This equation is sensitive to only v'. Experiments by Das were conduc- 
ted in a stationary flume in which only a horizontal bottom plate was 
oscillated to produce turbulence. The results showed some promise for 
the method. 
When Das' method of measuring v' was tried in the swing flume, it 
was not successful because of excessive vibrations of the sensor. These 
vibrations were mainly due to: (a) the long holder required to extend 
the sensor to the flume bottom, and (b) attaching this holder to the 
support frame which was indirectly subjected to the vibrations from the 
flume motion. The velocities of the sensor due to vibrations were 
greater than the velocities of the turbulent flow; therefore, no com- 
ponent of turbulent velocity could be distinguished. 
An approximation to v!' had to be obtained based on the following 
assumptions. It was assumed that at a given elevation the root-mean- 
square value of the three components of turbulent velocity fluctuations 
is proportional to each other. It was also assumed that the heat con- 
vected from the sensor due to velocities in the direction parallel to 
the sensor axis was insignificant compared to the heat convected by 
velocities perpendicular to the axis. This assumption is justified in 
that the hot-film has directional properties making the maximum sensi- 
tivity at right angles to the flow. Also, the aspect ratio (length- 
diameter) of the sensor is such that its properties approach those of 
an infinite wire where there is no effect of a longitudinal velocity. 
Based on these assumptions, the effective velocity causing heat convec- 
tion is, as an average: 
Uss=eiivic Ke yas) (19) 
e 
where Ug is the velocity corresponding to the output voltage of the 
hot-film bridge, K' is the constant of proportionality between the 
vertical component and one of the horizontal components of turbulent 
velocity fluctuation, and v' is the vertical component of the turbulent 
velocity causing heat convection from the sensor. It is apparent then 
that the sensor must be placed in the flume with its axis horizontal. 
The magnitude of the vertical component of velocity fluctuation can then 
be calculated and is, as an average: 
ECs TE (20) 
Although equation (20) is only an approximation, the assumptions used 
do not. affect the basic relationships (a) between the root-mean-square 
54 
