5th IAHR International Junior Researcher and … IAHR International Junior Researcher and Engineer Workshop on Hydraulic Structures ... CFX post analysis

5th IAHR International Junior Researcher and Engineer Workshop on Hydraulic

Structures (IJREWHS) Hydrodynamic Investigation of Free-Surface Turbulent Vortex

Flows with Strong Circulation in a Vortex Chamber

Spa, Belgium – 28-30 August 2014

S. Mulligan*, J. Casserly2, R. Sherlock3

CERIS, Institute of Technology ,Sligo (Ireland)

2 Department of Civil Engineering and Construction (IT Sligo)

3School of Science (IT Sligo)

*email : [email protected]

Overview of Presentation

Spa, Belgium 28-30 August 2014

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1. Introduction – Vortex Flow

2. Hydrodynamics of Vortex Flow

3. Numerical Modelling using ANSYS CFX

4. Overview of Numerical Model

5. Physical Model

6. Results

7. Discussion

8. Concluding Remarks

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• Previous Studies - Weak air-core vortex flows

• Previous investigations carried out to avoid or

eradicate their formation at intakes

• Minimum submergence or critical depth is

determined

Pump Intake vortex Formation

(http://www.pumpfundamentals.com/)

Hydropower Intake Vortex (The Rance

Tidal Hydropower Station) Schematic of Intake Vortex (Illustration by Author)

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• In recent years, technologies have began employing vortex flow behaviour for various hydro-

industrial applications

• Mixing tanks, hydro cyclone separators, vortex drop shafts and hydro electric power applications

• Generates a sizeable tangential velocity field and fully developed and relatively constant air-core

diameter

• Flow field is complex, three dimensional, multiphasic (air/water/particles) and highly turbulent in

nature

• Requires numerical simulation to quantify key flow parameters and carry out optimisation

parameters and to carry out system optimisation

Hydro Cyclone Separator

(www.bmind.in) Construction of the Tso Kung Tam Vortex

Dropshaft (http://www.dsd.gov.hk/) Vortex air core (IT Sligo)



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Elevated view of strong air core vortex in

vortex chamber

Plan view vortex chamber and velocity profiles

Video of vortex development to steady

state

1. Introduction – Project Aims


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Thesis Title - Hydrodynamic Investigation of Free-Surface Turbulent Vortex Flows with Strong Circulation in a Vortex Chamber

Investigate the relevant flow variables that govern vortex flow behaviour

Quantify the relationships for dependent parameters

Provide a general outlook on the three dimensional velocity profiles

Validate a numerical model for strong vortex flow simulation and analysis ANSYS CFX

2. Hydrodynamics of Vortex Flow


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Author Equation

Rankine Model

Scully Model

Vatistas n = 2 Model

Cross section of vortex chamber highlighting velocity models

3. Numerical Modelling using ANSYS CFX


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Geometry creation or external geometry

input (e.g. AutoCAD ACIS file)

Mesh generation or external mesh file

importation

Solver setup (physical model, boundary

conditions, turbulence models etc.)

Monitor solution and equation residuals

CFX post analysis (vectors, streamlines

etc.)

• ANSYS CFX uses the Finite Volume Method to model and simulate fluid flows in a range of

applications.

• It has a reputation to be focused on applications in turbo machinery

• The solver emphasises on the widely tested shear stress transport (SST) and Reynolds stress

models which have particular advantages in fidelity when separated flows, free shear and rapid

streamline curvature is experienced



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General Multiphase Modelling



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Reynolds Stress Model (BSL RSM)

• Reynolds stress components are modelled

using transport equations

• Generally more suited to complex three

dimensional turbulent flows with anisotropy and

large streamline curvature

• Model is more complex, computationally

intensive and more difficult to converge than

eddy viscosity models

• Turbulence consists of stochastic fluctuations in the flow field in space and time

• Without the need for direct numerical simulation (DNS), CFX allows us to model turbulence

statistically by averaging the flow quantities into what are known as the Reynolds averaged Navier-

Stokes equations (RANS)

• Two methods for solving the RANS equations are investigated in this work

Turbulence Modelling



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• Three test cases are analysed for

mass flows (flowrates) at the inlet

• Outlet set to opening with

entrainment (direction of flow is

unknown) with a relative pressure of

0

• Top of chamber is set to opening

with relative pressure of 0

• Walls modelled with a no slip

boundary condition

• Turbulence intensity set to 10% at

inlet and opening

Boundary Details

A Inlet Boundary type: Mass flow normal to boundary Other details: 10% turbulence intensity

Test Cases

A1 0.725kg/s H/d = 1 = 67 mm

A2 1.677kg/s H/d = 2 = 134 mm

A3 3.111kg/s H/d= 3.5 = 235 mm B Outlet Boundary type: Opening with entrainment

Other Details: Zero relative pressure, zero gradient turbulence intensity

C Walls Boundary type: No-slip smooth wall boundary D Opening Boundary type: Opening with pressure and direction

Other details: Zero relative pressure, 10% turbulence intensity

Boundary conditions

Test Cases



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Unstructured and structured mesh cases investigated using the SST model (due to robustness)

Unstructured Tetrahedral

• Coarse (100k cells, 10mm min size) –

Fine (499k cells, 5mm min size)

• Tetrahedral elements sized using

proximity and curvature

• Inflation applied at boundaries

• Difficult to refine mesh in the near field

region without using mesh adaptation

Structured Hexahedral

• Coarse (100k cells, 6mm min size) – Fine (499k

cells, 1.5mm min size)

• Domain separated into three multiple parts

• Mesh is dense at outlet and increased in size in the

far field

• Inflation applied at boundaries

• Efficient use of cells and possible to refine mesh in

the near field

Plan and cross section of finely unstructured mesh Plan and cross section of finely structured mesh

Mesh Details



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Turbulence Modelling

5. Overview of Physical Model


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Illustration of vortex test rig

Physical model investigated in this work

Inlet

Orifice

Camera

viewport

Spiral

casing

• Test facility designed at IT Sligo to physically

model vortex flows

• Consist s of an open top tank resting over a

hydraulic bench and reservoir

• Tank can be adapted with various geometric

boundaries for a range of open channel vortex

flow cases

• Physical model – Spiral, open channel vortex

chamber with a 67mm outlet

• Water circulated through apparatus during

testing (0 - 3.5l/s) and is monitored using a

magnetic flow meter

• A depth gauge is employed to traverse the flow

field and map the physical water surface

General Overview

5. Overview of Physical Model


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PTV Image highlighting tracer

particles entering the domain

Laser light sheet profile passing through

vortex during image acquisition

• Measurement of tangential and radial velocity

components of the flow field

• Neutrally buoyant particles are tracked at

various horizontal planes (z/Hin)

• Illumination provided by an Nd:YAG laser

pulsing at 10-30Hz passed through light

sheet optics

• Light sheet is imaged subsequently using

high speed camera housed on the tank

underside

• Velocity vectors are determined from

displacements determined in particle tracks

Particle Tracking Velocimetry (PTV)

6. Results


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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.5 1 1.5 2 2.5 3 3.5

V𝝷 (m/s)

r/do

Experimental

Vatistas n = 2

SST Coarse Mesh

SST Medium Mesh

SST Fine Mesh

SST with Curvature Correction

BSL Reynolds Stress Model

Vort

ex C

ore

Sensitivity of water surface profile

Sensitivity of tangential velocity profile

• Unstructured mesh is unsuitable for vortex

flow analysis due to excessive numerical

diffusion error (false diffusion)

• Solution becomes mesh independent at fine

mesh scenario (490k Elements, 1.5mm

minimum cell size)

• Curvature correction significantly improves

the shear stress transport model particularly at

the vortex core

• Most suitable model was found to be the BSL

Reynolds stress model

• Candidate turbulence model used for further

modelling

Solution Sensitivity

6. Results


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A3

A2

A1

CFX Physical Model Comparison of

results

Error = 22%

Error = 21%

Error = 26%

Water Surface Comparison

6. Results


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0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3

V𝝷

(m/s)

r/do

Experimental

Vatistas n=2

BSL ReynoldsStress

Vo

rtex

Co

re

Γ = 0.2144 m²/s

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 1 2 3r/do

Experimental

Vatistas n=2

BSL ReynoldsStress

Vo

rtex

Co

re

Γ = 0.229m²/s

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 1 2 3r/do

Experimental

Vatistas n=2

BSL ReynoldsStress

Vo

rtex

Co

re

Γ = 0.219m²/s

Tangential velocity profile

and water superficial

velocity for z/Hin = 0.26







Tangential Velocity Comparison

6. Results


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0

0.2

0.4

0.6

0.8

1

0 0.1 0.2 0.3 0.4

z/Hin

Vin (m/s)

Experimental

BSL ReynoldsStress Model

Vorticity field at the

vortex core

Inlet velocity profile

comparison

Visual comparison of water surface waves at the vortex core

Surface waves

• CFX post indicates that the vorticity field

is zero everywhere except for at a small

region at the orifice and water surface

• Tangential velocity curves also highlight

that there is no significant reduction of

tangential velocity at the vortex core as

suggested in previous models

• Therefore flow is fully irrotational i.e. no

viscous core

• Inlet velocity profile is overestimated

using the CFX BSL Reynolds stress

model

• Recognisable similarities in surface

wave formations

Other Results

7. Discussion


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• Unstructured grid is unsuitable for modelling of vortex flows

• Grid sensitivity analysis is a crucial, mandatory step in vortex modelling. In

this example a case for mesh independency is found when using a fine

mesh with a minimum cell size of 1.5mm at the outlet region

• Application of curvature correction (production multiplier of 1.25) makes a

significant improvement to the overall solution of the SST model

• Baseline Reynolds stress model (BSL RSM) was found to be the most

suitable candidate for further modelling

7. Discussion


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• The CFX model presents a good solution for the position of the vortex core

and the general shape of the free surface including the formation of surface

waves

• However it fails to resolve the position of the free surface by significantly

under predicting in all three cases; 22%, 21% and 26% for A3, A2 and A1

respectively

• The RSM shows fair to moderate agreement when compared to the

tangential velocity data in all cases by over predicting the velocity profile

• The Vatistas n = 2 analytical solution proves to agree well with experimental

data in all cases

• Limitations due to uncertainty of the models performance at the tank

boundaries and close to the water surface. The model also exhibits

axisymmetric behaviour.

7. Discussion


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Reasons for inaccuracies in the numerical

model?

• Errors in obtaining physical data are

concluded to be minimal

• Numerical uncertainty may arise from

discretisation error (truncation & diffusion

error), boundary conditions, improper time

step etc.

• Numerical uncertainty was practically

reduced/minimized during sensitivity analysis

• Failure of convergence and evidence of

fluctuating residuals and monitor point

• It is possible that unsteady features in the

flow system are preventing further resolution

of the numerical model (surface and sub

surface waves)

Example of fluctuating mass and momentum

residuals plot for case A2

7. Discussion


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Water surface (isosurface water volume

fraction 0.5) and velocity vectors in CFX

post

Discharge in a vortex drop shaft

structure (Hohl)

• Further transient analysis of flow system

is required

• Limitations of the particle tracking

velocimetry setup obstructs the

examination of tangential velocities in the

near field (vortex core)

• Work will be progressed in this direction

to further validate the numerical and

analytical model

• Results from a fully validated CFD model

is invaluable for the design and

optimisation of such structures (sizing

structures, velocities, pressures etc.)

• CFX bench mark report to be completed

in 2015


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Thank you for your attention!

5th IJREWHS

Documents

5th IAHR International Junior Researcher and … IAHR International Junior Researcher and Engineer Workshop on Hydraulic Structures ... CFX post analysis