Features of the flow structure in the boundary layer of a V-shaped flying wing model with controls and a distributed electric propulsion

. The study was carried out on a model of a small-sized aircraft of the type of a flying wing, which has a V-shape in plan. The structure of the flow on the wing surface was studied depending on the angle of attack and deflection of the controls. Experiments were carried out to study the influence of the work of the distributed electric propulsion on the flow structure on the wing surface. As a result, flow patterns were obtained using the "soot-oil" visualization method for each flow regime.


Introduction
Control and manoeuvring of the aircraft occurs due to the controls.These include control rudders, ailerons, ailerons-interceptors, trimmers and a controlled stabilizer.During the flight, the controls affect the flow around the wing, so the study of their influence on the flow around the wing is a new and actual task for scientists.
With the development of aviation, the use of distributed electric propulsion is actively promoted.The reason for the promotion was the task of improving the aerodynamic characteristics of the aircraft [1][2].The basic concept behind distributed propulsion is that the thrust generating components of the aircraft are now fully integrated into the vehicle's airframe.The presented experimental study is aimed at studying the influence of the operation of a distributed electric propulsion on the vortex flow structure in the boundary layer on a V-shaped model of a small-sized aircraft with controls.
Thus, the presented work had two objectives of the study.The first goal was related to the study of the influence of the controls on the change in the flow structure on the surface of the V-shaped wing.The second goal, as mentioned above, was to obtain data on the impact on the flow around the model of a distributed electric propulsion in working condition.

Methodology and experimental setup
This work took place in several stages of preparation and conducting experiments.The research is experimental in nature and was conducted at the Khristianovich Institute of Theoretical and Applied Mechanics SB RAS, Novosibirsk in Laboratory No. 8 of "Aerophysical Studies of Subsonic flows" in a subsonic wind tunnel T-324.This wind tunnel had a closed test section with a square section of 1000x1000x4000 mm.The degree of free flow turbulence in the test section was 0.04%.As a sample of an aircraft, we chose the basic model of an aircraft of the type of a flying wing (Fig. 1).In terms of the model had a V-shape.The wingspan was 750 mm, the length of the central and end chords was 250 mm.Wing sweep angle was 34 degrees.The elevon chord was 1/3 of all model chords (83 mm).To simulate the operation of the distributed electric propulsion, it was decided to use six cased high-speed small-sized impellers (Fig. 2).The flow velocity in the trace of the turned on engine at maximum rotational speed (32800 rpm) was measured using a hot-wire anemometer.The maximum velocity value was 50 m/s.2) The location of the axis of rotation of the impellers relative to the trailing edge (Fig. 3): • The axis of rotation was flush with the trailing edge; • The axis of rotation was above the trailing edge.As mentioned above, to obtain flow patterns on the model, the method of visualization of "soot-oil" coatings was used.This method gives an idea of the structure of the flow over the studied wing surface.The resulting visualization is a time-averaged flow pattern showing the limiting streamlines on the surface.
The experiments were carried out at an oncoming flow velocity of 25 m/s (Re = 4.4•10 5 ) and a change in the angle of attack from 5⁰ to 20⁰.The controls on the wing deviated up and down at an angle equal to 30⁰.

Results on the V-shaped wing
The purpose of the research on the V-shaped model was to study the separated flow structure directly on a given type and shape of the wing, as well as to study how the controls affect the separated flow.In the first mode, at an angle of attack α = 0⁰, a flow pattern was observed similar to that of the flow around a trapezoidal model [3].Two local separation bubbles formed on the wing surface, located in front of the elevons in the central part of the model parallel to the leading edge (Fig. 4a).The upward deflection of the left elevon caused the bubble to move closer to the leading edge.The downward deviation of the right elevon made it possible to partially reattach the flow in the region of separation formation (Fig. 4b).An increase in the angle of attack to 5⁰ led to a forward shift of the separation point and the bubble itself (Fig. 5a).Deviation of the right elevon down led to a decrease in the size of the bubble on the right wing console.In the left part of the wing, the deflection of the left elevon led to the displacement of the bubble towards the leading edge, as in the case of α = 0⁰ (Fig. 5b).In the flow regime at an angle of attack α = 14⁰, the local separation bubble shifted closer to the leading edge and became thinner compared to the regime at α = 5⁰ (Fig. 6а).An attached turbulent flow was observed behind the separation bubble.The structure of the separated flow around the V-shaped model is similar to the structure of the flow near the surface on a trapezoidal wing.Deviation of the right elevon down led to narrowing of the separation zone, deviation of the left elevon upward did not entail any changes (Fig. 6b).Increasing the angle of attack to 18⁰ did not lead to changes in the flow structure near the wing surface.The flow pattern is identical to the regime at an angle of attack α = 14⁰ (Fig. 7a).A thin locally detached bubble is located near the leading edge.Deviation of the right elevon down led to a significant narrowing of the separation area (Fig. 7b).In the flow mode at an angle of attack α = 20⁰, for the first time on this model, a global stall was formed (Fig. 8a).A bistable flow regime was observed, when one of the consoles had an attached flow, and the second one had a complete separation.In this case, another flow regime was possible -a symmetrical global stall from both consoles.It should be noted that on the trapezoidal wing model, the global flow separation from the leading edge was formed at lower angles of attack, starting from α = 17⁰.The deviation of the controls did not have a significant effect on the stall (Fig. 8b).  of local separation zones.The separation structure of the flow is identical to the flow without a distributed propulsion system.With an increase in the angle of attack to α = 5⁰, the displacement of the bubble towards the leading edge was observed similar to the regime shown in Figure 5a.

The results of the influence of the distributed electric propulsion
The location of the engines above the trailing edge also did not affect the formation of locally separated bubbles (Fig. 10a and 10b).With an increase in the angle of attack to α = 14⁰ in the mode of the engines on, the separation bubbles narrowed significantly and shifted to the leading edge (Fig. 11a).An attached flow is observed over most of the surface of the model.In the vicinity of the side edges, vortex structures (tip vortices) were formed, apparently due to the influence of the flow from bottom to top.Blocking of the engines led to the formation of a bistable flow regime (Fig. 11b).On the right console, the global stall mode with reverse flow is implemented.On the left console, there is predominantly an attached flow.In the flow mode at an angle of attack α = 18⁰, the influence of the work of the distributed electric propulsion led to the formation of a stable stall flow from the leading edge (Fig. 12a).Two large-scale vortices formed and a reverse flow appeared.Thus, it can be argued that the presence of engines led to a decrease in the critical angle of attack from 20⁰ to 18⁰.Blocking the engines did not change the flow pattern (Fig. 12b).Further movement of the distributed electric propulsion to the level of the trailing edge also did not affect the global stall in the operating mode at maximum rotation and blocking speeds (Fig. 12c).
Experimental researches were carried out to study the separation structure of the flow near the leeward side of the V-shaped wing model, taking into account such factors as the angle of attack, the angles of deflection of the controls and the operation of the distributed electric propulsion at various positions relative to the trailing edge.It was found that: • on the V-shaped wing, stall occurred at significantly higher angles of attack compared to the trapezoidal model under the same initial conditions.
• when the elevons were deflected downward at α = 5⁰, the local separation area decreased by more than 50% on the wing surface.
• at low angles of attack, the distributed electric propulsion increases the zone of the attached flow and prevents the development of tip vortices; • at an angle of attack of 14 degrees, due to the presence of running engines behind the model on the surface, the zone of locally separated bubbles is significantly reduced; • the presence of the distributed electric propulsion resulted in a decrease in the critical angle of attack from 20⁰ to 18⁰, at which a global stall is formed; • the maximum influence of the distributed electric propulsion was observed in the position of the engines above the trailing edge.
The study was supported by the Russian Science Foundation grant № 22-29-00309, https://rscf.ru/en/project/22-29-00309/ The work was carried out using the Equipment Sharing Center "Mechanics" of ITAM SB RAS.

Fig. 3 .
Fig. 3.The position of the engines behind the model: (a)above the trailing edge (left); (b) -flush with the trailing edge (right).

Fig. 6 .Fig. 7 .
Fig. 6.Visualization of the flow on the V-shaped model at an angle of attack α = 14°: (a) -elevons are not deflected; (b)with an asymmetric deviation of elevons by 30⁰ (left up, right down).

Fig. 8 .
Fig. 8. Visualization of the flow on a V-shaped model at an angle of attack α = 20°: (a) -elevons are not deflected; (b)with an asymmetric deviation of elevons by 30⁰ (left up, right down).

FiguresFig. 9 .
Figures 9a and 9b show the results of visualization of the flow around a V-shaped model of a flying wing with running engines at angles of attack α = 0⁰ and α = 5⁰.The axis of rotation of the engines was located flush with the trailing edge.It was found that the operating engines did not have a significant effect on the formation

Fig. 10 .Fig. 11 .
Fig. 10.Visualization of the flow on a V-shaped model at angle of attack α=5° with engines, which are located above the trailing edge: running engines (a); blocked engines (b).

Fig. 12 .
Fig. 12. Visualization of the flow on a V-shaped model with engines at angle of attack α=18°: running engines (a) and blocked engines (b), which are located above the trailing edge; blocked engines which are located flush with the trailing edge(c).