A Wind Tunnel Study of Thermally Stratified Boundary Layers over Rough Surfaces Marcio Cataldi Programa de Engenharia Mecânica (PEM/COPPE/UFRJ) C.P. 68503, 21945-970, Rio de Janeiro, Brasil. mcataldi@serv.com.ufrj.br Juliana B. R. Loureiro Programa de Engenharia Mecânica (PEM/COPPE/UFRJ) C.P. 68503, 21945-970, Rio de Janeiro, Brasil. jbrloureiro@serv.com.ufrj.br Luiz Cláudio G. Pimentel Departamento de Meteorologia (IGEO/UFRJ) Rio de Janeiro, Brazil. pimentel@lttc.com.br Atila P. Silva Freire Programa de Engenharia Mecânica (PEM/COPPE/UFRJ) C.P. 68503, 21945-970, Rio de Janeiro, Brasil. atila@serv.com.ufrj.br Abstract. The effect of stratification on the features of boundary layers developing over rough surfaces has been studied experimentally. The results are compared with the classical theories of other authors for two different stability regimes: stable, and unstable flows. A description of the apparatus together with the measuring technique is given in a specific section. The parameters of the boundary layer are qualified through: growth, structure, equilibrium, turbulent transport of heat and energy spectrum. Keywords. Stratification, roughness, turbulence, wind tunnel, thermal anemometry. 1. Introduction Thermal stratification is known to play a major role in the physical processes that take place in the atmosphere. Of particular concern to researchers has been the study of the diffusion of pollutants in the atmosphere under the combined effects of stratification and orography; this very often leads to hazardous environmental conditions. Because investigating naturally occurring density stratified flows is normally a very difficult and costly affair, one attractive alternative is to try to reproduce similar conditions in the laboratory. This, in theory, would favor the experimenter in view of the controlled conditions and easy access instrumentation. As a result, low-speed wind tunnels have largely been used in the past to study pollutant dispersion. Unfortunately, most studies have been carried out for flows under adiabatic conditions and over smooth surfaces. The purpose of this work is to investigate experimentally the effect of density stratification on the characteristics of boundary layers developing over rough surfaces. The effects of unstable and stable conditions on the flow will be examined. Requiring the following criteria to be met assessed the quality of the simulation: Reynolds number independence, mean velocity profiles and turbulence intensities. The paper will show how heating through electrical resistances both, the floor and the incoming air, controls the boundary layer that is formed in the wind tunnel. The resistances can furnish an increase in floor temperature of up to 100 o C above ambient temperature and can be applied over a 6000 mm long surface with a controlled variation of 2 o C. The incoming air is heated by forcing the flow through a set of 10 electrical heating elements that can be operated individually. The system can then be used to produce, unstable, neutral and stable boundary layers. The rough surface was constructed with a well-defined geometry. Previous authors (see, e.g., Perry and Joubert(1963), Antonia and Luxton(1971,1972), Wood and Antonia(1975)) have classified rough surfaces into two types of surfaces: 1) “K” type rough surfaces and, 2) “D” type rough surfaces. In cases where the nature of the roughness can be expressed with the help of a single length scale - the height of the protrusions, K - the surface is termed of type “K”. Flows, on the other hand, which are apparently insensitive to the characteristic scale K, but depend on other global scale of the flow, are termed “D” type flows. This is the case, for example, of a roughness geometrically characterized by a surface with a series of closely spaced grooves within which the flow generates stable vortical configurations. Here, only a K-surface will be investigated. The experimental investigation of turbulent boundary layers under unheated, steady flow conditions is a relatively common exercise in laboratories. For most studies, Hot-Wire Anemometry (HWA) is the principal research tool. The very well known advantages of this superb measuring technique make it a must for turbulence research. Thus, it is only natural to realize that a host of articles have been dedicated in literature to this technique. However, all measuring techniques have their problems and application restrictions and HWA is not free from them. For example, dealing with flows in which the temperature of the fluid varies with either time or position may pose severe difficulties, which are