Understanding Fluid Flow: Laminar, Turbulent, and Pressure
Understanding Fluid Flow
Non-viscous flow can be classified as:
- Laminar Flow: Flow without significant mixing of particles but with significant viscous shear. If a dye is injected, the flow does not mix with the fluid except for molecular activity. It retains its identity over a relatively long period of time.
- Turbulent Flow: The flow varies irregularly, so that its quantities show a random variation. A dye injected into the flow mixes immediately due to the random motion of particles.
Pressure in Fluids
Static Pressure: The pressure P is generally expressed as gauge pressure.
Dynamic Pressure: When fluids move in a duct, the inertia of motion produces an additional increase of the static pressure in a collision over an area perpendicular to the motion. This force is produced by the action of what is known as dynamic pressure. The dynamic pressure depends on the speed and density of the fluid.
Total Pressure: Also called stagnation pressure, it is the pressure in a stalemate.
Pitot Tube Flow Measurement
In a Pitot tube, flow is linked with the difference between the dynamic pressure (static pressure plus “high speed”) and the static pressure. The center hole is for measuring dynamic pressure, and the side holes measure static pressure at the wall. Its use is particularly widespread in cases where no load losses are accepted at the facility. The flow is obtained through the velocity and cross-section of the pipe, and the pressure difference measurement provides information on the speed at the measurement point. This speed is only representative of the flow if it is constant throughout the section.
Hydrometric Measurement
A hydrometer is an instrument used to determine the relative density of liquids without calculating its mass and volume. It is usually made of glass and consists of a hollow cylinder with a heavy bulb at its tip so it can float upright. The hydrometer is introduced gradually into the liquid to float freely. The reading is taken from the scale at the point where the surface of the liquid touches the stem of the hydrometer. Hydrometers usually contain a paper scale inside them so you can directly read the specific gravity in grams per cubic centimeter. It is based on the principle of buoyancy. When placed in water, the reading is 1.0.
Cavitation Phenomenon
Cavitation is caused by a sharp drop in pressure in a fluid below its vapor pressure. This is due to sudden changes in flow velocity or a partial vacuum effect, usually given by large fluid columns as in the case of pumps. This pressure drop causes steam bubbles, which travel through the fluid to a place where the pressure is restored, causing a sudden condensation of gases, generating an implosion. The elements that suffer the greatest damage from this phenomenon are propellers, pumps, hydroelectric turbines, rudders of ships, etc. It is not easy to avoid cavitation; it cannot be controlled in many cases. For these cases, what is done is to modify the geometry of the elements and/or perform a careful design of the equipment involved.
Bernoulli’s Equation vs. Energy Equation
Bernoulli’s equation, derived from Newton’s second law, is valid along a streamline. In contrast, the Energy Equation, derived directly from the first law of thermodynamics, is valid between two sections of a fluid flow.