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How to visualize flows in Experiments and Simulations?

Introduction to Flow Visualizations Technique

Why Flow Visualization is important

Humans can understand physically well compared to the theatrical concept of any subject.
A real picture of the phenomenon is considered to be equivalent to thousands of words
Visual examination of flow field has important before the evolution of numerical or CFD methods
- Physical understanding of flow phenomenon
- Validation of numerical model or theory results
- Establishing prototype of products
- Experimental issues beyond simulations
Different techniques have been used for flow visualization in experiments and CFD simulations

Commonly used methods in Experiments

1. Smoke Dye in the wind tunnel
2. Liquid dye in Water tunnel
3. Flexible threads (Tuffs)
4. China Clay
5. Shadowgraph Technique
6. Schlieren Photography
7. Oil Flow visualization
8. Laser sheet with seeded particles like Particle Image Velocimetry (PIV) in turbulent flow anlysis

Methods in CFD simulations

- Contours of instantaneous and averaged physical quantities
- Streamlines colored with scalar or vector
- Velocity Vector
- Vortex structure for instantaneous turbulent flows using gradient methods
- Particle tracking from inlet to outlet
- Recirculation ratio
- Iso – the surface of scalars

Smoke Dye in Wind Tunnel

A wind tunnel has been used in many aerospace, automobile industries and laboratory in academia using the prototype of object.

A wind tunnel is useful to study and understand the aerodynamics of aircraft and automobile. In such cases, object is fixed and fluid flows over the object.
Probes are placed around the object to measure velocity and local pressure around the surface of body
Smoke is generated to create streamlines around the model to be tested
The interaction of the smoke lines in fluid flow with the body to understand flow pattern around the body
Different colors of dye are used to depict flow pattern around objects in wind tunnel
Physics of turbulent flow for gaseous flow can be well understood using smokes for complex models of aero-planes and space vehicles

Position of the Model for space shuttle in wind tunnel

Smoke lines over an automobile

- Using smoke we can understand the flow pattern over the car.
- This will help to design streamline the body of the car and wake region behind the car
- Many automobile industries use this method to check the wake region formed behind the car. The car body can be well understood whether it is streamlined or bluff body.

Water tunnel Flow visualization using a liquid dye

Similar to wind tunnels, water tunnels are also popular in laboratories used to visualize and understand the hydrodynamic of floated or submerged objects
The model is fixed and water flows around the objects
Dye or colors are injected from the holes on the body to visualize the flow pattern
Vortices in turbulent flow can be visualized in wake region
The following figure represents the set-up of water tunnel to visualize the flow behind the car. In this experiment, dyes are injected at different level at the upstream portion of car. These dyes flow with water as streak lines. In wake regions, streak lines are broken due to vortices and we can observe random flow patterns. This is called a recirculation region.

Water flows over the airplane and colored dye is injected with nozzles on the surface of the body. Vortices generated due to wake region we can see using this method.
If streamlines are not straight or diffused in water which means strong vortices are there

Tufts

Tufts are flexible threads of yarn of string that are free to flow as per the direction of flow
These threads are easily available and glued to the surface of the object
This technique is helpful to understand and visualize flow near the surface to the body
The length and weight of the tuft should be selected properly without altering the flow pattern near the surface
It provides qualitative understanding for flow separation, cross-flow, and reverse flow near the surface
These flow behaviors influence the overall resistance offered

Flow over the surface of the fighter jet is understood using stuff

China clay

To get a clear view of flow pattern, an ultraviolet dye (pigment) fluoresces the streak lines around the surface
This method is easy to see flow pattern on the model and separation regions
The major issue of this techniques is that we cannot change the position of model during the experiment. Pigment can clog pressure taps.

Shadowgraph Technique

The shadowgraph technique is the simplest ways for observing a flow showing variations of the fluid density
The system may or may not use any optical part except a light source.
A recording plane onto which to project the shadow of the varying density field
The shadow effect is observed as light rays are deflected with refraction in fluid in such a way un-deflected rays keep the dark position on the recording plane
At the same time the position where the deflected rays arrive appear to be brighter than the undisturbed environment
A visible flow pattern of variations of the illumination is created in the recording plane.
From a flow analysis of the optics of the shadow effect, which follows that the visible signal is dependent on the second derivative of the refractive index (d2η/dn2) of the fluid. Hence, the shadowgraph can be called an optical diagnostic technique which is sensitive with changes in the second derivative of the fluid density (d2ρ/dn2).

The shadowgraph technique is used to visualize high speed flow over a bullet

Schlieren Photography

Schlieren photography of flow visualization is similar to the shadowgraph technique
This method uses light rays which are bent whenever they encounter changes in density of a fluid
This method differentiates fluid of changing densities
Schlieren system is used to visualize the instantaneous and high-speed supersonic flow pattern away from the surface of an object
The setup of Schlieren photography comprises a bright source of light, a sharp knife edge to focus the light and two concave mirrors are on both sides of flow being to be visualized. The rays continue on to a recording device like a video camera.

Schlieren photography is sued to visualize flow pattern of turbulent jet
Eddy structure of jets due to entrainment can be understood

Use of shadowgraph and schlieren photography to visualize compressible flow

Visualizing Coughs

The schlieren photography is useful to get the image exhaled human airflows
During covid pandemic, effect of face mask on coronavirus can be studied using this technique
Instantaneous images are taken for different times.

Flow due to a loose fiited face mask is presented below bases on Schlieren methods

Comparison of Shadowgraphs and Schlieren Visualization Methods

flow visitation for flow over body and bullet are given

Oil flow visualization Technique

A viscous oil is coated over the surface of the body. high- speed air flowing around the body transport some oil and creates streaks on the body surface.
The resulting oil streak lines formed over the surface represents the flow pattern around surface of the object which is to be analysed
Oil streak lines are different for impingement and flow separation region.
Initial film thickness of oil around the body is essential to obtain results

Streak lines of oil over the surface model is used to understand flow directions around the body

Limitation Experimental visualization techniques

Experimental visualization techniques cannot present the details of flow physics due to following limitations in instrumentations
- Number of points for dyes or colours
- Inaccuracies of flow visualization for high diffusive inks in water or air
- Dyes or colour can be injected in high temperature or pressure and multiphase flow
- Location of instrument
- Optical blockage
- Prohibitive use of instrumentation for high-speed flow or recirculation regimes
Experimental techniques are expensive and time consuming

Numerical Methods

Flow visualization in numerical simulation is quite easier compared to experimental methods. Hence, CFD modelling of complex flows is widely carried out in many industries.
Compared to experimental techniques of flow visualization, numerical flow field easy to calculate the date of velocity and locations of cells
There are following method to physical represent flow pattern
- Velocity Vectors
- Streamlines and stream tubes
- Path lines of particles
- Streak-lines
- Recirculation ratio
- Refractive technique
- Surface flow techniques
A volley of options is available in post-processing of CFD software to visualize and understand the details of a simulated flow field like pressure, velocity and temperature
- Contour plots of flow variables such as scalar, vector and tensor quantities
- Distribution of scalar and vector quantities
- Drawing of Streamlines, path-lines, particle tracks
- Reporting of Animations of flow variables
- Visualization of turbulent eddies (3D flow structures) based on vortex structure
- Use of 3D submerged virtual reality approach or virtual travel through the fluid domain

Streamlines

Streamlines are used to understand both instantaneous or steady flow using RANS models in continuous fluid (Eulerian frame work)

Particle Tracking

Particle based approach (langrage formulation) is useful to find concentration of particles for particle laden flows.

Vortex Structure

For instantaneous flow, eddies in turbulent are represented with second gradient tensor to find vortex structure in the flow using large eddy simulation or direct numerical simulations. The 3S structure vortex is coloured with magnitude of vorticity.

Comparison of Flow Pattern by Experimental and CFD

CRAFT did experiment of flow over an aircraft to visualize the flow. They compared their oil flow visualization with contours of pressure coefficients over the wing surface. Both method shows similar flow pattern.

https://crunch.craft-tech.com/applications/aerodynamics/high-lift-prediction-workshop/

Summary

Experimental visualization of fluid flows is essential to provide insights into flow physics and understand practical vide of fluid dynamics
In CFD or numerical simulation, Visualization of simulation results is calculated numerically with help of scalar, vector and tensor quantities which are difficult to get everywhere in flow domain