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10:45   Fluid-Structure Interaction 1 10:45 15 mins #477 Large-eddy simulation of sparse and dense rigid canopy regimes Mohammad Omidyeganeh, Alessandro Monti, Alfredo Pinelli Abstract: Highly resolved large eddy simulations (LES) of the incompressible, turbulent, open channel flows developing over four different canopy configurations have been carried out. The Reynolds number based on the bulk velocity and the channel height is set to 6,000. The four configurations only differ in value of the frontal area solidity obtained by varying the height of the rigid stems that compose the canopy. A detailed analysis of the four turbulent flows obtained for the different frontal area solidity values allowed to investigate the difference in the flow regimes that, according to the geometrical classification would fall within the so-called sparse, intermediate and dense canopy flow regimes. 11:00 15 mins #623 Comparison of Large Eddy Simulations and wind tunnel experiments of flow above rough surfaces Vladimír Fuka, Klára Jurčáková, Radka Kellnerová Abstract: The turbulent flow above four types of rough surfaces, generated by periodically repeating 1x4x4 mm roughness elements in a wind tunnel with a 250x250 mm crossection and 4m length, is measured using hot wire anemometry (HWA) and particle image velocimetry (PIV) and simulated by large eddy simulation (LES). The wind tunnel has a uniform inflow profile and the boundary layer is spatially developing. The measurements are performed using a high temporal resolution setup to capture the dynamics in the area of the highest interest located 3.4 m from the inlet. The LES is performed using the Extended Large Eddy Microscale Model (ELMM, [1]) with the second order finite difference method and the mixed time scale subgrid model[2]. The influence of the roughness layout on the turbulent boundary layer is examinated. The preliminary comparison of the mean profiles in the test sections shows good correspondence of the magnitudes mean velocity and the Reynolds stress between LES and HWA but the boundary layer height is underestimated. Further analysis will include spectra of the time series in several heights and proper orthogonal decomposition (POD). 11:15 15 mins #154 A NUMERICAL STUDY OF THE SPANWISE TURBULENCE PAST A CYLINDER FLOW Andrea Ferrero, Francesco Larocca, Guglielmo Scovazzi, Massimo Germano Abstract: Many important benchmark turbulent flows are two-dimensional in the mean sense, and are provided with a spanwise direction of homogeneity. The experimental and the numerical study of the turbulence that develops along this direction is not so easy to study. In particular from the computational point of view we have problems due to the limited grid resolution and domain along the spanwise direction. As remarked in [1], in the case of the flow past a square cylinder an insufficient spanwise extent can produce a overprediction of the streamwise and normal turbulent intensities and corrispondingly artificial suppression of the spanwise turbulent intensity. Another important cause of uncertainty as regards in particular the spanwise quantities is the statistical error in the time averages. The aim of our research is to analyze numerically the turbulent flow past a cylinder at a Reynolds number of 3900 with a particular interest in the spanwise turbulence. Many computations and experiments have been done at this particular Reynolds number [2], and the comparison with past data is facilitated. Our database is obtained by means of a compress- ible solver which is run with a low far field Mach number (M ∞ = 0.2) in order to get neglibigle compressibility effects. In particular, a parallel Discontinuous Galerkin finite element spatial discretisation is coupled with a RK2 time integration. The solution inside each element is described by means of a hierarchical orthonormal modal basis following the approach of Bassi et al. [3]. Convective fluxes are evaluated by an approximate Riemann problem solver [4] while diffusive fluxes are computed by means of a recovery based technique [5]. The simulation are performed by integrating the compressible Navier-Stokes equations without the introduction of any explicit subgrid term. For this reason, the simulations performed at low resolution can be seen as implicit Large Eddy Simulations while the computations at higher resolutions tend to Direct Numerical Simulations. The preliminary mesh contains approximately 3 · 10 5 elements. Some preliminary simu- lations have been carried out with second (DG1) and third (DG2) order accurate schemes on the chosen mesh. Since the DG1 and DG2 scheme introduce 4 and 10 modal degrees of freedom per element, these discretisations require 1.2 · 10 6 and 3 · 10 6 degrees of freedom per equation, respectively. Some preliminary results concerning the Reynolds stress tensor (R 11 , R 22 , R 33 ) are reported in Figure 1. They are compared with the available experimental data from [2], and we see a significative streamwise component R 33 . The filtering approach proposed in [6] will be adopted to study the turbulence in the spanwise direction. 11:30 15 mins #474 Analysis of Transient Flows over an NACA0015 Airfoil toward Better Flow Control Authority of Plasma Actuators Takuto Ogawa, Kengo Asada, Tomoaki Tatsukawa, Kozo Fujii Abstract: A dielectric barrier discharge plasma actuator (PA) which control large-scale flow by adding local momentum to flow field is extensively focused improving aerodynamic performances of airfoils. The PA generates plasma which induces flow when alternating current (AC) voltage is applied. The voltage, frequency and waveform of the AC voltage are important factors for the performance of the plasma actuator. Use of unsteady actuations such as ``burst mode actuation'' and ``feed-back control actuation'' changing the parameters in the AC voltage waveform gives better separation control effect as compared to the continuous AC voltage waveforms at moderate angles of attack and low Reynolds numbers. However, as gradually increasing the angle of attack, these actuation methods do not give separation control effect. As increase the angle of attack, a transient flow state, in which attached flows and separated flows alternately occur, is seen. Toward better flow controls, understanding these transient flow phenomena may be necessary and useful. For example, investigating the process from the separated flow to attached flow may help to improve the PA performance when the PA attracts the separated shear layer to the airfoil surface. On the other hand, investigating the process from the attached flow to separated flow may help to improve the PA performance of sustaining the attached flow. Therefore, in the present study, we investigate the transient flows by analysis of the flows obtained by large-eddy simulations (LES) to expand the applicable condition of the PA flow control. The NACA0015 airfoil is used at AoA=14deg, which is the post-stall angle at Re=63,000. The PA is installed at 5% of the chord length from the leading edge of the suction side of the airfoil surface. A high order scheme is employed and the validation of the numerical methods is shown in the several studies. Figure 1 shows the time variation of the lift and drag ratio (L/D). Figure 2 shows the instantaneous flow fields at each time. The streamlines are colored by the velocity component in the vertical direction. The iso-surfaces represent the spanwise second invariant of the velocity gradient tensor (Q*=5) colored by gray. The L/D oscillates between high value and low value. The fluctuation has a periodic cycle (T ~ 15 [tU∞/c]). The flow is massively separated from the leading edge when the L/D becomes the lowest value (T=15). The spanwise vortices advect away from the airfoil surface. Thereafter, the L/D increases gradually. The flow is attached gradually by the large spanwise vortices shedding from the leading edge (T=17.5). The flow is attached over most of the airfoil when the L/D becomes the highest value (T=20). The spanwise vortices are shed quasi- periodically from the leading edge. The time period of the vortex advection is equivalent to the time period of the burst mode actuation. After that, the L/D decreases gradually. The flow gradually separates generating the fine vortices (T=25). The some streamlines strongly bend along the vortices. Finally, the flow is separated once again completely. The obtained results show that the flow condition is strongly affected by the advection of the spanwise vortices. In the full paper, the effect of the vortices on the flow field is investigated by using a network analysis of the vortex structures. 11:45 15 mins #324 Bistability of a Pendulum in a Flow Ariane Gayout, Nicolas Plihon, Martin Obligado, Mickaël Bourgoin Abstract: Simple systems tend to prove themselves much more complex that they seem to be, in particular when they are coupled to turbulence. A clear example of this is the pendulum. Well-understood since the 17th century, yet once coupled with another pendulum or a flow, its behavior can change from the classic harmonic oscillation towards more chaotic responses. A previous thorough study of such a coupling has been done in 2013 by Obligado et al.. The experimental setup, which we have in common, is a simple disk pendulum placed in a wind tunnel. By measuring the equilibrium angle of this pendulum, we can trace back the force balance and get a hang of the angular dependency of the aerodynamical coefficients C_D and C_L. When the pendulum consists of a disk facing the flow, a striking phenomenon occurs where the pendulum presents two stable positions for a certain range of flow velocity determined by its mechanical characteristics. By increasing or decreasing the velocity step by step, we have an overview of the equilibrium landscape. The bistable zone is hysteretic, due to the asymmetry of the associated potential wells, established by the balance between gravity and aerodynamical forces (drag and lift). Interestingly, ambient turbulence and the pendulum own dynamics can trigger transitions from the two stable branches. Our study reports on the investigation of such transitions. They happen in both "laminar" (turbulence rate 0.2%) and low turbulent (turbulence rate 2%) environments. By increasing the turbulence rate up to 20\%, the bistability region (hence the transitions) reduces as far as to disappear, thus suggesting that ambient turbulence may have a significant influence on the aerodynamical properties of the disk, independently from the Reynolds number, simply based on mean flow velocity and disk diameter. The influence of the turbulence is also clear on the statistics of the transitions. An automated experiment to study the transitions over long time scale has enabled us to get statistically well-resolved escape-time distributions as well as an overview on the waiting time dynamics. Considering that, the variations of angular position of the pendulum are a direct visualization of the fluctuations of the aerodynamical (lift and drag) coefficients, the influence of the ambient turbulence is especially visible on the extreme events. Counterintuitively, we observe that these events are of great amplitude in laminar flow, while there are almost none of them at moderate turbulence. It might be the expression of a competition between vortex shedding and turbulence in dictating the dynamics of the pendulum. 12:00 15 mins #259 Simulation of Thin & Long Flexible Objects in a Turbulent Flow Daniel Meyer, Marco Hostettler Abstract: We focus on the simulation of elongated flexible structures in a turbulent flow. The structures are assumed to be thin with a thickness much smaller than the smallest turbulent eddies. The structures have, on the other hand, lengths that are comparable in size with turbulent length-scales. Possible application examples include strings, ropes, or long fibers in turbulence. Similar to small inertial particles, these structures induce considerable computational costs with their surrounding boundary layers properly resolved. Alternatively, zero-dimensional point particles are typically applied for the simulation of particle-laden flows. There, the fluid/particle momentum exchange is represented or modeled by Stokes drag-forces that are determined based on seen or undisturbed fluid velocities at particle locations. In the case of elongated structures, the resolution requirements become even more demanding, as is illustrated for example by the two-dimensional simulations reported in [Zhao et al., Women in Mathematical Biology, 8, Springer, 2017]. There, an immersed boundary method was applied to represent the fluid/structure coupling of spider silk in two-dimensional laminar flows. In the present work, similar to the cited point-particle techniques, we propose a less expensive alternative, being comprised of a one-dimensional structure model with transverse/longitudinal cylinder-based drag-models. Moreover, we account for structural deformation such as stretching and bending. In order to test the accuracy of the applied physical models and the numerical methods, we present validation results. Eventually we investigate the motion and deformation of spider silk attached to a point-like spider body, both immersed in a turbulent flow. Our investigation sheds light on the ballooning displacement of small spiders, which was recently studied experimentally [Cho et al., PLOS Biology, 16(6), 2018]. 12:15 15 mins #88 Turbulent wake of a freely rotating disk in a uniform flow: experiments and stochastic modelling Olivier Cadot, Edouard Boujo Abstract: The fluid-structure interaction between a thin circular disk and its turbulent wake is investigated experimentally and described with a low-order stochastic model. The disk faces a uniform flow at Reynolds number $Re=$133 000 and can rotate around one of its diameters (Fig.~\ref{fig:experiment}). It is equipped with instantaneous pressure measurements to give the aerodynamic loading; moments and centre of pressure of the front and base sides. % When the disk is fixed and aligned, symmetry-breaking vortex shedding is observed and all orientations are visited with equal probability, yielding axisymmetric long-term statistics. Very-low-frequency antisymmetric modes as recently observed by Rigas \textit{et al.} (\textit{J. Fluid Mech.}, 2014) %\cite{Rigas_2014-JFM} are not observed unless the disk shape is modified into that of a bullet, showing the importance of the separation angle. It suggests that these modes are linearly stable for the disk while they are unstable for the bullet shape. % When the disk is fixed and inclined, the base center of pressure (CoP) shifts preferentially towards one side of the rotation axis. % Finally, when the disk is free to rotate, the front moment is proportional to the disk inclination (aerodynamic stiffness), while the base moment subject to large fluctuations is well correlated with the disk inclination (Fig.~\ref{fig:PDFs}$a$). % From these observations, a low-order model is derived that couples (i)~the CoP, forced stochastically by the turbulent flow, and (ii)~the disk inclination, as a linear harmonic oscillator forced by the base moment. The model captures well the main statistic and dynamics features observed experimentally (Fig.~\ref{fig:PDFs}$b$). One difference lies in the distribution of the disk inclination (which features exponential tails in the measurements but is Gaussian in the model), suggesting a possible refinement with non-linearities or time delays. % The model is expected to capture coupled dynamics in other systems involving turbulent wakes behind freely rotating bluff bodies.