Aeronautics and astronautics – Aircraft sustentation – Sustaining airfoils
Reexamination Certificate
1998-08-13
2001-01-23
Swiatek, Robert P. (Department: 3643)
Aeronautics and astronautics
Aircraft sustentation
Sustaining airfoils
Reexamination Certificate
active
06176454
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an apparatus for influencing the separation of a flow from a body immersed in the flow, by which a shear layer of the flow at the body immersed in the flow is excitable by means of periodic blowing and suction in order to act against a further separation of the shear layer from the body immersed in the flow.
The wing of an airplane or the rotor blade of a helicopter or the impeller blade of a compressor, for example, can be applied as the body immersed in the flow.
BACKGROUND OF THE INVENTION
An apparatus of the type described above is known from EP 0 264 144 A2. Several cavities are arranged under the surface of the body immersed in the flow. Each cavity has an opening leading to the flow at the body immersed in the flow. A loudspeaker is positioned at the edge of each cavity, with which compressional vibrations can be created in the cavity. As a consequence of the compressional vibrations in the cavity, the flow medium, i.e. generally air, enters and leaves through the opening in the cavity. In this way, the flow at the immersed body is exited by means of blowing and suction. As a consequence of this excitation, the complete flow separation from the immersed body is delayed. This means that it results only at a larger angle of incidence and/or at lower speeds of the immersed body relative to the flow, i.e. at lower Reynolds' numbers. However, the known apparatus is complex in construction. Equipment includes not only the cavities and the loudspeakers but also an energy supply to the loudspeakers must be provided.
It is the object of the invention to provide an apparatus with extremely simple construction for influencing the separation of a flow from a body immersed in the flow, which can be realized at low costs.
SUMMARY OF THE INVENTION
The Invention provides an apparatus for influencing the separation of a flow from a body immersed in the flow, by which a shear layer of the flow at the body immersed in the flow is excitable by means of periodic blowing and suction in order to act against a further separation of the shear layer from the body immersed in the flow; said apparatus comprising at least one cavity in the immersed body, which has at least one opening leading to the surface of the body and which is part of a passive cavity resonator, said cavity resonator being passively excited to create compressional vibrations by the flow and said compressional vibrations of the cavity resonator exciting the shear layer.
In fulfilment of its object, the invention does not possess loudspeakers or any other active component for creating compressional vibrations in the cavity. The compressional vibrations in the cavity resonator, which is in accordance with the invention at least partially formed by the cavity, are created rather by the flow which is to be influenced itself. The compressional vibrations of the cavity resonator created by the flow over the opening lead, as a result, to the desired excitation of the shear layer of the flow at the immersed body by means of blowing and suction. In terms of the invention, it therefore suffices to provide cavities and to construct them as parts of passive cavity resonators in order to attain the desired influence of the flow separation from the body immersed in the flow. Neither loudspeakers nor any other active elements or energy supply to these active elements are required. Nevertheless, by application of the invention a significant influencing effect on the flow separation from the body immersed in the flow is attained. Therefore, the maximum angle of incidence of a NACA 0012 profile, for example, can be increased by several degrees. This is to be considered as absolutely astonishing in consideration of the expense which is incurred with respect to active excitation of the shear layer and which only leads to the same degree of influence on flow separation from the body immersed in the flow.
The appropriate cavity for the new apparatus may have a completely enclosed design with the exception of the openings leading to the free shear layer in order to make possible, in an especially simple manner, the defined build up of compressional vibration.
The opening of the cavity leading to the surface of the body may constitute in a slot transverse to the direction of flow of the flow. The slot can be interrupted by bars along the main course of direction of the opening in order to maintain the stability of the surface of the body immersed in the flow. It is also, however, possible that a row of individual holes with circular cross-sectional shapes, for example, runs transversely to the direction of flow of the flow and forms the openings of the cavity leading to the surface of the body. In concrete applications, the most advantageous shape is dependent on the relative size of the cavity resonators. Moreover, the flow conditions may vary decisively in a direction, as for example along the span of a rotor blade, and can require differently shaped cavity resonators along this direction in order to excite the shear layer optimally.
A cavity resonator preferred for the new apparatus includes a hollow cylinder arranged as a cavity transversely to the direction of flow of the flow.
If the cavity has several openings, these can lead to the surface of the body in at least two areas directly behind each other with respect to the direction of flow of the flow. In this case, they can be designed as the openings of two slots leading to a common hollow cylinder. The openings leading to the surface of the body at areas directly behind each other with respect to the direction of flow have the advantage that the excitation of the cavity resonator can be effected by the flow in the one area in which the flow shows more intensive pressure fluctuations or a higher speed and, as a consequence, makes possible a stronger excitation of the cavity resonator, whereas the excitation of the shear layer of the flow on the body immersed in the flow is created by the pressure fluctuations of the cavity resonator primarily in another area in which the influence on flow separation by alternating suction and blowing is particularly effective.
Preferably, at least a portion of the openings of the cavity resonator is to be located where the free shear layer of the flow begins to separate or has just separated from the body immersed in the flow. At this point, the influence on the shear layer of the flow is especially effective for influencing the characteristics of separation from the body immersed in the flow.
If, for example a wing is applied as the body immersed in the flow, the openings of the cavity resonator can, in concrete terms, be prescribed at a chord length x′ with x′/c=0.005 to 0.050, where c is the length of the body immersed in the flow. These specifications relate especially to the NACA profile of approx. NACA 0012. Of essence is the fact that the openings are located as close as possible to the separation point of the shear layer which is dependent on the profile shape of the body immersed in the flow and the flow conditions.
Additional openings of the cavity resonator can be located at a distance of x′ with x′/c=0.010 to 0.100 in the direction of flow behind the openings at x′ with x′/c=0.005 to 0.050. This area is located characteristically behind the starting points of a flow separation from the body immersed in the flow and the flow is especially suitable for a strong excitation of the cavity resonator, here.
The resonance frequency of the cavity resonator can be in the frequency range of the natural instability frequency of the free shear layer, the first subharmonic of the natural instability frequency of the free shear layer, or a tenth of the natural instability frequency of the free shear layer. These frequencies are especially well suited for excitation of a shear layer for the purpose of preventing the flow separation from a immersed body by a flow.
When tuning the cavity resonator to the above-mentioned specified frequency ranges, the resona
Bader Viktor
Grosche Fritz-Reinhard
Deutsches Zentrum fur Luft-und Raumfahrt e,V
Swiatek Robert P.
Womble Carlyle Sandridge & Rice
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