Microbial dynamics in maize-growing soil under different tillage and residue management

Non-Peer ReviewedMicroorganisms are involved in the fertility-related processes of agricultural fields. The long-term impact of tillage and residue management on soil microorganisms was studied over the growing season, in a sandy loam to loamy sand soil of southwestern Quebec. Tillage and residue treatments had been first imposed in fall 1991, on a maize (Zea mays L.) monoculture. Treatments consisted of no till, reduced tillage, and conventional tillage with crop residues either removed from (-R) or retained on (+R) experimental plots, laid out in a randomized complete block design. Soil microbial biomass carbon (SMB-C), soil microbial nitrogen (SMB-N) and phospholipid fatty acid (PLFA) concentrations were measured four times over the 2001 growing season i.e., in May 7 (preplanting), June 25, July 16, and September 29 (prior to corn harvest). The effect of time was larger than those of tillage or residue treatments. While SMB-C showed little seasonal change (160 μg C g-1 soil), SMB-N was responsive to post emergence mineral nitrogen fertilization, and PLFA analysis showed an increase in fungi and total PLFA throughout the season. The effect of residue was more pronounced than that of tillage, with increased SMB-C and SMB-N (61% and 96%) in +R plots compared to –R plots. This study illustrated that measuring soil quality based on soil microbial components must take into account seasonal changes in soil physical and chemical conditions

Effects of background rotation on a towed-sphere wake in a stably-stratified fluid

The wake of a towed sphere in a stable background density gradient can be considered a convenient model problem for studying the emergence and longevity of the coherent patches of alternate-signed vertical vorticity that comprise the late wake. In practical applications it is likely that the lifetime is such that rotation effects should also be considered, and strong rotation can also be used to deliberately disturb the inferred three-dimensional wake structure. Competing effects of stratification and rotation can be measured by the ratio of rotation to buoyancy timescales, fON, which varies from 0.05 to 0.22 in experiments conducted on the 14 m diameter rotating table at Coriolis. Wake anticyclones, with sense of rotation opposite to the background rotation, were spread out over a larger area, and were less strongly peaked than their cyclonic counterparts, with the magnitude of the asymmetry depending on fON. Deformation of anticyclones into highly elliptical shapes coincided with a broader range of wake vortex interactions than in the non-rotating case. At much longer times (hundreds of buoyancy periods, 5-10 rotation times), a recircularisation of elliptical anticyclones can occur. The observed asymmetries are consistent with existing data on homogeneous wake flows with rotation

On the possibility (or lack thereof) of agreement between experiment and computation of flows over wings at moderate Reynolds number

The flight of many birds and bats, and their robotic counterparts, occurs over a range of chord-based Reynolds numbers from 1 x 104 to 1.5 x 105. It is precisely over this range where the aerodynamics of simple, rigid, fixed wings becomes extraordinarily sensitive to small changes in geometry and the environment, with two sets of consequences. The first is that practical lifting devices at this scale will likely not be simple, rigid, fixed wings. The second is that it becomes non-trivial to make baseline comparisons for experiment and computation, when either one can be wrong. Here we examine one ostensibly simple case of the NACA 0012 airfoil and make careful comparison between the technical literature, and new experiments and computations. The agreement (or lack thereof) will establish one or more baseline results and some sensitivities around them. The idea is that the diagnostic procedures will help to guide comparisons and predictions in subsequent more complex cases.We are grateful to the Air Force Office of Scientific Research for equipment funding and partial funding of J.T. during this work under grants FA9550-15-1-0255 and FA9550-16-1-0392, both under the management of Doug Smith. L.S.’s work is supported by South African National Aerospace Center, and by a Research Completion Grant from the University of Pretoria, South Africa.http://rsfs.royalsocietypublishing.org2018-02-28hb2017Mechanical and Aeronautical Engineerin

The Aerodynamics of Hummingbird Flight

Hummingbirds fly with their wings almost fully extended during their entire wingbeat. This pattern, associated with having proportionally short humeral bones, long distal wing elements, and assumed to be an adaptation for extended hovering flight, has lead to predictions that the aerodynamic mechanisms exploited by hummingbirds during hovering should be similar to those observed in insects. To test these predictions, we flew rufous hummingbirds (Selasphorus rufus, 3.3 g, n = 6) in a variable–speed wind tunnel (0-12 ms-1) and measured wake structure and dynamics using digital particle image velocimetry (DPIV). Unlike hovering insects, hummingbirds produced 75% of their weight support during downstroke and only 25% during upstroke, an asymmetry due to the inversion of their cambered wings during upstroke. Further, we have found no evidence of sustained, attached leading edge vorticity (LEV) during up or downstroke, as has been seen in similarly-sized insects - although a transient LEV is produced during the rapid change in angle of attack at the end of the downstroke. Finally, although an extended-wing upstroke during forward flight has long been thought to produce lift and negative thrust, we found circulation during downstroke alone to be sufficient to support body weight, and that some positive thrust was produced during upstroke, as evidenced by a vortex pair shed into the wake of all upstrokes at speeds of 4 – 12 m s-1

Vortex wakes generated by robins Erithacus rubecula during free flight in a wind tunnel

The wakes of two individual robins were measured in digital particle image velocimetry (DPIV) experiments conducted in the Lund wind tunnel. Wake measurements were compared with each other, and with previous studies in the same facility. There was no significant individual variation in any of the measured quantities. Qualitatively, the wake structure and its gradual variation with flight speed were exactly as previously measured for the thrush nightingale. A procedure that accounts for the disparate sources of circulation spread over the complex wake structure nevertheless can account for the vertical momentum flux required to support the weight, and an example calculation is given for estimating drag from the components of horizontal momentum flux (whose net value is zero). The measured circulations of the largest structures in the wake can be predicted quite well by simple models, and expressions are given to predict these and other measurable quantities in future bird flight experiments

Wake structure and wingbeat kinematics of a house-martin Delichon urbica

The wingbeat kinematics and wake structure of a trained house martin in free, steady flight in a wind tunnel have been studied over a range of flight speeds, and compared and contrasted with similar measurements for a thrush nightingale and a pair of robins. The house martin has a higher aspect ratio (more slender) wing, and is a more obviously agile and aerobatic flyer, catching insects on the wing. The wingbeat is notable for the presence at higher flight speeds of a characteristic pause in the upstroke. The essential characteristics of the wing motions can be reconstructed with a simple two-frequency model derived from Fourier analysis. At slow speeds, the distribution of wake vorticity is more simple than for the other previously measured birds, and the upstroke does not contribute to weight support. The upstroke becomes gradually more significant as the flight speed increases, and although the vortex wake shows a signature of the pause phase, the global circulation measurements are otherwise in good agreement with surprisingly simple aerodynamic models, and with predictions across the different species, implying quite similar aerodynamic performance of the wing sections. The local Reynolds numbers of the wing sections are sufficiently low that the well-known instabilities of attached laminar flows over lifting surfaces, which are known to occur at two to three times this value, may not develop

Wing-body circulation control by means of a fuselage trailing edge

Ideal flight sheds the least amount of kinetic energy into a wake while imparting momentum sufficient to balance the vehicle weight. This combination defines a unique downwash distribution for the wake, which an aircraft designer should provide for. A central fuselage, as required for the typical flicht objective, presents an obstacle to this intent. A wing interrupted by a prominent fuselage is expected to shed inboard trailing vortices with central upwash harmful to the span efficiency of the aircraft. It is proposed here that a trailing edge on the fuselage can be used to control the circulation in the central region of the aircraft so that the central downwash deficiency can be avoided. Such a Kutta edge can further be applied as part of a high-o8ft system to increase cehtrao d9ownwashy by ihncreasi8ng the loading on the wing root and lift over the fuselage itself. Time-averaged flowfields behind a wing-body combination with and without a Kutta edge have been measured in wind-tunnel experiments. The results show that an edged aft-body does influence central circulation, as predicted. Flight with ideal wakes may be more readily attained than hitherto realized.http://dx.doi.org/10.2514/1.C031543http://arc.aiaa.org/doi/abs/10.2514/1.C03154