CN110686853A - Focusing laser differential interferometer and non-intrusive method for measuring density pulsation of flow field of wind tunnel - Google Patents
Focusing laser differential interferometer and non-intrusive method for measuring density pulsation of flow field of wind tunnel Download PDFInfo
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Abstract
The invention discloses a focusing laser differential interferometer and a method for measuring density pulsation of a wind tunnel flow field in a non-intrusive mode, wherein the focusing laser differential interferometer comprises an emitting light path and a receiving light path, and the emitting light path comprises: coherent light source generator, optical element, first polarizing plate P1A first prism W1And a first convex lens C2First convex lens C2The two separated light beams with mutually vertical polarization directions are focused into two separated focal points; the receiving optical path comprises a second convex lens C3A second prism W2A second polarizing plate P2And a photoelectric receiver D, a second convex lens C3For refocusing the diverging beam after passing through the focusing area A, a second prism W2And a first prism W1Forming a symmetrical reaction and for combining two separate beams of laser light, a second polarizer P2Combining the lightThe beams are mixed to enable phase interference, and the photoelectric receiver D is used to convert the light intensity information into a voltage signal.
Description
Technical Field
The invention belongs to the field of flow field measurement, and particularly relates to a focusing laser differential interferometer and a non-intrusive method for measuring density pulsation of a flow field of a wind tunnel.
Background
Wind tunnel experiment is one of the important means of current stage aerodynamic research, and the flow field information can not be captured in the experiment without the support of flow field measuring equipment. At the present stage, a related measurement method is not available for acquiring density pulsation information of a high-precision space point of an experimental flow field.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides an indirect measurement method of the density pulsation of a flow field, namely a focusing laser differential interferometer, and aims to obtain the density pulsation information of the flow field.
The invention provides a focusing laser differential interferometer, which comprises an emission light path and a receiving light path, wherein the emission light path comprises: coherent light source generator, optical element, first polarizing plate P1A first prism W1And a first convex lens C2The coherent light source generator is used for emitting a beam of parallel laser beams with the same phase and consistent polarization direction, the optical element is used for diverging the parallel laser beams into a conical beam, and the first polaroid P1The first prism W is used for filtering interference light rays with nonlinear polarization in the light beam1The first convex lens C is used for separating one beam of polarized light into two beams of polarized light with the same intensity and mutually vertical polarization directions according to the double refraction principle2The two separated light beams with mutually vertical polarization directions are focused into two separated focal points; the receiving optical path comprises a second convex lens C3A second prism W2A second polarizing plate P2And a photoelectric receiver D, the second convex lens C3For refocusing the diverging beam after passing through the focusing area A, the second prism W2And the first prism W1Forming a symmetrical reaction and for combining two separate beams of laser light, the second polarizer P2For mixing the combined light beams to enable phase interference, the photoelectric receiver D is arranged at the focal position of the light beams and is used forThe light intensity information is converted into a voltage signal.
Further, the optical element may be a concave lens C1Or a convex lens.
Further, the first convex lens C2And the second convex lens C3Are the same in both focal length and size.
Further, a concave lens C1First convex lens C2And a second convex lens C3The positional relationship therebetween conforms to the imaging theorem.
Wherein, 1/L1+1/L2＝1/f2，L3＝f2Concave lens C1And a first convex lens C2Is L from each other1Second convex lens C3The distance between the photoelectric receiver D and the photoelectric receiver is L1First convex lens C2A distance L from the focusing area A2Second convex lens C3A distance L from the focusing area A2First prism W1And a first convex lens C2Is L from each other3Second prism W2And a second convex lens C3Is L from each other3，f2Is a first convex lens C2The focal length of (c).
Further, the first prism W1And the second prism W2Is a birefringent prism and has the same characteristic parameters, i.e. the same splitting angle. Specifically, Wollaston prisms and Sanderson prisms may be employed.
The invention also provides a non-intrusive method for measuring the density pulsation of the flow field of the wind tunnel based on the focusing laser differential interferometer, which comprises the following steps:
(1) placing a focusing area A of an optical probe of a focusing laser differential interferometer on a space point to be measured in a flow field;
(2) starting the wind tunnel and recording a voltage signal of a photoelectric detector of the focusing laser differential interferometer;
(3) and obtaining the density pulsation information of the measured flow field according to the voltage signal.
Further, in the step (1), polymerizationConvex lens C at the position of focal region A2，C3At the midpoint of (a). When the optical device is arranged, the optical ray propagation direction is required to be perpendicular to the flow direction of the flow field.
Furthermore, in step (2), the output voltage of the focusing laser differential interferometer can be collected and stored in a computer through a data acquisition card.
Further, in step (3), according to a conversion relation between the output voltage of the focusing laser interferometer and the density pulsation at the measured flow field space point:
obtaining density pulsation information of the measured flow field; wherein, the delta rho is the density gradient of the position of a measuring point of the flow field; rhoLThe average density of the measuring points is; lambda [ alpha ]0Is the laser wavelength; pi is the circumference ratio; l is the length of the measurement sensitivity zone (FIG. 2); k is a Gladstone-Dale constant; v is a voltage value output by the photoelectric sensor; v0＝2I0RRL，I0To pass through a first prism W1The light intensity of the single laser beam after light splitting is R, and the sensitivity of the diode of the photoelectric sensor is R; rLIs the load resistance of the photosensor. Therefore, the density gradient of the flow field at the position of the system probe, namely the density pulsation at the point can be measured in real time.
Compared with the prior art, the technical scheme of the invention has the advantages that the density pulsation information on the internal space point of the flow field can be measured without influencing the flow field structure because the optical measurement probe is used for measuring the density pulsation of the flow field without intervention. Because the optical interferometry is adopted for measurement, due to the extremely high propagation speed of light, the response frequency of a measurement system is up to 100MHz under the condition that the length of a measurement optical path of the system is less than 3 meters. Meanwhile, as the optical measurement probe of the system is a light beam focusing point, higher spatial resolution (less than 0.5mm in the flow direction) can be realized under the condition of reasonably configuring an optical lens.
Drawings
Fig. 1 is a schematic structural diagram of a focusing laser interferometric differentiator according to an embodiment of the present invention.
Fig. 2 is a schematic diagram of a bifocal separating area of a focusing laser interferometer according to an embodiment of the present invention.
Fig. 3 is a flowchart of an implementation of a method for non-intrusive measurement of density pulsation of a flow field of a wind tunnel based on a focused laser differential interferometer according to an embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
The invention provides a flow field density pulsation indirect measurement method for obtaining density pulsation information of a flow field, which obtains the density pulsation information in the flow field by using the light splitting characteristic of a birefringent prism and the light interference principle. The method is used as a non-contact measurement means, has better accuracy and precision in analyzing the density information of the flow field, and has wide application range (high enthalpy flow field can be measured and the like). In addition, due to the high frequency response of the optical device, the method has high resolution frequency (more than 10 Mhz) of flow field density pulsation and good spatial resolution (10 micrometers).
The invention aims to provide a method for measuring density pulsation of a flow field of a wind tunnel in a non-intrusive mode. The relationship between the density pulsation of the flow field and the intensity of light is obtained by utilizing the refraction and interference principles of light, and then the density pulsation information of the flow field is analyzed.
In order to achieve the purpose, the invention adopts the following technical scheme to realize the purpose:
the invention provides a wind tunnel incoming flow density pulsation measurement system, and a certain implementation example of the system comprises: the device comprises a laser, a concave lens, a convex lens, a polaroid, a Wollaston prism, a photoelectric sensor and a data processing unit.
The method comprises the following steps:
firstly, a test system is set up, and the specific method is as follows:
the focusing laser interferometer is mainly divided into two parts, namely a transmitting light path and a receiving light path, as shown in fig. 1, the left part of a focusing area a is the transmitting light path, and the right part is the receiving light path.
The emitting light path part firstly emits a parallel laser beam with the same phase and consistent polarization direction by a laser. The light beam passes through the concave lens C1The back-divergence is a cone beam. The light path passes through the polarizing plate P1Then obtaining the divergent laser beam with completely consistent polarization direction. Then Wollaston prism W1The light beam is divided into two beams of divergent laser beams which have equal light intensity, mutually vertical polarization directions, equal frequency amplitude and phase and are mutually separated. Finally the light beam passes through a convex lens C2The focusing action of (2) forms two separate focal points in a focal area a, which is the sensitive area where the measuring probe is located.
Wherein, the laser in the emission light path mainly generates coherent light beams, and can be replaced by other coherent light source generators. The stability and noise of the coherent light beam directly affect the measurement stability and signal-to-noise ratio of the system. The laser beam can also be subjected to light path steering through a periscope or a reflector, so that the effects of reasonably arranging optical components and accurately adjusting the direction of the light beam are realized, and the usability of the system is improved.
Concave lens C1The effect of this is to diverge the parallel laser beam into a cone beam in preparation for subsequent beam focusing. The size of the focal length determines the divergence angle of the light path, and further influences the light path separation length at the focusing area A of the system. Thus, the concave lens C1Or a convex lens can be used for replacing the light beam, so that the light beam is focused and then diffused.
Polarizing plate P1The non-linear polarized interference light in the light beam is filtered, and the signal-to-noise ratio of the test system is improved.
Wollaston prism W1The function of the device is to separate a beam of polarized light into two beams of polarized light with the same intensity and mutually vertical polarization directions by the double refraction principle. Other birefringent prisms such as Sanderson prisms may be substituted. Furthermore, the separation angle of the Wollaston prism determines the distance of beam separation, which directly affects the spatial resolution of the system. Spatial division of the system with smaller splitting angle of the birefringent prismThe higher the resolution. The currently commonly used Wollaston prism has a splitting angle of 2 arc minutes.
Convex lens C2The diverging cone beam is focused, whereby two separate beams with mutually perpendicular polarization directions are gradually focused into two separate focal points, and the vicinity of the double focal point is the measurement region of the system, i.e. the focal region a (shown in fig. 2). In the figure, L is the length of the light path separation region, and Δ X is the separation distance of the bifocal points, namely the spatial resolution of the measurement probe of the system along the flow direction.
The receiving light path part is composed of a convex lens C3The emitted light after the focusing area is re-polymerized and then passes through a Wollaston prism W2Two laser beams with a certain separation angle are combined and pass through a polaroid P2The filtering of (a) results in a focused laser beam with a uniform polarization direction. A photoelectric receiver D is arranged at the focal position of the light beam to convert the light intensity information into a voltage signal.
Wherein, the convex lens C3The diverging light beams after passing through the focusing area a are refocused, thereby forming a measurement area of a bifocal point.
Wollaston prism W2The function of the prism is to combine two separated laser beams and the prism W1A symmetrical reaction is formed. Other birefringent prisms such as Sanderson prisms may be substituted.
Polarizing plate P2The function is to mix the combined beams to enable phase interference.
The concave lens C as described above1And a convex lens C2，C3Is the main component that diverges and focuses the laser beam. Convex lens C2And a convex lens C3The focal length and size are the same. The mutual position between the lenses complies with the imaging theorem, namely: 1/L1+1/L2＝1/f2，L3＝f2Wherein the concave lens C1And a first convex lens C2Is L from each other1Second convex lens C3The distance between the photoelectric receiver D and the photoelectric receiver is L1First convex lens C2A distance L from the focusing area A2Second convex lens C3And a focusing regionDistance between domains A is L2First prism W1And a first convex lens C2Is L from each other3Second prism W2And a second convex lens C3Is L from each other3，f2Is a first convex lens C2The focal length of (c).
And constructing a proper optical path measuring system according to actual measurement requirements and optical path space dimensions.
Secondly, collecting wind tunnel data, and the specific method is as follows:
(1) adjusting a light path to place a focusing laser interference differentiator probe in a flow field target measurement area;
(2) starting a wind tunnel to record a voltage signal of a photoelectric detector of the focusing laser interference differentiator;
in the data acquisition process, a high-precision data acquisition card is adopted to completely capture the micro disturbance signals output by the system, so that the test precision of the system is improved.
Thirdly, analyzing the wind tunnel test data, wherein the specific method comprises the following steps:
the focusing laser differential interferometer can convert density pulsation information of a flow field into light intensity change on the surface of the photoelectric receiver, and the light intensity change is converted into voltage signal change to be recorded by the data acquisition system.
The data analysis process is that mathematical expression is carried out on the principle of the focusing laser differential interferometer, the relation between the density pulsation of the flow field and the system output voltage value is found out, and then the signal data is converted according to the relation to obtain the density pulsation information of the measured flow field.
As shown in fig. 3, an embodiment of the present invention provides a non-intrusive method for measuring density pulsation of a flow field of a wind tunnel based on the above-mentioned focused laser differential interferometer, including the following steps:
(1) placing a focusing area A of an optical probe of a focusing laser differential interferometer on a space point to be measured in a flow field;
(2) starting the wind tunnel and recording a voltage signal of a photoelectric detector of the focusing laser differential interferometer;
(3) and obtaining the density pulsation information of the measured flow field according to the voltage signal.
Wherein, in the step (1), the optical probe, the position of the focus area A, namely the convex lens C2，C3At the midpoint of (a). When the optical device is arranged, the optical ray propagation direction is required to be perpendicular to the flow direction of the flow field.
In the step (2), the output voltage of the focusing laser differential interferometer can be collected and stored in a computer through a data acquisition card.
In the step (3), according to the conversion relation between the output voltage of the focusing laser interferometer and the density pulsation on the measured flow field space point:obtaining density pulsation information of the measured flow field; wherein, the delta rho is the density gradient of the position of a measuring point of the flow field; rhoLThe average density of the measuring points is; lambda [ alpha ]0Is the laser wavelength; pi is the circumference ratio; l is the length of the measurement sensitivity zone (FIG. 2); k is a Gladstone-Dale constant; v is a voltage value output by the photoelectric sensor; v0＝2I0RRL，I0To pass through a first prism W1The light intensity of the single laser beam after light splitting is R, and the sensitivity of the diode of the photoelectric sensor is R; rLIs the load resistance of the photosensor. Therefore, the density gradient of the flow field at the position of the system probe, namely the density pulsation at the point can be measured in real time.
The measurement principle of the focusing laser differential interferometer provided by the embodiment of the invention is as follows:
based on the linear relationship between the refractive index of the fluid and its density, given by the Gladstone-Dale equation (1): n-K ρ +1. (1), where n is the refractive index of the fluid, K is a constant, and ρ fluid is the density.
The laser is diverged into a cone beam through a concave lens, and the Wollaston prism divides the beam into two beams of orthogonal polarized light with a certain separation angle, and the frequency amplitude and the phase of the two beams of polarized light are the same. The convex lens focuses the light beam to form two different focal points. As shown in fig. 2.
From equation (1), the difference in density of the fluids in the focal zone results in twoThe optical path difference Δ OPL of the beam rays is as in equation (2): Δ OPL ═ n1-n2) (2), wherein n is an1，n2Is the refractive index of the fluid at two separate foci, and L is the focal separation region length.
The optical path difference in turn causes a phase difference as in formulas (3), (4):
In the process of restoring the optical path, the phase difference of the two beams will cause interference cancellation, thereby causing the change of the light intensity, and the formula (5):
wherein I1＝I2＝I0For a single focal point intensity, IdTo receive the light intensity of the light path.
Is the dot product of the unit vectors of the two polarized lights.
The relationship between the photodetector voltage and the light intensity is expressed as formula (6): v ═ IRRL... (6), wherein I is the laser intensity; r is the diode sensitivity; rLIs a load resistor.
In summary, the relationship between the flow field density and the sensor electrical signal can be derived as equation (7):
where ρ isLAs average density of measured points, V0＝2I0RRL。
Therefore, the density gradient of the flow field at the position of the system probe, namely the density pulsation at the point can be measured in real time.
The technical scheme of the invention comprises three parts: firstly, a light path system is built, and the light path system which can be used for actual measurement is constructed according to the measurement principle of a focusing laser interference differential instrument and the position and the size of a actually measured flow field space region. And secondly, data acquisition, namely acquiring voltage signals of a photoelectric sensor in the measuring system in the operation process of the wind tunnel. Thirdly, data post-processing, namely converting the acquired voltage signals into density pulsation information of the measured flow field according to a theoretical formula of the focusing laser interference differentiator.
The invention belongs to a non-intrusive flow field measurement system, and in the measurement process, no physical probe extends into the flow field, and only a focused laser beam passes through the flow field, so that the flow field structure cannot be changed in the measurement process, the measurement accuracy and precision are better, and the application range is wide (high enthalpy flow field can be measured, and the like). In addition, because the light propagation data is extremely fast, the response frequency of the system is high (when the length of the light path is less than 3 meters, the frequency response of the system measuring light path is as high as 100MHz), and in addition, because the diameter of a focal spot of the focused laser is small, the measurement spatial resolution of the system along the flow direction is good (can reach 0.5mm magnitude).
The focusing laser differential interferometer provided by the embodiment of the invention can be applied to the application fields of flow field measurement (from low speed to hypersonic speed), boundary layer measurement (from low speed to hypersonic speed), jet flow (subsonic speed and supersonic speed) measurement and the like.
The present invention will be described in further detail with reference to specific embodiments.
Example (b): in the method for measuring the density pulsation of the hypersonic wind tunnel, the flow field density pulsation measurement is carried out on a Ludwieg tube wind tunnel with a Mach 6 caliber of 500 mm. The wind tunnel has a unit Reynolds number of 8 x 106～1.6×107[1/m]In the meantime.
The specific measurement steps are as follows:
(1) and (3) building a measuring light path, and placing the focusing laser differential interferometer optical measuring probe at a position which is 200mm away from the outlet of the spray pipe on the axis of the wind tunnel experiment section. The optical path of the focusing laser interference difference meter is symmetrical about the optical measuring probe, so that the measuring probe is placed on the axis of the spray pipe for the convenience of building the optical path. The probe is placed 200mm away from the nozzle outlet in consideration of the relative position between the wind tunnel observation window and the nozzle.
(2) The air storage section of the wind tunnel is inflated and heated through the control platform, and the total pressure and the total temperature of the wind tunnel are controlled to be 430K and 10bar respectively at the general typical Mach 6 wind tunnel total temperature. And starting the wind tunnel, and recording an output voltage signal of the measuring system.
(3) The obtained voltage signal is converted into a flow field density gradient at a measuring point by voltage (formula (7)), and then the root mean square of the density gradient in a time domain is obtained to obtain the density pulsation of the wind tunnel.
In this embodiment, the focused laser differential interferometer performs density pulsation measurements on the hypersonic flow field through a non-intrusive optical probe. Compared with conventional flow field measuring equipment such as a hot wire meter, a pitot tube and the like, the non-intrusive optical measuring probe of the focusing laser differential interferometer does not damage a flow field structure and has no shock wave interference problem. In addition, the focusing laser differential interferometer is developed based on the optical interference principle, the measuring response frequency of the focusing laser differential interferometer is extremely high, and the measuring response frequency of the optical path is up to 100MHz under the condition that the length of the optical path is less than 3 meters. In addition, because the measuring probe part is a focused laser focus, the spatial resolution of the measuring probe part along the flow direction is high and is generally less than 0.5 mm.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (10)
1. A focusing laser differential interferometer is characterized by comprising a transmitting light path and a receiving light path,
the emission light path includes: coherent light source generator, optical element, first polarizing plate P1A first prism W1And a first convex lens C2The coherent light source generator is used for emitting a beam of parallel laser beams with the same phase and consistent polarization direction, the optical element is used for diverging the parallel laser beams into a conical beam, and the first polaroid P1The first prism W is used for filtering interference light rays with nonlinear polarization in the light beam1Used for separating a beam of polarized light into two beams with the same intensity according to the double refraction principleAnd polarized lights with mutually perpendicular polarization directions, the first convex lens C2The device is used for focusing two separated light beams with mutually vertical polarization directions into two separated focal points, and the two separated focal points are positioned in a focusing area A;
the receiving optical path comprises a second convex lens C3A second prism W2A second polarizing plate P2And a photoelectric receiver D, the second convex lens C3For refocusing the diverging beam after passing through the focusing area A, the second prism W2And the first prism W1Forming a symmetrical reaction and for combining two separate beams of laser light, the second polarizer P2The photoelectric receiver D is arranged at the focal position of the light beam and is used for converting light intensity information into a voltage signal.
2. The focused laser differential interferometer of claim 1, wherein the optical element is a concave lens C1Or a convex lens.
3. The focused laser differential interferometer of claim 1 or 2, wherein the first convex lens C2And the second convex lens C3Are the same in both focal length and size.
4. The focused laser differential interferometer of claim 2 or 3, wherein the concave lens C1First convex lens C2And a second convex lens C3The positional relationship therebetween conforms to the imaging theorem.
5. The focused laser differential interferometer of claim 4, wherein 1/L1+1/L2＝1/f2，L3＝f2，
Wherein, the concave lens C1And a first convex lens C2Is L from each other1Second convex lens C3The distance between the photoelectric receiver D and the photoelectric receiver is L1First convex lens C2A distance L from the focusing area A2Second convex lens C3A distance L from the focusing area A2First prism W1And a first convex lens C2Is L from each other3Second prism W2And a second convex lens C3Is L from each other3，f2Is a first convex lens C2The focal length of (c).
6. The focused laser differential interferometer of any of claims 1-5, wherein the first prism W1And the second prism W2Is a birefringent prism and has the same splitting angle.
7. The method for non-intrusive measurement of wind tunnel flow field density pulsation based on the focused laser differential interferometer of claim 1 is characterized by comprising the following steps:
(1) placing an optical probe of a focusing laser differential interferometer, namely a focusing area A, on a space point needing to be measured in a flow field;
(2) starting the wind tunnel and recording a voltage signal of a photoelectric detector of the focusing laser differential interferometer;
(3) and obtaining the density pulsation information of the measured flow field according to the voltage signal.
8. The method of claim 7, wherein in step (1), the optical device is arranged such that the optical light propagation direction is perpendicular to the flow direction of the flow field.
9. The method of claim 7, wherein in step (2), the output voltage of the focused laser differential interferometer is collected and stored by a data acquisition card.
10. The method of claim 7, wherein in step (3), the output voltage of the focused laser interferometer is converted to a density ripple at a point in space of the measured flow fieldCalculating relationships
Obtaining density pulsation information of the measured flow field;
wherein, the delta rho is the density gradient of the position of a measuring point of the flow field; rhoLThe average density of the measuring points is; lambda [ alpha ]0Is the laser wavelength; pi is the circumference ratio; l is the length of the measurement sensitivity area; k is a Gladstone-Dale constant; v is a voltage value output by the photoelectric sensor; v0＝2I0RRL，I0To pass through a first prism W1The light intensity of the single laser beam after light splitting is R, and the sensitivity of the diode of the photoelectric sensor is R; rLIs the load resistance of the photosensor.
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| CN111426446A (en) * | 2020-04-23 | 2020-07-17 | 空气动力学国家重点实验室 | Multichannel focusing laser differential interferometer |
| CN114509236A (en) * | 2022-02-25 | 2022-05-17 | 中国航空工业集团公司沈阳空气动力研究所 | Scanning type flow field two-dimensional plane measurement focusing laser differential interferometer |
| CN115325933A (en) * | 2022-08-25 | 2022-11-11 | 中国空气动力研究与发展中心设备设计与测试技术研究所 | Continuous scanning type focusing laser differential interferometer and measuring method |
| CN115325933B (en) * | 2022-08-25 | 2023-03-10 | 中国空气动力研究与发展中心设备设计与测试技术研究所 | Continuous scanning type focusing laser differential interferometer and measuring method |
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