1 Experiments and numerical simulation methods
1. 1 experimental method
The experiment of the axial static pressure coefficient was performed in a model wind tunnel as shown in FIG. The experimental wind speed was set to 25 ms -1. Six kinds of working conditions including three kinds of collecting port angles, two kinds of collecting mouth throat clearances, and one kind of collecting mouth angle and clearance were simultaneously performed. The first three kinds of working conditions are no gaps in the throat of the collection mouth, and the angles are 0, 10, and 15 respectively; and the next two kinds of working conditions are that the angle of the collecting mouth is 0, the throat clearance is 22, 32 mm, and the last one is The condition was a collection mouth angle of 10 and a throat clearance of 22 mm. Pitot tube was used in the experiment to measure the static pressure and total pressure, and the transfer rack was used to coordinate the pitot tube.
1.2 Numerical Simulation Methods
In the numerical simulation, 6 kinds of working conditions corresponding to the experiment were selected for simulation. The geometric model is based on the specific structure and size of the model wind tunnel, and a part of the model wind tunnel is selected for simulation. The calculation area is composed of a contraction section, an experimental section, a collection port, and a diffusion section, as shown in FIG. In order to make the downstream flow without backflow, an extension of 1 000 mm in length was artificially added. The calculation area was meshed with commercial software Gambit. The surface mesh of the surface of the cave wall is a minimum of 5 mm and a maximum of 20 mm. In order to better solve the boundary layer of the wall, a boundary layer mesh is created on the surface. Most of the computational regions create hexahedral meshes that increase numerical accuracy. Other regions create tetrahedrons and hybrid meshes. The total number of initial grids in the entire calculation area is approximately 1.8 million. The commercial software Fluent based on the finite volume method numerically solves the unstructured grid. The solution of the turbulent flow field is based on the realizable k-two equation eddy viscosity transfer model in the framework of RANS equations. The slip-free speed boundary condition is applied to all the walls of the cave. The Fluent software's own unbalanced wall function and the realizable k-turbulent model are used to solve the turbulent flow in the calculation area. Set the same speed as the experiment at the entrance of the contraction section and set the exit as free flow. The numerical calculation first selects a more stable first-order format. After several hundred iterations, a higher-precision second-order format is used. After satisfying the set residual and the monitored physical quantity as constants, the first iteration is stopped. After encrypting the mesh according to the y+ value that characterizes the near wall dimension as a distance, continue iterating until y+ is between 30 and 200 because this area is considered to be the effective area of ​​the wall function, and the monitored physical quantity must also remain unchanged. Stop the final iteration.
2 Results Analysis and Discussion
Both the axial static pressure coefficient and the axial static pressure gradient are the physical quantities that characterize the static pressure distribution in the experimental section. According to the design acceptance criteria for this type of wind tunnel and the wind tunnel in foreign countries, the axial static pressure coefficient of the experimental section is selected as the assessment index, and its definition is shown in Formula 1. Select the axis which is located at the center of the nozzle symmetry plane and the ground height of the experimental section is 50 mm as the reference curve. On this axis, 19 measuring points were created with a spacing of 50 mm. The measuring point with zero coordinates was located at the exit of the nozzle, the measuring point with a coordinate of 900 mm was close to the entrance of the collecting port, and the measuring point with a coordinate of 500 mm was selected as the reference point.
Cp( x ) = P i- PP t - P
(1) In the formula: P i is the static pressure value of the measuring point; P
The static pressure at the reference point; Pt is the total pressure at the reference point.
2. 1 Results from different collection port angles
In order to study the influence of the collection port angle on the axial static pressure coefficient of the experimental section, the angles of the three collection ports were studied experimentally and numerically, as shown. As can be seen from the figure, experiments and numerical simulations are relatively close in terms of trends and specific values. For example, when the collector angle is 0 and the axial position is greater than 650 mm, both the experimental and numerical simulations have the same result. That is, the static pressure coefficient at the measuring point increases with the distance from the nozzle. When the axial position of the measuring point is 900 mm, the static pressure coefficient of this point is measured as 0. 103, and the static pressure coefficient of this point is 0. 104. It can be seen that the experimental and numerical simulation results are very Close, so in the study of the experimental section of the axial static pressure coefficient, experimental and numerical simulation methods can be used, experimental and numerical simulation can play a role in mutual verification.
The axial static pressure coefficient of the experimental section presents a bowl-shaped structure. The size of the static pressure coefficient near the collecting port reflects the smoothness of the distribution of the axial static pressure coefficient and determines the length of the model zone. The wind tunnel not only requires the static pressure coefficient of the model area with an axial position of 300 700 mm to be less than 0.020, but also requires that the maximum static pressure coefficient near the collecting port be less than 0. 050. As can be seen from 3, various collecting ports At the angle of view, the axial static pressure coefficient of the experimental section can meet the static pressure coefficient requirements of the model zone, but some collection port angles cannot meet the requirements of the maximum static pressure coefficient.
In addition, it can also be seen from 3 that with the increase of the angle of the collection port, the axial static pressure coefficient has a tendency to decrease significantly. Specifically, when the collecting port angle is 0 and the axial position is 900 mm, the static pressure coefficient at this point is measured to be 0.130 by experiment, and when the collecting port angle is 15, the static pressure coefficient at this point is only 0. 048, a drop of 53%.
In order to understand its intrinsic mechanism, the projection of the three-dimensional streamline in the vertical and horizontal sections of the experimental section is shown. In the vertical midsection, a strong vortex is located in front of the top of the collection port. In the horizontal section, two pairs of vortexes of different sizes are located not far in front of the collection mouth and on the front side of the collection mouth. Comparing the streamline plots of the mid-vertical cross sections between 0 and 15, it can be seen that the streamline above the main jet changes significantly.
When the collecting port angle is 0, the fluids on both sides of the collecting port are collected from the bottom to the top of the experimental section, and then flow against the main jet direction to the top of the nozzle, driven by the main jet stream, descending from top to bottom, and returning to the main jet stream. .
However, when the angle of the collecting port increases to 15, the gas flow above the collecting port also flows to the nozzle against the direction of the main jet, but it does not fill all the space above the jet of the experimental segment like 0, but it reaches a certain distance from the jet. After that, it merges with the airflow downstream from the spout and returns to the main jet zone. In addition, as can also be seen from the figure, the vortex in front of the top plate of the collection port plays a role in suppressing the curvature of the streamline, that is, it can suppress the diffusion effect of the collection port to a certain extent. This suppression effect increases with the angle of the collection port. And it's more obvious. Because of this, the larger the angle of the collecting port, the smaller the static pressure coefficient of the measuring point located at the same position in the drawing, and it can be intuitively understood that the increase of the angle of the collecting port increases the effective area of ​​the main jet and reduces the static pressure coefficient. .
2. 2 results of different collection mouth throat clearance
Both experimental and numerical simulations have found that adjustment of the throat gap in the collection mouth can also change the axial static pressure coefficient of the experimental section. The throat gap was chosen to be 22, 32 mm and compared with the static pressure coefficient of the measurement point with zero throat gap. Given two kinds of throat gaps, the static pressure coefficient at the measurement point is changed. From the figure, it can be seen that as the throat clearance increases, the axial static pressure coefficient at the measurement point also shows a significant decrease. For example, when the throat gap is 32 mm, the axial position is 900 mm, the static pressure coefficient measured at this point is only 0. 044, and when there is no throat gap, the point static pressure coefficient is 0. 103 , It can be seen that the throat clearance can greatly reduce the axial static pressure coefficient of the experimental section.
When the throat of the collection mouth has a gap, the flow line in the section of the test section and the distance of 50 mm from the ground has changed. Three pairs of vortexes of different sizes appeared on both sides of the collection mouth, of which two pairs were distributed on both sides of the side plate of the collection mouth and one pair was located not far in front of the collection mouth. The vortex position at the top of the collection port also changes. When there is no gap in the collection mouth throat, the vortex is located in front of the top of the collection mouth, and when the throat of the collection mouth is clear, the vortex is located above the collection mouth. As can be seen from the changes in the flow lines between the two, the gap in the throat of the collection mouth can also reduce the static pressure coefficient, because the air flow from both sides reaches the side wall of the experimental section, and then merges into the vortexes on both sides of the side wall. Compared with no gap, the influence area on the vortex becomes larger, and therefore the diffusion of the collection port is better suppressed. The same flow from the top of the collection port to the vortex above the collection port can also play a certain role. The inhibitory effect, but compared to both sides, the effect may be slightly worse.
2.3 Results of two superposition methods for improving the axial static pressure coefficient
It can be seen from the analysis in the previous section that changing the angle of the collecting port and increasing the gap between the throat of the collecting mouth can all play a role in smoothing the axial static pressure coefficient of the wind tunnel experimental section. In order to evaluate the effect of the superposition of the two methods, a set of experiments with a combined state (gauge angle of 10, throat gap of 22 mm) was performed. The experimental results of the combined state are given and compared with the other three states. As can be seen from the figure, at an axial position of 900 mm, the collection port angles are 0, 10, the throat gap is 22 mm, the collection port angle is 10, and the throat clearance is 22 mm. The point static pressure coefficient is 0. 103, 0. 061, 0. 057, 0. 057. It can be seen that when the angle of the collecting port is 10, the opening clearance of the throat can be used to reduce the axial static pressure coefficient of the experimental section. The effect was reduced by approximately 6.6%. However, under the conditions of the collection mouth throat clearance, adjusting the angle of the collection mouth did not significantly improve the axial static pressure coefficient of the experimental section. It should be pointed out that the above two methods are only combined once. As for the combination of other conditions, it needs to be further studied.
3 Conclusion
Two methods for improving the axial static pressure coefficient of automotive wind tunnel test sections were tested and numerically simulated. The results obtained are consistent. That is, increasing the angle of the collecting port or increasing the clearance of the throat of the collecting port helps to improve the axial static pressure coefficient of the experimental section. The larger the angle of the collecting port or the larger the gap between the throats, the more significant the effect of improving the axial static pressure coefficient.
Rubber Ball Tray,Golf Rubber Ball Tray,Commercial Golf Accessories,Everlasting Rubber Ball Tray
Yantai UVT Sports Co.,Ltd. , https://www.uvtgolf.com