Vibration Suppression of a Harmonically Forced Oscillator via a Parametrically Excited Centrifugal Pendulum

Vibration suppression has been a widely studied topic for a long time, with various modifications in passive vibration mitigation devices to improve the efficacy. One such modification is the addition of the inerter. The inerter has been integrated into various vibration mitigation devices, whose mass amplification effect could be used to enhance the performance of dynamic vibration absorbers. In the current study, we consider an inerter based pendulum vibration absorber (IPVA) system and conduct a theoretical study on vibration suppression of the device. The IPVA system operates based on the principle of nonlinear energy transfer, wherein the energy of the primary structure is transferred into the pendulum vibration absorber. This is the result of parametric resonance of the pendulum, where the primary resonance of the system becomes unstable and a harmonic regime containing a frequency half the resonant frequency emerges (referred to as secondary regime). We use the harmonic balance method along with bifurcation analysis using Floquet theory to study the stability of primary resonance. It is observed that a pitchfork bifurcation and period-doubling bifurcation are necessary for nonlinear energy transfer to occur. Furthermore, we integrate the IPVA with a linear, harmonically forced oscillator to demonstrate its efficacy compared with a linear benchmark. We also examine the effects of various system parameters on the occurrence of the secondary regime. Moreover, we verify the nonlinear energy transfer phenomenon (due to the occurrence of the secondary regime) by numerical Fourier analysis.

Influence of a Non-Axisymmetric Endwall on the Flow Field in a Turbine Passage: High-Resolution LDV

The present paper focuses on the influence of a non-axisymmetric endwall on the flow field in a turbine passage. Therefore, laser Doppler velocimetry (LDV) is applied to both a non-axisymmetric endwall and a flat endwall as reference. Time mean values and fluctuating components of the velocity field are evaluated. Three-component LDV with up to 38000 valid coincident events per second is carried out up- and downstream of a linear cascade with high resolution, especially in the near-wall region of the suction surface, where secondary vortices are expected. The three velocity components recorded by the LDV are transformed into the orthogonal coordinate system of the turbine cascade, and several flow quantities, like turbulence parameters and parameters describing the secondary regime, are assessed. By comparing the data for the non-axisymmetric endwall and the flat endwall, the influence of the contouring on the secondary flow field is shown. To substantiate the measurements and gain a deeper insight, the data are compared to previous surface oil flow visualizations and specific results of former thermal studies.

Biaxial Thermal Creep of Alloy 617 and Alloy 230 for VHTR Applications

In this study, we employed pressurized creep tubes to investigate the biaxial thermal creep behavior of Inconel 617 (alloy 617) and Haynes 230 (alloy 230). Both alloys are considered to be the primary candidate structural materials for very high-temperature reactors (VHTRs) due to their exceptional high-temperature mechanical properties. The current creep experiments were conducted at 900 °C for the effective stress range of 15–35 MPa. For both alloys, complete creep strain development with primary, secondary, and tertiary regimes was observed in all the studied conditions. Tertiary creep was found to be dominant over the entire creep lives of both alloys. With increasing applied creep stress, the fraction of the secondary creep regime decreases. The nucleation, diffusion, and coarsening of creep voids and carbides on grain boundaries were found to be the main reasons for the limited secondary regime and were also found to be the major causes of creep fracture. The creep curves computed using the adjusted creep equation of the form ε=Aσcosh−1(1+rt)+Pσntm agree well with the experimental results for both alloys at the temperatures of 850–950 °C.

Biaxial Thermal Creep of Alloy 617 and Alloy 230 for VHTR Applications

In this study, we employed pressurized creep tubes to investigate the biaxial thermal creep behavior of Inconel 617 (Alloy 617) and Haynes 230 (Alloy 230). Both alloys have been considered to be the primary candidate structural materials for very high temperature reactors (VHTRs) due to their exceptional high-temperature mechanical properties. The current creep experiments were conducted at 900°C for the effective stress range of 15–35 MPa. For both alloys, complete creep strain development with primary, secondary, and tertiary regimes were observed in all studied conditions. The tertiary creep was found to be dominant in the entire creep lives of both alloys. With increasing applied creep stress, the fraction of the secondary creep regime decreases. The nucleation, diffusion, and coarsening of creep voids and carbides on grain boundaries was found to be the main reason for the limited secondary regime, and was also found to be the major cause of creep fracture. The creep curves computed using the adjusted creep equation of the form ε = Aσ cosh−1(1 + rt) + Pσntm agree well with the experimental results for both alloys at the temperatures of 850–950°C. Paper published with permission.

A Study of Fully-Developed, Laminar, Axial Flow and Taylor Vortex Flow by Means of Shear Stress Measurements

The results of a shear stress investigation for combined axial and rotational flow, under conditions of both laminar flow and laminar flow plus Taylor vortices, are presented. These results show that two distinct régimes of shear stress dependency exist. In the primary régime, the shear stress is constant and depends only on the imposed axial flow, the flow being either laminar (Poiseuille and Couette flows) or laminar with vortices, but with the effect of the vortices being negligible. In the secondary régime, the shear stress is shown to depend only on Ta0.735, irrespective of the imposed axial flow. At low axial flow rates, a tertiary régime was observed, but no satisfactory explanation can be found for this phenomenon. The demarcation between the primary and secondary flow régimes differs markedly from those of other investigators. In the present study, critical conditions are determined by the point at which vortices begin to have an effect on measurements taken at the outer annular wall rather than the conditions under which vortices are initially formed. Evidence is presented for the vortices being formed within a confined layer near to the inner cylinder and thereafter growing with increasing rotational speed until the outer annular wall is approached, resulting in the much higher critical conditions found in the present study.