15th European Turbulence Conference 2015
August 25-28th, 2015, Delft, The Netherlands

## Invited speakers:

Prof. Marc Brachet. Ecole Normale Superieure, Paris, France

Prof. Peter G. Frick, Institute of Continuous Media Mechanics, Perm, Russia

Prof. Bettina Frohnapfel,  Karlsruher Institut fur Technology, Germany

Prof. Andrea Mazzino, Dipartimento di Fisica, University of Genova, Italy

Prof. Bernhard Mehlig. Department of Physics, University of Gothenburg, Sweden

Prof. Lex Smits, Mechanical and Aerospace Engineering, Princeton University, USA

Prof. Chao Sun Physics of Fluids, University of Twente, The Netherlands

Prof. Steve Tobias, Applied Mathematics, University of Leeds, United Kingdom

 10:30 15 mins Investigation of a flow field generated by a fractal grid based on experimental data and CFD simulations André Fuchs, Wided Medjroubi, Nico Reinke, Gerd Guelker, Joachim Peinke Abstract: Fractal grids generate turbulence by directly exciting many length-scales of different sizes simultaneously, rather than using the nonlinear cascade mechanism to obtain multiscale excitation, as it is the case for classical grids. These scales influence each other and show very different properties compared to all previously documented turbulent flows. In this work we present experimental wind tunnel and computer fluid dynamics (CFD) studies of the turbulent flow generated by a fractal grid under the same conditions. We did an extensive statistical study and a direct comparison between the experimentally and numerically acquired time series in order to investigate and compare one-point- and two-point-statistics. In addition we present an application of a stochastic method, so-called Langevin approach, to the experimentally and numerically acquired velocity increment time series to examine three-point-statistics in terms of Kramers-Moyal coefficients. 10:45 15 mins Reynolds number effect on turbulent drag reduction Davide Gatti, Maurizio Quadrio, Bettina Frohnapfel Abstract: An analytic relationship that predicts the Reynolds number effect on turbulent drag reduction by active means is developed in analogy with riblets. It is applicable to all control techniques whose action result in an upward shift ∆B of the logarithmic region of the turbulent velocity profile. In particular, we use it to address the Re-effect affecting streamwise-traveling waves of spanwise wall velocity \cite{quadrio-ricco-viotti-2009}, aided by a new large dataset of Direct Numerical Simulations of turbulent channel flows at increasing Re. The main outcome of this study is that the control-induced upward-shift of the logarithmic region ∆B does not vary with Re along a large part of the wave parameter space, also where high drag reduction is achieved. Here, the analytical relationship allows to extrapolate low-Re drag reduction information to high-Re flows. In the narrow regions where ∆B does vary with Re, an additional Re-effect is deemed to exist, which depends on the present control technique only and which is investigated with a three-dimensional phase conditional averaging procedure. 11:00 15 mins Turbulent drag reduction by traveling waves of spanwise forcing Maurizio Quadrio, Wenxuan Xie Abstract: Several techniques based on spanwise focing were introduced in the last decade \cite{du-symeonidis-karniadakis-2002, quadrio-ricco-viotti-2009} in the form of traveling waves of spanwise forcing as generalization of the spanwise-wall oscillation technique to reduce skin-friction drag in a turbulent channel flow. Here we examine all their variants (including one that has never been considered before), by addressing the type of forcing (wall movement versus body force) as well as the traveling direction (streamwise versus spanwise) of the waves. We carry out a DNS-based study within an unified framework, to compare their capability to reduce skin-friction drag and, more importantly, net energy savings. The present results confirm the potential for drag reductions for every considered forcing. The best-performing spanwise traveling wave, in terms of either drag reduction or net energy saving, is found to be the one with infinite wavelength, i.e. still the spanwise wall oscillation. The streamwise-traveling waves consistently offer the best performance, especially in terms of net savings. The conditions under which body-force based control can be meaningfully compared to wall based control are discussed. 11:15 15 mins Stochastic analysis of the effects of inlet velocity conditions on the evolution of spatially evolving mixing layers Marcello Meldi, Maria Vittoria Salvetti, Pierre Sagaut Abstract: The sensitivity of the evolution of the spatially evolving mixing layer test case to inlet conditions parameters is studied in the present work. The analysis is performed combining DNS simulations and a response surface stochastic approach. The angle between the two asymptotic streams $\theta$ and the parameter $\alpha = (U1 - U2)/(U1 + U2)$ are identified as parameters of interest and they are considered as random variables. The uncertainty propagated over the output quantities, such as the average velocity $U$, is then investigated. The analysis of the results indicates that different physical regimes can evolve from the initial conditions investigated. In particular, non linear effects streaming from interactions between $\theta$ and $\alpha$ are observed. These effects are not captured by classical DNS analyses of the spatially evolving mixing layer test case 11:30 15 mins Experimental study of surface modification in a fully turbulent Taylor-Couette flow Arnoud Greidanus, Rene Delfos, Sedat Tokgoz, Jerry Westerweel Abstract: Friction measurements were performed in a Taylor-Couette setup. Drag reduction was obtained with a riblet surface and indicated a drag reduction for a wide range of shear Reynolds numbers, with a maximum of 5.3% at Re_s=47000 (s+=14). Tomographic PIV verified that the friction coefficients are strongly related to the flow regimes and structures. The bulk fluid rotation was changed by the application of the riblets, as the wall-bounded flow conditions at the inner cylinder wall were changed due to the surface modification and is called the rotation effect. A simple model was used to indicate the averaged bulk velocity shift (1.4%), after which the drag changes due to the rotation effect (-1.9%) and the riblet effect (-3.4%) were determined. The bulk velocity shift of 1.4% was verified by PIV measurements. Compliant surfaces will be further investigated to check their required conditions for drag reduction of wall-bounded flows 11:45 15 mins Massive turbulent separation: an investigation of shear-layer interfaces. Francesco Stella, Nicolas Mazellier, Azeddine Kourta Abstract: The massively separated flow behind a backward-facing ramp is investigated using particle image velocimetry, in the perspective of designing active separation control. Beyond the usual one-point statistical analysis, we address the issue of the characteristic length-scales corrugating the interfaces featuring the turbulent separation. Our preliminary results evidence the multi-scale nature of these interfaces. This fact might hold promise for new sizing criteria for fluidic actuators intended to alleviate separation effects. 12:00 15 mins Turbulent skin-friction drag reduction by travelling waves induced by spanwise Lorentz force Qiang Yang, Yongmann Chung Abstract: The streamwise and spanwise travelling waves induced by spanwise Lorentz force are studied for skin-friction drag reduction in a turbulent channel. The streamwise travelling wave by spanwise Lorentz force on drag reduction is compared to the with the spanwise wall motion. The drag reduction map shows a drag reduction region and a drag increase region, depending on a time scale $\mathscr{T}=\lambda/(\mathscr{U}_c-\omega/\kappa)$. For spanwise travelling wave, a large drag reduction appears at large oscillation frequencies and small spanwise wave numbers, while all stationary wave cases give a drag increase. When the wave travels at an oblique angle to the streamwise mean flow, the optimal drag reduction appears in backward travelling wave case. Generally, the backward streamwise travelling wave is found to be most efficient in drag reduction among all oblique travelling waves. Spanwise oscillation, forward streamwise travelling, spanwise travelling and backward streamwise travelling wave cases share a similar drag reduction mechanism: first, the spanwise motion directly breaks the near wall quasi-streamwise vortices structure array \cite{Jeong_etal1997}, which results in the shortening of streamwise streaks; second, the spanwise velocity layer maintains the asymmetry of the positive and negative quasi-streamwise vortices, which leads to a sustained drag reduction. 12:15 15 mins On the base pressure of 3D turbulent bluff body wakes with sharp separation Antoine Evrard, Olivier Cadot, Vincent Herbert, Denis Ricot, Remi Vigneron, Tony Ruiz, Fabien Harambat Abstract: Particle Image Velocimetry (PIV) and pressure measurements are used to study the turbulent wake of Ahmed bluff body. A cavity on the base is created to control the base pressure and modify the recirculating bubble and its equilibrium.