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Numerical Simulation about the Characteristics of the Store Released from the Internal Bay in Supersonic Flow

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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 k−ε turbulence model coupled with an equation with six degrees of freedom are used to simulate the store released from the internal bay. The motion trajectory and the attitude angle of the store separation under the conditions of different centroid, velocity, height and control measures are given by the calculated result. Through analysis, the position of the centroid will affect the separation of the store, which needs to be considered in the design. Increasing the launching height is conducive to the separation of the store. If the store has an initial velocity, it can leave the internal bay more quickly and reduce the probability of collision with the wall. Cylindrical rod and slanted aft wall control measures can improve the attitude of the store and make the store fall more smoothly.
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DOI: 10.32604/cmes.2023.022694
ARTICLE
Numerical Simulation about the Characteristics of the Store Released from
the Internal Bay in Supersonic Flow
Xiaohui Cheng1,HaiqingSi
1,*,YaoLi
1and Peihong Zhang2
1College of General Aviation and Flight, Nanjing University of Aeronautics and Astronautics, Nanjing, 21100, China
2China Aerodynamics Research and Development Center, Mianyang, 621000, China
*Corresponding Author: Haiqing Si. Email: sihaiqing@126.com
Received: 21 March 2022 Accepted: 02 September 2022
ABSTRACT
To understand the inuence of the initial release conditions on the separation characteristics of the store and
improve it under high Mach number (Ma=4) ight conditions, the overset grid method and the Realizable kε
turbulence model coupled with an equation with six degrees of freedom are used to simulate the store released
from the internal bay. The motion trajectory and the attitude angle of the store separation under the conditions of
dierent centroid, velocity, height and control measures are given by the calculated result. Through analysis, the
position of the centroid will aect the separation of the store, which needs to be considered in the design. Increasing
the launching height is conducive to the separation of the store. If the store has an initial velocity, it can leave the
internal bay more quickly and reduce the probability of collision with the wall. Cylindrical rod and slanted a wall
control measures can improve the attitude of the store and make the store fall more smoothly.
KEYWORDS
Separation of the store; motion trajectory; overset grid; equation with six degrees of freedom; control measures
1Introduction
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 [13].
However, this will make the flow field between the store and the internal bay complex. In addition,
there is aerodynamic interference between the store and the internal bay when the store falls, so it is
difficult to predict the motion trajectory of the store. If the store is separated in an unsafe way, its hit
rate will be reduced, and it may even collide with the internal bay [4]. Therefore, it is necessary to study
the separation of the store.
In the study of separation characteristics, Johnson et al. [5] and Davis et al. [6] simulated the
separation of the internal storage through numerical simulation and wind tunnel experiments. It is
found that the motion trajectory of the internal store will change at different time under the same
launch conditions, which is caused by the unsteady flow field of the internal bay. Some scholars
1730 CMES, 2023, vol.136, no.2
[712] have studied the hybrid complex variable element-free Galerkin (HCVEFG) method, which
can improve computational efficiency. The research of Westmoreland [13] showed that applying
appropriate ejection force to the store is conducive to the separation. Stallings [14] studied the
separation of the store by wind tunnel experiment. The experimental results show that the presence of
hatch will affect the pitching moment of store after leaving the internal bay. For the external store, the
research of Mizrahi et al. [15,16] suggested that the rolling motion in the separation process of external
store is affected by the relation between the ejection period and the low structural frequencies of the
wing. A similarity law between the separation of a heavy store and the unsteady wind tunnel free flight
tests of air-launch rockets was derived, is can be used to the research of the separation of a heavy store
[17,18]. A modeling method based reduced order model for external store separation is proposed,
which can make high fidelity predictions for both surface pressure and shear stress distributions at
minimal computational cost [19,20].
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 f low. For the passive control method, the flow field is usually controlled by
changing the geometry of the cavity, such as slanted aft wall [21,22], adding the spoiler of different
shapes [23,24], and passive resonant absorbers [25]. The active control method is to inject energy
into the shear layer and cavity f low field, such as leading edge blowing [26,27], mass-injection [28,29],
actuators [30], micro-jets [31]. For the influence of flow control on the separation characteristics of
the store, the research of Guo et al. [32] indicated that the cylindrical spoiler and leading edge blowing
are conducive to the safe separation of the internal bay with the Ma =3.5. To improve the separation
characteristics of the internal store, Song et al. [33,34] installed a cuboid control device at the leading
edge of the internal bay.
With the further and faster development of fighter airplanes, the engineering application back-
ground 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
kεsimulation method coupled with equation with six degrees of freedom has been proved that it can
well simulate the motion trajectory of the store through the calculation example verification. Then the
influence of different initial conditions on the separation of the store and whether the control measures
can improve the separation characteristics of the store are analyzed by using a validated numerical
simulation method.
2Numerical Method
A three-dimensional compressible flow solver based on an unstructured mesh technique is used for
numerical simulation. The overset grid method and the Realizable kεturbulence model coupled with
equation with six degrees of freedom are used to calculate the motion trajectory of the store released
from the internal bay. For spatial discretization, the second-order TVD (Total Variation Diminishing)
scheme is adopted. The dual time step method is used for time discretization.
CMES, 2023, vol.136, no.2 1731
The governing equation is the Navier-Stokes equation, which is expressed in the three-dimensional
rectangular coordinate system [35] is as follows:
Q
t+(FiGi)
x+FgGg
y+(FhGh)
z=·
S(1)
In Eq. (1),Qis a vector of conserved variables, ·
Sis a vector of source terms, Fis vectors of
convective flux, Gis vectors of viscous flux, and their definition forms are as follows:
Q=EρρuρvρwT(2)
Fi=(E+P)uρuρu2+Pρvu ρwuT
Fg=(E+P)vρvρuv ρv2+PρwvT
Fh=(E+P)wρwρuw ρvw ρw2+PT
(3)
Gi=x0τxx τxy τxzT
Gg=y0τyx τyy τyzT
Gh=z0τzx τzy τzzT
(4)
x=uτxx +vτxy +wτxz +kTx
y=uτyx +vτyy +wτyz +kTy
z=uτzx +vτzy +wτzz +kTz
(5)
k=Cp
μ
Pr
(6)
In Eqs. (2)(6),ρrepresents the gas density, Prepresents the pressure, Erepresents the internal
energy, v=(u,v,w)represents the velocity in the three coordinate axis directions of the rectangular
coordinate system, respectively. τis the viscous stress tensor. kis the thermal conductivity. Cpis the
constant pressure specific heat capacity Pris the number of Prandtl. Tis the temperature. Eq. (7) is
introduced to close the equations:
P=nRT,E=P
γ1+1
2ρu2+v2+w2(7)
where the value of specific heat ratio γis 1.4. When heat conduction and body force are not considered,
the integral form of N-S equation is expressed as Eq. (8)
t
Wd+∂
(FcFv)dS =0 (8)
where ∂ is the boundary of control body .dS is the unit area of ∂.n=nx,ny,nzis the unit
normal vector. Fc,Fvrepresent convective flux and viscous f lux in integral form, respectively.
1732 CMES, 2023, vol.136, no.2
Fc=
ρV
ρuV +nxP
ρvV +nyP
ρwV +nzP
(ρE+p)V
,Fv=
0
nxτxx +nxτxy +nxτxz
nxτyx +nxτyy +nxτyz
nxτzx +nxτzy +nxτzz
nxx+nxy+nxz
(9)
Vis normal velocity of dS which is expressed as Eq. (10).
V=v·n=u·nx+v·ny+w·nz(10)
Realizable kεturbulence model has been proven to be able to simulate different types of flows,
such as rotating uniform shear flow, jet flow, boundary layer flow, mixed flow, and flow separation.
Compared with the standard kεturbulence model, the Realizable kεturbulence model has
better performance in dealing with strong streamline bending, vortex and rotation [36]. Therefore,
the Realizable kεturbulence model is more suitable for simulating the flow field of the store with
complex shapes. The transport equation of the Realizable kεis as follows:
t(ρk)+
xiρkuj=
xiμ+μt
σkk
xj+Gk+Gbρε YM+Sk(11)
t(ρε)+
xjρεuj=
xjμ+μt
σε∂ε
xj+ρC1SερC2
ε2
k+vε+C1ε
ε
kC3εGb+Sε(12)
C1=max 0.43, η
η+5,η=Sk
ε(13)
Among them, C1ε=1.44, C2=1.9, C3ε=0.09, σk=1.0, σε=1.2, the turbulent kinetic energy
generated by the velocity gradient is denoted by Gk, the turbulent kinetic energy generated by buoyancy
is denoted by Gb, and the contribution of pulsation expansion in compressible turbulence is denoted
by YM,Skand Sεdepend on the specific calculation conditions [36].
Where
μt=ρCμ
k2
ε,Cμ=1
A0+As
kU
ε
(14)
U=SijSij +
ij
ij,
ij =ij 2εijkωk,ij =ij εijk ωk(15)
where ij is the mean rate-of-rotation tensor viewed in a rotating reference frame with the angular
velocity ωk. The model of constants A0and Asare as follows:
A0=4.04, As=6cosφ,φ=1
3arccos 6W(16)
W=SijSjk Skj
S,
S=SijSij ,Sij =1
2uj
xi+ui
xj(17)
CMES, 2023, vol.136, no.2 1733
For high Mach number flow, the influence of compressibility on turbulence is reflected in YM
YM=ρε2M2(18)
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:
fx=m·
vx+vzωyvyωz
fy=m·
vy+vxωzvzωx
fz=m·
vz+vyωxvxωy(19)
Mx=·
ωxIxx ·
ωzIzx ωyωzIzz IyyωxωyIzx
My=·
ωyIyy ωxωz(Ixx Izz)ω2
xω2
zIzx
Mz=·
ωzIzz ·
ωxIzx +ωxωyIyy Ixx+ωyωzIzx
(20)
The relationship between attitude angle and angular velocity is given by Eq. (21)
·
θ=ωycos γωzsin γ
·
ϕ=ωysin γ+ωzcos γ/cos θ
·
γ=ωx+tan θωysin γ+ωzcos γ
(21)
f=fxi+fyj+fzk
M=Mxi+Myj+Mzk
ω=ωxi+ωyj+ωzk
(22)
where fand Mare the resultant force and resultant moment, respectively. mis the mass. Iis moment
of inertia. vand ωthe linear velocity and angular velocity, respectively. θ,ϕand γare the pitch angle,
yaw angle, and roll angle, respectively. From these three equations, the attitude and motion trajectory
of the store released from the internal bay can be obtained.
3Numerical Modeling
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 1040kg. The moment of inertia around the X axis is 32kg·m2,and
the moment of inertia around the Y and Z axes is 1580 kg·m2. 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 Fig. 1.
1734 CMES, 2023, vol.136, no.2
Figure 1: The geometric dimensions of the two control measures
To understand the inf luence 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 Tabl e 1.
Table 1: Detailed parameter values under different conditions
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
CMES, 2023, vol.136, no.2 1735
The overset grid includes background grid and component grid which are generated indepen-
dently. 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 ×104s
and the calculation step is 1000. The total calculation time of internal store falling is 0.25s the grid
used for calculation is shown as Fig. 2.
Figure 2: The grid used for calculation
4Model Validation and Analysis of Numerical Simulation Results
The model in the literature [37] is selected as a verification example for investigating the accuracy
of the numerical simulation method. According to the experiment condition, the store is launched by
ejection which is subjected to a vertical downward ejection force and a pitching moment. The values
of these two parameters are 53388 N and 12140 N·m, respectively. The length and the diameter of the
store are 3.38 and 0.508 m, the position of centroid is 1.42 m away from the tip of the head of the store,
and the overall mass of the store is 907.2kg. The moment of inertia around the X axis is 27kg·m2,and
the moment of inertia around the Y and Z axes is 488 kg·m2.
The numerical simulation method and mesh division are same as the above. Fig. 3 gives the attitude
angle of the store at different time. It can be seen that the pitch angle at first increases which is cause
by the ejection force. After the ejection force is removed, the pitch angle decreases due to the effect
of gravity and aerodynamic force. The calculated results are consistent with the experimental values
quite well, which also increases the reliability of the numerical method.
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.
1736 CMES, 2023, vol.136, no.2
Figure 3: Comparison of calculated results and experimental values
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. Fig. 4 compares
the attitude angles of the store at different centroid positions. The store under the two conditions has
a certain degree of pitch when it released from the internal bay. It can be found that the pitch angel
of the store whose centroid moves forward at 0–0.4 s is greater, while the pitch angle of the store with
reference centroid after 0.4 s is greater. As for the yaw angle and roll angle, there is little different
between two conditions. In general, the position of centroid will affect the separation of the store, so
it should be considered in the design.
Figure 4: Comparison of the attitude angles of the store at different centroid positions
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. Fig. 5 compares attitude angle and motion trajectory of the store with different initial
CMES, 2023, vol.136, no.2 1737
velocity in which the time interval of the store falling is 0.1 s. It can be seen the attitude angle of the
store with initial velocity =3 m/s is slightly larger, which means that the aerodynamic interference of
the flow field of the internal bay to the store does not weaken with the increase of velocity. However,
the time for the store with velocity =3 m/s to leave the flow field of the internal bay is shortened, so the
effect time of aerodynamic interference on the store will be reduced which means that the probability of
collision between the store and the wall of the internal bay is reduced. Due to the initial velocity =0m/s,
the store released by gravity will leave the flow field of the internal bay for a long time, which may
lead to large attitude and even collision with the wall of the internal bay under strong aerodynamic
interference. Therefore, the store with an initial velocity is conducive to the separation.
Figure 5: Comparison of the motion trajectory of the store with different initial velocity
The only difference between condition 1 (20 km) and condition 4 (25 km) is that launching height
of the store is different. Fig. 6 shows the comparison of attitude angles of the store at different
launching heights. It is found that the pitch angle, yaw angle and roll angle of store at launching
height of 25 km are less than that of 20 km. Compared with the altitude of 20 km, the air density at
1738 CMES, 2023, vol.136, no.2
the altitude of 25 km is relatively low, so the aerodynamic force of the store is relatively small when
falling, and the influence of the flow field of the internal bay on the store will be weakened accordingly.
Consequently, the lower the height, the worse the launch environment of the store, which may cause
unsafe separation.
Figure 6: Comparison of attitude angles of the store at different altitude
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 Fig. 7. It is concluded that two control
measures improve the separation characteristics of the store to a certain extent. Cylindrical rod and
slanted aft wall make the store fall more smoothly. Among them, the more obvious is the improvement
of pitch angle. For the “clean” internal bay, the maximum pitch angle of the store is 13°, while the
maximum pitch angle of the store with slanted aft wall and cylindrical rod are 2.5° and 2.7°. These
two control measures are to improve the flow field of the internal bay, so as to obtain a better attitude
in the process of releasing and separating the store. The control measure of slanted aft wall is better
than that of cylindrical rod.
Figure 7: (Continued)
CMES, 2023, vol.136, no.2 1739
Figure 7: Comparison of the attitude angle of the store with different control measures
1740 CMES, 2023, vol.136, no.2
5Conclusions
In this paper, the realizable kεturbulence model coupled with the equation with six degrees of
freedom is used to simulate the separation of the store under different centroid, height, initial velocity
and control measure. It can be concluded that:
(1) The position of the centroid will affect the separation of the store, which needs to be considered
in the design.
(2) 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.
(3) The store leaves the internal bay faster with an initial velocity, and reduces the probability of
collision with the wall.
(4) 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.
Funding Statement: The authors received no specific funding for this study.
Conflicts of Interest: The authors declare that they have no conf licts of interest to report regarding the
present study.
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Article
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Article
Full-text available
The improved element-free Galerkin (IEFG) method is proposed in this paper for solving 3D Helmholtz equations. The improved moving least-squares (IMLS) approximation is used to establish the trial function, and the penalty technique is used to enforce the essential boundary conditions. Thus, the final discretized equations of the IEFG method for 3D Helmholtz equations can be derived by using the corresponding Galerkin weak form. The influences of the node distribution, the weight functions, the scale parameters of the influence domain, and the penalty factors on the computational accuracy of the solutions are analyzed, and the numerical results of three examples show that the proposed method in this paper can not only enhance the computational speed of the element-free Galerkin (EFG) method but also eliminate the phenomenon of the singular matrix.
Conference Paper
View Video Presentation: https://doi.org/10.2514/6.2022-0312.vid Store separation from aircraft and spacecraft has been a critical, and in some cases problematic, issue for the aerospace industry. However, the procedures currently used for identifying potential failure conditions are both resource intensive and time consuming. A potential remedy to reducing resources and to improve turn around time in this process is the construction of a mode based reduced order model (ROM) that can be utilized for fast and accurate modeling of store separation. The objective of this study was to utilize the mode based reduced order model for two subtask; load prediction and trajectory prediction. To analyze the ROMs capability for load prediction, a preliminary study for modeling hysteresis loops of an oscillating store was pursued first. This model was based on a series of twelve simulations covering a range of two oscillation parameters. Two models were constructed, one through a series of 2-D surface mappings and another through Kriging interpolation of the surface pressure distribution. Through a comparison of ROM and CFD-simulation at three combinations of reduced frequencies and amplitudes the resulting model was able to make accurate load predictions for intermediate reduced frequencies and amplitudes in the training data range at a negligible computational cost. In an attempt to investigate the ROMs ability to predict store trajectories of intermediate conditions, three cases of store separation at Mach 0.5, 0.8, and 1.2 were considered. Prediction of trajectories at an intermediate Mach numbers can be accomplished using the ROM-based model constructed from computed store separations at Mach 0.5, 0.8, and 1.2. Results showed a close agreement between CFD and ROM trajectory predictions. The model predictions can encompass more parameters and the computational cost of the model will still remain negligible compared to the full CFD simulation.
Conference Paper
View Video Presentation: https://doi.org/10.2514/6.2021-2548.vid Successful supersonic store separation has historically been, and continues to be, a challenging task. Processes currently used for identifying potential failure conditions of store separation are both resource intensive and time consuming. It is this time consuming nature of store separation modeling which currently limits the possibility for improving separation trajectory through use of store design optimization and flight controller training. To address the challenges associated with store separation modeling, this work presents a modeling method which is capable of making high fidelity predictions for both surface pressure and shear stress distributions at minimal computational cost. To build this model, three supersonic Mach numbers were selected at 1.2, 1.4, and 1.6 for fixed wing store separation simulations. Calculated surface pressures and shear stress distributions were recorded at equal time intervals. Load distributions were analysed with Proper Orthogonal Decomposition (POD) and reduced to a subspace of 64 modes. Kriging interpolation was then used along side the equations of motion to predict store trajectories at Mach numbers of 1.3 and 1.5. Comparison between POD reduced order model (ROM) and CFD simulation show that the resulting model was able to not only make accurate trajectory predictions but also make close predictions for surface pressure and shear stress distributions. By using the ROM, computational run times were reduced from 6 hours to 1 minute while retaining a sufficiently accurate store aerodynamic load and trajectory prediction.
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The deviations of trajectory and attitude angle for internal store separation are evaluated by two wind tunnel test methods. One is the Freedrop Test (FDT), which is known as unsteady and time-dependent method of scaled model. The other is the Captive Trajectory System (CTS) test, which is usually regarded as a quasi-steady and time-averaged test technology. The result shows that there is a streamwise adverse pressure gradient on the cavity resulting in a nose-up pitching moment coefficient (>0) acting on the store model. When the initial pitch angular velocity is 0, the store exits the shear-layer with a nose-up pitch attitude causing the store to climb back towards and collide with the aircraft. However, the store passes through the shear-layer into the freestream with a nose-down pitch, which causes a successful separation event when the initial pitch angular velocity < 0. The pitch angle obtained by unsteady test method is different from that by quasi-steady test method. The time-dependent test includes the aerodynamic force induced by pitch angular velocity whereas the time-averaged method (CTS) cannot reflect the effect of unsteady aerodynamic force. The deviation of vertical displacement is not obvious for FDT and CTS test since the store has an initial vertical velocity, which is dominant for the vertical displacement. This means that the highly unsteady flow can create unpredictability in aerodynamic pitching moment of the store, which can lead to the deviation of pitch angle for internal store separation.
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Prediction of aerodynamic noise is the key technology for the internal weapon bay of supersonic aircraft. Nonlinear numerical simulation is proposed as a new method to analyze noise recently. The Non-Linear Acoustic Solver (NLAS) is employed to study the cavity noise in supersonic combined with Reynolds-averaged Navier-Stokes (RANS) equation. It is shown by the comparison between the calculated results and the WICS wind tunnel tests when the Mach number is 2, the sound pressure level of every monitoring point is in good agreement with experiments. It indicates that the NLAS method which is based on turbulence reconstruction could well simulate the cavity noise in supersonic and reduce mesh numbers. Based on this, the contribution to aerodynamic noise suppression made by active control measure such as adding jet screen at the leading edge of cavity is investigated. It is found that jet screen improves flow condition and increases cavity safety, therefore, it is suitable for suppression of aerodynamic noise under supersonic condition.
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An evaluation criterion of aircraft-store compatibility for internal weapons separation is put forward and the drop tests are conducted in a high-speed wind tunnel with the Mach 1.5. The dynamic store models are designed by light Mach scaling (LMS) and are released to the freestream by an ejection mechanism driven by the high-pressure air from two rectangular cavities with a length-to-depth ratio of 7.76 and 11.86. The three affected factors of initial pitch angle, angle-of-attack of aircraft and length-to-depth ratio are considered and analysed. The result of the drop tests shows the aircraft-store compatibility of the internal weapons decreases along with increasing AOA of carrier aircraft and even changes from compatibility to incompatibility. The flow characteristics of the cavity have a great influence on the aircraft-store compatibility of internal weapons separation. The compatibility of aircraft-store separation tends to obviously decrease when the cavity change from open to transitional cavity flow. The initial pitch angle of the store is the key factor affecting the aircraft-store compatibility. With the increase of initial pitch angle, the aircraft-store compatibility of is obviously reduced, even unsafe.
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In this paper, combining the dimension splitting method with the improved complex variable element-free Galerkin (ICVEFG) method, we present a fast ICVEFG method for three-dimensional wave propagation problems. Using the dimension splitting method, the equations of three-dimensional wave propagation problems are translated into a series of two-dimensional ones in another one-dimensional direction. The new Galerkin weak form of the dimension splitting method for three-dimensional wave propagation problems is obtained. The improved complex variable moving least-square (ICVMLS) approximation is used to obtain the shape functions, and the penalty method is used to apply the essential boundary conditions, finite difference method is used in the one-dimensional direction, then the formulae of the ICVEFG method for three-dimensional wave propagation problems are obtained. The convergence and the corresponding parameters in the ICVEFG method are discussed. Some numerical examples are given to show that the new method has higher computational precision, and can improve the computational efficiency of the conventional meshless methods for three-dimensional problems greatly.
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The influence of a flow control device on store separation from an internal cavity is examined using a wind-tunnel drop-test (WTDT) method, which is based on dynamic similarity. In the wind-tunnel test, an open cavity with a length-to-depth ratio of R=7.76 is employed. An ejection mechanism driven using two cylinders is designed using the CATIA software and is virtually simulated using automatic dynamic analysis of mechanical systems (ADAMS). The store is designed using the light Mach scaling method. Conditions involving a cuboid control device (CCD) and no control device (NCD) for the open cavity are compared. Furthermore, both the scaled and higher ejection forces are employed in the test. The results indicate that a CCD installed in front of the cavity can effectively improve the aerodynamic characteristics of the flow field, which is beneficial for the store to rapidly pass through the interferential flow field of the cavity. Owing to the pitching moment acting on the internal store, the head of the store exhibits a nose-up configuration under both the CCD and NCD conditions; however, the value of the pitching moment coefficient in the CCD case is smaller than in the NCD case.
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Based on the dimension splitting method (DSM) and the improved complex variable element-free Galerkin (ICVEFG) method, the hybrid complex variable element-free Galerkin (HCVEFG) method for three-dimensional (3D) elasticity is proposed in this paper. The equilibrium equation of a 3D elasticity is written as three sets of equations, which contains the equilibrium equations of any two directions. By introducing the DSM, a set of the equilibrium equation of a 3D elasticity problem is transformed into a series of two-dimensional (2D) problems which are solved by using the ICVEFG method. In splitting direction, the finite difference method is used for coupling those 2D discretized equations. Similarly, the discretized equation of another set of the equilibrium equation can be derived. Then by coupling those two discretized system equations, the numerical solutions of the HCVEFG method for 3D elasticity problems can be obtained. The error and the convergence are analyzed by numerical examples, and the results show that the HCVEFG method has greater computational speed.
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Test-method research was carried out to consider the separation of a heavy store, and a similarity law for unsteady wind-tunnel free-flight tests of air-launch rockets was derived. The derivation of this similarity law considers the particular characteristics of a heavy store and aerodynamic interference with the carrier and focuses on solving the following problems: the separation of a heavy store causes a real carrier to have an acceleration and a velocity that cannot be ignored; the carrier in a wind-tunnel test is vertically fixed; and a wind-tunnel test cannot meet the Froude number (Fr) similarity condition. According to the special characteristics of heavy-store separation, a similarity law for wind-tunnel free-flight tests of heavy-store separation is derived. Computational fluid dynamics simulations are used to verify the new similarity law. The results show that the new similarity law can simulate the separation trajectory more realistically than existing methods, and the linear and angular displacement errors are decreased by an order of magnitude. The experimental accuracy of the new similarity law is even higher than that of a separation trajectory satisfying Fr matching. It is demonstrated that the new similarity law can be used to carry out unsteady experimental research on the separation of a heavy store such as an air-launch rocket, and this new law provides strong support for establishing the safety boundaries of heavy-store separation.