Wind Tunnel Model Research Articles

The time-evolving impact of tree size on nighttime street canyon microclimate: Wind tunnel modeling of aerodynamic effects and heat removal

Urban trees play a crucial role in urban climate in many aspects. However, existing research has not adequately explored the impact from a time-evolving perspective, that is, tree growth over time. To bridge this research gap, this study investigates in a wind tunnel the effects of tree-to-canyon foliage cover and relative height (0.32–1.1 times canyon height), mimicking growth of trees, on conditions in street canyons during moderate and extreme heat. The results reveal that trees may affect canyon-wide ventilation and heat removal in two different scenarios. First, when canyons are in isothermal conditions, medium and large trees, that fill half the canyon height or reach slightly above the canyon, decelerate the shear layer and weaken the vortical flow, as a result reducing the canyon-wide ventilation. Second, in extreme heat conditions, medium and large trees trap heat at the pedestrian level due to the blockage of air entrainment and the suppression of upward buoyancy-driven flow from the ground surface. An air temperature rise that corresponds to 1.5 °C in a full-scale urban setting is observed in measurements. These observations suggest that urban trees' foliage cover must be managed for a canyon's optimal ventilation and heat removal during nighttime.

Frequency domain identification of modal characteristics and loads from output‐only measurements

AbstractThis paper proposes a frequency domain framework for concurrently estimating the modal structural parameters and modal loads from the measured output response. A mathematical load model is theoretically developed and compared to the load determined by force identification. Unlike conventional modal identification schemes, the proposed approach can effectively accommodate the spectral shape of the external load in the frequency domain, enabling the determination of the modal parameters without explicit white noise assumption. The proposed approach is verified by applying it to a single‐degree‐of‐freedom system system representing a modal response, an aeroelastic model in an atmospheric boundary layer wind tunnel, a multi‐degree‐of‐freedom system, and the measured acceleration response of a 40‐story building. The simulation results demonstrate that the proposed technique can reliably estimate the modal parameters and load model and is robust against the noise present in the response and the order of the load model. Additionally, the proposed technique can be applied to various loading conditions such as vortex‐induced and buffeting effects since the modal parameters and the load model can be identified for any shape of the loading spectrum.

Wire-driven parallel robot suspension system for SDM in a low-speed wind tunnel

A wire-driven parallel robot with eight wires (WDPR-8) acting as the suspension system for an aircraft model in a low-speed wind tunnel is introduced in this paper. A kinematics model for WDPR-8 is established here. A theoretical basis is also provided, through which movement control of the model is achieved by changing the wire length. A model motion control subsystem, a monocular vision measurement subsystem for the model position, and a data acquisition subsystem were also developed for WDPR-8. A prototype of WDPR-8 was built and mounted in a low-speed wind tunnel with an open test section. A six-component strain-gauge balance was installed inside the test model to acquire aerodynamic parameters. The aerodynamic coefficients of the static test show reasonable agreement with reference results. The difference between the test result of WDPR-8 and the results from reference is smaller in the positive angle of attack than the negative angle of attack, and the total difference between WDPR-8 and other facilities is under 10%. Pitch dynamic tests with different amplitudes and frequencies were used to verify the aerodynamic hysteresis phenomena. The research indicates it is successful that the solution of measuring aerodynamic parameters with strain-gauge balance set into the standard dynamic model suspended by WDPR. It demonstrates that WDPR-8 is effective and feasible as a model suspension system in a low-speed wind tunnel. Furthermore, test results without interference correction in accordance with the reference data proves that the interference of Wire-Driven support system to flow field during wind tunnel tests is negligible. Finally, WDPR suspension system introduced in this paper will remarkably improve the efficiency and accuracy in wind tunnel test.

Prediction of wing buffet pressure loads using a convolutional and recurrent neural network framework

AbstractIn the present study, a hybrid deep learning reduced-order model (ROM) is applied for the prediction of wing buffet pressure distributions on a civil aircraft configuration. The hybrid model is compound of a convolutional variational neural network autoencoder (CNN-VAR-AE) and a long short-term memory (LSTM) neural network. The CNN-VAR-AE is used for the reduction of the high-dimensional flow field data, whereas the LSTM is applied to predict the temporal evolution of the pressure distributions. For training the neural network, experimental buffet data obtained by unsteady pressure sensitive paint measurement (iPSP), is applied. As a test case, the Airbus XRF-1 configuration is selected, considering two different experimental setups. The first setup is defined by a wind tunnel model with a clean wing, whereas the second setup includes an ultra high bypass ratio engine nacelle on each wing. Both configurations have been tested in the European Transonic Windtunnel, considering several transonic buffet conditions. Finalizing the training of the hybrid neural networks, the trained models are applied for the prediction of buffet flow conditions which are not included in the training data set. A comparison of the experimental results and the pressure distributions predicted by the hybrid ROMs indicate a precise prediction performance. Considering both aircraft configurations, the main buffet flow features are captured by the hybrid ROMs.

Effects of wind barriers on VIV performances of twin separated parallel decks for a long-span rail-cum-road bridge

Wind barriers are frequently used on the railway bridges to improve operational safety of high-speed trains. However, the effects of wind barriers along the railway on the separated decks' Vortex-induced vibration (VIV) for rail-cum-road bridges are not fully known, whose VIV characteristics are more complicated because of aerodynamic interference effect (AIE). A number of dynamic sectional model wind tunnel tests for VIV performance were carried out. Three different heights (H) varying from 2 m to 4 m and four different ventilation ratios (R) varying from 20% to 50% for wind barriers on railway decks were examined under two opposite incoming flow directions at a fixed spacing. Results indicated that twin separated decks' overall VIV performance are poor when the wind flows from the railway upstream to the highway downstream. The wind barrier on railway deck can weak the remarkable fluctuating wind pressure coefficients, change the correlation coefficients and reduce the contribution coefficients of local distribution aerodynamic forces to the overall aerodynamic forces. It was effective to suppress VIV of the railway deck, and it also had a greater impact on the adjacent highway deck. The VIV-suppress ability of wind barriers is determined by the parameters of H and R. The wind barriers with H = 3 m and R = 30% were recommended to satisfy the requirements of twin decks after a comprehensive evaluation.

Experimental investigation of wind effects on a 282-meter-high tower with complex aerodynamic shapes based on a rigid model and multi-degree-of-freedom aeroelastic model

Slender and flexible high-rise structures are very sensitive to large wind-induced vibration phenomena such as vortex-induced vibration and galloping under strong winds. However, pressure measurements on rigid models cannot consider coupling aeroelastic effects, and single-degree-of-freedom aeroelastic model wind tunnel tests can only consider the contribution of the first-order linear mode. For wind-sensitive structures with complex aerodynamic shapes, the effect of higher-order modes on the wind-induced response may not be ignored. In this study, a series of multi-degree-of-freedom (MDOF) aeroelastic model wind tunnel tests and pressure measurements of a rigid model were conducted to comparatively investigate the wind effects of a 282-meter-high tower with complex aerodynamic shapes. A detailed analysis of the aeroelastic effects on the high-rise tower is presented, and the effects of the structural damping ratio, wind speed, and wind direction are also discussed. The results show that the variation trends of the peak acceleration responses are generally consistent, while in those wind-sensitive directions (165-degree wind direction), the peak acceleration responses of the MDOF model are almost 1.5 times larger than those of the rigid model, indicating that the aeroelastic effects are remarkable. The corresponding response spectral peak values of the MDOF model are significantly larger than those of the rigid model by a factor of more than 3 and the correlations of responses in the two orthogonal directions also increase greatly by 43.5%. However, the “set-back” aerodynamic shape (180-degree wind direction) can considerably weaken aeroelastic effects and the peak acceleration responses are close to those of the rigid model. The generalized aerodynamic force spectrum also shows broadband features, implying its superior aerodynamic performance. The comfort assessments for different structural damping ratios are conducted based on various codes and criteria, suggesting that the structural damping ratio should be larger than 3%. The equivalent static wind loads on the upper floors are notably affected by aeroelastic effects while the lower floors are relatively insensitive. This study provides a detailed perspective of the aeroelastic effect for slender tower, and hence facilitates the wind-resistant design of flexible structures.

Designing a flight information collection system for calculating the aerodynamic characteristics of a UAV

The article describes the development of an onboard information collection system for calculating the aerodynamic characteristics of unmanned aerial vehicles (UAVs), which can only be obtained under free flight conditions. A functional diagram is presented, a flight parameters registration system operation algorithm is described. The proposed solutions make it possible to collect a large amount of information and improve system efficiency through the selection and compression of information. The collected information is used to determine the aerodynamic characteristics, eliminating the need to test UAVs models in wind tunnels, which can save money and time on the development of new UAVs. The features of the information exchange between the system elements are considered, which make it possible to speed up the data transfer, and therefore improve the accuracy of measurements by increasing the frequency of polling measuring devices. The results obtained may be of interest to developers of autonomous measuring systems. Of particular importancce for calculating the aerodynamic characteristics of an UAV using the proposed method is the high accuracy of measuring the parameters of the aircraft trajectory, including the linear accelerations and angular velocities that occur in the course of motion. In addition, the article considers a variant of the hardware implementation of the proposed system for UAVs.

Density estimation of an ultrahigh-speed rarefied flow field based on force analysis of a pendulum sphere

The similarity criterion for high speed or high enthalpy rarefied reaction flows is the binary scaling law, one of which is the product of the incoming flow density and the characteristic scale of the test model in a rarefied wind tunnel should be equal to that under the flight condition. This is to ensure that the ratio between the characteristic distance of the relaxation process and the characteristic length of the body is equal. However, in a rarefied wind tunnel, it is difficult to directly measure the flow density below 10−5 kg/m3 using common methods due to the relatively high rarefied degree, an indirect density estimation method based on force analysis of a pendulum sphere was proposed in this study. The oscillation trajectory of the pendulum sphere under ultrahigh-speed rarefied flow conditions was recorded and the corresponding aerodynamic drag force acted on the sphere along the trajectory was analyzed, based on which the density distribution of the flow was then estimated combined with DSMC simulations. The obtained results show that in the present study, the flow is in under-expanded condition and the measured density sharply decreased from 10−6 kg/m3 to 10−7 kg/m3 at 140–170 mm from the nozzle outlet.

Wind tunnel experiment and thermodynamic modeling for dry-snow accretion on a heated surface

Wind tunnel experiments and modeling of dry-snow accretion on a heated surface were performed to estimate the characteristics of dry-snow accretion occurring in sub-freezing temperatures by using cold-climate facilities. The experiments encompassed a wide range of meteorological conditions—air temperature, wind speed, and snow flux—and were conducted over a surface heat flux range of 300–1800 W/m2. The measurements were performed with respect to the snow properties, including growth rate, sticking efficiency, and liquid water content (LWC), and were used to analyze the impact of dry-snow accretion on a heated surface. The experimental results demonstrated the distinct influence of the surface heat flux on the snow accretion process. The sticking efficiency was found to highly correlate with the LWC of the accreted snow and meteorological parameters, and the accreted snow layer was shed from the surfaces when the LWC was slightly above 40%. Moreover, on the basis of heat and mass transfer during the accretion process, models for estimating sticking efficiency and LWC were developed for dry-snow accretion on a heated surface. The effects of convective cooling and evaporation of liquid water inside the accreted snow were incorporated, and the models were cross-validated with the experimental results. The equation was applied to the determination of the minimum value for sticking efficiency, and a linear proportional relation between sticking efficiency and meteorological parameters was demonstrated.

The effect of wind characteristics on tall buildings with porous double-skin façades

Wind loads on tall buildings with porous double-skin façade (PDSF) systems were studied for various wind characteristics. Experiments were performed on a small-scale building model in a boundary layer wind tunnel. A single-skin building model was used as a reference case alongside three PDSF systems of 25%, 50% and 65% porosity. Two atmospheric boundary layer (ABL) simulations were created corresponding to category I (rural) and category III (suburban) ABLs of the EN-1991-1-4:2005 standard. Wind loads on the building model were studied based on high-frequency force balance measurements of integral moments, while pressure distribution on the building model surfaces was analyzed based on pressure measurements. Experiments were performed for ten flow incidence angles 0° < α < 45° with a 5° increment. The obtained results are considered highly relevant both for research purposes and engineering applications, especially considering the lack of similar studies currently available. In particular, the rural ABL simulation yields larger mean pressure coefficients. The effect of the ABL characteristics on the pressure coefficient standard deviation depends substantially on α, while the most notable difference is observed on the windward building model surface at α = 0°, where the suburban ABL causes a substantially higher standard deviation. The mean across-wind moment coefficient of the suburban ABL simulation is lower at all α points, while its standard deviation is lower for α > 0°. The along-wind moment coefficient in the suburban ABL simulation is ∼20% lower than in the rural ABL simulation, a trend which is observed for all α points. In contrast, the along-wind moment standard deviation is ∼25% larger in the suburban ABL simulation. The power spectral density of the along-wind moment is considerably lower in the suburban ABL simulation.

Experimental and Numerical Investigation of the Aerodynamic Ventilation Drag of Heavy-Duty Vehicle Wheels

Due to current EU regulations, constant-speed testing on test tracks is used for aerodynamic certification of heavy-duty vehicles (HDV). However, the aerodynamic development of HDVs is performed using wind tunnels and computational fluid dynamics (CFD). Both techniques commonly neglect the rotational aerodynamic losses of the wheels—the so-called ventilation drag—that are present when driving on the road. This is due to the fact that there is no full-scale wind tunnel for this type of vehicle with a suitable belt system for the simulation of the wheel rotation. Furthermore, the ventilation drag of HDV wheels has been neglected in CFD due to their almost completely closed rim design. These constraints lead to an underprediction of the aerodynamic forces in comparison to the results under on-road conditions when performing constant-speed tests. In order to investigate the ventilation drag of HDV wheels, measurements were carried out on a 1:4.5 scale generic tractor-trailer model in the Model Scale Wind Tunnel of the University of Stuttgart. The measured aerodynamic forces as well as the measured flow field data provide the basis for the definition and validation of a procedure for analyzing the ventilation drag in CFD. Accordingly, the ventilation drag of a full scale HDV was investigated in CFD. The results show that the tire treading and rim geometry have a significant influence on ventilation drag that contributes to the total aerodynamic drag of the HDV. The present work shows that the ventilation drag has a relevant impact on the total aerodynamic drag of HDVs and should therefore not be neglected. The presented CFD approach thus allows to assess the aerodynamic drag under real on-road conditions in an early stage of the vehicle development.

Non-uniform wind excitation on dynamic responses of vehicle running on bridge

In order to investigate the effect of wind non-uniform characteristics in mountainous terrain on the dynamic analysis of a vehicle-bridge (VB) system, a typical bridge site for a terrain model was established in a wind tunnel. Six typical flow directions, such as parallel to the river direction and perpendicular to bridge axis and ridge, were adopted to study the mean wind speed of experimental monitoring points at the bridge deck. Owing to the local ridge at the bridge site, the distribution of wind speed along the bridge deck shows obvious non-uniform characteristics. Seven monitoring points were set on the bridge deck, and the linear interpolation method was adopted to obtain the time histories of the wind speed at every point. A scaled wind tunnel model was also applied to conduct the three aerodynamic coefficients of the vehicle and bridge. Based on the experimental wind characteristics of mountainous terrain, the numerical framework of the wind-vehicle-bridge (WVB) system was established considering the effect of non-uniform wind, where a railway cable-stayed bridge was simulated using the finite element model, and a multi-body dynamic model of the vehicle was used. The influence of fluctuating wind excitation on the WVB system was discussed, and it played an essential role in vehicle vibration.

Topology optimization for the design of a 3D-printed rotating shaft balance

Rotating shaft balances (RSBs) are devices that are used to measure rotor blade forces and moments of wind tunnel models during wind tunnel tests. The design of an RSB can be challenging, because it has to comply with many and sometimes contradicting requirements such as high stiffness and high strain gauge bridge outputs. The manufacturing of a conventional RSB consists of different subsequent steps, which can be time consuming, expensive and associated with many risks. Therefore, in this work, the authors investigate if a RSB can be designed by topology optimization and manufactured by 3D printing. A topology optimization method was developed with as design objective the minimization of strain energy with constraints for the volume of the RSB’s midsection, defined stresses at strain gauge locations used for the measurement of axial force and torque and an overhang constraint for additive manufacturing. The optimal preliminary design found by topology optimization was translated into a final printable design with the highest bridge sensitivity for axial force and torque, sufficient output for in-plane forces and moments and an acceptable safety factor on strength for combined loads. After adding extra supports required for printing, the RSB was successfully printed in metal by the laser powder bed fusion process, resulting in a product without external defects. The same topology optimization and manufacturing method can potentially be used for other balance types, leading to a reduction in total lead time and manufacturing costs while increasing the design freedom.

Underconstrained Cable-Driven Parallel Suspension System of Virtual Flight Test Model in Wind Tunnel

An underconstrained cable-driven parallel robot (CDPR) suspension system was designed for a virtual flight testing (VFT) model. This mechanism includes two identical upper and lower kinematic chains, each of which comprises a cylindrical pair, rotating pair, and cable parallelogram. The model is pulled via two cables at the top and bottom and fixed by a yaw turntable, which can realize free coupling and decoupling with three rotational degrees of freedom of the model. First, the underconstrained CDPR suspension system of the VFT model was designed according to the mechanics theory, the degrees of freedom were verified, and the support platform was optimized to realize the coincidence between the model’s center of mass and the rotation center of the mechanism during the motion to ensure the stability of the support system. Finally, kinematic and dynamical modeling of the underconstrained CDPR suspension system was conducted; the system stiffness and stability criteria were deduced. Thus, the modeling of an underconstrained, reconfigurable, passively driven CDPR was understood comprehensively. Furthermore, dynamic simulations and experiments were used to verify that the proposed system meets the support requirements of the wind tunnel-based VFT model. This study serves as the foundation for subsequent wind tunnel test research on identifying the aerodynamic parameters of aircraft models, and also provides new avenues for the development of novel support methods for the wind tunnel test model.

A Case Study on Redesign & Printing Large-scale Wind Tunnel Experiment Models via Combined Additive Manufacturing Processes

While additive manufacturing (AM) offers the ability to produce complex, customized structures, it still faces limitations when it comes to printing large structures with different material sections. For the production of big-sized structures or products, traditional processes still hold the advantage in terms of cost, manufacturing time, and material compatibility. Additionally, identifying suitable parts for AM, particularly for a single AM process, can be challenging, especially those with large sizes and different material compositions. This paper proposes a modular hybrid AM solution that combines different AM processes to address the challenges associated with large-scale structure printing in AM. The study presents a case in which a metallic powder-bed-fusion process and a stereolithography process were used in combination to produce large-scale wind tunnel experimental models. A comparative analysis was conducted between the proposed approach and traditional manufacturing methods. The results demonstrate that, for large-scale cases such as wind tunnel models, a combination of multiple AM processes has the potential to improve performance/quality, reduce costs, and save time when compared to traditional processes. However, it is crucial to emphasize that the success of the hybrid approach depends heavily on the design and process planning.

Virtual wind tunnel modelling and numerical calculation of forklift power compartment based on acoustic-heat-flow multi-physical field coupling

Virtual wind tunnel modelling and numerical calculation of forklift power compartment based on acoustic-heat-flow multi-physical field coupling

Virtual wind tunnel modelling and numerical calculation of forklift power compartment based on acoustic-heat-flow multi-physical field coupling

Virtual wind tunnel modelling and numerical calculation of forklift power compartment based on acoustic-heat-flow multi-physical field coupling

Assessment and application of optical flow in background-oriented schlieren for compressible flows

Optical flow provides an opportunity to elevate the resolution and sensitivity of deflection sensing in background-oriented schlieren (BOS). Despite extensive relevant literature within the field of computer vision, there is a lack of proper quantification of its abilities and limitations with regard to the state-of-the-art BOS experiments. Thus, this study performs an assessment of accuracy and resolution limits in different flow field scenarios utilizing background patterns generated with random dot and wavelet noise distributions. Accordingly, a synthetic assessment over a theoretically generated Prandtl–Meyer expansion fan is conducted with variations introduced in the background patterns and operational parameters of optical flow. A clear superiority of accuracy and resolvable range of density gradient amplitudes over cross-correlation is demonstrated. Moreover, an experimental assessment of supersonic flow features over multiple wind tunnel models is performed. The influence of experimental constraints, limitations and uncertainties related to the application of optical flow in BOS and its comparative performance against the block-matching counterpart is characterized.Graphical abstract

A new similarity criterion and design method for wind tunnel model tests of floating offshore wind turbines

Floating offshore wind turbines (FOWTs) have received strong attention in recent years, and numerous experimental studies have been conducted on them. However, the small-scaled wind turbine models designed by the criterion of traditional thrust coefficient similarity are unable to obtain similar power coefficients with the prototype, and they are not able to reflect the expected dynamic aerodynamics induced by the platform motions. To this end, this paper presents a new criterion for wind tunnel model tests based on the mapping of the optimal tip speed ratio (TSR) and the similarity of the changing rate of the relative TSR. The National Renewable Energy Laboratory's offshore 5-MW wind turbine is selected as the prototype wind turbine. The blade of the model wind turbine is generated by NACA 4412 airfoil, and the chord and twist are optimized by the genetic algorithm. The objective is to keep the thrust and power coefficients of the model wind turbine around the optimal TSR consistent with that of the prototype through a linear mapping relationship. Dynamic simulations with the Unsteady Blade-Element Momentum method are performed to verify the new similarity criterion at different surge frequencies. Compared with those of the model wind turbine designed by the traditional method, fluctuations of the mapped thrust and power coefficients of the new model are in better agreement with the prototype. Hysteresis effects with respect to the relative TSR are newly captured, and loops of the new model are also closer to that of the prototype. This paper provides a more reasonable criterion for wind tunnel model tests of FOWTs, and the dynamic aerodynamics of the prototype can be reflected more accurately compared with the traditional method, which is important for the performance evaluation and optimization design of large-scale FOWTs.

Wind turbine model-test method for achieving similarity of both model- and full-scale thrusts and torques

For model tests of a floating offshore wind turbine (FOWT) system, a great challenge is how to model the interaction between the wind turbine and floating platform with correctly-scaled aerodynamic and hydrodynamic loads because of a well-known contradiction between Reynolds and Froude scaling. Several approaches have been proposed in the literature to tackle the challenge but none of them can correctly and simultaneously model the scaled thrust and torque, and so the interaction between turbine and platform. This paper will present a new model-test method for achieving similarity of both thrust and torque. This is achieved by redesigning the model blades with keeping the blade twist angle same as that of the full-scale turbine and by adjusting the pitch angle of blades and rotational speed of wind turbine in model tests. Numerical simulations and wind tunnel model tests are carried out to validate the present method. Both numerical results and experimental data show that the present method can realize the similarity of thrust and torque simultaneously, and thus, making it possible to study full interaction between turbine and floating platform by physical testing in wind-wave basins.