Using this assumption， one can apply this flow visualization method to any working medium.
One designed apparatus consists of two large tanks on two different levels. The lower tank is constructed entirely out of plexiglass and receives a con-stant flow from the upper tank. The flow entering the lower tank comesthrough alarge， rectangular opening， which houses a number of screens so that no turbulence is created by water entering the lower tank. The center of the lower tank can be fitted with various boxes for the various flow visual-ization problems to be studied. This modular design enables a rapid inter-changing of models and work on more than one concept at a time.
To study the effect of laminarflow， the blades were slotted as shown in Figure7-.. For the blade treatment cascade rigexperiment， a plexiglass cascade was designed and built. Figure 7-10 shows the cascade. This cascade
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Figure 7-10. Cascade model in axial-flow test tank.
was then placed in the bottom tank and maintained at a constant head.Figure 7-11 shows the entiresetup， and Figure 7-12 shows the cascade flow. Note the large extent of the laminar-flow regions on the treated center blades as compared to the untreated blades.
The same water tunnel was used for tests to study the effect of casing treat-ment in axial-flow compressors. In thisstudy， the same Reynolds number and specific speeds were maintained as those experienced in an actual axial-flow compressor.
In an actual compressor the blade and the passage are rotating with respect to the stationary shroud. It would be difficult for a stationary observer to obtain data onthe rotating blade passage.However， if that observer wererotating with the blade passage， data would be easier to acquire. This was accomplished by holding the blade passage stationarywith respect to the observer and rotating the shroud. Furthermore， sincecasing treatment affects the region around the bladetip， it was sufficient to study only the upper portion of the blade passage. These were the criteria in the design of the apparatus.
The modeling of the blade passage required provisions for controlling the flow in and out of the passage. This control was accomplished by placing theblades， which partially form the blade passage， within a plexiglass tube. The tube had to be of sufficient diameter to accommodate the required flow through the passage without tube wall effect distorting the flow as it entered
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Figure 7-11. Apparatus for testing axial-flow cascade model.
or left the blade passage. This allowance was accomplished by using a tube three times the diameter of the blade pitch. The entrance to the blades was designed so that the flow entering the blades was a fully developed turbulent flow. The flow in the passage between the blade tip and the rotating shroud was laminar. This laminar flow was expected in the narrow passage.
A number of blade shapes could have beenchosen; therefore， it was necessary to pick one shape for this study which would be the most repre-sentative for casing treatment considerations. Since casing treatment is mosteffective from an acoustic standpoint in the initial stages of compression， the maximum point of camber was chosen toward the rear of the blade (Z二 .6 chord). This type of blade profile is most commonly used for transonic flow and is usually in the initial stages of compression.
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