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Avoid division by 0 by just setting railbutton pressure CD when moving very slow

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Note CD really doesn't matter when velocity is 0, since there's no drag force anyway.
This commit is contained in:
JoePfeiffer 2023-03-18 10:47:50 -06:00
parent 769ac69300
commit 206603e865
1 changed files with 46 additions and 38 deletions
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84
core/src/net/sf/openrocket/aerodynamics/barrowman/RailButtonCalc.java
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@ -58,48 +58,56 @@ public class RailButtonCalc extends RocketComponentCalc {
final double notchArea = (button.getOuterDiameter() - button.getInnerDiameter()) * button.getInnerHeight(); final double notchArea = (button.getOuterDiameter() - button.getInnerDiameter()) * button.getInnerHeight();
final double refArea = outerArea - notchArea; final double refArea = outerArea - notchArea;
// accumulate Cd contribution from each rail button // accumulate Cd contribution from each rail button. If velocity is 0 just set CDmul to a value previously
// competed for velocity MathUtil.EPSILON and skip the loop to avoid division by 0
double CDmul = 0.0; double CDmul = 0.0;
for (int i = 0; i < button.getInstanceCount(); i++) { if (conditions.getMach() > MathUtil.EPSILON) {
for (int i = 0; i < button.getInstanceCount(); i++) {
// compute boundary layer height at button location. I can't find a good reference for the
// formula, e.g. https://aerospaceengineeringblog.com/boundary-layers/ simply says it's the // compute boundary layer height at button location. I can't find a good reference for the
// "scientific consensus". // formula, e.g. https://aerospaceengineeringblog.com/boundary-layers/ simply says it's the
double x = (button.toAbsolute(instanceOffsets[i]))[0].x; // location of button // "scientific consensus".
double rex = calculateReynoldsNumber(x, conditions); // Reynolds number of button location double x = (button.toAbsolute(instanceOffsets[i]))[0].x; // location of button
double del = 0.37 * x / Math.pow(rex, 0.2); // Boundary layer thickness double rex = calculateReynoldsNumber(x, conditions); // Reynolds number of button location
double del = 0.37 * x / Math.pow(rex, 0.2); // Boundary layer thickness
// compute mean airspeed over button
// this assumes airspeed changes linearly through boundary layer // compute mean airspeed over button
// and that all parts of the railbutton contribute equally to Cd, // this assumes airspeed changes linearly through boundary layer
// neither of which is true but both are plenty close enough for our purposes // and that all parts of the railbutton contribute equally to Cd,
// neither of which is true but both are plenty close enough for our purposes
double mach;
if (buttonHt > del) { double mach;
// Case 1: button extends beyond boundary layer if (buttonHt > del) {
// Mean velocity is 1/2 rocket velocity up to limit of boundary layer, // Case 1: button extends beyond boundary layer
// full velocity after that // Mean velocity is 1/2 rocket velocity up to limit of boundary layer,
mach = (buttonHt - 0.5*del) * conditions.getMach()/buttonHt; // full velocity after that
} else { mach = (buttonHt - 0.5*del) * conditions.getMach()/buttonHt;
// Case 2: button is entirely within boundary layer } else {
mach = MathUtil.map(buttonHt/2.0, 0, del, 0, conditions.getMach()); // Case 2: button is entirely within boundary layer
mach = MathUtil.map(buttonHt/2.0, 0, del, 0, conditions.getMach());
}
// look up Cd as function of speed. It's pretty constant as a function of Reynolds
// number when slow, so we can just use a function of Mach number
double cd = MathUtil.interpolate(cdDomain, cdRange, mach);
// Since later drag force calculations don't consider boundary layer, compute "effective Cd"
// based on rocket velocity
cd = cd * MathUtil.pow2(mach)/MathUtil.pow2(conditions.getMach());
// add to CDmul
CDmul += cd;
} }
// look up Cd as function of speed. It's pretty constant as a function of Reynolds // since we'll be multiplying by the instance count up in BarrowmanCalculator,
// number when slow, so we can just use a function of Mach number // we want to return the mean CD instead of the total
double cd = MathUtil.interpolate(cdDomain, cdRange, mach); CDmul /= button.getInstanceCount();
// Since later drag force calculations don't consider boundary layer, compute "effective Cd"
// based on rocket velocity
cd = cd * MathUtil.pow2(mach)/MathUtil.pow2(conditions.getMach());
// add to CDmul } else {
CDmul += cd; // value at velocity of MathUtil.EPSILON
} CDmul = 8.786395072609939E-4;
}
// since we'll be multiplying by the instance count up in BarrowmanCalculator,
// we want to return the mean CD instead of the total
CDmul /= button.getInstanceCount();
return CDmul * stagnationCD * refArea / conditions.getRefArea(); return CDmul * stagnationCD * refArea / conditions.getRefArea();
} }