An integrated approach for the analysis and modeling of road tunnel ventilation. Part I: Continuous measurement of the longitudinal airflow profile

The knowledge of the flow field inside road tunnels under normal operation, let alone fire conditions, is only approximate and partial. The reason is that while the full three-dimensional, unsteady problem is out of reach of numerical methods, on the other hand accurate measurement of the airflow in road and railway tunnels constitutes an extremely demanding task. The present work, structured as a twofold study, takes up the challenge and proposes an original integrated experimental and numerical approach for the analysis and modeling of flow inside a road tunnel and its ventilation systems, aiming at defining a methodology for the creation of “digital twins” of the system itself, on which advanced ventilation and smoke control strategies can be tested and fine-tuned. In this first part, an innovative experimental facility for the continuous acquisition of the longitudinal velocity profile along the whole length of a road tunnel has been designed and built. The facility consists of a survey rake with five bidirectional vane anemometers, which is mounted on a small electric vehicle that can travel through the tunnel at constant speed. This paper reports the design procedure of the measurement facility, with particular focus on the conception and realization of the vehicle carrying the survey rake. Results of the first experimental campaign carried out under the 11611 meters long Mont Blanc road tunnel are presented to corroborate the validity of the approach adopted and the accuracy of the measurement chain

Development and calibration of a 1D thermo-fluid dynamic model of ventilation in tunnels

In complex, large civil infrastructures where ventilation has a crucial role for the safety of users in both normal operation and hazardous scenarios, the correct prediction of flow and heat transfer parameters is of fundamental importance. While full 3D simulation is applicable only to a limited extent, and the resort to 1D modeling is a common practice in both design and evaluation phases, the limitation of such models lies in the choice of transfer parameters, such as friction loss coefficients and heat transfer coefficients. In this work, an original approach based on the Finite Volume integration of the 1D flow and energy equations is presented. Such equations are to be solved on a network of ducts, representing the ventilation system in the 11.6 km long Mont Blanc Tunnel with a spatial resolution of 10 m. A preliminary calibration of a set of friction loss coefficients against a rich experimental dataset collected throughout a dedicated set of in situ tests is of particular concern here, as it is carried out by means of genetic optimization algorithms. Predictions of the flow field are in remarkable agreement with the experimental data, with an overall RMS error of - 0.42 m/s. Further refinements and possible parameter choices are also discussed

An integrated approach for the analysis and modeling of road tunnel ventilation. Part II: Numerical model and its calibration

The present work represents the second and final part of a twofold study aiming at the definition and validation of an integrated methodology for the analysis and modeling of road tunnel ventilation systems. A numerical approach is presented, based on the Finite Volume integration of the 1D mechanical and thermal energy conservation equations on a network of ducts, representing the ventilation system of the 11.6 km long Mont Blanc Tunnel. The set of distributed and concentrated loss coefficients, representing dissipation of mechanical energy by friction in each part of the ventilation system, is calibrated against a rich experimental dataset, collected throughout a dedicated set of in situ tests and presented in the first part of the work. The calibration of the model is carried out by means of genetic optimization algorithms. Predictions of the flow field using the calibrated parameters are in remarkable agreement with the experimental data, with an overall RMS error of \ub1 0.27 m/s, i.e. of the same order of the accuracy of the measurement probes. Further validation against a selection of field data recorded by the tunnel monitoring and control system is brought forward, highlighting the robustness and potential general applicability of the proposed approach

Robustness of tissue oxygenation estimates by continuous wave space-resolved near infrared spectroscopy

Significance: Continuous wave near infrared spectroscopy (CW-NIRS) is widely exploited in clinics to estimate skeletal muscles and brain cortex oxygenation. Spatially resolved spectroscopy (SRS) is generally implemented in commercial devices. However, SRS suffers from two main limitations: the a priori assumption on the spectral dependence of the reduced scattering coefficient [μs'(λ)] and the modeling of tissue as homogeneous. Aim: We studied the accuracy and robustness of SRS NIRS. We investigated the errors in retrieving hemodynamic parameters, in particular tissue oxygen saturation (StO2), when μs'(λ) was varied from expected values, and when layered tissue was considered. Approach: We simulated hemodynamic variations mimicking real-life scenarios for skeletal muscles. Simulations were performed by exploiting the analytical solutions of the photon diffusion equation in different geometries: (1) semi-infinite homogeneous medium and constant μs'(λ); (2) semi-infinite homogeneous medium and linear changes in μs'(λ); (3) two-layered media with a superficial thickness s1=5, 7.5, 10 mm and constant μs'(λ). All simulated data were obtained at source-detector distances ρ=35, 40, 45 mm, and analyzed with the SRS approach to derive hemodynamic parameters (concentration of oxygenated and deoxygenated hemoglobin, total hemoglobin concentration, and tissue oxygen saturation, StO2) and their relative error. Results: Variations in μs'(λ) affect the estimated StO2 (up to ±10%), especially if changes are different at the two wavelengths. However, the main limitation of the SRS method is the presence of a superficial layer: errors strongly larger than 20% were retrieved for the estimated StO2 when the superficial thickness exceeds 5 mm. Conclusions: These results highlight the need for more sophisticated strategies (e.g., the use of multiple short and long distances) to reduce the influence of superficial tissues in retrieving hemodynamic parameters and warn the SRS users to be aware of the intrinsic limitation of this approach, particularly when exploited in the clinical environment

Numerical Predictions for Stable Buoyant Regimes within a Square Cavity Containing a Heated Horizontal Cylinder

Buoyancy-induced flow regimes are investigated numerically for the basic case of a horizontal cylinder centred into a long co-axial square-sectioned cavity. In the frame of the 2D assumption, the treshold for the occurrence of time-dependent behaviour is explored. Stable symmetric and non-symmetric steady-state solutions, as well as unsteady regimes are observed, depending on the Rayleigh number, Ra, and the aspect ratio of the cavity, d. Four d-values are considered (d = .2, .4, .6, and .8). Heat transfer results are correlated by a single equation covering the full subcritical region

Numerical Analysis of Natural Convection Flow and Heat Transfer from an Enclosed Cylindrical Heat Source

A numerical study of the natural convection flow arising from a confined thermal source is presented. The physical system considered is a 2D cavity of square cross-section, containing a horizontal cylindrical source, the heat carrier fluid being air at Prandtl number Pr = .7. The heating body is placed in central position; the resulting flow is investigated with respect to the variation of the Rayleigh number and the ratio between the source diameter and the cavity side length. Either Dirichlet and Neumann thermal boundary conditions are imposed on the cylinder surface. The analyses make use of two Navier-Stokes, finite- volume codes: a general-purpose CFD package, and a Direct Numerical Simulation software. Main issues of the work are the rise of steady and transitional flows and the heat transfer performance of the system, as influenced by the different boundary conditions at the heat source wall. A correlating equation for the average Nusselt number on the cylinder, previously obtained for the isothermal case, is validated for the uniform heat flux condition

Reproducibility of identical solid phantoms

SIGNIFICANCE: Tissue-like solid phantoms with identical optical properties, known within tolerant uncertainty, are of crucial importance in diffuse optics for instrumentation assessment, interlaboratory comparison studies, industrial standards, and multicentric clinical trials. AIM: The reproducibility in fabrication of homogeneous solid phantoms is focused based on spectra measurements by instrument comparisons grounded on the time-resolved diffuse optics. APPROACH: Epoxy-resin and silicone phantoms are considered as matrices and both employ three different instruments for time-resolved diffuse spectroscopy within the spectral range of 540 to 1100 nm. In particular, we fabricated two batches of five phantoms each in epoxy resin and silicone. Then, we evaluated the intra- and interbatch variability with respect to the instrument precision, by considering the coefficient of variation (CV) of absorption and reduced scattering coefficients. RESULTS: We observed a similar precision for the three instruments, within 2% for repeated measurements on the same phantom. For epoxy-resin phantoms, the intra- and the interbatch variability reached the instrument precision limit, demonstrating a very good phantom reproducibility. For the silicone phantoms, we observed larger values for intra- and interbatch variability. In particular, at worst, for reduced scattering coefficient interbatch CV was about 5%. CONCLUSIONS: Results suggest that the fabrication of solid phantoms, especially considering epoxy-resin matrix, is highly reproducible, even if they come from different batch fabrications and are measured using different instruments

Relationship between sensory characteristics and optical properties in ‘Conference’ pears

Pears are appreciated by consumers for their juicy-buttery texture, sweet flavour and pleasant aroma. Sweetness and juiciness are the main drivers for consumer liking, while firm-grainy textures and lack of flavor are disliked. Thus, it is important that the industry can provide high quality fruit to encourage pears consumption. Nondestructive techniques, such as time-resolved reflectance spectroscopy (TRS), can help producers in discriminating pears with different sensory characteristics to satisfy consumer expectations. The aim of this work was to study the relationship between sensory characteristics, quality, and optical properties (absorption and scattering) measured by TRS, in ‘Conference’ pears. At harvest, fruit were measured by TRS for absorption coefficient at 650 nm (μa650), ranked by μa650 in three maturity classes (less, medium, more mature), and randomized in four samples according to 1-MCP treatment (treated, untreated) and storage atmosphere (air; CA: 2 kPa O2, 1 kPa CO2). After 4 months of storage at -1°C plus 7 days at 20°C, less and more mature pears were measured by TRS at 650, 780 and 940 nm, analyzed for firmness (FF), soluble solids (SS) and acidity (TA) and submitted to sensory analysis. Cluster analysis applied on sensory attributes produced three groups (W1, W2, W3), each one representing a specific sensory profile. W1 showed the lowest scores for sweetness and the highest for sourness and astringency. W2 grouped pears with the lowest scores for firmness and astringency and the highest for sweetness, aroma and acceptability together with the lowest FF, μa940 and μs780 values. W3 showed the highest scores for firmness and graininess and the lowest for juiciness and aroma, coupled with the lowest TA and the highest FF, μa650, μa940 and μs780 values. Discriminant analysis based on TRS optical properties correctly classified W1, W2 and W3 profiles in 27, 92 and 67% of the cases, respectively