Rolf Radespiel

Vortex Generation in a Low Speed Wind Tunnel and Vortex Interactions With a High-Lift Airfoil

This paper describes the design, construction and evaluation of a vortex generator in a closed wind tunnel, with the objective to produce well defined disturbances convecting over a high-lift airfoil. The vortex generator produces quasi two-dimensional vortices in transverse direction. The design criteria, the mode of use and the framework of the vortex generator are described. Furthermore, the produced vortices and their propagation are characterized along an otherwise empty test-section as well as their interaction with a high-lift airfoil. To acquire the presented data, several measurement methods, such as Particle Image Velocimetry (PIV), a newly developed time resolving Five Hole Probe (FHP), monitoring the position of the servo-actuators and time resolving as well as static pressure measurements were used. As a result, the vortex generator’s ability to create reproducible single vortices, as well as reproducible series of vortices, has been verified.

Separation Control on a High-Lift Airfoil using Vortex Generator Jets at High Reynolds numbers

This paper describes an active flow control wind tunnel experiment with a 2D two element high-lift airfoil at high Reynolds numbers of up to Re = 12:2 10 6 and for a corresponding Mach number of M = 0:2. Additional test cases with variations of the Reynolds and Mach number were also conducted. The measurements allow to study Reynolds and Mach number scaling effects. The wind tunnel experiments were performed in the cryogenic test facility DNW-KKK at Cologne, Germany. The main wing is equipped with vortex generator jets which are located close to the leading edge on the pressure side. The objective of the active flow control system is to suppress or delay a turbulent leading edge stall. The experiments clearly show that the active flow control system can prevent stall but the increase of the maximum lift coefficient decreases the higher the Reynolds number is. The benefits primarily depend on the flow rate. In most cases dynamic blowing is superior to static blowing. The actuation frequency seems to be more efficient with higher value. In range of the tested active flow control parameters the Mach number has a small influence.

Experimental and Numerical Investigations on the Control of Airfoil Stall using Vortex Generator Jets

This contribution discusses the implementation of active flow control with steady and pulsed vortex generator jets in a (nominally) 2D two-element high-lift airfoil to increase the maximum lift of such a configuration. The vortex generator jets are applied at the leading-edge of the airfoil to prevent a turbulent leading-edge stall-type. Experimental investigations were made with a medium-scale, medium-Re configuration in two different wind tunnels, the atmospheric tunnels DNW-NWB in Braunschweig and DNW-KKK in

Numerical Simulations of Streamwise Vortices on a Generic High-Lift Configuration

The results of numerical simulations on a generic high-lift configuration are shown. The high-lift airfoil features a finite slat, where the slat-end creates strong longitudinal vortices interacting with the flow along the suction side. For the simulations, two computational grids with varying density are created. The properties of the grids are presented. The simulations are performed with the DLR TAU-Code at different angles of attack up to stall. The Menter-SST eddy viscosity turbulence model and the JHh-v2 Reynolds-Stress-Model are applied. Two vortices exist at the spanwise end faces of a slat and the corresponding clean-nose, they trail downstream on the suction side of the wing and influence the high-lift behavior of the configuration. A strong interaction between both vortices is observed for higher angles of attack. The behavior of the vortex system and the stall mechanism is characterized. In particular the effect of the turbulence models of different types on the vortex behavior is shown.

Vortex interactions with a high-lift airfoil in a low speed wind tunnel

This publication describes experimental investigations on the effects of large scale disturbances on a two element airfoil. Quasi two-dimensional transversal vortices are created in a closed test-section shortly after the nozzle. Convecting along the test section, these vortices interact with the two-element airfoil in high lift configuration. Various measurement techniques, like Particle Image Velocimetry (PIV), time resolving static pressure measurements, time averaged measurements of the static pressure distribution of the high-lift airfoil and oilflow visualizations were utilized to identify and characterize these complex interactions. As a result, the influence of the induced vortices on the time resolving static pressure along the airfoil’s surface is described. Moreover the vortex interaction with the airfoil flap flow is discussed.

Flow structure and unsteadiness in the supersonic wake of a generic space launcher

The wake flow of a generic axisymmetric space-launcher model with and without propulsive jet is investigated. Measurements are performed at Mach 2.9 and a Reynolds number ReD = 1.3 · 10 based on model diameter D, and the nozzle exit velocity of the jet is at Mach 2.5. Velocity measurements in the wake flow by means of Particle Image Velocimetry and mean and unsteady wall-pressure measurements on the main-body base are performed simultaneously. This way, the evolution of the wake flow was observed along with its spectral content. In this paper, the mean flow topology and turbulence behavior of the wake is described based on PIV measurements, and the influence of the afterexpanding jet plume is discussed. For the case without propulsive jet, a large separated zone is forming downstream of the main body shoulder and the flow is reattaching downstream on the nozzle fairing. Under the influence of the jet plume, the separated region is increased and reattachment does not occur anymore. The flow is displaced away from the wall. The jet plume appears to have a stabilizing effect on the wake flow. The development of the shear layer and the magnitude of the turbulent intensities are damped, which we showed with the axial and radial components of the turbulent velocity fluctuations, as well the Reynolds shear stress evolution along the launcher afterbody.

Hypersonic PIV in a Ludwieg Tube Wind Tunnel at Mach 5.9

The present work describes particle image velocimetry at M = 5.9 in the Hypersonic Ludwieg Tube Braunschweig on a generic space launcher model. The wind tunnel allows high quality flows as well as high repetition rates and, thus, it is suited for PIV-measurements. The PIV-measurements were performed with oil based tracer particles. Two different aerosol facilities, which differ basically in the position of the aerosol inlet, are described and compared. The first aerosol facility is close to the Laval nozzle whereas the second one is at the begin of the storage tube. The influences of both aerosol facilities to the quality of the PIV-measurements as well as to the safety of the wind tunnel are discussed. The results show that oil based PIV-measurements in a hypersonic blow down facility with a heated and pressurized storage tube are possible. The first aerosol facility close to the Laval nozzle results in an inhomogeneous tracer particle distribution with a low repeatability from run to run. The second aerosol facility at the begin of the storage tube enhances the distribution of the tracer particles inside the storage tube. The resulting tracer particle distribution is homogeneous as well as reproducible but the number of the tracer particles is reduced.

Transient, Three-Dimensional Disturbances Interacting with a High-Lift Airfoil - Wind Tunnel Experiments

This publication presents experimental investigations on the aerodynamics of a highlift airfoil in disturbed inflow. Transient, three-dimensional disturbances are generated in a closed wind tunnel test-section. These disturbances are created by means of two vortex generating airfoils. The first creates quasi two-dimensional, transient, transversal vortices whereas the second airfoil generates a longitudinal wingtip vortex. Convecting along the test-section, these vortices interact with each other and with a two-element airfoil in high-lift configuration. Various measurement techniques, like Particle Image Velocimetry (PIV), time resolved static pressure measurements, measurements of the static pressure distribution of the high-lift airfoil, and time resolved Five-Hole-Probe (FHP) measurements of the induced disturbances are used to identify and characterize these interactions. These measurements are complemented by qualitative surface oilflow visualizations of the highlift airfoil. It is found that the interaction of the vortices with the high-lift airfoil exhibits a varying complexity with changing angle of attack. At zero incidence, the effective flow angles induced by the vortices prevail. For higher angles of attack, complex interaction effects can be observed in the time-resolving static pressure signals. By phase-locked PIV measurements, it can be shown that the flow field at the high-lift flap for small incidences is governed by the circulation effect of the main element rather than by the velocity effect of the passing vortices.

Sparse Model of the Lift Gains of a Circulation Control Wing with Unsteady Coanda Blowing

The present study investigates and models the lift gains and losses generated by the superposition of a periodic actuation component onto a steady component on an airfoil with a highly deflected Coanda flap. The periodic actuation is provided by two synchronized specially-designed valves that deliver actuation frequencies up to 30 Hz and actuation amplitudes up to 20% of the mean blowing intensity. The lift gains/losses response surface is modeled using a data-driven sparse identification approach. The results clearly demonstrate the benefits of superimposing a periodic component onto the steady actuation component for a separated or partially-attached flow, where up to \(\varDelta C_l=0.47\) lift increase is achieved. On the other hand, this same superimposition for an attached flow is detrimental to the lift, with up to \(\varDelta C_l=-0.3\) lift reduction compared to steady actuation with similar blowing intensity is observed.

A Generalized Reduced-Order Model of Flow around an Airfoil with Circulation Control

A low-dimensional generalized model is proposed for a flow around an airfoil with circulation control, describing natural vortex shedding and steady actuation. The suggested application range of the model extends from the un-actuated natural state to the strongly actuated near-steady state. The form of the dynamical system has been based on generalized mean-field consideration. Time resolved PIV snapshots are employed to derive the POD time coefficients and to calibrate the system parameters. The model is solely dependent on the actuation blowing momentum coefficient, and makes a good candidate for flow control applications.