Presentation on theme: "The Rayleigh-Taylor Instability Jenna Bratz Rachel Bauer."— Presentation transcript:
The Rayleigh-Taylor Instability Jenna Bratz Rachel Bauer
Introduction Rayleigh Taylor Instability occurs when a denser fluid is being accelerated by a lighter fluid This project deals with silicone oil as the denser fluid, and air as the less dense fluid Theory will attempt to explain the most unstable wavelength, critical wavelength, and 2-D pattern formation
Literature Fermigier et al. discussed patterns, dominant wavelength, investigated dripping Sir Geoffrey Taylor describes instability and imposes a vertical acceleration larger than gravity on the fluid (Verified by Lewis) Ott described a model assuming zero thickness Chen and Fried investigated a liquid-liquid interface and showed viscosity doesn’t change the ultimate formation of the instability
Procedure -A glass plate was placed on a picture frame with graph paper placed underneath -2 or 3 mL of silicon oil were poured at varying viscosities -6 Trials were conducted, each allowing the oil to spread for 5 minutes -Glass was flipped over and pictures were taken as the instability evolved
Procedure (cont’d) -One trial (3 mL at 1000 cst) was left for 4 hours to spread - Pictures were taken until dripping was observed
Data (cont’d) 350 cst, 3 mL, sitting for 5 minutes before flipping
Data Matlab was used for measurements Measurements were taken (when visible): –Between drops in each ‘layer’ –Diameter of drops –Distance between ‘layers’ –Thickness of ‘layers’
Data (cont’d) Diameter of drops were measured for up to 10 drops in each trial Std. Dev.=.0416 cm Mean= 1.014 cm
Data (cont’d) The outer ‘layer’ is the first visible ring formed in the fluid Distance between the centers of the drops was measured Std. Dev.=.2747 cm Mean= 1.6431 cm
Data (cont’d) Distance between the centers of the drops in the second layer were measured Std. Dev.=.19794 cm Mean= 1.5377 cm
Theory Most Unstable Wavelength (wavelength you actually see) Wavelength for which the system is stable/unstable 6-Axis symmetry in the 2-D pattern that forms
Most Unstable Wavelength Dimensional Analysis – depends on surface tension ( ), density ( ), and gravity (g). –Matching up the units yields the following equation:
Most Unstable Wavelength (cont’d) Matching up the exponents gives a system of equations, which leads to a result which still has a dimensionless constant: Though we could not obtain the value of C, it is expected to be 2*Pi*sqrt(2). (Fermigier )
Most Unstable Wavelength (cont’d) Use data to estimate c.
Most Unstable Wavelength (cont’d) Observed C was measured using Known C was Average Observed C was 8.8821 Error only.0036
Stability Consider the question, for what wavelength does the system become unstable? Compare energy of initial cylinder with perturbed cylinder First perturb the height by only looking at a radial perturbation
First, introduce a volume constraint since the volume is constant. Since volume is constant, all epsilon terms go to zero.
Stability (cont’d) Want to find difference between the energy of the perturbed (E[u(r)]) and the unperturbed system (E[h]). Energy is proportional to the surface area minus gravitational potential energy.
Stability (cont’d) Want to look at E[h]-E[u(r)]: But the volume constraint result gave:
Stability (cont’d) Since we expect the same outcome for each wavelength, we can just look at one wavelength and when the system is stable for this wavelength. So let Then
Stability (cont’d) Simplify to obtain: This is >0 when, and so when the system is unstable. And, the system is stable for
Stability (cont’d) Surprising result? –This critical value of lambda is approximately 2.3468 cm, which is greater than all of the experimental wavelengths measured (including the theoretically calculated most unstable wavelength which was 1.324 cm) –Would expect the most unstable wavelength to be greater than the critical wavelength since the system reaches instability after wavelength reaches this critical length
Stability (cont’d) Now consider perturbing in the theta direction, as well as with respect to the radius of the cylinder Set-Up is similar except now the perturbed height becomes And the Energy becomes
Stability (cont’d) Now, Similar to just perturbing the radius, we should consider E[h]-E[u(r,theta)], this would give a condition on both w and gamma for stability
Conclusion Data validated that the most unstable wavelength is equal to: Where is capillary length
Conclusion (cont’d) Although it may not coincide with measured data, when The system becomes unstable. So for wavelengths exceeding this wavelength, ‘rings’ will form, so the ‘pancake’ of fluid will not retain its shape.
Conclusion (cont’d) Considering the energy difference for a perturbation in both the direction of the radius and theta should give another stability condition, showcasing the symmetry of the 2-D pattern.
Further Work Over what time period does the instability occur? Verify the critical wavelength with more data. Theoretically obtain the constant C in the expected wavelength Explore directing the fluid into different shapes, and looking at the effect on the instability