Vogel and Patterson 



to use a trip ring made of 0.09 cm diameter wire placed 1.43 cm from the nose 

 of the body (between the nose slot and the 2nd slot). Observations using threads 

 attached to the body indicated that at tunnel speeds above 150 cm/sec the bound- 

 ary layer was completely turbulent from the trip ring to the tail. It is uncertain 

 whether the boundary layer from the nose slot to the trip ring is completely tur- 

 bulent at the lower speeds used. 



Before the drag runs were made the body was aligned by measuring the lift 

 and pitching moment on a three component force balance (Kempf and Remmers, 

 Hamburg) and rotating the body vertically until these forces were zero. The 

 balance holds the model so that the centre-line of the wing section is horizontal 

 and at right angles to the centre-line of the tunnel. 



Figure 4 shows the drag of the body-wing combination with the trip ring vs 

 the tunnel velocity used for this work. The Reynolds numbers, based on the 

 length of the body, are 6.5 x 10^ for 150 cm/sec and 2.6 x 10^ for 625 cm/sec. 



SOURCES OF ERRORS IN DRAG MEASUREMENTS 



There are several sources of error in determining the drag of the body. 

 Fluctuations in tunnel velocity during the run gave rise to fluctuations in the 

 drag reading by as much as plus or minus 1 to 2 grams. This error was mini- 

 mized during the runs with the polymer injection by reading the drag reduction 

 when the fluid was injected, and the drag increase when the fluid was stopped. 

 If there was a marked difference between the two readings the run was repeated. 

 Another source of error is a reading error of the meter recording the drag; this 

 is about plus or minus 0.25 grams. A third source of error is the gradual build- 

 up of the concentration of the polymer in the tunnel. The drag of the body was 

 measured before each run and if there was a difference between it and the data 

 in Fig. 4 the tunnel was drained and refilled with water. On only one occasion 

 was it necessary to drain the tunnel for this reason; the usual procedure was to 

 drain the tunnel after each day's runs when the higher concentrations were be- 

 ing used. At the lower concentrations the tunnel was drained after three day's 

 runs. The tunnel holds 23,000 litres of water. Most of the drag data reported 

 is an average of the data recorded for at least two runs, and the estimated error 

 is plus or minus 1 gram. 



EFFECT OF POLYMER CONCENTRATION AND 

 MOLECULAR WEIGHT 



A series of runs was carried out to determine the effect of polymer concen- 

 tration on the drag reduction for the three polymers. The fluid was injected into 

 the boundary layer through the nose slot which was 0.25 mm wide. Although the 

 nose slot is at right angles to the centre-line of the body, the fluid when injected 

 flows back over the body and does not appear to disturb the flow in the boundary 

 layer . 



Figures 5 to 7 show the drag reduction vs tunnel velocity obtained with the 

 three polymers when they are injected into the boundary layer at a rate of 30 

 ml/sec. The average velocity through the slot would be 200 cm/sec. These 



980 



