Implement full RailButton aerodynamics

core/src/net/sf/openrocket/aerodynamics/barrowman/RailButtonCalc.java

@@ -1,27 +1,36 @@
 package net.sf.openrocket.aerodynamics.barrowman;
 
+import java.util.List;
+import java.lang.Math;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import net.sf.openrocket.aerodynamics.AerodynamicForces;
 import net.sf.openrocket.aerodynamics.FlightConditions;
 import net.sf.openrocket.aerodynamics.Warning;
 import net.sf.openrocket.aerodynamics.WarningSet;
 import net.sf.openrocket.rocketcomponent.RailButton;
 import net.sf.openrocket.rocketcomponent.RocketComponent;
+import net.sf.openrocket.util.Coordinate;
 import net.sf.openrocket.util.MathUtil;
 import net.sf.openrocket.util.Transformation;
 
 public class RailButtonCalc extends RocketComponentCalc {
+	private final static Logger log = LoggerFactory.getLogger(RailButtonCalc.class);	
 
-	private double refArea;
+	// values transcribed from Gowen and Perkins, "Drag of Circular Cylinders for a Wide Range
+	// of Reynolds Numbers and Mach Numbers", NACA Technical Note 2960, Figure 7
+	private static final List<Double> cdDomain = List.of(0.0, 0.2,  0.3,  0.4, 0.5, 0.6, 0.7, 1.0, 1.6, 2.0,  2.8, 100.0);
+	private static final List<Double> cdRange =  List.of(1.2, 1.22, 1.25, 1.3, 1.4, 1.5, 1.6, 2.1, 1.5, 1.45, 1.33,  1.33);
+
+	private final RailButton button;
 
 	public RailButtonCalc(RocketComponent component) {
 		super(component);
 
-		final RailButton button = (RailButton) component;
+		// need to stash the button
-
+		button = (RailButton) component;
-		final double outerArea = button.getTotalHeight() * button.getOuterDiameter();
-		final double notchArea = (button.getOuterDiameter() - button.getInnerDiameter()) * button.getInnerHeight();
-
-		refArea = (outerArea - notchArea) * button.getInstanceCount();
 	}
 
 	@Override
@@ -39,16 +48,54 @@ public class RailButtonCalc extends RocketComponentCalc {
 	@Override
 	public double calculatePressureCD(FlightConditions conditions,
 									  double stagnationCD, double baseCD, WarningSet warnings) {
+		// grab relevant button params
 		
-		// this is reasonably close for Reynolds numbers roughly 10e4 to 2*10e5, which takes us to low supersonic speeds.
+		final int instanceCount = button.getInstanceCount();
-		// see Hoerner p. 3-9 fig 12, we summarizes a bunch of sources
+		final Coordinate[] instanceOffsets = button.getInstanceOffsets();
-		// I expect we'll have compressibility effects having an impact well below that, so this is probably good up
-		// to the transonic regime.
-		double CDmul = 1.2;
 
-		// warn the user about accuracy if we're transonic (roughly Mach 0.8) or faster
+		// compute button reference area
-		if (conditions.getMach() > 0.8) {
+		final double buttonHt = button.getTotalHeight();
-			warnings.add(Warning.RAILBUTTON_TRANSONIC);
+		final double outerArea = buttonHt * button.getOuterDiameter();
+		final double notchArea = (button.getOuterDiameter() - button.getInnerDiameter()) * button.getInnerHeight();
+		final double refArea = outerArea - notchArea;
+
+		// accumulate Cd contribution from each rail button
+		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());
+			}
+
+			// 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;
 		}
 		
 		return CDmul*stagnationCD * refArea / conditions.getRefArea();