Evaluating the Operation of Hydrodynamic Power Transmission With CFD
Key Takeaways

Hydrodynamic power transmission units utilize moving fluid to hydrodynamically transfer power.

Hydrodynamic power transmission can be done using fluid coupling units and hydrodynamic torque converters.

The power transmission can be calculated based on Bernoulli’s and Euler’s principles.
A hydrodynamic power transmission unit–a torque converter in an automotive vehicle
Many automotive systems use the concept of hydrodynamics, or a fluid’s motion, to work the machine. In many machines, this is done with the help of a hydrodynamic power transmission unit, which utilizes fluid to hydrodynamically transfer the power generated by driving the engine to drive the load. We can usually observe this in vehicles or propulsion systems where the automatic transmission system uses fuel (i.e., fluid) to transfer the power to drive the motor or rotate a turbine.
Since the fluid flow is the main driving force, CFD can be used to analyze the torque conversion and power generation in the system. This calculation can then be used in the simulation and validation of the torque converters and couplings. In this article, we will discuss hydrodynamic power transmission and the applicability of CFD in the design of transmission systems.
Hydrodynamic Power Transmission
Hydrodynamic power transmission is based on the Foettinger principle, which explains the transmission of power via moving fluid to drive the output component. A power transmission system primarily consists of two parts–the pump wheel and the rotor wheel. As the fluid flows through a pump wheel, it converts the mechanical energy of the pump to kinetic energy in the fluid. This highenergy fluid flows to the turbine wheel, where the kinetic energy changes back to mechanical energy again. This is an indirect transmission principle. The direct transmission principle involves the use of transmission gear units to operate. Based on these principles, the hydrodynamic transmission units can be fluid coupling and hydrodynamic torque converters.
Fluid Coupling
The fluid coupling in a hydrodynamic transmission system has two main components–a pump on the inlet side and a turbine on the outlet side. When the power is supplied to the pump, the velocity of the fluid passing through it increases. This flowing fluid reaches the turbine blade, rotating it to generate an output power.
Hydrodynamic Torque Converter
A hydrodynamic torque converter has three primary components–a pump, a turbine, and a reaction member (or a guide wheel or stator placed between the pump and turbine). When the power is supplied, the pump energizes the working fluid, which in turn operates the turbine. The power of the working fluid directly influences the output power to drive the hydrodynamic system. The power can be calculated using the formula below:
So, the available power is equal to the rotating speed times the torque. The torque converter is a convenient way of analyzing the power ratio between that of the driving and the driven machine.
Operating Hydrodynamic Principles
The power transmission system follows the basic principles of hydrodynamics–Bernoulli’s equation and Euler’s equation. We know the Bernoulli’s energy equation for a streamlined flow is as follows:
Note that, v is velocity, ρ is density, p is the pressure, g is the gravity, and z is the elevation from a reference plane. The term gz indicates the gravitational potential energy.
For a power transmission system, the Euler’s equation for turbomachinery dictates the calculation for power and machine efficiency:
Here, Y is the specific hydrodynamic energy, which is equal to the product of the pressure head and acceleration due to gravity, i.e., Y=H . g.
u1 and u2 are the velocities of the pump and turbine blade at inlet and outlet.
c1 and c2 are the absolute velocities of the fluid at inlet and outlet.
Using the above equations, the power transmission can now be calculated with the following equation, where m’ is the flow mass:
P=m'Y
Hydrodynamic Power Transmission Analysis Using CFD
In hydrodynamic power transmission analysis, CFD tools can prove to be instrumental in the design and optimization of fluid coupling and torque converters. CFD simulation of these hydrodynamic power transmission systems is helpful in simplifying the performance analysis and improving the accuracy of torque conversion and power generation calculations. Furthermore, the use of CFD is also beneficial in stimulating the flow within a transmission system. With flow modeling and design validation, CFD can improve the efficiency of the hydrodynamic power transmission system while reducing costs.
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