13:30
Control 4
Chair: Bettina Frohnapfel
13:30
15 mins
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Turbulent drag reduction by hydrophobic surfaces with shear-dependent slip length
Sohrab Khosh Aghdam, Pierre Ricco, Mehdi Seddighi
Abstract: The stabilisation of a parabolic equilibrium profile in a three-dimensional (3D) turbulent channel flow for an incompressible fluid is addressed with the objective of achieving drag reduction. The formulation of this problem stems from Balogh’s work [1] where Lyapunov stability analysis was used to devise prototype feedback laws and prove global stability of the solutions. This treatment only considers the controller as a mathematical artefact, but it can actually be linked to physical control strategies modelling hydrophobic surfaces and porous media. In the former, only linear slip velocity boundary conditions (BC) were considered [8]. However, experiments [2] have suggested that the slip length may be shear-dependent. Motivated by these, the effect on drag reduction of a shear-dependent slip length surface is examined in the present study using Direct Numerical Simulations (DNS) at Re τ0 = u τ0 δ/ν ≃ 180. δ is the channel half height, u τ0 the wall-shear velocity for regular no-slip walls channel and ν the kinematic viscosity. The theoretical analysis in [5], is extended to this new model. The proposed formulation shows that the skin-friction coefficient can be reduced by tuning the parameters in the shear-dependent slip length model. The results, which verified by DNS simulations, show that by taking a slip length
value based on a constant slip model [8] and combining it within a shear-dependent model, up to 50% drag reduction can be obtained. The effect of control is further assessed by formulating the Fukagata identity [4] with general boundaries; the weighted Reynolds shear-stress for each quadrant shows an enhanced reduction in the sweep/ejection events compared to the constant slip model.
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13:45
15 mins
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Identification of Variations of Angle of Attack and Lift Coefficient for a Large Horizontal-Axis Wind Turbine
Abdolrahim Rezaeiha, Maziar Arjomandi, Marios Kotsonis, Martin O.L. Hansen
Abstract: The current paper investigates the effects of various elements including turbulence, wind shear, yawed inflow, tower shadow, gravity, mass and aerodynamic imbalances on variations of angle of attack and lift coefficient for a large horizontal-axis wind turbine. It will identify the individual and the aggregate effect of elements on variations of mean value and standard deviation of the angle of attack and lift coefficient in order to distinguish the major contributing factors. The results of the current study is of paramount importance in the design of active load control systems for wind turbine.
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14:00
15 mins
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Decay of turbulence at high Reynolds numbers
Michael Sinhuber, Gregory P. Bewley, Eberhard Bodenschatz
Abstract: Using the unique capabilities of the Variable Density Turbulence Tunnel
at the Max Planck Institute for Dynamics and Self-Organization, we investigated
virtually homogeneous and isotropic grid turbulence over a wide range of Reynolds numbers,
$Re = UM/\nu$, between $10^4$ and $5\cdot 10^6$.
The choice of pressurizable Sulfur Hexafluoride as a working gas
makes it possible to reach extremely high Reynolds numbers without changing boundary conditions.
Indeed, the Reynolds number we reached were higher than any previous classical grid wind-tunnel experiment.
In this talk, we focus on the fundamental question of how fast turbulent energy decays
once it has been created, and
show that the Reynolds number plays no important role in setting the decay rate if it is high enough.
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14:15
15 mins
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Customized turbulent flow fields
Nico Reinke, Michael Hölling, Joachim Peinke
Abstract: A new approach is shown, which describes the interaction between an active grid excitation and a wind tunnel flow. With this approach we are able to find an excitation for the grid which reproduces an any turbulent flow field. Thereby we can bring free field measurements inside the wind tunnel. The presentation will show how the approach works and comparisons between reference data sets (outside measurements by ultrasonic anemometer) and wind tunnel flow fields generated by an active grid.
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14:30
15 mins
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Experimental application of a dynamic observer to capture and predict the dynamics of a flat-plate boundary layer
Eliott Varon, Juan Guzman Inigo, Denis Sipp, Peter Schmid, Jean-Luc Aider
Abstract: The recent approach, proposed by Guzman-Inigo et al. \cite{GuzmanInigo2014}, using System Identification to derive a Reduced Order Model from snapshots of a flow is applied to a transitional boundary layer growing over a flat-plate. It is shown that such an approach can indeed be applied to experimental PIV snapshots. Using a proper learning dataset and a proper local sensor, it is shown that the evolution of boundary layer can be properly estimated from the time evolution of the local probe and with no more than ten POD modes for the Reduced Order Model. The influence of the various parameters on the efficiency of the system identification technique is discussed.
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14:45
15 mins
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SHAPE OPTIMIZATION OF THE MAXIMIZING PROBLEM OF THE DISSIPATION ENERGY AND ITS EFFECT ON HYDRODYNAMIC STABILITY
Takashi Nakazawa
Abstract: This paper presents a numerical result for generalized eigenvalue problems on the optimum shape using the numerical method for a flow-field shape optimization problem. The main shape optimization problem addressed in this paper is defined as a two-dimensional lid-driven cavity flow. As an objective cost function, we use the dissipation energy. The domain volume is used as a constraint cost function. The shape derivative of the objective cost function with respect to the domain variation is evaluated using the solution of the main problem and the adjoint problem. Numerical schemes used to conduct the shape optimization problem using an iterative algorithm based on the traction method for reshaping are presented, where the shape of the boundaries aside the top boundary is optimized. Furthermore, by operating a generalized eigenvalue problem, linear neutral curves between the initial domain Ω0 and the optimum shape domain Ω1 are compared. Numerical results reveal that the shape is optimized by satisfying the volume constraint. Based on generalized eigenvalue problems, the critical Reynolds number of the optimum shape Ω1 is larger than that of the initial shape Ω0.
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15:00
15 mins
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DIRECT NUMERICAL SIMULATIONS OF DRAG REDUCTION IN TURBULENT CHANNEL FLOW OVER BIO-INSPIRED HERRINGBONE RIBLET TEXTURE
Henk Benschop, Jerry Westerweel, Wim-Paul Breugem
Abstract: The use of drag reducing surface textures is a promising passive method to reduce fuel consumption. Probably most wellknown
is the utilisation of shark-skin inspired ridges or riblets parallel to the mean flow. They can reduce drag up to 10%. Recently
another bio-inspired texture based on bird flight feather riblets has been proposed. It differs from the standard riblets in two ways.
First, the riblets are arranged in a converging/diverging or herringbone pattern. Second, the riblet height or groove depth changes
gradually. Drag reductions as high as 20% have been claimed [2]. The objective of the present work is to study the drag reducing properties and mechanisms of this texture. To that purpose Direct Numerical Simulations (DNSs) of turbulent plane channel flow have been performed. Structured roughness has been applied to both walls and several geometric parameters have been varied. Marginal drag reductions on the order of 2.5% and significant drag increases well beyond 100% were found. The latter is attributed to a strong secondary flow that mixes momentum through the whole channel. In future optimization studies we might look for conditions at which secondary motions affect the near-wall cycle of turbulence only.
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