ALBERT

Description: Using a simulated highly compressible isotropic turbulence field with turbulent Mach number around 1.0, we studied the effects of local compressibility on the statistical properties and structures of velocity gradients in order to assess salient small-scale features pertaining to highly compressible turbulence against existing theories for incompressible turbulence. A variety of statistics and local flow structures conditioned on the local dilatation-a measure of local flow compressibility-are studied. The overall enstrophy production is found to be enhanced by compression motions and suppressed by expansion motions. It is further revealed that most of the enstrophy production is generated along the directions tangential to the local density isosurface in both compression and expansion regions. The dilatational contribution to enstrophy production is isotropic and dominant in highly compressible regions. The emphasis is then directed to the complicated properties of the enstrophy production by the deviatoric strain rate at various dilatation levels. In the overall flow field, the most probable eigenvalue ratio for the strain rate tensor is found to be 3:1:2.5, quantitatively different from the preferred eigenvalue ratio of 4:1:3 reported in incompressible turbulence. Furthermore, the strain rate eigenvalue ratio tends to be 1:0:0 in high compression regions, implying the dominance of sheet-like structures. The joint probability distribution function of the invariants for the deviatoric velocity gradient tensor is used to characterize local flow structures conditioned on the local dilatation as well as the distribution of enstrophy production within these flow structures. We demonstrate that strong local compression motions enhance the enstrophy production by vortex stretching, while strong local expansion motions suppress enstrophy production by vortex stretching. Despite these complications, most statistical properties associated with the solenoidal component of the velocity field are found to be very similar to those in incompressible turbulence, and are insensitive to the change of local dilatation. Therefore, a good understanding of dynamics of the compressive component of the velocity field is key to an overall accurate description of highly compressible turbulence. © 2012 Cambridge University Press.

Description: We investigate how compressibility affects the turbulent statistics from a Lagrangian point of view, particularly in the parameter range where the flow transits from the incompressible type to a state dominated by shocklets. A series of three-dimensional simulations were conducted for different types of driving and several Mach numbers. For purely solenoidal driving, as the Mach number increases a new self-similar region first emerges in the Lagrangian structure functions at sub-Kolmogorov time scale and gradually extends to larger time scale. In this region the relative scaling exponent saturates and the saturated value decreases as the compressibility becomes stronger, which can be attributed to the shocklets. The scaling exponent for the inertial range is still very close to that of incompressible turbulence for small Mach number, and discrepancy becomes visible when the Mach number is high enough. When the driving force is dominated by the compressive component the shocklet-induced self-similar region occupies a much wider range of time scales than that in the purely solenoidal driving case. Regardless of the type of driving force, the probability density functions of the velocity increment collapse onto one another for the time scales in the new self-similar region after proper normalization. © 2015 Cambridge University Press A.

Description: SUMMARYA simulated grazing system was set up in Huanxian County, Gansu province, on the Loess Plateau of Northwest China, involving the purchase of 18-month-old wether lambs in June of each year at c. 20 kg body weight (BW) and sale 6 months later at c. 35 kg BW. Three stocking rate (SR) treatments of 2·7, 5·3 and 8·7 wether lambs/ha were evaluated on geographically separated warm season (WS) and cold season (CS) paddocks c. 1 km apart; 3 years' data are reported (2004–2006). The metabolizable energy (ME) yield of the grazing system, calculated from the weight of animals fed and their weight gain, averaged 1·7, 3·3 and 4·7 GJ/ha/year for 2004, 2005 and 2006, respectively, in the WS and 0·9, 1·9, and 2·7 GJ/ha/year, respectively, in the CS for SR of 2·7, 5·3 and 8·7 animals/ha. Detailed grazing behaviour records were kept in order to elucidate intake dynamics. In these grazing systems, bite weight was typically c. 0·04 g/bite, lower than for temperate grazing systems at comparable herbage mass. A hypothesis for further study is proposed that this may relate to the distribution of a similar herbage mass over a greater sward height range in steppe vegetation than in temperate grass pasture. Sheep increased their bite rate (bites/min) and the number of steps/min at higher SR to compensate, such that intake/animal was reduced by not more than 10% with a threefold increase in SR. At higher SR, herbage ground cover on grazed plots was still lower than on ungrazed plots, 1 year after a 90-day summer grazing period or a 48-day winter grazing period. The significance of the findings for management of these systems is briefly discussed.

Description: SUMMARYLow-temperature stress is an important environmental factor that severely disrupts plant respiration but can be alleviated by symbiotic arbuscular mycorrhizal fungi (AMF). In the current study, a pot experiment was performed to determine changes in the respiratory metabolic capacity of mycorrhizal rice (Oryza sativa) under low-temperature stress. The results demonstrated that low temperature might accelerate the biosynthesis of strigolactone in mycorrhizal rice roots by triggering the expression of genes for the synthesis of strigolactone, which acted as a host stress response signal. In addition, AMF prompted the host tricarboxylic acid (TCA) cycle by enhancing pyruvate metabolism, up-regulating the expression of genes of the TCA cycle under low-temperature stress and affecting the electron transport chain. The alternative oxidase pathway might be the main electron transport pathway in non-mycorrhizal rice under stress, while the cytochrome c oxidase (COX) pathway might be the predominant pathway in arbuscular mycorrhizal symbiosis. Mycorrhizal rice also had higher adenosine triphosphate production to maintain the natural status of respiration under stress conditions, which resulted in improved root growth status and alleviated low-temperature stress.