4,105 research outputs found
Experimental investigation of transitional flow in a toroidal pipe
The flow instability and further transition to turbulence in a toroidal pipe
(torus) with curvature (tube-to-coiling diameter) 0.049 is investigated
experimentally. The flow inside the toroidal pipe is driven by a steel sphere
fitted to the inner pipe diameter. The sphere is moved with constant azimuthal
velocity from outside the torus by a moving magnet. The experiment is designed
to investigate curved pipe flow by optical measurement techniques. Using
stereoscopic particle image velocimetry, laser Doppler velocimetry and pressure
drop measurements, the flow is measured for Reynolds numbers ranging from 1000
to 15000. Time- and space-resolved velocity fields are obtained and analysed.
The steady axisymmetric basic flow is strongly influenced by centrifugal
effects. On an increase of the Reynolds number we find a sequence of
bifurcations. For Re=4075 a supercritical bifurcation to an oscillatory flow is
found in which waves travel in the streamwise direction with a phase velocity
slightly faster than the mean flow. The oscillatory flow is superseded by a
presumably quasi-periodic flow at a further increase of the Reynolds number
before turbulence sets in. The results are found to be compatible, in general,
with earlier experimental and numerical investigations on transition to
turbulence in helical and curved pipes. However, important aspects of the
bifurcation scenario differ considerably
Subcritical versus supercritical transition to turbulence in curved pipes
Transition to turbulence in straight pipes occurs in spite of the linear
stability of the laminar Hagen--Poiseuille flow if the amplitude of flow
perturbations as well as the Reynolds number exceed a minimum threshold
(subcritical transition). As the pipe curvature increases centrifugal effects
become important, modifying the basic flow as well as the most unstable linear
modes. If the curvature (tube-to-coiling diameter ) is sufficiently large
a Hopf bifurcation (supercritical instability) is encountered before turbulence
can be excited (subcritical instability). We trace the instability thresholds
in the parameter space in the range by means
of laser-Doppler velocimetry and determine the point where the subcritical and
supercritical instabilities meet. Two different experimental setups were used:
a closed system where the pipe forms an axisymmetric torus and an open system
employing a helical pipe. Implications for the measurement of friction factors
in curved pipes are discussed
European populations of Diabrotica virgifera virgifera are resistant to aldrin, but not to methyl-parathion
The western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), is a major pest of cultivated corn in North America and has recently begun to invade Europe. In addition to crop rotation, chemical control is an important option for D. v. virgifera management. However, resistance to chemical insecticides has evolved repeatedly in the USA. In Europe, chemical control strategies have yet to be harmonized and no surveys of insecticide resistance have been carried out. We investigated the resistance to methyl-parathion and aldrin of samples from nine D. v. virgifera field populations originating from two European outbreaks thought to have originated from two independent introductions from North America. Diagnostic concentration bioassays revealed that all nine D. v. virgifera field populations were resistant to aldrin but susceptible to methyl-parathion. Aldrin resistance was probably introduced independently, at least twice, from North America into Europe, as there is no evident selection pressure to account for an increase of frequency of aldrin resistance in each of the invasive outbreaks in Europe. Our results suggest that organophosphates, such as methyl-parathion, may still provide effective control of both larval and adult D. v. virgifera in the European invasive outbreaks studied
INFLUENCE OF CRANK LENGTH ON PEDALLING ECONOMY IN THE ACCELERATION PHASE IN TRACKCYCLING – A SINGLE CASE STUDY
The purpose of this study was to identify different effects of crank-length on cycling performance in track cycling. Two different crank lengths (162,5mm and 170mm) were used in a single-blind and balanced order at a wooden indoor cycling track. Saddle height and the position of the handlebar were kept constant during all trials. The subject was asked to complete 100m with maximum load with every crank length in each test session. To avoid test effects and effects from fatigue, the order of crank length was changed throughout all test sessions. Measured variables were torque (Nm), power output (W), Force (F), Cadence (rpm) and time (s). Results showed that force output stayed constant, while the 170mm condition had advantages in time, torque and power output due to the longer lever arm. Longer distances than 100m were not analysed
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