*2.1. Propeller Jet*

Hamill et al. [3] started the research on propeller jets according to the axial momentum theory and plain jet theory. Hamill et al. [17] found that the flow structure of the propeller jet is symmetrical along the rotational axis. The jet can be divided into two development stages, which are the zone of flow establishment and zone of established flow and the whole jet is conical in 3D view. The zone of flow establishment is the initial development stage of flow velocity with more complicated nature compared to the second zone downstream. Low-velocity region was found near to the propeller jet outflow plane due to the hub e ffects with no contribution of velocity from blades. The maximum lateral velocity distribution occurred in the middle of the blade, where the maximum blade width occurred. The velocity distribution is symmetrical along the propeller axis. For the zone of established flow, the maximum velocity is lesser compared to the initial zone due to water entrainment to reduce the velocity gradient within the jet. The high-velocity flow is mixed with the low-velocity flow to balance the velocity distribution.

Jiang et al. [15] proposed the flow structure of the twin-propeller jet has two zones as a single propeller, as shown in Figure 1. The zone of flow establishment can be divided to be non-interference zone (ZFE-TP-NI) and inference zone (ZFE-TP-I). Two propeller jets in twin system have no contact in the non-interference zone and expand separately to meet at one point where the interference zone starts. The zone of flow establishment consists of the four-peak region (ZFE-TP-4P) and two-peak region (ZFE-TP-2P) according to the lateral velocity section. A mixing point happens downstream within the twin-propeller jet. The twin-propeller jet is the same as two separate jets in the upstream region of the mixing point. Four maximum speeds were found on the cross-section, which corresponds to the two velocity peaks for every single propeller. The four maximum velocities combined to be two maximum velocities located at the rotational axis of each propeller when two single jets mixed together. In the zone of established flow, only one maximum velocity was found on the symmetrical plane.
