To understand the influence of the initial release conditions on the separation characteristics of the store and improve it under high Mach number (Ma = 4) flight conditions, the overset grid method and the Realizable

In order to keep the fighter airplane smooth, reduce air resistance and radar scattering cross section, improve stealth performance, increase mobility and avoid the serious aerodynamic heating problem of the store under supersonic conditions, the store is embedded in the internal bay [

In the study of separation characteristics, Johnson et al. [

At present, the research at home and abroad mainly focuses on cavity flow control about the control devices, including passive control and active control. The internal bay is one of the typical applications of cavity flow. For the passive control method, the flow field is usually controlled by changing the geometry of the cavity, such as slanted aft wall [

With the further and faster development of fighter airplanes, the engineering application background of the separation of the store at high Mach number (Ma = 4) is wider and wider. Based on the research progress at home and abroad, the research on the store separation mainly focuses on the subsonic and transonic fields. Although several scholars have begun to study the store separation in supersonic flow, there are few researches on the store separation with Ma > 3. Whether the effect of initial launching conditions on the separation of the store at high Mach number is same as that at low Mach number and whether the control measures can improve the separation characteristics of the store at high Mach number need to be studied. In this paper, based on an unstructured grid Realizable

A three-dimensional compressible flow solver based on an unstructured mesh technique is used for numerical simulation. The overset grid method and the Realizable

The governing equation is the Navier-Stokes equation, which is expressed in the three-dimensional rectangular coordinate system [

In

In

Realizable

Among them,

Where

For high Mach number flow, the influence of compressibility on turbulence is reflected in

The motion trajectory of the store is solved by solving six-degree-of-freedom equation, in which the equation of motion of the centroid is as follows:

The relationship between attitude angle and angular velocity is given by

For the internal bay, the length the width and the depth are 5.32, 0.5, 0.5 m, respectively. The length of the store is 4.8 m, the position of centroid is 2.3 m away from the center point of the tail of the store, and the overall mass of the store is 1040 kg. The moment of inertia around the X axis is 32 kg⋅m^{2}, and the moment of inertia around the Y and Z axes is 1580 kg⋅m^{2}. Two different shape control measures are adopted to improve the separation characteristics of the store. Among them, the cylindrical rod control measure is installed on the front plate of the internal bay, and another control measure is to change the aft wall to slanted wall. The geometric dimensions of the two passive control measures are shown in

To understand the influence of centroid position, launching height, initial velocity and different control measures on the separation characteristics of store, a total of six conditions are calculated in which the Mach number of free stream flow is 4 and the angle of attack is 0°. The detailed parameter value of each condition is shown in

Condition | Ma | Altitude | Pressure | Temperature | Initial velocity | Centroid coordinate | Control measures |
---|---|---|---|---|---|---|---|

1 | 4 | 20 km | 5470 Pa | 217 K | 0 m/s | (2.65, 0.3754, 0.28) | Nothing |

2 | 4 | 20 km | 5470 Pa | 217 K | 0 m/s | (2, 0.3754, 0.28) | Nothing |

3 | 4 | 20 km | 5470 Pa | 217 K | 3 m/s | (2, 0.3754, 0.28) | Nothing |

4 | 4 | 25 km | 2540 Pa | 222 K | 0 m/s | (2.65, 0.3754, 0.28) | Nothing |

5 | 4 | 25 km | 2540 Pa | 222 K | 0 m/s | (2.65, 0.3754, 0.28) | Cylindrical rod |

6 | 4 | 25 km | 2540 Pa | 222 K | 0 m/s | (2.65, 0.3754, 0.28) | Slanted aft wall |

The overset grid includes background grid and component grid which are generated independently. Then the CFD++ software is used to combine the two sets of grids together to establish the topological relationship by “digging holes”. The grids inside the internal bay, the store and the place where the store is to fall are densified, and the total number of grids is 3.34 million. To ensure the accuracy of hole excavation and interpolation, the ratio of the size of the background grid and the grid of the store at the junction should be kept between 1–1.2. The spatial dimension of grid is 3D. The characteristic inlet/outlet boundary is adopted for the external boundary of the overall calculation domain, the internal bay and the surface of the store are set as adiabatic non slip wall boundary, and the overset boundary is adopted for the external boundary of the store. The time step is 2.5 × 10^{−4 } s and the calculation step is 1000. The total calculation time of internal store falling is 0.25 s the grid used for calculation is shown as

The model in the literature [^{2}, and the moment of inertia around the Y and Z axes is 488 kg⋅m^{2}.

The numerical simulation method and mesh division are same as the above.

The separation quality of the store can be divided into two types: safe separation and unsafe separation. Unsafe separation can be divided into two cases: one case is that the distance between the store and the internal bay decreases gradually due to the change of attitude angle and motion trajectory. Another case is the direct collision between the store and the internal bay after separation. These two situations should be avoided. The safe separation should achieve the conditions of small change of attitude angle, leave quickly from the flow field and no collision with the internal bay.

Compared with condition 1 (Reference centroid), the position of centroid has been moved horizontally forward along the X-axis by 0.65 m in the condition 2 (The position of centroid moves forward), in which the centroid of condition 1 is recorded as the reference centroid.

There are two methods for the separation of the store, one is gravity launch, the other is ejection launch. One of the differences between the two methods is that the ejected store has an initial velocity when falling.

The only difference between condition 1 (20 km) and condition 4 (25 km) is that launching height of the store is different.

The comparison of attitude angle among the cylindrical rod (condition 5) slanted aft wall (condition 6) and “clean” internal bay (condition 4) is shown as

In this paper, the realizable

The position of the centroid will affect the separation of the store, which needs to be considered in the design.

The higher the height, the smaller the air density, and the aerodynamic force on the store will become smaller. Therefore, increasing the launching height is conducive to the separation of the store.

The store leaves the internal bay faster with an initial velocity, and reduces the probability of collision with the wall.

The cylindrical rod and the slanted aft wall can improve the attitude of the store, and make it fall more smoothly. The pitch angle of the store falling can be reduced by about 10° by two control measures.

The authors received no specific funding for this study.

The authors declare that they have no conflicts of interest to report regarding the present study.