Downwash Angle Research Articles

Impact of the train heights on the aerodynamic behaviour of a high-speed train

The impact of train heights on train aerodynamic performance is studied by using an improved delayed detached-eddy simulation (IDDES) method. The correctness of the numerical method has been verified by the existing wind tunnel and moving model experiments data. The aerodynamic drag, lift, slipstream, and wake flow are compared for three train heights. The results presented that the drag and lift increased by 6.2% and 23.8% respectively, with an increase in train height from 3.89 m to 4.19 m. Compared with the 3.89 m case, the maximum time-averaged slipstream at the platform location for 4.04 and 4.19 m cases are increased by 2.0% and 4.3% respectively. Meanwhile, the wake topology for three cases is described and analyzed quantitatively. The downwash angle of the wake longitudinal flow is increased with the increasing train height, resulting in the mixing of the downwash flow and the ground flow in advance. The wake in the higher trains tends to develop outward and downward. Besides, the higher trains will also bring greater transient aerodynamic loads to the equipment above the train. It’s recommended to shorten the maintenance period of the electrical equipment above the higher trains to ensure the devices’ safety.Abbreviations: CFL: Courant–Friedrichs-Lewy; COT: Center of the track; FDR: Flow development region; FFT: Fast Fourier transform; GF: Ground-fixed reference system; ICE3: Intercity Express 3; IDDES: Improved delayed detached-eddy simulation; LES: Large-eddy simulation; LV: Longitudinal vortex; MME: Moving model experiments; NBL: Negative bifurcation line; PBL: Positive bifurcation line; PSD: Power spectral density; RANS: Reynolds averaged Navier – Stokes; SF: Stable focus; SP: Saddle point; STBR: Single-track ballast and rails; SV: Spanwise vortex; TF: Train-fixed reference system; TOR: Top of the track; TSI: Technical specification for interoperability; UN: Unstable node; WPR: Wake propagation region

Assessment of Blade Element Momentum Theory-based engineering models for wind turbine rotors under uniform steady inflow

Blade Element Momentum Theory (BEMT) -based approaches provide 15%–20% inaccurate load predictions under uniform steady inflow. Inaccuracies were related to unclear aerodynamic mechanisms causing the Prandtl Tip-Root Loss (TRL) correction to fail at high Tip-Speed Ratio (TSR) and the ambiguous drag coefficient treatment under Stall Delay (SD). With this study, we provide an in-depth flow analysis to explain what physical mechanisms are poorly captured in TRL and SD corrections and offer a potential solution for the improvement of the Prandtl model. Corrections are assessed by comparison with RANS CFD simulations of the NREL 5MW rotor at its design TSR, TSR 4, and TSR 10. The Prandtl tip-loss correction provides 5%–15% higher loading than CFD at TSR 10. The reason is a lower (0.5°) downwash angle of attack, providing a 5%–10% lift coefficient overestimation. We recommend employing the tip-loss correction of Zhong, as it considers the lift coefficient reduction due to the tip-vortex downwash. The investigated Eggers SD correction predicts an incorrect drag coefficient at TSR 4, while a better agreement is found for the lift coefficient. We conclude that only the latter requires a correction for SD in the inboard blade.

Tidal current turbine blade optimisation with improved blade element momentum theory and a non-dominated sorting genetic algorithm

Tidal current energy has the advantage of predictability over most of the other renewable energy resources. However, due to the harsh operating environment and complicated site conditions, developments in this domain have been gradual. Paramount to these points is device design and optimisation of hydrodynamic performance. Recent developments in the correction models of BEM theory have further improved the accuracy of the prediction model. Using an improved blade element momentum theory model that is capable of accurately capturing the downwash angle and combining it with a well-developed and reliable non-dominated sorting genetic algorithm model, an effective and efficient tidal current turbine blade optimisation tool has been developed and is presented in this paper. This novel work incorporated a NACA generator that is capable of reproducing any NACA profile, such a tool allows the solver to analyse each and every profile used in each spanwise blade element. As a result, the model is very effective at producing tidal current turbine blades that have been optimised not only for local twist angle and chord length, but also for the suitable NACA profiles to be used at a particular spanwise blade element. The use of the non-dominated sorting genetic algorithm in this work allows the model to efficiently explore a wide range of solutions, outputting a number of tidal current turbine blades suitable for a specified operating condition. The accuracy of the performance prediction of the improved BEM model is validated against an experimentally validated tidal current turbine blade. The coefficient of determination (R2) values for power and thrust coefficient are 0.99828 and 0.99488 respectively when comparing this work with experimental measurements found in the literature. Furthermore this proves that the improved BEM model is capable of efficiently predicting hydrodynamic performance of a tidal current turbine blade to a high degree of accuracy. Further work includes implementing computational fluid dynamics for further validation and evaluation.

Aeroelastic analysis of an air-to-air refueling hose–drogue system through an efficient novel mathematical model

This paper presents an accurate hose and drogue model for aerial refueling. This model includes the influence of the bending forces on the hose, the downwash angle induced by the tanker-role aircraft wing and the phase lag angle between the aerodynamic forces on the hose and its oscillating motion. A time linearization concerning the hose and drogue static equilibrium position is performed to investigate the aeroelastic stability of the resulting system. Flutter of the time linearized system is computed by the tracking of the roots method. The influence of the different parameters on the flutter characteristics is analyzed. In all cases considered, flutter always occurs by a single mode of oscillation corresponding to the first mode of the hose.

Effect of Piezo-Embedded Inverted Flag in Free Shear Layer Wake

The use of flexible inverted piezo embedded Polyvinylidene Difluoride (PVDF) as a simultaneous energy harvester and as a wake sensor is explored. The oscillation amplitude (characterized by voltage output) and oscillation frequency of the piezo-embedded PDVF was quantified in the wake of a 2D NACA 0012 model and SD7003 model at a Reynolds number of 100,000 and 67,000, respectively. The performance of the sensor was also quantified in the freestream without the presence of the wing. In order to quantify the sensor response to angle of attack and downstream distance, the amplitude and frequency of oscillations were recorded in the wing wake. Increase in angle of attack of the wing resulted in increase in oscillation frequency and amplitude of the PVDF. The results also indicated that the inverted flag configuration performed better in the wake under unsteady conditions when compared to freestream conditions. The results from Particle Image Velocimetry indicated that the wake signature was not affected by the presence of the PVDF in the wake. The root mean square voltage contours in the wake of SD7003 airfoil show remarkable free shear layer wake features such as upper and lower surface stratification and downwash angle which shows the sensitivity of the sensor to the unsteadiness in the wake. The capability of this device to act as a potential energy harvester and as a sensor has serious implications in extending the mission capabilities of small UAVs.

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.

The aerodynamic effects on a cornering Ahmed body

As a vehicle travels through a corner, the flowfield observed from the vehicle׳s frame of reference becomes curved. This condition results in the relative flow angle and freestream velocity changing both across the width and along the length of the body. Wall-resolved Large Eddy Simulations were used to simulate a simple vehicle shape through three different radii corners. The variable flow angle and acceleration affected the pressure distribution along either side of the body and caused an increase in the size of the outboard C-pillar vortex, and an inboard decrease. Furthermore, an outboard extension of the separation bubble at the bluff trailing face resulted in a gentler downwash angle off the backlight surface, with the opposite occurring inboard. At a Reynolds number of 1.7×106, a 19.2% increase in aerodynamic drag occurred for a five car-length radius corner when compared to the straight-line condition. In addition, a yawing moment acted against the rotation of the body through the corner, and a side force acted towards the centre of the corner. An exponential trend related the curvature of a vehicle׳s path to the increase in aerodynamic drag, with a linearity exhibited for the increase in yawing moment and side force.

Research on Modeling of Propeller in a Turboprop Engine

AbstractIn the simulation of engine-propeller integrated control system for a turboprop aircraft, a real-time propeller model with high-accuracy is required. A study is conducted to compare the real-time and precision performance of propeller models based on strip theory and lifting surface theory. The emphasis in modeling by strip theory is focused on three points as follows: First, FLUENT is adopted to calculate the lift and drag coefficients of the propeller. Next, a method to calculate the induced velocity which occurs in the ground rig test is presented. Finally, an approximate method is proposed to obtain the downwash angle of the propeller when the conventional algorithm has no solution. An advanced approximation of the velocities induced by helical horseshoe vortices is applied in the model based on lifting surface theory. This approximate method will reduce computing time and remain good accuracy. Comparison between the two modeling techniques shows that the model based on strip theory which owns more advantage on both real-time and high-accuracy can meet the requirement.

Experimental and numerical investigation of the power-on effect for a propeller-driven UAV

The power-on effect contributes to the performance and the stability of a propeller-driven aircraft. In the present study, the power-on effect for a propeller-driven unmanned aerial vehicle with a pusher propeller has been investigated through the wind tunnel test and the computational fluid dynamics simulation. The positive power-on effect is to increase the maximum lift and delay the stall of the aircraft. However, the most common power-on effect is to produce the slipstream drag and the nose-up pitching moment of the aircraft. The slipstream interaction with the wing and the fuselage increases the fuselage pressure drag leading to an overall increase in aircraft drag. Moreover, the slipstream interaction increases the downwash angle and dynamic pressure in the vicinity of the horizontal tail leads to an overall increase in aircraft nose-up pitching moment. Therefore, the power-on effects tend to reduce the performance and longitudinal stability of the propeller-driven aircraft. The power-on effect is pronounced at slower speeds and higher thrust levels. It is expected that the analysis results presented and discussed in this paper will contribute to the development of the propeller-driven aircraft with the pusher propeller.

Hydrodynamic interaction of roll stabilizing wings

The paper presents an investigation of hydrodynamic interaction of roll stabilizing wings, based on a special vortex system and known experimental results.Introducing a finite equivalent length of free tip vortices behind the leading wing allows to determine the downwash angle and lift force of the following wing over a wide range of aspect ratios and intervals between the wings.An example of applying the obtained results is considered in calculations of a roll damping moment generated by lift forces of the Short Wings Roll Stabilizer (SWIRS), installed on a high-speed ship.

Analytic and Experimental Investigation of Dihedral Configurations of Three-Winglet Planforms

An analytic and experimental effort was undertaken to assess the effectiveness and efficiency of three winglets mounted chordwise to the tip of a rectangular wing. The winglets, with an aspect ratio of 4.6, were mounted on a half-span wing having an effective aspect ratio of 6.29. 13 configurations of varying dihedral arrangements were analyzed with a vortex lattice method and tested in a low-speed wind tunnel at a Reynolds number of 600,000. While the analytic method provided fair agreement with the experimental results, the predicted trends in lift, drag, and (to a lesser degree) pitching moment were in good agreement. The analytic distributions of wake velocity, circulation, and downwash angle verified that highly nonplanar configurations tended to reduce and diffuse the regions of highest circulation and to create more moderate downwash angles in the wake. This was manifest as an overall drag reduction. More specifically, the results showed that the winglets could be placed in various optimum orientations to increase the lift coefficient as much as 65% at the same angle of attack, decrease the drag coefficient as much as 54% at the same lift coefficient, or improve the maximum L∕D by up to 57%. The most dramatic findings from this study show that positioning the winglet dihedral angles had the result of adjusting the magnitude and slope of the pitching moment coefficient. These observations suggest that multiple winglet dihedral variations may be feasible for use as actively controlled surfaces to improve the performance of aircraft at various flight conditions and to “tune” the longitudinal stability characteristics of the configuration.

Aileron and sideslip-induced unsteady aerodynamic modeling for lateral parameter estimation

Aileron inputs and sideslipping motion of an aircraft will give rise to a trailing vortex system that is a function of time. To account for such unsteady aerodynamic effects into the lateral equations of motion, a model is proposed based on a simple vortex system. The expressions for induced sidewash and downwash angles obtained for such vortex system are recast so that the resulting equations of motion can be used for parameter estimation. Maximum likelihood method in frequency domain is used to analyze simulated flight data of an example airplane to study the effects of inclusion and omission of unsteady aerodynamic modeling on estimated parameters. Sensitivity of the extracted parameters to different control input forms is shown to reduce with the inclusion of unsteady aerodynamics.

Two complementary approaches to estimate downwash lag effects from flight data

ALIDATION of the wind-tunnel and analytical estimates of the aerodynamic derivatives with estimates from flight test data is an important application of the system identification methodology. Reliable and accurate estimates of a large number of aerodynamic derivatives are obtained from flight data using the maximum likelihood method, although, routinely, explicit accounting for certain unsteady aerodynamic effects such as downwash and sidewash interferences pose difficulties. Recent advances in both parameter estimation methods and in flight test techniques have provided a new impetus for modeling and identification of such unsteady interference effects in aircraft dynamics.1'3 This Note addresses in some depth the problem of accounting for downwash lag effects in aircraft parameter estimation from flight data. One possibility of including such an effect is to proceed from the fundamental and to model the forces acting on the wing and tail plane separately. In such a case it becomes possible to estimate explicitly the downwash angle and also to include the lag effect. It leads, however, to a model that is nonlinear in parameters and necessarily requires an estimation program capable of handling nonlinear model postulates. The other approach is a simplification based on linearization. It leads to the first-order approximation of aerodynamic derivatives with respect to translational acceleration. The derivatives due to the body-fixed translational acceleration in the vertical direction are equivalent in the stability axis to those with respect to the rate of change of angle of attack. The second approach was first investigated in Ref. 1 to estimate from flight data the pitching moment derivatives with respect to pitch rate and rate of change of angle of attack of a highly maneuver able aircraft with large roll angle capabilities. A parameter estimation program capable of handling linear as well as nonlinear system models is used here to compare the two approaches of accounting for the downwash. Further, an attempt is made to investigate the possibility of using the linearized approach to estimate separately the two pitch damping derivatives from flight tests with a larger aircraft having limited roll angle capabilities.

Influence of downwash on the aerodynamic efficiency of fine-ribbed surfaces

A low-turbulence wind tunnel has been used to make an investigation by the weight method of the influence of downwash of the free stream relative to the direction of ribs on the reduction of the frictional drag of a ribbed surface. It was found that an increase of the downwash angle to 10–15° hardly reduced the efficiency of the ribbing. The effect of a relative bluntness Δ/s of the crests (s is the ribbing step) of the riblets on the frictional drag was considered, and an appreciable reduction of the efficiency of longitudinal ribbing was observed for Δ/s ≈ 0.05.

Numerical solution of the downwash associated with a blown-flap system

For blown-flap aircraft, large downwashes typically occur downstream of the wing due to the high downward momentum of the lifting jet. To determine the velocities induced by the wing- and jet-flap circulation, a threedimensional, nonlinear, finite-element model that can predict downwash angles at any desired location has been developed. Vortex lattices positioned on the wing, flap, and thin jet sheet provide vortex filaments and control points to satisfy kinematic and dynamic boundary conditions. To simplify the mixed-boundary value problem and to reduce computer run time, the rollup of the wake in the spanwise direction has been neglected. In addition, the spanwise distribution of circulation in the wake is assumed to be elliptic in form. A reasonably fast and stable iterative method that allowed a self-consistent wake path to be found was used. Predicted downwash and lift coefficients generally agree with experimental values for various test conditions; but further improvements, such as fuselage modeling, might enhance the accuracy of the model. Nomenclature AH = wing aspect ratio C = airfoil chord length Cy/C=flap chord ratio Cj = blowing coefficient Cr = root chord length CX = velocity induced in the x direction due to a unit vorticity Ff = turning force of jet in radial direction nij =jet mass flow rate over blown flap M = number of chord wise wing lattice divisions S = total wing area S' = equivalent blown wing area V = freestream velocity Vj - velocity of blown jet Vt = average tangential velocity surrounding the blown jet Vx = velocity in the x direction due to the wing lattice Vxw = velocity in the x direction due to the wake lattice Vz = velocity in the z direction due to the wing lattice Vzw = velocity in the z direction due to the wake lattice x = distance in the downstream direction parallel with the freestream

亜音速振動揚力面理論の解法の研究 ＩＩ 改善策の数値的検討

It is the purpose of this Part 2 of the paper to examine the two devices for improvement proposed in Part 1 numerically. We call these two devices Method 1 and Method 2. Downwash angle is calculated of a steady rectangular wing assuming a simple loading. It is shown that Method 1 makes it possible to greatly reduce the computing time in cases of oscillating wings, and that remarkable improvement in convergence with respect to the number of spanwise integration points is obtained by Method 2.

Trimmed drag considerations

The distribution of lift between the wing and tail surfaces of a conventional aircraft is examined in order to determine that which would produce the minimum drag for a given lift. Further, the center of gravity position is determined which gives the desired lift distribution and, at the same time, maintains aircraft trim. Analytical expressions are developed which clearly show the dependence of this optimal e.g. position on the various aerodynamic and geometric parameters. In particular, it is shown how large changes in position of the optimal e.g. position can occur for small changes in the tail downwash angle. Numerical results are presented for a small twin-engine aircraft.

The rolling up of a trailing vortex sheet

The rolling up of the vortex sheet shed from a finite aspect ratio wing into discrete vortices has been extensively investigated in the past. Most of these investigations have, however, considered wings having an elliptic loading.The early design charts of Silvertstein et al gave downwash angles for a variety of flapped and unflapped aerofoils by considering the trailing vortex sheet to be composed of discrete line vortices but ignored the rolling up process that takes place.

Lifting surface theory for V/STOL aircraft in transition and cruise. I

This is the second part of a two-part paper dealing with a large-tilt-angle lifting-surf ace theory for tilt-wing and tilt-propeller (or rotor) type V/STOL aircraft. Part I presented a new inclined actuator disk theory and a model for slipstream swirl. In Part II, the inclined actuator disk analysis is combined with a discrete-vortex Weissinger-type lifting-surf ace theory for application to wing-propeller combinations at arbitrary wing angle of attack, propeller tilt angle, and thrust coefficient. Configurations with one, two, or four slipstreams are considered, and effects of slipstream rotation are included in all but the single-slipstream cases. Agreement between theory and experiment is shown to be satisfactory for small slipstream inclination angles. However, at large tilt angles, the theory (with an undeformed, but displaced, slipstream and wake) is shown to predict lower downwash angles in the tail region than observed from a single set of test data, possibly due to slipstream deformation and wake rollup. Use of only one-half the calculated wake displacement is shown to give improved agreement. Insufficient downwash angle data are currently available for making a general evaluation of the theory at large slipstream angles.

Comments on "Limiting Circulatory Lift of a Wing of Finite Aspect Ratio"

The second point questions the direct application of Eq. (1) to the finite jet-flapped wing. In the derivation of Eq. (1), it is assumed tha t the wing has an elliptic spanwise loading distribution and tha t the trailing vorticity lies in a plane inclined to the free stream at the induced down wash angle calculated at the wing lifting line. But, the ultimate downwash angle far behind the wing will be twice the inclination at the wing. Thus, the effects of the curvature of the trailing vorticity in the stream direction are neglected. However, it is possible to include these effects by considering the forces on a control surface at infinit}^ in the usual manner. A wing, which has an elliptic load distribution given by r = r0(i v A) (2)