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# Calculation of Pressure Lift and Drag

How do I calculate the Lift and Drag due to Pressure?

This method works for EnSight 7.6.4(i) or later.

For both forces and moments, a force variable must first be computed. This
force variable is then vector summed over the parts, yielding a total force
vector. Moments require the computation of a moment variable, which is
then integrated to produce a total moment vector.

Don't make the mistake of integrating the force magnitude over the area
and divide the resulting scalar by the total area which is not the
vector sum of the forces.

EnSight is a 'dimensionless' package, meaning that it is the user's
responsibility to assure that consistent units are being employed in
these computations.

Overview of pressure force calculation

For pressure forces, the force variable is simply the surface pressure times
the surface normal vector on each polygonal face, which itself is the
incremental surface area times the unit normal vector at the center of each
polygon on the part.

This results in a pressure force vector at every element on the part or parts
of interest. EnSight provides a pre-defined 'Force' variable in the
calculator to compute the pressure force vector.

Once the force vector has been computed, the user-defined math function
udmf_sum can be used to sum up each of the components of the force vector
to find the sum of the forces in the x, y, and z directions respectively.

Step by step calculation of the pressure force vector

1. FIND THE NORMAL - For parts comprised of 1D elements
(e.g. bars), the function used to calculate the force, Force1D requires the
Normal as input, so we'll need
to use the Normal calculator function to calculate the normal vector field and
name the variable 'Normal'.
For parts with 2D elements (e.g. shells, plates, etc) the function used to
calculate the force, Force, doesn't requre a Normal vector, so
we won't need to calculate this variable.

For 1D parts, the normal function attempts to make sure that you
get consistent normals (in or out). It will not work if
you have discontinous segments (such as a wing cross section and
a flap in the same part). There is a possibility under these
conditions that you get inward pointing normals on one portion and
outward pointing normals on another. For 2D parts it is also possible
but unlikely that your elements may not have consistent normals.

For 1D parts, We highly recommend that after
you compute 'Normal' you go ahead and create a vector arrow part
using this variable to make sure it is oriented uniformly. For 2D parts,
the vector arrow
dialog also includes a variable for normal visualization that does not
show up in the calculator: 'Surface Normal'. Use this variable for 2D
parts to verify normal consistency.

If all of your 1D 'Normal' vectors or 2D 'Surface Normal' vectors
are not consistent (either all inward or all
outward), then you have a problem with your model and cannot continue.

For all parts, if all of your 'Normal' or 'Surface Normal' vectors are inward,
then create a 'Normalneg'
vector using the calculator which is -1*Normal and use it instead of
'Normal'.

2. CALCULATE THE PER ELEMENT FORCE

Important note:
If pressure is a nodal variable, then use NodeToElem to convert Nodal Pressure
to Element Pressure before proceeding. Always use the Element Pressure to
calculate a Force.

a. Use the 'Force' calculator function to
calculate the force vector using the per elment
pressure scalar on a 2-D part (e.g. shells, plates, etc)
or

b. Use the 'Force1D'
calculator function to calculate on a 1D part (bars) using element pressure,
and 'Normal'.

Note that if your normals are in the wrong direction, then your force
vector will be in the wrong direction, and when you sum up the force
vectors to get a resultant force it will also be in the wrong direction.
You can create a Force_neg = -1 * Force.

Possible reasons you will not get a force vector using Force1D:

The 1D part was structured (must have unstructured)

The part contains 2D or 3D elements - the part must contain only
1D elements

The part is not planar.

3. SUM THE FORCES.
a. Select your part or parts, and use udmf_sum and the
element force [X], [Y], and [Z] scalar components to calculate
the net force in the x, y, and z directions respectively.

4. LIFT & DRAG. - Once you have the force vector,
the lift and drag can
be resolved using the angle between the freestream
velocity and the force vectors. The component of the force
in the direction of the freestream velocity is drag, and the
component normal to the freestream velocity is lift.

5. PRESSURE MOMENTS - You can calculate the
MomentVector at every node on your part using Calculator
'MomentVector' function. This will enable you to visualize
trends in the moment only at part node locations but cannot show the moment
at any arbitrary point.

To calculate the moment about any arbitrary point,use
the predefined calculator function 'Moment'.
The location of the cursor tool is used to define the moment arm. The
cursor tool may be placed at a precise (x,y,z) location using the tranformation
editor for tools.

A force vector is required. For pressure moments, use the same pressure force
vector computed from integrating pressure * area in the normal direction (see above).

The [X], [Y] or [Z] component of the moment is computed based on the force
vector, the position of the cursor tool and the component desired. The Moment
function computes the sum of the cross product of the distance from the cursor
to each of the force vectors.

The center of pressure is the location where the sum of the
moments is zero. Finding this point requires some manual iteration.
First calculate the
MomentVector at every node to visualize the moment over the part. Using
this insight, then place the cursor tool and calculate the Moment in the
desired direction. Move the cursor around and recalculate the moment
until it is sufficiently close to zero.