Modeling Hydrodynamic Lubrication Based on Hydrodynamic Shear Stress
Key Takeaways
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Hydrodynamic lubrication is a lubrication regime in which a liquid lubricant is introduced between surfaces to prevent them from rubbing against each other.
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Hydrodynamic lubrication is extensively used in jet engine turbine blades, mechanical seals, bearings, gears, internal combustion engines, biomedicine, and nanotechnology.
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Depending on the yield shear stress, lubricant can be categorized into rigid or quasi-Newtonian.
In hydrodynamic lubrication, hydrodynamic shear stress characteristics are of great importance, as they influence the deformation of the lubricant.
Whenever there is friction between two surfaces–say a tool and a workpiece–it can lead to productivity issues. Hydrodynamic lubrication is an accepted lubrication regime that helps reduce friction between surfaces. In hydrodynamic lubrication, hydrodynamic shear stress characteristics are of great importance, as they influence the deformation of the lubricant. Depending on the hydrodynamic shear stress, material changes can be either permanent or temporary, which can impact the effectiveness of the lubrication.
Let’s explore what hydrodynamic lubrication is and why it is needed.
Hydrodynamic Lubrication
When two surfaces are in contact, friction force arises, which limits the ease of movement. In engineering, friction is a common phenomenon. In most engineering systems, lubrication is provided to prevent the wear and tear caused by two surfaces rubbing against each other.
Hydrodynamic lubrication is a lubrication regime in which a liquid lubricant is introduced between surfaces to prevent them from rubbing against each other. The lubricant is usually used to form a layer between the two surfaces. Hydrodynamic lubrication is also known as thick film or full film lubrication.
How Does Hydrodynamic Lubrication Reduce Friction?
We all know that even mirror-finished surfaces are composed of crests and troughs called hills and valleys. The imperfections on surfaces cause surface roughness. By introducing hydrodynamic lubrication, a proper lubricant is added to the contact surface, forming a thin layer. This film of lubricant prevents the surfaces from coming into direct contact with each other, thereby reducing friction.
Types of Lubricant
Any substance that can control the friction, wear, and tear of surfaces is called lubricant. Other functionalities of lubricants include the elimination of heat, power transmission, protection, additive supply to contacts, and sealing. Lubricants can be solid (such as molybdenum disulfide or graphene) or liquid (such as oil or water). They can take semi-solid forms in grease or gaseous forms as well (for example, air).
Applications of Hydrodynamic Lubrication
When the design of surfaces with optimal geometries becomes a challenging task, lubrication becomes necessary. Hydrodynamic lubrication is extensively used in jet engine turbine blades, mechanical seals, bearings, gears, internal combustion engines, biomedicine, and nanotechnology.
In all these applications, the fundamentals of hydrodynamic lubrication are utilized to establish a smooth surface and frictionless contact. The development of frictionless surface contacts in engineering systems is governed by the works of Navier and Stokes. Reynold’s equation is instrumental in validating the effectiveness of hydrodynamic lubrication. The fluid flow behavior of the lubricants can be modeled, and studies on such models describe the characteristics and fluid dynamics of the lubricants.
Let’s look at a model used in tribology called the Bingham Plastic Model.
Characterizing the Bingham Plastic Model Using Hydrodynamic Shear Stress
Grease is widely used as a lubricant, and the Bingham model is the model typically used to describe the behavior of grease. The mathematical basis of the model is Reynold’s equation. It is possible to predict the bearing behavior and core formation using this model.
There are two parameters by which the Bingham model is characterized:
- Viscosity
- Yield shear stress
Yield shear stress is the minimum hydrodynamic shear stress that must be applied to the lubricant to initiate flow. Depending on the yield shear stress, the lubricant can be categorized into rigid or quasi-Newtonian. When the hydrodynamic shear stress magnitude exceeds the yield shear stress, the lubricant flows as a Newtonian fluid. Otherwise, it is rigid.
It is important to have an understanding of the shear stress and yield shear stress of your lubricant when applying hydrodynamic lubrication to engineering systems. The fluid (lubricant) flow behavior, as well as the deformation rate, is dependent on the hydrodynamic shear stress acting on it.
Cadence’s tools can help you investigate and simulate flow behavior and shear stress distribution. Cadence offers a complete set of fluid dynamics simulation and analysis tools in Omnis 3D solver. Subscribe to our newsletter for the latest CFD updates or browse Cadence’s suite of CFD software, including Fidelity and Fidelity Pointwise, to learn more about how Cadence has the solution for you.