Background
In recent years, with the rapid development of the transportation industry, the number of bridges is increasing, and square piers are widely applied to the construction of viaducts along the trend of rivers. However, natural disasters caused by severe scours generated at piers and abutments are frequent, which has a great influence on national economy. Under the condition of complex incoming flow, the streaming of the square pier comprises complex flowing phenomena such as backflow, impact, separation, reattachment, vortex shedding and the like. Particularly, the water flow structure is changed by the water blocking effect of the square pier, and a horseshoe vortex system in front of the pier, a submerged water flow and a wake vortex system are generated, so that the local shear stress of the bed surface around the square pier is increased, and finally local scouring is formed. Currently, most bridge instability and collapse are caused by serious local scouring around the bridge pier. Therefore, the method has very important engineering significance for researching the local scouring problem of the square pier under the condition of complex incoming flow.
At present, the research aiming at the problem of local scouring mainly comprises three methods, namely semi-theoretical and semi-empirical, reduced-proportion hydraulic model test and numerical calculation. The three methods have advantages and disadvantages respectively, and the first two methods belong to the traditional methods, have the advantages of reality, intuition and high data reliability, but need a large amount of manpower and material resources, advanced measuring equipment and testing means. In this case, the numerical calculation can completely overcome the disadvantages of the conventional method, and the microscopic flow phenomenon and more detailed physical parameters can be obtained by the numerical calculation, but the requirement of the numerical calculation method on the theoretical level and the numerical model of the user is high. In terms of numerical models, in practical engineering application cases, most of the numerical models are calculation frameworks based on the reynolds time mean square range RANS, and the method is poor in calculation accuracy and relatively insufficient in capture capacity of micro-flow details. Therefore, it is necessary to develop an efficient numerical calculation method applied to the problem of local scour of piers.
Disclosure of Invention
The invention aims to solve the problems, and provides a method for calculating a streaming flow field of a square pier scouring problem.
The technical scheme of the invention is a method for calculating the flow-around flow field of the square pier scouring problem, which comprises the following steps:
step 1: constructing a physical model of the square pier and the scouring water flow of the square pier, namely a streaming model;
step 2: carrying out mesh division on the physical model in the step 1, and dividing boundary layer meshes in the area near the surface of the pier;
and step 3: importing the grid file obtained in the step 2 into fluid finite element software, and establishing a numerical model of the square pier;
and 4, step 4: setting conditions of a numerical model of the square pier, a solving format of a control method and numerical calculation precision;
and 5: calculating by using a numerical model to obtain a streaming flow field of the square pier, and extracting a calculation result by using post-processing software to obtain non-steady characteristics of the streaming flow field of the square pier;
step 6: and (3) establishing an entity model of the square pier, performing a water flow scouring experiment under the same condition in the step (3), comparing the water flow scouring experiment result with the calculation result of the streaming flow field in the step (5), and correcting the numerical model of the square pier according to the comparison result.
Further, in the step 1, a physical model of the square pier and the scouring water flow of the square pier is built, and the three-dimensional drawing software UG is adopted to model the square pier and the area near the surface of the pier.
Further, in step 4, all physical meters are setCalculating convergence residual error criterion to be 1 × 10-5And taking the convergence result of the constant calculation as the initial value of the unsteady calculation to accelerate the convergence efficiency.
Further, step 2 comprises the following substeps:
step 2.1: importing the physical model file in the step 1 into network partitioning software ANSYS ICEM, and defining the flow field region boundary;
step 2.2: and dividing the paste blocks into a flow field calculation domain, setting the number of nodes of each edge on each block according to the side length of the physical model, placing the first layer of grids inside the laminar flow bottom layer, repeating the operation to obtain four grids with different densities, which are named as a coarse grid, a medium grid, a fine grid and a superfine grid respectively, and outputting four grid files.
Further, step 3 comprises the following substeps:
step 3.1: using ANSYS FLUENT software to read the grid file obtained in the step 2 and checking whether the grid quality meets the requirement;
step 3.2: establishing a turbulence model of the square pier, and compiling;
step 3.3: setting the working medium as liquid water, setting the density and viscosity of the working medium, and calculating the boundary of a domain inlet as a speed inlet, the boundary of an outlet as a pressure outlet and the rest boundaries as solid wall surfaces;
step 3.4: setting a pressure-speed coupling equation and adopting a SIMPLEC algorithm; the pressure interpolation adopts a Standard format; the momentum equation, the turbulent kinetic energy, the dissipation rate and the cavitation volume fraction control equation adopt a second-order windward format; the non-fixed item selects bound Second order identifier format.
Preferably, in step 3.2, the turbulence model of the square pier adopts a separation vortex model based on grid scale correction, and on the basis of the existing separation vortex model, the kinetic energy dissipation term Y of turbulence caused by turbulence is calculatedkThe correction is carried out, and the control equation is as follows:
in the formula, the symbol < represents the mean value of the variables in the calculation domain; rho is density; k is the turbulence energy; u. ofjIs the velocity component; μ is the molecular viscosity; mu.stIs a turbulent viscosity; sigmakTurbulence prandtl number, which is the turbulence energy; gkGenerating a term for the kinetic energy of the turbulence; y iskA turbulent kinetic energy dissipation term; skCustom items for turbulence energy users; sigmaωA turbulent prandtl number that is the specific dissipation ratio; y isωIs the specific dissipation ratio dissipation term; gωGenerating terms for the specific dissipation ratio; sωCustomizing an item for a specific dissipation rate user; fAdaptive-DESIs a masking function; Δ is a sub-lattice scale; v is the first layer grid cell volume; l istThe turbulence scale of the two-stroke model; fSSTIs a mixing function; r isdIs a characteristic distance; climIs CDESA lower critical value; cdynIs CDESAn upper critical value; cDESSeparating vortex model coefficients; l iskIs the kolmogorov scale; ζ is the dissipation ratio; upsilon is a kinematic viscosity coefficient; f. ofdIs a delay function; Ω is strain rate tensor; s is a rotation rate tensor; h ismaxThe maximum side length of the wall surface grid is; h isminThe minimum side length of the wall surface grid is; v istIs turbulent kinematic viscosity; ν is the molecular kinematic viscosity; dw
The height of the grid from the wall surface;
is a decay function; r is the attenuation coefficient; gamma is the maximum grid transverse-longitudinal ratio; the power factor is used for representing filtering; u shape
i,jIs the velocity gradient tensor; constant α 25, β 0.05, λ 0.61, C
μ=0.09,C
d1=80,C
d2=3,K=0.41。
Further, step 6 comprises:
1) obtaining predicted values of horizontal component velocity and vertical component velocity at different positions under the same Reynolds number condition through data processing, and comparing the predicted values with water flow scouring experiment values under the same condition;
2) obtaining the predicted values of the vertical component velocity root mean square at different positions under the same Reynolds number condition through data processing, and comparing the predicted values with the water flow scouring experiment values under the same condition;
3) and verifying the accuracy and the feasibility of the numerical model of the square pier according to the comparison result, and correcting the numerical model of the square pier.
Compared with the prior art, the invention has the beneficial effects that:
1) the calculation method of the flow bypassing flow field realizes high-precision numerical calculation of the flow field of the actual engineering bridge scouring problem, obtains detailed characteristic parameters and unsteady flow structure of the flow field, and provides theoretical support for revealing a local scouring mechanism of the bridge;
2) the separation vortex model based on grid scale correction is adopted for numerical calculation, compared with the existing separation vortex model, the calculation precision is higher, and the problems of modeling stress loss and grid induced separation of the existing model are solved;
3) the method provided by the invention utilizes an experimental method to verify and correct the calculation result of the numerical model of the square pier, so that the accuracy of the calculation result of the numerical model is better guaranteed.
Detailed Description
The embodiment adopts the experiment models of the river-crossing bridge and the bridge piers thereof which are reduced in equal proportion to carry out experiments and verification on the method.
As shown in fig. 1, the method for calculating the flow-around flow field of the square pier scouring problem comprises the following steps:
step 1: constructing a physical model of the square pier and the scouring water flow of the square pier, namely a streaming model;
as shown in FIG. 2, the bypass flow model of the square pier comprises an Inlet Inlet, an Outlet Outlet, a wall surface Solid boundary and the square pier located in the calculation domain. The side length D of the square pier is 40mm, the total length of the calculation domain is 25D, the width of the calculation domain is 14D, the distance from the left side face of the square pier to an inlet is 5D, and in the center of the vertical direction, the center point of the square pier is used as the origin of coordinates, and an XOY coordinate axis is established. And modeling the overcurrent area by adopting three-dimensional drawing software UG, and storing the final physical model as an igs format file.
Step 2: performing mesh division on the physical model in the step 1, dividing boundary layer meshes with sufficient precision in a region near the surface of the pier, and performing mesh independence analysis to obtain flow field calculation meshes as shown in FIG. 3;
step 2.1: importing the igs file in the step 1 into mesh division software ANSYS ICEM, defining the flow field region boundary, and respectively defining a calculation region inlet, an outlet and a solid wall surface;
step 2.2: based on the topology idea, a patch block is divided into a flow field calculation domain, then the number of nodes of each edge on the block is set according to the side length of a physical model, a first layer of grids is arranged in a laminar flow bottom layer, four grids with different densities are obtained through repeated operation and named as a coarse grid, a medium grid, a fine grid and a fine grid respectively, and four msh files are output.
And step 3: importing the msh file obtained in the step 2 into ANSYS FLUENT v16.0, and establishing a numerical model of the square pier; step 3.1: reading the grid file obtained in the step 2 by using ANSYS FLUENT v16.0 software, and checking whether the grid quality meets the requirement;
step 3.2: establishing a turbulence model of a square pier, and adopting a separation vortex model based on grid scale correction;
step 3.3: setting the entrance boundary of the calculation domain as a speed entrance, wherein the size of the entrance boundary is U-0.535 m/s; the outlet boundary is a pressure outlet, the size of the outlet boundary is standard atmospheric pressure, and the rest boundaries are solid wall surfaces; the working medium is liquid water, and the density and the viscosity of the working medium are set, wherein the density rho is 1000kg/m3The kinetic viscosity coefficient [ mu ] is 1.139 × 10-3Pa.s;
Step 3.4: setting a pressure-speed coupling equation and adopting a SIMPLEC algorithm; the pressure interpolation adopts a Standard format; the momentum equation, the turbulent kinetic energy, the dissipation rate and the cavitation volume fraction control equation adopt a second-order windward format; the non-fixed item selects bound Second order identifier format.
And 4, step 4: and setting a solving format of a control equation and numerical calculation precision. And solving the format setting of the pressure-velocity coupling mode, the pressure item, the momentum, the turbulence energy, the dissipation rate and the non-constant item. Setting the convergence residual error standard of all physical quantity calculation as 1 × 10-5And taking the convergence result of the constant calculation as the initial value of the unsteady calculation to accelerate the convergence efficiency.
And 5: and calculating by using a numerical model to obtain a flow-around flow field of the square pier, and extracting a calculation result by using post-processing software. Step 6: and (3) establishing an entity model of the square pier, performing a water flow scouring test under the same condition in the step (3), comparing the water flow scouring test result with the calculation result of the streaming flow field in the step (5), and correcting the numerical model of the square pier according to the comparison result.
3.2, based on the Mesh-adaptive DES, the Mesh-adaptive DES is used for solving the problem that the original DES method is used, and the Mesh-adaptive DES dissipates the turbulent kinetic energy Y caused by the turbulent flowkAnd (6) correcting. In the original DES model, model coefficients CDES0.61 and the filter size is close to the wallThe maximum side length of the planar grid cell, i.e., Δ ═ max (Δ x, Δ y, Δ z). When the original DES model is applied to a complex grid, the conversion process from Reynolds average RANS to large vortex simulation LES is advanced due to the empirical coefficients, so that problems of modeling stress loss, grid induced separation and the like are caused in sequence, and finally simulation distortion is caused. The Mesh-adaptive DES can avoid the above problems, and the control equation of the Mesh-adaptive DES is as follows:
in the formula, the symbol < represents the mean value of the variables in the calculation domain; rho is density; k is the turbulence energy; u. of
jIs the velocity component; μ is the molecular viscosity; mu.s
tIs a turbulent viscosity; sigma
kTurbulence prandtl number, which is the turbulence energy; g
kGenerating a term for the kinetic energy of the turbulence; y is
kA turbulent kinetic energy dissipation term; s
kCustom items for turbulence energy users; sigma
ωA turbulent prandtl number that is the specific dissipation ratio; y is
ωIs the specific dissipation ratio dissipation term; g
ωGenerating terms for the specific dissipation ratio; s
ωCustomizing an item for a specific dissipation rate user; f
Adaptive-DESIs a masking function; Δ is a sub-lattice scale; v is the first layer grid cell volume; l is
tThe turbulence scale of the two-stroke model; f
SSTIs a mixing function; r is
dIs a characteristic distance; c
limIs C
DESA lower critical value; c
dynIs C
DESAn upper critical value; c
DESSeparating vortex model coefficients; l is
kIs the kolmogorov scale; ζ is the dissipation ratio; upsilon is a kinematic viscosity coefficient; f. of
dIs a delay function; Ω is strain rate tensor; s is a rotation rate tensor; h is
maxThe maximum side length of the wall surface grid is; h is
minThe minimum side length of the wall surface grid is; v is
tIs turbulent kinematic viscosity; ν is the molecular kinematic viscosity; d
wThe height of the grid from the wall surface;
is a decay function; r is the attenuation coefficient; gamma is the maximum grid transverse-longitudinal ratio; the power factor is used for representing filtering; u shape
i,jIs the velocity gradient tensor; constant α 25, β 0.05, λ 0.61, C
μ=0.09,C
d1=80,C
d2=3,K=0.41。
In step 6, the numerical calculation results of the horizontal component velocity and the vertical component velocity at different positions of the flow-surrounding flow field under the same reynolds number are obtained through data processing, and are compared with the experimental values, as shown in fig. 4(a), 4(b), 4(c), and 4(D), where fig. 4(a) shows the calculated values and the experimental values of the horizontal component velocity of the water flow at different vertical coordinates where x/D is 1.0 on the abscissa, fig. 4(b) shows the calculated values and the experimental values of the vertical component velocity of the water flow at different vertical coordinates where x/D is 1.0 on the abscissa, fig. 4(c) shows the calculated values and the experimental values of the vertical component velocity of the water flow at different vertical coordinates where x/D is 3.0 on the abscissa, and fig. 4(D) shows the calculated values and the experimental values of the horizontal component velocity of the water flow at different vertical coordinates where x/D is 8.0 on the abscissa. The numerical calculation results and experimental values of the root-mean-square of the vertical component velocities at different positions on the transverse section of the central axis of the square pier are shown in fig. 5. Therefore, the numerical calculation result is well matched with the experimental value, and the accuracy and the feasibility of the numerical method are verified.
The above detailed description is intended to illustrate the objects, aspects and advantages of the present invention, and it should be understood that the above detailed description is only exemplary of the present invention and is not intended to limit the scope of the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the scope of the present invention.