Viscous Flow, Reynolds Number & Boundary Layers Athul Viswam - - PowerPoint PPT Presentation

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Viscous Flow, Reynolds Number & Boundary Layers Athul Viswam - - PowerPoint PPT Presentation

Nov 22, 2022 1.07k likes •1.68k views

AE-705: Introduction to Flight Viscous Flow, Reynolds Number & Boundary Layers Athul Viswam Aerospace Engineering Department IIT Bombay AE-705 Introduction to Flight Lecture-05 Chapter-03 CONTENTS Introduction to Viscous Flow

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AE-705 Introduction to Flight Lecture-05 Chapter-03

AE-705: Introduction to Flight

Viscous Flow, Reynolds Number & Boundary Layers

Athul Viswam

Aerospace Engineering Department IIT Bombay

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AE-705 Introduction to Flight Lecture-05 Chapter-03

CONTENTS

 Introduction to Viscous Flow  Laminar and Turbulent Flow  Concept of Boundary Layer  Types of Boundary Layer  Flow Separation

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Lets start with a simple experiment

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?

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Which one is thicker ??

https://en.wikipedia.org/wiki/Viscosity#/media/File:Viscosities.gif

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 Resistance to relative motion ( friction??? )  Thicker  Higher Viscosity  Higher Viscosity  Lower flow rate 

Are gases viscous too??

Viscosity

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Well, Good to know about viscosity! Let us calculate the pressure using Bernoulli’s principle Oye! Go and read my assumptions

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Flow in a pipe

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What about external flows? Can we have Laminar flow over aeroplane wings?

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Laminar flow over a wing cross section

Source : **

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Laminar flow over cylinder

Source : **

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Laminar flow over a rectangular block

Source : **

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Laminar flow over a flat plate

Source : **

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Can we predict when would a Laminar flow becomes Turbulent?

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I have made some efforts to study this. You will learn most of my theories later !! Who am I ?

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A Short Quiz

Who is this famous scientist?

a) Ludwig Prandtl b) George Gabriel Stokes c) Osborne Reynolds d) Arnold Sommerfeld

Can you answer ?

https://en.wikipedia.org/wiki/Sir_George_Stokes,_1st_Baronet#/media/File:Ggstokes.jpg

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Reynolds Number:

 Re No =

Inertial force Viscous force = ρVL μ

 Transition Re No  Critical Reynolds No  Critical Re No for Internal flow  3000 - 5000  For external flow  300000 – 500000  Best measure to compare flows  Re No Laminar nature

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Re No : 100

http://www.neuroems.com/2014/03/15/blood-flow-through-the-brain-pt-1-overview/

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Re No ~ 4 million

https://breakingmuscle.com/fitness/7-essential-swimming-tips-for-even-the-strongest-athletes

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Re No ~ 109

http://www.cruisecritic.co.uk/news/news.cfm?ID=7096

Can you name this Ship ?

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What happens when Re No is 2900 in a pipe? Will it be purely Laminar?

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We usually compare the

  • rder of

magnitude

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This is non-viscous flow in a pipe What would be the case with viscosity?

Laminar? Or Turbulent? So, will Reynolds no be low or high?

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Viscous flow over a solid surface

Why didn’t the fluid in contact with solid surface move?

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Viscous flow over flat plate

https://github.com/su2code/SU2/wiki/Laminar-Flat-Plate

Viscosity spoiled my flow field

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AE-705 Introduction to Flight Lecture-05 Chapter-03 https://en.wikipedia.org/wiki/Boundary_layer

Why don’t you split the domain in 2 regions ? One where viscous effects are significant Another with negligible viscous effects?

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 First defined by the Father of Aerodynamics  Edge of BL – 99% of Freestream velocity

v δ = Boundary layer thickness Boundary layer

Boundary Layer

δ

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AE-705 Introduction to Flight Lecture-05 Chapter-03 Source **

I observed it first ! Who am I ? I formulated it first ! Who am I ?

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Velocity profile 99% of v Transition to turbulence Laminar BL Turbulent BL

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Transition Strips in WT Testing

M Mirzaei, MH Karimi, MA Vaziri, An investigation of a tactical cargo aircraft aft body drag reduction based on CFD analysis and wind tunnel tests, Aerospace Science and Technology, 2012

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Transition Strips in WT Testing

M Mirzaei, MH Karimi, MA Vaziri, An investigation of a tactical cargo aircraft aft body drag reduction based on CFD analysis and wind tunnel tests, Aerospace Science and Technology, 2012

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Sometimes, transition strips don’t work !

XL Wang, GY Fu, DP Duan, XX Shan, Experimental Investigations on Aerodynamic Characteristics of the ZHIYUAN-1 Airship, Technical Note, Journal of Aircraft, 2010,

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Experiment v/s Numerical Simulation

  • S. Suman, S. Lakshmipathy, R. S. Pant, Evaluation of the assumed-transition-point criterion in context of

RANS simulations around Lighter-Than-Air vehicles, Journal of Aircraft, 50(2), 2013

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Effect of Transition Location

Flow separation occurs CD ↓ as more surface is exposed to Laminar flow CD ↑ as more surface is exposed to flow separation

  • S. Suman, S. Lakshmipathy, R. S. Pant, Evaluation of the assumed-transition-point criterion in context of

RANS simulations around Lighter-Than-Air vehicles, Journal of Aircraft, 50(2), 2013

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Real flow field

?

Source ** Video source : NIT Suratkal

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Flow separation

 Viscous flow phenomenon  “Adverse pressure gradient” ?

Pressure decreases  favourable pressure gradient

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Pressure increases  unfavourable pressure gradient

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Distance Local pressure Direction of flow tendency

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Distance Local pressure Actual direction of flow

How??

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Because flow has momentum which drives the flow against pressure gradient But flow near the solid surface already lost most

  • f its momentum!!
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So, guess what happens!!

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What is happening in the separated flow region?

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Reversed Flow

Source **

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AE-705 Introduction to Flight Lecture-05 Chapter-03 https://en.wikipedia.org/wiki/Flow_separation

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Can you answer why is there a reversal of flow? Clue: pressure gradient

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HOMEWORK: Flow Separation and Recirculation

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Is laminar BL separation similar to turbulent BL separation?

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Turbulent BL delays separation – higher momentum

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Tired?

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Let us watch cricket!!

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SWING and REVERSE SWING?

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Let us talk about Golf!

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Why so many dimples?

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What we have learnt till now?

 Viscous flow  Types of flow – Laminar & Turbulent  Transition  Reynolds no – effects  Boundary layer  Separation

QUESTIONS??