Browsing by Author "Ubulom, Iroizan"

Now showing 1 - 3 of 3
  • Thumbnail Image
    ItemOpen Access
    Application of spectral method for vibration-induced high-cycle fatigue evaluation of an HP turbine blade
    (American Society of Mechanical Engineers, 2020-11-11) Ubulom, Iroizan
    A fluid-structure interaction coupling is implemented for a forced-response, vibration-induced fatigue life estimation of a high-pressure turbine blade. Two simulations approaches; fully-coupled and uncoupled methods, are implemented to investigate the influence of fluid-solid coupling on a turbine blade structural response. The fatigue analysis is performed using the spectral moments estimated from the response power spectral densities of the two cases. The method is compared against similar prediction using the time-domain approach with mean stress correction. Correspondingly, the mean stress and multiaxiality effects are also accounted for in the frequency domain spectral approach. In the mean stress case, a multiplication coefficient is derived based on the Morrow equation, while the case of multiaxiality is based on a criterion which reduces the triaxial stress state to an equivalent uniaxial stress using the critical plane assumption. The analyses show that while the vibration-induced stress histories of both simulation approaches are stationary, they violate the assumption of normality of the frequency domain approaches. The stress history profiles of both processes can be described as platykurtic with the distributions having less mass near its mean and in the tail region, as compared to a Gaussian distribution with an equal standard deviation. The fully-coupled method is right leaning with positive skewness while the uncoupled approach is left leaning with negative skewness. Noticeable differences were found in the peak distribution of the normal stresses for both methods, but the predicted Euler angle orientations were consistent in both cases.
  • Thumbnail Image
    ItemOpen Access
    Improved delayed detached eddy simulations applied to a NACA0015 aerofoil subject to acoustic excitation
    (AIAA, 2022-06-20) Coskun, Seyfettin; Pachidis, Vassilios; Ubulom, Iroizan; Bacic, Marko
    The effect of acoustic excitation on the aerodynamic performance of NACA0015 aerofoil at $\alpha=11^{\circ}$ and $Re_c = 1.0 \ x 10^6$ based on $c=0.35m$ chord length is investigated numerically using Improved Delayed Detached Eddy Simulations (IDDES). Flow separation is observed over the suction surface in the uncontrolled flow that forms a separated shear layer, resulting in aerodynamic performance degradation. External acoustic excitation is used in controlling the flow separation by exciting the separated shear layer. Acoustic excitation frequency and amplitude are the control parameters considered. The flow field is excited at a frequency of the most amplified disturbances and at first harmonic and sub-harmonic frequencies with a constant excitation amplitude. Unlike the harmonics, when the separated shear layer is excited at $F+=1$, the separation characteristics of the uncontrolled flow field are significantly altered. Unsteady disturbances are locked into the excitation frequency and transition is promoted within the shear layer. A separation bubble is generated over the suction surface with a reattachment region aft of the aerofoil surface. Vortex coherence and organized wake structures are also improved. The effect of excitation amplitude is investigated at the frequency of the most amplified disturbances. Increasing the excitation amplitude has two predominant impacts; it delays the boundary layer separation and advances the reattachment location, thereby reducing the separation bubble length. The results also suggest that there is a maximum effective acoustic excitation amplitude.
  • Thumbnail Image
    ItemOpen Access
    Influence of fluid-structure interaction modelling on the stress and fatigue life evaluation of a gas turbine blade
    (SAGE, 2020-10-27) Ubulom, Iroizan
    A computational method of fluid-structure coupling is implemented to predict the fatigue response of a high-pressure turbine blade. Two coupling levels, herein referred to as a “fully coupled” and “decoupled” methods are implemented to investigate the influence of multi-physics interaction on the 3 D stress state and fatigue response of a turbine blade. In the fully-coupled approach, the solutions of the fluid-flow and the solid-domain finite element problem are obtained concurrently, while in the decoupled approach, the independently computed aerodynamic forces are unilaterally transferred as boundary conditions in the subsequent finite element solution. In both cases, a three-dimensional unsteady stator-rotor aerodynamic configuration is modelled to depict a forced-vibration loading of high-cycle failure mode. Also analyzed is the low-cycle phenomenon which arises due to the mean stresses of the rotational load of the rotating turbine wheel. The coupling between the fluid and solid domains (fully-coupled approach) provides a form of damping which reduces the amplitude of fluctuation of the stress history, as opposed to the decoupled case with a resultant higher amplitude stress fluctuation. While the stress amplitude is higher in the decoupled case, the fatigue life-limiting condition is found to be significantly influenced by the higher mean stresses in the fully-coupled method. The differences between the two approaches are further explained considering three key fatigue parameters; mean stress, multiaxiality stress state and the stress ratio factors. The study shows that the influence of the coupling between the fluid and structures domain is an important factor in estimating the fatigue stress history.