Showing papers on "Drag coefficient published in 2016"

Turbulent drag reduction over air- and liquid- impregnated surfaces

Abstract: Results on turbulent skin friction reduction over air- and liquid-impregnated surfaces are presented for aqueous Taylor-Couette flow. The surfaces are fabricated by mechanically texturing the inner cylinder and chemically modifying the features to make them either non-wetting with respect to water (air-infused, or superhydrophobic case), or wetting with respect to an oil that is immiscible with water (liquid-infused case). The drag reduction, which remains fairly constant over the Reynolds number range tested (100 ≤ Reτ ≤ 140), is approximately 10% for the superhydrophobic surface and 14% for the best liquid-infused surface. Our results suggest that liquid-infused surfaces may enable robust drag reduction in high Reynolds number turbulent flows without the shortcomings associated with conventional superhydrophobic surfaces, namely, failure under conditions of high hydrodynamic pressure and turbulent flow fluctuations.

Multipoint Aerodynamic Shape Optimization Investigations of the Common Research Model Wing

Abstract: The aerodynamic shape optimization of wings in transonic flow is an inherently challenging problem. In addition to the high computational cost of solving the Reynolds-averaged Navier–Stokes equations, there is a complex interdependence between the cross-sectional shape, wave drag, and viscous effects. Furthermore, it is necessary to perform multipoint optimizations to ensure good performance for a range of flight conditions. The choice of which flight conditions should be considered in a multipoint optimization and how many of these should be considered is still not well understood. This paper addresses this issue by solving a series of seven benchmark optimizations developed by the AIAA Aerodynamic Optimization Discussion Group. These optimization cases include a single-point optimization, four three-point optimizations, a nine-point optimization, and a five-point optimization. The optimizations consist in minimizing the weighted drag coefficient subject to lift, moment, thickness, and volume constraints...

Sustained drag reduction in a turbulent flow using a low-temperature Leidenfrost surface

Abstract: Skin friction drag contributes a major portion of the total drag for small and large water vehicles at high Reynolds number (Re). One emerging approach to reducing drag is to use superhydrophobic surfaces to promote slip boundary conditions. However, the air layer or “plastron” trapped on submerged superhydrophobic surfaces often diminishes quickly under hydrostatic pressure and/or turbulent pressure fluctuations. We use active heating on a superhydrophobic surface to establish a stable vapor layer or “Leidenfrost” state at a relatively low superheat temperature. The continuous film of water vapor lubricates the interface, and the resulting slip boundary condition leads to skin friction drag reduction on the inner rotor of a custom Taylor-Couette apparatus. We find that skin friction can be reduced by 80 to 90% relative to an unheated superhydrophobic surface for Re in the range 26,100 ≤ Re ≤ 52,000. We derive a boundary layer and slip theory to describe the hydrodynamics in the system and show that the plastron thickness is h = 44 ± 11 μm, in agreement with expectations for a Leidenfrost surface.

Influence of afterbody rounding on the pressure distribution over a fastback vehicle

Abstract: Experimental analyzes were performed to understand the drag evolution and the flow field modifications resulting from afterbody rounding on the Ahmed body, a simplified vehicle model with 25 degrees rear slant. Curvature effects were investigated using balance measurements, flow visualizations, wall pressure, and particle image velocimetry measurements. The rear end of the original well-known Ahmed body has sharp connections between the roof and the rear window as well as squared rear pillars. Similarly to previous studies, rounding the roof/backlight intersection was shown to reduce drag up to 16 %. Surprisingly, additional rear curvature associated with side pillar rounding did not further modify the drag. However, the zero net effect was found to result from opposite drag effects on the slanted and vertical surfaces and to hide strong local modifications on the flow field. The tridimensional organization and vorticity transport in the near wake were analyzed and connected to the observed local increase in the pressure drag on the base. Finally, these results were shown to be generic for a realistic rounded-end car shape.

Direct numerical simulations of dynamic gas-solid suspensions

Abstract: Direct numerical simulation results for gas flow through dynamic suspensions of spherical particles is reported. The simulations are performed using an immersed boundary method, with careful correction for the grid resolution effect. The flow systems we have studied vary with mean flow Reynolds number, solids volume fraction, as well as particle/gas density ratio. On the basis of the simulation results, the effect of particle mobility on the gas-solid drag force is analyzed and introduced into the existing drag correlation that was derived from simulations of stationary particles. This mobility effect is characterized by the granular temperature, which is a result of the particle velocity fluctuation. The modified drag correlation is considered so-far the most accurate expression for the interphase momentum exchange in computational fluid dynamics models, in which the gas-solid interactions are not directly resolved. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1958–1969, 2016

Investigation on subsonic to supersonic flow around a sphere at low Reynolds number of between 50 and 300 by direct numerical simulation

Abstract: In this study, analysis of flow properties around a sphere and its aerodynamic coefficients in the high-Mach-and-low-Reynolds-numbers conditions is carried out by direct numerical simulations solving the three-dimensional compressible Navier–Stokes equations. The calculation is performed on a boundary-fitted coordinate system with a high-order scheme of sufficient accuracy. The analysis is conducted by assuming a rigid sphere with a Reynolds number of between 50 and 300, based on the diameter of the sphere and the freestream velocity and a freestream Mach number of between 0.3 and 2.0, together with the adiabatic wall boundary condition. The calculation shows the following yields: (1) unsteady fluctuation of hydrodynamic forces become smaller as the Mach number increases under the same Reynolds number condition, (2) the drag coefficient increases with the Mach number due to an increase in the pressure drag by the shock wave, and (3) an accurate prediction of the drag coefficient in the supersonic regime using traditional models might be difficult.

Estimating the impact of the changes in land use and cover on the surface wind speed over the East China Plain during the period 1980–2011

Abstract: Long-term changes in surface wind speed (SWS) are influenced by both large-scale circulation and relative resistance. The effects of large-scale circulation are embodied by the pressure-gradient force (PGF), which is mostly a natural factor, whereas the resistance is due to the drag between the air and the surface as well as in the different boundary layers, which is mainly caused by the anthropogenic land use and cover change (LUCC). We performed experiments using a simple dynamical method in which a balance among the PGF, Coriolis force, and drag is reached to separate the effects of the PGF and LUCC on the SWS, and then, to quantitatively estimate the influence of the LUCC on the SWS over the East China Plain (ECP) during the period 1980–2011. The results show a distinct decrease in the SWS in the station observation data with a rate of −0.13 m s−1 (10 year)−1, but there is no statistically significant long-term trend in the reanalysis data. At the same time, the drag coefficient induced by the LUCC shows an increasing trend, which is consistent with the 30 % increase in the rate of urbanization during the study period. In addition, the PGF fluctuates with distinct seasonal and interannual changes, and it has an insignificant long-term increasing trend during the period 1980–2011. At the same time, the spatial distribution of the linear trend coefficient of the normalized PGF is inconsistent with that of the SWS, but the linear trend coefficient of the normalized drag coefficient shows a similar spatial distribution as the SWS. Therefore, the increase in the drag coefficient induced by the LUCC should account for the long-term decrease in the SWS. The difference between the model wind speed, in which the drag coefficient is constrained to its value in the year 1980, and the observed wind speed at each station (SWSD) can reflect the influence of the LUCC on the SWS. Furthermore, the long-term changes in East Asian monsoons may not completely account for the observed wind speed decrease near the surface in the ECP region, but it is an important factor in the SWS.

Sedimentation of finite-size spheres in quiescent and turbulent environments

Abstract: Sedimentation of a dispersed solid phase is widely encountered in applications and environmental flows, yet little is known about the behaviour of finite-size particles in homogeneous isotropic turbulence. To fill this gap, we perform direct numerical simulations of sedimentation in quiescent and turbulent environments using an immersed boundary method to account for the dispersed rigid spherical particles. The solid volume fractions considered are , while the solid to fluid density ratio . The particle radius is chosen to be approximately six Kolmogorov length scales. The results show that the mean settling velocity is lower in an already turbulent flow than in a quiescent fluid. The reductions with respect to a single particle in quiescent fluid are approximately 12 % and 14 % for the two volume fractions investigated. The probability density function of the particle velocity is almost Gaussian in a turbulent flow, whereas it displays large positive tails in quiescent fluid. These tails are associated with the intermittent fast sedimentation of particle pairs in drafting–kissing–tumbling motions. The particle lateral dispersion is higher in a turbulent flow, whereas the vertical one is, surprisingly, of comparable magnitude as a consequence of the highly intermittent behaviour observed in the quiescent fluid. Using the concept of mean relative velocity we estimate the mean drag coefficient from empirical formulae and show that non-stationary effects, related to vortex shedding, explain the increased reduction in mean settling velocity in a turbulent environment.

Model Predictive Control for Hybrid Electric Vehicle Platooning Using Slope Information

Abstract: This paper presents a new model predictive control (MPC) system for hybrid electric vehicle (HEV) platooning using slope information to improve fuel economy. The new features of this study are as follows. First, a system for HEV platooning has been developed considering varying drag coefficients and road gradients. Second, the general model of the aerodynamic drag coefficient of different vehicles in a platoon is developed. Third, simulations and analysis (under different parameters, i.e., road conditions, prediction horizon, vehicle state of charge, etc.) are conducted to verify the effectiveness of the method to achieve higher fuel efficiency. Fourth, the spacing between the vehicles in the platoon is designed in the objective function to ensure driving safety. The MPC problem is solved using a discrete numerical computation method: the continuation and generalized minimum residual method.

CFD-PBM approach with modified drag model for the gas–liquid flow in a bubble column

Abstract: In this work, numerical simulations of cylindrical bubble column are performed using the Euler–Euler approach incorporated with a population balance model (PBM). First, three drag models and their corresponding modified models with the wake acceleration are incorporated into the coupled approach in order to evaluate the effectiveness of these drag models. The simulated time-averaged local gas holdups and normalized axial liquid velocities using different drag equations are compared with the experimental data, showing that only the PBM-customized drag model with the wake acceleration (cf., the application of a correction factor) can reproduce the measured flow field data. Subsequently, the applicability of the coupled approach with the effective drag model is further evaluated at various superficial gas velocities and gas distributors. The simulated results accord well with the experimental data at high gas velocities. However, the model greatly underestimates the radial local gas holdup and the total gas holdup at low gas flow rates. Additionally, the simulated results demonstrate that the opening area and orifice geometry play a significant role in total aeration and the triple-ring gas distributor produces more uniform radial profiles of local gas holdup and normalized liquid velocity than the multi-orifice one, thus leading to poor mixing efficiency in the bubble column.

Hydrodynamics of sediment threshold

Abstract: A novel hydrodynamic model for the threshold of cohesionless sediment particle motion under a steady unidirectional streamflow is presented. The hydrodynamic forces (drag and lift) acting on a solitary sediment particle resting over a closely packed bed formed by the identical sediment particles are the primary motivating forces. The drag force comprises of the form drag and form induced drag. The lift force includes the Saffman lift, Magnus lift, centrifugal lift, and turbulent lift. The points of action of the force system are appropriately obtained, for the first time, from the basics of micro-mechanics. The sediment threshold is envisioned as the rolling mode, which is the plausible mode to initiate a particle motion on the bed. The moment balance of the force system on the solitary particle about the pivoting point of rolling yields the governing equation. The conditions of sediment threshold under the hydraulically smooth, transitional, and rough flow regimes are examined. The effects of velocity fluctuations are addressed by applying the statistical theory of turbulence. This study shows that for a hindrance coefficient of 0.3, the threshold curve (threshold Shields parameter versus shear Reynolds number) has an excellent agreement with the experimental data of uniform sediments. However, most of the experimental data are bounded by the upper and lower limiting threshold curves, corresponding to the hindrance coefficients of 0.2 and 0.4, respectively. The threshold curve of this study is compared with those of previous researchers. The present model also agrees satisfactorily with the experimental data of nonuniform sediments.

An Exploration of Wind Stress Calculation Techniques in Hurricane Storm Surge Modeling

Abstract: As hurricanes continue to threaten coastal communities, accurate storm surge forecasting remains a global priority. Achieving a reliable storm surge prediction necessitates accurate hurricane intensity and wind field information. The wind field must be converted to wind stress, which represents the air-sea momentum flux component required in storm surge and other oceanic models. This conversion requires a multiplicative drag coefficient for the air density and wind speed to represent the air-sea momentum exchange at a given location. Air density is a known parameter and wind speed is a forecasted variable, whereas the drag coefficient is calculated using an empirical correlation. The correlation’s accuracy has brewed a controversy of its own for more than half a century. This review paper examines the lineage of drag coefficient correlations and their acceptance among scientists.