# 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.

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1. ### B Life at low Reynolds number

https://www2.gwu.edu/~phy21bio/Reading/Purcell_life_at_low_reynolds_number.pdf “It helps to imagine under what conditions a man would be swimming at, say, the same Reynolds number as his own sperm”
2. ### I Reynolds Number and Turbulence: Why Does Increasing Density Affect Flow?

hi. I have a question regarding rheology. the Reynolds number increases when density of a fluid increases. so the flow becomes more turbulent. but I wonder why it is that the flow becomes more turbulent when density increases. why is that?
3. ### Pressure drag and the Reynolds number

Ok, I am kind of lost in my fluid dynamics course on a specific topic. I understand the reynolds number and how it works. What i do not get:: if Re = VD/v, and if we increase V(velocity) which in turn will increase the inertial effects of the number, why is it called a pressure drag? pressure...
4. ### Nozzle loss coefficient that increases with Reynolds number?

Hi, I started to think about the drag coefficient of a sphere. At high Reynolds number Cd drops of suddenly when the boundary layer separates. If the Reynolds number is increased further, Cd increases with increasing Re. (I'm thinking about what is referred to as "post critical separated flow")...
5. ### Fluid flow around a wind turbine blade -- help please

I have to estimate the Reynolds number of the fluid flowing around a blade but I only have one detail: the length of the blade is 0.66m. I have no idea how to do it and I got to submit the assignment in a week, I am desperate for help here.
6. ### I Laminar-turbulent transition and Reynolds number

Hi all! This is my first post here, so hopefully I am not in violation of any rules or etiquette. I'm looking to derive an equation for Reynolds number as function of pressure for a pinhole leak in a pressurized gas line. The line is regular air and is pressurized to 1 atmosphere and is leaking...
7. ### Pipe Friction, Reynolds number & Bernoilli's Equation

First calc reynolds no. Re Flow = 1.5ms-2 Re = (density x mean velocity x diameter) / viscosity = (1007 kgm-3)(1.5ms-1)(0.1m) / 0.002 Pa s = 75,525 = Turbulent flow I used this chart Here And I am getting friction factor of 0.02 Would you agree with this?
8. ### I Reynolds number in biology

Recently I read an article that relates fish larval survival to Reynolds number. It is a new and very interesting concept to me. After some researching, the definition "ratio of inertial forces to viscous forces" still confuses me. As I understand it, both are resistant while the former to...
9. ### Dynamic Viscosity in Reynolds Number

I am trying to run a calculation to work out the overall heat transfer coefficient of an arbitrary ten plate exchanger where the fluid is not determined using Re=puD/mu where mu is dynamic viscosity. When mu is such a strong function of temperature how should I determine which valye to use, say...
10. ### Mass transfer coefficient from a numerical model

Dear all, For an assignment, I am trying to find the relationship between the Sherwood number and the Reynolds number in a channel for different laminar velocity profiles, where there is a concentration of a species at both the top and bottom wall which is transported to the fluid. For this, I...
11. ### Fully developed flow

Does the definition of fully developed flow is different for laminar and turbulent? I understand the fact that the entrance length are different in laminar and turbulent flows, but I believe the definition of fully hydrodynamically developed flow means that the velocity profile (hence momentum)...
12. ### Critical Reynolds number and characteristic length

What are the critical Reynolds number for fluid and characteristic length when a cylinder immerses and then rotates in the fluid (The fluid is initially static)? Please suggest the critical number for the transition from concentric flow to laminar and from laminar to turbulent flow. I would...
13. ### Uncertainty - Reynolds number and the friction factor

Homework Statement Correctly present the table of information. The values in the table are deliberately in a wrong format. The calculated Re values have been analysed to have an uncertainty of ± 0.4% and the calculated f values an uncertainty of ± 0.1%. Homework Equations The Attempt at...
14. ### Reynolds Number < 70 flow

Hello people, If we are working on the flow with reynolds number less than 70 as we all know the flow will be laminar in pipe. Now if the flow continues to flow through elbow fittings like 90 degrees, will we get any turbulence because of the elbow bend ? How to deal with this problem in ansys...
15. ### Aerofoil Reynolds number, meaning of a statement in my homework

Homework Statement hello, i have been given a piece of homework which is determining the characteristics on an aerofoils boundary layer. In the brief the following statement is made with no context “Aerofoil is flying in chord Reynolds number 0.2x10^6” am i correct in saying that is the point...
16. ### At what critical Reynolds number does vortex shedding begin?

In: "Fluid Dynamics", Chapter 3 (Turbulence), Section 26, Landau and Lifchitz analyze the problem of the stability of a steady flow past a body of finite size. The fluid is assumed to be incompressible and they reach the conclusion that perturbations that deviate from steady flows start to...
17. ### Reynolds number calculation

Hello, The particle Reynolds number makes me confused, and I hope someone can help me with this, please! Reynolds number is calculated by Re = ρfUD/μ where U is particle velocity, D can be particle diameter, and ρf and μ are density and viscosity of the continuous fluid. How I can Reynolds...
18. ### Hydrodynamic forces on a foil, Reynolds number, viscosity

Hi! I know little about physics, but I am very well educated , with an inquisitive mind. First the context, then my specific question A. Context The practice of windsurfing has brought me to try and understand the principles of hydrodynamics, in order to make my sailing more efficient. As...
19. ### Effective Reynolds Number for a swept wing

Suppose we have an infinite straight wing, using a given airfoil. Also, suppose for simplicity the B.L. is completely turbulent, and M<<1 (incompressible fluid). As we know, the forces per unit length are: L=q⋅c⋅cl, D=q⋅c⋅cd, where cl and cd are the coefficients of the 2D airfoil for the given...
20. ### How to replicate a realistic Reynolds number for an airfoil

Homework Statement Hi I am doing a project of designing and testing a wing in a wind tunnel, the wing is based a cessna 172 skyhawk wing and I am trying to determine what chord length it should be to try and replicate similar reynolds numbers that a cessna 172 wing would experience in real...
21. ### Intuitively, which of these coroplast fins has less drag?

Here's a crude model rocket fin made out of a material called Coroplast (corrugated plastic): And here's the exact same fin with the flutes cut out from all around the edges, allowing the edges to be squeezed shut and sealed with tape: It may look like there are small openings, but...
22. ### Particle Reynolds number / max entrained particle size

I am not a student; this is for work. Would some kind soul: a) direct me to an online calculator to enable answering the following or b) give me the answer and show how you arrived at it :) or c) advise me of the formula / process by which I can arrive at the answer? THANK YOU! What is the...
23. ### Characteristic lengh in Reynolds number

As we know $$Re= \frac{ρ.v.l}{μ}$$ The characteristic length (l) is said to be representative of the problem but according to what should we choose this length? I mean if we want to calculate Reynolds number of a wing of plane, should we choose the length or the width?
24. ### Working out heat transfer flux in a pipe

1. Question 2. The attempt at a solution Now I'm not sure where to go next? Do I work out the local heat transfer coefficient at 1.2m or just the heat transfer coefficient? Using h = Nu × k/x or h = Nu × k/d Thanks, Sam.
25. ### I Reynolds number of washing machine like object

What would the reynolds number be for a closed pipe spinning like a washing machine? would the characteristic length be the diameter... if so why. Everything I have read is so vauge on this. How can I determine it for strange situations? Can I skip it and figure out if flow is laminar or...
26. ### Reynolds number & Mach number for flow regime

"An adverse pressure gradient strongly favors transition to turbulent ﬂow. In contrast, strong favorable pressure gradients (where p decreases in the downstream direction) tend to preserve initially laminar ﬂow." "High values of M∞ and low values of Re tend to encourage laminar ﬂow" Looking at...
27. ### Particle Reynolds number

Hello, The particle Reynolds number makes me confused and I hope someone can help me on this please! Normally (as I read in every books and papers) that when a bubble or drop rises in a fluid, the bubble/drop Reynolds number is calculated by: Re = ρUD/μ where U is particle velocity, D can...
28. ### Confused about Reynolds Number- Fluid Mechanics

Hi, I am working on the laminar flow and during my calculation at the outlet of my nozzle my Re was even greater than 120,000 however my other calculations seemed legit to me. And someone told me that I am calculating the Reynolds number of an open channel and for that flow can remain in laminar...
29. ### Fluid Mechanics - Head Loss Due to Friction

Homework Statement This is simply a sanity check concerning the system described and attached. My concerns are detailed within my attempts at the solution. Any guidance will be greatly appreciated! The diagram attached represents a process for which a pump and associated pipe work require to...
30. ### Why does friction factor decrease with Reynolds number?

As the flow rate increases frictional force increases but friction factor(Darcy,Fanning) decreases with Re. Can anyone explain what is the concept behind it?
31. ### Boundary layer thickness confusion

Hi, PF! Recently, while reading chapter 6 of Incropera's Fundamentals of Heat and Mass Transfer I got into a confusion regarding the velocity boundary layer. The book first states that, as the flow becomes more turbulent, the boundary layer gets thicker, as indicated by both figures attached at...
32. ### Relationship between Mach number and Reynolds number

Is there any correlation between Mach number and Reynolds number? Both of these nondimensional numbers involve speed, but they don't seem all that related other than that. For high speed flow, we have high Mach number. Is it incorrect to say that "generally" high speed flow is turbulent?
33. ### Wall effect on falling sphere of small Reynolds number

If I were to drop a sphere down a tube of fluid. What are the effects of the wall of the tube on the sphere? Assuming the sphere has an extremely low Reynolds number of less than 1. Is it right to say that the ratio of the radius of the tube and the radius of the sphere has an effect on the...
34. ### Vortex Shedding, Cylinder vs. Cuboid

I have been reading papers now for the last few weeks and I have taken from all the information so far that vortex shedding from a cylinder happens periodically for reynolds numbers anywhere between 90 and 1000. It seems to me that there is no actual way of defining the specific parameters of...
35. ### Trouble calculating the Reynolds number

Hi, I am having trouble calculating the reynolds number for 2 spear shafts underwater. The two shafts vary in diameter and velocity. The tips are identical, stream lined to a fine point so the flow should be laminar. Both spears are 1.9304 m long. The diameters are 11/32" (8.73125mm) and 3/8"...
36. ### Fully developed flow, Reynolds number

Hi, For my experiment I was asked to determine the length of the tube for a fully developed flow to occur ( Entrance length as shown in the attachment) , with the following flow parameters. Flow rate : 1, 5, 15 lit/min Fluid: Water Tube Id: 1 inch. My question How do I find out the length of the...
37. ### Calculating Reynolds number for a wind tunnel

Hi All! as you can see I'm new here! i signed up in order to ask for a wee bit of help with my thesis! (now don't get me wrong I am not exactly asking anyone to write it for me!) I've found myself at a small bit of a loose end here, as a mechanical engineering student i am perfectly capable of...
38. ### How is the head-flow curve dependant on reynolds number

Homework Statement i trying to figure out why the head-flow curve changes at the speeds of 90, 80 and 70 revs and was told that i might want to look into reynolds number for this as the head coefficient is somewhat dependant on this. Homework Equations Ch=gH/(Nd)^2 Cf=Q/Nd^3 pi=f(Q/Nd^3, re)...
39. ### Understanding the Reynolds Number: A Guide

Homework Statement I don't fully understand the Reynolds number and it has arisen in a problem. It says: Fluid with viscosity µ and density ρ fills the gap between two parallel plates at z = 0 and z = h. The upper plate at z = h moves with speed V in the x direction, while the lower plate at...
40. ### Fluid dynamics: Reynolds Number, Drag Constant

I completed 4a successfully, and with 4b, i have 2 queries: a)why can't I let Reynolds # equal to 2.19 x 10^5 (from part a) then simply sub v=4 instead of 5m/s and rearrange for viscosity? I tried it this way first and got a very wrong answer. Why do we, essentially, need to work backwards to...
41. ### Matching Reynolds Number in Scale Model Wind Tunnel Testing

For my senior design project, I have to reduce the aerodynamic drag of a truck and trailer. I am doing CFD simulations and wind tunnel testing. The dimensions of the wind tunnel (test section) are 94 in long, 13.5 in tall and 20 in wide and a cross-sectional area of 270 in^2 and it could go up...
42. ### Fluid mechanics- High Reynolds number flows over bodies

A 40 km/hr wind is blowing parallel to a storage facility , as shown in the figure. estimate the normal force on a 0.5-m high, 1-m long window if the front door is left open. assume the windspeed along the outside of thewindow is 1.2 U. Air density is 1.22 kg/m^3. There's a picture that...
43. ### Haaland Equation, find the Reynolds Number

Hi Guys, I've just started a HND Building Services Engineering course (for Work) and I have been given my first assignment for hand in on the 11th Nov 2014. I've not completed any Maths or Physics since Secondary School - 18 years ago (and didn't really pay attention then!) and I'd appreciate...
44. ### Reynolds Number in open space

Hi people! I'm studying the problem of a house in a valley with a wind at 5m/s. If I'd like to compute Reynolds number here and know if the flow is laminar or turbulent which is the charateristic dimension? The height of the house?
45. ### Wind Turbine Reynolds number calculation verification

Homework Statement Hi guys, I just wanted to verify my calculation of a reynolds number of a wind turbine. I understand that the reynolds number will vary in a spanwise direction along the blade due to changes in the characteristic length(chord width) and velocity(due to the tip speed...
46. ### Reynolds number calculation verification

Hi guys, I just wanted to verify my calculation of a reynolds number of a wind turbine. I understand that the reynolds number will vary in a spanwise direction along the blade due to changes in the characteristic length(chord width) and velocity(due to the tip speed ratio). As a way to...
47. ### Reynolds number calculation for undergraduate

Calculate the Reynolds number at a location χ= 0.3m along the chord length of an aircraft wing at each of the following velocities;u= 20,40,60,80 and 100 knots. Assume International Standard Atmosphere (ISA) conditions for pressure and temperature. Take; R =287 J/KG;μ= 18 X10 -6 Kg/m s; 1 knot =...
48. ### Drag Coefficient and Reynolds Number Related to free fall

Homework Statement Hi, I am doing a theoretical investigation which will be compared to an experimental I'll do later. I am trying to calculate how much time will it take an object to fall a heigh H. the object is a 5x5x5 cm cube. I have the Reynolds number as this Re=3546*Velocity; and I...