ISSN:
eISSN:
1450-5584
2406-0925

## Theoretical and Applied Mechanics

Теоријска и примењена механика

## Articles in Press

 Variational model of scoliosis Igor Popov, Nikita Lisitsa, Yuri Baloshin, Mikhail Dudin, and Stepan Bober Available online 7 November 2017 Abstract Scoliosis, being one of the most widespread spine diseases among children, has been studied extensively throughout the history of medicine, yet there is no clear understanding of its initiating factors and the mechanogenesis of the monomorphic three-dimensional deformation due to its polyetiological nature. We present a novel mathematical model of the process of emergence of three-dimensional deformation of human spine based on variational principles. Typical scoliosis geometry is assumed to be described as minimal curves of a particular energy functional, which are shown to closely resemble actual scoliosis. The numerical properties of the first stage of scoliosis are investigated, which is shown to have the highest influence on the development of the disease. Mathematics Subject Classification 92C10; 74L15 Keywords spine, model, variational method An effect of a purely dissipative process of microstresses on plane strain gradient plasticity problems Adebowale Borokinni, Odunayo Fadodun, and Adegbola Akinola Available online 2 March 2018 Abstract This article considers a plane strain gradient plasticity theory of the Gurtin--Anand model [M. Gurtin, L. Anand, A theory of strain gradient plasticity for isotropic, plastically irrotational materials Part I: Small deformations, J. Mech. Phys. Solids 53 (2005), 1624--1649] for an isotropic material undergoing small deformation in the absence of plastic spin. It is assumed that the system of microstresses is purely dissipative, so that the free energy reduces to a function of the elastic strain, while the microstresses are only related to the plastic strain rate and gradient of the plastic strain rate via the constitutive relations. The plane strain problem of the Gurtin-Anand model for a purely dissipative process gives rise to elastic incompressibility. A weak formulation of the flow rule is derived, making the plane strain problem suitable for finite element implementation. Mathematics Subject Classification 74C10 Keywords plane strain gradient, microstresses, flow rule, weak formulation Asymptotic solution for the Darcy-Brinkman-Boussinesq flow in a pipe with helicoidal shape Igor Pažanin Available online 5 July 2018 Abstract We study the fluid flow and heat transfer in a helical pipe filled with a sparsely packed porous medium. Motivated by the engineering applications, pipe's thickness and the distance between two coils of the helix have the same small order of magnitude, whereas the fluid inside the pipe is assumed to be cooled (or heated) by the exterior medium. After writing the dimensionless Darcy-Brinkman-Boussinesq system in curvilinear coordinates, we employ the multi-scale expansion technique to formally derive the effective model valid for small Brinkman-Darcy number. The obtained asymptotic solution is given in the explicit form which is important with regards to numerical simulations. Comparison with our previous results on the straight-pipe flow is also provided. Mathematics Subject Classification 35B40; 35Q35; 76S05 Keywords helical pipe, Darcy-Brinkman-Boussinesq system, Newton cooling condition, curvilinear coordinates, asymptotic approximation On the influence of turbulent kinetic energy level on accuracy of 𝑘 − 𝜀 and LRR turbulence models Djordje M. Novković, Jela M. Burazer, Aleksandar S. Ćoćić, and Milan R. Lečić Available online 13 July 2018 Abstract This paper presents research regarding the influence of turbulent kinetic energy (TKE) level on accuracy of Reynolds averaged Navier--Stokes (RANS) based turbulence models. A theoretical analysis of influence TKE level on accuracy of the RANS turbulence models has been performed according to the Boussinesq hypothesis definition. After that, this theoretical analysis has been investigated by comparison of numerically and experimentally obtained results on the test case of a steady-state incompressible swirl-free flow in a straight conical diffuser named Azad diffuser.~Numerical calculations have been performed using the OpenFOAM CFD software and first and second-order closure turbulence models. TKE level, velocity profiles and Reynolds stresses have been calculated downstream in four different cross sections of the diffuser. Certain conclusions about modeling turbulent flows by $k-\varepsilon$ and LRR turbulence models have been made by comparing the velocity profiles, TKE distribution and Reynolds stresses on the selected cross sections. Mathematics Subject Classification 76F10 Keywords CFD, diffuser, OpenFOAM, RANS, turbulent kinetic energy Experimental and numerical analysis of flow field and ventilation performance in a traffic tunnel ventilated by axial fans Milan Šekularac and Novica Janković Available online 5 September 2018 Abstract To investigate air flow in longitudinally ventilated traffic tunnels, a scaled model of a typical road-traffic tunnel with an appropriate ventilation system based on axial ducted fans, is designed and built in the Lab. The focus of this paper is the airflow in a bi-directional traffic, two-lane tunnel. At the scale ratio of approx. 1:20, at 20.52m length it represents ≈400m of a real-scale tunnel. The model consists of two parallel tunnel tubes, where the main tunnel (with a hydraulic diameter of Dh1≈0.4m) has the geometry of a scaled road traffic-tunnel. The second tunnel (Dh2≈0.16m) has a smaller size and is circular in cross-section, used only to simulate airflow towards an evacuation tunnel tube. Thus the two tunnels are connected by the evacuation passages, equipped with adjustable escape doors. By a combination of experimental and numerical work, the air flow-field and the performance of the ventilation system are investigated. The velocity field and its turbulence properties exiting the fans were determined experimentally using hot-wire anemometry. These data were further processed to be used in the tunnel flow computations by CFD. The efficiency of momentum transfer (ηi, Kempf factor) of the longitudinal tunnel ventilation is determined. The effect that the imposed boundary conditions and the level of their detail, have within a CFD computation of tunnel airflow, with respect to accuracy, velocity distribution and computed ηi. Finally a traffic-loaded (traffic "jam") case of flow is studied through experiment and CFD. The difficulty in assessing the required thrust of the plant in traffic-jam tunnel conditions is discussed, and the ventilation efficiency is estimated. Based on later results, the two limiting shapes of axial velocity distribution with respect to height above the road, in this type of tunnel and traffic, are estimated. The last result can be used as a realistic boundary condition (as inlet b.c. and/or initial condition) for numerical studies of flow and fire scenarios in such tunnels with the traffic load critical for design. Mathematics Subject Classification 76-05; 76D99; 76F99 Keywords ventilation flows, tunnels, axial ducted fans, turbulence, numerical boundary conditions, CFD Constitutive modeling for FRP composite materials subject to extreme loading Robert J. Asaro and David Benson Available online 7 September 2018 Abstract A physically based, finite deformation, rate and temperature dependent theory and model have been developed to simulate the deformation and failure of FRP composite materials and structures. Failure modes include: inter alia, fiber crushing and kinking as occurs during extreme compressive loading; fiber fracture as occurs in for example fragmentation; interlaminar shear as occurs at elevated temperatures and that leads to kinking; debonding and delamination including the coupling with laminate kinking; and debonding as occurs in cored FRP panels. The theory/model is capable of describing quasi-static (including creep) as occurs at elevated temperatures, and dynamic deformation and failure as occurs during shock, blast or impact. The model is implemented within LS DYNA and specific example simulations are described that illustrate the theory/model capabilities. In Part I, fragmentation is not covered in detail. Fiber fracture and fragmentation are to be covered to detail with specific examples in Part II. Mathematics Subject Classification 74D10; 74E30 Keywords FRP composite buckling, FRP composite compressive failure
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