Wind Tunnel Flow Research Articles

Assessment of uncertainties in hot-wire anemometry and oil-film interferometry measurements for wall-bounded turbulent flows

In this study, the sources of uncertainty of hot-wire anemometry (HWA) and oil-film interferometry (OFI) measurements are assessed. Both statistical and classical methods are used for the forward and inverse problems, so that the contributions to the overall uncertainty of the measured quantities can be evaluated. The correlations between the parameters are taken into account through the Bayesian inference with error-in-variable (EiV) model. In the forward problem, very small differences were found when using Monte Carlo (MC), Polynomial Chaos Expansion (PCE) and linear perturbation methods. In flow velocity measurements with HWA, the results indicate that the estimated uncertainty is lower when the correlations among parameters are considered, than when they are not taken into account. Moreover, global sensitivity analyses with Sobol indices showed that the HWA measurements are most sensitive to the wire voltage, and in the case of OFI the most sensitive factor is the calculation of fringe velocity. The relative errors in wall-shear stress, friction velocity and viscous length are 0.44%, 0.23% and0.22%, respectively. Note that these values are lower than the ones reported in other wall-bounded turbulence studies. Note that in most studies of wall-bounded turbulence the correlations among parameters are not considered, and the uncertainties from the various parameters are directly added when determining the overall uncertainty of the measured quantity. In the present analysis we account for these correlations, which may lead to a lower overall uncertainty estimate due to error cancellation Furthermore, our results also indicate that the crucial aspect when obtaining accurate inner-scaled velocity measurements is the wind-tunnel flow quality, which is more critical than the accuracy in wall-shear stress measurements.

A New Approach to Low-Cost Open-Typed Subsonic Compressible Flow Wind Tunnel for Academic Purpose

The most challenging task concerning wind tunnel description is, to ascertain the supporting mechanism at high-speed flows inside test section with least complexity and low-cost. This paper presents the design and development of a simple and cost effective test section for a subsonic compressible wind tunnel for educational purpose. The objective of this open-type wind tunnel was to demonstrate flow around model without any interference at high speeds. In this work, that is study of flow analysis, a subsonic square nozzle was fabricated to achieve maximum velocity of 340 m/s. The test section is 25 mm x 25 mm x 70 mm and is externally attached with travers. It starts from length 25 mm to avoid dead zone and ends at 95 mm to avoid end effects, and on one of the surfaces pressure taps were provided for measuring pressure at different locations inside the wall of duct. The duct length was taken as 100 mm and the pressure taps were at the distance of 8 mm, 16 mm, 24 mm, 49 mm, 59 mm, 80 mm and 90 mm to measure wall pressure. They were connected to sensors through PVC tubes and further to DAQ using LabVIEW interface, and finally to the computer. In addition to this, the flow through the duct can be visualized using transparent glass to find the reattachment point for our re-circulation bubble. A new concept of attaching models in a 3 D traverse was found very easy for inserting it from behind in test section, and thereby interference and breaking of strings was eliminated. Calibration was done through pitot tube at the exit of test section in y and z direction. The nature of graph tells about the correctness of the designed wind tunnel test section. After calibration, we found that the exit velocity is constant for approximately 80 percent area. This area is the effective test section work area.

Experimental Study on the Wake Meandering Within a Scale Model Wind Farm Subject to a Wind-Tunnel Flow Simulating an Atmospheric Boundary Layer

The phenomenon of meandering of the wind-turbine wake comprises the motion of the wake as a whole in both horizontal and vertical directions as it is advected downstream. The oscillatory motion of the wake is a crucial factor in wind farms, because it increases the fatigue loads, and, in particular, the yaw loads on downstream turbines. To address this phenomenon, experimental investigations are carried out in a wind-tunnel flow simulating an atmospheric boundary layer with the Coriolis effect neglected. A \(3 \times 3\) scaled wind farm composed of three-bladed rotating wind-turbine models is subject to a neutral boundary layer over a slightly-rough surface, i.e. corresponding to offshore conditions. Particle-image-velocimetry measurements are performed in a horizontal plane at hub height in the wakes of the three wind turbines occupying the wind-farm centreline. These measurements allow determination of the wake centrelines, with spectral analysis indicating the characteristic wavelength of the wake-meandering phenomenon. In addition, measurements with hot-wire anemometry are performed along a vertical line in the wakes of the same wind turbines, with both techniques revealing the presence of wake meandering behind all three turbines. The spectral analysis performed with the spatial and temporal signals obtained from these two measurement techniques indicates a Strouhal number of \(\approx 0.20 - 0.22\) based on the characteristic wake-meandering frequency, the rotor diameter and the flow speed at hub height.

Numerical analysis of high speed wind tunnel flow disturbance measurements using stagnation point probes

Since supersonic test facilities have tunnel noise that strongly influences boundary layer transition experiments, the determination of tunnel noise is of great significance to properly evaluate and interpret experimental results. The composition of tunnel noise, which consists of acoustic, entropy and vorticity modes, highly influences the boundary layer receptivity. The measurement of the separate modes is a major goal of ongoing research. In this study, the properties of stagnation point probes for a newly developed modal decomposition method for tunnel noise are investigated by direct numerical simulation. Pressure and heat flux responses of a stagnation point probe to various entropy and acoustic mode input functions are analysed to investigate how tunnel noise is perceived by corresponding sensor types. The interaction of the incident mode and the shock wave upstream of the probe is analysed and the resulting wave pattern in the subsonic region between shock wave and probe is evidenced. It is found that pure incident acoustic or entropy modes cause acoustic and entropy waves downstream of the shock wave whose strengths differ depending on the incident mode. The resulting wave pattern downstream of the shock wave is determined by postshock acoustic waves propagating bidirectionally between shock wave and probe. Formulating a model equation linking pressure and heat flux fluctuations to the initially caused postshock acoustic and entropy wave, a criterion for the applicability of stagnation point probes measuring pressure and heat flux fluctuations in the new modal decomposition method can be deduced: to distinguish between the incident mode types based on their pressure and heat flux signal the perception of initially generated entropy waves downstream of the shock wave by the heat flux sensor is crucial. The transfer function between entropy waves and heat flux is shown to have low pass filter characteristics and the cutoff Strouhal number could be estimated by control theory. The analysis of the frequency response to continuous incident waves corroborated this cutoff Strouhal number.

Improving acoustic beamforming maps in a reverberant environment by modifying the cross-correlation matrix

A method to improve the quality of acoustic beamforming in reverberant environments is proposed in this paper. The processing is based on a filtering of the cross-correlation matrix of the microphone signals obtained using a microphone array. The main advantage of the proposed method is that it does not require information about the geometry of the reverberant environment and thus it can be applied to any configuration. The method is applied to the particular example of aeroacoustic testing in a hard-walled low-speed wind tunnel; however, the technique can be used in any reverberant environment. Two test cases demonstrate the technique. The first uses a speaker placed in the hard-walled working section with no wind tunnel flow. In the second test case, an airfoil is placed in a flow and acoustic beamforming maps are obtained. The acoustic maps have been improved, as the reflections observed in the conventional maps have been removed after application of the proposed method.

Numerical investigation of air flow in a supersonic wind tunnel

In the framework of TsAGI’s supersonic wind tunnel modernization program aimed at improving flow quality and extending the range of test regimes it was required to design and numerically validate a new test section and a set of shaped nozzles: two flat nozzles with flow Mach number at nozzle exit M=4 and M=5 and two axisymmetric nozzles with M=5 and M=6. Geometric configuration of the nozzles, the test section (an Eiffel chamber) and the diffuser was chosen according to the results of preliminary calculations of two-dimensional air flow in the wind tunnel circuit. The most important part of the work are three-dimensional flow simulation results obtained using ANSYS Fluent software. The following flow properties were investigated: Mach number, total and static pressure, total and static temperature and turbulent viscosity ratio distribution, heat flux density at wind tunnel walls (for high-temperature flow regimes). It is demonstrated that flow perturbations emerging from the junction of the nozzle with the test section and spreading down the test section behind the boundaries of characteristic rhomb’s reverse wedge are nearly impossible to eliminate. Therefore, in order to perform tests under most uniform flow conditions, the model’s center of rotation and optical window axis should be placed as close to the center of the characteristic rhomb as possible. The obtained results became part of scientific and technical basis of supersonic wind tunnel design process and were applied to a generalized class of similar wind tunnels.

Model predictive control synthesis algorithm based on polytopic terminal region for Hammerstein-Wiener nonlinear systems

An improved model predictive control algorithm is proposed for Hammerstein-Wiener nonlinear systems. The proposed synthesis algorithm contains two parts: offline design the polytopic invariant sets, and online solve the min-max optimization problem. The polytopic invariant set is adopted to replace the traditional ellipsoid invariant set. And the parameter-correlation nonlinear control law is designed to replace the traditional linear control law. Consequently, the terminal region is enlarged and the control effect is improved. Simulation and experiment are used to verify the validity of the wind tunnel flow field control algorithm.

A remote microphone technique for aeroacoustic measurements in large wind tunnels

The present study was devoted to the development and application of a remote microphone technique for aeroacoustic measurements in large aerodynamic wind tunnels. In this technique, the microphone and its connecting line to the sensing port are fitted within an aerodynamically streamlined fairing. A model-based calibration method was applied to account for the phase lag and dissipation within the microphone connecting line. The proposed system permits to carry out acoustic measurements when the wind tunnel surface cannot be altered, since the fairing can be simply glued on the wall. The fairing is streamlined to minimize flow disturbances as well as the risk of separation which would otherwise contaminate the wind tunnel flow quality and acoustic measurements. It was furthermore hoped that the acceleration of the flow over the fairing surface might reduce the turbulence fluctuations and thereby increase the signal-to-noise ratio of the acoustic measurements. Those aerodynamic effects were investigated with a set of hot-wire measurements performed in the subsonic L2B small wind tunnel of von Karman Institute (VKI). The mean velocity and the turbulence intensity profile were analyzed in the presence of different inflow turbulence characteristics at 5, 10 and 15m/s freestream velocity. The effect of incident turbulence on the acoustic measurement was investigated applying the calibration procedure and comparing the measurements with a wall-mounted reference microphone signal in similar flow conditions. The results indicate that the wished beneficial effects of the flow acceleration are not significant. As an application, this technique is used to measure the noise emitted from the contra-rotating fans of the large L1 subsonic wind tunnel of the von Karman Institute, where the microphone fairing is placed inside the diffuser and thus subjected to a thick turbulent boundary layer. The results indicate that in spite of the mitigated turbulence-reduction performance of the fairing, it provides a measurement solution that is suitable for large wind tunnels provided the signals are compensated using the measured dynamic calibration. While it should be stressed that the calibration procedure itself is not new, the originality of the present paper stands in the proposed fairing design, and in the aerodynamic/acoustic investigation of its effect on the flow.

Flow and Turbulence Control in a Boundary Layer Wind Tunnel Using Passive Hardware Devices

The Climatic Wind Tunnel (CWT) is a new experimental facility of the Institute of Theoretical and Applied Mechanics (ITAM) in Prague, Czech Republic. It is uniquely able to replicate precipitation and freezing effects on structural models in the climatic test section, and simultaneously perform aerodynamic and aeroelastic tests on structural models in the aerodynamic test section. Extensive measurements are carried out to investigate airflow and turbulence characteristics in the empty aerodynamic test section. Capability to experimentally simulate the atmospheric boundary layer (ABL) is studied using castellated barrier wall, Counihan vortex generators, and surface roughness elements. An individual influence of each of these passive hardware devices is separately tested as well, as they are commonly used combined, and their individual contributions to the ABL model characteristics are not well known. The analysis is based on hot-wire anemometer measurements carried out at various cross sections in the aerodynamic test section of the CWT. Experimental results are reported as mean velocity and turbulence intensity fields for the empty aerodynamic test section without passive hardware devices. Vertical profiles of mean velocity, turbulence intensity, and turbulence length scales, as well as power spectral density of velocity fluctuations, are reported for the ABL small-scale model. The barrier is observed to cause a decrease in flow velocity and an increase in turbulence close to surface. Vortex generators enhance turbulence throughout the entire height range under scope. Increased turbulence and decreased flow velocity are particularly exhibited close to surface due to surface roughness. Influence of each of the studied passive hardware devices on flow and turbulence is quantified in a dimensionless form to allow for separately accounting for their effects.

Extremal-point density of scaling processes: From fractional Brownian motion to turbulence in one dimension

In recent years several local extrema-based methodologies have been proposed to investigate either the nonlinear or the nonstationary time series for scaling analysis. In the present work, we study systematically the distribution of the local extrema for both synthesized scaling processes and turbulent velocity data from experiments. The results show that for the fractional Brownian motion (fBm) without intermittency correction the measured extremal-point-density (EPD) agrees well with a theoretical prediction. For a multifractal random walk (MRW) with the lognormal statistics, the measured EPD is independent of the intermittency parameter μ, suggesting that the intermittency correction does not change the distribution of extremal points but changes the amplitude. By introducing a coarse-grained operator, the power-law behavior of these scaling processes is then revealed via the measured EPD for different scales. For fBm the scaling exponent ξ(H) is found to be ξ(H)=H, where H is Hurst number, while for MRW ξ(μ) shows a linear relation with the intermittency parameter μ. Such EPD approach is further applied to the turbulent velocity data obtained from a wind tunnel flow experiment with the Taylor scale λ-based Reynolds number Re_{λ}=720, and a turbulent boundary layer with the momentum thickness θ based Reynolds number Re_{θ}=810. A scaling exponent ξ≃0.37 is retrieved for the former case. For the latter one, the measured EPD shows clearly four regimes, which agrees well with the corresponding sublayer structures inside the turbulent boundary layer.

DESIGN AND CONSTRUCTION OF A PROTOTYPE SOLAR UPDRAFT CHIMNEY IN ASWAN/EGYPT

This work is part of a joint project funded by the Science and Technology Development Fund (STDF) of the Arab republic of Egypt and the Federal Ministry of Education and Research (BMBF) of the Federal Republic of Germany. Continuation of the use of fossil fuels in electricity production systems causes many problems such as: global warming, other environmental concerns, the depletion of fossil fuels reserves and continuing rise in the price of fuels. One of the most promising paths to solve the energy crisis is utilizing the renewable energy resources. In Egypt, high insolation and more than 90 percent available desert lands are two main factors that encourage the full development of solar power plants for thermal and electrical energy production. With an average temperature of about 40 °C for more than half of the year and average annual sunshine of about 3200 hours, which is close to the theoretical maximum annual sunshine hours, Aswan is one of the hottest and sunniest cities in the world. This climatic condition makes the city an ideal place for implementing solar energy harvesting projects from solar updraft tower. Therefore, a Solar Chimney Power Plant (SCPP) is being installed at Aswan City. The chimney height is 20.0 m, its diameter is 1.0m and the collector is a four-sided pyramid, which has a side length of 28.5 m. A mathematical model is used to predict its performance. The model shows that the plant can produce a maximum theoretical power of 2 kW. Moreover, a CFD code is used to analyse the temperature and velocity distribution inside the collector, turbine and chimney at different operating conditions. Static calculations, including dead weight and wind forces on the solar updraft chimney and its solar collector, have been performed for the prototype. Mechanical loading and ambient impact on highly used industrial structures such as chimneys and masts cause lifetime-related deteriorations. Structural degradations occur not only from rare extreme loading events, but often as a result of the ensemble of load effects during the life-time of the structure. A Structural Health Monitoring (SHM), framework for continuous monitoring, is implemented on the solar tower. For the ongoing case study, the types of impacts, the development of the strategic sensor positioning concept, examples of the initially obtained results and further prospects are discussed. Additional wind tunnel tests have been performed to investigate the flow situation underneath the solar collector and inside the transition section. The flow situation in and around the SCPP has been simulated by a combination of the wind tunnel flow and a second flow inside the solar tower. Different wind tunnel velocities and volume flow rates have been measured respectively. Particle Image Velocimetry (PIV) measurements give some indication of the flow situation on the in- and outside of the solar tower and underneath the collector roof. Numerical simulations have been performed with the ANSYS Fluent to validate the experimental tests.

Measurement of the aerothermodynamic state in a high enthalpy plasma wind-tunnel flow

This paper presents spatially resolved measurements of absolute particle densities of N2, N2+, N, O, N+, O+, e− and excitation temperatures of electronic, rotational and vibrational modes of an air plasma free stream. All results are based on optical emission spectroscopy data. The measured parameters are combined to determine the local mass–specific enthalpy of the free stream. The analysis of the radiative transport, relative and absolute intensities, and spectral shape is used to determine various thermochemical parameters. The model uncertainty of each analysis method is assessed. The plasma flow is shown to be close to equilibrium. The strongest deviations from equilibrium occur for N, N+ and N2+ number densities in the free stream. Additional measurements of the local mass-specific enthalpy are conducted using a mass injection probe as well as a heat flux and total pressure probe. The agreement between all methods of enthalpy determination is good.

A parametric study on the influence of boundary conditions on the longitudinal pressure gradient in CFD simulations of an automotive wind tunnel

Computational fluid dynamics (CFD) is an important and extensively used tool for aerodynamic development in the vehicle industry today. Validation of virtual methods by comparison to wind tunnel experiments is a must because manufacturers aim to substitute physical tests on prototype vehicles with virtual simulations. An appropriate validation can be performed only if the wind tunnel geometry with representative boundary conditions is included in the numerical simulation, and if the flow of the empty wind tunnel is accurately predicted. One of the important flow parameters to predict is the longitudinal pressure distribution in the test section, which is dependent on both the wind tunnel geometry and the settings of the boundary layer control systems. This study investigates the effects of flow angularity at the inlet and different boundary layer control systems, namely, basic scoop suction, distributed suction, and moving belts, on the longitudinal pressure distribution in the full-scale aerodynamic wind tunnel of Volvo Cars using CFD and a systematic design of experiments approach. The study shows that the different suction systems used to reduce boundary layer thickness upstream of the vehicle have statistically significant effects on the longitudinal pressure distribution in the test section. However, the estimated drag difference induced on a typical vehicle by the difference in horizontal buoyancy between the tested settings is within the test-to-test uncertainty of the physical wind tunnel, thereby leading to the conclusion that force calculations in simulations are fairly insensitive to the tested parameters on the investigated intervals.

High-Order Numerical Simulations of Wind Turbine Wakes

Previous attempts to describe the structure of wind turbine wakes and their mutual interaction were mostly limited to large-eddy and Reynolds-averaged Navier–Stokes simulations using finite-volume solvers. We employ the higher-order spectral-element code Nek5000 to study the influence of numerical aspects on the prediction of the wind turbine wake structure and the wake interaction between two turbines. The spectral-element method enables an accurate representation of the vortical structures, with lower numerical dissipation than the more commonly used finite-volume codes. The wind-turbine blades are modeled as body forces using the actuator-line method (ACL) in the incompressible Navier–Stokes equations. Both tower and nacelle are represented with appropriate body forces. An inflow boundary condition is used which emulates homogeneous isotropic turbulence of wind-tunnel flows. We validate the implementation with results from experimental campaigns undertaken at the Norwegian University of Science and Technology (NTNU Blind Tests), investigate parametric influences and compare computational aspects with existing numerical simulations. In general the results show good agreement between the experiments and the numerical simulations both for a single-turbine setup as well as a two-turbine setup where the turbines are offset in the spanwise direction. A shift in the wake center caused by the tower wake is detected similar to experiments. The additional velocity deficit caused by the tower agrees well with the experimental data. The wake is captured well by Nek5000 in comparison with experiments both for the single wind turbine and in the two-turbine setup. The blade loading however shows large discrepancies for the high-turbulence, two-turbine case. While the experiments predicted higher thrust for the downstream turbine than for the upstream turbine, the opposite case was observed in Nek5000.

Experimental Investigation on Flutter Similitude of Thin-Flat Plates

This paper shows the experimental results of the flutter speed of thin-flat plates with free leading edge in axial flow as a function of plates’ geometry, fluid densities, and viscosities, as well as natural frequencies of the plates. The experiment was developed based on similitude theory using dimensional analysis and Buckingham Pi Theorem. Dimensional analysis generates four dimensionless numbers. Experiment was conducted by placing the thin-flat plates in a laminar flow wind tunnel in order to obtain the relationship among those dimensionless numbers. The flutter speed was measured by varying the flow velocity until the instability occurred. The dimensional analysis gives a map of the flutter Reynolds number as a function of a new type of dimensionless number that is hereby called flutter fluid structure interaction number, thickness-to-length, and aspect ratios as the correcting factors. This map is a very useful tool for predicting the flutter speed of thin-flat plates in general. This investigation found that the flutter Reynolds number is very high at the region of high flutter fluid structure and thickness-to-length ratios numbers; however, it is very sensitive to the change of those two dimensionless numbers. The sensitivity is higher at lower aspect ratio.

Influence of Ablation on Vacuum-Ultraviolet Radiation in a Plasma Wind Tunnel Flow

Plasma wind tunnel experiments have been performed simulating a Hayabusa reentry trajectory point at 78.8 km altitude with a velocity of corresponding to a local mass–specific enthalpy of and a stagnation pressure of 2.44 kPa. Ablation–radiation coupling is investigated using a carbon preform sample, a lightweight carbon phenolic ablator sample, and cooled copper. Optical emission spectroscopic measurements in the vacuum ultraviolet (VUV) regime (116–197 nm) have been conducted through a bore hole in the stagnation point of the different samples. Optical emission spectroscopic measurements in the UV/VIS spectral range (320–810 nm) have been conducted viewing the plasma from the side. The stagnation point VUV radiation to the carbon preform sample is strongest, whereas it is weakest for the carbon phenolic sample. In the UV/VIS both carbon-based material samples lead to stronger plasma radiation than copper. Atomic number densities in front of the samples are largest for the carbon preform sample and lowest for the carbon phenolic sample. Rotational and vibrational temperatures are lower for both the carbon-based materials compared with copper. The measurements show for the first time experimentally the radiative heat flux mitigation by carbon phenolic materials.

Application of the PITM Method Using Inlet Synthetic Turbulence Generation for the Simulation of the Turbulent Flow in a Small Axisymmetric Contraction

We investigate the turbulence modeling of second moment closure used both in RANS and PITM methodologies from a fundamental point of view and its capacity to predict the flow in a low turbulence wind tunnel of small axisymmetric contraction designed by Uberoi and Wallis. This flow presents a complex phenomenon in physics of fluid turbulence. The anisotropy ratio of the turbulent stresses τ 11/τ 22 initially close to 1.4 returns to unity through the contraction, but surprisingly, this ratio gradually increases to its pre-contraction value in the uniform section downstream the contraction. This point constitutes the interesting paradox of the Uberoi and Wallis experiment. We perform numerical simulations of the turbulent flow in this wind tunnel using both a Reynolds stress model developed in RANS modeling and a subfilter scale stress model derived from the partially integrated transport modeling method. With the aim of reproducing the experimental grid turbulence resulting from the effects of the square-mesh biplane grid on the uniform wind tunnel stream, we develop a new analytical spectral method of generation of pseudo-random velocity fields in a cubic box. These velocity fields are then introduced in the channel using a matching numerical technique. Both RANS and PITM simulations are performed on several meshes to study the effects of the contraction on the mean velocity and turbulence. As a result, it is found that the RANS computation using the Reynolds stress model fails to reproduce the increase of anisotropy in the centerline of the channel after passing the contraction. In the contrary, the PITM simulation predicts fairly well this turbulent flow according to the experimental data, and especially, the “return to anisotropy” in the straight section of the channel downstream the contraction. This work shows that the PITM method used in conjunction with an analytical synthetic turbulence generation as inflow is well suited for simulating this flow, while allowing a drastic reduction of the computational resources.

Turbulence Model and Validation of Air Flow in Wind Tunnel

Turbulence Model and Validation of Air Flow in Wind Tunnel

Turbulence Model and Validation of Air Flow in Wind Tunnel

Turbulence Model and Validation of Air Flow in Wind Tunnel

The Impact of Pressure Regulators on the Runtime and Energy Savings of Supersonic Blowdown Wind Tunnels

Pressure regulators have a great impact on the runtime and energy savings of supersonic wind tunnels. In order to investigate this impact quantitatively the theoretical isentropic (ideal) equations for the supersonic wind tunnel flow were compared with viscous CFD computations. The theoretical model was extended include the flow from the outflow from the pressure vessel up to the wind tunnel. This theoretical model was solved with and without pressure regulation valve using a Runge-Kutta method. The differences in runtime and energy consumption for both configurations with and without pressure regulator as well as the derivation of the analytical model and the numerical solution are presented in detail in this paper.