ISBN 9783843958011

978-3-8439-5801-1, Reihe Thermodynamik

Max Nüßle
Investigating Periodic Phenomena in a Mixing Nozzle Flow

148 Seiten, Dissertation Universität Stuttgart (2026), Softcover, A5

Zusammenfassung / Abstract

Mixing flows are of high technical relevance and occur in virtually all combustion and chemical process applications. In this thesis, mixing of a tracer gas by a central injector into an accelerating nozzle flow is considered. Often, high frequency oscillations occur at the trailing edge of the injector. Capturing these rapid phenomena experimentally or numerically demands substantial effort with conventional techniques such as particle image velocimetry (PIV) or laser induced fluorescence (LIF). This thesis introduces novel methods for resolving these high speed flow oscillations experimentally with phase-locking and event-based cameras and also presents numerical simulations of these phenomena.

To achieve phase-locking, the vortex shedding frequency at the injector base is measured by an improved laser schlieren method that provides high time resolution. Simultaneously, conventional PIV and LIF measurements are conducted. The information from the laser schlieren measurements is then used to achieve phase-locking for PIV and LIF, allowing the vortex shedding process to be resolved in terms of phase angle rather than time. This phase-locking is achieved in post processing only.

As a second method of resolving these high frequency phenomena, event-based cameras are used. Event-based cameras are used to measure the vortex shedding frequency in a schlieren setup, as well as for estimating the flow velocity with particle tracking. Due to their novel working principle, new processing algorithms are developed and presented in this thesis.

These experimental efforts are supported by numerical simulations using stress blended eddy simulations (SBES) and explicit algebraic Reynolds stress models. Both models are compared regarding their resolution of flow characteristics, like velocity, and their representation of the mixing wake. Especially the strong acceleration and its effect on the Reynolds stresses pose a special challenge to these models.