Chordwise Pressure Distributions Research Articles

Aerodynamic center of a wing at low Reynolds number

The location of the aerodynamic center, with respect to the center of gravity, determines the longitudinal static stability of an aerial vehicle. While most studies in the past focused on high Reynolds number (Re) flows, we consider flows at low Re (100≤Re≤6000) past an end-to-end wing and a finite wing of various semi-aspect ratio (0.25≤sAR≤5). It is shown that two-dimensional computations, even for an end-to-end wing, are inadequate for accurate prediction of aerodynamic coefficients. The chordwise pressure distribution as well as the variation of aerodynamic coefficients with angle of attack (α) at low Re are significantly different from that at high Re. Beyond a certain α, the flow is associated with a large recirculation zone, resulting in a non-linear variation of the aerodynamic coefficients with α. The conventional definition of the aerodynamic center is too restrictive at low Re. Owing to the non-linear variation of the pitching moment with change in α, it is not possible to determine one single location where it is invariant over a large range of α. The aerodynamic center is, therefore, determined over various sub-regimes of α. It varies significantly over the sub-regimes. The wing-tip vortex significantly affects the vortex shedding, flow structures, and pressure distribution on a finite wing. The locations of the aerodynamic center and center of pressure move fore, toward the leading edge, with a decrease in aspect ratio compared to that for an end-to-end wing.

Numerical investigation of non-planarity and relative motion for bionic slotted wings

Bird wings have split primary feathers that extend out from the wing surface. This structure is called the wingtip slot, which is recognized as a product of bird evolution to improve flight performance. In this paper, numerical simulations based on RANS (Reynolds-averaged Navier–Stokes) equations are conducted to examine and understand the influence of wingtip slots on six wings at Re = 100 000. The overlapping grid method, driven by an in-house UDF (User Defined Function), is used to model the motion of the bionic slotted wings. The motion law of the winglets is improved based on the law extracted from a level-flying bald eagle. Then the aerodynamic force, pressure distribution, vorticity contours, wake stream, and other flow structures of the slotted wings with different layouts were compared and analyzed. The results show a significant increase in aerodynamic force when the slotted wingtips are employed. The maximum lift-to-drag ratio is also improved in our designed wing model with a non-planar wingtip by a maximum of 34% from the base wing. Each winglet works as a single wing due to the existence of slots, with a chordwise pressure distribution similar to that of the main wing. The vortex structures of slotted wings show expressive changes in the tip vortex as compared with the base wing. Additionally, an innovative bionic slotted wing is proposed with a dynamic wingtip that forms varying gaps between winglets. Due to the collective mechanism of aerodynamic interaction among multiple winglets for the innovative wing, it acquires the optimal time-averaged force during a flapping period. As expected, the slotted wingtip reduces the main wingtip vortex intensity and creates weaker vortices. The non-planarity and relative motion of the wingtip strengthen its weakening effect on the wingtip vortex and wake.

Aerodynamic performance investigation of sweptback wings with bio-inspired leading-edge tubercles

The aerodynamic behavior of sweptback wing configurations with bio-inspired humpback whale (HW) leading-edge (LE) tubercles has been investigated through computational and experimental techniques. Specifically, the aerodynamic performance of tubercled wings with symmetric (NACA 0015) and cambered (NACA 4415) airfoils is validated against the baseline model at various angles of attack ([Formula: see text]. The [Formula: see text]/[Formula: see text] ratio of the HW flipper is strategically reduced to 0.15 for ascertaining the flow control potential of the bio-inspired wings with sweptback configuration. It is a novel effort to quantify the effect of the leading-edge protuberances on stall delay, flow separation control and distribution of streamline vortices at unique [Formula: see text]/[Formula: see text] ratio outside the thickness range of HW flipper morphology. Four tapered sweptback wing models (Baseline A, Baseline B, HUMP 0015, HUMP 4415) are used with the amplitude-to-wavelength ([Formula: see text] ratio of 0.24 and Reynolds number about [Formula: see text]. The chordwise pressure distributions are recorded at the peak, mid and trough regions of the tubercled wings through a detailed wind tunnel testing and validated with numerical analysis. Additionally, the flow characteristics over the bio-inspired surfaces have been qualitatively analyzed through the laser flow visualization (LFV) technique to reveal the influence of laminar separation bubbles (LSBs). The essential aerodynamic characteristics such as boundary layer trip delay, vortex mixing, stall delay, and flow control at different AoA are addressed through consistent experimental data. As the sweptback configuration is a primary choice for airplane wings, the improved aerodynamic characteristics of the tubercled wings can be effectively utilized for the design of novel lifting surfaces, hydroplanes and wind turbines in the near future.

Aeropropulsive Coupling Effects on a General-Aviation Aircraft with Distributed Electric Propulsion

As distributed electric propulsion has continued to gain popularity as an enabling aircraft technology, further research is required to understand the complete implications of aeropropulsive coupling effects for distributed propulsion systems integrated into actual aircraft geometries. A set of low-speed wind-tunnel tests was performed at the University of Illinois at Urbana-Champaign to characterize the aerodynamic performance of a semispan Cirrus SR22T wing-body model that was equipped with an array of four ducted fans integrated into the upper-surface, trailing edge of the wing. Aerodynamic performance and aeropropulsive coupling effects were investigated through six-component load-cell measurements of the forces and moments experienced by the model, supplemented by chordwise pressure distributions at four stations along the span of the wing. Additionally, a set of unsteady force and moment measurements were taken to investigate time constants associated with the response of aerodynamic forces and moments to step changes in fan speed. It was found that the fan tip-speed ratio dictates the aerodynamic performance of the model, with primary effects including an increased lifting force, yawing moment, rolling moment, and nose-down pitching moment as the tip-speed ratio increased. Additionally, the lift-curve slope changed with tip-speed ratio as a function of thrust setting in relation to thrust required.

Aerodynamics of a Common Research Model Wing with Leading-Edge Ice Shape

Icing on three-dimensional lifting surfaces has been shown to produce highly complex flowfields. However, there is a lack of experimental and computational information in the public domain regarding three-dimensional aerodynamics for iced commercial aircraft wing geometries. Recent work completed at NASA Glenn Research Center’s Icing Research Tunnel has developed realistic ice shapes for the leading edge of a 65% scale Common Research Model (CRM65). These ice shapes were then converted into artificial ice shapes that were used in aerodynamic testing at the Wichita State University’s Walter H. Beech Wind Tunnel on an 8.9% scale CRM65. RANS simulations were computed for one large leading-edge ice shape to assess the ability to predict the aerodynamic performance parameters and to better understand the flow physics. In general, RANS reasonably captures the complex “swept stall” flow features, which are dominated by strong spanwise flow and separation line breaks. Despite the flow complexity, the simulations are able to generally predict the chordwise pressure distributions even at high angles of attack. While the coefficients of lift, drag, and moment are more accurately captured at lower angles of attack than higher angles of attack, the agreement with experimental data is quite reasonable given the complexity of the flow around this iced swept wing.

Determination of the angle of attack on a research wind turbine rotor blade using surface pressure measurements

Abstract. In this paper, a method to determine the angle of attack on a wind turbine rotor blade using a chordwise pressure distribution measurement was applied. The approach used a reduced number of pressure tap data located close to the blade leading edge. The results were compared with the measurements from three external probes mounted on the blade at different radial positions and with analytical calculations. Both experimental approaches used in this study are based on the 2-D flow assumption; the pressure tap method is an application of the thin airfoil theory, while the probe method applies geometrical and induction corrections to the measurement data. The experiments were conducted in the wind tunnel at the Hermann Föttinger Institut of the Technische Universität Berlin. The research turbine is a three-bladed upwind horizontal axis wind turbine model with a rotor diameter of 3 m. The measurements were carried out at rated conditions with a tip speed ratio of 4.35, and different yaw and pitch angles were tested in order to compare the approaches over a wide range of conditions. Results show that the pressure tap method is suitable and provides a similar angle of attack to the external probe measurements as well as the analytical calculations. This is a significant step for the experimental determination of the local angle of attack, as it eliminates the need for external probes, which affect the flow over the blade and require additional calibration.

Numerical investigation of the separated and reattaching flow over a 5:1 rectangular cylinder in streamwise sinusoidal flow

Three-dimensional Large-Eddy Simulations (3D LES) are performed to study the separated and reattaching flow over a 5:1 rectangular cylinder in streamwise sinusoidal flow. The ratio of inflow frequency to natural vortex-shedding frequency is varied from 0 to 6 ​at five amplitudes up to 60% of the time-mean velocity. The effect on the two-dimensional (2D) and three-dimensional (3D) characteristics of the separation bubble and surface pressures are reported. The relationship between the time-averaged flow field and the statistics of the chordwise pressure distribution are obtained firstly. The flow-pressure relationships are qualitatively similar in uniform and sinusoidal flows. Then, the amplitude-frequency domain of the sinusoidal flow is divided into three regimes to describe the effects on the mean reattachment length. The simulations capture the interferences between leading-edge vortices and trailing edge vortices for longer separation bubbles. However, the interactions are not observed in shorter separation bubbles due to the diffusions of leading-edge vortices. Finally, the flow three-dimensionality is studied by performing proper orthogonal decomposition (POD) and spanwise correlation analysis on the surface pressures. The oncoming flow oscillations correspond to the most energetic structures dominating the pressure field, which are constant in the spanwise direction. While the local vortical structures mainly influence the 3D features of the flow dynamics.

A Low-Speed Compressor Test Rig for Flutter Investigations

A test facility for aeroeolastic investigations has been set up at the chair of Aero Engines at the Technische Universität Berlin. The test rig provides data for tool and code validation, and is used for basic aeroelastic experiments. It is a low-speed wind tunnel, which allows free and controlled flutter testing. The test section contains a linear cascade with eleven compressor blades. Nine of them are elastically suspended. The paper presents a detailed description of the test facility results to evaluate the overall flow quality alongside an aeroelastic model to predict the flutter velocity and critical interblade phase angles (IBPAs). Furthermore, chordwise pressure distributions, measured with traveling wave (TW) mode experimental tests, are presented. These measurements have been carried out for a wide range of IBPAs and have been compared to numerical results. Hot-wire anemometry has been applied to examine the inlet flow for several Mach numbers and Reynolds numbers. The results show small turbulence intensities. The blade surface pressure distribution and the flow field of the blade's suction and pressure sides have been obtained by oil flow visualization.

Effect of computational grid on accurate prediction of a wind turbine rotor using delayed detached-eddy simulations

The present study focuses on the impact of grid for accurate prediction of the MEXICO rotor under stalled conditions. Two different blade mesh topologies, O and C-H meshes, and two different grid resolutions are tested for several time step sizes. The simulations are carried out using Delayed detached-eddy simulation (DDES) with two eddy viscosity RANS turbulence models, namely Spalart-Allmaras (SA) and Menter Shear stress transport (SST) k-ω. A high order spatial discretization, WENO (Weighted essentially nonoscillatory) scheme, is used in these computations. The results are validated against measurement data with regards to the sectional loads and the chordwise pressure distributions. The C-H mesh topology is observed to give the best results employing the SST k-ω turbulence model, but the computational cost is more expensive as the grid contains a wake block that increases the number of cells.

Numerical Analysis on the Effect of Fluidic on Demand Winglet on the Aerodynamic Performance of the Wing

A numerical study was undertaken to study the effect of the span wise injection on the performance of a 3D wing at a velocity of 15 m/s and angle of attack of 6°, 8°, and 10°. A baseline configuration along with injection at tip was studied. A study was conducted to understand the flow field and the winglet control techniques. Based on the study, a wing configuration was chosen as baseline configurations and different injection velocities were applied to this configuration. The chord wise pressure distribution is seen to change with the span wise location from the root and this distribution is affected by the wing tip vortex. The wingtip was observed to change the pressure distribution near the tip. The velocity field, stream lines and the vortices were seen to be affected by the presence of the injection. The lift and drag values were seen to decrease with the angle of attack but the l/d ratio remained nearly constant for all the injection configurations. Maximum reduction in drag of nearly 19% could be achieved with the injection. This study proved the possibility of using span wise injection as a control method to control the wing tip vortex.

A coupling VWM/CFD/CSD method for rotor airload prediction

A coupling fluid-structure method with a combination of viscous wake model (VWM), computational fluid dynamics (CFD) and comprehensive structural dynamics (CSD) modules is developed in this paper for rotor unsteady airload prediction. The hybrid VWM/CFD solver is employed to model the nonlinear aerodynamic phenomena and complicated rotor wake dynamics; the moderate deflection beam theory is implemented to predict the blade structural deformation; the loose coupling strategy based on the ‘delt method’ is used to couple the fluid and structure solvers. Several cases of Helishape 7A rotor are performed first to investigate the effect of elastic deformation on airloads. Then, two challenging forward flight conditions of UH-60A helicopter rotor are investigated, and the simulated results of wake geometry, chordwise pressure distribution and sectional normal force show excellent agreement with available test data; a comparison with traditional CFD/CSD method is also presented to illustrate the efficiency of the developed method.

Discontinuous Galerkin methodology for Large-Eddy Simulations of wind turbine airfoils

This paper aims at evaluating the potential of the Discontinuous Galerkin (DG) methodology for Large-Eddy Simulation (LES) of wind turbine airfoils. The DG method has shown high accuracy, excellent scalability and capacity to handle unstructured meshes. It is however not used in the wind energy sector yet. The present study aims at evaluating this methodology on an application which is relevant for that sector and focuses on blade section aerodynamics characterization. To be pertinent for large wind turbines, the simulations would need to be at low Mach numbers (M ≤ 0.3) where compressible approaches are often limited and at large Reynolds numbers (Re ≥ 106) where wall-resolved LES is still unaffordable. At these high Re, a wall-modeled LES (WMLES) approach is thus required. In order to first validate the LES methodology, before the WMLES approach, this study presents airfoil flow simulations at low and high Reynolds numbers and compares the results to state-of-the-art models used in industry, namely the panel method (XFOIL with boundary layer modeling) and Reynolds Averaged Navier-Stokes (RANS). At low Reynolds number (Re = 6 x 104), involving laminar boundary layer separation and transition in the detached shear layer, the Eppler 387 airfoil is studied at two angles of attack. The LES results agree slightly better with the experimental chordwise pressure distribution than both XFOIL and RANS results. At high Reynolds number (Re = 1.64 x 106), the NACA4412 airfoil is studied close to stall condition. In this case, although the wall model approach used for the WMLES is very basic and not supposed to handle separation nor adverse pressure gradients, all three methods provide equivalent accuracy on averaged quantities. The present work is hence considered as a strong step forward in the use of LES at high Reynolds numbers.

Rotor wake and flow analysis using a coupled Eulerian–Lagrangian method

ABSTRACTA coupled Eulerian–Lagrangian methodology was developed in this paper in order to provide an efficient and accurate tool for rotor wake and flow prediction. A Eulerian-based Reynolds-averaged Navier–Stokes (RANS) solver was employed to simulate the grid-covered near-body zone, and a grid-free Lagrangian-based viscous wake method (VWM) was implemented to model the complicated rotor-wake dynamics in the off-body wake zone. A carefully designed coupling strategy was developed to pass the flow variables between two solvers. A sample case of a forward flying rotor was performed first in order to show the capabilities of the VWM for wake simulations. Next, the coupled method was applied to rotors in several representative flight conditions. Excellent agreement regarding wake geometry, chordwise pressure distribution and sectional normal force with available experimental data demonstrated the validity of the method. In addition, a comparison with the full computational fluid dynamics (CFD) method is presented to illustrate the efficiency and accuracy of the proposed coupled method.

Passive control of pitch-break of a BWB UCAV model using vortex generator

To examine the effect of vortex generator on pitch-break control for a BWB model having lambda wing configuration, wind tunnel experiments were conducted. Tests were carried out at two freestream velocities of 30 m/s and 60 m/s. Experimental results of the BWB model, which was modified slightly from UCAV 1303, at various angles of attack are presented. Aerodynamic force and moment coefficients and chordwise pressure distributions at several spanwise stations are included. Spanwise location of a vortex generator was selected based on pressure distributions of the BWB model at several angles of attack and through a preliminary test. Well known pitch-break or pitch-up phenomenon was identified to occur at a specific angle of attack for the case without vortex generator. When the vortex generator was installed, this pitch-break was found to be delayed to a much larger angle of attack, which might well be adopted for efficient flight control.

A parametric investigation of non-circular spiroid winglets

This paper will present the study about Spiroid winglets and will go through its theoretical significance as well as the processes involved in the selection of an optimum aerodynamic design (configuration) of the spiroid that produced efficient aerodynamic performance results in terms of mainly Lift to Drag ratio (L/D), Induced drag, Chordwise and Spanwise Pressure distributions, Vorticity formation and strength comparison (between a simple wing, simple winglet and the spiroid with the best performance results) by varying several parameters and by comparing and performing detailed Computational Fluid Dynamics Analyses on every design under cruise conditions. Consequently, the research conducted in this paper concluded that the spiroid exhibit better performance numbers upon comparison with its other options in terms of vortex reduction and overall drag reduction.

Understanding of unsteady pressure fields on prisms based on covariance and spectral proper orthogonal decompositions

This paper presents applications of proper orthogonal decomposition in both the time and frequency domains based on both cross spectral matrix and covariance matrix branches to analyze multi-variate unsteady pressure fields on prisms and to study spanwise and chordwise pressure distribution. Furthermore, modification of proper orthogonal decomposition is applied to a rectangular spanwise coherence matrix in order to investigate the spanwise correlation and coherence of the unsteady pressure fields. The unsteady pressure fields have been directly measured in wind tunnel tests on some typical prisms with slenderness ratios B/D=1, B/D=1 with a splitter plate in the wake, and B/D=5. Significance and contribution of the first covariance mode associated with the first principal coordinates as well as those of the first spectral eigenvalue and associated spectral mode are clarified by synthesis of the unsteady pressure fields and identification of intrinsic events inside the unsteady pressure fields. Spanwise coherence of the unsteady pressure fields has been mapped the first time ever for better understanding of their intrinsic characteristics.

Multiple laminar-turbulent transition cycles around a swept leading edge

Certain interesting flow features involving multiple transition/relaminarization cycles on the leading edge of a swept wing at low speeds are reported here. The wing geometry tested had a circular nose and a leading edge sweep of 60°. Tests were made at a chord Reynolds number of 1.3 × 106 with model incidence α varied in the range of 3°−18° in discrete steps. Measurements made included wing chord-wise surface pressure distributions and wall shear stress fluctuations (using hot-film gages) within about 10 % of the chord in the leading edge zone. Results at α = 16° and 18° showed that several (often incomplete) transition cycles between laminar-like and turbulent-like flows occurred. These rather surprising results are attributable chiefly to the fact that the Launder acceleration parameter K (appropriately modified for swept wings) can exceed a critical range more than once along the contour of the airfoil in the leading edge region. Each such crossing results in a relaminarization followed by direct retransition to turbulence as K drops to sufficiently low values. It is further shown that the extent of each observed transition zone (of either type) is consistent with earlier data acquired in more detailed studies of direct transition and relaminarization. Swept leading edge boundary layers therefore pose strong challenges to numerical modelling.

A Reverse Approach for Aerodynamic Analysis of a Typical UAV

A reverse approach has been followed to handle the problem of investigating the aerodynamic characteristics of a typical unmanned aerial vehicle model which are assumed unknown. Both numerical and experimental investigations have been conducted using wind tunnel and CFD facilities. The tests have been conducted in a low speed wind tunnel for a half section of real size tailless model. The aerodynamic loads have been measured and the loads were varied with pitch performed in range of (-2 to +26°) angles of attack with pre-selected test wind speeds between (25 – 40) m/s. The test was carried out with different elevon deflection angles selected as (0-10-20-30°). The pressure tabs to investigate the chord-wise pressure distribution at Mean Aerodynamic Chord (MAC) for the sake of comparison between wind tunnel and CFD results. The test section conditions have been taken as inputs for the simulation with CFD Fluent software. Results revealed that the model was statically stable and the variation of aerodynamic coefficients to wind speed, angle of attack and elevon deflection is displayed. The pressure distribution along the MAC demonstrated the expected behaviour of the chord-wise pressure distribution during stall and laminar flow around the airfoil.

Unsteady aerodynamics of two interacting yacht sails in two-dimensional potential flow

In this paper, unsteady thin aerofoil theory is extended to the case of two interacting aerofoils that oscillate harmonically perpendicular to the direction of the incident flow. The two aerofoils represent the headsail and mainsail of a yacht that sails upwind in waves. The developed theory is validated against theoretical data from the literature and results from wind tunnel tests with rigid, high-aspect ratio sail models oscillating at reduced frequencies fromk= 0 tok= 0.68. Good agreement is found between the predicted and measured chordwise pressure distributions. An application of the theory to the case of an International America's Cup Class yacht reveals that the time-varying components of the aerodynamic forces are small and that the thrust gain is minimal, i.e. only very little energy can be extracted from the unsteady flow about the sails. No attempt is made to investigate the influence of the flexibility of the sails, three-dimensional effects or phenomena related to dynamic stall.

Reduction of blade passing noise of a vertical take-off and landing (VTOL) aircraft.

Rotor-stator interaction has been identified as the dominant noise source of a vertical take-off and landing (VTOL) aircraft developed by Entecho. This paper reports field measurement results of blade passing noise of the VTOL air-craft together with its analysis and control. The blade passing event was simulated in a wind tunnel experiment. The flow speed, rotor position, and rotor-stator spacing were varied with the chord-wise pressure distribution of the lead-ing surface of the rotor blade being measured by an array of six flush mounted microphones. Results show useful features of the pressure distribution on the rotor blade influenced by an upstream stator, which are used for the analysis and prediction of the sound radiation from the VTOL aircraft. The preliminary result of the reduction in the blade passing noise using angled stator blades is also presented.