Tagged Questions
10
votes
1answer
83 views
Can Increasing the Turbulence Inside a Pipeline Economically Increase Flow?
"The velocity gradient in turbulent flows is steeper close to the wall and less steep in the center of the pipe than it is for laminar flows (Blatt p.97)."
Does this mean that some degree of ...
10
votes
2answers
273 views
Can vorticity be destroyed?
I have a professor that is fond of saying that vorticity cannot be destroyed. I see how this is true for inviscid flows, but is this also true for viscous flow? The vorticity equation is shown below ...
5
votes
2answers
337 views
Explanation that air drag is proportional to speed or square speed?
A falling object with no initial velocity with mass $m$ is influenced by a gravitational force $g$ and the drag (air resistance) which is proportional to the object's speed. By Newton´s laws this can ...
1
vote
1answer
471 views
Determine viscosity using falling sphere (Stokes Law, Ladenburg correction)
Introduction
I am trying to determine the viscosity of a fluid. Therefore, I let a sphere of known mass m and radius r fall ...
2
votes
1answer
189 views
Water vs Milkshake being sucked through a straw
Consider water in a glass being sucked through a straw. The water rises up in the straw because of a pressure gradient introduced by the sucking action.
Now, change the liquid from water to something ...
7
votes
3answers
176 views
How do I intuit viscosity in a rotating fluid?
Suppose I have two plates with a viscous fluid in between. I slide them in the same direction (a direction in their own plane), one at $5 \,\text{m/s}$ and the other at $6 \,\text{m/s}$. Due to the ...
0
votes
2answers
124 views
The viscous force between the layers of liquid is same, then why there is variation in the velocities of its layers?
I have learned in my textbook that when the liquid flows the bottom layer of the liquid never moves because of friction, but the upper layers move with increasing velocities how it is possible if the ...
5
votes
1answer
218 views
Calculating Reynolds number for a viscous droplet
I'm trying to develop a very basic scaling law/unit analysis for viscous droplet formation, and I'd like to get some rough numerical values of the Reynolds number to play with. To be specific, I'm ...
5
votes
4answers
302 views
Do we have viscous force acting between two layers
Frictional force between solids operates even when they do not move with respect to each other. Do we have viscous force acting between two layers even if there is no relative motion?
3
votes
3answers
168 views
In the classic viscosity definition, why does doubling the plate gap cause the force to halve (intuitive)?
I am puzzled by an artifact of the definition of viscosity and need an intuitive picture to help explain it. I know $\tau_{yx}=-\mu{dv_x \over dy}$ but I am looking for an intuitive picture of the ...
1
vote
1answer
45 views
Are there dedicated instruments to measurethe viscosity of shear thinning liquids?
Googling around for ways to measure the viscosities of shear thinning liquids, it seems to me that most of the time viscometers are used at different settings to measure different apparent ...
5
votes
1answer
247 views
Are coffee's properties different enough from water's to cause increased spillage while walking?
I recently found this article, which describes how...
It just so happens that the human stride has almost exactly the right frequency to drive the natural oscillations of coffee, when the fluid is ...
3
votes
3answers
375 views
Equations of fluid dynamics and differential geometry
Where can I look for equations of fluid motion written in terms of nifty things from differential geometry like exterior derivative, Hodge dual, musical isomorphism?
Preferably both with and without ...
3
votes
1answer
476 views
Is there an analytical solution for fluid flow in a square duct?
I couldn't find one but assumed it must exist. Tried to find it on the back of an envelope, but got to an ugly differential equation I can't solve.
I'm assuming a square duct of infinite length, ...
3
votes
2answers
616 views
Does irrotational imply inviscid?
Let us consider a 2D irrotational flow, such that $\nabla\times\boldsymbol u =\boldsymbol 0$. Defining the stream function such that $\boldsymbol u =\nabla\times\psi \boldsymbol n$ where $\boldsymbol ...
1
vote
1answer
119 views
Capillaries in series
The velocity of fluid of viscosity $\eta$ through a capillary of radius $r$ and length $l$ at a distance $x$ from the center of the capillary is given by; $v=\frac{P}{4l \eta }(r^2-x^2)$ (where $P$ is ...
15
votes
1answer
379 views
Minimum viscosity of liquids
In a lecture by Purcell he mentions that he notices that there aren't any liquids with viscosities much less than that of water, even though they go up seemingly unbounded. In an endnote (endnote 1 in ...
3
votes
2answers
353 views
Strict general mathematical definition of drag
Is there a formal definition of drag, say, as some surface integral of normal and shear forces? There seem to be a lot of formulas for specific cases, but is there a general one?
I need to accurately ...
1
vote
1answer
199 views
Reynolds number with hyper-viscosity
Is it possible to evaluate a Reynolds number when viscosity operator is substituted by hyper-viscosity operator at the power H (Laplacien to the power H) in the incompressible Navier-Stokes equations ...
2
votes
1answer
542 views
What is the shear stress of a fluid?
One book defines the shear stress $\tau$ of a (Newtonian) fluid as
$$\tau = \eta \frac{\partial v}{\partial r} $$
where $\eta$ is the viscosity. There is not much context, so I've made some guesses. ...
8
votes
1answer
503 views
Friction term in Navier-Stokes equation
The friction term in Navier-Stokes equation assumes that the viscosity coefficients are the same for the longitudinal and transverse directions. This doesn't seem intuitive, because the former is ...
