ejector -- a constant momentum device where the momentum in a given mass of high - velocity gas is transferred to a second mass of slower moving or non - moving gas , with the resulting mixture having the same momentum as the original high - velocity gas stream . choked flow condition -- a condition that results when the rate of change of the specific volume of the stream of gas divided by the velocity of the gas stream gives a minimum area beyond which the area must increase to maintain continuity of flow . as the area of a gas flowing in a stream tube is decreased , the velocity of the gas stream increases , while the static pressure and temperature of the gas stream drop . at the same time , the specific volume of the gas stream increases , more due to the drop in static pressure than it decreases due to the drop in temperature of the gas stream . a point is reached , thus , where the specific volume of the gas stream divided by the velocity equals a minimum area beyond which the specific volume changes faster than the increase in velocity of the gas stream can move the gas out of the way . when this condition is reached , i . e ., where the weight flow of the gas flow through a nozzle cannot be increased by decreasing the downstream pressure , a &# 34 ; choked flow &# 34 ; condition occurs . to maintain continuity of flow beyond this point , the area of the gas stream must be increased so that the velocity of the gas stream multiplied by the larger area will equal the increased specific volume . mach number = the ratio of the velocity of the fluid or gas stream over the speed of sound in the gas stream at that static temperature ; or ## equ1 ## where u equals the actual velocity of the gas stream , γ equals the specific heat ratio , g equals the gravity constant , r equals the gas constant , and t s equals the static temperature of the gas in the gas stream . the noise generated by the exhaust of a jet engine varies at the eighth power of the velocity of the ejected gas stream . ejectors have been used as a method of reducing the velocity of those gases without losing the momentum or thrust thereof and thereby reducing the noise level of the exhaust gas stream . while it appears to be a simple problem to reduce the noise a desired amount by use of an ejector , in practical application the necessary length of the mixing section of the ejector to get adequate mixing and noise reduction has been on the order of five to ten times the equivalent diameter of the ejector . such a long , cumbersome ejector cannot be mounted on the wings or in any other location on the aircraft in a practical manner for that reason , ejectors have not been widely employed on jet aircraft for reducing jet noise . if the length of the mixing section of the ejector could be reduced substantially beyond that which has been considered necessary , the ejector then becomes a means of reducing the noise of jet aircraft in an economical and practical way . this invention is directed to a method for controlling jet engine noise employing an ejector designed for highvelocity secondary air inlet flow into the mixing zone for mixing with the primary gases so that the mixed gases reach a choked flow condition in the mixing zone , the choked flow condition for the mixed flow occurring before the secondary gas stream velocity reaches mach 1 . 0 . the quantity , pressure and temperature of the primary and secondary gases determine at what secondary entrance mach number choking occurs for the mixed flow . the basic equation for an ejector , referring to fig2 a , is : where w 1 and u 1 equal the mass and velocity of the primary gas : w 2 and u 2 equal the mass and velocity of the secondary air ; p s2 , 3 equals the static pressure at stations along the length of the ejector , 2 being the entry of the ejector and 3 being the exit of the ejector ; and a 2 , 3 equals the area at the points 2 and 3 . in this ejector equation , the area of the ejector at the entrance is the same as the area at the exit , or a 2 = a 3 . in practice , the p s2 a 2 and p s3 a 3 terms are very large and the velocity terms are small . the typical ejector pulls secondary air into the mixing section of the ejector by reduced pressure at the entrance to the mixing section . the velocity energy in the primary gas stream w 1 is transferred to the secondary air stream w 2 in the mixing section , thereby building up the static pressure within the mixing section until it reaches atmospheric pressure at the exit of the ejector . when a diffuser section is added to the exit of the mixing section , as in fig2 b , it causes a reduction in the static pressure at the exit end of the mixing section , which , in turn , reduces the static pressure at the entrance of the mixing section , thereby making it possible to get higher secondary inlet velocities . one of the major problems in the application of the ejector to sound suppression on aircraft has been the necessary length of the mixing section between the entrance and the exit thereof . normally , this has needed to be between five and ten times the equivalent diameter of the mixing section in length , making the use of such ejectors impractical for aircraft installation . it has been observed that there is a very rapid rate of change in the specific volume of both the primary and secondary gas streams in an ejector as a choked flow condition for the mixed primary and secondary gas stream is approached . this rapid rate of change in specific volume causes the gas streams to expand laterally as well as increase in velocity in a falling pressure field within the mixing zone . this phenomenon , combined with the natural tendency of the supersonic primary gas stream to go to a mach 1 . 0 condition ( due to friction ) and the normal shock which occurs at the entrance end of the diffusion section , all combine to result in rapid mixing of the primary and secondary gas streams in a short length of mixing section for high incoming secondary gas stream velocities . fig3 , 5 and 6 illustrate graphically what happens under several different flow conditions typical of possible jet engine operating conditions . fig3 and 4 illustrate , somewhat fictitiously , how the specific volume of the primary and secondary gas streams increases in the mixing section as the inlet mach number , m 2 , is increased to a point giving a mach 1 . 0 mixed condition . the engine operating conditions assumed for fig3 are : the purpose of fig3 and 4 was to study any natural tendency of the primary and secondary gas streams to mix in the mixing section as the inlet mach number increased to a choked flow condition . the specific volume change , or tendency of the gas streams to expand laterlly as well as increase in velocity as the pressure drops , was considered the prime factor to cause rapid mixing . curves 1 and 2 of fig3 illustrate the ratio of the specific volume at the end of the mixing section ( station 3 ) over the specific volume at the primary nozzle exit ( station 1 ) plotted against the secondary inlet mach number at station 2 . in curves 3 and 4 of fig3 the ratio of the specific volume at station 3 over that at station 2 , rather than at the nozzle exit ( station 1 ), was plotted . in all cases , as the choked flow condition is approached , a sharp rise in the specific volume ratio occurs , indicating a strong tendency for the gas to expand laterally . fig4 illustrates a similar plot for the secondary flow . again , a sharp rise in the specific volume ratio occurs as the choked flow condition is approached . fig5 is a plot of the ratio of the static pressure at the end over that at the entrance of the mixing section versus the secondary inlet mach number . there is a sharp drop in the ratio just before the choked flow condition , which indicates the pressure forces available for generating mixing are high . the conditions assumed for fig5 are : p s3 is the static pressure at the exit end of the mixing zone and p s2 is the static pressure at the entrance to the mixing zone . at a choked flow condition of the mixed gas stream , the secondary flow at the entrance to the mixing section is below mach 1 . 0 while the primary gas flow at the entrance to the mixing zone may increase to a supersonic condition . approaching a choked flow condition due to friction , both a subsonic and a supersonic stream normally tend to go to a sonic condition , thereby assisting in rapid mixing of the primary and secondary gas streams . fig6 is a plot of the absolute static pressure at the entrance and exit of the mixing section as the inlet mach number , m 2 , is changed for an engine condition of 10 #/ s turbine gas @ 1660 ° r ., 29 . 4 psia , and 50 #/ s fan air @ 600 ° r ., 19 . 8 psia , and illustrates a buildup in static pressue in the mixing section up to about mach 0 . 56 . thereafter , the static pressure decreases , resulting in a 2 . 55 psi sucking action in the mixing section at the choked condition . up to m 2 = 0 . 635 , a converging nozzle will be needed on the ejector . beyond this point , a diffusing section will be required . fig1 illustrates a schematic representation of a vertical cross - section of a turbofan jet engine employing an ejector system of the type described . the primary gas stream 10 is discharged from a variable - sized area by movement of 10a into the mixing section 12 along with a secondary or fan airstream 14 . a diffuser section 16 is secured to the exit end of the mixing section 12 . the area of the mixing section 12 is preferably made variable by movement of 14a to accommodate the varying operating characteristics of jet engines to maintain a mach 1 . 0 condition of the mixed gas stream in the mixing section . at a choked flow condition in the mixing zone , rapid mixing occurs because of the high rate of change of the specific volume . in this operating range , there is a strong tendency for the gas to expand laterally as well as increase in velocity due to a drop in static pressure . normal shock occurs in the diffusing section . for best efficiency , the shock should occur close to the start of the diffusion section . the location of the normal shock is controlled by the amount of diffusion , which is varied by moving 16a . fig2 illustrates a turbojet without a fan wherein the primary gas stream 10 enters a mixing section 12 and ambient air is pulled into the mixing section 12 through opening 14 . again , a diffuser 16 is attached to the exit end of the mixing section . the diffuser can be varied in length by moving 16a to control the amount of diffusion and maintain the normal shock at or near the entrance of the diffusion section . in utilizing an ejector system for noise suppression for jet engines , if the ejector is designed to have high secondary air inlet velocity so that a choked flow or mach 1 . 0 condition exists at the end of the mixing section , it is possible to take advantage of the rapid change in the specific volume at close to a mach 1 . 0 flow condition , as well as the normal shock , to accomplish rapid mixing of the primary and secondary gas streams . this reduces the needed length of mixing section to get good mixing and makes the ejector a practical system for use on aircraft . by reducing the exit velocity of the jet exhaust , it is possible to get a reduction in the noise level across the whole range of the noise frequency spectrum and not just a shift from one frequency range to another , as often occurs with noise - suppression devices . there is little or no loss , and possibly a gain , in thrust as , in the ejector , the momentum in a high - velocity stream is transferred into a larger mass moving at a lower velocity . there may actually be an opportunity to get a slight increase in thrust by increasing the velocity of slow moving air in the aircraft or around it in the ejector .