Section Wind Tunnel Research Articles

Soap bubbles seeding for quantitative time resolved velocity measurements of a turbulent wake flow behind a cylinder

The wake flow behind a cylinder of 100mm diameter is investigated using time resolved 2D PIV technique applied to an air flow generated in a closed loop open test section wind tunnel. The flow is seeded using a micro soap bubble generator (BG-1000, TSI Inc.). The bubbles in the air flow were illuminated with a CW laser source and imaged using a high-speed camera. The main purpose of this study is to show features and advantages of using soap bubbles as seeding for a relatively large-scale PIV investigation under low power illumination conditions.

Design and characterization of a multifunctional low-speed anechoic wind tunnel at HKUST

The Hong Kong University of Science and Technology (HKUST) has installed and commissioned an Ultra-quiet Noise Injection Test and Evaluation Device (UNITED). This facility is the first closed-circuit anechoic wind tunnel in Hong Kong. Calibrations and improvements have been made to improve the aerodynamic and aeroacoustic performance since its commissioning in 2016, and various experiments have been conducted in it. The test section is enclosed within a 3.2 m × 3.1 m × 2 m anechoic chamber, and it can be configured as either open-jet or closed test section. The exit plane of the contraction section has a dimension of 0.4 m × 0.4 m, and the flow speed of the test section can be varied from 10 m/s up to 70 m/s. In this paper, the major design constraints and design principles of key wind tunnel sections are described. To achieve low background noise, a silencer network is carefully designed, and a low-noise axial fan is specially designed and manufactured for the wind tunnel circuit. It is shown that the metrics of aerodynamic and aeroacoustic performances are comparable to similar facilities across the world. Representative research activities are also briefly summarized in this paper. Particularly, the measurement of trailing edge noise of a flat plate is demonstrated. The measured trailing edge noise collapses well under the classical fifth power-law scaling, suggesting the good performance of the entire measurement system.

Experimental and numerical investigation of the effect of blade number on the aerodynamic performance of a small-scale horizontal axis wind turbine

This study presents experimental and numerical investigation for the effect of number of blade (solidity) on the power and thrust coefficients of a small-scale horizontal-axis wind turbine HAWT. An experimental set- up was installed on wind turbine rotors with different number of blades, i.e. three, five and six, and at different tip speed ratios, in the closed-circuit open-test section wind tunnel. The obtained results from CFD simulations were performed to compare numerical data with experimental measurements. In addition, there is a lack of information presented for the effect of rotor blade number on the flow field for HAWT like vertical axis wind turbine VAWT. Therefore, the current study extends to study the effect of solidity on the flow field using numerical calculation. The numerical simulation was performed using a steady-RANS method. The SST k-ω turbulence model was used to represent turbulence characteristics. It is found that decreasing the number of blades (which makes the turbine less sensitive to the change in tip speed ratio) the wind turbine with 3 blade configuration has the maximum power coefficient in respect to 5 and 6 blade turbines, higher by around 2 and 4 percent respectively.

Development of unsteady background-oriented schlieren system in an indraft supersonic wind tunnel

To visualize the flow in the test section of an indraft supersonic wind tunnel in the University of Glasgow as long as possible, a background-oriented schlieren system was built up preliminarily. A MATLAB program based on a random dot algorithm developed in this study provides a fully customizable tool to generate background patterns with different sizes and dot densities. Background patterns produced by the in-house developed program then can be printed by a common ink-jet printer. To enhance the signal-noise ratio of the measurement system, white reflective film sheets, or semi-transparent paper can be employed. The correlation algorithm base on fast Fourier transform that is also applicable for PIV was chosen to process background oriented schlieren images. A validation test was performed to visualize the flow structure around a Pitot tube at M = 2.0. The experimental result proves that the BOS system established in this study is capable of visualizing the supersonic flow structure around the Pitot tube and sensitive enough to reveal weak density changes produced by the boundary layer, expansion waves, and weak oblique shock waves. Next, the current BOS system will be improved further by increasing the intensity of light sources to shorten the exposure time, using new cameras with better spatial resolution, and optimizing the background pattern.

Time-domain model for prediction of generalized 3DOF buffeting response of tall buildings using 2D aerodynamic sectional properties

A time-domain method to calculate the buffeting response of a rectangular tall building (AR of B/D = 1.5 and H/D = 6) in along-wind, across-wind and torsional degrees of freedom using 2D aerodynamic sectional properties is presented here. The aerodynamic sectional properties were identified first using wind tunnel section models. Aerodynamic damping, which is critical in wind-induced response analysis of tall buildings, was calculated using the identified aeroelastic derivatives (flutter derivatives) and validated by comparing with those extracted directly from the response measurements of aeroelastic models of the tall building. A good agreement between the numerically simulated responses and the experimental measurements of an aeroelastic model (1:175 scale) shows the capability of the proposed method in the prediction of generalized 3DOF buffeting response of tall buildings in a wide range of wind speeds. Furthermore, the response of a tall building with two other cross-sectional shapes, circular and elliptical, was calculated using the developed method and compared amongst the three cross-sectional shapes to study the effect of cross section on buffeting response. The relative dependency of buffeting response of a tall building along each of the 3DOF on cross section, wind speed as well as angle of attack was assessed. Finally, the effect of taper on tall building’s buffeting response was studied using the developed method which showed that taper could significantly reduce the wind-induced response of tall buildings. The time-domain method to assess wind loads and buffeting response of tall buildings based on experimentally or theoretically derived aerodynamic properties of two-dimensional sections, as presented here, provides a viable and perhaps the only alternative to methods based on aeroelastic model tests and CFD simulations.

Aerodynamic characteristics of trains on a viaduct with non-uniform cross-section under crosswinds by wind tunnel tests

This paper focuses on the aerodynamic characteristics of trains on a non-uniform double-track railway bridge under crosswinds through a scaled 1:40 sectional model wind tunnel test. Pressure measurements of five cross-sections of two types of trains, one with round roof and one with blunt roof, at the upstream and downstream tracks of the bridge were conducted under crosswinds with wind attack angles between −12° and 12°. The mean wind speed and turbulence intensity profiles around the windward surface of the train in the downwind and upward directions were also measured using cobra probe to obtain the boundary layer above the bridge surface. The results show that the shapes of train and bridge, as well as the wind attack angle, affect the aerodynamic characteristic of the train on the non-uniform bridge girder. The mean and fluctuating pressure coefficients are similar for all five cross-sections of the trains while the train is at the upstream track. However, when the train is at the downstream track, the extreme mean and fluctuating pressure coefficients around the windward and top surfaces of each cross-section on the train are different. At the downstream track, the mean wind speed profile and the turbulence intensity profile around the top of the train vary dramatically due to the separation flow caused by the leading edge of the bridge girder.

Wind Tunnel Measurements of a Multibladed Horizontal-Axis Small Wind Turbine

This work experimentally investigates the effects that a different number of blades, three and five, has in the dynamic torque, acceleration, and power performance of a horizontal-axis small wind turbine. It presents the methodology used for calculating the temporal behavior of the aerodynamic torque generated by the blades during their acceleration phase. The tests were performed in an open section wind tunnel of 1.0 × 0.8 m. The model was produced by additive manufacturing and had two possible assemblies, with three and five blades. An in-line dynamic torque meter acquired the torque and angular speed data, and a National Instruments USB-6009 device processed the signal. The resistive torques were calculated by spin-down tests. The tests were executed at a wind speed of 10 m·s–1. The measurements displayed the five blades assembly having improved performance compared to the three blades: a higher static torque, which resulted in shorter stating time, and a power coefficient 39% higher.

Reynolds number and wind tunnel wall effects on the flow field around a generic UHBR engine high-lift configuration

RANS simulations of a generic ultra-high bypass ratio engine high-lift configuration were conducted in three different environments. The purpose of this study is to assess small scale tests in an atmospheric closed test section wind tunnel regarding transferability to large scale tests in an open-jet wind tunnel. Special emphasis was placed on the flow field in the separation prone region downstream from the extended slat cut-out. Validation with wind tunnel test data shows an adequate agreement with CFD results. The cross-comparison of the three sets of simulations allowed to identify the effects of the Reynolds number and the wind tunnel walls on the flow field separately. The simulations reveal significant blockage effects and corner flow separation induced by the test section walls. By comparison, the Reynolds number effects are negligible. A decrease of the incidence angle for the small scale model allows to successfully reproduce the flow field of the large scale model despite severe wind tunnel wall effects.

On Comparison Between 0–1 Test for Chaos and Attractor Reconstruction of an Aeroelastic System

This paper aims to investigate the post-flutter nonlinearities due to the wing high aspect ratio. Flutter is an aerodynamic auto-excited phenomenon which occurs due to the coupling of two or more different vibration modes. This coupling results from the interaction between aerodynamic, elastic, and inertial forces. The present investigation scope is the comparison between experimental aeroelastic analysis of high-aspect-ratio wings using subsonic wind tunnels with computational experiments for three different center of gravity conditions. The computational nonlinear aeroelastic analysis is performed using a formulation based on the variation of the energy functional and considering Peters’ unsteady aerodynamic model. The post-flutter analysis consists in the comparison between the attractor reconstructed using the Takens’ theory with the result of the 0–1 test for chaos for both computational and wind tunnel experiments. First, the 0–1 test is performed for both computational nonlinear analysis and wind tunnel experiment and they are in good agreement for each condition. After that, the procedure to obtain the reconstructed attractor is followed for both cases and they are again in good agreement. The difference between computational experiment reconstructed attractors and wind tunnel experiment reconstructed attractors is mainly because of wind turbulence in wind tunnel section. This research shows that all three experimented conditions presented periodic dynamic behavior. The 0–1 test predicted well the dynamic behavior; therefore, it is recommended to apply this test for all nonlinear experiments so that a first qualitative evaluation is possible. All Lyapunov exponents calculated are negative, confirming the periodicity of the three center of gravity conditions for both computational and wind tunnel experiments.

Quantitative evaluation of empirical models of vortex-induced vibration of bridge decks through sectional model wind tunnel testing

The capabilities of some commonly used single-degree-of-freedom (SDOF) VIV models for bridge decks are examined through wind tunnel experiment. The VIV responses of a sectional deck model at different Scruton numbers are measured. An instantaneous identification method is adopted for obtaining the properties of aeroelastic damping and stiffness of the VIV system. The simulated aeroelastic damping by these models as well as the predicted VIV responses of the deck at different Scruton numbers are compared with experiments. The results indicate that the more accurate the simulated aeroelastic damping by an empirical model, the smaller the prediction errors by this model will be. Through directly fitting the aeroelastic damping, the prediction errors by an empirical model can be further reduced.

Simulation of Transient On-Road Conditions in a Closed Test Section Wind Tunnel Using a Wing System with Active Flaps

<div class="section abstract"><div class="htmlview paragraph">Typical automotive research in wind tunnels is conducted under idealized, stationary, low turbulence flow conditions. This does not necessarily reflect the actual situation in traffic. Thus, there is a considerable interest to simulate the actual flow conditions. Because of this, a system for the simulation of the turbulence intensity I, the integral linear scale L and the transient angle of incidence β measured in full-scale tests in the inflow of a test vehicle was developed and installed in a closed-loop, closed test section wind tunnel. The system consists of four airfoils with movable flaps and is installed in the beginning of the test section. Time-series of the flow velocity vector are measured in the empty test section to analyze the system’s envelope in terms of the turbulence intensity and the integral length scales. It is shown that the length scales in spanwise and in driving (streamwise) direction can be varied from 0.15 m to 7.9 m and from 0.15 m to 2.5 m, respectively, depending on the frequency of the flap movement. The maximum obtained turbulence intensity in the driving direction x is 3% and in the spanwise direction y 9.8%, depending on the flap’s amplitude. It is further shown that the turbulence intensity in driving direction can be increased to 5.6% with passive turbulence generators. Additionally, a model for predicting the flap movement reproducing the transient angle of incidence β measured in the on-road tests during an overtaking maneuver was developed. Measurements of the forces acting on the vehicle revealed an influence of the non-stationary flow on the non-stationary force coefficients. Finally, changes of up to 0.002 in Δc<sub>d</sub> and 0.157 in Δc<sub>s</sub> were measured.</div></div>

Design and analysis of five probe flow analyser for subsonic and supersonic wind tunnel calibration

The flow angularity in a wind tunnel plays a major role in test section when testing models. The models are being tested at a mixture of flow velocity. Usually a subsonic wind tunnel is calibrated using a normal Pitot tube and supersonic wind tunnels with high efficiency probes. The calibration of a wind tunnel includes determining the Mach number range ad pressure range throughout the test section of the flow throughout the pattern of operating velocity. In the analysis it is noted that the sensitivities of the calibrating instruments are maximum when it is either cone or wedge shaped. When the wedge or cone angles are maximum, more accurate results are also obtained, but it’s not possible to have a maximum angle cone or wedge because of the detached wave and tunnel blockage factors. Hence in order to overcome this difficulties in measurement, a five probe flow analyzer has been designed. The five probe flow analyzer is used to determine the wind tunnel test section parameters like flow angularity, Pitot pressures, static pressures, wave angles, and the presence of test section noise. Over to these parameter, the stagnation states can also be determined using this analyzer. The proposed model is designed by getting optimum solutions from previous analysis and the existing data by considering all the aerodynamic and mechanical loading. This instrument designed is to be tested experimentally after carrying two and three dimensional computational analysis for Mach number ranging from 0.5 of 3. Also it is designed to fit successfully for 0.3m and 0.6m test section wind tunnel to obtain reasonably merging solution with theoretical and experimental results.

The correlation of the pulsations of flow in the settling chamber with the pulsations of supersonic flow

The relationship of the pulsations of flow in the settling chamber with the pulsations of supersonic flow in the test section of the T-325 wind tunnel of ITAM SB RAS was investigated. Experimental data on the correlation between flow pulsations in different sections of wind tunnel and efficiency of turbulence reducing system were obtained. The system of three-channel hot-wire measurements in subsonic and supersonic flows was tested.

Numerical and experimental investigation of flow in an open-type subsonic wind tunnel

A numerical and experimental investigation of flow, inside the test section of an open-type subsonic wind tunnel, has been incorporated in the present paper. Experimental data are collected at different selected locations along the wind tunnel length and inside the test section. For detail assessment of the spatial variation of flow variables, numerical analysis is carried out. Boundary conditions have a significant influence on the validation of numerical simulation. A novel approach of system curve generation by experimental analysis of wind tunnel is adopted, instead of using the conventional approach of fan-type boundary condition. Mass flow rate and pressure jump obtained by system curve are utilized for inlet and outlet boundary conditions, respectively. Comparison of numerical and experimental flow fields at the test section suggests maximum error of 9.84% in case of area-weighted average wind velocity along the length of the test section, whereas along the height of the test section a maximum error of 7.75% is observed.

Development of small propeller test bench system

A set of performance analysis was performed on Master Airscrew 10X7 Electric propeller. The airfoil geometry was extracted by cutting and scanning a number of blade cross-sections. Experimentally, the test was performed at Institut Teknologi Bandung using 40 cm x 40 cm rectangular test section wind tunnel. The experimental rig was built using 3 load cells designed to measure thrust and torque. For data comparison, theoretical and numerical analysis were also done using Momentum-Blade Element method and Computational Fluid Dynamics. Compared with the previous study, the experimental data shows good agreement with most of the reference data. The theoretical data overestimated the thrust value by around 20% and underestimated the torque value by 30% at max, but the performance curve follows the similar trend with the experimental data. Both the thrust and torque value of the numerical data shows better agreement when fitted to experimental data by around 10%. While it may need more propeller samples analyzed to draw general conclusion, the three methods performed in this research showed analogous results and could be used for a particular stage of research depending on the level of accuracy needed.

Effect of Wind Tunnel Blockage on the Performance of a Horizontal Axis Wind Turbine with Different Blade Number

Blockage corrections for the experimental results obtained for a small-scale wind turbine in a wind tunnel are required in order to estimate how the same turbine would perform in real conditions. The tunnel blockage is defined as the ratio of the wind turbine swept area to the wind tunnel cross-section area. Experimental measurements of the power coefficient were performed on a horizontal-axis wind turbine with two rotors of diameter equal to 2 m and different numbers of blades, namely three and five. Measurements were carried out for different tip speed ratios in the closed circuit open test section wind tunnel of the University of Perugia (Italy). The obtained experimental results were compared with the numerical ones carried out in free conditions by using a CFD approach based on the steady-RANS method with the SST k-ω turbulence model, adopting the multiple reference frame (MRF) strategy to reduce the computational effort. The comparison showed that the maximum value of blockage, which is reached in the asymptotic limit at very large tip speed ratio (TSR) values, does not depend appreciably on the number of blades. A higher number of blades, however, makes the occurrence of the maximum blockage come earlier at lower TSRs.

Flow quality analysis of contraction section and test section of low-speed wind tunnel based on CFD numerical simulation

On the basis of the overall structure design and aerodynamic calculation of the annular low-speed wind tunnel, this paper uses the CFD numerical simulation technique to study the influence of the contraction curve form, the contraction ratio and the open/closed loop form of the test section on the flow field quality of the wind tunnel test section, and then gives the three-dimensional geometric model and the numerical simulation results. It is found that the best flow field quality can be obtained by using the bicubic curve, the contraction ratio of 10.24 and the closed loop form of the test section under the same fan condition. The three-dimensional modeling method is adopted in this paper, which can obtain more accurate numerical simulation result than the two-dimensional modeling method. Thus, the numerical simulation result provides a solid theoretical basis for the design and construction of the actual low-speed wind tunnel.

SIMULATION OF 2D FLOW AROUND OF AIRFOILS AT LOW-SPEED WIND TUNNEL WITH OPEN JET TEST-SECTION

At present, there is a great interest in the development of new airfoils for wind turbines and high-lift wings of unmanned aerial vehicles (UAV). The requirements for such airfoils differ from conventional aircraft airfoils, because of structural reasons and extreme operating conditions. So, wind turbine airfoils operate frequently under fully separated flow when stall is used for power regulation at high wind speeds. At the same time design of airfoils for wings UAV poses the problem of availability of high-lift at low Reynolds number. Modern airfoils are to a large extent developed from numerical methods. However, the complex flow conditions such as separation at high angles of attack, laminar separation bubbles and the transition from laminar to turbulent flow are difficult to predict accurately. Hence, testing of airfoils at a two-dimensional condition is an important phase in airfoil design. The development and validation of a 2D testing facility for investigation of single and multi-element airfoils in the wind tunnel Т-102 with open test section are considered in this article. T-102 is a continuous-operation, closed-layout wind tunnel with two reverse channels. The test section has an elliptical cross-section of 4 ×2,33 m and a length of 4 m. Two big flat panels of the L × H=3 ×3,9 m size installed upright on balance frame aligned with the free stream are used for simulating two-dimensional flow in the tunnel test section. The airfoil section in the layout of a rectangular wing is mounted horizontally between flat panels with minimum gaps to ensure 2D flow conditions. The aerodynamic forces and pitch moment acting on the model were measured by wind tunnel balance. To determine boundary corrections for a new test section of wind tunnel, the experimental investigation of three geometrically similar models has been executed. The use of boundary corrections has provided good correlation of the test data of airfoil NACA 6712 with the results obtained from the wind tunnel except for lift and drag coefficient values at high angles of attack.

Characterization of luminescent mini-tufts in quantitative flow visualization experiments: Surface flow analysis and modelization

As a widely used surface flow visualization method, luminescent mini-tuft has become one challenging topic with its practical advantages in quantitative flow measurement. The luminescent mini-tufts method is preferred with its reduced size and increased luminescence, which is suitable for surface visualization measurement. To provide a standard method/procedure in quantitative analysis for luminescent mini-tuft measurement, the current study established an experimental characterization platform of luminescent mini-tufts method and conducted flat-pate model for flow analysis. The experimental system is consisted of wind tunnel and model section, high-speed image data recording system, digital image processing as well as the control system. The digital imaging processing method for result analysis is also explained, which includes the dark current image extraction, averaging, mini-tufts recognition, and tuft inclination angle/tuft angle estimation process. In this study, the steady flow characterization and quantitative flow analysis is conducted on a flat plate model (Re = 1.6 × 105–6.6 × 105), which is combined with hot-wire anemometry to investigate the basic surface flow topology and boundary layer behaviors. The method is shown capable of capturing both the steady and transient behaviors of a surface flow. Luminescent mini-tufts physical model is also established and found good agreement with the experimental results in this study, which in turn support the mini-tufts characterization and selection in practical applications.

Wind Tunnel Investigation of the Wind Patterns in the Launching Pad Area of the Brazilian Alcântara Launch Center

This paper presents a wind tunnel study of the flow patterns in the main Brazilian Space Port, the Alcântara Launch Center (ALC), located in the Northwest region, over an irregular coastal cliff of 40 m height. In the ALC region, constant winds of strong incidence are very common, which can change the characteristics of the atmospheric boundary layer and affect safety conditions during launching operations. This study was conducted using a scaled model of the Launching Pad Area (LPA) in ALC complex with the purpose of getting insights about the wind flow patterns, as zones of strong vorticity, in that region when the wind incidence angle and coastal cliff characteristics change. The experiments were carried out in an aerodynamic wind tunnel, and for this reason it was necessary to use some techniques described in the literature to simulate an atmospheric boundary layer in a short test section wind tunnel. Hot-wire anemometer measurements were carried out for turbulence level evaluation with empty wind tunnel test section, and a two-dimensional Particle Image Velocimetry (PIV) was used for flow field velocity measurements in different confi gurations of wind incidence angle (α), Reynolds number and coastal cliff slope angle (β).