Characteristics Of Aerodynamic Forces Research Articles

Experimental Measurements of Drag and Lift Coefficient on Building with an Elliptical Cross-section

Abstract The characteristics of aerodynamic forces acting on an elliptic cylinder with an aspect ratio of 0.5 with a wind attack angle from 0 to 90° and subjected to the boundary layer wind tunnel were investigated. The model was initially calibrated and compared with the existing work. The aspect ratio of the investigated model was 0.5, and the model was emerging in a turbulent flow. The mean and fluctuating drag and lift coefficients were investigated. The minimum drag coefficient occurred in the wind direction of 0° and the maximum at 90°. The lift coefficient was the largest in the 30° wind direction and the smallest at 0°. Fluctuating coefficients were similar profiles as the mean coefficients. Around the 30° wind direction, an inappropriate phenomenon occurred, caused by the generation of asymmetrical vortices structures and wake instabilities.

Unsteady Aerodynamics Around a Pitching Airfoil with Shock and Shock-Induced Boundary-Layer Separation

This study numerically investigated the characteristics of unsteady aerodynamic forces around a pitching airfoil in the transonic flow regime. The unsteady Reynolds-averaged compressible Navier–Stokes (URANS) equations were solved using a Spalart–Allmaras (SA) turbulence model. The pitching NACA64A010 airfoil at mean angles of attack of 0, 2, 4, and 6 deg with the amplitude of 1 deg and reduced frequency of 0.2 was parametrically simulated. The study found that the trend of the unsteady aerodynamic characteristics around the pitching airfoil, particularly that of the unsteady lift force, significantly changes with the mean-angle-of-attack conditions. The shock-induced boundary-layer separation and reattachment during the pitching oscillation have a crucial role in determining the trend of the unsteady aerodynamic forces, such as the phase-delayed or phase-advanced features for the variation in the angle of attack. The boundary-layer separation during the oscillation, observed at a high mean angle of attack of 6 deg, produces the phase-advanced feature of the lift force. Because the phase-advanced feature is associated with a positive damping effect, the result indicates that the boundary-layer separation may serve as a stabilizing effect on the pitching airfoil motion. The boundary-layer reattachment from separation during the airfoil oscillation, observed at an intermediate mean angle of attack of 4 deg, generates an unusual shock wave movement, resulting in a unique lift loop for the variation in the angle of attack. The study also demonstrated a good prediction accuracy of the URANS with the SA model for the unsteady aerodynamics around the pitching airfoil with the shock and shock-induced boundary-layer separation.

Experimental Investigation of Unsteady Aerodynamic Forces and Vortex-Induced Aeroelastic Effects of a Slender Structure in Uniform and Turbulent Flows

Slender structures are sensitive to wind loads, and aeroelastic effects during vortex-induced resonance have considerable influences on their survivability, serviceability, and habitability. However, the vortex-induced aeroelastic effects and the effects of turbulence intensity on unsteady aerodynamic forces of an aeroelastic prism remain far from been fully understood. In this study, a series of wind tunnel tests, including the pressure measurements of a rigid model and simultaneous measurements of the pressure and response of an aeroelastic model, were conducted in three different wind flows. The characteristics of unsteady aerodynamic forces and the effects of turbulence intensity on the unsteady aerodynamic forces were systematically investigated. The mechanism of aeroelastic effects and the variations of aerodynamic damping ratios were also discussed. Wind-induced responses were predicted by the unsteady aerodynamic forces and compared with the measured values. The experimental results show that vortex-induced aeroelastic effects on the wind-induced responses and the unsteady aerodynamic forces are significant. An increase in the turbulence intensity may result in a weak correlation of the unsteady aerodynamic forces and thus suppress the vortex-induced aeroelastic effects. Moreover, the amplitudes of the positive and negative peaks of the aerodynamic damping ratios decrease with the increase of the turbulence intensity. In addition, the wind-induced responses predicted by the unsteady aerodynamic forces measured from the aeroelastic model are the closest to the measured responses.

Study on Nonlinear Aerodynamic Characteristics of a Semi-Closed Box Bridge Deck Based on Coupled Amplitude Variation

This paper explores the nonlinear characteristics of the self-excited aerodynamic forces of a semi-closed box deck section to perfect the theory of aeroelastic response analysis. A numerical wind tunnel model was established based on the computational fluid dynamics (CFD) method. The heaving-pitching coupled motion is realized by loading user-defined function (UDF) and dynamic grid technology. The self-excited aerodynamic forces varying with amplitude are identified and analyzed, and the reliability of the aerodynamic results obtained by numerical simulation is verified in the wind tunnel test. In the heaving-pitching coupled motion, the results show that the nonlinear characteristics of aerodynamic forces, especially the aerodynamic moment, are mainly affected by the pitching motion. The phenomenon of high-order harmonic energy transfer is observed with the increase in pitching amplitude, and the main component of high-order harmonic can be determined by the pitching amplitude. The contribution of heaving motion to aerodynamic forces nonlinear components is small, but its influence on nonlinear characteristics is complex. Small amplitude heaving motion plays a positive damping role in heaving-pitching coupled motion, and its scope and effect of positive damping action are affected by pitching motion. The extreme value heaving amplitude of positive damping action is observed in the aerodynamic lift.

Study on Aerodynamic Nonlinear Characteristics of Semiclosed Box Deck Based on Variation of Motion Parameters

In order to study the nonlinear characteristics of self-excited aerodynamic forces of bluff body bridge section with the change of motion parameters, a numerical wind tunnel is established by the dynamic mesh technique of computational fluid dynamics (CFD). A state-by-state forced vibration method is used to identify the self-excited aerodynamic forces of single degree-of-freedom (DOF) heaving and pitching motion. Fast Fourier transform (FFT) is adopted to obtain frequency-domain data for analysis. The reliability of the obtained aerodynamic results is verified by wind tunnel tests. The results show that the high-order harmonic components are found in the self-excited aerodynamic forces of semiclosed box deck section, which are more significant in aerodynamic lift than in aerodynamic moment. The proportion of aerodynamic nonlinear components increases with amplitude. The effect of amplitude on the nonlinear components of heaving motion is generally higher than that of pitching motion, and aerodynamic moment is highly sensitive to the increase of vertical amplitude. The variation of the nonlinear components of the deck section with frequency is not a simple monotonic relationship, and there is a stationary point at 10 Hz frequency. The existence of wind attack angle makes the proportion of nonlinear components reach more than 30% and greatly increases the proportion of second harmonic. In addition, the high-order harmonic components, which are not integer multiples, are found at large amplitude and positive angle of attack.

Nonlinear motion-induced aerodynamic forces on large hyperbolic paraboloid roofs using LES

In this paper, the nonlinear characteristics of motion-induced aerodynamic forces on large-scale hyperbolic paraboloid roofs are numerically investigated using LES (large-eddy simulation). The mean and standard deviation (STD) of wind pressure coefficients on the rigid hyperbolic paraboloid roofs in the turbulent boundary layer are first simulated to validate the numerical models. Then the hyperbolic paraboloid roofs under forced excitation with different oscillation amplitudes are simulated to study the wind pressures on and the flow structures above the roofs. Moreover, the motion-induced aerodynamic forces, which are represented by the aerodynamic stiffness and damping forces, are also investigated with the harmonic balance method (HBM). It is found that the aerodynamic stiffness force is linearly proportional to the oscillation amplitude, but the aerodynamic damping force is nonlinearly dependent on the oscillation amplitude. The wind-induced responses of the large-scaled roof considering nonlinear motion-induced aerodynamic forces exhibit typically hardening non-Gaussian characteristics at the low reduced frequency, which are consistent with the probabilistic features of vortex-induced vibration (VIV) of slender structures.

Passive aerodynamic control of a single-box girder using self-issuing jets

In this study, wind tunnel experiments were used to evaluate the control effect of self-issuing jets on the aerodynamic characteristics of a streamlined single-box girder. Based on the deck thickness, the Reynolds number was set to 1.17 ​× ​105 ​at a zero angle of attack. The time-averaged and fluctuation characteristics of wake velocities, aerodynamic forces, and surface pressures were investigated along the streamwise and spanwise directions. The results indicated that jet slits arranged on the lower inclined panels of a bridge deck can effectively suppress the lift and moment fluctuations, and introduce three-dimensional variations in the wake velocity. The wake velocity profiles exhibit wave-like distributions in the near-wake region along the spanwise direction, and the length of the vortex formation region is elongated by the jet. Furthermore, the frequency along the spanwise direction of the wake decreases considerably. The changes in the near wake affect the upstream flow field around the deck, which reduces and varies the surface pressure along the deck span, resulting in appreciable reductions in the aerodynamic forces.

Characteristics of unsteady aerodynamic forces on an aeroelastic prism: A comparative study

Aerodynamic forces on a slender prism are frequently estimated by conventional measurements using rigid models, thus the characteristics of unsteady aerodynamic forces inherent to an aeroelastic prism often remain unclear. Erected upon empirical results measured from aeroelastic, forced vibration and rigid models, this work supplies a holistic investigation on the characteristics of unsteady aerodynamic forces acting on a slender prism. First, local and generalized aerodynamic forces, unsteady pressures, corresponding power spectra, and Strouhal numbers on the three models are comparatively presented and discussed. Subsequently, wind-induced responses predicted by aerodynamic forces on the three models are drawn parallel to experimental results for comparison. The comparison indicates that the aerodynamic forces measured from aeroelastic and forced vibration models include the effect of structural motion, whereas those observed from a rigid model do not. Furthermore, the responses predicted by aerodynamic forces on an aeroelastic model are in close agreement with experimental results, whereas remarkable discrepancies are found for both forced vibration and rigid models. In addition, the hybrid aeroelastic-pressure balance (HAPB) test technique proves more effective in the measurement of aerodynamic forces than the forced vibration test technique. Ensuing investigation, this work not only advances our understanding on the characteristics of unsteady aerodynamic forces on an aeroelastic prism, but also ascertains the effectiveness of the aforementioned wind tunnel techniques.

Characteristics of local wind pressure distribution and global aerodynamic forces on a vertical forest

Vertical forests comprise of high-rise buildings with trees planted on their balconies. When trees are planted on a building, they change its exterior, and may influence local wind pressure and global aerodynamic forces. Wind-borne debris caused by local extreme wind pressures lead to negative effects on living environments, and large acceleration responses induced by global aerodynamic forces result in occupants’ uneasiness. Therefore, it is important to investigate the local and global wind loads on vertical forests. In this study, wind tunnel tests of simultaneous multi-channel pressure measurement were conducted to investigate the effects of trees on local wind pressures (mean and fluctuating wind pressures on different positions of building facades). High-frequency-base-balance (HFBB) wind tunnel tests were performed to probe the impacts of trees on the global aerodynamic force of the building (base bending and torsional moments). Pressure measurements reveal that trees have minimal effects on mean and extreme values of wind pressure on windward and leeward facades; however, trees decrease the values of extreme wind pressure at the separation area. The results of HFBB show that vegetation slightly increases the intensity of mean global wind load at along-wind directions, decreases the peak value of power spectral density (PSD) of lift forces corresponding to vortex shedding. In general, trees on balconies reduce wind pressures at separation area as well as the across-wind fluctuating forces for the building with the side ratio of 1.67. However, there is no significant effects on the rest of wind pressures and loads. Therefore, to some extent, trees planted on balconies were found to improve wind-resistant performance of vertical forests compared with conventional high-rise buildings.

Characteristics of aerodynamic forces on high-rise buildings with various façade appurtenances

The appurtenances effects of high-rise building on wind loads are investigated through a series of wind tunnel experiments with 21 façade appurtenances simulated by different configurations of horizontal splitter plates. “Zigzag variations” of layer forces can be found in both along-wind and cross-wind when the vertical separation distance ratio of continuous plates is larger than 4%. The appurtenances can significantly reduce fluctuating pressures on the side façade but with different effect on the fluctuating cross-wind layer force. The reason of this will be discussed in this study. The power spectra of response of the building model indicate that the horizontal façade appurtenances have negligible effects on the period of vortex shedding. But the peaks in the power spectra become slightly higher than those from a smooth surface model, and the bandwidths are also narrower. Such results indicate that the horizontal plates may result in better organization of the vortex shedding by restricting the flow in vertical direction and reduce the intensity of vortex. Further check on the correlation coefficients between pressures at the same level reviews that the correlation of response from different positions of the building models with appurtenances is higher than that for building model with smooth surface. It is also noted that discontinuous appurtenances can reduce the cross-wind base moment by approximately 5%.

Identification method and application of aerodynamic damping characteristics of super high-rise buildings under narrow-band excitation

Aerodynamic damping is an important parameter for assessing the wind–structure interaction of super high-rise buildings. A new analysis method is proposed, which first uses the complex blind source separation technique to achieve the effective decoupling of complicated coupled signals and then the Bayesian spectral density approach (BSDA) considering the actual structural aerodynamic characteristics, namely, new Bayesian spectral density approach (NewBSDA), to conduct the parameter identification of decoupled signals effectively. The proposed method is applied to the modal parameter identification of the aeroelastic model test of an 838 m super high-rise building. Results show that the complex blind source separation technique can effectively decouple the coupled signals generated by similar sway modal frequencies in the aeroelastic model. Results of the proposed method are considerably more reasonable than those of the traditional BSDA due to the actual aerodynamic characteristics considered. Comparison shows that BSDA substantially underestimates the cross-wind aerodynamic damping of the structure near the critical wind speed. Given that along-wind aerodynamic loads are wide-band excitations, the aerodynamic damping ratios identified by BSDA and NewBSDA are close. Furthermore, the characteristics of aerodynamic forces identified by the proposed method are satisfactory, which further illustrates the rationality and validity of the method.

Excitation of the large-amplitude vibrations of a circular cylinder under normal wind conditions in the critical Reynolds number range

The specific flow regime in which the large-amplitude vibrations of dry circular cylinders are observed in the critical Reynolds number range remains unclear because several different flow regimes overlap in this narrow range. The aerodynamic forces on a stationary circular cylinder and the displacement of an elastically mounted cylinder were measured using the same model at same wind speeds corresponding to Reynolds number from 1.2×105 to 4.3×105 to reveal the characteristics of aerodynamic forces and potential vibrations in different regimes. The results show that the large amplitude vibrations of a dry circular cylinder under normal wind conditions can be excited by unsteady reattachment only during a transitional process between flow regimes rather than throughout the entire critical Reynolds number range.

Time-frequency evolutionary characteristics of aerodynamic forces around a streamlined closed-box girder during vortex-induced vibration

The mechanism of vortex-induced vibration (VIV) of long-span bridges is a key issue in bridge wind engineering. It's necessary to have an understanding about aerodynamic loads and then take corresponding countermeasures to mitigate VIV. Vortex-excited forces (VEFs) and the structural behaviors of a spring-suspended sectional model (SSSM) were synchronously measured in multi-scale levels in terms of distributed aerodynamic forces around a streamlined closed-box girder, general integral loadings, and structural wind-induced displacements in the VIV-development process, involving the pre-VIV stage, the ascent stage, extreme amplitude point, descent stage, and post-VIV stage. Based on the synchronized measurement technologies above, time-frequency evolutionary characteristics in those stages were analyzed considering numerous correlations between local and general aerodynamics around a bridge deck. The spatial distribution of mean, RMS values of pressure coefficients, predominant frequencies of the distributed aerodynamic forces, phase lags and correlation coefficients between the distributed aerodynamic forces at each tap and the general VEFs, pressure coefficients at predominant frequencies, etc., have close evolutionary relationships with displacements in the VIV-development process, especially near the downstream region of the upper surface and the corner region of the lower surface and tail wind fairing, showing dependency on the development stage of the VIV process. Aerodynamic forces near the downstream region of the upper surface make major contributions to VEFs, which are predominant and mainly responsible for the VIV phenomena.

Surface Roughness and Reynolds Number Effects on the Aerodynamic Forces and Pressures Acting on a Semicylindrical Roof in Smooth Flow

AbstractThe effects of surface roughness and Reynolds number on the characteristics of aerodynamic forces and pressures on a semicylindrical roof have been investigated in a wind tunnel. Simultaneo...

Large eddy simulation of unsteady aerodynamic behavior of long-span vaulted roofs

This paper discusses the unsteady aerodynamic behavior of long-span vaulted roofs. First, a forced vibration test in a turbulent boundary layer is conducted in a wind tunnel. The models are force vibrated in the first anti-symmetric mode to investigate the effects of wind speed, rise/span ratio, and the amplitude and frequency of forced vibration on the distributions of wind pressures and unsteady aerodynamic forces. Then, a large eddy simulation (LES) is carried out to clarify the physical mechanism of wind-roof interaction as well as to investigate the influences of a roof’s vibration on the flow field around the roof. From the results of the wind tunnel experiment and the LES, we discuss the characteristics of unsteady aerodynamic forces on a long-span vaulted roof over a wide range of the reduced frequency of vibration. The effect of unsteady aerodynamic forces on the dynamic response of the roof is also discussed. A comparison between the wind tunnel experiment and the LES indicates that the LES can be used effectively to evaluate the unsteady aerodynamic behavior.

Numerical Analysis of the Effects of Rotating Wind Turbine Blades on the Aerodynamic Forces Acting on Tower

We have investigated the effects of the rotating blades of an upwind-type three-blade horizontal-axis wind turbine (HAWT) on the basic characteristics of aerodynamic forces acting on its tower by conducting improved delayed detached-eddy simulations (DESs). Three tip-speed ratios were considered for the operating conditions of the HAWT: λ = 3 (low), λ = 6 (optimum), and λ = 10 (high). The diversion of the flow approaching the tower by the rotating blades and the low-pressure region that formed downwind of the blades significantly affected the aerodynamic forces acting on the tower. For example, the azimuth angle around the tower at which the pressure reached a maximum at each height shifted significantly in the direction of the movement of the blade passing the tower because of the diversion of the flow by the blades. Fluctuations in the lift force of the tower were significantly larger than those in its drag force because of the low-pressure region downwind of the blades.

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.

The influence of flow field and aerodynamic forces on a straight-bladed vertical axis wind turbine

This paper has attempted to compile the assessment of flow field and aerodynamic forces acting on a small straight-bladed vertical axis wind turbine (VAWT). Two dimensional unsteady flows around the VAWT, operating at three different tip speed ratios in a wind tunnel, were investigated through the use of a Laser Doppler Velocimeter (LDV) system. Furthermore, in order to explicate the characteristics of aerodynamic forces, pressures acting on the blade surface were measured during the rotation by a multiport scanner mounted on the hub and pressure signals were transmitted to the stationary system through a wireless LAN. Velocity distribution proved the wind velocity deficit. While, the geometrical angle of attack and resultant flow velocity change periodically due to local wind velocity and direction depending on the azimuth angle. The power coefficient, tangential force, lift and drag which are obtained by pressure distribution are discussed as a function of blade azimuthally position, achieving a numerical quantification of the influence of tip speed ratio on overall rotor performance. As a result, it is clarified that aerodynamic forces show the maximum values when the blade passes through the upstream region.

Unsteady aerodynamic forces on a vibrating long-span curved roof

The present paper discusses the characteristics of unsteady aerodynamic forces on long-span curved roofs. A forced vibration test is carried out in a wind tunnel to investigate the effects of wind speed, vibration amplitude, reduced frequency of vibration and rise/span ratio of the roof on the unsteady aerodynamic forces. Because the range of parameters tested in the wind tunnel experiment is limited, a CFD simulation is also made for evaluating the characteristics of unsteady aerodynamic forces on the vibrating roof over a wider range of parameters. Special attention is paid to the effect of reduced frequency of vibration. Based on the results of the wind tunnel experiment and CFD simulation, the influence of the unsteady aerodynamic forces on the dynamic response of a full-scale long-span curved roof is investigated on the basis of the spectral analysis.

The Effects of Wind and Posture on the Aerodynamic Performance During the Flight Stage of Skiing

Numerical simulation is conducted to evaluate the wind and posture effects on the aerodynamic performance of a skier during the flight stage. Both steady and unsteady models are applied on a 2D geometry. Using the Fluent code, the fundamental equations of fluid flow are solved simultaneously. In particular we focus on the influence of wind speed and direction on aerodynamic forces with several different postures of the skier in steady modeling. For a chosen case, the unsteady models are used to predict the transient characteristics of streamline distributions and aerodynamic forces. It is found that the skier's postures, wind speed, and direction play a significant role. The wind condition affects the pressure force (the form drag) on the skier and makes it a resistance or thrust regarding wind directions. The optimized posture with a minimization of resistance under a facing wind is determined as a moving-forward body of the skier. The unsteady modeling reveals that the wake around the skier and aerodynamic forces are strongly dependent on time. This initial study not only provides a qualitative and theoretical basis for the athletes to understand the effects of wind and postures, and then to optimize their postures according to the wind condition during the flight stage of skiing, but also builds the foundation for the systematic study of skiing process with more advanced CFD models in the future.