1,148 research outputs found
A Streamwise Constant Model of Turbulence in Plane Couette Flow
Streamwise and quasi-streamwise elongated structures have been shown to play
a significant role in turbulent shear flows. We model the mean behavior of
fully turbulent plane Couette flow using a streamwise constant projection of
the Navier Stokes equations. This results in a two-dimensional, three velocity
component () model. We first use a steady state version of the model to
demonstrate that its nonlinear coupling provides the mathematical mechanism
that shapes the turbulent velocity profile. Simulations of the model
under small amplitude Gaussian forcing of the cross-stream components are
compared to DNS data. The results indicate that a streamwise constant
projection of the Navier Stokes equations captures salient features of fully
turbulent plane Couette flow at low Reynolds numbers. A system theoretic
approach is used to demonstrate the presence of large input-output
amplification through the forced model. It is this amplification
coupled with the appropriate nonlinearity that enables the model to
generate turbulent behaviour under the small amplitude forcing employed in this
study.Comment: Journal of Fluid Mechanics 2010, in pres
On Orbits for a Particular Case of Axial Symmetry
A particular case of steady state and axial symmetry - the potentialformula proposed by Miyamoto and Nagai - is studied. A number of orbits of abound test particle is determined numerically, with both, the potentialparameters and initial conditions, varied. Unlike special cases, such asnearly circular and nearly planar orbits, in the case of "truly spatial orbits" the time dependence of the coordinates becomes very complicated and amathematical treatment including any known periodic functions is hardlypossible. Bearing in mind that orbits studied in the present paper aredetermined by three elements, the authors propose the mean values over time ofthe squares of velocity components to characterize them
Fuzzy practical exponential tracking of an electrohydraulic servosystem
Cilj ovog rada je da doprinese teorijskoj i praktiÄnoj primeni fazi logiÄkog upravljanja koriÅ”Äenjem koncepta praktiÄnog praÄenja. Predlaže se novi fazi upravljaÄki algoritam za ostvarivanje željenog kvaliteta praÄenja jednog elektrohidrauliÄkog pozicionog servosistema, koji se može naÄi u mnogim industrijskim ureÄajima. Fazi logiÄki kontroler je jedan od najjednostavnijih. On koristi samo jednu ulaznu veliÄinu, sa linearnom metodom zakljuÄivanja. Fazi prateÄi algoritam upravljanja je zasnovan na principu samoprilagodljivosti. Strukturna karakteristika takvog sistema upravljanja je postojanje dve povratne sprege: globalne, negativne po izlaznoj veliÄini i lokalne, pozitivne po upravljaÄkoj veliÄini. Takva struktura obezbeÄuje sintezu upravljanja bez poznavanja unutraÅ”nje dinamike objekta i bez merenja poremeÄajnih veliÄina. Predloženi fazi prateÄi algoritam upravljanja obezbeÄuje promenu greÅ”ke izlazne veliÄine po unapred definisanom eksponencijalnom zakonu. Prezentuju se rezultati simulacije nelinearnog matematiÄkog modela hidrauliÄkog servosistema.The aim of this paper is to contribute to the theoretical and practical applications of fuzzy logic control using practical tracking concept. A new fuzzy control algorithm is proposed to achieve the desired tracking performance of a nonlinear electrohydraulic position servo system, which can be found in many manufacturing devices. The fuzzy logic controller is one of the simplest. It employs only one input, with linear fuzzy inference method. The practical tracking control algorithm is based on the selfadjustment principle. The structural characteristic of such a control system is the existence of two feedback sources: the global negative of the output value and the local positive of the control value. Such a structure ensures the synthesis of the control without the internal dynamics knowledge and without measurements of disturbance values. The proposed fuzzy practical control algorithm ensures the change of the output error value according to a prespecified exponential law. The simulation results of the nonlinear mathematical model of the hydraulic servo system are presented
Fuzzy practical exponential tracking of an electrohydraulic servosystem
Cilj ovog rada je da doprinese teorijskoj i praktiÄnoj primeni fazi logiÄkog upravljanja koriÅ”Äenjem koncepta praktiÄnog praÄenja. Predlaže se novi fazi upravljaÄki algoritam za ostvarivanje željenog kvaliteta praÄenja jednog elektrohidrauliÄkog pozicionog servosistema, koji se može naÄi u mnogim industrijskim ureÄajima. Fazi logiÄki kontroler je jedan od najjednostavnijih. On koristi samo jednu ulaznu veliÄinu, sa linearnom metodom zakljuÄivanja. Fazi prateÄi algoritam upravljanja je zasnovan na principu samoprilagodljivosti. Strukturna karakteristika takvog sistema upravljanja je postojanje dve povratne sprege: globalne, negativne po izlaznoj veliÄini i lokalne, pozitivne po upravljaÄkoj veliÄini. Takva struktura obezbeÄuje sintezu upravljanja bez poznavanja unutraÅ”nje dinamike objekta i bez merenja poremeÄajnih veliÄina. Predloženi fazi prateÄi algoritam upravljanja obezbeÄuje promenu greÅ”ke izlazne veliÄine po unapred definisanom eksponencijalnom zakonu. Prezentuju se rezultati simulacije nelinearnog matematiÄkog modela hidrauliÄkog servosistema.The aim of this paper is to contribute to the theoretical and practical applications of fuzzy logic control using practical tracking concept. A new fuzzy control algorithm is proposed to achieve the desired tracking performance of a nonlinear electrohydraulic position servo system, which can be found in many manufacturing devices. The fuzzy logic controller is one of the simplest. It employs only one input, with linear fuzzy inference method. The practical tracking control algorithm is based on the selfadjustment principle. The structural characteristic of such a control system is the existence of two feedback sources: the global negative of the output value and the local positive of the control value. Such a structure ensures the synthesis of the control without the internal dynamics knowledge and without measurements of disturbance values. The proposed fuzzy practical control algorithm ensures the change of the output error value according to a prespecified exponential law. The simulation results of the nonlinear mathematical model of the hydraulic servo system are presented
Examination of weld defects by computed tomography
Defects in metal arc gas (MAG) welds made in S235JR low carbon steel of 6 mm thickness were examined. A sample containing lack of fusion (LOF) and pores was examined by computed tomography ā CT. The computed tomography examination was performed in order to define LOF size and position as well as dimensions and distribution of accompanying pores in the weld metal
Ispitivanje greŔaka zavara kompjutorskom tomografijom
Defects in metal arc gas (MAG) welds made in S235JR low carbon steel of 6 mm thickness were examined. A sample containing lack of fusion (LOF) and pores was examined by computed tomography ā CT. The computed tomography examination was performed in order to define LOF size and position as well as dimensions and distribution of accompanying pores in the weld metal.Ispitivane su greÅ”ke u zavarenom spoju niskougljiÄnog Äelika S235JR debljine 6 mm zavarenog MAG postupkom. Uzorak koji sadrži greÅ”ke naljepljivanja i pore je ispitivan kompjutorskom tomografijom - KT. Programskom analizom tomografa je odreÄena veliÄina i pozicija greÅ”aka naljepljivanja kao i dimenzije i raspored prateÄih pora u zavarenom spoju
Quasi-Chaplygin Systems and Nonholonimic Rigid Body Dynamics
We show that the Suslov nonholonomic rigid body problem can be regarded
almost everywhere as a generalized Chaplygin system. Furthermore, this provides
a new example of a multidimensional nonholonomic system which can be reduced to
a Hamiltonian form by means of Chaplygin reducing multiplier. Since we deal
with Chaplygin systems in the local sense, the invariant manifolds of the
integrable examples are not necessary tori.Comment: minor changes, to appear in Letters in Mathematical Physic
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