TR No. 22 
velocity component, 
= ! 
tao U;, +u 
Multiplying by T;, 
T= 19/62) eme= Us Ty ul ae 
The expression U; T; is the distance relative. to. the water which the 
current meter has advanced in the interval T... Hence if u'; is negligible 
compared to Uj, the values of u; are obtained at distances of x;, and are 
approximately equally spaced at intervals of AAx = 19.61 cm, regardless 
of the mean velocity. The error in assuming that the data are equally 
spaced is of the order of tu'y/Uj = +10/400 = 42.5% for the measurements 
reported herein, which is not greater than the existing ambiguity in 
establishing the correspondence between the values uj and the series of 
times 
a4 | 
+ =a 
J 
Jeo 
Such equally spaced data are of the appropriate form for digital spectral 
analysis with respect to wave number. 
Aliasing 
A discussion of the problem of aliasing is given by Blackman and 
Tukey (17) where it is shown that if there are significant contributions to 
the energy from velocity variations occuring at wave numbers greater than 
tne Nyquist wave number given by 
// 
a pale 
Kn a Sow [in interval Re 
Cad ZL: (11) 
then the computed ehergy spectrum is in error at all wave number. The 
Nyquist wave number for the data obtained from the current meter is tr /19.61 cm 
0.157 em71 
