Free-stream Speed Research Articles

Conditional sampling of transitional boundary layers in pressure gradients

AbstractStatistical analysis of transitional boundary layers in pressure gradients is performed using the flow fields from direct numerical simulations of bypass transition. Laminar–turbulent discrimination separates the streaky laminar flow from turbulent regions. Individual streaks are identified and tracked in the flow field in order to obtain statistics of the amplitude of the streak population. An extreme value model is proposed for the distribution of streak amplitudes. It is also possible to differentiate those streaks which break down into turbulent spots from innocuous events. It is shown that turbulence onset is due to high-amplitude streaks, with streamwise perturbation velocity exceeding 20 % of the free stream speed. The resulting turbulent spots are tracked downstream. The current analysis allows for the measurement of the lateral spreading angles of individual spots and their spatial extent and volumes. It is demonstrated that the volumetric growth rate of turbulent spots is insensitive to pressure gradient.

라이다 측정을 이용한 나셀 풍속계 보정식 제안

The nacelle anemometer mounted behind the blade roots of a wind turbine measures distorted wind speed comparable with free-stream wind because of the wake effects caused dependent upon the operation of the wind turbine and the rotation of its blades. The field campaign was carried out to measure free-stream wind speed at a height identical to the height of the nacelle anemometer by deploying a ground-based remote-sensing equipment, LIDAR. It is derived that a third-order polynomial equation for correcting wind speed measured by the nacelle anemometer to undistorted free-stream wind speed incident to a wind turbine. It is anticipated that the derived correction equation enables wind speed measured by the nacelle anemometer to be used as a precise input for a wind turbine performance test and for developing an active control logic.

On the scaling of air layer drag reduction

AbstractAir-induced drag reduction was investigated on a 12.9 m long flat plate test model at a free stream speed of$6. 3~\mathrm{m} ~{\mathrm{s} }^{- 1} $. Measurements of the local skin friction, phase velocity profiles (liquid and gas) and void fraction profiles were acquired at downstream distances to 11.5 m, which yielded downstream-distance-based Reynolds numbers above 80 million. Air was injected within the boundary layer behind a 13 mm backward facing step (BFS) while the incoming boundary layer was perturbed with vortex generators in various configurations immediately upstream of the BFS. Measurements confirmed that air layer drag reduction (ALDR) is sensitive to upstream disturbances, but a clean boundary layer separation line (i.e. the BFS) reduces such sensitivity. Empirical scaling of the experimental data was investigated for: (a) the critical air flux required to establish ALDR; (b) void fraction profiles; and (c) the interfacial velocity profiles. A scaling of the critical air flux for ALDR was developed from balancing shear-induced lift forces and buoyancy forces on a single bubble within a shear flow. The resulting scaling successfully collapses ALDR results from the current and past studies over a range of flow conditions and test model configurations. The interfacial velocity and void fraction profiles were acquired and scaled within the bubble drag reduction (BDR), ALDR and transitional ALDR regimes. The BDR interfacial velocity profile revealed that there was slip between phases. The ALDR results showed that the air layer thickness was nominally three-quarters of the total volumetric flux (per unit span) of air injected divided by the free stream speed. Furthermore, the air layer had an average void fraction of 0.75 and a velocity of approximately 0.2 times the free stream speed. Beyond the air layer was a bubbly mixture that scaled in a similar fashion to the BDR results. Transitional ALDR results indicate that this regime was comprised of intermittent generation and subsequent fragmentation of an air layer, with the resulting drag reduction determined by the fraction of time that an air layer was present.

Study of Gap Physics of Airfoils with Unsteady Flaps

Active trailing-edge flaps are a method of aerodynamic control under extensive research to reduce the detrimental effects of dynamic stall. Physical phenomena are poorly understood in the context of active flaps including vorticity and acoustics, separation, and transition. In addition, discrete trailing-edge flaps create a cavity-like flow within the airfoil-flap gap that can complicate these phenomena. This work has explored the physical response of a static airfoil with a discrete noncontoured oscillating flap over a range of freestream parameters. The effects of attached and separated flows, flap oscillation scheduling, airfoil-flap gap size, and freestream speed have all been investigated. Time-accurate predictions were performed using a hybrid Reynolds-averaged Navier–Stokes/large eddy simulation turbulence model. Trailing-edge stall suppression and an increase between aerodynamic response and deflection input were observed as the flap oscillation frequency increased. The lag between response and input also increased approximately linearly with airfoil-flap gap size. Results indicated the transition was unaffected by the flap oscillations. During the frequency content of flow the unsteadiness was consistent with separated flow driven by the flap. Discrete noncontoured flaps are not recommended; if they are required, the size of the gap should be minimized to maintain performance and reduce lag.

Mass transfer velocity across the breaking air–water interface at extremely high wind speeds

Mass transfer velocity k L across the wind-driven air–water interface was estimated at extremely high wind speeds (up to U 10=70 m s−1) in a high-speed wind-wave tank by measuring changes in CO2 concentration in the water. In addition, the volume flux of dispersing droplets lost from the tank and the wave height were measured. k L increases drastically with wind speed at extremely high wind speeds. The volume flux of dispersing droplets begins to increase drastically and the mean height of significant waves changes its rate of increase at almost the same wind speed as that at which the rate of increase of k L changed. These results suggest that intense wave breaking occurs at extremely high wind speeds and it has significant effects on mass transfer. k L is well correlated with the free-stream wind speed for both present laboratory and previous field measurements in the low and moderate wind speed regions. Present k L agrees well with the conventional correlation curves proposed by Wanninkhof (1992), Wanninkhof and McGillis (1999) and Wanninkhof et al. (2009) for low and moderate free-stream wind speeds. However, for extremely high free-stream wind speeds, the present data deviate upward from the correlation curves of Wanninkhof (1992) and Wanninkhof and McGillis (1999) and approach to that of Wanninkhof et al. (2009) as the wind speed increases. This indicates that the correlation curve of Wanninkhof et al. (2009) is more appropriate for the correlation between k L and free-stream wind speed than those of Wanninkhof (1992) and Wanninkhof and McGillis (1999) in extremely high wind speed region.

Low-speed Wind-tunnel Test Results of a BWB-UCAV Model

Wind-tunnel test results of a BWB-UCAV model which was modified slightly from UCAV 1303 at a free stream speed of 50m/s at various angles of attack and yaw angles are presented in the present paper. The six-component force and moment coefficients were measured by using internal balance. Well known pitch-break phenomenon was identified to occur at a rather low angle of attack. Yawing and rolling moment coefficients from the measurements data also were found to exhibit very irregular pitch-break type behavior at non-zero yaw angles. To elucidate some aerodynamic flow structures, surface oil flow visualization and PSP measurements were also carried out.

Galloping Characteristics of Conductor Accreted with Crescent-Shaped Ice

Accounting for the disastrous phenomena of ice-accreted conductor galloping, wind tunnel tests of LGJ630/45 conductor accreted with crescent-shaped ice are conducted. Based on the test results, conductor galloping is simulated by employing the PCL language of ANSYS commercial software package and then the influences of span lengths as well as free-stream speeds on galloping characteristics are studied. The results indicate that, conductor galloping consists several different frequency components. With the increase of span length, galloping energy moves from low-frequency component to its high-frequency counterpart, and finally high-frequency component dominates the galloping phenomenon. And with the movement of energy, galloping traces transforms from ellipse to that similar to a butterfly. With the increase of span length, the maximum cable tension first increases, next decreases sharply, and then increases again.

The modification of wind turbine performance by statistically distinct atmospheric regimes

Power production from wind turbines can deviate from the manufacturer’s specifications due to variability in atmospheric inflow characteristics, including stability, wind shear and turbulence. The practice of insufficient data at many operational wind farms has made it difficult to characterize this meteorological forcing. In this study, nacelle wind measurements from a wind farm in the high plains of central North America were examined along with meteorological tower data to quantify the effects of atmospheric stability regimes in the boundary layer on turbine power generation. The wind power law coefficient and the bulk Richardson number were used to segregate time periods by stability to generate regime-dependent power curves. Results indicated underperformance during stable regimes and overperformance during convective regimes at moderate wind speeds (8–12 m s−1). Statistical testing using the Monte Carlo approach demonstrated that these results were robust, despite potential deviations of the nacelle wind speeds from free-stream inflow values due to momentum loss from the turbine structure and spinning rotor. A hypothetical stability dependence between free-stream and nacelle wind speeds was generated that can be evaluated in future analyses. The low instrumentation requirement of our power analysis technique should enable similar studies at many wind sites formerly considered inappropriate.

Numerical study of the effects of human body heat on particle transport and inhalation in indoor environment

The inhalation of micron particles by a manikin standing in a ventilated indoor environment was numerically investigated using Computational Fluid Dynamics (CFD). Computations were conducted with various combinations of the free stream velocity (0.05–0.25 m/s representing typical indoor wind speeds.), occupant orientation relative to the free stream (back-to-the-wind or facing-the-wind) and heat transfer (isothermal or thermal flow). It was found that the body heat has a significant impact on the airflow field in the vicinity of the manikin by causing an upwards airflow on the downstream side of the manikin. It was also found that the effect of body heat on particle inhalation depends on the manikin orientation relative to the free stream. When the manikin is facing-the-wind, body heat has a little effect on particle inhalation and can be neglected. However for a back-to-the-wind orientation, the situation is much more complicated as the source height of inhaled particles depends on the speed of free stream. When the wind speed is low (0.05 m/s), the critical area is located near the floor level. The central height of the critical area then increases with increasing free stream speed until it reaches the nose height when the wind speed rises up to 0.25 m/s. This indicates that the body heat is an important consideration when investigating contaminant inhalation by human occupants in low-speed (typically less than 0.2 m/s) indoor environment.

Experimental study on flow hysteresis effect on NACA0015 airfoil using DBD plasma actuator

Experiments were performed on NACA0015 airfoil using dielectric barrier discharge (DBD) plasma actuator to investigate the influencing factors and regularity in flow hysteresis effect. The parameters investigated were free-stream speed, attack angle and actuation duration. Results demonstrate that the duration of flow hysteresis effect increases with increasing free-stream speed and actuation duration, while decreases with increasing attack angle nonlinearly in free-stream speed range 18≤v≤44m/s. More worthy of note is that the duration of flow hysteresis effect we got increases remarkably and exceeds 150s (it has been reported 8s), which makes it possible to effectively decrease the power consumption by using the non-steady plasma actuation.

Experimental Investigation on Stall Separation Control on NACA0015 Airfoil by Steady Plasma Aerodynamic Actuation

Experimental investigation was performed to study the influence law of the free-stream speed, angle of attack and actuation voltage on the control effectiveness of NACA0015 airfoil stall separation suppression by steady plasma aerodynamic actuation. It is found that plasma actuation can effectively suppress flow separation on the airfoil at free-stream speeds in the range from 20 m/s up to 65 m/s. When the speed is 25m/s, the stall angle will be delayed by 3°, lift coefficient will increase and drag coefficient will decrease by 10.4% and 28.9% respectively. Moreover, Experimental results indicate that the threshold voltage increases with the increasing free-stream speed and attack angle. Meanwhile, both of smoke flow visualization and flow control experiments were carried out, conclusions show that the direction of induce flow has little effect on plasma flow control effectiveness, which means that momentum transferring is not the basic cause of active flow control of plasma aerodynamic actuation.

Experimental investigation of nanosecond discharge plasma aerodynamic actuation

In this paper we report on an experimental study of the characteristics of nanosecond pulsed discharge plasma aerodynamic actuation. The N2 (C3Πu) rotational and vibrational temperatures are around 430 K and 0.24 eV, respectively. The emission intensity ratio between the first negative system and the second positive system of N2, as a rough indicator of the temporally and spatially averaged electron energy, has a minor dependence on applied voltage amplitude. The induced flow direction is not parallel, but vertical to the dielectric layer surface, as shown by measurements of body force, velocity, and vorticity. Nanosecond discharge plasma aerodynamic actuation is effective in airfoil flow separation control at freestream speeds up to 100 m/s.

Numerical Simulation of the Effect of Bionic Serrated Structures on the Aerodynamic Noise of a Circular Cylinder

Flow control can effectively reduce the aerodynamic noise radiated from a circular cylinder. As one of the flow control methods, a bionic method, inspired by the serrations at the leading edge of owls' wing, was proposed in this paper. The effects of bionic serrated structures arranged on the upper and lower sides of a cylinder on the aerodynamic and aeroacoustic performance of the cylinder were numerically investigated. At a free stream speed of 24.5 m·s −1, corresponding to Reynolds number of 1.58×10 4, the simulation results indicate that the bionic serrated structures can decrease the frequency of the vortex shedding and control the fluctuating aerodynamic force acting on the cylinder, thus reduce the aerodynamic noise. A qualitative view of the vorticity in the wake of the cylinder suggest that the serrated structures reduce aerodynamic sound by suppressing the unsteady motion of vortices

Aerodynamics of Fixed and Rotating Spoked Cycling Wheels

The performance of a semiracing spoked wheel is numerically and experimentally studied at full size in a wind tunnel. The numerical investigation is divided into two parts. In the first part, the wheel is considered to be fixed (no rotation) and the numerical results are compared to the experimental measurements. The flow past the wheel is treated as stationary and turbulent. The effects of cross wind and the wheel’s speed on the drag, side force, and yaw moment are investigated. Numerical results are presented via diagrams and plots at various yaw angles. Both the measurements and predictions agree quite well and they show a considerable increase in the yaw moment and side force at medium and high yaw angles. The axial drag force initially increases with yaw angle (up to 7.5 deg) and eventually decreases. Ground effects did not affect the overall loads, except for the vertical force at high yaw angles. In the second part, the effects of rotation have been taken into account. The wheel rotates at constant angular velocities and the flow is modeled as nonstationary and turbulent. The aerodynamic performance of the wheel is strongly affected by the rotational speed. In most of the cases, as the latter parameter increases, the loads nonlinearly increase. The rotation generates asymmetrical loading, since the flow is accelerated in one side and decelerated in the other (the Magnus effect). A vertical force is produced, which is dependent on the ratio of the rotational to the free-stream speed. Moreover, in an attempt to assess the effects of the number of spokes to the aerodynamic performance, two other models with 8 and 32 spokes have been numerically tested and compared to the original one (16 spokes). The results revealed, as expected, an increase in the axial drag and vertical force with the number of spokes.

Steady and rotating computational fluid dynamics simulations of a novel vertical axis wind turbine for small-scale power generation

A novel vertical axis wind turbine (VAWT) has been developed that consists of several asymmetric vertically-stacked stages. A computational investigation of the torque characteristics of the VAWT has been conducted using the commercial computational fluid dynamics (CFD) software CFdesign 2010. A validation study consisting of steady and rotating simulations was conducted using a Savonius rotor and good agreement was obtained with experimental data. Steady two-dimensional CFD simulations have demonstrated that the new VAWT has similar average static torque characteristics to existing Savonius rotors. Rotating three-dimensional CFD simulations were conducted at several tip speed ratios with a freestream speed of 6 m/s. The predicted dynamic torque generated by the rotor decays more rapidly with increasing tip speed ratio than the torque output of Savonius rotors due to its asymmetric design and the curvature of the outer rotor wall.

Ideal Ramjet: Optimum Freestream Mach Number for Minimum Thrust-Specific Fuel Consumption

ao = freestream speed of sound cp = specific heat at constant pressure gc = Newton’s constant hPR = fuel lower heating value Mo = Mach number at freestream conditions M o = freestream Mach number for minimum S S = thrust-specific fuel consumption T = temperature Tmax = material temperature limit To = freestream temperature Tto = freestream total temperature = ratio of specific heats b = = r r = inlet temperature ratio, Tto=To 1 1 M o=2 = material temperature limit to freestream temperature ratio, Tmax=To

Bluff body flow-induced noise control with sliding plasma actuators

Sliding plasma actuators were used to control the flow-induced broadband noise radiating from a bluff body. The structure consisted of a cylinder and a torque link that was installed on the lee side of the cylinder. The objective was to reduce the noise generated during the cylinder wake impinging on the torque link. By manipulating cylinder wake with the externally-imposed body force generated by the plasma actuators, the flow-structure interaction was reduced, leading to an attenuation of the broadband noise up to 3 dB in overall sound pressure level at a free stream speed of 30 m/s.

Performance and near wake measurements of a model horizontal axis wind turbine

ABSTRACTThe performance characteristics and the near wake of a model wind turbine were investigated experimentally. The model tested is a three‐bladed horizontal axis type wind turbine with an upstream rotor of 0.90 m diameter. The performance measurements were conducted at various yaw angles, a freestream speed of about 10 m s −1, and the tip speed ratio was varied from 0.5 to 12. The time‐averaged streamwise velocity field in the near wake of the turbine was measured at different tip speed ratios and downstream locations. As expected, it was found that power and thrust coefficients decrease with increasing yaw angle. The power loss is about 3% when the yaw angle is less than 10° and increases to more than 30% when the yaw angle is greater than 30°. The velocity distribution in the near wake was found to be strongly influenced by the tip speed ratio and the yaw angle. At the optimum tip speed ratio, the axial velocity was almost uniform within the midsection of the rotor wake, whereas two strong peaks are observed for high tip speed ratios when the yaw angle is 0°. As the yaw angle increases, the wake width was found to be reduced and skewed towards the yawed direction. With increasing downstream distance, the wake velocity field was observed to depend on the tip speed ratio and more pronounced at high tip speed ratio. Copyright © 2011 John Wiley & Sons, Ltd.

Effects of small-scale freestream turbulence on turbulent boundary layers with and without thermal convection

Effects of weak, small-scale freestream turbulence on turbulent boundary layers with and without thermal convection are experimentally investigated using a wind tunnel. Two experiments are carried out: the first is isothermal boundary layers with and without grid turbulence, and the second is non-isothermal boundary layers with and without grid turbulence. Both boundary layers develop under a small favorable pressure gradient. For the latter case, the bottom wall of the test section is heated at a constant wall temperature to investigate the effects of thermal convection under the effects of freestream turbulence. For both cases, the turbulence intensity in the freestream is Tu = 1.3% ∼ 2.4%, and the integral length scale of freestream turbulence, L∞, is much smaller than the boundary layer thickness δ, i.e., L∞/δ≪1. The Reynolds numbers Reθ based on the momentum thickness and freestream speed U∞ are Reθ = 560, 1100, 1310, and 2330 in isothermal boundary layers without grid turbulence. Instantaneous velocities, U and V, and instantaneous temperature T are simultaneously measured using a hot-wire anemometry and a constant-current resistance thermometer. The results show that the rms velocities and Reynolds shear stress normalized by the friction velocity are strongly suppressed by the freestream turbulence throughout the boundary layer in both isothermal and non-isothermal boundary layers. In the non-isothermal boundary layers, the normalized rms temperature and vertical turbulent heat flux are also strongly suppressed by the freestream turbulence. Turbulent momentum and heat transfer at the wall are enhanced by the freestream turbulence and the enhancement is notable in unstable stratification. The power spectra of u, v, and θ and their cospectra show that motions of almost all scales are suppressed by the freestream turbulence in both the isothermal and non-isothermal boundary layers.

Free vibration of a square cylinder at low Reynolds numbers

Free vibrations of a square cylinder at zero incidence are studied. A stabilized space-time finite-element method has been used to discretize the equations of fluid flow in two-dimensions. The cylinder mounted on elastic supports, with low mass ratio ( m ⁎=10), is allowed to undergo in-line as well as transverse vibrations. The coefficient of structural damping is assigned a zero value. The Reynolds number, Re, is based on the edge length of the cylinder and free-stream speed. The reduced natural frequency is defined as F N =14.39/Re. For a blockage of 0.05, results are presented for 60 ≤ Re ≤ 250 . As the Reynolds number is progressively increased, the regimes of flow observed are ‘primary’ lock-in, desynchronization and ‘secondary’ lock-in. It is found that the width of the ‘primary’ lock-in zone for a square cylinder with sharp corners is very narrow. The variation of transverse response with Re is associated with the initial, lower and ‘secondary’ lower branches. Transition between the initial and lower branches is hysteretic. Hysteresis in the cylinder response as well as aerodynamic coefficients is observed close to the lower-Re extremity of the ‘primary’ lock-in zone. A hysteretic jump at Re=87 marks the onset of lock-in regime. The 2S mode of vortex shedding is observed along the initial branch. Along the lower branch, the modes of vortex shedding are 2S and C(2S). Vortex shedding along the ‘secondary’ lower branch is via the 2S and 2P modes. The free vibration characteristics of a square differ, in several fundamental aspects, from those of a circular cylinder reported by Prasanth and Mittal (2008a,b). Compared to the circle, a much narrower lock-in zone along with smaller (about 50%) transverse amplitude is observed for the square for Re ≤ 150 . The lock-in extremities of a circular cylinder for Re ≤ 150 and B=0.05 are associated with two hysteresis loops as opposed to one for a square cylinder.