# Relationship b pressure and velocity

### What is the relationship between pressure and velocity for a liquid and gas? | How Things Fly

Velocity and Pressure are inversely proportional to the Area of cross section of the to cover “x” distance and particle at B is taking seconds for the same. The question is ill-defined. In what system are you looking for such a relation? The answers above (the correct ones, at least) pertain to different systems. For turbulent flow, the speed and or the direction of the flow varies. The pressure, speed, and height (y) at two points in a steady-flowing.

Example 1 A table tennis ball placed in a vertical air jet becomes suspended in the jet, and it is very stable to small perturbations in any direction. Push the ball down, and it springs back to its equilibrium position; push it sideways, and it rapidly returns to its original position in the center of the jet. In the vertical direction, the weight of the ball is balanced by a force due to pressure differences: To understand the balance of forces in the horizontal direction, you need to know that the jet has its maximum velocity in the center, and the velocity of the jet decreases towards its edges.

The ball position is stable because if the ball moves sideways, its outer side moves into a region of lower velocity and higher pressure, whereas its inner side moves closer to the center where the velocity is higher and the pressure is lower.

**Bernoulli's equation explained - Understanding the relation between velocity and pressure**

The differences in pressure tend to move the ball back towards the center. Example 3 Suppose a ball is spinning clockwise as it travels through the air from left to right The forces acting on the spinning ball would be the same if it was placed in a stream of air moving from right to left, as shown in figure Spinning ball in an airflow. A thin layer of air a boundary layer is forced to spin with the ball because of viscous friction.

At A the motion due to spin is opposite to that of the air stream, and therefore near A there is a region of low velocity where the pressure is close to atmospheric.

At B, the direction of motion of the boundary layer is the same as that of the external air stream, and since the velocities add, the pressure in this region is below atmospheric. The ball experiences a force acting from A to B, causing its path to curve. If the spin was counterclockwise, the path would have the opposite curvature.

The appearance of a side force on a spinning sphere or cylinder is called the Magnus effect, and it well known to all participants in ball sportsespecially baseball, cricket and tennis players.

Stagnation pressure and dynamic pressure Bernoulli's equation leads to some interesting conclusions regarding the variation of pressure along a streamline. Consider a steady flow impinging on a perpendicular plate figure There is one streamline that divides the flow in half: Along this dividing streamline, the fluid moves towards the plate. Since the flow cannot pass through the plate, the fluid must come to rest at the point where it meets the plate.

Bernoulli's equation along the stagnation streamline gives where the point e is far upstream and point 0 is at the stagnation point.

### Fluid dynamics and Bernoulli's equation

It is the highest pressure found anywhere in the flowfield, and it occurs at the stagnation point. It is called the dynamic pressure because it arises from the motion of the fluid. The dynamic pressure is not really a pressure at all: Pitot tube One of the most immediate applications of Bernoulli's equation is in the measurement of velocity with a Pitot-tube.

The second way is to make the pressure at one end of the pipe larger than the pressure at the other end. A pressure difference is like a net force, producing acceleration of the fluid. As long as the fluid flow is steady, and the fluid is non-viscous and incompressible, the flow can be looked at from an energy perspective. This is what Bernoulli's equation does, relating the pressure, velocity, and height of a fluid at one point to the same parameters at a second point.

The equation is very useful, and can be used to explain such things as how airplanes fly, and how baseballs curve. Bernoulli's equation The pressure, speed, and height y at two points in a steady-flowing, non-viscous, incompressible fluid are related by the equation: Some of these terms probably look familiar If the equation was multiplied through by the volume, the density could be replaced by mass, and the pressure could be replaced by force x distance, which is work.

Looked at in that way, the equation makes sense: For our first look at the equation, consider a fluid flowing through a horizontal pipe. The pipe is narrower at one spot than along the rest of the pipe. By applying the continuity equation, the velocity of the fluid is greater in the narrow section. Is the pressure higher or lower in the narrow section, where the velocity increases?

## Bernoulli’s Effect – Relation between Pressure and Velocity

Your first inclination might be to say that where the velocity is greatest, the pressure is greatest, because if you stuck your hand in the flow where it's going fastest you'd feel a big force. The force does not come from the pressure there, however; it comes from your hand taking momentum away from the fluid.

The pipe is horizontal, so both points are at the same height. Bernoulli's equation can be simplified in this case to: The kinetic energy term on the right is larger than the kinetic energy term on the left, so for the equation to balance the pressure on the right must be smaller than the pressure on the left.

It is this pressure difference, in fact, that causes the fluid to flow faster at the place where the pipe narrows. A geyser Consider a geyser that shoots water 25 m into the air. How fast is the water traveling when it emerges from the ground?

If the water originates in a chamber 35 m below the ground, what is the pressure there?

To figure out how fast the water is moving when it comes out of the ground, we could simply use conservation of energy, and set the potential energy of the water 25 m high equal to the kinetic energy the water has when it comes out of the ground. Another way to do it is to apply Bernoulli's equation, which amounts to the same thing as conservation of energy.

Let's do it that way, just to convince ourselves that the methods are the same. But the pressure at the two points is the same; it's atmospheric pressure at both places.

We can measure the potential energy from ground level, so the potential energy term goes away on the left side, and the kinetic energy term is zero on the right hand side. This reduces the equation to: The density cancels out, leaving: This is the same equation we would have found if we'd done it using the chapter 6 conservation of energy method, and canceled out the mass.

To determine the pressure 35 m below ground, which forces the water up, apply Bernoulli's equation, with point 1 being 35 m below ground, and point 2 being either at ground level, or 25 m above ground.