Dynamics of Free Jets Modulated by Plane Acoustic Waves : Part 2. Numerical Simulations

Abstract

The problem of flow modulation by upstream perturbations is considered in this article. This topic is of fundamental interest in the analysis of flow dynamics but it is also of technical importance since many practical fluid systems are modulated by external acoustic perturbations. The study focuses on numerical simulations of the response of free jets to plane acoustic modes. It is first carried out in the low Reynolds number range to allow a precise comparison with experiments. The jet behavior is then investigated at a higher Reynolds number. Calculations are carried out in both cases with a Navier-Stokes flow solver operating on unstructured meshes and providing second order accuracy in time and third order precision in space. The domain of calculation includes the upstream flow, and a large region surrounding the jet to allow for acoustic propagation. The useful region extends from the exhaust nozzle to a few diameters, and is finely meshed. The unconfined character of the flow is ensured by damping regions surrounding the jet, where viscosity is progressively increased. Non reflecting boundary conditions are set up on the lateral sides of the domain and on the downstream outlet. Three modulation Strouhal number are used to examine the propagation of the acoustic field and its coupling with the flow. The vortical structures are identified, and their interaction with the modulation is characterized for the dierent regimes of propagation. Simulations indicate that the dierent modes of propagation observed experimentally are well retrieved in the low Reynolds number range, and that additional information can be gathered by analyzing the numerical data in the space-frequency domain. In the high Reynolds number range, Large Eddy Simulations results indicate that the jet behavior is also tightly coupled to the perturbed vorticity field, and that dierent modes of propagation characterize the perturbed field in the free jet.