Local Deformation Rates Research Articles

Buffeting in transonic flow prediction using time-dependent turbulence model

In transonic flow conditions, the shock wave/turbulent boundary layer interaction and flow separations on wing upper surface induce flow instabilities, ‘buffet’, and then the buffeting (structure vibrations). This phenomenon can greatly influence the aerodynamic performance. These flow excitations are self-sustained and lead to a surface effort due to pressure fluctuations. They can produce enough energy to excite the structure. The objective of the present work is to predict this unsteady phenomenon correctly by using unsteady Navier–Stokes-averaged equations with a time-dependent turbulence model based on the suitable (k–ε) turbulent eddy viscosity model. The model used is based on the turbulent viscosity concept where the turbulent viscosity coefficient (Cμ) is related to local deformation and rotation rates. To validate this model, flow over a flat plate at Mach number of 0.6 is first computed, then the flow around a NACA0012 airfoil. The comparison with the analytical and experimental results shows a good agreement. The ONERA OAT15A transonic airfoil was chosen to describe buffeting phenomena. Numerical simulations are done by using a Navier–Stokes SUPG (streamline upwind Petrov–Galerkin) finite-element solver. Computational results show the ability of the present model to predict physical phenomena of the flow oscillations. The unsteady shock wave/boundary layer interaction is described. Copyright © 2005 John Wiley & Sons, Ltd.

Numerical investigation of particle migration in poiseuille flow of composite system

Shear-induced particle migration was investigated by using a continuum diffusive-flux model for the creep flow of nickel powder filled polymers which is viscous with shear-thinning characteristic. The original model of Phillips et al. [ Phys. Fluids A (1992) 4:30], for a Newtonian binder was extended to a generalized Newtonian binder, taking into consideration of local shear deformation rate. The model, together with flow equations, was employed for solving the non-Newtonian flow patterns and non-uniform particle concentration distribution of mono-modal suspensions in a pressure-driven tube flow. Particle volume fraction and velocity fields for the non-homogenous shear flow field were predicted. Effects of different viscosity model of binder system on particle migration during non-homogenous shear fluid flow were investigated.

Cavity growth in a superplastic Al–Mg alloy: II. An improved plasticity based model

The extent of cavity growth estimated from a combination of diffusional and plasticity based growth models generally underestimates the actual cavity growth in superplastic alloys. It has been shown that in a fine grain Al–Mg alloy, cavity growth begins by matrix/particle debonding at grain boundary particles (Mat. Sci. Forum, Trans. Tech. Pub. 304–306 (1999) 609), and also from pre-existing voids. In this study, cavity growth beyond interface decohesion is modeled in which deformation of the matrix surrounding the cavity is free from interface constraint, but it still experiences an accelerated local deformation rate. Stress and strain-rate in this region are intensified due to the perturbed flow field near the cavity, and not relaxed during the time frame for superplastic forming. This local deformation around the cavity is a function of strain-rate sensitivity, m, the level of strain concentration, and the cavity spacing. Two important effects not previously considered: (i) local stress concentration around the cavities, and (ii) continuous nucleation of new cavities, have been included in this work. Using this model that is suitable for low overall cavity volume (i.e. no cavity coalescence), faster growth rate is predicted for single cavities when strain-rate sensitivity is low and/or the population density of cavities is low (generally at slow strain-rates). By combining the predicted growth rate of individual cavities with the emerging cavity population density determined experimentally, a quantitative understanding of the various complex dependencies of cavitation has been obtained.

Extensive validation of computed laminar flow in a stirred tank with three Rushton turbines

AbstractHigh‐resolution CFD results, supplemented by extensive experimental validation, are presented for Newtonian laminar flow fields in a stirred tank equipped with three Rushton turbines. Flow fields are computed using the ORCA software suite for Reynolds numbers ranging from 20 to 200 with an unstructured tetrahedral mesh containing roughly 2 million tetrahedra. Each of the flow solutions takes less than 8 h to converge when running in parallel on eight desktop workstations. Excellent agreement is obtained between computed velocity fields and planar velocity vectors obtained using particle image velocimetry. Planar laser‐induced fluorescence was used to expose persistent poor‐mixing regions, in excellent agreement with numerical results. The computational results used illustrate strong flow compartmentalization and significant spatial heterogeneity with respect to local deformation rates within the vessel.

Les structures d'impact sur Europe : implications sur la rhéologie et la thermique de la croûte de glace

The depth of the brittle–ductile transition has been estimated under each graben rounding two ancient impact basins on Europa, Callanish and Tyre . Surface thermal gradients have been computed and are respectively equal to 4.3±0.4 and 2.4±0.25 K·km −1 . These results differ strongly from previous works obtained on other Europa's features. This can be explained by the variation of both the icy grain size and the chemical composition of the crust, but also by the variation of the local deformation rate. La profondeur de la transition ductile/cassant a été estimée à l'aplomb des grabens associés à deux bassins d'impact d'Europe, Callanish et Tyre . Les gradients thermiques de surface ont ainsi pu être calculés et sont respectivement de 4,3±0,4 et 2,4 ± 0,25 K · km −1 . Ces résultats, confrontés aux données antérieures obtenues sur d'autres structures d'Europe, illustrent bien la complexité de la croûte glacée. En plus des probables variations de composition et de taille des grains, nous soulignons ici l'influence de la vitesse de déformation du processus géologique mis en jeu localement sur la profondeur de la transition ductile–cassant.

Particle imaging technique for measuring the deformation rate of hydrogel microstructures

A technique for measuring the instantaneous deformation rate of hydrogel microstructures is introduced. In developing this technique, we have adapted microscopic particle image velocimetry, a method for measuring velocity fields in microfluidic devices. Small fluorescent seed particles are incorporated into the hydrogel microstructure, and as the structure swells or contracts, the displacement of the seed particles over some small time Δt is measured using a cross-correlation technique. By providing local deformation rate data in hydrogel microstructures, this technique will allow for optimization of device designs as well as providing a means for determining the validity of hydrogel expansion models.

The instrumental seismicity of the western Alps: spatio-temporal patterns analysed with the wavelet transform

We perform a spatial and temporal analysis of the instrumental seismicity of the Western Alps, between latitude 41°–48° N and longitude 5°–10° E, using a recently revised catalogue available for the period 1962–1995 containing 7500 events in the magnitude range 2–5.9. Taking into account the fact that the major difficulty of such an analysis in an area of moderate seismicity is the long return period of the events and the diffusive character of the seismic swarms, we first carry out a statistical analysis of the 3-D distribution of the foci with the help of a 3-D wavelet transform. This smooths the location errors, which are estimated to be 1 km in epicentral coordinates and 5 km for the depth for recent years, whereas they are more than 10 km in epicentral coordinates for the oldest events. The good agreement between the shape of the filtered volumes and geological/tectonic features supports this new methodology for defining seismogenic zones, which are outlined better than by simple observations of the seismicity map. The study of seismic energy release shows the major heterogeneity in the mechanical behaviour of these seismogenic zones. An evaluation of seismic deformation has been carried out for some of these regions and compared with published geodetic results. The cumulative slip estimated for the Durance fault is 0.01 mm yr−1 (1° per cent of the geodetic estimation), and for the Vuache fault it is 0.19 mm yr−1, whereas historical triangulation yields horizontal movements of up to 5 mm yr−1 in the neighbourhood of the Vuache fault. In the Valais region, the ‘seismic’ shear strain rate has been evaluated to be 5 × 10−4μrad yr−1 (0.5 per cent of the geodetic deformation); in the Ligurian Sea, the shortening rate deduced from seismological data is 1.1 mm yr−1 (20 per cent of the geodetic evaluation). The paradoxical result of this study is that areas where instrumental seismicity is low correspond to the location of strong historical seismicity or to regions of high geodetic deformation. Only two regions (Valais and the Ligurian Sea) seem to correspond in historical and present seismicity. These results show the difficulty of predicting seismic activity in such areas. The discrepancy between low seismic activity, high local deformation rate, and the moderate average velocity between Africa and Europe in the western Alpine region needs to be explained by any tectonic theory of the western Alps.

Constitutive equation for modeling mixed extension and shear in polymer solution processing

What is the simplest adequate model of shear thinning and extension thickening of polymer solutions at the intense local deformation rates in coating and related flows? We examine a combination of the Carreau equation of shear sensitivity with an analogous equation of extension sensitivity. The two contributions are mediated by the rate at which the principal straining axes turn with respect to the liquid itself, a relative rotation rate that is frame independent, vanishes in purely extensional flow, and harks back to Lumley’s (1969) idea of persistent molecular strain rate, which was adumbrated by Takserman‐Krozer (1963) and named by Frank and Mackley (1976). We indicate how the new equation can be used to extract viscosity‐in‐extension from the extension‐rich mixed flow between opposed nozzles. We illustrate how the new equation can be applied to the complex mixed flow in slide coating.

Some recent developments in VLBI in its application to crustal movement studies

Very Long Baseline Interferometry (VLBI), a technique developed by radio astronomers for the study of weak extragalactic radio sources, has been used for the determination of relative positions of very widely separated radio telescopes with an accuracy which cannot be attained by any other technique. Developments in VLBI have reached a stage in which continental drift and crustal deformations amounting to rates of change of position of a few cm a year can be measured with a high degree of confidence. The accuracy of such measurements may soon be expected to reach the level of a few mm. The technique is being applied also for the determination of relative positions of stations which are not very distant from each other. Measurements of local deformation rates in the earth's crust, and changes in the manner of their occurrence are beginning to provide better understanding of the mechanisms of the underlying processes. Obvious applications lie in the field of modelling earthquake generating processes, study of changes in the rate of rotation of the earth, and in the position of its rotation poles, earth-tide observations, and the like, all of which are being vigorously pursued. This paper is a review of some recent developments aimed at increasing the accuracy of measurements.

Kinematics, dynamics and stability of the tubular film extrusion of various polyethylenes

AbstractA basic study of the kinematics, dynamics, and heat transfer occuring during tubular film extrusion of polyethylene is outlined. Three rheologically characterized polyethylenes, a low‐density polyethylene (LDPE), a linear‐low‐density polyethylene (L‐LDPE), and a high‐density polyethylene (HDPE) were used in this study. The kinematics and stability of the tubular film process were investigated over a wide range of blow‐up ratios, drawdown ratios, and frost‐line heights. Local deformation rates along the bubble have been determined. Regions of stability and instability are described. Tensions and inflation pressures have been measured and expressed in terms of locol elongational viscosities. Temperature profiles along the bubble were determined and interpreted in terms of local heat transfer coefficients. Positions of crystallization and temperature profiles have been noted and used to estimate rates of crystallization. The characteristics of the LDPE, LLDPE, and HDPE are contrasted.

The earthquake deformation cycle, recurrence, and the time‐predictable model

Geodetic and geologic observations from a handful of well‐studied great plate boundary earthquakes provide a basis for exploring those features of the deformation cycle having the strongest influence on recurrence estimation. In ideal circumstances the time‐predictable model requires knowledge of only the seismic slip (and time) of the last earthquake and the rate of relative plate motion or fault slip. Practically, indirect measurements must usually suffice, local coseismic strain changes rather than seismic slip and local deformation rate rather than slip rate. Permanent, nonrecoverable deformation, often roughly half the coseismic strain offset near great thrust earthquakes, complicates the inference of interplate seismic slip. Owing to short‐ and long‐term postseismic transients, commonly 20–40% of the coseismic strain drop, deformation rates are variable and not simply related to the plate motion rate. At strike‐slip plate boundaries these complications can sometimes be avoided and seismic slip and slip rate can be obtained more or less directly. When these favorable circumstances are absent and when knowledge of the deformation cycle is incomplete, recurrence accuracy can be improved by empirically correcting for unknown elements of the cycle. Systematic features in the spatial distribution of the transients and permanent deformation near subduction zones help identify the regions where these corrections are largest and where the shortcomings of the empirical approach are most severe.