CN116432551B - River junction water quality distribution simulation method based on satellite image and CFD - Google Patents
River junction water quality distribution simulation method based on satellite image and CFD Download PDFInfo
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Abstract
The invention discloses a river junction water quality distribution simulation method based on satellite images and CFD. Compared with the existing technology for measuring the water quality distribution condition of the river junction area by driving the ship in the field, the method can effectively save time and economic cost, and improve the efficiency of water quality related research of the junction area and water environment prediction and treatment of related departments. Compared with the data obtained by the technology of measuring the water quality distribution condition of the river junction area by using the field ship, the method provided by the invention has the advantages that the dissolved oxygen distribution data at any position in the fluid in the river junction area can be extracted more accurately, and the operability of the follow-up scientific research analysis and water quality prediction management work is effectively improved.
Description
Technical Field
The invention belongs to the technical field of river water environment management and restoration, and particularly relates to a river intersection water quality distribution simulation method based on satellite images and CFD.
Background
The river junction area is used as a key node for communicating river networks of the river basin, and the inflow and outflow quantity of at least two rivers are collected, so that the river junction area is a water hazard frequent area, and is also a high-concentration pollution collection area and a core control area for water environment protection. If the water quality distribution condition of the intersection area can be obtained, the influence degree of the incoming flow pollution on the intersection area, the downstream and even the whole river basin can be judged, the method has direct guiding significance on the implementation of the water quality improvement and the remediation measures of the intersection area, and has important reference significance on the treatment and decision of a river basin pollution system. However, because two rivers are mutually jacked to form a complex flow velocity field in the intersection region, the hydrodynamic transport mixing process of the water quality factors in the intersection region is influenced to show great instability, and because the natural river channel has complex outline, changeable hydrologic conditions and irregular intersection forms, the water quality distribution rule of the intersection region is further difficult to be clear.
Dissolved oxygen is an essential living condition for water organisms, and microorganisms degrade organic pollutants by consuming the dissolved oxygen, so the dissolved oxygen is also an important index for reacting the water quality of rivers. The typical water flow structure in the junction area can influence the mixed transportation process of the dissolved oxygen of the main and branch flows on one hand, and the reoxygenation-oxygen consumption dynamic process of the water body can be influenced on the other hand. Therefore, the dissolved oxygen concentration distribution can be used for effectively indicating the response condition of the water quality change of the river junction to the hydrodynamic force of the junction. At present, the number of the sections of the main river water quality monitoring in China is gradually increased, and the coverage is relatively comprehensive. The water quality overall condition and specific index (dissolved oxygen concentration) data of a certain river can be obtained in real time. The data is the average level of the river cross section, has certain reference significance for judging the overall water quality condition of the river and the river basin, but the distribution condition of the water quality of the river cross section, especially the non-uniform distribution characteristic of the mixing of the water quality of the main and the branch of the river junction area, cannot be known through the data. At present, the most direct and effective method for acquiring the solubility distribution characteristics of the dissolved oxygen in the intersection area is to arrange monitoring sections and test points in the intersection area, obtain the measured concentration data of the dissolved oxygen at each test point in a driving ship mode, and then fit and acquire the water quality distribution in the intersection area. The data obtained by the technical method is accurate, but certain defects are also exposed, and the method is mainly characterized by large consumption of manpower and material resources, limited measured data points and the like.
Disclosure of Invention
The invention aims to provide a satellite image and CFD-based river junction water quality distribution simulation method, which can accurately extract dissolved oxygen distribution data at any position in a river junction fluid so as to conveniently and efficiently acquire the water quality distribution condition and the transportation mixing rule of the river junction, and reduce the water environment research and management cost of the river junction and river network of a river basin.
The technical scheme adopted by the invention is as follows: the river junction water quality distribution simulation method based on satellite images and CFD is implemented according to the following steps:
step 1, acquiring satellite images of a river junction region at target time according to the geographic position of the river junction region, and extracting and deriving the closed contour of the river junction region in the satellite images;
step 2, projecting the closed contour of the intersected river to a plane and generating a surface area, taking the surface area as a two-dimensional fluid area of the river intersection area, and discretizing the fluid area into a plurality of non-overlapping quadrilateral grid units;
step 3, acquiring water flow and water quality data of the upstream inlet section of the main flow and the upstream inlet section of the tributary of the fluid domain, and calculating boundary conditions of the fluid domain;
step 4, listing a control equation set at the central node of each quadrilateral grid of the fluid domain based on a two-dimensional turbulent fluid mathematical model and a dissolved oxygen transport mathematical model of the intersection region, combining the control equation sets of all quadrilateral grid units of the fluid domain, establishing a discrete algebraic equation set of the control equation set by adopting a finite volume method, substituting boundary conditions of the fluid domain, initializing data of the fluid domain, carrying out numerical calculation solving by a computer, and storing iterative calculation data at regular fluid time intervals;
and 5, inputting the stored calculation result into Tecplot software for post-processing, and obtaining hydrodynamic characteristics and dissolved oxygen distribution of the river junction region in a target period, namely, the water distribution in the junction region in the target period.
The invention is also characterized in that:
the closed contour of the intersected river comprises an intersection area, a downstream water bank boundary line, a main stream inlet section line, a tributary inlet section line and a downstream outlet section line.
The flow and water quality data of the inlet section water flow and the upstream of the branch flow in the step 3 comprise flow, water depth, dissolved oxygen concentration and BOD concentration.
The boundary conditions of the fluid domain comprise a main stream speed inlet boundary condition, a tributary speed inlet boundary condition, an outlet boundary condition and a river bank boundary condition, wherein the river bank boundary condition is defined as a slip-free boundary considering roughness, the roughness is determined according to the actual rough condition of a river channel, and a standard wall function method is adopted for calculation; the outlet boundary condition is free outflow; the main flow speed inlet boundary condition and the tributary speed inlet boundary condition comprise inlet flow speed, inlet turbulence energy dissipation rate, dissolved oxygen concentration and BOD concentration, wherein:
the inlet flow rate calculation formula is:
the inlet turbulence energy calculation formula is:
the calculation formula of the inlet turbulent energy dissipation ratio is as follows:
in the above, U inlet Inlet flow rate, m/s; q (Q) inlet For inlet flow, m 3 S; b is the river width of the inlet section, m; h is a inlet The average water depth of the inlet section is m; k (k) inlet For turbulent energy of inlet section, m 2 /s 2 ，ε inlet Is the dissipation rate of the inlet turbulent energy, and has no dimension.
In the step 4, the two-dimensional turbulent fluid mathematical model and the dissolved oxygen transport mathematical model of the intersection zone list a control equation set at the central node of each quadrilateral grid of the fluid domain, and the specific process is as follows:
according to the hydrodynamic conservation equation, there is the expression:
the RNG k-epsilon model is adopted to capture the turbulent motion characteristics of the water flow in the junction area, and the control equation is as follows:
and (c) equation:
epsilon equation:
the dissolved oxygen transport model is:
the dissolved oxygen mass transfer model is:
S DO ＝K r (C s -C DO )+S BOD (11)
S BOD ＝-K b C BOD (12)
K r ＝K r20 ×1.024 (T-20) (13)
C s ＝0.0035T 2 -0.33369T+14.407 (14)
in the above formula: ρ is density, kg/m 3 The method comprises the steps of carrying out a first treatment on the surface of the p is pressure, pa; t is time, s; u (u) i 、u j The flow velocity component of U in each direction is m/s; x is x i 、x j Is a coordinate component, m; epsilon is the turbulent energy dissipation ratio; mu is the molecular coefficient of viscosity; mu (mu) t Is the turbulent viscosity coefficient; g k For average flow velocity ladderGenerating term of turbulence k caused by degree, and taking C from empirical constant 1ε ＝1.42、C 2ε ＝1.42、α k ＝α ε ＝1.39、C μ ＝0.0845、η 0 ＝4.377、β＝0.012，K r Is the reoxygenation coefficient s -1 ；K b D is the degradation coefficient -1 ；K r20 Is the atmospheric reoxygenation coefficient at 20 ℃, s -1 ；K b20 Degradation coefficient s at 20 DEG C -1 ；C s Is saturated dissolved oxygen concentration, mg/L; c (C) DO DO concentration, mg/L; c (C) BOD BOD concentration, mg/L; t is water temperature, DEG C; θ is the temperature coefficient, θ=1.047.
The atmospheric reoxygenation coefficient is determined according to the current conditions of the actual river junction area, and is specifically one of the following conditions:
1) River with strong turbulence:
2) Deepwater rivers with slow flow rates:
3) High speed stream:
4) Shallow stream:
wherein U is a speed vector, m/s; k is the turbulence energy of the water body, m 2 /s 2 The method comprises the steps of carrying out a first treatment on the surface of the h is the water depth, m.
And 4, establishing a discrete algebraic equation set of a control equation set by adopting a finite volume method, wherein the specific process is as follows: integrating the control equation of the central node of each quadrilateral mesh on the quadrilateral mesh to obtain an algebraic equation set of the quadrilateral mesh, wherein the parameter value on the boundary of each quadrilateral mesh in the equation set is obtained by a second-order windward interpolation method.
Step 4, an algorithm used in numerical calculation solving by means of a computer is a SIMPLE iterative algorithm, and the iteration convergence condition is that the residual error is smaller than 10 -5 。
The data for initializing the fluid domain data includes: full area pressure value, flow velocity value, turbulence energy dissipation rate, dissolved oxygen concentration and BOD concentration.
The beneficial effects of the invention are as follows:
the river junction water quality distribution simulation method based on satellite images and CFD utilizes a small amount of data which is easy to acquire by means of satellite images and computer modeling technology, efficiently and conveniently simulates the coupling distribution of dissolved oxygen water quality indexes of any river junction so as to judge the water quality conditions of the river junction and downstream river channels. Compared with the existing technology for measuring the water quality distribution condition of the river junction area by driving the ship in the field, the method can effectively save time and economic cost, and improve the efficiency of water quality related research of the junction area and water environment prediction and treatment of related departments.
Because the method of the invention discretizes the whole area of the river junction so as to carry out numerical simulation calculation, the obtained calculation result covers the whole fluid area of the river junction. Compared with the data obtained by the technology of measuring the water quality distribution condition of the river junction area by using the field ship, the method provided by the invention has the advantages that the dissolved oxygen distribution data at any position in the fluid in the river junction area can be extracted more accurately, and the operability of the follow-up scientific research analysis and water quality prediction management work is effectively improved.
Drawings
FIG. 1 is a flow chart of a river junction water quality distribution simulation method based on satellite images and CFD;
FIG. 2 is a schematic diagram of an embodiment of satellite images and extracted closed contours of an intersecting channel;
FIG. 3 is a schematic diagram of a calculated fluid domain generated by projection of a closed contour of a river channel in an intersection region in an embodiment;
FIG. 4 is a schematic diagram of an embodiment computing a fluid domain discrete grid;
FIG. 5 is a flow velocity field distribution diagram of the river junction region calculated in the example;
FIG. 6 is a graph showing the turbulence energy distribution of the river junction area calculated by the embodiment;
FIG. 7 is a graph showing the concentration profile of dissolved oxygen in the river junction region as a result of calculation in the example;
FIG. 8 is a schematic diagram of the process of mixed transport of dissolved oxygen in the river junction area as a result of calculation in the example.
Detailed Description
The invention will be described in detail below with reference to the drawings and the detailed description.
The invention discloses a river junction water quality distribution simulation method based on satellite images and CFD, which is implemented according to the following steps as shown in figure 1:
step 1, acquiring satellite images of a river junction region at target time according to the geographic position of the river junction region, and extracting and deriving the closed contour of the river junction region in the satellite images; the closed contour of the intersected river comprises an intersection area, a downstream water bank boundary line, a main stream inlet section line, a tributary inlet section line and a downstream outlet section line.
Step 2, projecting the closed contour of the intersected river to a plane and generating a surface area, taking the surface area as a two-dimensional fluid area of the river intersection area, and discretizing the fluid area into a plurality of non-overlapping quadrilateral grid units; the larger the number of grids is, the higher the calculation accuracy is, the slower the calculation speed is, and the optimal grids meeting the calculation accuracy and speed can be determined by trial calculation through different grid numbers.
Step 3, acquiring data of water flow and water quality of an upstream inlet section of a main flow and a tributary of a fluid domain, wherein the data comprise flow, water depth, dissolved oxygen concentration and BOD concentration, and the data are obtained in three ways and are arranged according to a convenient sequence: the first is to obtain national examination section data through national hydrology or environmental protection department official websites; the second is to obtain related data for the existing related research literature data; the third is obtained for in-situ measurements.
Calculating boundary conditions of a fluid domain, wherein the component comprises a main stream speed inlet boundary condition, a tributary speed inlet boundary condition, an outlet boundary condition and a river bank boundary condition, the river bank boundary condition is defined as a slip-free boundary considering roughness, the roughness is determined according to the actual river course roughness, and the standard wall surface function method is adopted for calculation; the outlet boundary condition is free outflow; the main flow speed inlet boundary condition and the tributary speed inlet boundary condition comprise inlet flow speed, inlet turbulence energy dissipation rate, dissolved oxygen concentration and BOD concentration, wherein:
the inlet flow rate calculation formula is:
the inlet turbulence energy calculation formula is:
the calculation formula of the inlet turbulent energy dissipation ratio is as follows:
in the above, U inlet Inlet flow rate, m/s; q (Q) inlet For inlet flow, m 3 S; b is the river width of the inlet section, m; h is a inlet The average water depth of the inlet section is m; k (k) inlet For turbulent energy of inlet section, m 2 /s 2 ，ε inlet Is the dissipation rate of the inlet turbulent energy, and has no dimension.
Step 4, listing a control equation set at the central node of each quadrilateral grid of the fluid domain based on the intersection area two-dimensional turbulent fluid mathematical model and the dissolved oxygen transport mathematical model, wherein the specific process of the control equation set is as follows:
according to the hydrodynamic conservation equation, there is the expression:
the RNG k-epsilon model is adopted to capture the turbulent motion characteristics of the water flow in the junction area, and the control equation is as follows:
and (c) equation:
epsilon equation:
the dissolved oxygen transport model is:
the dissolved oxygen mass transfer model is:
S DO ＝K r (C s -C DO )+S BOD (11)
S BOD ＝-K b C BOD (12)
K r ＝K r20 ×1.024 (T-20) (13)
C s ＝0.0035T 2 -0.33369T+14.407 (14)
in the above formula: ρ is density, kg/m 3 The method comprises the steps of carrying out a first treatment on the surface of the p is pressure, pa; t is time, s; u (u) i 、u j The flow velocity component of U in each direction is m/s; x is x i 、x j Is a coordinate component, m; epsilon is the turbulent energy dissipation ratio; mu is the molecular coefficient of viscosity; mu (mu) t Is the turbulent viscosity coefficient; g k For the generation term of turbulence k caused by average flow velocity gradient, the empirical constant is taken to be C 1ε ＝1.42、C 2ε ＝1.42、α k ＝α ε ＝1.39、C μ ＝0.0845、η 0 ＝4.377、β＝0.012，K r Is the reoxygenation coefficient s -1 ；K b D is the degradation coefficient -1 ；K r20 Is the atmospheric reoxygenation coefficient at 20 ℃, s -1 ；K b20 Degradation coefficient s at 20 DEG C -1 ；C s Is saturated dissolved oxygen concentration, mg/L; c (C) DO DO concentration, mg/L; c (C) BOD BOD concentration, mg/L; t is water temperature, DEG C; θ is the temperature coefficient, θ=1.047.
K in the above formula (15) b20 The river number is different in different environments depending on the water quality and microorganism condition. The value of the target river junction can be determined by in-situ sampling laboratory measurement or reference to the data and literature.
The atmospheric reoxygenation coefficient is determined according to the current conditions of the actual river junction area, and is specifically one of the following conditions:
1) River with strong turbulence:
2) Deepwater rivers with slow flow rates:
3) High speed stream:
4) Shallow stream:
wherein U is a speed vector, m/s; k is the turbulence energy of the water body, m 2 /s 2 The method comprises the steps of carrying out a first treatment on the surface of the h is the water depth, m.
The control equation sets of all quadrilateral grid units of the fluid domain are combined, and a finite volume method is adopted to establish a discrete algebraic equation set of the control equation sets: integrating the control equation of the central node of each quadrilateral mesh on the quadrilateral mesh to obtain an algebraic equation set of the quadrilateral mesh, wherein the parameter value on the boundary of each quadrilateral mesh in the equation set is obtained by a second-order windward interpolation method.
Substituting the boundary condition of the fluid domain, and initializing the data of the fluid domain, wherein the initialized data comprises: the method comprises the steps of carrying out numerical calculation and solving by means of a computer on a full-area pressure value, a flow velocity value, a turbulence dissipation rate, a dissolved oxygen concentration and a BOD concentration, wherein an algorithm is a SIMPLE iterative algorithm, and an iteration convergence condition is that a residual error is smaller than 10 -5 Storing iterative calculation data at regular fluid time intervals;
and 5, inputting the stored calculation result into Tecplot software for post-processing, and obtaining hydrodynamic characteristics and dissolved oxygen distribution of the river junction region in a target period, namely, the water distribution in the junction region in the target period.
Examples
Taking the river junction as an example, the invention introduces a river junction water quality distribution simulation method based on satellite images and CFD and the effect thereof, and the method comprises the following steps:
step 1, acquiring satellite images of the river junction region on the 4 th month and the 25 th day of 2020 according to the geographic position of the target river junction region, and extracting and deriving the closed contour of the river junction in the satellite images. As shown in fig. 2, the closed contour includes an intersection region and downstream land boundary lines and main stream inlet section lines, tributary inlet section lines, downstream outlet section lines.
And 2, projecting the river channel contour in the step 1 to a plane and generating a surface area to form a two-dimensional calculation fluid area of the river junction area, as shown in fig. 3. Discretizing the computational fluid domain into a quadrilateral mesh assembly is shown in fig. 4. The quadrilateral grid assembly is composed of 669710 quadrilateral units, the quadrilateral grid assembly completely covers and does not exceed the computational fluid domain, and the quadrilateral grid units are not overlapped.
Step 3, calculating the water flow and water quality data of the inlet section of the upstream of the main flow and the tributary of the fluid domain by referring to national examination section data of national hydrology or official websites of environmental protection departments and reading related research literature data in the step 2, wherein the method specifically comprises the following steps: the cross section of the dry flow inlet is 276m, the average water depth is 2.5m, and the flow is 345.28m 3 And/s, the concentration of dissolved oxygen is 7.01mg/L, and the concentration of BOD is 4mg/L; the cross section of the tributary inlet is 86.17m, the average water depth is 1.5m, and the flow is 20.85m 3 And/s, the concentration of dissolved oxygen is 4.2mg/L, and the concentration of BOD is 6mg/L. The average water depth of the intersection area is 2m and the water temperature is 9 ℃. Calculating the boundary conditions of the fluid domain in the step 2 through the data comprises the following steps: the velocity of the dry flow inlet is 0.5m/s, and the turbulence is 2.04 multiplied by 10 -4 m 2 /s 2 Turbulent energy dissipation ratio is 6.96×10 -7 The concentration of the imported dissolved oxygen is 7.01mg/L, and the concentration of BOD is 4mg/L; the flow velocity of the inlet of the tributary is 0.2m/s, and the turbulence energy is 4.68X10-5 m 2 /s 2 Turbulent energy dissipation ratio 1.3X10 -7 . The dissolved oxygen concentration was 4.2mg/L and the BOD concentration was 6mg/L. The boundary roughness of the river bank is 0.02m.
And 4, constructing a two-dimensional turbulent fluid mathematical model and a dissolved oxygen transport mathematical model of the intersection region, and coupling the two to form a control equation. The control equation is specifically as follows:
mass conservation equation:
the conservation of momentum equation is:
the invention adopts an RNG k-epsilon model to capture the turbulent fluctuation characteristics of the water flow in the junction area, and the control equation is as follows:
and (c) equation:
epsilon equation:
the dissolved oxygen transport model is:
the dissolved oxygen mass transfer model is:
S DO ＝K r (C s -C DO )+S BOD
S BOD ＝-K b C BOD
K r ＝K r20 ×1.024 (T-20)
C s ＝0.0035T 2 -0.33369T+14.407
in the above formula: ρ is density, kg/m 3 The method comprises the steps of carrying out a first treatment on the surface of the p is pressure, pa; t is time, s; u (u) i 、u j The flow velocity component of U in each direction is m/s; x is x i 、x j Is a coordinate component, m; epsilon is the turbulent energy dissipation ratio; mu is the molecular coefficient of viscosity; mu (mu) t Is the turbulent viscosity coefficient; g k For the generation term of turbulence k caused by average flow velocity gradient, the empirical constant is taken to be C 1ε ＝1.42、C 2ε ＝1.42、α k ＝α ε ＝1.39、C μ ＝0.0845、η 0 ＝4.377、β＝0.012，K r Is the reoxygenation coefficient s -1 ；K b D is the degradation coefficient -1 ；K r20 Is the atmospheric reoxygenation coefficient at 20 ℃, s -1 ；K b20 Degradation coefficient s at 20 DEG C -1 The value is related to the microbial status of the water body and is determined to be 0.1d by referring to the data and the literature -1 。C s Is saturated dissolved oxygen concentration, mg/L; c (C) DO DO concentration, mg/L; c (C) BOD BOD concentration, mg/L; t is water temperature, DEG C; θ is the temperature coefficient, θ=1.047.
Listing a control equation set by taking each quadrilateral grid of the fluid domain calculated in the step 2 as a unit, and calculating the square of all quadrilateral grid units of the fluid domainCheng Zulian, a discrete algebraic equation set of the control equation set is established by adopting a finite volume method, and the discrete format is a second-order windward format. Substituting the boundary conditions of the step 3, initializing the data of the fluid domain, setting the global initial pressure, the flow speed, the turbulence energy and the turbulence energy dissipation rate of the fluid domain to be 0, and setting the initial values of the inlets of the main flow and the tributary to be consistent with the boundary conditions of the step 3. In order to observe the pollution transportation process of the branch flow into the main flow, the initial dissolved oxygen concentration of the global fluid domain is set to be 7.01mg/L, and the initial BOD concentration is set to be 4mg/L. Carrying out numerical calculation solving by means of a computer, wherein a calculation algorithm is a SIMPLE iterative algorithm (semi-implicit method for pressure-linked equations), and the iteration convergence condition is that the residual error is smaller than 10 -5 . The calculation results were saved every 200 seconds.
And 5, carrying out post-processing on the calculation result to obtain the hydrodynamic force characteristics, the dissolved oxygen distribution and the dissolved oxygen transportation process of the river junction region in the target period, and further judging the water quality distribution condition of the junction region.
Fig. 5 is a flow velocity field distribution of the river junction, fig. 6 is a turbulence energy distribution of the river junction, fig. 7 is a dissolved oxygen concentration distribution field of the river junction in a steady state, and fig. 8 is a transport process of a polluted water flow with low dissolved oxygen concentration carried by a tributary of the river junction. Therefore, the water quality distribution condition of the junction area can be judged as follows: when the quality of the tributary water is poor and is converged into the main stream, a pollution stagnation area with low flow speed exists at the tributary outlet of the intersection area, the quality of the area is poor, then the polluted water is mainly transported along the right bank of the main stream to the downstream and is transversely mixed, the pollution concentration is gradually reduced, and the dissolved oxygen concentration difference between the left bank and the right bank is gradually reduced.
For this embodiment, the method of the present invention is compared with the conventional field measurement technique in terms of time consumption, expense budget, instrumentation, data results, etc., as shown in the following table:
as can be seen from the table, the method is a convenient, efficient and low-cost method suitable for primarily judging the water quality condition of the river junction area. Can be used as an optimization supplement and important reference of the existing field measurement technology.
By means of the mode, the river junction water quality distribution simulation method based on the satellite image and the CFD utilizes a small amount of data which are easy to acquire by means of satellite image and computer modeling technology, and efficiently and conveniently simulates the coupling distribution of dissolved oxygen water quality indexes of any river junction so as to judge the water quality conditions of the river junction and downstream river channels. Compared with the existing technology for measuring the water quality distribution condition of the river junction area by driving the ship in the field, the method can effectively save time and economic cost, and improve the efficiency of water quality related research of the junction area and water environment prediction and treatment of related departments. Because the method of the invention discretizes the whole area of the river junction region so as to carry out numerical simulation calculation, the obtained calculation result covers the whole fluid area of the river junction region. Compared with the data obtained by the technology of measuring the water quality distribution condition of the river junction area by using the field ship, the method provided by the invention has the advantages that the dissolved oxygen distribution data at any position in the fluid in the river junction area can be extracted more accurately, and the operability of the follow-up scientific research analysis and water quality prediction management work is effectively improved.
Claims (7)
1. The river junction water quality distribution simulation method based on satellite images and CFD is characterized by comprising the following steps of:
step 1, acquiring satellite images of a river junction region at target time according to the geographic position of the river junction region, and extracting and guiding out a closed contour of the junction river channel in the satellite images;
the closed contour of the intersected river channel comprises an intersection area, a downstream water bank boundary line, a main stream inlet section line, a tributary inlet section line and a downstream outlet section line;
step 2, projecting the closed contour of the intersected river to a plane and generating a surface area, taking the surface area as a two-dimensional fluid area of the river intersection area, and discretizing the fluid area into a plurality of non-overlapping quadrilateral grid units;
step 3, acquiring water flow and water quality data of the upstream inlet section of the main flow and the upstream inlet section of the tributary of the fluid domain, and calculating boundary conditions of the fluid domain;
step 4, listing a control equation set at the central node of each quadrilateral grid of the fluid domain based on a two-dimensional turbulent fluid mathematical model and a dissolved oxygen transport mathematical model of the intersection region, combining the control equation sets of all quadrilateral grid units of the fluid domain, establishing a discrete algebraic equation set of the control equation set by adopting a finite volume method, substituting boundary conditions of the fluid domain, initializing data of the fluid domain, carrying out numerical calculation solving by a computer, and storing iterative calculation data at regular fluid time intervals;
the specific process of listing a control equation set at the central node of each quadrilateral grid of the fluid domain by the two-dimensional turbulent fluid mathematical model and the dissolved oxygen transport mathematical model of the intersection region is as follows:
according to the hydrodynamic conservation equation, there is the expression:
the RNG k-epsilon model is adopted to capture the turbulent motion characteristics of the water flow in the junction area, and the control equation is as follows:
and (c) equation:
epsilon equation:
the dissolved oxygen transport model is:
the dissolved oxygen mass transfer model is:
S DO ＝K r (C s -C DO )+S BOD (11)
S BOD ＝-K b C BOD (12)
K r ＝K r20 ×1.024 (T-20) (13)
C s ＝0.0035T 2 -0.33369T+14.407 (14)
in the above formula: ρ is density, kg/m 3 The method comprises the steps of carrying out a first treatment on the surface of the p is pressure, pa; t is time, s; u (u) i 、u j The flow velocity component of U in each direction is m/s; x is x i 、x j Is a coordinate component, m; epsilon is the turbulent energy dissipation ratio; mu is the molecular coefficient of viscosity; mu (mu) t Is the turbulent viscosity coefficient; g k For the generation term of turbulence k caused by average flow velocity gradient, the empirical constant is taken to be C 1ε ＝1.42、C 2ε ＝1.42、α k ＝α ε ＝1.39、C μ ＝0.0845、η 0 ＝4.377、β＝0.012，K r Is the reoxygenation coefficient s -1 ；K b D is the degradation coefficient -1 ；K r20 Is the atmospheric reoxygenation coefficient at 20 ℃, s -1 ；K b20 Degradation coefficient s at 20 DEG C -1 ；C s Is saturated dissolved oxygen concentration, mg/L; c (C) DO DO concentration, mg/L; c (C) BOD BOD concentration, mg/L; t is water temperature, DEG C; θ is the temperature coefficient, θ=1.047;
and 5, inputting the stored calculation result into Tecplot software for post-processing, and obtaining hydrodynamic characteristics and dissolved oxygen distribution of the river junction region in a target period, namely, the water distribution in the junction region in the target period.
2. The method for simulating water quality distribution in a river junction based on satellite images and CFD according to claim 1, wherein the main and tributary upstream inlet section water flow and water quality data in step 3 include flow, water depth, dissolved oxygen concentration, BOD concentration.
3. The river junction water quality distribution simulation method based on satellite images and CFD (computational fluid dynamics) according to claim 2, wherein the fluid domain boundary conditions comprise a main stream velocity inlet boundary condition, a tributary velocity inlet boundary condition, an outlet boundary condition and a river bank boundary condition, wherein the river bank boundary condition is defined as a slip-free boundary considering roughness, the roughness is determined according to actual river course roughness, and standard wall function method is adopted for calculation; the outlet boundary condition is free outflow; the main flow speed inlet boundary condition and the branch flow speed inlet boundary condition comprise inlet flow speed, inlet turbulence energy dissipation rate, dissolved oxygen concentration and BOD concentration, wherein:
the inlet flow rate calculation formula is:
the inlet turbulence energy calculation formula is:
the calculation formula of the inlet turbulent energy dissipation ratio is as follows:
in the above, U inlet Inlet flow rate, m/s; q (Q) inlet For inlet flow, m 3 S; b is the river width of the inlet section, m; h is a inlet The average water depth of the inlet section is m; k (k) inlet For turbulent energy of inlet section, m 2 /s 2 ，ε inlet Is the dissipation rate of the inlet turbulent energy, and has no dimension.
4. The method for simulating water quality distribution in a river junction region based on satellite images and CFD according to claim 1, wherein the atmospheric reoxygenation coefficient is determined according to the actual river junction region water flow conditions, specifically one of the following conditions:
1) River with strong turbulence:
2) Deepwater rivers with slow flow rates:
3) High speed stream:
4) Shallow stream:
wherein U is a speed vector, m/s; k is the turbulence energy of the water body, m 2 /s 2 The method comprises the steps of carrying out a first treatment on the surface of the h is the water depth, m.
5. The method for simulating water quality distribution in a river junction area based on satellite images and CFD according to claim 1, wherein the specific process of establishing a discrete algebraic equation set of the control equation set by using the finite volume method in step 4 is as follows: integrating the control equation of the central node of each quadrilateral mesh on the quadrilateral mesh to obtain an algebraic equation set of the quadrilateral mesh, wherein the parameter value on the boundary of each quadrilateral mesh in the equation set is obtained by a second-order windward interpolation method.
6. The method for simulating water quality distribution in river junction area based on satellite image and CFD according to claim 1, wherein in step 4, the algorithm used in the numerical calculation solution by means of computer is SIMPLE iterative algorithm, and the iterative convergence condition is that the residual is less than 10 -5 。
7. The method for simulating water quality distribution in a river junction based on satellite images and CFD according to claim 1, wherein the data for initializing the data of the fluid field comprises: full area pressure value, flow velocity value, turbulence energy dissipation rate, dissolved oxygen concentration and BOD concentration.
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