Increase In Drag Coefficient Research Articles

Impact of a Reverse Delta Type Add-on Device on the Flap-tip Vortex of a Wing

The effect of interactions of vortices produced by an outboard flap-tip of a half-span wing (NACA 23012 in landing configuration) and a slender reverse delta type add-on device, placed in the proximity of the outboard flap-tip, on the upper surface of the half-span wing is investigated using Particle Image Velocimetry in a closed loop low speed wind tunnel. Specifically the characteristics of the vortex interactions generated downstream in planes perpendicular to the free stream direction and their dependence on angles of attack at a chord-based Reynolds number of Rec=2.75×105 have been determined. It was found that the add-on device significantly reduces the tangential velocity magnitude and enlarges the vortex core of the resultant vortex by up to 36%. The aerodynamic performance of the half-span wing model was marginally affected by the use of a reverse delta type add-on device. The reduction in lift coefficient is 3.8% and the increase in drag coefficient is 14.9%.

Drag and Turbulent Characteristics of Mobile Bed Channel With Mixed Vegetation Densities Under Downward Seepage

The present study addresses the drag owing to the presence of vegetation and turbulent characteristics in a mobile bed channel, characterized by fully submerged vegetation formed by nonuniform vegetation densities. The influence of seepage on the velocity profiles, Reynolds stress, and turbulence intensities is discussed. Experimental results show that vegetation density is one of the important parameters that affect the flow resistance. It is found that higher vegetation density when placed at the downstream side leads to a reduction in velocity, Reynolds stress, and turbulent intensities. Downward seepage increases the near bed velocity, Reynolds stress, and turbulent intensities. Moment analysis shows that there is an increase in the inrush of flow, and sediment particles are transported more toward the streamwise direction with the application of seepage. The dominance of sweep events over ejection events increases more sediment transport. However, high vegetation density when placed at the downstream portion slightly decreases the dominance of sweep event. Drag coefficient decreases near the vegetation top and increases near the bed. Downward seepage decreases the effect of drag offered by the vegetation stems. The reduction in flow characteristics, viz., velocity, Reynolds stress, turbulent intensities, in the downstream portion of lesser spacing vegetation stems is attributed an increased drag coefficient.

The Influence of wave state and sea spray on drag coefficient from low to high wind speeds

Ocean waves alter the roughness of sea surface, and sea spray droplets redistribute the momentum flux at the air-sea interface. Hence, both wave state and sea spray influence sea surface drag coefficient. Based on the new sea spray generation function which depends on sea surface wave, a wave-dependent sea spray stress is obtained. According to the relationship between sea spray stress and the total wind stress on the sea surface, a new formula of drag coefficient at high wind speed is acquired. With the analysis of the new drag coefficient, it is shown that the drag coefficient reduces at high wind speed, indicating that the sea spray droplets can limit the increase of drag coefficient. However, the value of high wind speed corresponding to the initial reduced drag coefficient is not fixed, and it depends on the wave state, which means the influence of wave cannot be ignored. Comparisons between the theoretical and measured sea surface drag coefficients in field and laboratory show that under different wave ages, the theoretical result of drag coefficient could include the measured data, and it means that the new drag coefficient can be used properly from low to high wind speeds under any wave state condition.

Numerical Simulation of Two Different Flexible Bodies Immersed in Moving Flow**Supported by the National Natural Science Foundation of China under Grant Nos 51479007, 51309017, and 11102027, the Natural Science Foundation of Hubei Province under Grant No 2015CFA026, and the Fundamental Research Fund for State Public-Benefic Scientific Institutes of CRSRI under Grant No CKSF2015026/SL.

The coupled motion of two flexible bodies with different lengths immersed in moving fluid is studied numerically. The flapping frequency, flapping amplitude and average drag coefficient of each body are calculated and the influences of the arranging manner and separation distance are analyzed. In our simulation, when placed in the flow individually, the flexible body with a longer length will flap in period and the shorter one will maintain still straightly in the flow direction. The numerical results show that, two different flexible structures near placed in moving flow would strongly interact. When they are placed side by side, the existence of the stable shorter flexible body will restrain the flapping of the longer one while the existence of the longer flexible body may also induce the shorter one to flap synchronously. When placed in tandem with the shorter flexible body in upstream, the flapping of the longer one in downstream will be obviously enhanced. In the situation for the longer flexible body placed in upstream of the shorter one, the coupled flapping amplitude and average drag coefficients increase and decrease periodically with increasing the arranging space, and peak values appear as a result of the mediate of the tail wakes.

Supersonic flow control

Shock wave-induced boundary layer interactions are unavoidable in high-speed flows. The strong adverse pressure gradient imposed by the shock on the incoming undisturbed boundary layer may result in flow separation that is associated with low-frequency oscillations of the separation shock. The separation phenomenon, therefore, leads to undesirable effects such as wall pressure loading, high local wall heating, increased drag coefficient, reduced mass flow ingestion, lower-pressure recovery and inlet unstart conditions.

Experimental investigation of hydrodynamic characteristics of a submersible vehicle model with a non-axisymmetric nose in pitch maneuver

Experimental investigation is conducted to study the behavior of a submarine model with a non-axisymmetric nose in pitch maneuver. The submarine model with all its appendages is tested in a wind tunnel at Reynolds number of 6.6×106, based on the model length, and a range of pitch angles −10°≤α≤+27°. The Reynolds number effects on drag coefficient are investigated for a range of Reynolds numbers between 4.7×106 and 8.0×106. By increasing the pitch angle, a continuous increase in drag and lift coefficients is observed. However, the measured pitching moment approaches to almost an asymptotic constant value when the pitch angle increased beyond +10°. Smoke flow visualization tests are also conducted to explore the details of flow structure around the submarine model for various values of pitch angles. The visualized flow revealed the formation of cross flow vortices and flow separation over the submarine model. These results show that the location of the flow separation for the non-axisymmetric nose shape is closer to the nose tip than the symmetric nose shape at high angle of attack.

Numerical simulation of the effect of relative thickness on aerodynamic performance improvement of asymmetrical blunt trailing-edge modification

In this paper, the aerodynamic performance of wind turbine airfoils with different relative thicknesses and their modifications has been numerically investigated to facilitate a greater understanding of the effects of maximum relative thickness and its position on the aerodynamic performance improvement of asymmetrical blunt trailing-edge modification. The lift and drag coefficients of airfoil NACA4415 are calculated with the k-ω SST turbulence model, and are compared with experimental data to validate the simulation accuracy of the Computational Fluid Dynamics (CFD) approach. The airfoils with different relative thicknesses are modified to be asymmetrical blunt trailing-edge airfoils by means of the software Xfoil. The best trailing-edge thickness distribution ratio is obtained by comparing the aerodynamic performance of the modifications with different distribution ratios. The aerodynamic performance of original airfoils and their asymmetrical modifications with the best thickness distribution ratio being 1:3 is investigated to analyze the increments of lift and drag coefficients and lift-drag ratio. Results indicate that with the increasing of relative thickness, the lift coefficient increment of NACA4418 airfoil is the smallest for the angle of attack more than 9°, and the drag coefficient increment as a whole decreases first and then increases, but the average lift-drag ratio increment of NACA4412 airfoil is the largest, closely followed by NACA4415 airfoil. It is also showed that with the relative thickness position close to the leading-edge, the increments of lift and drag coefficients decrease and increase for the angle of attack more than a certain value, respectively, and the average lift-drag ratio increment of NACA4415 airfoil is positive and larger than those of NACA4415-mod25 and NACA4415-mod20 airfoils. Therefore, the medium thickness airfoil whose relative thickness position is away from the leading-edge is more suited to the asymmetrical blunt trailing-edge modification.

Laminar Flow and Heat Transfer Phenomena Across a Confined Semicircular Bluff Body at Low Reynolds Numbers

The present study is concerned with the simulation of incompressible Newtonian fluid flow and heat transfer over a long semicircular bluff body in a channel at low Reynolds numbers. In particular, wall effects on the forced convection from a (heated) semicircular cylinder confined in a horizontal channel are investigated for Reynolds number = 1–40 and blockage ratio = 16.67–50% for air as the working fluid. Flow and thermal fields are found steady for the preceding range of settings. The onset of flow separation increases as the wall confinement increases. The size of the recirculation zone downstream of a semicircular cylinder is seen to increase almost linearly with Reynolds number for a fixed blockage ratio, but it decreases with increasing blockage ratio for a fixed Reynolds number. As expected, total drag coefficient and its components decrease with increasing value of Reynolds number. However, with increasing blockage ratio, the values of these drag coefficients increase. On the basis of equal projected area, the total drag coefficient for the present flow system is found to be greater than the corresponding drag in the case of the unconfined semicircular cylinder. Similarly, the overall drag in the case of a confined semicircular cylinder is found to be greater than that of a confined circular cylinder for the appropriate range of dimensionless control parameters. The maximum augmentation in heat transfer for blockage ratios of 25% and 50% is found to be approximately 16% and 51% with respect to the corresponding value at the blockage ratio of 16.67% at Reynolds number = 1. Finally, the correlations of wake length, drag coefficient, and average Nusselt number are obtained.

Impact of the Waves on the Sea Surface Roughness under Uniform Wind Conditions: Idealized Cases for Uniform Winds (Part I)

The effect of the surface gravity waves over sea surface roughness length (z0) is investigated from several idealized numerical experiments with the Wave-Watch-III (WW3) model. The WW3 model is combined with a simplified model to estimate z0, CD, u* and U10 as function of the sea state. The impacts related to the presence of the ocean waves over z0 are obtained from conditions of growing (young waves) and mature seas (old waves). The wave spectrum is obtained from WW3 model for each idealized simulation under uniform wind conditions. Uniform wind experiments range from 15 to 45 m/s. The simplified algorithm determines z0, CD, u* and U10 for cases of young waves, old waves and by the Charnock method. The results show that when the ocean is characterized by young waves, both z0 and CD (drag coefficient) increase while U10 is reduced. In Charnock case, the values of z0, CD and U10 have no dependence with the presence of gravity waves. Experiments using winds higher than 30 m/s result in young waves’ CD values higher than the CD value for old waves. Even for young waves CD values are high for cases of strong winds. The results also show that in experiments using winds higher than 30 m/s the dependence between CD and wave age becomes stronger, which is in accordance with other studies.

Numerical investigation on drag reduction with hydrophobic surface by lattice Boltzmann method

In recent years, hydrophobic surface has attracted much attention for its potential applications in flow drag reduction. This article focuses on the drag reduction mechanism of hydrophobic surface by the multi-relaxation-time scheme and the Shan-Chen multiphase model of lattice Boltzmann method. At first, we validate our method through the multiphase cases of wall adhesion effect and the single-phase cases of flow around a square column, showing that the results from our method are in good consistence with those in previous literature. Then, we simulate and analyze the typical problem of flow around a square column with hydrophobic surface while Reynolds number is 100, in order to investigate the influences of contact angle and gas holdup of the inlet flow on drag coefficient and lift coefficient. The simulation results show that for a given contact angle, hydrophobic surface is capable of reducing drag when gas holdup of the inlet flow is in a certain range; otherwise, drag coefficient will increase. With an appropriate gas holdup of the inlet flow, both drag coefficient and lift coefficient will decrease as the contact angle becomes larger. Finally, we compare gas holdup contours and the corresponding streamline patterns under different drag coefficients. Analyses suggest that the increases of drag coefficient and lift coefficient are related to the gas mass shedding near the square column wall where the eddy forms. Increasing the gas holdup of the inflow is properly conducible to reducing the gas mass shedding and also both drag coefficient and lift coefficient greatly if contact angle is too large. However, if the near-wall gas holdup is saturated, it will aggravate the instability of gas holdup and change the near-wall gas holdup a little, which makes drag coefficient increase slightly. When gas holdup of the inlet flow is appropriate, the near-wall gas holdup becomes steadier with a larger contact angle. Through analysis we note that for hydrophobic surface, the key factor of drag reduction is to keep the near-wall gas layer stable, with which the effect of drag reduction becomes better as the contact angle becomes larger. However, the larger the contact angle, the more sensitive to the change of gas holdup both drag coefficient and lift coefficient are, so it is not recommended to adopt the hydrophobic surface with very large contact angle. With the analysis of the gas holdup near hydrophobic surface with different contact angles, in this article we put forward a new approach to the further exploration of the drag reduction mechanism of hydrophobic surface.

Numerical simulation of trailed vortex alleviation through chipped wingtip shapes

The shedding of strong vortices generated during take-off and landing poses serious hazards for the following aircraft. To circumvent the vortex wake hazard, a chipped wingtip shape was suggested, and a series of numerical simulations were conducted to verify the conceptual validity of such shape. Numerical simulation was performed by using the incompressible Navier-Stokes solver with the pressure-based semi-implicit method for pressure linked equations algorithm. The Reynolds stressmodel was employed to evaluatethe turbulence effects.The trailed vortices of the chipped wingtip shapes were found to be more dissipative than those of the baseline wingtip shape. The counter-rotating vortices produced by the chipped wingtip shape can weaken the primary wingtip vortex strength in the far field. Regardless of the slightly increasing the drag, the chipped wingtip shape can successfully mitigate the tangential velocity of the vortices. These types of wingtip shapes had various design parameters, such as the aspect ratio, spacing, and area of each chip. Systematic numerical simulations were conducted to evaluate the effect of these parameters. According to the parametric study results, the aspect ratio was sensitive to the increase in the drag coefficient, and the area was susceptible to the moment coefficient. Evidently, a trade-off relationship existed between the tangential velocity reduction and the drag increase.

A three-dimensional hydrodynamic model for aquaculture: a case study in the Bay of Fundy

Impacts of aquaculture on the local current field and the erosion of the bottom sedi- ment in the Bay of Fundy, Canada, have been investigated with a 3-dimensional hydrodynamic model. The model is evaluated against independent observations of the current. Model results show that the presence of fish cages restricts water flow and reduces the velocity in the surface layer occupied by the cages, but enhances the water velocity in the bottom layer beneath the cages. Sensitivity studies show that the change in the flow velocity beneath the cages is sensitive to variations in the drag coefficient and the height of the fish cages. As the drag coefficient increases, the bottom velocity also increases until a steady state value is reached. For the cage height, however, the tidal speed beneath the cages first increases with cage height and then sig- nificantly decreases with further increasing height. The maximum increase in velocity occurs when the cage height is about half the local water depth (H/H0 = 0.5, where H is the cage height and H0 is the water depth). The increase in bottom velocity significantly speeds up the erosion of the bottom sediment. The model results also indicate that there is an optimal drag coefficient and an optimal cage height for a specific farm site. By utilizing the optimal drag coefficient and height, it is possible to speed up sediment erosion beneath the cages and, thus, decrease the environ - mental problems caused by accumulated fish farm waste.

받음각 변화에 대한 수중익형의 캐비테이션 해석

Cavitation causes a great deal of noise, damage to components, vibrations, and a loss of efficiency in devices, such as propellers, pump impellers, nozzles, injectors, torpedoes, etc. Thus, the cavitating flow simulation is of practical importance for many engineering systems. In the present work, a two-phase flow solver based on the homogeneous mixture model has been developed. The solver employs an implicit preconditioning, dual time steppin g algorithm in curvilinear coordinates. The flow characteristics around Clark-Y hydrofoil were calculated and then validated by comparing with the experimental data. The lift and drag coefficients with changes of angle of attack and cavitation number were obtained. The results show that cavity length and lift, drag coefficient increase with increasing angle of attack.

Simulation of subice shelf melt rates in a general circulation model: Velocity-dependent transfer and the role of friction

Two parameterizations of turbulent boundary layer processes at the interface between an ice shelf and the ocean beneath are investigated in terms of their impact on simulated melt rates and feedbacks. The parameterizations differ in the transfer coefficients for heat and freshwater fluxes. In their simplest form, they are assumed constant and hence are independent of the velocity of ocean currents at the ice shelf base. An augmented melt rate parameterization accounts for frictional turbulence via transfer coefficients that do depend on boundary layer current velocities via a drag law. In simulations with both parameterizations for idealized as well as realistic cavity geometries under Pine Island Ice Shelf, West Antarctica, significant differences in melt rate patterns between the velocity-independent and velocity-dependent formulations are found. While patterns are strongly correlated to those of thermal forcing for velocity-independent transfer coefficients, melting in the case of velocity-dependent coefficients is collocated with regions of high boundary layer currents, in particular where rapid plume outflow occurs. Both positive and negative feedbacks between melt rates, boundary layer temperature, velocities, and buoyancy fluxes are identified. Melt rates are found to increase with increasing drag coefficient inline image, in agreement with plume model simulations, but optimal values of Cd inferred from plume models are not easily transferable. Uncertainties therefore remain, both regarding simulated melt rate spatial distributions and magnitudes.

A concept to reduce vibrations in steel catenary risers by the use of viscoelastic materials

One of the most important issues in steel riser systems is the VIV phenomenon (Vortex Induced Vibration). VIV may induce high levels of structural vibrations and stresses what eventually lead to considerable fatigue damage and reduction in structural lifetime. The use of suppression devices is a feasible solution to tackle this problem. Nevertheless, their disadvantage is the increase in drag coefficient; moreover, it is very expensive. This work proposes and investigates a concept aimed at suppressing vibrations in steel catenary risers by the use of viscoelastic sandwich layers and it is based on three steps. The first one considers the calibration of a viscoelastic constitutive model based on complex shear modulus measurements in a controlled experimental environment. The second one is a model validation scheme applied on the calibrated model provided by the first step and it is based on experimental data obtained out of a 1.6m long tubular sandwich structure. The third step assesses the effectiveness of viscoelastic sandwich layers using a small scale model of a steel catenary riser installed in P18 platform which is in operation in Campos Basin, Brazil. The results are quite compelling due to the great increase of damping that was observed.

Modelling of Drag Forces on a Closed Flexible Fish Cage

To cope with ecological challenges in the aquaculture industry, Closed Flexible Fish Cages (CFC) are proposed used in the sea. However, the existing knowledge about how the CFC will respond to external sea loads are limited. More knowledge is needed to understand the response of the cage if this technology is to be utilized in an industrial scale. In this paper a new method for mathematical modelling of the increase in drag, for decreasing filling level of a CFC is proposed. A model for a filling-level-dependent drag coefficient is presented. Experimental data are analysed related to forces and deformations on the bag for different filling levels. The analysed bag showed an increased tendency to deform for decreasing filling levels, leading to an increase in drag coefficient.

Influence of sea spray droplets on drag coefficient in high wind speed

Sea spray droplets change the distribution of air-sea momentum fluxes in the sea surface, and then influence sea surface drag coefficient considerably. In order to estimate the influence of sea spray droplets on drag coefficient exactly, a new sea spray generation function (SSGF) which depends on wind and wave, is deduced. Then the SSGF is integrated into the spray momentum flux formula to calculate the spray momentum flux. The result shows that revised spray momentum flux formula are sensitive to the wave state remarkably. It indicates that the new sea spray flux formulae can be used in any wave state. The total momentum flux of sea surface contains sea spray momentum flux and air-sea interface momentum flux. Based on this theory, the formula of drag coefficient which is influenced by sea spray droplets at high wind speed is acquired. The theoretical result of the drag coefficient formula shows that the drag coefficient reduces at the high wind speed, which indicates that the sea spray droplets can restrict the increase of drag coefficient at high wind speed. Comparison between theoretical value and measured value of sea surface drag coefficient in field and laboratory shows that the theoretical value of drag coefficient cover the measured value almost. Meanwhile, with the new sea surface drag coefficient, ocean wave model can model the significant wave height very well in typhoon condition. All the results show that the new drag coefficient can be used properly at high wind speed.

3D modeling and Computational Fluid Dynamics simulations of surface-attached CHO-K1 cells going to detach from a microchannel wall

Abstract A series of steady-state Computational Fluid Dynamics simulations has been carried out to estimate the liquid flow forces acting on Chinese Hamster Ovary-K1 cells, which are necessary to detach the cells from a non-coated wall surface in a microchannel. The simulation parameters were based on experimental work of Zhang et al. [16] . Simulations were performed for cell sizes of 5 μm, 12 μm and 20 μm with three simply modeled cell shapes respectively. Additionally, a drag coefficient for each modeled cell shape was estimated. The simulation results indicate that the surface-averaged total forces, which are necessary to detach those cells, were in the nanonewton range and increase with cell size. For the force components along and perpendicular to the direction of the flow, the viscous forces accounted for the major proportion. Concerning the velocity gradients, the local energy dissipation rates reach values of at least 4e + 05 W/m 3 at the top of the cell surfaces for each modeled cell shape. The drag coefficient calculations, with Reynolds numbers smaller than 1, have shown an increasing drag coefficient with a decreasing modeled cell size.

Wave mixed, wind-generated near-surface shear observed over a tidal flat

Around high tide over a flooded tidal flat, vertical shear measured 0.13–0.4m beneath the sea surface by upward-looking Acoustic Doppler Profilers (ADPs) was usually aligned with the wind. However, shear did not increase with increasing windspeed, likely owing to an increase in wave heights and wave-generated mixing with increasing windspeed. The associated increase in water-side drag coefficient with windspeed was predicted, with substantial scatter, by a model for near-surface mixing by breaking waves. A depth-independent eddy-viscosity model yielded only slightly less skill than a fitted depth-dependent model. Farther (0.3–0.6m) from the surface, model skill was low, suggesting that shear at depth was often determined by factors other than wind. Wave heights tended to increase with increasing water depth. Consequently, the simulated wave-generated surface eddy viscosity tended to increase with increasing depth.

Effects of surfactant on terminal velocity of a Taylor bubble in a vertical pipe

Effects of soluble surfactant on the terminal velocity of a Taylor bubble rising through a vertical pipe are investigated using an interface tracking method. A level set method is utilized to track the interface. Transport of surfactant in the bulk liquid and at the interface is taken into account. The amount of adsorption and desorption is evaluated using the Frumkin and Levich model. The normal component of surface tension force is computed using a ghost fluid method, whereas the tangential component, i.e., the Marangoni force, is evaluated by making use of the continuum surface force model. Simulations of small air bubbles contaminated with soluble surfactant are carried out for validation. The Marangoni effects on the bubbles, i.e., the surface immobilization and the increase in drag coefficient, are well predicted. Then Taylor bubbles rising through vertical pipes filled with contaminated water at a low Morton number are simulated for various Eötvös numbers, various bulk surfactant concentrations and two different surfactants, i.e., 1-pentanol and Triton X-100. As a result, the following conclusions are obtained: (1) the reduction of surface tension near the bubble nose is the cause of the increase in terminal velocity, (2) the surfactant does not affect the terminal velocities of high Eötvös number bubbles since the bubbles at high Eötvös numbers are independent of surface tension, (3) the terminal velocity of a low Morton number Taylor bubble can be evaluated by making use of available correlations for clean Taylor bubbles, provided that the degree of contamination near the bubble nose is known and the Marangoni effect in the nose region is negligible, and (4) the Hatta number, which is the ratio of the adsorption velocity to the bubble velocity, is a primary factor governing the degree of contamination in the bubble nose region.