Measured Drag Coefficients Research Articles

Investigation of High-Speed Train Drag with Towing Tank Experiments and CFD

In order to assess the accuracy of drag prediction methods for high-speed trains, experimental and numerical investigations were performed. Besides the drag coefficient, skin friction and pressure distributions on and near the model have been measured for a 1:22 model of the ICE/V. For the experiments, a moving model rig called DIWA (Drag measurement in water) was realised in a 120 m long towing tank to allow for a realistic simulation of the flow around the train, even in the underbelly region. Numerical investigations were performed using Partially-averaged Navier-Stokes (PANS) simulations based on the k-ω-SST turbulence model. Both experimental and numerical methods can be considered as a novelty in the field of train aerodynamics. The results are compared with data from full-scale tests. It is shown, that the moving model rig DIWA allows for the measurement of drag coefficients of trains with high accuracy. Furthermore, the data acquired using the PANS approach compares well with the experimental data.

Blockage corrections for wind tunnel tests conducted on a Darrieus wind turbine

In this study, the influence of the blockage ratio in three wind tunnel tests conducted on a small vertical-axis Darrieus wind turbine was quantitatively investigated. The vertical-axis Darrieus wind turbine was installed in three wind tunnels with different test-section sizes corresponding to blockage ratios of 3.5, 13.4, and 24.7%. The rotor torques, drag force, and upstream wind speeds were measured for the different blockage ratios. It is recommended from the tests that the reference wind speed of the wind turbine should be measured upstream at a distance of 3.5 times or more than the turbine diameter. The power coefficient of the wind turbine was severely distorted based on the blockage ratio. The power coefficients at blockage ratios of 13.4 and 24.7% were respectively 1.3 and 2 times higher than that at a blockage ratio of 3.5%. The optimal tip speed ratio also significantly shifted to a higher range. Based on the method proposed by Maskell (1963), a new correction coefficient is proposed for the Darrieus turbine from the measured drag coefficients obtained at three different blockage ratios. The validity of the proposed correction coefficient was confirmed by comparing it with the corrected power coefficients obtained using other correction methods.

A re-evaluation of design equations for the established flow region of vertical dilute-phase pneumatic conveyors

An analysis of data sets from the literature was completed for particle velocities of granular material in vertical, dilute-phase pneumatic conveyors. The generally accepted equation of motion used to describe such velocity data incorporates the solids friction factor, a variable term based on the additional pressure drop due to solids. The analysis determines that rather being a variable, the term is a constant related to the physical properties of the particle that influence its bouncing behaviour as it progresses along the pipeline. In addition, the analysis reveals that particle drag coefficients determined from the particle velocity data differ significantly from values taken from the standard drag coefficient curve. This observation was confirmed by drag coefficient measurements reported in the literature and made using particle-elutriating equipment. The drag coefficients from the pneumatic conveying experiments and the test equipment were determined under turbulent conditions, which is considered to account for the difference in values from the standard curve determined under quiescent conditions. Relationships are presented that predict the turbulence-influenced drag coefficients and the particle velocity data with good accuracy. These relationships are used to predict pressure drops for two data sets presented with the particle velocity data. The single-term pressure drop equation is used and predicts with good accuracy as well as producing the trough-shaped curve characteristic of the measured pressure drops.

Determining Electro-Osmotic Drag of Water in Anion Exchange Membrane Fuel Cells

In an effort to reduce the cost of proton exchange membrane fuel cells (PEMFCs), a significant amount of research into non-precious metal (NPM) oxygen reduction catalysts has been conducted in recent decades. Despite this effort, platinum remains the only material with suitable stability and activity for use in the harsh environment of PEMFCs. Anion exchange membrane fuel cells (AEMFCs) have been gaining increased attention in recent years, providing advantages of higher stability and activity of NPM catalysts in alkaline environments. Unfortunately, the relevant properties, e.g. conductivity and mechanical & chemical stability, of anion exchange membranes (AEMs) are inferior to their proton conducting counterparts. Many studies have been conducted which focus on improving membrane properties such as swelling ratio, conductivity, and stability. In comparison, relatively little attention has been paid to studying ion and water transport in anion exchange membranes. Previous work conducted in our group has indicated that the majority of AEMFC polarization arises from the anode. The performance is relatively insensitive to catalyst loading and initial results indicate that electrode flooding is the primary cause of poor performance. To improve the performance of AEMFCs and better design AEMs, it is therefore important to study the transport of water and ions within AEMs. One key parameter which has received very little attention in the literature is the electro-osmotic transport of water due to the conduction of anions from the cathode to the anode. In this work, we study the electro-osmotic drag (EOD) in anion exchange membranes. The cell design and concept for electro-osmotic drag coefficient (ε) measurements in this work follow from the work of Fuller and Newman as adapted by others1–3. Analysis is conducted on the membranes in pure hydroxide form and carbonate form. The conductivity, water uptake and water mobility of the membranes is also studied over a wide range of relative humidities and temperatures. Better understanding of the transport of water and ions within anion exchange membranes will facilitate the development of models for AEMFC behavior and offer insight for future AEM production. Figure 1: Effect of water activity on the EOD coefficient (ε) and water content (λ) in Tokuyama A201. T. F. Fuller and J. Newman, J. Electrochem. Soc., 139, 1332–1337 (1992).T. A. Zawodzinski, J. Davey, J. A. Valerio, and S. Gottesfeld, Electrochim. Acta, 40, 297–302 (1995).X. Wang, J. P. Mcclure, and P. S. Fedkiw, Electrochim. Acta, 79, 126–132 (2012) http://dx.doi.org/10.1016/j.electacta.2012.06.098. Figure 1

Drag Coefficient Prediction of a Single Bubble Rising in Liquids

The motion of single bubbles rising in 2-octanol solutions was investigated experimentally. By using a high-speed video system to follow the rising bubble, the sequences of the recorded frames were digitized and analyzed using image analysis software. The periodical fluctuation of the bubble terminal velocity was observed, which is indicative of a nonconstant bubble drag coefficient. Then, the measured drag coefficient was compared with correlations available in the literature. The comparison shows that these correlations cannot give fully satisfactory results in predicting the fluctuated drag coefficient. Using the extensive experimental data, the authors proposed a new correlation to predict the drag coefficient calculated from the fluctuated motion with the added mass force and history force included. In virtue of nonlinear curve fitting, the drag coefficient of the single bubble is correlated as a function of the Re number, Eo number, We number, and Mo number based on the equivalent bubble diameter. T...

Laboratory Measurements of Heat Transfer and Drag Coefficients at Extremely High Wind Speeds

AbstractHeat and momentum transfer across the wind-driven breaking air–water interface at extremely high wind speeds was experimentally investigated using a high-speed wind-wave tank. An original multi-heat-balance method was utilized to directly measure latent and sensible heat transfer coefficients. The results show that both heat transfer coefficients level off at low and normal wind speeds but increase sharply at extremely high wind speeds. The coefficients have a similar shape for wind speeds at a height of 10 m. Therefore, the wind speed dependence on the latent and sensible heat transfer coefficients can be represented by that of the enthalpy coefficient even in the extremely high-speed region. To show how significantly the drag and enthalpy coefficients affect the intensity of tropical cyclones, the coefficients were applied to Emanuel’s analytic model. The analytic model shows that the difference between the present laboratory and conventional correlations significantly affects the maximum storm intensity predictions, and the present laboratory enthalpy and drag coefficients have the remarkable effect on intensity promotion at extremely high wind speeds. In addition, the simulations of strong tropical cyclones using the Weather Research and Forecasting (WRF) Model with the present and conventional correlations are shown for reference in the appendix. The results obtained from the models suggest that it is of great importance to propose more reliable correlations, verified not only by laboratory but also by field experiments at extremely high wind speeds.

Velocity and Drag Evolution From the Leading Edge of a Model Mangrove Forest

AbstractAn experimental study of unidirectional flow through a model mangrove forest measured both velocity and forces on individual trees. The individual trees were 1/12th scale models of mature Rhizophora, including 24 prop roots distributed in a three‐dimensional layout. Thirty‐two model trees were distributed in a staggered array producing a 2.5 m long forest. The velocity evolved from a boundary layer profile at the forest leading edge to a vertical profile determined by the vertical distribution of frontal area, with significantly higher velocity above the prop roots. Fully developed conditions were reached at the fifth tree row from the leading edge. Within the root zone the velocity was reduced by up to 50% and the TKE was increased by as much as fivefold, relative to the upstream conditions. TKE in the root zone was mainly produced by root and trunk wakes, and it agreed in magnitude with the estimation obtained using the Tanino and Nepf (2008) formulation. Maximum TKE occurred at the top of the roots, where a strong shear region was associated with the change in frontal area. The drag measured on individual trees decreased from the leading edge and reached a constant value at the fifth row and beyond, i.e., in the fully developed region. The drag exhibited a quadratic dependence on velocity, which justified the definition of a quadratic drag coefficient. Once the correct drag length‐scale was defined, the measured drag coefficients collapsed to a single function of Reynolds number.

A new moving model test method for the measurement of aerodynamic drag coefficient of high-speed trains based on machine vision

A moving model test method has been proposed to measure the aerodynamic drag coefficient of a high-speed train based on machine vision technology. The total resistance can be expressed as the track friction resistance and the aerodynamic drag according to Davis equation. Cameras are set on one side of the track to capture the pictures of the train, from which the line marks on the side surface of the train are extracted and analyzed to calculate the speed and acceleration of the train. According to Newton’s second law, the aerodynamic drag coefficient can be resolved through multiple tests at different train speeds. Comparisons are carried out with the full-scale coasting test, wind tunnel test, and numerical simulation; good agreement is obtained between the moving model test and the full-scale field coasting test with difference within 1.51%, which verifies that the method proposed in this paper is feasible and reliable. This method can accurately simulate the relative movement between the train, air, and ground. The non-contact measurement characteristic will increase the test accuracy, providing a new experimental method for the aerodynamic measurement.

가시화 기법을 사용한 자유낙하하는 반구모델의 자세각 및 항력계수 측정

마하수 6 조건에서 반구 모델에 대해 자세각의 변화에 따른 항력계수를 연구하였다. 충격파 터널에서 실험이 진행되었으며 지지대 유동 간섭을 최소화하기 위해 자유낙하 기법을 사용하였다. 자유낙하하는 반구 모델의 자세와 항력계수를 측정하기 위해 계단식 모듈과 전자석을 이용한 자유낙하 기법을 구축하였고 초고속 카메라를 통한 shadowgraph 기법을 사용하였다. In this work, the effect of attitude angle variation on drag coefficients of hemisphere in a Mach 6 flow has been investigated. Experiments were conducted in a shock tunnel and a free-falling technique was used to minimize flow disturbance by a sting. For attitude and drag coefficient measurements of a free-falling hemisphere, a free-falling technique based on a releasing mechanism with a stair-typed module and an electromagnet was developed. A shadowgraph technique was used for flow visualization using a high-speed camera.

Gliding locomotion of manta rays, killer whales and swordfish near the water surface

The hydrodynamic performance of the locomotive near the water surface is impacted by its geometrical shape. For marine animals, their geometrical shape is naturally selective; thus, investigating gliding locomotion of marine animal under the water surface may be able to elucidate the influence of the geometrical shape. We investigate three marine animals with specific geometries: the killer whale is fusiform shaped; the manta ray is flat and broad-winged; and the swordfish is best streamlined. The numerical results are validated by the measured drag coefficients of the manta ray model in a towing tank. The friction drag of the three target models are very similar; the body shape affected form drag coefficient is order as swordfish < killer whale < manta ray; the induced wave breaking upon the body of the manta ray performs different to killer whale and swordfish. These bio-inspired observations provide a new and in-depth understanding of the shape effects on the hydrodynamic performances near the free surface.

Drag coefficient fluctuation prediction of a single bubble rising in water

The drag coefficient has a significant effect on the calculation of bubble motion. This paper experimentally investigated the drag coefficient of a single bubble rising in deionized water with the help of a high-speed video system. First, it is found that the terminal velocity presented periodical fluctuation, indicating that the drag coefficient is not a constant value. Then the measured drag coefficient was compared with correlations available in literatures. The comparison shows that these correlations cannot give fully satisfactory results in predicting the fluctuated drag coefficient. Based on massive data analysis, a new correlation combined Reynolds number, Eotvos number and Weber number was proposed to calculate the fluctuation of drag coefficient. The drag coefficient predicted by this new model is in excellent agreement with the experimental results over the range of 550⩽Re⩽1700, and the largest relative error is less than 0.4%. Comparing with data of water from existing literature under the condition of 8<Re<12,000 and 0.01<Eo<600, the proposed model gives better predictions than available drag models.

In situ drag coefficient measurements for rooftop trees

To evaluate the drag coefficients for roof-top tree crowns, previously developed field test methods for monitoring wind speed, wind direction, and stem deflection were improved using solar cells as a source of power. A field test was conducted for a Norway spruce and a shirakanba located on a rooftop. The drag coefficients showed large variations during low wind speeds, which were caused by the flexural vibration of the tree trunk due to the fluctuating wind speed. To compensate for this effect, the upper percentage of drag coefficients obtained for the 0.5 m/s wind speed classes were used for the exponential regression for drag coefficient as a function of wind speed. As a result, a conservative evaluation of drag coefficients for a greater range of wind speeds could be established instead of using all data. The drag coefficients at 30 m/s wind speed were estimated to be 0.594 for the Norway spruce, and 0.154 for the shirakanba using the upper 50 % of the data. The exponential function convergence was more gradual for the spruce than for the shirakanba, which may be affected by a difference in the streamlining behavior.

Evidence of an evolutionary-developmental trade-off between drag avoidance and tolerance strategies in wave-swept intertidal kelps (Laminariales, Phaeophyceae).

Kelps are a clade of morphologically diverse, ecologically important habitat-forming species. Many kelps live in wave-swept environments and are exposed to chronic flow-induced stress. In order to grow and survive in these harsh environments, kelps can streamline (reducing drag coefficient) to avoid drag or to increase attachment and breakage force to tolerate it. We aimed to quantify the drag tolerance and streamlining strategies of kelps from wave-swept intertidal habitats. We measured drag coefficient and tenacity of populations from eight kelp species over a wide range of sizes to determine whether kelps avoid dislodgement by reducing drag coefficient or by increasing tenacity as they grow, and whether these traits are traded off. We employed phylogenetic comparative methods to rule out potentially confounding effects of species' relatedness. There was a significant negative relationship between drag avoidance and tolerance strategies, even after incorporating phylogeny. Kelps that were more tenacious were less able to reduce drag, resulting in a continuum from "tolerators" to "streamliners," with some species demonstrating intermediate, mixed strategies. Drag and tenacity were correlated with geometric properties (i.e., second moment of area) of the stipe in large kelps. Results presented in this study suggest that kelps are either strong or streamlined, but not both. This continuum is consistent with avoidance and tolerance trade-offs that have been documented in many different biological systems and may have widespread implications for the evolution of large macroalgae, perhaps driving morphological diversity within this group.

伝播する衝撃波に誘起された非定常流中における球の抵抗係数の測定

伝播する衝撃波に誘起された非定常流中における球の抵抗係数の測定

Measurement of drag coefficients of non-spherical particles with a camera-based method

The current paper presents a novel experimental set-up which allows the automated determination of the drag coefficients of relatively large particles with complex shape. Typical examples of such types of particles are waste derived fuel (RDF) particles which are non-spherical and have a size up to a few centimeters. In contrast to conventional fossil fuel particles, where the particles may be considered as material points during the calculation of particle tracks in a reacting flow field, the spatial extent of RDF-particles and their lack of sphericity lead to pronounced self-induced movement and associated variations in the drag-coefficients.The experiments are based on a drop shaft equipped with two digital cameras. This allows to obtain time resolved stereo image sequences from which the settling velocity of particles, the self-induced velocity fluctuations and the corresponding drag and lift coefficients can be derived. As the system is automated, a large number of particles can be examined and statistical information on the distribution of drag coefficients can be obtained.In this publication, the methodology of these drop shaft measurements and their evaluation will be presented. Additionally drag coefficients of isometric spherical and non-spherical particle geometries (spheres, cubes, square plates and circular disks) were measured and compared with known correlations for drag coefficients. Probability density functions for the properties of typical RDF particles will be presented to highlight the potential of the new set-up.

Undulatory microswimming near solid boundaries

The hydrodynamic forces involved in the undulatory microswimming of the model organism C. elegans are studied in proximity to solid boundaries. Using a micropipette deflection technique, we attain direct and time-resolved force measurements of the viscous forces acting on the worm near a single planar boundary as well as confined between two planar boundaries. We observe a monotonic increase in the lateral and propulsive forces with increasing proximity to the solid interface. We determine normal and tangential drag coefficients for the worm, and find these to increase with confinement. The measured drag coefficients are compared to existing theoretical models. The ratio of normal to tangential drag coefficients is found to assume a constant value of 1.5 ± 0.1(5) at all distances from a single boundary, but increases significantly as the worm is confined between two boundaries. In response to the increased drag due to confinement, we observe a gait modulation of the nematode, which is primarily characterized by a decrease in the swimming amplitude.

Direct Measurements of Drag Forces in C. elegans Crawling Locomotion

With a simple and versatile microcantilever-based force measurement technique, we have probed the drag forces involved in Caenorhabditis elegans locomotion. As a worm crawls on an agar surface, we found that substrate viscoelasticity introduces nonlinearities in the force-velocity relationships, yielding nonconstant drag coefficients that are not captured by original resistive force theory. A major contributing factor to these nonlinearities is the formation of a shallow groove on the agar surface. We measured both the adhesion forces that cause the worm’s body to settle into the agar and the resulting dynamics of groove formation. Furthermore, we quantified the locomotive forces produced by C. elegans undulatory motions on a wet viscoelastic agar surface. We show that an extension of resistive force theory is able to use the dynamics of a nematode’s body shape along with the measured drag coefficients to predict the forces generated by a crawling nematode.

A comparison of Jabulani and Brazuca non-spin aerodynamics

Wind-tunnel experimental measurements of drag coefficients for non-spinning Jabulani and Brazuca balls are presented. The Brazuca ball’s critical drag speed is lower than that of the Jabulani ball, and the Brazuca ball’s super-critical drag coefficient is larger than that of the Jabulani ball. Compared to the Jabulani ball, the Brazuca ball suffers less instability due to knuckle-ball effects. Using drag data, numerically determined ball trajectories are created, and it is postulated that although power shots are too similar to note flight differences, goalkeepers are likely to note the differences between Jabulani and Brazuca ball trajectories for intermediate-speed ranges. This latter result may appear in the 2014 World Cup for goalkeepers used to the flight of the ball used in the 2010 World Cup.

Wind Tunnel Experiment of Bluff Body Aerodynamic Models Using a New Type of Magnetic Suspension and Balance System

We have measured drag coefficients of a sphere and a circular cylindrical aerodynamic model using a five axes and a six axes control magnetic suspension and balance system (MSBS) developed by us. This MSBS has the characteristics of large aperture relative to the weight, light weight, and small electric power consumption in comparison with the conventional ones. We had good agreements between the measured values of the drag coefficient and the values appearing in the common aerodynamic handbook or textbook. We also succeeded in measuring the aerodynamic influence of a supporting rod of the aerodynamic models making use of the characteristics of the MSBS. Conventionally, the MSBS can be used only in large scale laboratories because the size, weight, and electric power consumption are large. We think that successful measurements of various aerodynamic characteristics using this type of MSBS will stimulate the introduction of it into the wind tunnel experiments in small scale laboratories.

Quantifying wave attenuation to inform coastal habitat conservation

Coastal vegetation can protect people and property from erosion and flooding, potentially providing a win‐win solution for conservation and development. However, the conditions under which natural habitats provide protection have been controversial, partly because the geomorphic, ecological, and hydrodynamic factors that determine wave attenuation vary greatly among locations, times, and studies. We re‐analyzed existing wave attenuation studies in kelp, mangrove, marsh and seagrass habitats and found that much of the variation in wave attenuation can be explained by differences in vegetation characteristics and by the change in bulk drag with flow conditions. We found that vegetation can exert substantial drag on passing waves, but that the bulk drag coefficient declines in flow conditions characterized by high Reynolds numbers. This decline is important because storm conditions are highly turbulent (typical Reynolds numbers are greater than 104), and we lack empirical measurements of bulk drag coefficients from such conditions. Failing to account for the decline can over‐estimate wave attenuation in storms by 19% to 1600%. These results suggest that larger habitat areas will need to be set aside for coastal protection than previously thought. Our approach provides predictions for designing practical habitat conservation and restoration plans that also protect humans and property from flooding and erosion.