4,093 research outputs found
Drag Reduction by Applying Speedstrips on Rowing Oars
No full textAbstractThe objective of this study was to determine the advantage of the application of speedstrips to rowing oars for a lightweight single sculler. The research method comprehended three steps: (1) the analysis of the rowing oar movement, (2) the determination of the change in drag and (3) the composition of a rowing model to establish the advantage that could be achieved. The parameters needed for the model: boat velocity, oar angle velocity and power delivered by the rower, were recorded on a real single sculler. The change in drag due to speedstrips on cylinders was determined by performing wind tunnel experiments. The rowing model (Matlab) simulates a race by using real stroke data of a world-class rower as input, while calculating the drag with the coefficients determined by the wind tunnel experiments. The output of the model is the final advantage by the application of speedstrips to rowing oars. Speedstrips induce a 0.1% advantage over a 2000 m race under calm wind conditions. The advantage increases up to .4% with a headwind velocity of 5 m s-1. For bigger boats, the advantage could be even more significant
Drag and Power-loss in Rowing Due to Velocity Fluctuations
No full textAbstractThe flow motions in the turbulent boundary layer between water and a rowing boat initiate a turbulent skin friction. Reducing this skin friction results in better rowing performances. A Taylor-Couette (TC) facility was used to verify the power losses due to velocity fluctuations PV′ in relation to the total power , as a function of the velocity amplitude A. It was demonstrated that an increase of the velocity fluctuations results in a tremendous decrease of the velocity efficiency eV . The velocity efficiency eV for a typical rowing velocity amplitude A of 20 – 25% was about 0.92 – 0.95%. Suppressing boat velocity fluctuations with 60% will increase boat speed with 1.6%. Riblet surfaces were applied on the inner and outer cylinder wall to indicate the drag reducing ability of such surfaces. The results of the measurements at constant velocity are identical as the results reported earlier, while the experimental configuration was different. This confirms once more the consistency of the TC-system for drag studies. The maximum drag reduction DR was 3.4% at a Reynolds number Res 4.7 × 104, which corresponds to a shear velocity in this TC-system with water of V 4.7 m/s. For typical rowing velocity fluctuations, the riblets maintain to reduce the drag with 2.8% and corresponds to a averaged velocity increase of 0.9%. The drag reducing ability of riblets is partly lost due to velocity fluctuations with high amplitudes (A > 20%). From these results, it is concluded that the friction coefficient Cf will vary within one cycle. Higher acceleration/deceleration leads to a additional level of turbulent kinetic energy
A.J. Cronin. A doctor into lifelong writer
No full textReality and fiction might be strictly coexistent in the narrative world. The author of this article, after a deep reading of A.J. Cronin’s novels, has tried to find out the right key to penetrate into the novelist’s intricate world. After many interrogatives on A.J. Cronin both as a man and writer, the author , finally, has been able to grasp from the pages of the novelist, the suffering of a man who has made of his romance the history of his own life
Bivariate colour maps for visualizing climate data
No full textThe increasing availability of gridded, high-resolution, multivariate climatological data sets calls for innovative approaches to visualize inter-variable relations. In this study, we present a methodology, based on properties of common colour schemes, to plot two variables in a single colour map by using a two-dimensional colour legend for both sequential and diverging data. This is especially suited for climate data as the spatial distribution of the relation between different variables is often as important as the distribution of variables individually. Two example applications are given to illustrate the use of the method: one that shows the global distribution of climate based on observed temperature and relative humidity, and the other showing the distribution of recent changes in observed temperature and precipitation over Europe. A flexible and easy-to-implement method is provided to construct different colour legends for sequential and diverging data
Entropy and river meander planform
No full textThis study departs from the hypothesis that the often striifing geometric similarity and regularity of meanders is the result of the second law of thermodynamics applied to open dissipative systems. It is argued that along a meandering river the continuous production of entropy is as low and as uniform as possible. An expression of entropy production in a moderately meandering river is derived. A dimensioniess form of Odgaard's Meander flow model (1986a) is used to evaluate this expression along different meander bends described by the class of third order sine-generated curves. The results show a minimum variance of entropy production for a fattened curve with upvalley skewing, indicating that meander asymmetry described by Carson and Lapointe (1983) is in correspondence with the Theory of minimum variance (Langbein and Leopold, 1966)
Computing with cables: Towards massively parallel neuro computers
No full textElectrical Engineering, Mathematics and Computer Scienc
A state-dependent parameterization of saturated-unsaturated zone interaction
No full textThe relevance of groundwater as an important source of root zone moisture by means of capillary rise is increasingly being recognized. This is partly reflected in many current land surface schemes, which increasingly replace a one-way (i.e., downward) drainage of water by a two-way interaction flux between the root zone and a groundwater system. A fully physically correct implementation of this two-way saturated-unsaturated interaction flux requires transient simulations using the highly nonlinear Richards' equation, which is a computationally demanding approach. We test a classic simple approximation that computes the root zone¿groundwater interaction flux as the net effect of a downward drainage flux and an upward capillary rise flux against the Darcy equation for quasi steady state conditions. We find that for a wet root zone and/or shallow groundwater, the errors within this approximation are significant and of the same magnitude as the interaction flux itself. We present a new closed-form parameterization of the Darcy equation¿based fluxes that accounts both for root zone soil moisture and depth to the water table. Parameter values for this parameterization are listed for 11 different, widely applied soil texture descriptions. The high numerical efficiency of the proposed method makes it suitable for inclusion into demanding applications, e.g., a Monte Carlo framework, or high spatial resolution
Radio Frequency Interference Mitigation in Radio Astronomy
No full textThe next generation of radio telescopes is expected to be one to two orders of magnitude more sensitive than the current generation. Examples of such new telescopes are the Low Frequency Array (LOFAR), currently under construction in the Netherlands, and the Square Kilometer Array (SKA), currently in a concept study phase. Another trend is that technological advances in the fields of electronics and communications systems have led to a vast increase in radio communication applications and systems, and also to an increasing demand for radio spectrum. These two trends, more sensitive telescopes and a much denser spectrum use, imply that radio astronomy will become more vulnerable to interference from radio transmitters. Although protection criteria exist for radio astronomy, it becomes increasingly difficult to keep the radio astronomy frequency bands free from interference. In order to mitigate interference in radio astronomical data, filtering techniques can be used. In this thesis, modern array signal processing techniques have been applied to narrow-band multichannel interference detection and excision, and to narrow-band spatial interference filtering. By investigating the subspace structure of the telescope array output covariance matrices, new results were found, such as upper limits on interference residuals after excision and spatial filtering. The effect of bandwidth, extendedness of the interfering sources, and multipath effects on the detection and spatial filter effectiveness were studied as well. The advantage of a multichannel approach over a single telescope approach was demonstrated by using experimental data from the Westerbork Synthesis Radio Telescope (WSRT). As the performance of mitigation algorithms can be improved by calibration of the telescope gains and noise powers, calibration algorithms were developed. These algorithms were verified both for single and dual polarised arrays. Finally, a LOFAR interference mitigation strategy was developed.Electrical Engineering, Mathematics and Computer Scienc
Disentangling the response of forest and grassland energy exchange to heatwaves under idealized land–atmosphere coupling
No full textThis study investigates the difference in land–atmosphere interactions
between grassland and forest during typical heatwave conditions in order to
understand the controversial results of Teuling et al. (2010) (hereafter T10),
who found the systematic occurrence of higher sensible heat fluxes over
forest than over grassland during heatwaves. With a simple but accurate
coupled land–atmosphere model, we show that existing parametrizations are
able to reproduce the findings of T10 for normal summer and heatwave
conditions. Furthermore, we demonstrate the sensitivity of the coupled system
to changes in incoming radiation and early-morning temperature typical for
European heatwaves.
Our results suggest that the fast atmospheric control of stomatal resistance
can explain the observed differences between grassland and forest. The
atmospheric boundary layer has a buffering function therein: increases in
stomatal resistance are largely compensated for by increases in the potential
evaporation due to atmospheric warming and drying.
In order to disentangle the contributions of differences in several static
and dynamic properties between forest and grassland, we have performed a
virtual experiment with artificial land-use types that are equal to
grassland, but with one of its properties replaced by that of forest. From
these, we confirm the important role of the fast physiological processes that
lead to the closure of stomata. Nonetheless, for a full explanation of T10's
results, the other properties (albedo, roughness and the ratio of minimum
stomatal resistance to leaf-area index) play an important but indirect role;
their influences mainly consist of strengthening the feedback that leads to
the closure of the stomata by providing more energy that can be converted
into sensible heat. The model experiment also confirms that, in line with the
larger sensible heat flux, higher atmospheric temperatures occur over forest.
As our parametrization for stomatal resistance is empirical rather than
mechanical, our study stresses the demand for a better mechanistic
understanding of physiological processes in plants
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