Effects Of Freestream Turbulence Intensity Research Articles

Drag on circular cylinders with porous outer layers in turbulent cross-flow

The effects of free-stream turbulence intensity and porosity on the drag on cylinders with porous outer layers in cross-flow was investigated experimentally. This work is motivated by the need to better model spotting—a forest fire propagation mechanism in which burning branches and other debris (termed firebrands) are transported away from the main fire by the prevailing wind and ignite new fires. Multiple levels of background turbulence were studied by using no grid, passive grids, and an active grid to generate turbulence intensities of 0.4%, 1.7%, 2.7% and 12.4%. The porous char-layer on the outer surface of firebrands was mimicked by wrapping wire meshes (of 10, 20 and 40 pores per cylinder diameter or PPD) around the cylinders, each to three different layer-thickness fractions ( 1/16 , 1/8 and 1/4 ) of the cylinder’s outer diameter. The drag on one smooth cylinder and nine cylinders with porous outer layers was measured for Reynolds numbers in the range 7000–17 000, at the aforementioned four turbulence intensities. The results showed that (i) the free-stream turbulence intensity and PPD of the wire meshes affect the Reynolds number dependence of the drag coefficient; (ii) the drag coefficient increases with free-stream turbulence intensity when it is relatively low (0.4%–2.7%), then decreases at high intensities (12.4%), and this decrease is more pronounced as the PPD increases; (iii) the drag coefficient increases with the thickness of the porous layer, and asymptotes to an effectively constant value after a critical thickness of about 1/8 of the cylinder’s diameter; and iv) the drag coefficient exhibits a non-monotonic dependence on the PPD of the wire mesh. These results demonstrate the importance of accounting for free-stream turbulence intensity and the parameters that characterize the porous outer layers of cylinders when modeling the drag on firebrands, or in other applications in which there is cross flow over cylinders with porous outer surfaces.

On the wake characteristics of a model wind turbine and a porous disc: Effects of freestream turbulence intensity

Porous discs are the experimental counterparts of the numerical actuator disc concept and are frequently used in wind tunnel studies to reproduce the far-wake characteristics of wind turbines. The potential of using porous discs instead of wind turbines comes with the benefits such as operational easiness and design simplicity. This paper presents the results of an experimental study focusing on the wake development characteristics of a single porous disc and a single wind turbine under different freestream turbulence intensities but at similar Reynolds number conditions. The measurements are performed using two-dimensional two-component particle image velocimetry (2D2C PIV) technique up to seven diameters downstream distance in the wake of the models. The wake flows are analyzed in terms of the development of mean wake velocity deficit, wake decay, and wake turbulence characteristics. The results show that the wake recovery occurs faster for both the porous disc and the turbine with increasing freestream turbulence intensity. The wake width increases with the freestream turbulence intensity, indicating faster diffusion of the wake for both models. On the other hand, the wake width of the porous disc and the turbine is similar at the same turbulence intensity conditions, although they operate with slightly different thrust coefficients. Turbulent kinetic energy profiles in the far wake of the porous disc and the turbine, which are significantly different for the low turbulence intensity conditions, become similar for the high turbulence intensity case. The comparison of the turbulent kinetic energy, decay rate and production characteristics points to different turbulence mechanisms in the wake of the porous disc and the turbine. In the near wake, the wake decay rate and turbulent kinetic energy levels are higher in the porous disc case due to enhanced mixing levels associated with the grid-generated turbulence. In the far wake, the decay rate of the turbine wake is higher, most probably due to the breakdown of the tip vortices. When the freestream turbulence is increased, the differences in the turbulent wake flow characteristics diminish in the far wake, particularly after a 4.5-diameter downstream distance.

Proper Orthogonal Decomposition (POD) of the Wake Flow Field of a Model Wind Turbine and a Porous Disc under Different Freestream Turbulence Intensity Conditions

The effects of freestream turbulence intensity on the wake development of a model wind turbine and a porous disc are investigated through Proper Orthogonal Decomposition (POD) analysis. The capability of porous discs for reproducing far-wake characteristics of a model wind turbine is examined through coherent structures both in the near-wake and far-wake regions. Instantaneous velocity fields are obtained through wake measurements using two-dimensional two-component particle image velocimetry (2D2C PIV). These velocity fields are considered snapshots of the spatial domain. Results show inherent differences between coherent structures of a model wind turbine and porous disc in the near-wake region, especially when the freestream turbulent intensity level is low. However, these differences reduce, and coherent structures become more comparable when the freestream turbulence intensity level is higher. It is shown that the first five streamwise components of POD modes are paired for the model wind turbine and the porous disc cases under high freestream turbulence intensity conditions in the far-wake region.

Effects of free-stream turbulence intensity on transition in a laminar separation bubble formed over an airfoil

The laminar-to-turbulent transition process in a laminar separation bubble formed over a NACA 0018 airfoil is investigated experimentally. All experiments are performed for an angle of attack of 4 $$^{\circ }$$ , chord Reynolds numbers of 80,000 and 125,000, and free-stream turbulence intensities between 0.06 and 1.99%. The results show that increasing the level of free-stream turbulence intensity leads to a decrease in separation bubble length, attributed to a downstream shift in mean separation and an upstream shift in mean reattachment, the later ascribed to an upstream shift in mean transition. Maximum spatial amplification rates of disturbances in the separated shear layer decrease with increasing free-stream turbulence intensity, implying that the larger initial amplitudes of disturbances are solely responsible for the upstream shift in mean transition and as a result mean reattachment. At the baseline level of turbulence intensity, coherent structures forming in the aft portion of the bubble are characterized by strong spanwise coherence at formation, and undergo spanwise deformations leading to localized breakup in the vicinity of mean reattachment. As the level of free-stream turbulence intensity is increased, the spanwise coherence of the shear layer rollers is reduced, and spanwise undulations in the vortex filaments start to take place at the mean location of roll-up. At the highest level of turbulence intensity investigated, streamwise streaks originating in the boundary layer upstream of the separation bubble are observed within the bubble. These streaks signify an onset of bypass transition upstream of the separation bubble, which gives rise to a highly three-dimensional shear layer roll-up. A quantitative analysis of the associated changes in salient characteristics of the coherent structures is presented, connecting the effect of elevated free-stream turbulence intensity on the time-averaged and dynamic characteristics of the separation bubble.

Turbulence Intensity Effects on Laminar Separation Bubbles Formed over an Airfoil

The effects of freestream turbulence intensity on the mean topology and transition characteristics of laminar separation bubbles forming over the suction side of a NACA 0018 airfoil are investigated experimentally for angles of attack between 0 and 20 deg, chord Reynolds numbers between 100,000 and 200,000, and freestream turbulence intensities between 0.09 and 2.03%. The results show that increasing freestream turbulence intensity results in earlier transition and reattachment, contributing to an overall decrease in separation bubble length. At lower angles of attack, this is accompanied by a minor decrease in lift, whereas at prestall angles of attack and higher turbulence intensity levels, lift increases and stall is delayed. Spatial amplification rates of disturbances in the separated shear layer are shown to decrease at elevated levels of turbulence intensity, indicating that the earlier transition is attributed solely to the larger initial amplitude of perturbations. At elevated turbulence intensity levels, a broader range of unstable frequencies is detected in the separated shear layer, with the central frequency of the unstable band showing moderate variations with the level of freestream perturbations. The results indicate a change in the transition process at higher freestream turbulence intensity levels, indicative of bypass transition in the boundary layer. The degree of influence of freestream turbulence intensity on the separation bubble is shown to decrease as the chord Reynolds number is increased.

Effect of Free-Stream Turbulence Intensity on Transonic Airfoil with Shock Wave

Airplanes regularly operate switching between various flight modes such as take-off, climb, cruise, descend and landing. During these flight conditions the free-stream approaching the wings undergo fundamental changes. In transonic flow conditions, typically in the military or aerospace applications, existence of nonlinear and unsteady effects of the airflow stream significantly alters the performance of an airfoil. This paper presents the influence of free-stream turbulence intensity on transonic flow over an airfoil in the presence of a weak shock wave. In particular, NACA 0012 airfoil performance at Ma∞ = 0.7 is considered in terms of drag, lift, turbulence kinetic energy, and turbulence eddy dissipation parameters under the influence of varying angle of attacks and free-stream turbulence. The finite volume method in a commercial CFD package ANSYS-CFX is used to perform the numerical analysis of the flow. Mesh refinement using a mesh-adaption technique based on velocity gradient is presented for more accurate prediction of shocks and boundary layers. A Shear Stress Transport (SST) turbulence model is validated against experimental data available in the literature. Numerical simulations were performed, with free stream turbulence intensity ranging from low (1%), medium (5%) to high (10%) levels. Results revealed that drag and lift coefficients are approximately the same at every aforementioned value of turbulence intensity. However, turbulence kinetic energy and eddy dissipation contours vary as turbulence intensity changes, but their changes are disproportionally small, compared with values adopted for free-stream turbulence.

Inspection of the dynamic properties of laminar separation bubbles: free-stream turbulence intensity effects for different Reynolds numbers

The effects of free-stream turbulence intensity (FSTI) on the transition process of a pressure-induced laminar separation bubble have been studied for different Reynolds numbers (Re) by means of time-resolved (TR) PIV. Measurements have been performed along a flat plate installed within a double-contoured test section, designed to produce an adverse pressure gradient typical of ultra-high-lift turbine blade profiles. A test matrix spanning 3 FSTI levels and 3 Reynolds numbers has been considered allowing estimation of cross effects of these parameters on the instability mechanisms driving the separated flow transition process. Boundary layer integral parameters, spatial growth rate and saturation level of velocity fluctuations are discussed for the different cases in order to characterize the base flow response as well as the time-mean properties of the Kelvin–Helmholtz instability. The inspection of the instantaneous velocity vector maps highlights the dynamics of the large-scale structures shed near the bubble maximum displacement, as well as the low-frequency motion of the fore part of the separated shear layer. Proper Orthogonal Decomposition (POD) has been implemented to reduce the large amount of data for each condition allowing a rapid evaluation of the group velocity, spatial wavelength and dominant frequency of the vortex shedding process. The dimensionless shedding wave number parameter makes evident that the modification of the shear layer thickness at separation due to Reynolds number variation mainly drives the length scale of the rollup vortices, while higher FSTI levels force the onset of the shedding phenomenon to occur upstream due to the higher velocity fluctuations penetrating into the separating boundary layer.

The Effect of Freestream Turbulence on Deposition for Nozzle Guide Vanes

An evaluation of the effect of freestream turbulence intensity on the rate of deposit accumulation for nozzle guide vanes (NGVs) was performed using the turbine reacting flow rig (TuRFR) accelerated deposition facility. The TuRFR allowed flows up to 1350 K at inlet Mach numbers of 0.1 to be seeded with coal fly ash particulate in order to rapidly evaluate deposit formation on CFM56 NGVs. Hot film and particle image velocimetry (PIV) measurements were taken to assess the freestream turbulence with and without the presence of a grid upstream of the NGVs. It was determined that baseline turbulence levels were approximately half that of the flow exiting typical gas turbine combustors and were reduced by approximately 30% with the grid installed. Deposition tests indicated that the rate of deposition increases as the freestream turbulence is increased, and that this increase depends upon the particle size distribution. For ash with a mass median diameter of 4.63 μm, the increase in capture efficiency was approximately a factor of 1.77, while for ash with a larger median diameter of 6.48 μm, the capture efficiency increased by a factor of 1.84. The increase in capture efficiency is due to the increased diffusion of particles to the vane surface via turbulent diffusion. Based on these results, smaller particles appear to be less susceptible to this mechanism of particle delivery. Overall, the experiments indicate that the reduction of turbulence intensity upstream of NGVs may lead to reduced deposit accumulation, and consequently, increased service life. A computational fluid dynamics (CFD) analysis was performed at turbulence levels equivalent to the experiments to assess the ability of built-in particle tracking models to capture the physics of turbulent diffusion. Impact efficiencies were shown to increase from 21% to 73% as the freestream turbulence was increased from 5.8% to 8.4%. An analysis incorporating the mass of the particles into the impact efficiency resulted in an increase of the mass-based impact efficiency from 17% to 27% with increasing turbulence. Relating these impact efficiencies directly to capture efficiencies, the predicted increase in capture efficiency with higher turbulence is less than that observed in the experiments. In addition, the variation in the impact efficiencies between the two ash sizes was smaller than the capture efficiency difference from experiments. This indicates that the particle tracking models are not capturing all of the relevant physics associated with turbulent diffusion of airborne particles.

Effects of freestream turbulence on separated boundary layer in a low-Re high-lift LP turbine blade

The effects of inlet freestream turbulence intensity (FSTI) on aerodynamic performance as well as temporal and spatial evolution of coherent structures in separated shear layers are investigated in this paper. Large-eddy simulations (LES) are performed on the T106D-EIZ profile at Reynolds number (Re) of 60,154 (based on the chord and outflow velocity). Four cases are conducted on the conditions without upstream wakes but with FSTI of 0, 2.5%, 5.0% and 10.0% respectively. The results show that the open separations cannot be removed in these working conditions, but could be compressed by FSTI to a certain extent. Meanwhile, the coherent structures in separated shear layers are affected remarkably. The obvious three-dimensional perturbations appear earlier in separated shear layers as FSTI enhanced, which make the spanwise vortices roll up earlier and break down more quickly, therefore, the vortex pairing cannot occur even under relatively low FSTI conditions. The cases in this paper state clearly that the effects of FSTI are more prominent when it is lower than 5%. It is also found that the enhanced FSTI can increase the instability Strouhal numbers of separated shear layers. Nevertheless, the values of the instability Strouhal numbers are still within the typical level.

Wind loads on a residential solar water heater

This experimental study aims to address the effects of freestream turbulence intensity (TI) on wind loads on a residential solar water heater with and without a guide plate. Highly turbulent flow was produced by a turbulence generation grid at the inlet of the working section. Results show that the distributions of mean longitudinal and spanwise pressures are associated with tilt angle, guide plate, and freestream TI. Interactions between separation and corner vortices on the upper surface and impingement of shear layer from the tip of a guide plate on the lower surface would induce higher levels of fluctuating pressure, particularly in turbulent flow. A similarity parameter α/A* is also proposed to scale the uplift coefficients in both smooth uniform and turbulent flows.

Numerical Investigation of Laminar to Turbulent Boundary Layer Transition on a Naca 0012 Airfoil for Vertical-Axis Wind Turbine Applications

Laminar to turbulent transition in vertical-axis wind turbines has a dramatic effect on overall rotor performance, especially in fast rotating machines, where reliable prediction of the total drag coefficient for high values of tip speed ratio is one of the most critical topic in CFD simulations. This paper presents a 2D numerical investigation of the capability of the γ–θ transition model to predict the laminar to turbulent transition and consequent friction drag over a NACA 0012 airfoil for a Reynolds number of 360,000, which is typical of vertical-axis wind turbine blades during operation. The analized range of angles of attack varies from 0 deg to 10 deg. The commercial CFD solver ANSYS FLUENT® is used. In particular, the sensitivity to grid resolution is investigated for four different architectures: a completely unstructured mesh, a hybrid structured-triangular one and two distinct hybrid structured-triangular meshes where the wake region behind the airfoil is discretized using a fully structured grid. The effect of freestream turbulence intensity on the transition onset is also analyzed. Finally, CFD results are compared to experimental data, although affected by some uncertainty, and to the predictions of an interactive program for the design and analysis of subsonic isolated airfoils (XFOIL), showing a very good agreement provided that the value of freestream turbulence intensity is known.

Effect of Freestream Turbulence Intensity on Film Cooling Jet Structure and Surface Effectiveness Using PIV and PSP

An experimental investigation of film cooling jet structure using two-dimensional particle image velocimetry (PIV) has been completed for cylindrical, simple angle (θ=35 deg) film cooling holes. The PIV measurements are coupled with detailed film cooling effectiveness distributions on the flat plate obtained using a steady state, pressure sensitive paint (PSP) technique. Both the flow and surface measurements were performed in a low speed wind tunnel where the freestream turbulence intensity was varied from 1.2% to 12.5%. With this traditional film cooling configuration, the blowing ratio was varied from 0.5 to 1.5 to compare the jet structure of relatively low and high momentum cooling flows. Velocity maps of the coolant flow (in the streamwise direction) are obtained on three planes spanning a single hole: centerline, 0.25D, and 0.5D (outer edge of the film cooling hole). From the seeded jets, time averaged, mean velocity distributions of the film cooling jets are obtained near the cooled surface. In addition, turbulent fluctuations are obtained for each flow condition. Combining the detailed flow field measurements obtained using PIV (both instantaneous and time averaged) with detailed film cooling effectiveness distributions on the surface (PSP) provides a more complete view of the coolant jet-mainstream flow interaction. Near the edge of the film cooling holes, the turbulent mixing increases, and as a result the film cooling effectiveness decreases. Furthermore, the PIV measurements show the increased mixing of the coolant jet with the mainstream at the elevated freestream turbulence level resulting in a reduction in the jet to effectively protect the film cooled surface.

The effects of freestream turbulence on the drag coefficient of a sphere

The effects of freestream turbulence intensity and integral length scale as freestream turbulent parameters on the drag coefficient of a sphere were experimentally investigated in a closed circuit wind tunnel. The Reynolds number, Re = Ud/ ν, was varied from 2.2 × 10 4 to 8 × 10 4 by using spheres with diameter d of 20, 51 and 102 mm in addition to altering the freestream velocity, U. The freestream turbulence intensity Tu and flow integral length scale Λ were manipulated by the utilization of orifice perforated plates. The proper combination of orifice perforated plate hole diameter, sphere size, and sphere location along the center line of the wind tunnel enabled the independent alterations of turbulence intensity and relative integral length scale ( Λ/ d) from 1.8% to 10.7% and from 0.1 to 2.6, respectively, at each studied Reynolds number. Results show that over the range of conditions studied, the drag always decreases with increasing Tu and, the critical Reynolds number at which the drag coefficient is dramatically reduced is decreased by increasing Tu. Most interestingly, the drag at any particular Re and Tu may be significantly lowered by reducing Λ/ d; this is particularly the case at high Re and Tu.

Droplet Evaporation in a Turbulent Environment at Elevated Pressure and Temperature Conditions

A three-dimensional numerical model is developed to assess the effect of freestream turbulence on the vaporization of n-heptane droplet, which is exposed to a freestream of nitrogen at elevated pressure and subcritical temperature conditions. The freestream pressure, temperature and turbulence intensity are varied in the range of 0.5–10 MPa, 324–502 K, and 0%–60%, respectively. Variable thermophysical properties, the unsteadiness behavior of the gas and liquid phases, as well as heat transfer by radiation are all considered. In addition, non-ideality behavior of the gas phase, solubility of the gas into the droplet and pressure dependence of the gas-phase thermophysical properties are also accounted for. The turbulence terms in the conservation equations of the gas-phase are modeled by using the shear-stress transport (SST) model. The results show that, for the temperature range (T∞ < Tc) explored in the present study, the droplet lifetime increases, and thereby the vaporization rate decreases, as pressure rises. However, the effect of pressure gradually diminishes as the ambient temperature increases and vanishes when T∞ approaches the critical temperature of n-heptane, Tc. Moreover, the effect of freestream turbulence intensity, which is found to enhance droplet heat and mass transfer, weakens as pressure increases. Finally, droplet turbulent heat and mass transfer correlations are proposed which account for all the aforementioned parameters.

Effect of Turbulence Intensity and Periodic Unsteady Wake Flow Condition on Boundary Layer Development, Separation, and Reattachment Along the Suction Surface of a Low-Pressure Turbine Blade

This paper experimentally investigates the individual and combined effects of periodic unsteady wake flows and freestream turbulence intensity (FSTI) on flow separation along the suction surface of a low-pressure turbine blade. The experiments were carried out at a Reynolds number of 110,000 based on the suction surface length and the cascade exit velocity. The experimental matrix includes freestream turbulence intensities of 1.9%, 3.0%, 8.0%, and 13.0%, and three different unsteady wake frequencies with the steady inlet flow as the reference configuration. Detailed boundary layer measurements are performed along the suction surface of a highly loaded turbine blade with a separation zone. Particular attention is paid to the aerodynamic behavior of the separation zone at different FSTIs at steady and periodic unsteady flow conditions. The objective of the research is (i) to quantify the effect of FSTIs on the dynamics of the separation bubble at steady inlet flow conditions and (ii) to investigate the combined effects of Tuin and the unsteady wake flow on the behavior of the separation bubble.

원형단면 실린더를 지나는 유동에 대한 자유류 난류강도의 영향

In this study, the effects of freestream turbulence intensity on laminar-turbulent transition of separated shear layers In the wake of a circular cylinder are Investigated using an immersed boundary method and LES. It is shown that the present numerical results Without freestream turbulence for Re=3,900 based on bulk mean velocity and the cylinder diameter are In good agreement With other authors' experimental observations and numerical results, verifying our numerical methodology. Then a 'prescribed power spectrum method is Imposed to generate isotropic turbulence at the mlet of the computational domain at each time step The principal effects of freestream turbulence Intensity on flow statistics are investigated for Re=3,900 Statistical study reveals that the Reynolds stresses in the near-wake region gradually Increase, and transition occurs further upstream, as the turbulence intensity Increases. On the other hand, the bubble Size behind the cylinder decreases as the turbulence Intensity increases, which indicates that the freestream turbulence helps mean velocity be quickly recovered<br/>

ON THE AEROELASTIC BEHAVIOUR OF RECTANGULAR CYLINDERS IN CROSS-FLOW

An experimental and numerical study of the aeroelastic behaviour of elongated rectangular and square cylinders is presented. The main results are for a rectangular section with an aspect ratio of 2. The experiments were performed with a flexible cylinder clamped at both ends. This configuration leads to unusual lock-in of the vortex shedding with different bending modes, although the final steady oscillations occur in the fundamental mode. The galloping regime is also investigated, and the effect of free-stream turbulence intensity. Critical velocities are detected which do not correspond to calculations using the quasi-steady theory. A simple modelling of galloping is proposed to better fit the experiments, but it is shown that some of the configurations, in turbulent flow, are probably interacting with the vortex shedding and make the modelling inefficient. Numerical simulations on a 2-D rectangular section are presented and the resulting wall pressure distributions are analysed using the proper orthogonal decomposition technique. Indicators are proposed in order to link the proper functions with their contribution to the aerodynamic force components, and then a classification of the proper shapes of the decomposition is done. It is shown by comparison between the static case and forced oscillations, in the galloping range, that secondary vortices inside the shear layer become symmetrical and their effect on the forces is cancelled.

Turbulence Effect on Frequency Characteristics of Unsteady Motions in Wake of Wing

The ine uences of the freestream turbulence intensity on the frequency characteristics of the instability wave and vortex shedding in the wake of a NACA 0012 wing model are studied experimentally. The functional relationships among the Strouhal, Roshko, and Reynolds numbers in the viscous-effect-dominated and the inertialeffect-dominated regimesareobtained. Thefreestream turbulence hasapparenteffect on theStrouhal and Roshko numbers in the regimes of instability wave and laminar and subcritical vortex-shedding modes that are obtained at low Reynolds numbers. However, at large Reynolds numbers in the supercritical vortex-shedding mode, particularly at large angles of attack, the effect of freestream turbulence intensity on the frequency selection is not signie cant. For the instability wave, the Roshko number decreases with the increase of freestream turbulence and then approaches constant values when the freestream turbulence intensity is larger than a critical value between 0.45 and 0.5%. However, the Strouhal number of the vortex shedding of laminar and subcritical modes increases signie cantly with an increase of turbulence intensity and then decreases when the freestream turbulence intensity is larger than a critical value between 0.45 and 0.5%. The change of the characteristic surface e ow modes may contribute to the dominant mechanism of the existence of critical freestream turbulence intensity. Nomenclature C = chord length of wing, 6 cm d = length of wing-section projection on cross-stream plane f = frequency of instabilities in wake region, Hz ` = distance in X direction measured from mesh screen M = mesh size of turbulence generation screen Rec = Reynolds number based on chord length of wing, uwC/m Red = Reynolds number based on cross-stream projection of wing section, uwd/m Rod = Roshko number of vortex shedding, f d 2 /m Srd = Strouhal number of vortex shedding, f d/uw T = freestream turbulence intensity u = x component of local instantaneous velocity uw = average freestream velocity X = streamwise coordinate, originated from leading edge of wing model on root-plane Y = spanwise coordinate, originated from leading edge of wing model on root-plane Z = crosse ow coordinate, originated from leading edge of wing model on root-plane a = root angle of attack m = kinetic viscosity of air stream q = density of air stream U = power spectrum of e uctuating velocities

Effects of Freestream Turbulence on Wing-Surface Flow and Aerodynamic Performance

The effects of freestream turbulence intensity on the surface-e ow characteristics, including separation, bubble, leading-edge bubble, turbulent separation, and three-dimensional e ow, as well as the corresponding aerodynamic performance, including lift, drag, and stall, of a cantilever NACA 0012 wing model have been studied in a wind tunnel. Different wire-mesh screens were placed between the nozzleand test section to produce various freestream turbulence intensities. The characteristic regimes of surface e ow were identie ed by using the surface-oil e ow technique. The aerodynamic lift and drag were measured with a PC-based force/moment sensing system. The variationsofliftanddragwerefoundtobecloselyrelatedtothebehaviorofsurfacee ow.Thefreestreamturbulence signie cantlyaffectsthesurfacee owandaerodynamicperformance.Thesurfacee owandlift/dragatlowfreestream turbulenceareapparentlydifferentfrom thatatlargefreestreamturbulenceintensity.Theine uenceisdrasticwhen turbulence intensities are smaller than about 0.45%, whereas it becomes insignie cant when turbulence intensities are greater than 0.45%. Details of the ine uences are discussed and illustrated.

Probabilistic characteristics of pressure fluctuations in separated and reattaching flows for various free-stream turbulence

Abstract The statistical characteristics of the fluctuating pressures acting on bluff bodies in the region of flow separation are investigated in this paper. Probability density functions of the pressure fluctuations have shown a significant non-Gaussian behaviour. The lognormal density function is found to be in close agreement with the obtained experimental data. A probability distribution for the maximum magnitude of peak pressures expected during a period is proposed and the agreement between the experimental data and predicted results are good. The effects of free-stream turbulence intensity and integral scale on the statistical nature of pressure fluctuations are presented and discussed. The second part of this paper concerns the action of fluctuating wind pressure on building cladding. The effect of free-stream turbulence intensity and integral scale in model test on equivalent constant pressures for the purpose of cladding design is investigated, making use of direct integration of measured pressure data in the wind tunnels.