Showing papers on "Drag coefficient published in 2018"
01 Apr 2018
TL;DR: It is proved that the dynamical model of a quadrotor subject to linear rotor drag effects is differentially flat in its position and heading, and a method based on a gradient-free optimization to identify the rotor drag coefficients, which are required to compute the feedforward control terms.
Abstract: In this paper, we prove that the dynamical model of a quadrotor subject to linear rotor drag effects is differentially flat in its position and heading. We use this property to compute feed-forward control terms directly from a reference trajectory to be tracked. The obtained feed-forward terms are then used in a cascaded, nonlinear feedback control law that enables accurate agile flight with quadrotors. Compared to state-of-the-art control methods, which treat the rotor drag as an unknown disturbance, our method reduces the trajectory tracking error significantly. Finally, we present a method based on a gradient-free optimization to identify the rotor drag coefficients, which are required to compute the feed-forward control terms. The new theoretical results are thoroughly validated trough extensive comparative experiments.
235 citations
TL;DR: In this article, the authors investigated the effect of combinational opposing jet and spike concept on the aerodynamic drag and heat properties of a single spiked blunt body, and showed that the combinatorial thermal protection system has a great contribution to reduce the drag.
126 citations
TL;DR: In this article, the drag, lift and torque coefficients of non-spherical particles were determined using the lattice Boltzmann method with multi-relaxation time in the Reynolds number range 0.1 to 2.1 for different incident angles ϕ.
110 citations
TL;DR: The design space for wing design optimization with a fixed planform is largely convex, with a very small flat region that is multimodal because of numerical errors, but this region is so small, and the differences in drag so minor, that the design space can be considered unimodal for all practical purposes.
98 citations
TL;DR: This article designs the shroud and turbine shape as well as flange height through an optimization process that seeks to maximize the power while minimizing drag and thrust forces and demonstrates that the resulting optimal designs yield significant improvements in the output power coefficient.
75 citations
TL;DR: In this paper, the authors measured the terminal fall velocities of calcareous sand particles settling in liquids of different densities and viscosities, and correlated the drag coefficient with the shape descriptor and the particle Reynolds number.
65 citations
64 citations
TL;DR: In this article, the authors provide a systematic experimental study on the wake of two tandem cylinders of unequal diameters, including the time-mean drag coefficient, fluctuating drag and lift coefficients, and the Strouhal number.
Abstract: This work aims to provide a systematic experimental study on the wake of two tandem cylinders of unequal diameters. The fluid dynamics around a circular cylinder of diameter placed in the wake of another circular cylinder with a smaller diameter of is investigated, including the time-mean drag coefficient ( ), the fluctuating drag and lift coefficients ( and ), the Strouhal number ( ) and the flow structures. The Reynolds number based on is kept constant at . The ratios and vary from 0.2 to 1.0 and 1.0 to 8.0 respectively, where is the distance from the upstream cylinder centre to the forward stagnation point of the downstream cylinder. The ratios and are found, based on extensive hotwire, particle imaging velocimetry, pressure and flow visualization measurements, to have a marked influence on the wake dynamics behind the cylinders. As such, the flow is classified into the reattachment and co-shedding flow regimes, the latter being further subdivided into the lock-in, subharmonic lock-in and no lock-in regions. It is found that the critical spacing that divides the two regimes is dictated by the upstream-cylinder vortex formation length and becomes larger for smaller . The characteristic flow properties are documented in each regime and subdivided region, including the flow structure, , wake width, vortex formation length and the lateral width between the two gap shear layers. The variations in , , and the pressure distribution around the downstream cylinder are connected to the flow physics.
62 citations
TL;DR: In this article, the authors investigated the aerodynamic performances of four trains with different nose lengths (4, 7, 9 and 12 m) under strong crosswind using the detached eddy simulation (DES) method.
61 citations
53 citations
TL;DR: Atmospheric turbulence measurements made at the U.S. Army Corps of Engineers Field Research Facility (FRF) located on the Atlantic coast near the town of Duck, North Carolina during the CASPER-East Program (October-November 2015) are used to study air-sea/land coupling in the FRF coastal zone as discussed by the authors.
Abstract: Atmospheric turbulence measurements made at the U.S. Army Corps of Engineers Field Research Facility (FRF) located on the Atlantic coast near the town of Duck, North Carolina during the CASPER-East Program (October–November 2015) are used to study air–sea/land coupling in the FRF coastal zone. Turbulence and mean meteorological data were collected at multiple levels (up to four) on three towers deployed at different landward distances from the shoreline, with a fourth tower located at the end of a 560-m-long FRF pier. The data enable comparison of turbulent fluxes and other statistics, as well as investigations of surface-layer scaling for different footprints, including relatively smooth sea-surface conditions and aerodynamically rough dry inland areas. Both stable and unstable stratifications were observed. The drag coefficient and diurnal variation of the sensible heat flux are found to be indicators for disparate surface footprints. The drag coefficient over the land footprint is significantly greater, by as much as an order of magnitude, compared with that over the smooth sea-surface footprint. For onshore flow, the internal boundary layer in the coastal zone was either stable or (mostly) unstable, and varied dramatically at the land-surface discontinuity. The offshore flow of generally warm air over the cooler sea surface produced a stable internal boundary layer over the ocean surface downstream from the coast. While the coastal inhomogeneities violate the assumptions underlying Monin–Obukhov similarity theory (MOST), any deviations from MOST are less profound for the scaled standard deviations and the dissipation rate over both water and land, as well as for stable and unstable conditions. Observations, however, show a poor correspondence with MOST for the flux-profile relationships. Suitably-averaged, non-dimensional profiles of wind speed and temperature vary significantly among the different flux towers and observation levels, with high data scatter. Overall, the statistical dependence of the vertical gradients of scaled wind speed and temperature on the Monin–Obukhov stability parameter in the coastal area is weak, if not non-existent.
TL;DR: In this paper, a generic CD-KC relation for both wave-only and wave-current flows is proposed using direct measurement approach and a detailed comparison of these two approaches are given.
TL;DR: In this paper, the authors used the advanced CFD software STAR-CCM+ where the overset method is used so that the mesh local to the body moves within a stationary outer mesh.
TL;DR: For low submergence canopies, a universal scaling law is found that relates friction factor with canopy permeability and a rescaled bulk Reynolds number, which provides a valuable tool to assess habitats sustainability associated with hydro-dynamical conditions.
Abstract: Flow resistance caused by vegetation is a key parameter to properly assess flood management and river restoration. However, quantifying the friction factor or any of its alternative metrics, e.g. the drag coefficient, in canopies with complex geometry has proven elusive. We explore the effect of canopy morphology on vegetated channels flow structure and resistance by treating the canopy as a porous medium characterized by an effective permeability, a property that describes the ease with which water can flow through the canopy layer. We employ a two-domain model for flow over and within the canopy, which couples the log-law in the free layer to the Darcy-Brinkman equation in the vegetated layer. We validate the model analytical solutions for the average velocity profile within and above the canopy, the volumetric discharge and the friction factor against data collected across a wide range of canopy morphologies encountered in riverine systems. Results indicate agreement between model predictions and data for both simple and complex plant morphologies. For low submergence canopies, we find a universal scaling law that relates friction factor with canopy permeability and a rescaled bulk Reynolds number. This provides a valuable tool to assess habitats sustainability associated with hydro-dynamical conditions.
TL;DR: In this article, the authors present three-dimensional numerical simulations of shock propagation in air over a random bed of particles, where the flow is inviscid and governed by the Euler equations of gas dynamics, and the simulation is carried out by varying the volume fraction of the particle bed at a fixed shock Mach number.
Abstract: Propagation of a strong incident shock through a bed of particles results in complex wave dynamics such as a reflected shock, a transmitted shock, and highly unsteady flow inside the particle bed. In this paper we present three-dimensional numerical simulations of shock propagation in air over a random bed of particles. We assume the flow is inviscid and governed by the Euler equations of gas dynamics. Simulations are carried out by varying the volume fraction of the particle bed at a fixed shock Mach number. We compute the unsteady inviscid streamwise and transverse drag coefficients as a function of time for each particle in the random bed for different volume fractions. We show that (i) there are significant variations in the peak drag for the particles in the bed, (ii) the mean peak drag as a function of streamwise distance through the bed decreases with a slope that increases as the volume fraction increases, and (iii) the deviation from the mean peak drag does not correlate with local volume fraction. We also present the local Mach number and pressure contours for the different volume fractions to explain the various observed complex physical mechanisms occurring during the shock–particle interactions. Since the shock interaction with the random bed of particles leads to transmitted and reflected waves, we compute the average flow properties to characterize the strength of the transmitted and reflected shock waves and quantify the energy dissipation inside the particle bed. Finally, to better understand the complex wave dynamics in a random bed, we consider a simpler approximation of a planar shock propagating in a duct with a sudden area change. We obtain Riemann solutions to this problem, which are used to compare with fully resolved numerical simulations.
TL;DR: In this paper, a coupling conjugate heat transfer (CHT) approach has been applied to investigate the thermal protection, which takes the heat transfer of structure into consideration, and the influence of the spike length ratio, lateral jet pressure ratio and lateral jet location on the drag and heat reduction performance is analyzed comprehensively.
TL;DR: In this paper, the dynamics of light spheres rising freely under buoyancy in a large expanse of viscous fluid at rest at infinity is investigated numerically, and a comprehensive map of the rise regimes encountered up to Reynolds numbers is set up by varying independently the body-to-fluid density ratio and the relative magnitude of inertial and viscous effects in about $250$ distinct combinations.
Abstract: The dynamics of light spheres rising freely under buoyancy in a large expanse of viscous fluid at rest at infinity is investigated numerically. For this purpose, the computational approach developed by Mougin \& Magnaudet (Int. J. Multiphase Flow, 28:1837-1851, 2002) is improved to account for the instantaneous viscous loads induced by the translational and rotational sphere accelerations, which play a crucial role in the dynamics of very light spheres. A comprehensive map of the rise regimes encountered up to Reynolds numbers (based on the sphere diameter and mean rise velocity) of the order of $10^3$ is set up by varying independently the body-to-fluid density ratio and the relative magnitude of inertial and viscous effects in about $250$ distinct combinations. These computations confirm or reveal the presence of several distinct periodic regions on the route to chaos, most of which only exist within a finite range of the sphere relative density and Reynolds number. The wake structure is analyzed in these various regimes, evidencing the existence of markedly different shedding modes according to the style of path. The variations of the drag force with the flow parameters is also examined, revealing that only one of the styles of path specific to very light spheres yields a non-standard drag behaviour, with drag coefficients significantly larger than those measured on a fixed sphere under equivalent conditions. The outcomes of this investigation are compared with available experimental and numerical results, evidencing points of consensus and disagreement.
TL;DR: In this paper, boundary layer analysis of nanofluid flow over a bidirectional exponentially stretching sheet in the presence of transverse magnetic field and also in convective condition is studied.
Abstract: The principle aim of the present investigation is to study the boundary layer analysis of nanofluid flow over a bidirectional exponentially stretching sheet in the presence of transverse magnetic field and also in convective condition. The effects of Brownian motion and thermophoretic diffusion of nanoparticle are considered from the mathematical model. Governing partial differential equations are reduced into coupled non-linear ordinary differential equations using suitable similarity transformations, further the system of equations are solved by a new spectral relaxation method. Validation of the results is achieved by comparison with emitting case from previous studies in the literature. Also, it has been shown that the convergence rate of the spectral relaxation method is significantly improved by using the method in conjunction with the successive over relaxation method. The results reveal the existence of interesting Sparrow–Gregg-type Hills for temperature distribution pertinent to some range of parametric values. Moreover the numerical data of drag coefficient, local heat and mass transfer rates are evaluated and analyzed. Effects of local Biot number on temperature and concentration profiles are qualitatively similar. Both the temperature and concentration profiles are enhanced for higher values of local Biot number.
TL;DR: In this article, an experimental investigation is conducted on the flow properties of tandem cylinders for ranges of diameter ratio d/D = 0.25-1.0, spacing ratio L/d = 5.5-20, and Reynolds number Re =0.8 × 104-2.42 × 104, where d and D are the diameters of the upstream and downstream cylinders, respectively, L is the distance from the upstream cylinder center to the forward stagnation point of the downstream one.
Abstract: An experimental investigation is conducted on the ow around tandem cylinders for ranges of diameter ratio d/D = 0.25–1.0, spacing ratio L/d = 5.5–20, and Reynolds number Re = 0.8 × 104–2.42 × 104, where d and D are the diameters of the upstream and downstream cylinders, respectively, L is the distance from the upstream cylinder center to the forward stagnation point of the downstream one. The focus is given on examining the effects of d/D, L/d and Re on Strouhal number St, flow structures and fluid forces measured using hotwire, particle image velocimetry (PIV) and load cell measurement techniques, respectively. Changes in d/D and L/d in the ranges examined lead to five flow regimes, namely lock-in, intermittent lock-in, no lock-in, subharmonic lock-in and shear-layer reattachment regimes. Time-mean drag coefficient (CD) and fluctuating drag and lift coefficients (CD' and CL′ ) are more sensitive to L/d than d/D. The scenario is opposite for St where d/D is more prominent than L/d to change the St. The detailed facet of the dependence on d/D and L/d of CD, CD′, CL′ and St is discussed based on shear-layer velocity, approaching velocity, vortex formation length, and wake width.
TL;DR: In this paper, heat and momentum transfer across the wind-driven breaking air-water interface at extremely high wind speeds was experimentally investigated using a high-speed wind-wave tank.
Abstract: Heat and momentum transfer across the wind-driven breaking air–water interface at extremely high wind speeds was experimentally investigated using a high-speed wind-wave tank. An original m...
TL;DR: In this paper, numerical simulations are carried out on the flow over a realistic generic car geometry, the DrivAer-fastback car model, and compared to numerical and experimental results to assess the validity of these approaches when solving the flow field around complex automotive geometries.
TL;DR: In this article, the authors improved existing correlations for the drag coefficient and averaged Nusselt number of an ellipsoidal particle in a fluid by additionally considering its orientation by changing the shape (0.25 ≤ Ar ≤ 2.5) and incident angle (0°≤ θ≤ 90°) of the solid particle as well as the Reynolds number (10 ≤ Re≤ 200).
TL;DR: In this article, an experimental investigation of flow control over a square cylinder using an attached splitter plate was conducted at a Reynolds number of 485, where the splitter was kept constant at 10% of the cylinder width.
TL;DR: In this paper, the influence of the blockage ratio in three wind tunnel tests conducted on a small vertical-axis Darrieus wind turbine was quantitatively investigated, and a new correction coefficient was proposed for the wind turbine from the measured drag coefficients obtained at three different blockage ratios.
TL;DR: In this article, a thermal state model experimental study was performed to investigate the drag characteristic and thermal performance of non-uniform fillings for wet cooling towers under crosswind conditions, several valuable performance parameters, including cooling efficiency, drag coefficient, ventilation rate, heat transfer coefficient and Merkel number, etc.
TL;DR: The design aspects of force balance measurement system for aerodynamic vehicle flying at hypersonic speed, where additional aerodynamic heating creates difficulty in the system and needs change in the design geometry and material type is mainly focused on.
TL;DR: In this paper, a mean momentum equation based analysis of polymer drag-reduced channel flow is performed to evaluate the redistribution of mean momentum and the mechanisms underlying the redistribution processes, and it is shown that the state of maximum drag reduction is attained only after the inertial sublayer is eradicated, which coincides with the loss of a region of logarithmic dependence in the mean profile.
Abstract: Mean momentum equation based analysis of polymer drag-reduced channel flow is performed to evaluate the redistribution of mean momentum and the mechanisms underlying the redistribution processes. Similar to channel flow of Newtonian fluids, polymer drag-reduced channel flow is shown to exhibit a four layer structure in the mean balance of forces that also connects, via the mean momentum equation, to an underlying scaling layer hierarchy. The self-similar properties of the flow related to the layer hierarchy appear to persist, but in an altered form (different from the Newtonian fluid flow), and dependent on the level of drag reduction. With increasing drag reduction, polymer stress usurps the role of the inertial mechanism, and because of this the wall-normal position where inertially dominated mean dynamics occurs moves outward, and viscous effects become increasingly important farther from the wall. For the high drag reduction flows of the present study, viscous effects become non-negligible across the entire hierarchy and an inertially dominated logarithmic scaling region ceases to exist. It follows that the state of maximum drag reduction is attained only after the inertial sublayer is eradicated. According to the present mean equation theory, this coincides with the loss of a region of logarithmic dependence in the mean profile.
TL;DR: In this paper, the hydrodynamic coefficients of multiple heave plates are studied for offshore structures to reduce heave responses in oscillating flows, and the added mass and drag coefficients for a single and double heave plate with circular, octagonal and square cross-sections are proposed to cover a wide range of application.
TL;DR: In this paper, the aerodynamic characteristics of small trees are compared with the ones measured on larger mature trees from previous studies, and it is shown that the drag coefficients of model and natural trees are similar only if both types have similar aerodynamic porosity and if the model tree can undergo an aerodynamic reconfiguration similar to that of a natural tree.
TL;DR: Simulations and experiments using a quadrotor in both hovering and flight conditions have validated the proposed wind velocity estimation method, which derives the wind disturbance from rotors’ speeds and rotorcraft’s acceleration and position.
Abstract: Wind velocity (strength and direction) is an important parameter for unmanned aerial vehicle (UAV)-based environmental monitoring tasks. A novel wind velocity estimation method is proposed for rotorcrafts. Based on an extended state observer, this method derives the wind disturbance from rotors’ speeds and rotorcraft’s acceleration and position. Then the wind disturbance is scaled to calculate the airspeed vector, which is substituted into a wind triangle to obtain the wind velocity. Easy-to-implement methods for calculating the rotorcraft’s thrust and drag coefficient are also proposed, which are important parameters to obtain the wind drag and the airspeed, respectively. Simulations and experiments using a quadrotor in both hovering and flight conditions have validated the proposed method.