence phenomena associated to these engineering applications occur. Direct Numerical Simulations (DNS) are performed to investigate how skin friction and flow mixing are affected by introducing controlled perturbations, jet inlet pulsing, at the shear layer origin. The jet inlet pulse frequencies are varied in time and space to investigate its influence in the downstream domain. The forcing affects the energy of the large-scales which will consequently affect the small-scales allowing turbulence modulation. The interaction between inner and outer layer are of particular interest to understand the effects near the wall. Comparisons of experimental and computational data from previous studies, with and without external perturbation or co-flow, are used to quantify the effect of jet inlet pulsing on wall-jet characteristics, which include eddy production, velocity profiles, maximum velocity decay, half-width jet growth, and coherent structures. [Preview Abstract] |
Tuesday, November 26, 2019 8:24AM - 8:37AM |
Q26.00004: A 2-dimensional--3-component model of turbulent flow over riblets Davide Modesti, Sebastian Endrikat, Ricardo Garcia-Mayoral, Nicholas Hutchins, Daniel Chung Riblets are streamwise-aligned grooves that are designed to reduce drag by modifying the near-wall flow with respect to that of the smooth wall. Nevertheless, drag reduction breaks down when the viscous-scaled square root of the groove area $\ell_g^+>11$, and this breakdown has been attributed to the formation of time-averaged secondary flows over riblets, among other mechanisms. Here we propose to predict these secondary flows by adapting the 2-dimensional--3-component (2D--3C) model of Gayme \textit{et al.} (\textit{J. Fluid Mech.}, vol. 665, 2010, pp. 99--119), in which a sustained turbulent flow is obtained by modelling the incoherent turbulent fluctuations as random forcing. We conduct 2D--3C simulations of flow over several riblet geometries and sizes and compare the results with minimal direct numerical simulations. The 2D--3C model captures the onset and the topology of the secondary flows, suggesting that they are generated by a preferential distribution of near-wall turbulence pinned by the riblet grooves. The model can be used to predict the slip velocity at the riblet crest, providing a better estimate than Stokes (purely viscous) calculations for riblets of moderate sizes, $\ell_g^+<20$. [Preview Abstract] |
Tuesday, November 26, 2019 8:37AM - 8:50AM |
Q26.00005: One-shot methods for nonlinear optimization of turbulent flows with heat transfer Saleh Nabi, Piyush Grover, C. P. Caulfield We consider the optimization of buoyancy-driven flows governed by Boussinesq equations using i) Direct-Adjoint-Looping (DAL), and ii) one-shot methods. Various optimization scenarios are considered: first we solve a series of inverse-design problems for which the global optimal solution is known. We demonstrate that each optimization method is able to retrieve the optimal solution in a fully turbulent regime. Next, we consider the problem of maintaining a desired temperature field with specified input energy budget. The role of an approximate Hessian as a preconditioner as well as tuned step-size for the one-shot method iterations are highlighted. It is shown, by employing an efficient optimization algorithm, the one-shot method can solve the PDE-constrained optimization problem with a cost comparable (about fourfold) to that of the simulation problem alone, and substantially cheaper than using DAL, which requires $\mathcal{O}$ (10) direct-adjoint loops to converge. The optimization results arising from the one-shot method can be used for optimal sensor/actuator placement tasks, or to provide a reference trajectory to be used for online feedback control applications. [Preview Abstract] |
Tuesday, November 26, 2019 8:50AM - 9:03AM |
Q26.00006: Targetted modal turbulent flow control via localized heating Matthew Yao, Duosi Fan, Khaled Younes, Joseph Mouallem, Jean-Pierre Hickey Bidimensional empirical mode decomposition (BEMD) is an empirical method to decompose fluctuating signals into various intrinsic mode functions (IMF); these represent different scales of the turbulent fluctuations. The scale separation flow permits an analysis of their respective contributions towards the overall skin friction of the turbulent boundary layer. We quantify the effects of selective, localized wall heating on the formation and dynamics of turbulence structures at various scales, and consequently, the effect on the skin friction for turbulent flow control. The decomposition is applied to an unheated channel flow and is compared to a channel flow with streamwise aligned heated strips. The strip spacing is dependent on the length scale of the targeted turbulent structures. The individual contribution of the various eddy sizes to the overall skin friction is then calculated and compared to the unheated base case. [Preview Abstract] |
Tuesday, November 26, 2019 9:03AM - 9:16AM |
Q26.00007: Turbulence Control in Pipe Flow by Means of Unsteady Driving Davide Scarselli, Jos\'{e} M. Lopez, Atul Varshney, Bj\"{o}rn Hof Turbulent flows are responsible for huge energy losses in many diverse pumping applications ranging from heat exchange circuits to hydroelectric power plants. Several techniques to reduce frictional drag have been proposed over the last decades, however, very few have been tested experimentally and even less actually implemented. Based on the friction reducing properties observed in accelerating flows, we here propose a new approach to reducing drag by means of a pulsatile flow rate. We find 27\% drag reduction in fully turbulent pipe flow in experiments and this is confirmed in direct numerical simulations. The optimal Reynolds number modulation is discussed. Different from many other drag reduction techniques, this operation mode does not require feedback loops, fluid additives or any modification to an existing pipeline. [Preview Abstract] |
Tuesday, November 26, 2019 9:16AM - 9:29AM |
Q26.00008: Reynolds number e?ect on drag control via spanwise wall oscillation in turbulent channel ?ows. Xi Chen, Jie Yao, Fazle Hussain The e?ect of Reynolds number (Re$_{\mathrm{\tau }})$ on drag reduction (DR) by spanwise wall oscillation is studied through direct numerical simulation of incompressible turbulent channel ?ows with Re$_{\mathrm{\tau \thinspace }}$ranging from 200 to 2000. For the non-dimensional oscillation period T$^{\mathrm{+}} \quad =$ 100 with maximum velocity amplitude A$^{\mathrm{+}}$ $=$ 12, DR decreases from 35.3{\%} at Re$_{\mathrm{\tau }}=$200 to 22.3{\%} at Re$_{\mathrm{\tau }}=$2000. The oscillation frequency $\omega ^{\mathrm{+}}$ for maximum DR slightly increases with Re$_{\mathrm{\tau }}$, viz., from $\omega^{\mathrm{+}}=$0.06 at Re$_{\mathrm{\tau }}=$200 to 0.08 at Re$_{\mathrm{\tau }}=$ 2000, with DR$_{\mathrm{max}}$ $=$ 23.2{\%}. These results show that DR progressively decreases with increasing Re$_{\mathrm{\tau }}$. Turbulent statistics and coherent structures are examined to explain the degradation of drag control e?ectiveness at high Re$_{\mathrm{\tau }}$. FIK analysis in combination with the spanwise wavenumber spectrum of Reynolds stresses reveals that the decreased DR at higher Re$_{\mathrm{\tau \thinspace }}$is due to the weakened e?ectiveness in suppressing the near wall large-scale turbulence, whose contribution continuously increases due to the enhanced modulation and penetration e?ect of the large-scale and very large-scale motions. Based on the power-law model and the log-law model, we predict more than 10{\%} drag reduction at very high Reynolds numbers, say, Re$_{\mathrm{\tau }}=$ 10$^{\mathrm{5}}$.. [Preview Abstract] |
Tuesday, November 26, 2019 9:29AM - 9:42AM |
Q26.00009: Constant power input simulations of drag reduced viscosity stratified turbulent channel flow Oscar Nazarenko, Alessio Roccon, Francesco Zonta, Alfredo Soldati In this work, we analyze the energy budgets of a turbulent channel flow in which a thin layer of fluid is used to lubricate the flow. In particular, we consider a setup in which a thin layer of fluid (viscosity $\eta_1$) is used to lubricate the flow of a thicker fluid layer (viscosity $\eta_2$). The system dynamics is investigated numerically performing DNS of the Navier-Stokes equations coupled with a phase-field method. We consider a single-phase case and three multiphase cases. The multiphase cases differ by the value of the viscosity ratio employed: $\lambda=1.00$ (matched viscosity), $\lambda=0.50$ and $\lambda=0.25$ (less viscous lubricating layer). In order to obtain a more meaningful comparison among the different cases, simulations are performed using a Constant Power Input (CPI) framework and the power Reynolds number is kept fixed to $Re_{\Pi}=12220$ (corresponding in the single-phase reference case to $Re_\tau \simeq 300$). The results show that for all the cases considered, with respect to single-phase reference case, an increase of the thicker layer flow-rate is observed and Drag Reduction (DR) is obtained. The DR is linked to the interplay between inertial, viscous and surface tension forces which lead to an overall reduction of the turbulent dissipation. [Preview Abstract] |
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