### Abstract

Field tracer tests indicate that advective porosity, the quantity relating advective velocity to Darcy flux, may exhibit directional dependence. Hydraulic anisotropy explains some but not all of the reported directional results. The present paper shows mathematically that directional variations in advective porosity may arise simply from incomplete mixing of an inert tracer between directional flow channels within a sampling (or support) volume ψ of soil or rock that may be hydraulically isotropic or anisotropic. In the traditional fully homogenized case, our theory yields trivially a scalar advective porosity equal to the interconnected porosity φ, thus explaining neither the observed directional effects nor the widely reported experimental finding that advective porosity is generally smaller than φ. We consider incomplete mixing under conditions in which the characteristic time t_{D} of longitudinal diffusion along channels across ω is much shorter than the characteristic time t_{H} required for homogenization through transverse diffusion between channels. This may happen where flow takes place preferentially through relatively conductive channels and/or fractures of variable orientation separated by material that forms a partial barrier to diffusive transport. Our solution is valid for arbitrary channel Peclet numbers on a correspondingly wide range of time scales t_{D} ≤ t ≪ t_{H}. It shows that the tracer center of mass is advected at a macroscopic velocity which is generally not collinear with the macroscopic Darcy flux and exceeds it in magnitude. These two vectors are related through a second-rank symmetric advective dispersivity tensor Φ. If the permeability k of ω is a symmetric positive-definite tensor, so is Φ. However, the principal directions and values of these two tensors are generally not the same; whereas those of k are a fixed property of the medium and the length-scale of ω, those of Φ depend additionally on the direction and magnitude of the applied hydraulic gradient. When the latter is large, diffusion has negligible effect on Φ and one may consider tracer mass to be distributed between channels in proportion to the magnitude of their Darcy flux. This is made intuitive through a simple example of an idealized fracture network. Our analytical formalism reveals the properties of Φ but is too schematic to allow predicting the latter accurately on the basis of realistic details about the void structure of ω and tracer mass distribution within it. Yet knowing the tensorial properties of Φ is sufficient to allow determining it indirectly on the basis of ω-scale hydraulic and tracer data, including concentrations that represent homogenized samples extracted from (or sensed externally across) an ω-scale plume.

Original language | English (US) |
---|---|

Pages (from-to) | 149-159 |

Number of pages | 11 |

Journal | Advances in Water Resources |

Volume | 28 |

Issue number | 2 |

DOIs | |

State | Published - Feb 2005 |

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### Keywords

- Advective porosity
- Directional porous media
- Fractured rocks
- Tensor

### ASJC Scopus subject areas

- Earth-Surface Processes

### Cite this

**On the tensorial nature of advective porosity.** / Neuman, Shlomo P.

Research output: Contribution to journal › Article

*Advances in Water Resources*, vol. 28, no. 2, pp. 149-159. https://doi.org/10.1016/j.advwatres.2004.09.008

}

TY - JOUR

T1 - On the tensorial nature of advective porosity

AU - Neuman, Shlomo P

PY - 2005/2

Y1 - 2005/2

N2 - Field tracer tests indicate that advective porosity, the quantity relating advective velocity to Darcy flux, may exhibit directional dependence. Hydraulic anisotropy explains some but not all of the reported directional results. The present paper shows mathematically that directional variations in advective porosity may arise simply from incomplete mixing of an inert tracer between directional flow channels within a sampling (or support) volume ψ of soil or rock that may be hydraulically isotropic or anisotropic. In the traditional fully homogenized case, our theory yields trivially a scalar advective porosity equal to the interconnected porosity φ, thus explaining neither the observed directional effects nor the widely reported experimental finding that advective porosity is generally smaller than φ. We consider incomplete mixing under conditions in which the characteristic time tD of longitudinal diffusion along channels across ω is much shorter than the characteristic time tH required for homogenization through transverse diffusion between channels. This may happen where flow takes place preferentially through relatively conductive channels and/or fractures of variable orientation separated by material that forms a partial barrier to diffusive transport. Our solution is valid for arbitrary channel Peclet numbers on a correspondingly wide range of time scales tD ≤ t ≪ tH. It shows that the tracer center of mass is advected at a macroscopic velocity which is generally not collinear with the macroscopic Darcy flux and exceeds it in magnitude. These two vectors are related through a second-rank symmetric advective dispersivity tensor Φ. If the permeability k of ω is a symmetric positive-definite tensor, so is Φ. However, the principal directions and values of these two tensors are generally not the same; whereas those of k are a fixed property of the medium and the length-scale of ω, those of Φ depend additionally on the direction and magnitude of the applied hydraulic gradient. When the latter is large, diffusion has negligible effect on Φ and one may consider tracer mass to be distributed between channels in proportion to the magnitude of their Darcy flux. This is made intuitive through a simple example of an idealized fracture network. Our analytical formalism reveals the properties of Φ but is too schematic to allow predicting the latter accurately on the basis of realistic details about the void structure of ω and tracer mass distribution within it. Yet knowing the tensorial properties of Φ is sufficient to allow determining it indirectly on the basis of ω-scale hydraulic and tracer data, including concentrations that represent homogenized samples extracted from (or sensed externally across) an ω-scale plume.

AB - Field tracer tests indicate that advective porosity, the quantity relating advective velocity to Darcy flux, may exhibit directional dependence. Hydraulic anisotropy explains some but not all of the reported directional results. The present paper shows mathematically that directional variations in advective porosity may arise simply from incomplete mixing of an inert tracer between directional flow channels within a sampling (or support) volume ψ of soil or rock that may be hydraulically isotropic or anisotropic. In the traditional fully homogenized case, our theory yields trivially a scalar advective porosity equal to the interconnected porosity φ, thus explaining neither the observed directional effects nor the widely reported experimental finding that advective porosity is generally smaller than φ. We consider incomplete mixing under conditions in which the characteristic time tD of longitudinal diffusion along channels across ω is much shorter than the characteristic time tH required for homogenization through transverse diffusion between channels. This may happen where flow takes place preferentially through relatively conductive channels and/or fractures of variable orientation separated by material that forms a partial barrier to diffusive transport. Our solution is valid for arbitrary channel Peclet numbers on a correspondingly wide range of time scales tD ≤ t ≪ tH. It shows that the tracer center of mass is advected at a macroscopic velocity which is generally not collinear with the macroscopic Darcy flux and exceeds it in magnitude. These two vectors are related through a second-rank symmetric advective dispersivity tensor Φ. If the permeability k of ω is a symmetric positive-definite tensor, so is Φ. However, the principal directions and values of these two tensors are generally not the same; whereas those of k are a fixed property of the medium and the length-scale of ω, those of Φ depend additionally on the direction and magnitude of the applied hydraulic gradient. When the latter is large, diffusion has negligible effect on Φ and one may consider tracer mass to be distributed between channels in proportion to the magnitude of their Darcy flux. This is made intuitive through a simple example of an idealized fracture network. Our analytical formalism reveals the properties of Φ but is too schematic to allow predicting the latter accurately on the basis of realistic details about the void structure of ω and tracer mass distribution within it. Yet knowing the tensorial properties of Φ is sufficient to allow determining it indirectly on the basis of ω-scale hydraulic and tracer data, including concentrations that represent homogenized samples extracted from (or sensed externally across) an ω-scale plume.

KW - Advective porosity

KW - Directional porous media

KW - Fractured rocks

KW - Tensor

UR - http://www.scopus.com/inward/record.url?scp=11944262691&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=11944262691&partnerID=8YFLogxK

U2 - 10.1016/j.advwatres.2004.09.008

DO - 10.1016/j.advwatres.2004.09.008

M3 - Article

AN - SCOPUS:11944262691

VL - 28

SP - 149

EP - 159

JO - Advances in Water Resources

JF - Advances in Water Resources

SN - 0309-1708

IS - 2

ER -