Ideal vs Real Fluids: Study with Video Lessons, Practice Problems & Examples

Fluids in motion can be categorized into ideal and real fluids. Ideal fluids are incompressible, exhibit laminar flow, and have non-viscous flow, simplifying calculations. In contrast, real fluids can be compressible, experience turbulent flow, and have viscosity, which is a measure of fluid thickness and resistance to motion. Most problems focus on ideal fluids, while real fluid scenarios may involve viscosity but not turbulence or compression. Understanding these distinctions is crucial for solving fluid dynamics problems effectively.

Ideal vs Real Fluids

Everyone, so we're going to start talking about fluids in motion now, or sometimes referred to as fluid flow. The first thing we're going to cover is the distinction between an ideal fluid versus a real fluid. There are a couple of things that are really important here, but let's go ahead and get started. Right? So, the motion of flow, the flow of fluids can get pretty complicated. It's still an active area of research in physics. So what we do here is instead of a real fluid, we sort of simplify things with the model called an Ideal Fluid. So this is usually what you're going to see inside your problems. Okay? So, basically, it's kind of like how we use, you know, in motion problems, we said air resistance is negligible just to make all the calculations simple. It's kind of the same thing here. We just use ideal fluids to make everything a little bit simpler. Okay. So a couple of things you need to know. The first is that ideal fluids are always incompressible. They can't be compressed, which means that they have constant density. Alright. So real fluids on the other hand, like for instance, water at really high pressures, does have a slightly higher density. So it could be compressible under certain circumstances. Alright? So with an ideal fluid, you just always assume that the density remains constant and uniform throughout the whole thing. Okay. The second one has to do with the actual way that fluid flows. So, real fluid or sorry, ideal fluids always sort of flow really, really smoothly, and they have what's called laminar flow. This just means steady flow. Have you ever seen water flowing out of a pipe, it looks like really, really smooth and crisp? That's what laminar flow is. Whereas real fluids could have what's called turbulent flow. So turbulent or turbulence is basically what happens when the motion of water hits all of these imperfections and whatever it's flowing, and basically, it starts wiggling around like this, and this is generally bad. This just generally happens if the motion is too fast, but that's what turbulence is. Alright? Now luckily, you'll never have to do any calculations with turbulence. You might just have to know what this thing is conceptually. Alright? Now the third one has to do with the way that fluids actually flow and their viscosity. So I'm actually going to start here for just a second here. The defining characteristic of real fluids is they have what's called viscous flow. So viscous is viscous, and then basically, viscosity is sort of like a measure of the thickness of water. So what do I mean by this? Well, if you take water versus, like, for instance, honey or molasses, one of them sort of flows a little bit slower. And basically, it's because this thickness or this viscosity is essentially fluid friction. So there's this property of the fluid that kind of, like, makes it thick, and it makes it sort of, like, hard to flow. It's a resistance to motion. That's basically what friction is. Right? So that's the defining characteristic. Real fluids have viscous flow. Ideal fluids always have what's called non-viscous flow. Very creative, but that's just the name that we have here, which just means that there's no friction or anything like that. We're just sort of treating it as if there's no friction. Right? It's kind of like how we had motion problems. We said, neglect friction. It's the same exact thing here. Alright? Now most problems are going to be about ideal fluids. In fact, if your textbooks or your professors never even cover real fluids, then you can basically always assume that all your fluids will be ideal. Alright? And you can always assume that they are unless the problem says that they were a real fluid or if it mentions something related to viscosity. Alright? Now the last thing I want to point out here is that most real fluids problems, you're going to have viscosity. So if you ever do see a real fluid, you're going to have some viscosity. You might have some of that, but you're never going to have any turbulence. So you'll never have to worry about that. And you'll never have to worry about compression either. So basically, what happens here is if you look through, you'll never have this. You'll almost never have compressible fluids. You'll never have turbulent flow. You may see this every once in a while but still not super rare. But most of your problems are probably going to be about ideal fluids anyway. Alright? So that's just sort of an introduction. Let's go ahead and keep going.

Here’s what students ask on this topic:

What is the difference between ideal and real fluids?

Ideal fluids are theoretical models used to simplify fluid dynamics calculations. They are incompressible, meaning their density remains constant, and they exhibit laminar (smooth) flow without viscosity, which means no internal friction. Real fluids, on the other hand, can be compressible under high pressure, exhibit turbulent flow, and have viscosity, which is a measure of the fluid's resistance to flow. Understanding these differences is crucial for solving fluid dynamics problems effectively.

Why are ideal fluids considered incompressible?

Ideal fluids are considered incompressible because their density remains constant regardless of the pressure applied. This simplification makes calculations easier, as it eliminates the need to account for changes in density. In reality, most fluids can be slightly compressible under high pressures, but for many practical problems, assuming incompressibility provides a good approximation and simplifies the mathematical treatment.

What is laminar flow and how does it differ from turbulent flow?

Laminar flow is a type of fluid motion where the fluid flows in parallel layers with no disruption between them, resulting in smooth and orderly flow. In contrast, turbulent flow is characterized by chaotic and irregular fluid motion, often caused by high flow velocities or obstacles in the fluid's path. Laminar flow is typical of ideal fluids, while turbulent flow is more common in real fluids under certain conditions.

How does viscosity affect the flow of real fluids?

Viscosity is a measure of a fluid's resistance to flow, often described as its 'thickness.' High-viscosity fluids, like honey, flow more slowly and with greater resistance compared to low-viscosity fluids, like water. In real fluids, viscosity introduces internal friction, which affects the fluid's velocity and flow patterns. This is in contrast to ideal fluids, which are assumed to have no viscosity and thus no internal friction, simplifying the analysis of their flow.

When should you assume a fluid is ideal in problem-solving?

In most fluid dynamics problems, you should assume a fluid is ideal unless the problem specifically mentions real fluid characteristics such as viscosity, compressibility, or turbulent flow. Ideal fluid assumptions simplify calculations by neglecting these complexities. However, if the problem involves factors like fluid thickness or resistance to flow, it likely pertains to real fluids, and you should account for viscosity and other real fluid properties.