Tunnel Experiments Research Articles

Performance characterization of a laminar gas inlet

Abstract. Aircraft-based measurements enable large-scale characterization of gas-phase atmospheric composition, but these measurements are complicated by the challenges of sampling from high-speed flow. Under such sampling conditions, the sample flow will likely experience turbulence, accelerating and mixing of potential contamination of the gas-phase from the condensed-phase components on walls, and reduced vapor transmission due to losses to the inner walls of the sampling line. While a significant amount of research has gone into understanding aerosol sampling efficiency for aircraft inlets, a similar research investment has not been made for gas sampling. Here, we analyze the performance of a forward-facing laminar flow gas inlet to establish its performance as a function of operating conditions, including ambient pressure, freestream velocities, and sampling conditions. Using computational fluid dynamics (CFD) modeling we simulate flow inside and outside the inlet to determine the extent of freestream turbulent interaction with the sample flow and its implication for gas sample transport. The CFD results of flow features in the inlet are compared against measurements of air speed and turbulent intensity from full-sized high-speed wind tunnel experiments. These comparisons suggest that the Reynolds-averaged Navier–Stokes (RANS) CFD simulations using the shear stress transport (SST) modeling approach provide the most reasonable prediction of the turbulence characteristics of the inlet.

Fixed-time angle of attack constrained control for aircraft considering dynamic icing process

Aircraft icing deteriorates aerodynamic performance and reduces stall angle of attack, the fast convergence rate of tracking error is required to stabilize the aircraft when aircraft icing occurs. The state-of-the-art control methods for icing aircraft mostly assume that the icing of aircraft is instantaneous. Aiming at these issues, a fixed-time angle of attack-constrained control strategy is designed considering dynamic icing process. In order to explore the variation of aerodynamic coefficients in the process of dynamic icing, an ice wind tunnel experiment is implemented, and the relationship between lift coefficient, drag coefficient and pitching moment coefficient with angle of attack and icing intensity is obtained by fitting method. In order to prevent the stalling problem caused by the decrease of the stalling angle of attack in the process of dynamic icing, a method to determine the stalling angle of attack based on deep neural network is proposed. Considering the asymmetric and time-varying angle of attack constraint, a fixed-time convergent angle of attack-constrained robust control method is designed. The ice wind tunnel experiment shows the process of dynamic icing of the airfoil, and the simulation results verify the effectiveness of the proposed control method.

Floating wind turbine motion signature in the far-wake spectral content – a wind tunnel experiment

Abstract. The growing interest in floating offshore wind turbines (FOWTs) is rooted in the potential source of increased offshore energy production. As the technology is still in a pre-industrial state, several questions remain to be addressed where little field data are available. This study uses physical modelling at a reduced scale to investigate the signature of the floating motions into the wake spectral content of a simplified FOWT model. A wind turbine model based on the actuator disc concept is placed in an atmospheric boundary layer wind tunnel and subjected to a range of surge, heave and pitch motions. The signatures of idealised sinusoidal motion and realistic broadband motion on the model’s wake at distances of 4.6 D (D being the disc diameter) and 8 D are measured through the use of a rake of single hot wires. The spectral analysis shows that harmonic motion leaves clear signatures in the far wake's energy spectra, mainly in the top tip region, while broadband motion does not leave easily detectable signatures.

CFD analysis of wing-propeller interaction on the NASA X-57 Maxwell aircraft wing

Due to global warming concerns, the Aviation industry is trying to reduce its carbon footprint. Electric propulsion (EP) is one way of doing this, where the power is obtained from electrical sources. The concept of distributed electric propulsion (DEP) is in the focus now. NASA’s X-57 Maxwell, a high winged, all-electric experimental aircraft, uses this concept. The present work aims at developing a CFD model (ANSYS Fluent) to evaluate aerodynamic performance of two configurations of NASA’s X-57 aircraft wing; (i) wing and nacelle (clean wing) and (ii) wing, nacelle and one electric propeller under cruise condition; and compare it with the results of wind tunnel experiment performed by NASA/Armstrong X-57 research program. Parameters like lift, drag and pressure coefficients (CL, CD, CP) are compared for both cases. A good match is observed for CL, CD and CP, thus validating the model. The unsteady RANS solver is very efficient in capturing the effects of propeller slipstream on the wing. After validation, this model is further used to simulate aerodynamic performance of a wing with multi-propeller (DEP) configuration.

Manipulation of the flow induced by afterbody vortices using sweeping jets

In this study, the impact of oscillatory fluids on the flow induced by afterbody vortices, resembling those of a high-speed train, was investigated in wind tunnel experiments. The afterbody vortices were generated by a cylindrical model with a slanted base at Reynolds numbers ranging from 87 000 to 200 000. A fluidic oscillator was utilized to produce sweeping jets (SWJ) acting on the vortical flow, which is similar to those observed over the rear of a typical high-speed train. Our particle image velocimetry measurements on cross-stream planes reveal that the oscillatory flow produced by the sweeping jet significantly reduces the magnitude of induced flow, resulting in a 17.7% and 15.9% decrease in the total velocity and horizontal slipstream, respectively, at a velocity ratio of 6.4. This is attributed to the injection of the kinetic energy of sweeping jets into the core of the afterbody vortices, which leads to a higher dispersion of vorticity and incoherent energy distributions. The iso-surfaces of resultant velocities reveal that the fluid injection by the sweeping jet increased local velocities, resulting in a spanwise shrinkage of the induced velocity structure downstream. The study demonstrates the capability of sweeping jets in decreasing induced flow by manipulating afterbody vortices and their potential for reducing the slipstream produced by high-speed trains at the tracksides.

The wake of a large wind turbine in stable atmospheric boundary layer flow, simulated in the EnFlo stratified-flow wind tunnel

The EnFlo stratified-flow wind tunnel is described, and the parameterization of stable atmospheric boundary layers in the context of wind turbines and stability classification is given, as are the scaling constraints for stratified-flow wind tunnel experiments. Wake measurements of mean velocity, Reynolds stresses, and turbulent heat flux were made for three mild stable states, including overlying inversion, and the neutral state. The depth of the atmospheric boundary layer was kept constant, and so the results show the effect of change in stability alone, without the change in scale that would also arise in full scale measurements, an advantage provided by wind tunnel experiments. The simulated boundary-layer wind-speed profile is the same in each case, its height slightly exceeding the blade-tip top height, and the velocity deficit at the turbine is also the same in each, implying a constant thrust coefficient. In the near wake the momentum deficit rises more rapidly in the stable cases, and stays higher further downstream where it is subsequently reduced by turbulent mixing. The wake grows less rapidly in the vertical direction in the stable cases, both above and below the hub height, the height above growing still less rapidly with an imposed inversion, while the height below is unaffected by the inversion. The wake width is largely unaffected by stability. Stability reduces the Reynolds stresses in the wake over and above the reduction in the undisturbed flow; there is not a simple superposition. In the lower part of the wake the stresses are not affected by the inversion, while they are in the upper part. Turbulent heat flux is increased in the bulk of the wake, more so with an inversion, but is reduced to the surface and unaffected by an inversion.

Magnetic-Control Variable Stiffness and Damping Tail Support for a Wind-Tunnel Aircraft Model

To suppress random vibration of aircraft models during wind tunnel tests, a novel magnetorheological elastomer (MRE)-based device with variable stiffness and variable damping (VSVD) is proposed, which is installed on the tail support system. Firstly, the dynamic characteristics of the tail support system are established using finite element model simulation to obtain the vibration mode and stress distribution. Next, the vibration reduction mechanism of the MRE-based tail support system is analyzed by solving a finite element model. Then, the magnetic-controlled VSVD device with an annular sandwich structure is proposed, and the decisive rheology material and structural parameters in relation to the tail support system’s dynamics are determined. Subsequently, the magnetically generated element with multicoils is designed to guarantee a large enough magnetic field, and the optimal structure parameters are obtained. Finally, random wind flow experiments in the wind tunnel and frequency sweep tests in the ground laboratory for the tail support system equipped on the VSVD device are conducted to verify the effectiveness of the proposed device, respectively. The results under 15–200 Hz random excitation indicate the root mean square of acceleration at the centroid of the aircraft model with the VSVD device can be decreased by 46.8% compared to the system without the VSVD device.

Experiments for a power law of near-surface mean velocity profiles in stratified boundary layers

Through a series of experiments in a wind tunnel, we study the near-surface profiles of the mean streamwise velocity in thermally stratified boundary layers. The surface is horizontal and is smooth or roughened by rods. We heat or chill this surface to achieve that stratification. Temperature-compensated hot-film anemometry is used to measure the mean streamwise velocity. Within the transition sublayer, lying in between the surface and the constant-flux sublayer, we find that the velocity profile is always a power-law function of the height from the surface. Its exponent ranges from 0.01 to 0.2 and increases with the surface roughness and the surface temperature. We summarize these findings in a formula and discuss its applications to the atmospheric boundary layer via the so-called Reynolds number similarity.

Experimental Investigation of the Effect of Bio-Inspired Wavy Leading-Edges on Aerodynamic Performance and Flow Topologies of the Airfoil

The characteristic of delayed airfoil stalls caused by the bio-inspired Wavy Leading-Edges (WLEs) has attracted extensive attention. This paper investigated the effect of WLEs on the aerodynamic performance and flow topologies of the airfoil through wind tunnel experiments, while also discussing the flow control mechanism of WLEs. The result shows that, at small Angle of Attack (AOA), the flow through the WLEs exhibits periodic and symmetrical characteristics, where flow vortices upwash at the trough and downwash at the crest, resulting in flow from the crest to the trough. Upwash leads to the formation of a localized three-dimensional laminar separation bubble (LSB) structure at the leading edge of the trough section. At large AOA after baseline airfoil stall, the flow on the airfoil surface of WLEs presents a two-period pattern along the spanwise direction, and the separation zone and the attachment zone appear alternately, indicating that the control effect of delayed stall is accomplished by reducing the separation zone on the airfoil surface. The alternating occurrence of the separation and attachment zones is the result of intricate interactions among flows passing through multiple WLEs. This interaction causes the convergence of high-momentum attached airflows on both sides, thereby constraining the spread of the separation from the leading edge and enabling the re-attachment of separated air. The research results of this paper provide a reference for researchers to reveal the flow control mechanism of WLEs more comprehensively.

Experimental Study of Wake Evolution under Vertical Staggered Arrangement of Wind Turbines of Different Sizes

During the expansion of a wind farm, the strategic placement of wind turbines can significantly improve wind energy utilization. This study investigates the evolution of wake turbulence in a wind farm after introducing smaller wind turbines within the gaps between larger ones, focusing on aspects such as wind speed, turbulence intensity, and turbulence integral length scale. The flow field conditions are described using parameters like turbulence critical length and power spectral density, as determined through wind tunnel experiments. In these experiments, a single large wind turbine model and nine smaller wind turbine models were used to create a small wind farm unit, and pressure distribution behind the wind turbines was measured under various operating conditions. The results indicate that downstream wind speed deficits intensify as the number of small wind turbines in operation increases. The impact of these smaller turbines varies with height, with a relatively minor effect on the upper blade tip and increasingly adverse effects as you move from the upper blade tip to the lower blade tip. Through an analysis of power spectral density, the contribution of vortex motion to wake turbulence kinetic energy is further quantified. In the far wake region, the number of small wind turbines has a relatively small impact on wind speed fluctuations.

Research on UAV Flight Parameter Identification Method Based on Launch Force and Airspeed.

Flight parameters are crucial criteria for UAV control, playing a significant role in ensuring the safe and efficient completion of missions. Launch force and airspeed information are key parameters in the early and middle stages of flight, serving as important data for monitoring the UAV's flight status. In response to challenges such as weak launch force, low identification rates, small airspeed, and low recognition accuracy in UAVs, a method for identifying UAV flight parameters based on launch force and airspeed is proposed. From the aspect of launch force identification, a recognition method based on a low-g value accelerometer information source is proposed, utilizing a 'multi-level time window + threshold' approach. For airspeed identification, an optimization method for airspeed measurement under the Kalman filter architecture is introduced. A device for airspeed measurement based on pressure sensors is designed, and the recommended installation position is determined through simulation. Furthermore, the feasibility and robustness of the proposed launch force identification and airspeed measurement optimization methods are validated through simulation. Finally, the effectiveness of the design is verified through centrifuge and wind tunnel experiments. This research provides technical support for the identification of the launch force and airspeed measurement in UAVs.

Experimental and Numerical Analysis Over Power Series Nose Cone

While designing aerospace vehicles, one of the most important factors to consider is drag. Drag force opposes the vehicle and causes it to slow down, which in turn requires more fuel to maintain flight. The vehicle nose cone plays a vital role in minimizing the drag during flight. The shape of the vehicle's body, as well as the fluid characteristics and orientation, can all have a significant impact on drag. To determine the most effective nose cone design, a study was conducted to test the aerodynamic performance of power series nose cone profiles using both experimental and computational techniques. All nose cone profiles with the same L/D ratio were assessed at 25 m/s speed with no angle of attack. The experimental data, including pressure, velocity, and drag, were then compared with computational data to ensure the accuracy of the wind tunnel experiments. The primary factor in determining the best shape for the subsonic flow range among all the nose cone profiles was the drag coefficient. The nose cone with a power of 0.25 was found to have the lowest drag coefficient at low subsonic speeds, making it the best-performing nose cone profile among those investigated. During take-off, drag is the most undesirable factor, as it can significantly impact the fuel efficiency of the vehicle. Therefore, it is crucial to have the least amount of drag possible. In this case, the nose cone with a power of 0.25 can be used in the initial stages of the rocket to minimize drag and maximize fuel efficiency.

A multi-directional and multi-modal galloping piezoelectric energy harvester with tri-section beam

A traditional wind energy harvester based on galloping can only harvest wind energy from one specific direction, which fails to work efficiently in a natural erratic environment. In this study, we propose a galloping-based piezoelectric energy harvester that can collect energy from wind flow in a wide range of incident directions with multiple vibrational modes being excited. The proposed harvester is composed of a tri-section beam with bonded piezoelectric transducers and a square bluff body with splitters. Finite element analysis of the tri-section beam structure is first performed and confirms the clustered natural frequencies that ease the excitation of different modes. Then, the aerodynamic characteristics of various bluff bodies is conducted through computational fluid dynamics to compare the capability of galloping. Finally, the wind tunnel experiment is carried out to test the wind energy harvesting performance by utilizing the harvester’s multi-modal characteristics. The results of this study demonstrate that the proposed harvester can harvest wind energy in multiple directions with the capability of galloping in multiple vibrational modes, and superior performance is achieved when the second bending mode is triggered. The novel design of the harvester from this work provides a viable solution for harvesting wind energy in a natural environment with varying wind conditions.

Statistical assessment of the concentration fluctuations in street canyons via time-resolved wind tunnel experiments

Time-resolved wind tunnel experiments are performed for three periodic street canyon configurations: one of uniform (H/W = 1) and two of variable building height (consisting of 0.5H and 1.5H tall towers in aligned and staggered arrangements) for perpendicular wind direction. The velocity field is mapped using Laser Doppler Anemometry (LDA), and the tracer gas concentration is sampled using Fast Flame Ionization Detection (FFID). The concentration field is characterized by the mean concentration and the characteristic percentiles of the concentration-time series, such as the median and the peak, represented by the 99th percentile, in a total of 1138 sampling points. The probability distribution of the concentration in each point is modeled using the gamma distribution, which is fitted based on the mean and the variance of the measured time series. The results show that the mean pedestrian exposure to air pollutants is significantly lower in the case of heterogeneous roof height, although the mean concentration at the leeward corner of the source canyon can locally exceed those of the uniform street canyons for both tower arrangements of variable building height. Moreover, it is shown that the concentration peaks for both tower configurations can significantly exceed those of the uniform canyons case, especially near the sources.

Segmented layered bistable piezoelectric laminates for enhanced energy harvesting from wind‐induced vibration

AbstractIn this study, a segmented layered bistable piezoelectric energy harvester (SBPEH) has been developed and thoroughly investigated for its wind energy harvesting capabilities. The SBPEH's performance was comprehensively evaluated through the creation of a detailed 3D finite element model using Abaqus and XFlow software, and further validated via rigorous wind tunnel experiments. The results of this research demonstrate the SBPEH's remarkable potential for wind energy harvesting. With a wide operational wind speed range, it achieved a maximum measured power output of 5.476 mW, coupled with an outstanding power density of 68.45 mW/cm3, both of which were realized at a wind speed of 13 m/s. It is worth noting that as the wind speed surpasses a specific threshold, the energy harvesting efficiency of the SBPEH experiences a decline, emphasizing the importance of identifying the optimal operational wind speed range for maximizing energy output. These findings offer valuable insights for the enhancement and application of SBPEHs, contributing to the advancement of renewable energy technologies and providing an eco‐friendly solution for power generation.Highlights The study introduces a new segmented layered bistable piezoelectric energy harvester (SBPEH) design for wind energy harvesting, addressing the limitations of traditional harvesters. Performance is evaluated through 3D finite element modeling, ABAQUS, Xflow simulations, and wind tunnel experiments. The SBPEH operates efficiently across a broad wind speed range (6–14 m/s), showcasing optimal output power of 5.476 mW at 13 m/s.

Secondary Organic Aerosol Formation Potential from Vehicular Non-tailpipe Emissions under Real-World Driving Conditions.

Traffic emissions are a dominant source of secondary organic aerosol (SOA) in urban environments. Though tailpipe exhaust has drawn extensive attention, the impact of non-tailpipe emissions on atmospheric SOA has not been well studied. Here, a closure study was performed combining urban tunnel experiments and dynamometer tests using an oxidation flow reactor in situ photo-oxidation. Results show a significant gap between field and laboratory research; the average SOA formation potential from real-world fleet is 639 ± 156 mg kg fuel-1, higher than the reconstructed result (188 mg kg fuel-1) based on dynamometer tests coupled with fleet composition inside the tunnel. Considering the minimal variation of SOA/CO in emission standards, we also reconstruct CO and find the critical role of high-emitting events in the real-world SOA burden. Different profiles of organic gases are detected inside the tunnel than tailpipe exhaust, such as more abundant C6-C9 aromatics, C11-C16 species, and benzothiazoles, denoting contributions from non-tailpipe emissions to SOA formation. Using these surrogate chemical compounds, we roughly estimate that high-emitting, evaporative emission, and asphalt-related and tire sublimation share 14, 20, and 10% of the SOA budget, respectively, partially explaining the gap between field and laboratory research. These experimental results highlight the importance of non-tailpipe emissions to atmospheric SOA.

Aeroelastic metastructure for simultaneously suppressing wind-induced vibration and energy harvesting under wind flows and base excitations

This paper proposes an innovative dual-functional aeroelastic metastructure that effectively suppresses wind-induced structural vibrations under either pure aerodynamic galloping or concurrent galloping and base excitations, while simultaneously harnessing the vibratory energy to potentially allow for self-powered onboard low-power sensing applications. Two configurations are theoretically and experimentally analysed and compared, one consisting of simply regular locally resonating masses subjected to no external forces, while the other comprising locally resonating bluff bodies which experience additional aerodynamic galloping forces. Numerical investigation is conducted based on an established aero-electro-mechanically coupled model. Wind tunnel wind tunnel and base vibration experiments are carried out using a fabricated aeroelastic metastructure prototype to characterize the energy transfer mechanisms and validate the numerical results. The mutual effects of key system parameters, including the frequency ratio, mass ratio, load resistance and electromechanical coupling strength, on the dual-functional capabilities are examined, providing a comprehensive design guideline for efficiently enhancing the energy transfer and conversion. Experimentally, the galloping displacement of the primary structure is attenuated by 78% with a measured power output of 2.63 mW from a single auxiliary oscillator at a wind speed of 8 m s−1. This research opens new possibilities for designing novel metastructures in practical scenarios where both wind-induced vibration suppression and energy harvesting are crucial.

A frost ice formation method of horizontal axis wind turbine blade based on polynomial fitting

In this paper, a new direct calculation method of frost ice shape on the blade surface of horizontal axis wind turbine is proposed. Using linear interpolation algorithm, the airfoil ice shape obtained by LEWICE 2D software or ice wind tunnel experiment was fitted with equidistant step length in the first and fourth quadrants and equidistant step length in the second and third quadrants. The key point coordinates of ice shapes on cross-sections along the span-wise were mapped into lagging and flapping surfaces through the mathematical dimension reduction, respectively. The polynomial fitting was used to deal with ice projection points of multiple sections in lagging and flapping surfaces, and then the blade’s frost ice shape was obtained. By calculating the sum of squared residuals of the polar diameter at the same polar angle, the errors between experimental and airfoil frost shape fitting methods, experimental and FENSAP, and blade frost shape formation methods and FENSAP were analyzed. The results show that the new method is in good agreement with the ice shape of FENSAP simulation results and experimental results. The residual sum of squares is small. This method makes the analysis of frost ice morphology of wind turbine blades do not need to consider interdisciplinary. The calculation process is simple and reliable.

Influence of passive deformation in the lift coefficient of a NACA0012 wing model

The extensive use of lightweight materials in aerial vehicle wings involves structural flexibility phenomena that generate non-negligible deformation effects. This influence is not restricted to big aircraft but also plays a role in smaller aeroplanes and Unmanned Aerial Vehicles (UAVs). Here, we conduct wind tunnel experiments to analyze the effect of passive deformation on the wing model lift slopes. To isolate the deformation effect, we compare rigid wings with a NACA0012 airfoil imposing a prescribed spanwise deformation. We study three levels of deformation: non-deformed, around 2% and 4.5% of tip deflection. Also, we consider the effect of the wing length by using three different semi-aspect ratios (1, 2, and 4), so a total of nine rigid wing models have been analyzed for a range of Reynolds number from 80×103 to 160×103. Deformed wing models show an increase in lift coefficient compared to non-deformed wing cases. Both deformation levels exhibit a qualitatively similar lift increment. A correlation to predict lift coefficient slope in a flat plate is adapted for a NACA0012 airfoil and validated using our experimental results and literature data. The adjusted correlation can quantify the deformation effect on the lift slope, which is comparable to using a slightly longer wing model.

Wind-induced ventilation rate of single-sided ventilation in a building with internal partition

Wind-induced single-sided ventilation is a prevalent form of natural ventilation extensively used in buildings. Nevertheless, prior experimental investigations predominantly focused on single-zone buildings, neglecting the multizone buildings with internal partitions which is representative of more common scenarios. This study addresses this gap by investigating the impact of internal partitions on single-sided ventilation, employing a combination of wind tunnel experiment and numerical analysis. Airflow rate (AFR) was measured with a split-fibre probe and purging flow rate (PFR) was assessed by the tracer gas methodology. The PFR exhibits greater sensitivity to internal partitions in unidirectional airflow compared to bidirectional flow. Large Eddy Simulation (LES) was conducted to elucidate the intricate airflow characteristics in single-sided ventilation. The ventilation efficiency (ratio of PFR and AFR) derived from LES ranges between 0.74 and 0.79, which means that <80% of the AFR actively contributes to the removal of contaminants. Notably, the investigation discerned that the AFR of a single room approximates that of the entire room, whereas the PFR of a single room is smaller than that of the whole room. The disparities in AFR and PFR were caused by the recirculating flow, which was elaborated by the theoretical analysis.