Numerical analysis of non-excited and excited jets issuing from non-circular nozzles

Abstract

This paper concentrates on the application of passive and active flow control methods to jet flows. The passive control is obtained by shaping a nozzle cross-section (hexagonal, hexagram, square, triangular) and the active one by an external excitation. The research is performed by applying the large eddy simulation method (LES). The obtained results are thoroughly validated by comparisons with theoretical, experimental and numerical data for non-excited jets. The presented results comply well with the reference data, both in terms of the mean velocity profiles, velocity fluctuations profiles and the Strouhal number of the preferred oscillation frequency (Stp). Based on its value, the excitation frequencies (Stf) are chosen such to cover the range of Stp and its subharmonics. The applied excitation is defined as a sinusoidal function, which enforces the axial velocity oscillations at the level of 10% of the bulk velocity. It is shown that the dynamics of the excited non-circular jets is significantly different from the dynamics of their non-excited counterparts. It is demonstrated that by changing Stf one may modify both the instantaneous as well as their time-averaged behaviour, i.e., the centreline velocity decay, turbulence intensity, directional spreading rate and the entrainment rate. It is found that not all jets are prone to excitations to the same extent. In the far-field, the dependence of the velocity decay rate on Stf is the most pronounced in the jets issuing from circular and hexagonal nozzles. In the near-field, regardless of Stf the excitation causes a faster velocity drop, shortening of the potential core and increase of the centreline fluctuations in all jets. However, in the hexagram jet, the fluctuations turn out to be the least sensitive to change of Stf. Apparently, in this case, the complex nozzle shape determines the velocity oscillations so strongly that a relatively low amplitude excitation has only a small influence. In this respect, the jets issuing from square and triangular nozzles show the opposite, as for them the impact of the excitation is the strongest. Depending on Stf three qualitatively different scenarios of the fluctuations growth are identified by either manifesting their smooth increase or the occurrence of local maxima and minima. The entrainment in the square and triangular jets also turned out to be more dependent on the excitation rather than by the nozzle geometry. In the near-field, the excitation with Stp/2<Stf<Stp leads to almost doubling the entrainment maximum found for the non-excited jets.