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Influence of groundwater flow on the estimation of subsurface thermal parameters
No full textWe investigated the influence of groundwater flow on the thermal tests performed in borehole heat exchangers to infer the underground thermal properties. Temperature–time signals were simulated with a moving line source (MLS) model under different hypotheses of Darcy velocity. Periodic and random noise was included in the synthetic data obtained with this model in order to mimic high-frequency disturbances caused by several possible sources (e.g. equipment instability and changes in environmental conditions during the experiment) that often occur in real signals. The subsurface thermal conductivity, the Darcy velocity and the borehole thermal resistance were inferred by minimising the root-mean-square error between the synthetic dataset and the model. The calculated thermal and hydraulic parameters were consistent with the “a priori” values. The optimisation procedure results were then tested with the infinite line source (ILS) model. For a Darcy velocity exceeding 10−7 m s−1, ILS largely overestimates thermal conductivity. The approach relying on the MLS model was finally tested with real temperature–time data and produced reliable estimates of thermal conductivity, Darcy velocity and borehole thermal resistance. The inferred groundwater flow was cross checked by means of an independent method based on the analysis of temperature–depth logs recorded under thermal equilibrium conditions. Such a test validates the Darcy velocity inferred with the MLS approach
Heat-flow anomaly and residual topography in the Mascarene hotspot swell (Indian Ocean)
No full textWe review the sea-bottom heat-flow determinations and present a new heat-flow observation on the Mauritius island, which is part of the long-lived Reunion hotspot track. The marine heat flow is on average 66 ± 11 mW m−2 and is consistent with the on-land value of 61 ± 18 mW m−2 found in Mauritius. Since these values do not significantly deviate from the reference cooling-plate model, lithosphere erosion does not seem a likely mechanism for the swell formation. The lack of significant reheating due to a mantle plume impacting the lithosphere base is confirmed by thermal modelling. Moreover, the coherency between on-land and marine data is argument against advective redistribution of heat near the axis of the swell. We also analyse the large-scale features of the ocean lithosphere, which are not simply a function of the plate cooling and can reflect variations in mantle dynamic topography. The predicted topography variation along the swell shows amplitude and wavelength comparable to other hotspots. Both the topographic swell magnitude and the wavelength increase northwards with the increase of the age of volcanism. The estimated flux of material from the mantle follows the same trend, being larger in the northern part of the swell. The result that residual topography and the buoyancy flux are smaller at the active volcano of Reunion could be evidence that the activity of the plume has decreased with time
EXPERIMENTAL ANALYSIS OF TRANSITIONAL FLOWS UNDER TURBINE-LIKE CONDITIONS VIA APPLICATION AND DEVELOPMENT OF ADVANCED POST-PROCESSING TECHNIQUES
No full textThe present thesis is primarily devoted to developing and applying advanced post-processing techniques to inspect complex transitional boundary layer (BL) flows evolving under variable inflow conditions. A large amount of data has been experimentally acquired utilizing particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) in a test section consisting of a flat plate installed between two adjustable endwalls. Depending on the Reynolds number (Re), the free-stream (FS) turbulence intensity (Tu) and the adverse pressure gradient (APG) imposed to the flow, attached or separated boundary layer transition was obtained.
The effects of the inflow parameters variation have been studied in detail, focusing on the flow statistical and dynamical behavior.
Due to the complexity and variety of the transitional phenomena, data-driven modal decomposition techniques have been employed to reduce the large amount of experimental data collected here. Moreover, new variants of well-established post-processing techniques have been developed to identify the main features embedded in the extensive databases.
In the case of separated flow transition, the modal decomposition procedures allowed a deep insight into the instability mechanism developing in the shear layer. Dynamic Mode Decomposition (DMD) was used to analyze the most unstable wavelengths related to the Kelvin-Helmholtz (K-H) vortices driving transition. Proper Orthogonal Decomposition (POD) was applied to PIV data, inspecting the main flow structures developing in the different regions of the LSB. Subsequently, an Extended Proper Orthogonal Decomposition (E-POD) procedure was applied, highlighting the correlation between the main dynamics observed in the forward part of the bubble and the breakup events occurring in the reattachment region.
Regarding the data reduction, the extensive database was used to develop new empirical correlations predicting the transition process regarding the geometry of a LSB and the related shedding process.
The transition process was systematically analyzed using decomposition techniques in the context of the free-stream turbulence induced transition. In order to inspect BL receptivity to free-stream disturbances, a variant of the E-POD was proposed, based on the correlating events between the FS and the BL. Low-order reconstructions of the original data were used to highlight the most correlating events directly linked to the formation and the breakup of streaky structures. Moreover, a turbulent spot recognition algorithm was implemented to identify the BL statistical response to the inflow parameters through the probability density function (PDF) of spot nucleation. Thus, a model for the PDF of spot nucleation is proposed as a function of the main flow parameters involved in the transition process.
Based on the results of the previous analyses, engineering correlations for predicting the free-stream turbulence induced transition are also introduced.
Independently on the transition type, results obtained employing the aforementioned procedures allowed a fruitful characterization of the different instability mechanisms developing in the first stage of transition, the description and evolution of coherent structures, and the correlation between their dynamics
Tectonic regime dependence on the thermal state in the central-northern Apennines, Italy
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