Downwash Velocity Research Articles

Comprehensive Analysis of Factors Underpinning the Superior Performance of Ducted Horizontal-Axis Helical Wind Turbines

The societal and economic reliance on non-renewable energy sources, primarily fossil fuels, has raised concerns about an imminent energy crisis and climate change. The transition towards renewable energy sources faces challenges, notably in understanding turbine shear forces within wind technology. To address this gap, a novel solution emerges in the form of the ducted horizontal-axis helical wind turbine. This innovative design aims to improve airflow dynamics and mitigate adverse forces. Computational fluid dynamics and experimental assessments were employed to evaluate its performance. The results indicate a promising technology, showcasing the turbine’s potential to harness energy from diverse wind sources. The venturi duct aided in the augmentation of the velocity, thereby increasing the maximum energy content of the wind by 179.16%. In addition, 12.16% of the augmented energy was recovered by the turbine. Notably, the integration of a honeycomb structure demonstrated increased revolutions per minute (RPM) by rectifying the flow and reducing the circular wind, suggesting the impact of circular wind components on turbine performance. The absence of the honeycomb structure allows the turbine to encounter more turbulent wind (circular wind), which is the result of the movement of the fan. Strikingly, the downwash velocity of the turbine was observed to be equal to the incoming velocity, suggesting the absence of an axial induction factor and, consequently, no back force on the system. However, limitations persist in the transient modelling and in determining optimal performance across varying wind speeds due to experimental constraints. Despite these challenges, this turbine marks a significant stride in wind technology, highlighting its adaptability and potential for heightened efficiency, particularly at higher speeds. Further refinement and exploration are imperative for broadening the turbine’s application in renewable energy generation. This research emphasizes the turbine’s capacity to adapt to different wind velocities, signaling a promising avenue for more efficient and sustainable energy production.

Numerical study on the noise propagation characteristics of rotor in non-uniform downwash flowfield Based on Linearized Euler Equations

To study the influence of non-uniform flowfield on the propagation characteristics of helicopter rotor noise, a Hybrid Computational Aeroacoustics (HCAA) method is developed. The acoustic source region is simulated by Computational Fluid Dynamics (CFD) technique with the Unsteady Reynolds Averaged Navier-Stokes equations (URANS) as the governing equations. Acoustic near-field is simulated by Computational Aeroacoustics (CAA) technique with the Linearized Euler Equations (LEE) as the governing equations, and the numerical discretization of the LEE is accomplished by Runge-Kutta Discontinuous Galerkin (RKDG) method. A novel acoustic source extraction method based on pressure and pressure gradient is proposed to accomplish the one-way CFD-CAA weak coupling. The HCAA method is validated through comparisons with noise experimental data of the UH-1H model rotor and the BO-105 model rotor. Based on the proposed HCAA method, the convection and refraction effects of rotor noise under different collective pitch angles are analyzed. The results show that the distortion effect of the rotor noise is most affected by the non-uniformly distributed downwash velocity field, resulting in an increment of acoustic energy below the rotor plane. The effect of non-uniformly distributed downwash velocity on noise propagation increases with the increase of the collective pitch angle. For the UH-1H model rotor, the maximum change of the sound pressure level is 0.8 dB (about 10% change of the effective sound pressure).

Hovering flight in hummingbird hawkmoths: kinematics, wake dynamics and aerodynamic power.

Hovering insects are divided into two categories: 'normal' hoverers that move the wing symmetrically in a horizontal stroke plane, and those with an inclined stroke plane. Normal hoverers have been suggested to support their weight during both downstroke and upstroke, shedding vortex rings each half-stroke. Insects with an inclined stroke plane should, according to theory, produce flight forces only during downstroke, and only generate one set of vortices. The type of hovering is thus linked to the power required to hover. Previous efforts to characterize the wake of hovering insects have used low-resolution experimental techniques or simulated the flow using computational fluid dynamics, and so it remains to be determined whether insect wakes can be represented by any of the suggested models. Here, we used tomographic particle image velocimetry, with a horizontal measurement volume placed below the animals, to show that the wake shed by hovering hawkmoths is best described as a series of bilateral, stacked vortex 'rings'. While the upstroke is aerodynamically active, despite an inclined stroke plane, it produces weaker vortices than the downstroke. In addition, compared with the near wake, the far wake lacks structure and is less concentrated. Both near and far wakes are clearly affected by vortex interactions, suggesting caution is required when interpreting wake topologies. We also estimated induced power (Pind) from downwash velocities in the wake. Standard models predicted a Pind more than double that from our wake measurements. Our results thus question some model assumptions and we propose a reevaluation of the model parameters.

Mechanisms underlying flat surface forced convection enhancement by rectangular flexible strips

Employing turbulence generators, such as rectangular flexible strips, is a simple and effective approach to enhancing forced convection. In the current study, 12.7 mm wide rectangular flexible strips (with thickness of 0.1 mm, 0.2 mm and 0.25 mm, height of 25.4 mm, 38.1 mm and 50.8 mm, transversal spacing of 12.7 mm, 25.4 mm and 38.1 mm) were investigated at Reynolds number (Re) of 8500 and 6000, and freestream turbulence intensity (Tu) of 0.4%, 7%, 9%, and 11%. The convective heat transfer augmentation was analyzed in terms of the normalized Nusselt number (Nu/Nu0). The relationships between the averaged Nu/Nu0 and the strip thickness, height, spacing, Re and Tu were determined using multiple linear regression. The correlations between the local Nu/Nu0 and turbulent flow parameters, including turbulent kinetic energy, downwash velocity and near surface streamwise velocity were also illustrated. The freestream turbulence intensity (Tu) had the most significant effect in augmenting the averaged Nu/Nu0, and the local Nu/Nu0 correlated best with the local ke. The maximal averaged Nu/Nu0 over 23 W downstream, within ± 1 and ± 4 strip widths cross-stream was found for Tu = 7% case and Tu = 11% case, respectively.

Heat transfer enhancement of a heated flat surface via a flexible strip pair

A pair of 12.7 mm wide (W) and 25.4 mm tall rectangular flexible strips made of 0.1 mm thick aluminum sheet was employed to enhance convective heat transfer from a flat surface in a closed-loop wind tunnel. The strip pair was placed side-by-side with a spacing of 1W, 2W and 3W. The incoming wind velocity was fixed at 10 m/s, resulting in a Reynolds number based on the strip width (ReW) of 8.5 × 103. The heat transfer enhancement was represented in terms of the Nusselt number with respect to the reference Nusselt number in the absence of the strip pair (Nu/Nuo). A triple-sensor hotwire probe was employed to characterize the flow characteristics when the plate was unheated. Particularly strong vortex-vortex interactions, a large downwash velocity and intense flow fluctuations were obtained behind the 1W-spaced strip pair. These contributed to better mixing and transport of the thermal energy. The 1W-spaced strip pair generated the highest Nu/Nu0 values. Nu/Nu0 of approximately 1.50 was achieved at the middle of the strip pair at X=9W. At this location, the pairs separated by 2W and 3W resulted in Nu/Nu0 value of around 1.26 and 1.17, respectively. The 1W-spaced strip pair also produced the largest span-averaged Nu/Nu0.

High aerodynamic lift from the tail reduces drag in gliding raptors.

ABSTRACTMany functions have been postulated for the aerodynamic role of the avian tail during steady-state flight. By analogy with conventional aircraft, the tail might provide passive pitch stability if it produced very low or negative lift. Alternatively, aeronautical principles might suggest strategies that allow the tail to reduce inviscid, induced drag: if the wings and tail act in different horizontal planes, they might benefit from biplane-like aerodynamics; if they act in the same plane, lift from the tail might compensate for lift lost over the fuselage (body), reducing induced drag with a more even downwash profile. However, textbook aeronautical principles should be applied with caution because birds have highly capable sensing and active control, presumably reducing the demand for passive aerodynamic stability, and, because of their small size and low flight speeds, operate at Reynolds numbers two orders of magnitude below those of light aircraft. Here, by tracking up to 20,000, 0.3 mm neutrally buoyant soap bubbles behind a gliding barn owl, tawny owl and goshawk, we found that downwash velocity due to the body/tail consistently exceeds that due to the wings. The downwash measured behind the centreline is quantitatively consistent with an alternative hypothesis: that of constant lift production per planform area, a requirement for minimizing viscous, profile drag. Gliding raptors use lift distributions that compromise both inviscid induced drag minimization and static pitch stability, instead adopting a strategy that reduces the viscous drag, which is of proportionately greater importance to lower Reynolds number fliers.

Experimental and numerical studies of wingtip and downwash effects on horizontal tail

Studying wing downwash, which is caused by the wingtip effect, and its influence on horizontal tail is important for aircraft design. In this work, wing downwash was investigated using experimental and numerical methods. Sets of main wings and horizontal tails were fixed in a tunnel test chamber. For determining the wingtip effect and the wing downwash affecting the horizontal tail, experiments were performed, in which the pressure distributions near the main wingtip and on the upper and lower surfaces of the tail were measured. These experimental models were used in numerical calculations by the solving of differential equations for viscous flows and use of a singularity method for potential flows. The singularity method can be applied to determine the wing lift, as indicated by comparisons between the experimental and numerical results of the pressure distribution on the wing. Moreover, the wingtip and wing downwash effects influencing the horizontal tail should be determined with use of experimental and numerical methods that solve differential equations of viscous flow. In addition to the results regarding the pressure distributions near the main wing and on the horizontal tail, the longitudinal velocity, downwash velocity, and downwash angle distributions in the main wing wake were analyzed. We also investigated the kinetic parameters of the flow in mixed zones between the main wing downwash and the tail upwash.

Application of Ultrasonic Anemometer Array to Field Measurements of the Downwash Flow of an Agricultural Unmanned Helicopter

Abstract. Unmanned helicopters are widely used in agricultural spray applications; however, there is a lack of high-accuracy field data on the complex downwash flows caused by such helicopters to support efficient spraying. Therefore, we aimed to collect high-accuracy reference data by developing an ultrasonic anemometer array and related software to measure the downwash flow velocity field of a common agricultural unmanned helicopter (Copterworks AF-25B) during outdoor hovering and forward flight. Differential GPS and image scaling were used to determine the exact position and velocity of the helicopter to ensure repeatability of the tests. When the hovering height of the rotor above the ground plane was y/R = 2.5 (where y is the height above ground level and R is the radius of the rotor), the maximum downwash velocity reached 7 m s-1 at x/R = 0.5 (where x is the radial distance from the axis of the rotor). In addition, we obtained the spatial distribution of the turbulence kinetic energy. The attached flow velocity in the near-ground boundary layer increased radially and reached a maximum of 3 m s-1 at x = 3.6 m, after which it decreased. Comparatively, the maximum downwash velocity during forward flight occurred between x/R = 0.5 and 1.5 and was only about 3 m s-1, which was less than half the maximum downwash velocity while hovering. In addition, the circulation flow below the rotor tips had a velocity of 1.5 m s-1. The downwash flow lasted more than 1 s as the helicopter passed over a given point on the ground, the downwash velocity decreased from 3 m s-1 to 1.5 to 2.5 m s-1, and the symmetry of the flow field was not maintained. This study is the first application of an ultrasonic anemometer array to measure the airflow around an unmanned helicopter and provides high-accuracy field data that can be used as a reference for future indoor and simulation studies. Keywords: Agricultural unmanned helicopter, Downwash flow velocity field, Forward flight, Hovering, Ultrasonic anemometer.

Numerical Analysis of Hydrodynamics for Bionic Oscillating Hydrofoil Based on Panel Method.

The kinematics model based on the Slender-Body theory is proposed from the bionic movement of real fish. The Panel method is applied to the hydrodynamic performance analysis innovatively, with the Gauss-Seidel method to solve the Navier-Stokes equations additionally, to evaluate the flexible deformation of fish in swimming accurately when satisfying the boundary conditions. A physical prototype to mimic the shape of tuna is developed with the revolutionized technology of rapid prototyping manufacturing. The hydrodynamic performance for rigid oscillating hydrofoil is analyzed with the proposed method, and it shows good coherence with the cases analyzed by the commercial software Fluent and the experimental data from robofish. Furthermore, the hydrodynamic performance of coupled hydrofoil, which consisted of flexible fish body and rigid caudal fin, is analyzed with the proposed method. It shows that the caudal fin has great influence on trailing vortex shedding and the phase angle is the key factor on hydrodynamic performance. It is verified that the shape of trailing vortex is similar to the image of the motion curve at the trailing edge as the assumption of linear vortex plane under the condition of small downwash velocity. The numerical analysis of hydrodynamics for bionic movement based on the Panel method has certain value to reveal the fish swimming mechanism.

Transformation of flow structure on a rotating wing due to variation of radius of gyration

The flow structure on a rotating wing (rectangular plate) is characterized over a range of travel distance at different radii of gyration. Travel distance is defined as the length of the arc subtended by the radius of gyration. Stereoscopic particle image velocimetry is employed to determine the volumetric flow structure, in the form of three-dimensional surfaces of the q-criterion, helical density, and downwash velocity. These representations are complemented by sectional patterns of vorticity and tangential velocity. An increase in the radius of gyration reduces the influence of rotation on the flow structure. At small radius of gyration, a coherent leading-edge vortex develops rapidly and then persists over a range of travel distance. At moderate radius of gyration, this leading-edge vortex is replaced by an arch vortex, which develops relatively slowly over a larger travel distance, and is eventually swept into the wake of the wing. The foregoing classes of vortical structures are associated with distinctive patterns of: helical density, which represents the axial vorticity flux through the three-dimensional vortex system; downwash related to the strengths of the components of the vortex system; and tangential velocity associated with the extent of reverse flow, or stall.

The Bionic Wing with Winglet in Near Space Aerodynamic Analysis

With reference to a certain type of flying drones with imitation airfoil design a seagull flat wings and on the basis of its wing tip winglet in this paper. Through to the numerical simulation of two wings, it is concluded that the bionic wing aerodynamic performance is superior to the conventional airfoil wing, after adding wing tip winglet bionic wings effectively reduced the downwash velocity, reduce the induced drag, makes the wing aerodynamic performance is improved. Provide theoretical reference for the design of the uav wing

A coupled free-wake/panel method for rotor/fuselage/empennage aerodynamic interaction and helicopter trims

A coupled free-wake/panel method to predict the rotor downwash aerodynamic interaction and helicopter trims is presented. Free-wake method was developed to analyze aerodynamics interference of rotor wake, based on lifting-surface theory and vortex method, which relates the core radius, span station, and circulation of initial tip vortex with the blade-bound circulation distribution, and eliminates the empirical parameters during rotor free wake analysis. Helicopter fuselage with empennage was discretized into source panels. The vortex line mirror method was adopted to account for wake acceleration phenomenon that resulted from the close interaction between rotor wake and fuselage surface. The rotor wake geometry and downwash simulations were investigated including comparisons with the available measured data. Combined with the helicopter flight dynamics model and embedded in the trim procedure, the free-wake/panel coupled method was applied to calculate the rotor wake and its interference on other components of a full-scale UH-60A rotorcraft in level flight. Comparisons among predictions, referenced results, and experimental results are made for rotor wake geometries, blade-bound vortex, blade tip vortex, rotor downwash velocity, and control stick positions, and encouraging results were obtained. To validate the present method, this paper discusses the fundamental formulation, the numerical algorithms, and the simulation results.

Closed-Loop Active Flow Control of a Three-Dimensional Turret Wake

An experimental study has been performed to investigate the wake of a three-dimensional turret with aspect ratio of 1.17 at a diameter-based Reynolds number of . Stereoscopic particle image velocimetry was used to evaluate the spatial structure of the wake, and dynamic surface-pressure measurements were used to study the time-dependent behavior. The uncontrolled wake was seen to be strongly three-dimensional with two main structures on opposing sides of the center plane. Cross correlations from the surface pressure indicated that opposite sides of the wake had nearly zero correlation in time. With the application of active flow control, both the spatial structure and time-dependent behavior of the wake were significantly altered. The downwash velocity along the center plane was seen to increase significantly, causing the wake to spread more rapidly near the support plate, and the spectra from the pressure sensors contained multiple peaks at different frequencies throughout the wake that were not seen in the baseline case. Closed-loop control achieved similar control authority as steady open-loop forcing, but with a 45% reduction in the mean jet-momentum coefficient.

Laboratory study of the cross-shore flow structure in the surf and swash zones

[1] The flow structure of oscillatory broken waves within surf and swash zones was investigated using particle image velocimetry (PIV) in the laboratory. With the resolved spatial distribution of the velocity field, vorticity was computed directly. The results show that flow separation at the bed occurs during the interaction between the uprush bore front and the downwash flow. The separation point advances in the onshore direction with the bore front until the bore reaches the lowest point of the moving shoreline. The bore front continues to climb up onshore and collapses at the highest point of the moving shoreline. In the swash zone, flow attachment to the bed occurs during the transition from the uprush to downwash process. An internal flow circulation is generated at the flow reversal phase as the flow near the bed responds to the gravitational force earlier than the flow in the upper water column, where the uprush momentum is sustained later in phase. The magnitude of the downwash velocity in the swash zone is greater than that of the uprush process. The swash zone flow observed in the experiments should be erosive in the lower part of the swash and accretive in the upper part where flow attachment occurs.

Numerical Simulation of Unsteady Flow Around Forward Flight Helicopter with Coaxial Rotors

Three-dimensional unsteady Euler equations are numerically solved to simulate the unsteady flows around forward flight helicopter with coaxial rotors based on unstructured dynamic overset grids. The performances of the two coaxial rotors both become worse because of the aerodynamic interaction between them, and the influence of the top rotor on the bottom rotor is greater than that of the bottom rotor on the top rotor. The downwash velocity at the bottom rotor plane is much larger than that at the top rotor plane, and the downwash velocity at the top rotor plane is a little larger than that at an individual rotor plane. The downwash velocity and thrust coefficient both become larger when the collective angle of blades is added. When the spacing between the two coaxial rotors increases, the thrust coefficient of the top rotor increases, but the total thrust coefficient reduces a little, because the decrease of the bottom rotor thrust coefficient is larger than the increase of the top rotor thrust coefficient.

Stroke-averaged lift forces due to vortex rings and their mutual interactions for a flapping flight model

The stroke-averaged lift forces due to various vortex rings and their mutual interactions are studied using a flapping flight vortex model (Rayner,J. Fluid Mech., vol. 91, 1979, p. 697; Ellington,Phil. Trans. R. Soc. Lond.B, vol. 305, 1984b, p. 115). The vortex system is decomposed into the wing plane (wing-linked) vortex ring, a loop closed by the bound vortex and (arc-shaped) trailing vortex and the wake (the vortex rings shed previously). Using the vorticity moment theory (Wu,AIAA J., vol. 19, 1981, p. 432) we are able to identify the roles of vortex rings in lift production or reduction and express the lift as function of areal contraction or expansion of vortex rings. The wake vortex rings induce areal contraction of the trailing vortex, which should decrease the lift, but this decrease is exactly compensated by the inducing effect of the trailing arc on the wake. The wake reduces the lift through inducing a downwash velocity on the wing plane. The lift force is shown to drop to a minimum at the second half stroke, and then increases to an asymptotic value slightly below the lift at the first half stroke, in such a way following the experimental observation of Birch & Dickinson (Nature, vol. 412, 2001, p. 729). The existence of the negative peak of lift is due to the first shed vortex ring which, just at the second half stroke, lies in the close vicinity to the wing plane, leading to a peak of the wing plane downwash velocity.

Wake Measurement Downstream of a Hybrid Wing Body Model with Blown Flaps

Flow-field measurements were obtained in the wake of a full-span Hybrid Wing Body model with internally blown flaps. The test was performed at the NASA Langley 14 x 22 Foot Subsonic Tunnel at low speeds. Off-body measurements were obtained with a 7-hole probe rake survey system. Three model configurations were investigated. At 0deg angle of attack the surveys were completed with 0deg and 60deg flap deflections. At 10deg angle of attack the wake surveys were completed with a slat and a 60deg flap deflection. The 7-hole probe results further quantified two known swirling regions (downstream of the outboard flap edge and the inboard/outboard flap juncture) for the 60deg flap cases with blowing. Flowfield results and the general trends are very similar for the two blowing cases at nozzle pressure ratios of 1.37 and 1.56. High downwash velocities correlated with the enhanced lift for the 60deg flap cases with blowing. Jet-induced effects are the largest at the most inboard station for all (three) velocity components due in part to the larger inboard slot height. The experimental data are being used to improve computational tools for high-lift wings with integrated powered-lift technologies.

Phase-Locked Particle Image Velocimetry Measurements of a Flapping Wing

The unsteady aerodynamics of a biomimetic inspired flapping-wing mechanism has been analyzed by performing detailed phase-locked diagnostics of its flow field. Flow visualization and particle image velocimetry results have shown the presence of a shed dynamic stall vortex that spans across most of the wing span. The shedding of this type of leading-edge vortex was accompanied by the formation of another leading-edge vortex before the first vortex reached the midchord, resulting in multiple shedding leading-edge vortices on the top surface of the wing during each wing stroke. A strong starting vortex was also formed at the trailing edge of the wing during the early part of its translational stroke. This vortex continuously gained strength from shed vorticity as the wing accelerated into its stroke. The starting vortex remained close to the trailing edge until the wing reached midstroke. A pair of vortices that continuously trailed from the root and tip of the wing were identified, both of which induced a significant downwash velocity over the wing surface. These trailed vortices were found to exhibit a contracting wake structure as they convected into the wake below the wing, consistent with an increase in slipstream velocity. The evolution of the tip and root vortex pair showed rapid diffusive characteristics with an increase in time (wake age).

Dynamic Flow Response Due to Motion of Partial-Span Gurney-Type Flaps

Uninhabited air vehicles are commonly designed with high-aspect-ratio wings, which can be susceptible to significant aeroelastic vibrations. These oscillations can result in a loss of control or structural failure, and new techniques are necessary to alleviate them. A multidisciplinary effort at Stanford developed a distributed flow control method that used small trailing-edge actuators, known as micro-trailing-edge effectors (MiTEs) to alter the aerodynamic loads at specific spanwise locations along an airplane wing. The actuators were based on a Gurney flap, which is a trailing-edge flap of small size and large deflection, allowing an increase in lift with a small drag penalty. The transient response caused by relatively rapid MiTE actuation was studied using particle image velocimetry in the near wake. The transient response was quasi-steady for dimensionless actuation times (tU∞/c) near unity. A shorter dimensionless actuation time of 0.2 produced a transient response with significant overshoot of the downwash velocity in the near wake. This indicated a nonmonotonic response of the aerodynamic loads for rapid actuation.

Force and flow structures of a wing performing flapping motion at low Reynolds number

Aerodynamic force and flow structures of a wing performing a simplified flapping motion that emulates the wing motion of small insects in normal hovering flight are studied, using the method of numerically solving the Navier-Stokes equations. For a typical case (wing rotation-axis is at 0.25 chord position and wing rotation is symmetrical with respect to stroke reversal), large peaks inC L andC D are produced near the end of a stroke by the wing-rotation, but the wing-rotation also generates a vortical structure which induces strong downwash velocity, reducing the lift production in the early part of the following stroke; averaging over one flapping cycle,C L is 28% larger than the steady-state value and is about that needed to support the weight of a small insect. The timing of the wing-rotation at stroke reversal can change the size of the peaks inC L andC D and their averages; e.g. in the case of shifting the wing-rotation forward in time by 7.5% of a stroke period, the averageC L becomes 63% larger than the steady-state value, which is larger than that needed for weight supporting (and might provide extra force for control or maneuvers). When the rotation-axis is moved rearward to the middlechord position, the lift and drag peaks due to the wing-rotation become smaller, and in this case, the wing-rotation generates a vortical structure that tends to prevent the relative motion between positive and negative vorticities, also reducing the lift production in the early part of the following stroke, resulting in a smaller averageC L .