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Passive drag reduction in cross-flow rigid plates near the free surface
Abstract
Flat plate models of fixed aspect ratio incorporating either serrations or holes at the upper and lower plate edges, were towed normal to the flow and in proximity to the free surface. The hydrodynamics and the drag generated by the plates has been studied in detail for several configurations. In general, the models with holes presented greater drag reduction than those with serrations, with the hole angle playing an important role on the total drag measured. The fluid dynamic sources for drag reduction at low submergence depths have been investigated using quantitative flow visualization, and they have been related to the changes in the separating shear layers, as a result of the different edge features imposed to the plates.
... The drag force (C D ) of the sphere may be reduced by 50 % using this ventilation technique, as demonstrated by Suryanarayana andMeier, 1995 andGrosche andMeier, 2001. Other studies have also revealed that adding slots can reduce the C D acting on the bluff bodies, benefiting specific engineering applications (Satheesh and Huera-Huarte, 2020;Rashidi et al., 2016;Liu et al., 2021). In addition, vortex generators (VGs), when implemented in flexible riser configurations, serve the dual function of augmenting load resistance and facilitating pressure equalization along the length of the riser. ...
Optimizing flow patterns around cylinders is crucial for diverse engineering applications, aiming to mitigate vibrations induced by vortex shedding and minimize drag. Strategies, such as the implementation of vortex generators and modifications in the cylinder's surface geometry, are explored to optimize flow patterns and enhance the performance of structures exposed to fluid dynamics. This study investigated the flow characteristics of a smooth circular cylinder and cylinders equipped with triangular vortex generators placed at different position angles (α=15°, 30°, 60°, and 75°) and yaw angles (β=30°, 45°, 60°, and 75°) at the surface. The particle image velocimetry technique is used to measure the instantaneous and time-averaged velocities of the fluid flow over cylinders at Reynolds Number 8000. It is found that flow characteristics are highly dependent on the position angle of the triangular vortex generators. Specifically, at α=30° and β=60°, a substantial reduction of 17.3, 10.3, and 19.5 % occurred in normalized streamwise and root mean square of cross-stream velocity fluctuations and turbulent kinetic energy values, respectively, in comparison to that of the smooth circular cylinder. Furthermore, a noteworthy drag reduction of 33.3 % was achieved in the case of α=30° and β=60° through the elongation of the recirculation length and the establishment of a low-pressure zone away from the base of the circular cylinder. On the other hand, the lowest Strouhal number is obtained as 0.180 in the case of α=75° and β=30° where it was 0.211 at the smooth CC case. As a result, triangular vortex generators with an area of 1.1% of the total side projection area of the cylinder were found to be very effective in flow control.
Vortex distribution affects the flow field variation of offshore structures at different aspect ratios (ARs; lengthto-draft). This paper investigates the relationship between the vortex distribution and flow field around a semi-submerged rectangular cylinder (SRC) at a low Reynolds number (Re = 36300) using the OpenFOAM platform. First, vortex generation is explored, and then the influence of vortex distributions on the flow field around the SRC is analyzed. The numerical results show that when the SRC pierces the free surface, the wake region is different from the Kármán vortex street due to the free surface. AR mainly affects the range of the wake region and the SRC bottom vortex. In terms of vortex generation and differences in vortex structure, the variations of the flow field around the SRC are significantly affected by the vortex at the bottom of the SRC.
Paddle, padel or platform tennis is a rapidly growing sport derived from tennis, but played in smaller courts using a solid racquet, instead of a strung one. Most of the racquet manufacturers drill holes in the head arguing improved aerodynamic performances. The question is, if almost all manufacturers drill aerodynamic holes, are they optimally distributed from the fluid dynamics point of view? In this work, we use a generic racquet model to analyse what is the effect of the holes on the drag forces suffered by the racquet and how these forces are related to the wakes seen by the racquet during its motion. A porosity parameter is defined that takes into account not only the porosity itself but also how it is distributed within the racquet’s head. Digital particle image velocimetry is used to quantify the flow field around the racquet, allowing the identification of different wake topologies that can be related to different drag performances.
Wave motion over a submerged porous plate is studied by the boundary-element method based on the linear potential wave theory. The boundary condition at the porous plate is formulated under the assumption that the flow within the porous medium is governed by Darcy's law. The characteristics of variation of the reflection, the transmission, the loss of energy and the force versus the plate length, the water depth, the incident wavelength, the depth of submergence, and the porosity of the plate are discussed. It is shown that the reflection and transmission coefficients vary periodically as the relative length of the plate to the incident wavelength increases. Moderately large values of the porosity of the plate are found to produce the maximum dissipation of the incident wave energy. It is also demonstrated that a plate with proper porosity can significantly suppress the reflection and reduce the wave force acting on it while the transmission can be kept at a low level.
The flow in the wakes behind two-dimensional perforated plates has been investigated in the Reynolds number range 2·5 × 10 ⁴ to 9·0 × 10 ⁴ .
Measurements of drag and shedding frequency were made and a pulsed hotwire anemometer was used to measure the mean velocity and turbulent intensity variations in the highly turbulent regions immediately behind the plates.
The results indicate the existence of two distinct types of flows: one appropriate to high and the other to low values of plate porosity.
Rectangular foils of different flexural rigidities were towed normal to the flow at a fixed speed in a quiescent fluid in order to study the effect of the proximity of the upper edge of the models to free surface. It was found that flexibility ensured drag reduction due to the reconfiguration process at all submergence depths, with certain foils exhibiting depth-independent behavior. The study of Vogel exponents also showed that a sublinear or even a depth independent relationship between drag and velocity can be attained at specific flexural rigidity values. A modified classical beam theory model using a power-law based load distribution was utilized to obtain an empirical relationship between the loading exponent and Cauchy numbers and to identify the foil tip location. Particle image velocimetry was also undertaken to interpret and further understand the force results. The experiments showed the existence of 2 ranges of Cauchy numbers with a small degree of overlap in ranges wherein the drag coefficient and Vogel exponents are independent of submergence depth.
Flat rectangular plates of different aspect ratios were towed normal to the flow at various speeds in a quiescent fluid, with the focus being the study of the effect of the plate edge proximity to the free surface on drag force. The submergence depth was measured from the plate upper edge to the free surface and varied from zero to the centre of the tank. It was found that the drag increases abruptly prior subsiding with increasing submergence depth, with this jump in drag being more prominent in low aspect ratio plates. The abrupt rise in the drag is due to the existence of a gap-flow at the free surface resulting in the formation of a recirculating flow in close proximity to the base region of plate. Overall, the trends are Reynolds number independent, except when the aspect ratios are in the range from 0.75 to 1.33, and the plate was near the free surface. Flow visualization was also employed on specific plates and at certain depths to understand the flow features and link them to the observed force trends.
Porous disks are commonly encountered in experimental studies dealing with flow through objects such as wind turbines, parachutes, and fluidic devices to regulate pressure and/or downstream turbulence. Perforations are typically staggered and only porosity is altered to attain the required disk drag coefficient, despite a documented influence of topology. Few works have reported, however, to which extent the spatial distribution of the circular perforations affect the mean flow pertaining freestanding disks, and for this reason, this work presents a first, more systematic study focused on the effect of azimuthally varying hole topology and porosity on disk drag and near-wake characteristics. An experimental study performed in airflows of negligible freestream turbulence at Reynolds numbers in the order of 10⁵ is reported and related to the existing literature to ensure reliability. Complementary to drag measurements, near-wake surveys have been performed on a variety of perforation layouts using two-component laser Doppler velocimetry and two-component particle image velocimetry. It is shown that minor changes in perforations can cause drastic changes in near-wake flow topology and no perforation layout can be consistently associated with highest drag. Explicit empirical expressions for drag coefficient linked with the simplified topologies considered have been derived.
The evolution of vortices produced in the wake of impulsively-started axisymmetric and non-axisymmetric plates is investigated using force and Particle Image Velocimetry (PIV) measurements. At a Reynolds number of 40,000, circular, square and elliptical plates were impulsively-towed through a towing tank to investigate vortex pinch-off for each plate. The PIV measurements were used to calculate circulation growth and formation numbers for each shape. Good agreement with formation numbers reported for vortex rings produced in isolation was observed for the circular and square plates, but not for the ellipse. Force data reveals a strong coupling between vortex evolution and the forces generated on the plates. The presence of a vortex ring results in a vortex force that modifies the instantaneous drag on the plate. This vortex force is related to the self-induced velocity of the vortex ring as well as the proximity of the vortex ring relative to the plate, and therefore stable vortex rings result in the greatest reduction in the overall drag force. As a means to explain pinch-off, the pressure field surrounding the vortices was also investigated. Contour plots of the streamwise pressure gradient show that interaction of a favourable pressure gradient on the leeward side of the plates results in the redirection of the shear layer away from the vortex. This results in a termination of mass flux into the vortex, and subsequent plateau in vortex circulation.
Digital particle image velocimetry (DPIV) is the digital counterpart of conventional laser speckle velocitmetry (LSV) and particle image velocimetry (PIV) techniques. In this novel, two-dimensional technique, digitally recorded video images are analyzed computationally, removing both the photographic and opto-mechanical processing steps inherent to PIV and LSV. The directional ambiguity generally associated with PIV and LSV is resolved by implementing local spatial cross-correlations between two sequential single-exposed particle images. The images are recorded at video rate (30 Hz or slower) which currently limits the application of the technique to low speed flows until digital, high resolution video systems with higher framing rates become more economically feasible. Sequential imaging makes it possible to study unsteady phenomena like the temporal evolution of a vortex ring described in this paper. The spatial velocity measurements are compared with data obtained by direct measurement of the separation of individual particle pairs. Recovered velocity data are used to compute the spatial and temporal vorticity distribution and the circulation of the vortex ring.
This is a joint experimental and computational research effort on the aerodynamics of perforated surfaces inclined to a freestream. The goal is to characterize the key parameters affecting the flowfield in order to construct a macroscopic computational model that obviates resolution of the detailed perforations. The model simulates the effects of the perforated surfaces by locally applying a body force term in the momentum equation. The study considers wedge-shaped deflectors used for jet noise reduction in turbofan engine nozzles in the form of flaps with varying angles and perforation shapes. The experiments measured the mean velocity field inside and in the wake of the flaps while the numerical analysis comprised direct computations of simplified perforated flaps and computations using the body force model. It is found that the effective porosity of the surface is affected by flow separation within the perforations. The flow blockage of the separation regions is primarily a function of the angle to the freestream and the ratio of the thickness to hole diameter. Accordingly, a formulation for the body force model is proposed that incorporates this finding. The resulting velocity field is in good agreement with the experiments and with the direct computations.
Perforated plates are widely used in pipeline systems either to reduce flow nonuniformities or to attenuate the onset and the development of cavitation. This experimental work aims at investigating the dependence of the pressure losses through sharp-edged perforated plates with respect to the geometrical and flow key parameters. The data, collected in two large experimental campaigns carried out on different pilot plants, are reported and discussed. Several plates with different geometrical characteristics were tested. More precisely, perforated plates whose equivalent diameter ratio varies between 0.20 and 0.72; relative hole thickness between 0.20 and 1.44; and number of holes between 3 and 52. Experimental data from literature are also considered in order to ensure the reliability of the parametric investigation. The dependence of the pressure loss coefficient upon the Reynolds number, the equivalent diameter ratio, the relative thickness, and the number and disposition of the holes is studied. A comparison to different empirical equations, as available by the technical literature, and to the standard ISO 5167-2 single-hole orifice is also provided.
The effective size of holes is considered, taking into account three-dimensional and two-dimensional examples. Attention is given to double slits and split plates, laws of combination for finite sparse distributions of holes, blockage and permeability coefficients, Green's function for a rectangular pipe, applications involving vehicles moving close to a plane surface, and aspects of water-wave transmission through holes in walls. The Helmholtz resonator is discussed and an investigation of real-fluid effects is conducted.
To investigate the effect of porosity in ribbon parachutes, the flow over a series of flat, circular disks has been studied in detail. These disks have been fabricated with 1-18 concentric slots covering a geometric porosity range from 2% to 33% open-area ratio. On one disk the rear edges of each slot have been chamfered at 45 deg to fix the separation points at all test Reynolds numbers. All outside diameters were chamfered. The disk drag and pressure distributions have been measured, and the results compared to those for a solid disk. Base pressure coefficients behind the solid, or ribbon, elements show surprisingly low values. © 1980 American Institute of Aeronautics and Astronautics, Inc., All rights reserved.
Using a pulsed-wire anemometer, extensive measurements have been made in the wakes of two-dimensional solid and porous fences immersed in the constant-stress region of a simulated rural atmospheric boundary-layer. The porosity (ratio of open to total area) of the perforated fences ranged from 0.0 to 0.5 and each porosity was modelled using three forms of openings, i.e. vertical slats, horizontal slats and circular holes.Comparative measurements made using the more conventional hot-wire anemometer are given. The results show the superiority of the pulsed-wire anemometer in correctly measuring the highly turbulent and sometimes recirculating wake flows. This indicates that previously reported experiments may be in considerable error.Results from the present experiment are given in the form of dimensionless mean and turbulence velocity profiles. In addition, contour plots of shelter parameters behind the fences are given for comfort and shelter analyses.Downstream of the re-attachment region, velocity deficits and excess shear and normal stress perturbations (quantities useful in shelter analyses) are plotted in a self-preserving form and simple equations that fit the experimental points are given.Power spectra measured in the presence of the solid fence are given and the structure of the turbulent wake deduced from these is discussed.
The interaction of oblique monochromatic incident waves with horizontal/inclined/dual porous plates is investigated in the context of two-dimensional linear potential theory and Darcy's law (the normal velocity of fluid passing through a thin porous plate is linearly proportional to the pressure difference across it). The developed theory is verified by both small-scale and full-scale experiments. First, matched eigenfunction expansion (MEE) solutions for a horizontal porous plate are obtained. The relationship between the plate porosity and the porous parameter is obtained from systematic model tests by using six porous plates with different sizes and spacing of circular holes. Secondly, a multi-domain boundary-element method (BEM) using simple-sources (second-kind modified Bessel function) is developed to confirm the MEE solutions and to apply to more general cases including inclined or multiple porous plates. The BEM-based inner solutions are matched to the eigenfunction-based outer solutions to satisfy the outgoing radiation condition in the far field. Both analytical and BEM solutions with the developed empirical porous parameter agree with each other and correlate well with both small-scale data from a two-dimensional wave-tank test and full-scale measurement in a large wave basin. Using the developed predictive tools, wave-absorption efficiency is assessed for various combinations of porosity, water depth, submergence depth, wave heading, and plate/wave characteristics. In particular, it is found that the performance can be improved by imposing the proper inclination angle near the free surface. The optimal porosity is near porosity P=0.1 and the optimal inclination angle is around 10° as long as the plate length is greater than 20% of the wavelength. Based on the selected optimal parameters (porosity=0.1 and inclination angle=11.3°), the effective wave-absorption system for MOERI's square basin is designed.
The hole diameter effect on the flow characteristics of wake behind porous fences has been investigated experimentally in a circulating water channel having a test section of . Three porous fences having different hole diameters of were tested in this study, but they have the same ε=38.5% geometric porosity. One thousand instantaneous velocity fields for each fence were measured consecutively by the hybrid PTV system employing a high-speed CCD camera. Free stream velocity was fixed at and the corresponding Reynolds number based on the fence height was Re=2,985. Consequently, the fence with the smallest hole diameter ) decreases the streamwise velocity component and increases the vertical velocity component. Among the three hole diameters tested in this study, the d1.4 fence has the largest turbulence intensity in the shear layer developed from the fence top. Regardless of the hole diameter, however, all three fences having the same porosity reduce the reduction of turbulent intensity in the lower region below the fence height (y/H<1).
Shallow two-dimensional turbulent wake flows have been studied experimentally on a large water table. In the experiments, the ambient Reynolds number Reh = UaH/ν, in which Ua is the depth-averaged ambient velocity, H the water depth, and ν the kinematic viscosity, is large, well above a lower critical value of the order of 500 for open-channel flows so that the ambient base flow is fully turbulent. Different types of blunt bodies extending over the full depth are inserted in that base flow, including cylinders and flat solid and porous plates oriented transversely to the ambient flow. In all cases, the transverse body dimension D greatly exceeds the water depthy, . With that condition, the wake Reynolds number Red = UaD/ν is very large, greater than 104. The shallow near-wake characteristics of plane wakes from blunt bodies extending over the full water depth have been found to fall into one of three classes: (i) the vortex street (VS) type with an oscillating vortex shedding mechanism, (ii) the unsteady bubble (UB) wake type with flow instabilities growing downstream of a recirculating bubble attached to the body, and (iii) the steady bubble (SB) wake type with an attached bubble followed by a turbulent wake that contains no growing instabilities. When Reh > 1500, the flow classification is uniquely dependent on a shallow wake parameter, in which cf is a quadratic law friction coefficient. For circular cylindrical bodies the VS-UB transition is characterized by a critical value, Sca ≈ 0.2, and the UB-SB transition by Scc ≈ 0.5. Solid plates, oriented transversely, differ by a factor of 1.25.The shallow far-wake behavior has been investigated with a special variable porosity wake device that reduces the wake velocity deficit and completely suppresses the VS instabilities in the near-field. Thus, only UB and SB wake types are found in that case. Furthermore, the shallow plane wake is obsserved to “stabilize” for large downstream distances, , in the sense that the growth and maintenance of the large scale structures in the wake flow become suppressed and the wake collapses into a more ordered flow that, however, still contains small scale (of scale H) turbulence. This wake stabilization is controlled by two factors: first, the usual evolution in a turbulent wake that reduces the velocity deficit while increasing the wake parameter S, and secondly, the exponential loss of the momentum deficit flux in the wake due to bottom friction.