3,000 research outputs found
Nonlinear elastic polymers in random flow
Polymer stretching in random smooth flows is investigated within the
framework of the FENE dumbbell model. The advecting flow is Gaussian and
short-correlated in time. The stationary probability density function of
polymer extension is derived exactly. The characteristic time needed for the
system to attain the stationary regime is computed as a function of the
Weissenberg number and the maximum length of polymers. The transient relaxation
to the stationary regime is predicted to be exceptionally slow in the proximity
of the coil-stretch transition.Comment: 10 pages, to be published in J. Fluid Mec
Shear effects on passive scalar spectra
The effects of a large-scale shear on the energy spectrum of a passively
advected scalar field are investigated. The shear is superimposed on a
turbulent isotropic flow, yielding an Obukhov-Corrsin scalar
spectrum at small scales. Shear effects appear at large scales, where a
different, anisotropic behavior is observed. The scalar spectrum is shown to
behave as for a shear fixed in intensity and direction. For other
types of shear characteristics, the slope is generally intermediate between the
-5/3 Obukhov-Corrsin's and the -1 Batchelor's values. The physical mechanisms
at the origin of this behaviour are illustrated in terms of the motion of
Lagrangian particles. They provide an explanation to the scalar spectra shallow
and dependent on the experimental conditions observed in shear flows at
moderate Reynolds numbers.Comment: 10 LaTeX pages,3 eps Figure
A model for alignment between microscopic rods and vorticity
Numerical simulations show that microscopic rod-like bodies suspended in a
turbulent flow tend to align with the vorticity vector, rather than with the
dominant eignevector of the strain-rate tensor. This paper investigates an
analytically solvable limit of a model for alignment in a random velocity field
with isotropic statistics. The vorticity varies very slowly and the isotropic
random flow is equivalent to a pure strain with statistics which are
axisymmetric about the direction of the vorticity. We analyse the alignment in
a weakly fluctuating uniaxial strain field, as a function of the product of the
strain relaxation time and the angular velocity about
the vorticity axis. We find that when , the rods are
predominantly either perpendicular or parallel to the vorticity
Efficient two-step procedure for parameter identification and uncertainty assessment in model updating problems
The model updating procedures employed in vibration-based health monitoring need to be reliable and computationally efficient. The computational time is a fundamental task if the results are used to evaluate, in quasi-real-time, the safe or the unsafe state of strategic and relevant structures. The paper presents an efficient two-step procedure for the identification of the mechanical parameters and for the assessment of the corresponding uncertainty in model updating problems. The first step solves a least squares problem, providing a first estimate of the unknown parameters. The second (iterative) step produces a refinement of the solution. Moreover, by exploiting the error propagation theory, this article presents a direct (non-iterative) procedure to assess the uncertainty affecting the unknown parameters starting from the experimental data covariance matrix. To test the reliability of the procedure as well as to prove its applicability to structural problems, the methodology has been applied to two test-bed case studies. Finally, the procedure has been used for the damage assessment in an existing building. The results provided in this article indicate that the procedure can accurately identify the unknown parameters and properly localize and quantify the damage
Interference fit effect on holed single plates loaded with tension-tension stresses
This paper deals with the influence of interference fit coupling on the fatigue strength of holed plates. The effect was investigated both experimentally and numerically. Axial fatigue tests have been carried out on holed specimens made of high performance steel (1075MPa of Ultimate strength and 990MPa of Yield strength) with or without a pin, made of the same material, press fitted into their central hole. Three different conditions have been investigated: free hole specimens, specimens with 0.6% of nominal specific interference and specimens with 2% of nominal specific interference. The experimental stress-life (S–N) curves pointed out an increased fatigue life of the interference fit specimens compared with the free hole ones. The numericalinvestigation was performed in order to analyse the stress fields by applying an elastic plastic 2D simulation witha commercial Finite Element software. The stress history and distribution along the contact interference of the fitted samples indicates a significant reduction of the local stress range due to the externally applied loading (remote stress) since a residual and compressive stress field is generated by the pin insertion
Interference fit effect on holed single plates loaded with tension-tension stresses
This paper deals with the influence of interference fit coupling on the fatigue strength of holed plates. The effect was investigated both experimentally and numerically. Axial fatigue tests have been carried out on holed specimens made of high performance steel (1075MPa of Ultimate strength and 990MPa of Yield strength) with or without a pin, made of the same material, press fitted into their central hole. Three different conditions have been investigated: free hole specimens, specimens with 0.6% of nominal specific interference and specimens with 2% of nominal specific interference. The experimental stress-life (S–N) curves pointed out an increased fatigue life of the interference fit specimens compared with the free hole ones. The numerical investigation was performed in order to analyse the stress fields by applying an elastic plastic 2D simulation with a commercial Finite Element software. The stress history and distribution along the contact interference of the fitted samples indicates a significant reduction of the local stress range due to the externally applied loading (remote stress) since a residual and compressive stress field is generated by the pin insertion
Evaluation of carcass and meat traits of Muscovy duck fed with black soldier fly partially defatted meal
The aim of this study was to evaluate the carcass characteristics and breast meat quality in Muscovy duck (Cairina moschata domestica) fed different inclusion levels of a partially defatted black soldier fly larva (BSF) meal. A total of 256 Muscovy ducklings (average live weight, LW: 71.32\ub12.70 g) were reared from day 3 to day 48 and randomly allotted in 32 pens (8 replicates/treatment). Four different diets were formulated with increasing substitution level of corn gluten meal with BSF larva meal (0, 3, 6 and 9%; BSF0, BSF3, BSF6 and BSF9, respectively) and divided in 3 feeding phases: starter (1-14 days), grower (14-35 days) and finisher (35-48 days). At day 48, 2 animals/replicate were slaughtered and dissected to determine their carcass yields. The weights of spleen, bursa of Fabricius, liver, heart and abdominal fat were recorded. Breast and thigh muscles were then excised from 16 ducks/treatment and weighted. Ultimate pH (pHu) and L*, a*, b* colour values were then measured on breast muscle. The collected data were tested by means of oneway ANOVA evaluating the effect of dietary BSF inclusion level by polynomial contrasts. Significance was declared
at P<0.05. The inclusion of BSF did not affect final LW (2,515.68\ub192.42 g on average). Hot and cold carcass weights showed a quadratic response (P<0.05) to increasing BSF larva meal, with a minimum corresponding to BSF6; however, refrigeration losses were not affected by treatments. Weight of spleen, bursa of Fabricius, liver and heart did not differ among treatments. The weight of abdominal fat showed a quadratic response to increasing BSF meal with a minimum corresponding to BSF6 group (P<0.05). Breast and thigh yields, pHu and L*, a*, b* colour values did not differ among
groups. With the exception of BSF6, the inclusion of BSF meal did not affect meat traits and carcass characteristics, confirming the potential use of BSF meal in Muscovy duck diets
Parametric and numerical modeling tools to forecast hydrogeological impacts of a tunnel
The project of interest involving a hydroelectrical diversion tunnel through a crystalline rock massif in the Alps required a detailed hydrogeological study to forecast the magnitude of water inflows within the tunnel and possible effects on groundwater flow The tunnel exhibits a length of 9.5 km and is located on the right side of the Toce River in Crevoladossola (Verbania Province, Piedmont region, northern Italy). Under the geological framework of the Alps, the tunnel is located within the Lower Penninic Frappes in the footwall of the Simplon Normal Fault, and the geological succession is mostly represented by Antigorio gneiss (metagranites) and Baceno metasediments (metacarbonates). Due to the presence of important mineralized springs for commercial mineral water purposes, the above mentioned hydrogeological study focused on both quantity and quality aspects via rainfall data analysis, monitoring of major spring flow rates, monitoring of hydraulic heads and pumping rates of existing wells/boreholes, hydrochemical and isotopic analysis of springs and boreholes and hydraulic tests (Lefranc and Lugeon). The resulting conceptual model indicated dominant low-permeability (aquitard) behavior of the gneissic rock masses, except under conditions of intense fracturing due to tectonization, and aquifer behavior of the metasedimentary rocks, particularly when interested by dissolution. Groundwater flow systems are mainly controlled by gravity. The springs located near the Toce River were characterized by high mineralization and isotopic ratios, indicating long groundwater flow paths. Based on all the data collected and analyzed, two parametric methods were applied: 1) the Dematteis method, slightly adapted to the case study and the available data, which allows assessment of both potential inflows within the tunnel and potential impacts on springs (codified as the drawdown hazard index; DHI); 2) the Cesano method, which only allow assessment of potential inflows within the tunnel, thereby discriminating between major and minor inflows. Contemporarily, a groundwater flow model was implemented with the equivalent porous medium (EPM) approach in MODFLOW-2000. This model was calibrated under steady-state conditions against the available data (groundwater levels inside wells/piezometers and elevation and flow rate of springs). The Dematteis method was demonstrated to be more reliable and suitable for the site than was the Cesano method. This method was validated considering a tunnel through gneissic rock masses, and this approach considered intrinsic parameters of rock masses more notably than morphological and geomorphological factors were considered. The Cesano method relatively overestimated tunnel inflows, considering variations in the topography and overburden above the tunnel. Sensitivity analysis revealed a low sensitivity of these parametric methods to parameter values, except for the rock quality designation (RQD) employed to represent the fracturing degree. The numerical model was calibrated under ante-operam conditions, and sensitivity analysis evaluated the influence of uncertainties in the hydraulic conductivity (K) values of the different hydrogeological units.The hydraulic head distribution after tunnel excavation was forecasted considering three scenarios, namely, a draining tunnel, tunnel as a eater loss source, and tunnel sealed along its aquifer sectors, considering 3 levels of K reduction. Tunnel impermeabilization was very effective, thus lowering the drainage rate and impact on springs. The model quantitatively defined tunnel inflows and the effects on spring flow at the surface in terms of flow rate decrease. The Dematteis method and numerical model were combined to obtain a final risk of impact on the springs. This study likely overestimated the risk because all the values assigned to the parameters were chosen in a conservative way, and the steady-state numerical simulations were also very conservative (the transient state in this hydrogeological setting supposedly lasts 1-3 years). Monitoring of the tunnel and springs during tunnel boring could facilitate the feedback process
Modeling soil nitrate accumulation and leaching in conventional and conservation agriculture cropping systems
Nitrate is a major groundwater inorganic contaminant that is mainly due to fertilizer leaching. Compost amendment can increase soils' organic substances and thus promote denitrification in intensively cultivated soils. In this study, two agricultural plots located in the Padana plain (Ferrara, Italy) were monitored and modeled for a period of 2.7 years. One plot was initially amended with 30 t/ha of compost, not tilled, and amended with standard fertilization practices, while the other one was run with standard fertilization and tillage practices. Monitoring was performed continuously via soil water probes (matric potential) and discontinuously via auger core profiles (major nitrogen species) before and after each cropping season. A HYDRUS-1D numerical model was calibrated and validated versus observed matric potential and nitrate, ammonium, and bromide (used as tracers). Model performance was judged satisfactory and the results provided insights on water and nitrogen balances for the two different agricultural practices tested here. While water balance and retention time in the vadose zone were similar in the two plots, nitrate leaching was less pronounced in the plot amended with compost due to a higher denitrification rate. This study provides clear evidence that compost addition and no-tillage (conservation agriculture) can diminish nitrate leaching to groundwater, with respect to standard agricultural practices
An Analytical Method for the Determination of the Climatic Distance between Different Microclimates for the Conservation of Wooden Cultural Heritage Objects
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