Národní úložiště šedé literatury Nalezeno 19 záznamů.  1 - 10další  přejít na záznam: Hledání trvalo 0.00 vteřin. 
Wind tunnel tests for lifetime estimation of bridge and mast cables exposed to vortex induced vibrations
Trush, Arsenii ; Pospíšil, Stanislav ; Kuznetsov, Sergeii
A significant number of TV and radio broadcasting masts in the Czech Republic was built in the 70-80s of the last century. At the moment is an actual issue is the reconstruction and determination of residual life of these structures. Guyed masts and particularly guy ropes have significant dimensions and comparatively low mass and damping with high flexibility. Therefore, aerodynamic and aeroelastic loads, such as vortex induced vibrations, galloping, wind gusts, etc., are key for them. As a tensile construction elements (guy ropes) for guyed masts the traditional open wire spiral strand cables are used. This type of cable has a characteristic helical surface roughness pattern that can act as vortex suppressor, high fatigue endurance, although somewhat lower corrosion resistance comparing to modern locked coil cables with non-circular shaped wires of outer layer and cables with protective polymer coatings. At the same time, on numerous bridges with the above-mentioned modern cable types the fatigue damage to wires in anchorage zones and destruction of protective coatings was detected. Present paper provides results of wind tunnel testing of three models of helical strake cable in order to evaluate separately impact of lay angle and surface roughness factors and reference smooth cylinder model in flow with grid generated turbulence of different intensities. The reduction of the lock-in range of helical strand cables comparing to reference smooth model was observed whereby the greatest impact was an increase of lay angle.
Measurements of dynamic wind pressures on gable roofs – comparison with ČSN EN 1991-1-4
Pospíšil, Stanislav ; Macháček, Michael ; Hračov, Stanislav ; Kuznetsov, Sergeii ; Trush, Arsenii
The contribution is dealing with the wind tunnel measurements of dynamic pressures on the canopy roofs. It compares experimentally determined pressure coefficients Cp with the values in ČSN EN 1991-1-4 and former code ČSN 73 0035 as well. The pressures are expressed by the dynamic values. Thus, the averaged values are presented together with the fluctuating part, which significantly contributes to the overall loading. The measurement is carried out at two roof models, two basic directions, and two turbulence intensities.
Investigation of Strouhal number of ice-accreted bridge cables at moderate flow turbulence
Górski, P. ; Tatara, M. ; Pospíšil, Stanislav ; Kuznetsov, Sergeii
This paper is concerned with the investigation of the Strouhal number (St) for a stationary iced cable model of cable-supported bridges using wind tunnel tests with respect to three principal angles of wind attack. The investigations were carried out in the Climatic Wind Tunnel Laboratory of the Czech Academy of Sciences in Telč. The St values were determined at the flow turbulence intensity of the order of 12%, within the range of the Reynolds number from 2.4·10^4 to 12.9·10^4, based on the dominant vortex shedding frequencies measured in the wake of the model. The obtained experimental St values of the iced cable of a cable-supported bridge will allow to determine the critical wind velocity as well as will be helpful for prediction of the cable response due to the vortex excitation phenomenon at moderate flow turbulence condition.
Wind-tunnel simulation of thermally unstable atmospheric flow in complex terrain
Pospíšil, Stanislav ; Kuznetsov, Sergeii ; Kozmar, H. ; Michalcová, V.
The paper presents the characteristics of thermal effects in wind-tunnel simulations of the atmospheric boundary layer (ABL) flow developing above complex terrain. The differences and instable stratification effects between the windward and leeward sides of a hill model were investigated for different wind characteristics. Particularly, the buoyancy effects on the structure of the ABL simulation in a wide range of stability conditions expressed by Richardson number are studied.
Development of Boundary Layer on a Heated Flat Plate
Antoš, Pavel ; Kuznetsov, Sergeii
An experimental study of boundary layer on a flat plate with heat transfer was done. Temperature and velocity boundary layer development was studied on a uniformly heated flat plate with zero pressure gradient by means of hot-wire anemometry. Several levels of heat flux at the wall were used. The difference between the wall temperature and free-stream temperature was set up to 180 K. Measurements were performed in the closed-circuit wind tunnel with zero pressure gradient at free-stream velocity of 3-5 m/s and turbulence intensity of 0.8 percent. The velocity and the temperature profiles of mean values and fluctuations are presented.
Variation of Strouhal number on iced cable in sub-transitional range
Pospíšil, Stanislav ; Górski, P. ; Kuznetsov, Sergeii ; Tatara, M. ; Marušić, A.
The influence of ice accretion, angle of attack and Reynolds number on the flow field around iced cables of the cable-supported bridges is not clearly understood. The paper presents results of a wind tunnel investigations of Strouhal number of stationary iced cable model of cable-supported bridges. The investigations were carried out in a Climatic Wind Tunnel Laboratory of ITAM. The methodology leading to the experimental icing of the inclined cable model in the climatic section of the laboratory was prepared. For the aerodynamic investigations the iced cable model in a smaller scale was reproduced with combination of photography and 3-D printing procedure. The Strouhal number (St) was determined within the range of the Reynolds number (Re) between 2.4×10^4 and 16.4×10^4, based on the dominant vortex shedding frequency measured in the flow behind the model. The model was orientated at three principal angles of wind attack for each of the Re. Good agreement with the generally reported value in the sub-critical Re range for a circular cylinder was obtained.
Comparison of flutter derivatives for Kao Pin Hsi Bridge and flat plate
Buljac, A. ; Pospíšil, Stanislav ; Kozmar, H. ; Kuznetsov, Sergeii ; Král, Radomil
Focus of this work is put on evaluation of dimensionless aeroelastic coefficients, i.e. flutter derivatives, which are considered to be indicators of the aeroelastic stability of bridges. A comprehensive experimental study on dynamic wind-induced behavior of the Kao-Pin-Hsi bridge in Taiwan is described. The results are compared to those of the thin flat plate with high width to height ratio, which previously proved to be aeroelastically stable.
Influence of ice accretion at bridge rope on Strouhal number
Pospíšil, Stanislav ; Gorski, P. ; Kuznetsov, Sergeii
The paper is concerned with the method and results of wind tunnel investigations of the/nStrouhal number (St) of a stationary iced cable model of cable-supported bridges with/nrespect to different angles of wind attack. The methodology leading to the experimental/nicing of the inclined cable model was prepared in a climatic section of the laboratory./nThe St values were determined within the range of the Reynolds number (Re) between/n2.4·104 and 16.5·104, based on the dominant vortex shedding frequencies measured in/nthe wake of the model.
The stability of the cable supported bridge deck during wind and traffic
Král, Radomil ; Buljac, Andrija ; Pospíšil, Stanislav ; Kuznetsov, Sergeii
With nowadays trends of elongation of span length in bridge structures, they are becoming more and more sensitive to wind loads, therefore analysis of their aero elastic stability is of great concern. These structures have low eigen frequencies and low damping, which makes them highly susceptible to actions of the wind. Studies of flow-induced vibrations, which are excited by complex interaction between airflow and moving bridge girder, are of great importance for estimation of dynamic response of bridge structures.
Transient heat transfer coefficient on precooled circular cylinder at various reynolds numbers
Král, Radomil ; Beran, Pavel ; Kuznetsov, Sergeii
In this work, the experimental investigation of unsteady averaged heat transport coefficient is carried out. Using precooled concrete circular cylinder being immersed into the air cross-flow at constant temperature level, the heat convection for various wind speed ranging from 5 to 15 m/s is determined. The attention is also given to the temperature distribution over the circular cross-section with respect the time evolution. Instantaneous energy lost is evaluated in average together with its distribution over the cylinder. The heat transfer effect expressed by the non-dimensional Nusselt numvber is identified and graphically interpreted.

Národní úložiště šedé literatury : Nalezeno 19 záznamů.   1 - 10další  přejít na záznam:
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