EP0732513B1 - Method and device for active damping of oscillations in detached unstable flows - Google Patents

Description

The invention relates to a method for active steaming global flow oscillations in a flowing medium and a device for Application of the procedure.

Global flow oscillations are self-excited vertebral disorders that periodically detached in unstable currents arise and downstream propagate. A detached flow denotes one Current that is limited by at least one of them Detaches from the interface, d. H. due to the shape of the The flow lines follow an interface from a so-called Draw point not at the interface, they do not run more parallel to this. Associated with the replacement of the Flow is often a tendency to unstable behavior, d. H. the flow has at least one after detachment unstable layer following a flow line characterized in that a small deflection of the Downstream by absorbing energy from the Flow is steadily amplified until nonlinear Processes limit this growth. As a result of In the end, nonlinearities go into one Whirl about. According to this scenario, the initials are formed called flow oscillations from small disturbances an unstable layer near a peel point. A known sufficient condition for an unstable For example, layer is a turning point of the Velocity profile of the flow along a line orthogonal to the flow lines. Such an unstable one Layer is called shear layer. Currents with Multiple peel points can have multiple unstable layers who, each forming a source of eddies for themselves, collectively to a common 'global' Flow oscillations interact.

It is characteristic of these flow oscillations that Vertebrae periodically due to a self-excitation mechanism arise and they are a source of acoustic waves with the Frequency that corresponds to the generation rate of the vertebrae, represent. Because of this acoustic radiation, they are global flow oscillations in technical Flow systems undesirable, not only because they are mostly in the low frequency range <10 kHz occur and therefore in Are annoying in terms of noise pollution, but because they become so intense in special configurations can that z. B. to material fatigue in the sound exposed solids can lead. Such a Material fatigue in flow systems can be serious Consequences, it is not avoided from scratch or you will not get early through rouliem inspection and repaired. Steam and cooling water pipes in Power plants or gas-flowed missiles such as airplanes are just two examples of where global flow oscillations are can occur and possibly through them the Operational safety is endangered due to material fatigue.

It is not always when using all knowledge about the conditions for the appearance of global Flow oscillations possible, a flow system like this interpret that global flow oscillations avoided become. In these cases there is an interest in active ones Control methods that allow existing ones Flow oscillations using suitable compensatory To dampen feedback in a targeted manner.

A method of active steaming globally Flow oscillations using feedback is already there known. The article 'On the active control of shear layer oscillations across a cavity in the presence of pipeline acoustic resonance 'by X. Y. Huang et al., Journal of Fluids and Structures 5, 207-219 (1991) Procedure for active damping of global Flow oscillations in a flow system for air, in which a part of the air is trapped Walls as an acoustic resonator for acoustic Wave is formed due to global Flow oscillations in the resonator is generated. By doing described methods for active damping of the global Flow oscillations become compensatory Feedback achieved by the acoustic wave in the Resonator is compensated with that in the resonator coupled sound radiation from a speaker, wherein the loudspeaker with the appropriately frequency-filtered, amplified and phase-shifted signals from a sensor, which detects the global flow oscillations, driven becomes. Indirect is the compensation of the acoustic Wave also dampened global flow oscillation.

This method is based on a special situation tailored to the presence of one acoustic resonator, whose frequency is based on that of radiated by the global flow oscillations acoustic waves is tuned and because of Interaction between the detached unstable flow and the acoustic wave for self-excitation specific global flow oscillations. This Concept is therefore not transferable from the approach Cases where global flow oscillations are excited are through completely different self-excitation mechanisms. For example, it is known that an obstacle is introduced into a detached unstable flow, the appearance of global flow oscillations can cause specific details of the generated vortices such as the frequency the emitted acoustic wave or the geometric Arrangement of vortices in succession relative to one another to each other from details such as B. the shape of the Obstacle and the flow velocity and the Viscosity of the flowing medium depends. An acoustic In this example, resonance is not a trigger for Flow oscillation. Rather, fluctuations affect unstable layers after hitting the obstacle upstream to the unstable strata nearby the transfer points back. This retroactive effect ensures Self-excitation from global flow oscillations, i. H. reproduce through permanent feedback like vertebrae periodically again and again.

• universell, d. h. unabhängig von spezifischen Anregungsmechanismen für die Strömungsoszillationen funktioniert und
• möglichst effizient ist, d. h. möglichet wenig Loictung zum Dämpfen verbraucht,

und eine Vorrichtung zur Anwendung dieses Verfahrens zu schaffen.It is the object of the present invention to provide a method for actively damping global flow oscillations

• universal, ie works independently of specific excitation mechanisms for the flow oscillations and
• is as efficient as possible, ie uses as little solution as possible for steaming,

and to provide an apparatus for applying this method.

The inventive method is characterized by the features of claim 1 and the inventive Device for applying the method is by the Features of claim 11 characterized.

The idea underlying the invention is based on the Idea that in general the global oscillations periodically reproducing disturbances of a detached one are unstable flow because of some not specified cause in the immediate vicinity of the Separation point of the flow are generated and that there is a The characteristic of an unstable flow is these disturbances measured by their extent and by that in them amplify stored energy until nonlinear Processes stand in the way of further reinforcement.

The inventive method for global steaming Flow oscillations in a flowing medium in the Area of one of at least one interface replacing unstable flow is now the global oscillations with a sensor system and the flowing medium one with the signals of the Sensor system controlled compensatory oscillation in a separation zone of the detached unstable flow aufzugeprägen. Accordingly, the invention contains Device a generator that the flowing medium compensatory oscillation in a separation zone of the detached unstable flow, and a Control system that evaluates the signals from the sensor system and regulates the compensatory oscillation so that the Amplitude of a given global oscillation by one predetermined factor is damped.

Under separation zone there is one at a separation point beginning and downstream area understood the unstable flow in which a disturbance of the Flow is strengthened downstream. The compensatory oscillation is ideally a direct one Influencing the detached unstable flow in such a way that one in a predetermined part of the Peeling zone existing small disturbance, which itself unaffected due to the amplifying effect of the unstable Increase current and develop into a vortex would be compensated exactly. Naturally, at exact compensation of the disturbance before amplification extensive vertebrae develop because of the effect the cause is taken. Under a compensatory oscillation is in the following generally a direct influence on the detached unstable flow can be understood in such a way that one in a predetermined part of the Peeling zone existing small disturbance, which itself unaffected due to the amplifying effect of the unstable Increase current and develop into a vortex would be reduced in amplitude. Then it is global flow oscillation due to the compensatory Oscillation in general not perfectly suppressed, but only in their intensity, measured by that of the Energy absorbed or the intensity of the disorder the acoustic wave emanating from the flow oscillation, attenuated.

The method according to the invention consequently consists in using Help capture an existing global Flow oscillation using a sensor system approximate for a given area of the separation zone to identify a perturbation that the observed Flow oscillation would result, and this the flow in the given area of the separation zone in phase opposition and possibly with a weakened amplitude, d. H. With compensatory effect for the existing disturbances, too overlap. Because global flow oscillations unite Representing a periodic process allows the recording a flow oscillation at a given time Determination of a compensatory disturbance which, in phase of the flow in a given range of Peeling zone imprinted, only the next or one of the can dampen the following vortex formation.

Since this consideration of how the The method according to the invention does not depend on one special self-excitation mechanism, is this procedure universally applicable to dampen global Flow oscillations. Its efficiency is therefore high, because they dampen a flow oscillation energy to be applied only corresponds to the energy that is needed to move the part of the peeling zone to generate compensating interference. But this energy is small compared to the energy that the undamped Flow oscillation represents since the Flow oscillation due to the described Amplification mechanism absorbs energy from the flow. The high efficiency of the method according to the invention results in influencing an unstable flow at their most sensitive region, where the current, considered downstream, becomes unstable and the Reinforcement mechanism begins to work.

FIG. 1A

eine Konfiguration für abgelöste instabile Strömung ohne globale Strömungsoszillation;

FIG. 1B

eine Konfiguration für abgelöste instabile Strömung mit schwach ausgeprägter globaler Strömungsoszillation;

FIG. 1C

eine Konfiguration für abgelöste instabile Strömung mit ausgeprägter antisymmetrischer globaler Strömungsoszillation;

FIG. 1D

eine Konfiguration für abgelöste instabile Strömung mit ausgeprägter symmetrischer globaler Strömungsoszillation;

FIG. 1E-F

Frequenzspektren der Signale eines Sensors zur Erfassung der Strömungsoszillationen der Konfigurationen in FIG. 1A-D;

FIG. 2

eine Anordnung zur aktiven Dämpfung von globalen Strömungsoszillationen mittels Rückkopplung: Querströmung als kompensatorische Oszillation;

FIG. 3

eine Anordnung zur Erzeugung einer kompensatorischen Oszillation: Oszillation eines Ablösepunktes;

FIG. 4A-H

Frequenzspektren von Sensorsignalen zur Erfassung von Strömungsoszillationen für verschiedene in der Ablösezone aufgeprägte kompensatorische Oszillationen;

FIG. 5A-C

Frequenzspektren von Sensorsignalen zur Erfassung von Strömungsoszillationen für schmalbandig phasenangepasste kompensatorische Rückkopplung;

FIG. 5D-F

Frequenzspektren von Sensorsignalen zur Erfassung von Strömungsoszillationen für breitbandig phasenangepasste kompensatorische Rückkopplung;

FIG. 6A-B

Sensorsignale zur Erfassung von Strömungsoszillationen als Funktion der Zeit beim An- und Abschalten der aktiven Dämpfung.

FIG. 7

Anordnung zur aktiven Dämpfung von Strömungsoszillationen an einem akustischen Resonator mit einer Öffnung, die einer Strömung längs einer Grenzfläche zugewandt ist.

The method according to the invention and a device for its use are explained in detail below with reference to the following schematic figures. Show it:

FIG. 1A

a configuration for detached unstable flow without global flow oscillation;

FIG. 1B

a configuration for detached unstable flow with weak global flow oscillation;

FIG. 1C

a configuration for detached unstable flow with pronounced antisymmetric global flow oscillation;

FIG. 1D

a configuration for detached unstable flow with pronounced symmetrical global flow oscillation;

FIG. 1E-F

Frequency spectra of the signals from a sensor for detecting the flow oscillations of the configurations in FIG. 1A-D;

FIG. 2

an arrangement for active damping of global flow oscillations by means of feedback: cross flow as compensatory oscillation;

FIG. 3

an arrangement for generating a compensatory oscillation: oscillation of a separation point;

FIG. 4A-H

Frequency spectra of sensor signals for recording flow oscillations for various compensatory oscillations impressed in the separation zone;

FIG. 5A-C

Frequency spectra of sensor signals for the detection of flow oscillations for narrow-band phase-adjusted compensatory feedback;

FIG. 5D-F

Frequency spectra of sensor signals for the detection of flow oscillations for broadband phase-adjusted compensatory feedback;

FIG. 6A-B

Sensor signals for the detection of flow oscillations as a function of time when the active damping is switched on and off.

FIG. 7

Arrangement for actively damping flow oscillations on an acoustic resonator with an opening which faces a flow along an interface.

FIG. 1 shows examples of flow systems that form different global flow oscillations can, and the associated for each flow system Frequency spectrum of the signals P of a pressure sensor 13, the is intended to be at a point in the range of one detached unstable flow global To detect flow oscillations without the flow to influence itself significantly. These examples serve as a starting point for a description of how it works of the inventive method and for the description of Embodiments of devices for its use.

The FIG. 1A-D shows 4 flow systems, each of which same source for a detached unstable flow 10 have a slot 9 of width h that any flowing medium, e.g. B. a liquid, a gas or a Gas-liquid mixture, with the density ρ of Flow velocity V crosses. The flow systems are each perpendicular in cross section through the slot 9 shown, d. H. the slot 9 is defined by two Limiting elements 9a, 9b perpendicular to the paper plane. It it is also assumed that the slot width h is very small is opposite the height of the slot and that the cross sections shown in the central region of the Slot are located where the flow conditions in the vertical direction are considered to be invariant can. In the 2-dimensional representation in FIGS. 1A-D the flow separates from 2 separation points 11 and 12 two boundary surfaces of the slot 9 delimiting them. To the flow moves, if none Hindornis stands in the way (FIG. 1A), free of others directional interfaces and is at their Propagation only through inner friction, described by the kinematic toughness ν, limited. It is assumed that with this free propagation none Self-excitation mechanisms for global Flow oscillations are realized (e.g. a acoustic resonance). The frequency spectrum of the pressure sensor 13 then only points to a weak, broadband Fluctuation near the frequency f = 0 (curve a in FIG. 1E). A condition for self-excitement of Flow oscillations occur when downstream after detachment, an obstacle to the further course the flow influences as in the arrangements in FIG. 1B-D. A characteristic of these cases is an increase in the Broadband fluctuation levels near the Frequency f = 0 and the occurrence of narrowband maxima at higher frequencies. The narrow-band maxima represent pressure pulsations, the amplitudes of which are a measure for the intensities of global flow oscillations are. Obviously the frequencies f and die Intensity of flow oscillations other than that Flow velocity V from the geometric arrangement of the obstacle with respect to slot 9 (FIG. 1B: Gap 7; FIG. 1C: wedge 14; FIG. 1D: gap 8 with Side walls). Another property of Flow oscillations necessary for the implementation of the The method according to the invention is relevant, but not the Frequency spectra of a pressure sensor can be taken is the spatial arrangement of the one after the other Vortex viewed at a given time. To a given time can under different Conditions generated a different vortex have spatial geometry. The in FIG. 1 shown Flow systems have e.g. B. 2 transfer points, of which each the edge point one downstream extending unstable layer form. The one in the two unstable layers generated separately, but to one common vortex oscillation contributing vortices differ in their sense of rotation by the Time difference between the creation of a vortex in the one unstable layer and generating the next Vortex in the other unstable layer, this one Time difference a different distance between the two Vortex from separation points 11 and 12 at slot 9 mean. In the two layers, vertebrae can do more or less simultaneously (symmetrical modes, e.g. vortex 6a, 6b in FIG. 1D) or in phase opposition, d. H. in same Time intervals alternately either at the transfer point 11 or at separation point 12 (antisymmetric modes, e.g. vortex 5a, 5b in FIG. 1C) arise, the perfectly symmetrical or perfectly antisymmetric flow oscillations usually have the greatest intensity. This temporal Processes must be in a phase-appropriate synthesis of a compensatory oscillation to dampen the global Flow oscillation are taken into account.

Taking into account the basic properties mentioned global flow oscillations should now be based on one of the in FIG. 1 listed flow systems that Method according to the invention for damping the Flow oscillations and preferred devices too its application are explained. This serves as an example System in FIG. 1C, consisting of slot 9 as the origin a detached unstable flow and wedge 14 as Obstacle. This special selection means none Limitation of generality, since the procedure is general is designed to be independent of the mechanisms, those for the excitation of specific flow oscillations lead, works.

• Erfassung der globalen Strömungsoszillationen mit einem Sensorsystem;
• Erzeugung einer kompensatorischen Oszillation in einer Ablösezone;
• Aufbereitung der Sensorsignale und Regelung der kompensatorischen Oszillation.

The process according to the invention consists of the following three process steps:

• Detection of global flow oscillations with a sensor system;
• Generating a compensatory oscillation in a separation zone;
• Processing of the sensor signals and control of the compensatory oscillation.

An arrangement for executing the method is shown in FIG. Second

I. Detection of global flow oscillations with a sensor system

In FIG. 2 is the one causing the flow oscillation Obstacle, the wedge 14, symmetrical to the perpendicular the slot 9, the x-axis. In this example it is global oscillation characterized by vortices 20a.1, 20a.2, ... and vortex 20b.x (x = 1, 2, ...), each around half a period of flow oscillation in time offsets either at separation point 11 (y <0) or on Detachment point 12 (y> 0) of the detached unstable flow arise and, rotating in opposite directions, in the x direction move. This is an example of one antisymmetric flow oscillation. The essential Parameters with regard to the active damping of the Flow oscillations are recorded, their frequency and a measure of their intensity. One is also useful Information about the phase position of the different Vortex trains 20a.x and 20b.x (x = 1, 2, ...). Latter However, phase information is not absolutely necessary to carry out the procedure.

Preferred sensors for detecting the frequency and the The intensity of the flow oscillation is a pressure sensor or a sensor for measuring the flow velocity. Preferred positions for such sensors are points in the flowing medium, on the one hand the sensor itself influenced the flow only little and not itself to the The trigger of flow oscillations. Are on the other side to optimize sensor sensitivity points in the Area of influence of the vertebrae with the greatest expansion or Points close to the flow oscillation causing obstacle advantageous. The sensor can also be built into the obstacle.

An alternative sensor for detecting the frequency and the Intensity of flow oscillation is a force sensor, which detects the force that the flow has on the Obstacle 14 exercises.

Suitable sensors are commercially available: as Pressure sensor is suitable, for example, as a microphone Sensor for measuring the flow rate Hot wire instrument, strain gauges as force sensors or piezoelectric or piezoresistive sensors.

II. Generating a compensatory oscillation

FIG. 2 shows one possibility for a generator for one compensatory oscillation in a separation zone: on compensatory flow field in the area of the separation zone. The compensatory flow field corresponds to one acoustic wave and affects the detached unstable Flow 10 immediately after separation of the flow. It is ideally designed so that interference is more unstable Layers through with the compensatory flow field connected pressure gradients are exactly compensated. In the arrangement in FIG. 2 becomes a compensatory Flow field composed approximately with the help of two Excitation sources 17 and 18, each having a cross flow 15, 16 across the detached flow 10 and along two Generate interfaces 17a and 18a, the cross flows independently of each other in their flow velocities are controllable. The cross flows are shown in FIG. 2 through double-sided arrows next to the separation points 11 and 12 indicated. The outlet openings for the cross flows are placed so that each separation point 11 and 12 has one Exit opening comes as close as possible without the Outlet opening itself the flow essential affected. Because of the proximity of the outlet openings to the Transfer points is the work to be done Generation of compensatory oscillation in particular low. The independent controllability of both Excitation sources allow a flow field to be closed overlay that with respect to amplitude and phase two lines is controllable. The x position of the Interfaces 17a and 18a determine the width of the compensatory flow field. It is according to the invention sufficient, the expansion of the compensatory Flow field to the detachment zone of the detached unstable flow or even a portion of the Restrict separation zone.

As excitation sources 17, 18 for the compensatory Flow field come mechanical driven Vibration systems, e.g. B. a speaker in question, the part of the flowing medium towards the Move interfaces 17a and 18a. The interfaces 17a and 18a then force currents running parallel to them 15 and 16, which emerge from the outlet openings between the Interfaces 17a, 18a and the separation points 11 and 12 escape.

This concept of the compensatory flow field leaves generalize. On the one hand, the flow field not be designed so that there is a flow perpendicular to it the unstable layers of the detached unstable flow represents. It is enough to stabilize one unstable layer that with the compensatory Flow field a sufficient component of the Pressure gradients connected perpendicular to the unstable layer is. in the example according to FIG. 2 is the interpretation of the compensatory flow field as cross flow not the only possible solution; the direction across In the example considered, flow causes only one particularly efficient influencing the unstable Layers that are at least near the separation points 11 and 12 approximately perpendicular to the flow 10 through slot 9 run. A second generalization of the concept of is compensatory flow field, starting from Example in FIG. 2, regarding the preferred number of Sources of excitation necessary for its production to install. In the example in FIG. 2 are 2 sources of excitation provided because there are two downstream unstable layers, starting from the separation points 11 and 12, there and both layers to compensate for the occurring antisymmetric global Flow oscillations can be influenced in phase opposition have to. The situation would be different, would be for others Feedback ratios (e.g. different geometry of the Obstacle) a symmetrical flow oscillation dampen. In this case, the two speakers must be in Phase operated. Both for symmetrical as well antisymmetric flow oscillations can be a single Excitation source for generating the compensatory Oscillation is sufficient to dampen the Achieve flow oscillation. The maximum achievable In this case, damping is usually lower. Has naturally in principle, with a further increase in the number of Excitation sources even more degrees of freedom (amplitude and Phase) available to attenuate the Optimize flow oscillations.

An efficient alternative to this acoustic method to generate a compensatory oscillation shows FIG. 3. In this example the stabilization is a unstable layer achieved by an oscillation of the corresponding separation point. This oscillation can be caused by a mechanical movement of a Element of the interface from which the unstable Current takes over. In the example in FIG. 3 are each one Detachment point 11 and 12 of the detached unstable flow 10 at an end point of the boundary elements 34 and 35, the in turn around points 36 and 37 using conventional controllable actuators (not shown) can be tilted can. Such a tilt leads to a shift the separation points perpendicular to the boundary elements and thus to a lateral deflection of an unstable one Layer across the current. This deflection is by means of Control signals of the sensor system so that a malfunction the unstable layer near a peel point is compensated.

The examples in FIG. 2 and 3 refer to 2-dimensional Flow profiles related to a third orthogonal direction are invariant, so unstable Layers should always be considered as layers. Both However, examples can be found in the 3-dimensional case generalize with curved unstable layers. If necessary, individual segments of the curved unstable Layers are stabilized independently of one another.

III. Processing of the sensor signals and control of the compensatory oscillation

In the following it is assumed that all controllable Parameters for determining the compensatory oscillation, z. B. the amplitudes and phases of the excitation sources Generation of a compensatory flow field or the Oscillation of separation points in terms of amplitude and Phase, set using conventional control systems and that all amplitudes and Phases are regulated by means of signals which are generated by the following described preparation from the above (I) discussed sensor signals are obtained.

The flow system in FIG. 2. The amplitudes and phases of the two excitation sources 17, 18 are controlled by a signal that results from the signal of the sensor 13 by frequency filtering and / or amplification and / or phase shift. The signals from sensor 13 are fed to a frequency filter 25 (24). This frequency filtering is optional and only serves to suppress noise. The frequency-filtered signal is fed via line 27 to an amplifier element 29 and from there via line 31 to excitation source 18. This signal determines the amplitude and phase of the excitation source 31. Accordingly, the amplitude and phase of the excitation source 17 is derived from the signal of the sensor 13 modified by the frequency filter 25 and the amplifier element 28 and transmitted via the connecting lines 26, 30. The function of the amplifier elements 28, 29 is it is on the one hand to amplify the supplied signals by a (generally frequency-dependent) factor G _i and to shift the phase by a (generally frequency-dependent) value Φ _i (i: index for amplifier element).

This example can be generalized analogously to Systems with any number of excitation sources or oscillating separation points. For driving everyone independently controllable element that contributes to the compensatory oscillation in the separation zone, is an amplifier element like elements 28, 29 and appropriate connections for signal transmission provided.

For a complete description of the method according to the invention, it suffices to specify a regulation for the selection of suitable amplifications G _i and phases Φ _i for the individual amplifier elements. It is sufficient to use the example in FIG. 2 to be treated with 2 amplifier elements. Additional amplifier elements can be set up using the same regulations.

The system in FIG. 2 shows an antisymmetric flow oscillation. Since the vortices that originate from the two separation points are generated in phase opposition with the same intensity, it is obvious to use the same phase gain for both amplifier elements, i.e. G = G1 = G2 and Φ ₁ - Φ ₂ = ± π, and G and Φ ₁ should be selected so that the flow oscillation is damped by a predetermined factor.

The FIG. 4A-H show an arrangement according to FIG. 2 shows the frequency spectrum of the signal of the sensor 13 with the compensatory oscillation switched on for different gains G and different frequency filters 25, a bandpass filter with maximum permeability at the frequency of the flow oscillation (FIG. 4A-D) and a high pass filter (FIG. 4E-H). Φ ₁ is selected in all cases in such a way that the global flow oscillation available for gain G = 0 (in this example at f = 100 Hz) is optimally damped with increasing gain. As the FIG. 4A-H show, the initially existing mode at 100 Hz for both filter types is attenuated with increasing gain and disappears for G≥1.3. Destabilization takes place for larger amplifications: the flow oscillation at 100 Hz remains suppressed, but flow oscillations occur at other frequencies, the exact value of which depends on the choice of the frequency characteristic of the frequency filter 25. In this case, the flow field generated by the excitation sources 17, 18 is compensatory only in a limited spectral range; Outside of this spectral range, global flow oscillations, the intensity of which increases with the gain G, can even be excited above a system-specific limit for the gain.

This example is specially tailored for an antisymmetric flow oscillation. In general, the phases Φ _i in Bich measurements must be selected independently of one another.

The in FIG. 4 documented destabilization for larger amplifications is characteristic of amplifier elements 29 and 28, in which the phases Φ _i cannot be controlled in a controlled manner over the entire frequency range effective for the amplification. In this case, the phases Φ _{i can} generally only be adjusted such that the feedback of the signals from the sensor 13 to the detached unstable flow has a damping effect on global flow oscillations only within a limited frequency range. Outside this frequency range, the feedback has a reinforcing effect for flow oscillations. These become dominant if the feedback is sufficiently strong to sufficiently suppress the flow oscillation that is present without feedback of the signals from the sensor 13 in comparison to the amplified flow oscillation. Consequently, with this type of generating compensatory feedback, the intensity of global flow oscillations integrated over all frequencies has a minimum for certain values for G _i > 0.

That this type of feedback occurs only within one finite frequency bandwidth attenuating for global Flow oscillations is limiting in Flow systems in which the Flow rate V changes. Because with one Change in flow velocity V also the frequency of global flow oscillation changes is damping of the flow oscillations even over a finite one Ensure range of flow velocities; if the flow velocity deviations are too great the feedback becomes unstable.

The aforementioned instability problems can be remedied by adapting the gain G _i and / or the phases Φ _i over a wide frequency range. With amplifier elements for which the frequency response of G _i and / or Φ _i can be adjusted in a controlled manner, an optimization of the damping of a flow oscillation can be automated according to predetermined criteria. With conventional search strategies, e.g. B. starting from G _i = 0, all parameters of G _i and / or Φ _i (z. B. maximum values, frequency response) for a predetermined frequency range are selected so that the intensity of the flow oscillation is minimal or falls below a predetermined value. A frequency analyzer for the sensor signals is used to control the optimization. Commercially available amplifier elements are suitable for carrying out this optimization. There are e.g. B. adaptive amplifier elements are known in which the gain and phase are automatically changed over a predetermined frequency range so that a predetermined error signal is minimal. With the signal from sensor 13 as an error signal and as a signal to be amplified, such an adaptive amplifier element can be used for the automatic, dynamic optimization of the damping of flow oscillations.

The improvement of the stability of the control loop for generating the compensatory oscillation when using adaptive amplifier elements in comparison to conventional amplifier elements without adjusting the frequency response of the gain and the phase is shown in FIG. 5A-F. FIG. 5A-F show experimental results for an arrangement according to FIG. 2. Frequency spectra of the signals from sensor 13 (with an arbitrary zero point) are compared for undamped (dashed lines) and, under various conditions, attenuated (solid lines) by compensatory feedback (solid lines) flow oscillations. In the cases of FIG. 5A-C, conventional amplifier elements 28, 29, in the cases of FIG. 5D-F uses adaptive amplifier elements 28, 29 over the frequency range 0-500 Hz. Different figures represent different flow velocities V, measured by the Reynolds number Re = Vh / ν (h: width of the flow at the separation points 11, 12; ν: kinematic toughness): 1) Re = 3.9 10 ⁴ (FIG. 5A, D) ; 2) Re = 6.7 10 ⁴ (FIG. 5B, E); 3) Re = 7.9 10 ⁴ (FIG. 5C, F). The flow oscillations are represented by peak maxima over a noise background, whereby a dominant maximum lies between 50 and 150 Hz depending on the flow speed. Obviously, adaptive amplifier elements lead to broadband damping of flow oscillations for all flow velocities, while in the case of conventional amplifier elements, flow oscillations in the range above 150 Hz and in the vicinity of the flow oscillation dominating without feedback in the range 50-100 Hz are excited due to the instabilities of the feedback mentioned. Apart from the improved stability, the adaptive amplifier elements enable a stronger damping of the flow oscillations by more than 30 dB and an additional damping of the low-frequency noise for frequencies below the frequencies of the flow oscillations.

One for the efficiency of the method according to the invention revealing property shows FIG. 6A-B, which the Course of the signals from sensor 13 (in arbitrary Units) as a function of time t when switching on (FIG. 6A) and switching off (FIG. 6B) the damping using represents adaptive amplifier elements. According to FIG. 6A finds when switching on the damping within a few cycles the frequency of global flow oscillation is a transition instead of a strong, periodic sensor signal, according to the intensity of the undamped Flow oscillation, to a weak noise signal. Conversely, when the feedback is turned off of the sensor signals from the weak noise signal within fewer cycles with the frequency of the flow oscillation the strong, periodic signal of the undamped Flow oscillation back (FIG. 6B). Because of the damping the flow oscillation required compensatory Oscillation by amplification from the signal of the sensor 13 is also derived for damping the Flow oscillation does not require constant power: you decreases during fewer periods of flow oscillation a maximum value at the beginning of the damping down to one minimal power that is needed to make a new one To prevent self-excitation of a flow oscillation (FIG. 6A). This effect improves the efficiency of the method according to the invention additionally in comparison to the peculiarity already discussed above, that only one minimal fraction of a detached unstable flow must be stabilized while consuming power in order to a global flow oscillation and its acoustic To dampen side effects.

• die Rückwirkung der Strömung strömungsaufwärts, ausgelöst durch die Wechselwirkung der Strömung mit der Begrenzung 41 (Hindernis);
• die Wechselwirkung einer vom Ablösepunkt 11 ausgehenden instabilen Schicht mit einer akustischen Resonanz des Raumes 40.

FIG. 7 shows an application of the method according to the invention in a system with a detached unstable flow, in which global flow oscillations are not excited or not exclusively by obstacles in the flow, but rather with the participation of acoustic resonances which interact with the detached unstable flow. The flow system in FIG. 7 consists of a flow 10 along an interface with an opening with a front or rear boundary 11 or 41 relative to the direction of flow otherwise, space 40 closed off by interfaces. Because of the opening, space 40 is accessible to the flowing medium. Furthermore, in the area of the opening, the parts of the flowing medium enclosed in space 40 can interact with the part of the flowing medium moving along the interface. Because of this coupling, the parts of the flowing medium delimited by the space 40 can be excited to acoustic vibrations, for which the space 40 acts like an acoustic resonator. The flow 10 separates from an interface at the boundary 11. The boundary 11 therefore has the function of a separation point with an adjacent separation zone for the flow 10 and serves as the starting point for generating the eddies of a global flow oscillation 60. In this case, two mechanisms ensure the selection of a global flow:

• the reaction of the flow upstream, triggered by the interaction of the flow with the restriction 41 (obstacle);
• the interaction of an unstable layer starting from the separation point 11 with an acoustic resonance of the space 40.

The flow system in FIG. 7 is a model system which in equals many technical applications. Flugund Floats (e.g. planes, missiles, ships, Submarines) and land-based vehicles such as B. High-speed trains often have in their surface Indentations that trigger when moving quickly of global flow oscillations. The effect of Indentations as acoustic resonators lead to special intense flow oscillations. Such are common Indentations provided as a practical space for the Housing items that are not normally should be directly exposed to the current, but in If necessary, contact the flowing medium need, for example sensors and measuring instruments in Airplanes, weapons in military aircraft. Another Examples are electrically driven High-speed trains. They usually point in indentations retractable pantographs that contact while driving to a power line near the surface of the train must have and therefore one at high speed are exposed to relatively strong currents. Such Objects can with extreme flow speeds unacceptable burdens due to the occurring Flow oscillations or the acoustic resonance of the Indentation for such problems is the Use of an active damping method Flow oscillations are particularly advantageous because of passive measures such as a special shape of the Indentation the flow oscillations mostly not can be prevented.

Like the method according to the invention in these cases can be used to advantage, shows the model system in FIG. 7. In the detachment zone or part of it a compensatory oscillation imprinted in this Example one from a speaker 46 as an excitation source generated cross flow 15 between the separation point 11 and the limit 65. To realize the for the active Damping the flow oscillations requires feedback the loudspeaker 46 is driven by the control system 44 suitable frequency filtered and / or amplified and / or phase-shifted signals from a sensor 50 or 42 for Detection of flow oscillations. Regarding the sensor result in comparison to those previously discussed Examples of alternatives. A sensor is suitable as a sensor 50, the most direct the speed fluctuations or the pressure pulsations of the generated vertebrae 60 probed, e.g. B. one of the sensors mentioned, mounted in close to the boundary 41. A sensor is also suitable, that associated with the flow oscillation 60 acoustic wave detected, e.g. B. a microphone 42 on the the opening between the boundaries 11 and 41 opposite side of room 40.

It should be emphasized that the acoustic radiation of the Loudspeaker 46 essentially in the cross flow 15 is implemented and is not intended by the Flow oscillation 60 acoustic 40 excited in room 40 To compensate for vibration and so indirectly also To suppress flow oscillation 60. Furthermore, that Embodiment in FIG. 7 can be modified according to the different implementation options for the compensatory oscillation, the detection of the global flow oscillation, the preparation of the Sensor signals and the regulation of the compensatory Oscillation according to the explanations in the Sections I-III.

Claims (17)

1. Method for damping global flow oscillations (20a.x, 20b.x, 5a-b, 6a-b, 60) in a flowing medium in the region of at least one separation zone located at an unambiguously defined position of a boundary surface (11, 12) at which an unstable flow (10) separates, wherein the global flow oscillations are detected in said method with a sensor system (13, 42, 50) disposed donwstream of the separation zone and wherein a compensatory oscillation (15, 16, 34, 35) controlled via the signals of the sensor system (28, 29, 44) is superimposed onto the flowing medium in the separation zone of the flow.
2. Method in accordance with claim 1, characterized in that the global flow oscillation is detected at a predetermined point in the flowing medium via measurement of the pressure or of the flow speed.
3. Method in accordance with claim 1, characterized in that the global flow oscillation is caused by influencing the separated unstable flow with an obstacle (7, 8, 14, 41) and is measured by measuring the force which the flow exerts on the obstacle.
4. Method in accordance with one of the claims 1 to 3, characterized in that a flow field (17, 18, 46) is generated as the compensatory flow oscillation in at least one separation zone and/ or an oscillation (34, 35) of at least one separation point (11, 12) of the separated flow is excited.
5. Method in accordance with one of the claims 1 to 4, characterized in that the global flow oscillation is measured by evaluating the signals of the sensor system (13, 42, 50) and, with the aid of this evaluation, global flow oscillations are characterized having regard to their frequency and/or intensity and/or phase.
6. Method in accordance with one of the claims 1 to 5, characterized in that the signals of the sensor system are modified by amplification (28, 29, 44) and/or frequency filtering (25) and/or phase shifting (28, 29, 44) and in that the signals modified in this way are used for producing the compensatory oscillation.
7. Method in accordance with claim 6, characterized in that the amplification and the phase shifting are matched with adaptive amplification elements over a prespecified frequency range in accordance with prespecified rules so that the intensity of the flow oscillation lies below a predetermined value.
8. Method in accordance with one of the claims 1 to 7, characterized in that the separated unstable flow and/or the global flow oscillation interact with an acoustic wave.
9. Method in accordance with claim 8, characterized in that the acoustic wave is influenced by an acoustic resonator (40).
10. Method in accordance with one of the claims 8 to 9, characterized in that the sensor system measures the global flow oscillation by means of a sensor (42) which characterizes the acoustic wave.
11. Apparatus for use in the method in accordance with one of the claims 1 to 10 on global flow oscillations (20a.x, 20b.x, 5a-b, 6a-b, 60) in a flowing medium in the region of at least one separation zone located at an unambiguously defined position of a boundary surface (11, 12) at which an unstable flow separates, the apparatus comprising a sensor system (13, 42, 50), a generator (17, 18, 46) and a control system (28, 29, 40)
    wherein the sensor system with which the global flow oscillations are detected is arranged downstream of the separation zone,
    wherein the generator (17, 18, 46) superimposes a compensatory oscillation (15, 16, 34, 35) on the flowing medium in the separation zone of the flow and
    wherein the control system evaluates the signals of the sensor system and controls the compensatory oscillation so that the amplitude of the global flow oscillation is damped by a predetermined factor.
12. Apparatus in accordance with claim 11 comprising an obstacle (7, 8, 14, 41) in the separated unstable flow, with the obstacle causing the global flow oscillations.
13. Apparatus in accordance with claim 12, characterized in that the sensor system comprises a sensor for measuring the force exerted by the flow onto the obstacle.
14. Apparatus in accordance with one of the claims 11 to 13, characterized in that

    the compensatory oscillation is a flow field (15, 16) and the generator contains at least one excitation source (17, 18) for producing the flow field, or that

    the compensatory oscillation is an oscillation of at least one separation point (11, 12) and that mobile boundary elements (34, 35) for the separated flow are provided for exciting the oscillation of the separation points, the boundary elements defining the position of the separation points (11, 12), with the generator containing an apparatus for producing a movement of the boundary elements, the movement effecting the oscillation of the separation points.
15. Apparatus in accordance with one of the claims 11 to 14, characterized in that the control system comprises a frequency filter (25) and/or a frequency analyser and/or an amplifier (28, 29, 44) and/or a phase shifter (28, 29, 44) for processing the signals of the sensor system.
16. Apparatus in accordance with one of the claims 11 to 15, characterized in that the separated unstable flow occurs at a recess (40) in a boundary surface which is remote from the flowing medium.
17. An aeronautical or amphibious body or a vehicle having an apparatus in accordance with one of the claims 11 to 16 for damping a global flow oscillation at the surface.

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