ME POST key terms

The principle that every term in a valid physical equation must reduce to the same combination of base dimensions. It lets engineers check formulas for errors and derive relationships between variables without full analysis.

A method that rewrites a physical relationship among n variables into n minus the number of independent dimensions dimensionless groups. These groups allow experiments on small models to predict full-scale behavior.

A scalar by which a nonzero vector is multiplied when a square matrix acts on it. In vibration and stability problems, eigenvalues give natural frequencies and indicate whether a system grows or decays.

The ratio of inertial to viscous forces in a flow, equal to density times velocity times length divided by viscosity. Low values indicate laminar flow, high values turbulent flow, and it governs model scaling.

An energy statement for steady, incompressible, frictionless flow along a streamline: pressure head plus velocity head plus elevation head stays constant. It converts between pressure and speed in nozzles, venturis, and pitot tubes.

Two regimes of fluid motion. Laminar flow moves in orderly parallel layers dominated by viscosity; turbulent flow has chaotic eddies and mixing. The regime changes friction factor and heat transfer behavior sharply.

A formula for head loss in a pipe: friction factor times length over diameter times velocity head. It is the standard basis for sizing pipes and comparing friction losses in series and parallel networks.

Net positive suction head, the margin of absolute pressure above a liquid's vapor pressure at the pump inlet. Too little causes cavitation, which erodes impellers and degrades performance.

The formation and violent collapse of vapor bubbles in a liquid where local pressure falls below vapor pressure. It produces noise, vibration, and surface pitting, and it limits pump suction conditions and valve operation.

Scaling rules for a geometrically similar pump: flow varies with speed, head with speed squared, and power with speed cubed. They let engineers predict performance at a new speed or impeller diameter.

A dimensionless or conventional index describing pump or turbine geometry relative to flow and head at best efficiency. It guides selection among radial, mixed-flow, and axial machines.

The conservation of energy for a system: heat added minus work done equals the change in internal energy plus flow and kinetic energy terms. It underpins every energy balance in cycles and devices.

A property combining internal energy with the product of pressure and volume. It equals the heat exchanged in a constant-pressure process and appears directly in steam tables and turbine energy balances.

The ratio of actual work output to ideal work for a turbine, or ideal to actual for a compressor, when the process is compared with a reversible adiabatic one. It quantifies irreversibility in turbomachinery.

A statement that spontaneous processes increase total entropy and that heat cannot flow from cold to hot without work input. It sets the maximum efficiency of engines and refrigerators.

The ratio of useful cooling or heating delivered to the work input of a refrigeration or heat pump cycle. Higher values mean more efficient operation, and it is bounded by the Carnot limit.

The effective temperature driving force in a heat exchanger, computed from the hot and cold terminal differences. It accounts for the changing temperature difference along parallel and counterflow passages.

A single value combining convective and conductive resistances across a heat exchanger wall, usually referenced to one surface area. It converts area and temperature difference into heat rate.

A heat exchanger rating approach using number of transfer units and effectiveness, useful when outlet temperatures are unknown. Effectiveness relates actual heat transfer to the thermodynamic maximum.

The ratio of peak local stress at a geometric discontinuity, such as a hole or fillet, to nominal stress in the same section. It drives fatigue and brittle fracture at notches.

The stress amplitude below which a material survives an indefinitely large number of load cycles without failure. Ferrous metals often show one, while most nonferrous metals do not.

The ratio of a material's strength or allowable stress to the expected working stress. It covers uncertainty in loads, material properties, and analysis methods.

A graphical construction showing how normal and shear stress vary with plane orientation at a point. It gives principal stresses and maximum shear stress for combined loading.

The normal stress acting on a plane where shear stress is zero. The largest and smallest principal stresses set the failure criterion for ductile and brittle materials under multiaxial loading.

The permitted dimensional variation of a part and the resulting clearance or interference when two parts assemble. Standard fit systems let designers specify interchangeability without custom matching.

Controlled heating and cooling of a metal to alter its microstructure and properties, such as hardening, tempering, or annealing. It changes strength, hardness, and toughness without changing shape.

The capacity of a material to be joined by welding into a sound joint under a given process. It depends on composition, especially carbon equivalent, and on preheat and cooling practice.

A measure of the small-scale roughness left on a machined surface, usually expressed as average roughness. It affects friction, wear, fatigue, and sealing performance.

A sensor whose electrical resistance changes when stretched or compressed with a test surface. Bonded to a structure, it converts mechanical strain into a measurable voltage through a bridge circuit.

A four-arm resistive circuit that produces a voltage proportional to a small resistance imbalance. It is the standard readout for strain gauges and resistance temperature detectors.

A temperature sensor made of two dissimilar metals joined at a measuring junction, producing a voltage that depends on the junction temperature difference. Cold-junction compensation is required for accurate readings.

The process of comparing an instrument's output with a known reference and adjusting it to reduce error. It establishes traceability and quantifies uncertainty in measurements.

A feedback strategy that combines proportional action on current error, integral action on accumulated error, and derivative action on error rate. Tuning these gains balances speed of response against overshoot and steady-state offset.

The ratio of a system's output to its input in the Laplace domain, assuming zero initial conditions. It describes dynamics and is used to analyze poles, zeros, and response.

The property that a system's response to a bounded disturbance dies away rather than growing. It is judged from the location of closed-loop poles in the complex plane.

The idealized steam power cycle of pump, boiler, turbine, and condenser. Its efficiency depends mainly on boiler pressure and condenser pressure, and it is improved by reheat and regeneration.

The thermal energy input a power plant consumes per unit of electrical output, usually in kilojoules per kilowatt-hour. Lower values mean higher plant efficiency and lower fuel cost.

The ratio of average load over a period to the peak load in that period. It indicates how fully capital equipment is used and influences the economics of generation and distribution.

A working fluid that absorbs heat by evaporating at low pressure and releases it by condensing at high pressure in a refrigeration cycle. Selection considers ozone depletion, global warming potential, and safety.

A graph of moist-air properties showing dry-bulb and wet-bulb temperature, humidity ratio, relative humidity, and enthalpy. It is used to analyze air-conditioning and cooling-tower processes.

The equivalent value today of a stream of future cash flows discounted at a chosen interest rate. It allows alternatives with different timing of costs and benefits to be compared on one basis.

The time required for cumulative savings or returns to recover an initial investment. It is simple to compute but ignores cash flows after payback and the time value of money.

A situation in which a professional's personal interest, financial or otherwise, could improperly influence engineering judgment. It must be disclosed and, where necessary, the engineer must withdraw from the decision.

The degree of care and skill that a reasonably competent engineer would exercise under similar circumstances. It is the benchmark used to judge negligence in professional practice.

A formal set of principles governing professional conduct, typically covering public safety, competence, honesty, and fairness. Licensed engineers are bound by their jurisdiction's code.