Low Turbulence Intensity Research Articles

Experimental investigations of boundary layer impact on the airfoil aerodynamic forces of Horizontal Axis Wind Turbine in turbulent inflows

In order to check whether the developed airfoil UMY02-T01-26 can improve aerodynamic performance, a multiport device is used to investigate the pressure distribution acting on a single blade surface under inflow conditions. In this experiment, aerodynamic forces are discussed with different Reynolds numbers of Re = 0.5 × 105, 1.0 × 105, 1.5 × 105 and 2.0 × 105. Furthermore, for the purpose of clarifying the impact of turbulence intensity, the aerodynamic forces characteristics of HAWT airfoil are estimated in the case of different turbulence intensities generated by static turbulence grids. According to the wind tunnel experimental analysis, it can be known that the blade boundary layer tape has no significant effects on the drag coefficient with or without turbulence grids. For the low turbulence intensity (without grid), the airfoil performance is improved when the boundary layer tape is attached near the leading edge. For the high turbulence intensity (with grid), in the case of the Re = 0.5 × 105, the pressure coefficient presents a steady value from some back positions of x/c = 0.20, where the boundary layer tape is attached on the trailing edge position.

An experimental study of the wake of a model wind turbine using phase-averaging

The wake of a model wind turbine with a rotor diameter of 0.9 m has been investigated at the design condition (tip speed ratio = 6) in a closed return wind tunnel with a cross-section of 1.8 by 2.7 m. The three bladed rotor was operated in a low turbulence intensity uniform flow. Velocity data were obtained in the wake using a four-wire hot-wire probe at 18 radial traverses from x/D=0.22 to 5. All three components of the velocity vector were resolved instantaneously for a wide range of flow angles and velocity magnitudes. In addition to time averaged data, phase-averages with respect to rotor position have been used to extract periodic phase-averaged motions from the flow. In this way the downstream development of the tip vortices was extracted. The phase-averaging method allows the local phase-averaged and turbulent parts of the stresses associated with the vortical motion to be studied, as well as their production terms. The results show that the periodic phase-averaged structures initially contains most of the turbulent kinetic energy and dominate the edge of the wake. The helical vortex structures interact and pair-up, merge and finally breaks up within x/D=3. From this point the phase-averaged motion is lost and the radial transport of momentum across the wake is seen to increase significantly. By x/D=5 the large scale diffusion has removed the sharp edge of the wake that was characteristic in the initial region, and developed the approximately Gaussian velocity defect which is characteristic of bluff body wakes.

Calculations of flow around airfoils using two-dimensional RANS: an analysis of the reduction in accuracy

Flows around sets of airfoils with different shapes and thicknesses have been numerically investigated at relatively high Reynolds numbers (Re ≥ 106) and low turbulence intensity (I ≤ 0.1%) using two-dimensional (2D) Reynolds-Averaged Navier-Stokes equations (RANS) closed by different turbulence models. The effects of a set of factors such as wind tunnel walls, the compressibility and the influence of the laminar-turbulent transition were investigated. The most likely reasons for the systematic disagreement between simulation and experimental data were established to be 3D effects ignored by 2D simulation, and imperfection of the existing turbulence models.

Blind test comparison of the performance and wake flow between two in-line wind turbines exposed to different turbulent inflow conditions

Abstract. This is a summary of the results of the fourth blind test workshop that was held in Trondheim in October 2015. Herein, computational predictions on the performance of two in-line model wind turbines as well as the mean and turbulent wake flow are compared to experimental data measured at the wind tunnel of the Norwegian University of Science and Technology (NTNU). A detailed description of the model geometry, the wind tunnel boundary conditions and the test case specifications was published before the workshop. Expert groups within computational fluid dynamics (CFD) were invited to submit predictions on wind turbine performance and wake flow without knowing the experimental results at the outset. The focus of this blind test comparison is to examine the model turbines' performance and wake development with nine rotor diameters downstream at three different turbulent inflow conditions. Aside from a spatially uniform inflow field of very low-turbulence intensity (TI = 0.23 %) and high-turbulence intensity (TI = 10.0 %), the turbines are exposed to a grid-generated highly turbulent shear flow (TI = 10.1 %).Five different research groups contributed their predictions using a variety of simulation models, ranging from fully resolved Reynolds-averaged Navier–Stokes (RANS) models to large eddy simulations (LESs). For the three inlet conditions, the power and the thrust force of the upstream turbine is predicted fairly well by most models, while the predictions of the downstream turbine's performance show a significantly higher scatter. Comparing the mean velocity profiles in the wake, most models approximate the mean velocity deficit level sufficiently well. However, larger variations between the models for higher downstream positions are observed. Prediction of the turbulence kinetic energy in the wake is observed to be very challenging. Both the LES model and the IDDES (improved delayed detached eddy simulation) model, however, consistently manage to provide fairly accurate predictions of the wake turbulence.

Experimental study of the effects of free stream turbulence on characteristics and flame structure of bluff-body stabilized conical lean premixed flames

This article describes an experimental investigation of the effect of different levels of free stream turbulence intensity on the structure of an unconfined, lean propane–air flame stabilized on an axisymmetric bluff body. Simultaneous imaging of hydroxyl (OH) and formaldehyde (CH2O) by planar laser induced fluorescence and particle image velocimetry (PIV) was used to study the interaction between the flame and the flow field. CH2O fluorescence and the pixel-by-pixel multiplication of OH and CH2O fluorescence signals were utilized to mark preheat and heat release regions respectively. At low turbulence intensity, flame front featured weak corrugations and mostly symmetric flame structures with continuous heat release regions. Pronounced formation of cusps and unburnt mixture fingers were observed as the turbulence intensity was increased from 4% to 14% without discontinuities in the heat release front. The local flame structure was observed to be strongly modified for the high turbulence intensities with the occurrence of flamelet merging, localized extinctions along the shear layer and flame fragmentations. The general shape of the flame was observed to intermittently switch from a symmetric (varicose) to asymmetric (sinuous) mode. Several flame properties were determined to characterize the effects of turbulence–flame interaction which includes the average preheat and reaction zone thicknesses, strain rates and curvature along the flame front, flame brush thickness, flame surface density, flame area ratio and turbulent flame speed. With increasing turbulence levels, the preheat zone was observed to become thicker while reaction zone thickness reduced. The curvature and strain rates pdfs broadened with increasing levels of turbulence intensity. The flame brush thickness showed an initial increase with increasing turbulence levels but saturated beyond 24% turbulence intensity. Measurements showed a reduction in flame surface density with increasing levels of turbulence intensity. The turbulent flame area ratio increased with turbulence intensity but at a reduced rate at higher turbulence intensities.

Effect of turbulence intensity and particle characteristics on the deposition of submicron particles enhanced by the ionic air purifier

A negative air ionizer (NAI) is a common indoor air purifier for aerosol particles. The turbulence intensity can influence the performance of NAI. Besides, the dielectric constants (p) of particles can affect the electric mobility and deposition of particles in the electric field. Hence, this study aims to exam the difference between the deposition rate of NaCl (p = 6.1) and sucrose (p = 3.3) submicron particles when an NAI is operating under various turbulence intensities.The experiments were conducted in a stainless steel chamber under 50% relative humidity. Polydisperse submicron particles (PSPs) of 30–300 nm and monodisperse submicron particles (MSPs) of 30, 50, 100, 170 and 300 nm were used for testing. In the experiments of particle deposition, the aerosol particle number concentration and size distribution were monitored continuously by a Scanning Mobility Particle Sizer. The decay constant of particle concentration (k) and effective cleaning rate (ECR) were determined from the time profiles of particle concentration.When the NAI was off, k of NaCl and sucrose particles was similar. However, when the NAI was operating, the deposition rate of NaCl particles was higher than that of sucrose particles and the NAI performed better under lower turbulence intensity. Because the coagulation coefficient of PSP was larger than MSP, the decay constant of PSP was superior to that of MSP. The ECR was negatively correlated with particle size and was higher under lower turbulence intensity.Conclusively, the NAI is more efficient in charging particles with higher dielectric constants and performance better under lower turbulence intensity.

Best location of the small wind turbine around a single high-rise building

ABSTRACTThis research is a three-dimensional investigation about the aerodynamic interaction between the wind flow and a single high-rise building. In order to find location(s) with high potential of velocity around the building, a wide variety of wind speeds ranging from 2 to 10 m/s is studied. On the other hand, a high-rise building with the ratio of height to width of H/W = 3 is considered. Computations are performed numerically by means of the finite volume approach. Several results are obtained in the present numerical study. For example, it is found that due to wind-structure vertical interaction, locations with enhanced velocities are developed on the building roof in which the rate of this enhancement increases with increasing the wind speed. In addition, over the building, “lines C and D” are realized as the best locations having high power potentials and low turbulence intensities. In addition, lateral wind-structure interaction revealed that for all wind speeds, location of L/W = 0.5 is the best for the small wind turbine installation.

Thermal Field Measurements for a Shaped Hole at Low and High Freestream Turbulence Intensity

Shaped holes are increasingly selected for airfoil cooling in gas turbines due to their superior performance over that of cylindrical holes, especially at high blowing ratios. The performance of shaped holes is regarded to be the result of the diffused outlet, which slows and laterally spreads coolant, causing coolant to remain close to the wall. However, few thermal field measurements exist to verify this behavior at high blowing ratio or to evaluate how high freestream turbulence alters the coolant distribution in jets from shaped holes. The present study reports measured thermal fields, along with measured flowfields, for a shaped hole at blowing ratios up to three at both low and high freestream turbulence intensities of 0.5% and 13.2%. Thermal fields at low freestream turbulence intensity showed that the coolant jet was initially attached, but far downstream of the hole the jet lifted away from the surface due to the counter-rotating vortex pair. Elevated freestream turbulence intensity was found to cause strong dilution of the coolant jet and also increased dispersion, almost exclusively in the lateral as opposed to the vertical direction. Dominance of lateral dispersion was due to the influence of the wall on freestream eddies, as indicated from changes in turbulent shear stress between the low and high freestream turbulence cases.

Transition to turbulence by interaction of free-stream and discrete mode perturbations

Mixed mode transition is studied by direct numerical simulation. Low frequency streaks are induced within the boundary layer by free-stream turbulence and an Orr-Sommerfeld discrete mode eigenfunction is introduced at the inlet. Amplitudes are selected such that the interaction of these modes can cause transition. Aside from the highest amplitude of free-stream turbulence, neither disturbance alone is sufficient to cause transition within the flow domain. Results are classified into three routes to transition, depending upon the 2D Tollmien-Schlichting (TS) mode strength and free-stream turbulence intensity. (1) At low turbulence intensities, secondary instabilities instigate transition. On a strong TS mode, Λ vortices develop, but they are neither H nor K type. The pattern and spanwise size of Λ vortices depend upon the frequency and spanwise width of Klebanoff streaks by which they are generated. (2) When the TS mode amplitude is low, transition is via streak breakdown. The streaks are induced by the free-stream turbulence, but this case differs from conventional bypass transition in the mechanism of inception of turbulent spots. Three dimensional visualizations of the perturbation flow field show growing, helical undulations similar to n = 1 instability modes observed in axisymmetric jets and wakes. (3) At high turbulence intensities, the flow undergoes bypass transition. The TS wave has a small effect, but its influence is seen at the larger of the two amplitudes studied.

Experimental Investigation of the Unsteady Flow Behavior on a Film Cooling Flat Plate

An experimental study was conducted to investigate the unsteady behavior of film cooling for cylindrical holes on a flat plate wind tunnel. Tests have been carried out at low speed and low inlet turbulence intensity level, with blowing ratios varied in the range 0.5–1.5. Aerodynamic investigations have been performed through the measurement of discharge coefficients in order to characterize the blowing conditions. A Laser Doppler Velocimetry (LDV) was then used to study the boundary layer behavior just downstream of the holes for variable injection condition. A high resolution PIV system was finally used for flow visualization and flow field measurement to investigate the unsteady mixing process taking place between coolant and main flow. For low blowing ratios, the jet stays close to the surface; traces of coherent vortical structures are detected only far from injection, which appear as clockwise vortices. Increasing the blowing ratio leads to the appearance of counter-clockwise vortical structures due to the breakdown of the Kelvin Helmholtz instability of the shear layers.

Experimental investigation of the influence of freestream turbulence on an anti-vortex film cooling hole

Film cooling is used in gas turbines to thermally protect combustor and turbine hot section components by creating a layer of relatively cooler air to insulate the components from the hot freestream gases. This relatively cooler air is taken from upstream in the compressor section at a loss to the engine efficiency and therefore must be used as effectively as possible. A novel anti-vortex hole (AVH) geometry has been investigated experimentally through a transient infrared thermography technique to study the film cooling effectiveness and heat transfer coefficient by varying blowing ratio and freestream turbulence intensity. The AVH geometry is designed with two secondary holes stemming from a main cooling hole that attempts to diffuse the coolant jet and mitigate the vorticity produced by conventional straight holes. The AVH geometry data collected showed improved cooling performance at low freestream turbulence intensities compared to conventional straight holes. Three turbulence intensities of 1, 7.5, and 11.7% were investigated for the AVH geometry at blowing ratios of 0.5, 1.0, 1.5, and 2.0 for a total of twelve different conditions. Results showed that higher freestream turbulence conditions were beneficial to the performance of the AVH. Increasing the blowing ratio at all turbulence levels also improved film cooling effectiveness, both span-averaged and on the centerline. The highest performing case was at a turbulence intensity of 7.5% and a blowing ratio of 2.0. The 11.7% cases outperformed the 1% cases, but it appears that at the 11.7% cases the higher freestream turbulence reduces the performance of the secondary holes compared to the 7.5% case. Increasing the blowing ratio and turbulence intensity will result in a higher heat transfer coefficient, which must be taken into account for future designs and overall reduction of heat load.

Direct Numerical Simulation of the bending effect in turbulent premixed flames

In turbulent premixed flames, much experimental evidence points to a strong influence of pre-mixture turbulence intensity on the turbulent burning velocity. The linear enhancement of turbulent burning velocity in low-intensity turbulence is predicted accurately by current models. In contrast, the deviation from linearity in high-intensity turbulence, known as the “bending effect,” remains to be explained. The present work has employed Direct Numerical Simulation (DNS) to investigate the bending effect. An initially laminar methane-air premixed flame was subjected to increasing levels of turbulence across five different simulations which maintained all parameters except the turbulence intensity constant. The bending effect was captured within these simulations. Subsequently, plausible explanations were investigated using the framework of the Flame Surface Density (FSD) approach. From the ensuing analysis, it is evident that flame surface area reflects distinctly the variation of turbulent burning velocity with turbulence intensity. Local flame quenching does not appear to be the primary mechanism behind the bending effect. Instead, the observed bending effect results from a shift in balance, under high-intensity turbulence, towards mechanisms that favour destruction of flame surface area. These mechanisms tend to preserve the reaction layer and, thereby, ensure the validity of Damköhler’s hypothesis and flamelet models in conditions that cause the bending effect that is observed here to occur.

Experimental study of the effect of turbulence on the structure and dynamics of a bluff-body stabilized lean premixed flame

The structure of an unconfined, lean propane–air flame stabilized on an axisymmetric bluff body is studied for different levels of turbulence intensity ranging from 4 to 30% in the approach flow. Simultaneous planar laser induced fluorescence imaging of hydroxyl radical (OH) and formaldehyde (CH2O) was performed to determine the effects of turbulence on local flame structure. In an accompanying experiment high speed particle image velocimetry at 5kHz was used to obtain the time resolved velocity field and evaluate flame surface density, brush thickness and probability density functions of curvature and strain rates conditioned on the flame front. The low turbulence intensity flame featured wrinkling and mostly symmetric flame structures with thin regions of heat release along the flame front. For moderate turbulence intensity (14%), pronounced formation of cusps and unburnt mixture fingers was observed with continuous heat release regions. However, the local flame structure was observed to be strongly altered for the 30% turbulence intensity. Localized extinction occurred along the flame surface creating isolated pockets of OH with heat release occurring along the boundary. Pockets of preheated reactants along with fresh reactants were observed inside the flame envelope. The overall flame appearance was observed to intermittently switch from varicose (symmetric) to sinuous (asymmetric) type (vortex shedding mode). The curvature and strain rates pdfs broadened with increasing levels of turbulence intensity. The flame brush thickness showed an initial increase with increasing turbulence levels but saturated beyond 24% turbulence intensity. Measurements showed reduction in flame surface density with increasing levels of turbulence intensity.

Design and performance of a small-scale aeroacoustic wind tunnel

The D5 aeroacoustic wind tunnel at Beihang University is a newly commissioned small-scale closed-circuit wind tunnel with low turbulence intensity and low background noise. The wind tunnel is built to study both aerodynamic and aeroacoustic performance of aircraft components or scaled models. The wind tunnel has two types of test sections, the closed type test section is used for aerodynamic tests while the open type test section is mainly used for aeroacoustic experiments. Both types of test section are 1m in height and 1m in width, and the maximum wind velocity in the test section can be up to 80m/s. An anechoic chamber is built surrounding the test section to provide the non-reflecting condition. This paper provides an overview of design criteria and performance of the small-scale wind tunnel. The layout of the wind tunnel and some critical design treatments to improve aerodynamic and acoustic performance are discussed in detail. Some experiments are conducted to verify the performance of D5 wind tunnel, results confirm that the turbulence intensity is less than 0.08% in the core of test section and the background noise is comparable with other aeroacoustic wind tunnels. A scaled simplified nose landing gear model is also measured as a benchmark test, results reveal that noise radiated from the model is adequately higher than the background noise for a wide frequency range and remarkably consistent with other results from literatures.

Exploring the wakes of large offshore wind farms

Offshore meteorological characteristics set specific conditions for the operation of offshore wind farms. One specific feature is low turbulence intensity which on the one hand reduces loads on turbines but on the other hand is the reason for much longer turbine and farm wakes than over land. The German Government is presently funding a research project called WIPAFF (Wind PArk Far Field) which heads for the analysis of properties and impacts of offshore wind park far fields. The focus is on the analysis of wind farm wakes, their interaction among each other and their regional climate impact. This is done by in-situ, extensive aircraft and satellite measurements and by operating meso-scale wind field models and an analytical wind farm model.

Optimization of the poro-serrated trailing edges for airfoil broadband noise reduction.

This paper reports an aeroacoustic investigation of a NACA0012 airfoil with a number of poro-serrated trailing edge devices that contain porous materials of various air flow resistances at the gaps between adjacent members of the serrated-sawtooth trailing edge. The main objective of this work is to determine whether multiple-mechanisms on the broadband noise reduction can co-exist on a poro-serrated trailing edge. When the sawtooth gaps are filled with porous material of low-flow resistivity, the vortex shedding tone at low-frequency could not be completely suppressed at high-velocity, but a reasonably good broadband noise reduction can be achieved at high-frequency. When the sawtooth gaps are filled with porous material of very high-flow resistivity, no vortex shedding tone is present, but the serration effect on the broadband noise reduction becomes less effective. An optimal choice of the flow resistivity for a poro-serrated configuration has been identified, where it can surpass the conventional serrated trailing edge of the same geometry by achieving a further 1.5 dB reduction in the broadband noise while completely suppressing the vortex shedding tone. A weakened turbulent boundary layer noise scattering at the poro-serrated trailing edge is reflected by the lower-turbulence intensity at the near wake centreline across the whole spanwise wavelength of the sawtooth.

Roughness and Turbulence Effects on the Aerodynamic Efficiency of a Wind Turbine Blade Section

Numerous experiments were conducted on a section of a 660kw wind turbine blade in a subsonic wind tunnel. The selected airfoil was tested with a clean and distributed contamination roughness surface, also with high and low tunnel turbulence intensity. Surface contamination was simulated by applying 0.5 mm height roughness over the entire upper surface of the airfoil. The surface pressure distribution is measured in the steady and unsteady condition at three Reynolds numbers, 0.43, 0.85, and 1.3 million, and over a range of angles of attack, AOA=7o-19o. Unsteady data were acquired by both pitch and plunge-type oscillation of the model about its quarter chord at a reduced frequency of 0.07. Results show that the surface roughness reduces section aerodynamic efficiency and the lift coefficient, but increases the drag coefficient for all the Reynolds numbers. The application of roughness reduces upper surface pressure coefficient and extends a separation region at high angles of attack. Increasing the tunnel turbulence intensity resulted in delay of the stall, an increase of maximum lift coefficient, and smoothness of the stall behavior. However, the drag coefficient increased significantly. Furthermore, turbulence intensity affected the predicted power output of the blade.

Effect of turbulent inflows on airfoil performance for a Horizontal Axis Wind Turbine at low Reynolds numbers (part I: Static pressure measurement)

This paper presented the results of a study to investigate the effect of turbulent inflows on the blade performance of a Horizontal Axis Wind Turbine (HAWT) by employing the experimental measurement at low Reynolds numbers. In order to observe the stall phenomenon, pressures on the rotor surface were measured by a multiport pressure device. Furthermore, for understanding the effects of different turbulence intensities on the aerodynamic performance characteristics of HAWT airfoil, static turbulence grids were used to adjust the turbulence intensity in wind tunnel. In this experiment, aerodynamic forces were discussed with different turbulence intensities and low Reynolds numbers. From comparing the results, it was found that the flow was separated at the leading edge in the case of without grids. On the other hand, in the case of turbulent flow field (with grids), the flow separated region was expanded gradually with the increase of the angle of attack. In the case of Re ≥ 1.5 × 105, compared with the low turbulence intensity, the lift coefficient was increased significantly at the high turbulent flow field. In addition, the drag coefficient in the turbulent flow field was reduced as compared to the low turbulence.

A D-Shaped Bileaflet Bioprosthesis which Replicates Physiological Left Ventricular Flow Patterns.

Prior studies have shown that in a healthy heart, there exist a large asymmetric vortex structure that aids in establishing a steady flow field in the left ventricle. However, the implantation of existing artificial heart valves at the mitral position is found to have a negative effect on this physiological flow pattern. In light of this, a novel D-shaped bileaflet porcine bioprosthesis (GD valve) has been designed based on the native geometry mitral valve, with the hypothesis that biomimicry in valve design can restore physiological left ventricle flow patterns after valve implantation. An in-vitro experiment using two dimensional particle velocimetry imaging was carried out to determine the hemodynamic performance of the new bileaflet design and then compared to that of the well-established St. Jude Epic valve which functioned as a control in the experiment. Although both valves were found to have similar Reynolds shear stress and Turbulent Kinetic Energy levels, the novel D-shape valve was found to have lower turbulence intensity and greater mean kinetic energy conservation.

Free-stream turbulence effects on the boundary layer of a high-lift low-pressure-turbine blade

The suction side boundary layer evolution of a high-lift low-pressure turbine cascade has been experimentally investigated at low and high free-stream turbulence intensity conditions. Measurements have been carried out in order to analyze the boundary layer transition and separation processes at a low Reynolds number, under both steady and unsteady inflows. Static pressure distributions along the blade surfaces as well as total pressure distributions in a downstream tangential plane have been measured to evaluate the overall aerodynamic efficiency of the blade for the different conditions. Particle Image Velocimetry has been adopted to analyze the time-mean and time-varying velocity fields. The flow field has been surveyed in two orthogonal planes (a blade-to-blade plane and a wall-parallel one). These measurements allow the identification of the Kelvin-Helmholtz large scale coherent structures shed as a consequence of the boundary layer laminar separation under steady inflow, as well as the investigation of the three-dimensional effects induced by the intermittent passage of low and high speed streaks. A close inspection of the time-mean velocity profiles as well as of the boundary layer integral parameters helps to characterize the suction side boundary layer state, thus justifying the influence of free-stream turbulence intensity on the blade aerodynamic losses measured under steady and unsteady inflows.