Bachelor's thesis Pascal Weber

 

Investigation of the start-up and decay behaviour of turbulent free jets using CFD simulation

Velocity of a turbulent free jet Copyright: EBC Velocity of a turbulent free jet

Pulsed free jets have a high potential to optimise residential ventilation systems in partial load conditions. Contrary to the well described free jets in the literature, pulsed free jets are still very unclear. Therefore, the aim of this paper is to gain further insights into pulsed free jets. For this purpose, a simulation model was developed in Ansys and validated with literature values, whereby the sensitivity of time and location discretization was also investigated. Large Eddy Simulation (LES) was chosen for the turbulence modelling in Ansys.

The pulsation of the free jets was divided into start-up and decay processes, which can be de- scribed by ramp functions. These start-up and decay processes were simulated with different speeds and ramp gradients, then evaluated and compared with each other. The simulations of the start-up process show that up to a certain speed the free jet consists only of the laminar core area. After this critical speed is exceeded, the free jet starts to turn turbulent from the tip of the core until the complete free jet is turbulent. Analogous to this, an ever increasing core area is formed during the decay process at a certain speed. Finally, approximative formulas were developed to calculate the core lengths as a function of the inlet velocity.