CONSIDERATION OF AERODYNAMIC IMPACT IN SETTING THE MAXIMUM PERMISSIBLE SPEEDS OF HIGH-SPEED TRAIN

Purpose. Studies of the effect of aerodynamic pressure on the maximum permissible speeds of a high-speed train on the existing railway infrastructure. Methodology. The study of the magnitude and direction of the aerodynamic pressure, its effect on the maximum speeds of a high-speed train was carried out on a train model composed of axisymmetric bodies with conical forms of head and tail parts. Findings. Determined the values of the aerodynamic pressure at different distances from the train are, when the high-speed train moves at a speed of 200 km/h or more. The maximum speeds of a high-speed train are determined taking into account the state of the infrastructure of the existing railway, ensuring the safe operation of a high-speed railway. Originality. Theoretical studies of aerodynamic pressure from secondary air currents formed during the movement of high-speed trains are performed on a model of a train composed of identical axisymmetric bodies with conical forms of head and tail moving in a compressible medium. The results of the research allow the regularity of the change in aerodynamic pressure during the movement of a high-speed train. Practical value. The obtained results allow to establish: 1) the maximum permissible speeds of a high-speed train taking into account the technical condition of permanent devices and structures of the existing railway infrastructure; 2) technical parameters of individual objects and structural elements of the infrastructure of high-speed iron subjected to the effect of aerodynamic pressure for a given maximum speed of high-speed trains.


Introduction
Half a century of experience in the operation of high-speed railroads, the results of numerous theoretical and experimental studies have shown that with an increase in speed, the character of the aerodynamic field around a moving high-speed train, the magnitude and vector of air currents change dramatically. The resulting air flows have a negative impact on the environment, worsening the safe functioning of the system «high-speed railway -the environment (or surrounding objects)».
The study of air flows generated by trains at high speeds is an urgent task of ensuring safety on high-speed railways.
Conducted the experimental and theoretical research directed carried out are aimed at obtaining information on the speed, direction of the secondary air currents and aerodynamic pressure arising from the movement of high-speed trains; assessment of their impact on the railway infrastructure facilities, surrounding nature, as well as on people; Security and comfort to passengers [1][2][3][4]. According to the results of experimental studies, determined the dependences of the aerodynamic drag on the speed [5]; the interaction of a moving high-speed train with objects located along the railway, including trains moving along a parallel path [3,4,[6][7][8]; the physics of the formation of aerodynamic flows in separate parts of a moving high-speed train [5,9].
Theoretical studies performed on the model of a high-speed train as an axisymmetric body made it possible to establish the magnitude and velocity vector of secondary air currents, as well as aerodynamic effects on railway infrastructure objects and people [10].
Providing traffic safety for trains and passengers; uninterrupted operation of the entire infrastructure of high-speed railway is the main condition for the organization of high-speed and high-speed passenger train traffic. A high degree of security is usually provided at all stages of creating a high-speed highway, i.e. is laid during the design, is provided during construction and is implemented in the course of operation of the infrastructure of high-speed railways. This task is relevant in the design of high-speed train traffic on existing railways, which were designed for the maximum speed of passenger trains of 120-160 km/h.
Since, on existing lines, high-speed traffic is possible after a large-scale reconstruction and modernization of permanent facilities and structures of the existing infrastructure, when designing a high-speed traffic organization using the existing railway infrastructure, the maximum permissible speeds for a high-speed train for each facility should be set separately taking into account their technical condition.

Purpose
To ensure the safe operation of a high-speed railway, it is necessary to consider the aerodynamic impact on people and railway infrastructure objects as one of the main safety criteria for high-speed passenger train traffic, since a train moving at high speed exerts aerodynamic impact on each i -th object by the max i P value. In this case, the technical state of the i -th object allows him to perceive the impact with the maximum permissible value of peri P without reducing the safety level of movement high-speed trains.
In connection with the foregoing, in order to ensure the safe operation of a high-speed railway on all sites or structural elements of the infrastructure of the existing path, the condition max i p e r i Since, according to Bernoulli's law, the aerodynamic pressure varies directly in proportion to the speed of the air flow f V created by the movement of a high-speed train, let us consider the theoretical aspects of establishing the value of the maximum train speed ensuring the fulfillment of condition (1).

Methodology
The aerodynamic impact, strength and directivity of pressure on an object depend on the maximum speed and duration of the air flow, the spatial location, availability and proximity of the railway infrastructure objects relative to a moving high-speed train.
For each i-th object (or its structural element) of the infrastructure of the existing railway, it is possible to compute a design scheme of the effect of aerodynamic pressure on it (Fig. 1). In all calculation cases, for a known distance B from a moving high-speed train to an object and the maximum permissible impact that a given peri P object can perceive, it is necessary to determine the speed of the high-speed train max i V and hence the airflow rate f V that satisfies condition (1).
Thus, the task of determining the aerodynamic impact on object i is reduced to determining the speed of the secondary airflow fi V directly at the i-th object when the high-speed train moves at a speed max i V . As the development of earlier studies [10] the distribution of the air flow and the determination of its velocity along a moving high-speed train, we investigate on a model of a train consisting of a locomotive and 2n wagons. Locomotive and wagons are presented as an axisymmetric body with streamlined forms of the head and tail parts moving in a compressible (acoustic) medium [10].
To simplify the calculations, we assume that the locomotive and all wagons in the cross section have the same shape, i.e. Consist of a circular cylinder with the same shape of the head and tail in the form of cones (Fig. 2).
The axisymmetric wave equation of the aerodynamic field near a high-speed train consisting of a locomotive and 2n wagons is solved both for a train consisting of one single wagon [8]. ( 1 j  corresponds to the cylindrical part, 2 j  the tail part, 3 j  the head part). The propagation of an acoustic wave in the air medium can be represented by the equation (2) with the boundary conditions (3) and (5) [10]. The condition that the component along the velocity axis of the medium on the boundary of the halfspace be zero, in contrast to equation (4) [11], takes the following form To find the solution of the equation, the method of sources was used [11]. Considering the function ( , ) r z  satisfying (2), the boundary conditions (3), and (2), the solution in [10] can be represented in the form Where ( ) q z -is the power of the source distributed over the surface of the moving body within 0 ( ) For an axisymmetric body from formula (3) to [12], it can be asserted that Since the problem is symmetrical with respect to the axis z 0 , a high-speed train consists of a locomotive and 2n wagons, the equation of the surface of the body ( ) ij r f z  , as well as the power of the source from each car ( ) q z and its components can be recorded separately.
For the cylindrical part of the car: For the head and tail parts of the wagon located up to the middle of the train: For the head and tail parts of the wagon located behind the middle of the train: at For example, for a wagon 1 i  and its part 2 j  as: . Taking into account the symmetry of the problem with respect to the variable z , the power of the source from each car and its components ( ) q z , equation (3) can be represented in the following form 0 0 And consider the total potential presented in the form 1 1 ( , ) [ ( . , ), ) The function ( , , ) n x y z  satisfies the boundary condition (2), and the function , we can expand in powers of this parameter as In the sum of potentials (9), the first approximation is a function 01  that satisfies equation (6) [10] and boundary condition (2).
The components of the velocity vector of air particles can be determined by the following formulas  1 11 ( , ) ( , ) The absolute speed of the air flow constitute by the system of high-speed train cars when it moves at a steady speed at an arbitrary point M (x, y, z) can be defined as The airflow pressure can be determined from formula  

Findings
As an example, the establishment of the maximum permissible speed of a high-speed train for a protected rail crossing is considered.
In this case, the most vulnerable object to the effect of aerodynamic pressure is the duty guard on the guarded crossing. In the calculations it is assumed that a high-speed train consists of a locomotive and four wagons, i.e.  Analysis of the graphs shows that in both cases the negative pressure is greater than the positive one. The reliability of these calculations is confirmed by the results of previous experiments in the US, Russia, Sweden. The effect between the wagons space on the magnitude of the negative (suction) aerodynamic pressure is clearly visible in the graph shown in Fig. 3, b. On the railways of individual states, the movement of dual high-speed trains is practiced. With sufficient streamlining of the head and tail wagons, in places the pairing of trains produces a negative aerodynamic pressure, the value of which considerably exceeds the value of the excess pressure. Similar graphs can be constructed for other velocities and distances.
Thus, it can be argued that in order to ensure the safe operation of the railway infrastructure, it is necessary to take into account the aerodynamic flows and pressures in their independence.
Using the results of calculations, it is also possible to construct a curve for the dependence of the magnitude of aerodynamic pressure on the speed of trains and the distance to the considered point P max (Fig. 4), similarly to Fig. 3 [7]. Using these dependencies, you can set the maximum permissible speed of a high-speed train along an object. Suppose that in the considered calculation case the permissible value of the aerodynamic impact on the moving attendant is known, which according to the sanitary norms should not exceed 100 PA. In this case, the distance on which the office space is located is regulated by the C dimension in accordance with GOST 9238 and should be at least 2.45 m from the track axis. Using the curve P max shown in Fig. 4, one can find the intersection point of the lines corresponding to the aerodynamic pressure of 100 PA and a distance of 2.45 m, i.e. Train speed.
Thus, when a high-speed train moves at a maximum speed of 150 km/h, the corresponding condition (1) will be satisfied by the value of this point, i.e. safety of the shift attendant.

Originality and practical value
Theoretical studies of aerodynamic pressure from secondary air currents formed during the movement of high-speed trains are performed on a model of a train composed of identical axisymmetric bodies with conical forms of head and tail moving in a compressible medium. The results of the research allow us to establish the patterns of variation in aerodynamic pressure during the movement of a high-speed train.

Conclusions
The obtained results allow to establish: -the maximum permissible speed of a highspeed train, taking into account the technical condition of permanent devices and structures of the existing railway infrastructure; -technical parameters of individual objects and structural elements of high-speed iron infrastructure subjected to the effect of aerodynamic pressure for a given maximum speed of high-speed trains.
The proposed method can be used in the practice of designing high-speed train traffic both on existing and newly constructed railways.