Free Surface Bubbly Flows

David Jeon | Lydia Trevino

The Free-Surface and Bubbly Flow group looks at flow phenomena relating to the interaction of the water, air, and solid surface interfaces. Since virtually all free-surface flows at ship scale are laden with air bubbles, the action of bubbles in boundary layers is of particular interest. A typical ship scale flow is shown below, a picture of the USS Ticonderoga (a Ticonderoga class guided missile cruiser). There is a clear region of white water around the hull and wake, indicting large areas of high bubble concentration. We are looking at three particular aspects of this flow: surface piercing phenomena, bubbly boundary layer flow, and shear stress measurement in harsh environments.

Surface Piercing Phenomena
Our experiments are centered on a 1:10 scale model of a wedge model used by Naval Surface Warfare Center in the David Taylor Model Basin. We are doing various tests to see which factors affect the geometry of the bow and stern waves. For example, we are particularly interested by how ship motion controls the shape of these waves, and hence the amount of air entrained by them.

Bubbly Boundary Layer Flow
These experiments are being done on a horizontally mounted flat plate with bubble injection. To better model the near-wall action of bubbles, we are using the idea of forced turbulence. Forced turbulence is the use of external turbulence to force a nominally laminar boundary layer. This lets us simulate the inner layers of a boundary layer, but at physical scales that are amenable to laboratory analysis. We then study how the injection of bubbles affects the wall shear stress, and how the manner of injection affects the bubbly flow. We feel these measurements are essential towards understanding how bubbles in the boundary layer affect skin friction drag.

Shear Stress Measurement
Quantifying shear stress has been a particular field of great interest to engineers due to its impact on hydrodynamics, aerodynamics and micro-fluidics among other applications. The amount of drag on a solid surface, whether inside a small channel or over large vessel surfaces, is influenced by the shear experienced at the surface. Over the past few years, various techniques have been developed to measure skin friction. All of these methods suffer in different degrees of limitation when utilized to measure the wall shear stress in underwater flows which are characterized by microbubbles and turbulent boundary layers. Such flows are present around ships in virtually all conditions and are an essential part of naval hydrodynamics. PIV, for instance, requires indirect measurements of shear stress via flow velocity near the wall. This method is impractical when used on moving underwater vehicles as probes disturb the flow in the boundary layer and are inaccurate when bubbles or contamination are present. We are interested in developing a method to reliably measure wall shear stress in high void fraction and contaminated flows. Factors to be considered in the design include reproducibility, reusability, robustness and compact size so that many may be placed along the walls of vessels. This sensor must also be designed to minimize the effects of other forces such as those due to the pressure gradient.

 

Vortex Flows in Nature and Technology
Optimal Flapping

Michele Milano | Matthew Ringuette

We use Machine Learning techniques to assess from a fluid dynamics perspective the trajectory and the shape of a flapping appendage. We formulated a novel vortex formation parameter to help interpreting the results of trajectory optimization for a pitching and heaving flat plate. Our findings show that to maximize the average lift produced, flapping appendages must generate vortices of maximal circulation during a flap.

Shown above: three still images from an animation. DPIV measurements taken at 50% span of the vorticity generated by a pitching and heaving flat plate maximizing the average lift produced: the plate turns right before the leading edge vortex pinches off. (Click on each still to see larger graphic.)

To show that similar constraints hold for the appendage shapes also, we consider Drosophila Melanogaster as a case study, extending further the vortex formation parameter to account for general shapes and flapping trajectories.

Shown left: left graph: elliptic wing following the trajectory of a fruit fly wing; right graph: corresponding generalized formation parameter. Click here or on image to view animation.

The evolutionary algorithm evolves families of wing shapes that optimize vortex formation, for a given flapping trajectory. When the flapping trajectory is fixed to that of Drosophila melanogaster (fruit fly), our evolutionary algorithm evolves a family of wing shapes that correlate very closely to natural fruit fly wings. (Picture, above right, of an actual Drosophila wing.)

Figure shown at left: blue: digitized shape of a real fruit fly wing; red and black: two ellipting wings belonging to the family evolved by our evolutionary algorithm to produce the same maximal formation parameter as the real fruit fly wing. See larger image of graphic.

 

Atmospheric Boundry Layer Project

Brad Dooley

The Atmospheric Boundary Layer Project being conducted by the Gharib Group spans two distinct areas of research:

• Characterization of boundary layer flows along the surface of the earth, and in particular the influence of turbulence on the near-surface velocity profile and surface shear stress; and
• Experimental investigation of liquid evaporation from non-porous and porous substrates beneath a surface-parallel freestream and boundary layer.

These two avenues of research converge at such topics as "secondary evaporation" of chemical agents: Having soaked into the ground after deployment, the rate of evaporation into the atmosphere is vital information for the safety of personnel in the vicinity. Our goal with this research is to gain greater understanding of this very complex and poorly-characterized process.

Boundary Layer Flows
A series of experiments has begun and is ongoing in the John W. Lucas Adapative Wall Wind Tunnel at GALCIT. We utilize the tunnel's ground plane to generate a flat-plate boundary layer at various Reynolds numbers, and install various devices upstream of the leading edge to increase the freestream turbulence in the flow. At select stations downstream of the leading edge we measure both wall shear stress and the velocity profile; results to date demonstrate that relatively small variations in freestream turbulence level produce significant changes in the wall shear stress. These wall shear changes are significant in the process of liquid evaporation from the surface. (Set-up shown above left. See larger image.)

Wall shear stress measurements are made using an optical interferometry technique; the deformation of a small droplet of silicone oil placed on a glass surface may be related quite precisely to wall shear. (Shear set-up shown at left. See larger image.)

Liquid evaporation
A technique has been developed within the group to quantitatively measure the evaporation rate of a liquid soaked into a porous material. Previous attempts to deal with this problem have directly measured mass loss, or have extrapolated evaporation rates from spectrometry measurements in the airflow downstream. The innovative technique being refined in the Gharib group involves measuring the very small pressure drop in a sealed cavity beneath an initially-soaked sample of porous substrate; as liquid evaporates a wicking action within the porous sample necessarily increases the volume within the cavity, lowering the pressure slightly.

Obtaining precise correlations between pressure drops and mass loss is the current subject of intense work.

Universal Time Scale for Vortex Ring Formation

L / D = 2	L / D = 14

The formation of vortex rings generated through impulsively started jets is studied experimentally. Utilizing a piston/cylinder arrangement in a water tank, the velocity and vorticity fields of vortex rings are obtained using Digital Particle Image Velocimetry (DPIV) for a wide range of piston stroke to diameter (L/D) ratios.

The results indicate that the flow field generated by large L/D consists of a leading vortex ring followed by a trailing jet. The vorticity field of the formed leading vortex ring is disconnected from that of the trailing jet. On the other hand, flow fields generated by small stroke ratios show only a single vortex ring. The transition between these two distinct states is observed to occur at a stroke ratio of approximately 4, which, in this paper, is referred to as the "formation number". In all cases, the maximum circulation that a vortex ring can attain during its formation is reached at this non-dimensional time or "formation number". The universality of this number was tested by generating vortex rings with different jet exit diameters and boundaries, as well as with various non-impulsive piston velocities.

It is shown that the "formation number" lies in the range of 3.6 - 4.5 for a broad range of flow conditions. An explanation is provided for the existence of the "formation number" based on the Kelvin-Benjamin variational principle for steady axis-touching vortex rings. It is shown that based on the measured impulse, circulation and energy of the observed vortex rings, the Kelvin-Benjamin principle correctly predicts the range of observed "formation numbers".

 

Heat Transport Processes
Heat Transport Processes in the Wake of Stationary and Oscillating Circular Cylinders

Joshua Adams

(Image at left) Temperature of the wake behind a heated circular cylinder (Re=610) as visualized by Thermochromic Liquid Crystals (TLC) particles. Blue denotes hot; red, cold; green, warm. The hot fluid from the boundary layer form the shear layers which then roll-up into the vortices.

We are developing the technique of Digital Particle Image Velocimetry/Thermometry (DPIV/T) to study the process of heat transport in unsteady flows. DPIV/T allows us to measure simultaneously both the instantaneous velocity and temperature fields of the flow. The temperature is measured by seeding the flow with thermochromic liquid crystal (TLC) particles which change their reflected wavelength as function of temperature. By calibrating reflected wavelength versus temperature using a color multi-CCD camera, the local temperature of the flow may be deduced. The velocity is measured by using the same particles as Lagrangian flow tracers, and local velocity or displacement of the flow may be measured by cross-correlating two sequential images as in standard DPIV.

Some of the findings from a study of the heat transport in the near wake of a stationary and oscillating circular cylinder in cross-flow using DPIV/T are:

The direction of the turbulent heat flux vectors, the vectors are found not to be co-linear with the gradient of mean temperature. This misalignment implies that the gradient transport models are inappropriate for modeling the turbulent heat transport in the near wake of a circular cylinder. parision of the direction of the phase averaged molecular (green vectors) and incoherent heat flux vectors (blue vectors) in the wake of a stationary cylinder.

The kinetic energy production occurs in the saddle regions (regions where the fluid is being stretched in one direction and compressed in another), while the temperature fluctuations are produced at the edges of center regions (regions where the fluid is rotating), i.e., the edges of the vortex cores. Besides the previously known increase near the natural vortex shedding frequency for transverse oscillations, there also exist large increase in the heat transfer at certain super-harmonics.

(Image at left) Surface heat transfer versus cylinder oscillation frequency for Re=550.

At the frequencies corresponding to roughly two and three times the unforced vortex shedding frequency, the wake pattern may become synchronized by processes of period doubling and tripling with respect to the cylinder oscillation period, respectively. The increase in the heat transfer rate is found to correlate with the distance at which vortices roll-up behind the cylinder. Therefore, the near wake is found to play a critical role in the heat transfer from the surface of a circular cylinder, and the cause of the increase in heat transfer is believed to the removal of the stagnant and low heat convecting fluid at the base of the cylinder during the roll-up of the vortices.

Stagnant and non-heat convecting fluid near the base of cylinder for stationary case (low heat transfer). The velocity near the base of the cylinder is small as compared to case of oscillation (Image at left).

 

 

 

Vortices scrubbing away fluid near the base of the cylinder for case of cylinder oscillating near the vortex shedding frequency. Note the close roll-up of the vortex and the large velocity near the base of the cylinder as compared to the stationary case (Image at right).

 

This material is based upon work supported by the National Science Foundation under Grant No. 9417520 and 9903346. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF).