Wind Tunnel Research Articles

Industrial hemp biomass negatively affected by herbicide drift from corn and soybean herbicides.

The establishment of industrial hemp (Cannabis sativa L.) fields near row crops has raised concerns about the potential adverse effects of herbicide drift on hemp production. This study examined hemp susceptibility to drift of herbicides registered for use in corn and/or soybeans. Herbicide solutions (2,4-D, dicamba, glufosinate, glyphosate, imazethapyr, lactofen, mesotrione) were applied separately in the wind tunnel (3.6m s-1 airspeed), simulating drift scenarios, with conventional TP95015EVS (TP) and air inclusion AI95015EVS (AI) flat fan nozzles calibrated to deliver 140L ha-1 carrier volume at 230kPa. Mylar cards and hemp plants (20-25cm tall) were placed downwind up to 12m. Spray deposition from mylar cards was quantified using fluorometry and hemp biomass was collected 21 post application. Results indicated the nozzle design influenced downwind deposition; 5% of spray deposits from the TP nozzle reached 5.9 m downwind versus 2.0 m for the AI nozzle. Glyphosate, glufosinate, and mesotrione caused the highest biomass reductions, with 50% reductions observed at 19.3 (inferred), 8.7, and 9.3m downwind for TP nozzle, and 4.1, 4.0, and 2.9m for AI nozzle. These findings suggest herbicide applications at airspeeds of 3.6m s-1 or greater present a risk to nearby hemp fields.

Investigation of the influence of rain on pitot-static measuring systems of unmanned aerial vehicles

AbstractFor unmanned aerial systems with a maximum take-off mass of up to 25 kg, simple pitot-static tubes are predominantly used to measure the aerodynamic speed. In this work, the influence of rain on this measurement is investigated using representative pitot tubes of different diameters. In wind tunnel tests, water droplets with a diameter of about 3 mm are generated at a flow velocity of 10 m/s, which hit the pitot tubes at the total pressure bore. The observed disturbances sometimes last several hours and are equivalent to a total failure of the measuring system during flight operations. Phenomena are identified that influence the measurement of dynamic pressure in rain. It can be shown that both capillarity and the momentum of the raindrops play a decisive role in the type, severity and duration of the disturbance due to their relative movement to the tube. The further course of the disturbance is also influenced by the evaporation of liquid, both on and in the tube. In flight tests under rain conditions, the findings of the wind tunnel tests were qualitatively confirmed and additional disturbance patterns were observed.

Exploration of bio-inspired wingtip devices for low aspect ratio wing

Abstract This study investigates the performance of low aspect ratio wing by incorporating bio-inspired wingtip devices, aiming to enhance the flying characteristics of micro air vehicles. The S5010 profiled wing, with an aspect ratio of 1.0, is selected as the reference wing. The wingtip devices are designed as flat plates, with a taper ratio of 0.5, featuring rounded leading and trailing edges. These devices are attached to the wingtip in a planar manner, thereby creating slots on the wingtip. Such an approach is intended to replicate the wingtip slot observed in the structure of primary feathers of soaring birds during flight, potentially providing aerodynamic benefits. In this study, four different winglet configurations are fabricated, and investigations are carried out in a subsonic wind tunnel at a Reynolds number range of 70000 to 110000. The results show significant improvements in lift slope, maximum lift coefficient, drag, lift-to-drag ratio, and pitching moment for all winglet configurations compared to the baseline. Furthermore, the study also investigates the effectiveness of winglet configurations by varying the number of attachments to the wingtip and their lengths. It is observed that configurations with a higher number of attachments show a more significant reduction in induced drag and upward pitching tendency than configurations with fewer attachments. Additionally, the performance of wing configurations is strongly affected by the Reynolds number, and it improves as the Reynolds number increases.

Flow disturbance by intrusive instrumentation located on the surface of a compressor blade

In this paper, we present an experimental and numerical investigation into the flow disturbance by intrusive instrumentation in the form of spanwise pressure tubes mounted on the pressure and suction side of a compressor blade in a linear cascade wind tunnel at a moderate subsonic Mach number of 0.5 and a Reynolds number of 790,000. The pressure tubes were modelled as simplified bumps extending over the entire span with a width of 24% of the chord length and a height of 67% of the maximum blade thickness. The leading edge of the bumps is located at 30% of the streamwise surface length. We found that the total pressure losses generated by these pressure tubes are considerably larger when they are placed on the suction side of the blade. The suction-sided bump increases the total pressure loss coefficient by a factor of four, while the pressure-sided bump approximately doubles it. CFD simulations reveal that the suction-sided bump causes the flow to form a large, open recirculation zone, while for the pressure-sided bump, the flow reattaches near the trailing edge. In conclusion, we provide insights into the effects of pressure tubes on the losses generated by compressor blades. The mounted tubes significantly impact the flow and should be carefully considered during instrumentation design.

Optimizing vortex-induced vibration suppression in π-shaped girders using wind tunnel testing and CFD analysis

This investigation meticulously explores the vortex-induced vibration (VIV) characteristics in Π-shaped girders within long-span cable-stayed bridges, presenting novel VIV mitigation techniques. Employing detailed wind tunnel evaluations of a 1:50 scale model alongside computational fluid dynamics (CFD) analyses to model the two-dimensional flow around the girder, this study reveals the formation and shedding of vortices behind the railings and main steel girder, which significantly contribute to VIV in such structures. The introduction of horizontal splitter plates, one m in width, in conjunction with two-m-wide V-shaped guide vanes, was found to substantially reduce the VIV amplitude of the Π-shaped girder, minimizing vortex generation on the deck’s sides and dramatically reducing the maximum vertical VIV amplitude from 208.75 mm to only 5.56 mm. CFD simulations confirm the effectiveness of these measures in suppressing Karman vortex street formation, thereby significantly improving downstream airflow characteristics. These findings offer pivotal insights for the advancement of bridge design and the proactive management of VIV, showcasing the potential of integrated aerodynamic modifications to enhance structural resilience and performance.

Wind tunnel and flight testing of a lamb wave-based ice accretion sensor

Abstract Severe icing conditions are still a risk in aviation. Heavy ice accretion on aircraft can affect mass and aerodynamic performance in a way, that a catastrophic loss of control may occur. Detecting icing conditions and the extent of ice accretion is therefore essential to react instantly and to assess the severity. This eases the decision to either activate ice protection systems and continue the flight or to leave the icing conditions immediately. These functions can be provided by an icing sensor. Within the EU funded SENS4ICE project, several ice sensors have been developed. One of them is the so-called local ice layer detector (LILD). It uses lamb waves which are guided by the aircraft structure. If an ice accretion happens, the wave guide behavior is affected changing amplitude and lag time of the lamb wave pulses. This can be measured. If the sensor is mounted to a surface on the aircraft which is prone to icing, e.g. the leading edge of the wing, ice can be detected where it is relevant for the aerodynamic performance. Within the project, the sensor hard- and software have been developed to create and measure lamb wave pulses in a wide frequency range. To investigate the influence of ice accretion on the lamb waves, wind tunnel tests were undertaken. Finally, the sensor was investigated in a flight test. In this paper, an overview about the function of the sensor as well as the general signal behavior are given. The results of the wind tunnel test and some preliminary results from the flight tests will be shown. In both tests, the sensor was able to detect even very thin ice layers with minimum delay from the beginning of the ice accretion.

Numerical Investigation and Validation of Noise Sources of a Distributed Propulsion System

This study delves into the exploration of a distributed propulsion system consisting of three co-rotating propellers in front of a high-lift wing system, with a primary focus on understanding its acoustic characteristics. Employing high-fidelity computational fluid dynamics simulations, the flow field is simulated in climb condition. Subsequently, acoustic transport to far-field observers is conducted using the Ffowcs–Williams and Hawkings equation. Because the noise mechanisms of such a configuration are not fully understood yet, several permeable and impermeable surfaces as input for the acoustic simulation are used, aiming to separate the various noise source contributions. Results reveal that in addition to the rotor-alone noise from the propellers, noise arising from propeller–wing interaction emerges as dominant, emitting mostly at the blade passing frequency. Moreover, a boundary element method tool is applied to observe the scattering effect of propeller-generated noise by the wing. Furthermore, this study incorporates consideration of structural components in close proximity to the model, thereby achieving a more realistic acoustic field. With this knowledge, the simulation is validated with experimental data from an open wind tunnel test campaign. An overall good agreement is observed between the simulated and experimental results, substantiating the reliability of both aerodynamic and acoustic predictions.

Shock-Induced Fan Cowl Separation During Aeroengine Windmilling at Diversion from Cruise

When a civil aircraft engine is operated at windmill during the cruise flight phase, there is supersonic flow acceleration around the leading edge of the fan cowl toward the external surface. The terminating normal shock wave can separate the turbulent boundary layer developing on this external surface. A series of experiments at a flight-relevant Reynolds number (1.2 million based on lip thickness) are performed in a quasi-two-dimensional wind tunnel rig to investigate the underlying flow physics. At a nominal inflow Mach number of 0.65 and a nacelle incidence angle of 4.5 deg, as the equivalent engine mass-flow rate is reduced, an increase in shock strength results in flow separation when the shock exceeds Mach 1.4. Over a 10% range in the notional engine mass-flow rate, the boundary layer developing on the external fan cowl thickens by a factor of three on the onset of separation. A reduction in the incoming Mach number from 0.65 to 0.60 weakens the shock wave and thus delays separation. An increase in surface roughness has no significant effect in situations where the boundary layer remains attached. However, for separated cases, an increased local roughness height causes a greater separation extent and a thicker boundary layer downstream of the shock wave.

Retrofit Measures for Aircraft Noise Reduction: Simulation Benchmark and Impact Assessment

DLR, German Aerospace Center, has developed a low-noise retrofit to reduce noise generation of a conventional midrange transportation aircraft, i.e., measures to the airframe and engine exhaust nozzle. Selected measures have been installed onboard of DLR’s flying testbed Advanced Technology Research Aircraft and been subject to a flyover noise campaign. The aircraft in its original configuration and the modified aircraft have been tested and measured in flyover campaigns in 2016 and 2019, respectively. Initial simulation models have been derived from the results and previous component simulations and wind tunnel tests to account for the new reduction measures within DLR’s system noise prediction tool Parametric Aircraft Noise Analysis Module (PANAM). This paper presents the developed measures as selected for the flight test campaigns. An overview of the conducted flight tests is provided. The overall noise simulation process, including aircraft and engine design, flight simulation, and noise prediction, is presented. Simulation results obtained from this process are then compared to the measured levels from the flight campaigns. Estimated simulation uncertainties from PANAM can now be directly compared to differences between measured and simulated levels. Finally, a parameter study is conducted in order to assess the reduction potential of the novel retrofit measures along different simulated flight procedures. Spatial noise distribution in sound exposure level contours is assessed.

Film-Cooling Performance Comparison of Blade Tips With a Trailing Edge, Pressure Side Cutback, or a Suction Side Squealer in a Transonic Linear Cascade

Abstract Film-cooling effectiveness and leakage flow play crucial roles in turbine blade heat transfer. In contrast to the conventional trailing edge cutback squealer blade tip (CB tip), this study aims to compare the film-cooling effectiveness of a suction side squealer tip (SS tip) under different blowing ratios and density ratios. This experiment is conducted in a three-blade cascade wind tunnel under transonic conditions. While applying the pressure-sensitive paint (PSP) technique for film-cooling effectiveness measurements, both top-view and side-view perspectives were employed to illustrate the distribution of local cooling effectiveness across various regions of the blade. Furthermore, PSP can be used to estimate the leakage flow across the blade tip by analyzing the local static pressure distribution. For the CB tip, the film-cooling effectiveness within the cavity increases as both the blowing ratio and density ratio increase. The PSP technique clearly captures the accumulation of coolant within the cavity and the coolant discharge near the trailing edge in the vicinity of the squealer cutback. When the blade tip is outfitted with only a suction side squealer, the film-cooling effectiveness on the blade tip is comparable to that of the full squealer with the trailing edge cutback. This similarity is achieved as the CB tip design requires 14–20% more coolant to achieve the same blowing ratios as the SS tip. The advantage of the SS tip is further demonstrated as leakage flow across the tip is reduced compared to the CB tip.

Performa Termal Kondensor Udara pada Wind Tunnel: Evaluasi Perpindahan Panas

Heat transfer in air condenser systems is a critical aspect in various industrial applications, especially refrigeration and air conditioning systems. The thermal efficiency of condensers is greatly influenced by airflow characteristics, but airflow velocity optimization remains a challenge due to the trade-off between improved heat transfer and energy consumption. Although previous research has examined various aspects of condenser design, there is still a need to explore the optimal limit of airflow velocity that can maximize thermal efficiency without excessively increasing system workload. This study aims to evaluate the thermal performance of an air condenser in a wind tunnel, focusing on the effects of airflow velocity variations on heat transfer rate and thermal efficiency. The experimental method uses a wind tunnel with dimensions of 280x28x28 cm, equipped with a precise temperature and flow velocity measurement system. The flow velocity was varied from 0.8 to 1.8 m/s. The results showed a significant increase in the heat transfer coefficient (h) and heat transfer rate (Q) as the flow velocity increased. Analysis of Reynolds and Nusselt numbers revealed the transition of the flow from laminar to turbulent, which contributed to the increase in heat transfer efficiency. Comparison of experimental results with simulations showed good agreement in the middle velocity range, but there were deviations at extreme velocities. In conclusion, increasing the airflow velocity proved effective in improving the thermal performance of the condenser, but further optimization is required to balance the thermal efficiency with the energy consumption of the system.

Machine Learning‐Based Wind Classification by Wing Deformation in Biomimetic Flapping Robots: Biomimetic Flexible Structures Improve Wind Sensing

Flying animals such as insects and birds have strain receptors on their wings. The functions and information processing capabilities of these strain receptors, however, are largely unknown. Herein, the potential ability of wing strain sensors to detect wind direction in flapping flight is experimentally explored. Seven strain gauges are attached to the biomimetic wing shafts of flexible wings. The wings flap through an electric mechanism emulating hovering flight in a wind tunnel with gentle wind, and a convolutional neural network model for wind direction classification is developed. The results show that with only one strain gauge, accurate wind classification is achieved using segmented data of ≈1 flapping cycle. Even with segmented data of only 0.2 flapping cycles, wind classification can be achieved with all seven strain gauges. Moreover, the use of wing shafts improves the classification accuracy, especially with shorter segmented data. Additional flapping phase information does not substantially improve classification performance. These results suggest that hovering animals determine the wind direction via strain sensing, which is useful for flight control. The implementation of wing strain sensors in flapping‐wing aerial robots may improve flight control ability.

Comparative study on deep and machine learning approaches for predicting wind pressures on tall buildings

Wind-structures interaction has been extensively examined in the last few decades using field measurements, full scale measurements and wind tunnel testing. These experimental approaches are considered costly and time consuming. The need for a reliable analytical approach that can be used for examining wind-effects on buildings is clear. Although Computational Fluid Dynamics (CFD) is one of the other alternative numerical options yet might not reached the level of confidence to be reliably used to finalize the structural design. On the other hand, a limited number of studies have been carried out using soft computing methods to examine wind-induced loads on structures. However, its promising results, more work is still required towards achieving the full analytical prediction of wind effects on structures. This study investigates the use of different soft-computing techniques in predicting wind pressures on tall buildings. Two deep learning methods viz deep belief network (DBN) and deep neural network (DNN), and five machine learning methods namely feedforward neural network, extreme learning machine, weighted extreme learning machine, random forest, and gradient boosting machine were evaluated, and compared in predicting the design wind pressures on tall buildings. Wind tunnel datasets, used in the current study to develop the proposed computing models, were collected from testing three tall buildings having the same full-scale horizontal dimensions of (40 m and 80 m) and different heights of (80 m, 120 m and 160 m). The buildings were tested at a scale of 1:400 in urban terrain exposure. Mean and fluctuating wind pressure coefficients on the building with the height of 120 m are herein predicted using the seven computing methods and the results were compared to the corresponding measured pressures. Overall, the examined methods performed well in the wind pressure prediction process. Furthermore, the employed DNN was found to have the best performance in predicting mean and fluctuating wind pressures with the highest correlation coefficients. Hence, the DNN was also used in predicting the mean and fluctuating wind pressures on the two other buildings with heights of 80 m and 160 m. Experimental results indicate that the employed DNN model can be effectively used in predicting wind-induced pressures on tall buildings.

Three-Dimensional Flutter Numerical Simulation of Wings in Heavy Gas and Transonic Flutter Similarity Law Correction Method

Wind tunnel testing is a crucial method for studying aircraft flutter. Using heavy gas as the wind tunnel medium can mitigate the escalating issue of test models being overweight as advanced aircraft develop. This paper employs an analytical method for numerical calculations of three-dimensional (3D) wing flutter based on fluid–structure interaction (FSI). Flutter calculations for the Goland wing are conducted, and the results in the air medium are consistent with the literature. In contrast, significant differences in flutter behavior are observed in the heavy gas R134a medium. Compared to air, when the model reaches a critical state in R134a, the incoming flow velocity is lower, the incoming flow density is approximately 3 to 5 times air, and the incoming flow dynamic pressure is about 1.1 to 1.2 times that of air. The correction of heavy gas flutter data is crucial for wind tunnel testing. This paper proposes a correction method based on the unsteady transonic flow similarity law proposed by Bendiksen under quasi-steady conditions. Attempts are made to revise relevant published wind tunnel tests and heavy gas flutter calculation results. The transonic flutter similarity law effectively explains the flutter similarity of rigid models in both heavy gas and air media. Still, it fails in cases with highly reduced frequencies and low mass ratios, such as those encountered with flexible wings.

Multi-directional and multi-modal vortex-induced vibrations for wind energy harvesting

A conventional vortex-induced vibration (VIV)-based energy harvester is typically restricted to capturing wind energy from a very limited range of wind directions, making it inefficient in varying wind conditions. This Letter proposes a tri-section beam configuration for VIV-based piezoelectric energy harvester to enable harnessing wind energy from varying incident angle with different vibration modes being triggered. The finite element analysis investigates the tri-section beam harvester's mode shapes and natural frequencies. A wind tunnel experiment is conducted for a comparative study of the energy output performance of the harvesters with straight and tri-section beams. The findings show that the proposed harvester with the tri-section beam can efficiently capture wind energy from a much wider range of incident angles, as opposed to the specific limited directions of its counterpart with the straight beam. The proposed harvester can also widen the lock-in speed range with a higher bending mode being triggered and achieve the optimal output power of 1.388 mW when the proposed harvester works in the second mode at a higher natural frequency, superior to that of its counterpart (0.386 mW) that can only work in the first mode. The proposed configuration sheds light on developing multi-directional and multi-modal VIV-based energy harvesters adapted to wind conditions in natural environments.

THE COOLING EFFECT OF COMBUSTOR EXIT LOUVER SCHEME ON A TRANSONIC NOZZLE GUIDE VANE ENDWALL

Abstract The ever-increasing combustor exit temperature in modern turbine engine designs raises challenges for the nozzle guide vane cooling. Due to the complexity of NGV cooling design, the cooling effect from the upstream combustor cooling features can prove valuable. This study investigates, experimentally and numerically, the cooling effect of a louver cooling scheme near the combustor exit on the NGV endwall. The wind tunnel testing and CFD simulation are carried out with engine-representative conditions of an exit Mach number of 0.85, an exit Reynolds number of 1.5 × 106, an inlet turbulence intensity of 16%, and a density ratio of 2.1. Various coolant mass flow ratios from 1% to 4% are tested to demonstrate the effect of the coolant rate. For the geometry studied, the results found a critical mass flow ratio between 1%~2%. The coolant forms a uniform film only by exceeding this value to provide good coverage upstream of the NGV passage inlet. As for the cooling of the NGV passage, the mass flow ratio of the range investigated is insufficient for desirable cooling performance. The pressure side endwall proves most difficult for the coolant to reach. In addition, the fishmouth cavity at the combustor-NGV passage causes a three-dimensional cavity vortex that transports the coolant in the pitch-wise direction. The transport pattern is dependent on the coolant mass flow ratio. Therefore, the authors propose combining this louver scheme with the upstream jump cooling scheme for a desirable NGV cooling system.

Influence of exposure time on the aero-optical effects of a high-speed aircraft optical window

Under high-speed conditions (Mach number > 3), the turbulent shear layer formed on the optical window causes phase changes in the light, resulting in the aero-optical effects, which significantly affect the imaging quality at different exposure times, τ. In this study, imaging data of an optical window were collected in a Mach 3.8 supersonic low-noise wind tunnel for a facula and a USAF 1951 resolution board, as well as distorted wavefront data of visible light. As τ increases from 0.2 ms to 5 ms, the centroid jitter of the facula shows an overall decreasing trend and the increase of τ mainly inhibits the jitter in the spanwise direction. While τ varies from 0.2 ms to 28 ms, structural analysis indicated that the structural similarity and stability of imaging improves and this improvement has an upper limit of τ0 = 22 ms, coined as the saturation exposure time. Normalized modulation transfer function curves derived from USAF 1951 images show a decrease in the imaging resolution as τ increases. The average wavefront distortion can be reduced by up to 71.7% as τ increases from 5 ms to 30 ms, while the standard deviation of wavefront distortion initially decreases and then stabilizes at 0.0025λ.

Experimental Study on Identification of Aerodynamic Damping Matrix for the Tower of an Operating Wind Turbine by Artificial Excitation

ABSTRACTQuantitation of damping is of great significance for the design and condition assessment of wind turbines. The authors' previous theoretical and numerical studies showed that compared with damping ratios, a modal aerodynamic damping matrix can better describe the damping coupling in the fore‐aft (FA) and side‐side (SS) tower motions. In the present study, an improved damping identification method was first proposed to identify this damping matrix with artificial exciters and then verified by using OpenFAST simulations under different excitation frequencies, excitation force amplitudes, and turbulent wind fields. Following the numerical study, a scaled wind turbine model with a geometric scale ratio of 1/75 was carefully designed based on the NREL 5‐MW wind turbine prototype, in which the scaled blade design follows the rule in thrust coefficient similarity. An identification study was performed with this scaled model by a series of wind tunnel tests. The modal aerodynamic damping matrix was identified under steady‐state harmonic excitation in the operating state and compared with the identified results by a free decay method and the theoretical values. The results experimentally confirm the correctness of the aerodynamic damping matrix theory under uniform wind and the feasibility of the improved identification method in practice.

Supercooled Water Droplets Impacting at High Speed on Superhydrophobic Surfaces: Open Questions Related to Icephobicity

It is posited that impact ice adhesion strength on superhydrophobic surfaces is related to droplet impalement. Several gaps still exist in the understanding of how different types of textured surfaces resist the impalement of high‐speed impacting droplets (<102 μm diameter at 102–103 m s−1). A previous study reveals that a pressure balance calculation can predict the wetting state of an impacting drop (≈103 μm diameter at ≈101 m s−1). However, the ability of this model to predict the wetting state of small droplets impacting at high speed or of droplets in a supercooled state has not been reported. To better understand supercooled microdroplet impalement, and to verify the hypothesis that droplet impalement is directly related to ice adhesion strength, herein, the impact behavior of microdroplets in a cold airflow onto surfaces of varying topography and wettability in an icing wind tunnel with support of high‐speed video images are investigated. Although the results do not allow for validating the above hypothesis about impalements and ice adhesion, they show that the difference between supercooled droplet spreading and retraction can be a predictor of ice adhesion strength. However, this relation must be validated with a greater body of data from samples with very diverse surface properties.

Synthetic Wind Estimation for Small Fixed-Wing Drones

Wind estimation is crucial for studying the atmospheric boundary layer. Traditional methods such as weather balloons offer limited in situ capabilities; besides an Air Data System (ADS) combined with inertial measurements and satellite positioning is required to estimate the wind on fixed-wing drones. As pressure probes are an important constituent of an ADS, they are susceptible to malfunctioning or failure due to blockages, thus affecting the capability of wind sensing and possibly the safety of the drone. This paper presents a novel approach, using low-fidelity aerodynamic models of drones to estimate wind synthetically. In our work, the aerodynamic model parameters are derived from post-processed flight data, in contrast to existing approaches that use expensive wind tunnel calibration for identifying the same. In sum, our method integrates aerodynamic force and moment models into a Vehicle Dynamic Model (VDM)-based navigation filter to yield a synthetic wind estimate without relying on an airspeed sensor. We validate our approach using two geometrically distinct drones, each characterized by a unique aerodynamic model and different quality of inertial sensors, altogether tested across several flights. Experimental results demonstrate that the proposed cross-platform method provides a synthetic wind velocity estimate, thus offering a practical backup to traditional techniques.