A new three-dimensional mathematical model is proposed to predict the shape of accreted ice on helicopter rotors. The model assumes that ice accretion on a rotor’s surface occurs by flow and solidification of a thin water film on its surface. The model is developed by analyzing conservation of mass, momentum, and energy in the thin water film on a rotating curved surface. The model is expressed in rotating body-fitted nonorthogonal curvilinear coordinates, and the Coriolis-force term in the momentum equations is neglected because its effect is relatively small compared with that of the centrifugal force. The computational method is presented to solve the model, and the model is validated by comparing its results with published experimental data. The results show that the proposed model accurately predicts both the water-film thickness and the ice-layer thickness on a rotating surface. Additionally, the model has been used to study how the rotor angular speed and MVD (Median Volumetric Diameter) of the droplets affect ice accretion. The results show that an increase in both angular speed and MVD can lead to a thicker layer of ice, while the effect of MVD on ice accretion near the stagnation point becomes unobvious when MVD is larger enough.

Piston engines fueled by kerosene have a strong application prospect in special vehicles and small aircrafts, but the low amount of octane in kerosene fuel causes its knock combustion phenomenon to be particularly serious. A knock will deteriorate the power and economy of the engine and will damage the engine in serious cases. Therefore, knocking is the key problem with kerosene engines. We propose a knock-prediction system for kerosene engines based on in-cylinder pressure signals. Firstly, the intrinsic mode function (IMF) caused by knock resonance is extracted from the in-cylinder pressure signal via empirical mode decomposition (EMD) and a time–frequency domain analysis. A time-domain statistical analysis (TDSA) combined with a principal component analysis (PCA) is used to extract features from the IMF. Finally, the data collected from the test bench are trained by a support vector machine to obtain the knock-prediction model. Compared with other technical combinations for training, the proposed scheme achieved more accurate results in knock prediction. Considering the working characteristics of kerosene engines, a slight knock can increase the power of a kerosene engine. Therefore, some incorrectly predicted cycles (slight-knock cycles) do not affect the normal operation of the engine.

Laminar separation bubbles (LSBs) on low-Reynolds-number airfoils are sustained by intrinsic unsteadiness driven by Kelvin–Helmholtz (K-H) growth in the separated shear layer. Using incompressible 2D URANS with the SA-$\gamma$ transition model for a NACA 0012 airfoil at $Re=5.3\times {10}^4$, we reveal that numerical dissipation behaves as a critical bifurcation parameter. Validated against the recent Jardin (2025) experimental benchmark, the physical state correctly resolves the LSB-induced pressure plateau ($C_p$) and local negative skin friction ($C_f<0$). However, when numerical dissipation exceeds the K-H instability growth rate, the physical limit-cycle oscillation collapses into a spurious fixed-point attractor—a phenomenon defined as numerical quenching. This pseudo-convergence triggers a catastrophic ∼30% deficit in mean lift ($C_l$). Furthermore, at $\alpha =6^{\circ }$, a drag-mechanism inversion is identified: while the physical branch is dominated by LSB-induced pressure (form) drag, the quenched branch exhibits a non-physical drag surge that exceeds the fully turbulent baseline. Phase portraits and power spectral densities ($St\approx 0.2$) provide objective diagnostics, demonstrating that standard residual convergence is a deceptive indicator of physical fidelity in transitional separated aerodynamics.