a [m]	Assumed bubble length	T [s]	Transit time
c [m/s]	Speed of sound	T₀ [s]	Time travelled through the TDX
d [m]	Horizontal bubble length	T_c [s]	Reference transit time for calibration
$D (U_{i}^{1}, U_{j}^{2})$ [s]	Distance function between trajectories from TDX1 and TDX2	u [-]	Number of points in an interpolated trajectory
$E (U_{i}, ℍ_{j})$ [s]	Distance function between pulse-echo and HSC trajectories	$U$	Pulse echo trajectory
f₀ [s⁻¹]	Basic frequency of the ultrasound	VF [-]	Void fraction
f_PRF [s⁻¹]	Pulse repetition frequency	w [s/m]	Scaling factor
F_a [m³/s]	Air flow rate	W' [s/m]	Scaling factor
F_w [m³/s]	Water flow rate	x [m]	Distance between bubble and TDX
h [-]	HSC1 trajectory length	x' [m]	Distance between TDX and the opposite side of the bubble
$ℍ$	HSC trajectory	x₁₂ [m]	Distance between TDX surfaces
M [V²]	Signal strength	x_c [m]	Reference distance for calibration
N [-]	Number of cycles in a pulse	y [m]	Horizontal distance between the bubble and the TDX axis
n, m [-]	Number of points in a trajectory	y_max [m]	Maximum estimated y
l [-]	HSC2 trajectory length	y_min [m]	Minimum estimated y
$R_{2}^{y z}$ [-]	Tilt of bubble trajectory on HSC2	z [m]	Vertical bubble position
t [s]	Measurement time (pulse count)	$σ_{R}^{y z}$ [-]	Standard deviation of bubble tilt
Subscripts and Superscripts
i, j	Trajectory counter	1, 2	TDX or HSC counter
Abbreviations
HSC	High-speed camera	TDX	Ultrasonic transducer