2024-11-26
10:48 |
Úplný záznam
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2024-11-26
10:48 |
Úplný záznam
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2024-11-26
10:48 |
Úplný záznam
|
2024-11-26
10:48 |
Úplný záznam
|
2024-11-26
10:48 |
Úplný záznam
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2024-11-10
00:01 |
The influence of the blockage effect on the aerodynamic characteristics of the bluff body in the air flow
Ledvinková, Blanka ; Hračov, Stanislav ; Macháček, Michael
Experiments on airflow around bluff bodies were conducted in a confined space in a wind tunnel. The presence of the body induces the blockage effect—reducing the cross-sectional area of the tunnel and causing an increase in flow velocity. 2D simulations of flow around sharply edged bodies (a rectangle with a 2:1 side ratio and a U-profile with a non-porous barrier) were performed at various angles of attack using the RANS k-ω SST model to evaluate the influence of the blockage effect. Two different computational domains were used: the first, with spacing between confining walls matching the actual dimensions of the wind tunnel at our institute (1.9 m), and the second, with a width of 7.5 m, assuming the blockage effect to be negligible in this case. The mean drag and lift coefficients, fluctuating lift coefficient, and Strouhal number were evaluated for both cases.
Úplný záznam
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2024-11-10
00:01 |
Influence of torsional stiffness on the galloping instability of a low-voltage insulated power line
Macháček, Michael ; Hračov, Stanislav
Our research contribution focuses on a three-dimensional nonlinear numerical study of the galloping phenomenon in a specific bundled overhead power line affected by ice accumulation. We investigated the susceptibility to this self-induced oscillatory behavior at critical wind speeds necessary to initiate the dynamic response in a low-tension cable with simulated icing that parallels observations on analogous real-world conductors. Given the highly nonlinear mechanical properties of such flexible cables, we employed the Newmark integration technique in conjunction with the iterative Newton-Raphson approach. We analyzed two numerical models of the overhead line under wind loading: one assumes linear mechanical behavior under nonlinear wind loading, while the other incorporates full nonlinearity, including geometrical aspects. Our study assessed the impact of torsional stiffness on galloping instability, finding that considering torsional stiffness and the three-dimensional movement of cables under wind can amplify the dynamic response during galloping.
Úplný záznam
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2024-11-10
00:01 |
Numerical analysis of non-stationary response types in the stochastic van der Pol equation
Fischer, Cyril ; Náprstek, Jiří
Slender structures subjected to external excitation with a dominant frequency near, but outside, the lock-in frequency region exhibit quasi-periodic behaviour in deterministic cases and highly irregular responses when random perturbations are introduced. The nature of the stochastic response changes significantly as the excitation frequency moves further from the lock-in region. This contribution explores theoretical and numerical methods for characterizing the stochastic behaviour of these responses.
Úplný záznam
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2024-11-10
00:01 |
Case study of galloping in a low-voltage insulated bundled overhead line
Hračov, Stanislav ; Macháček, Michael
An analysis of the susceptibility of a bundled overhead line to galloping is presented, focusing on a case study of an aerial cable composed of four conductors insulated with polyethylene, used in low-voltage power lines. The susceptibility to instability is analyzed for the cable both with and without simulated icing, as observed on similar real conductors. In the first case, the proneness to galloping was ruled out based on the results of a CFD simulation and the Den Hartog criterion. In the second case, the potential for galloping was confirmed. The critical wind velocity for the ice-covered cable was calculated using quasi-steady theory. Finally, the amplitudes of limit cycle oscillation for supercritical wind speeds were estimated through a simplified numerical analysis. Numerical simulations were also carried out for several different angles of attack—i.e., various rotational angles of the line's cross-section relative to the wind direction—particularly close to the critical angles, to determine and analyze the maximum possible dynamic response.
Úplný záznam
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2024-11-10
00:01 |
Vibration and stability of a self-excited aeroelastic SDOF system with random coefficient perturbations
Náprstek, Jiří ; Fischer, Cyril
A system with aerodynamic self-excitation and random parametric and additive perturbations can be stabilized by non-linear damping, within certain limits, in the sense of stability in probability. Although the system remains unstable in terms of asymptotic stability, its response will exhibit an elevated but still finite variance. The potential for stabilization depends on achieving a critical level of non-linearity and the degree of correlation between parametric and additive noise. This contribution demonstrates this effect by analyzing the stochastic response computed using the stochastic linearisation method.
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