Ktisis at Cyprus University of Technologyhttp://ktisis.cut.ac.cyThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Sat, 23 Sep 2017 19:58:26 GMT2017-09-23T19:58:26Z5051Quasi-steady flow in sloping aquifershttp://ktisis.cut.ac.cy/handle/10488/9239Title: Quasi-steady flow in sloping aquifers
Authors: Akylas, Evangelos; Gravanis, Elias; Koussis, Antonis D.
Abstract: Mass conservation links the storage S and the outflow Q of an aquifer. A relation between them (an S-Q relation) provides then a model governing the evolution of these quantities. In this work we construct an analytical quasi-steady state model which exploits the properties of the exact S-Q relation associated with steady state solutions of the Boussinesq equation for the sloping aquifer (that is, the Henderson and Wooding [1964] solutions). The model is derived by matching the asymptotic forms of the exact S-Q relation which arise for small and large values of the Henderson and Wooding parameter λ. These asymptotic forms provide a novel rederivation of well-known semiempirical S-Q relations of the form Q ∝ S and Q ∝ S2, and they lead to soluble quasi-steady state models. The quadratic asymptotic relation turns out to hold for surprisingly low values of λ. This characteristic and its formal properties allow smooth matching with the linear relation at λ=π2/4=2.47. The obtained model holds over the entire parameter space. An important characteristic of the model, stemming from its derivation, is that it involves only the geometric and hydraulic quantities present in the exact Boussinesq equation. The model is tested by best fitting four data sets from experiments simulating aquifer drainage. The derived curves for the drained volume are in excellent agreement with the data. The estimated values for k and n are also in overall very good agreement with their reference values.
Sun, 01 Nov 2015 00:00:00 GMThttp://ktisis.cut.ac.cy/handle/10488/92392015-11-01T00:00:00ZGeneralized Batchelor functions of isotropic turbulencehttp://ktisis.cut.ac.cy/handle/10488/9389Title: Generalized Batchelor functions of isotropic turbulence
Authors: Gravanis, Elias; Akylas, Evangelos
Abstract: We generalize Batchelor's parameterization of the autocorrelation functions
of isotropic turbulence in a form involving a product expansion with multiple
small scales. The richer small scale structure acquired this way, compared to
the usual Batchelor function, is necessary so that the associated energy
spectrum approximate well actual spectra in the universal equilibrium range. We
propose that the generalized function provides an approximation of arbitrary
accuracy for actual spectra of isotropic turbulence over the universal
equilibrium range. The degree of accuracy depends on the number of higher
moments which are determinable and it is reflected in the number of small
scales involved. The energy spectrum of the generalized function is derived,
and for the case of two small scales is compared with data from high-resolution
direct numerical simulations. We show that the compensated spectra (which
illustrate the bottleneck effect) and dissipation spectra are encapsulated
excellently, in accordance with our proposal.
Sun, 11 Jan 2015 00:00:00 GMThttp://ktisis.cut.ac.cy/handle/10488/93892015-01-11T00:00:00ZAn analytical solution in probabilistic rock slope stability assessment based on random fieldshttp://ktisis.cut.ac.cy/handle/10488/4918Title: An analytical solution in probabilistic rock slope stability assessment based on random fields
Authors: Gravanis, Elias; Pantelidis, Lysandros; Griffiths, D. V.
Abstract: An analytical solution for calculating the probability of failure of rock slopes against planar sliding is proposed. The method in based on the theory of random fields accounting for the influence of spatial variability on slope reliability. In this framework, both the cohesion and friction coefficient along a discontinuity are treated as Gaussian random fields which are fully described by their mean values View the MathML source, standard deviations View the MathML source, spatial correlation lengths View the MathML source, and the parameters View the MathML source which account for the cross-correlation between cohesion and coefficient of friction. As shown by the examples presented herein, the spatial correlation of shear strength can have an important influence on slope performance expressed by the probability of failure. This is a significant observation, since ignoring the influence of spatial correlation in design may lead to unconservative estimations of slope reliability.
Wed, 01 Oct 2014 00:00:00 GMThttp://ktisis.cut.ac.cy/handle/10488/49182014-10-01T00:00:00ZInfluence of spatial variability on rock slope reliability using 1-D random fieldshttp://ktisis.cut.ac.cy/handle/10488/4931Title: Influence of spatial variability on rock slope reliability using 1-D random fields
Authors: Pantelidis, Lysandros; Gravanis, Elias; Griffiths, D.V.
Abstract: In this work, the theory of random fields is used to account for the influence of spatial
variability on slope reliability. Within this framework the friction coefficient along a
discontinuity is treated as a Gaussian random field which is fully described by its mean value,
standard deviation and spatial correlation length. The random field is simulated using the
Local Average Subdivision (LAS) method. As shown by the examples presented herein, the
spatial correlation of shear strength along a failure plane can have an important influence on
slope performance, as expressed by the failure probability. This is a significant observation
since ignoring the influence of spatial correlation in design may lead to non-conservative
estimations of slope reliability. The planar mode of failure is considered.
Thu, 01 Jan 2015 00:00:00 GMThttp://ktisis.cut.ac.cy/handle/10488/49312015-01-01T00:00:00ZStationarity of linearly forced turbulence in finite domainshttp://ktisis.cut.ac.cy/handle/10488/7684Title: Stationarity of linearly forced turbulence in finite domains
Authors: Gravanis, Elias; Akylas, Evangelos
Abstract: A simple scheme of forcing turbulence away from decay was introduced by Lundgren some time ago, the "linear forcing," which amounts to a force term that is linear in the velocity field with a constant coefficient. The evolution of linearly forced turbulence toward a stationary final state, as indicated by direct numerical simulations (DNS), is examined from a theoretical point of view based on symmetry arguments. In order to follow closely the DNS, the flow is assumed to live in a cubic domain with periodic boundary conditions. The simplicity of the linear forcing scheme allows one to rewrite the problem as one of decaying turbulence with a decreasing viscosity. Its late-time behavior can then be studied by scaling symmetry considerations. The evolution of the system in the description of "decaying" turbulence can be understood as the gradual symmetry breaking of a larger approximate symmetry to a smaller symmetry that is exact at late times. The latter symmetry implies a stationary state: In the original description all correlators are constant in time, while, in the "decaying" turbulence description, that state possesses constant Reynolds number and integral length scale. The finiteness of the domain is intimately related to the evolution of the system to a stationary state at late times: In linear forcing there is no other large scale than the domain size, therefore, it is the only scale available to set the magnitude of the necessarily constant integral length scale in the stationary state. A high degree of local isotropy is implied by the late-time exact symmetry, the symmetries of the domain itself, and the solenoidal nature of the velocity field. The fluctuations observed in the DNS for all quantities in the stationary state can be associated with deviations from isotropy that is necessarily broken at the large scale by the finiteness of the domain. Indeed, to strengthen this conclusion somewhat, self-preserving isotropic turbulence models are used to study evolution from a direct dynamical point of view. Simultaneously, the naturalness of the Taylor microscale as a self-similarity scale in this system is emphasized. In this context the stationary state emerges as a stable fixed point. We also note that self-preservation seems to be the reason behind a noted similarity of the third-order structure function between the linearly forced and freely decaying turbulence, where, again, the finiteness of the domain plays a significant role.
Sat, 01 Jan 2011 00:00:00 GMThttp://ktisis.cut.ac.cy/handle/10488/76842011-01-01T00:00:00Z