Reynolds Stress Components in the Flow Field of a Motored Reciprocating Engine 950725

Coincident 3-D velocity measurements have been made in a single-cylinder, motored research engine using a six-beam, three-wavelength LDV system. The engine had a pancake combustion chamber, a compression ratio of 8.0 and was operated with a fixed intake shroud valve position. Measurements have been performed at 600, 1000 and 1500 RPM and at three distinct locations within the combustion chamber.

Software coincidence filtering and ensemble averaging have been thereby used for data processing. Results obtained so far indicate that: Mean flow velocities and turbulence intensities scale about linearly with engine speed and decay rapidly through the compression and expansion strokes; a complex interaction between the overall swirling and locally tumbling motion takes place however at late compression and leads to a short-living but significant turbulence enhancement close and prior to compression TDC; though qualitative patterns of turbulent fluctuations are similar among the three velocity components, quantitatively, turbulence is rather not isotropic even until late expansion; a certain degree of non-homogeneity is also observed between the three locations investigated. The influence of the in-take shroud position on the induced flow field in terms of both, mean and fluctuating velocities is quite important; the associated differences of intake conditions do moreover persist, in part to a significant extent even around and after compression TDC.

In addition, estimates of the Reynolds stress tensor could be obtained for the first time in an engine flow field showing that: All three shear stress components scale approximately with the square of the mean piston speed; for all conditions investigated the axial-radial and axial-tangential components have the highest values of about 0.2 ÷ 0.4 times the product of the corresponding fluctuation intensities, while the magnitude of the tangential-radial component is very close to zero; with concern to their temporal behaviour peaks of the first two components are typically around early and late compression respectively. The influence of the intake shroud position on the details of the turbulent shear stress patterns is however quite apparent in almost all cases.