Local Force Coefficients Research Articles

Streamwise vortices, cellular shedding and force coefficients on finite wing at low Reynolds number

Numerical simulations for flow past a finite rectangular wing with a NACA 0012 section at $Re=1000$ for various semi-aspect ratios ( $0.25\le sAR \le 7.5$ ) over a range of angles of attack ( $0^{\circ }\le \alpha \le 14^{\circ }$ ) reveal streamwise vortices, which increase in strength and number to occupy an increasing spanwise extent with increase in $\alpha$ . They result in non-monotonic spanwise variation of local force coefficients and increased strength of wing-tip vortex for $\alpha >8^{\circ }$ . Viscous and pressure drag dominate for low and high sAR, respectively. The time-averaged drag coefficient first decreases and then increases with increase in $sAR$ . Vortex shedding for $\alpha =14^{\circ }$ is single cell and parallel for $sAR<3$ . Shedding is in two cells with an oblique angle that varies with time, leading to large spanwise variation in the root mean square of local force coefficients for higher $sAR$ . Various types of dislocations, reported earlier in wakes of bluff bodies, are seen for different $\alpha$ and $sAR$ . Dislocations for $\alpha =14^{\circ }$ appear at the same spanwise location for $sAR=3$ and at different spanwise locations for $sAR\ge 4$ . Vortex shedding for $\alpha =12^{\circ }$ and $sAR=5$ exhibits one cell structure in the near wake and two cells in the far wake due to splitting and reconnection of vortices near the mid-span in the moderate wake. Linkages form between counter-rotating spanwise vortices for $sAR\ge 1$ . Additional linkages between shed- and wing-tip vortices are observed for lower $sAR$ . At each $\alpha$ , the strength of the wing-tip vortex and radius of its core, estimated using Rankine and Lamb–Oseen models, increases up to a certain $sAR$ beyond which it is approximately constant.

Aeroelastic and Aerodynamic Tests of Wind Turbine with Various Polygonal Towers

Traditionally, circular cross-section towers have been used as supporting systems of wind turbines, but weaknesses have become apparent with recent upsizing of wind turbines. Thus, polygonal cross-section towers have been proposed and used in Europe. In this study, the effects of polygonal cross-sections on the aeroelastic and aerodynamic characteristics of wind turbines were examined through a series of wind tunnel tests. Aeroelastic tests showed that a square cross-section tower showed instability vibrations, and polygonal cross-section towers showed limited vibrations for tower-only cases. However, for wind turbines with various polygonal cross-section towers, no instability vibrations were observed, and displacements increased proportionally to the square of mean wind speed. Furthermore, pressure measurements showed that local force coefficients changed largely depending on wind direction and azimuth angle. Local drag force coefficients decreased with increasing number of tower sides, approaching those of the tower-only case, and local lift force coefficients showed larger absolute values than those of the tower-only case. The maximum mean and fluctuating drag force and the maximum fluctuating lift coefficients at each height decreased with increasing number of tower sides.

Experimental study of wave kinematics and wave load distribution on a vertical circular cylinder

In this paper, we present measurements of force distribution on a vertical, circular cylinder exposed to shoaling regular waves. The cylinder was divided into four individual sections on which the wave loads were measured. Furthermore, we measured the wave kinematics with Particle Image Velocimetry (PIV) and with Laser Doppler Velocimetry (LDV) in a few reference cases. The force distribution was measured for twenty regular wave conditions and in all the cases the maximum force did not occur simultaneously on the individual sections of the cylinder. A decomposition of the total force in force harmonics showed the same tendencies for the total force as presented in literature. A decomposition of the force on individual cylindrical sections showed that the section at the surface had a significantly higher second force harmonic compared to the sections closer to the bed. Furthermore, we studied the local force coefficients on the individual sections on the cylinder. The inertia coefficient exhibited the same trend for all the individual sections. Estimated force coefficients from measured wave kinematics corresponded well with force coefficients in the literature.

Characteristics of aerodynamic forces exerted on a twisted cylinder at a low Reynolds number of 100

Laminar flow over a twisted cylinder is numerically simulated at a Reynolds number of 100. The flow past a smooth cylinder is calculated for comparison. Jung and Yoon (J. Fluid Mech., 759, 2014) showed the considerable suppression of force coefficients at the subcritical Reynolds number of 3000. The present results verify that the twisted shape of the cylinder can be used to reduce the force coefficients at a low Reynolds number containing the laminar flow. This suppression of the force coefficients is supported by a longer vortex formation length produced by the twisted cylinder. We investigated the characteristics of the local force coefficients for a twisted cylinder. The simple periodic oscillation of the time histories of total and local lift coefficients for the twisted cylinder exhibits the same pattern as that of a smooth cylinder. However, the time history of drag for the twisted cylinder reveals the presence of multi-frequency oscillations, resulting in harmonic behavior of the power spectra. This is confirmed by a time sequence of the instantaneous flow fields during the half-period of the time histories of force coefficients. The nature of the three-dimensional (3-D) twisted shape forms the 3-D vortical structures along the spanwise direction, which leads to the harmonic behavior of the drag time trace power spectra.

Interference effects on aerodynamic wind forces between two buildings

Wind-induced interference effects on overall wind loads and local wind loads on two buildings were studied in a series of the wind tunnel test on typical tall building models using a pressure measurement technique. In this study, five types of adjacent building models were considered with height ratios (Hr=0.5, 0.7, 1, 1.5 and 2) and wind directions were considered from 0° to 355° in 5° steps. Aerodynamic interference effects on base moments and local wind forces along nine height levels of the principal building are presented and discussed. As a result, interference effects (MIFMD and RIFMD) on along-wind base moment coefficients with Hr=1 and 1.5 significantly increased when the interfering building was close to the principal building. Moreover, when a higher interfering building was located at (X/B, Y/B)=(1.5, 1), mean wind loads and fluctuating wind loads in the along-wind and across-wind directions simultaneously increased. Mean and RMS along-wind local force coefficients at z/H=0.975 were significantly increased when the interfering building was located at (X/B, Y/B)=(1.5, 0). Variations of along-wind and across-wind local force coefficients along the height levels depended on the locations and height ratios of the interfering building.

An actuator line method with novel blade flow field coupling based on potential flow equivalence

AbstractA Reynolds‐averaged Navier–Stokes‐embedded actuator line model for wind and tidal turbine simulation has been implemented and validated using the National Renewable Energy Laboratory Phase VI wind tunnel experimental results. Actuator line models, first introduced by Sørensen and Shen, represent the blades virtually, enabling time‐resolved rotor simulations without requiring blade boundary layer discretization. This results in a lower computational cost than blade‐resolved simulations while preserving the predominant features of the rotor flow. The present method introduces a novel technique, based on potential flow equivalence, to determine the local flow velocity at the blade, and a method of projecting the resulting momentum sources to the flow field. These methods circumvent the requirement for smearing techniques used in other actuator line models. In addition, the model is adapted for use with an unstructured mesh, thereby enabling turbine components such as the tower and nacelle to be explicitly included in the domain. The model is validated through comparison of computed integrated loads and local force coefficients with the National Renewable Energy Laboratory Phase VI experimental results. Results for local force coefficients indicate general agreement with experiment, although discrepancies associated with three‐dimensional flow effects are observed at the tips. Copyright © 2014 John Wiley & Sons, Ltd.