Hans-Jürgen Niemann

A review of recent experiments on the flow past circular cylinders

Abstract The flow around circular cylinders still is one of the challenges in fluid mechanics. Moreover, it is of great interest for practical applications, such as wind loads on cylindrical structures. For the two-dimensional problem in a uniform flow, research since the beginning of this century has produced a good if still incomplete insight into the phenomena. This does not apply, if three-dimensionality, and in particular freestream turbulence and shear of the flow are included in the consideration. For these more general but also more significant conditions a large amount of work remains to be done.

The L.R.C. (load-response-correlation) - method a general method of estimating unfavourable wind load distributions for linear and non-linear structural behaviour

Abstract In current design practice, for quasi-static structures, wind load effects often are defined by the equivalent steady gust model or by a pseudo-steady approach, neglecting the influence of the correlation of the fluctuating pressures over the whole structure. The application of these load patterns may result in an unconservative design when the minima of the wind loads on parts of the structure are more critical for a response considered, depending on its influence function. This problem particularly occurs when the wind load has to be combined with e.g. dead load. Then, for the design an extreme load pattern which causes the maximum interactive response has to be defined. These load patterns may be obtained from wind tunnel tests by a conditional sampling technique. A more efficient approach, the L.R.C.-Method, is presented in this paper enabling systematic studies e.g. on the influence of the static system and on the influence of geometrical non-linearities. In an example of practical application it is shown that the extreme load patterns obtained by the L.R.C.-Method are producing accurate linear peak responses and excellent approximate values for non-linear responses. So to the designing engineer as well as to the wind engineer, an effective tool is presented to describe realistically wind load effects for linear and weakly non-linear structures with quasi-static behaviour.

Towards quality assurance for wind tunnel tests: A comparative testing program of the Windtechnologische Gesellschaft

Recently, comparative wind-tunnel tests have been initiated by the Windtechnologische Gesellschaft WTG. Twelve institutes are participating in this program and have performed surface pressure measurements on a cubic model and some dispersion experiments. The elaborated results exhibit a certain scatter which is partly due to different simulations of the given full-scale situations. A reduced sample of all pressure measurements has been selected ignoring those results with major deviations to the average. The pressure difference between the front and rear face of the cube has been applied as an indicator for erroneously used dynamic and static reference pressures. The statistics of the reduced sample is listed.

Mechanical design in spines of diadematoid echinoids (Echinodermata, Echinoidea)

SummaryThe long and slender spines of Diadema are highly flexible, although their skeleton consists mainly of CaCO3 and behaves optically like a single monocrystal of calcite. The flexibility is due to the shape of the spine skeleton as well as to the material properties of the echinoderm calcite.The spine skeletons are hollow beams consisting of radial wedges or septs. The shape of the septs shows a broad base situated at the periphery of the cross section, producing a high load-bearing capacity with minimum weight. Furthermore, material is concentrated at the base of the spine in such a way that the strain of the structure is kept constant along the axis. The septs are connected with one another by a few transverse bars positioned as closely as possible to the axis. The load-bearing parts of the septs are free. They have small diameters similar to flexible glass fibres. The stiff spines of other echinoids are also mainly built by radial wedges, but the spaces in between are closely filled with transverse bars. On the surface of stiff spines there are low grooves between the septs. The echinoid spines are covered with an epithelium which shows a basiepithelial nerve plexus. In the stiff spines this plexus forms cords which lie protected within the superficial grooves mentioned. In the flexible spines of Diadema the cords are deeply sunken in the spaces between the septs. In this manner the nerve cords are largely free from the tensile stresses to which the spines surface is exposed.The flexible spines were used to determine the material properties of echinoderm calcite. Youngs modulus was determined for fresh (live) spines, dry spines, and cleaned spine skeletons. Fresh spines show the highest elasticity, and their Youngs modulus is significantly below the Youngs modulus of the other test groups. The echinoderm calcite does not show the cleavage planes of mineral calcite, and probably this feature contributes to the high flexibility of echinoderm calcite.

Numerical study of the bleeding effect on the aerodynamic characteristics of a circular cylinder

Abstract Numerical results on the influence of a jet issued into a near wake on body drag and lift force of a cylinder in cross-flow are presented. The effect of base bleed through two narrow symmetrical slots on the cylinder rear surface and ventilation is considered. Calculations are based assuming two-dimensional subcritical flow.

Is Aeolian risk as significant as other environmental risks

Abstract Wind is the origin of many risks. These are identified and defined in 1 Introduction , 2 What is Aeolian risk? of this paper. The effects of climate changes are discussed in Section 3 , while structural vulnerability to wind actions is the object of Section 4 . In addition to the heavy damages that can be caused by windstorms, the wind climate affects significantly the quality of life through phenomena like wind-borne pollution that are addressed in Section 5 . Section 6 addresses specifically experimental research on wind effects. The final sections discuss some current and forthcoming research on assessment and reduction of wind risks.

Wind tunnel experiments on micro-scale dispersion of exhausts from motorways

Since exhaust gases from road traffic are mainly released in the immediate vicinity of urban areas, constructive measures are required to prevent unfavourable immissions. Up to now, the micro-scale dispersion of exhausts could not be sufficiently predicted by purely theoretical methods. At the Ruhr-University Bochum, wind tunnel experiments have been conducted. Aerodynamic similarity criteria for the atmospheric boundary layer and in particular the traffic-induced turbulence have to be considered. Comparison of time averaged concentrations, measured in situ and in the corresponding wind tunnel model, prove the reliability of this experimental tool. With this background, the influence of different noise abatement barriers on the spread of exhaust gases is systematically investigated.

Full-scale and model tests on wind-induced, static and dynamic stresses in cooling tower shells

Abstract The response of the shell of concrete cooling towers to turbulent wind is investigated by experimental methods. Calculations based on membrane theory are used to verify the reliability of the test results with respect to the membrane forces. The tests include measurements of wind load and shell response in model and full-scale conditions. For the model tests, dynamic similarity of the model was observed, and the atmospheric boundary layer was simulated. The Schmehausen cooling tower was equipped with special strain and pressure pick-ups for the full-scale observations. The results suggest that there is considerable bending in circumferential stresses, and that dynamic stresses are not always correctly calculated by assuming a static design wind load, affecting in particular the buckling safety of the shell.

Some properties of fluctuating wind pressures on a full-scale cooling tower☆

Abstract The fluctuating components of wind pressures on hyperbolic cooling towers are sensitive to Reynolds number and surface roughness in a similar way to the average pressures. They are random and may be described by r.m.s. pressure distributions, cross correlations and spectra. Full-scale measurements are necessary to obtain reliable data. Such measurements, originally used for the determination of average pressures, were interpreted with respect to the fluctuating pressures. A general difficulty of full-scale tests is the consideration of the reference pressure. If it is fluctuating randomly it superimposes an additional process on the pressures in question. Its influence is discussed and a method is given to extract as much information as possible in spite of the disturbing process. Some results are presented, such as cross-correlation coefficients and r.m.s. pressure distributions. They show the influence of separation and of neighbouring buildings, which induce additional turbulent energy. Further work is necessary to obtain a better understanding.

The boundary layer wind tunnel: an experimental tool in building aerodynamics and environmental engineering

Abstract Some of the major difficulties in wind tunnel experiments arise from the blockage effect, the Reynolds number disparity, and the possible errors induced by the response of the tubing system to pressure fluctuations. These are discussed and methods to solve the problems are presented, including typical examples of application.