CFD insights for the design and analysis of hydraulic structures under complex free-surface flow conditions.
Open Channel Flow refers to fluid motion with a free surface exposed to the atmosphere and driven primarily by gravity. It is a fundamental phenomenon in many engineering applications, including hydraulic structures such as spillways, stilling basins, canals, and energy dissipators, as well as river engineering, flood management, irrigation, mining, and environmental flows.
A key parameter in open channel hydraulics is the Froude number, which compares inertial and gravitational effects, and is defined as the ratio of the characteristic flow velocity (U) to the square-root of the product of the gravitational acceleration (g), and the hydraulic depth (h). It clasifies open channel flows and free-surface flows as:
From a design and analysis perspective, open channel flows are challenging due to strong free-surface deformation, rapidly varying flow regimes, and highly turbulent structures. Phenomena such as hydraulic jumps, flow separation, recirculation, and air–water interaction are inherently unsteady and sensitive to geometric details, making reliable prediction of local velocities, pressure loads, and energy dissipation mechanisms difficult. While empirical correlations, design charts, and standardized guidelines remain effective for preliminary design, they offer limited insight into local flow features and transient behavior, particularly for non-standard geometries or operating conditions.
Computational Fluid Dynamics (CFD) complements classical hydraulic methods by enabling a detailed, physics-based description of open channel flow. In particular, Ansys FreeFlow, based on a Smoothed Particle Hydrodynamics (SPH) approach, is well suited for free-surface-dominated applications, as it naturally captures large surface deformations and strongly transient phenomena. This enables robust representation of flow separation, splashing, and recirculation, providing deeper insight into free-surface evolution, velocity fields, and energy dissipation mechanisms than traditional design tools.
Methods
To address this type of hydraulic design problem, engineers rely on a combination of analytical methods, empirical design guidelines, and numerical simulations. Classical hydraulic relations and standards are typically used for preliminary sizing and conceptual layouts of energy dissipation and flow control structures. For more complex geometries and flow conditions, Computational Fluid Dynamics (CFD) provides a powerful tool to analyze velocity fields, pressure distribution, turbulence, and recirculation zones in applications like:
CFD allows engineers to visualize the flow behavior in detail, evaluate alternative designs, and assess the effectiveness of energy dissipation structures under different operating scenarios, reducing uncertainty before construction or physical testing. Some examples are shown below using Ansys Tools [1] such as Ansys LS-Dyna, Fluent, CFX and FreeFlow. The capabilities not only include CFD, but Fluid-Structure Interaction as well [2]. Application examples are listed as follows:
Methods
Ansys FreeFlow is a new product that provides simple setup and fast solutions for free-surface flows using SPH (smoothed-particle hydrodynamics). SPH is a mesh-free Lagrangian method for simulating fluid flow and other physical phenomena. It represents fluids as a collection of particles and uses smoothing to interpolate properties between neighboring particles. This allows it to model complex fluid behavior and easily handle complex motion. In this blog, a simulation of a USBR Type III is presented for demonstration purposes.
USBR Type III. USBR stilling basins, developed by the United States Bureau of Reclamation, comprise a set of standardized energy dissipator designs (Type I, II, III, among others) intended for different approach flow conditions. The Type III configuration features baffle blocks on a horizontal apron, which enhance turbulence, stabilize the hydraulic jump, and promote effective energy dissipation within the basin.
Geometry. The picture below shows the geometry of the USBR Type III for the simulation. The dimensions are not the result of a specific design process, but the geometry is used for demonstration purposes. The file is available for download at the end of this blog.
Boundary conditions. For this simulation, the SPH setup requires the definition of inlet and outlet sections. The inlet is a rectangular section located at the origin and rotated to match the inclination of the channel walls. Its dimensions are equal to the channel width and the specified water depth. For this demonstration, the inlet water velocity was set to 5.5 m/s. The outlet is also defined as a rectangular section located at the end of the channel, with dimensions equal to the channel width and height. More boundary conditions are presented as follows:
Results. FreeFlow is able to capture the hydraulic jump occurring in this structure, which is a strong dissipative mechanism where the flow transitions from a supercritical to a subcritical regime. It is important to note that the results are highly dependent on the geometry and boundary conditions. Designers or analyists can import different geometries and rerun the simulation to evaluate alternative configurations. The side walls were defined as transparent to allow visualization of the flow behavior including bubble formation, complex motion, and free surface dynamics.
Hardware. The simulation was solved using the GPU NVIDIA RTX A6000. The simulation time was 103 min to represent 30 s of physical time.
Downloadable Files. The geometry files used for this Demo are available in this link.
Watch the following video to see the simulation setup in action and learn more about the key modeling details.
We offer support, mentoring, and consulting services to enhance the performance and reliability of your systems. Trust our proven track record to accelerate projects, optimize performance, and deliver high-quality, cost-effective results for both new and existing water control systems. For more information, please visit https://ozeninc.com.
Suggested blogs