Specify Initial and Ambient Conditions
Procedure
- Specify the Value of
each initial condition Parameter.
Double-click a Value cell to edit the cell contents or select the appropriate parameter type.
If you want to specify a coordinate-dependent or time-dependent value, click Dependency
in
the corresponding Value box
(see “Dependency”).- Under Parameter Definition select
whether you want to manually specify initial (ambient) conditions
or apply another project’s results as initial (ambient) conditions
for the current project:
Select User Defined for manual specification.
Select Transferred for using results from another calculation (see “Select Results to Transfer” for details).
TipTo use the previous project calculation results as initial (ambient) conditions, use the Take previous results option (see “Select Results to Transfer” for details).
- Under Thermodynamic
Parameters (the Fluid Flow option is
enabled on the Analysis
type tab), specify a combination of independent flow
parameters (static pressure, static temperature, static density).
For liquids you can specify the initial static Pressure and Temperature.
For gases you can select a pair of Thermodynamic Parameters to specify and the third parameter will be calculated automatically.
Pressure, Temperature. Specify the initial static Pressure and Temperature values. The density is calculated from these parameters.
Pressure, Density. Specify the initial static Pressure and Density values. The temperature is calculated from these parameters.
Temperature, Density. Specify the initial static Temperature and Density values. The pressure is calculated from these parameters.
For the pre-defined air you can also specify the Altitude above the sea level and the corresponding Pressure and Temperature will be calculated and displayed (according to the ISO 2533:1975 standard, applicable range is from 0 to 100 km).
Additionally, a Temperature deviation from the temperature standard for this altitude can be specified. The resulting temperature value is calculated as T = Temperature + Temperature deviation.
If you enable a rotating reference frame (the Rotation option is enabled on the Analysis type tab), you can select the Pressure potential (Rotation) check box to specify the pressure distribution relative to the rotating reference frame (see “Rotation”).
If the Pressure potential (Rotation) option is selected, you can also select the Refer to the origin option (see “Relative to rotating frame”).
If you enable gravitational effects (the Gravitation option is enabled on the Analysis type tab), you can select the Pressure potential (Gravity) check box to assign the spatial distribution of pressure (see “Pressure Potential for Gravitational Effects”).
If the Pressure potential (Gravity) option is selected, you can also select the Refer to the origin option (see “Refer to the origin (Gravitation)”).
- Under Velocity Parameters (the Fluid Flow option
is enabled on the Analysis
type tab), specify the Velocity vector,
for gases you can also specify the Mach Number instead
of the absolute velocity.
The absolute velocity (or Mach number) vector can be specified in terms of:
3D Vector
Specify the X, Y and Z components of the velocity vector with respect to the reference Coordinate System (see below).
Aerodynamic angles (for an External analysis only).
Specify the Velocity magnitude and the vector direction in terms of the aerodynamic angles.
Figure 1. Aerodynamic angles definition: Vr - relative wind, X -longitudinal axis, Y - vertical axis, Z - lateral axis, XY - longitudinal plane, α - angle of attack, β - angle of sideslip.
The Relative wind (Vr) is the “wind” vector established by flight path vector. It describes the oncoming flow. The direction of the oncoming flow (specified with respect to the Global Coordinate System) is indicated in the graphics area by the blue arrow.
The Longitudinal axis (X) is an axis drawn through the body of the vehicle from nose to tail in the normal direction of flight. Rotation about this axis is called roll or bank. Roll changes the orientation of the aircraft's wings with respect to the downward force of gravity.
The Lateral axis (Z), also called transverse axis, passes through the plane from wingtip to wingtip. Rotation about this axis is called pitch. Pitch changes the vertical direction the aircraft's nose is pointing.
The Vertical axis (Y), also called yaw axis, is perpendicular to the other two axes and it is defined to be perpendicular to the body of the wings with its origin at the center of gravity and directed towards the bottom of the aircraft. Rotation about this axis is called yaw. A yaw motion is a movement of the nose of the aircraft from side to side.
The Longitudinal plane (XY), also called pitching plane, is the aircraft's symmetry plane which describes the position of the aircraft's nose in relation to its tail and the horizon.
The Lateral plane (XZ) is the plane formed by the longitudinal and lateral axes.
The Angle of attack (α) is the angle between the projection of the relative wind on the longitudinal plane and the longitudinal axis of the body.
The Angle of sideslip (β) is the angle between the relative wind and its projection on the longitudinal plane. It relates to the rotation of the aircraft centerline (the longitudinal axis) from the relative wind.
If you enable a rotating reference frame (the Rotation option is enabled on the Analysis type tab), you can select the Relative to rotating frame check box to specify the velocity distribution relative to the rotating reference frame (see “Relative to rotating frame”).
- Under Turbulence Parameters (if
the Fluid
Flow option is enabled on the Analysis type tab and
the Laminar and turbulent or Turbulent Only flow type
is selected on the Fluids tab),
you can adjust the default turbulence parameters if you are fully
confident in your turbulent values.
The default turbulence parameters are used as initial conditions (or ambient conditions for external analyses) in the Computational Domain and as a default inlet boundary condition in Boundary Conditions and Fans.
You can set either Turbulent intensity and Turbulent length or Turbulent energy and Turbulent dissipation.
- Under Concentration (if
the Fluid
Flow option is enabled on the Analysis type tab and
the number of fluids selected as default fluids on the Fluids tab is greater
than one), specify the relative concentrations of the project's
default fluids either by Mass or
by Volume.
By default, Flow Simulation uses equal concentrations for all fluids. Flow Simulation uses the specified concentrations as initial conditions (or ambient conditions for external analyses) for the entire computational domain and as a default inlet boundary condition in Boundary Conditions and Fans.
TipIf you have several fluids piped into a volume, you can decrease the total calculation time by specifying the initial fluid concentrations within a pipe equal to the concentrations at the pipe’s inlet. To do this, replace a pipe fluid volume (the void) with a solid part, disable this part in the Component Control dialog, and using the Initial Condition dialog specify the appropriate initial fluid concentrations for the fluid region represented by this part.
If you model a flow of two immiscible fluids with an interface (if the Free Surface option is enabled on the Analysis type tab), you must define the Initial Fluid distribution by specifying how fluids are distributed throughout the computational domain (see “Free Surface”).
The height is measured from the origin of the reference Coordinate System (see below).
- Under Solid Parameters for Conjugate Heat Transfer problems (if the Conduction option is enabled on the Analysis Type tab), specify the Initial solid temperature assigned by default to all model components. This temperature is needed to start the calculation when solving a steady-state problem or to define the initial state when solving a time-dependent problem. However, you can specify a different initial solid temperature to a particular model component by using the Initial Condition dialog (see “Initial Conditions”).
- Under Humidity for project
involving calculation of the relative humidity of gas or mixture
of gases (if the Fluid Flow option is
enabled on the Analysis
type tab and the Humidity option
is enabled on the Fluids tab),
specify the Relative humidity of
the gas flow and the Humidity reference pressure and Humidity reference temperature values,
under which the relative humidity has been determined (see “Relative to rotating frame” for details).
By default, the Relative humidity is equal to 0%.
Flow Simulation uses the specified values as an initial condition (or ambient condition for external analysis) for the entire computational domain and as the default boundary condition in Boundary Conditions for the Flow Opening and Pressure Opening types.
- Under Parameter Definition select
whether you want to manually specify initial (ambient) conditions
or apply another project’s results as initial (ambient) conditions
for the current project:
- You can select a reference coordinate system,
if you intend to specify initial conditions with non-uniform flow
parameters (velocity vector and height) distribution:
The reference Coordinate System
selected by default is the Global Coordinate System (see “Global Coordinate System” for details).To replace the default coordinate system, click Coordinate system button and then select your coordinate system in the flyout FeatureManager design tree.
TipTo create a coordinate system, click Insert > Reference Geometry > Coordinate System.
If you want to use a cylindrical Coordinate System, select the axis of the specified Coordinate System as the Reference axis. The selected Coordinate System and Reference axis also define a local spherical coordinate system.