Drag Crisis Research Articles

Sharp Transition in the Lift Force of a Fluid Flowing Past Nonsymmetrical Obstacles: Evidence for a Lift Crisis in the Drag Crisis Regime

Bluff bodies moving in a fluid experience a drag force which usually increases with velocity. However in a particular velocity range a drag crisis is observed, i.e., a sharp and strong decrease of the drag force. This counterintuitive result is well characterized for a sphere or a cylinder. Here we show that, for an object breaking the up-down symmetry, a lift crisis is observed simultaneously to the drag crisis. The term lift crisis refers to the fact that at constant incidence the time-averaged transverse force, which remains small or even negative at low velocity, transitions abruptly to large positive values above a critical flow velocity. This transition is characterized from direct force measurements as well as from change in the velocity field around the obstacle.

Leidenfrost Vapor Layers Reduce Drag without the Crisis in High Viscosity Liquids

The drag coefficient C_{D} of a solid smooth sphere moving in a fluid is known to be only a function of the Reynolds number Re and diminishes rapidly at the drag crisis around Re∼3×10^{5}. A Leidenfrost vapor layer on a hot sphere surface can trigger the onset of the drag crisis at a lower Re. By using a range of high viscosity perfluorocarbon liquids, we show that the drag reduction effect can occur over a wide range of Re, from as low as ∼600 to 10^{5}. The Navier slip model with a viscosity dependent slip length can fit the observed drag reduction and wake shape.

Calculation of the drag and heat transfer from a sphere in the gas flow in a cylindrical channel

A numerical experiment on the simulation of heat transfer from a sphere to a gas flow in a cylindrical channel in the Stokes and transient flow regimes has been described. Radial and axial profiles of the gas temperature and the dependences of drag coefficient Cd of the body and Nusselt number Nu on Reynolds number Re have been calculated and analyzed. The problem of the influence of the early drag crisis for a sphere on its heat transfer to the gas flow has been considered. The estimation of this phenomenon has shown that the early drag crisis of the sphere in a strongly turbulent flow causes a reduction in heat transfer from the sphere to the gas by three to six times (in approximately the same proportion as for its drag coefficient).

Physics of knuckleballs

Zigzag paths in sports ball trajectories are exceptional events. They have been reported in baseball (from where the word knuckleball comes from), in volleyball and in soccer. Such trajectories are associated with intermittent breaking of the lateral symmetry in the surrounding flow. The different scenarios proposed in the literature (such as the effect of seams in baseball) are first discussed and compared to existing data. We then perform experiments on zigzag trajectories and propose a new explanation based on unsteady lift forces. In a second step, we exploit wind tunnel measurements of these unsteady lift forces to solve the equations of motion for various sports and deduce the characteristics of the zigzags, pointing out the role of the drag crisis. Finally, the conditions for the observation of such trajectories in sports are discussed.

The intermittent nature of the laminar separation bubble on a cylinder in uniform flow

Abstract Flow past a circular cylinder in a uniform flow is investigated for 1 × 10 4 ≤ Re ≤ 4 × 10 5 . A stabilized finite element method is used to solve the incompressible flow equations in primitive variables in three dimensions. The computations capture the phenomenon of drag crisis: a significant reduction in drag with increase in Re in the critical regime. The mechanism for this decrease in drag during the drag crisis is explored. It is found that the transition of the boundary layer from a laminar to turbulent state, as well as the formation of the laminar separation bubble (LSB), is intermittent. The LSB does not exist in the sub-critical regime while it appears at all times beyond the critical regime. The frequency of its appearance as well as the duration of its stay, in the critical Re regime, increases with increase in Re . This is established by studying the rms of the high pass filtered fluctuations in the surface pressure and flow close to the cylinder. A procedure to capture these fluctuations due to the shear layer activity responsible for the formation of LSB is proposed and implemented. It is utilized to estimate the intermittency factor of the LSB at various Re . It is found that the intermittency can be utilized to explain the variation of mean drag with Re . Weakening of vortex shedding is observed in critical regime and beyond.

スポーツウェア布地の空力特性

Some sports wears are used at a top speed of about 25 m/s in a specific sport. This means that some sports wears are used in harsh environments. Therefore, reduction of air resistance is in great demand. A high-performance sportswear contributes to improvement of a record. This study is investigated the aerodynamic behavior of functional fabric on a cylinder model. A cylinder model imitates a trunk of a human body and arms by connecting a cylinder different in the diameter. Seams are imitated in this cylinder model. This fabric was tasted in a wind tunnel from 3 to 30 m/s. The wake of this fabric was visualized by the PIV measurement. The single cylinder showed drag crisis in Re =0.6～0.8×105. However, the Cylinder model didn't exhibit drag crisis. These results are correlated with Visualization of the wake. The arms of human body are important for studying aerodynamic characteristics of sportswear.

Large eddy simulations of a circular cylinder at Reynolds numbers surrounding the drag crisis

Large eddy simulations of the flow around a circular cylinder at high Reynolds numbers are reported. Five Reynolds numbers were chosen, such that the drag crisis was captured. A total of 18 cases were computed to investigate the effect of gridding strategy, turbulence modelling, numerical schemes and domain width on the results. It was found that unstructured grids provide better resolution of key flow features, when a ‘reasonable’ grid size is to be maintained.When using coarse grids for large eddy simulation, the effect of turbulence models and numerical schemes becomes more pronounced. The dynamic mixed Smagorinsky model was found to be superior to the Smagorinsky model, since the model coefficient is allowed to dynamically adjust based on the local flow and grid size. A blended upwind-central convection scheme was also found to provide the best accuracy, since a fully central scheme exhibits artificial wiggles, due to dispersion errors, which pollute the solution.Mean drag, fluctuating lift Strouhal number and base pressure are compared to experiments and empirical estimates for Reynolds numbers ranging from 6.31×104 to 5.06×105. In terms of the drag coefficient, the drag crisis is well captured by the present simulations, although the other integral quantities (rms lift and Strouhal number) show larger discrepancies. For the lowest Reynolds number, the drag is seen to be more sensitive to the domain width than the spanwise grid spacing, while at the higher Reynolds numbers the grid resolution plays a more important role, due to the larger extent of the turbulent boundary layer.

Aerodynamic study of modern sepak takraw ball: Flow visualization test approach

<p>It has been shown in previous studies that the flight trajectories of sports balls are influenced by their aerodynamic characteristics. These aerodynamic characteristics are primarily dependent on the physical shape and surface texture of the balls. Even though <em>sepak takraw</em> is well established as a sport, little is known regarding the aerodynamic characteristics of the <em>sepak takraw</em> ball, which has a rather complex shape and surface texture. Hence, the main objective of this research is to investigate the aerodynamic characteristics (specifically the drag and lift coefficients) and flow features of a modern <em>sepak takraw</em> ball commercially available in the market by means of numerical simulations and wind tunnel experiments using the smoke flow visualization technique. The aerodynamic characteristics and flow features of the ball are determined for non-spinning conditions at a wind speed of 3 m/s. It is found that the drag coefficient and lift coefficient of the <em>sepak takraw</em> ball is 0.4868400 and -0.0130915, respectively. The images captured from the smoke flow visualization experiments reveal that the <em>sepak takraw</em> ball is in the subcritical flow regime at a wind speed of 3 m/s, which is the regime before the drag crisis. The laminar boundary layer separates from the upper and lower surfaces of the ball at points upstream of the equator of the ball, creating a large wake region downstream of the <em>sepak takraw</em> ball and resulting in high drag. This in turn, influences the trajectory of the <em>sepak takraw</em> ball in flight. The flow features observed from the smoke flow visualization experiments are representative of the flow during a <em>sepak takraw </em>game. Owing to the complexity of <em>sepak takraw</em> ball, it is recommended that the aerodynamic characteristics of the <em>sepak takraw</em> ball are investigated for spinning conditions in future studies.</p>

Drag Moderation by the Melting of an Ice Surface in Contact with Water.

We report measurements of the effects of a melting ice surface on the hydrodynamic drag of ice-shell-metal-core spheres free falling in water at a Reynolds of number Re~2×10^{4}-3×10^{5} and demonstrate that the melting surface induces the early onset of the drag crisis, thus reducing the hydrodynamic drag by more than 50%. Direct visualization of the flow pattern demonstrates the key role of surface melting. Our observations support the hypothesis that the drag reduction is due to the disturbance of the viscous boundary layer by the mass transfer from the melting ice surface.

Effects of cylinder Reynolds number on the turbulent horseshoe vortex system and near wake of a surface-mounted circular cylinder

The turbulent horseshoe vortex (HV) system and the near-wake flow past a circular cylinder mounted on a flat bed in an open channel are investigated based on the results of eddy-resolving simulations and supporting flow visualizations. Of particular interest are the changes in the mean flow and turbulence statistics within the HV region as the necklace vortices wrap around the cylinder’s base and the variation of the mean flow and turbulence statistics in the near wake, in between the channel bed and the free surface. While it is well known that the drag crisis induces important changes in the flow past infinitely long circular cylinders, the changes are less understood and more complex for the case of flow past a surface-mounted cylinder. This is because even at very high cylinder Reynolds numbers, ReD, the flow regime remains subcritical in the vicinity of the bed surface due to the reduction of the incoming flow velocity within the bottom boundary layer. The paper provides a detailed discussion of the changes in the flow physics between cylinder Reynolds numbers at which the flow in the upstream part of the separated shear layers (SSLs) is laminar (ReD = 16 000, subcritical flow regime) and Reynolds numbers at which the transition occurs inside the attached boundary layers away from the bed and the flow within the SSLs is turbulent (ReD = 5 ∗ 105, supercritical flow regime). The changes between the two regimes in the dynamics and level of coherence of the large-scale coherent structures (necklace vortices, vortex tubes shed in the SSLs and roller vortices shed in the wake) and their capacity to induce high-magnitude bed friction velocities in the mean and instantaneous flow fields and to amplify the near-bed turbulence are analyzed. Being able to quantitatively and qualitatively describe these changes is critical to understand Reynolds-number-induced scale effects on sediment erosion mechanisms around cylinders mounted on a loose bed, which is a problem of great practical relevance (e.g., for pier scour studies).

Publisher’s Note: “Statistics and dynamics of the boundary layer reattachments during the drag crisis transitions of a circular cylinder” [Phys. Fluids 27, 014101 (2015)

First Page

Statistics and dynamics of the boundary layer reattachments during the drag crisis transitions of a circular cylinder

Time series of pressure measured on the periphery of the central section of a circular cylinder of aspect ratio 22 are used to investigate the dynamics of the local reattachments during the drag crisis transitions. A succession of multi-stable dynamics are identified and characterized through conditional statistical analysis as the Reynolds number is increased. The first transition marking an abrupt weakening of the periodic pressure fluctuations associated to the global shedding dynamics is accompanied by the appearance of symmetric and bistable perturbations. Afterwards, two scenarii of asymmetric and symmetric boundary layer reattachments are found. The asymmetric scenario leads to two transitions. During the first transition, the flow explores randomly three stable states to eventually stabilize on the state corresponding to the permanent reattachment on one side of the cylinder. The second transition is bistable and leads to the permanent reattachment on both sides. For the second symmetric scenario, the boundary layer reattachments occur simultaneously on both side of the cylinder. In that case, the flow explores randomly two stable states to eventually stabilize on the state of full reattachment. Pressure distributions of all of these states are characterized as well as their corresponding probabilities during the drag crisis transitions of the critical regime.

Unsteady forces on a circular cylinder at critical Reynolds numbers

It is well known that the flow past a circular cylinder at critical Reynolds number combines flow separation, turbulence transition, reattachment of the flow, and further turbulent separation of the boundary layer. The transition to turbulence in the boundary layer causes the delaying of the separation point and an important reduction of the drag force on the cylinder surface known as the drag crisis. In the present work, large-eddy simulations of the flow past a cylinder at Reynolds numbers in the range 2.5 × 105-6.5 × 105 are performed. It is shown how the pressure distribution changes as the Reynolds number increases in an asymmetric manner, occurring first on one side of the cylinder and then on the other side to complete the drop in the drag up to 0.23 at Re = 6.5 × 105. These variations in the pressure profile are accompanied by the presence of a small recirculation bubble, observed as a small plateau in the pressure, and located around ϕ = 105∘ (measured from the stagnation point). This small recirculation bubble anticipated by the experimental measurements is here well captured by the present computations and its position and size measured at every Reynolds number. The changes in the wake configuration as the Reynolds number increases are also shown and their relation to the increase in the vortex shedding frequency is discussed. The power spectra for the velocity fluctuations are computed. The analysis of the resulting spectrum showed the footprint of Kelvin-Helmholtz instabilities in the whole range. It is found that the ratio of these instabilities frequency to the primary vortex shedding frequency matches quite well the scaling proposed by Prasad and Williamson [“The instability of the separated shear layer from a bluff body,” Phys. Fluids 8, 1347 (1996); “The instability of the shear layer separating from a bluff body,” J. Fluid Mech. 333, 375–492 (1997)] (fKH/fvs ∝ Re0.67).

The flow along a wall moving reversely to a parallel non-Newtonian, power-law stream

The flow induced by a flat plate, with finite length, moving reversely to a parallel ambient stream of a non-Newtonian, power-law fluid is considered in the present paper. This problem has been investigated partly in the past (Klemp and Acrivos, 1972, 1976) but only for a Newtonian fluid and using a modified boundary layer approximation. In the present work the investigation is based on the solution of the complete equations using the commercial code ANSYS FLUENT. It is found that for low Reynolds numbers and low power-law index (n < 0.5) the drag coefficient obeys the relationship cD = A/Re. It was also found that the drag coefficient, in general, increases with the velocity ratio parameter λ but at λ around 2 the drag coefficient shows a local sharp decrease (drag crisis). An explanation for this crisis is given.

Leidenfrost vapour layer moderation of the drag crisis and trajectories of superhydrophobic and hydrophilic spheres falling in water.

We investigate the dynamic effects of a Leidenfrost vapour layer sustained on the surface of heated steel spheres during free fall in water. We find that a stable vapour layer sustained on the textured superhydrophobic surface of spheres falling through 95 °C water can reduce the hydrodynamic drag by up to 75% and stabilize the sphere trajectory for the Reynolds number between 10(4) and 10(6), spanning the drag crisis in the absence of the vapour layer. For hydrophilic spheres under the same conditions, the transition to drag reduction and trajectory stability occurs abruptly at a temperature different from the static Leidenfrost point. The observed drag reduction effects are attributed to the disruption of the viscous boundary layer by the vapour layer whose thickness depends on the water temperature. Both the drag reduction and the trajectory stabilization effects are expected to have significant implications for development of sustainable vapour layer based technologies.

0303 LESによる臨界レイノルズ数付近の球周り流れ解析 : 滑面と粗面の比較

The present study has conducted large-eddy simulation (LES) of smooth and golf ball around the critical Reynolds number. Drag coefficients and the flow field around the sphere and the golf ball have been analyzed. The result shows that the surface roughness on the golf ball surface doesn't strongly affect the flow behavior in the subcritical regime. However, the surface roughness could result in the drag crisis of golf ball occurring at a lower Reynolds number in the supercritical regime compared with the smooth sphere. This is because the flow becomes instable when it passes the dimples at the supercritical Reynolds number and could consequently travel further downstream.

Flow of colloid particle solution past macroscopic bodies and drag crisis

The motion of colloid particles in a viscous fluid flow is considered. Small sizes of colloid particles as compared to the characteristic scale of the flow make it possible to calculate their velocity relative to the liquid. If the density of a colloid particle is higher than the density of the liquid, the flow splits into regions in which the velocity of colloid particles coincides with the velocity of the liquid and regions of flow stagnation in which the colloid velocity is higher than the velocity of the fluid. This effect is used to explain qualitatively the decrease in the drag to the flows past macroscopic bodies and flows in pipes.

O efeito da "crise do arrasto" no mergulho de altura

Neste trabalho será investigado o movimento de queda de uma esfera através do ar seguida de um mergulho através da água. Consideramos a esfera sob a ação da força gravitacional, arrasto hidrodinâmico e força de empuxo, tanto no ar quanto na água. Como o empuxo na água torna-se maior que a força gravitacional, a esfera atinge uma profundidade máxima de mergulho antes de retornar à superfície. Considerando a expressão de arrasto quadrático com coeficiente constante nos dois fluídos, obtivemos uma expressão analítica relacionando a altura da queda com a profundidade máxima do mergulho. Em seguida comparamos esta expressão com resultados numéricos considerando o coeficiente de arrasto dependente da velocidade e o empuxo variável à medida que a esfera penetra através da interface ar-água. Comparações entre os resultados analíticos e numéricos demonstram que a "crise do arrasto" desempenha um papel importante nos primeiros instantes após o mergulho, influenciando significativamente na profundidade máxima alcançada.

Influence of Fabric Structural Attributes on Their Aerodynamic Behavior

The aerodynamic properties of 15 knitted fabrics of varying cover factor, yarn, and fiber compositions were investigated for their aerodynamic properties on circular cylinders in a wind tunnel. Measurements of the drag force, pressure distribution, and the Particle Image Velocimetry (PIV) technique were used in order to obtain a better understanding of the effects of yarn, fiber composition, cover factor, and elastic deformation on the flow field and drag coefficient. It was clearly demonstrated from the drag force measurements that the yarn construction and fiber composition have a substantial effect on the drag coefficient (CD), with fabrics composed of spun yarn experiencing no CD-drop as opposed to those composed of filament yarn, and being almost unaffected by the cover factor in the range of Reynolds numbers investigated. Hairiness of the spun yarn was found to minimise the drag-reducing effect of the boundary layer transition and increase the trans-critical drag. The hairy surface layer also appeared to retard the turbulent boundary layer as almost no pressure recovery was observed prior to separation on the cylinder model. The effect of elastic deformation was investigated by image analysis of scanned textile samples, and demonstrated that surface roughness might not be directly correlated to cover factor when the fabrics are stretched. Different elastic behavior of fabrics with different cover factors was also found to affect the structure of the knit surface and thus their aerodynamic behavior. The onset of drag crisis found in drag measurements confirmed the deviation from a sequence determined solely by cover factor.

Dynamic Measurements and Drag Crisis Hysteresis in Garment Aerodynamics

The advantage of tripping boundary layer turbulence around bluff bodies in order to reduce the aerodynamic drag and stabilize the flow has been known for many decades and applied in a number of engineering fields. However, its use in the sport technology area is quite recent. The first practical application of modeled surfaces used to reduce drag has been in golf balls, with the introduction of a dimpled surface. This solution allowed the ball to experience a drag value that is nearly half of the drag value of the same ball with a smooth surface. More recently, with the introduction of the NIKE swift suit in 2002 the focus of sports companies and athletes moved to the so called “low drag suits”. Recent published research proved that low drag suits with panels with different types of roughness clearly improve the athletes’ performances. Currently, most of the research carried out on textiles aerodynamics relies on static measurements on cylindrical models and limited research considers a possible hysteresis in the drag crisis phenomenon for low drag suits applications. The hysteresis phenomenon in the drag crisis is however a well-known phenomena and previous authors have been addressing it. In the present work, the instant drag on a cylinder model mounted in the wind tunnel and covered with different fabrics has been measured and compared with static measurements made on the same model. The experiments were carried out with a sampling frequency of 200Hz using an AMTI BP-600400 HF with a natural frequency of 470Hz and thus capable of measuring dynamic forces. Results show that a hysteresis process exists and that dynamic measurements could be a good alternative to static measurements requiring less testing time and giving more accurate results.