Jack E. Cermak

Applications of Fluid Mechanics to Wind Engineering—A Freeman Scholar Lecture

Wind has always had a strong influence, both unfavorable and favorable, upon man and his activities. Within the last decade needs for treatment of wind effects from an engineering point-of-view have increased tremendously. Losses due to wind (

Laboratory Simulation of the Atmospheric Boundary Layer

500,000,000 in property damage, 240 deaths and 2600 injuries annually), increased demand and concern for human comfort, serious attempts to control air pollution, and the development and expansion of energy-production capabilities have resulted in applications of engineering to problems for which a body of knowledge has only started to emerge in the United States. The primary elements of this body of knowledge are found in the disciplines of meteorology, fluid mechanics, aerodynamics, and structural mechanics—organizing this knowledge to form a coherent subject-matter base for wind engineering is a real challenge for fluids engineers. The objectives of this review are to establish an initial subject-matter base for wind engineering, to demonstrate current capabilities and deficiencies of this base for an engineering treatment of wind-effect problems, and to indicate areas of research needed to broaden and strengthen the subject-matter base. Focusing of subject matter for wind engineering is accomplished through a historical summary of relevant scientific and technological material, an examination of information on wind characteristics, and a review of current capabilities for physical modeling of winds and wind effects in the laboratory. Current methods and capabilities in wind engineering are demonstrated by a review of problems related to atmospheric advection and dispersion of air pollutants, wind forces on buildings and structures, and control of winds. Research needs are specified separately for each area reviewed -wind characteristics, simulation of the wind, atmospheric transport of air pollutants, wind forces, and wind control. Physical modeling of boundary-layer-type winds and wind effects by measurements on small-scale models placed in long-test-section, meteorological wind tunnels currently provides the most reliable source of data for wind engineering. Coordinated measurements on full-scale systems and their small-scale models are necessary for continued confirmation of similarity for the laboratory data and for development of new modeling capabilities. In particular, development of a tornado simulator is an urgent need to support structural design for nuclear-power-plant facilities. Intensive analytical investigations of three-dimensional, thermally-stratified, turbulent boundary layers; separation of turbulent, unsteady flows; turbulent shear flow over bluff bodies; and interacting turbulent flows with a variety of turbulence characteristics are needed to ensure future progress in wind engineering. These investigations are needed to provide a framework for correlation of both laboratory and full-scale data, to support efforts to develop numerical modeling as a practical tool, and to develop a better understanding of the physical processes involved. These flow problems represent formidable frontiers of turbulent fluid motion. Therefore, investigations in the fluid-mechanics laboratory coupled with measurements on full-scale systems are expected to be the primary sources of information for wind engineering in the immediate future.

PRESSURE FLUCTUATIONS ON A SQUARE BUILDING MODEL IN BOUNDARY-LAYER FLOWS

Similarity criteria are given for micro-, small-, and meso-scale motion of the atmospheric boundary layer. Requirements for simulation of dispersion of passive contaminants in the atmosphere are discussed. The characteristic features of a unique meteorological wind tunnel with a capability for simulating thermally stratified boundary layers are described. Mean wind speed, mean temperature and turbulence statistics measured in this laboratory facility are found to be similar to corresponding data obtained from measurements in the atmosphere. Examples of simulated dispersion over a variety of surface features including urban areas and complex topography are described.

A Wind Tunnel Study of Gaseous Pollutants in City Street Canyons

Abstract Spatio-temporal measurements of a fluctuating pressure field acting on the side faces of a square prism of finite height in boundary-layer flows are presented for 0° angle of attack. Two typical neutral atmospheric flow conditions were simulated in the wind tunnel to represent open country and urban flow environments. The fluctuating pressure field data allowed computations of the mean and r.m.s. (root mean square) pressure coefficients, power spectral density, autocorrelations, co-spectra, cross-correlations, orthogonal eigenfunction expansions and statistical dependence. Increased levels of turbulence in the incident flow have a marked influence on the fluctuating pressure field, through modifications which take place in the structure of the separated shear layers. The periodic vortex-shedding process is vitiated in the presence of high levels of turbulence intensity in the incident flow, resulting in redistribution of the energy associated with pressure fluctuations over a wider frequency range.

Wind-tunnel development and trends in applications to civil engineering

Steady state mean concentrations of tracer gas were measured in a 400:1 scale model of an idealized city with variable geometry placed within a wind tunnel at various orientations to the mean flow for a free stream velocity of 6.8 ft/sec. The tracer gas was released from two parallel line sources to simulate lanes of traffic in an effort to quantify the persistence of pollution as well as the mean values realized at street levels. An aerodynamically rough turbulent boundary layer of neutral thermal stratification was employed to simulate the atmosphere. Values of concentration measured in the model city were converted to prototype concentrations in ppm and compared to National Ambient Air Quality Standards. It was shown that single isolated structures may cause favorable mixing of pollution downwind but very high concentrations exist in the immediate leeward vicinity of the building. Two favorable geometries for city blocks tested were found to reduce pedestrian exposure to pollution both near heavy traff...

Full- and model-scale cladding pressures on the Texas Tech University experimental building

Abstract A review is presented on wind tunnels capable of simulating natural winds, the boundary-layer wind tunnel (BLWT), and trends in their extensive use in civil-engineering practice. BLWTs and data-acquisition systems, as they evolved to meet needs in civil engineering, are described. Advancements are highlighted for the types of wind-load information now available to structural engineers and architects by BLWT tests using the advanced data-acquisition systems–the high-frequency base balance (H-FBB) and the synchronous multi-pressure sensing system (SM-PSS). Trends in applications of BLWT tests to determine wind effects on structures and to investigate wind-related environmental problems are described. Technical details of the BLWT, the H-FBB, the SM-PSS, and their proper use are available in references cited and are not restated in this review.

Wall pressures of separation—reattachment flow on a square prism in uniform flow

Abstract Extensive pressure data have been collected on a 1:100 and 1:50 model of the Texas Tech University (TTU) experimental building in a wind-tunnel simulated Atmospheric 3urface Layer (ASL). Sample data are presented and discussed. Comparison with the full-scale results from TTU are presented and show generally good agreement; however, the largest peak suctions are often underestimated by the model data measured at edge and corner roof locations. Some ideas are presented here to possibly explain these model-scale, full-scale anomalies.

Wind tunnel evaluation of a modified andersen impactor and an all weather sampler inlet

Abstract Mean and fluctuating pressures on a two-dimensional square prism in uniform flow at a Reynolds number of 1.4 × 10 5 were measured to investigate the nature of the separation—reattachment phenomenon. Measurements were taken on the side-wall face and in the wake region to quantify the behavior of the fluid in the reattachment zone. The experiment was performed with and without a turbulence producing grid for angles of approach flow ranging from −50° to +90°.

Mean force and moment coefficients for buildings in turbulent boundary layers

Abstract Modifications were made to a one-cfm (4.72 × 10 −4 m 3 / s ) Andersen impactor to permit the collection of larger particles than previously possible. An All Weather Sampler Inlet was developed for the unit which enables acquisition of unbiased aerosol samples for varying meteorological parameters. When tested with a uranine-tagged oleic acid aerosol in a wind tunnel operated at a speed of 4.6 m/s (15 ft/sec), the standard Andersen design precludes collection of more than 50 percent of particles larger than approximately 7 μm diameter whereas the corresponding particle size for the modified system is 14 μm. Wind tunnel tests with the modified unit show the performance to be relatively unaffected by wind speeds over the range of 5–15 ft/sec or by turbulence with intensities up to eight percent. The geometrical configuration precludes wind directional effects.

Sediment-laden velocity profiles developed in a long boundary-layer wind tunnel

Abstract Mean force and moment coefficients for a series of flat-roofed rectangular buildings are presented for a range of wind directions. The coefficients were determined by integrating mean pressures measured on model structures immersed in thick turbulent boundary layers simulating four typical neutral atmospheric flow conditions (power-law exponents of 0.12, 0.27, 0.34, and 0.38). A series of thirteen different building geometries were studied. The ratios of adjacent sides were 1, 0.5, 0.25, and the aspect ratio, the ratio of the height to the smaller side, ranged from 1 to 8. The effects of side ratio, aspect ratio, incident turbulence intensity, longitudinal integral scale, and the ratio of building height to boundary-layer thickness are discussed.