When both cross-sectional area and flow rate are fixed, the fluid velocity must remain constant according to the continuity equation. Lowering pressure does not change velocity because velocity equals flow rate divided by area, and both of these parameters are fixed.
The continuity equation states that flow rate equals area times velocity. When flow rate is constant, velocity is inversely proportional to area. A smaller pipe has higher velocity, while a larger pipe has lower velocity, but the flow rate remains the same.
This is the crucial point: when both flow rate and cross-sectional area are fixed constraints, the velocity must remain constant regardless of pressure changes. The continuity equation locks the velocity at Q divided by A. Pressure variations cannot alter this relationship when Q and A are predetermined.
Consider a practical example: a water pump system delivering 100 liters per minute through a 5 centimeter diameter pipe. The velocity calculates to 8.49 meters per minute. Even if system pressure drops from 5 bar to 3 bar, the velocity remains constant because the pump maintains the fixed flow rate.
In conclusion, the answer is definitively NO. Lowering fluid pressure does not reduce fluid velocity when both cross-sectional area and flow rate are fixed. The velocity is determined solely by the ratio of flow rate to area, making it independent of pressure changes in this constrained scenario.