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Number of found documents:
310
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Dependence of saltation parameters on bed roughness and bed porosity
Kharlamova, Irina; Vlasák, Pavel
2012 - English
In numerical models of bed load transport a bed structure of channel could be described by two parameters, a size of bed particles and a standard deviation of normal distribution of bed particles in the vertical direction. The present paper deals with the effect of bed parameters on average length and height of one jump of saltating particle. A new formula was proposed for bed roughness based on size and standard deviation of the normal distribution of the bed particles. The dependences of length and height of the jump on the diameter of the saltating and bed particles were determined for different variation of vertical distribution of the bed particles.
Keywords:
saltation parameters; saltation length; saltation height; bed structure; normal distribution of bed particles; bed roughness
Available on request at various institutes of the ASCR
Dependence of saltation parameters on bed roughness and bed porosity
In numerical models of bed load transport a bed structure of channel could be described by two parameters, a size of bed particles and a standard deviation of normal distribution of bed particles in ...
LDA measurements and turbulence spectral analysis in an agitated vessel
Kysela, Bohuš; Konfršt, Jiří; Chára, Zdeněk
2012 - English
During the last years considerable improvement of the derivation of turbulence power spectrum from Laser Doppler Anemometry (LDA) has been achieved. The irregularly sampled LDA data is proposed to approximate by several methods e.g. Lomb-Scargle method, which estimates amplitude and phase of spectral lines from missing data, methods based on the reconstruction of the auto-correlation function (referred to as correlation slotting technique), methods based on the reconstruction of the time series using interpolation between the uneven sampling and subsequent resampling etc. These different methods were used on the LDA data measured in an agitated vessel and the results of the power spectrum calculations were compared. The measurements were performed in the mixing vessel with flat bottom. The vessel was equipped with four baffles and agitated with a six-blade pitched blade impeller. Three values of the impeller speed (Reynolds number) were tested. Long time series of the axial velocity component were measured in selected points. In each point the time series were analyzed and evaluated in a form of power spectrum.
Keywords:
LDA/LDV; turbulence; mixing
Available on request at various institutes of the ASCR
LDA measurements and turbulence spectral analysis in an agitated vessel
During the last years considerable improvement of the derivation of turbulence power spectrum from Laser Doppler Anemometry (LDA) has been achieved. The irregularly sampled LDA data is proposed to ...
Motion of rotating spherical particles touching a wall
Chára, Zdeněk; Vlasák, Pavel; Keita, Ibrahima
2012 - English
The paper deals with an analysis of motion of rotating spherical particle in calm water, when the particle is in contact with a smooth, horizontal wall. The motion was visualized by a fast digital camera at 1000 frames/second. Based on software analysis the particle trajectories as well as rotational velocities were determined. Values of vertical, horizontal and rotational velocities were used as input parameters for numerical model and the results are compared with experimental data. Experiments were performed with glass particle of diameter 25 mm, initial values of rotational speeds varied from 500 to 3000 revolution per minute.
Keywords:
particle rotation; particle trajectory; Magnus force
Available on request at various institutes of the ASCR
Motion of rotating spherical particles touching a wall
The paper deals with an analysis of motion of rotating spherical particle in calm water, when the particle is in contact with a smooth, horizontal wall. The motion was visualized by a fast digital ...
Sedimentation of dilute suspension in intermediate region
Vlasák, Pavel; Chára, Zdeněk
2012 - English
The present paper describes the results of experimental investigation of the continuous sedimentation of a dilute model of fine-grained suspension in the intermediate region of settlement. The effects of particle size (glass beads of average diameter 150–850 μm), concentration, and inclination of the vessel axis on particle fall velocity were evaluated. It was confirmed that the local relative particle-liquid velocity has a practically constant value across the vessel cross-section. The settling pattern is strongly affected even by a very gentle slope of the vessel axis, which causes an asymmetrical absolute velocity profile and significant increase of the local concentration and absolute fall velocity of particles near the upward-facing wall of the vessel.
Keywords:
sedimentation; intermediate settlement; particle size; concentration; vessel inclination
Available on request at various institutes of the ASCR
Sedimentation of dilute suspension in intermediate region
The present paper describes the results of experimental investigation of the continuous sedimentation of a dilute model of fine-grained suspension in the intermediate region of settlement. The ...
Influence of the spheroid prolongation on the drag force
Lukerchenko, Nikolay; Keita, Ibrahima; Kvurt, Y.; Miles, Jiří
2011 - English
The drag force acting on a spheroid moving perpendicularly to its axis of rotation in water was studied experimentally. Along the spheroid axis, which is normal to its axis of rotation, a round narrow hole was bored. The spheroid moved along a thin vertical thread stretched in water. A video system recorded the spheroid motion and the spheroid velocity was determined from the record. The drag force coefficient was calculated from the balance of forces acting on the spheroid. Two oblate, two prolate spheroids and one sphere with ratio of the axes 0.67; 0.81; 1.33; 2 and I (sphere), respectively, with approximately the same volumes, were used. The friction coefficient between the thread and spheroid was determined from the comparison of the experimental and calculated motions of the sphere, for which the drag force coefficient is known. The dependence of the drag force coefficient of the spheroid on the ratio of its semi-axes was obtained.
Keywords:
spheroid; drag force coefficient
Available in digital repository of the ASCR
Influence of the spheroid prolongation on the drag force
The drag force acting on a spheroid moving perpendicularly to its axis of rotation in water was studied experimentally. Along the spheroid axis, which is normal to its axis of rotation, a round narrow ...
Motion of multiple cylinders in potential flow of ideal fluid
Kharlamov, Alexander A.; Filip, Petr
2011 - English
The flow field past multiple arbitrarily moving cylinders is calculated using a generalisation of the conventional images method. The flow is presented by a flow from an infinite set of dipoles. For calculation purposes the infinite series must be, however, truncated. The kinetic energy of fluid is presented as a sum over all modelling dipoles. A rather good convergence of the method is observed. However, if the distances between any cylinders tend to zero, the method fails in zones close to the contact point. Nevertheless, even for the case of touching cylinders the series for the kinetic energy remain convergent. In case of just two cylinders the method projects on the classical method of images. Method is validated by several comparisons with solutions known from the literature.
Keywords:
generalised images method; potential flow; ideal fluid; circular cylinders
Available on request at various institutes of the ASCR
Motion of multiple cylinders in potential flow of ideal fluid
The flow field past multiple arbitrarily moving cylinders is calculated using a generalisation of the conventional images method. The flow is presented by a flow from an infinite set of dipoles. For ...
Modelling of bed in process of particle saltation in channel
Kharlamova, Irina; Kharlamov, Alexander A.; Chára, Zdeněk; Vlasák, Pavel
2011 - English
For numerical modelling particle saltation in channel with rough bed is important to define a bed configuration. The paper deals with the bed consisting of spherical particles of the different size then the saltating particle. In horizontal x-z plane the particles are arranged hexagonally. In vertical direction the particles are distributed according to Gaussian distribution. Before each collision of the saltating particle with bed the bed is shifted on a random distance and it is rotated by a random angle, so there is uniform probability to find a bed particle in any point of the x-z plane. The bed structure is chosen with aim to represent the natural bed as much as possible, thus the known information about distribution of the bed particles along y-direction is used and the location of bed particles in the x-z plane is controlled by principles of equal probability and minimal dense packing.
Keywords:
bed structure; normal distribution; hexagonal packing
Available in digital repository of the ASCR
Modelling of bed in process of particle saltation in channel
For numerical modelling particle saltation in channel with rough bed is important to define a bed configuration. The paper deals with the bed consisting of spherical particles of the different size ...
Coarse-particles conveying in pipes
Vlasák, Pavel; Kysela, Bohuš; Chára, Zdeněk
2011 - English
Flow behavior of coarse-grained slurry depends generally on particle size, shape, density and concentration. The present paper describes the results of experimental investigation of the model coarse-grained slurries on the pipe loop of inner diameter 36 mm with smooth pipes. Graded pebble gravel and glass balls conveyed in water were studied. Research was focused on evaluation of the effect of slurry velocity and concentration on hydraulic gradient versus slurry velocity relationship and the slurry flow behavior. Particle distribution in the pipe vertical cross section and motion of particles along the pipe bottom were studied in a transparent pipe section.
Keywords:
coarse-grained slurry; gravel-water mixture; pressure drops; flow structure
Available in digital repository of the ASCR
Coarse-particles conveying in pipes
Flow behavior of coarse-grained slurry depends generally on particle size, shape, density and concentration. The present paper describes the results of experimental investigation of the model ...
Simulation of water and heat transport considering preferential flow. Experimental site Na Lizu
Votrubová, J.; Dohnal, M.; Vogel, T.; Tesař, Miroslav
2011 - English
Coupled water and heat transport in soil profile at the experimental site Na Lizu (Volynka headwater catchment, Sumava Mountains, southern Bohemia) is studied. The site is located on a mild slope cov-ered with grass. The soil is Cambisol developed upon biotite paragneiss bedrock. One-dimensional dual-continuum model with semi-separate flow domains representing the soil matrix and the network of preferential pathways (S1D) is employed. Hydraulic parameters of the model are set based on labo-ratory retention curve measurements and in-situ measurements of hydraulic conductivity. Thermal pa-rameters are estimated using methodology of Cote and Konrad (2005). Simulation is conducted for the summer season of 2009. The in-situ monitored precipitation and soil surface temperatures are used as the model upper boundary condition. Model performance was evaluated through comparison with soil water pressure and soil temperatures monitored in situ.
Keywords:
soil heat transport; advective heat flux; preferential flow; dual-continuum approach
Available in digital repository of the ASCR
Simulation of water and heat transport considering preferential flow. Experimental site Na Lizu
Coupled water and heat transport in soil profile at the experimental site Na Lizu (Volynka headwater catchment, Sumava Mountains, southern Bohemia) is studied. The site is located on a mild slope ...
On the electrospinning of PVB solutions
Švrčinová, Petra; Filip, Petr; Lubasová, D.
2010 - English
Extension rheological characteristics belong to the principal description of polymer jet formation during a process of electrospinning. However, in spite of nearly absence of shear deformation of polymer solutions during the process of their spinnability, it seems that shear electrorheological characteristics can serve as an indicator of good or bad quality of an electrospun template. The experiments were carried out with four polyvinylbutyral (PVB) solutions in concentration of 10 wt%. Shear viscosities in absence (η0) and presence (η) of electric field were measured using a rotational rheometer equipped with an electrorheological cell. It was shown that the good solvents of PVB exhibit an almost constant curve of η/η0 vs. electric field strength, whereas the viscosity curves for the poor solvents exhibit enhancement. This enhancement corresponds to better quality of electrospun templates (obtained for polymer solutions of PVB with poor solvents).
Keywords:
electrospinning; elongational viscosity; polyvinylbutyral (PVB)
Available on request at various institutes of the ASCR
On the electrospinning of PVB solutions
Extension rheological characteristics belong to the principal description of polymer jet formation during a process of electrospinning. However, in spite of nearly absence of shear deformation of ...
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