DIFFERENTIAL STRESS MODELING OF TURBULENT FLOWS IN MODEL RECIPROCATING-ENGINES

A study is made here of the application of a differential stress model (DSM) of turbulence to flows in two model reciprocating engines. For the first time this study includes compressive effects. An assessment between DSM and k-epsilon results is made comparing with laser Doppler anemometry experimental data of the mean flow and turbulence intensit y levels during intake and compression strokes. A well-established two -dimensional finite-volume computer code is employed. Two discretizati on schemes are used, namely the HYBRID scheme and the QUICK scheme. Th e latter is found to be essential if differentiation is to be made bet ween the turbulence models. During the intake stroke the DSM results a re, in general, similar to the k-epsilon results in comparison to the experimental data, except for the turbulence levels, which the DSM ser iously underpredicts. This is in contrast to a parallel set of calcula tions of steady in-flow, which showed significant gains from using the DSM, particularly at the turbulence field level. The increased number of grid lines employed in those calculations contribute to this appar ent difference between steady and unsteady flows, but cycle-to-cycle v ariations are more likely to be the primary cause, resulting in too hi gh levels of turbulence intensity being measured. However, during the compression stroke the DSM returns vastly superior results to the k-ep silon model at both the mean flow and turbulence intensity levels. Thi s is because the DSM generates an anisotropic shear stress field durin g the early stages of compression that suppresses the main vortical st ructure, in line with the experimental data.