A method for measuring density pulsation in a flat plate boundary layer

Abstract

The invention discloses a density pulsation measurement method for a flat plate boundary layer, which comprises the following steps: a parallel coherent light beam emitted unidirectionally by a laser is divided into a plurality of coherent fan-shaped light beams A by a beam splitter; each fan-shaped light beam A is separated into two light beams B; each divergent light beam B is converted into a parallel light beam again, and then focused on a target area to be measured in a flat plate boundary layer; a wind tunnel is started to make the focused light beam pass through the area to be measured; each focused light beam B is converted into mutually parallel light beams B again; the two light beams B separated from each fan-shaped light beam A are refocused and merged into a coherent light beam C; each coherent light beam C is filtered and then converted into a voltage signal by a corresponding photoelectric detector, and density pulsation information in a flat plate boundary layer is inferred according to the voltage signal; and density pulsation information of multiple measuring points in a flat plate boundary layer can be measured simultaneously.

Description

Technical Field

[0001] The invention relates to the technical field of fluid mechanics measurement, and in particular to a method for measuring the density pulsation of a flat plate boundary layer. Background Art

[0002] Wind tunnel testing is one of the most important methods for current aerodynamic research. Its method of obtaining flow field state information relies on the support of flow field measurement equipment. At present, quantitative wind tunnel experimental measurement methods include pitot probe, hot wire anemometer, pulsating pressure sensor, etc.

[0003] However, the above-mentioned invasive measurement methods are limited by the probe size and have low spatial resolution, which will interfere with the flow field itself and affect the measurement accuracy.

[0004] At present, the density pulsation information of the flow field in the flat plate boundary layer can be obtained through the cylindrical focusing laser differential interferometer. Due to the use of cylindrical lenses, the focusing point of the sheet light can be close to the surface of the flat plate to measure the information of the density pulsation of the flow field inside the flat plate boundary layer. There is no flat plate shading phenomenon, which has a strong advantage.

[0005] However, existing cylindrical focusing laser differential interferometers are all single-point measurements, and can only obtain information on one measurement point each time they are run, resulting in very low measurement efficiency. Summary of the invention

[0006] The object of the present invention is to provide a method for measuring density pulsation of a flat plate boundary layer, which can simultaneously measure the density pulsation information of multiple measuring points in a flat plate boundary layer to solve the technical problems existing in the prior art.

[0007] In order to solve the above technical problems, the present invention specifically provides the following technical solutions: a method for measuring density pulsation of a flat plate boundary layer, comprising the following steps:

[0008] Step 100, a parallel coherent light beam emitted unidirectionally by a laser is expanded and separated into a plurality of coherent fan-shaped light beams A;

[0009] Step 200, splitting each of the fan-shaped light beams A into two light beams B;

[0010] Step 300, after converting the divergent light beams B back into parallel light beams B, the light beams B are focused on the area to be tested in the boundary layer of the flat plate; and the wind tunnel is started so that the focused light beams B pass through the area to be tested;

[0011] Step 400, converting the focused light beams B back into mutually parallel light beams B;

[0012] Step 500, refocusing and merging the two beams B separated from each fan-shaped beam A into a coherent beam C;

[0013] Step 600: After filtering, each coherent light beam C is converted into a voltage signal by a corresponding photodetector, and the density pulsation information in the flat plate boundary layer is inferred based on the voltage signal.

[0014] Further, in step 100, the coherent light beam sequentially passes through a first polarizer, a first cylindrical plano-convex lens and a beam splitter;

[0015] The first polarizer is used to filter out noise light in the coherent light beam; the convex mirror of the first cylindrical plano-convex lens is set towards the propagation direction of the light beam, and is used to focus the coherent light beam and then expand it to diverge into a fan-shaped light beam; the beam splitter is used to split a fan-shaped light beam into N identical coherent fan-shaped light beams A.

[0016] Furthermore, in step 200, the N coherent fan-shaped light beams A pass through a first Wollaston prism, and each fan-shaped light beam A is further separated into two light beams B with polarization directions perpendicular to each other, so that all the light beams B are in a horizontal plane.

[0017] Furthermore, in step 200, the two light beams B separated by the first Wollaston prism satisfy the following relationship:

[0018]

[0019] Wherein, ΔX1 is the separation distance between the two light beams B, f2 is the focal length of the second biconvex lens, and σ is the separation angle of the first Wollaston prism.

[0020] Further, in step 300, the light beam B passes through the second cylindrical plano-convex lens and the first biconvex lens in sequence;

[0021] The convex mirror of the second cylindrical plano-convex lens is arranged toward the propagation direction of the light beam, and the second cylindrical plano-convex cylindrical mirror converts the light beam B back into a parallel light beam; the first biconvex lens focuses all the converted parallel light beams B into the test area of ​​the flat plate boundary layer.

[0022] Furthermore, in step 400, the focused light beam B passes through a second biconvex lens and is converted back into a parallel light beam.

[0023] Furthermore, in step 500, the light beam B converted into a parallel light beam passes through a third cylindrical plano-convex lens and a second Wollaston prism in sequence;

[0024] The convex mirror of the third cylindrical plano-convex lens is arranged toward the propagation direction of the light beam, and is used to convert the parallel light beam into a focused light beam; the second Wollaston prism is used to refocus and merge the two focused light beams B separated by the fan-shaped light beam A into a coherent light beam C.

[0025] Furthermore, in step 600, each of the coherent light beams C passes through the second polarizer and the plano-concave lens in sequence;

[0026] The second polarizer is used to filter the coherent light beam C to obtain a focused light beam with a consistent polarization direction;

[0027] The plano-concave lens is used to expand the filtered coherent light beam C, which is received by the corresponding photodetector.

[0028] Furthermore, in step 600, according to the formula of density in the flat plate boundary layer, the density pulsation information in the flat plate boundary layer is inverted according to the voltage signal;

[0029] The formula for the density in the flat plate boundary layer is as follows:

[0030]

[0031] Among them, Δρ is the density gradient at the flow field measurement point; ρ L is the average density of the measuring points; λ0 is the laser wavelength; π is the circumference of a circle; L is the length of the sensitive area; K is the Gladstone-Dale constant; V is the voltage output by the photodetector; V0 = 2I0RR L , I0 is the intensity of a single laser beam after being split by the first prism, R is the diode sensitivity of the photodetector; R L is the load resistance of the photodetector.

[0032] Compared with the prior art, the present invention has the following beneficial effects: the flat plate boundary layer density pulsation measurement method provided by the present invention utilizes a diffraction optical lens beam splitter to divide a single laser beam emitted by a single laser into multiple laser beams, which are then focused at multiple measuring points. Density pulsation information of multiple measuring points in the flat plate boundary layer can be obtained simultaneously each time, which greatly reduces the number of experiments, improves the test efficiency of the wind tunnel, and saves test costs.

[0033] The test beam of each channel of the interferometer of the present invention has good consistency. By performing correlation processing on experimental data of different measuring points, the correlation of each measuring point can be analyzed, and the understanding of the temporal coherence and spatial coherence of the flat plate boundary layer flow field structure can be deepened. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the implementation methods of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the implementation methods or the description of the prior art. Obviously, the drawings in the following description are only exemplary, and for ordinary technicians in this field, other implementation drawings can be derived from the provided drawings without creative work.

[0035] Figure 1 A schematic diagram of the structure of a method for measuring density pulsation of a flat plate boundary layer provided by an embodiment of the present invention;

[0036] Figure 2 A top view of the focus distribution in a model boundary layer of a flat plate boundary layer density pulsation measurement method provided by an embodiment of the present invention;

[0037] Figure 3 A front view of the focus distribution in a model boundary layer of a flat-plate boundary layer density pulsation measurement method provided in an embodiment of the present invention.

[0038] The numbers in the figure represent the following:

[0039] P1-first polarizer; C1-first cylindrical plano-convex lens; E-beam splitter; W1-first Wollaston prism; C2-second cylindrical plano-convex lens; S1-first biconvex lens; S2-second biconvex lens; C3-third cylindrical plano-convex lens; W2-second Wollaston prism; P2-second polarizer; V-plano-concave lens; D1-first photodetector; D2-second photodetector; D3-third photodetector; D4-fourth photodetector; D5-fifth photodetector; D6-sixth photodetector. DETAILED DESCRIPTION

[0040] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0041] The present invention provides a method for measuring the density pulsation of a flat plate boundary layer, comprising the following steps:

[0042] Step 100, a laser unidirectionally emits a parallel coherent light beam;

[0043] The coherent light beam passes through a first polarizer, a first cylindrical plano-convex lens and a beam splitter in sequence;

[0044] The first polarizer is used to filter out noise light in the coherent light beam; the convex mirror of the first cylindrical plano-convex lens is set towards the propagation direction of the light beam, and is used to focus the coherent light beam and then expand it to diverge into a fan-shaped light beam; the beam splitter is used to split a fan-shaped light beam into N identical coherent fan-shaped light beams A.

[0045] Step 200, the N coherent fan-shaped light beams A pass through a first Wollaston prism, and each fan-shaped light beam A is further separated into two light beams B with mutually perpendicular polarization directions, so that the light beams A and the light beams B are in the same plane;

[0046] The two beams B separated by the first Wollaston prism satisfy the following relationship:

[0047]

[0048] Where ΔX1 is the separation distance between the two beams B, f2 is the focal length of the second biconvex lens, and σ is the separation angle of the first Wollaston prism.

[0049] Step 300, the light beam B passes through the second cylindrical plano-convex lens and the first biconvex lens in sequence;

[0050] The convex mirror of the second cylindrical plano-convex lens is arranged toward the propagation direction of the light beam, and the second cylindrical plano-convex cylindrical mirror converts the light beam B back into a parallel light beam; the first biconvex lens focuses all the converted parallel light beams B into the to-be-measured area of ​​the flat plate boundary layer;

[0051] and start the wind tunnel so that the focused light beam B passes through the area to be tested;

[0052] Step 400: the focused light beam B passes through a second biconvex lens and is converted back into a parallel light beam;

[0053] Step 500, the light beam B converted into a parallel beam passes through the third cylindrical plano-convex lens and the second Wollaston prism in sequence;

[0054] The convex mirror of the third cylindrical plano-convex lens is arranged toward the propagation direction of the light beam, and is used to convert the parallel light beam B into a focused light beam B; the second Wollaston prism is used to refocus and merge the two focused light beams B separated by the fan-shaped light beam A into a coherent light beam C;

[0055] Step 600, each of the coherent light beams C passes through the second polarizer and the plano-concave lens in sequence;

[0056] The second polarizer is used to filter the coherent light beam C to obtain a focused light beam with a consistent polarization direction; the plano-concave lens is used to expand the filtered coherent light beam C, which is received by the corresponding photodetector. The photodetector converts the light signal into a voltage signal and infers the density pulsation information in the flat plate boundary layer based on the voltage signal.

[0057] like Figure 1-Figure 3 As shown, the present invention also provides a specific implementation method of the flat plate boundary layer density pulsation measurement method, using relevant optical lens components to build a measurement interferometer. The measurement light path generated by the measurement interferometer includes a transmitting light path and a receiving light path, the airflow passes between the two, and the measurement light path passes through the area to be measured.

[0058] The optical lens components constituting the emission light path include a laser generator, and a first polarizer, a first cylindrical plano-convex lens, a beam splitter, a first Wollaston prism, a second cylindrical plano-convex lens and a first biconvex lens which are sequentially arranged along the propagation direction of the light beam emitted by the laser;

[0059] The heat-generating optical lens components constituting the receiving light path include a second biconvex lens, a third cylindrical plano-convex lens, a second Wollaston prism, a second polarizer, a plano-concave lens and a photodetector which are sequentially arranged along the propagation direction of the light beam.

[0060] The laser generator emits a parallel coherent light beam, which passes through the first polarizer P1 to filter out noise light in the coherent light beam.

[0061] The coherent light beam passes through the first plano-convex cylindrical mirror C1, and then is expanded after being focused, and the light beam is diverged in all directions and is allowed to diverge in a single direction, forming a fan-shaped light beam.

[0062] The fan-shaped beam passes through the beam splitter E, which splits a coherent beam into six identical coherent fan-shaped beams A. The separation angles between the six coherent fan-shaped beams A are adjustable, and the separation angles determine the distances between the focal points formed after adjacent coherent lights pass through the lens.

[0063] Six coherent fan-shaped light beams A pass through the first Wollaston prism W1, and each coherent fan-shaped light beam A is further separated into two fan-shaped light beams B with mutually perpendicular polarization directions, and the separation angle between the two fan-shaped light beams B meets the preset requirements; wherein the mutually perpendicular polarization directions are determined by the characteristics of the first Wollaston prism W1, so that all separated light beams B are in a horizontal plane, so that the airflow passes through perpendicular to the optical axis direction, and passes through the two light beams in sequence.

[0064] Each coherent fan-shaped beam A is separated into two fan-shaped beams B with mutually perpendicular polarization directions, so that there is an optical path difference between the two fan-shaped beams B. The optical path difference further leads to a phase difference, which is reflected as a density difference. The relevant data is used for subsequent voltage inversion of density pulsation information.

[0065] The light beams pass through the second plano-convex cylindrical mirror C2, converting the 12 unidirectionally divergent light beams B back into parallel light beams B; the parallel light beams B pass through the first biconvex lens S1, focusing the 12 parallel light beams B in all directions; and starting the wind tunnel so that the focused light beams B pass through the area to be tested.

[0066] The light beam passes through the second double convex lens S2, which converts the focused light beam B back into a parallel light beam B; the light beam passes through the third cylindrical convex lens C3, which converts 12 parallel light beams B into unidirectional focused light beams B; the light beam passes through the second Wollaston prism W2, which recombines the 12 unidirectional focused light beams B into 6 coherent light beams C; the coherent light beam C passes through the second polarizer P2, which filters the coherent light beam C to obtain a focused light beam with consistent polarization direction; the two light beams interfere to generate interference light intensity, and the relevant data is used for subsequent voltage inversion of density pulsation information.

[0067] The light beam passes through the concave lens V, expanding the filtered coherent light beam C; the first photodetector D1, the second photodetector D2, the third photodetector D3, the fourth photodetector D4, the fifth photodetector D5 and the sixth photodetector D6 receive the corresponding light beam C and linearly convert the light intensity of the light beam C into a voltage signal.

[0068] The two fan-shaped beams B separated by the first Wollaston prism satisfy the following relationship:

[0069]

[0070] Wherein, ΔX1 is the separation distance between the two fan-shaped light beams B, f2 is the focal length of the second biconvex lens, and σ is the separation angle of the first Wollaston prism.

[0071] In order to make the light beam pass through the center of the optical lens accurately, the required optical lenses are installed in a coaxial system. This ensures that the center of each optical lens is at the same height. At the same time, the coaxial system can be used to easily adjust the axial position of each lens and the distance between them.

[0072] In this embodiment, the spacing between the optical lens elements must also meet the following requirements:

[0073] Wherein, L1 is the distance between the first cylindrical plano-convex lens and the second cylindrical plano-convex lens, is the focal length of the first cylindrical plano-convex lens, is the focal length of the second cylindrical plano-convex lens;

[0074] Wherein, L2 is the distance between the first biconvex lens and the second biconvex lens, is the focal length of the first biconvex lens, is the focal length of the second biconvex lens;

[0075] Wherein, L3 is the distance between the third cylindrical plano-convex lens and the plano-concave lens, is the focal length of the third cylindrical plano-convex lens;

[0076] L4=f V ; Where L4 is the distance between the plano-concave lens and the photodetector, f V is the focal length of the plano-concave lens.

[0077] The formula for the density in the flat plate boundary layer is as follows:

[0078]

[0079] Among them, Δρ is the density gradient at the flow field measurement point; ρ L is the average density of the measuring points; λ0 is the laser wavelength; π is the circumference of a circle; L is the length of the sensitive area; K is the Gladstone-Dale constant; V is the voltage output by the photodetector; V0 = 2I0RR L , I0 is the intensity of a single laser beam after being split by the first prism, R is the diode sensitivity of the photodetector; R L is the load resistance of the photodetector.

[0080] According to the Gladstone-Dale formula: n=Kρ+1; wherein n is the refractive index; ρ is the density; and K is the Gladstone-Dale constant.

[0081] The change in refractive index leads to a change in the optical path difference, which is:

[0082] △OPL=(n1-n2)L=△nL; where L is the size of the sensitive area.

[0083] The change of optical path difference leads to the change of phase difference, which is:

[0084]

[0085] Substituting the optical path difference into the formula, we can get the relationship between phase difference and density change:

[0086]

[0087] The formula for calculating light intensity change is:

[0088]

[0089] Where α is the polarizer angle; β is the phase difference change caused by other factors. (Note: Adjust the polarizer angle to 45 degrees; adjust Wollaston to -90°, and when I1=I2, let I1=I2=I0, then:

[0090]

[0091] And because the output voltage U: U = IRR L ;

[0092] The output voltage U0 of the photodetector when the phase difference caused by the flow is 0: U0=I0RR L , where R is the diode sensitivity; RL is the load resistance.

[0093] Then the relationship between phase difference and voltage is:

[0094]

[0095] Then the density difference and voltage:

[0096]

[0097] Divide both sides by ρ L , we get:

[0098]

[0099] Therefore, the change of density gradient in the flow field can be reflected by the change of voltage in the photodetector.

[0100] A single laser beam emitted by a single laser is divided into multiple laser beams using a diffraction optical lens beam splitter, which are then focused at multiple measuring points. The density pulsation information of multiple measuring points in the flat plate boundary layer can be obtained simultaneously each time, greatly reducing the number of experiments, improving the test efficiency of the wind tunnel, and saving test costs.

[0101] The above embodiments are only exemplary embodiments of the present application and are not intended to limit the present application. The protection scope of the present application is defined by the claims. Those skilled in the art may make various modifications or equivalent substitutions to the present application within the essence and protection scope of the present application, and such modifications or equivalent substitutions shall also be deemed to fall within the protection scope of the present application.

Claims

1. A method for measuring the density pulsation of a flat plate boundary layer, characterized in that: The following steps are included Step 100, a parallel coherent light beam emitted unidirectionally by a laser is expanded and separated into a plurality of coherent fan-shaped light beams A; The coherent light beam passes through the first polarizer, the first cylindrical plano-convex lens and the beam splitter in sequence; the first polarizer is used to filter out noise light in the coherent light beam; the convex mirror of the first cylindrical plano-convex lens is arranged toward the propagation direction of the light beam, and is used to focus the coherent light beam and then expand it to diverge into a fan-shaped light beam; the beam splitter is used to split a fan-shaped light beam into N identical coherent fan-shaped light beams A; Step 200, splitting each of the fan-shaped light beams A into two light beams B; Step 300, converting the divergent light beams B back into parallel light beams B, and focusing them toward the area to be measured in the boundary layer of the flat plate; and start the wind tunnel so that the focused light beam B passes through the area to be tested; Step 400, converting the focused light beams B back into mutually parallel light beams B; Step 500, refocusing and merging the two beams B separated from each fan-shaped beam A into a coherent beam C; Step 600, after filtering, each coherent light beam C is converted into a voltage signal by a corresponding photodetector, and the density pulsation information in the flat plate boundary layer is inferred based on the voltage signal; Among them, each coherent light beam C passes through the second polarizer and the plano-concave lens in sequence; the second polarizer is used to filter the coherent light beam C to obtain a focused light beam with a consistent polarization direction; the plano-concave lens is used to expand the coherent light beam C after filtering interference, and is received by the corresponding photodetector.

2. The method for measuring the density pulsation of a flat plate boundary layer according to claim 1, characterized in that: In step 200, the N coherent fan-shaped light beams A pass through a first Wollaston prism, and each fan-shaped light beam A is further separated into two light beams B with polarization directions perpendicular to each other; Make all beams B in one horizontal plane.

3. The method for measuring the flat plate boundary layer density pulsation according to claim 2, characterized in that: In step 200, the two light beams B separated by the first Wollaston prism satisfy the following relationship: Wherein, ΔX1 is the separation distance between the two light beams B, f2 is the focal length of the second biconvex lens, and σ is the separation angle of the first Wollaston prism.

4. The method for measuring the density pulsation of a flat plate boundary layer according to claim 1, characterized in that: In step 300, the light beam B passes through the second cylindrical plano-convex lens and the first biconvex lens in sequence; The convex mirror of the second cylindrical plano-convex lens is arranged toward the propagation direction of the light beam, and the second cylindrical plano-convex cylindrical mirror reconverts the light beam B into a parallel light beam; The first biconvex lens focuses all the light beams B converted into parallel light beams into the to-be-measured region of the flat plate boundary layer.

5. The method for measuring the density pulsation of a flat plate boundary layer according to claim 1, characterized in that: In step 400, the focused light beam B passes through a second biconvex lens and is converted back into a parallel light beam.

6. The method for measuring the density pulsation of a flat plate boundary layer according to claim 1, characterized in that: In step 500, the light beam B converted into a parallel beam passes through the third cylindrical plano-convex lens and the second Wollaston prism in sequence; The convex mirror of the third cylindrical plano-convex lens is arranged toward the propagation direction of the light beam, and is used to convert the parallel light beam into a focused light beam; The second Wollaston prism is used to refocus and merge the two focused light beams B separated from the fan-shaped light beam A into a coherent light beam C.

7. The method for measuring the density pulsation of a flat plate boundary layer according to claim 1, characterized in that: In step 600, according to the formula of density in the flat plate boundary layer, the density pulsation information in the flat plate boundary layer is inferred according to the voltage signal; The formula for the density in the flat plate boundary layer is as follows: Among them, Δρ is the density gradient at the flow field measurement point; ρ L is the average density of the measuring points; λ0 is the laser wavelength; π is the circumference of a circle; L is the length of the sensitive area; K is the Gladstone-Dale constant; V is the voltage output by the photodetector; V0 = 2I0RR L , I0 is the intensity of a single laser beam after being split by the first prism, R is the diode sensitivity of the photodetector; R L is the load resistance of the photodetector.

8. The method for measuring the density pulsation of a flat plate boundary layer according to claim 1, characterized in that: In step 100, the beam splitter is used to split a fan-shaped light beam into six identical coherent fan-shaped light beams A.

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