The given question asks about a specific type of fluid flow where fluid particles rotate about their mass center while moving along the flow direction. Let's go through the options and identify the correct answer:

1. Steady flow: This type of flow implies that the flow parameters (such as velocity, pressure, etc.) at any point in the fluid do not change over time. It does not necessarily concern the rotation of particles. Therefore, it is not the correct choice.
2. Uniform flow: This indicates that the flow parameters are consistent across any cross-section of the flow. It only concerns spatial variation and does not involve rotation of particles. Thus, this is also incorrect.
3. Laminar flow: This is characterized by the smooth and regular paths of fluid particles, often moving in layers without much mixing between them. While laminar flow can involve steady or unsteady flow, it does not specifically denote the rotation of fluid particles about their mass center.
4. Rotational flow: In this type of flow, fluid particles exhibit rotation about their mass center while moving along the flow. This is the key characteristic described in the question and is indeed the correct option.

Based on the explanations above, the correct answer to the question is Rotational flow. Rotational flow is categorized by the presence of vorticity, where the fluid elements rotate as they flow, which matches the description given in the question.