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11.7.5 Using the In-Cylinder Model

This section describes the problem setup procedure for an in-cylinder dynamic mesh simulation.


Consider the 2D in-cylinder example shown in Figure  11.7.23 for a typical pent-roof engine.

Figure 11.7.23: A 2D In-Cylinder Geometry

In setting up the dynamic mesh model for an in-cylinder problem, you need to consider the following issues:

Defining the Mesh Topology

FLUENT requires that you provide an initial volume mesh with the appropriate mesh topology such that the various mesh update methods described in Section  11.3.2 can be used to automatically update the dynamic mesh. However, FLUENT does not require you to set up all in-cylinder problems using the same mesh topology. When you generate the mesh for your in-cylinder model (using GAMBIT or other mesh generation tools), you need to consider the various mesh regions that you can identify as moving, deforming, or stationary, and generate these mesh regions with the appropriate cell shape.

The mesh topology for the example problem in Figure  11.7.23 is shown in Figure  11.7.24, and the corresponding volume mesh is shown in Figure  11.7.25.

Figure 11.7.24: Mesh Topology Showing the Various Mesh Regions

Figure 11.7.25: Mesh Associated With the Chosen Topology

Because of the rectilinear motion of the moving surfaces, you can use dynamic layering zones to represent the mesh regions swept out by the moving surfaces. These regions are the regions above the top surfaces of the intake and exhaust valves and above the piston head surface, and must be meshed with quadrilateral or hexahedral cells (as required by the dynamic layering method).

For the chamber region, you need to define a remeshing zone (triangular cells) to accommodate the various positions of the valves in the course of the simulation. In this region, the motion of the boundaries (valves and piston surfaces) is propagated to the interior nodes using the spring-based smoothing method. If the cell quality violates any of the remeshing criteria that you have specified, FLUENT will automatically agglomerate these cells and remesh them. Furthermore, FLUENT will also remesh the deforming faces (based on the minimum and maximum length scale that you have specified) on the cylinder walls as well as those on the sliding interfaces used to connect the chamber cell zone to the layering zones above the valve surfaces.

For the intake and exhaust port regions, you can use either triangular or quadrilateral cell zones because these zones are not moving or deforming. FLUENT will automatically mark these regions as stationary zones and will not apply any mesh motion method on these cell zones.

The dynamic layering regions above the piston and valves are conformal with the adjacent cell zone in the chamber and ports, respectively, so you do not have to use sliding interfaces to connect these cell zones together. However, you need to use sliding interfaces to connect the dynamic layering regions above the valves and the remeshing region in the chamber. This is shown in Figure  11.7.26 with the exhaust valve almost at full extension. Notice that cells on the chamber side of the interface zone are remeshed (i.e., split or merged) as the interface zone opens and closes because of the motion of the exhaust valve.

Figure 11.7.26: The Use of Sliding Interfaces to Connect the Exhaust Valve Layering Zone to the Remeshing Zone

Defining Starting Position Mesh for the In-Cylinder Model

If you are solving an in-cylinder flow problem, it is recommended that you generate your initial mesh to coincide with the TDC (top-dead-center) position. You can then use FLUENT to position the valves and piston to correspond to the starting crank angle of your simulation using the position-starting-mesh text-interface command.

define $\rightarrow$ models $\rightarrow$ dynamic-mesh-controls $\rightarrow$ in-cylinder-parameter $\rightarrow$ position-starting-mesh

/define/models/dynamic-mesh-controls/in-cylinder-parameter> position-starting-mesh
Start Crank Angle (deg) [0] 340

Updating mesh position ... Done

Subdividing mesh layers ... Done

Remeshing cell zone fluid-chamber (id = 18)
 remeshing face zone cyl-wall-deform (id = 30) with length scale = 1.65000e-03
 remeshing face zone intf-exv-lt-ob-t (id = 72) with length scale = 8.66000e-04
 remeshing face zone intf-exv-rt-ob-t (id = 77) with length scale = 8.66000e-04
 remeshing face zone intf-inv-lt-ob-t (id = 92) with length scale = 8.66000e-04
 remeshing face zone intf-inv-rt-ob-t (id = 97) with length scale = 8.66000e-04
Remeshing cell zone fluid-export (id = 41)
 remeshing face zone exvalve-deform-lt (id = 7) with length scale = 1.60000e-03
 remeshing face zone intf-exv-rt-ib-t (id = 67) with length scale = 8.66000e-04
 remeshing face zone intf-exv-lt-ib-t (id = 46) with length scale = 8.66000e-04
 remeshing face zone exvalve-deform-rt (id = 39) with length scale = 1.60000e-03
Remeshing cell zone fluid-inport (id = 43)
 remeshing face zone invalve-deform-rt (id = 8) with length scale = 1.60000e-03
 remeshing face zone intf-inv-lt-ib-t (id = 82) with length scale = 8.66000e-04
 remeshing face zone intf-inv-rt-ib-t (id = 87) with length scale = 8.66000e-04
 remeshing face zone invalve-deform-lt (id = 40) with length scale = 1.60000e-03

This technique has the following restrictions:

FLUENT will automatically remesh any deforming face zones and the adjacent cell zones (both remeshing and layering) based on the remeshing and dynamic layering parameters that you have set up for your model. In the above example, the starting crank angle for the in-cylinder simulation is 340 degrees (20 degrees before TDC). Figure  11.7.27 and Figure  11.7.28 show the initial and the starting mesh generated by FLUENT.

Figure 11.7.27: In-Cylinder Initial Mesh

Figure 11.7.28: In-Cylinder Starting Mesh Generated by FLUENT at Crank Angle of 340 degrees

Defining Motion/Geometry Attributes of Mesh Zones

As the piston moves down from the TDC to the BDC position, you need to expand the remeshing region such that it can accommodate the valves when they are fully extended. To accomplish this, you need to specify the dynamic layering zone adjacent to the piston surface to move with the piston until some specified distance from the TDC position. Beyond this cutoff distance, the motion of the layering zone is stopped and the piston wall is allowed to continue to the BDC position. Because there is relative motion between the piston head surface and the now non-moving dynamic layering zone, cell layers will be added when the ideal layer height criteria is violated. Figures  11.7.29 to 11.7.34 show the sequence of meshes before and after the onset of cell layering when the motion in the layering zone above the piston surface is stopped (shown with $\Delta\theta$ = 5 $^\circ$).

Figure 11.7.29: Mesh Sequence 1

Figure 11.7.30: Mesh Sequence 2

Figure 11.7.31: Mesh Sequence 3

Figure 11.7.32: Mesh Sequence 4

Figure 11.7.33: Mesh Sequence 5

Figure 11.7.34: Mesh Sequence 6

FLUENT provides built-in functions to handle the full piston motion and the limited piston motion for the dynamic layering zone above the piston surface. When you define the motion attribute of the dynamic layering zone above the piston surface, you need to use the limited piston motion function ( **piston-limit** in the C.G. Motion UDF/Profile field in the Dynamic Mesh Zones panel). Note that you must define the parameters used by these functions before you can use them. In the current example, the piston stroke is 80 mm and the connecting rod length is 140 mm. The piston stroke cutoff is assumed to happen at 25 mm from TDC position. The lift as a function of crank angle between $344^\circ$ and $1064^\circ$ is shown in Figure  11.7.35 for both limited and full piston motion.

Figure 11.7.35: Piston Position (m) as a Function of Crank Angle (deg)

To define the motion of the valves, you need to use profiles that describe the variation of valve lift with crank angle. FLUENT expects certain profile fields to be used to define the lift and the crank angle. For example, consider the following simplified profile definition:

((ex-valve 5 point)
 (angle 0   180  270  360   720)
 (lift 0.05 0.05 1.8  0.05 0.05))

((in-valve 5 point)
 (angle 0   355   440  540  720)
 (lift 0.05 0.05 2.0 0.05 0.05))

FLUENT expects the angle and lift fields to define the crank angle and lift variations, respectively. The angle must be specified in degrees and the lift values must be in meters. The actual valve lift profiles that you will use for the current example are shown in Figure  11.7.36. Notice that there is an overlapped period where both the intake and exhaust valves are open.

Figure 11.7.36: Intake and Exhaust Valve Lift (m) as a Function of Crank Angle (deg)

The valve lift profiles and the built-in functions will describe how each surface moves as a function of crank angle with respect to some reference point. For example, the valve lift is zero when the valve is fully closed and the valve lift is maximum when it is fully open. In order to move the surfaces, FLUENT requires that you specify the direction of motion for each surface. FLUENT will then update the "center of gravity'' of each surface such that

 \vec{x} = \vec{x}_{\rm ref} - l\vec{e}_{\rm axis} (11.7-10)

where $\vec{x}_{\rm ref}$ is some reference position, $\vec{e}_{\rm axis}$ is the unit vector in the direction of motion, and $l$ is either the valve or the piston distance with respect to the reference position $\vec{x}_{\rm ref}$. Note that the unit vector of the direction of motion is specified to point in the negative direction. For example, the correct intake valve axis for this example is $(-0.3421,~ 0.9397)$, as shown in Figure  11.7.37.

Figure 11.7.37: Definition of Valve Zone Attributes (Intake Valve)

Defining Valve Opening and Closure

FLUENT assumes that once you have set up the mesh topology, the mesh topology is unchanged throughout the entire simulation. Therefore, FLUENT does not allow you to completely close the valves such that the cells between the valve and the valve seat become degenerate (flat cells) when these surfaces come in contact (removing these flat cells would require the creation of new boundary face zones). To prevent the collapse, you need to define a minimum valve lift and FLUENT will automatically stop the motion of the valve when the valve lift is smaller than the minimum valve lift value. The minimum valve lift value can be specified in the Dynamic Mesh Parameters panel. For the current example, a minimum valve lift value of 0.1 mm is assumed.

When the valve position is smaller than the minimum valve lift value, it is normal practice to assume that the valve is closed. The actual closing of the valves is accomplished by deleting the sliding interfaces that connect the chamber cell zone to the dynamic layering zones on the valves. The interface zones are then converted to walls to close off the "gaps'' between the valves and the valve seats.

The valve opening is achieved by the reverse process. When the valve lift has reached beyond the minimum valve lift value, the valve is assumed to be open and you can redefine the sliding interfaces such that the chamber zone is now connected to the dynamic layering zones above the valves.

Defining Events for In-Cylinder Applications

FLUENT will automatically limit the valve lift values depending on the specified minimum valve lift value. However, the conversion of the sliding interface zones to walls (and vice versa) is accomplished via the in-cylinder events (see Section  11.7.4). For example, if the exhaust valve closes at $-5^\circ$ before TDC position, you must define a Delete Sliding Interface event at the crank angle of $-5^\circ\!$. You need to define similar events for the intake valve opening (using the Create Sliding Interface event), the intake valve closing ( Delete Sliding Interface event), and the exhaust valve opening ( Create Sliding Interface event) at the respective crank angles.

For the current example, the exhaust valve is assumed to be open between $131^\circ$ and $371^\circ$ and the intake valve is open between at $345^\circ$ and $584^\circ\!$.

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© Fluent Inc. 2006-09-20