Additives Injected Into the Boundary Layer of an Underwater Body 



of 100:1 takes place at the centre of the wake 7 centimetres behind the body. A 

 rough calculation based on the observed boundary layer thickness at the down- 

 stream end of the cylindrical portion of the body showed that the dilution through 

 the boundary layer would be of this order. The boundary layer is too thin to 

 probe so no direct measurements in the boundary layer have been made. 



The effect of increasing the polymer concentration has been shown for the 

 three polymers used. For POLYOX WSR-301 a peak in the drag reduction curve 

 is reached somewhere between a concentration of 500 ppm and 1,000 ppm. It is 

 suggested that polymer molecular entanglement and interaction at the higher con- 

 centration is responsible for this effect. It is possible that the time interval the 

 fluid is flowing over the body is too short for the molecules to become untangled 

 even though the fluid is being diluted as it flows over the body. In extending this 

 work we plan to carry out more dye runs at the higher concentrations to deter- 

 mine whether the fluid is being mixed through the boundary layer or concentrat- 

 ing near the body. 



The turbulence studies in the wake have indicated that both the mean veloc- 

 ity and the turbulence level are being affected by the injection of the polymer 

 solution into the boundary layer. We have not measured these parameters at 

 other distances downstream nor in the boundary layer of the body itself so do not 

 know whether the measurements are only applicable at this one position. Also 

 the turbulence probe measures only the component of the turbulence in the direc- 

 tion of the mean flow. It would appear though that the polymer is interacting 

 with the turbulence and with the shear stresses. The increase in the turbulence 

 level for the low wave numbers for the 500 ppm solution of WSR-301 could be a 

 result of the interaction of the polymer with the turbulence components in the 

 two directions not measured and energy being fed into the downstream component. 

 It could also be a result of the change in the shear stresses which occur with the 

 change in the average velocity profile. 



The one spectrum shown for the 100 ppm solution and the high wave number 

 portions of the 500 ppm solution spectra indicate that the polymer does interact 

 with the downstream component of the turbulence. Further information is re- 

 quired before a more definite statement on the significance of these results can 

 be made. 



In order to obtain more detailed information we are in the process of inves- 

 tigating the wake of a cylinder at right angles to the flow. The polymer solution 

 is injected into the wake through holes in the rear of the cylinder and measure- 

 ments of the effect of the polymer on the turbulence and mean flow of the wake 

 are being investigated. Because the wake of a cylinder is quite well known it is 

 hoped that some of the questions raised with this experiment will be answered. 



SUMMARY OF RESULTS 



The effects of injecting solutions of three linear, high molecular weight 

 polymers into the boundary layer of a three-dimensional streamlined model 

 are being investigated. The following are the preliminary results of this ex- 

 periment: 



995 



