Unit One – Sources of Noise Flashcards
Involute gear tooth shape
a gear tooth shape that provides constant rotational velocity during rotation of the gear.
Reversal of sliding velocity
the change in direction of sliding velocity between contact surfaces along the line of action of meshed gears.
Tooth-meshing frequency
the frequency that controls tonal noise harmonic frequencies for two meshed gears, related to the number of teeth in the smaller pinion gear and its angular speed in RPM.
Transmission errors
the combination of variations in the number of teeth in contact and the geometry of the gear teeth from perfect involute profiles which results in a departure from a constant angular velocity.
Asynchronous alternating current (AC) electric motors
an electric motor also known as an induction motor configured so that AC current is applied to the windings in the fixed stator to create a rotating magnetic field which induces a current in the rotor windings. These motors experience ‘slip’ which is the difference in speeds between the speed of the rotor and the rotating magnetic field created by the stator.
Synchronous alternating current (AC) electric motors
an electric motor configured so that in addition to applying AC current to the stator, AC current is applied to the rotor via armature windings through brushes or slip rings. These motors experience no ‘slip’ because the stator and rotor fields align and maintain position.
Direct current (DC) electric motors
an electric motor configured so that DC current is applied to the stator to produce a stationary magnetic field, and to the rotor through a commutator that reverses polarity to keep its magnetic field aligned with the stator magnetic field.
Magnetostriction
changes in the core shape in a transformer caused by the alternating electrical field which results in the creation of noise.
Indirect injection diesel engine combustion
diesel engine combustion that starts in a prechamber in the cylinder head of the engine, causing a slower rate of combustion.
Direct injection diesel engine combustion
diesel engine combustion that occurs through direct fuel injection into the combustion chamber leading to a faster rate of combustion and more noise.
Engine block transfer function
the difference between the sound pressure level inside the cylinder of an internal combustion engine and the sound pressure level of the noise radiated by the engine block as measured at a distance of one meter from the surface of the block.
Piston slap
the impact of the piston in a combustion engine cylinder against the cylinder wall as the piston is forced from one side of the cylinder to the other as it passes through the top of the stroke.
Lighthill’s stress tensor
a term added to the wave equation for sound to simulate flow noise added to a system due to movement of fluid over a stationary structure.
Causality condition
the characterization describing the delayed effect of noise transmitted from the source to the receiver until the sound pressure generated at the source reaches the receiver.
Reynolds stresses
a component of the total shear stress in a fluid which accounts for turbulent fluctuations in the fluid, based on the ratio of the inertial to viscous stresses.
Mach number
the magnitude of velocity (v) m/s in a fluid flow, divided by the speed of sound (c) m/s in the fluid, 343 m/s for air.
Choked jet flow
the condition of flow in a nozzle when the velocity of the air flow in the nozzle reaches the speed of sound, i.e. becomes sonic, occurring when the pressure inside the nozzle is greater than 1.89 times the ambient pressure outside the jet.
Flow tones
tonal noise created by airflow transverse to an opening caused by pressure fluctuations at the trailing edge of the opening coming into phase with the flow oscillations at the leading edge. The tone may be amplified if there is a cavity under the opening.
Boundary layer displacement thickness
the distance away from a flat surface which the inviscid (low to no viscosity) flow is spaced by the boundary layer flow.
Turbulent boundary layer wavenumber spectrum
the Fourier transform of wall pressures caused by turbulent flow over a surface, taken over space at a fixed time.
Convective ridge
the point in the turbulent layer boundary flow where the amplitude of the pressure at the wall peaks.
Convective wavenumber
the acoustic wavenumber where the spectrum of the turbulent boundary layer peaks, coincident with the peak in the boundary layer wall pressure as flow passes over a surface.
Axial-flow fans
a fan where airflow through the fan is in the direction of the axis of rotation of the fan blades, with no change in direction of the airflow through the fan.
Centrifugal fans
a fan where air is drawn into the fan wheel in the direction parallel to the axis of rotation and thrown out in the radial direction perpendicular to the axis of rotation.
Blade volume-displacement or ‘thickness’ noise
fan noise created when a fan blade displaces air.
Harmonic analyses of wake fields
the use of a Fourier series expansion to estimate the magnitudes of unsteady lifts and radiated noise resulting from blade rotation through the wakes left behind obstructions upstream of the rotating blade.
Blade number mismatching
ensuring the number of blades of an axial flow fan are not an integer multiple of the number of obstructions upstream of the fan blades.
Turbulence ingestion noise
broadband noise caused by turbulence generated upstream of the fan rotor as it passes into the rotor and impacts the blades.
In-line pumps
pumps that move fluid through piston motion in cylinders placed parallel to the rotating shaft, caused by attachment to an angled yoke plate.
Vane pumps
pumps that move fluid through movable vanes mounted on a rotor that is offset in a bore so that on one side, the vanes move outward to collect fluid and on the other side, the vanes retract, and fluid is pushed out of the bore.
Gear pumps
pumps that move fluid through counterrotating gears that pull fluid into the pump through the meshing gears and expel it on the other side as the gears unmesh.
Cavitation
collapse of vapor bubbles in a fluid that have been formed due to localized low static pressure regions caused by turbulence in the flow stream.
Fan laws
ratios relating fan speed, fan size, volume flow and static pressure between two operating conditions, used to estimate performance for operating conditions other than a known performance.
Critical frequency of a rotating component
when the rotational frequency of a rotor-shaft system coincides with the shaft’s natural frequency.
Static balancing
adding or removing mass to a component until the center of gravity of the component falls on the axis of rotation so that the component remains at any given rotational position when placed there and released.
Dynamic balancing
adding masses in pairs about the rotational axis so that the static balance is not changed but the rotational balance is aligned with the principal axis of rotation.
Principal axes of rotation
an axis of rotation where the moments generated by centrifugal forces during rotation sum to zero.
Modal balancing
balancing of a flexible rotating component by adding masses distributed to counter the mode shape at each frequency of resonance.
