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 reciprocating engine under motoring conditions. Two non-simultaneous velocity components have been recorded at three different locations on the mid-plane of the TDC 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 exhaust valves). The combination of different engine speeds and different chamber locations enables one to look both at the global behavior of the flow and 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 analyze the variation of such a frequency with the measuring location and with the engine speed. The analysis has been carried out by using different tools; 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 give 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 the ordered jet coming our of the intake valves.