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In physics, a fluid is defined as a substance that deforms continuously when subjected to a tangential or shear stress, regardless of how small that stress may be. This means that fluids will flow or change shape when any force is applied parallel to their surface. Both liquids and gases are considered fluids because they share this property of continuous deformation under shear stress. Solids, in contrast, resist deformation and maintain their shape unless a significant force is applied. Understanding Fluid Mechanics and Stress
Fluids are commonly treated as a continuum in fluid mechanics, meaning their properties (such as density, pressure, and velocity) are assumed to be distributed smoothly and continuously throughout the region of interest. This allows for the analysis and calculation of fluid behavior using mathematical models. Examples of fluids include water, air, oil, and mercury. Basics of Fluid Mechanics | Ansys Innovation Courses
Fluids can be further classified as Newtonian or non-Newtonian. Newtonian fluids, such as water and air, have a linear relationship between shear stress and shear rate, while non-Newtonian fluids do not follow this linear relationship and may exhibit behaviors like shear thinning or thickening. Understanding Fluid Mechanics and Stress
Would you like more details on the differences between Newtonian and non-Newtonian fluids?
March 27, 2026 at 2:22 am
directcustomer
Subscriber
what is fluids?
March 27, 2026 at 2:22 am
AnsysGPT
Question to Ansys Assistant
what is fluids?
Response from Ansys Assistant
Definition of Fluids in Physics
In physics, a fluid is a substance that deforms continuously under the application of any shear (tangential) stress, no matter how small. This property distinguishes fluids from solids, which resist deformation and maintain their shape unless a significant force is applied. Both liquids and gases are considered fluids because they flow and change shape in response to applied forces. Recap and Conclusion
Classification: Newtonian vs. Non-Newtonian Fluids
Fluids are classified based on how their shear stress relates to the rate of deformation (shear rate). Newtonian fluids obey Newton’s law of viscosity, where shear stress is directly proportional to shear rate, and the proportionality constant is called viscosity. Common examples include water, air, oil, and mercury. Non-Newtonian fluids do not follow this linear relationship; their viscosity can change depending on the shear rate or other factors. Non-Newtonian fluids include categories such as shear-thinning (pseudoplastic), shear-thickening (dilatant), and plastic fluids, which are defined by how their shear stress responds to deformation. Recap and Conclusion
For Newtonian fluids, the relationship is expressed as τ = μ(du/dy), where τ is shear stress, μ is dynamic viscosity, and du/dy is the velocity gradient. For non-Newtonian fluids, the relationship can be generalized as τ = M(du/dy)N, where M is the consistency index and N is the flow behavior index. If N = 1, the fluid is Newtonian; if N < 1, it is pseudoplastic; if N > 1, it is dilatant; and if N = 0, it is plastic. Recap and Conclusion
Would you like a deeper explanation of the mathematical models used for non-Newtonian fluids?
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The Ansys Learning Forum is a public forum. You are prohibited from providing (i) information that is confidential to You, your employer, or any third party, (ii) Personal Data or individually identifiable health information, (iii) any information that is U.S. Government Classified, Controlled Unclassified Information, International Traffic in Arms Regulators (ITAR) or Export Administration Regulators (EAR) controlled or otherwise have been determined by the United States Government or by a foreign government to require protection against unauthorized disclosure for reasons of national security, or (iv) topics or information restricted by the People's Republic of China data protection and privacy laws.
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