Scattering and diffraction of plane SH-waves by periodically distributed canyons

A new method is presented to study the scattering and diffraction of plane SH-waves by periodically distributed canyons in a layered half-space. This method uses the indirect boundary element method combined with Green＇s functions of uniformly distributed loads acting on periodically distributed inclined lines. The periodicity feature of the canyons is exploited to limit the discretization effort to a single canyon, which avoids errors induced by the truncation of the infinite boundary, and the computational complexity and the demand on memory can be significantly reduced. Furthermore, the total wave fields are decomposed into the free field and scattered field in the process of calculation, which means that the method has definite physical meaning. The implementation of the method is described in detail and its accuracy is verified. Parametric studies are performed in the frequency domain by taking periodically distributed canyons of semi-circular and semi-elliptic cross-sections as examples. Numerical results show that the dynamic responses of periodically distributed canyons can be quite different from those for a single canyon and significant dynamic interactions exist between the canyons.

Improving uniform scattering device for straw-smashing,back-throwing,no-tillage planter under complete straw mulching condition

With the objective of obtaining a completely straw-mulched field,when no-tillage mechanical sowing is implemented with straw smashing,delivering,and back-throwing approaches,it may be difficult to scatter the smashed straw uniformly during a succeeding wheat sowing step.This is because the previous rice straw is substantial in quantity and has a high humidity and toughness,which may easily result in non-uniform straw mulching and thus sparse and weak seedlings of wheat.Therefore a force-dispersing and uniform-scattering device was designed.With the number of scattering impellers,impeller angle,and impeller rotation speed as the main factors and the percentage of pass for the scattering width and non-uniformity of the straw mulching as the assessment indices,single-factor experiments and orthogonal regressive tests were performed,and a dual-index(percentage of pass for the scattering width and non-uniformity of the straw mulching)fitted regression equation was established.The test results suggested that the main factors(from primary to secondary)that influence the indices were the impeller rotation speed,number of scattering impellers,and impeller angle.The optimal parameter combination for the uniform scattering device was four rows of impellers with an angle of 15°,rotation speed of 1015 r/min,percentage of pass of 72.65%for the scattering width,and a non-uniformity of 13.8%in the straw mulching.This combination can be used to realize a uniform scattering of the smashed straw along the seedling rows on the after-sowing ground.According to the field investigation of the wheat growth,the wheat emergence rate was 90.7%.The research results can provide a reference for improving the uniform scattering device for a straw-smashing,back-throwing,no-tillage planter for obtaining a completely straw-mulched field,enhancing the quality of the machinery operation,and ensuring good and strong seedlings after sowing.