Volume 10, number 2 (2016)
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Item Numerical analysis of a pedestrian to car collision: Effect of variations in walk(University of West Bohemia, 2016) Špička, Jan; Vychytil, Jan; Hynčík, LuděkThis work is focused on the modelling of car to pedestrian crash scenario. Virtual hybrid human body model VIRTHUMAN as well as a simplified model of car chassis is modelled under Virtual Performance Solution software. The main idea of the work is the investigation and sensitivity analysis of various initial conditions of the pedestrian during frontal car crash scenario, such as position of the extremities due to different step phases or turning of the pedestrian around his own axis. The experimental data of human gait measurement are used so that one human step is divided into 9 phases to capture the effect of walk when the pedestrian crosses a road. Consequently, the influence of different initial conditions on the kinematics, dynamics of the collision together with injury prediction of pedestrian is discussed. Moreover, the effect of walk is taken into account within translational velocities of the full human body and rotational velocities of the extremities. The trend of the injury prediction for varying initial conditions is monitored. The configurations with zero and non-zero initial velocities are compared with each other, in order to study the effect of walking speed of the pedestrian. Note that only the average walking speed is considered. On the basis of the achieved results, the importance or redundancy of modelling the walking motion and the consideration of different step phases in the car-pedestrian accident can be examined.
Item Single variable refined beam theories for the bending, buckling and free vibration of homogenous beams(University of West Bohemia, 2016) Sayyad, Atteshamuddin Shamshuddin; Ghugal, Yuwaraj M.In this paper, single variable beam theories taking into account effect of transverse shear deformation are developed and applied for the bending, buckling and free vibration analysis of thick isotropic beams. The most important feature of the present beam theories is that unlike any other higher order theory, the proposed class of theories contains only one unknown variable and does not require shear correction factor. The displacement field of the present theories is built upon the classical beam theory. The theories account for parabolic distribution of transverse shear stress using constitutive relations, satisfying the traction free conditions at top and bottom surfaces of the beam. Governing differential equation and boundary conditions of these theories are obtained using the principle of virtual work. Results obtained for the displacements, stresses, fundamental frequencies and critical buckling loads of simply supported isotropic solid beams are compared with those obtained by other theories to validate the accuracy of the present theories.Item Enhancement of centrifugal compressor stable operation using an internal recirculation channel(University of West Bohemia, 2016) Růžička, MiroslavCentrifugal compressors with a high pressure ratio arewidely used in small aircraft turbine engines and turbocharges. At high rotational speeds they have a narrow stable operating region and the commonly used impellers with back swept blades are not able to ensure requested stability. In order to achieve a wider stable operating region, some other anti-surgemeasures can be used such as an Internal RecirculationChannel (IRC) located in the inlet area of the compressor impeller. This paper analyses the influence of various IRC inlet slot geometries on air flow parameters. The flow velocity profiles downstream of the IRC outlet slot are evaluated and the influence of various channel configurations on compressor performance map is shown. Results from experiments as well as CFD simulations enable us to analyse the airflow in IRC and find out its most suitable geometry.Item Influence of mesh density on calculated extreme stresses in aortic aneurysms(University of West Bohemia, 2016) Man, Vojtěch; Polzer, Stanislav; Burša, JiříThe paper deals with evaluation of the influence of finite element mesh density on the resulting extreme stresses in models of abdominal aortic aneurysms. In most patient-specific computationalmodels published recently, a free mesh of tetrahedrons is used and any information on density of the applied mesh is often missing. In this study a comparison of differentmesh densities has been realized with four patient-specificmodel geometries, all based on a numerical reconstruction of the unloaded geometry of the aneurysm, and with two different Yeoh-type constitutive models. It has been shown that resulting maximumstresses are not mesh independent; due to a better description of the stress gradient in the critical location, the maximum wall stress increases with increasing number of elements throughout the wall thickness, especially in models without residual stresses. This effect is more pronouncedwhen using Vande Geest constitutive model with higher strain stiffening than for Raghavan-Vorp material parameters. Although the mesh density requirements were not so high when the stress gradient was reduced by taking residual stresses into consideration, even in this case low numbers of elements throughout the wall thickness may givemesh dependent results. Although for a rigorous recommendation of the mesh density more analyses are needed, it was shown that the time consuming procedure of taking residual stresses into consideration cannot be replaced by a simpler model with rough mesh.Item Numerical investigation of diffuser solidity effect on turbulent airflow and performance of the turbocharger compressor(University of West Bohemia, 2016) Chehhat, Abdelmadjid; Si-Ameur, Mohamed; Boumeddane, Boussad; Abo-Serie, Essam; Boulahrouz, SalimLow solidity diffuser in centrifugal compressors can achieve both high efficiency and wide operating ranges which is of great importance for turbocharger compressor. Low solidity is achieved by using a low chord to pitch ratio. In this work, a CFD simulation is carried out to examine the effect of solidity on airflow field of a turbocharger centrifugal compressor which consists of a simple-splitter impeller and a vaned diffuser. By changing the number of diffuser vanes while keeping the number of impeller blades constant, the solidity value of the diffuser is varied. The characteristics of the compressor are evaluated for 6, 8, 10 and 12 stator vanes which correspond to solidity of: 0.78, 1.04, 1.29 and 1.55, respectively. The spatial distribution of the pressure, velocity and turbulent kinetic energy show that the diffuser solidity has significant effect on flow field and compressor performance map. The compressor with a 6 vanes diffuser has higher efficiency and operates at a wider range of flow rate relative to that obtainedwith larger vans number.However a non-uniform flow at the compressor exit was observedwith relatively high turbulent kinetic energy.
