Total Drag Force Research Articles

A multiobjective aerodynamic optimization design of a high-speed train head under crosswinds

In this work, a multiobjective aerodynamic optimization of a high-speed train head was performed to improve the aerodynamic performance of the high-speed train running on an embankment under crosswinds. Seven optimization design variables were selected to control five regions on the train head. The total aerodynamic drag force, aerodynamic lift force, and aerodynamic side force of the head coach were set as the optimization objectives. The optimal Latin hypercube sampling method was used to obtain the values of the design variables of the sample points. The high-speed train head was deformed using the free-form deformation approach through which the mesh morphing was performed without remodeling and re-meshing. Then, the aerodynamic performances of the high-speed trains at the sample points were calculated using the computational fluid dynamics method. A Kriging surrogate model between the design variables and their optimization objectives was constructed. Then, the multiobjective aerodynamic optimization of the high-speed train head was performed using multiobjective genetic algorithms based on the Kriging model. Based on the results of the sample points, the relationships between the optimization design variables and the optimization objectives were analyzed, and the contributions of the primary factors to the optimization objectives were obtained. After optimization, a series of Pareto-optimal head shapes were obtained. The steady and unsteady aerodynamic performances of the train with an optimal head, which was selected from the Pareto-optimal head shapes, were compared with those of the original train.

Evaluation of the dual-feed rotor spinning unit based on airflow dynamics and blended yarn properties

In fiber processing on a rotor spinning machine, the fiber orientation gained in pre-processing stages like carding and drawing is mainly lost as the fibers enter the rotor interior for further processing. To improve fiber orientation and facilitate fiber blending, a new dual-feed rotor spinning unit is proposed. We present a comprehensive numerical evaluation of the airflow dynamics in the original and the proposed rotor spinning unit based on finite volume methods. The velocity profiles, pressure distribution, vortices, streamlines, and drag force in the two prototypes are evaluated. Results reveal that the velocity magnitude and pressure in the dual-feed unit adopt an even pattern and the total drag force reduction of up to 60–80% is achieved. Comparison of the yarn quality properties of the two systems showed that the dual-feed spun yarns possess superior quality compared to the conventional yarns.

Wake stabilization mechanism of low-drag suppression devices for vortex-induced vibration

Inspired by the drag reduction mechanism in a tandem cylinder arrangement, the objective of this study is to numerically investigate the characteristic response of shear layer reattachment for the suppression of vortex-induced vibration (VIV). A new device termed as connected-C is introduced, whereby its geometry can be constructed by attaching a C-shaped foil at the end of a splitter plate behind a circular cylinder. We investigate the significance of each part of the connected-C device and compare the proposed device with conventional fairings, the splitter plate, and one of its variant disconnected-C device without the connector plate between the main cylinder and the C-shaped foil. To simplify the numerical study and to concentrate on the wake stabilization mechanism, the cylinder-device systems are only allowed to move in the transverse direction. There is no relative rotation between the cylinder and the attached devices. Two- and three-dimensional studies are carried out at low Reynolds number Re=100 and subcritical Reynolds numbers Re∈[6150,7400]. We observe the counter-rotating pair of recirculations and the shear layer reattachment for the vibrating connected-C device. These flow characteristics of the connected-C device result into a similar VIV performance as the fairing with respect to the suppression of VIV. Similar to fairings, there is a net reduction in the total drag force exerted over the combined cylinder-device system. Based on the results from the low Reynolds number study, while the presence of C-shaped device is essential to prevent the occurrence of galloping at high reduced velocity, the presence of connector plate is not necessary to suppress VIV.

Influence of pantograph fixing position on aerodynamic characteristics of high-speed trains

To study the influence of the pantograph fixing position on aerodynamic characteristics of high-speed trains, the aerodynamic models of high-speed trains with eight cars were established based on the theory of computational fluid dynamics, and eight cases with pantographs fixed on different positions and in different operational orientations were considered. The pantographs were fixed on the front or the rear end of the first middle car or fixed on the front or the rear end of the last middle car. The external flow fields of the high-speed trains were numerically simulated using the software STAR-CCM+. The results show that the pantograph fixing position has little effect on the aerodynamic drag force of the head car and has a large effect on the aerodynamic drag force of the tail car. The influences of the pantograph fixing position on the aerodynamic lift forces of the head car, tail car and pantographs are obvious. Among the eight cases, considering the total aerodynamic drag force of the train and the aerodynamic lift force of the lifted pantograph, when the pantographs are fixed on the rear end of the last middle car and the lifted pantograph is in the knuckle-upstream orientation, the aerodynamic performance of the high-speed train is the best.

The wake and thrust by four side-by-side cylinders at a low Re

The flow around four identical side-by-side circular cylinders placed normal to the oncoming flow is numerically simulated using the finite volume method (FVM) at a low Reynolds number of 100 based on cylinder diameter D and freestream velocity. How the wake structure, forces, and vortex shedding patterns are contingent on the spacing ratio g* (=g/D, where g is the gap spacing between the cylinders) is studied systemically when g* varies from 0.0 to 2.0. Based on the intrinsic features of the flow, four distinct flow regimes are identified in the range of g* examined. The total time-averaged drag force acting on the four cylinders escalates exponentially with a decrease in g*, as does the lift force, repulsive, on the outer cylinders. The lift forces of the inner cylinders are also repulsive but very weakly sensitive to g*. The Strouhal number is identical for each of the four cylinders in single body flow, different for the outer and inner cylinders in flip-flopping and quasi-interlocked flows, and again identical for interlocked flow.

Flow visualisation of downhill skiers using the lattice Boltzmann method

In downhill alpine skiing, skiers often exceed speeds of 120 km h−1, with air resistance substantially affecting the overall race times. To date, studies on air resistance in alpine skiing have used wind tunnels and actual skiers to examine the relationship between the gliding posture and magnitude of drag and for the design of skiing equipment. However, these studies have not revealed the flow velocity distribution and vortex structure around the skier. In the present study, computational fluid dynamics are employed with the lattice Boltzmann method to derive the relationship between total drag and the flow velocity around a downhill skier in the full-tuck position. Furthermore, the flow around the downhill skier is visualised, and its vortex structure is examined. The results show that the total drag force in the downhill skier model is 27.0 N at a flow velocity of 15 m s−1, increasing to 185.8 N at 40 m s−1. From analysis of the drag distribution and the flow profile, the head, upper arms, lower legs, and thighs (including buttocks) are identified as the major sources of drag on a downhill skier. Based on these results, the design of suits and equipment for reducing the drag from each location should be the focus of research and development in ski equipment. This paper describes a pilot study that introduces undergraduate students of physics or engineering into this research field. The results of this study are easy to understand for undergraduate students.

Direct measurements of the drag force over aligned arrays of cubes exposed to boundary-layer flows

Wind tunnel measurements of the total drag force for aligned arrays of cubes exposed to two different boundary-layer flows at three flow velocities are discussed. The drag force for eight different building packing densities λp (from 0.028 to 1) is measured with a standard load cell generating a novel dataset. Different λp are reproduced by increasing the number of buildings on the same lot area; this represents a real situation that an urban planner is faced with when a lot area of a given (fixed) size is allocated to the development of new built areas. It is assumed that the surrounding terrain is uniform and there is a transition from a given roughness (smooth) to a new roughness (rough). The approaching flow will adjust itself over the new surface within a distance that in general may be larger than the horizontal length covered by the array. We investigate the region where the flow adjustment occurs. The wide range of packing densities allowed us to analyse in detail the evolution of the drag force. The drag force increases with increasing packing densities until it reaches a maximum at an intermediate packing density (λp = 0.25 in our case) followed by a slight decrease at larger packing densities. The value of the drag force depends on the flow adjustment along the array which is evaluated by introducing the parameter “drag area” to retrieve information about the drag distribution at different λp. Results clearly suggest a change of the distribution of the drag force, which is found to be relatively uniform at low packing densities, while most of the force acts on first rows of the arrays at large packing densities. The drag area constitutes the basis for the formulation of a new adjustment length scale defined as the ratio between the volume of the air within the array and the drag area. The proposed adjustment length scale automatically takes into account the change in drag distribution along the array for a better parameterization of urban effects in dispersion models.

Effect of posture on the aerodynamic characteristics during take-off in ski jumping

The purpose of this study was to investigate the effects of posture of a ski jumper on aerodynamic characteristics during the take-off using computational fluid dynamics (CFD). The CFD method adopted for this study was based on Large–Eddy Simulation. Body surface data were obtained by 3-D laser scanning of an active ski jumper. Based on video analysis of the actual take-off movement, two sets of motion data were generated (world-class jumper A and less-experienced jumper B). The inlet flow velocity that corresponds to the in-run velocity in actual ski jumping was set to 23.23m/s in the CFD. The aerodynamic force, flow velocity, and vortexes for each model were compared between models. The total drag force acting upon jumper A was lower than that acting upon jumper B through the whole movement. Regarding the total lift force, although jumper A׳s total lift force was less in the in-run posture, it became greater than that of jumper B at the end of the movement. In the latter half of the movement, low air-speed domain expansion was observed at the model׳s back. This domain of jumper B was larger. There were two symmetric vortexes in the wake of jumper A, but the disordered vortexes were observed behind the jumper B. In the case of jumper A, these two distinct vortexes generated by the arms produced a downwash flow in the wake. It is considered that the positioning of the arms in a very low position strongly influences the flow structure.

Dynamic interaction of edge dislocations with point defects and prismatic dislocation loops at high-strain-rate deformation of crystals

The motion of an ensemble of edge dislocations at high-strain-rate deformation of a crystal with a high concentration of prismatic dislocation loops and point defects has been analyzed. It has been shown that, under certain conditions, the drag of an edge dislocation by prismatic dislocation loops has the character of dry friction, and the magnitude of the drag force of the dislocation is determined by the relationship between the concentration of prismatic dislocation loops and the density of mobile dislocations. An increase in the density of mobile dislocations leads to an enhancement of their collective interaction, thus facilitating the overcoming of prismatic dislocation loops by edge dislocations. The total drag force of an edge dislocation is a nonmonotonic function of the concentration of point defects, which, under certain conditions, has a minimum.

A Discussion on the Interpretation of the Darcy Equation in Case of Open-Cell Metal Foam Based on Numerical Simulations.

It is long known that for high-velocity fluid flow in porous media, the relation between the pressure drop and the superficial velocity is not linear. Indeed, the classical Darcy law for shear stress dominated flow needs to be extended with a quadratic term, resulting in the empirical Darcy–Forchheimer model. Another approach is to simulate the foam numerically through the volume averaging technique. This leads to a natural separation of the total drag force into the contribution of the shear forces and the contribution of the pressure forces. Both representations of the total drag lead to the same result. The physical correspondence between both approaches is investigated in this work. The contribution of the viscous and pressure forces on the total drag is investigated using direct numerical simulations. Special attention is paid to the dependency on the velocity of these forces. The separation of the drag into its constituent terms on experimental grounds and for the volume average approach is unified. It is shown that the common approach to identify the linear term with the viscous forces and the quadratic term with the pressure forces is not correct.

Computational aerodynamic modeling for flight dynamics simulation of ram-air parachutes

This work presents a step toward bridging the gap between flight dynamics simulation of ram-air parachutes and high-fidelity computational fluid dynamics. Today's parachute design codes mainly rely on the empirical or semi-empirical methods generated from wind tunnel experiments and drop tests. The outcome of this study will hopefully help to reduce the cost of experiments and drop testing in the design of future canopies and to better understand the aerodynamic characteristics of these geometries. In this work, the parachute geometries were modeled as rigid rectangular wings with an aspect ratio of two and zero anhedral angle. The wings have seven opening cells and the trailing edge is deflected or not deflected. To validate computational methods, the aerodynamic predictions of similar wings, but with closed and round inlets, are compared with experimental data available from the Subsonic Wind Tunnel at United States Air Force Academy. Total lift and drag force coefficients were measured at a Reynolds number of 1.4 million. The results show that computational predictions of fine (closed-inlet) grids match the experimental data very well up to the stall angle. Both experiments and simulations show that closed wings have sharp stalling characteristics. The aerodynamics of closed wings up to stall can be approximated by linear functions and their derivatives. The closed wings show a negative static stability with respect to changes in the angle of attack. The open wings, on other hand, have positive static stability in the longitudinal and lateral directions. The open wings exhibit highly nonlinear unsteady aerodynamic characteristics; they also stall earlier and have higher drag values than the closed wings. The aerodynamic derivatives of open and closed wings were estimated using a linear regression method and training data simulated in small-amplitude oscillations in pitch, yaw, and roll directions. While the open wings have large oscillations in aerodynamic coefficients over the yawing and rolling hysteresis loops, lateral aerodynamic derivatives of the open and closed wings are similar. Finally, the results show that model predictions are reasonably accurate for use in flight-dynamics simulations.

Computational aero-acoustic modelling of external rear-view mirrors on a mid-sized Sedan

Ever-rising fuel costs necessitate design of fuel-efficient vehicles. Consequently, modern vehicle manufacturers are focused on designing low aerodynamic drag vehicles which would in-turn reduce the fuel consumption. This study analyses the contribution of external rear-view mirrors to the total drag force and the overall sound pressure level at the A, B and C pillars, while optimising the external rear-view mirror design accordingly. Solid Works renditions of external rear-view mirror models mounted on a reference luxury sedan were analysed using a commercially available computational fluid dynamic package ANSYS FLUENT. A different approach was followed to carry out the empirical flow visualisation and predict sound pressure levels. The aerodynamic characterisation of the vehicle was done utilising the widely used shear stress transport turbulence model, while the analysis of wind noise and the contributing vortices employed a large eddy simulation. This approach significantly reduced computational time without compromising on accuracy.

Introduction of parameterized sea ice drag coefficients into ice free-drift modeling

Many interesting characteristics of sea ice drift depend on the atmospheric drag coefficient (C a) and oceanic drag coefficient (C w). Parameterizations of drag coefficients rather than constant values provide us a way to look insight into the dependence of these characteristics on sea ice conditions. In the present study, the parameterized ice drag coefficients are included into a free-drift sea ice dynamic model, and the wind factor α and the deflection angle θ between sea ice drift and wind velocity as well as the ratio of C a to C w are studied to investigate their dependence on the impact factors such as local drag coefficients, floe and ridge geometry. The results reveal that in an idealized steady ocean, C a/C w increases obviously with the increasing ice concentration for small ice floes in the marginal ice zone, while it remains at a steady level (0.2–0.25) for large floes in the central ice zone. The wind factor α increases rapidly at first and approaches a steady level of 0.018 when A is greater than 20%. And the deflection angle θ drops rapidly from an initial value of approximate 80° and decreases slowly as A is greater than 20% without a steady level like α. The values of these parameters agree well with the previously reported observations in Arctic. The ridging intensity is an important parameter to determine the dominant contribution of the ratio of skin friction drag coefficient (Cs’/Cs) and the ratio of ridge form drag coefficient (C r’/C r) to the value of C a/C w, α, and θ, because of the dominance of ridge form drag for large ridging intensity and skin friction for small ridging intensity among the total drag forces. Parameterization of sea ice drag coefficients has the potential to be embedded into ice dynamic models to better account for the variability of sea ice in the transient Arctic Ocean.

Flow Visualization of Downhill Ski Racers Using Computational Fluid Dynamics

In downhill alpine skiing, racers often exceed speeds of 120km/h, with air resistance substantially affecting the overall race times. To date, studies on air resistance in alpine skiing have used wind tunnels and actual skiers to examine the relationship between the gliding posture and magnitude of drag, as well as for the design of skiing equipment. However, these studies have not revealed the flow velocity distribution and vortex structure around the skier. In the present study, we used computational fluid dynamics with the lattice Boltzmann method to derive the relationship between flow velocity in the full tuck position (the downhill racer's speed) and total drag. Furthermore, we visualized the flow around the downhill racer and examined its vortex structure. The results show that the total drag force in the downhill racer model is 27.0N at a flow velocity of 15 m/s, increasing to 185.8N at 40 m/s. Moreover, the visualization of the flow field indicates that the primary drag locations at a flow velocity of 40 m/s are the head, upper arms, lower legs, and thighs (including the buttocks).

A numerical study of microburst-like wind load acting on different block array configurations using an impinging jet model

A numerical investigation on the microburst-like wind characteristics in block array configurations has been performed using Computational Fluid Dynamics (CFD). The CFD modelling of impinging jet mimics a microburst wind shear. Effects of plan and frontal area densities on the drag and lift force acting on the arrays are studied by investigating the wall shear stress and pressure distributions. A semi-empirical model based on Poreh et al. (1967) is derived to estimate the spatially-averaged wall shear stress of the finite urban array located near the microburst storm centre. Moreover, the pressure and viscous drag force acting on obstacles in the arrays with different plan and frontal area densities are discussed and compared with the published results regarding the arrays placed in a neutrally stratified Atmospheric Boundary Layer (ABL) flow. The present results show that the viscous drag is insignificant relative to the total drag force for all the cases with different frontal and plan area densities (i.e. roughness packing densities). The mean vertical lift force acting on the arrays for various packing densities is discussed, and the lift force is compared with drag and resultant forces. The averaged lift force acting on a block in the array is 0.3–0.6 times of the magnitude of the resultant force. Therefore, it should be taken into account for the design and maintenance of high-rise buildings in cities.

Study of composite plate traveling in water containing Ice Equivalent Objects

An experimental study was conducted for a composite plate traveling in water containing Ice Equivalent Objects (IEO) in order to investigate the drag force exerted on the composite plate as well as its structural response. This study considered both fluid–structure interaction (FSI) as well as solid body interaction. The plate was partially submerged so as to interact with the IEO. As test parameters, the traveling speed and the orientation angle of the plate were changed, and the coverage density of the IEO was also varied. Both drag force and the strain response of the plate were measured when the plate was in either the steady-state or accelerating transient motion. In addition, a high speed camera was utilized to capture the interaction among the composite plates, IEO, and water flow. Computer modeling and simulation was also conducted for some simple cases in order to better aid understanding. The results showed that with the inclusion of the IEO, not only was there an increase in the total drag forces but also a difference in the resulting deformed shapes of the plates. As a result, strain measurements at different locations did not vary proportionally resulting from the IEO.

Numerical investigation of flow and turbulence structure through and around a circular array of rigid cylinders

This numerical study investigates flow and turbulence structure through and around a circular array of solid circular cylinders of diameter$d$. The region containing the array of rigid cylinders resembles a porous circular cylinder of diameter$D$. The porous cylinder Reynolds number defined with the steady incoming flow velocity is$\mathit{Re}_{D}=10\,000$. Fully three-dimensional (3D) large eddy simulations (LES) are conducted to study the effects of the volume fraction of solids of the porous cylinder ($0.023<\text{SVF}<0.2$) and$d/D$on the temporal variation and mean values of the drag/lift forces acting on the solid cylinders and on the porous cylinder. The effects of the bleeding flow through the circular porous cylinder on the wake structure and the influence of the SVF and$d/D$on the onset of flow three-dimensionality within or downstream of the porous cylinder and transition to turbulence are discussed. Results are compared with experimental data, predictions of theoretical models available in the literature and also with the canonical case of a solid cylinder ($\text{SVF}=1,d/D=1$). Three-dimensional LES predict that large-scale wake billows are shed in the wake of the porous cylinder for$\text{SVF}>0.05$, similar to the von Karman vortex street observed for solid cylinders. As the SVF decreases, the length of the separated shear layers (SSLs) of the porous cylinder and the distance from the back of the porous cylinder at which wake billows form increase. For sufficiently low volume fractions of solids (e.g. $\text{SVF}=0.05$, 0.023), no wake billows are shed and the interactions among the wakes of the solid cylinders are weak. Even for$\text{SVF}=0.023$, SSLs containing large-scale turbulent eddies form on the two sides of the porous cylinder, but their ends cannot interact to generate wake billows. In both regimes, the force acting on some of the solid cylinders within the array is highly unsteady. As opposed to results obtained based on 2D simulations, no intermediate regime in which the force acting on the solid cylinders is close to steady is present. Interestingly, an energetic low frequency corresponding to a Strouhal number defined with the diameter of the porous cylinder of approximately 0.2 is present within the porous cylinder and near-wake regions not only for cases where wake billows are generated but also for cases where no wake billows form. In the latter cases, this frequency is due to an instability acting on the SSLs which induces in-phase large-scale undulatory deformations of the two SSLs. A combined drag parameter for the porous cylinder${\it\Gamma}_{D}=\overline{C}_{d}\,aD/(1-\text{SVF})$is introduced, where$aD$is the non-dimensional frontal area per unit volume of the porous cylinder. This parameter characterizes by how much the velocity of the bleeding flow at the back of the porous cylinder is reduced compared with the incoming flow velocity for a given total drag force acting on the porous cylinder. Results from simulations conducted with different values of the SVF,$d/D$and mean time-averaged solid cylinder streamwise drag parameter,$\overline{C}_{d}$, show that${\it\Gamma}_{D}$increases monotonically with increasing$aD$. Several ways of defining the spatial extent of the wake region in a less ambiguous way are proposed.

An improved correlation for pressure drop in a tunnel under traffic jam using CFD

Undoubtedly, an accurate estimation of pressure drop due to vehicles can contribute to correctly predicting the number of jet fans which ultimately leads to choosing the optimum ventilation system in a tunnel. The main objective of the present study is to improve the 1D (one-dimensional) model, which currently is unable to consider the effects of combination of cars in traffic jam on the total drag force. The CFD technique has been applied to model several tunnels in different lengths under severe traffic jam conditions; this way, the difference between pressure drop predicted by CFD simulation and 1D model can be investigated. The CFD simulation results were used to develop a new correlation for pressure drop due to cars inside the tunnel as a function of tunnel average air velocity and tunnel length. Additionally, the differences between the pressure drop due to the cars predicted by the CFD and the 1D model for different traffic conditions were investigated. It was shown that the existing 1D model underestimates or overestimates the pressure drop depending on the traffic condition and the drag-area value used in analysis. Finally, 1D model is modified by a simple equation for the pressure drop in a tunnel in congested traffic conditions using an equivalent friction factor resembling the stopped vehicles.

Theoretical Analysis and Model Test of Rock Berm Protection

Yan, S.; Guo, B.; Sun, L., and Lei, Z., 2015. Theoretical analysis and model test of rock berm protection.Submarine pipelines are exposed to the risk of damage from dragged anchors of ships that lose control during severe storms. Rock armor berms are commonly employed to protect the pipelines by deflecting the dragged anchors. Theoretical work and model tests are implemented in this study to understand the protection mechanism provided by rock berms. A limit equilibrium method is proposed to address the interaction between the dragged Hall anchor and the developed soil wedges. The total drag force on the anchor is obtained as the sum of the drag force on the anchor crown and that on the anchor fluke. Model tests are performed with three model anchors with different linear scales. The drag forces measured in the model tests agree well with those calculated by the theoretical approach. Results show that when the anchor is dragged into the rock armor berm from the soil foundation, an unbalanced moment is induced on the anchor; this moment causes the dragged anchor to rotate and the fluke tip to rise up. Such prediction may be helpful to berm designers.

Distributed forcing flow control in the wake of a blunt trailing edge profiled body using plasma actuators

A modern flow control technique for reducing the drag associated with the periodic shedding of von Kármán vortices in the wake of a blunt trailing edge profiled body is presented. The technique involves distributed forcing of the wake flow using an array of dielectric barrier discharge plasma actuators, with a spanwise spacing matched to the spanwise wavelength of the dominant secondary wake instability. The experiments include measurement of the velocity field in multiple vertical and horizontal planes in the wake using particle image velocimetry, as well as base pressure, at Reynolds numbers of 2000, 3000, and 5000 based on trailing edge thickness. The flow control technique causes elongation of the vortex formation region across the span, and significant reduction of the fluctuating and total drag forces, up to a maximum of 94% and 18%, respectively. The effectiveness of the flow control technique is shown to be dependent on the induced momentum coefficient. Proper orthogonal decomposition analysis is used to investigate the mechanism of interaction of the flow control technique with the wake flow. Two distinct flow regimes are observed depending on the induced momentum coefficient. The effect of the control on the wake flow structure in the first regime is similar to those observed in previous studies involving mild spanwise-periodic geometric perturbations at the trailing edge, where control leads to streamwise displacement of the vortices and a shift in shedding frequency. However, an incremental increase in the momentum coefficient leads to a second flow regime similar to those previously observed in the case of large-amplitude geometric perturbations, with an almost complete attenuation of vortex shedding in the near-wake region.