Wake Asymmetry Research Articles

Numerical and experimental investigation of conventional and un-conventional preswirl duct for VLCC

This paper shows the study of preswirl duct as an effective energy saving devices that have been devised and reviewed to support the propeller performance, especially for the ship of VLCC with large block coefficients. From the bare hull wake measurements, typical upper/lower asymmetry of hull wake at the propeller disk was found. The 2 kinds of pre-swirl duct, Unconventional half circular duct and Conventional circular pre-swirl duct have been designed and reviewed to recover the loss of propeller running in that condition. The general function of the pre-swirl duct was set to work against this asymmetry of wake and generate pre-swirled flow into the propeller against the propeller ro-tating direction. The optimum self propulsion tests with various angle configurations were carried out and the best configuration was decided. Accordingly, cavitation test was carried out with best configuration of unconventional half circular duct. The blade surface and tip vortex cavitation behaved smoother when the duct was mounted. The hull pressure amplitudes reflected this difference, so the hull pressure amplitude with duct was smaller than that of without duct.

선박용 연료절감장치 Pre-Swirl Duct의 설계 및 평가방법 연구

Recently, with oil price jumping and environmental issues, Green ship is paid deep attention to by ship owner, operator, builder, class and government. Fuel efficiency and reduction of <TEX>$CO_2$</TEX> emissions are expected to have a strong influence on the design and operation of merchant ships. Many ship owners and operators are seeking the more economic method by the best operating route and the application of reliable and effective energy saving devices. With the Energy Efficiency Design Index (EEDI) in 2013 attention will more than ever be focused at achieving maximum fuel economy in the hydrodynamic design of hull forms, their appendages and propellers. IMO requirements for <TEX>$CO_2$</TEX> emission for ships will now be implemented for vessels ordered from 1st January 2013. So far, a lot of new idea and patents have been proposed, tested, claimed and applied for various kinds of ship type. This paper shows numerical and experimental work related to a study on a energy saving devices particularly for fuller ship such as merchant vessel of Tanker and Bulker. From the bare hull wake measurements, typical upper/lower asymmetry of hull wake at the propeller disk was found. The pre-swirl duct have been designed and reviewed to recover the loss of propeller running in that condition. The general function of the pre-swirl duct was set to work against this asymmetry of wake and generate pre-swirled flow into the propeller against the propeller rotating direction.

A large-eddy simulation study of wake propagation and power production in an array of tidal-current turbines

This paper presents our initial work in performing large-eddy simulations of tidal turbine array flows. First, a horizontally periodic precursor simulation is performed to create turbulent flow data. Then those data are used as inflow into a tidal turbine array two rows deep and infinitely wide. The turbines are modelled using rotating actuator lines, and the finite-volume method is used to solve the governing equations. In studying the wakes created by the turbines, we observed that the vertical shear of the inflow combined with wake rotation causes lateral wake asymmetry. Also, various turbine configurations are simulated, and the total power production relative to isolated turbines is examined. We found that staggering consecutive rows of turbines in the simulated configurations allows the greatest efficiency using the least downstream row spacing. Counter-rotating consecutive downstream turbines in a non-staggered array shows a small benefit. This work has identified areas for improvement. For example, using a larger precursor domain would better capture elongated turbulent structures, and including salinity and temperature equations would account for density stratification and its effect on turbulence. Additionally, the wall shear stress modelling could be improved, and more array configurations could be examined.

Les fonctions de réponses aux chocs dans les modèles VAR structurels à changements de régimes markovien

Depuis l&#8217;article fondateur de Koop et al. [1996] sur les mod&#232;les non lin&#233;aires, il existe tr&#232;s peu de travaux portant sur les fonctions de r&#233;ponses impulsionnelles pour les mod&#232;les VAR structurels &#224; changements de r&#233;gimes markoviens. Cette question est pourtant importante pour qui veut &#233;tudier l&#8217;existence d&#8217;asym&#233;tries d&#8217;un syst&#232;me &#233;conomique en r&#233;ponse &#224; un choc &#233;conomiquement identifi&#233;. Cet article pr&#233;sente l&#8217;&#233;volution des diff&#233;rentes fonctions propos&#233;es dans la litt&#233;rature depuis le d&#233;but des ann&#233;es&#160;2000. Je discute de leurs propri&#233;t&#233;s et montre, au travers d&#8217;une application sur les ?ux bruts d&#8217;emplois, que le choix d&#8217;une fonction de r&#233;ponse n&#8217;est pas neutre en termes de r&#233;sultats obtenus.

Cerebral asymmetries in sleep-dependent processes of memory consolidation

Preference for previously seen, unfamiliar objects reflects a memory bias on affective judgment, known as the "mere exposure effect" (MEE). Here, we investigated the effect of time, post-exposure sleep, and the brain hemisphere solicited on preference generalization toward objects viewed in different perspectives. When presented in the right visual field (RVF), which promotes preferential processing in the left hemisphere, same and mirrored exemplars were preferred immediately after exposure. MEE generalized to much dissimilar views after three nights of sleep. Conversely, object presentation in the left visual field (LVF), promoting right hemisphere processing, elicited a MEE for same views immediately after exposure, then for mirror views after sleep. Most importantly, sleep deprivation during the first post-exposure night, although followed by two recovery nights, extinguished MEE for all views in the LVF but not in the RVF. Besides demonstrating that post-exposure time and sleep facilitate the generalization process by which we integrate various representations of an object, our results suggest that mostly in the right hemisphere, sleep may be mandatory to consolidate the memory bias underlying affective preference. These interhemispheric differences tentatively call for a reappraisal of the role of cerebral asymmetries in wake- and sleep-dependent processes of memory consolidation.

Finite size vortices in the wake of an accelerated bent lamina

A theory of an unsteady incompressible inviscid flow past an accelerated lamina, moving across the fluid, is extended to address a bent (rather than flat) lamina and finite size (rather than point) vortices in its wake. It is shown that when the velocity of the lamina varies with time t as t m , the distance of the vortices from the lamina and the size of their cores grow, initially, as t ( 2 m + 2 ) / 3 and t ( 2 m + 2 ) / 3 exp ( - Ct - ( 8 m + 2 ) / 3 ) , respectively, where C is a strictly positive constant. The growth rate of the core dimensions with time seems to provide a plausible energy argument for the experimentally observed wake asymmetry that culminates the initial symmetric vortex arrangement.

An experimental investigation of symmetric and asymmetric turbulent wake development in pressure gradient

This paper examines and contrasts the response of planar turbulent wakes with initially symmetric and asymmetric mean velocity profiles to identical imposed streamwise pressure gradients. The focus on near wake behavior, profile asymmetry, and pressure gradient is motivated by their relevance to high-lift systems for commercial transport aircraft. In the experiments, the symmetric wake is generated by a flat plate with a tapered trailing edge. Active and passive flow control are applied on an identical second plate in order to generate the asymmetric wake. Both favorable and adverse constant pressure gradients are imposed on the wakes by means of a downstream diffuser section with fully adjustable top and bottom wall contour. For both symmetric and asymmetric wakes, the effect of pressure gradient on wake spreading, mean velocity defect decay, and the streamwise evolution of the turbulent stresses are documented in detail. In this manner, the role of asymmetry in strained wake development is isolated. We also gauge the ability of commonly used one- and two-equation turbulence models to predict the streamwise wake evolution observed in the experiments.

Wake-boundary layer interaction subject to convex and concave curvatures and adverse pressure gradient

Measurements of mean velocity and turbulent quantities have been carried out when the wake of a symmetrical airfoil interacts with the boundary layer on the (i) walls of a straight duct/diffuser and (ii) convex and concave walls of a curved duct/diffuser. The effects of adverse pressure gradient and of curvatures on the interaction are studied separately and in combination. Six cases are considered, viz. with (i) neither pressure gradient nor curvature, (ii) adverse pressure gradient and no curvature, (iii) and (iv) convex curvature with zero and adverse pressure gradients, respectively, (v) and (vi) concave curvature with zero and adverse pressure gradients, respectively. For the flows with curvature, the curvature parameter δ/R is 0.023, and for the flows with adverse pressure gradient, the Clauser pressure gradient parameter β is 0.62. The individual influences of adverse pressure gradient and convex and concave curvatures on the boundary layer are similar to those observed by earlier investigations. It is further observed that the combined effect of concave/convex curvature and the adverse pressure gradient causes higher turbulence intensities than the sum of the individual effects. The effect of curvature is to make the wake asymmetric, and in combination with adverse pressure gradient the asymmetry increases. It is observed that the adverse pressure gradient causes faster wake–boundary-layer interaction. Comparing measurements in a straight duct, a curved duct, a curved diffuser and a straight diffuser, it is seen that the convex curvature reduces the boundary layer thickness. The asymmetry in wake development compensates for this effect and the wake–boundary-layer interaction on a convex surface is almost the same as that on a straight surface. In the case of interaction with the boundary layer on a concave surface, the curvature increases the boundary layer thickness and causes enhanced turbulence intensities. However, the asymmetry in wake is such that the extent of wake is lower towards the boundary layer side. As a result, the wake–boundary-layer interaction on concave surface is almost the same as on a straight surface. The interaction is faster in the presence of adverse pressure gradient.

SYMMETRIC AND ASYMMETRIC SEPARATION PATTERNS OVER A HEMISPHERE CYLINDER AT LOW REYNOLDS NUMBERS AND HIGH INCIDENCES

A careful documentation of the development of leeward vortices, horn vortices and separation bubbles over a hemisphere-cylinder is presented. Flow visualization, static pressure measurements and seven-hole-probe velocity measurements were obtained to provide a comprehensive picture of the flow at angles of attackα=0 to 45°. Skin-friction line topographies and static pressure data are presented. The results indicate that the development of the leeward vortices is strongly affected by the nose structures. Small disturbances on the nose result in steady wake asymmetries.

LDV Measurements of Spatially Periodic Flows Over a Detached Solid-Rib Array

Measurements of mean velocities, turbulence intensities, and Reynolds stresses are presented for spatially periodic flows in a duct of width-to-height ratio 2 with a detached solid-rib array. The Reynolds number based on the duct hydraulic diameter and cross-sectional bulk mean velocity (Ub), the pitch to rib-height ratio, and the rib-height to duct-height ratio were 2 × 104, 10, and 0.133, respectively. The rib-detached-distance to rib-height ratio was varied from 0 to 3.25 (duct axis) to study its effect on wake length and asymmetry, convective velocity and turbulent kinetic energy immediately behind the rib, maximum turbulent shear stress, and turbulence anisotropy. The results showed that the dominant fluid dynamic factors responsible for the reported peak values of local Nusselt number around the detached rib could be identified. Moreover, the turbulence structure parameter distribution and anisotropy were analyzed to examine the basic assumptions embedded in the turbulence models. Furthermore, the secondary-flow mean velocities were found to be one to two order of magnitude smaller than Ub.

Elliptic tip effects on the vortex wake of an axisymmetric body at incidence

The effectiveness of a new version of an elliptic cross-section tip in controlling the asymmetry of the vortex wake of an axisymmetric body at angle of attack has been studied. The elliptic cross sections were generated using two sixth-degree polynomials such that the tip radius, slope, and curvature would match those of a right circular cone at the point where the polynomial became tangent to a cone generator. The tip was found to be effective in varying the vortex wake geometry of a right circular cone at large angle of attack. The measured side force coefficient varied smoothly with tip roll angle for the two lowest angles of attack studied and exhibited square-wave and more undesirable variations for the larger angles of attack studied. These square-wave and peak, reduction-in-magnitude, and change-in-sign variations were caused by vortex breakaway, which allowed vortex crossover to occur. Ahead of vortex breakaway, the elliptic cross-section tip yielded smooth variations of vortex wake asymmetry with tip roll angle, indicating that the tip would possibly be a feasible control device for high-performance fighter aircraft at high angle of attack.

Nonimpulsively started steady flow about a circular cylinder

The effect of constant acceleration, prior to the establishment of a steady uniform flow, on some of the characteristics of the resulting time-dependent flow about a circular cylinder is investigated experimentally. It is shown that the occurrence of a local maximum drag, the onset of wake asymmetry, and the evolution of the transverse force are dependent on the parameters characterizing the nonimpulsive nature of the ambient flow.

Experimental studies of lift and drag forces upon cylindrical obstacles in homogeneous, rapidly rotating fluids

A series of laboratory measurements is described, in which simultaneous drag and lift forces have been measured directly on full and truncated cylinders moving uniformly through homogeneous rotating fluids. For the full cylinder experiments, it has been demonstrated that within the Reynolds ( Re ) and Rossby ( Ro ) number ranges 9.8 × 10 3 ≤ Re ≤5.6 × 10 4 and 0.1 ≤ Ro ≤0.9, the mean total lift coefficient and the mean lift coefficient because of wake asymmetry both increase with decreasing Rossby number Ro . For cases in which the fluid is non-rotating, the total lift coefficient is shown to be zero, and the values of the drag coefficient for the cylinder are shown to be in accord with existing drag for this geometry. When the value of the Reynolds number is sufficiently small, the presence of background rotation is shown to result in a two- to three-fold increase in the drag coefficient. Experiments with truncated cylinders show similar behaviour to the full cylinder cases with regard to the dependence of the drag coefficient upon Ro and Re . However, values of the lift coefficient are shown to be increased considerably over corresponding full cylinder cases throughout the Reynolds number range, particularly at low and intermediate values of the inverse Rossby number Ro −1 . The increase in lift is attributed to (1) the accentuation of wake asymmetry caused by vortex tube stretching as fluid passes below the truncated cylinder, and (2) the generation of an unbalanced Coriolis force on the volume of fluid displaced by the cylinder.

Flow past a circular cylinder in a rotating stratified fluid

Experimental results are presented to show the development of the flow past a cylinder in a rotating stratified fluid. Both transient and fully developed flow states are investigated, for a range of values of Rossby number Ro and Burger number Bu and for full depth and truncated obstacles. The fully developed flows are shown to be of three main types, delineated respectively as attached eddy pairs, transitional, and vortex wakes, with the boundaries between each flow type being determined by the values of the Reynolds number, the Burger number, the aspect ratio and the ratio of the height of the obstacle to the depth of the fluid. For cases of homogeneous fluid flow, the critical Rossby and Reynolds numbers for vortex shedding are significantly lower than the values found by Boyer. The discrepancy is ascribed to aspect ratio effects and differences in the cylinder shear layer structures in the two studies. The weakening effects of density stratification upon characteristic features of rapidly rotating homogeneous flows are seen to be two-fold: firstly, the wake asymmetries associated with the unstratified flows are seen to be destroyed by the stratification, and, secondly, the retention times of starting eddies, the eddy growth times and the isolated eddy formation times are shown to no longer tune with the background rotation timescale but to scale with the advective timescale of the motion and the aspect ratio of the cylinder. Cross-stream velocity profiles taken at several downstream stations in the wake of the cylinder are used to show a linear increase in the momentum flux coefficient with increasing Re , in the range 55 &lt; Re &lt; 135. Velocity profile measurements taken above and downstream of the truncated mcylinder are presented. These show that the vertical distance over which the upper level fluid is influenced by the presence of the obstacle depends strongly upon the value of Bu , and is typically one-tenth of the obstacle height at the highest values of Bu . Finally, flow visualization studies of the motion in the vertical shear layers for Bu = 0 are compared and contrasted with other similar observations, and with theoretical understandings of the flow structure. The importance of the upper boundary condition is clarified.

A laboratory study of the lift forces on a moving solid obstacle in a rotating fluid

Abstract A series of experiments is described in which a novel technique is used to measure directly the lift force on a right cylynder circular moving through a homogeneous rotating fluid. These experiments have been conducted on the large rotating table facility at the Institut de Mecanique de Grenoble, in order to investigate simultaneously flows at high Reynolds number ( Re ), low Rossby ( Ro ), and low Ekman ( Ek ) number. The measurements have shown that within the range of parameters 0.25 Ro −6 Ek −6 ; 1.50 × 10 4 Re 5 , the cylinder experiences a mean lift due to (i) the structural asymmetry between cyclonic and anticyclonic eddies in its wake, and (ii) the Coriolis force on its excess mass. The total mean lift coefficient C ∗ shows no significant dependence upon Ek , but exhibits a linear relationship with Ro −1 . Similarly, the contribution of the mean lift coefficient C p due to the wake asymmetry alone is shown to vary approximately linearly with Ro −1 . In addition to the net mean lift, the cylinder experiences a fluctuating lift force generated by periodic vortex shedding. The amplitude of the fluctuating lift coefficient also increases linearly with increasing values of Ro −1 , though the Strouhal frequency shows no variation with either Ro or Ek . Measured values of the Strouhal number do not differ significantly from values typical of non-rotating flow past circular cylinders.

Occurrence and Inhibition of Large Yawing Moments during High-Incidence Flight of Slender Missile Configurations

This paper deals with the occurrence of asymmetric wake configurations, and associated side forces and yawing moments, during high-incidnce flight of slender, axisymmetric missile configurations. The relevant phenomena are characterized experimentally, and the root causes of wake asymmetries and means of inhibiting their occurrence are explored. It is concluded that certain flowfield disturbance devices may be effective in significantly reducing the maximum values of yawing moments induced by asymmetric disposition of wake vortices.