Investigation of Hydrodynamic Dispersion and Intra-pore Turbulence Effects in Porous Media

Abstract

The aim of the present paper is to evaluate and compare the pore level hydrodynamic dispersion and effects of turbulence during flow in porous media. In order to compute these quantities, large eddy simulations of turbulent flow in five unit cells comprised of spherical particles are performed and the results are averaged over the cells. Visualizations of vortical structures reveal that the size of the turbulence structures is of the size of the pores. Investigations furthermore yield that volume-averaged values of the hydrodynamic dispersion are of the same order as the Reynolds stress within the pores. It is also shown that the effect of intra-pore turbulence and hydrodynamic dispersion on the redistribution of macroscopic momentum within the porous medium is negligible compared to Forchheimer term. A discussion is provided on the accuracy of the eddy viscosity hypothesis in the modeling of the volume-averaged intra-pore Reynolds stresses. Finally, the effect of variation in the pore-scale geometry on the turbulence structures and averaged values of hydrodynamic dispersion and Reynolds stress is investigated.

Technical Note: Novel triple O<sub>2</sub>-sensor aquatic eddy covariance instrument with improved time-shift correction reveals central role of microphytobenthos for carbon cycling in coral reef sands

“…Continuous flow may be more effective than oscillating flow in driving advective pore water exchange in permeable sediments. In contrast to the steady pressure gradients continuous flows generate, wave orbital motion produces oscillating gradients, which enhance turbulence in the pore space of the sand (Horton and Pokrajac, 2009;Jouybari et al, 2020). This turbulence and inertial losses associated with the acceleration and deceleration of the pore flows may lessen the effects of pressure gradients driving the advective pore flows and interfacial water exchange.…”

Technical note: Novel triple O<sub>2</sub> sensor aquatic eddy covariance instrument with improved time shift correction reveals central role of microphytobenthos for carbon cycling in coral reef sands

“…5b) is in line with rates reported for reef lagoon sediments in Moorea (P max 6.8 ± 0.5 mmol m −2 h −1 ; Boucher et al, 1998), New Caledonia (P max ∼ 10 mmol m −2 h −1 ; Clavier and Garrigue, 1999), and the Great Barrier Reef (P max ∼ 11 mmol m −2 h −1 ; Eyre et al, 2013). To put these rates into perspective, eddy covariance flux measurements over dense Mediterranean Posidonia seagrass meadows (13 m depth, PAR 300-400 µmol photons m −2 s −1 ) revealed daytime O 2 fluxes of 6.8 ± 0.7 mmol m −2 h −1 and nighttime fluxes of −3.6 ± 0.4 mmol m −2 h −1 (Koopmans et al, 2020), i.e. rates of the same magnitude as measured in the microphytobenthos communities.…”