INTEGRATED INVESTIGATION OF FLOW-INDUCED DRAG REDUCTION IN STEPPED CYLINDERS AT SUPERCRITICAL REYNOLDS NUMBERS

On the flow around two circular cylinders in tandem arrangement at high Reynolds numbers

Unsteady numerical investigations of flow past a partially rotating stepped cylinder have been performed. The objective of the study was to investigate whether the wake characteristics could be controlled with rotation of one cylinder while the other remains stationary and how partial rotation impacts the aerodynamic forces. The stepped cylinder was 2 m in length where the first meter was a round cylinder 5 cm in diameter followed by a 2:1 step down cylinder. Two round end plates, 0.1 cm thick and 40 cm in diameter, were placed at each end. The end plates were positioned at 5 degrees with respectto the incoming flow to remove the end effect on vortex shedding. All simulations were performed using the Siemens PLM STAR-CCM+ CFD software with K-ω turbulence model. The time step was 0.00083 second to resolve the flow for each 10 degrees rotation. 1200 time steps were used. The investigations were performed with one cylinder rotating while the other remains stationary. Four cases were investigated. When either cylinder was rotating, the RPM was maintained at either 2000 or 4000 while the free stream velocity was maintained at 10 m/sec. The Reynolds number for the large and small cylinders were approximately 32,258 and 16,129, respectively. The corresponding velocity ratios λ for the large cylinder rotating were 0.5 and 1.0, and 0.25 and 1.0 for the small cylinder. Previous investigations have classified vortical structure in the wake of a step cylinder in terms of L-cell (for large cylinder), S-cell (for small cylinder) and N-cell (the region in between). When the large cylinder is rotating, at λ = 1.0, the velocity and vorticity in the wake of the large cylinder is increased. The N-cell initially has a larger velocity than the L-cell and is at a slanted angle. A suction effect was observed in the near wake region, causing the flow in the L-cell to coalesce near its midsection. The vortices originated at the step were connected to the S-cell at a lower speed. The overall lift to drag ratio (L/D) for this case was 1.14. When λ = 0.5, vortex structures were maintained through the three different cells with increased variations in cell frequency across the large cylinder, the L/D was reduced to 0.36. When the small cylinder was rotating, at λ = 0.5, vortex shedding was suppressed within the S-cell and considerable distortion was observed in the vortical structure in the wake of the large cylinder. However, the N-cell had similar structure as when large cylinder was rotating, but connecting to the L-cell at a larger slanted angle. When λ was reduced to 0.25, shedding was observed across the length of the cylinder with increased variations. The corresponding L/D ratios for these cases were both at 0.2.

This paper presents the results of an extensive experimental investigation of the in-line and transverse forces acting on smooth and rough circular cylinders placed in oscillatory flow at Reynolds numbers up to 700,000, Keulegan-Carpenter numbers up to 150, and relative roughnesses from 0.002 to 0.02. The drag and inertia coefficients have been determined through the use of the Fourier analysis and the least squares method. The transverse force (lift) has been analyzed in terms of its maximum, semi peak-to-peak, and root-mean-square values. In addition, the frequency of vortex shedding and the Strouhal number have been determined. The results have shown that (a) for smooth cylinders, all of the coefficients cited above are functions of the Reynolds and Keulegan and Carpenter numbers, particularly for Reynolds numbers larger than about 20,000; (b) for rough cylinders, the force coefficients also depend on the relative roughness k/D and differ significantly from those corresponding to the smooth cylinder; and that (c) the use of the 'frequency parameter' D2/vT and the roughness Reynolds number Umk/v allow a new interpretation of the present as well as the previously obtained data and the establishment of model laws for oscillatory flow about cylinders at supercritical Reynolds numbers. INTRODUCTION The design of structures for the marine environment requires the prediction of the forces generated by waves and currents. Much of the present knowledge has been obtained by means of model tests at Reynolds numbers generally two to three orders of magnitude smaller than prototype Reynolds numbers. These model tests have relied heavily on the so-called Morison formula for expressing the force as the sum of a drag and inertia force. The values of the drag and inertia coefficients to be used in the Morison equation became the subject of many experimental studies in the last twenty years. The correlation of these coefficients with the relative amplitude of the waves (or the Keulegan-Carpenter number) has been generally inconclusive. Furthermore, lift forces which are associated with vortex shedding have received relatively little attention. It thus became clear that much is to be gained by considering plane oscillatory flow about cylinders at high Reynolds numbers in order to isolate the influence of individual factors such as relative amplitude, Reynolds number, and the relative roughness on vortex shedding and resistance. It is with this realization that the present investigation was undertaken and the preliminary results obtained with smooth cylinders in a small U-shaped water tunnel operating at relatively low Reynolds numbers (2,500 to 25,000) have been previously reported []. The present paper deals with in-line and transverse forces acting on smooth and artificially- roughened circular cylinders in harmonic flow at critical and supercritical Reynolds numbers.

Flow and aerodynamic characteristics of a pair of circular cylinders arranged side by side in a uniform flow were experimentally studied at high Reynolds numbers near and over the critical one. Pressure distributions, lift and drag coefficients and Strouhal numbers were measured at different spacings in a wind tunnel. There are good agreements between the present results and others at subcritical Reynolds numbers. At high Reynolds numbers over critical one, we examined the relationships among the formation of separation bubbles, the aspect of wakes, and the lift and drag coefficients. Separation bubbles are formed only on the outward sides of two cylinders at small spacings and this leads to a positive and significant increase in lift coefficient, and biased flow with a high Strouhal number like at subcritical Reynolds numbers cannot be observed to appear.

A badminton shuttlecock flies in a high-drag, and thus, the sport has been a subject of research from the point of view of aerodynamics. A badminton shuttlecock generates significant aerodynamic drag and has a complex flight trajectory. It also has the smallest ballistic coefficient and exhibits the largest in-flight deceleration of any airborne sporting projectile. The ballistic coefficient of a projectile is a measure of its ability to overcome air resistance in flight and is inversely proportional to deceleration. The primary objectives of this study were to measure the aerodynamic properties of feather shuttlecocks under a range of the wind speed (10–60 m/s) and pitch angle (0°–25°). In particular, measurements of aerodynamic forces were performed at high Reynolds numbers (more than Re = 210,000), and the effect of shuttlecock deformation on aerodynamic properties was also investigated, because it is presumed that the flight dynamics is affected by the deformation of the shuttlecock skirt. A shuttlecock skirt is composed of an array of diverging stems, the ends of which are at the convergent end of the skirt, joined together in an end ring. The shuttlecock rotates about its major axis in actual flight, and thus, the experiments were performed on shuttlecocks with and without rotation (spin). Furthermore, the effect of the flow passing through the gaps between the slots (stiffeners) located at the leg portion of the shuttlecock skirt on aerodynamic characteristics is demonstrated by means of a shuttlecock model without gaps, which was completely covered with cellophane tape. The free rotation rate of a shuttlecock increased with an increase in the Reynolds number, and the drag coefficient gradually decreased above Re = 86,000 for a non-rotating shuttlecock. The reduction of drag can be explained by the deformation of the skirt observed in wind tunnel experiments at high speed. In this study, for a rotating shuttlecock, a reduction of drag was not observed over a whole range of Reynolds numbers, because deformation of the skirt for a rotating shuttlecock becomes smaller than that for a non-rotating shuttlecock. However, there was no significant difference in drag coefficient between rotating and non-rotating shuttlecocks, in contrast to the difference in drag coefficient between shuttlecocks with and without gaps. The drag coefficient for a shuttlecock without gaps was significantly smaller than that for a standard shuttlecock (with gaps). For a standard shuttlecock, the air flowed through the gaps into the shuttlecock skirt, and this flow was related to high aerodynamic drag.

Magnetic field effects on Newtonian and non-Newtonian ferrofluid flow past a circular cylinder

Due to increasing fuel cost and emphasis on energy conservation as well as pollution control, there has been considerable interest in improving propulsive efficiency of road vehicles. Reduction in aerodynamic resistance is one aspect of it. Although aerodynamically contoured automobiles has become a standard design practice. Trucks have changed little over the past three decades. The thesis presents results of an organized and extensive wind tunnel test-program, complemented by full-scale road tests, aimed at assessing the effectiveness of two boundary-layer control procedures for reduction of the pressure drag of a cube-van. Wind tunnel results, obtained using 1/6th scale models, at a subcritical Reynolds number of 105, suggest that both the Moving Surface Boundary-layer Control (MSBC) as well as the tripping of the boundary-layer using fences reduce the pressure drag coefficient. Although both the concepts are promising, application of the entirely passive fence procedure appears more attractive from an economic consideration as well as the ease of implementation. The road tests with a full-size cube-van substantiated the trends indicated by the fence data although the actual drag reduction observed was lower (yet quite significant, = 16.6%) than that predicted by the wind tunnel tests. This may be attributed to a wide variety of factors including the differences in the geometry (models; fences and their orientation),operating conditions (Reynolds number; yaw; wind variations in magnitude and direction; turbulence; road boundary-layer; road surface condition), and measurement errors. However, the objective of the study was to assess potential of the concepts which, indeed, is quite promising. Fuel consumption results also substantiated the drag reduction trend. As expected they depend on the gearing condition and hence no general expression applicable to all speed ranges is available. As anticipated the data show a rapid increase in the fuel consumption efficiency at the top end of the speed range. It is concluded that fences can lead to a significant improvement in drag reduction and fuel consumption when applied to flat-faced trucks if positioned correctly. They represent a more elegant, versatile, and cheaper alternative to the 'nose cones' and deflectors available in the market. It is recommended that further road tests should be conducted using both boundary-layer control devices.

Coupled investigation on drag reduction and thermal protection mechanism induced by a novel combinational spike and multi-jet strategy in hypersonic flows

Experimental investigation on the drag reduction characteristics of traveling wavy wall at high reynolds number in wind tunnel

Fluctuating pressure on two circular cylinders at high Reynolds numbers

In this study, passive and active flow control methods were used together to manipulate the flow around a circular cylinder. The experiments were conducted in a wind tunnel for the Reynolds number range of 4000 and 10000 based on the diameter of the circular cylinder (D). A splitter plate was used as passive flow control device and its length was chosen to be about 3.75D. Plasma actuators were placed on the circular cylinder at a position of ±90° as an active flow control device. Combining the active and passive flow control methods, a greater reduction of the drag coefficient was achieved compared to that of the cases when using these methods separately. For Reynolds numbers of 5000 and 10000, the hybrid method gives a reduction in drag of 48% and 45%, respectively. The velocity measurements were carried out by using the hot-wire anemometry and velocity profiles were obtained in the wake region. The flow was visualized by using a smoke wire method. The results revealed that the wake region of the circular cylinder with plasma actuator and splitter plate has a narrower width than the plain cylinder and with splitter plate. Also, it can be seen from spectral analysis that the vortex shedding frequency was suppressed significantly by usage of the hybrid flow control method was used.

Imai suggested a new method to estimate the pressure drag coefficients of bluff bodies at high Reynolds number. He supposed that the flow at high Reynolds number might be obtained by solving a modified Navier-Stokes equation at low Reynolds number which was determined by the assumption of the eddy viscosity in the wake region. In this paper authors discuss the details of the two-dimensional viscous fluid flows past the blunt bodies of arbitrary shape at low Reynolds number (R=40) by solving numerically Navier-Stokes equations, and investigate the validity of Imai's hypothesis above mentioned.The results obtained are that Imai's idea concerning the flow pattern can be acceptable, and that the pressure drag coefficients of bluff bodies at low Reynolds number agree approximately with those in the high but subcritical Reynolds number range. But that Imai's idea gives invalid informations as to the pressure distribution along the body surface at high Reynolds number.

Large-eddy simulations (LES) of the flow past a circular cylinder are used to investigate the characteristics of the near wake region at Reynolds numbers Re = 2.5 × 105 - 8.5 × 105. This range encompasses both the critical and super-critical regimes. Wake characteristic lengths are measured and compared between the different Reynolds numbers. It is shown that the super-critical regime is characterised by a plateau in the drag coefficient at about CD ˜ 0.22, and a quasi-stable wake which has a width of dw /D ˜ 0.4. The analysis also shows a steep decrease in the Reynolds stresses when entering the super-critical regime. Furthermore, the overall analysis shows that after the changes occurring at critical Reynolds numbers, the wake enters a regime where its dynamics is quite similar regardless of the Reynolds number.

PurposeTo study the steady viscous incompressible electrically conducting fluid flow past a circular cylinder under the influence of an external magnetic field at high Reynolds numbers (Re).Design/methodology/approachThe finite difference method is applied to solve the governing non‐linear Navier‐Stokes equations. First order upwind difference scheme is applied to the convective terms. The multigrid method with coarse grid correction is used to enhance the convergence rate. The defect correction technique is employed to achieve the second order accuracy.FindingsA non‐monotonic behavior in separation angle when N≥5 and separation length when N≥3 is found with the increase of external magnetic field. The drag coefficient is found to increase with increase of N. The pressure drag coefficient, total drag coefficient and rear pressure are found to exhibit a linear dependence with N0.5. The pressure Poisson equation is solved to find pressure fields in the flow region. It is found that the upstream base pressure increases with increase of external magnetic field while the downstream base pressure decreases with the increase of the external magnetic field.Originality/valueThe non‐monotonic behaviors in the separation angle and separation length at high Re are explained through pressure fields which are found first time for this problem. The linear dependence of the pressure drag coefficient, total drag coefficient and the pressure at rear stagnation point with N0.5 is in agreement with experimental findings.

This study investigates the drag reduction potential of a cylinder with combinations of dimpled and smooth surfaces, examining the effects of various dimple placements on drag coefficient (CD) across a range of Reynolds Numbers (Re). Experimental results reveal that strategic dimple placements on the cylinder’s windward side effectively reduce drag by influencing boundary layer behavior. Specifically, the configurations with 60o dimpled and 150o smooth and 120o dimpled and 60o smooth surfaces consistently demonstrated the lowest drag coefficients, especially at higher Reynolds Numbers, where delayed flow separation was achieved. Conversely, configurations with limited dimple coverage, such as 30o dimpled and 150o smooth, produced higher drag coefficients, indicating less effective boundary layer control. Full dimple coverage (180o dimpled) showed significant drag reduction only at higher Reynolds Numbers, while higher drag persisted at lower speeds. These findings suggest that partial dimple coverage on the windward side can optimize drag reduction by promoting early boundary layer transition, making such configurations advantageous for applications requiring efficient aerodynamic and hydrodynamic performance. The overall results indicate that dimple placement is crucial in influencing drag, providing insights for optimizing surface modifications for improved aerodynamic efficiency.