Increase In Drag Research Articles

Energy-based drag decomposition analyses for a turbulent channel flow developing over convergent–divergent riblets

Direct numerical simulations of a turbulent channel flow developing over convergent–divergent (C–D) riblets are performed at a Reynolds number of Reb = 2800, based on the half channel height δ and the bulk velocity. To gain an in-depth understanding of the origin of the drag generated by C–D riblets, a drag decomposition method is derived from kinetic energy principle for a turbulent channel flow with wall roughness. C–D riblets with a wavelength, Λ, ranging from 0.25δ to 1.5δ, are examined to understand the influence of secondary flow motions on the drag. It is found that as Λ increases, the intensity of the secondary flow motion increases first and then decreases, peaking at Λ/δ=1. At Λ/δ≥1, some heterogeneity appears in the spanwise direction for the turbulent kinetic energy (TKE) and vortical structures, with the strongest enhancement occurring around regions of upwelling. All the riblet cases examined here exhibit an increased drag compared to the smooth wall case. From the energy dissipation/production point of view, such a drag increase is dominated by the TKE production and the viscous dissipation wake component. While the drag contribution from the TKE production shear component decreases as Λ increases, the drag contribution from the wake component of both the TKE production and viscous dissipation follows the same trend as the intensity of the secondary flow motion. From the work point of view, the drag increase in the riblet case at Λ/δ=0.25 comes mainly from the work of the Reynolds shear stresses, whereas at Λ/δ≥1, the drag augmentation is dominated by the work of the dispersive stresses. At Λ/δ=0.5, both components play an important role in the increase in the drag, which also exhibits a peak.

Trailing-edge noise reduction potential of a locally applied shallow dimpled surface

There is a strong need for airframe noise reduction methods that preserve the aerodynamic performance of lift-generating airframe components. Inspired by the drag reduction studies of van Nesselrooij et al. (2016) and Spalart et al. (2019), the current study explored the trailing-edge noise reduction potential of shallow dimples applied near the trailing edge of a NACA0012 airfoil (approximately the latter 18% of the chord). The effect of shallow dimples on the aerodynamic coefficients is studied by Reynolds Averaged Navier–Stokes (RANS). To investigate the noise reduction credentials of dimples, high-fidelity Overset-Large Eddy Simulations are carried out for a chord based Reynolds number Rec of 4.2 ×105 at zero angle of attack. Near-field analysis revealed that this local application of dimples eliminated the tonal vortex shedding noise, which is otherwise present in the reference configuration without dimples. Furthermore, the application of shallow dimples resulted in the breakdown of the span-wise coherence in the low-to-mid frequency range and a reduction in the convective velocity of the stream-wise convecting eddies. These effects have a favourable impact on the trailing edge noise reduction with a modest 1.8% increase in the overall drag (less than 1 drag count). The far-field sound characteristics revealed that the dimple treatment resulted in the reduction of overall sound pressure levels in the downstream radiation direction (≈ 5.5 dB) and an increase in the upstream radiation direction (≈ 3.5 dB). In a nutshell, this manuscript argues for the inclusion of shallow dimples to the list of potential candidates for passive noise reduction methods, such as serrations, porous materials, and finlets.

Influence of Different Plates Arrangements on the Car Body

The purpose of this study was to investigate whether small plates covering the roof and the hood of the DrivAer estate vehicle can be used as airbrakes and increase its drag as well as the downforce. The presented results were obtained with the use of the commercial computational fluid dynamics software ANSYS® Fluent. The main findings of the article are that the aerodynamic devices such as flaps covering surfaces of the vehicle can have a significant impact on drag increase and can be used not only to make the design of the car more striking but also beneficial when utilized as a part of an active aerodynamic setup.

Unsteady aerodynamic characteristics of a bionic flapping wing in three-dimensional composite motion

Owing to continual advances in computational fluid dynamics, simulations of fluids have emerged as an important means of analyzing the aerodynamic characteristics of flapping-wing flight. This study establishes an aerodynamic model of bionic flapping-wing flight and uses dynamic hybrid grid technology to divide its flow field and other parts. We formulate equations to control the discretized flow field and combine the Navier–Stokes equation (N–S equation) with the theory of dynamic vortices to solve the flow field of the motion of flapping wings. We analyzed the lift and drag generated by the flapping wing at different wind speeds, amplitudes, frequencies, and chordal torsion angles. The results show the following: (1) wind speed had a significant influence on the lift resistance of the flapping wing. When the wind speed was 1 m/s, the lift force was 1.544 N and increased 4.3 times to 6.636 N when the wind speed was 5 m/s. The resistance increased from 0.39 to 0.88 N. (2) Changes in the amplitude of flutter had little effect on the average lift resistance. When the amplitude was increased from 15° to 45°, the lift force increased to only 0.757 N and drag changed by little. (3) The increase in the flapping frequency improved flight lift. When the frequency was increased from 1 to 5 Hz, the lift increased by 2.9 N (1.78 times) and resistance increased by only 0.08 N. (4) Increasing the chord torsion angle increased flight lift. The lifts at β = 5° and 15° were 6.636 and 6.654 N, respectively, 0.85 and 0.87 N greater than those at β = 0°. As the torsion angle continued to increase, the lift decreased, while the resistance increased more quickly. When β = 15°, the resistance increased by 1.53 N, 9.6 times larger than that at 0°. Increasing the flight speed and flapping frequency can increase flight lift. A small increase in the chord torsion angle increased flight lift, but an excessively large angle led to a substantial increase in drag. Increasing the amplitude of flapping can increase the instantaneous lift generated but has a smaller effect on the average lift. By revealing the influence of different parameters on the lifting resistance of a flapping wing during flight, this paper provides a theoretical foundation for the design and control of bionic flapping-wing aircraft.

Drag of circular cylinders in viscoelastic fluid flow

Drag forces on a circular cylinder placed in a rotating cylindrical annulus filled with a viscoelastic wormlike micellar fluid are measured using a load cell. The flow patterns are extracted from the PIV (particle image velocimetry) measurements of the flow fields around the circular cylinder. In viscoelastic fluids, shear waves can be propagated, and the viscoelastic Mach number M is defined as the ratio of the speed of an object in a fluid to the speed of shear waves in the fluid. In this study, the effect of viscoelastic Mach number on the drag characteristics of the circular cylinder is investigated. The results show that when the viscoelastic Mach number M > 1, an increase of drag over that expected from the growth rate at viscoelastic Mach numbers M < 1 is observed. From the results of PIV measurements, a dead water region is observed, which is formed symmetrically around the circular cylinder when M < 1 and behind the circular cylinder when M > 1. It plays an important role in the drag increase on a circular cylinder.

自動車のフロントエンド形状とエンジン冷却風による空力特性

The purpose of this research is to experimentally investigate how increase of the air inlet area of the engine cooling air affects the aerodynamic characteristics depending on the shape of the front-end of the vehicle body. A simplified 1/5 scale vehicle model was used with reproduction of the engine compartment where FF and FR type engines could be installed. In past researches, we used a rectangular front-end model, and in this research we used a round front-end model with a shape closer to the actual vehicle. In the wind-tunnel experiments, the moving-belt ground board was used to capture ground effects, measuring drag force, lift force and body surface pressure. The experiments were conducted at a wind speed and a moving belt speed of 20 m/s. As a result, the reason for the increase in drag due to the increase in air inlet area was the damming effect caused by the interference between the scavenging air flow in the engine compartment and the underfloor flow. The drag force of the round model was lower than that of the rectangular model, but the lift force was not significantly reduced.

ФЛУКТУАЦИИ ЗАПЫЛЕННОГО ПОТОКА И ИХ ВЛИЯНИЕ НА СОПРОТИВЛЕНИЕ ЗОЛОУЛАВЛИВАЮЩЕГО АППАРАТА

Despite the all-Russian gasification of power plants, the issue of high-quality ash collection is still relevant. More than 50 % of domestic stations operate on coal fuel. From the point of view of modernization of the station, it is important to switch to more efficient ash cleaning devices with a perfect capture system, which require less additional costs, both water and electricity.The inertial-vacuum ash collector is a competitive example of inertial gas cleaning devices. In a full-scale experiment conducted at a power plant, a decrease in efficiency and an increase in the aerodynamic drag of the apparatus were revealed. This article will consider the local component of the total pressure drop across the apparatus, compare areas with and without ash concentration, and formulate prospects for further research into the design of the ash collector.

Evaluation of Drag Reduction Effect and Torque Generated at the Base of Riblet with Direct Numerical Simulation in Turbulent Channel Flow

Reducing fluid drag and the energy required for fluid transport is an important issue. Riblet is used as a method to reduce frictional drag. Direct numerical simulation (DNS) was used to research the drag reduction effect of the riblet and the torque generated at the base of the riblet. The riblet is a three-dimensional riblet whose height and span vary with respect to the flow direction. The riblet is represented by a superposition of two sine waves, with amplitude and phase determined randomly. The torque and drag reduction effects were investigated for 1108 cases of riblets with different geometries. The friction Reynolds number, defined by the friction velocity uτ [m/s] and the kinematic viscosity ν [m2/s], is 110. The results show that torque is a shape parameter and is most dependent on pressure of the riblet surface. A Paretooptimal solutions that minimizes both the torque and the drag increase ratio was investigated. As a result, four Pareto solutions were found. The drag force generated on each face of the riblet was also analyzed and compared to the Pareto solution in all cases. The results show that the Pareto optimal solution group has a smaller fraction of drag due to pressure than all cases.

Modal Characterization, Aerodynamics, and Gust Response of an Electroactive Membrane

Flexibility and electric field sensitivity of dielectric elastomer membranes motivate the consideration of their applications in the wings of unmanned aerial vehicles. Applied electric field excitation expands the dielectric elastomer in the in-plane direction by compressing it into the thickness direction, which causes a change in the tension and ultimately affects the dynamic behavior of the elastomer. This study outlines the modal characteristics and fluid-structural dynamic behavior of the very high bond 4910 membrane, a type of dielectric elastomer, especially in an abrupt flow environment. An experimental shaker arrangement along with the digital image correlation system is used for experimental vibration testing. A finite element model is developed to study the modal characteristics and validate the experimental results. A fluid–structure interaction model of the electroactive membrane and its surrounding airflow is also developed to investigate the aerodynamic responses of the structure. Gust environments are predicted using the von Kármán and Dryden gust models, and the effect of different gust velocities on the aerodynamic lift and drag are obtained from the numerical fluid–structure interaction models. A wind tunnel, equipped with a data acquisition system, is used to study the aerodynamic responses and validate the numerical results experimentally. The modal analysis shows that the resonance frequencies decreased as the applied electric field excitation is increased. The coefficients of lift and drag fluctuate with the stochastic gust distribution, and the effects of von Kármán and Dryden gust profiles on aerodynamic characteristics are comparable. The values of the coefficients of lift and drag increase with the increase of applied voltage. The numerical results for the modal and aerodynamic responses show a good agreement with their corresponding experimental findings.

Secondary Flow in Smooth and Rough Turbulent Circular Pipes: Turbulence Kinetic Energy Budgets

Direct Numerical Simulations have been performed for turbulent flow in circular pipes with smooth and corrugated walls. The numerical method, based on second-order finite discretization together with the immersed boundary technique, was validated and applied to various types of flows. The analysis is focused on the turbulence kinetic energy and its budget. Large differences have been found in the near-wall region at low Reynolds number. The change in the near-wall turbulent structures is responsible for increase of drag and turbulence kinetic energy. To investigatselinae the effects of wall corrugations, the velocity fields have been decomposed so as to isolate coherent and incoherent motions. For corrugated walls, we find that coherent motions are strongest for walls covered with square bars aligned with the flow direction. In particular, the coherent contribution is substantial when the bars are spaced apart by a distance larger than their height. Detailed analysis of the turbulence kinetic energy budget shows for this set-up a very different behavior than for the other types of corrugations.

Investigation of wheelhouse shapes on the aerodynamic characteristics of a generic car model

As vehicle speed increases, the aerodynamic drag reduction becomes increasingly significant. The aim of this paper is to find out the effects of the wheelhouse shapes on the aerodynamics of an Ahmed body with a 35 slant angle. In this paper, based on the detached-eddy simulation method, the effects of the three classic different wheelhouse on the aerodynamic performance and near wake of the Ahmed body are presented. The mesh resolution and methodology are validated against the published test results. The results show that the front wheelhouse has a significant impact on the aerodynamic performance of the Ahmed body, leading to different aerodynamic drag forces and flow fields. Enlarging the wheelhouse cavity volume could result in a gradual increase in aerodynamic drag coefficients, the ratio of the wheelhouse cavity volume increased by 2.9% and 9.8%, the drag coefficients increased by 2.5% and 4.5% respectively. The increase in aerodynamic drag was primarily caused by flow separation in the large cavity volume wheelhouse.

Investigating Schmidt number effects in turbulent electroconvection using one‐dimensional turbulence

AbstractTurbulent electroconvection denotes a fluctuating multiphysical flow in which hydrodynamics and electrokinetics interact on multiple scales. The dynamical processes at work are entangled down to the molecular‐diffusive scales that are determined by the Schmidt (Sc) and Reynolds (Re) number. Turbulence properties are generally nonuniversal which leads to high numerical resolution requirements. We address the numerical challenges associated with accuracy and feasibility by utilizing a stochastic one‐dimensional turbulence (ODT) model. Here, ODT is applied to turbulent Couette flow of dilute electrolytes as canonical problem for turbulent electroconvection. For Sc ⩾ O(10), ODT predicts an increase of the skin friction drag due to electrohydrodynamically (EHD) enhanced small‐scale eddy production once the flow is sufficiently turbulent.

Computational analyses of tail fin configurations for a sounding rocket

Missiles and sounding rockets usually deviate from the trajectory due to unstable roll. Fins with cant angles are generally used to provide a rolling moment in sounding rockets and missiles to minimize the instability. Inducing a rolling moment also leads to an increase in the rocket motor’s power consumption due to the rise in drag, so inducing an optimal rolling moment with a minimal increase in drag is a crucial design criterion. It is crucial to maintain the similarity parameters while testing a scaled-down model in a wind tunnel. Therefore, computational fluid dynamics (CFD) is more efficient than extensive wind tunnel tests. In this paper, three-dimensional, incompressible simulations were performed on different models of sounding rockets using commercial CFD package fluent. The simulations were performed with the help of k-epsilon standard turbulence model. The results obtained were tabulated and graphically represented, and the trends of aerodynamic coefficients like C_{text {d}} and C_{text {m}} were analyzed. The purpose of this study is to analyze the dependency of aerodynamic coefficients on different fin configurations with emphasis on the cant angle. This study will be helpful to researchers designing a sounding rocket and help in maximizing apogee. The experimental and computational results show a favourable comparison. The results will show a particular configuration of fin having greater C_{text {m}}/C_{text {d}} which yields in a greater rolling moment and least amount of drag.

CFD Analysis of the Influence of the Front Wing Setup on a Time Attack Sports Car’s Aerodynamics

In time attack races, aerodynamics plays a vital role in achieving short track times. These races are characterized by frequent braking and acceleration supported by aerodynamic downforce. Usually, typical cars are modified for these races by amateurs. Adjusting the aerodynamic solutions to work with bodies developed for other flow conditions is difficult. This paper presents the results of a numerical analysis of the effects of installing a straight wing in front of or above the body on the modified vehicle system’s aerodynamic characteristics, particularly on the front wheels’ aerodynamic downforce values. The paper presents the methodology and results of calculations of the aerodynamic characteristics of a car with an additional wing placed in various positions in relation to the body. The numerical results are presented (Cd, Cl, Cm, Clf, Clr), as well as exemplary pressure distributions, pathlines, and visualizations of vortex structures. Strong interactions between the wing operation and body streamline structure are shown. An interesting and unexpected result of the analysis is that the possibility of obtaining aerodynamic downforce of the front wheels is identified, without an increase in aerodynamic drag, by means of a wing placed in a proper position in front of the body. A successful attempt to balance the additional downforce coming from the front wing on the front axle is made using a larger spoiler. However, for large angles of attack, periodically unsteady flow is captured with frequency oscillations of ca. 6–12 Hz at a car speed of 40 m/s, which may interfere with the sports car’s natural suspension frequency.

The Effect of Afterbody Geometry on Passenger Vehicles in Platoon

It is well known that platoons of closely spaced passenger cars can reduce their aerodynamic drag yielding substantial savings in energy consumption and reduced emissions as a system. Most published research has focused on platoons of identical vehicles which can arguably be justified by some evidence that geometric variety has little to no effect on the overall flow characteristics in platoons of three vehicles or more. It is known that much of the aerodynamic benefit from platooning is gained by the leading two cars, so operating as vehicle pairs could potentially achieve similar environmental benefits whilst addressing many of the practical challenges associated with the safe operation of long platoons on public roads. However, it has been reported that unlike long platoons, the effect of geometry and arrangement is critical if the drag reduction of a pair is to be optimised. This paper describes a parametric study based on three geometric variants of the popular DrivAer model with different combinations and spacings. It is confirmed that vehicle geometry crucially affects the results with the best combinations matching those of long platoons and others creating a net drag increase.

Modelling spanwise heterogeneous roughness through a parametric forcing approach

Inhomogeneous rough surfaces in which strips of roughness alternate with smooth-wall strips are known to generate large-scale secondary motions. Those secondary motions are strongest if the strip width is of the order of the half-channel height and they generate a spatial wall shear stress distribution whose mean value can significantly exceed the area-averaged mean value of a homogeneously smooth and rough surface. In the present paper it is shown that a parametric forcing approach (Busse &amp; Sandham, J. Fluid Mech., vol. 712, 2012, pp. 169–202; Forooghi et al., Intl J. Heat Fluid Flow, vol. 71, 2018, pp. 200–209), calibrated with data from turbulent channel flows over homogeneous roughness, can capture the topological features of the secondary motion over protruding and recessed roughness strips (Stroh et al., J. Fluid Mech., vol. 885, 2020, R5). However, the results suggest that the parametric forcing approach roughness model induces a slightly larger wall offset when applied to the present heterogeneous rough-wall conditions. Contrary to roughness-resolving simulations, where a significantly higher resolution is required to capture roughness geometry, the parametric forcing approach can be applied with usual smooth-wall direct numerical simulation resolution resulting in less computationally expensive simulations for the study of localized roughness effects. Such roughness model simulations are employed to systematically investigate the effect of the relative roughness protrusion on the physical mechanism of secondary flow formation and the related drag increase. It is found that strong secondary motions present over spanwise heterogeneous roughness with geometrical height difference generally lead to a drag increase. However, the physical mechanism guiding the secondary flow formation, and the resulting secondary flow topology, is different for protruding roughness strips and recessed roughness strips separated by protruding smooth surface strips.

A model of an inflatable elastic aerofoil

A novel method is presented to calculate the deformation of a simple elastic aerofoil with a view to determining its aerodynamic viability. The aerofoil is modelled as a thin two-dimensional elastic sheet whose ends are joined together to form a corner of prescribed angle, with a simple support included to constrain the shape to resemble that of a classical aerofoil. The weight of the aerofoil is counterbalanced exactly by the lift force due to a circulation set according to the Kutta condition. An iterative process based on a boundary integral method is used to compute the deformation of the aerofoil in response to an inviscid fluid flow, and a range of flow speeds is determined for which the aerofoil maintains an aerodynamic shape. As the flow speed is increased the aerofoil deforms significantly around its trailing edge, resulting in a negative camber and a loss of lift. The loss of lift is ameliorated by increasing the inflation pressure but at the expense of an increase in drag as the aerofoil bulges into a less aerodynamic shape. Boundary layer calculations and nonlinear unsteady viscous simulations are used to analyse the aerodynamic characteristics of the deformed aerofoil in a viscous flow. By tailoring the internal support the viscous boundary layer separation can be delayed and the lift-to-drag ratio of the aerofoil can be substantially increased.

Modification of flow behind a circular cylinder by steady and time-periodic blowing

We apply steady and time-periodic blowing, respectively, to flow over a circular cylinder at a Reynolds number of 21 000 for the modification of flow in the wake. The blowing is applied near the separation point on the upper and lower surfaces from either a slit (uniform blowing) or eight holes (discrete blowing) along the spanwise direction at each surface. For steady blowing, the blowing momentum coefficient ranges from 0.008 to 0.269, and, for time-periodic blowing, it does from 0.013 to 0.162. The changes in the drag are estimated by a momentum theorem with the velocity measurements in the wake. The steady discrete blowing is more effective and efficient for drag reduction than the steady uniform blowing, providing a maximum drag reduction of 38%. Counter-rotating vortices induced by the optimal discrete blowing change the Kármán vortex cores into three-dimensional vortices and weaken their strength. On the other hand, steady uniform blowing significantly increases the wake width and leads to drag increase. With time-periodic blowing, the maximum drag reductions by uniform and discrete blowing are 68% and 28%, respectively, indicating that the time-periodic uniform blowing is more effective than the steady discrete blowing. For the time-periodic blowing, two different vortex lock-on phenomena (symmetric and asymmetric vortex shedding in the wake) are found, and they lead to a significant drag decrease and increase, respectively.

Treatment for gap and step nonconformity of slat on tail-hang civil aircraft

Slat aerodynamic contour nonconformity often occurs during the manufacture of civil aircraft, which can cause appearance influence, drag increase and even manoeuvrability & stability problem. This article introduces 4 types of gaps and 3 types of step nonconformity and reason for them. Then the article studies treatment principle for wing slat contour nonconformity. In the end, the article figures out acceptable criteria and 5 types of standard repair methods for slat gap and step nonconformity.

Effect of Air Pressure on Aerodynamic Characteristics of the HTS Maglev Running in a Tube

The high-temperature superconducting maglev-evacuated tube transport (HTS Maglev-ETT) is a green, energy-saving, and high-speed future transit mode. The initial tube pressure is a vital parameter of the ETT system. To investigate the impact of the initial tube pressure on the aerodynamic characteristics of the HTS maglev, a two-dimensional scaled simulation model of 1:10 was established. The aerodynamic drag of the train and the shock wave phenomenon in the tube under different initial tube pressures were studied. The study found that the shock wave formed in the tube contains oblique shock wave and normal shock wave. There is a linear relationship between the range of the shock wave and the initial tube pressure. The range of the shock wave and aerodynamic drag increase with the increases of initial tube pressure. Additionally, the pressure drag plays a dominant role in the total drag. By lowering the pressure in the tube, the pressure difference between the front and rear of the train was reduced, thus reducing the aerodynamic drag.