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

Powered by
© Fyper VOF
Conference Websites
15:00   Instability and Transition 2
Chair: Geert Brethouwer
15 mins
Taylor-Couette flow with asymmetric end-walls boundary conditions
Torsten Seelig, Kamil Kielczewski, Ewa Tuliszka-Sznitko, Uwe Harlander, Christoph Egbers, Patrick Bontoux
Abstract: In the paper the authors present the results obtained during a numerical (Direct Numerical Simulation/Spectral Vanishing Viscosity method - DNS/SVV) and experimental investigations (Kalliroscope, PIV) of the Taylor-Couette flow with asymmetric boundary conditions. In the paper attention is focused on the laminar-turbulent transition process. The main purpose of the research is to investigate the influence of different parameters (aspect ratio, curvature parameter, end-walls boundary conditions) on the flow structure and on the flow characteristics. The transverse current Jω is computed from the velocity field obtained numerically. The λ2 criterion has been used for numerical visualization.
15 mins
Reduced modeling of transitional exact coherent states in shear flow
Cedric Beaume, Greg Chini, Keith Julien, Edgar Knobloch
Abstract: In parallel shear flows, the lower branch solution follows simple streamwise dynamics. A decomposition of this solution into Fourier modes in this direction yields modes whose amplitudes scale with inverse powers of the Reynolds number. We use this scaling to derive a reduced model for exact coherent structures in general parallel shear flows. The reduced model is regularized by retaining higher order viscous terms. Both lower branch and upper branch solutions are captured and studied.
15 mins
Nonlinear dynamics of large-scale coherent structures in free shear layers
Xuesong Wu, Xiuling Zhuang
Abstract: It is well known that fully developed turbulent free shear layers exhibit a high degree of order, characterized by large-scale coherent structures, i.e. spanwise vortex rollers. Extensive experimental investigations show that such organised motions bear remarkable resemblance to inviscid instability waves, and their main characteristics, including the length scales, propagation speeds and transverse structure, are reasonably well predicted by inviscid linear stability analysis of the mean flow. In this paper, we present a mathematical theory to describe the nonlinear dynamics of coherent structures. The theory is adapted from the nonlinear non-equilibrium critical-layer approach for laminar-flow instabilities by accounting for (a) the enhanced non-parallelism associated with fast spreading of the mean flow, and (b) the influence of small-scale turbulence on coherent structures. The combination of these factors with nonlinearity leads to an interesting evolution system, consisting of the coupled amplitude and vorticity equations, in which non-parallelism contributes the so-called translational critical-layer effect. Numerical solutions of the evolution system captures vortex roll-up, which is the hallmark of turbulent mixing layer, and the predicted amplitude development closely mimics what was measured in experiments. Key words: turbulence, coherent structures, instability, nonlinearity
15 mins
Andrea Sansica, Neil David Sandham, Zhiwei Hu
Abstract: Direct numerical simulations are carried out for different forcing techniques to trigger transition during the interaction between an oblique shock-wave and a laminar boundary-layer at M = 1.5. Three forcing methods are used: a) forcing of oblique unstable modes, whose shape and behaviour are determined by the local linear stability theory, b) broadband free-stream acoustic disturbances, and c) a cold plasma flow control device. While the oblique-mode breakdown is dominant for low-amplitude forcing, long streaky structures drive the transition process in a high-amplitude disturbance environment. LES are also performed on the experimental setup by the Institute of Theoretical and Applied Mechanics (ITAM) from Novosibirsk State University with cold plasma actuation. As well as the disturbance type, the effect of Reynolds number and forcing amplitude will be investigated.
15 mins
Flow regimes of inertial suspensions of finite size particles
Iman Lashgari, Francecso Picano, Wim-Paul Breugem, Luca Brandt
Abstract: Inertial regimes in a channel flow of suspension of finite-size neutrally buoyant particles are studied for a wide range of Reynolds numbers: $500 \le Re\le 5000$, and particle volume fractions: $0 \le \Phi \le 0.3$. The flow is classified in three different regimes according to the phase-averaged stress budget across the channel \cite{Lashgari2014}. The laminar viscous regime at low $Re$ and $\Phi$ where the viscous stress is the dominating term in the budget, the turbulent regime at high $Re$ and relatively low $\Phi$ where the momentum is mainly transferred by the action of the Reynolds stress and the inertial shear-thickening regime where the particle stress contributes the most to the significant enhancement of the wall shear stress. Particle distribution and dispersion properties provide additional evidence for the existence of the three different regimes.
15 mins
Shashank H J, Sreenivas K R
Abstract: The effect of drag reducing polymers on the onset of instability in a pipe with reverse flow is investigated. Reverse flow is established by using a piston cylinder mechanism, the programmed motion of which imparts a known impulse to the fluid. An inflection point is formed, leading to flow instability above a critical Reynolds number. The effect of the drag reducing polymers (Poly-ethylene oxide, dissolved in water) is observed using dye visualization and PIV. The time of onset of instability and the wavelength of the instability are studied in systems with and without Poly-ethylene Oxide (PEO). The experiments show that the wavelength increases in systems with PEO when compared to water. The flow due to forward motion of the piston is solved analytically to obtain the velocity profiles after the piston stops. Stability analysis is also performed to compare the behaviour of systems with and without PEO.
15 mins
An Investigation of Transitional Phenomena from Laminar to Turbulent Natural Convection using Compressible Direct Numerical Simulation
ChungGang Li, Tsubokura Makoto
Abstract: The transitional phenomena from laminar to turbulent natural convection and the development process inside the channel are investigated using compressible direct numerical simulation (DNS). Numerical method of Roe scheme with preconditioning and dual time stepping are used for addressing natural convection flows with large temperature differences, which are low speed but the densities are variable. The results are qualitatively well consistent with the experimental data [1] and the transition point can be accurately captured. In addition, the development process respected to time can be clearly identified for four stages, which are laminar, unstable process, reliminarization and turbulence. After reaching the quasi-steady state, it can be observed that the laminar, transition and turbulence coexist in the same flow filed. Most important of all, the transitional phenomena are naturally induced by the effects of interactions between the buoyancy and shear stress without adding any fluctuations at inlet. It means that the numerical scheme and physical model adopted in this study has the potential to be a universal case for estimating the accuracy of turbulence model because the characteristics of parameters-free and independence from inlet condition.
15 mins
Yunfei Zhao, Wei Liu, Xiaoliang Yang, Shihe Yi, Xiaogang Deng
Abstract: The laminar-turbulent transition of a supersonic flat-plate boundary layer with isolated roughness element is investigated both numerically and experimentally. Experiments are conducted in a Mach 3 low-noise wind tunnel for three different roughness heights of 1mm, 2mm and 4mm respectively. The flow structures in the transitional boundary layer are measured by a nano-based planar laser scattering (NPLS) flow visualization technique. Calculations are implemented in the same wind tunnel conditions using both second-order scheme and fifth-order weighted compact nonlinear scheme (WCNS-E-5) for comparison. Good agreements are achieved between experimental data and high-order solutions, including the turbulent boundary layer structures and quantitative pressure distribution along the plate centerline. However, the second-order scheme is found to be too dissipative to resolve the unsteadiness and small-scale structures in the transitional flow field. It is observed that the shear layer instability appears to be the leading mechanism for transition to turbulence in the wake of roughness element. With increasing height of roughness, the shear layer breaks up earlier and the transition tends to move forward.