2. Tune Airbag Vent Hole Area for Occupant Chest Deflection

Why Vent Area Is the Primary Chest-Loading Lever

In a frontal airbag model, the vent hole is the dominant path for gas to escape the cushion after peak inflation. Its area controls how quickly bag internal pressure decays, and therefore how the cushion behaves against the occupant's chest during the ride-down phase. Too small a vent area keeps the bag stiff and pushes chest deflection up; too large a vent area lets the bag collapse early, allowing the chest to travel further forward and potentially reach the steering-wheel or bottom-out condition. The tuning task is therefore a bounded search: find the vent area that produces the lowest chest deflection while the bag still retains enough pressure to prevent bottom-out.

This case uses the baseline model from the previous exercise as the starting point. The airbag is defined with a fabric material and a vent hole whose area is currently a single scalar value. We will parametrize that area, run a small sweep, extract chest deflection from the dummy, and identify the optimum with a bottom-out guard.

Parametrizing the Vent Hole Area

In LS-DYNA the vent hole is normally defined inside the airbag definition using *AIRBAG_SIMPLE_AIRBAG_MODEL or, for a fabric-based bag, through the *MAT_FABRIC card combined with a vent definition. The vent area is entered as a length-based parameter (the effective vent diameter or area depending on the keyword variant). To sweep it cleanly, promote the vent area to a parameter and drive it from the command line rather than editing the deck by hand.

*PARAMETER
$#   name        value
     VENT_AREA   0.0080
$#   name        value
     VENT_DIA    0.0101

*MAT_FABRIC
$#      mid        ro         ea         eb        ec
         1   8.0e-7     2.0e11     2.0e11     2.0e11
$#     prba      cse      cse      ...
      0.30       0.0       0.0
$#  vent_id   vent_area
         1   &VENT_AREA

The baseline vent area in this example is 0.0080 m² (about 80 cm², roughly a 101 mm equivalent diameter). We will sweep from 0.0040 to 0.0140 m² in five steps. Each run is a full frontal 50 km/h impact with the same dummy, seatbelt, and inflator mass flow curve, so the only variable is vent area.

Sweep Setup and Execution

Write a small driver script that substitutes VENT_AREA, runs the solver, and collects the chest deflection history. The example below uses a shell loop with a parameter file; adapt the solver invocation to your local LS-DYNA executable and license setup.

#!/bin/bash
# vent_sweep.sh - parametric vent area sweep
AREAS="0.0040 0.0065 0.0080 0.0105 0.0140"
for A in $AREAS; do
  RUN=vent_${A}
  mkdir -p $RUN
  cp baseline.k $RUN/model.k
  sed -i "s/&VENT_AREA/$A/" $RUN/model.k
  cd $RUN
  ls-dyna i=model.k ncpu=4 memory=200m > run.log 2>&1
  cd ..
done

After each run, extract the chest deflection from the dummy's chest accelerometer or chest deflection transducer output. In the baseline model the chest deflection is written to nodout or a dedicated *DATABASE_HISTORY_NODE set. The peak chest deflection is the maximum absolute value of the chest compression signal over the impact window (typically 0–120 ms).

# extract_peak.py - read peak chest deflection from nodout
import re, sys
def peak_chest(path):
    peak = 0.0
    with open(path) as f:
        for line in f:
            parts = line.split()
            if len(parts) >= 2:
                try:
                    v = float(parts[1])
                except ValueError:
                    continue
                if abs(v) > abs(peak):
                    peak = v
    return peak

for run in sys.argv[1:]:
    print(run, peak_chest(f"{run}/nodout"))

Run the extraction over the five output directories. The expected shape of the response is a U-curve: chest deflection falls as vent area increases from the stiff baseline, reaches a minimum, then rises again as the bag becomes too soft and the chest travels forward into the bottom-out region.

Reading the Response and Guarding Against Bottom-Out

The table below shows representative example values for this sweep. These are illustrative numbers for the teaching case, not measured results from a specific run; your model will produce its own values, but the trend and the decision logic are the same.

The minimum chest deflection in this example sits near 0.0080 m². The largest vent area, 0.0140 m², shows a bottom-out flag: the bag pressure at 60 ms has fallen far enough that the cushion no longer separates the chest from the steering wheel, and the chest deflection rises sharply. That run must be rejected even though it is on the soft side of the curve, because bottom-out is a hard constraint, not a soft trade-off.

Vent Area vs. Chest Deflection and Bottom-Out Boundary

The figure below summarizes the sweep as a parameter-response curve with the bottom-out boundary overlaid. It is the decision chart for this case: the acceptable region is the interval where chest deflection is low and the bottom-out flag is clear.

The curve is shallow near the optimum, which is useful: a small manufacturing tolerance on vent area will not move chest deflection much. The steep rise on the soft side, however, means the bottom-out boundary is the real constraint. In practice you would pick the vent area at or just below the minimum, then confirm the bottom-out margin with a second run at the tolerance limit.

Verification and Acceptance Criteria

Accept the tuned vent area only when all of the following hold for the selected run:

• Peak chest deflection is at or near the minimum of the sweep and below the injury threshold you are targeting (for example, 40 mm for this teaching case).
• No bottom-out: bag-to-wheel contact force stays zero, or the bag surface does not reach the wheel rim before the chest has rebounded.
• Bag pressure at 60 ms remains above the minimum that keeps the cushion inflated against the chest (roughly 15–20 kPa in this example).
• The result is stable when the vent area is perturbed by ±10%, confirming the choice is not sitting on a cliff edge.

If the sweep shows no clear minimum, or the minimum coincides with a bottom-out flag, the vent area alone cannot solve the problem. In that case the next lever is the inflator mass flow curve, which is the subject of the following exercise. For this case, the deliverable is the vent area value, the sweep table, and the bottom-out check that justifies the choice.