Questions tagged [turbulence]

A regime of nonlinear viscous flow characterized by random, rotational motion with a wide range of length scales. Its study is critical to many fields, such as aerospace, atmospheric science, chemical engineering, and astrophysics.

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If the divergence of an isotropic vector field (in 3d Euclidean space) is a constant, what is it's dependence on the co-ordinates?

Basically the title. In symbols, I have \begin{equation} \nabla \cdot F = - 4 \varepsilon \ , \end{equation} where $\varepsilon$ is a constant, and there is isotropy. My guess was $F$ can depend ...
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Ideal Turbulence

I'm studying the turbulence in ocean and i started to study a mathematic introduction of turbulence. We define $ R_{ij}(\underline{x},\underline{r},t) := \overline{u_i(\underline{x},t)u_j(\underline{x}...
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How to understand non-uniqueness of solutions of the Navier-Stokes Equations?

In the book of boundary layer theory: "The solutions of the Navier–Stokes equations do not have to be unique for given initial and boundary conditions. Primarily because of the nonlinearity of ...
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How to tell if a system has a direct or reverse energy cascade?

We know, in 3D turbulence one observes a direct energy cascade, where the energy flows from the large scales to small scales (see wiki 1,1), usually attributed to vortex stretching. We also know that ...
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Why fast trains make a white noise sound?

I have read that because turbulence has a fractal structure then all frequencies are excited equally and that makes the familiar whistling noise. Is this explanation correct?
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Why does blowing air through pursed lips make the air cooler than if we exhale through wide open mouth? [duplicate]

When we try to cool something through blowing on it, we purse our lips to exhale air through a narrow opening, which produces cooler air. However, if we open our mouths wide and exhale , then the air ...
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135 views

Particle velocity field in turbulence

Consider solid particles in a turbulent flow. When is it appropriate to use a velocity field for the particles? To elaborate: In fluid mechanics, one uses the velocity field of the fluid as it is ...
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How to express eddy vorticity according to Kolmogorov's model for turbulence?

I would like to express the typical eddy vorticity $\omega_l$ [s-1]of an eddy with length $l$ [m] in terms of the energy dissipation rate $\epsilon$ [m2 s-3] and the characteristic time scale of that ...
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How a shower head can affect water temperature? [closed]

This is a real-life case, that I simply don't understand :) A shower set is composed of a faucet $-$ that contains a mixer, a hose and a showerhead. The water comes from 2 pipes $-$ hot and cold. The ...
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Is there such thing as a non-turbulent wake?

We learnt in our fluids course that boundary layer separation can occur even for laminar non-turbulent flow (for high viscosity fluid). In this case, shown as ‘steady separation’ in the diagram below. ...
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How does the eddy velocity scale with its time scale?

According to Kolmogorov's energy casacade model, if we have a flow with inertial velocity scale $\mathcal{V}$, inertial length scale $\mathcal{L}$ then we can calculate the eddy velocity of an eddy ...
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Does wind turbulence increase or decrease with height from ground?

Above is a picture of laminar to turbulent transition on a flow past a plate. Towards the right edge, we see that the layer closer to the surface is more laminar than turbulent, while away from the ...
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Is there a version of aerodynamics that incorporates and intergrates the complete physics of sound?

In learning about aerodynamics for aviation, it has struck me to what extend it is based on a variety of assumptions and presentations, that may one day have served to create a notion of what happens, ...
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Can turbulence be predicted from entropy?

Frozen water has lower entropy than liquid water. This is true for most solid/liquid transitions. Clearly a solid can't flow with turbulence ( although glass does flow over many years as can be seen ...
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Piston speed calculation for Votex rings generation?

I'm trying to design an apparatus to generate vortex rings, and I've run in a number of problems so basically the classic apparatus is a piston pushing a slug of fluid trough a circular opening as ...
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Are results of a turbulent flow experiment repeatable?

Say I perform a simple pipe flow experiment. I make sure that the Reynolds No. is sufficiently high so that the flow is turbulent. I measure the velocity field using probes placed at different points ...
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Why don't we see turbulence on the surface of liquids

By turbulence I mean a chaotic field full of eddies. As far as I'm aware this is the state of most fluids, like the airflow in an room, unless effort has been taken to make the flow laminar. Shouldn't ...
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Is it possible to determine the position of the laminar-turbulent transition of a boundary-layer?

Can the exact position of the transition point from laminar to turbulent flow be calculated when working with flat plates? Or can the position only be estimated based on the common Reynold's numbers ...
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Interpretation of wind power spectral density

I'm reading this article on Dryden Wind Turbulence Model, and also similar Von Karman wind model. The Dryden model gives power spectral densities of turbulent wind. But why is power spectral density a ...
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Which direction do whirlpools on a river eddy line spin?

In a turbulent flowing river, an eddy is created when water is forced upstream. The area where the upstream and downstream flows meet is called the "eddy line", and whirlpools often form ...
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Is there such a thing as laminar-turbulent transition in free flow?

Free flow in the sense of "without any objects or obstacles influencing the flow". High altitude wind or ocean currents could probably be real-world examples. There is the well known ...
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Free falling water from tap turns laminar to turbulent

Why does water flowing (falling freely) from a tap become turbulent after some distance? Radius at tap = 0.4 cm Distance from tap where turbulence start = 15 cm approx Radius where turbulence start ...
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Pressure in wake of turbulent vs laminar flow

I understand that a turbulent flow creates a smaller wake than a laminar flow, as explained here. Does the pressure inside this wake also depend on the flow regime (turbulent/laminar) or doesn't the ...
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Why is astronomy intensity interferometry immune to atmospheric turbulence?

I have read there is renewed interest in intensity interferometry in astronomy. I read that intensity interferometry is immune to atmospheric turbulence, which plagues astronomy and regular (amplitude)...
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Why does falling water become less and less focused as it falls down?

So, I was observing my water purifier fill up my water bottle and I noticed that the beam of water coming through it became less and less focused as it falls down more. Now, I do understand the ...
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Locality of the Turbulent Energy Cascade

Locality appears to be an omnipresent feature of physics, including within turbulent fluid dynamics. In 1941 Kolmogorov postulated that the turbulent energy cascade is driven by scale-local ...
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Wall shear stress definitions

Good afternoon everyone. In defining the shear velocity at the wall $u_\tau=\sqrt{\frac{\tau_w}{\rho}}$ for the incompressible turbulence, $w$ being the wall, I found in some literature that this ...
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646 views

Does flow separation starts earlier at higher airflow speeds?

At same AoA,will flow separation starts earlier (point of separation more upstream) at higher airflow speeds (=higher air inertia) compare to lower airflow speeds? High speed airflow has more air ...
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Understanding the Viscous Sublayer in Turbulent Flow

I'm slightly confused at how/why the viscous sublayer forms in a turbulent boundary layer. From what I understand, the boundary layer is a region where there is momentum transport through the planes ...
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Determining the Reynolds number from 3D velocity data

I have some 3D velocity data $\{v_x,v_y,v_z\}$ recorded over time at a very high frequency in an environment with anisotropic turbulence. I know that for a single (i.e. 1D) variable, we may use the ...
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Why is the production of turbulent kinetic energy maximum in the lowest layers of the turbulent boundary layer?

I have been studying the basics of CFD from a book titled 'An Introduction to Computational Fluid Dynamics' by H.K. Versteeg. In the turbulence modelling section, the author shows how the production ...
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761 views

Concept behind Reynolds number

I would be glad if someone can explain to me the concepts and the applications of Reynolds number in real-life. My handout has this explanation and I cannot understand it clearly... Reynolds number ...
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From where does irreversibility arise in the Navier-Stokes momentum equation?

A form of the Navier-Stokes momentum equation can be written as: $$ \rho \left( \frac{\partial \mathbf{u}}{\partial t} + \mathbf{u} \cdot \nabla \mathbf{u} \right) = - \nabla \bar{p} + \mu \, \nabla^2 ...
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What is instability in the context of fluid dynamics?

I have been reading a paper about the production of respiratory droplets in human respiratory tract and learned that they are produced due to instabilities, which includes the Rayleigh Taylor ...
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$C_d$ is bigger at laminar or turbulent flow?

Bodies can be subjected to pressure drag or friction drag. But let's consider the overall drag coefficient $C_d$. My question is: $C_d$ is bigger at laminar or turbulent flow? This image represents ...
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A technique to distinguish the mean magnetic field from the fluctuations in the solar wind?

My question is: Is there a technique to distinguish the mean magnetic field from the fluctuations in the solar wind? Thanks
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Can we differentiate flows with identical Reynolds numbers but with different turbulence intensities?

The Reynolds number is given by $$\mathrm{Re} = \frac{v_{rms} L}{\nu}$$ Now imagine we fix $L$, $\nu$ and $v_{rms}$ - so the Reynolds number is constant. Imagine two different flows: $$(1): \bar{v} = ...
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Transition from laminar to turbulent flow

Is there a mathematically rigorous definition of turbulent or laminar flow? As far as I understand, laminar flow - means, that the velocity fields varies smoothly along the distance and time. Layers ...
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Turbulent boundary layer development

I'm currently working on my bachelor's thesis about the numerical simulation of a turbulent air flow through a rectangular channel. As part of that I'd like to compare my results to empirical ...
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Why is turbulence dissipation $\epsilon$ defined this way?

When deriving the turbulence kinetic energy equation $k$ by taking the trace of Reynolds stress transport PDE, this term appears : $$ 2\nu \overline{\frac{\partial u'_i}{\partial x_k}\frac{\partial u'...
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In turbulence what to we mean that an eddy has a wavenumber k?

In turbulence what to we mean that an eddy has a wavenumber k?
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Converting a turbulence spectrum in frequency to wavelength

So, I have a spectrum calculated from an underwater sensor, $S_{vv}(f)$, and want to convert it to $S_{vv}'(k)$, where $k$ is wavelength and $f$ frequency. I assume Taylor hypothesis for frozen ...
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Explanation of figure:turbulent magnetic field lines + spatial distribution of out-of-plane electric current density

The following graph is supposed to present turbulent magnetic field lines along with the spatial distribution of out-of-plane electric current density. However I am not sure I completely understand ...
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Can a Styrofoam box bounce back into a moving van?

I just came across this video. A styrofoam box falls out of a moving van, bounces off the road and lands back in the van. How can the box land back so precisely inside the van?! Usually, I would ...
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Initial Conditions for no-slip velocity (Walls)

For CFD initial condition, I want to know if i have to apply a very high velocity gradient on the wall of a pipe (internal flow) as it's the case when you study the freestream flow which surrounds an ...
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Is there a non-dimensional argument for a fluid’s characteristic force?

What the characteristic force is Dimensional analysis has an interesting result in fluid mechanics: the quantity $\chi=\eta^2/\rho$ has the units of force, where $\eta$ is the usual dynamic viscosity. ...
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Is there any relation between turbulent energy dissipation and pressure loss?

It is known that pressure loss depends on viscosity of fluid. However I learned today that energy dissipation rate in turbulent flow is independent of viscosity. It sounds like there is a ...
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Is it possible to solve for pressure loss in turbulent flow along a pipe purely analytically?

I understand it's possible to find pressure loss using formulas such as the Darcy Weisbach equation works with turbulence - but to my knowledge the friction factor is derived using empirical means. Is ...
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Why are aerodynamic / streamlined shapes always stumpy at the front?

I'm building an autonomous boat, to which I now add a keel below it with a weight at the bottom. I was wondering about the shape that weight should get. Most of the time aerodynamic shapes take some ...
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Reynolds number determines turbulent or laminar but Moody Chart has different interpretation

One can calculate the Reynolds number to determine whether or not flow is turbulent or not inside a pipe with circulating water. The formula is: $Re = \frac{\rho VD}{\mu}$ Let's say one gets $Re = 10,...

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