Aiming at the problem that the effect of two-typical wing forms on aerodynamic characteristics of double-ellipsoidal airship, the aerodynamic characteristics of airship with X-shaped tail and inverted Y-shaped tail are studied respectively. Based on the method of Computational Fluid Dynamics (CFD), the aerodynamic characteristics of airships with the same hull shape parameters are simulated, at an angle of attack ranging from -30 to 30 degrees and angle of sideslip ranging from 0 to 20 degree. The result shows that the double-ellipsoidal airship with inverted Y-shaped tail has a certain advantage in reducing hull resistance, compared with the airship with X-shaped tail. The double-ellipsoidal airship with X-shaped tail has better ability to provide lift force and maintain hull stability, compared with the airship with inverted Yshaped tail. The simulation results provide an effective reference for aerodynamic shape design and flight control strategy of double-ellipsoidal airship.
Airship is a kind of aircraft which can fly by air buoyancy and propulsion system [
Aiming at the influence of tail airfoil on tethered balloons, Zhang Guifu [
The Reynolds number of the flow field studied in this paper is in the order of 107, so the turbulence
model is used. The main function of turbulence model is to connect the new unknowns with the average
velocity gradient. The standard model id based on the model of two transport equations to solve k and
ε. The coefficient is given by empirical formula, which has many applications, moderate calculation
amount and more data accumulation. Convergence and calculation accuracy can meet the general
engineering calculation requirements [
The eddy viscosity model of Reynolds stress is [
tij = − uu j = 2t (Sij − Snm ij / 3) − 2 k ij / 3
Where t is the eddy viscosity and Sij is the mean-velocity strain-rate tensor, is the fluid density, k is the turbulent kinetic energy, and ij is the Kronecker delta. Eddy viscosity is defined as a function of turbulent kinetic energy k and turbulent dissipation rate .
t = C f k 2 / (2)
Based on dimensional analysis, eddy viscosity is scaled by fluid density , turbulent velocity scale k2 and length scale k3/2/ . The attenuation function f is modeled by turbulent Reynolds number Ret = k 2 / ( ) . The turbulent transport equation can be expressed as turbulent energy transport equation (3) and energy dissipation transport equation (4).
(tk) + x j ( u j xki − ( + kt ) xkj ) = tij Sij − +k ( ) t + x j ( u j − ( + t ) )
x j 2 = c 1 k tij Sij − c 2 f2 k + The right end term represents the production term, dissipation term and wall term respectively. The values of the constants in the mode are as follows.
C = 0.09 c 1 = 1.45 c 2 = 1.92 The wall term is
Where us is the flow velocity parallel to the wall. The slip free boundary condition integrated to the wall is k=0 and =0.
For complex flow problems, the numerical simulation method has the advantages of low cost, short time-consuming and easy to obtain the data in the flow field. In this paper, the double ellipsoid hulls with X-shaped tail and inverted Y-shaped tail are numerically simulated by CFD flow simulation software ANSYS FLUENT, and the flow characteristics under different angles of attack and sideslip are analyzed.
The simulation models of double-ellipsoidal airship with X-shaped tail and inverted Y-shaped tail are established respectively. As shown in Figure 1 and Figure 2, the length of the hulls is 100 m and the slenderness ratio is 4. NACA0018 airfoil with relatively large thickness is selected for the tail wings. The trailing edge of the airfoil is the tangent circle of the airfoil curve at about 95% of the theoretical chord length. The chord length of the tail wing tip is 9.4 m, the sweep angle of the leading edge is 1 degree, the span height is 10.4 m, and the trailing edge of the wing root is 8.4 m from the tail of the airship.
According to the overall dimension of airship, the spatial scale of flow field calculation domain is set as 1000 m × Φ 250 m, so that the flow can be fully developed. Tetrahedral can mesh complex geometric models quickly and efficiently. Tetrahedral mesh is used to topology the computational domain, and prismatic mesh is added near the wall. It can produce continuous character points in the transition zone. Take the front end of the airship as the origin, and establish the Cartesian coordinate system as shown in the Figure 3.
The drag coefficient is the ratio of the drag on an object to the product of the air pressure and the reference area. When the angle of attack α changes in the range of -30 degrees to 30 degrees, the change of drag coefficient of double-ellipsoidal airships with X-shaped tail, inverted Y-shaped tail and without tail are shown in Figure 4. When the sideslip angle β changes in the range of 0 degrees to 20 degrees, the change of drag coefficient of double-ellipsoidal airships with X-shaped tail, inverted Y-shaped tail and without tail are shown in Figure 5. tail and inverted Y-shaped tail are about 0.05. When the angle of attack exceeds this range, the drag coefficient will increase significantly. When 0 degrees≤β≤10 degrees, the drag coefficient of airships with X-shaped tail and inverted Y-shaped tail increase at a lower rate. When β≥10 degrees, the growth rate of drag coefficient increases gradually.
The lift coefficient is the ratio of the lift force on an object to the product of the aerodynamic pressure and the reference area. When the angle of attack α changes in the range of -30 degrees to 30 degrees, the change of lift coefficient of double-ellipsoidal airships with X-shaped tail, inverted Y-shaped tail and without tail are shown in Figure 6. When the sideslip angle β changes in the range of 0 degrees to 20 degrees, the change of lift coefficient of double-ellipsoidal airships with X-shaped tail, inverted Yshaped tail and without tail are shown in Figure 7.
The pitching moment coefficient is ratio of the pitching moment received by the object to the product of aerodynamic pressure, reference area and average aerodynamic chord length. When the angle of attack α changes in the range of -30 degrees to 30 degrees, the change of pitching moment coefficient of double-ellipsoidal airships with X-shaped tail, inverted Y-shaped tail and without tail are shown in Figure 8. When the angle of attack α changes in the range of 0 degrees to 30 degrees, the pitching moment comparison of hull and tail wing of double-ellipsoidal airship is shown in Figure 9.
Through the simulation of double-ellipsoidal airships with X-shaped tail and inverted Y-shaped tail at different angles of attack and sideslip, the wind load on the hull surface under the set boundary conditions is obtained.
For double-ellipsoidal airship with X-shaped tail, the simulation results of surface wind load under the condition of angle of attack of 0 degrees, 10 degrees and 20 degrees are shown in the Figure 10, Figure 11 and Figure 12.
The surface wind load simulation is carried out for double-ellipsoidal airship with X-shaped tail with an angle of attack of 0 degrees, 10 degrees, 20 degrees and 30 degrees. The distribution of wind pressure on the upper and lower tail surfaces of the airship is shown in the Figure 16 to Figure 22.
The simulation results show that when the angle of attack is 0 degrees, the surface wind pressure of the upper and lower tail of double-ellipsoidal airship with X-shaped tail is basically the same, and the pitching moment generated by aerodynamic force is basically 0. With the increase of the angle of attack, the distribution trend of wind pressure on the upper and lower tail surfaces changes. The positive pressure region gradually moves from the front end of the tail to the lower part, and the negative pressure area gradually moves from the upper and lower sides of the tail to the upper part of the front end of the tail and the lower part of the rear end of the tail. The intensity of positive pressure and negative pressure is gradually increasing, which makes the airship subject to positive pitching torque.
Compared with double-ellipsoidal airship with X-shaped tail, the airship with inverted Y-shaped tail has certain advantages in reducing hull resistance. Compared with airship with inverted Y-shaped tail, the airship with X-shaped tail has stronger ability to provide lift and maintain hull stability.
From the perspective of flight control, the strategy of flying at a small angle of attack can not only provide a certain lift for the double-ellipsoidal airship, but also maintain its resistance and pitching moment at a low level.
Based on the research results of this paper, the combination research of different wing forms and different hull shapes can be carried out to obtain the optimal combination of aerodynamic characteristics of airship shape.