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Aeroelastic instability of differently porous U-profiles in crosswind direction
Hračov, Stanislav ; Macháček, Michael
Flow-induced vibrations of the flexibly mounted slender U-shaped beams allowed to oscillate in the crosswind direction only are studied experimentally in the wind tunnel. All beams are characterized by a cross section having a side ratio of along-wind to across-wind dimension equal to two. The effects of two depths of U profiles and two porosities of their flanges ( 0 % and 75 %) onto a loss of aeroelastic stability are investigated under the smooth flow conditions and for low Scruton numbers. The results indicate almost similar proneness of the non-porous beams to galloping-type oscillations to a rectangular prism with the same side ratio regardless their depth. The onset of across-wind galloping occurred in these cases at wind velocity very close to von-Kármán-vortex-resonance flow speed, even though the critical velocity predicted by the quasisteady theory is much lower. For porous and shallower U profile this asynchronous quenching also takes\nplace. However, the higher flange porosity reduces significantly not only the vortex-shedding effect, but also causes an increase in the onset galloping velocity above the critical speed determined for non-porous profiles. In the case of deeper U-shaped beam, the effect of higher porosity even suppresses the proneness to galloping
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Aerodynamic wind tunnel testing of U-beams
Hračov, Stanislav ; Macháček, Michael
The paper presents the outcomes from the experimental testing of the set of slender U-beams in the climatic wind tunnel. All analysed beams have identical basic geometry with the U-shaped cross section given by the side ratio equal to 2 (having the short side perpendicular to the flow), but they differ in the porosity of their flanges and in the depth of their profile. Two depths of the U-profile combined with six different levels of flange porosity are analysed. The U-beams were tested in the smooth flow in order to determine their aerodynamic coefficients for various angles of wind attack. The influences of the depth and porosity onto these coefficients are studied in detail. Moreover, the susceptibility of each individual case to transversal galloping is assessed based on the classical quasi-steady theory. The comparison with the results from the aerodynamic tests of the prisms with rectangular cross-sections having side rations equal to two, four and six is also given and discussed.
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Numerical study of the air flow around the U-profile
Ledvinková, Blanka ; Hračov, Stanislav ; Macháček, Michael
Our contribution deals with the numerical simulations of the air flow around u-profiles at various angles of attack with the aim to investigate their proneness to galloping. The Unsteady Reynolds Averaged Navier- Stokes (RANS) simulations were performed in the COMSOL Multiphysics software, k-ω SST turbulence model was used. The values of the aerodynamic coefficients and Strouhal number were evaluated and the results were compared with the results obtained experimentally in the wind tunnel.
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Study of aeroelastic interference effect among four cylinders arranged in rectangular configuration
Hračov, Stanislav ; Macháček, Michael ; Michálek, Petr
The paper describes the analysis of the destructive oscillation of a skewed steel rod with the length 21 m. The steel rod is a part of roof supporting bundle (rectangular array) of cylindrical hangers at the airport building; it is therefore exposed to frequent moderate and sometimes strong winds with the low turbulence. The vibration is analyzed from the aeroelastic perspective, i.e. the cause of the oscillation is analyzed as wake-induced vibration because the collapse was observed at the bottom downwind hanger in that array configuration. The normalized distance of hangers in horizontal and vertical directions is equal to 3.15D, and 3.8D, respectively, where D is the diameter of the hanger cross section. Experimental testing of 1:1 scaled sectional model of four-cylinder array in the wind tunnel indicates, that the cause of the high level vibration is most probably the interference galloping. The unstable behavior was detected both for the downwind hanger as a part of rectangular array and for the hanger as a part of a two hangers in a row only. The results indicate the strong influence of the top pair of the hangers on the decrease of the critical wind velocity, which is even below the value of the critical wind velocity obtained for two hangers only. The strong hysteresis effect resulting in the preservation of the high vibration amplitudes even after the decrease of wind speed far below the critical one, was observed.
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Souhrnná výzkumná zpráva z měření vibrací v objektu ELI Beamlines
Pospíšil, Stanislav ; Bayer, Jan ; Hračov, Stanislav ; Urushadze, Shota
Souhrnná zpráva popisuje postupy a výsledky série měření mikro-vibrací podlah a stěn laserových hal v objektu ELI. Úroveň změřených vibrací konstrukcí je posouzena s ohledem na kritéria NIST, která se aplikují pouze prostory s nejpřísnějšími vibračními požadavky. Speciální pozornost je zaměřena také na zhodnocení různých úprav podlah provedených lokálně, ve vybraných místech. Tyto úpravy byly realizovány za účelem snížení hladiny vibrací i z krátkodobého hlediska, které klientem zvolená metodika posouzení nepostihovala.
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