tailieunhanh - Environmental Fluid Mechanics - Chapter 6

Khái niệm của lớp ranh giới đã được giới thiệu vào đầu thế kỷ 20 bởi Prandtl và Pohlhausen để đơn giản hóa việc phân tích và tính toán của các hiện tượng dòng chảy và tương tác với các ranh giới vững chắc. Khái niệm này đã được mở rộng cho các tính toán nhiệt và chuyển khối lượng trong các loại khác nhau của lĩnh vực. Một vài ví dụ trong các chương trước đó đã được giới thiệu có liên quan đến khái niệm của các lớp ranh giới, nơi nó được thể hiện thường xuyên, và. | 6 Boundary Layers INTRODUCTION The concept of the boundary layer was introduced in the beginning of the 20th century by Prandtl and Pohlhausen to simplify the analysis and calculation of flow phenomena and interaction with solid boundaries. This concept has since been extended for calculations of heat and mass transfer in various types of domains. Several examples in the previous chapters have already been introduced that involve the concept of boundary layers where it was shown that often and especially in environmental flows only a minor portion of the flow domain is subject to viscous laminar or turbulent flow with significant velocity gradients and shear stresses. Other portions of the domain are practically at rest or subject to potential flow. For example in Chaps. 3 and 4 we considered the flow domain on top of an oscillating plate and the case of flow around a cylinder as examples in which the concept of the boundary layer was applicable. For the present chapter we cover only several cases in which the boundary layer concept is applicable for flow calculations in environmental fluid mechanics though some consideration of other issues of transport phenomena also are included. If fluid with small viscosity like water or air flows with high Reynolds number around a solid body then even at a very small distance from the solid body the effect of the inertial forces is much more significant than the effect of the viscous forces. Therefore it is possible to consider that in major portions of the domain the flow is virtually unaffected by friction. Frictionless flow is described by potential flow theory Chap. 4 . However close to the solid boundary the effect of the viscous shear stresses is significant and cannot be ignored. In order to exemplify the application of the boundary layer concept to flow phenomena we first consider the buildup of a boundary layer and the effect of shear stresses associated with the development of the flow field Copyright 2001 by .