CN107480834B - Method for predicting occurrence of karman vortex street at tail end of river submerged vegetation group - Google Patents
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Abstract
The invention discloses a method for predicting the occurrence of karman vortex street at the tail end of a river submerged vegetation group, which comprises the steps of firstly measuring the width D and the height h of the high-density submerged vegetation group and calculating the height-to-width ratio h/D of the submerged vegetation group; and judging whether the karman vortex street at the tail end of the submerged vegetation group occurs or not according to the height-to-width ratio h/D of the submerged vegetation group. The method only needs to measure the width (D) and the height (h) of the submerged vegetation group, judges whether the karman vortex street at the tail end of the submerged vegetation group occurs or not through the height-to-width ratio h/D of the submerged vegetation group, is simple and accurate, does not need to carry out flow rate measurement work or dye test to detect the karman vortex street, and has wide universality in the field.
Description
Technical Field
The invention belongs to the fields of hydraulics and river dynamics, and relates to a method for predicting the occurrence of karman vortex street at the tail end of a river submerged vegetation group.
Background
In a natural river, a vegetation group is generally an initial growth form of an aquatic plant community, the vegetation grows intensively, and the overlooking shape is approximately circular. These aquatic plants grow in the area of river channels, and their vegetation height is small at their initial growth stage, usually showing a state of being submerged by who. The emergence of submerged vegetation clusters can alter the water flow structure around them, particularly creating wake vortices at the tail of the vegetation cluster.
The karman vortex street is a hydrodynamic phenomenon generated by viscous incompressible fluid, and is characterized in that when a constant incoming flow bypasses some objects (such as vegetation groups), two sides of the objects periodically shed double-row line vortexes which rotate in opposite directions and are arranged regularly. Generally speaking, karman vortex street appears at the tail end of high-density non-submerged vegetation group, which is CdaD>4 of the vegetation group, wherein CdThe coefficient of drag force of vegetation is a unit water blocking area (n × D, n is the number of vegetation in unit area, D is the diameter of the individual vegetation), and D is the width of vegetation. This is because of the high density of non-floodingThe water flow around the vegetation can be considered as an approximately two-dimensional water flow (only the lateral variation of the water flow is considered), and the flow velocity inside the vegetation group is greatly different from the flow velocity outside the vegetation group, so that the karman vortex street occurs. Karman vortex street appears in the wake zone (L) of vegetation groupw) On the rear side, the flow velocity of the tail end of the vegetation group is increased, the turbulent fluctuation intensity is increased rapidly, and river bed scouring is caused, and the change of the water and sand motion characteristics is not beneficial to the ecological stability and the balanced development of the river channel.
When the vegetation group is submerged, the water flow structure at the tail end of the vegetation group becomes extremely complex, and is a very typical three-dimensional water flow structure (the change of the water flow in the transverse direction and the vertical direction is considered at the same time). When the density of the submerged vegetation is high (ah >0.1), the tail end of the submerged vegetation can generate obvious karman vortex street under certain conditions, and can not generate the karman vortex street under other conditions (as shown in figure 1). However, based on the current research means, no simple and accurate discrimination method is found at home and abroad, and the occurrence of the karman vortex street at the tail end of the submerged vegetation group can be directly judged.
Disclosure of Invention
Aiming at the technical current situation that whether the karman vortex street occurs at the tail end of the river submerged vegetation group is difficult to effectively predict in the prior art, the invention aims to provide a method for predicting the karman vortex street at the tail end of the river submerged vegetation group so as to judge whether the karman vortex street at the tail end of the submerged vegetation group occurs by utilizing the geometric dimensions (height and width of the vegetation group) of the vegetation group.
The basic idea of the invention is to aim at the flow rate U0The method comprises the steps that high-density submerged vegetation groups (ah is larger than 0.1) in a river channel with the height being larger than 3.1cm/s are detected whether karman vortex streets occur at the tail ends of a large number of river channel vegetation groups through a coloring agent test or a flow velocity measurement operation, a judgment index related to the geometric dimension of the submerged vegetation groups is given according to a statistical result, and whether the karman vortex streets occur at the tail ends of the unknown river channel submerged vegetation groups is predicted through the judgment index.
Based on the above invention thought, the invention provides a method for predicting the occurrence of karman vortex street at the tail end of a river submerged vegetation group, which comprises the following steps:
(1) measuring river courseWater flow rate U0If U is present0If U is more than 3.1cm/s, entering the step (2)0Ending the program when the concentration is less than or equal to 3.1 cm/s;
(2) measuring the transverse width D, the height h, the number n of vegetation in unit area and the diameter D of the individual vegetation of the submerged vegetation group in the direction perpendicular to the river, and calculating the unit water blocking area a of the submerged vegetation group as n multiplied by D and the height-to-width ratio h/D of the submerged vegetation group;
(3) judging whether the vegetation density ah is greater than 0.1, if ah is greater than 0.1, indicating that the submerged vegetation group is a high-density vegetation group, entering the step (4), if ah is less than or equal to 0.1, indicating that the submerged vegetation group is a low-density vegetation group, and ending the program;
(4) judging whether the karman vortex street occurs at the tail end of the submerged vegetation group according to the following judgment indexes: when the height-to-width ratio h/D of the submerged vegetation group is less than 0.7, the tail end of the submerged vegetation group does not generate a karman vortex street; when the height-to-width ratio h/D of the submerged vegetation group is more than 0.9, the tail end of the submerged vegetation group stably and continuously generates the karman vortex street; when the height-to-width ratio h/D of the submerged vegetation group is more than or equal to 0.7 and less than or equal to 0.9, the tail end of the submerged vegetation group can not stably and continuously generate the karman vortex street; the determination process is completed and the routine ends.
The method for predicting the occurrence of the karman vortex street at the tail end of the river submerged vegetation group comprises the following steps of (4):
(i) image acquisition: in the river channel with the height-width ratio h/D of the submerged vegetation group known, the movement period T according to the karman vortex streetkvSeparately collect intervals T kv2, pictures at two moments comprise submerged vegetation groups and water flow movement paths within the range of 0-10D behind the submerged vegetation groups;
(ii) and (3) qualitatively determining whether the Karman vortex street occurs at the tail end of the submerged vegetation group: comparing the two pictures obtained in the step (i), if the movement paths of water flow in the two pictures are basically overlapped, the tail end of the submerged vegetation group does not generate the karman vortex street, and if the vortexes in the two pictures are mirror symmetry, the tail end of the submerged vegetation group generates the karman vortex street;
(iii) and (3) flow rate data processing: and (3) flow rate data processing: measuring the instantaneous flow rate in the range of 2D-4D at the rear side of the wake zone of the vegetation group submerged in the river channel, converting the instantaneous flow rate changing along with time into a flow rate frequency spectrum changing along with frequency, and drawing the flow rate frequency spectrum as a flow rate frequency spectrum graph;
(iv) quantitatively judging whether the Karman vortex street occurs at the tail end of the submerged vegetation group: (iv) according to the flow rate spectrogram obtained in the step (iii), if a significant peak value exists in the spectrogram, it indicates that karman vortex street occurs at the tail end of the submerged vegetation group, and if no peak value exists in the spectrogram, it indicates that karman vortex street does not occur at the tail end of the submerged vegetation group;
(v) repeating the steps (i) to (iv) in the river channels of the submerged vegetation groups with different height-to-width ratios h/D, summarizing the karman vortex street occurrence of the tail ends of the submerged vegetation groups of the river channels with the known height-to-width ratios h/D along with the change of the height-to-width ratios h/D of the submerged vegetation groups, and obtaining a judgment index for judging whether the karman vortex street occurs at the tail ends of the submerged vegetation groups, namely when the height-to-width ratios h/D of the submerged vegetation groups are less than 0.7, the karman vortex street does not occur at the tail ends of the submerged vegetation; when the height-to-width ratio h/D of the submerged vegetation group is more than 0.9, the tail end of the submerged vegetation group stably and continuously generates the karman vortex street; when the height-to-width ratio h/D of the submerged vegetation group is more than or equal to 0.7 and less than or equal to 0.9, the karman vortex street can not stably and continuously occur at the tail end of the submerged vegetation group.
According to the method for predicting the karman vortex street at the tail end of the river submerged vegetation group, the width D of the submerged vegetation group is the diameter of the submerged vegetation group which is approximately circular. According to the prediction method for the karman vortex street at the tail end of the vegetation group submerged in the river channel, in order to observe the water flow movement path, the coloring agents can be added to the edges of the two sides of the vegetation group to form the water flow movement path containing the coloring agents, so that the movement track of the karman vortex street at the tail end of the vegetation group can be traced.
The method for predicting the occurrence of karman vortex street at the tail end of the river submerged vegetation group is T-intervalkvThe pictures of two moments of/2 are obtained by the following method: injecting coloring agents into water bodies on two sides of the vegetation group submerged in the river channel to trace the water flow motion track at the tail end of the vegetation group, and shooting a water flow motion video at a overlooking angle by utilizing a camera (such as a high-definition camera), wherein the video acquisition time at least comprises two karman vortex street motion periods; the pictures can also be obtained by directly taking the pictures by a high-definition camera.
In the method for predicting the occurrence of karman vortex street at the tail end of the river submerged vegetation group, the method for determining the wake zone of the submerged vegetation group in step (iii) may refer to conventional means already disclosed in the art, for example, the method for measuring the length of the wake zone of the river submerged vegetation group disclosed in application No. 201710005042.8.
In order to enable the summary result to be more accurate, the method for predicting the occurrence of the karman vortex street at the tail end of the river submerged vegetation group can perform qualitative analysis realized by the steps (i) and (ii) and quantitative analysis realized by the steps (iii) and (iv) on a plurality of groups of submerged vegetation group watercourses with different height-to-width ratios h/D; however, in order to shorten the test period, a qualitative judgment can be made through the steps (i) and (ii), and quantitative analysis can be made through the steps (iii) and (iv) for the river channel judged as "the karman vortex street does not occur at the tail end of the submerged vegetation group".
The traditional vegetation group tail karman vortex street distinguishing condition is only suitable for the riverway covered by the high-density non-submerged vegetation group, and the tail water flow of the high-density non-submerged vegetation group in the riverway is approximate to two-dimensional water flow (only the transverse change of the water flow is considered); the water flow at the tail end of the high-density submerged vegetation group in the river channel is a very typical three-dimensional water flow, the change of the water flow in the transverse direction and the vertical direction needs to be considered at the same time, and the judgment condition of the karman vortex street at the tail end of the traditional vegetation group cannot be applied. The invention provides a simple and effective discrimination method for predicting the occurrence of the karman vortex street at the tail end of the river channel submerged vegetation group on the basis of researching the occurrence of the karman vortex street of the river channel high-density submerged vegetation group.
Interpretation of terms:
karman vortex street movement period Tkv: the time that the transverse vortex at the rear side of the vegetation group moves from one side of the vegetation group to the other side of the vegetation group along with the water flow and then moves back to the side is referred to.
Submerging the vegetation group tail end: refers to the position of the most edge of the submerged vegetation group in the direction of water flow.
Compared with the prior art, the invention has the following beneficial effects:
1. the invention provides a simple and accurate prediction method, which can predict the occurrence of the karman vortex street at the tail end of the submerged vegetation group only according to the height-to-width ratio h/D of the high-density submerged vegetation group in the river;
2. the method for predicting the occurrence of the karman vortex street at the tail end of the river submerged vegetation group only needs to measure the width (D) and the height (h) of the submerged vegetation group, judges whether the karman vortex street at the tail end of the submerged vegetation group occurs or not through the height-to-width ratio h/D of the submerged vegetation group, does not need to carry out flow rate measurement work or dye test detection, and has wide universality in the field;
3. in the method for acquiring the judgment index provided by the invention, a coloring agent is used for matching with an image pick-up device (such as a high-definition camera) to shoot the overlooking angle with the time about two karman vortex street periods TkvThe Karman vortex street motion video can obtain the phase difference TkvThe two pictures of the/2 are compared to judge whether the karman vortex street at the tail end of the submerged vegetation group occurs or not, and the shooting time is short, so that the test efficiency can be improved;
4. in the judgment index obtaining method provided by the invention, the judgment accuracy is reduced when the height-width ratio of the submerged vegetation group is close to the critical height-width ratio of the karman vortex street, and qualitative analysis is carried out by adopting picture comparison; if the situation can not be judged, the instantaneous flow rate in the range of 2D-4D at the rear side of the wake area of the submerged vegetation group is measured and converted into a flow rate spectrogram, and the karman vortex street at the tail end of the submerged vegetation group is further judged by adopting a quantitative means, so that the judgment accuracy is improved.
Drawings
FIG. 1 is a schematic view of the motion of a coloring agent at the tail end of a submerged vegetation group in a river; wherein, (a) the submerged vegetation group has a karman vortex street at the tail end, (b) the submerged vegetation group has no karman vortex street at the tail end, and (c) the submerged vegetation group is in a side view.
FIG. 2 is a diagram showing how different height-to-width ratio (h/D) submerged vegetation groups are indicated by coloring agent according to an embodiment of the present invention, and the top 5 sub-diagrams show the initial time t0The movement locus of the staining agent at the tail end of each submerged vegetation group is shown by the following 5 sub-images after half karman vortex street movement period (t)0+TkvAnd/2) the movement track of the coloring agent at the tail end of each submerged vegetation group.
FIG. 3 is a graph of the instantaneous flow rate spectrum of the rear side of the wake of the submerged vegetation group with different aspect ratios in accordance with an embodiment of the present invention; the spectral graph of the rear side of the submerged vegetation group is (a) h/D is 0.3, (b) h/D is 0.5, (c) h/D is 0.7, (D) h/D is 0.9, and (e) h/D is 1.1.
FIG. 4 is a summary diagram of the occurrence or non-occurrence of Karman vortex streets at the tail ends of submerged vegetation groups in 25 test conditions in the example of the present invention.
Detailed Description
The technical solutions of the embodiments of the present invention will be described clearly and completely with reference to the accompanying drawings, and it is to be understood that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Example 1
In this embodiment, the specific description is made by determining the judgment index of the karman vortex street at the tail end of the river submerged vegetation group through a water tank simulation experiment.
① purpose of the test
The movement process of the vortex at the tail end of the submerged vegetation group is recorded through a water tank test, whether the karman vortex street occurs at the tail end of the submerged vegetation group is judged, and whether the karman vortex street occurs in each set of working conditions is further detected through flow velocity spectrum analysis. Summarizing the obtained test results, and providing a judgment index of the occurrence of the karman vortex street at the tail end of the submerged high-density vegetation group.
② test equipment
The main equipment is shown in table 1 below.
Table 1 instrument and equipment for stain test and flow rate measurement test
③ test conditions
The 25 groups of submerged vegetation group model operating parameters are shown in table 2.
TABLE 2 stain tracing and ADV flow Rate measurement Conditiona
Wherein,aU0the average upstream flow velocity after the model is stabilized is shown, D is the diameter of the vegetation group, and H and H are the height and the depth of the vegetation group respectively;bwhether the karman vortex street at the tail end of the submerged vegetation group occurs or not is a test observation result.
④ test method
The experimental acquisition and data processing procedures are further illustrated using conditions 19-23.
The process for acquiring the discrimination index provided by the embodiment includes the following steps:
(i) image acquisition:
step 1: arranging the submerged vegetation group on a PVC plate, slowly adding water to reach the expected water depth H according to the parameter setting of different working conditions, starting a pump to enable the water flow to flow, and stabilizing the upstream average flow speed U0As shown in table 2;
step 2: injecting coloring agents at the edges of two sides of the submerged vegetation group to trace the movement track of the karman vortex street at the tail end of the vegetation group, and arranging a high-definition camera at a position 80cm downstream of the submerged vegetation group so as to record the movement process video of the karman vortex street in a overlooking angle; according to the movement period T of the karman vortex streetkv(about 10s), the video acquisition time is not less than 20s, so that at least two karman vortex street movement periods are completely recorded, and the result analysis is accurate.
And step 3: selecting any moment in the video collected in the step 2 as an initial moment t0Intercepting the video picture at the moment, and marking the vortex formed by the Karman vortex street by arrow lines, such as the upper 5 subgraphs of FIG. 2; through half karman vortex street movement cycle (T)kv2 ≈ 5 seconds), intercept time t0+Tkv[ 2 ] ofVideo pictures, in which the vortexes formed by the karman vortex street are marked by arrow lines, such as the lower 5 subgraphs of fig. 2; the picture comprises submerged vegetation groups and water flow movement paths within a length range of 0-10D from the rear sides of the submerged vegetation groups.
(ii) And (3) qualitatively determining whether the Karman vortex street occurs at the tail end of the submerged vegetation group: two moments in time (t)0And t0+TkvThe same submerged vegetation group tail end is correspondingly placed in the video screenshot of the/2), compared with the movement characteristics of the vortex, the vortex structure which presents mirror symmetry is the karman vortex street, namely the karman vortex street occurs at the submerged vegetation group tail end; if the movement paths of the coloring agents are basically overlapped, the karman vortex street does not occur, namely, the karman vortex street does not occur at the tail end of the submerged vegetation group.
It should be noted that, when the height-depth ratio (h/D) of the submerged vegetation group is very close to the critical height-depth ratio h/D of the karman vortex street, the karman vortex street is in an unstable state which does not occur when the karman vortex street occurs, so the accuracy of the judgment of the video image analysis means is reduced, and a more quantitative method needs to be considered to judge whether the karman vortex street occurs.
(iii) And (3) flow rate data processing:
step 1: flow rate collection
In order to ensure that the test data in each group of working conditions are collected at the same vertical position z is 1cm, the vegetation height direction is taken as the z-axis direction, and z is 0cm and is taken as the bottom of the vegetation group. Adopting an ADV (advanced digital video) flow meter to carry out flow velocity measurement on 2D-4D at the rear side of the vegetation group wake flow area, wherein the sampling frequency and time of each point are respectively 25Hz and 4min, so that at least 6000 sample points can be collected at each point, and the measurement precision is ensured; flooding vegetation group wake zone (L)w) Reference is made to the method for measuring the length of the wake area of the vegetation group in the river channel disclosed in application No. 201710005042.8.
Step 2: raw data processing
The acquired raw data of the flow velocity is converted into a flow velocity spectrum changing with frequency according to the instantaneous flow velocity changing with time, and the flow velocity spectrum is drawn as a flow velocity spectrogram (shown in figure 3). Wherein the abscissa is frequency and the ordinate is flow rate spectrum Svv。
(iv) Quantitatively judging whether the Karman vortex street occurs at the tail end of the submerged vegetation group:
according to the flow velocity spectrogram obtained in the step (iii), the spectrum peaks are marked by arrow lines in fig. 3, and obvious peaks appear in subgraphs c, d and e in fig. 3, which proves that karman vortex street appears at the rear side of the submerged vegetation group (wherein, the position where the spectrum peak appears is about 0.1Hz, which is consistent with the movement period about 10 seconds of the karman vortex street), and conversely, the subgraphs a and b in fig. 3 do not have the spectrum peaks, which proves that the karman vortex street does not occur. The sub-graphs a, b, c, d and e are ordered from left to right and from top to bottom.
(v) Summarizing test results, and giving out a discrimination index:
the results of the 25 sets of tests in table 2 are summarized in fig. 4, where the solid origin is the non-occurrence karman vortex street at the tail end of the submerged vegetation group, and the hollow point is the occurrence of karman vortex street at the tail end of the submerged vegetation group. As can be seen from fig. 4, when the aspect ratio h/D of the vegetation group is less than 0.7, karman vortex street does not occur at the tail end of the submerged vegetation group (solid point); when the height-to-width ratio h/D of the vegetation group is greater than 0.9, the karman vortex street stably and continuously occurs at the tail end of the submerged vegetation group (hollow point); when the height-to-width ratio h/D of the submerged vegetation group is more than or equal to 0.7 and less than or equal to 0.9, the karman vortex street can not stably and continuously occur at the tail end of the submerged vegetation group, and the karman vortex street is not in an unstable state when occurring.
⑥ method test
In order to test the practicability of the proposed method for judging the occurrence of the karman vortex street at the tail end of the high-density submerged vegetation group, 5 additional groups of tests are further designed for verification.
TABLE 3 comparison of the discrimination and observation results of Karman vortex street at the tail end of submerged vegetation group
aJudging results of the discrimination method of the karman vortex street at the tail end of the submerged vegetation group;
bflooding byAnd (5) observing results of a stain tracing test of the karman vortex street at the tail end of the vegetation group.
For five groups of working conditions, firstly, calculating the ratio h/D of the height to the diameter of the submerged vegetation group, predicting according to the judgment indexes given in the front, and giving a judgment result (shown in a table 3) whether the karman vortex street at the tail end of the submerged vegetation group occurs or not; and (3) obtaining a test result of whether the karman vortex street at the tail end of the submerged vegetation group occurs under the five working conditions through a dye tracer test (referring to the steps (i) to (ii)) given in the test method (as shown in table 3). As can be seen from Table 3, the result of determining whether the karman vortex street occurs at the tail end of the submerged vegetation group is consistent with the test result, which shows that the prediction effect of the determination index provided by the embodiment is very good.
Example 2
In order to further test the practicability of the discrimination and prediction method for the occurrence of the Karman vortex street at the tail end of the high-density submerged vegetation group, 2 groups of tests of the wild natural river channels are developed.
The test is carried out in a changing water return area at the upper stream of a purple plateau water laying hydropower station of a weir city of the city of rivers in Sichuan province, the water storage area is submerged, most parts are exposed when water is drained, sufficient sunlight and water sources are provided, and the vegetation grows rapidly. The test is carried out when a reservoir drains water (the flow velocity of a water body is 14-17 cm/s), only one main river channel with the width of about 2m is arranged at the time, the water depth is about 30cm, and some vegetation groups grow in the main river channel. Selecting 2 vegetation groups for carrying out the test, firstly measuring the diameter (d) of the vegetation group single plant vegetation to be 3-6mm, and the density (n) to be about 2 roots/cm2Accordingly, the unit water blocking areas (a ═ nd) of the vegetation groups No. 1 and No. 2 were 0.6cm each-1And 1.2cm-1. Then, measuring the height (h) and the width (D) of 2 vegetation groups, wherein the height and the width of the No. 1 vegetation group are h-9 cm and D-8 cm respectively; the height and width of the No. 2 vegetation group are h-13 cm and D-11 cm respectively. Thirdly, calculating the vegetation density of the ah, No. 1 and No. 2 vegetation groups to be 5.4 and 15.6 respectively (>0.1), both vegetation groups are high density vegetation groups. Finally, the aspect ratios (h/D) of the vegetation groups No. 1 and No. 2 are 1.1 and 1.2 respectively, according to the judgment indexes given in the embodiment 1, the aspect ratios of the vegetation groups No. 1 and No. 2 both meet the condition that h/D is more than 0.9, and the karman vortex street is predicted to occur at the tail ends of the vegetation groups 2. Finally, inAnd (3) carrying out a stain tracing test on the test site (referring to the steps (i) to (ii) given in the test method) to confirm that the karman vortex street occurs at the tail ends of the 2 vegetation groups, and the result is consistent with the result given by the prediction method.
Claims (6)
1. A prediction method for the occurrence of Karman vortex street at the tail end of a river submerged vegetation group is characterized by comprising the following steps:
(1) measuring river water flow rate U0If U is present0If U is more than 3.1cm/s, entering the step (2)0Ending the program when the concentration is less than or equal to 3.1 cm/s;
(2) measuring the transverse width D, the height h, the number n of vegetation in unit area and the diameter D of the individual vegetation of the submerged vegetation group in the direction perpendicular to the river, and calculating the unit water blocking area a of the submerged vegetation group as n multiplied by D and the height-to-width ratio h/D of the submerged vegetation group;
(3) judging whether the vegetation density ah is greater than 0.1, if ah is greater than 0.1, indicating that the submerged vegetation group is a high-density vegetation group, entering the step (4), if ah is less than or equal to 0.1, indicating that the submerged vegetation group is a low-density vegetation group, and ending the program;
(4) judging whether the karman vortex street occurs at the tail end of the submerged vegetation group according to the following judgment indexes: when the height-to-width ratio h/D of the submerged vegetation group is less than 0.7, the tail end of the submerged vegetation group does not generate a karman vortex street; when the height-to-width ratio h/D of the submerged vegetation group is more than 0.9, the tail end of the submerged vegetation group stably and continuously generates the karman vortex street; when the height-to-width ratio h/D of the submerged vegetation group is more than or equal to 0.7 and less than or equal to 0.9, the tail end of the submerged vegetation group can not stably and continuously generate the karman vortex street; the determination process is completed and the routine ends.
2. The method for predicting the occurrence of the karman vortex street at the tail end of the river submerged vegetation group according to claim 1, wherein the method for acquiring the judgment index in the step (4) comprises the following steps:
(i) image acquisition: in the river channel with the height-width ratio h/D of the submerged vegetation group known, the movement period T according to the karman vortex streetkvSeparately collect intervals Tkv2, pictures at two moments comprise submerged vegetation groups and water flow movement paths within the range of 0-10D behind the submerged vegetation groups;
(ii) and (3) qualitatively determining whether the Karman vortex street occurs at the tail end of the submerged vegetation group: comparing the two pictures obtained in the step (i), if the movement paths of water flow in the two pictures are basically overlapped, the tail end of the submerged vegetation group does not generate the karman vortex street, and if the vortexes in the two pictures are mirror symmetry, the tail end of the submerged vegetation group generates the karman vortex street;
(iii) and (3) flow rate data processing: measuring the instantaneous flow rate in the range of 2D-4D at the rear side of the wake zone of the vegetation group submerged in the river channel, converting the instantaneous flow rate changing along with time into a flow rate frequency spectrum changing along with frequency, and drawing the flow rate frequency spectrum as a flow rate frequency spectrum graph;
(iv) quantitatively judging whether the Karman vortex street occurs at the tail end of the submerged vegetation group: (iv) according to the flow rate spectrogram obtained in the step (iii), if a significant peak value exists in the spectrogram, it indicates that karman vortex street occurs at the tail end of the submerged vegetation group, and if no peak value exists in the spectrogram, it indicates that karman vortex street does not occur at the tail end of the submerged vegetation group;
(v) repeating the steps (i) to (iv) in the river channels of the submerged vegetation groups with different height-to-width ratios h/D, summarizing the karman vortex street occurrence of the tail ends of the submerged vegetation groups of the river channels with the known height-to-width ratios h/D along with the change of the height-to-width ratios h/D of the submerged vegetation groups, and obtaining a judgment index for judging whether the karman vortex street occurs at the tail ends of the submerged vegetation groups, namely when the height-to-width ratios h/D of the submerged vegetation groups are less than 0.7, the karman vortex street does not occur at the tail ends of the submerged vegetation; when the height-to-width ratio h/D of the submerged vegetation group is more than 0.9, the tail end of the submerged vegetation group stably and continuously generates the karman vortex street; when the height-to-width ratio h/D of the submerged vegetation group is more than or equal to 0.7 and less than or equal to 0.9, the karman vortex street can not stably and continuously occur at the tail end of the submerged vegetation group.
3. The method of claim 2, wherein the coloring agent is added to the two side edges of the submerged vegetation population to form a water flow path containing the coloring agent.
4. The method for predicting the occurrence of Karman vortex street at the tail end of river submerged vegetation group according to claim 2 or 3, wherein T intervals are providedkvThe pictures of two moments of/2 are obtained by the following method: submerged planting in riverWater bodies on two sides of the vegetation group are injected with coloring agents to trace the movement track of the water flow at the tail end of the vegetation group, a water flow movement video is shot at a overlooking angle by utilizing a camera device, and the video acquisition time at least comprises two karman vortex street movement periods.
5. The method for predicting the occurrence of the karman vortex street at the tail end of the river channel submerged vegetation group according to claim 2 or 3, wherein the flow data processed in step (iii) is the river channel judged as "the karman vortex street does not occur at the tail end of the submerged vegetation group" in step (ii).
6. The method for predicting the occurrence of the karman vortex street at the tail end of the river channel submerged vegetation group according to claim 4, wherein the flow data processed in step (iii) is the river channel judged as "the karman vortex street does not occur at the tail end of the submerged vegetation group" in step (ii).
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