90 



TUNNEL PRESSURE 

 CONTROL VALVE - 



AIR REMOVAL- 



J^?^^,:^>^ ' ..u„^.Jw. | . ' ■^ ' ■ l J.^ ! ■.■.i■!l!.^^^ll'^^.^■ v■^^».;,lJ.- ^■..^■^^^v- ' ^^^■ ' .. 



-VACUUM PUMP 



I ft. 



FIGURE 6. Diagram of the Low Turbulence Water Tunnel 

 (LTWT) . 



Low Turbulence Water Tunnel (LTWT) both at the 

 California Institute of Technology. Since the 

 HSWT has been described in detail elsewhere [see 

 Knapp et al. (1948) or Knapp, Daily, and Hammit 

 (1970) ], it will only be noted here that one, it 

 has a resorber and two, the freestream turbulence 

 level has been measured to be about 0.2 percent by 

 Professor S. Barker. The LTWT [Vanoni et al . 

 (1950)] is also a closed loop recirculating tunnel; 

 but, as can be seen in Figure 6, it has no resorber. 

 In this facility the maximum test section velocity 

 and minimum cavitation number are approximately 8 

 meters per second and 0.3 respectively. The unique 

 feature of the LTWT is that the freestream turbu- 

 lence level in the test section can be varied from 

 a very low value (for water tunnels) of 0.05 per- 

 cent to a high value of 3.6 percent. The low tur- 

 bulence level is obtained by use of small turning 

 vanes in each elbow of the circuit, a yery gradual 

 diffuser (included angle is 3°13'), a nozzle with a 

 16:1 contraction ratio, and by turbulence - damping 

 screens and honeycombs in the "stagnation" section 

 of the tunnel just upstream of the nozzle. The 

 configuration of screens and honeycombs which pro- 

 duces the 0.05 percent turbulence level is shown 

 schematically in Figure 7 (with the exception that 

 no turbulence generating grid is installed) and is 

 based upon the results of Loehrke and Nagib's (1972) 

 report. 



By inserting different turbulence generating 

 grids into the tunnel circuit the turbulent intensity 

 can be gradually increased from 0.05 to approxi- 

 mately 3.6 percent. The description of these grids 

 is as follows: 



HONEYCOMB 



I"x7" TRIA,IGULAR CELLS 



1 



-TURBULENCE DAMPING SCREENS 

 0.0075" DIA. WIRE, 22 meshes/lineal inch 



TURBULENCE 

 GENERATING GRID 



-SECOND HONEYCOMB 

 l/8"x2" HEXAGONAL CELLS 



grid No. 

 grid No. 

 grid No. 

 grid No . 

 Grids 1 , 2 , 



1: 12.7mm diameter bars with 50.8mm 

 mesh 



2: 6.35mm diameter bars with 25.4mm 

 mesh 



3: three 25.4mm diameter horizontal 

 bars on 76.2mm centers 



4: 0.635mm diameter fishing line with 

 19.05mm mesh 



and 4 are located at the entrance to 

 the test section as is shown in Figure 7 (the 

 distance from these grids to the test model is 

 approximately 1.2 meters). Grid No. 3 is located 

 in the "stagnation" section immediately after the 

 final turbulence damping screen. Grid No. 3 has 

 this particular configuration because (after much 

 trial and error) it was found to produce a turbu- 

 lence level which is close to the levels measured 

 in a number of other facilities — see Table 1. 



A DISA constant temperature anemometer was used 

 to measure the turbulence levels in the test section. 

 The probe was a wedge-shaped hot film type and was 

 firmly mounted on the tunnel center-line at the 

 model position (1.2 meters from the test section 

 entrance) . The results of these measurements have 

 been summarized in Figure 8. 



Polymer Injection System 



The injection approach of introducing the polymer 

 into the boundary layer versus filling the tunnel 

 with a polymer solution (polymer ocean) was chosen. 

 After considering a number of injection configura- 

 tions [Wu (1971) ] it was decided to follow van der 



I — 



3 



Ld 

 > 



UJ 



o 



cr 



0.5 



0.1 



0.05 



O o 



O O O 



a D 



D n a n 



\ r 



GRID # I 



GRID #2 



GRID #4 

 A A A A A A 



GRID #3 ; 



FREESTREAM 



_L 



5 10 15 20 



TUNNEL VELOCITY -U , ft/sec 



25 



FIGURE 7. Sketch of LTWT contraction nozzle showing 

 the turbulence manipulators. 



FIGURE 8. Summary of turbulence intensity measurements 

 in the LTWT. 



