An invisibility cloak placed in a porous medium background and its application

Abstract

The present invention provides a stealth cloak placed in a porous medium background and its application. The stealth cloak includes a stealth area and a stealth cloak shell layer covering the stealth area; the permeability of the material of the stealth cloak shell layer satisfies any one of the following formula I or formula II. In the present invention, by designing the permeability of the material of the stealth cloak shell layer and the hydraulic permeability of the porous medium background, a stealth cloak with a relatively thin stealth cloak shell layer is prepared.

Description

Technical Field

[0001] The invention belongs to the field of stealth technology, and in particular relates to an invisible cloak placed in a porous medium background and its application. Background Art

[0002] Making objects invisible has always been a tireless pursuit of mankind. Since the advent of transformation optics theory in 2006, the dream of realizing invisibility cloaks in different physical fields has become no longer far-fetched. If the governing equation of a transport field remains unchanged under coordinate transformation, then its transport path can be modulated by our carefully designed metamaterials with spatial anisotropic parameters to bypass the invisible object, thereby avoiding the occurrence of scattering and achieving the invisibility function. Based on this theory, researchers can design an invisibility cloak that can prevent the internal space from being detected by the outside world, making the internal objects invisible under certain circumstances, such as optics, magnetism, acoustics, thermal and even time.

[0003] CN107114833A discloses a controllable two-dimensional thermal stealth cloak based on multi-layer nanofluids. The controllable two-dimensional thermal stealth cloak is formed by extending the nanofluid ring layer layer by layer in the x and y axis directions with the center line of the xy horizontal plane as the axis. By controlling the thermomagnetic conversion or thermophysical properties of the nanofluids in different ring layers, the characteristics of the nanomagnetic particles contained therein, the structure of the fluid, and the chemical composition of the nanoparticles, each layer of the nanofluid can correspond to a different thermal conductivity coefficient, and the two-dimensional thermal conductivity distribution required for thermal stealth can be obtained. Then, after the heat flow bypasses the cloak area, the temperature field and the heat flow line restore the original distribution, and the thermal stealth function is realized, and the external heat flow interference is shielded for the object in the center of the thermal stealth cloak, while the external heat flow distribution is not affected. At the same time, by cyclically controlling the thermal conductivity coefficient of the nanofluid in each ring layer, the real-time on / off performance of the thermal stealth cloak is realized, thereby overcoming the disadvantage that the two-dimensional thermal stealth cloak cannot be cyclically switched.

[0004] CN106983191A discloses a controllable three-dimensional thermal stealth cloak based on multi-layer nanofluids. The controllable three-dimensional thermal stealth cloak is realized by a surface covering shell composed of nanofluids. The surface covering shell is composed of multiple nanofluid ring layers stacked from bottom to top. By controlling the thermomagnetic conversion or thermophysical properties of the nanofluids in different ring layers, the characteristics of the nanomagnetic particles contained therein, the structure of the fluid, and the chemical composition of the nanoparticles, each layer can correspond to a different thermal conductivity coefficient, and the three-dimensional thermal conductivity distribution required for thermal stealth can be obtained, so that after the heat flow bypasses the cloak area, the temperature field and isotherms restore the original distribution, and the thermal stealth function is realized. At the same time, by cyclically controlling the thermal conductivity of the nanofluid in each ring layer, the real-time on / off performance of the thermal stealth cloak is realized, thereby overcoming the disadvantage that the three-dimensional thermal stealth cloak cannot be cyclically switched on and off.

[0005] Generally, such stealth devices are designed as a shell that can wrap the object to be stealthily hidden. Similarly, in fluid mechanics, an invisibility cloak based on transformation optics has also been proposed to hide an object without disturbing the surrounding fluid flow field, so that an observer observes a uniform flow field near the obstacle outside the stealth shell, which is indistinguishable from the flow field without an obstacle. In addition, the fluid invisibility cloak is of great significance in target stealth and fluid drag reduction. Since the invisibility cloak based on the transformation optics theory requires anisotropic or even extremely varying material parameter distributions, usually this type of invisibility cloak uses metamaterials to design the stealth shell. However, this design makes the cloak structure very complex and poses high requirements for processing accuracy, which makes the cloak design with a simple structure and homogeneous materials more attractive.

[0006] Although many researchers have conducted experimental and theoretical studies on invisibility cloaks with different structures, however, the thickness of the cloak, as an important measure of the cloak's stealth performance, has never been systematically studied. You know, for any transport field, as long as the distance from the object to be stealthily hidden is far enough, the interference of the object to the field will be so small that the observer cannot detect it. Therefore, an overly thick invisibility cloak has no meaning. For the design of an invisibility cloak, the thinner the shell layer, the better. The thinner the invisibility cloak, the more difficult it is for a near-field observer to detect it. However, the price of making the invisibility cloak extremely thin is that the material parameter distribution becomes more extreme, and it is very difficult to find materials with such extreme parameters in many physical fields. Therefore, it is also more difficult to achieve experimentally.

[0007] Therefore, how to design an invisibility cloak with a relatively thin shell layer of the invisibility cloak has become a technical problem to be solved urgently at present. Summary of the Invention

[0008] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide an invisibility cloak placed in a porous medium background and its application. In the present invention, by designing the permeability of the material of the invisibility cloak shell layer and the hydraulic permeability of the porous medium background, and making the two satisfy a certain relational expression, an invisibility cloak with a relatively thin shell layer of the invisibility cloak can be prepared.

[0009] To achieve this purpose, the present invention adopts the following technical solutions:

[0010] In a first aspect, the present invention provides an invisibility cloak placed in a porous medium background, and the invisibility cloak includes a stealth area and an invisibility cloak shell layer covering the stealth area;

[0011] The permeability of the material of the invisibility cloak shell layer satisfies any one of the following formula I or formula II:

[0012] k s =(R2 2 +R1 2 ) / (R2 2 -R12 ) × k b Formula I;

[0013] k s = (2R2 3 + R1 3 ) / (2(R2 3 - R1 3 )) × k b Formula II;

[0014] Wherein, k s is the hydraulic permeability of the invisibility cloak shell material, k b is the hydraulic permeability of the porous medium background, R1 is the distance between the centroid of the invisibility region and the boundary of the invisibility region; R2 is the distance between the centroid of the invisibility region and the side of the invisibility cloak shell far from the invisibility region.

[0015] Since the invisibility cloak based on transformation optics requires anisotropic distribution of parameters within the invisibility shell, for experimental realization, metamaterials are usually used to construct the invisibility shell. The use of metamaterials makes the structural design of the invisibility cloak complex and poses high requirements for processing technology, which also makes the invisibility shell of the cloak not very thin. The invisibility cloak of the present invention is based on the bilayer theory, that is, a bilayer combination of one layer that repels physical fields and one layer that attracts physical fields can achieve the invisibility function, avoiding material anisotropy. The so-called repelling or attracting physical fields means that, for example, the invisibility cloak provided by the present invention is used in a flow field environment. The background field of the flow field environment is a fixed porous medium. When the fluid in the porous medium encounters a solid obstacle, since the permeability of the solid obstacle is 0, its streamlines are shown to bypass the obstacle and pass through, which is defined as a repelling flow field; similarly, for the fluid in the porous medium, passing through a blank area without porous medium and without any structure, where the permeability is greater than that of the background field, the fluid will converge in this area and the streamlines become dense in this area, so it can be called an attracting flow field.

[0016] In addition, if the thickness of the stealth cloak based on the bilayer theory is required to be very thin, the ratio of the parameters of the shell material to the background material needs to be extremely extreme, and it is very difficult to find materials with suitable parameters in many physical fields. The porous medium material in the fluid flow field, due to the exponential distribution relationship between its porosity and hydraulic permeability, can have a hydraulic permeability change of up to three orders of magnitude, meeting the design requirements of extremely extreme material parameters, and thus bringing hope for the realization of an ultra-thin stealth cloak. In the present invention, by directly solving the Laplace equation, a shell-type stealth cloak with a relatively thin thickness has been successfully realized in the porous medium flow field. That is to say, in the present invention, by placing the stealth cloak in a porous medium background and regulating the permeability of the stealth cloak shell material and the hydraulic permeability of the porous medium background to meet certain conditions, a stealth cloak with a relatively thin shell can be prepared, solving the problem that it is difficult to prepare a stealth cloak with a relatively thin thickness due to the extremely extreme ratio of the parameters of the shell material to the background material.

[0017] It should be noted that the thickness of the stealth cloak shell in the present invention is (R2 - R1), and R1 and R2 have the same protection scope as described above.

[0018] At the same time, it should be noted that in the present invention, there are no special limitations on the size of the stealth area, the material of the porous medium background, and the material of the stealth cloak shell, as long as it satisfies k s =(R2 2 +R1 2 ) / (R2 2 -R1 2 )×k b or k s =(2R2 3 +R1 3 ) / (2(R2 3 -R1 3 ))×k b 's relational expression is sufficient, where k s is the hydraulic permeability of the stealth cloak shell material, k b is the hydraulic permeability of the porous medium background, R1 is the distance between the centroid of the stealth area and the boundary of the stealth area; R2 is the distance between the centroid of the stealth area and the side of the stealth cloak shell far from the stealth area.

[0019] The following are the preferred technical solutions of the present invention, but do not limit the technical solutions provided by the present invention. Through the following preferred technical solutions, the purpose and beneficial effects of the present invention can be better achieved and realized.

[0020] As a preferred technical solution of the present invention, the stealth cloak is a two-dimensional stealth cloak or a three-dimensional stealth cloak.

[0021] Preferably, the invisibility cloak is a two-dimensional invisibility cloak, and the permeability of the shell material of the invisibility cloak is:

[0022] k s =(R2 2 +R1 2 ) / (R2 2 -R1 2 )×k b ;

[0023] wherein, k s , k b , R1, and R2 have the same scope of protection as described above.

[0024] As a preferred technical solution of the present invention, the invisibility cloak is a three-dimensional invisibility cloak, and the permeability of the shell material of the invisibility cloak is:

[0025] k s =(2R2 3 +R1 3 ) / (2(R2 3 -R1 3 ))×k b ;

[0026] wherein, k s , k b , R1, and R2 have the same scope of protection as described above.

[0027] As a preferred technical solution of the present invention, the porous medium background is an isotropic porous medium background.

[0028] As a preferred technical solution of the present invention, the porous medium background is obtained by arranging background unit arrays or is composed of heterogeneous porous materials.

[0029] Preferably, the heterogeneous porous material is selected from any one or a combination of at least two of soil, rock, and mineral layers.

[0030] It should be noted that when the porous medium background is obtained by arranging background unit arrays, in the present invention, there are no special limitations on the material of the background unit and the shape of the cross-section of the background unit, as long as it has a hydraulic permeability matching that of the shell of the invisibility cloak. Exemplarily, the shape of its cross-section can be rectangular, triangular, elliptical, etc.

[0031] When the size of the invisible area (R1) and the thickness of the shell of the invisibility cloak (R2 - R1) are determined, in the present invention, the hydraulic permeability of the porous medium background can be adjusted by adjusting the size of the background unit and the distance between the background units, so that it matches the hydraulic permeability of the shell of the invisibility cloak.

[0032] Similarly, the porous medium background is composed of heterogeneous porous materials, and the heterogeneous porous materials also need to have a hydraulic permeability that matches the stealth region. Moreover, the hydraulic permeability of the heterogeneous porous materials does not refer to the hydraulic permeability at a certain point of the heterogeneous porous materials, but rather refers to the average hydraulic permeability of the heterogeneous porous materials in the observation region of the flow field environment that matches the hydraulic permeability of the stealth region (satisfying Equation I or Equation II).

[0033] As a preferred technical solution of the present invention, the stealth region is a closed region composed of a smooth curve or a smooth surface.

[0034] It should be noted that if the stealth region is a closed region composed of a smooth curve, a two-dimensional stealth cloak will be obtained in the end; if the stealth region is a closed region composed of a smooth surface, a three-dimensional stealth cloak will be obtained in the end.

[0035] As a preferred technical solution of the present invention, the stealth region is selected from any one of a circle, an ellipse, a sphere, or an ellipsoid.

[0036] As a preferred technical solution of the present invention, the hydraulic permeability of the stealth region is 0.

[0037] Since the hydraulic permeability of the stealth region is 0, according to Darcy's law, the flow direction will always bypass the region with a permeability of 0. That is to say, the stealth region (obstacle) "repels" the streamlines, and external observers will observe the distortion of the flow field, thereby detecting the existence of the obstacle. Therefore, in order to achieve invisibility, a background region with a hydraulic permeability higher than that which "attracts" the streamlines is necessary to offset the "repelling" effect of the obstacle.

[0038] As a preferred technical solution of the present invention, (R2 - R1) / R1 < 0.02, for example, it can be 0.001, 0.003, 0.005, 0.007, 0.01, 0.012, 0.014, 0.016, or 0.018, etc.

[0039] Preferably, (R2 - R1) / R1 < 0.01, for example, it can be 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, or 0.009, etc.

[0040] Wherein, R1 and R2 have the same protection scope as described above.

[0041] It should be noted that the invisibility cloak provided by the present invention includes three parts: an invisibility region, an invisibility cloak shell layer covering the invisibility region, and a porous medium background. The sizes between the three parts can be scaled proportionally without affecting the product effect, and the size scaling is valid from the microfluidic scale (1 nm - 1 mm scale) to the macroscopic scale. The influence of different scales on the effect lies in: within what range of observation the system can maintain the invisibility effect. That is, as long as the observer's observation range is greater than the size of the smallest structural unit (the smallest structural unit includes the invisibility region, the invisibility cloak shell layer covering the invisibility region, and the porous medium background), it can be considered that the system remains invisible in the flow field (the existence of the structure has no influence on the external field).

[0042] Meanwhile, it should be noted that the present invention does not make any special limitations on the preparation method of the invisibility cloak placed in the porous medium background. Exemplarily, it includes but is not limited to: soft lithography technology, 3D printing processing technology, etc.

[0043] In the second aspect, the present invention provides an application of the invisibility cloak as described in the first aspect in a flow field environment, where the Reynolds number of the flow field environment < 1, for example, it can be 0.5, 0.6, 0.7, 0.8, or 0.9, etc.

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] (1) Different from the fields of using other common natural materials to construct metamaterials, the porous medium background of the invisibility cloak provided by the present invention can be composed of natural materials or can be artificially constructed, and its parameters can be determined by the structure designed by humans. Therefore, this system gives the porous medium background material and the invisibility cloak the greatest design freedom;

[0046] (2) There is an exponential relationship between the controllable parameter (porosity) in the designed porous medium background flow field and the relevant parameters (hydraulic permeability) of the invisibility cloak provided by the present invention. Therefore, the parameter span of this invisibility cloak is large, usually spanning several orders of magnitude, meeting the necessary conditions for an extremely thin and invisible cloak with extreme parameters;

[0047] (3) The design of the present invention is simple. An invisibility cloak with an extremely thin invisibility cloak shell layer thickness is designed by using the porous medium background in fluid mechanics, and this invisibility cloak has a simple design and can be prepared without materials with extreme parameters. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 is a schematic structural diagram of the cross-section of the invisibility cloak provided in Embodiment 1 of the present invention;

[0049] Figure 2 is Figure 1 an enlarged structural diagram of the cross-section within the black square;

[0050] Figure 3 is a physical diagram of the microfluidic device provided in Embodiment 1 of the present invention;

[0051] Figure 4 is a velocity field picture of the microfluidic device provided in Comparative Example 1 of the present invention through parametric simulation by COMSOL Multiphysics;

[0052] Figure 5 is a velocity field picture of the microfluidic device provided in Comparative Example 2 of the present invention through parametric simulation by COMSOL Multiphysics;

[0053] Figure 6 is a velocity field picture of the microfluidic device provided in Embodiment 1 of the present invention through parametric simulation by COMSOL Multiphysics;

[0054] Figure 7 is a velocity field picture of the microfluidic device provided in Comparative Example 1 of the present invention through simulation by COMSOL Multiphysics;

[0055] Figure 8 is a velocity field picture of the microfluidic device provided in Comparative Example 2 of the present invention through simulation by COMSOL Multiphysics;

[0056] Figure 9 is a velocity field picture of the microfluidic device provided in Embodiment 1 of the present invention through simulation by COMSOL Multiphysics;

[0057] Figure 10 is by using an electric microscope for Figure 7 fluid velocity field photograph of fluorescent tracer particles within the square box;

[0058] Figure 11 is by using an electric microscope for Figure 8 fluid velocity field photograph of fluorescent tracer particles within the square box;

[0059] Figure 12 is by using an electric microscope for Figure 9 fluid velocity field photograph of fluorescent tracer particles within the square box;

[0060] Figure 13 is the test result of the normalized velocity of the fluid at 0.5 cm from the bottom end of the microfluidic devices provided in Embodiment 1 of the present invention and Comparative Example 2;

[0061] Figure 14 is a three-dimensional structure schematic diagram of the three-dimensional invisibility cloak placed in a porous medium background provided in Embodiment 2 of the present invention;

[0062] Figure 15It is a velocity field picture of the three-dimensional invisibility cloak provided in Embodiment 2 of the present invention through parametric simulation by COMSOL Multiphysics;

[0063] Figure 16 It is a velocity field picture of the three-dimensional cloak provided in Comparative Example 3 of the present invention through parametric simulation by COMSOL Multiphysics;

[0064] Figure 17 It is a pressure field picture of the three-dimensional invisibility cloak provided in Embodiment 2 of the present invention through parametric simulation by COMSOL Multiphysics;

[0065] Figure 18 It is a pressure field picture of the three-dimensional cloak provided in Comparative Example 3 of the present invention through parametric simulation by COMSOL Multiphysics;

[0066] Among them, 1 - invisibility region, 2 - invisibility cloak shell layer, 3 - porous medium background. Specific Embodiments

[0067] The technical solutions of the present invention will be further described below in conjunction with the accompanying drawings and through specific embodiments. Those skilled in the art should understand that the described embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention.

[0068] It should be noted that the structural dimensions in the present invention can all be scaled proportionally. Therefore, only the relative proportions or dimensionless dimensions of each structural dimension are described in this specification.

[0069] In the following embodiments and comparative examples: k s is the hydraulic permeability of the material of the invisibility cloak shell layer, k b is the hydraulic permeability of the porous medium background, R1 is the distance between the centroid of the invisibility region and the boundary of the invisibility region; R2 is the distance between the centroid of the invisibility region and the side of the invisibility cloak shell layer far from the invisibility region.

[0070] Embodiment 1

[0071] This embodiment provides a two-dimensional invisibility cloak and a microfluidic device placed in a porous medium background. The schematic structural diagram of the cross-section of the two-dimensional invisibility cloak placed in the porous medium background is as shown in Figure 1 and Figure 2 shown, where Figure 2 is Figure 1 the enlarged structural diagram of the cross-section within the black square;

[0072] The two-dimensional invisibility cloak includes an invisibility region 1 (made of polydimethylsiloxane, a cylinder with a height of 10 μm and a diameter of 40 mm, i.e., R1 is 20 mm), an invisibility cloak shell layer 2 that covers the invisibility region (a thin, unstructured, water-permeable layer, i.e., an air layer, k s is 3×10^(-9) m^2, and its thickness is 60 μm, i.e., R2 is 20.06 mm), and cylinders arranged in an array around the invisibility cloak shell layer. The diameter of the cylinders is 800 μm, and the minimum distance between any two adjacent cylinders is 10 μm. The cylinders arranged in an array around the invisibility cloak shell layer form a porous medium background 3, k b is 9×10^(-12) m^2.

[0073] The microfluidic device has a length of 8 cm, a width of 6 cm, and a height of 10 μm; the microfluidic device includes the above two-dimensional invisibility cloak.

[0074] The preparation method of the above microfluidic device is as follows:

[0075] A microfluidic device is prepared by soft lithography technology. The microfluidic device includes an invisibility region 1 placed at the center of the microfluidic device (a cylinder made of polydimethylsiloxane, with a height of 10 μm and a diameter of 40 mm), an invisibility cloak shell layer 2 that covers the invisibility region (a thin, unstructured, water-permeable layer, i.e., an air layer, with a thickness of 60 μm), and cylinders arranged in an array around the invisibility cloak shell layer and uniformly filling the microfluidic device (forming a porous medium background 3). The diameter of the cylinders is 800 μm, and the minimum distance between any two adjacent cylinders is 10 μm.

[0076] Comparative Example 1

[0077] This comparative example provides a porous medium background material and a microfluidic device. The porous medium material is composed of cylinders arranged in an array. The diameter of the cylinders is 800 μm, and the minimum distance between any two adjacent cylinders is 10 μm;

[0078] The microfluidic device has a length of 8 cm, a width of 6 cm, and a height of 10 μm and is composed of the above porous medium background material.

[0079] Comparative Example 2

[0080] This comparative example provides a two-dimensional cloak placed in a porous medium background and a microfluidic device. The difference from Example 1 is that the two-dimensional cloak does not include an invisibility cloak shell layer that covers the invisibility region, i.e., it is only composed of an invisibility region (also called an obstacle) and a porous medium background;

[0081] The microfluidic device does not include an invisibility cloak shell layer that covers the invisibility region;

[0082] Other conditions are the same as those in Example 1.

[0083] The flow field distributions of the microfluidic devices provided in Example 1 and Comparative Examples 1-2 were tested, and the test results are as follows Figure 4 - 12 as shown, where:

[0084] Figure 4 - 6 are respectively the photos of the velocity fields in three cases: homogeneous porous medium (the microfluidic device provided in Comparative Example 1), porous medium with obstacles (stealth region) (the microfluidic device provided in Comparative Example 2), and stealth cloak (the microfluidic device provided in Example 1) through parametric simulation by COMSOL Multiphysics. It can be seen from Figure 4 - 6 that: in the homogeneous porous medium ( Figure 4 ), the velocity field is parallel and uniform; if there are obstacles ( Figure 5 ) in the microfluidic device, the velocity field bends and is disturbed around the obstacles; when the obstacles are completely covered by the stealth shell layer ( Figure 6 ), the velocity field is parallel and uniform. It shows that through parametric simulation (i.e., after modeling in COMSOL Multiphysics, specific numerical values are input in specific regions for simulation), as long as R1, R2, k s , k b satisfy the formula k s = (R2 2 + R1 2 ) / (R2 2 - R1 2 ) × k b , the effect of an ultrathin stealth cloak for fluids can be achieved.

[0085] Fluorescent tracer particles with a diameter of 2 μm (obtained by staining polymethyl methacrylate particles (density 1.03 g / cm 3 ) with rhodamine B) were dispersed in water, and the volume concentration of the fluorescent tracer particles in the water was 0.5 μL / mL to obtain a working fluid. The working fluid was stably injected into the microfluidic devices provided in Example 1 and Comparative Examples 1-2 through an injection pump, and the photos of the velocity fields of the microfluidic devices provided in Comparative Examples 1-2 and Example 1 were simulated by COMSOL Multiphysics, as Figure 7 - 9 shown. It can be seen from Figure 7 - 9 that: by adjusting the size of the cylinders and the distance between any two cylinders, a porous medium background with a specific hydraulic permeability can be formed, and thus the relationship between the hydraulic permeabilities k s and k b necessary for the ultrathin stealth cloak can be achieved, and it is also confirmed that the effect of ultrathin stealth can be achieved through a specific design, that is, the combination of a porous medium background and an unstructured water-permeable thin layer.

[0086] An electric microscope (Nikon TI-E) was used for observation to obtain streamline photographs of fluorescent tracer particles in the microfluidic devices provided in Comparative Examples 1-2 and Example 1. The results are as Figure 10 - 12 shown. It can be seen from Figure 10 - 12 that when there are only cylinders arranged in an array in the microfluidic device (Comparative Example 1, Figure 10 ), the streamlines in the microfluidic device are parallel and uniform; if there are obstacles in the microfluidic device (Comparative Example 2, Figure 11 ), the streamlines will bend and be disturbed around the obstacles; when the obstacles are completely covered by the stealth shell layer (Example 1, Figure 12 ), the streamlines in the microfluidic device are parallel and uniform, indicating that the obstacles are well hidden and a good stealth effect is achieved.

[0087] The normalized velocities of the fluid at 0.5 cm from the bottom of the microfluidic devices provided in Example 1 and Comparative Example 2 (denoted as the observation line) were tested. The test results are as Figure 13 shown. It can be seen from Figure 13 that the fluid velocity near the stealth object in the microfluidic device provided in Example 1 remains almost constant. In contrast, the flow velocity near the obstacle without the stealth cloak fluctuates greatly, indicating that through the design of the stealth cloak in the present invention, a good stealth effect is achieved.

[0088] Among them, the normalized velocity is: the velocity value at each point 0.5 cm from the bottom of the microfluidic device is divided by the velocity value measured at x = 0 on the observation line. The point x = 0 is the intersection of the center of the stealth area and the perpendicular distance of the test line. The value of x < 0 represents the distance from the point x = 0 to the left on the test line. Similarly, the value of x > 0 represents the distance from the point x = 0 to the right on the test line;

[0089] The normalized x is: all the values of x are divided by 1 mm.

[0090] Example 2

[0091] This example provides a three-dimensional stealth cloak placed in a porous medium background. Its three-dimensional structure schematic diagram is as Figure 14 shown, including a porous medium background 3, a stealth area 1, and a stealth cloak shell layer 2 covering the stealth area;

[0092] The stealth area is a sphere with a diameter of 20 cm, that is, R1 is 10 cm;

[0093] The thickness of the stealth cloak shell layer is an air thin layer of 0.03 cm, that is, R2 is 10.03 cm, and k s is 3×10^(-9) m^2;

[0094] The porous medium background is composed of homogeneous spherical particles arranged isotropically, and k bis 1.8×10^(-11) m^2.

[0095] Comparative Example 3

[0096] This comparative example provides a three-dimensional cloak placed in a porous medium background. The difference from Example 2 is only that the three-dimensional cloak does not include a stealth cloak shell layer covering the stealth area, and other conditions are the same as those in Example 2.

[0097] Use COMSOL Multiphysics parametric simulation to simulate the velocity field of the three-dimensional cloaks provided in Example 2 and Comparative Example 3. The results are as Figure 15 and 16 shown. From Figure 15 and 16 it can be seen that: around the stealth cloak provided in Example 2 of the present invention, the velocity field does not bend and is not disturbed, and is parallel and uniform, indicating that under the precursor satisfying k s =(2R2 3 +R1 3 ) / (2(R2 3 -R1 3 ))×k b , the designed three-dimensional stealth cloak of the present invention has achieved a good stealth effect.

[0098] Use COMSOL Multiphysics parametric simulation to simulate the pressure field of the three-dimensional cloaks provided in Example 2 and Comparative Example 3. The results are as Figure 17 and 18 shown. From Figure 17 and 18 it can be seen that: around the stealth cloak provided in Example 2 of the present invention, the pressure field does not bend and is not disturbed, and is parallel and uniform, indicating that under the precursor satisfying k s =(2R2 3 +R1 3 ) / (2(R2 3 -R1 3 ))×k b , the designed three-dimensional stealth cloak of the present invention has achieved a good stealth effect.

[0099] In summary, the present invention uses a porous medium material as the background, giving the maximum design freedom to the porous medium background material and the stealth cloak; by regulating the hydraulic permeability of the porous medium background, the necessary conditions for an ultra-thin stealth cloak with extreme parameters are met; at the same time, by regulating the permeability of the stealth cloak shell layer material and the hydraulic permeability of the porous medium background to match them, a stealth cloak with an ultra-thin stealth cloak shell layer thickness is designed, and this stealth cloak has a simple design and can be prepared without materials with extreme parameters.

[0100] The applicant declares that the detailed structural features of the present invention are illustrated by the above embodiments, but the present invention is not limited to the above detailed structural features, that is, it does not mean that the present invention must rely on the above detailed structural features to be implemented. Those skilled in the art should understand that any improvement to the present invention, the equivalent replacement of the components selected for the present invention, the addition of auxiliary components, the selection of specific methods, etc. all fall within the protection scope and the disclosure scope of the present invention.

Claims

1. An invisibility cloak placed in the background of a porous medium, characterized in that, The invisibility cloak includes an invisible region and an invisibility cloak shell layer covering the invisible region; The permeability of the material of the invisibility cloak shell layer satisfies any one of the following formula I or formula II: k s =(R2 2 +R1 2 ) / (R2 2 -R1 2 )×k b Equation I; k s =(2R2 3 +R1 3 ) / (2(R2 3 -R1 3 ))×k b Formula II; where (R2 - R1) / R1 < 0.02; k s is the hydraulic permeability of the invisibility cloak shell material, k b is the hydraulic permeability of the porous medium background, R1 is the distance between the centroid of the invisibility region and the boundary of the invisibility region; R2 is the distance between the centroid of the invisibility region and the side of the invisibility cloak shell away from the invisibility region.

2. The invisibility cloak according to claim 1, characterized in that, The invisibility cloak is a two-dimensional invisibility cloak or a three-dimensional invisibility cloak.

3. The invisibility cloak according to claim 2, characterized in that, The invisibility cloak is a two-dimensional invisibility cloak, and the permeability of the material of the invisibility cloak shell layer is: k s =(R2 2 +R1 2 ) / (R2 2 -R1 2 )×k b 。 4. The invisibility cloak according to claim 2, characterized in that, The invisibility cloak is a three-dimensional invisibility cloak, and the permeability of the material of the invisibility cloak shell layer is: k s =(2R2 3 +R1 3 ) / (2(R2 3 -R1 3 ))×k b 。 5. The invisibility cloak according to claim 1, characterized in that, The porous medium background is an isotropic porous medium background.

6. The invisibility cloak according to claim 5, characterized in that, The porous medium background is obtained by arranging a background unit array or is composed of a heterogeneous porous material.

7. The invisibility cloak according to claim 6, characterized in that, The heterogeneous porous material is selected from any one or a combination of at least two of soil, rock, and mineral layer.

8. The invisibility cloak according to claim 1, characterized in that, The invisible region is a closed region formed by a smooth curve or a smooth surface.

9. The invisibility cloak according to claim 8, characterized in that, The invisible region is selected from any one of a circle, an ellipse, a sphere, or an ellipsoid.

10. The invisibility cloak according to claim 1, characterized in that, The hydraulic permeability of the invisible region is 0.

11. The invisibility cloak according to claim 1, characterized in that, (R2 - R1) / R1 < 0.

01.

12. An application of the invisibility cloak according to any one of claims 1-11 in a flow field environment, characterized in that, The Reynolds number of the flow field environment < 1.

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