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.

# Calculation of Pressure Lift and Drag

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