Increase In Drag Research Articles

Effect of Rain on Vertical Axis Wind Turbines

Results are presented that demonstrate that rain will have a significant effect on the output of a vertical axis wind turbine. The experiments were carried out in the climatic wind tunnel at the University of Nottingham where water was sprayed into the wind tunnel to simulate several rainfall rates. The rain had the effect of increasing the drag, slowing the rotational speed of the wind turbine and decreasing the Power for the equivalent wind speed. The increasing in the drag has the additional effect of decreasing the optimal coefficient of performance as the rainfall rate is increased. Similar studies in airfoil performance in the rain have shown that the increase in drag and decrease in lift is related to the chord length of the airfoil and so could potentially be larger for larger turbine blades. This could have an effect on the control strategy necessary for controlling wind turbine performance and will need to be studied further.

Effect of plant shapes on sand transport rate and aerodynamic particle entrainment rate from the perspective of plant drag

Plant shape is an important factor affecting sand transport rate and aerodynamic particle entrainment rate. Two different types of flexible plant models (one is slender and the other has a large upper part and a small lower part) and a rigid tree-like plant model are selected with approximately zero basal-to-frontal area ratio. The sand transport rate, aerodynamic entrainment rate and shear stress partitioning on vegetated surfaces were measured under different plant densities and incoming wind speeds in a wind tunnel. The results show that the sand transport rate and aerodynamic entrainment rate on vegetated surfaces decrease with an increase in lateral cover and form drag of plant models. The relationship between sand transport rate, aerodynamic entrainment rate and drag parameter β (defined as the drag coefficient ratio of a plant model to the bare surface) is established by introducing the attenuation function of effective wind speed, which decays exponentially with an increase in lateral cover and linearly with an increase in drag coefficient ratio β. The present findings contribute to our understanding of how different vegetation types affect sediment transport rate and aerodynamic entrainment rate.

The Dominant Role of the Summer Hemisphere in Subtropical Lower Stratospheric Wave Drag Trends

AbstractIt is well established that the shallow branch of the Brewer‐Dobson circulation accelerates in a warming climate due to enhanced wave drag in the subtropical lower stratosphere. This has been linked to the strengthening of the upper flanks of the subtropical jets. However, the seasonality of the zonal wind trends, peaking in the winter hemisphere, is opposite to that of the Eliassen‐Palm flux convergence trends, peaking in summer. We investigate the seasonality in the wave drag trends and find a different behavior for each hemisphere. The Shepherd and McLandress (2011, https://doi.org/10.1175/2010jas3608.1) mechanism, involving transient wave dissipation at higher levels following the rise of the critical lines, is found to maximize in austral summer. On the other hand, in the Northern Hemisphere the wave drag increase peaks in summer primarily due to the changes in the stationary planetary waves (monsoonal circulations) associated with enhanced deep convection.

Analysis of Aerodynamics Load of a Flapping Wing by Vortex Lattice Method

The aim of this study is to investigate the effect of changing the angle of attack and flapping angle on the aerodynamic characteristics of an unsymmetrical wing with the NACA 2412 profile. This work uses the Vortex Lattice Method. The results of this study show that increasing the flapping angle causes the pitching moment to decrease while induced drag and moment coefficient increase, meanwhile, the lift coefficient increases at positive angles of attack and then decreases at negative angles of attack. The pitching moment decreases with increasing angle of attack at positive flap angles, but this value increases at negative flap angles. Meanwhile, the induced drag decreases from an angle of attack -15° to a minimum at an angle of attack -5° and then increases. The moment coefficient increases with increasing angle of attack at positive flapping angles, but decreases at negative flapping angles. Meanwhile, the lift coefficient increases with increasing angle of attack.

Movement of and drag force on slender flat plates in an array exposed to combinations of unidirectional and oscillatory flow

When flexible structures deflect under unidirectional flow or sway under oscillatory flow, their hydrodynamic drag forces are reduced. The flow-induced motion of a structure is governed by the balance between hydrodynamic forces and restoring forces due to stiffness (a ratio known as the Cauchy number, Ca). Movement of and drag on structures under unidirectional (current) and oscillatory (wave) flow have often been studied separately, but such flows frequently coexist. In this study, a simple numerical model of a slender flexible plate, which can represent a seagrass blade, was used to investigate how the posture of and drag on the plate in oscillatory flow was impacted by collinear currents moving in the same (following) and opposite (opposing) directions, relative to the wave propagation. The rate of work done against hydrodynamic drag forces defines the rate of wave energy dissipation, ED,w. The velocity acting on the blade was modeled as if the blade existed in a canopy of blades. For conditions in which blade motion was observed, added currents influenced ED,w in two ways. When the ratio of the time-averaged velocity within the canopy, U1, to the wave velocity amplitude, Uw, was less than 0.33±0.05, ED,w was reduced relative to the pure wave condition, which was attributed to current-induced deflection. However, when U1/Uw exceeded 0.33±0.05, ED,w increased. While the blade increasingly deflected with increasing current, higher currents also limited unsteady wave-induced reconfiguration, increasing the relative motion between the blade and fluid and therefore increasing drag. While Ca can predict the drag reduction associated with weak current, it cannot predict the increase in drag for U1/Uw>0.33. Finally, the model was used to explore the influence of current on wave amplitude evolution across a seagrass meadow. Relative to pure wave conditions, opposing currents enhanced wave damping, but following currents had little impact on wave damping.

On the aerodynamic performance of redundant propellers for multi-rotor eVTOL in cruise

As multi-rotor and convertible configurations gain compelling popularity in Advanced Air Mobility designs, the handling of redundant propellers during cruise flight regimes becomes a new challenge. This paper presents a systematic study of the aerodynamic performance of feathered and windmilling redundant propellers on a simplified multi-rotor, multi-wing configuration. High-fidelity CFD simulations were performed with a systematic test matrix including the blade feathering angle, azimuth shift, coning angle, and windmilling pitch angle and RPM. For the feathered blades, the feathering angle had a strong impact on the aerodynamic performance. The feathered blades could substantially reduce the overall lift and increase the overall drag at high feathering angles (up to 25% lift reduction and 70% drag increase). This is correlated with changes in the sectional wing inflow angles induced by the upstream feathered blades. Windmilling propellers, in ideal cases, were found capable of considerable wind energy extraction (50% of required cruise power) and could create slowed inflows for downstream thrusting propellers, at the cost of excessive drag (up to 90% more). Comparisons of vehicle performance with feathered and windmilling blades, showed similar aerodynamic forces and energy consumption. This shows that windmilling could be a feasible option for redundant propellers, given careful balancing of the drag and energy conversions. The presented results provide valuable guidance for the handling of redundant blades, for future multi-rotor aircraft designs for sustainable aviation.

Viscous Loading Regulates the Flagellar Energetics of Human and Bull Sperm.

The viscoelastic properties of the female reproductive tract influence sperm swimming behavior, but the exact role of these rheological changes in regulating sperm energetics remains unknown. Using high-speed dark-field microscopy, the flagellar dynamics of free-swimming sperm across a physiologically relevant range of viscosities is resolved. A transition from 3D to 2D slither swimming under an increased viscous loading is revealed, in the absence of any geometrical or chemical stimuli. This transition is species-specific, aligning with viscosity variations within each species' reproductive tract. Despite substantial drag increase, 2D slithering sperm maintain a steady swimming speed across a wide viscosity range (20-250 and 75-1000 mPas for bull and human sperm) by dissipating over sixfold more energy into the fluid without elevating metabolic activity, potentially by altering the mechanisms of dynein motor activity. This energy-efficient motility mode is ideally suited for the viscous environment of the female reproductive tract.

Direct numerical simulations of the drag degradation mechanism in channel flow over trapezoidal riblets

Direct numerical simulations of turbulent channel flows with trapezoidal riblets were conducted to analyze the characteristics of the near-wall flow structures in the drag reduction, drag neutral, and drag increase regimes. The trapezoidal riblets possess a tip angle of α =15°and a height–to-width ratio of k/s = 0.335 for cross-section-area based sizes lg+=Ag+= 11.7, 17, 23.2, and 29 at a friction Reynolds number of Reτ=395. The Reynolds stresses are compared with the smooth wall case, and it is observed that the intensity of the turbulent activities in the boundary layer is diminished for drag reduction riblet and enhanced for drag increase riblets. The roughness sublayer indicates that riblet only directly affect the near-wall region of the flow. By applying the Fukagata, Iwamoto, and Kasagi (FIK) identity in Phys. Fluids, vol. 14, 2002, L73, it is found that the modification of the overall drag is mainly due to the turbulent shear stress. The variation of the dispersive stress is only evident within the roughness sublayer for the drag increasing riblets. The enhanced mean wall-shear stress in the valley is responsible for the drag increase for oversized riblets. The production and convection terms of the turbulent stress reveal that although the total drag contribution is trivial, the convection inside the riblet valleys caused by secondary flow is crucial for the redistribution of the turbulent stress and drag increase of oversized riblets.

A Numerical Study on the Influence of Transverse Grooves on the Aerodynamic Performance of Micro Air Vehicles Airfoils

Micro Air Vehicles (MAVs) airfoils usually operate at low Reynolds number conditions, where viscous drag will consume a large amount of propulsion power. Due to the small dimensions, many drag reduction methods have failed, resulting in limited current research. To develop an effective method of reducing viscous drag, transverse grooves were placed on the surface of MAVs airfoils in this study, and a numerical investigation was implemented to uncover the corresponding flow control law as well as the mechanism. Research has shown that transverse grooves have an impact on the drag and lift of airfoils. For drag, properly sized transverse grooves have the effect of reducing drag, but under high adverse pressure gradients or when the continuous arrangement of grooves is excessive, the optimal drag reduction effect achieved by the grooves is weakened, and even the drag increases due to the significant increase in pressure difference. In severe cases, it may also cause strong flow separation, which is not conducive to MAV flight. For lift, the boundary vortex in the groove has the ability to reduce the static pressure near the groove. However, high adverse pressure gradients or too many grooves will thicken the boundary layer and increase the blockage effect, resulting in a large static pressure on the grooved side of the airfoil (with an increase in drag). From the perspective of circulation, the static pressure changes on the suction and pressure surfaces have opposite effects on lift. Considering the comprehensive aerodynamic performance of the airfoil, we designed a high lift-to-drag ratio airfoil with grooves, which increased the lift-to-drag ratio by 33.747% compared to the smooth airfoil. Based on the conclusions, we proposed preliminary design criteria for grooved airfoils, providing guidance for subsequent research and applications.

Effect of Unsteady Fan-Intake Interaction on Short Intake Design

Abstract The next generation of ultrahigh bypass ratio civil aero-engines promises notable engine cycle benefits. However, these benefits can be significantly eroded by a possible increase in nacelle weight and drag due to the typical larger fan diameters. More compact nacelles, with shorter intakes, may be required to enable a net reduction in aero-engine fuel burn. The aim of this paper is to assess the influence of the design style of short intakes on the unsteady interaction under crosswind conditions between fan and intake, with a focus on the separation onset and characteristics of the boundary layer within the intake. Three intake designs were assessed, and a hierarchical computational fluid dynamics (CFD) approach was used to determine and quantify primary aerodynamic interactions between the fan and the intake design. Similar to previous findings for a specific intake configuration, both intake flow unsteadiness and the unsteady upstream perturbations from the fan have a detrimental effect on the separation onset for the range of intake designs. The separation of the boundary layer within the intake was shock driven for the three different design styles. The simulations also quantified the unsteady intake flows with an emphasis on the spectral characteristics and engine-order signatures of the flow distortion. Overall, this work showed that is beneficial for the intake boundary layer to delay the diffusion closer to the fan and reduce the preshock Mach number to mitigate the adverse unsteady interaction between the fan and the shock.

Effect of Stokes number and particle-to-fluid density ratio on turbulence modification in channel flows

Two-way momentum-coupled direct numerical simulations of a particle-laden turbulent channel flow are addressed to investigate the effect of the particle Stokes number and of the particle-to-fluid density ratio on the turbulence modification. The exact regularised point-particle method is used to model the interphase momentum exchange in presence of solid boundaries, allowing the exploration of an extensive region of the parameter space. Results show that the particles increase the friction drag in the parameter space region considered, namely the Stokes number $St_+ \in [2,80]$ , and the particle-to-fluid density ratio $\rho _p/\rho _f \in [90,5760]$ at a fixed mass loading $\phi =0.4$ . It is noteworthy that the highest drag occurs for small Stokes number particles. A measurable drag increase occurs for all particle-to-fluid density ratios, the effect being reduced significantly only at the highest value of $\rho _p/\rho _f$ . The modified stress budget and turbulent kinetic energy equation provide the rationale behind the observed behaviour. The particles’ extra stress causes an additional momentum flux towards the wall that modifies the structure of the buffer and of the viscous sublayer where the streamwise and wall-normal velocity fluctuations are increased. Indeed, in the viscous sublayer, additional turbulent kinetic energy is produced by the particles’ back-reaction, resulting in a strong augmentation of the spatial energy flux towards the wall where the energy is ultimately dissipated. This behaviour explains the increase of friction drag in particle-laden wall-bounded flows.

Optimal swimming locomotion of snake-like robot in viscous fluids

Snake-like robots have several joints that enable them to swim in various types of viscous fluids such as water, mud, and clay by varying the undulation. Because the optimal swimming motion is assumed to be different in fluids with different viscosities, we constructed an equation of motion for a snake-like robot swimming in viscous fluids and attempted to find the optimal swimming motion via numerical analysis. We constructed a snake-like robot comprising eight links in this study that could swim in four fluids with different kinematic viscosities, and measured their joint positions and relative angles during swimming. Accordingly, we proposed a method for identifying multiple unknown parameters of fluid forces acting on a body using the unscented Kalman filter, which could determine the unknown added mass and drag coefficient. Subsequently, we clarified that the inertial drag model is effective in water (small viscosity, μ∼1mPas), while the friction drag model is effective in oil (large viscosity, μ>100mPas). By considering the increase in frictional drag due to boundary layer thinning in the proposed model, we identified the constant drag coefficient regardless of the swimming frequency. Next, we employed the constructed numerical model to explore the trade-off relationship between consumed power and velocity during swimming using exhaustive search. Consequently, we confirmed that the swimming frequency decreased as the fluid viscosity increased under the same swimming power consumption. Moreover, the wavelength of undulation was confirmed to be between half and one wavelength, regardless of the viscosity.

How the distribution and morphology of interfacial gas bubbles enhancing the boundary slip of fluid flow

A comprehensive investigation on how distribution and morphology of interfacial gas bubbles (IGBs) affecting the boundary slip of fluid flow was conducted by using finite element simulation. Results showed that the IGB dispersion, protrusion angle, coverage area, lateral size, height and quantity play crucial roles in determining the slip length. The slip length shows an decrease with the increase of dispersion index. Especially for IGB spacing distance smaller than 0.1 times IGB lateral size, the slip length shows an obvious increase. Moreover, a larger IGB lateral size and fewer IGB quantity are more effective for drag reduction (protrusion angle θ smaller than critical angle θc) or for drag increase (θ > θc) when IGB coverage area and protrusion angle are fixed.

Unsteady aerodynamic behaviour of high-speed maglev trains during plate braking in tailwind and headwind opening configurations

ABSTRACT High-speed maglev trains are important future rail transportation systems that require high braking performance. One approach to braking in high-speed trains is using aerodynamic plate brakes, which offer improved braking performance at high speeds but can impact the train’s aerodynamic characteristics. In this study, dynamic grid technology was used to achieve plate motion, and unsteady Reynolds-averaged Navier-Stokes (URANS) equations and shear stress transfer (SST) k-ω turbulence model were adopted to simulate the unsteady aerodynamic behaviour of a 5-car maglev train running at 600 km/h. Two wind tunnel experiments validated the method, and grid independence analysis was also conducted. The unsteady aerodynamic performance and transient flow field around the train during plate braking in tailwind and headwind opening configurations were studied and analysed. As the plates opened, there was a rapid increase in both aerodynamic drag and lift, creating a pulse-like change. The tailwind opening configuration generated relatively large peak aerodynamic forces, whereas the peak forces decreased successively downstream along the train length. As the flow stabilized, periodic fluctuations in the aerodynamic forces of the plates occurred. The interference with the convective field and the impact on the aerodynamic force of the tail car were more significant for the tailwind opening configuration. In addition, the tailwind opening plate caused significant fluctuations in the aerodynamic force of the tail car, with similar effects observed in other cars. From an aerodynamic perspective, the headwind opening configuration yielded better results. These findings can guide the design and implementation of plate brakes for high-speed maglev trains.

Frictional Resistance Increase Due to Hull Roughness: An Analysis of the Hull Form Parameters Influence

An awareness of the drag increase brought on by biofouling's roughness on the ship hull is one technique to cut emissions aboard ship. However, predicting the increased drag on ships poses significant challenges. When predicting the rise in frictional resistance brought on by roughness, the hull is considered flat. In fact, ship hulls have a variety of shapes, and it is not certain whether this is a factor influencing the magnitude of the increase in resistance due to roughness. In this article, the effect of the hull's form parameters— (ratio of length per breadth), (coefficient block), and (Length of center buoyancy)—on the increase in frictional resistance brought on by roughness have been investigated. The method used to calculate the ship resistance is Computational Fluid Dynamics (CFD) simulation, complemented by roughness modelling using the wall function approach method. The Design of Experiment (DOE) method has been used to vary the shape of the hull model as a variation of the test specimen in this study. The verification and validation tests have been carried out on the CFD simulation results, where the results have been compared with proven empirical methods. Based on the study results, the value of frictional resistance and increased frictional resistance () of all specimens has shown no significant difference in value, evidenced by the variance values, ranging only 1.57-2.1%. Thus, these results prove that the increase in frictional resistance due to roughness is sufficient to assume the ship's hull as a flat plate. The other finding is that roughness can also increase the pressure resistance, and hull shape parameters also contribute towards changes in the value of resistance.

Effect of upstream flow characteristics on the wake topology of a square-back truck

The influence of upstream flow characteristics on the bi-stable flow structure in the wake region of a simplified square-back heavy vehicle model at a Reynolds number of 2.7 × 104 was investigated by using the improved delayed detached eddy simulation method. The asymmetric wake structure of this model and its corresponding aerodynamic response were examined, aiming to identify the effect mechanism of three inlet profiles on the asymmetric wake structure of the named ground transportation system (GTS) model in simulations. The accuracy of the numerical method used in this study was validated by comparison with wake structure data, including the flow states, vortex core's location, and aerodynamic drag obtained from previous large eddy simulations and water channel experiments. The numerical results show that different turbulent inlet velocity profiles lead to different wake topologies. When the turbulent velocity profile with a turbulence intensity of 15% generated by TurbSim, a stochastic inflow turbulence tool for generating turbulent velocity inlet on an atmospheric boundary layer profile, is used, the expected bi-stable flow topology is still observed, but it is not shown in the case by means of the turbulence generator incorporated into ANSYS Fluent. Those turbulent inlet velocity profiles contribute to the increase in GTS model's aerodynamic drag forces. Compared to the uniform velocity profile, the TurbSim velocity profile can achieve a drag increase in 7.23%. In addition, this turbulent profile intensifies the flow fluctuations in the wake region and enhances the transient response frequency of the wake region. Thus, when assessing the vehicle aerodynamic performance in open air, especially under crosswinds, the real turbulence velocity profile, e.g., the profile generated by TurbSim in the current study, is recommended to be used for a more accurate prediction in numerical simulations.

A systematic investigation into the effect of roughness on self-propelled swimming plates

This study examines the effects of surface topography on the flow and performance of a self-propelled swimming (SPS) body. We consider a thin flat plate with an egg-carton roughness texture undergoing prescribed undulatory swimming kinematics at Strouhal number $0.3$ and tail amplitude to length ratio $0.1$ ; we use plate Reynolds numbers $Re=6$ , 12 and $24\times 10^3$ , and focus on $12\,000$ . As the roughness wavelength is decreased, we find that the undulation wave speed must be increased to overcome the additional drag from the roughness and maintain SPS. Correspondingly, the extra wave speed raises the power required to maintain SPS, making the swimmer less efficient. To decouple the roughness and the kinematics, we compare the rough plates to equivalent smooth cases by matching the kinematic conditions. We find that all but the longest roughness wavelengths reduce the required swimming power and the unsteady amplitude of the forces when compared to a smooth plate undergoing identical kinematics. Additionally, roughness can enhance flow enstrophy by up to 116 % compared to the smooth cases without a corresponding spike in forces; this suggests that the increased mixing is not due to increased vorticity production at the wall. Instead, the enstrophy is found to peak strongly when the roughness wavelength is approximately twice the boundary layer thickness over the $Re$ range, indicating the roughness induces large-scale secondary flow structures that extend to the edge of the boundary layer. This study reveals the nonlinear interaction between roughness and kinematics beyond a simple increase or decrease in drag, illustrating that roughness studies on static shapes do not transfer directly to unsteady swimmers.

Drag reduction in turbulent channel flow by gas perfusion through porous non-hydrophobic and hydrophobic interfaces: Part I

Skin-friction drag in liquid flows can be reduced by several methods including bubble injection into the near-wall region, air layer creation, air cavities, and applied super-hydrophobic coatings. The first three of these methods each has major drawbacks. In the fourth method, the depletion of the trapped air pockets on the super-hydrophobic surfaces (SHSs) in turbulent flows may cause a significant drag increase. To improve FDR in turbulent flows, a novel method is investigated that perfuses air through a porous medium with and without a hydrophobic treatment. A set of experiments has been conducted in a recirculating water tunnel at downstream-distance-based Reynolds numbers to 8.2million. The test model was a flat porous plate and the total wall shear stress was measured by a load cell apparatus. Air was perfused through such interfaces at mass flow rates to 50 L/min. It is noted that downstream persistence is not an issue with this technique as long as perfusion occurs near uniformly along the surface. The results showed that the method with maximum airflow exhibited a total drag reduction of around 10–25% with an untreated surface and about 20–30% with a treated surface.

Flow control over a circular cylinder using a slot and axially arranged holes

Circular cylinder flow is controlled using a slot and axially arranged holes (AAH) at a Reynolds number of 32000. When the slot and AAH are placed at small incidence angles from the free-stream direction, the passive jet generated on the leeward cylinder side pushes the wake vortex downstream, reducing the drag compared to the baseline cylinder. If the incidence angle is larger than the critical angle, the alternate blowing and suction induced by the slot and AAH promote a boundary-layer transition on one side and a shear-layer transition on the other. The earlier transition to turbulent flow substantially shortens the separated shear layers on both sides, shifting the wake formation region toward the base and leading to an increase in drag. The AAH has a higher critical incidence angle than the same-size slot, as the passive jet developed from the AAH undergoes a transition to alternate blowing and suction at a higher incidence angle than the slot. Just before the incidence angle of the AAH reaches the critical angle, the weak jet flow induced on the windward cylinder side delays the boundary-layer separation through early separation and laminar reattachment without shortening the separated shear layer, thereby allowing additional drag reduction.

DESIGN OF UNDIRECT GUY WIRE TENSION METER BASED ON STRAIN GAUGE

The safety of the tower is depend the tension of guy wire, where it must have the same tensile stress at all positions. To meet this requirement, the load cell guy wire is designed based on strain gauge. Load cell guy wire is designed portable and it can detect stress of the guy wire indirectly. The main component of load cell is a beam, two hooks and a cylinder to form a bending moment force in the beam, the value of the bending moment on the beam will be directly proportional to the increase or decrease in force drag on guy wire. Design process of load cell doone using mathematical analysis, and then the load cell is calibrated by standard load cell, based on the data result of calibration is known that the stress at the guy wire load cell is close and under the yield stress of the load cell material, it is proved that load guy wire cell’s design result is safe to use.