Turbulent Flows in Reciprocating Internal Combustion Engines

Abstract

In this section some of the fundamental assumptions underlying the description of a turbulent flow are presented.¹ First, it is useful to discuss the basic features of a turbulent flow. Turbulent flows are highly diffusive. This high diffusivity results in increased rates of momentum, heat and energy transfer. For example, turbulent flames propagate faster than laminar flames and turbulent heat transfer allows the heating of a room to occur in seconds rather than hours. Another feature of turbulent flows is that they always occur at high Reynolds numbers. This implies that turbulence originates from instabilities in the laminar flow. These instabilities originate from the interaction of viscous and inertial forces which are highly non-linear. Turbulent flows are always three-dimensional and rotational. They contain distributed regions of vorticity which undergo vortex stretching in order to maintain or increase the amount of vorticity. Without this vortex stretching mechanism, the turbulence would decay rapidly. This implies that turbulent flows are highly dissipative: that is, without a supply of energy to maintain the loss to viscosity, the turbulence will decay rapidly. Finally, turbulence is a characteristic of the flow. It is not a property of the fluid like viscosity or density but rather a feature of a particular flow field.