Roof rainwater accumulated water depth analysis method based on numerical simulation

Abstract

The invention discloses a roof rainwater accumulation depth analysis method based on numerical simulation, and the method comprises the steps: S1, building a roof three-dimensional model, and carrying out the grid division; s2, selecting local rainfall data, and determining a rainwater falling mass flow rate by adopting a Chicago rain pattern; s3, importing the grid model and the rainwater falling mass flow rate into CFD software, and calculating the volume fraction at of water in a two-phase flow formed by water and air in each grid at the monitoring target at any moment t, which occupies the grid; s4, selecting the maximum value amax of the at of each grid; and S5, detecting whether amax = 1 of any grid exists or not, and if yes, modifying the height of the monitoring target and repeatedly executing the steps S1 to S5 until amax = 1 does not exist. According to the method, the maximum ponding height of the monitored target is calculated, and reference basis can be provided for roof ponding judgment.

Description

Technical Field

[0001] The invention relates to the field of water supply and drainage, and in particular to a method for analyzing the depth of rainwater accumulation on a roof based on numerical simulation. Background Art

[0002] According to the current national standard "Building Water Supply and Drainage Design Standard" GB50015 (hereinafter referred to as the "Standard"), the design rainwater flow is related to the design rainstorm intensity and the catchment area. When the design recurrence period and rainfall duration are fixed, the design rainstorm intensity is a constant value in a certain area. For complex roofs, affected by the roof shape layout, roof slope changes, and roof materials, the catchment area of ​​the outlet is difficult to accurately calculate through the roof shape. When the slope direction and slope of the irregular curved roof change greatly, the water flow velocity, water film thickness and catchment time of the roof rainwater runoff calculated by conventional hydraulic formulas will produce large errors compared with the relevant data during the actual rainfall process. The "Standard" has no special provisions for slopes far greater than 2.5% or complex roofs. In addition, the static rainwater flow calculation method is used. The height of special roof nodes (i.e., monitoring targets, including but not limited to skylights, roof equipment, water retaining boards, etc.) judged according to conventional empirical values ​​may be lower than the depth of water accumulation, resulting in damage to the structural safety and water accumulation at the building connection, affecting the safety of the building. The Chicago rainfall pattern is a typical rainfall process curve generated according to the rainstorm intensity formula and the rain peak coefficient. Based on the rainfall process curve, the numerical simulation method is used to study the rainwater confluence of roofs with complex rainfall durations, dynamically monitor the water accumulation depth on the roof during rainfall, verify the drainage flow of the outlet, and ensure the safety of the roof structure. Summary of the invention

[0003] The technical problem to be solved by the present invention is to overcome the shortcomings of the existing technology and provide a method for analyzing the depth of rainwater accumulation on a roof based on numerical simulation. The depth of rainwater accumulation can be obtained through simulation, which provides a reference basis for setting the monitoring target height and ensures the safety of the rainwater system.

[0004] The specific solution to the problem is to use the finite element differential equation method to simulate the real-time rainwater runoff velocity on the roof, the real-time rainwater accumulation depth on the roof, the set rainwater outlet instantaneous flow and the outlet catchment area during the rainfall process that conforms to the Chicago rainfall pattern rainfall duration characteristics.

[0005] The present invention specifically adopts the following technical solutions to solve the above technical problems:

[0006] A method for analyzing the depth of rainwater accumulation on a roof based on numerical simulation includes the following steps:

[0007] S1: Establish a roof geometry model, select a rainfall height, enclose the roof to form a geometric body, and perform mesh division;

[0008] S2: Select local rainfall data to determine the mass flow rate of rainwater falling;

[0009] S3: Import the grid model and the mass flow rate of rainwater into the CFD software, set the monitoring target height, and use the software to calculate the volume fraction a of water in each grid in the two-phase flow formed by water and air at the monitoring target at any time t. t ;

[0010] S4: Select a for each grid within the calculation time step t The maximum value a max ;

[0011] S5: Check whether all grids have a max =1, if yes, adjust the monitoring target height and repeat S1 to S5 until there is no a max =1.

[0012] Furthermore, in step S1, after completing the selection of a section of rainfall height and enclosing a geometric body with the roof, the rainwater pipe on the roof or the sump at the rainwater pipe is defined as the outlet surface. In step S3, the drainage volume of each outlet surface is monitored, and a time-flow curve of each outlet surface is established. If the flow in the time-flow curve reaches the maximum drainage volume designed for the outlet surface, return to step S1 to change the set number of outlet surfaces or flow.

[0013] Furthermore, the sum of the mass flow rate of rainwater at each outlet integrated over time is counted. If the error between the sum of the mass flow rate of rainwater at each outlet and the total amount of rainwater input from the sky above the calculation domain exceeds a set value, the process returns to step S1 to encrypt the grid.

[0014] Further, step S4 includes:

[0015] S41: Make all volume meshes a max =0;

[0016] S42: Executed when the iterative calculation of each time step is completed, looping through all wall grids, and for a given wall grid, reading the volume fraction of water a in the grid at the current time t t , judge a t with a max The size of a max Less than a t Then a max =a t , otherwise a max constant.

[0017] Furthermore, the grid division is encrypted near the building surface so that there are multiple grids at the monitoring target for distinguishing the depth of water accumulation.

[0018] Further, in step S2, the mass flow rate of the rainwater falling is determined by using the Chicago rain type; where the rainstorm duration is 2 hours, the recurrence interval is 100 years, and the rain peak coefficient is 0.4.

[0019] Further, the mass flow rate Y is calculated by the following formula:

[0020] Y = (2.5a*(1 - c)(48 - t) + ab) / ((48 - t) / 0.4 + b) (1+c) , t <= 48;

[0021] Y = (1.667*(1 - c)(t - 48) + ab) / ((t - 48) / 0.6 + b) (1+c) , 48 < t <= 120, where Y is the rainfall, t is the time, and a and b are constants.

[0022] Further, in step S2, the rainstorm intensity formula is i = a / (b + t)^c, where t is the time, and a, b, and c are constants.

[0023] Further, the corresponding relationship between the rainfall Y and the time t is as follows:

[0024]

[0025] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0026] The present invention combines the grid with the CFD software to solve the problem of difficult calculation of rainwater drainage on large and large-slope complex roofs, and can show the water accumulation situation at the monitoring target and the maximum flow rate of any drainage outlet under actual rainstorm conditions, providing a reference basis for the safe design of the building rainwater system and ensuring the safety of the drainage system, structure, and building.

[0027] By analyzing the model at the monitoring target, the backwater height at the monitoring target is obtained, providing a basis for setting the height of the monitoring target. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a flowchart of the method for analyzing the rainwater accumulation depth on the roof based on numerical simulation in the present invention in the specific implementation manner;

[0029] Figure 2 It is a rainfall state diagram simulated in the embodiment of the present invention. SPECIFIC IMPLEMENTATION MANNER

[0030] The technical solution of the present invention will be described in detail below with reference to the drawings:

[0031] Please refer to Figure 1 and Figure 2, a roof rainwater accumulation depth analysis method based on numerical simulation includes the following steps:

[0032] S1: Establish a roof geometry model, select a rainfall height, enclose the roof to form a geometric body, and perform mesh division;

[0033] S2: Select local rainfall data to determine the mass flow rate of rainwater falling;

[0034] S3: Import the grid model and the mass flow rate of rainwater into the CFD software, set the monitoring target height, and use the software to calculate the volume fraction a of water in each grid in the two-phase flow formed by water and air at the monitoring target at any time t. t ;

[0035] S4: Select a for each grid within the calculation time step t The maximum value a max ;

[0036] S5: Check whether all grids have a max =1, if yes, adjust the monitoring target height and repeat S1 to S5 until there is no a max =1.

[0037] In this embodiment, the modeling software used is Rhino, Sketchup, AutoCAD, ZWCAD, etc., and the meshing software used is Gambit, ICEM, Meshing, Gmsh, etc.

[0038] The present invention performs a fine analysis of the local grid on the monitoring target with the largest catchment area at the starting point, and records the maximum value a of the water component in each grid area. max The results show that a max The grid area where <1 is located is located at the lower part of the monitoring target, that is, the water level at the monitoring target is lower, and the water flow will not overflow the monitoring target during heavy rain, thereby determining the height of the monitoring target.

[0039] It can be understood that the monitoring target is generally a place where water easily accumulates, such as a skylight on a roof. In other embodiments, the location of the monitoring target can be selected according to actual needs.

[0040] Furthermore, in step S1, after completing the selection of a section of rainfall height and enclosing a geometric body with the roof, the rainwater pipe on the roof or the sump at the rainwater pipe is defined as the outlet surface. In step S3, the drainage volume of each outlet surface is monitored, and a time-flow curve of each outlet surface is established. If the flow in the time-flow curve reaches the maximum drainage volume designed for the outlet surface, return to step S1 to change the set number of outlet surfaces or flow.

[0041] Further, sum the time integrals of the mass flow rates of the rainwater at each outlet. If the error between this sum and the total amount of rainwater input from the sky above the computational domain exceeds the set value, return to step S1 to refine the mesh division.

[0042] In this embodiment, the set value is 1%, and if the error is less than 1%, it is considered that the calculation is correct.

[0043] Further, step S4 includes:

[0044] S41: Set a of all volume meshes to 0; max = 0;

[0045] S42: Execute when the iterative calculation for each time step is completed. Traverse the meshes of all walls in a loop. For the meshes of a given wall, read the volume fraction a of the water in the mesh at the current time t, t , and judge the magnitude of a t and a max . If a max is less than a t , then a max = a t , otherwise a max remains unchanged.

[0046] Further, during mesh division, encryption is performed near the building surface so that there are multiple meshes at the monitoring target to distinguish the accumulated water depth.

[0047] Further, in step S2, the Chicago rain pattern is selected to determine the mass flow rate of the rainwater falling; where the rainstorm duration is 2 hours, the return period is 100 years, and the rain peak coefficient is 0.4.

[0048] Further, in step S2, the rainstorm intensity formula is i = a / (b + t)^c, where t is time, and a, b, and c are constants.

[0049] Further, in step S2, the Chicago rain pattern is selected to determine the mass flow rate of the rainwater falling,

[0050] Y = (2.5a*(1 - c)(48 - t) + ab) / ((48 - t) / 0.4 + b) (1+c) , t <= 48;

[0051] Y = (1.667*(1 - c)(t - 48) + ab) / ((t - 48) / 0.6 + b) (1+c) , 48 < t <= 120. In the formula, Y is the rainfall amount, t is time, and a, b, and c are constants, which vary according to the local rainstorm intensity formula.

[0052] Further, the correspondence between the rainfall amount Y and time t is as follows:

[0053]

[0054]

[0055] In this embodiment, in the CFD software, the rainfall is calculated by writing UDF, and the specific content of UDF is as follows: variable declaration and definition:

[0056] define boundary macro mass flow rate rainfall_mass_flow_rate;

[0057] Declare variable rainfall intensity rainfall_rate;

[0058] Declare the variable mass flow rate;

[0059] Assign the formula for calculating the rainstorm intensity to the rainstorm intensity rainfall_rate.

[0060] run:

[0061] In each time step, the mass flow rate rainfall_mass_flow_rate (rainstorm intensity * area * water density) is calculated according to the current time

[0062] Assign the mass flow rate to F_PROFILE(f,thread,position).

[0063] In this embodiment, in the CFD software, the flooding water level is calculated by writing UDF, and the specific content of the UDF is as follows:

[0064] Variable declaration and definition:

[0065] Select the target wall to mount the UDF;

[0066] Define thread pointer;

[0067] Define face pointer;

[0068] Define UDMI.

[0069] initialization:

[0070] Loop through the wall surface, search for the volume mesh that contains each surface mesh, and assign a value of -1 to the selected UDMI.

[0071] run:

[0072] At the end of each time step iteration, loop each surface mesh on the selected wall, find the volume mesh containing the surface mesh, and read the volume fraction α of water in the volume mesh t , if the α of the previous time step recorded by UDMI in the volume gridt-1 Value < α t , then replace UDMI with α t , until the end of the simulation. If 1 is used as the boundary, it is considered that the area with UDMI>1 will be flooded during the rain.

[0073] In a specific embodiment, modeling software (Rhino, Sketchup, AutoCAD, Zhongwang CAD, etc.) is selected to create a sports center model. The roof area is about 15,000 square meters, divided into two parts: the middle roof and the outer circle roof. The plane of the middle roof is an oblong, with a long axis of 106.5 meters and a short axis of 80.2 meters. The maximum slope of the roof along the long axis is 14%, and it is slightly arched along the short axis, slightly like a turtle back. The material is a vertical lock-edge aluminum-magnesium-manganese alloy plate, 85 monitoring targets are set, the height of the monitoring target is 340mm, the maximum size of the monitoring target is 3.7mx4.7m, and the minimum size is 2.7mx3.7m. The middle roof does not have a drainage gutter; the outer circle of the middle roof is a truncated cone, with a projection width of 24 meters to 35 meters in the south, east, and west, and a slope of 21% to 40%. The minimum width on the north side is 4.5 meters, and the slope is 21% to 38%. Two groups of six monitoring targets are set on the outer circle roof, with a length of about 12 meters, a width of 2.5 meters, and a flange height of 400mm. The entire roof is set with drainage ditches along the outer circle, and rainwater from the middle roof is discharged to the drainage ditch through the outer circle roof. The drainage ditch is 1 meter wide, 0.4 meters deep, with a total length of 471 meters and a height difference of about 20 meters. It is made of stainless steel and adopts the principle of zoning water collection. The rainwater buckets are sparsely arranged at higher places and densely arranged at lower places. According to this principle, a total of 16 water collection pits are set up, with a minimum length of 1.2 meters and a maximum length of 7 meters.

[0074] The roof model from the modeling software (Rhino, Sketchup, AutoCAD, ZWCAD, etc.) is processed and meshed in the mesh processing software (Gambit, ICEM, Meshing, Gmsh, etc.). The mesh form adopts a hybrid mesh, which is mainly a polyhedral mesh and contains a hexahedral mesh. The input rainfall data adopts the Chicago rainfall type, such as Figure 2 As shown in the figure, a more realistic rainfall state is simulated. The calculation uses the data of a once-in-a-century rainstorm, a three-hour rainstorm duration, and is calculated from the beginning of a rainfall to the end of the rainfall. The closed area for calculation is the entire roof including the drainage ditch. The simulation is performed using air and water two-phase flow. During the solution and calculation process, the flow velocity, flow rate, pressure, and the maximum value of the water component in the side wall grid of the monitoring target are observed in real time, the speed of key points is detected, and whether there is divergence is observed. The flow rate of each outlet is monitored to verify whether the flow is conserved.

[0075] After the calculation is completed, check whether the inlet and outlet flow rates are conserved during the analysis process, and sum the flow rates of all discharge points to equal the sum of rainfall. If the error is less than 1%, the calculation is considered correct.

[0076] The present invention performs a fine analysis of the local grid on the monitoring target with the largest catchment area at the starting point, and records the maximum value a of the water component in each grid area. max The results show that a max The grid area where ≥1 is located is located in the lower part of the monitoring target, that is, the water level at the monitoring target is lower, and the water flow will not overflow the monitoring target during heavy rain, thereby determining the height of the monitoring target.

[0077] The present invention combines modeling software with fluid analysis software to solve the problem of difficult calculation of rainwater drainage on large, large-slope complex roofs containing monitoring targets. It can display the water accumulation at the monitoring target and the maximum flow of any outlet under actual rainstorm conditions, providing a reference basis for the safe design of building rainwater systems and ensuring the safety of drainage systems, structures, and buildings.

[0078] By analyzing the model at the monitoring target, the water level at the monitoring target was obtained, which provided a basis for setting the monitoring target height.

[0079] Finally, it should be noted that the above are only preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments or to make equivalent substitutions for some of the technical features therein. However, any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for analyzing the depth of rainwater accumulation on a roof based on numerical simulation, characterized in that: Includes steps: S1: Establish a roof geometry model, select a rainfall height, enclose the roof to form a geometric body, and perform mesh division; S2: Select local rainfall data to determine the mass flow rate of rainwater falling; S3: Import the grid model and the mass flow rate of rainwater into the CFD software, set the monitoring target height, and use the software to calculate the volume fraction a of water in each grid in the two-phase flow formed by water and air at the monitoring target at any time t. t ; S4: Select a for each grid within the calculation time step t The maximum value a max ; S5: Check whether all grids have a max =1, if yes, adjust the monitoring target height and repeat S1 to S5 until there is no a max =1.

2. The method for analyzing the depth of rainwater accumulation on a roof based on numerical simulation according to claim 1 is characterized in that: In step S1, after completing the selection of a section of rainfall height and enclosing a geometric body with the roof, the rainwater pipe on the roof or the sump at the rainwater pipe is defined as the outlet surface. In step S3, the drainage volume of each outlet surface is monitored, and a time-flow curve of each outlet surface is established. If the flow in the time-flow curve reaches the maximum drainage volume designed for the outlet surface, return to step S1 to change the set number of outlet surfaces or flow.

3. The method for analyzing the depth of rainwater accumulation on a roof based on numerical simulation according to claim 2 is characterized in that: The sum of the mass flow rate of rainwater at each outlet integrated over time is counted. If the error between the total amount of rainwater input from the sky above the computational domain and the total mass flow rate of rainwater at each outlet exceeds a set value, the process returns to step S1 to encrypt the grid.

4. The method for analyzing the depth of rainwater accumulation on a roof based on numerical simulation according to claim 1 is characterized in that: Step S4 includes: S41: Make all volume meshes a max =0; S42: Executed when the iterative calculation of each time step is completed, looping through all wall grids, and for a given wall grid, reading the volume fraction of water a in the grid at the current time t t , judge a t with a max The size of a max Less than a t Then a max =a t , otherwise a max constant.

5. The method for analyzing the depth of rainwater accumulation on a roof based on numerical simulation according to claim 1 is characterized in that: When dividing the grid, the grid is encrypted near the building surface so that there are multiple grids at the monitoring target to distinguish the depth of water accumulation.

6. The method for analyzing the depth of rainwater accumulation on a roof based on numerical simulation according to claim 1, characterized in that: In step S2, the Chicago rain type is selected to determine the mass flow rate of falling rainwater; the rainstorm duration is 2 hours, the recurrence period is 100 years, and the rain peak coefficient is 0.

4.

7. The method for analyzing the depth of rainwater accumulation on a roof based on numerical simulation according to claim 1, characterized in that: The mass flow rate Y is calculated by the following formula: Y=(2.5a*(1-c)(48-t)+ab) / ((48-t) / 0.4+b) (1+c) ,t<=48; Y = (1.667 * (1 - c)(t - 48) + ab) / ((t - 48) / 0.6 + b) (1+c) , 48 < t <= 120, where Y is the rainfall, t is the time, and a and b are constants.

8. The method for analyzing the depth of rainwater accumulation on a roof based on numerical simulation according to claim 1, characterized in that: In step S2, the rainstorm intensity formula is i=a / (b+t)^c, where t is time, and a, b, and c are constants.

9. The method for analyzing the depth of rainwater accumulation on a roof based on numerical simulation according to claim 1, characterized in that: After step S5, the method further includes the steps of obtaining the allowable water accumulation depth h of the structural load and checking whether the water accumulation height at the monitoring target is less than h.

10. The method for analyzing the depth of rainwater accumulation on a roof based on numerical simulation according to claim 7, characterized in that: The corresponding relationship between rainfall Y and time t is as follows:

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