IRIS Università degli Studi di Bolognahttps://cris.unibo.itIl sistema di repository digitale IRIS acquisisce, archivia, indicizza, conserva e rende accessibili prodotti digitali della ricerca.Thu, 15 Apr 2021 15:08:47 GMT2021-04-15T15:08:47Z10331UNSTEADY COUETTE FLOW OF A BINGHAM FLUIDhttp://hdl.handle.net/11585/62646Titolo: UNSTEADY COUETTE FLOW OF A BINGHAM FLUID
Abstract: Many fluids which are of interest in engineering, as bentonite, slurries, fresh concrete, ceramic past, molten polymers, behave as Bingham fluids, i.e. fluids which present a yield stress. This paper investigates numerically the unsteady motion between two coaxial cylinders when the internal one rotates about its axis. The constitutive law of the fluid presents a discontinuity for zero shear rate which introduces severe difficulties in solving any problem of unsteady motion, both analytically and numerically. A suitable way to avoid the obstacle is to regularize the constitutive equation using a smooth function to approximate the Bingham law: in this paper the adopted law is based on the error function. Different procedures of start-up and cessation of flow and some periodical flows are examined numerically using an implicit finite difference method.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/11585/626462008-01-01T00:00:00ZFLUID FLOW AND HEAT TRANSFER OF A NON-NEWTONIAN FLUID IN A MICRO -ANNULUS IN THE PRESENCE OF VISCOUS DISSIPATIONhttp://hdl.handle.net/11585/572504Titolo: FLUID FLOW AND HEAT TRANSFER OF A NON-NEWTONIAN FLUID IN A MICRO -ANNULUS IN THE PRESENCE OF VISCOUS DISSIPATION
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/11585/5725042016-01-01T00:00:00ZUnsteady simple shear flow in a viscoplastic fluid:
comparison between analytical and numerical solutionshttp://hdl.handle.net/11585/83451Titolo: Unsteady simple shear flow in a viscoplastic fluid:
comparison between analytical and numerical solutions
Abstract: Abstract In this paper, an unsteady flow of a viscoplastic
fluid for simple shear flow geometry is solved numerically
using two regularizing functions to overcome
the discontinuity for zero shear rate of the Bingham
constitutive law. The adopted models are the wellknown
Papanastasiou relation and one based on the
error function. The numerical results are compared
with the analytical solution of the same problem obtained
by Sekimoto (J Non-Newton Fluid Mech 39:107–
113, 1991). The analysis of the results emphasizes
that the errors are much smaller in the yielded than
in the unyielded region. Themodels approximate closer
the ideal Bingham model as the regularization parameters
increase. The differences between the models tend
to vanish as the regularization parameters are at least
greater than 105.
Fri, 01 Jan 2010 00:00:00 GMThttp://hdl.handle.net/11585/834512010-01-01T00:00:00ZCessation of pipe flow of a Bingham fluidhttp://hdl.handle.net/11585/91195Titolo: Cessation of pipe flow of a Bingham fluid
Abstract: The aim of this paper is to examine closely the cessation of the flow of Bingham fluids in a circular pipe. The constitutive law, which has a singularity for zero shear rate, has been regularised using four different relations: that of Papanastasiou, a bi-viscous one and two new models proposed by the authors, based on the error function erf and the hyperbolic function tanh respectively. The results have been obtained with an implicit finite difference method and show that for the cessation of motion there are appreciable differences, particularly for the values of the stopping time, which for the proposed models are intermediate between the Papanastasiou and the bi-viscous ones. The stopping time as function of the Bingham number has been evaluated for the Erf model, emphasizing the very strong agreement between the numerical results and the theoretical upper limit given by Glowinski.
Fri, 01 Jan 2010 00:00:00 GMThttp://hdl.handle.net/11585/911952010-01-01T00:00:00ZPipe flow of fluids with time-dependent rheological behaviourhttp://hdl.handle.net/11585/48657Titolo: Pipe flow of fluids with time-dependent rheological behaviour
Abstract: Several technical applications make use of fluids whose rheological characteristics change with time owing to the external shearing stress, even if it is steady. They are the so called rheopectic or thixotropic fluids, depending on whether the fluid viscosity increases or decreases significantly with time owing to an imposed shear stress or shear rate. In thixotropic fluids the applied stress breaks gradually the molecular structure decreasing the viscosity, which, as the applied stress is removed and the structure rebuilds, increases again. The time-scales can range from many minutes in the case of breakdown to many hours in rebuilding.
Thixotropy has been widely studied; even recently several models have been proposed, e.g. by Dullaert and Mewis: the latter produced in 1979 a well informed review which is still worth consulting. A more recent detailed description of the phenomenon, an extensive review of time effects on viscosity and of the thixotropic materials is due to Barnes which gives an exhaustive list of references. Nakaishi and Yasutomi examine theoretically and experimentally the thixotropic properties of clay-water systems, showing that they depend on the length of the measurement time.
A typical application of thixotropic fluids is electronic packaging (Chen et al.) where epoxy and adhesive are used for encapsulation and surface mounting; the dispensed amounts of materials have to be well controlled, and generally vary with time: typically in a first time interval the fluid is delivered, and then the motion is stopped; the fluid viscosity changes during the delivery time and when the force inducing the flow is removed. Other remarkable applications concern the behaviour of fluids used in paint industry: the rheological properties of paints depend on the polymer matrix; the thixotropy plays a fundamental role in sagging and levelling during and after the application process (Armelin et al.); or fluids used in the drill industry, which are subjected to cyclic pressure and temperature loads when circulating in the bore; many of these fluids (e.g. xanthan) exhibit a viscosity degradation and are not able to re-acquire the original viscosity (Fiber et al., Billingham and Ferguson). Many organic fluids too present thixotropic characteristics; the rheological properties of human blood depend on its time-variable microstructure: the aggregation and the disaggregation process of red cells, which collide rarely, take place over different time scales, giving rise to thixotropic behaviour (Owens). Fluids used in food industry such as yogurt, butter and mayonnaise have often a gel structure where the micelles are linked to each other in the form of a tridimensional matrix which includes the liquid phase; the application of a shear modifies the structure and gives rise to a time-dependent viscosity (O’Donnel and Butler, Gopalakrishnan et al.). The aim of this paper is to solve the equation of the unsteady motion in circular pipes of a Newtonian fluid with time-dependent viscosity. Two different situations are considered: in the first the hydraulic slope is applied for a finite time interval whereas in the second the time interval is infinite and an asymptotic steady flow can be reached. Simple expressions are given, suitable for the numerical calculation of velocity and flow discharge in both cases.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/11585/486572007-01-01T00:00:00ZPoiseuille flow of a Giesekus fluid with non-zero solvent viscosityhttp://hdl.handle.net/11585/566303Titolo: Poiseuille flow of a Giesekus fluid with non-zero solvent viscosity
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/11585/5663032016-01-01T00:00:00ZMoto periodico di un fluido di Ostwald-de Waele fra due tubi coassialihttp://hdl.handle.net/11585/299913Titolo: Moto periodico di un fluido di Ostwald-de Waele fra due tubi coassiali
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/11585/2999132013-01-01T00:00:00ZCouette-Poiseuille flow of the Giesekus model between parallel plateshttp://hdl.handle.net/11585/67419Titolo: Couette-Poiseuille flow of the Giesekus model between parallel plates
Thu, 01 Jan 2009 00:00:00 GMThttp://hdl.handle.net/11585/674192009-01-01T00:00:00ZUnsteady Flow of Fluids With Arbitrarily Time-Dependent Rheological Behaviorhttp://hdl.handle.net/11585/590674.2Titolo: Unsteady Flow of Fluids With Arbitrarily Time-Dependent Rheological Behavior
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/11585/590674.22017-01-01T00:00:00ZNumerical solution for unsteady Couette flow of viscoplastic fluidshttp://hdl.handle.net/11585/73923Titolo: Numerical solution for unsteady Couette flow of viscoplastic fluids
Abstract: Many fluids which are of interest in engineering, as bentonite, slurries, fresh concrete, ceramic past, molten polymers, behave as Bingham fluids, i.e. fluids which present a yield stress. This paper investigates numerically the unsteady motion between two coaxial cylinders when the internal one rotates about its axis. The constitutive law of the fluid presents a discontinuity for zero shear rate, which introduces severe difficulties in solving any problem of unsteady motion, both analytically and numerically. A suitable way to avoid the obstacle is to regularize the constitutive equation using a smooth function to approximate the Bingham law: in this paper the suggested law is based on the error function. Different procedures of start-up and cessation of flow are examined numerically using an implicit finite difference method
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/11585/739232008-01-01T00:00:00Z