How to get RMS for MONPNT1 points in Nastran

Introduction

When conducting dynamic structural analysis in Nastran, understanding the response at specific, non-structural points is often crucial. These points, defined using MONPNT1 entries, allow you to monitor responses like acceleration or displacement at locations where no grid points exist. A common requirement is to calculate the Root Mean Square (RMS) value of these responses, which provides a statistical measure of their magnitude over a frequency or time range.

This guide provides a comprehensive walkthrough of how to define MONPNT1 points, extract their output data, and calculate the RMS values. Whether you’re working on a random vibration analysis or a transient response simulation, these steps will help you get the critical data you need for your design and verification process.

RMS and its Importance in Nastran

The Root Mean Square (RMS) is a statistical measure that represents the effective magnitude of a varying quantity. In structural dynamics, the RMS value of a response, like acceleration or displacement, provides a single, meaningful number that quantifies the overall level of vibration.

Why is this important? In a random vibration analysis (SOL 111), the input is defined by a Power Spectral Density (PSD) curve, and the output is also in the form of a PSD. While the full PSD curve gives detailed information across frequencies, an RMS value summarizes this entire response into one number. This is incredibly useful for:

• Fatigue Analysis: RMS stress values are often used as input for fatigue life predictions.
• Performance Specification: Many engineering specifications require that the RMS acceleration or displacement at key locations remain below a certain threshold.
• Test Correlation: Experimental vibration tests often measure and report RMS values, making it a critical metric for correlating your simulation results with physical test data.

Calculating RMS values helps you quickly assess whether your structure meets its design requirements without needing to interpret complex frequency response plots for every single point of interest.

Understanding MONPNT1

MONPNT1 entries in Nastran are used to define “monitor points” where you want to request output but don’t have a structural grid point. These are essentially virtual observation points. They are particularly useful for monitoring responses at locations like a payload’s center of gravity or a specific sensor location that isn’t explicitly part of your finite element mesh.

A MONPNT1 entry is defined by a name, a label, and a set of referenced grid points with corresponding coefficients. The response at the MONPNT1 point is calculated as a weighted average of the responses at these referenced grid points.

The basic format on a MONPNT1 card is:

MONPNT1, NAME, 'LABEL', G1, C1, G2, C2, ...

• NAME: A unique identifier for the monitor point set.
• LABEL: A text label for the specific monitor point within the set.
• Gi: The ID of a referenced structural grid point.
• Ci: The coefficient (weighting factor) for the corresponding grid point.

When you run a dynamic analysis, Nastran will compute the displacement, velocity, or acceleration at these MONPNT1 locations based on the motion of the referenced structural points.

Step-by-Step Guide to Extracting MONPNT1 Data

Here’s how to set up your Nastran input file to get the output you need for your MONPNT1 points. We’ll use a random vibration analysis (SOL 111) as the primary example.

Step 1: Define MONPNT1 Points in the Bulk Data Deck

First, you need to define your monitor points in the Bulk Data section of your Nastran .dat file.

For example, let’s say you want to monitor the response at a point whose motion is the average of four grid points (101, 102, 103, 104).

$ Monitor Point Definition
MONPNT1, MYMONS, 'CG_ACCEL', 101, 0.25, 102, 0.25, 103, 0.25, 104, 0.25

Here, we’ve created a monitor point set named MYMONS with one point labeled CG_ACCEL. Its response will be the average of the responses at the four specified grid points.

Step 2: Request Output for MONPNT1

In the Case Control section, you need to explicitly ask Nastran to calculate and output the results for your monitor points. For a random analysis, you’ll want to request PSD and RMS output.

SUBCASE 1
  LABEL = RANDOM VIBRATION ANALYSIS
  ACCELERATION(PUNCH, PSD, RMS) = MYMONS
  DISPLACEMENT(PLOT, PSD) = MYMONS

• ACCELERATION = MYMONS: This tells Nastran to calculate acceleration results for the monitor point set named MYMONS.
• (PUNCH, PSD, RMS): This is the key part.
• • PUNCH: This requests the output to be written to the .pch (punch) file, which is a text-based, column-formatted file that is easy to parse.
  • PSD: This requests the Power Spectral Density function.
  • RMS: This requests Nastran to calculate the total RMS value based on the integrated PSD.

Step 3: Run the Nastran Analysis

With your input file configured, run the Nastran job. After the analysis completes successfully, you will have several output files, but the one we are most interested in is the punch file (.pch).

Calculating RMS Values from Nastran Output

When you request RMS output directly in the Case Control deck, Nastran does the calculation for you. The result will be printed in the .pch file.

Finding the RMS Value in the .pch File

Open the .pch file in a text editor. You will need to search for the output block corresponding to your MONPNT1 results. The output for RMS values from a random analysis typically looks something like this:

$                             R M S   V A L U E S   F O R   A C C E L E R A T I O N 
$
SUBCASE ID =           1
MONPNT1 NAME = MYMONS
LABEL = CG_ACCEL
         POINT ID.         TYPE          T1            T2            T3            R1            R2            R3
               1             A      1.534E+01     2.118E+01     9.876E+00     0.000E+00     0.000E+00     0.000E+00

Here’s how to interpret this block:

• The header indicates these are RMS values for acceleration.
• MONPNT1 NAME and LABEL confirm this is the result for your defined point CG_ACCEL.
• POINT ID. 1: This is the internal ID for your monitor point.
• TYPE A: Indicates this is an acceleration result.
• T1, T2, T3: These are the RMS values for the translational degrees of freedom (X, Y, Z). In this example, the RMS acceleration in the X-direction is 15.34 g, in the Y-direction is 21.18 g, and in the Z-direction is 9.876 g.
• R1, R2, R3: These are the RMS values for the rotational degrees of freedom. They are zero here because rotational acceleration wasn’t requested or isn’t applicable.

This method is the most direct way to get the RMS value, as Nastran performs the integration of the PSD curve for you.

Troubleshooting Common Issues

1. No MONPNT1 Output:
2. • Check that you have correctly requested the output in the Case Control section. Ensure the MONPNT1 set name matches between Case Control and Bulk Data.
   • Verify that your MONPNT1 definition is syntactically correct in the Bulk Data deck.
2. RMS Values Seem Incorrect:
3. • Confirm the coefficients (Ci) on your MONPNT1 card are correct. If you want a simple average, the sum of coefficients should be 1.0.
   • Check the units of your model and your input PSD. If your input PSD is in g²/Hz, your RMS acceleration output will be in g.
3. Output is not in the .pch File:
4. • Ensure you used the PUNCH keyword in your Case Control request. If you only use PLOT, the results will be written to a binary .op2 file, which requires a post-processor to read.

Final Thoughts: Streamlining Your Analysis

Using MONPNT1 points is an effective technique for monitoring critical responses at non-structural locations in your Nastran models. By directly requesting RMS output in the .pch file, you can quickly obtain the essential data needed for design validation and test correlation. This approach simplifies your post-processing workflow and allows you to efficiently evaluate the performance of your structure under dynamic loading.

For more advanced workflows, you can write simple scripts in Python or MATLAB to automatically parse the .pch file and extract these RMS values, further integrating Nastran’s powerful analysis capabilities into your automated design processes.

Frequently Asked Questions (FAQs)

Q: Can I get RMS values for MONPNT1 points in a transient analysis (SOL 112)?
A: No, Nastran does not have a direct RMS output request for transient analysis. RMS is typically a statistical measure for random or steady-state signals. For a transient response, you would typically look at peak values over time. You could, however, export the time history data for the MONPNT1 point and calculate the RMS value yourself in a post-processing tool like Python or MATLAB.

Q: What’s the difference between MONPNT1, MONPNT2, and MONPNT3?
A: They serve similar purposes but are defined differently.

• MONPNT1 references structural grid points.
• MONPNT2 defines monitor points in an aerodynamic analysis.
• MONPNT3 defines monitor points by referencing other monitor points, allowing you to create complex combinations.

Q: Can I request stress or strain output for a MONPNT1 point?
A: No, MONPNT1 points are non-structural and do not have physical properties like stress or strain. They can only report motion-based quantities like displacement, velocity, and acceleration.