Self-similar decay and mixing of a high-Schmidt-number passive scalar in an oscillating boundary layer in the intermittently turbulent regime

2013 ◽  
Vol 726 ◽  
pp. 338-370 ◽  
Author(s):  
Carlo Scalo ◽  
Ugo Piomelli ◽  
Leon Boegman
Keyword(s):  
Boundary Layer ◽  
Scalar Field ◽  
Equilibrium State ◽  
Turbulent Mixing ◽  
Schmidt Number ◽  
Wall Region ◽  
Turbulent Regime ◽  
High Schmidt Number ◽  
Oscillating Boundary ◽  
Non Equilibrium

AbstractWe performed numerical simulations of dissolved oxygen (DO) transfer from a turbulent flow, driven by periodic boundary-layer turbulence in the intermittent regime, to underlying DO-absorbing organic sediment layers. A uniform initial distribution of oxygen is left to decay (with no re-aeration) as the turbulent transport supplies the sediment with oxygen from the outer layers to be absorbed. A very thin diffusive sublayer at the sediment–water interface (SWI), caused by the high Schmidt number of DO in water, limits the overall decay rate. A decomposition of the instantaneous decaying turbulent scalar field is proposed, which results in the development of similarity solutions that collapse the data in time. The decomposition is then tested against the governing equations, leading to a rigorous procedure for the extraction of an ergodic turbulent scalar field. The latter is composed of a statistically periodic and a steady non-decaying field. Temporal averaging is used in lieu of ensemble averaging to evaluate flow statistics, allowing the investigation of turbulent mixing dynamics from a single flow realization. In spite of the highly unsteady state of turbulence, the monotonically decaying component is surprisingly consistent with experimental and numerical correlations valid for steady high-Schmidt-number turbulent mass transfer. Linearly superimposed onto it is the statistically periodic component, which incorporates all the features of the non-equilibrium state of turbulence. It is modulated by the evolution of the turbulent coherent structures driven by the oscillating boundary layer in the intermittent regime, which are responsible for the violent turbulent production mechanisms. These cause, in turn, a rapid increase of the turbulent mass flux at the edge of the diffusive sublayer. This outer-layer forcing mechanism drives a periodic accumulation of high scalar concentration levels in the near-wall region. The resulting modulated scalar flux across the SWI is delayed by a quarter of a cycle with respect to the wall-shear stress, consistently with the non-equilibrium state of the turbulent mixing.

scholarly journals Lagrangian wall shear stress structures and near-wall transport in high-Schmidt-number aneurysmal flows

2016 ◽  
Vol 790 ◽  
pp. 158-172 ◽  
Author(s):  
Amirhossein Arzani ◽  
Alberto M. Gambaruto ◽  
Guoning Chen ◽  
Shawn C. Shadden
Keyword(s):  
Boundary Layer ◽  
Mass Transfer ◽  
Shear Stress ◽  
Vector Field ◽  
Wall Shear Stress ◽  
Residence Time ◽  
Vessel Wall ◽  
Schmidt Number ◽  
High Schmidt Number ◽  
Near Wall

The wall shear stress (WSS) vector field provides a signature for near-wall convective transport, and can be scaled to obtain a first-order approximation of the near-wall fluid velocity. The near-wall flow field governs mass transfer problems in convection-dominated open flows with high Schmidt number, in which case a flux at the wall will lead to a thin concentration boundary layer. Such near-wall transport is of particular interest in cardiovascular flows whereby haemodynamics can initiate and progress biological events at the vessel wall. In this study we consider mass transfer processes in pulsatile blood flow of abdominal aortic aneurysms resulting from complex WSS patterns. Specifically, the Lagrangian surface transport of a species released at the vessel wall was advected in forward and backward time based on the near-wall velocity field. Exposure time and residence time measures were defined to quantify accumulation of trajectories, as well as the time required to escape the near-wall domain. The effect of diffusion and normal velocity was investigated. The trajectories induced by the WSS vector field were observed to form attracting and repelling coherent structures that delineated species distribution inside the boundary layer consistent with exposure and residence time measures. The results indicate that Lagrangian WSS structures can provide a template for near-wall transport.

scholarly journals Recovery of wall-shear stress to equilibrium flow conditions after a rough-to-smooth step change in turbulent boundary layers

2019 ◽  
Vol 872 ◽  
pp. 472-491 ◽  
Author(s):  
Mogeng Li ◽  
Charitha M. de Silva ◽  
Amirreza Rouhi ◽  
Rio Baidya ◽  
Daniel Chung ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Equilibrium State ◽  
Smooth Transition ◽  
Wall Region ◽  
Smooth Wall ◽  
Equilibrium Conditions ◽  
Buffer Region ◽  
Wall Shear

This paper examines the recovery of the wall-shear stress of a turbulent boundary layer that has undergone a sudden transition from a rough to a smooth surface. Early work of Antonia & Luxton (J. Fluid Mech., vol. 53, 1972, pp. 737–757) questioned the reliability of standard smooth-wall methods for measuring wall-shear stress in such conditions, and subsequent studies show significant disagreement depending on the approach used to determine the wall-shear stress downstream. Here we address this by utilising a collection of experimental databases at $Re_{\unicode[STIX]{x1D70F}}\approx 4100$ that have access to both ‘direct’ and ‘indirect’ measures of the wall-shear stress to understand the recovery to equilibrium conditions of the new surface. Our results reveal that the viscous region ($z^{+}\lesssim 4$) recovers almost immediately to an equilibrium state with the new wall conditions; however, the buffer region and beyond takes several boundary layer thicknesses before recovering to equilibrium conditions, which is longer than previously thought. A unique direct numerical simulation database of a wall-bounded flow with a rough-to-smooth wall transition is employed to confirm these findings. In doing so, we present evidence that any estimate of the wall-shear stress from the mean velocity profile in the buffer region or further away from the wall tends to underestimate its magnitude in the near vicinity of the rough-to-smooth transition, and this is likely to be partly responsible for the large scatter of recovery lengths to equilibrium conditions reported in the literature. Our results also reveal that smaller energetic scales in the near-wall region recover to an equilibrium state associated with the new wall conditions within one boundary layer thickness downstream of the transition, while larger energetic scales exhibit an over-energised state for several boundary layer thicknesses downstream of the transition. Based on these observations, an alternative approach to estimating the wall-shear stress from the premultiplied energy spectrum is proposed.

The geometric properties of high-Schmidt-number passive scalar iso-surfaces in turbulent boundary layers

2007 ◽  
Vol 588 ◽  
pp. 253-277 ◽  
Author(s):  
L. P. DASI ◽  
F. SCHUERG ◽  
D. R. WEBSTER
Keyword(s):  
Scalar Field ◽  
Boundary Layers ◽  
Schmidt Number ◽  
Passive Scalar ◽  
Turbulent Boundary Layers ◽  
Length Scale ◽  
Geometric Properties ◽  
Convective Regime ◽  
High Schmidt Number ◽  
Kolmogorov Scale

The geometric properties are quantified for concentration iso-surfaces of a high-Schmidt-number passive scalar field produced by an iso-kinetic source with an initial finite characteristic length scale released into the inertial layer of fully developed open-channel-flow turbulent boundary layers. The coverage dimension and other measures of two-dimensional transects of the passive scalar iso-surfaces are found to be scale dependent. The coverage dimension is around 1.0 at the order of the Batchelor length scale and based on our data increases in a universal manner to reach a local maximum at a length scale around the Kolmogorov scale. We introduce a new parameter called the coverage length underestimate, which demonstrates universal behaviour in the viscous–convective regime for these data and hence is a potentially useful practical tool for many mixing applications. At larger scales (in the inertial–convective regime), the fractal geometry measures are dependent on the Reynolds number, injection length scale, and concentration threshold of the iso-surfaces. Finally, the lacunarity of the iso-surface structure shows that the instantaneous scalar field is most inhomogenous around the length scale corresponding to the Kolmogorov scale.

Investigation of Turbulent Boundary Layer Flow Over 2D Bump Using Highly Resolved Large Eddy Simulation

2011 ◽  
Vol 133 (11) ◽  
Author(s):  
Dalibor Cavar ◽  
Knud Erik Meyer
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Boundary Layer Flow ◽  
Wall Region ◽  
Internal Layers ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Layer Flow ◽  
Near Wall ◽  
Non Equilibrium

A large eddy simulation (LES) study of turbulent non-equilibrium boundary layer flow over 2D Bump, at comparatively low Reynolds number Reh=U∞h/ν=1950, was conducted. A well-known LES issue of obtaining and sustaining turbulent flow inside the computational domain at such low Re, is addressed by conducting a precursor calculation of the spatially developing boundary layer flow. Those results were subsequently used as turbulent inflow database for the main non-equilibrium boundary layer flow computation. The Sagaut (Rech. Aero., pp. 51–63, 1996) sub grid scale (SGS) turbulence model, based on a local estimate of the subgrid scale turbulent kinetic energy ksgs and implicit damping of turbulent SGS viscosity νt(sgs) in the near-wall region, was selected as a suitable basis for the present LES computations due to the fact that block structured MPI parallelized CFD code used in the current computations did not provide a direct possibility for wall-damping of, e.g., the Smagorinsky constant in the near-wall region. The grid utilized in the main calculation consisted of approximately 9.4 × 106 grid points and the boundary layer flow results obtained, regarding both mean flow profiles and turbulence quantities, showed a good agreement with the available laser Doppler anemometry (LDA) measurements. Analysis of the flow was directly able to identify and confirm the existence of internal layers at positions related to the vicinity of the upstream and downstream discontinuities in the surface curvature and also partially confirm a close interdependency between generation and evolution of internal layers and the abrupt changes in the skin friction, previously reported in the literature.

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