<i>y</i><sup>+</sup>值对MIRA模型气动参数计算精度的影响

摘要: 为研究量纲为1的参数y⁺值对车辆气动参数计算精度的影响, 以阶梯背MIRA模型为基础, 在保证模型网格数量与质量相近的情况下调整近壁网格尺寸, 构建不同y⁺值的流场仿真模型; 考虑到不同的湍流模型对车辆外流场仿真的y⁺值具有不同的适用范围, 选取SST κ-ω和LES两种常用的湍流模型对阶梯背MIRA模型外流场进行稳态和非稳态仿真分析; 将气动参数仿真结果与试验结果进行对比分析, 得出合适的y⁺值取值范围; 结合仿真速度云图和车身表面受力曲线分析了边界层首层网格厚度对仿真精度的影响; 建立了方背MIRA模型在2种湍流模型下的外流场仿真模型, 进行不同流速下气动参数的计算, 从而对y⁺值取值范围进行验证。研究结果表明: 针对车辆外流场数值仿真, 采用SST κ-ω模型时对应的合适平均y⁺值取值范围为20~50, 而采用LES模型时对应的合适平均y⁺值取值范围为5~10;当边界层首层近壁网格厚度过大时, 数值仿真无法准确捕捉边界层中速度梯度的变化, 造成边界层流场流动信息丢失, 而当边界层首层近壁网格厚度过小时, 边界层网格会严重畸变, 2种情况下气动参数计算误差都超过5%, 从而影响车辆外流场数值仿真精度; 根据所获得的y⁺值取值范围, 方背MIRA模型计算的气动参数误差小于5%, 说明了2种湍流模型平均y⁺值取值范围的正确性。
- 车辆工程 /
- 流体数值仿真 /
- y⁺值 /
- 湍流模型 /
- 气动参数 /
- MIRA模型 /
- 误差分析
Abstract: In order to study the influence of dimensionless parameter y⁺ value on the calculation accuracy of aerodynamic parameters of vehicles, based on the step-back MIRA model, the size of near-wall grids was adjusted under the condition that the number and quality of model grids were similar, and the flow field simulation models with different y⁺ values were constructed. Considering that different turbulence models have different applicable ranges of y⁺ values for the external flow field simulation of vehicles, two common turbulence models of shear stress transmission (SST) κ-ω and large eddy simulation (LES) were selected to simulate the steady and unsteady external flow field of the step-back MIRA model. The simulation results of aerodynamic parameter were compared with the experimental results, and the appropriate ranges of y⁺ value were obtained. Combining the velocity nephogram and the carbody surface stress curve from the flow field simulation results, the influence of grid thickness of the first layer of boundary layer on the simulation accuracy was analyzed. The external flow field simulation models for the square-back MIRA model under the two turbulence models were established, the aerodynamic parameters were calculated at different flow velocities, and the ranges of y⁺ value were verified. Analysis result shows that the appropriate average y⁺ value range of SST κ-ω model is 20-50 for the external flow field numerical simulation of vehicles, and the appropriate average y⁺ value range of LES model is 5-10. When the thickness of the first near-wall grid of boundary layer is too large, the numerical simulation can not accurately capture the change of velocity gradient in the boundary layer, which leads to the loss of flow information in the flow field of boundary layer. When the thickness of the first near-wall grid is too small, the boundary layer grid will be seriously distorted. In both cases, the calculation errors of aerodynamic parameter exceed 5%, which will affect the numerical simulation accuracy of external flow field of vehicles. According to the obtained ranges of y⁺ value, the errors of aerodynamic parameter calculated by the square-back MIRA model are less than 5%, which illustrates the correctness of ranges of average y⁺ value for the two turbulence models.
- vehicle engineering /
- fluid numerical simulation /
- y⁺ value /
- turbulence model /
- aerodynamic parameter /
- MIRA model /
- error analysis

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图 1 边界层结构

Figure 1. Structure of boundary layer

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图 2 阶梯背MIRA模型(单位: mm)

Figure 2. Step-back MIRA model (unit: mm)

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图 3 虚拟风洞几何模型

Figure 3. Geometric model of virtual wind tunnel

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图 4 外流场仿真网格模型

Figure 4. Simulation grid model of external flow field

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图 5 不同边界层首层近壁网格厚度的网格模型

Figure 5. Grid models with different thicknesses of first near-wall grid of boundary layers

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图 6 阻力系数曲线

Figure 6. Curves of drag coefficient

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图 7 阻力系数相对误差曲线

Figure 7. Relative error curves of drag coefficient

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图 8 SST κ-ω模型的车头附近流速分布

Figure 8. Flow velocity distributions around vehicle head of SST κ-ω model

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图 9 LES模型的车头附近流速分布

Figure 9. Flow velocity distributions around vehicle head of LES model

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图 10 SST κ-ω模型的整车外流场流速分布

Figure 10. Flow velocity distributions of vehicle external flow field of SST κ-ω model

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图 11 LES模型的整车外流场流速分布

Figure 11. Flow velocity distributions of vehicle external flow field of LES model

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图 12 车身表面压力曲线(对称截面)

Figure 12. Carbody surface pressure curves (symmetrical section)

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图 13 车身表面切应力曲线(对称截面)

Figure 13. Carbody surface shear stress curves (symmetrical section)

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图 14 方背MIRA模型

Figure 14. Square-back MIRA model

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表 1 SST κ-ω模型气动参数仿真结果

Table 1. Simulation results of aerodynamic parameters of SST κ-ω model

首层网格厚度/mm	平均y⁺值	气动阻力/N			阻力系数
首层网格厚度/mm	平均y⁺值	压差阻力	摩擦阻力	总阻力	阻力系数
0.05	2	32.03	3.86	35.89	0.303 7
0.10	5	32.76	4.11	36.87	0.311 9
0.30	9	32.89	4.28	37.17	0.314 5
0.50	20	33.58	4.27	37.85	0.320 3
1.00	30	33.24	4.48	37.72	0.319 1
2.00	50	33.07	4.64	37.71	0.319 0
5.00	300	30.51	4.49	35.01	0.296 2
10.00	500	26.93	3.96	30.89	0.261 3
15.00	700	27.03	3.72	30.75	0.260 2
20.00	900	27.61	3.47	31.08	0.263 0
40.00	1 500	29.87	3.28	33.15	0.280 5

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表 2 LES模型气动参数仿真结果

Table 2. Simulation results of aerodynamic parameters of LES model

首层网格厚度/mm	平均y⁺值	气动阻力/N			阻力系数
首层网格厚度/mm	平均y⁺值	压差阻力	摩擦阻力	总阻力	阻力系数
0.05	2	32.92	1.98	34.90	0.295 2
0.10	5	33.30	3.56	36.87	0.311 9
0.30	9	33.93	4.31	38.24	0.323 5
0.50	20	35.60	3.81	39.41	0.333 4
1.00	30	36.77	3.95	40.72	0.344 4
2.00	50	37.44	3.68	41.12	0.347 9
5.00	300	38.86	3.62	42.48	0.359 4
10.00	500	35.84	3.75	39.59	0.334 9
15.00	700	35.29	3.72	39.01	0.330 0
20.00	900	31.17	3.82	34.99	0.296 0
40.00	1 500	29.87	3.88	33.75	0.285 5

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表 3 SST κ-ω模型仿真结果(方背MIRA)

Table 3. Simulation result of SST κ-ω model (square-back MIRA)

流速/ (m·s^-1)	总阻力/N	阻力系数仿真结果	阻力系数试验结果	误差/%
24.88	24.88	0.389 7	0.380 9	2.30
30.10	30.10	0.387 4	0.381 0	1.70
34.77	34.77	0.393 0	0.383 3	2.53

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表 4 LES模型仿真结果(方背MIRA)

Table 4. Simulation result of LES model (square-back MIRA)

流速/ (m·s^-1)	总阻力/N	阻力系数仿真结果	阻力系数试验结果	误差/%
24.88	28.99	0.363 9	0.380 9	4.46
30.10	45.48	0.390 2	0.381 0	2.41
34.77	61.84	0.397 5	0.383 3	3.71

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