Gearbox Sloshing effect

Introduction to Gearbox :

• A gearbox is used as a transmission system in vehicles. The gearbox is a mechanical method of transferring energy from one device to another and is used to increase torque while reducing speed.
• In the most basic sense, a gearbox functions like any system of gears; it alters torque and speed between a driving device like a motor and a load.
• Oil splash lubrication is often used for gearboxes. This method is also referred to as an oil bath, because it uses a reservoir is filled (or partially filled) with oil. As the gears rotate, they dip into this oil bath and splash the oil onto the other gears and bearings.
• Adequately lubricating these elements of your industrial gearbox will result in mechanical efficiency, reliability, low maintenance, and a long equipment life span

• Generally the gearboxes are filled with lubricants upto a desired level, so that as the gears rotate it takes up the lubricant to spread all around the gears. By this way all the gears are lubricated.

Geometry Used for Simulation :

• Most commonly a gearbox consists of number of gears with different sizes connected to each other using shafts. But to simplify our calculations, we are considering 2 gears of the same size rotating in opposite directions.

• As we are concentrated in the spread of lubricant over the gear we can do simulation in 2 Dimensional plane for faster calculations. We can consider the plane view on the XY plane for the simulation.

• We can create a region in this 2D plane and fill the region with respective lubricant and can simulate the flow as both the gears start to rotate.

Meshing :

• The base line mesh consists of quadratic elements, which may not be advisable to use in case of simulation involving motions.
• Thus we change the meshing methods to triangular mesh which is suitable for motion simulations.
• The element size is taken to be 30 mm for the motion to be free with proximity curvatures are captured to increase the accuracy near the teeth of the gears.
• The values considered to delicate the mesh are given below.

Setup & Simulation :

• In this project we are simulating and comparing four cases. The fluid region & the fluid material is going to vary in each case. This can be done by changing the settings in fluent.

Case 1: 20% Immersion, Fluid - Engine oil.

Case 2: 30% Immersion, Fluid- Engine oil.

Case 3: 20% Immersion,Fluid-n-heptane (c7h16).

Case 4: 30% Immersion,Fluid-n-heptane (c7h16).

• Let us see how the fluid lubricant flows between the motion of the gears. By further simulations & comparisons we can understand the characteristics of differnt fluid materials & fluid levels.
• Each case can be varied by just changing the fluid material & adjusting the coordinates of the fluid level region.
• Here we will be using Two different fluids, Thus select the respective fluids from the fluent database.

• The basic phase 1 fluid is air. We can change the phase 2 fluid for the respective cases from the multiphase dialogue box.

• The region of fluid levels are marked from Domain - Adopt - Adoption Control. We are asked to fill 20% & 30% of the whole region for the fluid lubricant. This varies for each of the cases. Thus we have to create 2 different regions and choose the appropriate region for the respective case to be simulated.

20% of the Region Coordinates :

30% of the Region Coordinates :

User Defined Function :

• To specify the rotating motion of the gears we are uploading an User Defined Function into the fluent. By loading the UDF into the server we can access the calculations specified in the UDF in the fluent through visual C++.
• These functions here are specified to rotate a body along an axis in left or right direction.

• After loading this function, we can specify the rotation of the gears around their center of gravity points.
• We will be rotating the left gear in the left direction which means right gear should be rotating in right direction.
• The center of gravity for left gear is (0,0) & for right gear is (0.115,0).

• After assigning the motions to the respective gears, we can simulate the flow.
• But we will get an error of negetive cell volume which does not allow for further calculations.
• To neglect the error we can adjust the values in the Dynamic mesh. The smoothing & Remeshing values can help us to adjust the update of the meshes after rotation. The values used for the simulation is shown in the figure.

• By Applying the conditions mentioned above in the fluent, we are able to get our results for all the four cases.
• These results can be compared for further conclusions.

Case 1: 20% Immersion, Fluid - Engine oil.

Case 2: 30% Immersion, Fluid- Engine oil.

Case 3: 20% Immersion,Fluid-n-heptane (c7h16).

Case 4: 30% Immersion,Fluid-n-heptane (c7h16).

Conclusion on Comparison :

• By increasing the fluid level from 20% to 30%, the fluid volume increases. Thus the fluid reaches more area and covers a larger amount of gear area.
• Even at the start of the rotation the gears tend to carry more fluid if the fluid level is higher as the height of immerssion of the gear in the fluid is greater.
• In the case of 20% fluid level, As the gaps around the gears are enough for the fluid to reach the bottom, after the circulation the fluid reaches the surface due to gravity. Thus by increasing the level of fluid we can make the flow circulation to last for a longer time which increases the time for lubrication.
• We can see some flow variations when the fluid material is changed. This happens mostly because of the viscosity & density variation of both the fluid materials.

Engine oil Density : 889 kg/m^3

Engine oil Viscosity : 1.06 Kg/m-s

n-heptane (c7h16) Density : 684 kg/m^3

n-heptane (c7h16) Viscosity : 0.000409 Kg/m-s

• The viscosity of n-heptane is less compared to engine oil. This allows the flow of n-heptane more faster than the engine oil.
• The difference can be noticed when the fluid reches the point where the two gears are meeting each other. This can be seen at 210th itration of the simulation.

Engine Oil : (Itration number 210)

n-heptane : (Itration number 210)

• In case of engine oil, majority of the fluid is being splashed off at the point of meeting. Whereas in case of n-heptane, some of the fluid leaks through the gap between the gears & only minimum amount of fluid is splashed off. This is because of the low density & low viscosity of n-heptane which allows the flow to penetrate the small gaps between the gears.

Dynamic Meshing :

• The dynamic mesh model can be used to model flows where the shape of the domain is changing with time due to motion on the domain boundaries.
• The building blocks for dynamic mesh capabilities within ANSYS Fluent are three dynamic mesh schemes, namely, smoothing, layering, and remeshing. A combination of these three schemes is used to tackle the most challenging dynamic mesh problems.
• Dynamic meshing refers to situations in which the computational grid changes dynamically during the run of the CFD simulation.
• As the body changes its motion, the meshes around them will also tend to move, which breaks the meshing. Thus by using Dynamic meshing we can update the mesh after some intervels during the simulation. Lets see how the mesh is changing in this simulation.

• In this simulation for every 20 itrations the mesh is being updated. By adjusting the Remeshing & smoothing options in the Dynamic meshing, we can adjust the update of the mesh.

Examples Where Dynamic mesh can be used :

We can use the dynamic mesh capability of ANSYS Fluent to solve a simple flow-driven rigid-body motion problem. Some of the examples are,

• Gate valve simulation, where the valve plate is moved linearly.
• Simulation involving piston movements.
• Aerodynamic simulation around a moving airfoil.
• Engine combusion simulations.
• Turbine motion simulation. etc.....

Sloshing effect :

• In fluid Dynamics, slosh refers to the movement of liquid inside another object. Sloshing is the motion of a free-surface liquid in a partially filled vessel, experienced in marine, automotive, rail, and aerospace industries.
• The sloshing motion is a highly nonlinear and random process depending on vessel motion, liquid depth, liquid properties, and vessel geometry.
• During sloshing, the liquid exerts a dynamic force on the surrounding vessel, which may cause leakage or damage to the vessel or its supporting structure. This can cause damage to the surrounding vessel or tank. Thus we can conclude that sloshing effect is bad.
• In our simulation we can see the sloshing effect more effectively in case 3 where the fluid level is high & the viscosity of the fluid material is high.

User Defined Function :

A user-defined function, or UDF, is a function that you program that can be dynamically loaded with the ANSYS FLUENT solver to enhance the standard features of the code.

The Fluent solver is a general-purpose code. In order to customize the Fluent solver, users can use their own C-codes called user-defined functions (UDFs) to accomplish:

• Special boundary conditions
• Customized or solution dependent material properties
• New physical models
• Reaction rates
• Source terms
• Customized post-processing
• Solving user-supplied partial differential equations.

In this simulation we use the UDF to specify the rotating motion of the gear.

Two functions are described to specify the motion on the left & right rotation.

Errors Occured in the simulation :

• Before adjusting the values in the dynamic meshing, we encounter some errors during the simulation. The main error is the Fail of Dynamic mesh & negetive cell volume detection.

• As the gear rotates the meshes around them also encounters a change which leads to a breakage in the mesh connection.
• At some point we can see the meshes enter the open areas where there is no surface.
• This causes the formation of negetive cell volume.

• In the above picture we can clearly see the meshes coming out from the teeth of the gear.
• This errors can be corrected by adjusting the values in smoothing & remeshing.