Physical models have long been used by engineers to gain a better under-standing of complex phenomena. Such models probably constitute the oldest method of structural design. Physical models have also been used for many years in the fields of hydraulics, hydrodynamics, and aerodynamics.
Examples of studies made with physical models include:
1. Dispersion of pollutants throughout a lake.
2. Behavior of waves within a harbor. (See Figure 5.5.)
3. The underwater performance of submarines of different shapes.
Full-scale models are sometimes built, but they are often built to a smaller scale. Typical scales for physical models range from 1 : 4 to 1 : 48 (6).
Perhaps the greatest value of physical models is that they allow the engineer to study a device, structure, or system with little or no prior knowledge of its behavior or need to make simplifying assumptions.
Figure 5.2 The results of an empirical study used for a simulation model. The figure shows distributions of times that vehicles occupy an intersection.
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Figure 5.3 A general flowchart for a simulation model.
An Example of Physical Models Some of the most beneficial research with physical models has been accomplished with wind tunnels (see Figures 5.6 and 5.7). A brief description of wind tunnels, abstracted from Reference 6, is given in the following paragraphs.
Wind tunnels are based on a fundamental law of fluid dynamics, namely, that a body immersed in a moving fluid experiences the same forces as if the body were moving and the fluid stationary, assuming that the relative speed of the fluid and solid object is the same in both cases. This means, for example, that the conditions surrounding an airplane in flight can be replicated by hold-ing the plane stationary and movhold-ing the air past it at a velocity comparable to flight speeds.
Advantages of wind tunnels over flight testing are economy, safety, and research versatility. A model airplane can be tested in a wind tunnel at a frac-tion of the cost of building and operating a full-scale prototype, and the air-worthiness of new and experimental designs can be tested without risking a Figure 5.4 Output from a computer simulation model. The
figure shows the average times cars wait to cross a main street.
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Figure 5.5 A physical model used to study the behavior of waves in a harbor. (Courtesy of the U.S. Army Corps of Engineers.)
Figure 5.6 Some of the most bene-ficial research with physical models has been accomplished with wind tunnels. Here a ram jet model is being studied in a 10 ft. 10 ft. test section. (Courtesy of the National Aeronautics and Space
Administration.)
pilot’s life. Wind-tunnel testing can simulate flight under conditions more con-trolled and measurable than would be possible in a flight test. Even before the Wright brothers’ first flight, the wind tunnel was the principal tool of the aero-nautical engineer.
All wind tunnels have common features that circumscribe their characteris-tics and capabilities. All have a test section in which a model, component, or system can be fixed or suspended. The cross section may be round, oval, rect-angular, or polygonal. Test sections may vary in width and height from a few inches up to 100 feet or more.
Wind tunnels may be either return or nonreturn. Nonreturn tunnels draw air from the atmosphere, pass it through a tube that includes a test section, and discharge it into the atmosphere. Such tunnels are simple and inexpensive to build but are inefficient and limited in the types of flow they can generate.
Many sophisticated tunnels use a return-type circuit in which the same air is moved around a closed loop.
A major advantage of closed tunnels is that they can be pressurized, a tech-nique that allows objects to be studied that are a fraction of the full scale.
Comparability between conditions of wind-tunnel tests on models and condi-tions experienced by a full-scale aircraft in flight depends on a dimensionless mathematical quantity known as the Reynolds number (named for the nine-teenth century British engineer Osborne Reynolds). The Reynolds number is a Figure 5.7 This giant air pas-sage houses a massive set of stationary guide vanes in one elbow of a wind tunnel which force the air to make a right angle smoothly. (Courtesy of the National Aeronautics and Space Administration.)
flow-similarity parameter that describes forces acting on a body in motion with respect to the fluid in which it is immersed. The number is directly propor-tional to the size of the body and the density and relative speed of the fluid, and inversely proportional to the viscosity of the fluid. Other things being equal, a model “moving” with respect to an airstream would have a smaller Reynolds number than a full-scale plane in flight. The easiest way to equalize the Reynolds numbers—and thus to obtain comparable flow conditions for the plane and the model—is to increase the speed or density of the airstream in which the model is immersed.
Almost all wind tunnels employ a complex array of balances and other mea-suring devices designed specifically for the purpose. Most closed-circuit tun-nels use tunnel vanes to guide the airflow smoothly around the corners in the circuit. Most tunnels use complex arrays of settling chambers, screens, and throat contractions to smooth and straighten the airstream as it accelerates into the test section. A variety of model-support systems is used, depending on the configuration of the test object. Some tunnels use smoke to help visualize air-flow. Some are rigged with special photographic devices that record shock waves produced at high speeds. Some tunnels are refrigerated to produce ice on the models like that encountered under certain flight conditions. In fact, wind tunnels have been designed to replicate nearly every condition encoun-tered by airplanes in flight (6).
5.5 EVALUATION AND SELECTION OF PREFERRED SOLUTION
As the engineering design process evolves, the engineer may evaluate again and again alternate ways of solving the problem at hand. Typically, the engi-neer winnows the unpromising design choices, yielding a progressively smaller set of options. Feedback, modification, and evaluation may occur repetitively as the device or system evolves from concept to final design.Depending on the nature of the problem to be solved, evaluation may be based on any number of factors. If it involves a product, safety, cost, reliability, and consumer acceptability are often of paramount importance.
Perhaps the most straightforward way to evaluate a product is to develop a prototype and simply test it in operation. In some cases the prototype might not work due to one or more components of the design. The designer should try to identify all of the weak links of a prototype before accepting or discard-ing the design idea. Many great ideas have been discarded prematurely, and many working prototypes have failed to operate as expected when turned into a product. No idea should be evaluated solely on the basis of one prototype or one test.
There are many indirect methods for evaluating a proposed design. For example, wind-tunnel testing a scale model of an aircraft design can reveal good and bad features of the design at a small fraction of the cost and risk of a full-scale assembly. Alternatively, the aerodynamics of the new aircraft design can be evaluated using computer simulation of the expected flight conditions.
The associated mathematical equations used in the computer simulation may
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not accurately account for some of the complicating factors such as the com-ponent interference and turbulence. However, the computer simulation can indicate approximate features of the design, which makes it easier to design the first scale model for wind-tunnel testing.
The optimization scheme can become very difficult when the design requires a human operator and a man-machine interface. This difficulty exists because no two human beings are the same. The basic anatomical and physio-logical differences between humans makes the human factors of the design dif-ficult to quantify. One human user may find a design very acceptable and efficient, while another may consider it to be intolerable; therefore, the opti-mization of human factors becomes a matter of statistical comparisons. Hence, the user population must be identified and characterized before attempting to optimize any designs that involve human operators.
In addition to the routine judgments that engineers make about a device or system, more formal and structured evaluations are often needed. This is espe-cially true of public works projects, which must be judged from the viewpoints of competing and often conflicting groups. Such evaluations have traditionally relied on economic analysis, but recent concerns with social and environmen-tal impacts of public projects have produced much broader evaluation tech-niques. Let us now examine some of these formal evaluation techtech-niques.