Overview

​

For many astronomical objects spatially resolved observations of the velocity structure are a key constraint to find their 3-D structure. Therefore it is essential to have a way to model the velocity structure of an objects in Shape. This is provided in the form of a Velocity Modifier that can be assigned to any geometric object.

​

Since velocity is a vector, the velocity modifier is probably the most complex modifier and, in some aspects, conceptually different from scalar modifiers such as the density modifier.

​

The main control panel of the velocity modifier is similar to other modifiers exposing the f0 scaling factor that is also incorporated in the Magnitude graph.

​

The main difference to other modifiers lies in this Magnitude graph, which we are going to discuss in some detail.

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

Similar to other modifiers there is the set of graphs on the right and the options on the left.  We will therefore focus on the drop-down lists on the left side for the General options, the types of Vector Field and the Dependencies. 

​

General:
The general options are again similar to other modifiers where you can choose as modes Custom and Analytic, as well as the type of coordinate system (Cartesian, Spherical, Cylindrical). Here you can also set the f0 scalar factor that multiplies the final vector. f0 is also exposed at the higher level main panel for the velocity modifier. 

​

Vector Field: 

This is one of the two key differences compared to other, scaler modifiers.  This drop-down list allows you to choose from a variety of predetermined vector fields, including Radial, Disk Rotation, Elliptical, Collimated, Random, Custom, and Path.

​

The practical importance of these options is that all except the Custom field preset the direction of the velocity vectors in space. For these one only needs to set the magnitude as a function of position using the graphs on the right. For these fields the graphs on the right work the same as in other modifiers yielding the scalar magnitude of the velocity vector.

 

Except for the case of the Custom field the coordinate tabs do not represent the velocity components! The Custom field is described below.

​

There is however an important difference. By default it is the same function, separable in their coordinate directions, as for other modifiers. In the velocity modifier it is, however, possible to choose on which spatial variable (u,v,w) each of the vector components (vu, vv, vw) depend on. Here (u,v,w) corresponds to the coordinates in the different types of coordinate systems. In Cartesian coordinates, for instance, the vx component may be defined as a function of the y-coordinate. It is important to note, however, that the actual variable in the analytic expression always x. What changes is its meaning according to the options that have been chosen.  This allows to set up quite complex velocity fields. Additional complexity can be obtained by adding several velocity modifiers in the modifier stack.

​

Radial Type Velocity Field:

The Radial Field sets all the velocity vectors to point away from the local coordinate system of the velocity modifier. lf the magnitude is negative, the vector points towards the coordinate origin.  A typical use case for this vector field are expanding nebulae such as supernovae and planetary nebulas. 

​

Disk Rotation Type Velocity Field:

The Disk Rotation Field sets all the velocity vectors in the direction perpendicular to the cylindrical radial and the z-direction from the local coordinate origin of the velocity modifier. This velocity field is suitable for rotating disks such as spiral galaxies or accretion disks. 

​

Collimated Type Velocity Field:

The Collimated Field is an easy way to set up a jet-like outflow with an opening angle as a parameter, which by default is zero. 

​

Custom Type Velocity Field:

The Custom Field is fundamentally different to the others in that each of the graph tabs on the right actually represents the corresponding vector component instead of just a factor to the magnitude. Hence, the total magnitude and direction of the vector is given by the vector addition of these components.

​

An important tool help with the velocity field modeling is the Field modifier, which allows one to visualize the vectors in the 3-D views of the 3-D Module.

​