As the Mach numbers achieved in the test section of a transonic tunnel rise, there is a corresponding boundary layer build up. The boundary layer build up results in blockage and an increased energy consumption that imposes a practical limit of about Mach number 0.9. To reduce the blockage, the test section is provided with porous or slotted walls through which boundary layer removal takes place. Conventionally, as test section Mach numbers approach the transonic range, auxiliary suction is introduced in the plenum chamber surrounding the porous test section walls, so as to remove boundary layer build up and thus to relieve blockage. The operating range of transonic tunnels has been extended by this procedure but the tunnel operating limits in Reynolds numbers are imposed generally by main drive power and boundary layer removal at higher Mach numbers. These limits are the result of the inefficiency of boundary layer removal since low energy air is being operated on and reintroduced into the tunnel to maintain the air mass of the system. Thus, a large amount of power is required to operate the removal process. These large power requirements occur in the main drive fan system when the removal is induced with flaps or other means at the trailing edge of the test section and in the compressor when an auxiliary removal circuit is used. Accordingly, much of the time the compressor must operate at "off design" inefficient conditions. The operating pressure limits of testing in the tunnel are therefore restricted by these limitations.
Reynolds number effects on skin friction, on boundary layer transition, and on shock and boundary layer interaction have been identified as a primary cause of difficulty in scaling wind tunnel data to full-scale drag and other aerodynamic characteristics.
It is an object of this invention to extend the Reynolds number and Mach number capability of a transonic tunnel.
It is a further object of this invention to reduce the main drive fan power load by boundary layer removal and ejector augmentation. These objects, and others as will become apparent hereinafter, are accomplished by the present invention.
According to the teachings of the present invention, a high pressure ejector system is located in a closed circuit wind tunnel downstream of the test section and in the forward end of the diffuser where it acts as an injector as well as an ejector. High energy air is injected into the diffuser boundary layer thereby tending to reduce the boundary layer thickness and, simultaneously the ejector accelerates the main flow and creates a corresponding pressure rise at the downstream end of the diffuser, so that the pressure rise requirements of the conventional fan system are reduced. Because test section static pressures at the desired Reynolds numbers are above atmospheric, boundary layer removal flow can exhaust directly to the atmosphere, and this removal flow is replenished by the flow introduced through the ejector. Thus, the large amount of power required to operate the boundary layer removal system is eliminated, and the operating limits of the tunnel are extended. The simultaneous occurence of these processes produce transonic flow at Reynolds numbers appreciably greater than are presently obtained with conventional power systems.
BRIEF DESCRIPTION OF THE DRAWINGS
For a fuller understanding of the present invention, reference should now be had to the following detailed description thereof taken in conjunction with the accompanying drawings wherein:
FIG. 1 is a schematic plan view of a closed circuit transonic wind tunnel with ejector augmentation; and
FIG. 2 is a sectional view taken along line 2--2 of FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In FIG. 1 the numeral 10 generally designates a closed circuit transonic wind tunnel including a variable pitch constant speed main drive fan 12 driven by motor 14, turning vanes V, a sphere plenum 26 surrounding test section 18, a diffuser plenum 28, and diffuser 30. Exhaust line 60 which contains throttle valve 62 and exhaust valve 64 is in fluid communication with sphere plenum 26 and leads to atmospheric exhaust stack 66.
Test section 18 has a model support strut 20 which supports model 22. Porous walls 24 separate test section 18 from sphere plenum 26 with holes 25 permitting fluid communication therebetween. Sphere plenum 26 is in fluid communication with diffuser plenum 28 through bypass valve 27 which is used for Mach number control. Fluid communication between diffuser plenum 28 and diffuser 30 is controlled by doors 29 which permit further boundary layer removal. Ejector nozzles 32 are mounted on the downstream side of the model support strut 20 to minimize blockage. Additional peripheral ejector nozzles 34 are mounted in the diffuser 30 so that a portion of the high energy primary air supplied thereto will be injected into the local boundary layer. Ejector nozzles 32 and 34 having exit Mach numbers in the range of 1.6 to 3.0 have proven to be satisfactory.
Supply tanks 36, 37 and 38 which contain air at high pressures such as 3,000 PSI are connected to manifold 45 by lines 39, 40 and 41 containing valves 42, 43 and 44, respectively. Manifold 45 supplies nozzles 34 via lines 46 and 47 which contain valves 48 and 49 respectively. Manifold 45 additionally supplies nozzles 32 via line 50 which contains pressure regulator 51 and valve 52 and which connects with manifold 53 which supplies nozzles 32 via lines 54, 55 and 56.
As best shown in FIG. 1, variable pitch constant speed main drive fan 12 driven by electric motor 14 supplies high pressure air to the test section 18 where model 22 is located. The air passes from the test section 18 to diffuser 30 from which it is returned to the main drive fan 12. The vanes V minimize friction losses in the wind tunnel 10 as the circulating high pressure air undergoes directional changes. Under the test conditions under consideration, such as a Reynolds number of 11 × 106 per foot at Mach number 1.00, the static pressure in the test section 18 is above atmospheric and, therefore, boundary layer removal takes place through porous walls 24 without the requirement for auxiliary power. With throttle valve 62 and exhaust valve 64 open, as illustrated, the boundary layer air removed exhausts to atmosphere from test section 18 via holes 25, exhaust line 60, throttle valve 62, exhaust valve 64 and exhaust stack 66. Doors 29 are located in the upstream portion of diffuser 30 and rather than permitting the conventional return of the boundary layer air removed from the test section as in the case of conventional re-entry doors, doors 29 permit further boundary layer removal. The boundary layer air removed from the upstream portion of diffuser 30 passes through doors 29 into diffuser plenum 28 and past bypass valve 27 into sphere plenum 26 where it joins with the boundary layer air removed from test section 18 and passes to exhaust. The total amount of boundary layer removal is not increased by opening bypass valve 27 to permit boundary layer removal but rather bypass valve 27 serve as a vernier control for controlling boundary layer removal and thereby the Mach number in the test section 18. Vernier control is possible because bypass valve 27 is much smaller than valve 62, e.g., 30 inches and 8 feet, respectively, and a 30° opening of bypass valve 27 would correspond to approximately a 1° opening of valve 62.
Since the air mass of the system must be maintained to achieve the desired Reynolds numbers, the exhausted air must be replaced. To replace the relatively low energy boundary layer air exhausted to the atmosphere, ejector nozzles 32 and 34 are connected to a source of high pressure air defined by supply tanks 36, 37 and 38. Ejector nozzles 32 are mounted in the wake of model support strut 20 to minimize blockage and so are located in the mainstream where the high energy flow mixes with the mainstream. Ejector nozzles 34 are located in the start of the diffuser, their location being so chosen that a portion of the high energy primary air would be injected into the local boundary layer. The energizing of this boundary layer averts separation and thus improves diffuser efficiency. An appreciable increase in the energy level of the flow is thus obtained by mixing a relatively small mass of high-energy air supplied via ejector nozzles. 32 and 34 with the main stream. After the diffusion process this additional energy is manifested as a pressure increase at the diffuser exit. The fan pressure rise is applied as a further increment above this value, so that tunnel operation is possible at greater stagnation pressures and hence higher Reynolds numbers without increasing the main drive power input.
The ejector tunnel is not a continuous flow system, but rather resembles a blow-down tunnel. Since the blow-down time is very short compared to the pump-back time, high-speed sequencing is required to coordinate all phases of the operation of the ejector system. The main fan system is rapidly unloaded during the start of the ejection system. The amperes to the electric motor 14 are maintained during this period by changing the fan blade pitch of fan 12, which is reduced again prior to turning the system off. The nozzle valves and regulator system is sequenced by a timing control panel and the data-gathering process is controlled by a minicomputer or the like. Mach control is done with the computer operated servo-controlled bypass valve 27 which regulates the boundary layer removal by sensing Mach number.
Although the above description is directed to a transonic wind tunnel employing a main drive fan 12 with only supplemental or replacement flow provided by the ejector system, if desired, the ejector system can also be employed to provide the main flow. Such a system would eliminate main drive fan 12 and its motor and would require an ejector system of increased capacity but otherwise would be similar to the system illustrated and described above. Other modifications such as changing the location from which the control boundary layer removal take place are also possible. Additionally, the teachings of this invention may be applied to existing tunnels to extend their range of operation. The structure of conventional wind tunnels may be modified or adapted for using ejector augmentation. For example, doors 29 may be the conventional re-entry doors and exhaust line 60 may be incorporated into an auxiliary compressor circuit used to pressurize and return the boundary layer air removed from the test section to the system. The number of bypass valves 27 the size of the various valves, the capacity of the storage tanks 36, 37 and 38 and the number and location of nozzles 32 and 34 will be dictated by design considerations such as the mass flow rate of the air, the rate of air mass removal at the boundary layer, the duration of operation required or desired and the intended range of operation.
Although a preferred embodiment of the present invention has been illustrated and described, other changes will occur to those skilled in the art. It is therefore intended that the scope of the present invention is to be limited only by the scope of the appended claims.