This paper presents a numerical study of the evolution of a velocity enhancement disturbance in the solar wind in terms of a one-dimensional, isentropic MHD flow model. It is shown that the disturbance steepens and evolves into a double shock pair while propagating outward away from the Sun. The double shock pair consists of a reverse fast shock, a reverse slow shock, a forward slow shock, and a forward fast shock in order of distance away from the Sun. The formation time of the double shock pair is nearly inversely proportional to the average velocity gradient of the disturbance. When the double shock pair is fully developed, the strength of the fast shocks is essentially determined by the disturbance amplitude, while the slow shocks behave differently. Their strength increases first with the disturbance amplitude but starts to decrease once the disturbance amplitude exceeds a certain value. However, the fully developed slow shocks will retain their identity up to 1 AU and even farther, though their propagation speed in the solar wind frame and the jump in velocity and total pressure across them decrease substantially with heliocentric distance. Theoretically, double shock pairs would occur frequently in the inner heliosphere, since the solar wind there is characterized by various large-scale structures and disturbances, which provide an appropriate ground for the formation of double shock pairs. Such a prediction remains to be confirmed by observations and data interpretation.
