Design and Optimization of Strength type Negative Pressure Suction Force Pluck Port based on Tesla Valve

: An apple pluck port based on negative pressure suction force can realize contactless apple plucking and also reduce possible damage to the apple. Accordingly, in this study, a strength type pneumatic pluck port was designed on the basis of a Tesla valve. First, a low air pressure block for mechanization of the Tesla valve structure at the intersection between the main and curved air passageway was theoretically modelled and analyzed. Then, the air pressure and the flow speed distribution were analyzed for three different types of structure parameters under various distances of the Tesla pluck port from the apple; on the basis of a fluent simulation, the maximum pressure difference at both sides of the apple was also simulated. Finally, the structure parameters under an optimal negative pressure field according to the simulation analysis were proposed, and a manufactured experimental test was conducted to compare the results with the simulation. The simulation and experimental data prove that when the included angle between the main and curved air passageway of the Tesla pluck port is lower than 45°, the low air pressure block at the intersection between the main and curved air passageway of the Tesla valve affects the flow of the pluck port and extends the length of the low air pressure block. The Tesla pluck port guarantees a flow in the pipe when the pipe port diameter is 10 – 15 mm larger than the apple diameter, ensuring the negative strengthening effect of the Tesla pluck port. The experiment proves that the Tesla pluck port designed in this study exhibits a better negative pressure strengthening effect than that achieved via previously existing methods, which can strengthen the plucking effect.


Introduction
Significant manpower is required for fruit plucking; in the context of apple plucking, several domestic and overseas specialists and scholars have proposed methods to improve the plucking process. German enterprises have adopted the vibration pluck method, for which an umbrella is used for fruit collection (Lu et al.2018) .Applying this method enables rapid apple plucking from the trees; however, standardized planting is required for this method to be effective.
Most importantly, owing to the longer maturation periods of apple breeds in China, apple plucking needs to be divided into three lots, where the pluck time of each lot is 7 days, with an interval of approximately 10 days between each lot. The vibration pluck method cannot distinguish unripe fruits, which cannot be sold out, leading to economic loss. Xindong Ni proposed a cutting pluck device which can determine the degree of apple ripeness and is also suitable for other spherical fruits; .However, its revolving blades cause mechanical damage to the apple surface during plucking, also leading to economic loss (Ni et al.2018). Wang Heng designed a computer simulated moving mechanical arm.However, the direct contact plucking causes mechanical damage to the apple surface (Wang et al.2018). Yang Zhang designed a rear carrying apple plucking and collecting integral machine, which improved the plucking and collection efficiency of fruits; however, the pluck head shields the visibility of the pluck workers, further increasing the possibility of mechanical damage (Zhang et al.2020). In addition, further studies regarding apple plucking were conducted from the standpoint of discrimination of the fruits (André Klostermann 2019 ; Li et al.2016). However, these have not solved the problem of mechanical damage being caused to the fruit during the process. Aiming to resolve the aforementioned issue, which requires the consideration of plucking apples in batches and the relative mechanical damage, we consider the advantages of a negative pressure suction force for contactless plucking and achieving an even load.
Through negative pressure suction force, different suction forces can be generated accordingly. The suction force exhibits a fluid motion, which applies an even force load on the object surface in the fluid field and is suitable for plucking of fruits such as apples.
Through the negative pressure suction force, a negative pressure field should be created at the pluck port, which is impacted by factors such as the intensity of pressure and fluid speed, among others, based on the mechanical shape of the pluck port; therefore, a suitable strength-type negative pressure suction force pluck port is needed. Jiashou Yang designed a fan/pressurize level that can achieve multiple levels of pressure and improves the working stable margin of fans; however, this design is applied in a situation involving a high altitude and low Reynolds number (Yang et al.2020). Senthil Kumar Raman analyzed a reentry empty body with a flap, which affects the pneumatic dynamics; we can consider using the flap to create a negative pressure field which favors the aim of negative plucking (Senthil Kumar Raman et al.2020). Nazanin Ansarifard utilized the computational fluid dynamics (CFD) method, modified the turbine design, and achieved an optimal steady state efficiency; this CFD method and experiment can be used in this study (Nazanin Ansarifard et al.2020 (Yamabe et al. 2020;Gerland et al. 2016). In addition, scholars have utilized the limit volume method to analyze the pressure distribution of Tesla valve which use water as the fluid; however, the Reynolds coefficient of water is larger than that of air under the same conditions (de Vries et al. 2017;Zhang et al. 2007;Truong et al. 2003  that is presented in this study (Yu et al.2013). In addition, the hydrodynamics of the wing was previously analyzed, and the fluid dynamics were determined to be similar to those obtained in our study (Mohammed et al.2018 ;Orlov et al.2015).
Considering the aforementioned problems, the intensity of pressure and flow speed distribution of the inlet and outlet of the Tesla pluck port, the low air pressure block of the negative pressure peak value, and the position of the Tesla pluck port need to be analyzed to optimize the position for collection of apples by strengthening the suction force of the negative pressure pluck port. In this study, a method that uses the Fluent software is adopted to process the hydrodynamics simulation, which is combined with an actual experience, to develop a fluid field for the Tesla valve under a negative pressure suction force to obtain an improved Tesla valve structure and optimized apple space condition, and to determine an optimal collecting position for the negative pressure suction force plucking system.
1 Theoretical model of Tesla valve negative pressure pluck port

Basic structure and characteristics of Tesla valve
The Tesla valve shown in Fig. 1 was used in this study, which consists of the following: an inlet on the left, outlet on the right a main air passageway directly from the inlet to the outlet, and a curved air passageway at both sides of the main air passageway.  Table   1. In this study, a fixed flow speed negative pressure source was added at the inlet of the Tesla valve pluck port, and the generated intensity of pressure and the flow speed distributed at the outlet of the Tesla valve structure was analyzed, along with the minimum value, size and distribution of the negative pressure of the low air pressure block at the intersection between the main and curved air passageways.

Numerical model of Tesla valve
An equation that presents the motion can be deduced through the equation of the mass conservation and momentum conservation law (Wang et al.2004). The equation of mass conservation can be expressed as follows: where ρ is the density of the fluid, V is the resultant velocity vector, consisting of u, v, and w in the x, y, and z directions, respectively, and div is the calculated divergence; for a given vector α, The momentum conservation law can be obtained in the x, y, and z directions according to momentum theorem as follows: where, grad() is the calculated gradient; for a given scalar α, where ϕ is a common variable, which indicates u, v, w, and other solvable variables, Γ is the generalized diffusion coefficient, and S is the generalized source item. In consideration of solving the numeric common equation (5), a suitable given expression and boundary condition of Γ and S are needed to solve for various ϕ.
2 Simulation analysis of Tesla valve negative pressure pluck port

Grid model of Tesla valve negative pressure pluck port
In this study, the ICEM CFD software was used to divide the grids of the Tesla valve at α=45°with D1=110 mm, as shown in Fig. 2.  Table 1.

Simulation contrast between Tesla valve negative pressure pluck port and common pluck ports
The reduced diameter of the common pluck port raised the intensity of pressure, whereby a peak value of -1.56e+03 was achieved at the pluck port, and the occurrence of a wall surface turbulent flow was enabled. Fig 3(a) presents the intensity of the low air pressure block, which was between -2.74e+03 and -2.80e+03; a higher flow speed existed at the center of the pluck port, which was affected by the reduced diameter, and a peak value of 6.18e+01 was achieved at the inlet of the pluck port.
The distribution of the intensity of pressure at the Tesla valve pluck port did not change in the main air passageway and was concentrated between -1.28e+03 and -1.57e+03. A larger pressure intensity range was observed in the curved air passageway, ranging from -3.90e+02 to -3.05e+03. The peak value of the low air pressure block at the intersection between the main and curved air passageways was -6.01e+03. Considering the flow speed distribution, the flow speed of the main air passageway ranged between 4.25e+01 and 4.96e+01, whereas the curved air passageway involved a lower flow speed. The flow speed distribution at the blocks ranged from 0 to 3.19e+01, and the curved air passageway at the intersection involved a high flow speed between the main and curved air passageways. Owing to the effect of the curved air passageway fluid, a peak value of 7.09e+01 was observed for a larger low flow speed area. At the intersection of the main air passageway, the size ranged from 4.61e+01 to 4.96e+01. 3 Simulation analysis of Tesla valve negative pressure pluck port with target apples 3.1 Grid division of Tesla valve negative pressure pluck port with target apple and external air pressure field This study was divided according to the national apple grade, grade A apples (maximum diameter larger than 80 mm [27] ). The intensity of the pressure and flow speed of the Tesla negative pressure pluck port was analyzed for h values of 30 mm, 10 mm, 0 mm, and -10 mm. The grid division in the ICEM CFD, indicated in article 2 above, adopted a non-structure grid division. The apple diameter was set to 97 mm, using an external 0 type grid, which was then deleted, and the external atmosphere field was drawn with a length of 287 mm and width of 100 mm, following which the structure grid was divided. The process grid was concentrated at the outlet of the apple and pluck port, with a maximum grid diameter of 0.1, and the internal boundary at the pluck port ensured that the air flow direction was vertical to the grid and that the tail flow effect was reduced. The detailed grid division is shown in Fig. 4. Considering the Tesla valve pluck port under the condition that h was 0 mm, the intensity of pressure distribution of the pluck port at α=45° and D1=110 mm is shown in section 3.3. For the pluck port at α=30° and D1=110 mm, the intensity of the pressure distribution of the main and curved air passageways were approximately the same, which is -9.81e+04 to -1.21e+05; for the apples affected by streaming, the main and curved air passageway intersection, the rear side has a smaller area with a pressure intensity of -6.04e+04 to -1.13e+05. For the pluck port at α=30° and D1=140 mm, the intensity of pressure distribution for the main and curved air passageways were approximately the same, which is -1.44e+04 to -1.70e+04; for the apples affected by streaming, the main and curved air passageway intersection, the rear side had a smaller area with a pressure intensity of -9.18e+03 to -1.57e+04. Streaming affected the area behind the apples, both sides of the apples had a low air pressure block because it was affected by the throttle formed by the apples and pipe port; the intensity of pressure was -1.70e+04 to -2.62e+04.
Considering the three Tesla valve pluck ports under the conditions of h being -10 mm, 10 mm, 20 mm, and 30 mm, the intensity of the pressure distribution of the pluck port at α=45° is shown in section 1.3.2; and the intensity of the pressure distributions of the pluck port at α=30° and D1=110 mm, as well as D1=140 mm, are listed in Table 2.   Table 3. and D1=110 mm is smaller, and rapidly collects after passing through the apples. Comparing the port at α=30° and D1=110 mm, the high-speed streaming of the Tesla pluck port at α=30° and D1=140 mm increased, and rapidly collected after passing through the apples.
Compared to the pluck port at α=45°, the flow speed direction in the curved air passageway of the Tesla valve pluck port at α=30° is more parallel to the axis, and better affects the fluid in the main air passageway. The high-speed streaming rapidly collects at the axis of the pluck port after bypassing the apples and acted on by the fluid that flows along the curved air passageway. The flow in the pluck port after h increased, did not completely reduce the speed of the curved air passageway to high-speed streaming, and the area increased. The flow increased when D1=140 mm; the high-speed streaming flow also increased, and did not completely reduce the speed in the curved air passageway; the area of high-speed streaming increased.
Considering the aforementioned simulation analysis, the angle of the curved air passageway reduces the performance of the Tesla valve pluck port, and an increase in the pluck port inlet diameter will restrain the performance of the Tesla valve curved air passageway. Therefore, the topology structure at α=30° is better than the structure at α=45°; an optimal curved air passageway should be smaller. The inlet diameter of the pluck port should not be significantly large, and should be 10-15mm larger than the target fruits being collected.  The throttle port formed by the apple and pipe port  Table 4. The flow speed of the Tesla valve gradually 4 The negative pressure pluck port experiment of Tesla valve with target apples

Experiment of common pluck port and three type Tesla valve pluck port
In this study, an experimental pluck platform was built, in which a culvert motor was used as the power source. The Tesla valve pluck port was experimentally tested with three different topology structures. The built pluck system is shown in Fig. 9.  mm, 20 mm, 10 mm, 0 mm, and -10 mm. Fig. 11 presents an apple at h=30 mm.

Experimental analysis when apples at different position of Tesla valve pluck port
The experimental results were contrasted and analyzed because the flow speed can be more directly measured when contrasted.
Considering the Tesla valve negative pressure pluck port with different topology structures indicated in section 1.3.2, the tendency of the simulation and experimental data of the Tesla valve pluck port at α=45°and D1=110 mm are similar, as shown in Fig.   12; they are the highest when h is 10 mm. working distance for this type of Tesla valve pluck port is determined to be 10-20 mm.  As shown in Fig.12, the suction force is maximum when h=0 because a smaller flow caused the Tesla valve pluck port to be reduced, thereby reducing the suction force when h was smaller than 0 mm. The optimal working distance for this type of Tesla valve pluck port is determined to be 0-10 mm.
For the Tesla valve pluck port at α=30° and D1=140 mm, owing to the smaller pipe port diameter, a flow remains in the pluck port under a smaller h. At the same time, the intersection between the main air passageway and the curved air passageway draws back, and restrains the effect of the Tesla valve structure, as shown in Fig. 15. The experimental and simulated flow speed does not increase when h ranges between 10-30 mm; a larger slope is observed if h is smaller than 10 mm. At the same time, as shown in Fig. 13, considering the suction force of the Tesla valve pluck port at D1=140 mm, the apple loading suction force does not significantly change when h ranges between 10-30 mm because the throttle reduces after h is smaller than 10 mm, causing the curved air passageway and suction force to increase. The optimal working distance of this type of Tesla valve pluck port is determined to be smaller than -10 mm.

Results and Discussion
Considering the flow speed experiment of the Tesla pluck port having three various topology structures, the flow speed of the Tesla valve pluck port at α=45° and D1=110 mm obtained a peak value at the 10-20 mm position; the flow speed decreases when h is reduced. However, the slope decreases when h=0 mm.
The flow speed of the pluck port at D1=140 mm decreases, and aa lower slope is obtained when h is larger; the slope increase when h is reduced.
Considering the transverse three-type Tesla valve pluck port, the pluck port at D1=140 mm can generate a higher negative intensity of pressure but a smaller performance range, still not achieving the peak value when h=-10 mm. This increased the difficulty of control for the pluck system. The peak value range of the pluck port at α=45° is 10-20 mm, which presents a larger performance range but not an apparent effect.
The Tesla valve pluck port at α=30° and D1=110 mm presents a more suitable range of performance in the experiment, a better negative pressure generation effect, and an improved Tesla valve topology structure.
Considering the three types of Tesla pluck ports simulated in this study, the following can be obtained: 2) For the Tesla pluck port at α=30° and D1=110 mm, the outlet diameter of the pluck port is smaller, closer to the apple diameter, and a significantly small flow exists in the pluck port when the apple is closer to the pipe port. Considering the small impact of the curved air passageway of the Tesla valve, and a larger intensity of pressure difference at both sides of the apples, the pipe port diameter of the Tesla pluck port should be set to 10-15 mm larger than the apple diameter.
3) For the Tesla pluck port at α=30° and D1=140 mm, the outlet diameter of the pluck port is larger, the curve air passageway has a larger flow when collecting grade A fruits and a larger low air pressure area of the pipe port of the pluck port, but the effect was lower than that of the Tesla valve pluck port at D1=110 mm.

Conclusions
Aiming to create the negative pressure required for apple plucking, this study proposes a method that creates a larger intensity pressure difference at both sides of apples by the low air pressure block which affects the area of the Tesla valve. Achieving a larger suction force and determined that the pipe port diameter of the Tesla pluck port should be set to 10-15 mm larger than the apple diameter, including the angle between the main air passageway and the curve air passageway which should be smaller than 45°, a larger negative pressure suction force effect can then be obtained.
1) A negative pressure field with an intensity of negative pressure 3 times that of the common pluck port can be obtained when adopting the Tesla pluck port to process the negative pressure pluck.
2) Reducing the curved air passageway angle α effects the strength of the Tesla valve, but increasing the pluck port pipe diameter restrains the effect. In conclusion, the pipe port diameter of the Tesla pluck port should be 10~15 mm larger than the apple diameter, which includes that the angle between the main air passageway and curved air passageway should be smaller than 45°.
3) The Tesla pluck port at α=30°and D1=110 mm has a better structure, with a 10-20 mm range in which the apple center distant to the pipe port h is the best action range.
This study contrasts the size of the negative pressure field between the common pluck port and the Tesla pluck port through CFD and experimental tests, and obtained a better topology structure for the Tesla pluck port. However, this study does not consider pluck grade B fruits. Because the apple leaves and branches were left out of the pipe port, we did not process the simulation at the intensity of pressure and the flow speed distribution of an actual apple entering the pipe, which will be processed in a future study.

Availability of data and materials:
The datasets used or analysed during the current study are available from the corresponding author on reasonable request.

Competing interests:
The authors declare that they have no competing financial interests Mesh generation of negative pressure collecting port of Tesla valve with target Apple and external pressure eld Figure 5 Pressure distribution of the distance between the apple center and Tesla collector with three structural parameters of 0 mm Figure 6 Velocity distribution with the distance of 0 mm between Apple center and Tesla collector with three structural parameters Figure 7 Pressure distribution of an apple with the distance from the center to the Tesla valve collecting port h of -10 mm, 0 mm, 10 mm, and 30 mm Figure 8 Velocity distribution with the distance between the apple center and Tesla collecting port h of -10 mm,0 mm, 10 mm, and 30 mm Negative pressure suction picking system Figure 10 Measure of the ow rate at the collecting port The rst Tesla valve negative pressure collection port at different h simulation ow rate data and experimental ow rate data line chart The second type of Tesla valve negative pressure collection port at varying h simulation ow rate data and experimental ow rate data line chart Figure 15 Third type of Tesla valve negative pressure collection port at different h simulation ow rate data and experimental ow rate data line chart

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