Progress in Wall Turbulence 2

Progress in Wall Turbulence 2
Author: Michel Stanislas
Publisher: Springer
Total Pages: 430
Release: 2015-08-17
Genre: Technology & Engineering
ISBN: 3319203886

This is the proceedings of the ERCOFTAC Workshop on Progress in Wall Turbulence: Understanding and Modelling, that was held in Lille, France from June 18 to 20, 2014. The workshop brought together world specialists of near wall turbulence and stimulated exchanges between them around up-to-date theories, experiments, simulations and numerical models. This book contains a coherent collection of recent results on near wall turbulence including theory, new experiments, DNS and modeling with RANS, LES. The fact that both physical understanding and modeling by different approaches are addressed by the best specialists in a single workshop is original.

A Numerical Study of the Effects of Superhydrophobic Surfaces on Skin-friction Drag Reduction in Wall-bounded Shear Flows

A Numerical Study of the Effects of Superhydrophobic Surfaces on Skin-friction Drag Reduction in Wall-bounded Shear Flows
Author: Hyun Wook Park
Publisher:
Total Pages: 117
Release: 2015
Genre:
ISBN:

Recent developments of superhydrophobic surfaces (SHSs) have attracted much attention because of the possibility of achieving substantial skin-friction drag reduction at high Reynolds number turbulent flows. An SHS, consisting of a hydrophobic surface combined with micro- or nano-scaled topological features, can yield an effective slip length on the order of several hundred microns. In this numerical study, direct numerical simulations of turbulent channel flows and turbulent boundary layers (TBLs) developing over SHSs were performed. An SHS was modeled through the shear-free boundary condition, assuming the sustainable gas-liquid interface remained as a flat surface. For the considered Reynolds number ranges and SHS geometries, it was found that the effective slip length normalized by viscous wall units was the key parameter and the effective slip length should be on the order of the buffer layer in order to have the maximum benefit of drag reduction. The effective surface slip length can be interpreted as a depth of influence into which SHSs affect the flow in the wall-normal direction. This result demonstrates that an SHS achieves its drag reduction by affecting the turbulence structures within the buffer layer of wall-bounded turbulent flow. It was also found that the width of an SHS, relative to the spanwise width of near-wall turbulence structures, was also a key parameter to the total amount of drag reduction. Significant suppression of near-wall turbulence structures were observed, which resulted in large skin-friction drag reduction due to the lack of the shear over SHSs. A comparison between TBLs and turbulent channel flows over SHSs were also examined. In contrast to fully developed turbulent channel flows, the effective slip velocity and hence the effective slip length varied in the streamwise direction of TBL, implying that total drag reduction of TBL would depend on the streamwise length of a given SHS. The present numerical study was compared with recent experimental results and showed good agreement. In addition to flow and SHS geometry conditions, the streamwise length of SHSs was also a key factor to understand the underlying physics of wall-bounded shear flows. Finally, it was found that the amount of drag reduction was theoretically estimated as a function of the effective slip length normalized by viscous wall units.

Turbulent Flow Over a Superhydrophobic Surface with Isotropic Slip

Turbulent Flow Over a Superhydrophobic Surface with Isotropic Slip
Author: Babak Vajdi Hokmabad
Publisher:
Total Pages: 107
Release: 2015
Genre: Hydrophobic surfaces
ISBN:

Superhydrophobic surfaces are proven to be capable of reducing the skin friction in laminar and turbulent flows. These surfaces consist of micro/nano-scale hydrophobic roughness features which make the surface render a non-wetting property due to the entrainment of air pockets between the solid surface and the liquid phase. This shear free air-water interface reduces the frictional drag force. This flow control method has two distinct effects in turbulent flow: drag reduction due to effective slip velocity and drag reduction associated with the modification of the turbulent flow structures (Rastegari & Akhavan, 2015). In the current research, the turbulent structure of the inner layer of a turbulent channel flow over a non-wetted superhydrophobic (SHO) surface with random pattern is experimentally studied. The results are compared with the wetted counterpart and also a smooth reference surface. Two planar particle image velocimetry (PIV) measurements are carried out in the streamwise/spanwise and streamwise/wall-normal planes. The vector fields are obtained from both ensemble averaging and individual cross-correlations of double-frame PIV images. The results showed a small increase (~5%) of the mean velocity profile at y+=10 over the non-wetted surface in comparison with the wetted and the smooth surfaces. Up to 15% reduction of normal and shear Reynolds stresses is observed in the inner layer over the non-wetted SHO surface. The wetted SHO counterpart demonstrates no effect on the mean velocity and Reynolds stresses in comparison with the smooth surface implying that the surface is hydrodynamically smooth. A noticeable suppression of the sweep and ejection events, increase of the spanwise spacing of the low and high speed streaks, and attenuation of vortical structures are observed over the non-wetted SHO. These indicate attenuation of the turbulence regeneration cycle due to the slip boundary condition over the non- iii wetted SHO surfaces with random texture. Tomographic PIV (tomo-PIV) and 3D particle tracking velocimetry (3D-PTV) as three-dimensional flow measurement techniques can unravel the relevant physics by revealing the flow modifications across the third dimension. The performance of these measurements is evaluated through comparison with DNS data in the literature. The results show that 3D-PTV is more accurate compared to tomo-PIV especially in near-wall region where noise increases for all PIV measurements.

Advances in Turbulence XII

Advances in Turbulence XII
Author: Bruno Eckhardt
Publisher: Springer Science & Business Media
Total Pages: 973
Release: 2010-03-17
Genre: Science
ISBN: 3642030858

This volume comprises the communications presented at the EUROMECH European Turbulence Conference ETC12, held in Marburg in September 2009. The topics covered by the meeting include: Acoustics of turbulent flows, Atmospheric turbulence, Control of turbulent flows, Geophysical and astrophysical turbulence, Instability and transition, Intermittency and scaling, Large eddy simulation and related techniques, Lagrangian aspects, MHD turbulence, Reacting and compressible turbulence, Transport and mixing, Turbulence in multiphase and non-Newtonian flows, Vortex dynamics and structure, formation, Wall bounded flows.

Advanced Approaches in Turbulence

Advanced Approaches in Turbulence
Author: Paul Durbin
Publisher: Elsevier
Total Pages: 554
Release: 2021-07-24
Genre: Technology & Engineering
ISBN: 0128208902

Advanced Approaches in Turbulence: Theory, Modeling, Simulation and Data Analysis for Turbulent Flows focuses on the updated theory, simulation and data analysis of turbulence dealing mainly with turbulence modeling instead of the physics of turbulence. Beginning with the basics of turbulence, the book discusses closure modeling, direct simulation, large eddy simulation and hybrid simulation. The book also covers the entire spectrum of turbulence models for both single-phase and multi-phase flows, as well as turbulence in compressible flow. Turbulence modeling is very extensive and continuously updated with new achievements and improvements of the models. Modern advances in computer speed offer the potential for elaborate numerical analysis of turbulent fluid flow while advances in instrumentation are creating large amounts of data. This book covers these topics in great detail. - Covers the fundamentals of turbulence updated with recent developments - Focuses on hybrid methods such as DES and wall-modeled LES - Gives an updated treatment of numerical simulation and data analysis

Turbulent Flows

Turbulent Flows
Author: Jean Piquet
Publisher: Springer Science & Business Media
Total Pages: 778
Release: 2001-03-26
Genre: Technology & Engineering
ISBN: 9783540654117

obtained are still severely limited to low Reynolds numbers (about only one decade better than direct numerical simulations), and the interpretation of such calculations for complex, curved geometries is still unclear. It is evident that a lot of work (and a very significant increase in available computing power) is required before such methods can be adopted in daily's engineering practice. I hope to l"Cport on all these topics in a near future. The book is divided into six chapters, each· chapter in subchapters, sections and subsections. The first part is introduced by Chapter 1 which summarizes the equations of fluid mechanies, it is developed in C~apters 2 to 4 devoted to the construction of turbulence models. What has been called "engineering methods" is considered in Chapter 2 where the Reynolds averaged equations al"C established and the closure problem studied (§1-3). A first detailed study of homogeneous turbulent flows follows (§4). It includes a review of available experimental data and their modeling. The eddy viscosity concept is analyzed in §5 with the l"Csulting ~alar-transport equation models such as the famous K-e model. Reynolds stl"Css models (Chapter 4) require a preliminary consideration of two-point turbulence concepts which are developed in Chapter 3 devoted to homogeneous turbulence. We review the two-point moments of velocity fields and their spectral transforms (§ 1), their general dynamics (§2) with the particular case of homogeneous, isotropie turbulence (§3) whel"C the so-called Kolmogorov's assumptions are discussed at length.