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Experiment and Numerical Analysis of a Rotating Hollow Cylinder in Free Flight
Abstract
The present purpose is to reveal the mechanism of a flying pipe from an aerodynamic point of view. At first, we conduct field observations of a flying pipe using a pair of high-speed video cameras, together with three-dimensional motion analyses. In addition, we conduct numerical analyses by a finite difference method based on the MAC scheme. As a result, the observed orbit is approximated to be not an obvious parabolic curve but rather a straight line, after an initial instable and complicated curve. The stable flight with this approximately-straight orbit suggests the importance of aerodynamics in flying mechanism. More specifically, the model is in an unstable and complicated flight during an initial flight, afterwards becomes in a stable and approximately-straight flight. In the initial instable and complicated flight, the model flies fluctuating its posture upward, downward, left-ward and right-ward. As flight distance increases, the absolute value and the amplitude of moment becomes small to zero. During such a decaying and stabilising process, the gyroscopic effect plays a primary role balancing not angular acceleration of the model but aerodynamic fluid moment. In the stable and approximately-straight flight, the flow in the stable and approximately-straight flight is nearly the velocity-potential one, and accompanies very-small drag force. And, we could ignore the influence of model’s rotation upon the flow and the orbit. In this context, the model’s rotation is only to stabilise its posture, and gives negligible contribution upon its aerodynamics.
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The development of a subharmonic three-dimensional instability mode in a vortex street is investigated both numerically and experimentally. The flow past a ring is considered as a test case, as a previous stability analysis has predicted that for a range of aspect ratios, the first-occurring instability of the vortex street is subharmonic. For the flow past a circular cylinder, the development of three-dimensional flow in the vortex street is known to lead to turbulent flow through the development of spatio-temporal chaos, whereas subharmonic instabilities have been shown to cause a route to chaos through the development of a period-doubling cascade. The three-dimensional vortex street in the flow past a ring is analysed to determine if a subharmonic instability can alter the route to turbulence for a vortex street. A linear stability analysis and non-axisymmetric computations are employed to compute the flow past a ring with an aspect ratio AR = 5, and comparisons with experimental dye visualizations are included to verify the existence of a subharmonic mode in the wake. Computations at higher Reynolds numbers confirm that the subharmonic instability does not initiate a period-doubling cascade in the wake.
The present purpose is to reveal the mechanism of a flying pipe from an aerodynamic point of view. At first, we conduct field observations of a flying pipe using a pair of high-speed video cameras, together with three-dimensional motion analyses. In addition, we conduct numerical analyses by a finite difference method based on the MAC scheme. As a result, the observed orbit is approximated to be not an obvious parabolic curve but rather a straight line, after an initial instable and complicated curve. The stable flight with this approximately-straight orbit suggests the importance of aerodynamics in flying mechanism. More specifically, the model is in an unstable and complicated flight during an initial flight, afterwards becomes in a stable and approximately-straight flight. In the initial instable and complicated flight, the model flies fluctuating its posture upward, downward, left-ward and right-ward. As flight distance increases, the absolute value and the amplitude of moment becomes small to zero. During such a decaying and stabilising process, the gyroscopic effect plays a primary role balancing not angular acceleration of the model but aerodynamic fluid moment. In the stable and approximately-straight flight, the flow in the stable and approximately-straight flight is nearly the velocity-potential one, and accompanies very-small drag force. And, we could ignore the influence of model's rotation upon the flow and the orbit. In this context, the model's rotation is only to stabilise its posture, and gives negligible contribution upon its aerodynamics.
The present aim is to clarify the wake structure of a ring with rectangular cross sections, which is immersed perpendicularly to the main flow. In a wind tunnel, the authors carry out flow visualisation and the measurement using a hot-wire anemometer, synchronously. Flow is visualised by a smoke-wire method, and the authors take consecutive picutures using a high-speed camcorder. Analyses of these pictures by a PIV bring us flow-field informations, namely, velocity vectors and vorticity contours. Tested ranges are Re=5.0 × 10 3, d/w=3.0-8.0 and t/w=0.28-1.30, where Re, d, w and t are the Reynolds number, mean diameter, cross-section width and cross-section thickness of the ring, respectively. Consequently, the authors reveal the corresponding wake structures with four flow modes. In particular for two axi-symmetrical modes, the authors discuss the effect of t/w on the development, the diffusion and the advection of ring vortices in near wake.
The present aim is to clarify the streamwise-thickness effect of a ring with rectangular cross sections, which is immersed perpendicularly to uniform flow. In a wind tunnel, we carried out measurements using a pressure transducer and a hotwire anemometer, and flow visualisations by a somkewire method, for Re = 5.0×103-1.5× 104, d/w=4.0-8.0. and t/w = 0.29-1.30, where d, if and t are mean diameter, cross-section width and cross-section thickness of the ring. respectively. Consequently, Strouhal number St, which is" a reduced frequency of regular vortex shedding from the ring, is independent of t/w, d/w and Re. On the other hand, we can observe the maximum base suction and the maximum drag at a certain critical value of the depending on d/w. We can clearly classify flow into two modes. riz., sub- and over-critical modes. Round the critical is a transition range of the modal shift. Over-critical leeward extension of the after body drastically enhances the formation of vortex rings, and weakens a ring-curvature effect. Only the existence of an inner sidewall is essential for the modal shift.
The present aim is to clarify the combination effect of curvature and streamwise thickness of a ring with rectangular cross sections, which is immersed perpendicularly to uniform flow. In a wind tunnel, the authors carry out measurements using a pressure manometer and hot-wire anemometers.and flow visualisations by a smoke-wire method, for Re=5.0×103-1.5×104, d/w=3.0-16.0 and t/w=0.28-1.80, where Re, d, w and t are the Reynolds number, mean diameter, cross-section width and cross-section thickness of the ring, respectively. Consequently, the Strouhal number St, which is a reduced frequency of regular vortex shedding from the ring, is independent of d/w, t/w and Re. On the other hand, base suction depends not on Re, but on d/w and t/w. Considering d/w and t/w as governing parameters, and regarding to base suction, flow visualisation, velocity fluctuation, mean-velocity and turbulence-intensity profiles and four-points velocity correlation, we can classify flow into four flow modes; viz., an over-critical axisymmetrical mode, a sub-citical axisymmetrical mode, a sub-critical non-axisymmetrical mode and a sub-critical less-correlation mode.
The present aim is to reveal the flow past a pipe which is immersed parallel to the mainstream at high Reynolds numbers. In a wind tunnel, we carry out (1) base-pressure measurements, (2) velocity-fluctuation measurements using a hot-wire anemometer and (3) flow visualisations by a smoke-wire method with PIV analyses, where we take consecutive picutures using a high-speed camcorder to obtain quantitative flow-field information such as velocity vector and vorticity. The tested parameter ranges are Re = 2.0-10 3 - 1.3-10 4, d/t = 4.0 - 10.0 and l/t = 1.0 - 10.0, where Re, d, t and / are the Reynolds number, mean diameter, thickness and length of the pipe, respectively. As a result, the Re effects are negligible. The base-suction coefficient -C pb monotonously decreases with decreasing d/t, or with increasing l/t. We propose a unified formula to predict -C pb, which are consistent with both a two-dimensional prism and a rod for l/t < 4.0 in addition to a ring. In contrast, the Strouhal number St almost coincides with that for a two-dimensional prism at any l/t, if we can detected any dominant frequencies. In addition, we conduct flow visualisations, and reveal the effects upon axisymmetry of wake. Finally, we classify the flow into three modes based on both periodicity and axisymmetry. Such a modal classification reveals that the enhancement of flow irregularity corresponds to the decrease of -C pb.
We have developed a new wind turbine system that consists of a diffuser shroud with a broad-ring brim at the exit periphery and a wind turbine inside it. The brimmed-diffuser shroud plays the role of a device for collecting and accelerating the approaching wind. Emphasis is placed on positioning the brim at the exit of the diffuser shroud. Namely, the brim generates a very low-pressure region in the exit neighborhood of the diffuser by strong vortex formation and draws more mass flow to the wind turbine inside the diffuser shroud. To obtain a higher power output of the shrouded wind turbine, we have examined the optimal form for the brimmed diffuser, such as the diffuser open angle, brim height, hub ratio, centerbody length, inlet shroud shape and so on. As a result, a shrouded wind turbine equipped with a brimmed diffuser has been developed, and demonstrated power augmentation for a given turbine diameter and wind speed by a factor of about five compared to a standard (bare) wind turbine.
Inaug.-diss.--Göttingen. Lebenslauf.
This paper seeks to evaluate the swimming flow around a typical slender fish whose transverse cross-section to the rear of its maximum span section is of a lenticular shape with pointed edges, such as those of spiny fins, so that these side edges are sharp trailing edges, from which an oscillating vortex sheet is shed to trail the body in swimming. The additional feature of shedding of vortex sheet makes this problem a moderate generalization of the paper on the swimming of slender fish treated by Lighthill (1960a). It is found here that the thrust depends not only on the virtual mass of the tail-end section, but also on an integral effect of variations of the virtual mass along the entire body segment containing the trailing side edges, and that this latter effect can greatly enhance the thrust-making.
The optimum shape problem considered here is to determine the transverse oscillatory movements a slender fish can make which will produce a prescribed thrust, so as to overcome the frictional drag, at the expense of the minimum work done in maintaining the motion. The solution is for the fish to send a wave down its body at a phase velocity c somewhat greater than the desired swimming speed U, with an amplitude nearly uniform from the maximum span section to the tail. Both the ratio U/c and the optimum efficiency are found to
depend upon two parameters: the reduced wave frequency and a 'proportional-loading parameter', the latter being proportional to the thrust coefficient and to the inverse square of the wave amplitude. The basic mechanism of swimming is examined in the light of the principle of action and reaction by studying the vortex wake generated by the optimum movement.
The vortex street behind a ring of circular cross section and large aspect ratio is investigated experimentally. Different modes of annular and helical vortex shedding are identified by phase and frequency measurements. Their stability domains overlap in a large interval in Reynolds number where mode selection depends on initial conditions only. A new instability of the vortex shedding process involving characteristic mode transitions has been observed. This instability can be explained in the context of a Ginzburg-Landau model by a mechanism resembling formally the Eckhaus instability of spatially periodic patterns.
A study of the wake structure behind bluff rings
- M Takamoto