Noise suppressing exhaust mixer assembly for ducted-fan, turbojet engine

A mixer assembly for suppressing noise emanating from the exhaust of a ducted-fan turbojet propulsion engine of the type having an exhaust end for receiving said assembly which defines an annular turbine outlet and an annular fan duct outlet, said fan duct outlet coaxially surrounding said turbine outlet, and said assembly comprising:

• a sleeve, a bulb-shaped plug having an axis of symmetry and being disposed coaxially within said sleeve so that the exterior surface of said plug is spaced radially inwardly from the interior surface of said sleeve, a mixer duct of generally annular shape disposed coaxially between said plug and said sleeve, and said sleeve, plug and mixer duct having concentric, radially spaced apart forward ends adapted for attachment to the exhaust end of the engine so that said plug and said mixer duct define therebetween a rearward extension of said turbine outlet and such that said mixer duct and said sleeve define therebetween a rearward extension of said fan duct outlet;
• said plug having a neck portion at its forward end and having a circular cross section that varies in diameter along the axis of said plug between said neck portion and a rear end of said plug, said circular cross section first increasing in diameter to the rear of said neck portion to form a radially outwardly flared offset portion that diverges rearwardly and terminates at a circular cross section of maximum diameter, said circular cross section thereafter decreasing in diameter to form an aft tapered portion that converges radially inwardly and rearwardly and terminates at said rear end of said plug;
• said mixer duct being formed with corrugations that are oriented parallel to the axis of said duct, said corrugations commencing at the forward end of said mixer duct and generally increasing in radial dimension toward the rear end of said mixer duct, said corrugations of said mixer duct defining first and second sets of channels, the first set of channels being defined between mutually opposed surfaces of said mixer duct and said plug and said second set of channels being defined between mutually opposed surfaces of said mixer duct and said sleeve, and channels of said first set alternating circumferentially with channels of said second set, said mixer duct being sized and arranged relative to said plug such that said first set of channels are in radial registration with said offset portion of said plug and such that the radially outermost boundaries of said first set of channels diverge radially outwardly and rearwardly substantially matching the rearward divergence of said offset portion of said plug such that said first set of channels redirects primary flow from the turbine outlet causing such flow to diverge radially outwardly and rearwardly, said mixer duct being sized and arranged relative to said sleeve such that the radially innermost boundaries of said mixer duct that define said second set of channels slope radially inwardly and rearwardly at a slope that generally matches an angle of convergence of the tapered portion of said plug, said second set of channels redirecting the fan flow to cause it to converge radially inwardly and rearwardly and to intersect with the primary flow at an angle of intersection that substantially equals the sum of the angle of divergence of the primary flow and of the angle of convergence of the fan flow, and said radially innermost boundaries of the mixer duct that define said second set of channels being generally aligned with the tapered portion of said plug so that fan flow exiting from said second set of channels flows along said tapered portion of said plug and thereby inhibits the formation of pressure voids along said tapered portion of said plug due to a tendency for the primary flow to separate from the surface of said plug as the primary flow passes across the maximum diameter cross section of the plug.