Step 1: Basic idea of subsonic flow.
When a fluid moves at speeds lower than the speed of sound (Mach number less than 1), compressibility effects are very small. In such cases, the flow behavior around solid objects like airfoils can be explained using energy conservation.
Step 2: Energy relation for low-speed flow.
For steady, incompressible, and frictionless flow, Bernoulli’s equation applies:
\[ P + \frac{1}{2}\rho v^2 + \rho g h = \text{constant} \]
Here, \(P\) is the static pressure, \(\rho\) is fluid density, and \(v\) is the flow velocity.
Step 3: Applying the equation to flow over an airfoil.
For airflow over an airfoil, changes in height are very small, so the term \(\rho g h\) can be treated as constant. This reduces the relation to a balance between pressure energy and kinetic energy.
As the air accelerates over the curved surface of the airfoil, its velocity increases. An increase in velocity raises the kinetic energy term \(\frac{1}{2}\rho v^2\).
Since the total energy remains constant, the pressure term \(P\) must decrease when velocity increases.
Step 4: Final conclusion.
In subsonic flow, higher fluid velocity corresponds to lower static pressure.
\[ \boxed{\text{Increase in velocity causes a decrease in pressure}} \]