ES2994808B2 - Fluid dynamic friction resistance reduction system - Google Patents

Description

[0001] DESCRIPTION

[0003] Fluid dynamic friction resistance reduction system

[0005] Technical field of the invention

[0007] The present invention describes a system for reducing fluid dynamic resistance in a fluid flow. This system can be used in the transportation sector (aeronautics, marine, automotive, rail), in the industrial sector (fluid transport in pipelines), or in the energy sector, for example, in wind turbines.

[0009] Background of the invention

[0011] Aerodynamic drag is the force experienced by a body moving through the air, and in particular, the component of that force in the direction of the body's relative velocity with respect to the medium. Drag is always in the opposite direction to this velocity, so it is usually said, analogous to friction, to be the force that opposes the movement of a body through the air. More generally, for a body moving within any fluid, this component is called fluid dynamic drag. In the case of water, for example, it is called hydrodynamic drag.

[0013] It is therefore one of the most relevant factors when moving inside a fluid, for example, in the transport sector, or when transporting a fluid in contact with a surface, for example, a pipe.

[0015] In the particular example of a vehicle comprising a fixed surface subjected to an external fluid flow, or main fluid flow, of velocity U, a fluid velocity profile can be defined. In a reference frame anchored to the vehicle, the fluid velocity varies from zero (on the vehicle surface) to the external velocity, U, generating a boundary layer of very small thickness (typically proportional to the inverse of the Reynolds number raised to a power, the Reynolds number being the ratio of fluid density to velocity divided by viscosity).

[0016] The high velocity gradient generates a frictional stress (force per unit area, Tw) on the car's wall. This frictional stress is the force required to decelerate the fluid from U to zero at the wall, and its value can be determined as:

[0019]

[0022] being

[0023] p: the density of the fluid,

[0024] U: the external speed and

[0025] Rex: Reynols number

[0027] This Reynolds number can be determined as:

[0030]

[0033] where

[0034] x: the longitudinal dimension and

[0035] gla fluid viscosity.

[0037] This frictional stress (Tw) on the vehicle surface can be correlated with its fluid dynamic frictional resistance. If we consider a flat plate of surface area A, with length L and width W, i.e., A = L ■ W; the frictional resistance (D) exerted by the external fluid moving at a velocity U is:

[0040]

[0043] Since this fluid dynamic friction resistance is an opposing force, the system's efficiency is reduced as it must overcome this resistance. Depending on the application, fluid dynamic friction resistance can represent between 20 and 100% of the losses. Specifically, it is estimated that this resistance accounts for 40% of the drag in an aircraft, 80-90% in ships and trains, 20% in automobiles, or 100% of the pressure losses in fluid flow in pipes. Therefore, when designing surfaces in contact with a fluid, the goal is to reduce fluid dynamic resistance.

[0045] Currently, there are solutions that aim to reduce fluid dynamic resistance through boundary layer control. Several of these also use sliding surfaces to reduce resistance.

[0047] For example, in application US1785300, Rolling Apron For Airpiane Wings, a device designed for airfoils is described, where the wing surface is replaced by a rolling carpet that promotes the magnus effect, pursuing in this case the control of lift rather than reducing friction.

[0049] A similar solution is proposed in US patent 6622973B2, Movable Surface Plane, and in US patent 9394046B2, Fluid Interface Device As Well As Apparatus And Methods Including The Same, where two movable surface planes are introduced that move independently and in opposite directions on the wing surface. The physical effect pursued by these inventions is to move one surface in the same direction as the fluid flow around the plane, and the opposite surface in a direction opposite to the fluid flow. The flow is accelerated across the surface moving in the same direction to produce lower pressure, and slowed across the surface moving in the opposite direction to produce higher pressure. Specifically, US patent 6622973B2 replaces the surface of an airfoil with a series of movable surfaces that can be actuated at will to accelerate or decelerate the fluid, thereby modifying the pressure change. The net result is a force that propels the plane towards the surface, moving in the direction of the ambient fluid flow.

[0051] The use of an actively movable roller to control the boundary layer has also been regularly used in several inventions, for example, in application US3092354, Aerodynamic System And Apparatus, which discloses a rotating cylinder that reduces air friction on a folding aircraft wing. A similar mechanism can be found in patent application US5590854A, Movable sheet for laminar flow and deicing. The invention includes a movable sheet mounted in a scroll shape on two motor-driven rollers; it is an active mechanism that aims to re-laminarize the boundary layer and remove ice layers.

[0053] Finally, there are also example solutions documented in documents, such as US patent 10583872B1, Flow rollers, or application CN102777450A, Novel resistance-reducing structure of high-speed surface; where a series of rollers are attached to the surface of automobiles. Alternatively, the Chinese patent CN108167282B, Device for reducing frictional resistance and method for reducing viscous frictional resistance of fluid by means of device, combines different spheres on the surface, pursuing the same effect as US patent 10583872B1.

[0055] Application CN1730950A, Multiple moveable wall surface drag reduction device for fluid, introduces a series of movable surfaces (in the form of multiple layers) that it claims reduce resistance in each of them by introducing lubricant between the different layers. Although the mechanism is designed for a similar purpose to the present solution, it uses a different physical concept by reducing resistance through the generation of a series of “gears” or a succession of fluid cylinders. According to this solution, the resistance reduction achieved is barely 15%.

[0057] Application US2019/0047640A1 describes an apparatus and method for reducing viscous resistance. The apparatus includes a pair of rollers attached to a support surface on the vehicle roof, a belt with a friction surface partially wrapped around the pair of rollers, such that the rollers allow the belt to rotate in response to the airflow generated around the vehicle when it is in motion. The roller pair has an axial length at least equal to the width of the belt. The roller and belt assembly is at least partially recessed with respect to the top line of the roof, and a reverse-flow cover is attached to the front end of the vehicle roof to block the backflow of air generated by the rotating belt.

[0059] Application US4502724A describes the exterior of a vehicle, for example, the roof of a land vehicle such as the cab of a trailer, semi-trailer, truck, bus, car, van, train car, or the like, wherein said exterior is provided with an exposed moving surface that moves backward relatively faster than the vehicle moves forward. As a result, the vehicle can exert less pressure on the ground, allowing it to carry more weight. In a preferred embodiment, the moving surface is formed by the exposed upper section of an endless belt guided on front and rear rollers, provided with a drive mechanism, for example, to rotate at least one of the rollers. The lower section of the belt may be concealed.

[0061] US patent 11679869B2 describes a low-drag surface for an object exposed to fluid flow, said low-drag surface being an aerodynamic surface comprising a cut-out region, and a continuously movable surface including a surface portion. The surface portion is positioned in the cut-out region such that the aerodynamic surface and the surface portion form a fluid contact surface, and said surface portion is movable relative to the aerodynamic surface.

[0063] Despite the numerous solutions currently available, some problems remain unresolved. These problems include:

[0064] - Low efficiency in reducing frictional resistance

[0065] - High cost of known solutions

[0066] - Limitation in applications

[0068] It is therefore necessary to develop new solutions that address the aforementioned problems.

[0070] Summary of the invention

[0072] As a solution to the aforementioned problems, the present invention relates to a fluid dynamic resistance reducing system.

[0074] Said reduction system comprises a fixed chassis and a sliding means.

[0076] The fixed chassis in turn comprises a first fixed wall and a second fixed wall, configured to come into contact with a fluid stream, wherein the first and second fixed walls are joined by an inner wall defining a cavity between said first and second fixed walls in the direction of the fluid stream.

[0078] On the other hand, the sliding means comprising a front cylinder and a rear cylinder, which together with a rolling belt give rise to an endless sliding belt.

[0079] Specifically, the front and rear cylinders are supported on the chassis. The front cylinder faces the first fixed wall, while the rear cylinder faces the second fixed wall. Their chassis support allows them to rotate around a central axis perpendicular to the direction of the fluid flow for which resistance reduction is desired.

[0081] The rolling belt, for its part, is a passive element that slides in the presence of the fluid flow. It is defined at its ends by the front and rear cylinders. Thus, an inner face of the belt can be distinguished, which contains the set of cylinders, that is, at least the front and rear cylinders in contact with a first and second end, respectively.

[0083] Consequently, an external face can be defined. This external face comprises an outer surface, facing the fluid flow, and an inner surface, facing the inner wall of the fixed chassis.

[0085] The sliding medium is arranged in the cavity present in the chassis, defining a fluid inlet between the second end of the rolling belt and the second fixed wall and an outlet between the first end of the rolling belt and the first fixed wall, joined by an internal channel such that, in use, it is traversed by a Couette flow in the opposite direction to the fluid flow. By replacing a rigid surface, for example, on a vehicle wall or a pipe, with a system as described comprising a sliding medium with a rolling belt, the frictional force will exert a traction function on the sliding medium.

[0087] Because initially said sliding medium is stationary, there will be no other force preventing movement, other than the rigidity of the medium itself or the resistance to movement of the bearings of the front and rear cylinders, which will cause a small loss of efficiency of the system.

[0089] As previously stated, the inner surface of the outer face of the treadmill is faced by the inner wall of the chassis. Therefore, when the treadmill gains speed (V) due to its sliding motion, a backflow is generated in the opposite direction to the flow of fluid entering the system. This flow creates a fluid dynamic resistance that tends to slow down the sliding medium.

[0090] Unlike other solutions, such as application CN1730950A, where a velocity distribution equivalent to the pattern defined by a standard or conventional boundary layer is determined, the system is configured to allow a Couette layer to flow in the system's inner channel. This Couette layer exhibits different fluid properties than a standard or conventional boundary layer of a fluid, as used in previous solutions, thus simplifying the system's mechanical design, eliminating the need for complex fluid gears, and resulting in a much more efficient solution in terms of reducing fluid dynamic friction resistance.

[0092] Therefore, the displacement of a sliding medium, in the configuration according to the present invention, generating a Couettte flow, produces a substantial reduction of the fluid dynamic friction resistance of the fluid stream.

[0094] Thus, unlike previous solutions, for an external fluid flow velocity of U = 100 m/s, with both laminar and turbulent boundary layers, fluid dynamic resistance reductions exceeding 95% are obtained for Couette layer thicknesses less than 1 cm. The reduction in fluid dynamic resistance is surprisingly high compared to previous solutions, where only 15% was achieved, providing a solution to the problems present in current solutions.

[0096] List of references used

[0098] 100. Reduction system

[0099] 1. First fixed wall

[0100] 2. Second fixed wall

[0101] 3. Interior wall

[0102] 4. Cavity

[0103] 5. Front cylinder

[0104] 6. Rear cylinder

[0105] 7. Rolling carpet

[0106] 8. First end of the tapestry

[0107] 9. Second end of the tapestry

[0108] 10. Fluid inlet

[0109] 11. Fluid outlet

[0110] 12. Interior Channel

[0111] Brief description of the figures

[0113] Figure 1 shows a perspective view of a particular embodiment of the reducing apparatus according to the present invention comprising two cylinders, a rear cylinder and a front cylinder, supporting a rolling belt of a sliding medium disposed in the cavity of the fixed chassis of a reducing system.

[0115] Figure 2 shows a profile of a particular embodiment of the reducing apparatus according to the present invention comprising two cylinders supporting a rolling belt, where the speed profile is shown

[0117] Figure 3 shows a velocity profile obtained in a numerical simulation, according to an upper boundary layer profile and a lower Couette flow profile, arranged between the lower outer face of the sliding medium and the inner wall of the fixed chassis.

[0119] Figure 4 shows the velocity obtained on a gradient from a numerical simulation of the flow near the rear cylinder, where the streamlines are detailed.

[0121] Figure 5 shows the velocity obtained in gradient from a numerical simulation of the flow near the previous cylinder, where the streamlines are detailed.

[0123] Figure 6 shows a comparison of the dimensionless velocity (v) profiles of a Couette flow and boundary layer versus the dimensionless distance (y) to the wall.

[0125] Figure 7 shows the percentage reduction of fluid dynamic resistance due to friction and the ratio of the speed of the sliding medium to the external flow as a function of the height of the inner channel (h), in meters, obtained in an embodiment of the reducing system contacted by a fluid flow with a speed of 100 m/s and kinematic viscosity 10-6 m2/s, in the case of laminar flow (a) and in the case of turbulent flow (b).

[0127] Figure 8 shows the percentage reduction of fluid dynamic resistance due to friction and the ratio of the speed of the sliding medium to the fluid stream of turbulent flow, as a function of the fluid stream speed for a generic value of the inner channel height (h) of 0.01 m and kinematic viscosity 10-6 m2/s.

[0128] Figure 9 shows the mechanical energy generated as a function of the external velocity for an embodiment of the reduction system comprising an inner channel with a height h = 0.01 m and is contacted by a fluid of kinematic viscosity 10-6 m2/s.

[0130] Figure 10 shows the detail of a computational mesh according to a particular embodiment of the reducing system where the fluid inlet and outlet have a straight inner wall.

[0132] Figure 11 shows the detail of a computational mesh according to a particular embodiment of the reducing system where the fluid inlet and outlet have a rounded inner wall.

[0134] Detailed description of the invention

[0136] As shown in Figure 1, the reduction system (100) according to the present invention comprises a fixed chassis and a sliding medium. Unlike other prior solutions, the present invention replaces standard or conventional boundary layer friction of the fluid flow on an outer surface with Couette layer friction, which is much less resistant than the standard boundary layer, thereby reducing the fluid dynamic resistance due to friction of the reduction system (100).

[0138] To this end, the fixed chassis of the system comprises in turn a first fixed wall (1) and a second fixed wall (2), configured to come into contact with a fluid stream, wherein said first and second fixed walls (1, 2) are joined by an inner wall (3) defining a cavity (4) between said first and second fixed walls (1,2) in the direction of the fluid stream.

[0140] A sliding means is arranged in said cavity (4). Said sliding means consists of at least a front cylinder (5) and a rear cylinder (6), which, together with a rolling belt (7), give rise to an endless sliding belt.

[0142] Specifically, the front cylinder (5) and the rear cylinder (6) are supported on the system chassis. The front cylinder (5) faces the first fixed wall (1), while the rear cylinder (6) faces the second fixed wall (2). Their chassis support allows them to rotate around a central axis, corresponding to the axis of revolution, positioned perpendicular to the direction of the fluid flow for which resistance reduction is desired.

[0143] The rolling belt (7) is a passive element that slides in the presence of the fluid flow. Its ends are defined by its contacts with the front cylinder (5) and rear cylinder (6). An inner face of the rolling belt (7) can be distinguished, which may optionally contain a set of cylinders, defined at its ends by the front cylinder (5) and the rear cylinder (6). That is, the set of cylinders of the sliding element may have at least one inner cylinder, positioned between the front cylinder (5) and the rear cylinder (6). Regardless of the total number of cylinders in the set, the front cylinder (5) and the rear cylinder (6) are in contact with a first end (8) and a second end (9), respectively.

[0145] Consequently, an external face can be defined. This external face comprises an outer surface, facing the fluid flow on which resistance is to be reduced, and an inner surface, facing the inner wall (3).

[0147] As previously stated, the sliding medium is arranged in the cavity (4) present in the chassis, defining a fluid inlet (10) between the second end (9) of the rolling belt (7) and the second fixed wall (2) and an outlet (11) between the first end (8) of the rolling belt (7) and the first fixed wall (1), joined by an inner channel (12), configured to, in use, be traversed by a Couette flow in the opposite direction to the fluid flow.

[0149] As can be seen in Figure 2, which shows the profile of the reducing system (100), the inner channel (12) has a height h so that it allows to be traversed by a Couette flow.

[0151] To achieve the generation of a Couette flow as described in the present invention, it is necessary to choose the appropriate configuration of the reducing system (100), which can be represented by the Reynolds h (Reh) design parameter.

[0154]

[0156] being

[0157] p: the density of the fluid,

[0158] p: the dynamic viscosity of the fluid,

[0159] V: the speed of the treadmill (7) and

[0160] h: the thickness of the inner channel (12).

[0162] This dimensionless parameter is critical when designing the reduction system (100), specifically the arrangement of the sliding medium, so that an appropriate thickness (h) of the inner channel (12) is defined, allowing the advantages in terms of fluid dynamic resistance of the solution to be obtained.

[0164] To corroborate the importance of the Reh parameter, a simulation was performed, using the StarCCm+ computational tool according to a Navier-Stokes fluid equation model, of the inner channel (12) of the reduction system (100), defining a rolling carpet (7) and the inner wall (3).

[0166] This simulation allows comparing velocity profiles for different values of Reh, and checking when they deviate from the Couette profile (linear) to a typical boundary layer profile (parabolic) with much greater resistance.

[0168] Figure 3 shows a velocity profile (vel.) obtained from a numerical simulation, based on an upper boundary layer profile and a lower Couette flow profile passing through the inner channel (12), located between the lower outer surface of the sliding medium and the inner wall of the fixed chassis. Figure 3 clearly illustrates the significant difference in fluid flow between the boundary layer (upper part) and a Couette layer (lower part). The rotation of the conveyor belt (7) is represented by arrows, indicating the direction of rotation of the outer and inner surfaces of the conveyor belt (7). The y-axis represents a normal coordinate (y) measured in meters. This represents a reduction system (100) comprising an inner channel (12) of 0.01 m and a sliding medium of 3.0 m. The x-axis shows the linear profile of the horizontal fluid velocity at a specific point within the inner channel (12).

[0170] Similarly, the fluid flow velocity at the inlet (10), as shown in Figure 4, and at the outlet (11), as shown in Figure 5, can be illustrated. The streamlines are visible in these figures. It can be seen how the fluid velocity decreases once it passes through the inlet (10) and into the inner channel (12), resulting in a velocity profile, as shown in Figure 3, of less than 5 m/s.

[0172] Figure 6 shows a comparison of the dimensionless velocity profiles of a Couette flow and a boundary layer. It can be seen that the drag force (proportional to the gradient at the wall of the velocity profile) is much lower in the Couette flow than in the flat plate, which allows for the effect of reducing fluid dynamic resistance.

[0173] Thus, according to this simulation, the numerical result obtained allows the identification of a critical Reynolds number (Reh) of approximately 1300, beyond which the velocity profile adopts a shape more similar to a conventional boundary layer, losing the fluid properties achieved by the present invention. Therefore, in one embodiment of the solution, the Reynolds number (Reh) of the flow as it passes through the inner channel (12) of the reducing system (100) is less than 1300. That is, in a preferred embodiment, the height h of the inner channel (12) is in the range of 1 to 100 mm. In this way, a sliding medium can be configured that can generate a Couette flow in said inner channel (12), achieving a reduction in fluid dynamic resistance.

[0175] Additionally, in another particular embodiment, the sliding medium may have a set of additional cylinders, i.e., at least one cylinder located between the front cylinder (5) and the rear cylinder (6). These cylinders may thus be defined as intermediate cylinders or inner cylinders. These inner cylinders may be used to maintain the tension of the conveyor belt (7) if the conveyor belt (7) has a dimension that would cause this problem.

[0177] By way of example, a configuration of an embodiment of a reducing system (100) comprising: has been considered:

[0178] - a plate or a treadmill (7) of length, L = 3 m,

[0179] - an interior channel (12) of height, h = 0.01 m, and

[0180] - an external fluid with an external velocity U = 10 m/s.

[0182] The contact between a fluid stream and a plate as described generates a frictional stress (Tw), where, defining a Couette layer, it can be determined as:

[0185]

[0188] being

[0189] p: the dynamic viscosity of the fluid,

[0190] V: the speed of the treadmill (7),

[0191] h: the inner channel thickness (12),

[0192] dp/dx: the pressure gradient to which it is subjected.

[0193] Since it is a flat plate, assuming that the pressure gradient is zero, the circumference of the cylinders is very small compared to the length of the rolling belt (7) and, therefore, its effect is negligible, the total resistance (D) exerted on the lower part of the belt of area A, through which a Couette layer flows, can be simplified as:

[0196]

[0199] Furthermore, the fluid adjacent to the upper layer of the rolling belt (7) will move at a belt speed, of value V, generating a boundary layer such that the speed varies from the value V to the speed of the external flow U. Taking a reference system anchored to a rolling belt (7) moving at a speed V, the fluid dynamic resistance (D) exerted by the external flow can be defined as:

[0201] 2

[0203]

F

[0205] being

[0206] ReL, the Reynolds number of the outside stream:

[0209]

[0212] where

[0213] L is the length of the tread plate (7).

[0215] The final equilibrium speed at which the treadmill (7) will move is determined by the balance of forces between the fluid dynamic resistance (D) of the lower part (which tends to slow the treadmill) and the fluid dynamic resistance (D) of the upper part (which tends to accelerate the treadmill), i.e., from the solution of the equation:

[0216]

[0219] According to the particular embodiment, an initial value Rei_= 3-105, and Reh=1000 are achieved. In this particular embodiment, the reduction of the fluid dynamic resistance (AD) of an external laminar flow can be determined, and estimated at a reduction greater than 60%.

[0221] For a broader scope, Figure 7 shows the percentage of resistance reduction (AD) and the ratio of the sliding medium's velocity to the external flow as a function of the inner channel height (h), in meters, obtained in an embodiment of the system contacted by a fluid flow with a velocity of 100 m/s. Two specific examples of the use of this solution are shown, where the contacting fluid flow exhibits laminar flow (a) and where the flow exhibits turbulent flow (b). Specifically, the results are shown for a 1 m² square flat plate, contacted by a flow with a kinematic viscosity of 10⁻⁶ m²/s (typical values for water or air) as a function of the Couette layer thickness, equivalent to the height h of the inner channel (12).

[0223] For an outside speed value of 100 m/s, with both laminar and turbulent boundary layers, resistance reduction values greater than 95% are obtained for Couette layer thicknesses less than 1 cm, which provides a Reh value within the critical Reynolds margins to maintain Couette flow in the inner channel (12) of the reducing system (100).

[0225] Also in Figure 8, for a turbulent boundary layer and for an inner channel (12) with a height of h = 1 cm, the percentage reduction of fluid dynamic resistance (D) as a function of the external velocity (U), as well as the percentage of the sliding medium's velocity relative to the external velocity (U), is shown. For all values of external velocity (U), the reduction of fluid dynamic resistance (D) is greater than 99%.

[0226] In calculations obtained by analytical methods and confirmed by computational fluid mechanics, reductions in fluid dynamic resistance exceeding 90% are produced. Although these numbers do not take into account the loss of efficiency due to sliding belt transmission (7), which are estimated to be around 3%, for example, using a timing belt working at a minimum torque of 20 Nm.

[0228] In one particular embodiment, the remaining resistance of the sliding medium can be used to power an electric generator. This generator is mechanically powered by the rotation of the cylinder. In this particular embodiment, the rotor would be the cylinder of the sliding medium itself.

[0230] This residual energy would be obtained from the resistance of the Couette layer multiplied by the velocity. Since the residual resistance is small, the potential energy extracted is also small. An example is shown in Figure 9, which illustrates the mechanical energy for a configuration of h = 0.01 m and a reference fluid with a kinematic viscosity of 10⁻⁶ m²/s. It can be seen that the extracted power varies cubically with the velocity, and it is possible to extract on the order of 1 W per square meter of tape.

[0232] Additionally, Figure 10 shows a detailed computational mesh for a particular embodiment of the reducing system (100). This reducing system (100) has a fluid inlet (10) and outlet (11) where the inner wall (3) is straight. Alternatively, Figure 11 shows a detailed computational mesh for a particular embodiment of the reducing system (100) where the fluid inlet (10) and outlet (12) have a rounded inner wall (3). Both embodiments produce similar results, and both embodiments of the inner wall (3) are acceptable.

[0234] Therefore, the solutions according to the present invention can be configured for various applications, for example:

[0236] 1) Application in fluid transport in pipes.

[0238] Almost 100% of the costs of transporting fluids in pipelines are due to pressure losses from friction against the walls. These losses require the use of pressure pumps that move the fluid (from higher to lower pressures). Reducing friction losses using a system like the one described in the present invention would involve reducing the size and performance of the pressure pumps, thus reducing costs. Furthermore, the specific implementation of a system connected to an electric generator would allow some of the remaining energy to be used to power the pressure pump motor, thereby reducing its consumption. The solution proposed here would be applied to pipelines, although, since the geometry is usually circular, it would require a specific study.

[0240] 2) Reduction of pressure fluid dynamic resistance. Control of detached boundary layers.

[0242] The invention has focused primarily on reducing frictional drag. However, by increasing the boundary layer speed, it would have more energy. More energetic boundary layers are less likely to separate. If the flow does not separate and remains adhered, a very positive effect would be produced in reducing pressure drag (dominant in automobiles and aircraft), also allowing for a reduction in high-lift surfaces, such as tabs or flaps, with a huge impact on the weight and efficiency of the aircraft or vehicle.

Claims (6)

1. CLAIMS 1 A fluid dynamic friction resistance reduction system for a fluid stream of velocity U, wherein said reduction system (100) comprises: - a fixed chassis comprising a first fixed wall (1) and a second fixed wall (2), configured to come into contact with a fluid stream, wherein said first and second fixed walls (1,2) are joined by an inner wall (3) defining a cavity (4) between said first and second fixed walls (1,2) in the direction of the fluid stream; and - a sliding means comprising: a front cylinder (5) and a rear cylinder (6) supported on the chassis such that the front cylinder (5) faces the first fixed wall (1) and the rear cylinder (6) faces the second fixed wall (2), being rotatable about a central axis arranged perpendicular to the direction of the fluid flow, and a passive sliding tread (7) in the presence of a fluid flow, defined at its ends by the front cylinder (5) and rear cylinder (6), wherein said tread (7) comprises: an inner face containing the rotatable front cylinder (5) and rear cylinder (6) in contact with a first end (8) and a second end (9) of the tread (7), respectively, and an outer face comprising an outer surface, facing the fluid flow, and an inner surface, facing the inner wall (3); where said sliding medium is disposed in the cavity (4), defining a fluid inlet (10) between the second end (9) of the rolling belt (7) and the second fixed wall (2) and an outlet (11) between the first end (9) of the rolling belt (7) and the first fixed wall (1), joined by an internal channel (12) of height h; and characterized in that the central axis of at least one cylinder of the sliding medium is connected to an electric generator actuated by the movement of the rolling belt (7). 2. The system according to claim 1, wherein the height h of the inner channel is in the range of 1 to 10 mm between the inner surface of the tread (7) and the inner wall (3). 3. The system according to any one of claims 1 to 2, wherein the sliding means comprises at least one inner cylinder disposed between the front cylinder (5) and the rear cylinder (6). 4. The system according to any one of claims 1 to 3, wherein the inner wall (3) of the inlet (10) or outlet (11) has a rounded profile. 5. The system according to any one of claims 1 to 3, wherein the inner wall (3) of the inlet (10) or outlet (11) has a straight profile. 6. A method for reducing the fluid dynamic friction resistance of a fluid stream of velocity U and generating electricity, employing a reduction system according to any one of claims 1 to 5, characterized in that the inner channel (12) is traversed by a Couette flow.