Wake Distortion Research Articles

Aerodynamic Modeling and System Identification from Flight Data-Recent Applications at DLR

This paper provides an overview of a few selected system identification activities carried out during the last decade at DLR, the German Aerospace Center. After a brief account of parameter estimation methods and of consistency checking of recorded flight data, an emphasis is placed on: i) development of global, high-fidelity aerodynamic databases for training simulators, ii) models for in-flight simulator iii) modeling of stall hysteresis, iv) estimation of aerodynamic flow field characteristics and validation of aircraft reaction models from wake vortex encounter flights, and v) a parametric wake distortion model to improve helicopter coupling off-axis response. These examples cover a variety of flight vehicles with widely different characteristics, namely the military transport aircraft Transall C-160, commuter aircraft Dornier 328, in-flight simulator VFW-614 ATTAS, high altitude research aircraft G850 Strato 2C, Airbus type aircraft, fly-by-wire helicopter BO-105, and fly-by-light helicopter EC-135. Besides parameter estimation aspects, those related to model validation are discussed. Through succinct examples, it is brought out that system identification has become an indispensable tool to support not only research but also industry activities in various key areas such as model validation, handling qualities evaluation, control law design, and flight vehicle design and certification. Thus, it contributes significantly to risk and cost reduction.

Helicopter Flight Dynamic Simulation with Refined Aerodynamics and Flexible Blade Modeling

Acoupled rotor ‐fuselagee ight dynamicmodel thatincludesa maneuveringfreewakemodel and acouplede ap ‐ lag‐torsione exibleblademodel,aswellastheresultsofaninvestigationonsomeeffectsofine owandblademodeling onthefree-e ightresponseofahingelessrotorhelicoptertopilotinputs,aredescribed.Thewakemodelisarelaxation typefreewake, capableofmodelingthewakegeometry changesdueto maneuvers; no assumptionsaremadeonthe wakegeometry,whichisfreetoevolvebased on themaneuver.Theoreticalpredictionsarecomparedwith e ight-test data. The results show that the free-e ight, on-axis response to pilot pitch and roll inputs can be predicted with good accuracy with a relatively unsophisticated model. It is possible to predict the off-axis response from e rst principles, that is, without empirically derived correction factors and without assumptions on the wake geometry. To do so, however, requires sophisticated modeling. Both a free wake model that includes the wake distortions caused by the maneuver and a ree ned e exible blade model must be used. Most features of the off-axis response can be captured using a dynamic ine ow theory extended to account for maneuver-induced wake distortions, for a fraction of the cost of using a free wake model. The most cost-effective strategy, for typical e ight dynamic analyses and if vibratory loads are not required, is probably to calibrate such a theory using the more accurate free wake based model, and then use it in all calculations.

Investigation of Unsteady Aerodynamic Blade Excitation Mechanisms in a Transonic Turbine Stage—Part I: Phenomenological Identification and Classification

Based on the results of time-dependent 3-D viscous computations the aerodynamic mechanisms that cause the unsteady pressure fluctuations on the vane and rotor blade surface of a high-pressure transonic turbine are identified and separately classified in a phenomenological manner. In order to be able to describe separately the influence of wake, potential and shock distortions on the blade surface pressure at design operation conditions, the stator exit Mach number is increased as to enhance the shock distortions and lowered as to enhance potential and wake distortions. In a comprehensive approach the observations from the off-design conditions are utilized to classify every major perturbation observed in the perturbation space-time maps at design operation conditions. The spanwise variations caused by the inherent 3-D nature of the flow field and promoted by the 3-D shape of the rotor blade are addressed.

On aerodynamic modelling of an insect–like flapping wing in hover for micro air vehicles

This theoretical paper discusses recent advances in the fluid dynamics of insect and micro air vehicle (MAV) flight and considers theoretical analyses necessary for their future development. The main purpose is to propose a new conceptual framework and, within this framework, two analytic approaches to aerodynamic modelling of an insect-like flapping wing in hover in the context of MAVs. The motion involved is periodic and is composed of two half-cycles (downstroke and upstroke) which, in hover, are mirror images of each other. The downstroke begins with the wing in the uppermost and rearmost position and then sweeps forward while pitching up and plunging down. At the end of the half-cycle, the wing flips, so that the leading edge points backwards and the wing's lower surface becomes its upper side. The upstroke then follows by mirroring the downstroke kinematics and executing them in the opposite direction. Phenomenologically, the interpretation of the flow dynamics involved, and adopted here, is based on recent experimental evidence obtained by biologists from insect flight and related mechanical models. It is assumed that the flow is incompressible, has low Reynolds number and is laminar, and that two factors dominate: (i) forces generated by the bound leading-edge vortex, which models flow separation; and (ii) forces due to the attached part of the flow generated by the periodic pitching, plunging and sweeping. The first of these resembles the analogous phenomenon observed on sharp-edged delta wings and is treated as such. The second contribution is similar to the unsteady aerodynamics of attached flow on helicopter rotor blades and is interpreted accordingly. Theoretically, the fluid dynamic description is based on: (i) the superposition of the unsteady contributions of wing pitching, plunging and sweeping; and (ii) adding corrections due to the bound leading-edge vortex and wake distortion. Viscosity is accounted for indirectly by imposing the Kutta condition on the trailing edge and including the influence of the vortical structure on the leading edge. Mathematically, two analytic approaches are proposed. The first derives all the quantities of interest from the notion of circulation and leads to tractable integral equations. This is an application of the von Kármán-Sears unsteady wing theory and its nonlinear extensions due to McCune and Tavares; the latter can account for the bound leading-edge vortex and wake distortion. The second approach uses the velocity potential as the central concept and leads to relatively simple ordinary differential equations. It is a combination of two techniques: (i) unsteady aerodynamic modelling of attached flow on helicopter rotor blades; and (ii) Polhamus's leading-edge suction analogy. The first of these involves both frequency-domain (Theodorsen style) and time-domain (indicial) methods, including the effects of wing sweeping and returning wake. The second is a nonlinear correction accounting for the bound leading-edge vortex. Connections of the proposed framework with control engineering and aeroelasticity are pointed out.

An Investigation of Helicopter Dynamic Coupling Using an Analytical Model

Many attempts have been made in recent years to predict the off-axis response of a helicopter to control inputs, and most have had little success. Since physical insight is limited by the complexity of numerical simulation models, this paper examines the off-axis response problem using an analytical model, with the goal of understanding the mechanics of the coupling. A new induced velocity model is extended to include the effects of wake distortion from pitch rate. It is shown that the inclusion of these results in a significant change in the lateral flap response to a steady pitch rate. The proposed inflow model is coupled with the full rotor/body dynamics, and comparisons are made between the model and flight test data for a UH-60 in hover. Results show that inclusion of induced velocity variations due to shaft rate improves correlation in the pitch response to lateral cycle inputs.

Aerodynamic interaction study of the propeller/wing under different flow configurations

Interactional aerodynamics could significantly affect the rotary wing performance, the airframe loads, as well as the handling qualities of aircraft. It has become necessary for the nature of the vortical slipstream and its interference with aircraft components to be understood over a wide range of flight operating conditions. The propeller/wing aerodynamic interaction phenomena have been experimentally investigated at low subsonic speeds on a propeller/nacelle/half-wing configuration (P/N/W2). The P/N/W2 ensemble has been studied under different wind-tunnel flow configurations to simulate different conditions of the whole flight envelope including axial, conversion/reconversion, and hovering flights. The experimental investigation has been based on extensive overall and local data measurements performed on the propeller, the wake geometry, and the half-wing. The results exhibited the considerable influence of the flow configuration and the tilt angle parameter on the nature of the wake distortion due to the half-wing. It is also shown that such tip vortex trajectories distortions strongly affect both the instantaneous and mean lift and drag distributions along the half-wing span.

3D Structure of Vortical Flow about a Stern of a Full Ship

A finite-volume method with the curvilinear grid system fitted to the hull surface is applied to the elucidation of the 3 D structure of the flow about a full hull form. After examining the grid effects with two different systems of which number of grid points are 1 × 105 and 4 × 105, the simulated flow field is carefully examined using a variety of drawings of the simulated flow. It is demonstrated that the grid dependency can not be fully removed with these number of grid points and that the Reynolds stress by the simulation with the finer grid system qualitatively agrees with the measurement. It is revealed that the stern flow about a full hull form is composed of complicated 3 D vortices. Both spanwise and streamwise vortices are included in the bilge vortices and the former plays an important role for the generation of the wake distortions.

Simplification of nonlinear indicial response models - Assessment for the two-dimensional airfoil case

Simplifications to the functional form of the nonlinear indicial response model and application to the aerodynamic response due to arbitrary motion inputs are discussed. Numerical results for a thin two-dimensional airfoil with wake distortion nonlinearities are used to control and justify the assumptions employed. Useful relationships between indicial response parameters and steady-state oscillatory force and moment data are obtained by representing the indicial response with a Taylor series and approximating the superposition integral with an asymptotic expansion

Inviscid modeling of turbomachinery wake transport

The problem of unsteady wake-airfoil interaction and wake transport is of importance in nearly all fluid mechanical devices involving rotating machinery. In this study, an inviscid airfoil-to-airfoil potential flow analysis is used to predict the transport of the wakes of upstream stators through both rotating compressor and turbine airfoil passages. In the analysis, the wakes are assumed to have an infinitesimal velocity defect and are represented by inviscid flow streamlines. The dependence of wake rotation on airfoil lift and the dependence of wake distortion on airfoil geometry are both shown. Flow visualization results obtained in both a large-scale compressor and a large-scale turbine show good comparison with the predictions, of the inviscid model.

Study on Propulsion by Partially Elastic Oscillating Foil (2 nd Report)

In a previous paper, the authors analyzed partially elastic oscillating foils by a linearized theory and idenfied its various characteristics.In this paper, the authors propose here a nonlinear theory using singularity distribution method. In this theory, Hess & Smith's method is extended in order to treat unsteady foil motion and wake distortion is taken into account by discrete vortices. In the case of partially elastic oscillating foil, the solution is obtained by satisfying also the equation of motion which can be derived from the equation of elastic curve.The results obtained by the present theory was found to be in good agreement with experimental ones using a large amplitude rigid oscillating foil. From this fact, it can be said that this calculation method is effective for quantitative analyses of oscillating foil propulsors.Some simple oscillating foil propulsors were designed for practical application to ships. The results showed that adequate thrust force can be achieved and efficiencies as high as 65% can be envisaged in case of elastic foils.