Christian Stemmer

Navier-Stokes Simulation of Harmonic Point Disturbances in an Airfoil Boundary Layer

Laminar-turbulenttransitionmechanismsinducedbyaharmonicpointsourcedisturbanceinae at-plateboundary layerwithadversepressuregradientareinvestigated by fourth-orderaccuratespatialdirectnumericalsimulation based on the complete three-dimensional Navier ‐Stokes equations for incompressible e ow. The disturbance is introduced into thetwo-dimensionalbasee owbytime-periodicsimultaneousblowing andsuction within acircular spot at the wall to quietly mimic the momentum input by an active loudspeaker below a hole in the surface in respective experiments. Thus a wave train consisting of pure Tollmien ‐Schlichting waves of a singlefrequency and a large number of obliqueness angles is stimulated, and its downstream evolution in both physical and spectral space is investigated. A breakdown scenario dominated entirely by oblique modes is observed that shows a spanwise peak/valley amplitude splitting with the valley plane at the centerline of the wave train. Dominant vorticity structures develop off centerline, and a clear-cut -shaped structure is formed in e nal stages related to the wall shear being a footprint of a e rst pair of K -vortices in the e ow.

DNS of harmonic poijnt source disturbances in an airfoil boundary layer flow

Laminar-turbulent transition mechanisms induced by a harmonic point source disturbance in a flatplate boundary layer with adverse pressure gradient are investigated by spatial Direct Numerical Simulation (DNS) based on the complete threedimensional Navier-Stokes equations for incompressible flow. A local disturbance is introduced into the two-dimensional (2-D) base flow at the wall by a harmonic point source. Thus TollmienSchlichting waves of a single frequency and a large number of obliqueness angles are stimulated and propagate downstream simultaneously, undergoing amplification by primary and subsequent instabilities, and eventually lead to breakdown of the laminar flow. The results of in-flight experiments, performed by other researchers within a joint research project (DFG-Hochschulgruppe), on a wing glove mounted on a motorglider are compared with the DNS results. The linear and non-linear development of the wave train in the boundary layer is investigated in detail by the spectral amplitude evolution and the vorticity/shear-layer dynamics.

Experimental and numerical investigations of controlled transition in low-speed free flight

This paper reports on the joint work of several university research groups, where scientists of five German universities tackled the problem of controlled laminar-turbulent transition on an unswept motor-glider wing with different low disturbance measurement methods on respective wing-gloves as well as with Direct Numerical Simulations. To establish a comparable measurement environment for the different measurement methods as well as to facilitate the comparison between measurement and numerical results, a controlled continuous disturbance wave train excited by a harmonic point source was introduced into the boundarylayer flow. A successful comparison between the controlled experiment and simultaneous computations is an essential step towards the understanding of more complex transition scenarios like natural transition. The investigation of the downstream development of the three-dimensional point-source disturbance was the main objective of the mutual research effort.  2001 Editions scientifiques et medicales Elsevier SAS boundary layer / laminar-turbulent transition / harmonic point source / in-flight measurements / surface sensors / direct numerical simulation Zusammenfassung Experimentelle und numerische Unsuchung der kontrollierten Transition im Freiflug bei niederen Anstromgeschwindigkeiten. Die vorliegende Arbeit berichtet uber Resultate aus Freiflugexperimenten und Direkter Numerische Simulation im Rahmen eines kooperativen Forschungsprojektes zwischen mehreren Universitaten. Kontrollierte Experimente zum laminar-turbulenten Stromungsumschlag auf dem ungepfeilten Flugelhandschuh eines Motorseglers wurden mit Hilfe verschiedener storungsarmer Oberflachenmestechniken durchgefuhrt und mit Direkter Numerischer Simulation verglichen. Um eine Vergleichbarkeit zwischen den Experimenten und der Simulation zu gewahrleisten, wurde ein kontinuierlicher Wellenzug durch eine harmonische Punktstorquelle in die Grenzschicht eingebracht. Erfolgreiche Vergleiche zwischen den kontrollierten Experimenten und der zeitgleichen Simulationen sind ein unverzichtbarer Schritt auf dem Weg zur Untersuchung und dem Verstandnis komplexerer Umschlagsszenarien wie der freien Transition. Die Untersuchung der Stromabentwicklung des dreidimensionalen Wellenzuges war das Hauptziel des vorgestellten gemeinsamen Forschungsprojektes.  2001 Editions scientifiques et medicales Elsevier SAS

Interference of Wave Trains with Varying Phase Relations in a Decelerated Two-Dimensional Boundary Layer

The investigation of laminar-turbulent boundary-layer transition for adverse pressure gradient flows is of particular interest for the evaluation of transition scenarios with respect to natural transition on an airfoil and the prediction of the transition location. An examination of transition scenarios initiated by a spanwise array of harmonic point sources shall be given in this article. The differences caused by the harmonic excitation of the adjacent point sources with a phase shift forms the major part of the work presented here. Direct numerical simulations are used to investigate the spatial downstream development of the excited wave trains in a two-dimensional boundary layer adapted to the flow present on the wing glove of an experimental sailplane of the TU Berlin.

Investigation of Hypersonic Flat-plate Boundary-layer Transition by Direct Numerical Simulation

Investigations on laminar-turbulent transition for high-speed flows at hypersonic Mach-numbers will be presented. Dissociation takes place above a temperature of T>2000K within the boundary layer, a temperature which is reached easily at Mach-numbers above M=5. Additional degrees of freedom for the energy must be taken into account by employing a vibrational energy equation. Chemical reactions take place which are modeled by a 5-species model proposed by Park [Par89]. Further details on the chemical modeling can be found in [Ste02, Ste03].

Point-source induced transition in free flight

This paper reports on experimental and numerical results, which were obtained during the course of work of the joint university research group “Naturliche Transition”. Main target of the investigations was the downstream development of an artificially induced point source disturbance in a decelerated transitional boundary layer in free flight. Experimental results obtained through in-flight measurements and numerical results from Direct Numerical Simulations of the twodimensional adverse pressure gradient flow over a wing glove on a motorglider are presented and compared qualitatively and quantitatively.

Disturbance Development in an Obstacle Wake in a Reacting Hypersonic Boundary Layer

The presented work is a continuation of the investigation of the influence of an isolated roughness on the laminar-turbulent transition for hypersonic boundary-layer flows. The first part of the in...

Wall Modelling for Implicit Large Eddy Simulation of Favourable and Adverse Pressure Gradient Flows

In order to perform Implicit Large Eddy Simulation (ILES) on complex geometries at high Reynolds numbers, a wall model based on the simplified Thin Boundary Layer Equations (TBLE) is designed in the framework of ILES with a cut-cell finite-volume immersed boundary method. This wall model is validated for turbulent channel flow at friction Reynolds number up to Re τ =2,000 on very coarse grids. The results compared with DNS and LES without wall model show that the wall model has the potential to improve the mean velocity in the outer flow region well at high Reynolds number. The wall model is applied to a complex converging diverging channel flow at Reynolds number Re=7,900 on very coarse meshes. Improved mean velocities and Reynolds stresses are obtained, which shows that the wall model has the ability to perform ILES on complex geometries at high Reynolds numbers.

DNS of Point-Source Induced Transition in an Airfoil Boundary-Layer Flow

Laminar-turbulent transition induced by a harmonic point source disturbance in a flat-plate boundary layer with adverse pressure gradient is investigated by spatial Direct Numerical Simulation (DNS) based on the complete three-dimensional Navier—Stokes equations for incompressible flow. A local disturbance is introduced into the two-dimensional (2-D) base flow at the wall by a harmonic point source. Thus Tollmien—Schlichting waves of a single frequency and a large number of obliqueness angles are stimulated and propagate downstream simultaneously, undergoing amplification by primary and subsequent instabilities, and eventually lead to breakdown of the laminar flow. The development of the wave train in the boundary layer is investigated by the spectral amplitude evolution and the vorticity/shearlayer dynamics. The computational aspects of this LAMTUR project are discussed in detail for runs on the NEC SX-4 and the CRAY T3E supercomputers.

Hypersonic Boundary-Layer Flow with an Obstacle in Thermochemical Equilibrium and Nonequilibrium

The hypersonic flow around a tip of a rocket-mounted Hypersonic Boundary-Layer Transition experiment has been simulated with direct numerical simulations. The numerical results reveal the local flo...