davide poggi | Politecnico di Torino (original) (raw)

Papers by davide poggi

Geophysical Research Letters

Journal of Hydraulic Engineering

Agu Fall Meeting Abstracts, Dec 1, 2009

ABSTRACT The modulation of turbulent energetics and their canonical length scales inside tall and... more ABSTRACT The modulation of turbulent energetics and their canonical length scales inside tall and dense canopies on complex terrain remains largely an unexplored topic though interest in these properties is now proliferating. These interests are motivated by footprint analysis when interpreting eddy-covariance derived evapo-transpiration (ET) measurements from towers, or when coupling land-surface fluxes to the various dynamical regions within the atmospheric boundary layer (ABL). Both applications require clear quantification of how topography modifies higher order statistics such as the velocity variances (and spectra). To investigate how these velocity variances (and spectra) are modified by the simultaneous action of topography and canopy, two flume experiments were carried out on a train of gentle cosine hills differing only in surface cover. The first experiment was conducted above a bare surface while the second experiment was conducted within and above a densely arrayed rod canopy. The velocity variances (and spectra) from these two experiments were compared in the middle, inner, and near-surface layers of the ABL with particular attention paid to phase-relationships between the velocity variances (and covariances) and the topography. Phase-predictions from rapid distortion theory, equilibrium theories, and canopy flow theories within these various layers are discussed. The addition of a canopy on hills alters the energy spectrum of turbulence inside the canopy by short-circuiting the energy cascade and by injecting energy by wake production. Moreover, inside the canopy and on the lee side of the hill, a re-circulation region occurs that is absent in the bare surface experiment. Hence, by comparison to the bare surface case, we show that these canopy processes modify significantly the phase-relationships between topography and higher-order flow statistics via novel mechanisms not previously explored, and the effects of these processes propagates to dynamical regions well beyond the immediate canopy sub-layer vicinity.

ABSTRACT The ergodic hypothesis is central to the statistical theory of turbulence and is implici... more ABSTRACT The ergodic hypothesis is central to the statistical theory of turbulence and is implicitly assumed when interpreting many laboratory and field experiments. The hypothesis states that the time/space averaging of flow statistics converge to that of an ensemble of flows with independent realizations but similar initial and boundary conditions. Despite recent theoretical and experimental support for ergodicity in turbulence statistics, flows within roughness elements such as canopies exhibit features that distinguish them from their classical boundary layer counterparts. These differences are commonly attributed to short-circuiting of the energy cascade and the prevalence of von Karman vortex streets in the deeper layers of the canopy. Using Laser Induced Florescence (LIF) measurements at two different depths within a rod canopy, the space-time statistical properties and concomitant necessary conditions for ergodicity of passive scalar turbulence statistics were evaluated. The integral time and length scales were first analyzed and their corresponding maximum values were used to guide construction of an ensemble of independent realizations from repeated space-time concentration measurements. A Kolmogorov-Smirnov test on the distributions of temporal and spatial concentration series against the ensemble revealed that the ergodic hypothesis is reasonable except close to the rod elements where wake-induced inhomogeneity and damped turbulence prevail. The spatial concentration statistics within a repeated rod-cell appeared less ergodic than their temporal counterparts given the periodicity and persistence of von Karman vortices in the flow field. Using lagged cross-correlations of scalar concentration time series at different spatial locations, a local advection velocity of dominant eddies was inferred. The computed longitudinal velocity probability density function of this advection velocity agreed with independently reported laser Doppler anemometery measurements at the same two levels inside the same rod canopy.

Boundary Layer Meteorology, May 31, 2004

Second-order closure models for the canopy sublayer (CSL) employ a set of closure schemes develop... more Second-order closure models for the canopy sublayer (CSL) employ a set of closure schemes developed for 'free-air' flow equations and then add extra terms to account for canopy related processes. Much of the current research thrust in CSL closure has focused on these canopy modifications. Instead of offering new closure formulations here, we propose a new mixing length model that accounts for basic energetic modes within the CSL. Detailed flume experiments with cylindrical rods in dense arrays to represent a rigid canopy are conducted to test the closure model. We show that when this length scale model is combined with standard second-order closure schemes, first and second moments, triple velocity correlations, the mean turbulent kinetic energy dissipation rate, and the wake production are all well reproduced within the CSL provided the drag coefficient (C D) is well parameterized. The main theoretical novelty here is the analytical linkage between gradientdiffusion closure schemes for the triple velocity correlation and non-local momentum transfer via cumulant expansion methods. We showed that second-order closure models reproduce reasonably well the relative importance of ejections and sweeps on momentum transfer despite their local closure approximations. Hence, it is demonstrated that for simple canopy morphology (e.g., cylindrical rods) with well-defined length scales, standard closure schemes can reproduce key flow statistics without much revision. When all these results are taken together, it appears that the predictive skills of secondorder closure models are not limited by closure formulations; rather, they are limited by our ability to independently connect the drag coefficient and the effective mixing length to the canopy roughness density. With rapid advancements in laser altimetry, the canopy roughness density distribution will become available for many terrestrial ecosystems. Quantifying the sheltering effect, the homogeneity and isotropy of the drag coefficient, and more importantly, the canonical mixing length, for such variable roughness density is still lacking.

Geophysical Research Letters, 2009

The exchange of water and nutrients between rivers and hyporheic zones has been recognized as a v... more The exchange of water and nutrients between rivers and hyporheic zones has been recognized as a very important process for the stream ecosystem. This water exchange is generally explained by pressure gradients on the riverbed that are induced by fluvial geomorphological features or by turbulent coherent structures. In this work we discuss a different exchange mechanism due to density gradients between the in-stream and the hyporheic water. We present laboratory results that show how weak density gradients are able to induce significant hyporheic fluxes. This gravity-induced exchange is expected to play a major role in streams with low-permeability sediments or with small topographic features.

Advances in Water Resources, Feb 1, 2006

Modeling scalar transport within canopies remains a vexing research problem in eco-hydrology and ... more Modeling scalar transport within canopies remains a vexing research problem in eco-hydrology and eco-hydraulics. Canopy turbulence is inhomogeneous, non-Gaussian, and highly dissipative, thereby posing unique challenges to three-dimensional Lagrangian Dispersion Models (LDM). Standard LDM approaches usually satisfy the well-mixed condition and account for turbulence inhomogeneity but not for its non-Gaussian statistics and enhanced dissipation. While numerous studies evaluated the importance of the former (with mixed results), few studies to date considered the latter. In this paper we present new data and explore: (1) the skill of LDM in reproducing mean scalar concentration distributions within dense and rigid canopies for source releases near the canopy top and near the ground, and (2) the extent to which these estimates are sensitive to the formulation of the mean turbulent kinetic energy dissipation rate () profile. Toward this end, Laser Induced Florescence (LIF) and Laser Doppler Anemometry (LDA) were used to measure scalar concentration and Eulerian flow statistics within a dense model canopy in a rectangular flume. It is shown that LDM concentration predictions are sensitive to how is estimated. Good agreement between measured and modelled mean concentration distributions were obtained when was estimated from the mean squared longitudinal velocity gradients and isotropic turbulence principles. However, when was estimated from the widely used scaling arguments that employ a constant Lagrangian time scale (T l) and a specified vertical velocity variance ðr 2 w Þ profile, the predicted concentrations diverged significantly from the LIF measurements. Better agreement was obtained when a constant mixing length scale was used with the r 2 w profile.

Geophysical Research Letters

Journal of Hydraulic Engineering

Agu Fall Meeting Abstracts, Dec 1, 2009

ABSTRACT The modulation of turbulent energetics and their canonical length scales inside tall and... more ABSTRACT The modulation of turbulent energetics and their canonical length scales inside tall and dense canopies on complex terrain remains largely an unexplored topic though interest in these properties is now proliferating. These interests are motivated by footprint analysis when interpreting eddy-covariance derived evapo-transpiration (ET) measurements from towers, or when coupling land-surface fluxes to the various dynamical regions within the atmospheric boundary layer (ABL). Both applications require clear quantification of how topography modifies higher order statistics such as the velocity variances (and spectra). To investigate how these velocity variances (and spectra) are modified by the simultaneous action of topography and canopy, two flume experiments were carried out on a train of gentle cosine hills differing only in surface cover. The first experiment was conducted above a bare surface while the second experiment was conducted within and above a densely arrayed rod canopy. The velocity variances (and spectra) from these two experiments were compared in the middle, inner, and near-surface layers of the ABL with particular attention paid to phase-relationships between the velocity variances (and covariances) and the topography. Phase-predictions from rapid distortion theory, equilibrium theories, and canopy flow theories within these various layers are discussed. The addition of a canopy on hills alters the energy spectrum of turbulence inside the canopy by short-circuiting the energy cascade and by injecting energy by wake production. Moreover, inside the canopy and on the lee side of the hill, a re-circulation region occurs that is absent in the bare surface experiment. Hence, by comparison to the bare surface case, we show that these canopy processes modify significantly the phase-relationships between topography and higher-order flow statistics via novel mechanisms not previously explored, and the effects of these processes propagates to dynamical regions well beyond the immediate canopy sub-layer vicinity.

ABSTRACT The ergodic hypothesis is central to the statistical theory of turbulence and is implici... more ABSTRACT The ergodic hypothesis is central to the statistical theory of turbulence and is implicitly assumed when interpreting many laboratory and field experiments. The hypothesis states that the time/space averaging of flow statistics converge to that of an ensemble of flows with independent realizations but similar initial and boundary conditions. Despite recent theoretical and experimental support for ergodicity in turbulence statistics, flows within roughness elements such as canopies exhibit features that distinguish them from their classical boundary layer counterparts. These differences are commonly attributed to short-circuiting of the energy cascade and the prevalence of von Karman vortex streets in the deeper layers of the canopy. Using Laser Induced Florescence (LIF) measurements at two different depths within a rod canopy, the space-time statistical properties and concomitant necessary conditions for ergodicity of passive scalar turbulence statistics were evaluated. The integral time and length scales were first analyzed and their corresponding maximum values were used to guide construction of an ensemble of independent realizations from repeated space-time concentration measurements. A Kolmogorov-Smirnov test on the distributions of temporal and spatial concentration series against the ensemble revealed that the ergodic hypothesis is reasonable except close to the rod elements where wake-induced inhomogeneity and damped turbulence prevail. The spatial concentration statistics within a repeated rod-cell appeared less ergodic than their temporal counterparts given the periodicity and persistence of von Karman vortices in the flow field. Using lagged cross-correlations of scalar concentration time series at different spatial locations, a local advection velocity of dominant eddies was inferred. The computed longitudinal velocity probability density function of this advection velocity agreed with independently reported laser Doppler anemometery measurements at the same two levels inside the same rod canopy.

Boundary Layer Meteorology, May 31, 2004

Second-order closure models for the canopy sublayer (CSL) employ a set of closure schemes develop... more Second-order closure models for the canopy sublayer (CSL) employ a set of closure schemes developed for 'free-air' flow equations and then add extra terms to account for canopy related processes. Much of the current research thrust in CSL closure has focused on these canopy modifications. Instead of offering new closure formulations here, we propose a new mixing length model that accounts for basic energetic modes within the CSL. Detailed flume experiments with cylindrical rods in dense arrays to represent a rigid canopy are conducted to test the closure model. We show that when this length scale model is combined with standard second-order closure schemes, first and second moments, triple velocity correlations, the mean turbulent kinetic energy dissipation rate, and the wake production are all well reproduced within the CSL provided the drag coefficient (C D) is well parameterized. The main theoretical novelty here is the analytical linkage between gradientdiffusion closure schemes for the triple velocity correlation and non-local momentum transfer via cumulant expansion methods. We showed that second-order closure models reproduce reasonably well the relative importance of ejections and sweeps on momentum transfer despite their local closure approximations. Hence, it is demonstrated that for simple canopy morphology (e.g., cylindrical rods) with well-defined length scales, standard closure schemes can reproduce key flow statistics without much revision. When all these results are taken together, it appears that the predictive skills of secondorder closure models are not limited by closure formulations; rather, they are limited by our ability to independently connect the drag coefficient and the effective mixing length to the canopy roughness density. With rapid advancements in laser altimetry, the canopy roughness density distribution will become available for many terrestrial ecosystems. Quantifying the sheltering effect, the homogeneity and isotropy of the drag coefficient, and more importantly, the canonical mixing length, for such variable roughness density is still lacking.

Geophysical Research Letters, 2009

The exchange of water and nutrients between rivers and hyporheic zones has been recognized as a v... more The exchange of water and nutrients between rivers and hyporheic zones has been recognized as a very important process for the stream ecosystem. This water exchange is generally explained by pressure gradients on the riverbed that are induced by fluvial geomorphological features or by turbulent coherent structures. In this work we discuss a different exchange mechanism due to density gradients between the in-stream and the hyporheic water. We present laboratory results that show how weak density gradients are able to induce significant hyporheic fluxes. This gravity-induced exchange is expected to play a major role in streams with low-permeability sediments or with small topographic features.

Advances in Water Resources, Feb 1, 2006

Modeling scalar transport within canopies remains a vexing research problem in eco-hydrology and ... more Modeling scalar transport within canopies remains a vexing research problem in eco-hydrology and eco-hydraulics. Canopy turbulence is inhomogeneous, non-Gaussian, and highly dissipative, thereby posing unique challenges to three-dimensional Lagrangian Dispersion Models (LDM). Standard LDM approaches usually satisfy the well-mixed condition and account for turbulence inhomogeneity but not for its non-Gaussian statistics and enhanced dissipation. While numerous studies evaluated the importance of the former (with mixed results), few studies to date considered the latter. In this paper we present new data and explore: (1) the skill of LDM in reproducing mean scalar concentration distributions within dense and rigid canopies for source releases near the canopy top and near the ground, and (2) the extent to which these estimates are sensitive to the formulation of the mean turbulent kinetic energy dissipation rate () profile. Toward this end, Laser Induced Florescence (LIF) and Laser Doppler Anemometry (LDA) were used to measure scalar concentration and Eulerian flow statistics within a dense model canopy in a rectangular flume. It is shown that LDM concentration predictions are sensitive to how is estimated. Good agreement between measured and modelled mean concentration distributions were obtained when was estimated from the mean squared longitudinal velocity gradients and isotropic turbulence principles. However, when was estimated from the widely used scaling arguments that employ a constant Lagrangian time scale (T l) and a specified vertical velocity variance ðr 2 w Þ profile, the predicted concentrations diverged significantly from the LIF measurements. Better agreement was obtained when a constant mixing length scale was used with the r 2 w profile.