Tom Bajcar

Numerial simulation and experimental study of non-newtonian mixing flow with a free surface

The object of this work was to evaluate the capability of numerical simulation to predict different features of non-Newtonian fluid mixing process. A relatively simple impeller (six bladed vane rotor) was used for the mixing of carboxymethyl cellulose. A LDA method was used to measure the tangential velocity at two points inside the mixing vessel. Using visualization, a significant vortex above the impeller was observed. The shape of the free surface was determined by a geometrical reconstruction of the images of the illuminated section. Torque on the impeller shaft was measured to determine the characteristics of the fluid. Fluent program package was used for the simulation. The problem is challenging since the effects of non-Newtonian fluid, mixing process and free surface have to be included in the simulation. The comparison between the experimental and numerical results confirms the accuracy of the simulations.

Study of velocity field at model sideweir using visualization method

A visualization method was employed for accurate non-intrusive measurement of velocity fields at a physical model of a sharp-crested rectangular sideweir under subcritical flow. The experimental observation of velocity vectors at various horizontal planes over the entire width of the main channel confirms that the flow conditions at sideweir are non-uniform. The coefficients of non-uniform velocity distribution were in the range from 1 to 1.1. The present study focuses on the relation between the longitudinal components of the overflow velocities and the corresponding cross-sectional average velocities in the main channel, detailed as a function of flow depth and of location along the sideweir crest. For different sideweir geometries, these coefficients varied between 1 and 1.2.

Kinematic properties of the jellyfish Aurelia sp.

A new, relatively simple method for determining the kinematic properties of jellyfish is presented. The bell movement of the scyphomedusa (Aurelia sp.) during its pulsation cycle was analysed using computer-aided visualization. Sequences of video images of individual Aurelia in a large aquarium were taken using a standard video camera. The images were then processed to obtain time series of the relative positions of selected points on the surface of the medusa’s bell. The duration of the bell relaxation was longer than that of the bell contraction, thereby confirming published results. In addition, the area of the exumbrellar surface of Aurelia increased during bell relaxation by more than 1.3-times that of the exumbrellar surface area during the maximum contraction of the bell. The volume change during the bell pulsation cycle was also measured using the same visualization method. Significant changes, of up to 50%, in the subumbrellar cavity volume were revealed while, in contrast, the volume between the exumbrellar and subumbrellar surfaces generally remained unchanged during the entire pulsation cycle of the bell. Comparison of the time series of the exumbrellar surface area and of the subumbrellar cavity volume indicated that the change of volume takes place before the change of the surface area of the bell.

Simultaneous study of pressure pulsation and structural fluctuations of a cavitated vortex core in the draft tube of a Francis turbine

The paper represents a simultaneous structural dynamics analysis and static pressure pulsation of a cavitated vortex core in the draft tube of a Francis turbine. Comparison of the static pressure oscillation signal and the dynamics of the cavitated vortex core structure, which were quantified by the computer-aided visualisation led to the experimental modelling of the cavitated vortex core. Through the wavelet analysis significant correlation of the observed processes and their interaction was confirmed. The obtained results of the experiments enables us to create a numerical prediction of the cavitated vortex core dynamics based on the well-known Fanelli differential equation [1]. Measured pressure and structure fluctuations of the cavitated vortex core were introduced into the differential equation. Consequently, using the method of least squares, the characteristic coefficients of the model at different operating conditions in a Francis model turbine could be determined. Appropriate regression coefficient with its typically high values (r2 > 0.7) confirms the correct choice of the applied type of differential equation and gives the opportunity to set up a phenomenological model that shows the interdependence between the complex cavitated vortex core dynamics and the performance characteristics of a turbine.

Numerical Investigation of Flow in the Vicinity of a Swimming Jellyfish

Abstract The paper deals with the simulation of unsteady flow around jellyfish Aurelia sp. with consideration of the cyclic behaviour of the jellyfish bell. The bell cyclic movement was extracted from the video recordings of a swimming jellyfish while its displacement was simultaneously calculated from the numerically predicted forces of the bell on the fluid. Numerical simulation enabled us to predict the time evolution of the velocity field in the vicinity of the jellyfish and the formation of vortex rings that were observed during experiments. Comparison with experimental investigations revealed a good correlation in the flow pattern between forces and swimming velocity. Because of a relatively simple problem set-up the study also indicates a good possibility for further numerical studies of locomotion of organisms where experiments are hard or impossible to perform. Additionally a hypothesis that the organism uses a slightly asymmetric swimming technique to preserve a symmetric flow field was formulated.

Thermovision method for diagnostics of local characteristics of natural draft cooling towers

Abstract Increased and more rigorous demands for power station operation in order to minimize both the cost of electrical energy production and environment pollution, addressed the necessity for optimisation of all segments of the process. As a consequence, there are certain needs for the introduction of new experimental methods which would ensure effective diagnostics. A newly developed experimental method for thermovisional detection of the temperature field in natural draft cooling towers is presented in this paper. The method is adapted to the real conditions that are present inside cooling towers and enables spatial‐ and time‐dependent detection of local temperatures in the region of drift eliminators of natural draft cooling towers. Combined with the previously developed methods for detection of velocity and temperature fields, the thermovision method enables quick detection of the local efficiency of cooling towers. Results of the method can be used for diagnostics of local and integral characteristics of cooling tower operation. Apart from this, they enable prediction of local corrections in order to increase the cooling tower efficiency.

Flow kinematics in a rotating axial diffuser

Abstract A study of the flow velocity field inside a rotating axial diffuser with a circular cross-section is presented in the paper. Measurements of the flow velocity components were performed by the LDA system and confirmed the existence of two types of the flow: a rotating region near the spinning wall and a non-rotating region near the longitudinal axis of the diffuser. Significant increase of all three-velocity components was observed in a thin layer at the rotating diffuser wall. Influence of the diffuser rotation and its geometry on the radial outflow was observed as well. In addition, a phenomenological model for prediction of the distribution of the velocity components in the diffuser near-wall region was developed. Exponential law was applied to the phenomenological model, which included the dimensionless velocity component as a function of two Reynolds numbers––axial and tangential. The developed model confirmed experimental results and showed substantial influence of axial and tangential Reynolds numbers on all velocity components in the vicinity of the diffuser wall.