Click on a title or picture for more details. For an update on each of the following projects, please check out our publication list.

stable hovering of asymmetric bodies in oscillatory airflows
We explore the stability of flapping flight in a model system that consists of a pyramid-shaped object hovering in a vertically oscillating airflow. Such a flyer not only generates sufficient aerodynamic force to keep aloft but also robustly maintains balance during free flight. Flow visualization reveals that both weight support and orientational stability result from the periodic shedding of vortices. We explain these findings with a model of the flight dynamics, predict increasing stability for higher center of mass, and verify this counterintuitive fact by comparing top- and bottom-heavy flyers.

cornstarch, a shear-thickening suspension delivers focused forces
We investigate force transmission through a layer of shear-thickening fluid, here a concentrated aqueous cornstarch suspension. When a solid body is pushed through this complex fluid and approaches its containing wall, a hardened volume of the suspension is observed that adds to the leading side of the body. This volume leads to an imprint on the wall which is made of molding clay. By studying the geometry of the hardened volume, inferred by the imprint shapes, we find that its geometry is determined by the size and speed of the body. By characterizing the response of the clay to deformation we show that the force transmitted through the suspension to the wall is localized. We also study other aspects of this dynamical hardening of the suspension, such as the effect of the substrate and body shape, and its relaxation as the imposed straining is stopped.

inverted hydrodynamic drafting of interacting flexible bodies
In aggregates of objects moving through a fluid, bodies downstream of a leader generally experience reduced drag force. This conventional drafting holds for objects of fixed shape, but interactions of deformable bodies in a flow are poorly understood, as in schools of fish. In our experiments on ``schooling" flapping flags we find that it is the leader of a group who enjoys a significant drag reduction (of up to 50 %), while the downstream flag suffers a drag increase. This counterintuitive inverted drag relationship is rationalized by dissecting the mutual influence of shape and flow in determining drag. Inverted drafting has never been observed with rigid bodies, apparently due to the inability to deform in response to the altered flow field of neighbors.

free boundaries in thermal convection: simulating continental drift
At high Rayleigh numbers, thermal turbulence often organizes itself to form large-scale circulations. Such flow structures can be induced and modified by localized thermal disturbances. In our experiment, we introduce a freely-moving floating boundary in Benard convection to study the dynamical coupling between a free-moving boundary and a thermally convective fluid. We found that the boundary (the model continent) can be driven to execute a periodic motion. This result is consistent with the geophysical observation that the Atlantic Ocean has been closed and then re-opened several times in the past 2.5 billion years. For more details click on the picture.

spontaneous thrust generation and forward flight from harmonic flapping
This work was conducted in collaboration with Nicolas Vandenberghe and Steve Childress.
We study how reciprocal flapping motion of a rigid, symmetric wing is capable to generate lateral thrust as a result of spontaneous symmetry breaking bifurcation. This instability involves a free boundary (the flapping wing) and its surrounding fluid, reveals that even a symmetric wing would be able to "fly" unidirectionally. Thus, the spontaneous generation of the lateral thrust can be regarded as the physical origin of flapping flight. We further examine the relationship between the flapping frequency, amplitude, wing geometry and forward flight speed. For detail, click on the picture.

flag flapping in laminar flows: the dynamics of passive swimmers
A flapping filament (a 1D flag) in a quasi-two-dimensional soap-film flow tunnel (a 2D wind): the resulting motion is similar to a 3D flapping flag. We study the dynamics of a single filament as well as multiple interacting filaments under the action of an incoming laminar flow. Several stable, dynamical states are observed in this system. We also compare these passive dynamical behaviors with actively swimming bodies: such as fish or birds. For more details click on the picture.

to reduce the drag, be flexible (flexible bodies yields lower drag in flows)
This work was conducted in collaboration with Silas Alben and Michael Shelley.
Through an archetype experiment, we study how flexible objects streamline themselves and experience less drag (compared with rigid bodies) under fluidic forces. In nature, tree branches, broad leaves can survive gusty winds possibly because they are flexible and able to passively deform their shapes. Click on the picture for more details.

a valveless pump that tansfers fluid
This work was conducted in collaboration with Tom Bringley et. al. A valveless pump is constructed after the suggestion of Prof. Peskin and Dr. Jung. It is a impedance induced flow system in a closed circle. A directional flow can be realized when a periodic forcing is applied to the flexible portion of the circle. The direction and speed of flow depends upon the driving frequency and amplitude. The detailed mechanism is still a mystery. For more details click on the picture.

forest-fire fronts and dynamic roughening of growing interfaces
Spreading fire fronts, either started from a single point or a straight line, will not end up with smooth geometries. Instead, they become progressively rough as time passes. We have carried out the first laboratory realization of forest-fire by a paper-burning experiment and studied in detail the dynamical, interfacial roughening process. The scaling aspects of the fire fronts, which are shown to be self-affine, are compared with the contemporary growth models. For more details click on the picture.

the first realization of tubeless siphons using flowing soap films
This work was conducted in collaboration with Julien Psaute and Jonathan Kamler. In daily life, we sometimes need to use a siphon (a hose) to empty a tank of fluid. The siphon will not work if the hose has an opening (a sizable leak) somewhere in the middle. In our lab, we have just created a working siphon that is totally exposed to the atmosphere. The serect? Soap!

a friction-driven oscillator
Solid-on-solid friction is studied in a friction driven oscillator. We observe a stochastic process where sticking and rolling alternate with slipping. The system is driven externally to a self-organized state that is not critical. For more details click on the picture.

3-D visualization of thermal turbulence
We have studied a technique for visualizing 3D thermal convection. Traditional shadowgraphs collapse the three dimensional (3D) information in a convection cell onto a two dimensional screen. Here, we adapt the principles of stereo vision to recover the 3D structures in thermal convection using two sets of shadowgraphs taken from slightly different angles. For more details click on the picture.