Aerodynamic Force Measurements Research Articles

EXPERIMENTAL STUDY OF DELAYING THE STALL OF THE NACA 0020 AIRFOIL USING A SYNTHETIC JET ACTUATOR ARRAY WITH DIFFERENT ORIFICE GEOMETRIES

An experimental study on the effect of active flow control using a synthetic jet mechanism on stall delay for the NACA 0020 airfoil is conducted. The experiments are carried out at an open-suction type wind tunnel at Reynolds number 5x104. In the presented experimental study, aerodynamic force measurements of the airfoil with having different orifice geometries (cylindrical, rectangular, sinusoidal, v-type, inclined rectangular) are examined by using a speaker type actuator in synthetic jet mechanism. It is observed that all different orifice geometries are effective in delaying the stall angle of NACA 0020 airfoil. However, it is observed that the inclined rectangular type of synthetic jet geometry is the most advantageous in delaying the stall of the airfoil. The effects of geometric parameters of the actuator on lift and drag coefficient of the NACA0020 airfoil are investigated. Experimental results show that among the all-orifice geometries the rectangular orifice geometry is the most effective in increasing lift coefficient of the airfoil. It is observed that there is a maximum decrease in drag at 10⁰ where the stall occurs. In addition, the decrease in drag is observed after 10 in rectangular, v-type and inclined rectangular orifice geometries.

Self-tuning model predictive control for wake flows

This study presents a noise-robust closed-loop control strategy for wake flows employing model predictive control. The proposed control framework involves the autonomous offline selection of hyperparameters, eliminating the need for user interaction. To this purpose, Bayesian optimization maximizes the control performance, adapting to external disturbances, plant model inaccuracies and actuation constraints. The noise robustness of the control is achieved through sensor data smoothing based on local polynomial regression. The plant model can be identified through either theoretical formulation or using existing data-driven techniques. In this work we leverage the latter approach, which requires minimal user intervention. The self-tuned control strategy is applied to the control of the wake of the fluidic pinball, with the plant model based solely on aerodynamic force measurements. The closed-loop actuation results in two distinct control mechanisms: boat tailing for drag reduction and stagnation point control for lift stabilization. The control strategy proves to be highly effective even in realistic noise scenarios, despite relying on a plant model based on a reduced number of sensors.

Various Blowing-Suction Schemes for Manipulating Turbulent Boundary Layers

ABSTRACT The methodology of efficiency analysis of turbulent flow modification by making permeable sections in the streamlined wing surface with the aim to reduce aerodynamic drag is the principal subject of the presented research. The numerical analysis of the effect of laterally and longitudinally located permeable sections on boundary-layer properties showed the following flow features: (1) the most effective place for permeable surface is an upwind side of the wing; (2) multi-sectional blowing can simply be organised as non-uniform (especially in the case of laterally arranged permeable sections) that brings additional flexibility to change the blowing intensity depending on flight mode and, first of all, on angle of attack; and (3) arrays of longitudinal permeable sections allow to intensify turbulent vortical structures exchange in the lateral direction and improve flow stability to stall. Moreover, due to creating the regular anisotropy of the boundary layer in the lateral direction, this modified blowing technique can potentially have some synergistic properties, which can give the additional benefit. All these effects are too delicate and their experimental study cannot be performed with the use of directed measurements of aerodynamic forces. The comparison of the obtained flow properties with the corresponding experimental data demonstrates an appropriate level of agreement.

Investigation of individual strategies in the aerial phase in ski jumping

The purpose of this investigation was to examine the performance strategy of three ski jumpers during the steady glide phase and explain how different strategical solutions can lead to jumps of roughly the same length. In this study, a total of 24 jumps performed by two World Cup (WC) athletes and one Continental Cup (COC) athlete were measured with a differential Global Navigation Satellite System (dGNSS) on a large ski jumping hill. For each athlete, the continuous position data, velocity, aerodynamic forces and lift-to-drag ratio (LD-ratio) were averaged and compared for the steady glide phase to examine individual jump strategies. The dGNSS yielded accurate measurements of trajectory, velocity and aerodynamic forces, revealing clear differences between the athletes. The largest differences were found between the WC athletes and the COC athlete. The WC athletes focused on maximizing horizontal velocity while the COC athlete minimized vertical velocity. This difference may be explained by the different hill sizes the athletes normally compete on. One of the WC athletes consistently increased their horizontal velocity during the end of the steady glide phase by maintaining a high LD-ratio, which highlights the effect of aerodynamics on the resulting velocity, trajectory and jump length.

Experimental design for a novel co-flow jet airfoil

The Co-flow Jet (CFJ) technology holds significant promise for enhancing aerodynamic efficiency and furthering decarbonization in the evolving landscape of air transportation. The aim of this study is to empirically validate an optimized CFJ airfoil through low-speed wind tunnel experiments. The CFJ airfoil is structured in a tri-sectional design, consisting of one experimental segment and two stationary segments. A support rod penetrates the airfoil, fulfilling dual roles: it not only maintains the structural integrity of the overall model but also enables the direct measurement of aerodynamic forces on the test section of the CFJ airfoil within a two-dimensional wind tunnel. In parallel, the stationary segments are designed to effectively minimize the interference from the lateral tunnel walls. The experimental results are compared with numerical simulations, specifically focusing on aerodynamic parameters and flow field distribution. The findings reveal that the experimental framework employed is highly effective in characterizing the aerodynamic behavior of the CFJ airfoil, showing strong agreement with the simulation data.

Active flutter suppression on a flexible wing via leading-edge blowing and circulation control

Flutter is a classical aeroelastic phenomenon that seriously affects the performance of flexible wings. This study investigates flutter suppression through flow control for a flexible wing. Aerodynamic force, flow field, and dynamic aeroelastic response measurements are conducted to analyze the mechanism of flutter suppression through flow control on a flexible wing modified to include leading-edge blowing (LEB) and circulation control (CC) actuators around the wing tip. Furthermore, the flutter control effects of two control strategies, i.e., steady state control and proportion–integral–differential (PID) control, are compared. The results show that steady LEB and steady CC can effectively reduce the flutter amplitude and increase the critical flutter velocity. When the mass flow coefficient CQ≥1.46×10−3, the Coanda jet has better flutter suppression effect than the leading-edge jet. Moreover, a closed-loop CC control based on the PID algorithm demonstrates that PID control can effectively improve the flutter control efficiency. Compared to steady flow control, PID control increases the critical velocity by more than 52.60% and reduces the air consumption by 90.44%.

Temperature Compensation of Wind Tunnel Balance Signal Detection System Based on IGWO-ELM.

The wind tunnel balance signal detection system is widely employed in aerospace applications for the accurate and automated measurement of aerodynamic forces and moments. However, measurement errors arise under different environmental temperature. This paper addresses the issue of measurement accuracy under different temperature conditions by proposing a temperature compensation method based on an improved gray wolf optimization (IGWO) algorithm and optimized extreme learning machine (ELM). The IGWO algorithm is enhanced by improving the initial population position, convergence factor, and iteration weights of the gray wolf optimization algorithm. Subsequently, the IGWO algorithm is employed to determine the optimal network parameters for the ELM. The calibration decoupling experiment and high-low temperature experiment are designed and carried out. On this basis, ELM, GWO-ELM, PSO-ELM, GWO-RBFNN and IGWO-ELM are used for temperature compensation experiments. The experimental results show that IGWO-ELM has a good temperature compensation effect, reducing the measurement error from 20%FS to within 0.04%FS. Consequently, the accuracy and stability of the wind tunnel balance signal detection system under different temperature environments are enhanced.

Effect of Flare Angle and Natural Ventilation on The Aerodynamic Characteristics of a Typical Re-Entry Body At Subsonic and Transonic Mach Numbers

Aerodynamic force and moment measurements were carried out on models of a typical re-entry body, featuring a blunt nose, conical body and flare, to assess the effect of flare angle on the overall aerodynamic characteristics. In one of the configurations, the junction between the cone and the flare was vented out to the base of the model to assess the effect of ventilation on the aerodynamic characteristics. It was expected that the stability characteristics would improve, by virtue of improved contribution from the flare to the normal force of the configuration. However, measurements did not show the expected trends. Instead, there was a remarkable reduction in the axial force with ventilation, by as much as 25% all across the Mach number and angle of attack range. Base pressure measurements indicated that in the low Mach number range pressure rise occured. However, in the high transonic Mach number range, even though the force measurements showed drag reduction, there was no significant rise in base pressure. Ventilation showed delay in transonic drag rise. It is therefore suggested that the overall pressure field is modified by ventilation, which results in drag reduction of the configuration.

An experimental study of rime ice accretion on bundled conductors

The dynamic ice accretion process on bundled electrical power conductors and icing-induced effects on the aerodynamic forces acting on the conductors were examined experimentally. An Icing Research Tunnel was used to generate a typical atmospheric rime icing condition experienced by power transmission cables. Two pieces of Aluminum-Conductor-Steel-Reinforced (ACSR) cables were mounted in the icing wind tunnel with different spacing and angular displacements with respect to the incoming airflow. While ice accretion was found to take place primarily on the frontal surfaces of the bundled conductors, the accreted ice layers resembled well the twisted outer strands of the ACSR conductors. Averaged outer profiles of the iced conductors were elliptical with the semi-minor axis being the radius of the conductors and the semi-major axis passing through the foremost frontal points of the accreted ice layers. The icing process on the windward conductor was almost unaffected by the existence of the leeward conductor. The ice accretion and resultant aerodynamic force acting on the leeward conductor were found to vary significantly, depending on the spacing and angular displacements between the bundled conductors. The windward conductor was found to induce a “shadowing effect” by intercepting airborne supercooled water droplets to prevent them from impinging onto the leeward conductor, resulting in much less ice accretion and smaller aerodynamic drag acting on the leeward conductor. The aerodynamic force measurements were correlated with the acquired ice accretion images and wake flow field around the bundled conductors to elucidate the underlying physics.

On the Validity of the Normal Force Model for Steadily Revolving Wings: An Experimental Investigation

Aerodynamic characteristics of revolving wing models were investigated to assess the validity of the normal force model. Aerodynamic force and torque measurements were conducted for six wing planforms (with aspect ratios of 2 and 3, and area centroid locations at 40%, 50%, and 60% of the wing length) at three different Reynolds numbers (0.5 × 104, 1 × 104, and 1.5 × 104) and three thickness-to-chord ratios (3%, 4%, and 5%). Both early and steady phase measurements were extracted for a range of angles of attack relevant to insect flight. It was shown that the so-called “normal force” model conveniently captures the variation of the lift and drag coefficients along the first quadrant of angles of attack for all cases tested. A least squares best fit model for the obtained experimental measurements was used to estimate the key parameters of the normal force model, namely the lift curve slope, the zero-lift drag coefficient, and the peak drag coefficient. It was shown that the knowledge of only the lift curve slope and the zero-lift drag coefficient is sufficient to fully describe the model, and that clear trends of these two parameters exist. Notably, both parameters decreased with the increase in area centroid location. For instance, for steady measurements and on average, the lift curve slope for a wing with an area centroid location at 40% span was 15.6% higher compared to an area centroid location at 60% span. However, the increase in the zero-lift drag coefficient for wings with a lower area centroid location had a detrimental effect on aerodynamic efficiency assessed via glide ratio. Wings with a lower area centroid location consistently led to a lower glide ratio regardless of the change in aspect ratio, thickness-to-chord ratio, or Reynolds number. Increasing the aspect ratio decreased the zero-lift drag coefficient but generally had a slighter increasing effect on the lift curve slope. Increasing the Reynolds number within the range experimented decreased both the lift curve slope and the zero-lift drag coefficient. Finally, the effect of the thickness-to-chord ratio was mainly pronounced in its effect on the zero-lift drag coefficient.

The effect of spoilers on flow around tandem circular cylinders

Examining the flow characteristics around the cylindrical elements, such as offshore (submarine) pipelines which can be used in single or multiple arrangements, has a prominent place in fluid mechanics. The use of spoilers for self-embedding of these structures has been a subject that researchers have studied for many years. In this study, (a) the flow around a cylinder without and with a spoiler and (b) the effect of adding spoiler(s) to the tandem cylinders on the flow was experimentally investigated. In these experiments, where the ratio of the distance between the cylinders to the cylinder diameter is 2, the Reynolds number is 14000, which remains in the subcritical region. Four experiments were performed: the smoke-wire method was used for flow visualization, aerodynamic force measurement, velocity measurement by hot-wire anemometer, and pressure measurement to determine the pressure distribution on the cylinders. Pressure, velocity, and force results were recorded with the time histories in this study for the first time. Experimental studies show that when a spoiler is added to a single cylinder, an opposing lift force acts on that and the drag force increases due to the enlargement of the low-pressure region at the wake of the cylinder. In a tandem situation, when the upstream cylinder has a spoiler, no drag force acts on the downstream cylinder. The forces exerted on the upstream cylinder are not affected by whether the downstream cylinder has a spoiler. In the case of the downstream cylinder with the spoiler, the fluctuations in the aerodynamic forces of the upstream cylinder decrease owing to the downstream cylinder with the spoiler. The force fluctuations are more in the downstream cylinder, and unlike other tandem and single-cylinder cases, the vortex shedding becomes complex.

Experimental investigation of aerodynamic forces and vortex-induced vibrations of wavy cylinders at subcritical Reynolds numbers

Wavy cylinders have been reported to exhibit satisfactory aerodynamic performance at low Reynolds numbers. However, aerodynamic characteristics are still ambiguous at higher Reynolds numbers. A series of wind tunnel tests were conducted for different wavy cylinders with Reynolds numbers (Re) from 0.75 × 104 to 2.00 × 105, including measurements of the overall aerodynamic forces, local surface static pressure, and vortex-induced vibrations (VIVs). The experiments yielded the following results. First, the mean drag and fluctuating lift coefficients of wavy cylinders were generally lower than those of a circular cylinder in the tested Re range. The maximum drag reduction was up to 18%. Second, a very slight waviness of a/D = 0.03 could improve aerodynamic performance, which could be taken as a reference for practical application. Third, the aerodynamic characteristics of wavy cylinders exhibited obvious three-dimensional (3D) features. This is reflected by various parameters varying along the spanwise direction. Fourth, the mechanisms of stronger 3D properties and less forces might be different. The former is dependent on the degree of obliqueness, while the latter is more related to the spanwise wavelength. Fifth, compared to a circular cylinder, the VIV amplitude was reduced and the lock-in range was shifted to higher wind velocities for wavy cylinders.

Proposal of a Method of Aerodynamic Force Measurement of Aircraft Model without Using Force Sensors in Wind Tunnel Testing

Recently, the use of unmanned air vehicles (UAV) at disaster sites has increased, and research and development has been active. Since these vehicles fly at low Reynolds number, they have different aerodynamic characteristic from conventional aircraft. Therefore, it is expected that research as flight test, wind tunnel tests and CFD will be accelerated in order to improve aerodynamic performance in this area. This research focused on Hand Launch Glider with high performance in the Re=104 regions. This paper proposes a new aerodynamic force measurement method based on positional measurements suitable for wind tunnel tests on a full-scale model of a Hand Launch Glider. Validation and experimentation of this method were conducted by measuring the lift and drag forces generated on the aircraft every A.O.A. and comparing the results of Koike et al. The results show that although the accuracy of the model's weight measurement has a significant effect on the aerodynamic force measurement, the validity of the new aerodynamic force measurement method based on positional measurements was considered. The proposed new aerodynamic force measurement method is available for evaluating the aerodynamic characteristics of slender fuselage airframes which cannot be inserted force sensors, or small loads that make it preciously difficult to measure aerodynamic forces with sensors, like the Hand Launch Glider.

Measurement of aerodynamic force and moment acting on a javelin using a magnetic suspension and balance system

The rules governing the dimensions of the Javelin were substantially changed in 1986. It was considered that this new design guaranteed there was zero pitching moment at 0° angle of attack and that the pitching moment decreased (became negative) with increasing angle of attack. The objective of this study is to investigate if the pitching moment remains always negative (nose-down rotation). To measure accurate aerodynamic forces acting on a Javelin, the world’s largest 1 m magnetic suspension and balance system was used. The magnetic suspension and balance system was able to measure aerodynamic forces without support interference in the wind tunnel. In addition, computational fluid dynamics were carried out to estimate the pitching moment coefficients. It was found that the pitching moment coefficient of a commercially available Javelin becomes positive (nose-up rotation) at lower angles of attack, less than 12°. The pitching moment becomes positive if the upstream side of the center of gravity receives more inflow than the downstream side. This situation can be attained by, for example, increasing the thickness of the upstream side when compared with that of the downstream side.

Time scales of dynamic stall development on a vertical-axis wind turbine blade

Vertical-axis wind turbines are excellent candidates to diversify wind energy technology, but their aerodynamic complexity limits industrial deployment. To improve the efficiency and lifespan of vertical-axis wind turbines, we desire data-driven models and control strategies that take into account the timing and duration of subsequent events in the unsteady flow development. Here, we aim to characterise the chain of events that leads to dynamic stall on a vertical-axis wind turbine blade and to quantify the influence of the turbine operation conditions on the duration of the individual flow development stages. We present time-resolved flow and unsteady load measurements of a wind turbine model undergoing dynamic stall for a wide range of tip-speed ratios. Proper orthogonal decomposition is used to identify dominant flow structures and to distinguish six characteristic stall stages: the attached flow, shear-layer growth, vortex formation, upwind stall, downwind stall and flow reattachment stage. The timing and duration of the individual stages are best characterised by the non-dimensional convective time. Dynamic stall stages are also identified based on aerodynamic force measurements. Most of the aerodynamic work is done during the shear-layer growth and the vortex formation stage which underlines the importance of managing dynamic stall on vertical-axis wind turbines.

Wind-tunnel tests of synthetic jet control effects on airfoil flow separation

To find out the comprehensive principles of synthetic jet control effects on airfoil stall characteristics, fundamental contrastive wind-tunnel tests were conducted systematically. By using six-component balance, Particle Image Velocimetry (PIV) technology and boundary layer probe, the measurements of model aerodynamic forces, the whole velocity field over airfoil and velocity profiles in the boundary layer were conducted, respectively. Based upon the experimentally parametric analyses of synthetic jet control on airfoil, it is concluded that the incline angle of synthetic jet has an important impact on both the flow in boundary layer and aerodynamic forces of airfoil. A tangential jet inject energy and accelerate the velocity of flow in inner boundary layer thus has a better control effects on delay stall of airfoil when the momentum coefficient of jet is relatively large, and the normal jet helps to enlarge the thickness of boundary layer which is proved to have better control effects on flow separation control and improving the aerodynamic characteristics of airfoil when the momentum coefficient of a jet is relatively small.

Experimental flow control investigation over suction surface of turbine blade with local surface passive oscillation

Impact of the local flexible membrane (LFM) on aerodynamic phenomena including the formation of a laminar separation bubble (LSB) and transition to turbulence was experimentally investigated over the suction surface of a Clark-Y airfoil first time in literature. The experiments such as aerodynamic force measurement, smoke-wire flow visualization and hot-film tests were carried out at the free-stream velocity of U∞ = 3.2 m/s, U∞ = 6.4 m/s, U∞ = 9.6 m/s, U∞ = 12.8 m/s, and Reynolds number based upon on the chord length was Rec = 3.5 × 104, Rec = 7.0 × 104, Rec = 1.05 × 105 and Rec = 1.4 × 105, respectively. The experimental angle of attack was set at 0° = α ≤ 20°. In detailed intermittency analysis by the hot-film sensor over the uncontrolled airfoil, it was seen that the LSB and transition to turbulence formed close to the trailing edge at a lower angle of attack, and it moved towards the leading edge when increasing the angle of attack simultaneously. Employing LFM on the suction surface obviously affected the progress of these flow phenomena. In the results of smoke-wire flow visualization, either the size of the laminar separation bubble (LSB) was reduced or its presence was suppressed at lower incidences. The aerodynamic force measurement results also supported those behaviors. In particular, at lower incidences, the negative effects of LSB were mitigated, resulting in the presence of a more stable lift curve. Additionally, it was clearly observed that utilizing LFM ensured positive effects, especially at the pre- and the post-stall regions in terms of fewer fluctuations at the CL curve, meaning that less aerodynamic vibration and noise on wind/hydro turbine could be obtained.

Aerodynamic Metrologists Guide to Dynamic Force Measurement

Static aerodynamic force measurement is generally of interest in both low- and high-speed ground test facilities, but dynamic force measurement is typically associated with higher-speed facilities. This work will show that dynamic force measurement is relevant to all aerodynamic metrologists because wind tunnel system dynamics can change the force transducer’s dynamic sensitivity resulting in differences between the static and dynamic sensitivity. In addition, the field of dynamic force measurement will be introduced using realistic wind tunnel scenarios, wherein the wind tunnel system dynamics contribute to increased uncertainty in what would otherwise be considered a static force measurement. A design approach focused on estimating the dynamic response is presented to achieve measurement objectives through the use of static force measurement techniques where possible. To that end, a reduced-order model is developed to characterize the dynamic response of the system, which can serve as a predictive tool in the design process. Then, strategies for characterizing wind tunnel system dynamics to determine acceptable bandwidths wherein the static measurements may be sufficient are discussed. In the event that systems have undesirable dynamic characteristics, which cannot be avoided in the design process, two dynamic compensation techniques to correct the measured signals are discussed.

Unsteady aerodynamic force modelling for 3-DoF-galloping of four-bundled conductors

Galloping of overhead transmission lines is characterised by large-amplitude, low-frequency, vertical–horizontal–torsional three degree-of-freedom (DoF) oscillations, which can potentially induce interphase short circuits and fatigue in the conductors and support structures. Numerical analyses are useful for estimating the galloping response of transmission lines, and unsteady aerodynamic forces should be evaluated for accurate simulation. In this study, the unsteady aerodynamic force modelling of four-bundled conductors is investigated based on aerodynamic force measurement test results. Three types of unsteady aerodynamic force measurement tests were performed: under constant torsional angular velocity and under torsional and vertical 1-DoF sinusoidal oscillations. Evaluating the dependence of the aerodynamic forces on the various motions enabled an improved unsteady aerodynamic force model to be developed, considering the effect of angular velocity. First, two-variable aerodynamic coefficients were proposed as a function of the angle of attack and non-dimensional angular velocity. These coefficients were determined via tests performed at constant torsional angular velocities; results with different combinations of wind speed and angular velocity confirmed that the non-dimensional angular velocity can be used to define the unsteady aerodynamic forces. Then, the validity of the two-variable aerodynamic force formulation was confirmed for predicting torsional or vertical 1-DoF oscillations, by comparing the unsteady aerodynamic force obtained via the sinusoidal forced-vibration tests and via the constant angular-velocity tests. Finally, results of the time-history​ analyses using the two-variable aerodynamic coefficients were compared with those of previous wind-response measurement tests to obtain the best unsteady aerodynamic force model for simulating large-amplitude 3-DoF galloping. These results indicate that the unsteady aerodynamic forces for 3-DoF-galloping of four-bundled conductors can be evaluated using the relative angle of attack, the rate of change of the relative angle of attack, and the relative wind speed, using two-variable aerodynamic coefficients measured under constant angular velocity conditions.

Modernization of the working part of the wind tunnel for tensometric measurements of aerodynamic forces in supersonic flow

Modernization of the working part of the wind tunnel for tensometric measurements of aerodynamic forces in supersonic flow