  We study a variant of Kuramoto-Sakaguchi model in which oscillators are
divided into two groups, each characterized by its coupling constant and phase
lag. Specifically, we consider the case that one coupling constant is positive
and the other negative, and calculate numerically the traveling speed of two
clusters emerging in the system and average separation between them as well as
the order parameters for positive and negative oscillators, as the two coupling
constants, phase lags, and the fraction of positive oscillators are varied. An
expression explaining the dependence of the traveling speed on these parameters
is obtained and observed to fit well the numerical data. With the help of this,
we describe the conditions for the traveling state to appear in the system.
