What Is Fluid Flow?

Fluid flow is the motion of a liquid or gas under the influence of forces, typically pressure gradients, gravity, or external pumping, through a conduit, over a surface, or within an open channel. It is the fundamental physical phenomenon that fluid dynamics and hydraulics seek to describe, predict, and control. Flow regimes range from the orderly, predictable motion of laminar flow at low velocities to the chaotic, energy-dissipating motion of turbulent flow at high Reynolds numbers. The transition between these regimes depends on the ratio of inertial to viscous forces, expressed as the dimensionless Reynolds number, which Osborne Reynolds characterized experimentally in 1883.

Fluid flow encompasses both internal flows, those bounded by solid walls as in pipes and ducts, and external flows, which occur over open surfaces such as aircraft wings, ship hulls, or building facades. The distinction matters because boundary conditions and the development length for fully developed profiles differ substantially between the two configurations. Flow may also be classified as steady (time-invariant mean properties) or unsteady, single-phase or multiphase, and compressible or incompressible.

Electrohydraulics

Electrohydraulics combines electrical signal processing with hydraulic power transmission to achieve precise, high-force actuation. In an electrohydraulic system, a proportional valve or servo valve meters hydraulic fluid flow in response to an electrical control signal, allowing the mechanical position, velocity, or force of a hydraulic actuator to be commanded with the resolution of the electrical input. Solenoid-operated valves open and close in discrete steps, while proportional valves modulate flow continuously across a range. The bandwidth and accuracy of electrohydraulic control systems are determined by the response speed of the valve, the compressibility of the hydraulic fluid, and the inertia of the load. Electrohydraulics is used in applications where pneumatic or purely electrical actuation cannot provide the required force density, including aircraft flight control surfaces, heavy industrial presses, and construction machinery.

Fluid Flow Measurement

Measuring fluid flow accurately is a prerequisite for process control and resource accounting. The principal measurement approaches are differential pressure sensing, which infers velocity from the pressure drop across a restriction; positive displacement, which counts discrete filled volumes passing through the meter; velocity sensing using turbines, vortex shedders, or electromagnetic induction; and acoustic methods, which derive velocity from transit-time differences of ultrasonic pulses. The selection criterion typically involves the fluid's physical properties, the allowable pressure drop, the required rangeability (the ratio of maximum to minimum measurable flow), and whether the pipe may be penetrated. Emerson's flow measurement resource library details how each technology's operating principle constrains its applicability to specific fluid types and process conditions.

Hydraulic Systems

Hydraulic systems transmit power through pressurized fluid, converting mechanical energy at a pump into useful work at an actuator. The governing relationship is Pascal's law: pressure applied to an enclosed fluid is transmitted undiminished throughout the fluid. This allows a small piston to generate a large force at a larger piston, giving hydraulic systems their characteristic force multiplication. In industrial machinery, hydraulic circuits incorporate filters, accumulators, pressure-relief valves, and heat exchangers to maintain fluid cleanliness and temperature within operating limits. IFPS hydraulic system engineering resources address the design of electrohydraulic components including proportional valves, servo valves, and their associated control electronics, covering the bridge between fluid-power engineering and electronic control systems.

Applications

Fluid flow has applications in a range of fields, including:

• Industrial process control, including chemical reactors, refinery pipelines, and water treatment
• Aerospace and automotive hydraulics for actuation of flight surfaces and braking systems
• Civil engineering, for water distribution networks, irrigation channels, and stormwater management
• Biomedical devices, including blood pumps, dialysis machines, and ventilators
• HVAC systems, where chilled water and air flow are balanced across building zones