Merge pull request #1333 from JoePfeiffer/refine-tube-drag

Refine tube drag

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

@@ -51,4 +51,19 @@ public abstract class RocketComponentCalc {
 	 */
 	public abstract double calculatePressureCD(FlightConditions conditions, 
 			double stagnationCD, double baseCD, WarningSet warnings);
+
+
+
+	/**
+	 * Calculation of Reynolds Number
+	 * 
+	 * @param length			characteristic length
+	 * @param conditions		Flight conditions taken into account
+	 * @return                  Reynolds Number
+	 */
+	public double calculateReynoldsNumber(double length, FlightConditions conditions) {
+		return conditions.getVelocity() * length /
+			conditions.getAtmosphericConditions().getKinematicViscosity();
+	}
+	
 }

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

@@ -6,8 +6,17 @@ import net.sf.openrocket.rocketcomponent.RocketComponent;
 import net.sf.openrocket.rocketcomponent.Tube;
 import net.sf.openrocket.util.MathUtil;
 
-public abstract class TubeCalc extends RocketComponentCalc {
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public abstract class TubeCalc extends RocketComponentCalc {
+	
+	private final static Logger log = LoggerFactory.getLogger(TubeFinSetCalc.class);
+
+	// air density (standard conditions)
+	private final double rho = 1.225; // kg/m^3
+	
 	private final double diameter;
 	private final double length;
 	protected final double innerArea;
@@ -29,29 +38,49 @@ public abstract class TubeCalc extends RocketComponentCalc {
 	@Override
 	public double calculatePressureCD(FlightConditions conditions,
 			double stagnationCD, double baseCD, WarningSet warnings) {
+		
+		// These calculations come from a mix of theoretical and empirical
+		// results, and are marked with (t) for theoretical and (e) for empirical.
+		// The theoretical results should not be modified; the empirical can be adjusted
+		// to better simulate real rockets as we get data.
+
+		// For the sources of the empirical formulas, see Carello, Ivanov, and Mazza,
+		// "Pressure drop in pipe lines for compressed air: comparison between experimental
+		// and theoretical analysis", Transactions on Engineering Sciences vol 18,
+		// ISSN 1743-35331998, 1998.
+		
+		// For the rockets for which we have data, the effect of the stagnation CD appears to be
+		// overstated.  This code multiplies it be a factor of 0.7 to better match experimental
+		// data
 
 		// Need to check for tube inner area 0 in case of rockets using launch lugs with
 		// an inner radius of 0 to emulate rail buttons (or just weird rockets, of course)
-		final double deltap;
+		double deltap;
 		if (innerArea > MathUtil.EPSILON) {
-			// calculation of pressure drop through pipe from "Atlas Copco Air Compendium",
-			// 1975, quoted as equation 14 in Carello, Ivanov, and Mazza, "Pressure drop
-			// in pipe lines for compressed air: comparison between experimental and
-			// theoretical analysis", Transactions on Engineering Sciences vol 18,
-			// ISSN 1743-35331998, 1998.
-
-			// Volume flow rate
+			// Temperature
+			final double T = conditions.getAtmosphericConditions().getTemperature();
+			
+			// Volume flow rate (t)
 			final double Q = conditions.getVelocity() * innerArea;
 			
-			// pressure drop
-			deltap = 1.6 * Math.pow(Q, 1.85) * length /
-				(Math.pow(diameter, 5) * conditions.getAtmosphericConditions().getPressure());
+			// Reynolds number (note Reynolds number for the interior of a pipe is based on diameter,
+			// not length (t))
+			final double Re = conditions.getVelocity() * diameter /
+				conditions.getAtmosphericConditions().getKinematicViscosity();
+			
+			// friction coefficient (for smooth tube interior) (e)
+			final double lambda = 1/MathUtil.pow2(2 * Math.log(0.5625 * Math.pow(Re, 0.875)) - 0.8);
+			
+			// pressure drop (e)
+			final double P0 = 100; // standard pressure
+			final double T0 = 273.15; // standard temperature
+			deltap = (lambda * 8 * length * rho * MathUtil.pow2(Q) * T * P0) /
+				(MathUtil.pow2(Math.PI) * Math.pow(diameter, 5) * T0 * conditions.getAtmosphericConditions().getPressure());
 		} else {
 			deltap = 0.0;
 		}
-		
+		   
 		// convert to CD and return
-		return (deltap * innerArea + stagnationCD * frontalArea) / conditions.getRefArea();
+		return (deltap * innerArea + 0.7 * stagnationCD * frontalArea) / conditions.getRefArea();
 	}
-
 }

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

@@ -101,9 +101,9 @@ public class TubeFinSetCalc extends TubeCalc {
 		// body tube, only consider the other half of the area
 		final double outerArea = tubes.getLength() * Math.PI * tubes.getOuterRadius();
 			
-		// Surface area of the portion of the body tube masked by the tube fins
+		// Surface area of the portion of the body tube masked by the tube fins, per tube
 		final BodyTube parent = (BodyTube) tubes.getParent();
-		final double maskedArea = tubes.getLength() * 2.0 * Math.PI * parent.getOuterRadius();
+		final double maskedArea = tubes.getLength() * 2.0 * Math.PI * parent.getOuterRadius() / tubes.getFinCount();
 		
 		wettedArea = outerArea - maskedArea;
 		log.debug("wetted area of tube fins " + wettedArea);