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Fix for Issue #175. Use a larger number to test equality of fin point y

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positions to reduce numerical stability in computation of chord lengths.
This commit is contained in:
kruland 2014-01-17 20:36:46 -06:00
parent a3a36a316d
commit b39cd8a016
3 changed files with 166 additions and 104 deletions
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23
core/src/net/sf/openrocket/aerodynamics/barrowman/FinSetCalc.java
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@ -17,8 +17,14 @@ import net.sf.openrocket.util.LinearInterpolator;
import net.sf.openrocket.util.MathUtil;
import net.sf.openrocket.util.PolyInterpolator;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class FinSetCalc extends RocketComponentCalc {
private final static Logger logger = LoggerFactory.getLogger(FinSetCalc.class);
private static final double STALL_ANGLE = (20 * Math.PI / 180);
/** Number of divisions in the fin chords. */
@ -112,7 +118,7 @@ public class FinSetCalc extends RocketComponentCalc {
// One fin without interference (both sub- and supersonic):
double cna1 = calculateFinCNa1(conditions);
// logger.debug("Component cna1 = {}", cna1);
// Multiple fins with fin-fin interference
double cna;
@ -134,6 +140,7 @@ public class FinSetCalc extends RocketComponentCalc {
cna = cna1 * finCount / 2.0;
}
// logger.debug("Component cna = {}", cna);
// Take into account fin-fin interference effects
switch (interferenceFinCount) {
@ -223,6 +230,7 @@ public class FinSetCalc extends RocketComponentCalc {
tau = 0;
cna *= 1 + tau; // Classical Barrowman
// cna *= pow2(1 + tau); // Barrowman thesis (too optimistic??)
// logger.debug("Component cna = {}", cna);
@ -234,7 +242,11 @@ public class FinSetCalc extends RocketComponentCalc {
// Calculate CP position
// logger.debug("Component macLead = {}", macLead);
// logger.debug("Component macLength = {}", macLength);
//FIXME - macLength is incorrect!
double x = macLead + calculateCPPos(conditions) * macLength;
// logger.debug("Component x = {}", x);
@ -342,7 +354,9 @@ public class FinSetCalc extends RocketComponentCalc {
double x2 = points[point].x;
double y2 = points[point].y;
if (MathUtil.equals(y1, y2))
// Don't use the default EPSILON since it is too small
// and causes too much numerical instability in the computation of x below
if (MathUtil.equals(y1, y2, 0.001))
continue;
int i1 = (int) (y1 * 1.0001 / span * (DIVISIONS - 1));
@ -412,6 +426,7 @@ public class FinSetCalc extends RocketComponentCalc {
double y = i * dy;
macLength += length * length;
logger.debug("macLength = {}, length = {}, i = {}", macLength, length, i);
macSpan += y * length;
macLead += chordLead[i] * length;
area += length;
@ -427,6 +442,7 @@ public class FinSetCalc extends RocketComponentCalc {
}
macLength *= dy;
logger.debug("macLength = {}", macLength);
macSpan *= dy;
macLead *= dy;
area *= dy;
@ -606,6 +622,7 @@ public class FinSetCalc extends RocketComponentCalc {
*/
private double calculateCPPos(FlightConditions cond) {
double m = cond.getMach();
// logger.debug("m = {} ", m);
if (m <= 0.5) {
// At subsonic speeds CP at quarter chord
return 0.25;
@ -624,7 +641,7 @@ public class FinSetCalc extends RocketComponentCalc {
val += poly[i] * x;
x *= m;
}
// logger.debug("val = {}", val);
return val;
}

12
core/src/net/sf/openrocket/util/MathUtil.java
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@ -227,17 +227,19 @@ public class MathUtil {
}
public static boolean equals(double a, double b) {
public static boolean equals(double a, double b, double epsilon) {
double absb = Math.abs(b);
if (absb < EPSILON / 2) {
if (absb < epsilon / 2) {
// Near zero
return Math.abs(a) < EPSILON / 2;
return Math.abs(a) < epsilon / 2;
}
return Math.abs(a - b) < EPSILON * absb;
return Math.abs(a - b) < epsilon * absb;
}
public static boolean equals(double a, double b) {
return equals(a, b, EPSILON);
}
/**
* Return the sign of the number. This corresponds to Math.signum, but ignores

43
core/test/net/sf/openrocket/rocketcomponent/FinSetTest.java
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@ -3,6 +3,10 @@ package net.sf.openrocket.rocketcomponent;
import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;
import net.sf.openrocket.aerodynamics.AerodynamicForces;
import net.sf.openrocket.aerodynamics.FlightConditions;
import net.sf.openrocket.aerodynamics.WarningSet;
import net.sf.openrocket.aerodynamics.barrowman.FinSetCalc;
import net.sf.openrocket.material.Material;
import net.sf.openrocket.material.Material.Type;
import net.sf.openrocket.rocketcomponent.ExternalComponent.Finish;
@ -124,6 +128,45 @@ public class FinSetTest extends BaseTestCase {
}
@Test
public void testWildmanVindicatorShape() throws Exception {
// This fin shape is similar to the aft fins on the Wildman Vindicator.
// A user noticed that if the y values are similar but not equal,
// the compuation of CP was incorrect because of numerical instability.
//
// +-----------------+
// \ \
// \ \
// + \
// / \
// +---------------------+
//
FreeformFinSet fins = new FreeformFinSet();
fins.setFinCount(1);
Coordinate[] points = new Coordinate[] {
new Coordinate(0, 0),
new Coordinate(0.02143125, 0.01143),
new Coordinate(0.009524999999999999, 0.032543749999999996),
new Coordinate(0.041275, 0.032537399999999994),
new Coordinate(0.066675, 0)
};
fins.setPoints(points);
Coordinate coords = fins.getCG();
assertEquals(0.00130, fins.getFinArea(), 0.00001);
assertEquals(0.03423, coords.x, 0.00001);
assertEquals(0.01427, coords.y, 0.00001);
BodyTube bt = new BodyTube();
bt.addChild(fins);
FinSetCalc calc = new FinSetCalc(fins);
FlightConditions conditions = new FlightConditions(null);
AerodynamicForces forces = new AerodynamicForces();
WarningSet warnings = new WarningSet();
calc.calculateNonaxialForces(conditions, forces, warnings);
System.out.println(forces);
assertEquals(0.023409, forces.getCP().x, 0.0001);
}
@Test
public void testFreeFormCGWithNegativeY() throws Exception {
// This particular fin shape is currently not allowed in OR since the y values are negative