Tone noise reduction in turbomachines

A turbomachine including:

• a casing;
• and
• a cascade of circumferentially spaced blades located in the casing and rotatable about the axis of the turbomachine;
• wherein:
• the turbomachine has at least one operating condition which generates supersonic fluid flow at the blades such that tone noise is produced at the blade passing frequency of the operating condition or at a harmonic frequency thereof;
• the casing has a set of radial modes with respective attenuation rates for the upstream propagation of the tone noise at said frequency along the casing;
• and
• the blades are shaped so that, at said operating condition and at a plane which is normal to the axis of the turbomachine and which is upstream of the radially outermost point of the blade leading edge relative to the overall direction of fluid flow through the turbomachine, the acoustic power of the tone noise at said frequency in the least attenuated radial mode is more than 5 dB lower than the total acoustic power of the tone noise at said frequency in the set of radial modes.

A turbomachine including:

• a casing;
• and
• a cascade of circumferentially spaced blades located in the casing and rotatable about the axis of the turbomachine;
• wherein:
• the turbomachine has at least one operating condition which generates supersonic fluid flow at the blades;
• the blades are shaped such that at the supersonic operating condition each blade produces first and second pressure shocks in the working fluid, the shocks extending out to and being radially spaced at a plane which is normal to the axis of the turbomachine and which is upstream of the radially outermost point of the blade leading edge relative to the overall direction of fluid flow through the turbomachine, and the shocks producing tone noise at the blade passing frequency of the operating condition or at a harmonic frequency thereof;
• and
• the casing has a set of radial modes with respective attenuation rates for the upstream propagation of the tone noise at said frequency along the casing, each radial mode within the set having a radially varying amplitude;
• wherein the maximum amplitude of the least attenuated radial mode is at a radial position between the shocks.

A turbomachine including:

• a casing;
• and
• a cascade of circumferentially spaced blades located in the casing and rotatable about the axis of the turbomachine;
• wherein:
• the turbomachine has at least one operating condition which generates supersonic fluid flow at the blades;
• wherein the blades are shaped such that, at the supersonic operating condition, each blade produces a pressure shock in the working fluid, the shock extending out to a plane which is normal to the axis of the turbomachine and which is upstream of the radially outermost point of the blade leading edge relative to the overall direction of fluid flow through the turbomachine, and the shock having first and second portions at said plane, which portions are radially spaced and circumferentially spaced such that, taking the axis of the turbomachine as the origin, the circumferential angle between the portions is at least a quarter of the angle in the circumferential direction between neighbouring blades, and the shock producing tone noise at the blade passing frequency of the operating condition or at a harmonic frequency thereof;
• and
• the casing has a set of radial modes with respective attenuation rates for the upstream propagation of the tone noise at said frequency along the casing, each radial mode within the set having a radially varying amplitude;
• wherein the maximum amplitude of the least attenuated radial mode is at a radial position between the shock portions.

A method of designing a blade which, in use, is one of a cascade of circumferentially spaced blades located in the casing of a turbomachine, the blades being rotatable about the axis of the turbomachine, the casing having an acoustic liner which covers an inner surface thereof and which extends upstream of the blades, and the turbomachine having at least one operating condition which generates supersonic fluid flow at the blades;

• the method comprising the steps of:
• (a) determining the flow field produced at the operating condition in the casing upstream of the blade relative to the overall direction of fluid flow through the turbomachine;
• (b) from the flow field, calculating the level of noise exiting the casing caused by the supersonic fluid flow;
• and
• (c) adjusting the shape of the leading edge of the blade and repeating steps (a) and (b) to reduce the level of the noise.