ISRO Scientist Mechanical key terms

States that entropy of an isolated system never decreases and that no cyclic device can convert all heat input into work, setting a directional limit on energy conversion in engines and refrigerators.

A thermodynamic property measuring molecular disorder and unavailable energy; it increases in irreversible processes and stays constant in reversible adiabatic ones, governing the direction of spontaneous change.

A reversible cycle of two isothermal and two isentropic processes operating between two reservoirs; it gives the maximum possible thermal efficiency for given source and sink temperatures.

For a refrigerator or heat pump, the ratio of desired heat transfer to net work input; higher values indicate a more effective cycle for the same temperature lift.

The standard refrigeration cycle using compressor, condenser, expansion valve and evaporator, exploiting latent heat of a refrigerant as it alternately vaporises and condenses.

An adiabatic flow through a restriction such as a valve or orifice with no work transfer; enthalpy remains essentially constant while pressure drops and temperature may change.

An idealised adiabatic process that is also reversible, so entropy stays constant; used as the reference compression and expansion path in air-standard cycle analysis.

The ratio of net work output to heat supplied in a power cycle; it quantifies how effectively fuel energy is converted into useful shaft work.

The maximum useful work obtainable from a system as it reversibly reaches equilibrium with its environment; also called availability and used to locate true inefficiencies.

Along a steady, incompressible, frictionless streamline, the sum of pressure head, velocity head and elevation head stays constant; a statement of energy conservation in fluids.

A dimensionless ratio of inertial to viscous forces in a flow; it predicts whether flow is laminar or turbulent and controls friction factor and heat transfer behaviour.

The thin region near a solid surface where velocity rises from zero at the wall to the free-stream value, dominating skin friction and convective heat transfer.

A dimensionless ratio of convective to conductive heat transfer across a fluid layer; correlations express it as a function of Reynolds and Prandtl numbers.

The ratio of momentum diffusivity to thermal diffusivity of a fluid; it indicates how quickly velocity and temperature profiles develop relative to each other.

A dimensionless coefficient relating pressure drop in a pipe to flow velocity and pipe geometry; it depends on Reynolds number and relative roughness.

Expresses conservation of mass for a control volume: the rate of mass entering equals the rate leaving plus the rate of accumulation inside.

The momentum equations for viscous fluid motion, balancing inertia, pressure gradient, viscous forces and body forces; they form the basis of most computational fluid dynamics.

A second-order tensor describing the state of stress at a point; its components give normal and shear stresses on any plane through that point.

The normal stress acting on a plane where shear stress is zero; the three principal stresses at a point define the extreme normal stresses and are eigenvalues of the stress tensor.

A graphical construction plotting normal stress against shear stress to find principal stresses, maximum shear stress and stresses on any inclined plane under plane stress.

An equivalent stress combining all stress components, used to predict yielding of ductile materials; yielding is assumed when it reaches the yield strength in uniaxial tension.

The ratio of material strength or allowable stress to the actual working stress; it accounts for uncertainty in loads, properties and analysis.

Localised increase in stress near geometric discontinuities such as holes, notches and fillets; quantified by a factor multiplying nominal stress.

Progressive damage and failure of a material under repeated or fluctuating stress at levels below the static strength, often starting at a surface defect.

A plot of alternating stress amplitude against number of cycles to failure; it shows the endurance limit below which many steels survive indefinitely.

The critical axial compressive load at which a slender ideal column becomes unstable, varying inversely with the square of its effective length.

The effective length of a compression member divided by its least radius of gyration; it determines whether buckling or direct yielding governs failure.

The number of independent coordinates needed to fully describe the configuration of a mechanism or vibrating system.

A mobility formula giving the degrees of freedom of a planar mechanism as three times the number of links minus two times the number of lower pairs, minus higher pairs.

A planar mechanism of four rigid links connected by four turning pairs; variants include crank-rocker and double-rocker, governed by Grashof's condition.

A mechanism converting rotary motion of a profiled cam into prescribed oscillating or translating motion of a follower, defined by displacement, velocity and acceleration diagrams.

The frequency at which a system oscillates freely after disturbance, determined by its mass and stiffness; resonance occurs when excitation matches it.

A dimensionless measure of how quickly oscillations decay, equal to actual damping divided by critical damping; it distinguishes underdamped, critically damped and overdamped response.

The rotational speed at which a shaft's operating frequency matches a natural frequency of its lateral vibration, causing large amplitude whirl and possible damage.

The ratio of flow speed to local speed of sound; it classifies flow as subsonic, transonic or supersonic and controls compressibility effects in nozzles and diffusers.

The pressure a flowing fluid would reach if brought to rest isentropically; it equals static pressure plus dynamic pressure contribution and stays constant in loss-free flow.

The ideal cycle for steam power plants, comprising pump, boiler, turbine and condenser processes; modified with reheat and regeneration to raise efficiency.

The ordered set of angles and dimensions defining a single-point tool, including back rake, side rake, relief and nose radius, which control cutting forces and finish.

A force diagram relating cutting force, thrust force, shear force and friction on the rake face during orthogonal machining, used to derive the shear angle relation.

Gradual loss of cutting edge material through abrasion, adhesion, diffusion or chipping; flank and crater wear are monitored against tool-life criteria.

The ease with which a material can be cut, judged by tool life, surface finish, cutting forces and power consumption under given conditions.