Justin and Shem present their thesis seminars

Last Friday, Justin Tang and She-Ming (Shem) Lau-Chapdelaine presented their thesis seminars, taking one step closer to finishing their degrees.  Congrats to both students and to their thesis supervisor, Dr. Matei Radulescu.

She-Ming Lau-Chapdelaine’s presentation can be seen here.  Justin Tang’s presentation can be seen here.

Click on the photos for larger images.

26th October: graduate seminar double header!

Coming up next week:  a graduate seminar double header!  Presenting next week , Justin Tang will talk about “”Non-linear dynamics and route to chaos in Fickett’s detonation analogue”.   Also presenting is S. She-Ming Lau-Chapdelaine on the subject of “Numerical Simulations of Detonation Re-initiation following Mach Reflection ”

The seminar abstracts follow below.

Date: Friday  26th October

Time: 2:30pm

Room: SITE B0138

Non-linear dynamics and route to chaos in Fickett’s detonation analogue

Justin Tang

Abstract

The present study investigates the spatiotemporal variability in the dynamics of self-sustained supersonic reaction waves propagating through an excitable medium. The model is an extension of Fickett’s detonation model with a state-dependent energy addition term. Stable and pulsating supersonic waves are predicted. With increasing sensitivity of the reaction rate, the reaction wave transits from steady propagation to stable limit cycles and eventually to chaos through the classical Feigenbaum route. The physical pulsation mechanism is explained by the coherence between internal wave motion and energy release. The results obtained clarify the physical origin of detonation wave instability in chemical detonations previously observed experimentally.

Numerical Simulations of Detonation Re-initiation following Mach Reflection

S. She-Ming Lau-Chapdelaine

Abstract

Rapid chemical reactions within the detonation structure periodically create over-driven detonations. Small time and size scales involved in detonations make it difficult to determine the cause of these rapid chemical reactions.

A detonation which diffracts over an obstacle is weakened by expansion of the gas. The lowered temperature behind the weakened shock wave increases the ignition time of combustion. This can lead to the decoupling of the shock and reaction fronts, essentially quenching the detonation. The detonation can be re-initiated downstream of the obstacle upon reflection off walls or planes of symmetry. This experimental setup is similar to the magnification of the detonation structure. This re-initiation phenomenon , the creation of an over-driven detonation, has previously been attributed to five possible causes of rapid reaction: shock compression from the incident and reflected shocks, shock compression from the Mach stem of the reflection, or turbulent mixing of burnt gas with the shocked, yet unburnt, gas through Kelvin-Helmholtz or Richtmyer-Meshkov instabilities or the wall-jetting effect. The precise cause of this phenomenon has, however, remained elusive and difficult to predict.

This study develops a numerical model capable of predicting detonation re-initiation events. This is done by solving the two-dimensional reactive Euler equations with one-step Arrhenius chemistry. The model is calibrated with a real chemistry model by replicating the post-shock conditions. The shock compression of the Mach stem and wall-jetting effect are found to play important roles in detonation re-initiation. The pressure required for re-initiation is accurately predicted and the flow-field is found to be in good agreement with experimental results with one exception: the reflected (transverse) detonation (“super detonation”) is not reproduced in the simulations.

JL presents and a thankyou for James

Last Friday, Jean-Louis presented his MASc thesis seminar in the presence of his grad student colleagues and supervisor Dr. Jodoin.  Afterwards, the grad students had a little social gathering and also presented a lovely parting gift to James, who is leaving us to go on to other adventures.

Click on the photos for larger images.

Special Guest Seminar 19th October

Next week (October 19th at 2:30pm in SITE B0138) we have a special guest speaker who will be giving a talk.  Sina Ghaemi from the Department of Aerodynamics, Delft University of Technology, will be giving a talk on the kinematics and dynamics of turbulant trailing-edge flow.

Kinematics and dynamics of turbulent trailing-edge flow

Sina Ghaemi
Department of Aerodynamics, Delft University of Technology,
2629 HS, Delft, The Netherlands, s.ghaemi@tudelft.nl

Abstract 

The prominent challenge of the trailing-edge noise of aircrafts is associated to the acoustic emissions of a large range of pressure fluctuations within the adjacent turbulent flow. In this study, the source of the trailing-edge noise is tackled through (a) identification of the coherent structures of the turbulent flow at a sharp symmetric trailing-edge followed by (b) the evaluation of PIV-based pressure technique to (c) characterize one of the relevant acoustic sources known as the high-amplitude pressure fluctuations.

The evolution of the turbulent boundary layer into the subsequent wake at the trailing-edge of NACA0012 airfoil is investigated using tomographic particle image velocimetry (Tomo-PIV). The high-speed streaks are observed to persist over a longer distance than the low-speed streaks within the wake region due to the vortical activities introduced here as “counter-hairpin” vortices. These vortices exhibit opposite features relative to the typical hairpin vortices of the turbulent boundary layer.

Identification of the acoustic sources requires access to the unsteady pressure field within the turbulent flow. In order to evaluate the pressure field, time-resolved three-dimensional velocity measurement using Tomo-PIV is apply in combination with the Poisson pressure equation. The pressure field is obtained within a turbulent boundary layer as a benchmark flow and is locally validated against surface pressure measurement using electret microphones.

Finally, the high-amplitude pressure fluctuations within the turbulent boundary layer are investigated as the major acoustic source contributing to the trailing-edge noise. Three-dimensional measurement of the velocity field along with the evaluated pressure field demonstrated the correspondence of the positive and negative high-amplitude pressure peaks to the shear layers and vortical structures of the turbulent boundary layer, respectively. The results point out possible control strategies for trailing-edge noise attenuations.

October 12 Seminar

Jean-Louis Pelletier will be presenting his thesis seminar today.  The abstract of his talk follows.

Study of Titanium Alloy Coating for Aviation Parts Repair Using Low Pressure Cold Spray and Pulsed Gas Dynamic Spray

Supervisor : Dr. B. Jodoin

Abstract

The objective of this study is to successfully repair Ti-6Al-4V substrates by depositing Ti-6Al-4V layers using two new commercially available Cold Spray processes such as Low Pressure Cold Spray (LPCS) and Pulsed Gas Dynamic Spray (PGDS). The examination of both feedstock powder and coatings were performed by different techniques such as Optical Microscopy (OP) and Scanning Electron Microscopy (SEM).

Porosity, hardness, adhesion strength, wipe test, fracture surface and wear test have been evaluated. Cold spray has shown to be a promising technique for the deposition and repair of heat sensitive particles such as titanium alloy. LPCS and PGDS both produced high quality coatings.