The turbulent boundary layer thickens more rapidly than the laminar boundary layer as a result of increased shear stress at the body surface. Transition may occur earlier, but it is dependent especially on the surface roughness. Transition from laminar to turbulent boundary layer occurs when Reynolds number at x exceeds Re x ~ 500,000. As the Reynolds number increases (with x) the flow becomes unstable and finally for higher Reynolds numbers, the boundary layer is turbulent and the streamwise velocity is characterized by unsteady (changing with time) swirling flows inside the boundary layer. In which V is the mean flow velocity, D a characteristic linear dimension, ρ fluid density, μ dynamic viscosity, and ν kinematic viscosity.įor lower Reynolds numbers, the boundary layer is laminar and the streamwise velocity changes uniformly as one moves away from the wall, as shown on the left side of the figure. The Reynolds number is the ratio of inertia forces to viscous forces and is a convenient parameter for predicting if a flow condition will be laminar or turbulent. The stages of the formation of the boundary layer are shown in the figure below:īoundary layers may be either laminar, or turbulent depending on the value of the Reynolds number. The concept of boundary layers is of importance in all of viscous fluid dynamics and also in the theory of heat transfer.īasic characteristics of all laminar and turbulent boundary layers are shown in the developing flow over a flat plate. The region in which flow adjusts from zero velocity at the wall to a maximum in the main stream of the flow is termed the boundary layer. the flat plate, the bed of a river, or the wall of a pipe, the fluid touching the surface is brought to rest by the shear stress to at the wall. Subbase courses are generally constructed out of crushed aggregate or engineered fill.In general, when a fluid flows over a stationary surface, e.g. A subbase course is not always needed or used. The subbase generally consists of lower quality materials than the base course but better than the subgrade soils. It functions primarily as structural support but it can also minimize the intrusion of fines from the subgrade into the pavement structure and improve drainage. The layer between the base course and subgrade. Base courses are usually constructed out of crushed aggregate or HMA. It provides additional load distribution and contributes to drainage. The layer immediately beneath the surface course. Surface courses are most often constructed out of HMA. This top structural layer of material is sometimes subdivided into two layers: the wearing course (top) and binder course (bottom). In addition, it prevents entrance of surface water into the underlying base, subbase and subgrade ( NAPA, 2001 ). It provides characteristics such as friction, smoothness, noise control, rut resistance and drainage. A typical flexible pavement structure (see Figure 2) consists of: Material layers are usually arranged within a pavement structure in order of descending load bearing capacity with the highest load bearing capacity material (and most expensive) on the top and the lowest load bearing capacity material (and least expensive) on the bottom. Thus, the further down in the pavement structure a particular layer is, the less load (in terms of force per area) it must carry (see Figure 1). A flexible pavement structure is typically composed of several layers of material each of which receives the loads from the above layer, spreads them out, then passes them on to the layer below. Flexible pavements are so named because the total pavement structure deflects, or flexes, under loading.
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