Chordwise Location Research Articles

Self-adaptive flaps on low aspect ratio wings at low Reynolds numbers

Self-adaptive flap is a bio-inspired passive flow control device which emulates the covert feathers of birds in a most rudimentary form. An exploratory study to ascertain the effectiveness of these self-adaptive flaps in delaying flow separation on low aspect ratio wings of three different planforms were carried out in a low speed wind tunnel at a Reynolds number of 105 based on their root chord. Flap widths between 0.08c and 0.15c; placement location between 0.4x/c and 0.8x/c were investigated. The flapped configurations exhibit good stall margin and higher CLmax in most cases. The chordwise placement location of adaptive flap for better lift enhancement varies with planform. Adaptive flap placed at the maximum span location on varying span planform exhibits better lift characteristics than flap placed at other chordwise location. However, for a constant span planform like rectangle, chordwise location close to trailing edge seems to be optimal for flap placement. No detrimental effects are observed in the drag characteristics of the single flapped configurations of rectangular and inverse Zimmerman planform. However, significant drag increase is observed for the flapped configurations of Zimmerman model and double flapped configurations. The temporal behavior of the adaptive flap is aperiodic. Spectral analysis of the flap oscillation shows multiple dominant frequencies between 10 Hz and 50 Hz for different angles of attack for both rectangular and inverse Zimmerman planform. The peaks occur at same frequencies for most angles of attack irrespective of planform. Skin friction pattern obtained from oil flow visualization experiments shows that the separation bubble which extends the entire chord for clean model at post-stall angles of attack, is contained upstream of the flap which leads to better post-stall lift characteristics.

Streamwise vortex generator for separation reduction on wind turbine profile

High angles of attack of the wind turbine blades induce severe flow conditions which lead to flow separation and, as the consequence, aerodynamic performance reduction. Implementation of a new type of passive streamwise vortex generator (Rod Vortex Generator - RVG), on a wind turbine profile in order to reduce the flow separation is presented. Numerical model validation is carried out for the S809 aerofoil and a wide range of angles of attack (AoA) employed as reference for flow control cases. Investigation of proposed passive control method involves attached as well as incipient and massive flow separation. A study of chordwise location of RVGs for different inflow conditions is performed. The numerical and experimental results are in good agreement. Obtained numerical results based on the RANS approach reveal a large potential of selected passive devices in reduction of flow separation and increase of aerodynamic performance.

A New Design Concept for Wind Turbine Airfoil

Wind energy has been attracting more and more attentions due to its clean and renewable source. The aerodynamic characteristic of wind turbine airfoil directly affects the turbine efficiency. In order to improve the airfoil aerodynamic characteristic, a new concept airfoil configuration for wind turbine is presented. A cave on the upper surface near the trailing edge is designed to generate a trapped vortex in the cave. The trapped vortex is used to stabilize the separated flow when the airfoil at high angle of attack. Combining with the Gurney flap, the airfoil with the cave behaves very good aerodynamic characteristics at wide range of incidences, especially at high angles of attack. The method is used on the well-known FFA-W3-301 turbine airfoil. By using numerical simulation, it is shown that the new airfoil has a higher lift than the original airfoil at the same angle of attack, the stall angle of attack increases from 12 degree to 17 degree, and the maximum lift coefficient increases approximately 64 percents. In addition, the effects of the chord-wise location of starting point of the designed cave are discussed. Therefore, it is believed that the new-designed concept can be investigated and explored further for wind turbine.

Combustion-Powered Actuation for Dynamic-Stall Suppression: High-Mach Simulations and Low-Mach Experiments

An investigation on the dynamic-stall-suppression capabilities of combustion-powered actuation applied to a tabbed VR-12 airfoil is presented. In the first section, the results from the computational fluid dynamics simulations carried out at Mach numbers of 0.3 and 0.4 are presented. Several geometric parameters are varied, including the slot chordwise location and angle. Actuation-pulse amplitude, frequency, and timing are also varied. The two-dimensional simulations suggest that cycle-averaged lift increases of approximately 4 and 8% with respect to the baseline airfoil are possible at Mach numbers of 0.4 and 0.3 for deep and near-deep dynamic-stall conditions. In the second section, the static-stall results from low-speed wind-tunnel experiments are presented. The low-speed experiments and high-speed computational fluid dynamics suggest that slots oriented tangential to the airfoil surface produce stronger benefits than slots oriented normal to the chord line. The low-speed experiments confirm that chordwise slot locations suitable for Mach 0.3–0.4 stall suppression (based on computational fluid dynamics) will also be effective at lower Mach numbers.

Impact of Surface Roughness on the Turbulent Wake Flow of a Turbine Blade

Roughened aeroengine blade surfaces lead to increased friction losses and reduced efficiency of the individual blades. The surface roughness also affects the wake flow of the blade and thus the inflow conditions for the subsequent compressor or turbine stage. To investigate the impact of surface roughness on a turbulent blade wake, we conducted velocity field measurements by means of stereo particle image velocimetry in the wake of a roughened turbine blade in a linear cascade wind tunnel. The turbine blade was roughened at different chordwise locations. The influence of the chordwise location of the added surface roughness was examined by comparing their impact on the width and depth of the wake and, the positions and distribution of vortical structures in the wake. Additionally, the friction loss coefficients for different surface roughness positions were estimated directly from the velocity field.

Research on heavy-duty gas turbine vane high efficiency cooling performance considering coolant phase transfer

Conjugate simulation for heavy-duty gas turbine vane of mist/air phase transfer cooling is carried out to evaluate the cooling enhancement. The Eulerian–Lagrangian particle tracking method is adopted to investigate the two-phase cooling for a gas turbine vane. The accuracy of numerical simulation program for conjugate heat transfer methodology is verified with the C3X gas turbine vane cooled with leading film holes. The effect of various parameters including mist concentration and mist diameter on the cooling performance improvement is investigated in this paper. Results show that the average blade surface temperature reduction about 90 K, 119 K and 94 K are achieved at hub section, midspan section and tip section with 8% mist injection. This translates into a significant blade surface cooling effectiveness enhancement of 11%, 15% and 11%, respectively. The disparate trends are shown for the influence of mist diameter on the blade cooling performance at different spanwise and chordwise location, the results indicate that the degree and the location of cooling performance can be controlled by manipulating different sizes of injected water mist.

Pectoral fin morphology of batoid fishes (Chondrichthyes: Batoidea): explaining phylogenetic variation with geometric morphometrics.

The diverse cartilaginous fish lineage, Batoidea (rays, skates, and allies), sister taxon to sharks, comprises a huge range of morphological diversity which to date remains unquantified and unexplained in terms of evolution or locomotor style. A recent molecular phylogeny has enabled us to confidently assess broadscale aspects of morphology across Batoidea. Geometric morphometrics quantifies the major aspects of shape variation, focusing on the enlarged pectoral fins which characterize batoids, to explore relationships between ancestry, locomotion and habitat. A database of 253 specimens, encompassing 60 of the 72 batoid genera, reveals that the majority of morphological variation across Batoidea is attributable to fin aspect-ratio and the chordwise location of fin apexes. Both aspect-ratio and apex location exhibit significant phylogenetic signal. Standardized independent linear contrast analysis reveals that fin aspect-ratio can predict locomotor style. This study provides the first evidence that low aspect-ratio fins are correlated with undulatory-style locomotion in batoids, whereas high aspect-ratio fins are correlated with oscillatory locomotion. We also show that it is phylogeny that determines locomotor style. In addition, body- and caudal fin-locomotors are shown to exhibit low aspect-ratio fins, whereas a pelagic lifestyle correlates with high aspect-ratio fins. These results emphasize the importance of phylogeny in determining batoid pectoral fin shape, however, interactions with other constraints, most notably locomotor style, are also highlighted as significant.

Tip Clearance Investigation of a Ducted Fan Used in VTOL Unmanned Aerial Vehicles—Part II: Novel Treatments Via Computational Design and Their Experimental Verification

Ducted fan based vertical lift systems are excellent candidates to be in the group of the next generation vertical lift vehicles, with many potential applications in general aviation and military missions. Although ducted fans provide high performance in many “vertical take-off and landing” (VTOL) applications, there are still unresolved problems associated with these systems. Fan rotor tip leakage flow adversely affects the general aerodynamic performance of ducted fan VTOL unmanned aerial vehicles (UAVs). The current study utilized a three-dimensional Reynolds-averaged Navier–Stokes (RANS) based computational fluid dynamics (CFD) model of ducted fan for the development and design analysis of novel tip treatments. Various tip leakage mitigation schemes were introduced by varying the chordwise location and the width of the extension in the circumferential direction. Reduced tip clearance related flow interactions were essential in improving the energy efficiency and range of ducted fan based vehicles. Full and inclined pressure side tip squealers were also designed. Squealer tips were effective in changing the overall trajectory of the tip vortex to a higher path in radial direction. The interaction of rotor blades and tip vortex was effectively reduced and the aerodynamic performance of the rotor blades was improved. The overall aerodynamic gain was a measurable reduction in leakage mass flow rate. This leakage reduction increased thrust significantly. Experimental measurements indicated that full and inclined pressure side tip squealers increased thrust obtained in hover condition by 9.1% and 9.6%, respectively. A reduction or elimination of the momentum deficit in tip vortices is essential to reduce the adverse performance effects originating from the unsteady and highly turbulent tip leakage flows rotating against a stationary casing. The novel tip treatments developed throughout this study are highly effective in reducing the adverse performance effects of ducted fan systems developed for VTOL UAVs.

Relationship between aerodynamic forces, flow structures and wing camber for rotating insect wing planforms

AbstractWing deformation is observed during the flight of some insect species; however, the effect of these distorted wing shapes on the leading edge vortex (LEV) is not well understood. In this study, we investigate the effect of one of these deformation parameters, (rigid) wing camber, on the flow structures and aerodynamic forces for insect-like wings, using a numerical model of an altered fruit fly wing revolving at a constant angular velocity. Both positive and negative camber was investigated at Reynolds numbers of 120 and 1500, along with the chordwise location of maximum camber. It was found that negatively cambered wings produce very similar LEV structures to non-cambered wings at both Reynolds numbers, but high positive camber resulted in the formation of multiple streamwise vortices at the higher Reynolds number, which disrupt the development of the main LEV. Despite this, positively cambered wings were found to produce higher lift to drag ratios than flat or negatively cambered wings. It was determined that a region of low pressure near the wing’s leading edge, combined with the curvature of the wing’s upper surface in this region, resulted in a vertical tilting of the net force vector for positively cambered wings, which explains how insects can benefit from wing camber.

Experimental Investigation into the Effect of Gurney Flaps on Various Airfoils

The aerodynamic effects of the application of Gurney flaps of different heights and chordwise locations to five airfoils have been investigated in the Pennsylvania State University low-speed, low-turbulence wind tunnel. The effectiveness of each Gurney flap/airfoil combination is measured by the change in the maximum lift coefficient of the airfoil. When grouped by flap height and plotted against chordwise location, there is considerable scatter in the data, indicating that the effectiveness of the Gurney flap is strongly influenced by airfoil shape. Two anomalous cases are considered in detail. In the first case, the increase in is considerably different for two airfoils with the addition of a Gurney flap having the same height and mounted at the same chordwise location. The second case is one in which a Gurney flap of a specific height and mounting location is found to increase on one airfoil and decrease it on another. For these cases, pressure distributions, lift curves, and drag polars are provided. An explanation for the mechanism for the change in lift due to a Gurney flap is presented.

Bending-Torsional Flutter of a Composite Rectangular Cantilever Wing Subjected to Engine Thrust

The bending-torsional flutter characteristics of a cantilever composite wing subjected to engine thrust are presented. The engine thrust modeled as a follower force and the wing modeled as a two degree of freedom beam. In order to consider the spanwise and chordwise location and the properties of the engine location, the generalized function theory is used. Unsteady Theodorsen aerodynamic theory in time domain, is used. The general laminate composite theory is used for modeling the effects of the ply angles on the flutter boundary. The Ritz method is subsequently applied to convert the partial differential equations into a set of ordinary differential equations. In order to precisely consider the location of the engine Dirac delta function is used. Moreover, the numerical results are compared with the published results and excellent agreement is observed. Numerical results highlighting the effect of engine thrust on the wing instability, is presented.

Experimental and numerical investigations of tip injection on tip clearance flow in an axial turbine cascade

This investigation presents detailed experimental measurements of an active tip-clearance control method based on tip injection in a high-turning axial turbine cascade. Besides that, numerical investigations are also conducted to study phenomena which are not easily measured in the experiments. It aims to study the influence of tip injection on tip clearance flow, with emphasis on the effects of injection locations. Detailed flow field measurements were made downstream of the cascade using a three-hole probe. Static pressure distributions were also measured on the blade surface at 50% and 97.5% span, respectively. The results suggest that tip injection can weaken tip clearance flow, reducing the tip clearance mass flow and its associated losses. Meanwhile, the heat transfer condition on the blade tip surface can be also improved significantly. It also can be found that injection chordwise location plays an important role in the redistribution of secondary flow within the cascade passage. When the same number of injection holes and injection mass flow are applied, holes located in the aft part of blade can perform much better than that in the front part.

Axial Flow Fan Tip Leakage Flow Control Using Tip Platform Extensions

Performance of an axial flow fan unit is closely related to its tip leakage mass flow rate and level of tip/casing interactions. The present experimental study uses a stereoscopic particle image velocimeter to quantify the three dimensional mean flow observed near the blade tip, just downstream of a ducted fan unit. After a comprehensive description of the exit flow from the baseline fan, a number of novel tip treatments based on custom designed pressure side extensions are introduced. Various tip leakage mitigation schemes are introduced by varying the chordwise location and the width of the extension in the circumferential direction. The current study shows a proper selection of the pressure side bump location and width are the two critical parameters influencing the success of each tip leakage mitigation approach. Significant gains in the axial mean velocity component are observed when a proper pressure side tip extension is used. It is also observed that a proper tip leakage mitigation scheme significantly reduces the tangential velocity component near the tip of the axial fan blade. Reduced tip clearance related flow interactions are essential in improving the energy efficiency of ducted fan systems. A reduction or elimination of the momentum deficit in tip vortices is also essential to reduce the adverse performance effects originating from the unsteady and highly turbulent tip leakage flows rotating against a stationary casing.

Absolute stability mechanism of a swept cylinder laminar boundary layer with imposed spanwise periodic conditions

The linear impulse response of a laminar three-dimensional boundary layer with imposed spanwise periodic conditions is theoretically analyzed. Because of the imposed spanwise periodicity, a unidirectional absolute instability is physically significant. The base flow is obtained through a direct numerical simulation of the flow along a swept cylinder, with periodic boundary conditions enforced at the spanwise boundaries. It is shown that this flow is absolutely stable. However, it exhibits crossflow modes of zero group velocity whose damping rate in time is quite slow. Moreover, these crossflow modes are spatially amplified. A direct numerical simulation of the same configuration but in the presence of surface roughness has enabled the observation of these traveling crossflow waves. The introduction of the roughness element in the computation creates initial transients that play the role of an impulselike forcing, which generates the traveling crossflow waves as predicted by the theoretical analysis. Depending on the roughness chordwise location, the frequency and damping rate in time of the crossflow waves change in agreement with the theoretical results.

Experimental Investigation of Gurney Flaps

The aerodynamics of a Gurney-flap-equipped airfoil has been explored by means of low-speed wind-tunnel experiments performed at a chord Reynolds number of 1:0 10. Various chordwise locations and sizes of Gurney flaps were tested. Surface-pressure distributions and the wake momentum deficit were measured and used to determine lift, pitching moment, and drag. Compared with the clean airfoil, the measured maximum lift coefficient can be increased by nearly 30%with these simple devices. The amount of lift increase has a nearly linear dependency on the chordwise location and size of the Gurney flap. Minimum drag is primarily affected by the flap size and, to a lesser extent, by the chordwise location. The Gurney flap increases in maximum lift are obtained by increasing the lower-surface pressures over the aft part of the airfoil. At the same time, themagnitude of pressure peak on the upper surface near the leading edge is reduced such that the upper-surface pressures over themiddle parts of the airfoil are reduced and the separation point is moved aft by the reduced pressure-recovery gradients. As expected, this increases the aft loading and results in an increased nose-down pitchingmoment. As the angle of attack is decreased, the influence of a Gurney flap extending from the lower surface likewise decreases as the flap is increasingly immersed in the thickening boundary layer. A Gurney flap mounted to the upper surface behaves in the opposite way: increasing the negative lift at low angles of attack and having less and less influence as the angle of attack is increased. AlthoughGurney flaps result in significantly higher drags for airfoils with extensive runs of laminar flow, this disadvantage disappears as the amount of turbulent boundary-layer flow increases, as is the case with fixed transition near the leading edge of the airfoil.

Plasma-Based Flow-Control Strategies for Transitional Highly Loaded Low-Pressure Turbines

Recent numerical simulations have indicated the potential of plasma-based active flow control for improving the efficiency of highly loaded low-pressure turbines. The configuration considered in the current and earlier simulations correspond to previous experiments and computations for the flow at a Reynolds number of 25,000 based on axial chord and inlet conditions. In this situation, massive separation occurs on the suction surface of each blade due to uncovered turning, causing blockage in the flow passage. It was numerically demonstrated that asymmetric dielectric-barrier-discharge actuators were able to mitigate separation, thereby decreasing turbine wake losses. The present investigation extends this work by investigating a number of plasma-based flow control strategies. These include the chordwise location of actuation, spanwise periodic arrays of actuators, multiple actuation in the streamwise direction, and spanwise-direct actuation. The effect of alternate plasma-force models is also considered. Solutions were obtained to the Navier–Stokes equations, which were augmented by source terms used to represent plasma-induced body forces imparted by an actuator on the fluid. The numerical method utilized a high-fidelity time-implicit scheme, employing domain decomposition to carry out calculations on a parallel computing platform. A high-order overset grid approach preserved spatial accuracy in locally refined embedded regions. Features of the flowfields are described, and resultant solutions are compared to each other, with a previously obtained control case, and with the base line situation where no control was enforced.

Experimental Studies on Unsteady Lateral Blowing on NACA 0012

T HE initial study of steady lateral blowing was conducted by Dixon in 1969 [1]. The mechanism of this concept is analogous to the flow over slender delta wing platforms, which produces a leading-edge vortex and nonlinear lift curves. Steady lateral blowing might be thought of as providing the sweep effects such as those of the delta wings, for which the effective sweep is a function of the jet momentum. The lateral blowing jet can be described in terms of the chordwise position along the airfoil section, the nozzle height, and the nozzle size. Dixon et al. [2] andClarke [3] suggested that the best vertical nozzle positionmay be a function of the nozzle diameter and that the jet from a nozzle placed too near the airfoil surface may have a deleterious effect on the flow over the airfoil. Wong and Kontis [4] performed a comprehensive study on the steady spanwise blowing on a NACA 0012 airfoil section. The force measurements showed that the lift coefficient increasedwhen the steady lateral blowingwas applied, and the most effective blowing location was found to be at x=c 0:25 (x is the chordwise location and c is the airfoil chord), because the lift augmentation ratio Cl=C was always above one for all positive angles of attack [ Cl is the increment in the lift coefficient due to blowing, andC is the blowing momentum coefficient; the definition ofC is shown inEq. (1)].However, largemomentumwas required to generate the lift enhancement on the airfoil and this became a major issue in the effectiveness of blowing. As a consequence, Meyer and Seginer [5] performed some initial tests for generating the same lift increase using lower momentum through pulsed blowing. One of the main disadvantages of lateral blowing is due to the fact that air for blowing is bled from the compressor stage of the engine. This reduces the amount of air for combustion, hence reducing thrust. Unsteady blowing provides the beneficial effect of reducing the mass-flow requirement from the engine. Another adverse effect of the technology arises from the need to install plumbing, control valves, regulators, oscillators, etc., to supply and control the compressed air from the engine. This adds weight to the aircraft, and a higher lift is required. C thrust

Parameter Sensitivities Affecting the Flutter Speed of a MW-Sized Blade

With the current trend toward larger and larger horizontal axis wind turbines, classical flutter is becoming a more critical issue. Recent studies have indicated that for a single blade turning in still air the flutter speed for a modern 35 m blade occurs at approximately twice its operating speed (2 per rev), whereas for smaller blades (5–9 m), both modern and early designs, the flutter speeds are in the range of 3.5–6 per rev. Scaling studies demonstrate that the per rev flutter speed should not change with scale. Thus, design requirements that change with increasing blade size are producing the concurrent reduction in per rev flutter speeds. In comparison with an early small blade design (5 m blade), flutter computations indicate that the non rotating modes which combine to create the flutter mode change as the blade becomes larger (i.e., for the larger blade the second flapwise mode, as opposed to the first flapwise mode for the smaller blade, combines with the first torsional mode to produce the flutter mode). For the more modern smaller blade design (9 m blade), results show that the non rotating modes that couple are similar to those of the larger blade. For the wings of fixed-wing aircraft, it is common knowledge that judicious selection of certain design parameters can increase the airspeed associated with the onset of flutter. Two parameters, the chordwise location of the center of mass and the ratio of the flapwise natural frequency to the torsional natural frequency, are especially significant. In this paper studies are performed to determine the sensitivity of the per rev flutter speed to these parameters for a 35 m wind turbine blade. Additional studies are performed to determine which structural characteristics of the blade are most significant in explaining the previously mentioned per rev flutter speed differences. As a point of interest, flutter results are also reported for two recently designed 9 m twist/coupled blades.

Parametric Studies of Transonic Aeroelastic Effects of an Aircraft Wing/Tip Store

This research investigates the effect of store parameters variation on store-induced flutter and limit-cycle oscillation phenomena of an aircraft wing in the transonic regime. The primary store parameters were its mass and the chordwise location of its center of gravity. The effect of including store aerodynamics on the wing/tip store configuration was also investigated. The tip store center of gravity (c.g.) was varied and positioned at three different locations: 32.5, 40, and 50%, with respect to aerodynamic tip chord. Automated Structural Optimization System and Computational Aeroelasticity Program‐Transonic Small Disturbance were used in the linear and nonlinear region to perform this research. Studies showed that flutter speed increases as the store c.g. was moved forward toward the leading edge. This gives an indication that store c.g. must be placed as far forward as possible with respect to the elastic axis to delay the occurrence of flutter, while satisfying other design constraints. It was observed that the increase in tip store mass significantly reduced the flight operating speed range of the aircraft. The effect of inclusion of store aerodynamics for different wing/store configurations was found to be insignificant compared to their corresponding mass-only models in the transonic regime. The limit-cycle oscillation (LCO) onset speed was found to be sensitive to both store mass and store c.g. parameters and varied significantly for different store mass configurations. The LCO onset speed decreased significantly with increase in store mass and aft movement of store c.g.

Preliminary Design of a Structural Wing Box Under a Twist Constraint Part I

We present a simple beam-type formulation to determine the structural weight of a wing-box to carry aerodynamic loads for a wing in a transonic flow-field. The formulation employs non-dimensional or scaled parameters and thus can be easily integrated with aerodynamic codes that often provide aerodynamic loads using scaled variables. Specifically, the aerodynamic code, TOPS, developed at NASA Ames Research center, is used as a representative aerodynamic code. The formulation first uses a simple double-plate model to determine the material required to withstand the bending caused by aerodynamic loads. This double plate model is then transformed into a hexagonal wing box using an empirical model made available to us earlier by the Lockheed Martin Company as part of our collaborative effort on the design of a strut-braced wing. Under certain design conditions, the empirical model was found to give excessive twist near the wing tip. To prevent large twist of the wing, a constraint that restricts the maximum rate of twist in a wing segment to 0.1 radian per wing semi- span is used. The constraint is found to be both effective and simple to implement. The angle of twist near the tip is also found to be significantly impacted by two parameters: the sweep of the elastic axis and the chord-wise location of the structural wing box. Two gradient based optimization approaches, namely the method of steepest descent and the conjugate gradient method are used to find values of these two parameters so that the overall aerodynamic torsion acting on the elastic axis is minimized. Side constraints were needed so that the obtained values of the two parameters are physically meaningful, i.e. these two parameters should be located within the respective range for each of these variables. The results from these gradient-based methods are compared with those given by a genetic algorithm. A good agreement is obtained.