Christian Breitsamter

Turbulent flow structure associated with vortex-induced fin buffeting

Selected results from an extensive experimental investigation showing the flowfield in the fin region of a deltacanard-configuration are presented. Results obtained include detailed flowfields of the time-dependent velocity components for various angles of attack (0-31.5 deg) at a test Reynolds number of 1.0 x 10 6 using hot-wire anemometry. The structure of the highly turbulent vortex dominated flow is clearly shown by time-averaged, root-mean-square (rms) and spectral distributions. Thus, strong interference effects between the canard and the wing vortex systems around the fin region are found. With increasing incidence the wing and the canard leading-edge vortices move inboard resulting in an increase of the velocity fluctuations in the area of the midsection. The power spectral density depicts significant frequency peaks related to the concentration of kinetic turbulent energy in the flow of the wing/canard vortex sheet. At high incidences the narrow-band turbulent energy induced by the vortex sheet affects the plane of symmetry because of the shift of the leading-edge vortex. This substantiates that fin oscillations at high-a are excited by narrow-band fluctuations due to vortical flow conditions.

Numerical and Experimental Analysis of a Generic Fan-in-Wing Configuration

The present investigation focuses on assessing the predictive capabilities of state-of-the-art computational fluid dynamics for a generic fan-in-wing configuration by comparison with experimental data. The objective is to reproduce a short take off and landing or a transition-flight situation without ground effect. A rotating fan is placed in the wing plane, inside of the wings rear part and close to the root section. The obtained experimental data include force measurements, surface pressure measurements, flowfield mapping using particle image velocimetry and wool-tufts visualization. A structured mesh of the entire configuration with minimum geometrical simplifications is used to perform unsteady Reynolds-averaged Navier-Stokes computations. The area surrounding the rotor blades is set up as sliding mesh to simulate the rotation. Because of the abrupt deflection of the flow by the fan, an unsteady recirculation area is generated above the rotor blades resulting in a highly distorted inflow. The blocking of the freestream by the jet exiting the fans nozzle creates a back pressure affecting the internal aerodynamics. The jet rolls up in a counter-rotating vortex pair with considerable impact on the wings aerodynamic performance. Time-averaged unsteady results over one fan revolution show a good agreement with the experimental data. The major turbulence phenomena are well predicted by the simulation.

Numerical simulation of the lateral aerodynamics of an orbital stage at stage separation flow conditions

Abstract The unsteady flow of an idealized two-stage hypersonic vehicle during the separation process is investigated. In particular, the aerodynamic characteristics of the orbital stage performing yaw and roll oscillations with defined frequencies at a certain distance above the lower stage are addressed. The numerical simulations of the flowfield are based on an explicit finite-volume method for solutions of the three-dimensional unsteady Euler equations. The discretization of the computational domain around the entire configuration is performed using a flexible multiblock grid generator. The results focus on the flowfields and the pressure distributions as well as on the aerodynamic coefficients. The latter provide an essential mean for the stability and control evaluations for lateral motion in hypersonic flight. The analysis show that unsteadiness cannot be neglected.

Lateral-Directional Coupling and Unsteady Aerodynamic Effects of Hypersonic Vehicles

Results from a joint research program of the Institutes of Fluid Dynamics and of Flight Mechanics and Flight Control of the Technical University of Munich on a two-stage space transportation system are presented. The unsteady aerodynamics resulting from an orbital stage at stage separation e ow conditions at Mach number 6.8 are e rst discussed. Unsteady e owe elds associated with yaw and roll oscillations of the orbital stage at a certain distanceabovethecarrierstageareinvestigated.Thecalculationsarebased ona e nitevolumemethodforreal-time solutions of the unsteady Euler equations. Theresultsfocus on pressuredistributions and aerodynamic coefe cients providing an essentialmeansforcarrying outstabilityand control investigations.Inthesecond part, e ightmechanics stability and control problems are addressed considering key issues of lateral ‐directional dynamics. Inherent vehicle characteristics can show specie c stability and e ying quality dee ciencies in hypersonic e ight, concerning a partially unstable dutch roll mode with high roll ‐yaw coupling and weak roll damping. There is also a coupling of the roll and spiral poles for certain cone gurations, yielding a slow, eventually unstable oscillation called lateral phugoid. Reasons and effects for the stability dee ciencies are discussed, including conditions of existence for the lateral phugoid.

Leading-Edge Roughness Effects on the Flow Separation Onset of the AVT-183 Diamond Wing Configuration (Invited)

Experimental investigations on a 53◦ leading-edge sweep diamond wing configuration with rounded leading-edge contour as part of the NATO Science and Technology Organization (STO) task group AVT-183 (Applied Vehicle Technology panel) are presented. The results obtained in a low speed wind tunnel facility include aerodynamic forces and moments and steady surface pressures. Special emphasis is laid on the effects of different leading-edge roughness, which is applied in the experimental analyses to ensure turbulent boundary layer characteristics on the entire diamond wing surface. The results show that the flow separation onset and the emerging leading-edge vortex are very sensitive to the roughness height applied to the diamond wing leading-edge. Both reasonably-tripped and over-tripped cases are obtained compared to the free transition case, which is discussed in detail. Moreover, comprehensive analyses on both the short term and the long term repeatability of the related cases are presented. Based on this analysis, one specific leadingedge roughness is defined as the target flow case, which provides the baseline for the flow field investigations and general CFD validation within the task group AVT-183.

Fin Buffet Pressure Evaluation Based on Measured Flowfield Velocities

A method to predict fin buffet aerodynamic loads is developed based on measured fluctuating velocity fields only. This has been investigated for a model of a high-performance, single-finned, delta-canard configuration. The time-dependent velocity field is measured in the fin region, resulting in a detailed description of the properties of the buffet-inducing quantities. A modified lifting surface method is used to evaluate the unsteady fin surface pressures. It is based on the amplitude spectra of local fin incidence calculated from the measured velocities. Consequently, rms and spectral densities of fluctuating surface pressure and normal force are obtained

Neurofuzzy-Model-Based Unsteady Aerodynamic Computations Across Varying Freestream Conditions

This paper presents a reduced-order modeling approach based on recurrent local linear neurofuzzy models for predicting generalized aerodynamic forces in the time domain. Regarding aeroelastic applications, the unsteady aerodynamic loads are modeled as a nonlinear function of structural eigenmode-based disturbances. In contrast to established aerodynamic input/output model approaches trained by high-fidelity flow simulations, the Mach number is considered as an additional model input to account for varying freestream conditions. To train the relationship between the input parameters and the corresponding flow-induced forces, the local linear model tree algorithm is adopted in this work. The proposed method is tested exemplarily with respect to the AGARD 445.6 configuration in the subsonic, transonic, and supersonic flight regimes. It is shown that good conformity is obtained between the reduced-order model results and the respective full-order computational-fluid-dynamics solution. A further comparative an...

Numerical and Theoretical Considerations for the Design of the AVT-183 Diamond-Wing Experimental Investigations

A diamond-wing configuration has been developed to isolate and study blunt-leadingedge vortex separation with both computations and experiments. The wing has been designed so that the results are relevant to a more complex Uninhabited Combat Air Vehicle concept known as SACCON. The numerical and theoretical development process for this diamond wing is presented, including a view toward planned wind tunnel experiments. This work was conducted under the NATO Science and Technology Organization, Applied Vehicle Technology panel. All information is in the public domain.

Vortical Flowfield Structure at Forward Swept-Wing Configurations

Extensive aerodynamic investigations have been carried out on forward swept-wing cone gurations with a wing sweep of i 40 deg. A generic wing-body model with removable aft swept canards is used to measure the instantaneous velocities in several crosse ow planes applying advanced hot-wire anemometry. The tests were made at 10-, 20-, and 30-deg angle of attack at a Reynolds number of 0 :46 £ 10 6 . Detailed surveys of mean and rms velocities show that at moderate angles of attack strong wing leading-edge vortices aregenerated rotating opposite in a sense to the wing tip vortices. At higher incidences trailing-edge vortices are shed at the inner-wing part with the same sense of rotation as the wing tip vortices. The canard vortices pass the wing leading-edge relatively high, and after theonset of wing vortices they are moved upward and outboard. Theinterferencebetween these vortices is studied in detail by analyzing the associated turbulent e ow structure. Vortex bursting over the wing occurs already at moderate angles of attack. Downstream, the highest turbulence intensities are found in an annular region around the burst vortex core, where e uctuations are signie cantly channeled into a narrow band.

Aerodynamic Active Control For Fin-Buffet Load Alleviation

The efficiency of an active auxiliary rudder system in alleviating fin buffeting of modern fighter aircraft is investigated. Low-speed wind-tunnel tests are performed on a detailed 1/15-scale model of canard-delta-wing type. A digitally controlled auxiliary rudder is installed on a specific fin model providing harmonic oscillations at varying frequency and deflection angle. The vertical tail is instrumented to measure unsteady surface pressures, fin-tip accelerations, and auxiliary rudder moments. At open-loop tests the fin unsteady pressure field is fed with energy at the frequencies of the auxiliary rudder motions. The corresponding rms values increase with increasing frequency and deflection angle at all angles of attack tested. The motion-induced rms pressures reach values well above the levels of the nonoscillating case. Thus, a potential to reduce rms buffet loads by approximately 18% for closed-loop operations exists. There is no decrease in the rudder moment with increasing angle of attack substantiating the effectiveness of the auxiliary rudder concept also for high angles of attack. The active control system uses single-input single-output control laws to alleviate buffeting in the fin first bending and torsion mode, respectively. With active control the spectral density peaks of fin-tip accelerations at the frequencies of the considered eigenmodes can be reduced by as much as 60% at angles of attack up to 31 deg.