The separation between turbulence and mean flow in ICE LDV data: The complementary point-of-view of different investigation tools

LDV measurements have been taken in a disc chamber four-stroke reciprocatin g engine under motoring conditions. Two non-simultaneous velocity component s have been recorded at three different locations on the mid-plane of the T DC clearance during the intake and compression strokes for three. different speeds (600, 800, 1000 rpm). The locations are characterized by different flow conditions (near the intake valves; on the cylinder axis; near the exh aust valves). The combination of different engine speeds and different cham ber locations enables one to look both at the global behavior of the flow a nd at the derails of the turbulence rime-evolution. The aim of the research is to identify the frequency which can be considered a separation between "true" turbulence and cycle-by-cycle variation of the mean flow and to anal yze the variation of such a frequency with the measuring location and with the engine speed. The analysis has been carried out by using different tool s; the non-stationary velocity autocorrelation function, the power spectrum and the cycle resolved analysis bused on the frequency filter The various approaches offer complementary perspectives of the same phenomenon, which g ive a clear perception of the physical meaning of the most frequently used investigation tools. The results show that the cut-off frequency increases as the engine speed increases and as the measuring point moves away from th e ordered jet coming our of the intake valves.