ME POST flashcards

41 cards. Read the term and the definition here. The deck at the top of this page is optional.

Reynolds Number

Dimensionless ratio of inertial to viscous forces, Re = ρVD/μ. It predicts whether pipe flow is laminar (below about 2300) or turbulent (above about 4000).

Bernoulli Equation

For steady, incompressible, frictionless flow along a streamline, p/ρg + V²/2g + z is constant. It converts between pressure, velocity, and elevation head.

Darcy–Weisbach Head Loss

Friction head loss in a pipe equals f(L/D)(V²/2g), where f is the friction factor, L the pipe length, D the diameter, and V the mean velocity.

Pump Affinity Laws

For a geometrically similar pump at fixed impeller diameter, flow scales with speed, head with speed squared, and power with speed cubed.

NPSH

Net positive suction head is the absolute pressure margin above vapor pressure at the pump inlet. If available NPSH falls below required NPSH, cavitation damages the impeller.

Carnot Efficiency

Maximum possible heat engine efficiency between absolute temperatures T_H and T_L is 1 − T_L/T_H. No real cycle between the same reservoirs can exceed it.

Isentropic Ideal Gas Relation

For an ideal gas undergoing a reversible adiabatic process, pV^γ is constant and T₂/T₁ = (p₂/p₁)^((γ−1)/γ), where γ is the specific heat ratio.

Stefan–Boltzmann Law

Radiant heat transfer from a gray body is εσA(T_s⁴ − T_surr⁴), with σ = 5.67×10⁻⁸ W/(m²·K⁴) and ε the surface emissivity.

LMTD

Log-mean temperature difference equals (ΔT₁ − ΔT₂)/ln(ΔT₁/ΔT₂) for a heat exchanger. It drives heat transfer as q = UA·LMTD in counterflow or parallel flow.

Heat Exchanger Effectiveness

Effectiveness is actual heat transfer divided by the thermodynamic maximum possible for given inlet conditions. It rises with NTU = UA/C_min and depends on flow arrangement.

Bending Stress Formula

Normal stress in a beam is σ = My/I, where M is bending moment, y is distance from the neutral axis, and I is the second moment of area.

Torsion of Circular Shaft

Maximum shear stress is τ = Tc/J, where T is torque, c the outer radius, and J = πd⁴/32 the polar moment. Twist angle is θ = TL/(GJ).

Principal Stress Formula

For plane stress, σ₁,₂ = (σx+σy)/2 ± √[((σx−σy)/2)² + τxy²]. Maximum in-plane shear stress equals half the difference of principal stresses.

Poisson's Ratio

Poisson's ratio is the negative ratio of lateral to axial strain, ν = −ε_lateral/ε_axial. For isotropic materials, G = E/[2(1+ν)].

Endurance Limit

The stress amplitude below which a ferrous material survives an indefinitely large number of fatigue cycles. For steels it is roughly half the ultimate tensile strength.

Taylor Tool Life Equation

Tool life relates to cutting speed as VTⁿ = C, where n and C are empirical constants. Higher speed sharply shortens tool life.

Chvorinov's Rule

Solidification time in a casting equals B(V/A)², where V is volume and A is heat-transfer surface area. Thicker sections cool more slowly and feed shrinkage.

Eutectoid Reaction

In the iron–carbon system, austenite of 0.76% carbon transforms at 727 °C into pearlite, an alternating lamellar mixture of ferrite and cementite.

Quenching and Tempering

Quenching rapidly cools austenite to form hard, brittle martensite. Tempering reheats it to relieve stress and restore toughness at some strength cost.

Gauge Factor

For a strain gauge, gauge factor is the fractional resistance change divided by fractional length change, GF = (ΔR/R)/(ΔL/L). Metal foil gauges are near 2.

Wheatstone Bridge

A four-arm resistive bridge that converts small resistance changes, such as from strain gauges, into a measurable voltage. It is balanced when arm resistance ratios are equal.

Seebeck Effect

A temperature difference between two junctions of dissimilar metals produces an emf proportional to that difference. Thermocouples use this effect and need cold-junction compensation.

First-Order Time Constant

The time for a first-order instrument to reach 63.2% of its final response after a step input. Settling to within 2% takes about four time constants.

Transfer Function

The ratio of Laplace-transformed output to input with zero initial conditions. Its poles, the denominator roots, determine stability and response shape.

Damping Ratio

For a second-order system, ζ sets the response character: ζ > 1 overdamped, ζ = 1 critically damped, 0 < ζ < 1 underdamped and oscillatory, ζ = 0 undamped.

PID Control

Control action u = K_p e + K_i ∫e dt + K_d de/dt. Proportional action responds to present error, integral removes offset, derivative adds damping.

Routh–Hurwitz Criterion

A tabular test on the characteristic polynomial. The number of sign changes in the first column equals the number of right-half-plane poles, so no changes means stability.

Rankine Cycle

The ideal steam power cycle of pump, boiler, turbine, and condenser. Efficiency improves with higher steam temperature and pressure and lower condenser pressure.

Brayton Cycle

The ideal gas turbine cycle of compressor, combustor, and turbine. Efficiency rises with pressure ratio and is improved by regeneration, intercooling, and reheat.

Heat Rate

Thermal energy input per unit of electrical output, usually in kJ/kWh. A lower heat rate means a more efficient power plant.

Relative Humidity

The ratio of actual water vapor pressure in air to the saturation pressure at the same dry-bulb temperature, expressed as a percentage.

Ton of Refrigeration

A cooling capacity equal to 3.517 kW, the rate of heat removal needed to freeze one short ton of water in 24 hours.

Net Present Value

The sum of all project cash flows discounted to time zero at the minimum attractive rate of return. A positive NPV means the project is economically acceptable.

Internal Rate of Return

The discount rate at which a project's net present value equals zero. A project is acceptable when its IRR exceeds the minimum attractive rate of return.

Break-Even Quantity

The production volume at which total revenue equals total cost, computed as fixed cost divided by unit price minus unit variable cost.

Engineer's Seal

A licensed engineer's stamp on drawings and reports certifying that the work was prepared or directly supervised by that engineer and meets applicable standards.

Conflict of Interest

A situation where personal or financial interest could bias professional judgment. It must be disclosed in writing and avoided when it could compromise client or public interest.

Paramountcy Clause

The ethical principle that an engineer's first obligation is to protect public health, safety, and welfare, which takes precedence over employer or client loyalty.

Quadratic Formula

Roots of ax²+bx+c=0 are [−b ± √(b²−4ac)]/(2a). The discriminant b²−4ac indicates two real, one repeated, or two complex roots.

Laplace Transform Pairs

Common pairs: L{1}=1/s, L{e^(at)}=1/(s−a), L{sin ωt}=ω/(s²+ω²), L{cos ωt}=s/(s²+ω²). These convert differential equations into algebra.

Coefficient of Variation

The ratio of standard deviation to mean, s/x̄, expressing scatter as a fraction of the average. It allows comparison of variability between data sets of different scale.