Wind tunnels

It is not always possible to employ a perfect model for which all Pi quantities are the same as those in the prototype. In such a case, the least important Pi quantities must be identified and these are the ones that are not modeled. Model testing in a wind tunnel illustrates this difficulty. 

The drag force on a prototype airfoil operating at 325 mph is to be determined from wind tunnel measurements on a 40 to 1 scale model. The dimensionless equation for this case is  

There are three dimensionally independent quantities and three scale factors may be fixed  

To have a perfect model, the model speed must be such as to make the Reynolds Number the same for protot)q)e and model  

The required air speed in the wind tunnel will thus be  

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IK as a cooling medium for nuclear reactors, and as a gas for supersonic wind tunnels. 

Vanes may be used to improve velocity distribution and reduce frictional loss in bends, when the ratio of bend turning radius to pipe diameter is less than 1.0. For a miter bend with low-velocity flows, simple circular arcs (Fig. 6-37) can be used, and with high-velocity flows, vanes of special airfoil shapes are required. For additional details and references, see Ower and Pankhurst The Mea.surement of Air Flow, Pergamon, New York, 1977, p. 102) Pankhurst and Holder Wind-Tunnel Technique, Pitman, London, 1952, pp. 92-93) Rouse Engineering Hydraulics, Wiley, New York, 1950, pp. 399 01) and Joreensen Fan Engineerinp, 7th ed., Buffalo Forge Co., Buffalo, 1970, pp. Ill, 117, 118). 

Case study 2 compressed air tanks for a supersonic wind tunnel... 

The supersonic wind tunnels in the Aerodynamic Laboratory at Cambridge University are powered by a bank of twenty large cylindrical pressure vessels. Each time the tunnels are used, the vessels are slowly charged by compressors, and then quickly discharged through a tunnel. How should we go about designing and checking pressure vessels of this type to make sure they are safe ... 

Two coefficients have been defined. Cl and Co, relating velocity, density, area, and lift or drag forces. These coefficients can be calculated from wind-tunnel tests and plotted as shown in Figure 7-6b versus the angle of attack... 

The NACA 65 blade profiles were tested in a systematie manner by Herrig, Emery, and Erwin. The easeade tests were earried out in a easeade wind tunnel with boundary-layer suetion at the end walls. Tip effeets were studied in a speeially designed water easeade tunnel with relative motion between wall and blades. 

A detailed method of determining pressure coefficients is to perform experiments with a wind tunnel facility. Cochran and Cermak compared wind tunnel pressure coefficient measurements with field measures on a test building and found excellent results, with the exception of small areas beneath the vortices near the upwind roof corner for winds approaching at 45 . For infiltration and natural ventilation designs, wind tunnel results should be sufficiently accurate. 

Another method is a series of exhaust dilution equations based on Wilson and Lamb " and a series of earlier papers summarized in ASHRAE. This method is based on wind tunnel tests on simplified buildings and is intended to provide conservative (low dilution) results. Wilson and Lamb compared the model to actual field data collected at a university campus and found that the model did indeed predict dilutions similar to measured worst-case dilutions suitable for a screening model. However, many cases resulted in conservative Linderpredictions of dilutions. ... 

Snyder, W. H., and R. E. Lawson. 1994. Wind tunnel measurements of flow fields m the vicinity of buildings. In Sth Joint Conference on Applications of Air Pollution Meteorology.. Anien-can Meteorological. Society and the Air and Waste Management Association. 

Murakami, S., A. Mochida, R. Ooka, S. Kato, and S. lizuka. 1996. Numerical prediction of flow around a building with various turbulence models Comparison of fe-e, EVM, ASM, DSM, and LES with wind tunnel tests. ASHRAE Transactions, vol. 102, no. I. 

Higson, R. L., R. F. Griffiths, C. D. Jones, and D. J. Hall. 1994. Concentration measurements around an isolated building A comparison between wind tunnel and field data. Atmospheric Environment, vol. 28, no. 11, pp. 1827-1836. 

Data are available only for simple building geometries. In Allard," a tool for the calculation of wind pressure coefficients for simple geometries is made available, and another tool is described in Knoll et al. Existing wind pressure data have to be examined carefully, because many data represent peak pressure values needed for static building analysis. Real cases with obstructions and buildings in the close surroundings are difficult to handle. Wind-tunnel tests on scale models or CFD analysis will be required. 

To calibrate larger sensors/instruments such as vane anemometers, a wind tunnel is required. A calibration wind tunnel consists of an open or closed tunnel, a fan to deliver the air, a nozzle to shape the velocity profile, and a mesh arrangement to uniform and reduce the flow turbulence. It may be necessary to control the air temperature in the tunnel by means of a heating/cooling sys-... 

Physical A model system in which tests are carried out on the emission and dispersion of a pollutant, e.g., a wind tunnel. 

Wind tunnel A fan-assisted test rig used to determine the air forces and flow patterns acting on model buildings or components. 

They chose an airfoil shape that, based on the collective data from their wind tunnel tests, would give a high L/D. The airfoil used on the... 

The drag coefficient for an antomohile body is typically estimated from wind-tunnel tests. In the wind tunnel, the drag force acting on a stationaiy model of the vehicle, or the vehicle itself, is measured as a stream of air is blown over it at the simulated vehicle speed. Drag coefficient depends primarily on the shape of the body, but in an actual vehicle is also influenced by other factors not always simulated in a test model. 

To a considerable extent, operations research as a formal discipline is occupied with the construction of models. This is closely related to the analysis of alternatives for decision-making. It is generally assumed that it is preferable to have a model to represent an operation, even though it is oversimplified and perhaps imperfect, than to have none. A model may be purely logical or it may be a physical analogue. A mathematical formula is an example of the former, a wind tunnel an illustration of the latter. In both cases, the model provides a ooherent framework for coping with the complexities of a problem. 

They point out that at the heart of technical simulation there must be unreality otherwise, there would not be need for simulation. The essence of the subject linder study may be represented by a model of it that serves a certain purpose, e.g., the use of a wind tunnel to simulate conditions to which an aircraft may be subjected. One uses the Monte Carlo method to study an artificial stochastic model of a physical or mathematical process, e.g., evaluating a definite integral by probability methods (using random numbers) using the graph of the function as an aid. 

Nitrogen monoxide is used as an oxidizer in biproplnt systems with carbon monoxide or methanol-w as fuels, and it is added to N2O4 to advantageously modify the frp and bp of this oxidizer. It is also employed as a high enthalpy flow medium in hypersonic wind tunnels (Ref 10), and as an oxidizing gas in atomic absorption spectroscopy (Ref 12). Qf -19.7 kcal/mole (Ref 3)... 

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