Functions
	test_cd_reduction_defaults_to_off ()
	test_the_old_default_produced_the_observed_error ()
	test_cd_stays_in_the_physical_band (d_mm)
	test_zero_reynolds_is_the_no_flow_sentinel_only ()

Variables
	_DISCHARGE

Detailed Description

The injector discharge coefficient must not be used as a lever for other constraints.

`solver.closure.Cd_reduction_factor` used to default to 0.95. When the spray constraints
were violated the closure loop multiplied the injector Cd by it and tried again, up to
`max_iterations` times. That is not a discharge coefficient: a drilled orifice does not
flow less because its spray is long. Cd is orifice geometry and Reynolds number, full stop.

Measured on the 8 kN ethalox design point before this was fixed:

    Cd_geom(d=2.9 mm)            = 0.6055   (the physical answer)
    Cd after 5 reductions        = 0.6055 * 0.95**5 = 0.4685
    Cd the solver actually used  = 0.4686

A 23% error that propagates straight into orifice sizing. Size holes for Cd 0.47 and real
hardware at Cd 0.70 flows ~50% more: the same engine then makes +11.9% thrust and Pc, and
injector dP/Pc falls to 0.164, below the 0.20 chug floor.

And it never worked. The feedback has the WRONG SIGN -- lower Cd gives lower jet velocity,
which gives a larger SMD, which gives a LONGER evaporation length. Measured: x* went from
0.1256 m with the loop off to 0.1338 m with it on, against the 0.05 m limit it was chasing.
The loop simply exhausted its iterations having made the constraint worse.

An x* violation belongs in the infeasibility report (it is already in diagnostics as
`x_star`), not in the discharge coefficient.

Function Documentation

◆ test_cd_reduction_defaults_to_off()

test_cd_is_not_a_free_variable.test_cd_reduction_defaults_to_off ( )

1.0 means the multiply is a no-op. Anything below 1.0 corrupts Cd.

Definition at line 41 of file test_cd_is_not_a_free_variable.py.

◆ test_cd_stays_in_the_physical_band()

test_cd_is_not_a_free_variable.test_cd_stays_in_the_physical_band ( d_mm )

At real injector Reynolds numbers Cd must sit near its geometry value, not at Cd_min.

Definition at line 62 of file test_cd_is_not_a_free_variable.py.

◆ test_the_old_default_produced_the_observed_error()

test_cd_is_not_a_free_variable.test_the_old_default_produced_the_observed_error ( )

Guard the guard: 0.95^5 is exactly the 23% error that was measured in the field.

Definition at line 49 of file test_cd_is_not_a_free_variable.py.

◆ test_zero_reynolds_is_the_no_flow_sentinel_only()

test_cd_is_not_a_free_variable.test_zero_reynolds_is_the_no_flow_sentinel_only ( )

cd_from_re(0) returns Cd_min by design -- that is the NO-FLOW branch, not a real Cd.

Four call sites pass a literal 0.0 when inlet pressure is below chamber pressure. That is
correct in context (mdot is zero anyway), but it means Cd_min doubles as a sentinel, so a
Cd landing on Cd_min in a FLOWING case is a signal that something is wrong.

Definition at line 73 of file test_cd_is_not_a_free_variable.py.

Variable Documentation

◆ _DISCHARGE

test_cd_is_not_a_free_variable._DISCHARGE

protected

Initial value:

=  dict(Cd_inf=0.60, a_Re=0.18, Cd_min=0.35, use_geometry_cd=True,
                  d_ref_m=0.002, cd_small_hole_exponent=0.20, cd_large_hole_log_gain=0.015,
                  cd_inf_max=0.62, cd_inf_min_geom=0.48, d_min_m=0.0004)

Definition at line 36 of file test_cd_is_not_a_free_variable.py.

Functions

Variables