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Merge pull request #2122 from JoePfeiffer/fix-2115

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Railbutton pressure drag tweaks
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Joe Pfeiffer 2023-03-19 18:58:32 -06:00 committed by GitHub
commit 98fb726c4b
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2 changed files with 113 additions and 34 deletions
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80
core/src/net/sf/openrocket/aerodynamics/barrowman/RailButtonCalc.java
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@ -58,44 +58,56 @@ public class RailButtonCalc extends RocketComponentCalc {
final double notchArea = (button.getOuterDiameter() - button.getInnerDiameter()) * button.getInnerHeight();
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;
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
// "scientific consensus".
double x = (button.toAbsolute(instanceOffsets[i]))[0].x; // location of button
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
// 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) {
// Case 1: button extends beyond boundary layer
// Mean velocity is 1/2 rocket velocity up to limit of boundary layer,
// full velocity after that
mach = (buttonHt - 0.5*del) * conditions.getMach()/buttonHt;
} else {
// Case 2: button is entirely within boundary layer
mach = MathUtil.map(buttonHt/2.0, 0, del, 0, conditions.getMach());
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
// "scientific consensus".
double x = (button.toAbsolute(instanceOffsets[i]))[0].x; // location of button
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
// 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) {
// Case 1: button extends beyond boundary layer
// Mean velocity is 1/2 rocket velocity up to limit of boundary layer,
// full velocity after that
mach = (buttonHt - 0.5*del) * conditions.getMach()/buttonHt;
} else {
// 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
// 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());
// 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();
// add to CDmul
CDmul += cd;
}
} else {
// value at velocity of MathUtil.EPSILON
CDmul = 8.786395072609939E-4;
}
return CDmul * stagnationCD * refArea / conditions.getRefArea();
}

67
core/test/net/sf/openrocket/aerodynamics/BarrowmanCalculatorTest.java
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@ -20,11 +20,13 @@ import net.sf.openrocket.rocketcomponent.FlightConfiguration;
import net.sf.openrocket.rocketcomponent.NoseCone;
import net.sf.openrocket.rocketcomponent.ParallelStage;
import net.sf.openrocket.rocketcomponent.PodSet;
import net.sf.openrocket.rocketcomponent.RailButton;
import net.sf.openrocket.rocketcomponent.Rocket;
import net.sf.openrocket.rocketcomponent.Transition;
import net.sf.openrocket.rocketcomponent.TrapezoidFinSet;
import net.sf.openrocket.startup.Application;
import net.sf.openrocket.util.Coordinate;
import net.sf.openrocket.util.MathUtil;
import net.sf.openrocket.util.TestRockets;
public class BarrowmanCalculatorTest {
@ -496,5 +498,70 @@ public class BarrowmanCalculatorTest {
testCP = testCalc.getCP(testConfig, testConditions, warnings).x;
assertEquals("should be warning from podset airframe overlap", 1, warnings.size());
}
/**
* Tests railbutton drag. Really is testing instancing more than actual drag calculations, and making
* sure we don't divide by 0 when not moving
*/
@Test
public void testRailButtonDrag() {
// minimal rocket with nothing on it but two railbuttons
final Rocket rocket = new Rocket();
final AxialStage stage = new AxialStage();
rocket.addChild(stage);
// phantom tubes have no drag to confuse things
final BodyTube phantom = new BodyTube();
phantom.setOuterRadius(0);
stage.addChild(phantom);
// set up test environment
WarningSet warnings = new WarningSet();
final FlightConfiguration config = rocket.getSelectedConfiguration();
final FlightConditions conditions = new FlightConditions(config);
final BarrowmanCalculator calc = new BarrowmanCalculator();
// part 1: instancing
// Put two individual railbuttons and get their CD
final RailButton button1 = new RailButton();
button1.setInstanceCount(1);
button1.setAxialOffset(1.0);
phantom.addChild(button1);
final RailButton button2 = new RailButton();
button2.setInstanceCount(1);
button2.setAxialOffset(2.0);
phantom.addChild(button2);
final AerodynamicForces individualForces = calc.getAerodynamicForces(config, conditions, warnings);
final double individualCD = individualForces.getCD();
// get rid of individual buttons and put in a railbutton set with two instances at same locations as original
// railbuttons
phantom.removeChild(button1);
phantom.removeChild(button2);
final RailButton buttons = new RailButton();
buttons.setInstanceCount(2);
buttons.setAxialOffset(1.0);
buttons.setInstanceSeparation(1.0);
final AerodynamicForces pairForces = calc.getAerodynamicForces(config, conditions, warnings);
final double pairCD = pairForces.getCD();
assertEquals("two individual railbuttons should have same CD as a pair", individualCD, pairCD, EPSILON);
// part 2: test at Mach 0
conditions.setMach(MathUtil.EPSILON);
final AerodynamicForces epsForces = calc.getAerodynamicForces(config, conditions, warnings);
final double epsCD = epsForces.getCD();
conditions.setMach(0);
final AerodynamicForces zeroForces = calc.getAerodynamicForces(config, conditions, warnings);
final double zeroCD = zeroForces.getCD();
assertEquals("drag at mach 0 should equal drag at mach MathUtil.EPSILON", epsCD, zeroCD, EPSILON);
}
}