Design optimization of buttress type premium casing connection by modifying lower corner radius of stab flank.

Full-scale drilling for shale gas has been deepening, and the horizontal definitions have been increasing for drilling efficiency. Pressures and bending stresses are generated in the joints of gas-cored pipes, which seriously affect their durability. Premium casing connections are primarily forced at the interfaces of two parts, so the design of the threads is crucial, and the choice of optimal parameters is precisely related to their resistance to stresses. This article proposes a novel premium connection design, and its performance is validated through simulations to demonstrate good noise, vibration, and harshness (NVH) and durability performance. A parametric study is especially performed with the change of lower corner radius to observe maximum stress changes of the system and it is advised that a parameter be optimized. This study demonstrates that it is possible to design a premium connection capable of reducing the stress concentrated on the stab flank contact portion when drilling deep gas wells in response to parameter changes.

Harnessing adaptive bistable stiffness of hair-cell-bundle structure for broadband vibration applications.

This study presents an initial study on the adaptive bistable stiffness of the hair cell bundle structure in a frog cochlea, and aims to harness its bistable nonlinearity that features a negative stiffness region for broadband vibration applications such as vibration-based energy harvesters. To this end, the mathematical model for describing the bistable stiffness is first formulated based on the modeling concept of piecewise type nonlinearities. The harmonic balance method was then employed to examine the nonlinear responses of bistable oscillator, mimicking hair cells bundle structure under the frequency sweeping condition, and their dynamic behaviors induced by bistable stiffness characteristics are projected on phase diagrams, and Poincare maps concerning the bifurcation. In particular, the bifurcation mapping at the super- and sub-harmonic regimes provides a better perspective to examine the nonlinear motions which occur in the biomimetic system. The use of bistable stiffness characteristics of hair cell bundle structure in frog cochlea thus offers physical insights to harness the adaptive bistable stiffness for metamaterial-like potential engineering structures such as vibration-based energy harvester, and isolator etc.

Examination of sub-harmonic responses along with various initial conditions induced by multi-staged clutch damper system.

Using the harmonic balance method to investigate the nonlinear dynamic behaviors pertaining to sub-harmonic responses is difficult compared with that of super-harmonic cases because of the limitations of the HBM. Since sub-harmonic motions differ under various initial conditions, difficulties can arise when this method is used to calculate all possible solutions within sub-harmonic resonances. To explore complex dynamic behaviors in sub-harmonic resonant areas, this study suggests mathematical and numerical techniques to estimate sub-harmonic responses depending on various initial conditions. First, sub-harmonic responses are calculated under various excitation conditions relevant to the sub-harmonic input locations of the HBM formula. Second, the HBM results are verified by comparing them with the results of the numerical simulation (NS) under various initial conditions with respect to different frequency up-sweeping paths. Finally, the positive real part of the eigenvalues is examined to anticipate bifurcation characteristics, which reflect the relevance of the complex dynamic behaviors in the eigenvalues' unstable solutions. Overall, this study successfully proves that the techniques and methods described are suitable for examining complex sub-harmonic responses, and suggests basic ideas for analyzing nonlinear dynamic behaviors in sub-harmonic resonances using the HBM.

Investigation of complex nonlinear dynamic behaviors observed in a simplified driveline system with multistage clutch dampers.

The results of the harmonic balance method (HBM) for a nonlinear system generally show nonlinear response curves with primary, super-, and sub-harmonic resonances. In addition, the stability conditions can be examined by employing Hill's method. However, it is difficult to understand the practical dynamic behaviors with only their stability conditions, especially with respect to unstable regimes. Thus, the main goal of this study is to suggest mathematical and numerical approaches to determine the complex dynamic behaviors regarding the bifurcation characteristics. To analyze the bifurcation phenomena, the HBM is first implemented utilizing Hill's method where various local unstable areas are found. Second, the bifurcation points are determined by tracking the stability variational locations on the arc-length continuation scheme. Then, their points are defined for various bifurcation types. Finally, the real parts of the eigenvalues are analyzed to examine the practical dynamic behaviors, specifically in the unstable regimes, which reflect the relevance of various bifurcation types on the real part of eigenvalues. The methods employed in this study successfully explain the basic ways to examine the bifurcation phenomena when the HBM is implemented. Thus, this study suggests fundamental method to understand the bifurcation phenomena using only the HBM with Hill's method.

Stability and bifurcation analysis of super- and sub-harmonic responses in a torsional system with piecewise-type nonlinearities.

The nonlinear dynamic behaviors induced by piecewise-type nonlinearities generally reflect super- and sub-harmonic responses. Various inferences can be drawn from the stability conditions observed in nonlinear dynamic behaviors, especially when they are projected in physical motions. This study aimed to investigate nonlinear dynamic characteristics with respect to variational stability conditions. To this end, the harmonic balance method was first implemented by employing Hill's method, and the time histories under stable and unstable conditions were examined using a numerical simulation. Second, the super- and sub-harmonic responses were investigated according to frequency upsweeping based on the arc-length continuation method. While the stability conditions vary along the arc length, the bifurcation phenomena also show various characteristics depending on their stable or unstable status. Thus, the study findings indicate that, to determine the various stability conditions along the direction of the arc length, it is fairly reasonable to determine nonlinear dynamic behaviors such as period-doubling, period-doubling cascade, and quasi-periodic (or chaotic) responses. Overall, this study suggests analytical and numerical methods to understand the super- and sub-harmonic responses by comparing the arc length of solutions with the variational stability conditions.