Yes, in many situations, velocity is inversely related to the cross-sectional area for flowing fluids. This means that if you decrease the cross-sectional area, the velocity of the fluid increases, and vice versa. This relationship is often observed in:
Fluid flow in pipes and tubes: Imagine squeezing a garden hose. As you narrow the opening, the water shoots out faster because the same amount of water must pass through a smaller area in the same amount of time.
Blood flow in arteries: Similar to the garden hose, when arteries narrow due to plaque buildup, blood velocity increases in that region.
Airflow in wind tunnels: As air approaches a narrower section of the tunnel, its velocity increases, explaining why airplane wings experience higher lift near their tips.
However, it's important to note that this relationship holds true only for incompressible fluids under certain conditions. For example, with compressible fluids like air, velocity changes can also affect pressure and density, making the relationship more complex.
Now, regarding your second question: how pressure changes if the area is doubled keeping force constant?
In an incompressible fluid with constant flow rate, doubling the area decreases the pressure. This is because for the same flow rate to pass through a larger area, the individual fluid particles need less force pushing them, resulting in lower pressure. Think of it like spreading the same amount of butter on a larger piece of bread; the pressure applied per unit area decreases.
However, if the force remains constant but the flow rate changes, increasing the area can have different effects on pressure depending on the specific situation.
It's crucial to consider the specific context and governing principles (like conservation of mass and energy) when analyzing how changes in cross-sectional area affect pressure and velocity in fluid flow.