Emma Alenius

Swirl switching in turbulent flow through 90 degrees pipe bends

Turbulent flow through 90° pipe bends, for four different curvatures, has been investigated using large eddy simulations. In particular, the origin of the so-called swirl switching phenomenon, which is a large scale oscillation of the flow after the bend, has been studied for different bend curvature ratios. A classification of the phenomenon into a high and a low frequency switching, with two distinct physical origins, is proposed. While the high frequency switching stems from modes formed at the bend, and becomes increasingly important for sharp curvatures, the low frequency switching originates from very-large-scale motions created in the upstream pipe flow.

A large eddy simulation study of bluff body flame dynamics approaching blow-off

ABSTRACT The mechanisms leading to blowoff were investigated numerically by analyzing bluff body stabilized flame at two conditions: a condition far from blowoff to a condition just prior to blowoff. Large eddy simulations have been used to capture the time dependent, three-dimensional evolution of the field. The results were first validated to available experimental data, showing very good agreement for the flow and overall good agreement for the flame. Changes in the large-scale structures are investigated by means of proper orthogonal decomposition and the wavelet method, elucidating the underlying dynamics of the complex flow-flame interaction of a flame approaching blowoff. Our results reveal that, when the flame approaches blowoff conditions, significant changes are found in the large-scale structures responsible for entrainment of species into the recirculation zone located downstream of the bluff body. Possible causes of this shift in large-scale structures are also discussed, which may be useful for extending the blowoff limits of bluff body stabilized burners.

LES of Acoustic-Flow Interaction at an Orifice Plate

The scattering of plane waves by a thick orifice plate, placed in a circular or square duct with flow, is studied through Large Eddy Simulation. The scattering matrix is computed and compared to measurements, showing reasonably good agreement except around one frequency (

Sound Generating Flow Structures in a Thick Orifice Plate Jet

St \approx 0.4

Numerical simulation of flow-induced sound generation from an orifice in a low Mach number ducted flow

). Here a stronger amplification of acoustic energy is observed in the circular duct simulations than in the measurements and the square duct simulations. In order to improve the understanding of the interaction between an incoming wave, the flow, and the plate, a few frequencies are studied in more detail. A Dynamic Mode Decomposition is performed to identify flow structures at significant frequencies. This shows that the amplification of acoustic energy occurs at the frequency where the jet in the circular duct has an axisymmetric instability. Furthermore, the incoming wave slightly amplifies this instability, and suppresses background flow fluctuations.

Scattering of Plane Waves by a Constriction

The aim of thiswork is to study sound generating flowstructures in a thick circular orifice plate jet, placed in a circular duct. Large eddy simulations (LES) are performed for two jet Mach numbers, 0.4 and 0.9. Characteristic frequencies in the flow, and their corresponding flow structures, are identified with dynamic mode decomposition (DMD). The results show that a tonal noise is generated at frequencies where the jet displays strong ring vortices, in the plane wave range. The main sound generating mechanisms seems to be a fluctuating mass flow at the orifice opening and a fluctuating surface force at the plate sides, caused by the ring vortices. The frequencies are believed to be chosen, and strengthened, by a feedback mechanism between the orifice in- and outlet.

Study of Plasma Actuator Efficiency by Simulation of the Detached Flow Over a Half-Cylinder

Aero-acoustic simulations are performed for an orifice plate mounted in a straight duct in a low-Mach number flow. The flow field is calculated by solving the filtered Navier-Stokes equations by means of direct numerical simulation (DNS), using a high-order finite difference scheme. The scheme uses summation-by-parts (SBP) finite difference operators with simultaneous approximation terms (SAT) to impose boundary conditions. Both the scattering of the sound (passive part) as well as the sound generation (active part) are studied in the low frequency plane wave range. An acoustic two-port model is applied to describe the sound in the duct. The results are compared with experimental data for the same configuration. The efficiency and robustness of the numerical technique are also examined.

Aero-acoustic simulations of an orifice plate mounted in a low-Mach-number ducted flow

Liner scattering of low frequency waves by an orifice plate has been studied using Large Eddy Simulation and an acoustic two-port model. The results have been compared to measurements with good agreement for waves coming from the downstream side. For waves coming from the upstream side the reflection is over-predicted, indicating that not enough of the acoustic energy is converted to vorticity at the upstream edge of the plate. Furthermore, the sensitivity to the amplitude of the acoustic waves has been studied, showing difficulties to simultaneously keep the amplitude low enough for linearity and high enough to suppress flow noise with the relatively short times series available in LES.

Large eddy simulations of acoustic-flow interaction at an orifice plate

In this paper, the effect of a numerical model for plasma actuators, in the form of single dielectric barrier discharge, is evaluated. One such plasma actuator is modeled by a steady body force dis ...

Large Eddy Simulation of lean blow off

Aero-acoustic simulations are performed for an orifice plate mounted in a straight duct in a low-Mach number flow. A two-dimensional flow-field is calculated by solv- ing the Navier-Stokes equation ...