The detached vortex isvortexOne of them is due towingThe pressure difference between the upper and lower wing surfaces leads to the transverse flow of the fluid, and the longitudinal movement of the wing relative to the fluid results in the spiral flow after leaving the wing.
The detached vortex has been widely used in aircraft to generate vortex lift since the 1970s. Its most typical application is the canard configurationCanard front wingThe generated detached vortex and the appendage vortex of the main wing form favorable interference, improve the flow field of the main wing, increase the lift of the main wing, delay the vortex breakdown, and improve the aerodynamic performance of the aircraft.The detached vortex belongs to the category of unsteady high angle of attack aerodynamics research, which has used the generalized dipole grid method, Euler equation method and N-S equation method.
The form of the detached vortex is closely related to the angle of attack and airspeed of the wingAngle of attackAs the pressure difference between the upper and lower wing surfaces increases, the strength of the detached vortex will increase, but when the angle of attack increases to a certain extent, the detached vortex will suddenly become asymmetric, or even break.
Due to the characteristics of the vortex itself, when the detached vortex flows across the upper surface of the main wing surface, it will continuously absorb the energy of the wing boundary, so that the aerodynamic pressure on the wing surface is very low, that is, the total pressure coefficient has a large negative value.This negative pressure has the effect of upward suction, which actually gives the wing a positive lift.Because this lift is completely different from the linear lift generated by the attached vortex, the lift contribution generated by the detached vortex is called nonlinear lift.The use of detached vortex lift isaerodynamicsA great achievement of development.On the one hand, the aircraft design has changed from simply absorbing the attached vortex lift to using both attached vortex lift and detached vortex lift;On the other hand, it has great guiding significance for the design of modern high-performance aircraftSupersonic aircraftA series of difficult problems that cannot be solved by classical design principles.In engineering practice, a direct product of the detached vortex lift theory is the "Concorde" supersonic airliner jointly developed by Britain and France.Although this aircraft has been criticized all the time, no one can deny that it is technically successful and has epoch-making revolutionary significance.[1]
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Application of detached vortex in canard layout
The detached vortex belongs to the category of unsteady high angle of attack aerodynamics research, which has used the generalized dipole grid method, Euler equation method and N-S equation method.However, there is basically no perfect theoretical system for the study of detached vortices above 35 degrees of attack, especially for asymmetric abrupt changes and breakages. In practical work, conclusions are still mainly drawn from wind tunnel blowing tests.
Since the 1970s, detached vortex has been widely used to generate vortex lift for aircraft, and its most typical application isCanard layoutIt forms favorable interference between the detached vortex generated by the canard front wing and the appendage vortex of the main wing, improves the flow field of the main wing, increases the lift of the main wing, delays vortex breakdown, and improves the aerodynamic performance of the aircraft.
Airplanes only realize nonlinear vortex lift, and there is basically no research and application on the asymmetry and rupture of the detached vortex. The research direction is to use the asymmetric mutation characteristics of the detached vortex to increase the aircraft heading maneuverability, so that it is possible for the aircraft nose to point horizontally in an instant. At that time, the aircraft will have 3-axis super maneuverability.
Northwestern Polytechnical UniversityTaking the slender body as the object, the numerical simulation method based on the detached vortex numerical simulation (DES) method, which is suitable for the flow field analysis of unsteady motion at large angles of attack, is developed firstly;By means of numerical simulation, the mechanism of the motion of slender body on the asymmetric vortex shedding flow at high angles of attack is studied;Analyze the lateral forceYaw、Rolling momentThe law of the influence of the air pressure on the air pressure of the missile lays a theoretical foundation for the realization of the subsonic and high angle of attack maneuvering flight of the missile.In addition, the characteristics of the influence of the arc geometry of the aircraft forebody cross section on the stability of the detached vortex were also studied;The equilibrium position of the detached vortex and the distribution of vortex intensity vary with the expansion of the wing and the angle of attack;Dynamic stability analysis of the detached vortex during the symmetrical stretching and retracting of the wings to both sides: the stability analysis of the asymmetrical detached vortex generated by the asymmetrical stretching and retracting of the wings to both sides and the influence of the stability of the detached vortex on the aerodynamic performance of the aircraft.[2]
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The use of detached vortex lift (i.e. vortex lift) is an ideological revolution in aircraft aerodynamic design.In terms of the basic flow pattern in aerodynamics, it is completely different from the classic attached flow pattern, and uses a new flow pattern that generates lift - the detached flow pattern.A new branch of aerodynamics, vortex dynamics, is formed by studying the use of vortex lift, the stability and control of vortex, and so on, with the detached flow pattern as the core.It should be said that the out of body flow pattern is just a late discovery by human beings. A large number of birds and insects in nature began to use it hundreds of millions of years ago.[1]
It is found through experiments that:sweep angleWhen the angle of attack of the thin wing is very small, the airflow will separate from the leading edge and roll into an detached vortex.The pressure at the vortex center of the detached vortex is very low, and the lift of the wing surface is increased due to the pressure difference between the upper and lower pressure.As we know, the total lift of delta wing is equal to the sum of potential flow lift and vortex lift.
The potential flow lift is calculated according to the potential flow theory.The dotted line in Figure 2 represents the total lift, while the dotted line represents the potential lift (the circle is the experimental point). The difference between the two lines is the theoretical vortex lift.It can be seen that due to the vortex lift, the delta wingLift line slopeAnd the maximum lift coefficient.If the canard with large sweep angle is closely coupled with the main wing, favorable interference will be generated, and the efficiency of the detached vortex will be higher and the vortex lift will be greater (Figure 3).When the canard is placed above the front of the main wing, the detached vortex of the front wing enters the low-pressure area on the upper surface of the main wing, which is conducive to the stability of the vortex center, delays the vortex breakdown and improves the stall angle of attack of the front wing.In addition, the detached vortex of the front wing not only induces vortex lift on the front wing, but also induces a vortex lift on the main wing when it sweeps over the upper surface of the main wing.The presence of the front wing vortex also helps to control the leading edge vortex formed on the main wing, which delays the stall of the main wing.Since the main wing is washed down by the front wing (inner wing section) on the one hand and washed up by the front wing (outer wing section) on the other hand, the total amount of downwash is reduced.Due to the existence of these favorable disturbances, the lift of close coupled canard aircraft is higher at high angles of attack, and the stall angle of attack is also larger (up to 30 degrees, while the stall angle of attack of ordinary rear tail aircraft is only 10 degrees).This is of great significance for expanding the maneuvering flight range of aircraft and improving the takeoff and landing performance of high-speed aircraft.
In the interaction between the front and rear wings, except that the downwash of the front wing to the main wing is adverse, other interference is beneficial, which makes the lift of the closely coupled canard aircraft much larger than that of the ordinary canard aircraft with the same wing area.In takeoff status, closely coupled canards can be compared to tailless aircraftDelta wing aircraftThe lift coefficient of is twice as high.
Of course, due to the large amount of down wash interference, the favorable interference is not enough to offset the unfavorable interference at small angles of attack.Even so, at a small angle of attack, the maximum lift drag ratio of a closely coupled canard aircraft is equivalent to that of a rear tail aircraft of the same class.With the increase of angle of attack, the favorable interference is gradually larger than the unfavorable interference.When the angle of attack reaches about 16 degrees, the favorable interference of the close coupled canard aircraft exceeds the unfavorable interference, and the lift coefficient of the whole aircraft is higher than the sum of the lift coefficients of the single front wing and the single main wing, which is beyond the reach of ordinary rear tail aircraft.Because for rear tail aircraft.There is also the problem that the main wing washes down the tail, and this adverse interference also increases with the increase of the angle of attack.Even if the tail also generates positive lift, its lift coefficient of the whole aircraft is always lower than the sum of the lift coefficients of the two separate airfoils.[3]
