Extensive Wind Tunnel Tests Research Articles

Predicting Wall Pressure Fluctuations on Aerospace Launchers Through Machine Learning Approaches

Artificial intelligence (AI) can be used to optimize the prediction of pressure fluctuations over the external surfaces of aerospace launchers and minimize the number of wind tunnel tests. In the present research, various machine learning (ML) techniques capable of predicting the acoustic load were tested and validated. The methods included decision trees, Gaussian Process Regression (GPR), Support Vector Machines (SVMs), artificial neural networks (ANNs), linear regression, and ensemble methods such as bagged and boosted trees. These algorithms were trained using experimental data from an extensive wind tunnel test campaign conducted to support the design of a VEGA (Advanced Generation European Vehicle) launcher vehicle and provide wall pressure fluctuations in many configurations. The main objective of this study was to identify, among several algorithms, the most suitable method able to process such complex databases efficiently and to provide reliable predictions. Different statistical indices, including the root mean square error (RMSE), the mean square error (MSE), and a correlation coefficient (R-squared), were employed to evaluate the performance of the ML methods. Among all the methods, the bagged tree algorithm outperformed the others, providing the most accurate predictions, with low RMSE and high R-squared values across all test cases. Other methods, such as the ANNs and GPR, exhibited higher errors, indicating their reduced suitability for this dataset. The results demonstrate that ensemble decision tree methods are highly effective in predicting acoustic loads, offering reliable predictions, even for configurations outside the training database. These findings support the application of ML-based models to optimize experimental campaigns and enhance the design of aerospace launch vehicles.

Wind-tunnel experimental investigation on rotor-rotor aerodynamic interaction in compound helicopter configuration

In recent years, the compound helicopter configuration, featuring the addition of lateral propellers to a helicopter main rotor, has gained renewed interest for its ability to achieve higher flight speed. The aerodynamic behaviour of compound rotorcraft is dominated by mutual interactions between the rotors and the wakes they generate, which can affect their performance and the handling qualities of the aircraft. In order to study the effects of the rotor-wake and wake-wake interactions, a test rig was developed and an extensive wind-tunnel test campaign was carried out measuring the performance of rotor and propellers under different flight conditions. Hovering flight and forward flight at different advance ratios were considered, as well as crosswind flight under various wind directions. In general, an increase in thrust for both main rotor and propellers is found due to the interaction. In particular, the significant increase in the propeller thrust is attributed to the influence of the main rotor downwash, which impacts edgewise on the propellers. In some specific conditions, a decrease in the propeller's thrust is observed, which might be related to blade-vortex interaction (BVI) effects. To aid in the interpretation of the experimental results, numerical simulations with a mid-fidelity aerodynamic code were also performed.

Experimental investigation on influence of terrain complexity for wind pressure of low-rise building

This study conducted extensive wind tunnel tests to evaluate the impact of terrain complexity on wind pressure across low-rise buildings. A series of wind tunnel tests was performed using 50 actual terrain morphologies in the US. The findings were compared with results obtained from testing on homogeneous terrain to discern variations in pressure coefficients. A notable increase in turbulence intensity was observed in complex heterogeneous terrains, even with similar effective roughness lengths. The heightened turbulence property was a crucial factor in explaining changes in Cp,mean. The magnitude of Cp,mean demonstrated a continuous rise in the windward wall and roof 1 regions with increasing turbulence intensity. This correlation held true even for Iu,eave values surpassing 0.3. In contrast, while Cp,RMS exhibited a tendency to increase with rising Iu,eave, it did not exhibit the same continuous increase phenomenon. Consequently, no significant disparity in magnitude was noted between homogeneous and heterogeneous terrains in this regard. These findings underscore the importance of accounting for terrain complexity in wind load assessments, particularly in scenarios of heightened terrain diversity, to mitigate potential errors in wind load evaluations.

Modelling the Wall Pressure Fluctuations on the VEGA-C Launcher in Supersonic Conditions

The prediction of pressure fluctuations generated over the external surface of aerospace launchers during the atmospheric flight remains a challenging task due to the complexity of the geometry and the effects of compressibility at high supersonic Mach numbers. An experimental database is here analysed to the scope of providing a procedure to model and predict the relevant statistics of the wall pressure fluctuations generated by a supersonic flow overflowing the VEGA-C Launcher Vehicle. Data have been obtained in an extensive Wind Tunnel test campaign carried out in the trisonic wind tunnel of the National Institute for Aerospace Research (INCAS) in Bucharest. Wall-mounted pressure transducers allowed for the computation of the pressure Auto- and Cross-spectra over the fourth (the payload region) and third stages of the launcher model. Coherence functions are modelled through exponential-like analytical functions following the approaches usually adopted in canonical boundary layer flows, whereas the auto-spectra models are based on polynomial fits. The approach adopted for the achievement of proper non-dimensional quantities as well as the procedure implemented for the full-scale extrapolation is presented and discussed.

Experimental study on wind characteristics and prediction of mean wind profile over complex heterogeneous terrain

This study investigates the impact of complex heterogeneous terrain on mean wind speed and turbulence intensity, highlighting the significance of terrain configuration in the wind loading on buildings and airflow over urban areas. Extensive wind tunnel tests were conducted for 60 different roughness configurations, obtained by processing aerial images across the United States. The study makes two main contributions. First, a model was proposed to predict mean wind profiles, using the morphological information of complex heterogeneous terrain. The Deaves and Harris model was utilized along with a novel algorithm for the automatic characterization of roughness transitions. The proposed model exhibited less than 2% average prediction error compared to the measured wind speed. Second, the study investigated the impact of terrain complexity on near-surface wind characteristics. By comparing the experimental results with those obtained from a homogeneous terrain with a similar roughness length, we quantified the potential errors that may arise when assuming a homogeneous terrain for wind speed assessment. It was observed that increasing variability in roughness length led to a decrease in mean wind speed and an increase in turbulence intensity. The influence of terrain complexity, however, was found to be secondary compared to roughness length. Consequently, the relationships between terrain complexity and wind characteristics were quantified, and a simplified model was proposed.

Skew wind actions on vehicles crossing bridges with solid parapets

This work focuses on the effect that the angle of incidence of the wind has on the flow around bridge decks with low-rise edge parapets, and how it affects the aerodynamic actions on vehicles. First, a generic deck model with different barrier configurations is studied using computational fluid dynamic (CFD) analysis, and it is observed that for very skew winds even relatively low barriers can deviate the flow to make it aligned with the direction of the deck, which is referred to as channelling effect in this study. The work continues with an extensive wind tunnel (WT) testing programme on a deck model that represents a realistic bridge with a conventional configuration of short side barriers. The flow visualisation and the aerodynamic forces measured on a high-sided vehicle show the existence of three different zones in terms of the skew angle of the wind, which are in agreement with the CFD results. It is concluded that skew winds can significantly increase the aerodynamic actions on the vehicles due to the reduction of the shielding area across the width of the deck, and also because of the along-deck wind channelling.

Engineering pivoting shade structures for the Dubai Expo

This article focuses on the engineering of pivoting sunshade structures for the Dubai Expo 2020, which won a Structural Award 2022. Synopsis Most engineers will be familiar with regular-shaped, fixed buildings or structures and how to design to ensure loads can safely be transferred to the ground. But what happens when your structure is bespoke, irregular, permeable, flexible and moveable? This article outlines the unique challenges involved in developing a successful design for the shade structures at the Dubai Expo, which involved extensive wind tunnel testing, prototyping and engineering well outside the Eurocodes and usual day-to-day work of most structural engineers. The project won a Structural Award 2022 under the Process attribute that recognises technical achievement and innovation.

Effects of boundary-layer bleed parameters on supersonic intake buzz

Extensive wind tunnel tests were conducted to study effects of the location, entrance slant angle, entrance width, and type (slot or porous) of boundary layer bleed on the buzz phenomenon in a supersonic air intake. The intake was a mixed-compression axisymmetric intake for the design Mach number of 2.0 that was studied at zero degrees angle of attack at three free stream Mach numbers of 1.8, 2.0, and 2.2 through shadowgraph visualization system and pressure recordings. For every free stream Mach number and bleed geometry, eight different back pressures were examined. Results showed that the bleed can significantly postpone the buzz onset and increase the stability range of the intake. The bleed location was the most effective parameter among the examined parameters of the bleed system. According to the results, the bleed restricts the domain of shock oscillations to downstream of the bleed entrance and consequently, it increases the frequency of buzz as compared with the no-bleed case. In addition, the slot bleed is more effective in stabilizing the intake flow field than the porous bleed.

Computational analyses of tail fin configurations for a sounding rocket

Missiles and sounding rockets usually deviate from the trajectory due to unstable roll. Fins with cant angles are generally used to provide a rolling moment in sounding rockets and missiles to minimize the instability. Inducing a rolling moment also leads to an increase in the rocket motor’s power consumption due to the rise in drag, so inducing an optimal rolling moment with a minimal increase in drag is a crucial design criterion. It is crucial to maintain the similarity parameters while testing a scaled-down model in a wind tunnel. Therefore, computational fluid dynamics (CFD) is more efficient than extensive wind tunnel tests. In this paper, three-dimensional, incompressible simulations were performed on different models of sounding rockets using commercial CFD package fluent. The simulations were performed with the help of k-epsilon standard turbulence model. The results obtained were tabulated and graphically represented, and the trends of aerodynamic coefficients like C_{text {d}} and C_{text {m}} were analyzed. The purpose of this study is to analyze the dependency of aerodynamic coefficients on different fin configurations with emphasis on the cant angle. This study will be helpful to researchers designing a sounding rocket and help in maximizing apogee. The experimental and computational results show a favourable comparison. The results will show a particular configuration of fin having greater C_{text {m}}/C_{text {d}} which yields in a greater rolling moment and least amount of drag.

Provisions and comparison of Chinese wind load standard for roof components and cladding

This paper presents the main theoretical foundation and innovation of winad load provisions for wind loads on components and cladding in the newly-revised Standard for Wind Loads on Roof Structures (JGJ/T 481-2019). Generally, by introducing the concept of peak wind pressure coefficients, a more reasonable and convenient wind load determination for roof components and cladding can be obtained for structural designers. Moreover, based on extensive wind tunnel test data of external wind pressures on components and cladding of typical roofs, the corresponding peak external wind pressure coefficients are proposed, with more specific wind load zones. Regarding those large-span curved roofs, innovative external wind pressure coefficient values on detailed roof zones are firstly presented. In addition, according to comparisons, it is seen that the roof zones in the Chinese standard are similar to those in the Canada wind load code. The resulting peak external wind pressure coefficients in corner, edge and interior zones reach the international advanced level. Finally, optional three methods to determine peak wind pressure coefficient are also clearly specified, which are applicable to cases with different wind pressure time history lengths. All the development and improvement of the standard provide necessary supplement and reference for the international and Chinese wind-resisting design.

Flow Asymmetry in a Y-Shaped Diverterless Supersonic Inlet: A Novel Finding

Extensive wind-tunnel tests were performed on a Y-shaped diverterless supersonic inlet (DSI). All tests were conducted at a free stream Mach number of , the design Mach number for this inlet, and at both zero degrees angle of attack (AOA) and angle of sideslip (AOS). The experiments were performed at various inlet operating conditions comprising supercritical, critical, and subcritical conditions that covered almost all ranges of the engine operating for this DSI. The results showed that the DSI had relatively acceptable performance characteristics when operating at its design condition. A symmetric supersonic flow pattern was observed at both supercritical and critical operating conditions of the present Y-shaped DSI. However, both symmetric and asymmetric supersonic flow patterns were observed during the subcritical operating conditions of the present inlet. At low subcritical operating conditions, the supersonic flow pattern remained symmetric, similar to that of the critical operating condition case. As the inlet experienced higher subcritical operating conditions, the supersonic flow structures became suddenly asymmetric and a new flow pattern was noticed. This flow pattern was seen to be highly unstable; and significant asymmetric shock wave oscillations, boundary-layer thickening, and shock-/boundary-layer interaction were observed for these situations. To the authors’ knowledge, this novel phenomenon of asymmetric flow behavior in a perfectly manufactured symmetric Y-shaped DSI encountering symmetric flow (0 deg AOA and AOS) has not been noticed by previous researchers and, if so, the results have not been published yet.

Wind tunnel measurements of the surface pressure fluctuations on the new VEGA-C space launcher

An extensive wind tunnel test campaign devoted to the characterization of the aerodynamic and aeroacoustic behaviour of the new Space Launcher VEGA-C has been carried out in the trisonic wind tunnel available at the National Institute for Aerospace Research (INCAS) in Bucharest. The present paper summarizes the main results of the aeroacoustic investigation with a specific focus on the buffeting analysis. For this reason, the results presented herein are limited to the transonic conditions which are the most critical in terms of occurrence of flow instabilities. The scaled instrumented model of the VEGA-C launcher has been equipped with flush mounted Kulite sensors that provided the distribution of the wall pressure fluctuations. Pressure measurements at the wall of the wind tunnel have been carried out as well, with the scope of measuring reference signals to be used for cleaning the model pressure data from the background noise. Numerical simulations have been also performed to facilitate the physical interpretation of the experimental outcomes by qualitatively identifying the position of the shockwaves and tracking their evolution along the launcher for increasing Mach number. Data processing provides temporal statistics of the wall pressure signals as well as spectral quantities that give clear indications about the absence of buffeting.

Dynamic Stability Analysis of Aircraft Flight in Deep Stall

Beyond aerodynamic stall, the analysis of aircraft flight requires models accurately representing nonlinearities and instabilities. Previous bifurcation analysis studies therefore had a priori knowledge of the aircraft dynamics including dynamic damping. Yet, unlike airliners, extensive wind-tunnel tests and high-fidelity models are rarely available for small unmanned aircraft. Instead, continuous fluid dynamic simulations may provide basic insights into the aerodynamics, however limited to static conditions. In this paper, bifurcation analysis is presented as a tool to discuss the effects of unidentified pitch-damping dynamics during deep-stall transition that allows development of a nonlinear pitch-damping model for a small unmanned aircraft. By preliminary study of the static case, the model is extended based on deep-stall flight data to predict dynamics and stability. As a result, the deep-stall modes of the extended model are investigated in full envelope.

Simulation and flight experiments of a quadrotor tail-sitter vertical take-off and landing unmanned aerial vehicle with wide flight envelope

This paper presents the modeling, simulation, and control of a small-scale electric powered quadrotor tail-sitter vertical take-off and landing unmanned aerial vehicle. In the modeling part, a full attitude wind tunnel test is performed on the full-scale unmanned aerial vehicle to capture its aerodynamics over the flight envelope. To accurately capture the degradation of motor thrust and torque at the presence of the forward speed, a wind tunnel test on the motor and propeller is also carried out. The extensive wind tunnel tests, when combined with the unmanned aerial vehicle kinematics model, dynamics model and other practical constraints such as motor saturation and delay, lead to a complete flight simulator that can accurately reveal the actual aircraft dynamics as verified by actual flight experiments. Based on the developed model, a unified attitude controller and a stable transition controller are designed and verified. Both simulation and experiments show that the developed attitude controller can stabilize the unmanned aerial vehicle attitude over the entire flight envelope and the transition controller can successfully transit the unmanned aerial vehicle from vertical flight to level flight with negligible altitude dropping, a common and fundamental challenge for tail-sitter vertical take-off and landing aircrafts. Finally, when supplied with the designed controller, the tail-sitter unmanned aerial vehicle can achieve a wide flight speed envelope ranging from stationary hovering to fast level flight. This feature dramatically distinguishes our aircraft from conventional fixed-wing airplanes.

In-flight tracking and vibration control using the DLR’s multiple Swashplate system

This paper discusses the design, integration, and test of a higher harmonic control algorithm capable of both vibration control and in-flight blade tracking in conjunction with DLR’s multiple swashplate control system (META), while honoring predefined limits in usable control authority. The design of the control algorithm is described in detail and the results of coupled numerical investigations with both a general purpose multibody code by Politecnico di Milano and DLR’s comprehensive rotor code to determine the algorithm’s performance are presented. The integration of the control algorithm into the real-time control software is shown for the META system, where, for safety reasons, a semi-open loop approach was implemented. First tests of the controllers in-flight tracking mode to reduce 1/rev loads during hover yielded an almost complete reduction in 1/rev vibratory loads while maintaining constant rotor thrust. Following the experiments at DLR’s own facility, extensive wind-tunnel tests were performed in 2016 with META and a 5-bladed rotor system at the large low-speed facility of the German Dutch Wind Tunnels. The control algorithm was adapted to the 5-bladed rotor and successfully applied for in-flight blade tracking as well as the reduction of 5/rev hub loads using multi-harmonic pitch inputs with frequencies from 4/rev to 6/rev in cruise and high-speed flight condition. In both cases, the controller showed excellent performance and yielded satisfactory reductions of 1/rev rotor imbalances as well as a reduction of 5/rev hub vibrations by more than $$80\%$$ , while, at the same time, adhering to user set limits for the higher harmonic control amplitudes.

A procedure for calibrating the spinning ultrasonic wind sensors

The accuracy of wind speed and direction measurements with spinning ultrasonic wind sensors is important than ever in today’s wind industry in which, they are usually installed on the hub of the wind turbines to measure the wind speed and direction for optimized power. In this paper, extensive wind-tunnel tests have been performed to calibrate the wind speed and direction measured by a spinning single-axis ultrasonic anemometer for both static and spinning conditions. This has been carried out with static measures at various stationary angles of transducers signal path with wind direction, and with dynamic measures in which the anemometer is rotating with various rotational speeds. The velocity measured by ultrasonic anemometer in static tests is calibrated with pitot-tube data, and the interpolation of obtained calibration coefficients is used to correct the ultrasonic velocity measured in dynamic tests. It is observed that the calibrated ultrasonic wind speed measurements in dynamic tests are in a good agreement with the reference velocity. According to the results, the ultrasonic velocity measurements in both static and dynamic tests are affected by the transducers head distortions, and the shifting in acoustic pulse trajectory due to the rotational motion does not affect the anemometer measurements. The uncertainty of the calibration process for the spinning tests was found to be about 0.3%.

CFD Study of Solid Wind Tunnel Wall Effects on Wing Characteristics

Most of airplane structures include the components such as Fuselage, Wings, Empennage, Landing gear and Power plant etc. Wings are the important part of aircraft and these are airfoils attached to each side of the fuselage and are the main lifting surfaces that support the airplane in flight. SSD-2 airfoil, a modified version of NACA23015 airfoil is used to study the aerodynamic characteristics such as lift, drag, moment and wall effects with different wind tunnel blockage ratios. Generally, any new airfoil characteristics are derived from the extensive wind tunnel tests, however experiments are costly. Hence, to get the idea of aerodynamic characteristics and wall effects over airfoil with different wind tunnel blockage ratios, CFD tools are used such as ICEM CFD and ANSYS FLUENT. This paper presents the results of simulation of flow past SSD-2 airfoil. The study is carried out on a 2D airfoil section for free stream condition. Further the computations are made on 3D rectangular wing of solid wind tunnel simulation by varying wind tunnel height to get the different wind tunnel blockage ratios, keeping the other parameters constant. In solid wind tunnel simulation the lift and moment are less compared to free stream simulation results due to blockage effects. Finally, it is observed that increase in the wind tunnel blockage ratio, lift and moment decreases, drag increases with less difference in pressure; whereas decrease in wind tunnel blockage ratio, the lift and moment increases, drag decreases with more difference in pressure.

An Evaluation of Multi-Rotor Unmanned Aircraft as Flying Wind Sensors

This paper examines the possibility of using a Multi-Rotor Unmanned Aircraft System (MUAS) for atmospheric flow measurements around a tall building. This novel sensing approach is proposed, whereby we attempt to determine the oncoming flow velocity magnitude and direction from measurements of the power required by each of the MUAS rotors. Extensive wind-tunnel testing was completed to determine the power required by the fore and aft rotor-pairs at varying flow velocities and directions. The results show that it is possible to map between rotor power consumption and the oncoming flow vectors, however, a unique and accurate mapping is only possible over a very small region of the measurement space. Thus, it is concluded that the practical use of this sensing method is limited. Examination of power consumption curves also revealed that the conditions under which a Vortex Ring State (VRS) develops for small MUAS. The characteristics of VRS development are similar to those of full-size helicopters, indicating that the VRS is Reynolds number independent. The reduction in power consumption due to the presence of updraft flows of various magnitudes was also quantified, indicating that significant endurance improvements of MUAS are possible and can be achieved when operating windward of large buildings.

Canard flow improvement in a split canard configuration

Extensive low-speed wind-tunnel tests are performed to study the flow field structure over a dual-surface canard, known as split canard, in comparison to the conventional configurations. Surface pressure for both steady deflections and unsteady oscillations of the canard are measured and compared for both canard-alone and split-canard cases. The split canard shows a superior advantage at high angles of attack, where a delay in the vortex breakdown phenomena on the canard surface is observed. For the split-canard configuration, the downwash of the front canard reduces the effective angle of attack at the inboard section of the rear element which postpones formation of the leading-edge vortex. Furthermore, a region of considerably high suction was observed at front portion of the rear canard when compared with the isolated canard case. This can be thought of as the main mechanism of performance enhancement in split canards comparing to the conventional configurations.

Correlation and combination of wind force components and responses

This paper summarizes the findings from extensive wind tunnel tests carried out by the authors' group for the evaluation of wind load combination effects for various types of building models. Characteristics of correlations of wind force components are examined using the absolute ratio of wind forces, phase–plane trajectories and (absolute) cross-correlation, and then wind load combinations are examined. The necessity to consider wind load combinations is inferred from the instantaneous pressure distributions, and the cross-correlation coefficients of the absolute values of wind force components are found to be more important when examining wind force combinations. Wind load combination effects are directly examined using frame models. Based on the peak normal stresses in columns under various loading conditions, combination factors for low-, middle- and high-rise buildings are proposed. Lastly, effects of wind direction on wind load combinations are discussed.