What is Reynolds number: Definition and 98 Discussions
The Reynolds number (Re) helps predict flow patterns in different fluid flow situations. At low Reynolds numbers, flows tend to be dominated by laminar (sheet-like) flow, while at high Reynolds numbers flows tend to be turbulent. The turbulence results from differences in the fluid's speed and direction, which may sometimes intersect or even move counter to the overall direction of the flow (eddy currents). These eddy currents begin to churn the flow, using up energy in the process, which for liquids increases the chances of cavitation. Reynolds numbers are an important dimensionless quantity in fluid mechanics.
The Reynolds number has wide applications, ranging from liquid flow in a pipe to the passage of air over an aircraft wing. It is used to predict the transition from laminar to turbulent flow, and is used in the scaling of similar but different-sized flow situations, such as between an aircraft model in a wind tunnel and the full size version. The predictions of the onset of turbulence and the ability to calculate scaling effects can be used to help predict fluid behaviour on a larger scale, such as in local or global air or water movement and thereby the associated meteorological and climatological effects.
The concept was introduced by George Stokes in 1851, but the Reynolds number was named by Arnold Sommerfeld in 1908 after Osborne Reynolds (1842–1912), who popularized its use in 1883.
When calculating Reynolds number, when do I use dynamic viscosity and when do I use kinematic viscosity?
In a pipe with air flowing inside it, what would you use when calculating Reynolds number and why?
I am trying to work out a reynolds number for an impinging jet using air. If I have:
μ=1.8*10-5 kg/ms
ρ=1.2 kg/m3
Velocity = 90m/s
Nozzle Diameter=15mm
is it Re = ρ u dh / μ
Which is (1.2 kg/m3*90m/s*0.015m)/1.8*10-5 kg/ms
Thanks
M
Lately, I have conducted an experiment where I dragged various circular cylinders through water in order to find the resistance, and hopefully also the drag coefficient. It seems to me that this coefficient is dependent on what is called the Reynolds number. Using some sources, I can easily find...
Hi I am a little confused as to what length to use as the characteristic length of the Reynolds number for a body in and incompressible flow that has no boundary conditions(ie there all open).
im going to try and draw a few examples to help me out..(i have no idea why..) from what i have read...
So I understand that Reynolds number is the ratio of intertial forces to viscous forces but how exactly does this relate to flow? Why is it that when viscous forces dominate one gets a laminar flow? How to conceptualize?
Just as the thread says, I am looking for some good flow visualization techniques at low Reynolds numbers. I am working on the design of a small recirculating water tunnel that I think can be built for ~$100. I cannot get a specific RE at the moment as I am still doing a preliminary feasibility...
Hello,
I have a question regarding Reynolds numbers.
As I understand, the higher a Reynolds number, the more turbulent a flow becomes, with Re>4000 considered turbulent.
I have a model where I've calculated a Reynolds number of 10^7, and when going to set my model boundary conditions I'm...
hey guys, I would like to know about reynold's number for a wing
I have designed a NACA 4412 wing which has wingroot = a , wingtip = b and winglet tip = c (a,b,c are any integers or decimals)
please tell me the formula of calculating D for the eqn of reynolds number = ρVD/μ
additional...
Homework Statement
If the Reynolds number is 4000, what is the Power (P) required in units of watts at a shaft speed (n) of 0.03 Hz? Assume the system contains acetone, with a kinematic viscosity of 0.419 stokes. The density of acetone is 0.785 grams per cubed centimeter. Us Curve B in the...
Hello
I am preparing for my aerodynamics exam and I'm having trouble with this question.
Why is the Reynolds number of the airflow over a wing important in determining the value of lift and dragThe Reynolds number is important for determining laminar and turbulent flows over a body. It is a...
Homework Statement
From dimensional analysis it is found that the drag force F that a fluid (density ρ, viscosity μ) exerts on a sphere (diameter d) moving through a fluid at a velocity u is given by
cD = f(Re),
where
.
The table below gives the relationship between the drag...
I'm trying to calculate the reynolds number for flow inside an air duct with an obstruction. Basically the set up involves cooling electronics by placing aerodynamic "shrouds" over the device to improve the heat transfer. So far a cube is being used to approximate the shape of the device. The...
when calculating a Reynolds number for an obstacle in wind tunnel
Re = air density * air velocity * D / Dynamic viscosity
D here is the characteristic length of the immersed body ( frontal length ) 2 dimensional problem
Or the Hydraulic diameter which relates the tunnel dimensions to the...
Hi
I know that for an airfoil, it's typical to describe stall in terms of angle of attack.
I'm wondering, how does Re play a role in flow separation of an airfoil (and for objects in general such as a plate, sphere, or cylinder)? One Wikipedia (http://en.wikipedia.org/wiki/Flow_separation) it...
Wikipedia: "Reynolds Number is a dimensionless number that gives a measure of the ratio of inertial forces to viscous forces."
What is meant by the inertial forces?
I have a much better intuition of viscosity. But it seems to be like the Reynolds Number predicts turbulent flow for a...
Homework Statement
For the following situations of steady flow, determine whether the flow is laminar or
turbulent based on Reynolds number: water at 10oC flowing at an average velocity of
2 m/s in a 100-mm pipe, (Viscosity of water at 10oC: 1.31 cP, density: 1000 kg/m3)
Homework...
Is there a simple mathematical way of proving the left hand side equals the right hand side (see attached image)?
Any help will be much appreciated as I'm desperately trying to work out exactly where the magnetic Reynolds number comes from. I can derive the left hand side (its simply the ratio...
So, I understand that when reynolds number increases, the fluid becomes more turbulent, and there is a greater energy loss due to formation of eddies. However, shouldn't this increase in reynolds number cause an increase in friction as well?
But, according to the moody diagram...
Homework Statement
This is a part of an assignment for a computational modelling paper requiring the modelling of couette flow between two (infinite) parallel plates in MATLAB. The lower plate is stationary while the upper plate moves at 0.1m/s, the plates are 0.01m apart and the no slip...
Homework Statement
Afluid with a viscosity of 9.1 x 10-4 Pa.s and density 1.1 g/cm3 flows through a tube with an inner diameter of 10 mm and fills a 10 mL container in 29 s. What is the Reynolds number for this flow?
Homework Equations
I am assuming its this equation for a tube...
Can someone explain to me how XFoil calculates the Reynolds Number for computational analysis. I have the XFoil manual which explains the method used in calculating the Reynolds Number. But what I don't understand is that it says that the "Reynolds Number is defined with the freestream velocity...
Could somebody please clarify a few things about Reynolds number for me? In class the lecturer approximated |u. \nabla |u by (u^2)/L where L is the characteristic length. In a specific example considering flow around a cylinder of diameter D, it was said that L=D.
What does the characteristic...
Homework Statement
I don't understand why flow coefficient increases with Re and why it becomes independent after a certain Re number for orifice plates and venturi meter!
Homework Equations
The Attempt at a Solution
It seems than when the flow becomes turbulent, the flow...
The wake is made because the inertia forces in the system ,right? So if i have low Reynolds number the inertia forces are negligible and the flow doesn't separate ,right?
So In every low Reynolds number the flow doesn't separate (no wake) but low Reynolds number also indicate a viscous flow...
Hello! First time poster, long time lurker.
I've been doing my own independent study of fluid dynamics (not recommended, i know!) and I've hit a wall. I'm trying to imagine problems of fluid dynamics other than the "river and pipe" variety to confirm that I'm really getting this drag stuff. I'm...
Hello, can someone tell me how the reynolds number for flow through a pipe change along it's length? Is the only method that the boundary layer gets thicker along the pipe so the freestream velocity increases? Assuming no gas temperature change along the pipe's length.
From thinking of the...
Hello,
I have a problem I am trying to model analytically (please note: I am NOT a fluid mechanics expert -- I come from a chemistry background):
A semi-infinite plate is oriented axially in the direction of flow.
Please refer to the following page for an illustration of my problem...
Homework Statement
At the moment, I am planning my I.B extended essay (last 2 years at school), and have chosen to do my EE on physics. I've done loads of background research about Reynolds Number and how it affects the type of flow present.
There is an experiment I can recreate (Reynolds...
Homework Statement
A pipe 0.15m in diameter has a roughness ratio (k/d) of 0.002 and carries water at a flow rate Q.
Using the moody diagram, show that at high flow rates (where the friction factor is independent of reynolds number), the head loss per m length due to friction is approx...
Homework Statement
Estimate a reynolds number for a pipe of 0.3m internal diameter, carrying water at a rate of 71 m/s, and say what that says about the likely nature of the flow.
Homework Equations
Re = (density x L x velocity of flow)/coefficient of dynamic viscosity for water...
I have the density (ρ) of the particles and the fluid, as well as the complete dimensions of the non-spherical particles. I know the Force of drag (F) and the Coefficient of drag (C). But how do you get the Reynolds number from this? I can't use Stokes' equations because they only apply to spheres.
Question from Text
Gas (rho = 5.25 kg/m^3, v = 2.0 x 10^-5 m^2/s) is flowing in a 20-mm-diameter pipe. When a gas flowmeter measures the flow as being 0.064 kg/s, it registers a pressure drop of 8.5 kPa. Investigators plan to test an enlarged model that is geometrically similar in a...
Hi,
I am interested in the relationship between changes in flow direction (and changes in flow velocity) and the Reynolds number.
Reynolds number = (fluid density) * (velocity) * (tube length) / (viscosity)
Reynolds number = (flow rate) * (tube length) / [(viscosity) * (cross-sectional...
Does anyone here know how to calculate the reynolds number for a pulsating swirl flow? I've been trying to find the right expression for ages, I can't locate it.
I'm sure it will be a integral equations based upon the frequency of swirl (i.e. 1Hz, 5Hz or 10Hz etc)
Thanks
hi
i have to find out the reynolds number for a missile launched from a sub.
i only have the temperature of the water, the dimensions of the missile and the velovity of a model of the of the actual missile at a scale of 1:10
i havn't given any quantities because i don't want a...
I have a question about Reynolds Number and wind tunnel tests. Last semester a few students at my university were conducting wind tunnel tests on a small model of a truck, they were trying to find ways to reduce the drag in order to save fuel. I would guess they were testing at a Reynolds Number...
1.I have been asked to:
Calculate the density of a fluid if a ctitical velocity of 2m/s at Re=2000 in a pipe of bore 90mm at a dynamic viscosity oc 0.09cP
I have calcualted the variables to SI units and found
\rho=?
Um (V) =2m/s
D=0.09m
\mu=0.09X10^-3
I am rearranging this...
Hey guys,
I'm looking for a little help understanding Reynold's Number. I know that its the ratio of the inertial forces versus the viscous forces. With that being said, everywhere I look, the Reynold's Number is calculated for flow in a pipe or duct or across some sort of surface. Why can't...
Homework Statement
Hi everyone,
This problem concerns calculating Reynold's Number of flow after exiting a duct. The duct that the flow exits is rectangular. The flow is measured directly underneath the center of the duct at a distance of 10 cm below the exit (Y-axis). Flow is measured...
Are there any reference tables for what to use as the characteristic dimension of various shapes when calculating the Reynolds number?
I am very interested in ballistics, and have done research and calculations for various ways of estimating the drag coefficient of various bullet shapes. I...
The question gives this ..
The Reynolds number, Re, is a dimensionless group which characterizes the intensity of a flow. For large Re, a flow is turbulent; for small Re , it is laminar. For pipe flow, Re=u(density)D/mu, where D is pipe diameter and mu is dynamic viscosity.
It asks ...
Reynolds number prominence to slip/stick the gases on the walls
Dears
A major question has arisen during the comparison of experimental evidence and numerical results. The problem is also recognized by some of the researchers but it’s not fully exhausted yet [1,2]. I know from the slip flow...
I understand most of the equation.
[tex] Re=\rho *D *V
over a symbol I don't recognize
I don't know what the symbol is the denominator, and I can't find its name so I don't know what it means. I just need one definition, thanks.
How does one relate the Reynolds number to the Drag Coeffient?
It seems the drag coefficient for different velocities must be determined experimentally per set. I know the Reynolds number is a method to determine laminar or turbulent flow, but can it be used to determine the drag coefficient?
hai all,
I'm now doing a research about castor oil. I've a got a few a confusing problems..
1. to flow castor oil in a 12 mm acrylic pipe i use a reservoir 1 metres from the outlet. Because castor oil is very thick (dyn visc. = 0.5 Pa-m), the velocity of the flow is very low just about 0.02...
Hi all!
In a pipe flow, when the flow rate of water is increasing, reaching a critical flow rate, the flow will switch from laminar to transition flow, right? And continue the increase of flow rate will further change it to turbulent flow. The Reynolds number for these 2 transitions are...