None.
Not Applicable
1. Field of the Invention
This invention relates to the field of flow control in a fluid. More specifically, the invention comprises the use of a multi-stage microjet-based actuator to create a highly unsteady flow field.
2. Description of the Related Art
Active control of fluid flow has many applications. One particular application involves noise suppression for aircraft. Another application is the control of flow separation over airfoils and lifting bodies. Because such flows typically involve rapid fluctuations, an actuator intended to achieve active control must be very responsive. Such an actuator must be able to create rapidly changing (highly unsteady) fluctuations in the flow.
Large scale supersonic impinging jets are known to create a highly unsteady flow field with a high mean and unsteady momentum. This flow field contains periodic pressure variations centered on certain frequencies. The same is true for small scale impinging jets. A supersonic microjet having a nozzle pressure ratio of 5.8 impinging upon a plate produces strong impinging tones in the range of 25-55 kHz. The resonance loop seen in larger jets is therefore also present in microjets.
The inventors have previously studied the effects of a microjet directed through a hole in a plate. Such a flow produces edge/hole tones. If the microjet's shear layer grazes the edge of the hole large amplitude tones—referred to as “hole tones” are produced. The hole tones tend to be lower in amplitude than simple impingement.
Still others have investigated the effects of a microjet directed into a cylindrical cavity having a closed bottom (a “blind hole”). This flow produced high amplitude tones in a suitable range of frequencies (“suitable” in terms of their possible application to active flow control). These prior results led the inventors to create the present invention, which serves the need for an actuator which can produce high-amplitude disturbances over a wide range of frequencies.
The present invention comprises a multi-stage microjet actuator. The actuator can produce large amplitude flow disturbances over a broad range of frequencies. The disturbance frequency can be varied by altering the geometry of the device, altering the pressure ratio(s) within the device, and combinations of the two. The actuator has many potential applications, including noise abatement for jet aircraft, and flow control over a moving airfoil.
The inventive method proposes to create a microjet having an oscillating pressure, where the variable component is a significant portion of the total pressure. Further, the inventive method proposes to alter the device creating the microjet so that the frequency of oscillation can be grossly and finely adjusted.
One or more small passages—designated as micronozzles 22—pass from cavity floor plane 28 to micronozzle exit plane 30. These are substantially parallel to source jet 16 (The center axis of each micronozzle is within 5 degrees of the center axis of the primary jet). However, in other configurations, one may design actuators with miconozzles which are offset from the center axis of the primary jet by more than 5 degrees). Thus, when primary nozzle 12 directs source jet 16 into impingement block 18, micronozzles 22 generate microjets. These extend downward in the orientation shown in the view.
The small view on the right side of
In order to be useful in the flow control and noise attenuation applications for small to mid-scale models (by no means the only applications for the present invention) the microjet frequency of oscillation should lie between about 1 KHz and about 60 KHz. The microjets themselves are supersonic, though the pressure oscillation may cause them to become subsonic for a portion of the cycle. The mean velocity of the microjet is typically 300-400 meters per second, with the unsteady component being about 50 to 100 meters per second.
Several geometric features are significant to the operation of the device. Referring again to
In order to produce the aforementioned frequency range, it is preferable to vary the ratio h/d from about 1.0 to about 2.0. L is preferably varied from about 1 mm to about 5 mm. The nozzle pressure ratio is preferably varied from about 1.9 to about 6.5. The reader should bear in mind that geometric and flow parameters lying in a different range may be used as required by the specific application.
The reader may wish to know representative dimensions for a particular embodiment. For one example, the source jet issued from a 1.0 mm converging nozzle (The source jet is preferably moderately to strongly underexpanded). An array of four micronozzles (as in
The main parameters governing the behavior of the resulting flow were h, L, and the source jet pressure ratio (nozzle pressure ratio). The device will operate over a wide range of these parameters. At some values steady flow is produced, while at others highly oscillating flow is produced. High amplitude peaks occur at discrete frequencies. Significantly, a modest variation in h/d produces a relatively large shift in the frequency of the peak amplitude. As an example, at h/d=1.3 and a constant nozzle pressure ratio of 4.8, a spectral peak of about 157 dB occurred at a frequency of about 58 kHz. When h/d was shifted to 1.8, the amplitude was about 141 dB at a frequency of about 42 kHz. Furthermore, the spectral peaks became broader with increasing h/d and beyond an h/d of 1.8 there was no distinct spectral peak.
Variations in the primary nozzle pressure ratio also significantly alter the amplitude and resonant frequency of the microjets produced. Looking at
The use of impinging jet resonance allows the actuator to produce highly unsteady flow.
With these thoughts in mind, the reader will note that in
In
The inlet to the cavity is preferably placed within the region of instability, which is the pressure recovery region of the first shock cell of the primary jet. Variations in the placement of the cavity inlet with respect to the shock cells of the primary jet are primarily responsible for the variations seen in
The phenomena illustrated in
The microjets produced by this configuration possess very high momentum (mean velocities generally greater than 300 m/s). Additionally, they can contain a substantial periodic variation (about 70-100 m/s). By using very small variations in the actuator dimensions (typically only a few hundred microns) the frequency of the unsteady component could be tuned over intervals of 10-15 kHz. These actuators are therefore suitable for many flow control applications.
As mentioned previously, varying the ratio L/d produces a large shift in the frequency of oscillation. Varying nozzle pressure ratio or the ratio h/d produces a smaller shift.
Those skilled in the art will realize that many different mechanisms could be used to actually vary the geometry of the actuator.
The micronozzles 22 are mounted on movable insert 44. This component can move up and down within cavity housing 44, thereby changing the distance “L” and changing the ratio L/d.
The foregoing description and drawings comprise illustrative embodiments of the present invention. Having thus described exemplary embodiments of the present invention, it should be noted by those skilled in the art that the within disclosures are exemplary only, and that various other alternatives, adaptations, and modifications may be made within the scope of the present invention. Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Accordingly, the present invention is not limited to the specific embodiments illustrated herein, but is limited only by the claims.
This application is a non-provisional application claiming the benefit of an earlier-filed provisional application pursuant to 37 C.F.R. '1.53 (c). The provisional application was assigned Ser. No. 61/215,625. It listed the same inventors and was filed on May 7, 2009.
Number | Date | Country | |
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61215625 | May 2009 | US |