Approach To Tunnel Flow Improvement

Approach to Tunnel Flow Improvement 1. Eliminate separation on the first diffuser 2. Flow in the second diffuser should fill the duct 3. Using an axisymmetric contraction cone with a contraction ration of 11, it was shown that the turbulence at the contraction cone exit is essentially independent of the screen Reynolds number based on either wire diameter or mesh size a. For longitudinal turbulence, the values behind the screen ahead of the inlet varies from 0.029 to 0.052, while the exit variation was 0.00500.0054 for five different screens. b. The lateral variation at the inlet was 0.032-0.063, and 0.0060-0.0070 at the exit. c. The pressure loss coefficient for four of the screens varied from K=0.65 to K=2.34 and the porosity varied from 0.62 to 0.75 d. The screen with K=2.34, inlet turbulence of 0.029, and exit turbulence of 0.0051 gave the same results at the exit or test section as the one with K=0.65, inlet turbulence of 0.052, and exit turbulence of 0.0050 e. These are all longitudinal values. f. This implies that one should use screens with the smallest pressure loss coefficient, and if multiple screens are used, again, screens with the smallest loss should be adopted. g. Ramjae and Hussain also determined that the ratio of rms exit to inlet turbulence as a function of contraction ratio is over-predicted in the longitudinal direction and under-predicted in the lateral direction. h. These predictions are: u component turbulence reduction = 1c2 v component turbulence reduction = 1c12 Linear theory of turbulence reduction due to a contraction predicted an increase in the lateral component Axial and lateral turbulence should be determined at several speeds, at the start of both contraction and the test section. Screens and/or honeycombs are installed, the reduction in turbulence should be checked as each device is installed. a. Will ensure that the minimum number of devices us used, thus holing losses to minimum b. The following values for turbulence are suggested c. Thus, the ideal values would of course be 0. For tunnels intended for research use in boundary layers and boundary layer transition, the lateral values of the turbulence, which is usually the largest, must be kept as small as possible. a. Values of about 0.05% have been suggested. Tunnels used for developmental testing can have larger turbulence values, perhaps as high as 0.5% in the axial direction. i.

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The Drive System

1. Various drive systems described subsequently can also be achieved by electrical methods that do not require rotating machinery. 2. Considering the drives capable of variable-speed control, we have the following: a. Generator and DC Motor b. Tandem Drive c. Variable Frequency d. Magnetic Coupling e. Multispeed Squirrel Cage f. Wound-Rotor Induction Motor g. Doubly Fed Induction Motor h. Internal Combustion Drive Wind Tunnel Construction 1. The structural loadings on the various sections of a low-speed wind tunnel are usually less critical than the strength needed to avoid vibration, a significant exception being the assurance that the drive motor will stay in place should it lose one-half of its blades. a. The rest of the tunnel may be examined to withstand the maximum stagnation pressure with a safety factor of perhaps 4.0. 2. Vibration of the parts of the wind tunnel contributes to noise, discomfort of the tunnel crew and possible fatigue failures, and usually adds to the turbulence in the wind stream a. Good practice to have the natural frequencies of all tunnel parts well above any exciting frequencies 3. All types of materials are used for tunnel construction: a. Wood b. Plywood (Small research and instruction tunnels) c. Thin metal d. Heavy metal e. Cast concrete f. Gunnite g. Plastics 4. Fan blades for low-speed tunnels are frequently made of wood, although modified aircraft propellers are sometimes used with trailing edge flaps to provide uniform pressure rise across the fan disk. 5. Immense cranes are needed during the construction of a metal tunnel