Biological Fluid Dynamics

Instabilities in flowing gyrotactic microorganism suspensions

 Pattern prediction of bioconvection under uniform shear

Microorganisms are present in almost every part of temperate aqueous environments, and they play a critical role in pathogenic infection, digestion, reproduction, and food chains in the oceans. Many swimming microorganisms often exhibit collective behaviors, and it often originates from interaction of the microorganisms' motility with surrounding fluid flows. The focus of the current research is aimed at understanding how these collective dynamics of the swimming microorganisms (particularly biflagallate algae, e.g. Chlamydomonas) interact with instabilities in shear flows.

See also.

Bioconvection under unform shear: linear stability
Y. Hwang & T. J. Pedley, 2013, In preparation for J. Fluid Mech.


Cellular mechanotransduction in vascular flow systems

Signaling cascade over cytoplasm in a cell  predicted by a reaction-diffusion system

The cardiovascular system is particularly interesting in the context of fluid mechanics because hemodynamics (fluid mechanics of blood flow) plays a crucial role in early development of atherosclerosis, the leading cause of heart attacks and strokes. Early atherosclerotic lesions localize preferentially in arterial regions that are exposed to low flow rate and/or oscillatory flow. Development of these lesions involves dysfunction of the vascular endothelium, the monolayer of cells lining the inner surfaces of all blood vessels. Therefore, understanding processes by which mechanical stimuli are translated into intracellular biochemical responses  (mechanotransduction) is a central research issue. My research was devoted to understanding how mechanical force is transmitted through intracellular structures.

See also:

Intracellular regulation of cell signaling cascades: how location makes difference
Y. Hwang, P. Kumar & A. I. Barakat, 2013, revised  for J. Math. Biol. 

Mechanisms of cytoskeleton-mediated mechanical signal transmission in cells
Y. Hwang & A. I. Barakat, 2012, Comm. Int. Biol., 5(6) p1.

Dynamics of mechanical signal transmission through prestressed stress fibers
Y. Hwang & A. I. Barakat, 2012, PLoS ONE, 7(4) e35343.


Physical Fluid Dynamics

Physics and control in wall turbulence

Very-large-scale motions in a turbulent channel with (left) and without (right) the near-wall and log-layer eddies

Recent discovery of very-large-scale motions (large-scale streaky structures) in wall-bounded turbulent flows has has attracted great interest to this field as they carry a signicant amount of turbulent kinetic energy and Reynolds stresses in the outer region. These motions are very energetic particularly at high Reynolds numbers. Therefore, understanding their origin and interaction with the other coherent motions has a significant impact in developing in innovativeenergy utilization processes, vehicle design, and so on.

See also:

Near-wall turbulent fluctuations in the absence of the wide outer motions
Y. Hwang, 2013, To appear, J. Fluid Mech.

Self-sustained processes in the logarithmic layer of turbulent channel flows
Y. Hwang & C. Cossu, 2011, Phys. Fluids, 23, 061702.

On the stability of large-scale streaks in turbulent Poiseulle and Couette flows
 J. Park, Y. Hwang & C. Cossu, 2011, C. R. Mecanique, 339(1), p1.

Optimally amplified large-scale streak and drag reduction in the turbulent pipe flow
 A. P. Willis, Y. Hwang & C. Cossu, 2010, Phys. Rev. E 83, 036321.

Self-sustained process at large scale in turbulent channel flow
 Y. Hwang & C. Cossu, 2010, Phys. Rev. Lett. 105, 044505. 

Linear non-normal energy amplification of harmonic and stochastic forcing in the turbulent channel flow
Y. Hwang & C. Cossu, 2010, J. Fluid Mech. 664, p51.

Amplification of coherent streaks in the turbulent Couette flow: an input-output analysis at low Reynolds numbers
Y. Hwang & C. Cossu, 2010, J. Fluid Mech. 643, p333.


Physics and control of instabilities in bluff-body wakes


Vortex shedding controlled by spanwise wavy blowing/suction (left) and the related vortex dynamics (right)

The wake behind a bluff body is an important canonical flow for engineering applications, as vortex shedding in wakes is a main source of drag, structural vibration and aeroacoustic noise.  Controlling vortex shedding using spanwise varying passive or active actuation  has recently been reported as a very efficient method for regulating bluff-body wakes. My recent research on this issue has been focused on eluminating the mechanism by which this type of controls stabilizes vortex shedding using hydrodynamic stability theory. To this end, I apply Floquet theory to spatiao-temporal dynamics of the near-wake instabilities.

See also:

Stabilization of absolute instability in spanwise wavy two-dimensional wake
Y. Hwang, J. Kim & H. Choi, 2013, Submitted to J. Fluid Mech.

Sensitivity of global instability of spatially developing flow in weakly and fully nonlinear regimes
Y. Hwang & H. Choi, 2008, Phys. Fluids 20, 071703 

Control of absolute instability by basic-flow modification in a parallel wake at low Reynolds number
Y. Hwang & H. Choi, 2006, J. Fluid Mech. 560 , p465.