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When an object of a given shape falls very slowly in a fluid, the steady-state velocity of the object is observed to depend only on the size of the object and the density and viscosity of the fluid. Use dimensional analysis to determine the functional relationship between the steady-state velocity and the influencing variables.

Short Answer

Expert verified
The velocity \( v = k \frac{\eta}{\rho d} \).

Step by step solution

01

Identify Variables Involved

First, identify the variables involved in the problem. Here we have:- Steady-state velocity of the object, \( v \), with dimensions \([L T^{-1}]\)- Characteristic size of the object, \( d \), with dimensions \([L]\)- Density of the fluid, \( \rho \), with dimensions \([M L^{-3}]\)- Viscosity of the fluid, \( \eta \), with dimensions \([M L^{-1} T^{-1}]\)
02

Dimensional Homogeneity

Write the relationship using dimensional analysis: \( v \propto d^{a} \rho^{b} \eta^{c} \). The dimensions of each term must satisfy dimensional homogeneity:- \([L T^{-1}] = [L]^{a} [M]^{b} [L]^{-3b} [M]^{c} [L]^{-c} [T]^{-c}\)- Simplify: \( [L T^{-1}] = [L]^{a-3b-c} [M]^{b+c} [T]^{-c} \)
03

Setting Up Equations

Equate the powers of fundamental dimensions from both sides:1. For \( L \): \( a - 3b - c = 1 \)2. For \( M \): \( b + c = 0 \)3. For \( T \): \( -c = -1 \)
04

Solve the Equations

From equation 3, solve for \( c \): \( c = 1 \).Substitute \( c = 1 \) into equation 2: \( b + 1 = 0 \) leads to \( b = -1 \).Substitute \( b = -1 \) and \( c = 1 \) into equation 1: \( a - 3(-1) - 1 = 1 \) or simplified \( a + 2 = 1 \), so \( a = -1 \).
05

Write the Final Relationship

Using the solved values of \( a \), \( b \), and \( c \), the functional relationship can be expressed as: \[ v = k \frac{d^{-1} \eta^{1}}{\rho^{1}} \]Or equivalently, \[ v = k \frac{\eta}{\rho d} \]Where \( k \) is a dimensionless constant.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Steady-State Velocity
When we talk about steady-state velocity, we're discussing the speed at which an object falls through a fluid without any further acceleration or deceleration. This occurs when the forces acting on the object are balanced. For an object falling through a fluid, like a small sphere descending through oil, its weight, buoyancy, and drag forces reach equilibrium.
This balance is often a requirement in many engineering and scientific applications, such as in sedimentation processes or when measuring properties of different materials. Understanding this concept helps in predicting how quickly particles settle or how liquids of varying viscosities behave.
This constant speed through the medium allows calculations that determine other properties of the fluid or the objects moving within it.
Fluid Viscosity
Fluid viscosity represents the internal friction within a fluid. It describes how thick or resistant a fluid is to flowing. Imagine honey; it's more viscous than water. The viscosity impacts how an object moves through the fluid.
  • Higher viscosity means the fluid is thicker, creating more resistance and slowing down the object.
  • Lower viscosity means the fluid is thinner, allowing the object to move more freely and faster.
In dimensional analysis, fluid viscosity is a key parameter that combines with others to define the equation governing the fluid's behavior. The relationship between viscosity and velocity is where we often find a proportional inverse relationship, crucial for understanding flow dynamics and processes.
Fluid Density
Fluid density is another crucial factor when examining how objects interact with fluids. It is defined as the mass per unit volume of a fluid. Think of density as how much material is packed into a specific space.
  • High-density fluids, like syrup, have many molecules packed closely together, affecting how objects move through them.
  • Low-density fluids, like air, have fewer molecules, providing less resistance.
The density of a fluid directly affects buoyancy and drag forces acting on an object, thus altering the steady-state velocity. The interplay between density and other parameters like viscosity allows us to predict and calculate movement through different media.
Dimensional Homogeneity
Dimensional homogeneity is a principle crucial in ensuring that all terms in an equation equate dimensionally. Essentially, it means that every term must be consistent in its dimensional units across the equation, ensuring mathematical and physical correctness.
In our problem, we use dimensional analysis to relate steady-state velocity to other variables. It involves equating dimensions on both sides of a proposed relationship through a combination of the terms:
  • Length \(L\)
  • Mass \(M\)
  • Time \(T\)
This ensures the resulting expression is valid and interpretable. By enforcing dimensional homogeneity, we guarantee that our calculations and formula derivations remain universally applicable, independent of the measurement system used.

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Most popular questions from this chapter

It is well known that tiny objects can be supported on the surface of a liquid by surface-tension forces. For an object of a particular shape, the weight of the object that can be supported is a function of the size of the object, the surface tension and density of the fluid, and the gravity constant. Determine a nondimensional functional relationship that relates the weight of the supported object to the influencing variables.

The Froude number, Fr, at any cross section of an open channel is defined by the relation $$\mathrm{Fr}=\frac{\bar{V}}{\sqrt{g D_{\mathrm{h}}}}$$ where \(\bar{V}\) is the average velocity, \(g\) is the acceleration due to gravity, and \(D_{\mathrm{h}}\) is the hydraulic depth. The hydraulic depth is defined as \(A / T,\) where \(A\) is the flow area and \(T\) is the top width of the flow area. (a) Show that Fr is dimensionless. (b) Determine the value of Fr in a trapezoidal channel that has a bottom width of \(3 \mathrm{~m}\), side slopes \(2.5: 1(\mathrm{H}: \mathrm{V}),\) an average velocity of \(0.4 \mathrm{~m} / \mathrm{s},\) and a flow depth of \(1.5 \mathrm{~m} .\)

A cable of length \(L\) and diameter \(D\) is strung tightly between two poles. A fluid of density \(\rho\) and viscosity \(\mu\) flows at a velocity \(V\) past the cable, producing a deflection \(\delta\). The modulus of elasticity of cable material is \(E,\) and the cable is sufficiently long that the geometry of the end poles does not affect the cable deflection. Determine a functional expression relating dimensionless groups that would be appropriate for studying the relationship between the cable deflection and the given independent variables.

A particular submarine is capable of cruising at \(20 \mathrm{~km} / \mathrm{h}\) at the surface of the ocean and far below the surface of the ocean. Tests of both of these conditions are to be performed using a 1: 40 scale model of the submarine. Determine the required model speed and the ratio of prototype drag to model drag for the following model conditions: (a) a model of cruising on the surface and (b) a model of cruising far below the surface. Assume that the prototype ocean water properties can be duplicated in the model tests.

An atomizer is a common term used to describe a spray nozzle that produces small droplets of a liquid that is drawn into the nozzle by the low pressure that exists in the nozzle. Consider a nozzle of diameter \(D\) that generates droplets of diameter \(d\) when the velocity at the nozzle exit is \(V\). The relevant liquid properties are the density, \(\rho\), the viscosity, \(\mu,\) and the surface tension, \(\sigma .\) With the objective of predicting the size of the droplets generated by an atomizer, express the relationship between the relevant variables in nondimensional form where, to the extent possible, dimensionless groups representing force ratios are used with each of the fluid properties.

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