Gearbox Sloshing effect using Ansys Fluent

Challenge on Gearbox Sloshing effect

Aim: Analysis of the different fluids(lubricants) sloshing in a Gearbox at a different Immersion depth.

Objectives:

For this challenge, you have to perform the four simulations in the Gearbox and then you have to compare the results among all the four cases.

Part 1:

1. 20% Immersion, Fluid - Engine oil.
2. 30% Immersion, Fluid- Engine oil.
3. 20% Immersion,Fluid-n-heptane (c7h16).
4. 30% Immersion,Fluid-n-heptane (c7h16).

Part 2:

1. What is Dynamic meshing? Give some other examples where dynamic meshing can be used.
2. What is the fluid Sloshing effect? Discuss whether the sloshing effect is good or bad? Explain.
3. What is the use of UDF?
4. Discuss the common errors that occurred in the simulation. A] 'Dynamic mesh failed' error. B] 'Negative cell volume detected' error.

Theory:

Introduction:

The efficiency of gear transmissions is an important study object in the field of energy‐saving. The total energy losses in splash‐lubricated gearboxes are mainly categorized in two ways the first is load-dependent losses, which are a result of the inter‐tooth meshing friction of gears and inter‐surface friction of bearings and seals; the second category is independent of load and is known as churning power losses, which are losses due to the fluid resistance torque between the gear and the surrounding oil, which is also known as simply Oil churning. Oil churning losses are generated by the combination of interactions between rotating gears and the surrounding oil, and the oil pumping effect between the meshing gear pair. When the rotational speeds of the gear speed are low, churning losses represent a tiny proportion of the total energy losses and can generally be ignored; however, several researchers have pointed out that when the gears rotate at higher speeds, the influence of the churning losses increases significantly. In a parallel‐shaft gearbox with splash lubrication, 70% of the total losses of gears are meshing friction loss and 30% is oil churning loss at low speed.

On the other hand, no‐load loss of rotating parts accounted for the majority of the overall loss of a gearbox under high‐speed and light‐load conditions. Under high speed operating conditions (32,000 rpm), the gear churning loss is 1.28 times that of the meshing friction loss. Therefore, reducing churning losses can significantly improve the transmission efficiency of the gear transmission and effectively save energy.

By selecting the appropriate lubrication and Immersion depth, the churning losses can be minimized.

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SIMULATION:

Steps in Simulation:

Open Ansys Workbench. Drag and drop Fluid Flow (Fluent) onto Project Schematic.

Here, we will see Steps we need to follow to solve a particular engineering system from a CFD standpoint.

1. Geometry: Spaceclaim, where we import the geometry.
2. Mesh: Ansys Mesh, where Boundary naming, element sizing and mesh generation with required grid size will be done.
3. Setup: Ansys Fluent, where actual case setup will be done.
4. Solution: Ansys Fluent, where runtime results visualization done.
5. Results: CFD-Post, where Post Processing of Results will be done.

CAD Cleanup:

Import Graphics card geometry into Spaceclaim, Spaceclaim is CAD cleanup software that can be used to prepare CAD models for simulation setup. It could be for CFD or FEA.

As we can see above figure is the Solid model. We need to carry out analysis on the fluid volume, so extract the fluid volume. Select Prepare, select volume extract, select edges. Select front and back edges of both the gears.

Split the body in order to extract the 2D geometry right in the middle plane as there is a limitation of the Software version of creating 3D model. Create Planes in x,y,z axis.

Extract the 2D geometry and paste it in a new file.

To assign the Immersion of gear portion inside the lubricant, we need to know its co-ordinates that will be use in Fluent while assigning Input co-ordinates in the Adapt/ Mark cell region,

• First Check the height of the Gears. Calculate 20% and/or 30% of its height i.e Immersion depth.
• Assign fluid region by creating a rectangle in such a manner that the gears should immersed 20% or 30% immersed in it.
• Then note the coordinates of the bottom left corner of the rectangle as Xmin and Ymin and the coordinates of the bottom left corner of the rectangle as Xmax and Ymax. As our origin lies in the middle of the left gear, then only Xmax will be positive and the rest coordinates will be negative.

As our origin lies in the middle of the left gear, then the Centre of rotation of the left gear will be (0,0) and to know the Centre of rotation for the right gear, open the 3D geometry -> Measure-> Mass properties-> select right gear. COR of the right gear is (0.115,0,0).

Mesh Generation

Name Selection: Name left gear as left_gear and right gear as right_gear.

Assign triangular Meshing.

Mesh element size 1mm.

Enable proximity(control of Mesh sizing) and the number of cells across the gap is 1 to capture fluid flow across the gap.

Fluent Setup:

Setting up physics: Select k-epsilon, Realizable (to provide superior performance for flows involving rotation, boundary layers under strong adverse pressure gradients, separation, and recirculation.) with enhanced wall treatment.

Solver: Pressure-based as Mach number is less than 3.

Assign Time as Transient Solver. Enable gravity in y-axis -9.81 m/s^2

Model: Multiphase-> Volume of Fluid(as there are two-phase. Liquid & air). Set formulation scheme Implicit (It will not diverge for CFL number greater than 1)

Material Selection:

For Gas- air

For liquid- Engine oil and n-heptane

Properties of Material:

For Air: Density = 1.22 kg/m^3

Viscosity = 0.0000181 kg/m.s

For Engine Oil: Density = 889 kg/m^3

Viscocity = 1.06 kg/m-s.

For h-heptane(c7h16): Density = 684 kg/m^3

Viscocity = 0.000409 kg/m-s.

Load the UDF file

#include "udf.h"

DEFINE_CG_MOTION(right_motion, dt, vel, omega, time, dtime)
{
	vel[0] = 0.0;
	vel[1] = 0.0;
	vel[2] = 0.0;

	omega[0] = 0.0;
	omega[1] = 0.0;
	omega[2] = 2.0e2; /* [rad/s]*/
}
DEFINE_CG_MOTION(left_motion, dt, vel, omega, time, dtime)
{
	vel[0] = 0.0;
	vel[1] = 0.0;
	vel[2] = 0.0;

	omega[0] = 0.0;
	omega[1] = 0.0;
	omega[2] = -2.0e2; /* [rad/s]*/
}

Setting up physics: Dynamic Mesh assign. Assign Centre of rotation for left gear as (0,0) and for right gear as (0.115, 0). The Centre of rotation has been calculated from SpaceClaim as shown above.

Adapt/Mark the cells and provide the Maximum and Minimum input coordinates as per the immersion depth which are calculated from the SpaceClaim.

For 20% immersion: For 30% immersion:

Solving: Hybrid Initialization->Patch ->Phase(engine oil/n-heptane)->variable(Volume fraction)-> value(1)-> Register to patch(hexahedron-r0)->patch->close.

Create Phase contour.

Create animation.

Calculate.

Plots and Results:

• 20% Immersion, Fluid - Engine oil.

Animation:

https://www.youtube.com/watch?v=8--9X62o0GQ

• 30% Immersion, Fluid- Engine oil.

Animation:

https://www.youtube.com/watch?v=73nb6afwajc

• 20% Immersion,Fluid-n-heptane (c7h16).

Animation:

https://www.youtube.com/watch?v=pIRYagee2kQ

• 30% Immersion,Fluid-n-heptane (c7h16).

Animation:

https://www.youtube.com/watch?v=5gJsNscilIA

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Part 2:

What is the fluid Sloshing effect? Discuss whether the sloshing effect is good or bad? Explain.

Sloshing can be described as back and forth motions of the liquid in a partially filled tank. Slosh is a phenomenon where the liquid in a container moves irregularly with a splashing sound.

The large liquid movement during sloshing creates highly localized impact pressure on tank walls which may in turn cause structural damage and may even create sufficient moment to affect the stability of the vehicle that carries the container. Sometimes, the dynamic motion of the liquid in tanks influences the wave-induced motions on ships.

The effect of sloshing in a moving liquid container causes safety and comfort issues such as undesirable forces on brake systems in liquid carrying tankers like LNG, high impact loads upon the containment system, and structural parts. Further, under earthquake conditions, it creates fatigue, safety problems in nuclear reactors, and power generating plants. Sloshing also produces unnecessary noises referred to as “clinking“ and safety issues in propellant tanks in the spacecraft under microgravity conditions (Snyder 1999). As compared to rigid loads, the number of road accidents related to liquid loads is significantly higher. This is due to the fact that the dynamic behavior of a liquid in a transporting container significantly affects the stability and controllability of the vehicle. Liquid sloshing occurs in several marine engineering applications including vessels transporting liquids in hold tanks. Water waves in the tank sloshing have similarities with green water on deck. Violent free surface motion can be expected in these two cases and a similar resonance phenomenon will occur in both problems. Hence, the problem of sloshing gains more attention and practical importance in the engineering community.

One of the positive sides of fluid sloshing is that it is used to generate electricity from tidal waves of Ocean using some devices.

Application: Elevated water towers and industrial storage tanks under excited ground motion, sloshing effects on large dams, liquefied natural gas (LNG) ship containers and moving oil tanks, to generate electricity from ocean waves.

IMMERSION

One of the difficulties for making a good measurement at high speed is to verify the right immersion factor for the gear. This factor is defined as follows: It is the ratio of the submerged depth of a pinion to its pitch radius. This will be difficult because of the waving behavior of the oil.

Dynamic Meshing

The dynamic mesh model allows you to move the boundaries of a cell zone relative to other boundaries of the zone, and to adjust the mesh accordingly. The boundaries can move rigidly with respect to each other (that is, linear or rotational motion), and/or deform.

The motion of the boundaries can be rigid, such as pistons moving inside an engine cylinder or a flap deflecting on an aircraft wing, or deforming, such as the elastic wall of a balloon during inflation or a flexible artery wall responding to the pressure pulse from the heart. In either case, the nodes that define the cells in the domain must be updated as a function of time, and hence the dynamic mesh solutions are inherently unsteady.

Why Use UDFs?

UDFs allow you to customize ANSYS FLUENT to fit your particular modeling needs. UDFs can be used for a variety of applications. Some examples include

• customization of boundary conditions, material property definitions, surface and volume reaction rates, source terms in ANSYS FLUENT transport equations, source terms in user-defined scalar (UDS) transport equations, diffusivity functions, etc.
• adjustment of computed values on a once-per-iteration basis
• initialization of a solution
• asynchronous (on-demand) execution of a UDF
• execution at the end of an iteration, upon exit from ANSYS FLUENT, or upon loading of a compiled UDF library
• postprocessing enhancement
• enhancement of existing ANSYS FLUENT models (e.g., discrete phase model, multiphase mixture model, discrete ordinates radiation model)

Discuss the common errors that occurred in the simulation. A] 'Dynamic mesh failed' error. B] 'Negative cell volume detected' error.

The above error occurring due to the high Skewness of the cells with respect to the adjacent cells. This issue can be rectified by providing less Cell Skewness in the Dynamic Mesh(0.4) and Maximum length scale as 0.01m under the Remeshing option, and/or lowering the Timestep size(dt). The maximum length scale is the maximum cell size.

Skewness: Skewness is the angular measure of a cell with respect to its adjacent cell. Highly skewed cells can decrease accuracy and destabilize the solution.

There are two methods to determine Skewness:

• Based on The Equilateral volume deviation

Applies only to triangle and Tetrahedra.

• Based on the deviation from a Normalized Angle deviation

Applies to all cell and face shapes

Where theta_e is the equiangular cell (60 for tets and tress and 90 for quad and hexas)

Conclusion:

• The Engine oil sticks to the surface of Gears more than the n-heptane(c7h16) because the Engine oil has more Viscocity than n-heptane. Hence Sloshing and the Churning losses of the oil are minimum than n-heptane.
• In Engine oil, there no significant change in oil sloshing for 20% and 30% of the immersion factor.