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Fix mistake of pre-multiplying aerodynamic coefficients

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Increase glider zero-lift drag
This commit is contained in:
Ludvig Böklin 2021-08-06 17:36:00 +02:00 committed by Treeco
parent 7039bf7a0c
commit b667ba86f9
2 changed files with 30 additions and 32 deletions
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1
CHANGELOG.md
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@ -67,6 +67,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
- Map will now zoom around the cursor's position and drag correctly
- No more jittering while running down slopes with the glider out
- Axe normal attack rewards energy without skill points
- Gliders no longer suffer from unreasonable amounts of induced drag
## [0.10.0] - 2021-06-12

31
common/src/comp/fluid_dynamics.rs
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@ -140,7 +140,6 @@ impl Body {
Vec3::zero()
} else {
let rel_flow_dir = Dir::new(rel_flow.0 / v_sq.sqrt());
// All the coefficients come pre-multiplied by their reference area
0.5 * fluid_density
* v_sq
* match wings {
@ -163,7 +162,7 @@ impl Body {
let aoa = angle_of_attack(&ori, &rel_flow_dir);
// c_l will be positive when aoa is positive (we have positive lift,
// producing an upward force) and negative otherwise
let c_l = lift_coefficient(ar, planform_area, aoa);
let c_l = lift_coefficient(ar, aoa);
// lift dir will be orthogonal to the local relative flow vector.
// Local relative flow is the resulting vector of (relative) freestream
@ -187,25 +186,24 @@ impl Body {
ori.pitched_down(aoa_eff).up()
};
// drag coefficient
let c_d = {
// induced drag coefficient (drag due to lift)
let cdi = {
// Oswald's efficiency factor (empirically derived--very magical)
// (this definition should not be used for aspect ratios > 25)
let e = 1.78 * (1.0 - 0.045 * ar.powf(0.68)) - 0.64;
// induced drag coefficient (drag due to lift)
let cdi = c_l.powi(2) / (PI * e * ar);
zero_lift_drag_coefficient(planform_area)
+ self.parasite_drag_coefficient()
+ cdi
c_l.powi(2) / (PI * e * ar)
};
// drag coefficient
let c_d = zero_lift_drag_coefficient() + cdi;
debug_assert!(c_d.is_sign_positive());
debug_assert!(c_l.is_sign_positive() || aoa.is_sign_negative());
c_l * *lift_dir + c_d * *rel_flow_dir
planform_area * (c_l * *lift_dir + c_d * *rel_flow_dir)
+ self.parasite_drag() * *rel_flow_dir
},
_ => self.parasite_drag_coefficient() * *rel_flow_dir,
_ => self.parasite_drag() * *rel_flow_dir,
}
}
}
@ -214,7 +212,7 @@ impl Body {
/// Skin friction is the drag arising from the shear forces between a fluid
/// and a surface, while pressure drag is due to flow separation. Both are
/// viscous effects.
fn parasite_drag_coefficient(&self) -> f32 {
fn parasite_drag(&self) -> f32 {
// Reference area and drag coefficient assumes best-case scenario of the
// orientation producing least amount of drag
match self {
@ -330,11 +328,10 @@ pub fn angle_of_attack(ori: &Ori, rel_flow_dir: &Dir) -> f32 {
/// Total lift coefficient for a finite wing of symmetric aerofoil shape and
/// elliptical pressure distribution.
pub fn lift_coefficient(aspect_ratio: f32, planform_area: f32, aoa: f32) -> f32 {
pub fn lift_coefficient(aspect_ratio: f32, aoa: f32) -> f32 {
let aoa_abs = aoa.abs();
let stall_angle = PI * 0.1;
planform_area
* if aoa_abs < stall_angle {
if aoa_abs < stall_angle {
lift_slope(aspect_ratio, None) * aoa
} else {
// This is when flow separation and turbulence starts to kick in.
@ -356,7 +353,7 @@ pub fn lift_coefficient(aspect_ratio: f32, planform_area: f32, aoa: f32) -> f32
/// The zero-lift profile drag coefficient is the parasite drag on the wings
/// at the angle of attack which generates no lift
pub fn zero_lift_drag_coefficient(planform_area: f32) -> f32 { planform_area * 0.004 }
pub fn zero_lift_drag_coefficient() -> f32 { 0.026 }
/// The change in lift over change in angle of attack¹. Multiplying by angle
/// of attack gives the lift coefficient (for a finite wing, not aerofoil).