Flow around a line obstacle

Shear layer flows which are strongly disturbed, often occur in nature as well as in engineering practice. Up to now little is known about this class of flows. This is Partly explained by the complexity of these flows and partly by a lack of experimental data. The objective of this study is: first, to carry out a full-scale experiment around a two-dimensional barrier which is attached to the earth surface; second, to compare the obtained data with existing models which describe such strongly disturbed shear layer flows.In Chapter 1, the goal of this study is described in more detail. A review is given of the most important outdoor experiments that were carried out in the past. Besides, in Chapter 1 a qualitative description of these flows is given.In Chapter 2, more details are given about the measurement program and the terrain conditions. To ensure that the measured flow disturbances are caused by the erected barrier and are not caused by other terrain disturbances, special attention must be paid to the terrain conditions of the measuring site. The surface condition must be homogeneous and free of other obstacles to an upstream distance of about 100 times the highest measuring level from the most windward location. Such a terrain was found along an aircraft runway, located in the south east of the Netherlands.In Chapter 3 the data will be discussed. The data of the undisturbed reference location were analyzed and compared with those in the literature. Furthermore , with these data the homogeneity of the windward terrain was analyzed. From these results it appeared that the windward terrain had an overall roughness length of z o = 35 mm.The disturbed mean wind field was analyzed in: first, thermally neutral condition and perpendicular flow direction; second, non- neutral conditions and perpendicular flow direction; third, neutral condition and oblique flow directions. From the data obtained in oblique flow directions it appeared that the width of the barrier has an important influence on the sheltering effect near the surface. The disturbed r.m.s. values of the speed fluctuations are analyzed in order to gain more insight into the turbulence of the distorted flow field. From these results it appeared that the turbulence near the surface is strongly dependent on thermal stratification.In Chapter 4, special attention is given to the aerodynamic characterization of the barrier. A physically attractive way to characterize an obstacle is to express the effect of an obstacle in terms of its resistance to the fluid flow, or, in dimensionless form, in terms of a drag coefficient. This coefficient was estimated in two different ways. First, by direct force measurements. More or less in the center of the barrier a section was removed and replaced by a measuring plate. The drag on this plate, exerted by the fluid flow was measured with force sensors. Second, by application of momentum conservation to a control volume around the barrier. To use this indirect method, just around the barrier the velocity profiles and the static pressure profiles were measured. Moreover, in Chapter 4 we give an analysis of the influence of thermal stratification and flow direction upon the drag coefficient.In Chapter 5, the data in near-neutral conditions and perpendicular flow direction are compared with existing models. One of the earliest attempts to describe the disturbed shear flow behind a barrier, is the diffusive model of Kaiser (1959). Because of the weak physical foundation of Kaiser's model and the bad agreement with experimental evidence, we decided not to analyze this model any further in our study. A physically more realistic model describing the disturbed flow behind a barrier is the self-preserving model of Townsend (1965). The results of this model for the near wake region, i.e. the region between the barrier and the leeward reattachment point, do not coincide at all with our data. Beyond the reattachement point, Townsend's model overestimates the data considerably but with increasing distance from the barrier the differences become gradually smaller. The most sophisticated model up to now which describes the disturbed flow behind a barrier in the far wake region, is that by Counihan, Hunt and Jackson (1974). The far wake region is the region beyond the reattachment point. This model (referred to as the C.H.J. model) consists of three layers: first, the surface layer in which the flow is in local equilibrium;second, the mixing layer in which the flow is self-preserving; third, the external layer. We checked the C.H.J. model against our data and found an excellent agreement.In Chapter 6, the final conclusions are presented and suggestions are made for future research.