Maarten van Reeuwijk

The turbulence boundary of a temporal jet

We examine the structure of the turbulence boundary of a temporal plane jet at

Simplified numerical and analytical approach for solutes in turbulent flow reacting with smooth pipe walls.

Re=5000

Turbulent transport and entrainment in jets and plumes: A DNS study

using statistics conditioned on the enstrophy. The data is obtained by direct numerical simulation and threshold values span 24 orders of magnitude, ranging from essentially irrotational fluid outside the jet to fully turbulent fluid in the jet core. We use two independent estimators for the local entrainment velocity

Asymptotic solutions for turbulent mass transfer at high Schmidt number

v_n

The turbulent/nonturbulent interface in penetrative convection

based on the enstrophy budget. The data show clear evidence for the existence of a viscous superlayer (VSL) that envelopes the turbulence. The VSL is a nearly one-dimensional layer with low surface curvature. We find that both its area and viscous transport velocity adjust to the imposed rate so that the integral entrainment flux is independent of threshold, although low-Reynolds-number effects play a role for the case under consideration. This threshold independence is consistent with the inviscid nature of the integral rate of entrainment. A theoretical model of the VSL is developed that is in reasonably good agreement with the data and predicts that the contribution of viscous transport and dissipation to interface propagation have magnitude

Systematic investigation of non-Boussinesq effects in variable-density groundwater flow simulations

2 v_n

Small-scale entrainment in inclined gravity currents

and

Visualization of three pathways for macromolecule transport across cultured endothelium and their modification by flow

-v_n

Clustering of particles in turbulence due to phoresis.

, respectively. We further identify a turbulent core region (TC) which is a region that shrinks over time and a buffer region connecting the VSL and the TC that grows in time and where the inviscid enstrophy production is important. The BR shows many similarities with the turbulent-nonturbulent interface (TNTI), although the TNTI seems to extend into the TC. The average distance between the TC and the VSL, i.e.\ the BR thickness is about 10 Kolmogorov length scales or half a Taylor length scale, indicating that intense turbulent flow regions and viscosity-dominated regions are in close proximity.

Direct simulation of turbulent entrainment due to a plume impinging on a density interface

A large group of reactions that affect water quality in distribution networks occur on the pipe wall surface. Existing simulation models are usually based on cross-sectionally averaged variables that use mass-transfer coefficients derived for constant-concentration (Dirichlet) boundary conditions to account for cross-sectional variations. In the case of a first-order wall-demand problem, the boundary condition is however of Robin type. We derive a simple one-dimensional (1D) model for the radial concentration profile of a solute of arbitrary Schmidt number ( Sc ) reacting with pipe walls in a fully developed turbulent flow. A modified van Driest mixing length model was used to approximate the Reynolds-averaged velocity and eddy diffusivity. Numerical solutions of the 1D model agree well with a two-dimensional mass transport model and experimental data. An asymptotic solution for high Sc is derived, which is in excellent agreement with the 1D model for Sc>100 . A comparison with the mass-transfer coefficie...