A method for locally updating and iteratively calculating a chemical non-equilibrium flow field

Abstract

The application belongs to the technical field of aircraft flow phenomenon, and discloses a chemical non-equilibrium flow field local updating iterative calculation method. The method is based on the general CFD calculation flow of the chemical non-equilibrium flow NS equation and the LU-SGS implicit time advancing method, increases a flow state identification function module, modifies the relaxation iteration process, obtains the steady flow parameter distribution through time advancing and relaxation iteration, only carries out the flow field variable updating iterative calculation on the non-steady state grid element, reduces a large number of invalid repeated calculation problems, thereby reduces the resource consumption, and further improves the efficiency of the chemical non-equilibrium flow simulation calculation.

Description

Technical Field

[0001] This invention belongs to the technical field of aircraft flow phenomena, and specifically relates to a method for calculating the local update and iteration of chemical non-equilibrium flow fields. Background Technology

[0002] Under high-speed flight conditions, complex chemical reaction processes such as gas dissociation, ionization, recombination, and exchange occur within the high-temperature shock layer surrounding the aircraft. These processes interfere with the high-speed flow, forming a complex flow phenomenon with coupled multi-physical effects, known as a chemical nonequilibrium effect. The chemical nonequilibrium Navier-Stokes equations are a tightly coupled computational equation system based on the classical Navier-Stokes equations and multiple component continuity equations. This system has numerous computational equations, and the calculations of mass, momentum, and energy transport systems and chemical source terms are complex. In engineering simulation applications, it suffers from significant problems such as high memory resource consumption, long computation time, and long cycles. Furthermore, due to the wide and extensive time distribution of elementary chemical reactions, and the order-of-magnitude difference between the flow characteristic time and the chemical reaction characteristic time, the equation system exhibits strong numerical rigidity, severely affecting the convergence of numerical iterations and further restricting the computational efficiency of flow simulation. Therefore, the computational efficiency of chemical nonequilibrium flow simulation has always been a "hot topic" and one of the key issues urgently needing resolution in engineering simulation applications.

[0003] In analyzing the spatiotemporal dynamic evolution of flow fields, chemical nonequilibrium flow fields actually involve many unnecessary and ineffective repetitive calculations. Taking a cylindrical model simulation of chemical nonequilibrium flow (using incoming flow parameters to provide initial flow field values) as an example, in the flow region outside the shock layer, the flow field remains unaffected by disturbances, and the flow variables remain almost unchanged. However, inside the shock layer and in the downstream region, the flow field is affected by disturbances, and the flow variables continuously change. But as the flow tends towards convergence, the flow variables will also stabilize. Therefore, based on whether the flow variables change significantly, chemical nonequilibrium flow fields can be divided into steady-state and unstable regions. In the unstable region, flow field variables can be updated iteratively using conventional methods; while in the steady-state region, repeated iterative solutions to update the flow field variables are unnecessary.

[0004] However, current traditional computational fluid dynamics methods require uniform iterative solutions across the entire computational domain, resulting in numerous unnecessary and repetitive calculations that limit computational efficiency. Therefore, it is necessary to develop corresponding local flow field update iterative calculation methods to avoid repeated iterative solutions in the steady-state region, thereby reducing unnecessary repetitive calculation time and improving the overall computational efficiency of flow simulation. Summary of the Invention

[0005] The purpose of this invention is to provide a method for local update and iterative calculation of chemical nonequilibrium flow fields, which addresses the problems mentioned above and aims to improve the problem of invalid and repetitive calculations in existing chemical nonequilibrium flow fields.

[0006] The technical solution adopted in this invention is as follows: A method for local update and iterative calculation of chemical nonequilibrium flow fields. This method is based on the general CFD calculation process of the NS equations for chemical nonequilibrium flow and the LU-SGS implicit time-progression method, adds a flow state identification function module, modifies the relaxation iteration process, and obtains the steady-state flow parameter distribution through time progression and relaxation iteration. The specific steps include the following:

[0007] A three-dimensional array data structure is set up to identify the flow state of all grid cells, and the initial flow state corresponding to the grid cell is marked as the steady state; all grid cells include boundary layer grid cells, etc.

[0008] During the flow field initialization process, boundary layer mesh elements are identified and their flow states are marked as unsteady.

[0009] During the update process of the right-hand term, the velocity gradient values ​​at the center of each interface of the mesh cell are calculated and stored;

[0010] Before the inviscid flux calculation module, a flow state identification function is added to update the flow state identification value of each grid cell by traversal.

[0011] The function module after modifying the inviscid flux calculation module ignores the repeated calculation process of the corresponding grid cell based on the flow state identifier value.

[0012] Furthermore, the flow status identifier value is set to ,and It has two values, 0 and 1, where 0 represents a stable state and 1 represents an unstable state; assigning values ​​to the three-dimensional array... and will As the default value.

[0013] Furthermore, during the flow field initialization process, the number of boundary layers in the boundary layer grid cells is set to... ,exist The flow field variables are updated and iterated within the boundary layer mesh cells. The flow state flag value of the boundary layer mesh element is set to .

[0014] Furthermore, during the update process of the right-hand term, the velocity gradient value is calculated as follows:

[0015] The velocity gradient was calculated using the Green-Gauss method. :

[0016] ;

[0017] in, An auxiliary control volume unit constructed around the center of the mesh cell interface. The bounding surface of the auxiliary control unit. For auxiliary control unit volume, For auxiliary control unit The rate at the center, Enclosing surface area, Enclosing surface The unit normal vector, The direction is based on The center points outward; For auxiliary control unit The numbering of each discrete bounding surface, Discrete bounding surface The rate at the center, Discrete bounding surface area, Discrete bounding surface The unit normal vector, The direction is based on The center points outwards.

[0018] Furthermore, the flow state identification function updates the flow state identification value of each grid cell using the velocity gradient value, and the specific process is as follows:

[0019] In two adjacent grid cells and At the center of the interface, calculate the dot product of the velocity gradient and the direction vector, i.e.:

[0020] ;

[0021] in, This represents the velocity gradient at the center of the interface. Let be the unit normal vector at the center of the interface, with direction from point to .

[0022] Furthermore, according to Modify the value of the grid cell and Flow status identifier value:

[0023] when , grid cell and The flow status identifier value does not need to be updated;

[0024] when This indicates that the disturbance has been propagated to the grid cell. As a result, the flow parameters change, thus affecting the mesh elements. The flow status identifier value is updated to ;

[0025] when This indicates that the disturbance has been propagated to the grid cell. The flow parameters change accordingly, thus affecting the mesh elements. The flow status identifier value is updated to ;

[0026] Based on the modified grid cells and The method for determining the flow state identifier value involves traversing the interfaces of all adjacent grid cells until all interfaces have been calculated and identified.

[0027] Furthermore, the flow status identifier value is set to... The number of grid cells is denoted as ,when Once the iterative calculations cease to change, the flow state identifier value of the grid cells is no longer updated.

[0028] Furthermore, the method for modifying the function module following the viscosity-free flux calculation module includes the following steps:

[0029] Add a query to all grid cells during the loop traversal of all grid cells. The statement for judging the identifier value, for a specified grid cell, if its flow state identifier value... If the subsequent flow field variable update iteration calculation process is ignored, the next grid cell is directly traversed.

[0030] If the flow state identifier value of the specified grid cell is specified. Then, the subsequent flow field variable update and iteration calculations will continue.

[0031] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0032] This invention identifies and updates the flow state identifiers of grid cells by traversing the grid, filters and excludes stable grid cells, and only performs flow field variable update and iteration calculations on unstable grid cells. This reduces a large number of invalid and repetitive calculations, thereby reducing resource consumption and further improving the efficiency of chemical nonequilibrium flow simulation calculations.

[0033] This invention updates the flow state identifier value of the grid cell by velocity gradient value. Based on the change of flow variables and the flow state identifier value, the chemical non-equilibrium flow field is divided into a steady state region and an unstable state region. In the unstable state region, the flow field variables can be updated and iterated according to conventional methods. However, in the steady state region, there is no need to repeatedly iterate to update the flow field variables, thereby improving the overall computational efficiency of flow simulation.

[0034] This invention reduces unnecessary repetitive calculation time by identifying local flow states and ignoring the numerical iteration process of steady-state flow, thereby improving the computational efficiency of flow simulation without affecting the computational accuracy of conventional LU-SGS implicit methods. Attached Figure Description

[0035] Figure 1 This is a flowchart of the method of the present invention;

[0036] Figure 2 The wall pressure distribution curve is calculated based on Example 7;

[0037] Figure 3 The wall heat flux distribution curve is calculated based on Example 7;

[0038] Figure 4 The residual variation curve is calculated based on Example 7 using the fully catalytic condition;

[0039] Figure 5 The residual variation curve is calculated based on Example 7 using non-catalytic conditions. Detailed Implementation

[0040] The present invention will now be described in detail with reference to the accompanying drawings.

[0041] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0042] Under high-speed flight conditions, complex chemical reaction processes such as gas dissociation, ionization, recombination, and exchange occur within the high-temperature shock layer surrounding the aircraft, which interfere with the high-speed flow and form a chemical non-equilibrium effect. Furthermore, the chemical non-equilibrium Navier-Stokes equations have prominent problems in engineering simulation applications, such as large memory resource consumption, long computation time, and long cycle. In addition, the equation system has strong numerical rigidity, which seriously affects the convergence of numerical iteration and further restricts the computational efficiency of flow simulation.

[0043] Taking a cylindrical model for simulating chemical nonequilibrium flow (using incoming flow parameters to provide initial flow field values) as an example, in the flow region outside the shock layer, the flow field remains unaffected by disturbances, and the flow variables remain almost unchanged. However, inside the shock layer and downstream of the flow, the flow field is affected by disturbances, and the flow variables continuously change. But as the flow converges, the flow variables also stabilize. Therefore, based on whether the flow variables change significantly, the chemical nonequilibrium flow field can be divided into a steady-state region and an unstable-state region. In the unstable-state region, flow field variables can be updated iteratively using conventional methods; while in the steady-state region, repeated iterative solutions to update the flow field variables are unnecessary.

[0044] Therefore, this invention proposes a local update and iteration calculation method for chemical nonequilibrium flow fields, aiming to improve the outstanding problems of large memory resource consumption, long calculation time and cycle of chemical nonequilibrium NS equations in engineering simulation applications. In addition, the equation system has strong numerical rigidity, which seriously affects the convergence of numerical iteration, thus restricting the computational efficiency of flow simulation.

[0045] like Figure 1 As shown, the local update iterative calculation method for chemical nonequilibrium flow fields is based on the general CFD calculation process of the chemical nonequilibrium flow NS equation and the LU-SGS implicit time-progression method. It adds a flow state identification function module and modifies the relaxation iteration process. The steady-state flow parameter distribution is obtained through time-progression and relaxation iteration. The specific steps include the following:

[0046] A three-dimensional array data structure is set up to identify the flow state of all grid cells, and the initial flow state corresponding to the grid cell is marked as the steady state;

[0047] During the flow field initialization process, boundary layer mesh elements are identified and their flow states are marked as unsteady.

[0048] During the update process of the right-hand term, the velocity gradient values ​​at the center of each interface of the mesh cell are calculated and stored;

[0049] Before the inviscid flux calculation module, a flow state identification function is added to update the flow state identification value of each grid cell by traversal.

[0050] The function module after modifying the inviscid flux calculation module ignores the repeated calculation process of the corresponding grid cell based on the flow state identifier value.

[0051] Example 1

[0052] One embodiment of the present invention is to set the flow state identifier value to... ,and It has two values, 0 and 1, where 0 represents a stable state and 1 represents an unstable state; assigning values ​​to the three-dimensional array... and will As the default value.

[0053] When the flow state indicator value of a certain grid cell is When the grid cell is in a stable state, subsequent flow field variable updates and iterations for that grid cell can be ignored, and the next grid cell can be directly traversed. The flow field variables for that grid cell do not need to be updated, thus avoiding wasting computational resources in that grid cell.

[0054] When the flow state indicator value of a certain grid cell is If the grid cell is in an unstable state, then the subsequent flow field variable update iteration calculations will continue.

[0055] By selectively updating and iterating the flow field variables of grid cells, computational efficiency can be improved while reducing the consumption and waste of computational resources.

[0056] Example 2

[0057] In another embodiment of the present invention, during the flow field initialization process, the number of boundary layer grid cells is set to 1. ,exist The flow field variables are updated and iterated within the boundary layer mesh cells. The flow state flag value of the boundary layer mesh element is set to .

[0058] The boundary layer mesh element is extrapolated from the wall. Layered mesh, this The flow field variables are updated iteratively using boundary layer mesh elements; this improves computational efficiency as it eliminates the need to perform calculations for each mesh element.

[0059] Example 3

[0060] In another embodiment of the present invention, the velocity gradient value is calculated during the update of the right-hand term as follows:

[0061] The velocity gradient was calculated using the Green-Gauss method. :

[0062] ;

[0063] in, An auxiliary control volume unit constructed around the center of the mesh cell interface. The bounding surface of the auxiliary control unit. For auxiliary control unit volume, For auxiliary control unit The rate at the center, Enclosing surface area, Enclosing surface The unit normal vector, The direction is based on The center points outward; For auxiliary control unit The numbering of each discrete bounding surface, Discrete bounding surface The rate at the center, Discrete bounding surface area, Discrete bounding surface The unit normal vector, The direction is based on The center points outwards.

[0064] The velocity gradient at the center of the interface between grid cells is calculated using the Green-Gaussian method. The flow state identifier of each grid cell is then updated based on the velocity gradient. The flow state identifier is then used to determine whether further calculations are needed for that grid cell.

[0065] Example 4

[0066] Another embodiment of the present invention is that the flow state identification function updates the flow state identification value of each grid cell through the velocity gradient value, and the specific process is as follows:

[0067] In two adjacent grid cells and At the center of the interface, calculate the dot product of the velocity gradient and the direction vector, i.e.:

[0068] ;

[0069] in, This represents the velocity gradient at the center of the interface. Let be the unit normal vector at the center of the interface, with direction from point to .

[0070] according to Modify the value of the grid cell and Flow status identifier value:

[0071] when , grid cell and The flow status identifier value does not need to be updated;

[0072] when This indicates that the disturbance has been propagated to the grid cell. The flow parameters change accordingly, thus affecting the mesh elements. The flow status identifier value is updated to ;

[0073] when This indicates that the disturbance has been propagated to the grid cell. The flow parameters change accordingly, thus affecting the mesh elements. The flow status identifier value is updated to ;

[0074] Based on the modified grid cells and The method of state identifier value is to traverse the interfaces of all adjacent grid cells until all interfaces have been calculated and identified.

[0075] The steady and unstable states of grid cells can be determined based on the flow state identifier value. Only the flow field variables of the unstable grid cells are updated and iterated, thereby improving computational efficiency.

[0076] Example 5

[0077] Another embodiment of the present invention is to set the flow state identifier value as... The number of grid cells is denoted as ,when Once the iterative calculations cease to change, the flow state identifier value of the grid cells is no longer updated.

[0078] Set a stopping condition for the traversal, that is, during the traversal, when the flow state flag value is... Number of grid cells It no longer increases and tends to stabilize, and Stop traversing the grid cells when the value reaches the maximum value after incrementing.

[0079] This frees up computing resources for iterative calculations of flow field variables, further improving the computational speed of flow field variable update and iteration calculations.

[0080] Example 6

[0081] Another embodiment of the present invention is that the method for modifying the function module after the viscosity-free flux calculation module includes the following steps:

[0082] Add a query to all grid cells during the loop traversal of all grid cells. The statement for judging the identifier value, for a specified grid cell, if its flow state identifier value... If the subsequent flow field variable update iteration calculation process is ignored, the next grid cell is directly traversed.

[0083] If the flow state identifier value of the specified grid cell is specified. Then, the subsequent flow field variable update and iteration calculations will continue.

[0084] First, the grid cells are divided into unsteady and steady states for easier classification and processing. For steady-state grid cells, it is not necessary to repeatedly iterate and solve for the flow field variables; for unsteady states, the flow field variables are updated and iterated according to conventional (existing) methods.

[0085] Example 7

[0086] like Figures 2-5 As shown, another embodiment of the present invention is to use high-temperature non-equilibrium CFD software, select a cylindrical model of HEG shock tunnel test, and apply the method of the present invention to carry out high-temperature high-speed chemical non-equilibrium flow simulation.

[0087] The inflow velocity was set to 4776 m / s, the temperature to 690 K, and the pressure to 687 Pa. The numerical simulation adopted the thermodynamic single-temperature 5-component Dunn-Kang chemical model, the convection flux was selected using a mixed scheme of Steger and AUSMPw+, the CFL number was set to 200, and the wall boundary was set to two conditions: isothermal fully catalytic wall and isothermal non-catalytic wall (wall temperature 300 K). The relaxation iteration step was 10,000.

[0088] Figure 2 and Figure 3 The wall pressure distribution curve and wall heat flow distribution curve obtained by the method of this embodiment and the conventional method under the condition of calculation convergence are shown. It can be seen that the pressure and heat flow curves calculated by the method of this embodiment are basically consistent with the calculation results of the conventional method. Therefore, the method used in this embodiment does not change the accuracy of the original calculation results.

[0089] Figure 4 and Figure 5 The residual variation curves obtained by the method of this embodiment and the conventional method under fully catalytic and non-catalytic conditions are presented respectively. The figures show that, with the same iteration count, the method of this embodiment reduces the computation time by approximately 16% compared to the conventional method, indicating that the method of this embodiment has higher computational efficiency and a positive and significant effect on improving the computational performance of high-temperature nonequilibrium CFD software solvers.

[0090] In summary, the method of this embodiment has good computational convergence when dealing with typical chemical nonequilibrium flow simulations, and the prediction accuracy of aerodynamic / thermal characteristics is reliable. It can achieve compatibility with and replacement of conventional methods.

[0091] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for calculating the local update and iteration of a chemical nonequilibrium flow field, characterized in that, The local update iterative calculation method for chemical nonequilibrium flow fields is based on the general CFD calculation process of the NS equations for chemical nonequilibrium flow and the LU-SGS implicit time-progression method. It adds a flow state identification function module and modifies the relaxation iteration process. The steady-state flow parameter distribution is obtained through time-progression and relaxation iteration. The specific steps include the following: A three-dimensional array data structure is set up to identify the flow state of all grid cells, and the initial flow state corresponding to the grid cell is marked as the steady state; During the flow field initialization process, boundary layer mesh elements are identified and their flow states are marked as unsteady. During the update process of the right-hand term, the velocity gradient values ​​at the center of each interface of the mesh cell are calculated and stored; Before the inviscid flux calculation module, a flow state identification function is added to update the flow state identification value of each grid cell by traversal. The function module after modifying the inviscid flux calculation module ignores the repeated calculation process of the corresponding grid cell based on the flow state identifier value.

2. The method for local update and iterative calculation of a chemical nonequilibrium flow field according to claim 1, characterized in that, The flow state identifier value is set to ,and It has two values, 0 and 1, where 0 represents a stable state and 1 represents an unstable state; assigning values ​​to the three-dimensional array... and will As the default value.

3. The method for calculating the local update and iteration of a chemical nonequilibrium flow field according to claim 2, characterized in that, During the flow field initialization process, the number of boundary layers in the boundary layer grid cells is set to... ,exist The flow field variables are updated and iterated within the boundary layer mesh cells. The flow state flag value of the boundary layer mesh element is set to .

4. The method for calculating the local update and iteration of a chemical nonequilibrium flow field according to claim 2, characterized in that, During the update of the right-hand term, the velocity gradient value is calculated as follows: The velocity gradient was calculated using the Green-Gauss method. : ； in, An auxiliary control volume unit constructed around the center of the mesh cell interface. The bounding surface of the auxiliary control unit. For auxiliary control unit volume, For auxiliary control unit The rate at the center, Enclosing surface area, Enclosing surface The unit normal vector, The direction is based on The center points outward; For auxiliary control unit The numbering of each discrete bounding surface, Discrete bounding surface The rate at the center, Discrete bounding surface area, Discrete bounding surface The unit normal vector, The direction is based on The center points outwards.

5. The method for calculating the local update and iteration of a chemical nonequilibrium flow field according to claim 2, characterized in that, The flow state identification function updates the flow state identification value of each grid cell using the velocity gradient value. The specific process is as follows: In two adjacent grid cells and At the center of the interface, calculate the dot product of the velocity gradient and the direction vector, i.e.: ； in, This represents the velocity gradient at the center of the interface. Let be the unit normal vector at the center of the interface, with direction from . point to .

6. The method for calculating the local update and iteration of a chemical nonequilibrium flow field according to claim 5, characterized in that, according to Modify the value of the grid cell and Flow status identifier value: when , grid cell and The flow status identifier value does not need to be updated; when This indicates that the disturbance has been propagated to the grid cell. As a result, the flow parameters change, thus affecting the mesh elements. The flow status identifier value is updated to ; when This indicates that the disturbance has been propagated to the grid cell. As a result, the flow parameters change, thus affecting the mesh elements. The flow status identifier value is updated to ; Based on the modified grid cells and The method for determining the flow state identifier value involves traversing the interfaces of all adjacent grid cells until all interfaces have been calculated and identified.

7. The method for calculating the local update and iteration of a chemical nonequilibrium flow field according to claim 6, characterized in that, Set the flow status identifier value The number of grid cells is denoted as ,when Once the iterative calculations cease to change, the flow state identifier value of the grid cells is no longer updated.

8. The method for calculating the local update and iteration of a chemical nonequilibrium flow field according to claim 2, characterized in that, The method for modifying the function module following the viscosity-free flux calculation module includes the following steps: Add a query to all grid cells during the loop traversal of all grid cells. The statement for judging the identifier value, for a specified grid cell, if its flow state identifier value... If the subsequent flow field variable update iteration calculation process is ignored, the next grid cell is directly traversed. If the flow state identifier value of the specified grid cell is specified. Then, the subsequent flow field variable update and iteration calculations will continue.

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