Journals Published by Vilnius Tech
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Sound absorption qualities of a cylinder-shaped isolated acoustic resonator
No full textIn the article, an isolated acoustic resonator consisting of a cylinder-shaped hole of variable volume and with large and variable diameter is theoretically analyzed.Theoretical formulaes by which sound absorption of such resonator is computed are presented. The computations incorporate the influence exerted by overtones, i.e. odd harmonics, and supplementary resonator mass upon sound absorption. A change in the sound absorbtion depending on the resonator hole diameter, the angle of sound wave incidence and its volume, i.e. the distance to the rigid surface, has been established. Sound absorption reaches its maximum at low frequencies and it depends strongly on the hole diameter. Under constant hole diameter, sound absorption does not depend on the resonator volume, i.e. the distance to the rigid surface. Absorption notably depends on the angle of sound wave incidence. When this angle increases, absorption also increases at resonant frequency, while the frequency itself moves towards higher frequencies. When the sound wave incidence is 90 degrees with respect to resonator, maximum absorption is already observed at high frequencies.Real and imaginary parts of the radiation impedance of cylindric hole and of the impedance of the hole itself, as well as their dependency on frequency are computed. In this case, hole impedance consists of the impedances of the hole and air resiliency. The influence exerted by the real and imaginary parts upon sound absorption has been established. The resonance takes place when the imaginary parts of radiation and hole impedances intersect.
Article in Lithuanian.
Cilindrinės formos vienetinio akustinio rezonatoriaus garso absorbcinės savybės
First Published Online: 26 Jul 201
Semi-analytically based finite elements and their application to thin-walled beams
No full textPusiauanalizinės teorijos bazėje suformuluoti baigtiniai elementai ir jų taikymas plonasienėms sijoms
Santrauka. Pateikiami naujo tipo pusiauanalizinės teorijos pagalba suformuluoti sijų baigtiniai elementai. Šie elementai išvedami iš pusiauanalizinės aukštesnės eilės sijų teorijos, nagrinėjančios sijos skerspjūvį kaip pusiauanalizinių elementų ansamblį. Elemento modelis yra sudaromas prisilaikant hierarchinės dviejų pakopų schemos, kur išskiriami pirminiai ir antriniai kintamieji. Aptartos abiejų pakopų funkcinės (pusiaudiskretinės) priklausomybės. Jos diskretizuojamos mišriais baigtiniais elementais, panaudojant Lagranžo daugiklių metodą. Bendrosios baigtinio elemento priklausomybės panaudotos plonasienių sijų elementams sudaryti. Gauti elementai išbandyti skaitiškai sprendžiant statikos ir dinamikos uždavinius bei lyginant su kitais sprendiniais.Pasiūlyti elementai turi laisvės laipsnius, suderinamus su kontinualiais kūnais, bei leidžia spręsti uždavinius, kur apkrovų bei atramų išsidėstymas išeina iš klasikinių sijų teorijų rėmų.
First Published Online: 26 Jul 201
A non-linear third-order triangular laminated plate element for analysis of cracked rc slabs and beams
No full textTrečios eilės trikampio sluoksniuoto elemento panaudojimas supleišėjusių gelžbetoninių plokščių ir sijų analizei
Santrauka. Straipsnyje pateiktas universalus sluoksniuotos plokštės netiesinės medžiagos baigtinių elementų modelis, pritaikytas supleišėjusios gelžbetoninės plokštės skaičiavimui. Pasirinktas sudėtingas analitiškai suformuotas trečios eilės trikampis sluoksniuotas Reisnerio-Mindlino plokštės elementas, turintis 50 laisvės laipsnių. Programa įvertina atskirų plokštės sluoksnių netiesines deformacijas, supleišėjimą. Betono fizikiniame modelyje įvertinta tempiamo ir gniuždomo betono įtempimų-deformacijų diagramos žemyn krentanti dalis. Skaičiavimo modelis ir programa patikrinti, atlikus turimų ir teorinių rezultatų palyginimą. Straipsnyje pateiktų gelžbetoninės sijos ir plokštės paskaičiuoti įlinkiai gerai atitiko eksperimentinius.
First Published Online: 26 Jul 201
Experimental determining of time and temperature strength parameters on which calculations of fire resistance can be based
No full textThe author presents results of his own experiments for determining strength parameters of steel in thermal conditions of fire on which calculations of fire resistance can be based. The author suggests the calculation of the thermal strength parameters on the basis of noncoventional tensile test in thermal conditions described by the “time-temperatures” curve. Test bars were loaded with the constant force, presented as a steady stress level. The author also investigated the influence of various thermal conditions, described by some “time-temperature” curves, on strength parameters.
Article in Lithuanian.
Laiko ir temperatūros parametrų, taikomų skaičiuoti atsparumą ugniai, eksperimentinis nustatymas
First Published Online: 26 Jul 201
Procedures of city transport systems\u27 planning
No full textThe article starts with looking into a treatment of the objects of city transport systems planning: administrative (determined) and systematic approach, that are of importance for the principles of planning and its quality.The major shortages of the traditional planning are the following: its determination, narrow functional attitude towards the processes of transportation, lack of evaluation of the interrelations between a city structure and its transport system.There are several obstacles to shift to the planning of higher quality, namely poorly arranged information supply, shortage of mathematical models for transport processes and relevant soft ware. Considering the gained experience of transport system planning and professional knowledge on transport systems, a new methodology of planning is presented. A major stress here is put on the complex of transport system functioning, the consequences and the price.
Article in Lithuanian.
Miestų susisiekimo sistemų planavimo proceso procedūros
First Published Online: 26 Jul 201
Distorsion in structural mechanics
No full textThe behaviour of elastic perfectly plastic structures under given loadings and distorsion is examined until plastic failure (analysis problem). “Distorsion” is here for the strain which does not satisfy the compatibility equations. A distorsion can be caused by prestressing, supports displacements or lack of precision during fabrication stage (the plastic strain is the example of distorsion, too). The mathematical models for analysis problems are created by the help of extremum energy principles—principles of complementary energy and total potential energy minimum. The emphasis is put on traditional to the structural mechanics bar and plate bending systems. The main unknowns are the self-equilibrium forces, displacements and strains. In the case of bar systems the analysis problem is formulated by means of linear yielding conditions as a quadratic programming problem. The analysis of Lagrange\u27s generalised problem shows that the displacements and forces from given distorsions and plastic strains should be obtained by the same expressions. For the algoritmisation of Lagrange\u27s problem different methods can be used: finite difference or finite element methods. Very convenient are well-known procedures of the equilibrium finite elements (with forces as main unknowns; the conditions of equilibrium satisfied) for building of stiffness and flexibility matrices. In the present paper only on the estimation of distorsion in the self-equilibrium finite elements and in the structural shakedown problems are given. The numerical example of plate analysis is presented.
Article in Russian.
First Published Online: 26 Jul 201