In this blog post, we will summarize the concept of Hydrocode, its differences from general CFD, as well as real-world application examples and research trends.

The Role of Computer Simulation

Computer simulation plays a crucial role when attempting to quantitatively analyze a phenomenon from a thermal and fluid dynamics perspective. In the past, physical test facilities such as wind tunnels or water tanks required the direct installation of test objects and measurement equipment for observation; this process consumed significant budget and time and was subject to experimental errors. In contrast, Computational Fluid Dynamics (CFD) can significantly reduce both budget and time, while also minimizing uncertainties caused by variables such as equipment aging or researcher proficiency.
CFD has already become an essential tool in the design and analysis of various fields, including aircraft, high-speed trains, and automobiles. In the process of predicting the required levels of lift, drag, and fuel consumption, CFD helps concretize the conceptualization of machines and devices and enables rapid iterative design.

What is a hydrocode, and how does it differ from CFD?

Unlike CFD, which deals with everyday fluid flow problems, a hydrocode is a general term for a family of simulation software developed to analyze phenomena—including physical and chemical reactions—that occur in extreme pressure and temperature environments. For example, in environments such as military explosions or the high-temperature, high-pressure conditions during a spacecraft’s atmospheric reentry, nonlinear and abrupt changes—such as objects melting or breaking apart—occur, making it difficult to make sufficient predictions using the simple models employed in general CFD.
The fundamental equations of fluid dynamics are the Navier–Stokes equations, derived from Newton’s second law, which are combined with the mass conservation equation and the equation of state to solve problems. However, hydrocodes must also account for changes in material properties under extreme conditions, solid fracture and phase transitions, chemical reactions, and rapid energy release; therefore, they cannot rely solely on the ideal gas equation of state.
Therefore, hydrocodes select and apply appropriate Equations of State (EOS), combustion models, and material failure and phase-change models depending on the problem. These models are essential for accurately simulating material behavior under high-temperature and high-pressure conditions, and selecting the wrong model can significantly distort the prediction results.

Actual Development Cases and Applications

A research laboratory has developed its own hydrocode that generates an orthogonal or cylindrical coordinate system internally and analyzes the flow field by applying the equations of mass conservation, momentum conservation, the equation of state, and combustion model equations. The physical quantities obtained from this code include pressure, velocity, density, and temperature at specific points, and these results allow designers to quantitatively evaluate the response of structures or protective devices.
Visualizing the actual analysis results provides important information such as the shock wave distribution around the re-entry vehicle and the maximum pressure and heat flux experienced by the object. These numerical values are directly utilized to establish design criteria or predict behavior under extreme conditions.
Another study addressed the supersonic flame propagation characteristics of mixtures containing AN-series propellants; such experimental and theoretical research provides crucial evidence for validating and tuning the combustion models incorporated into Hydrocode.
While hydrocode research tends to be utilized primarily in the military and defense sectors, its importance is growing in commercial and civilian applications, such as high-speed aircraft (Mach 5 or higher) and hyper-speed trains. For example, quantitative analysis of thermal loads or explosive events in atmospheric re-entry vehicles is useful for safety design and accident reconstruction, and it also aids in the technical analysis of past incidents, such as marine accidents.
Ultimately, to accurately predict the behavior of increasingly complex mechanical designs in extreme environments, the development of hydrocodes capable of integrating context-specific material property, combustion, and failure models is essential. Future research and expanded applications will contribute to improving the safety of hypersonic transportation and strengthening our ability to respond to extreme situations.