14.4
流体の種類
流体は、せん断応力に対する反応に基づいて、ニュートン流体と非ニュートン流体に分類できます。ニュートン流体は、せん断応力とせん断ひずみ速度の間に直線関係があり、ニュートンの粘性法則に従います。その粘度はせん断速度に関係なく一定であるため、挙動を予測でき、分析が容易です。一般的な例としては、水、空気、油…
流体は、ニュートンと非ニュートンの2つの主要なタイプに分類されます。ニュートン流体は粘度が一定で、そのせん断応力はせん断ひずみ速度に線形に関連しています。
ニュートン流体の振る舞いは予測可能です。例としては、水、空気、油、ガソリンなどがあります。
ただし、非ニュートン流体は、せん断速度に基づいて粘度が変化します。それらは、せん断減粘、せん断増粘、およびビンガムプラスチックに分けられます。
ラテックス塗料やポリマー溶液などのせん断減粘液は、せん断速度が増加すると見かけの粘度が低下し、せん断時に流動性が高くなります。
このプロパティは、ブラシ ペイントやインクジェット印刷など、滑らかで均一な塗布が必要なアプリケーションで役立ちます。
水中のコーンスターチなどのせん断増粘液は、より高いせん断速度で見かけの粘度が増加し、攪拌すると流れに対してより抵抗力が高くなります。
このユニークな特性により、保護具やダンピングデバイスに役立ちます。
歯磨き粉やマヨネーズなどのビンガムプラスチックは、降伏応力を超えると固体として動作し、その後流体のように流れるため、押し出しや広がりが必要な製品に最適です。
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Q1: What is the main difference between Newtonian and non-Newtonian fluids?
Newtonian fluids maintain constant viscosity with a linear relationship between shear stress and shear strain rate, making their behavior predictable. Non-Newtonian fluids exhibit viscosity that changes with varying shear rates. Examples of Newtonian fluids include water, air, oil, and gasoline, while non-Newtonian fluids include latex paint, cornstarch mixtures, and toothpaste.
Q2: How do shear-thinning fluids behave under stress?
Shear-thinning fluids, also called pseudoplastic fluids, decrease in apparent viscosity as shear rate increases, allowing them to flow more easily when subjected to force. Examples include latex paint and polymer solutions. This property makes them ideal for applications like brush painting and inkjet printing, where smooth and even application under stress is required.
Q3: What makes shear-thickening fluids useful in protective applications?
Shear-thickening fluids increase in apparent viscosity under higher shear rates, becoming more resistant to flow when agitated or stirred. Cornstarch in water is a common example. This unique property makes them valuable in protective gear and damping devices, where greater resistance under stress provides enhanced protection and energy absorption.
Q4: How do Bingham plastics differ from other non-Newtonian fluids?
Bingham plastics behave as solids until a specific yield stress is exceeded, after which they flow like fluids. Toothpaste and mayonnaise are common examples. This dual behavior makes them ideal for products requiring extrusion or spreading, as they maintain shape under low stress but flow when sufficient force is applied.
Q5: Why is understanding fluid classification important for engineering design?
Classifying fluids as Newtonian or non-Newtonian is crucial for designing and analyzing fluid systems, from pipelines to industrial processes. Newtonian fluids follow predictable viscosity patterns, simplifying calculations, while non-Newtonian fluids require specialized analysis based on their shear-dependent behavior. Proper classification ensures efficiency and functionality in fluid system applications.
Q6: What role does viscosity play in distinguishing fluid types?
Viscosity is the key property differentiating fluid types. Newtonian fluids maintain constant viscosity regardless of shear rate, following Newton's law of viscosity. Non-Newtonian fluids exhibit variable viscosity dependent on shear rate, creating three categories: shear-thinning, shear-thickening, and Bingham plastics. Understanding viscosity behavior is essential for predicting fluid performance.
Q7: How does the relationship between shear stress and shear rate define fluid behavior?
In Newtonian fluids, shear stress is linearly related to shear strain rate, creating predictable and consistent behavior. Non-Newtonian fluids deviate from this linear relationship, with viscosity changing as shear rate changes. This fundamental difference determines how fluids respond to applied forces and is essential for selecting appropriate fluids for specific engineering applications.