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Research Topics and Projects
Instability and laminar-turbulent transition.
Bifurcation, periodicity and chaos of non-linear dynamical systems.
Taylor-Couette flow with heat transfer.
Instability and turbulence modelling in stellar and planetary interiors.
Supersonic and hypersonic compressible flows.
Shear flows and vortices in stratified and rotating fluids
Urban meteorology: realistic urban flow simulation
Large-scale turbulent flow structures
Research has been externally funded by
Mathematical Sciences Small Grants scheme by Engineering and Physical Sciences Research Council (EPSRC), Modelling turbulence induced by hydrodynamic instability in differentially-rotating flow, May 2022–Feb 2024, total cost: £97,422, PI: Dr Junho Park (EP/W019558/1).
astro4dev technical grant by Royal Astronomical Society and Office of Astronomy for Development (RAS-OAD), Development of a new turbulent viscosity model for numerical simulations of the evolution of rotating stars, Sep 2021–Aug 2022, total cost: £2,230, PI: Dr Junho Park.
Research Highlight
Taylor-Couette flow
The Taylor-Couette flow is a flow between two concentric cylinders rotating independently. The flow becomes centrifugally unstable in the regime where there is an imbalance between pressure gradient and centrifugal force, the instability known as the centrifugal instability (CI). CI can further develop towards secondary instability and eventually become turbulence as the Reynolds number increases. In the presence of stable density stratification in the axial direction, there is a new type of instability called the strato-rotational instability (SRI), which occurs due to the resonance between inertia-gravity waves confined near the cylinders.
We see on the right (top) the competition between the SRI and CI when two instabilities have growth rates of the same order of magnitude. At the bottom, we see how the two instabilities develop simultaneously and break down to turbulence.
See also: Park and Billant (2013), Park et al. (2017, 2018), Park (2025).
Shear instabilities in stellar radiation zone
Radiation zone in the interior of stars is stably stratified while its thermal diffusivity is high. Turbulence in the radiation zone is considered to be weak while observations have suggested that turbulent dissipation should be strong enough to explain efficient transport of angular momentum within stellar interiors.
We recently found that the inclusion of rotational shear, in particular the rotation beyond the traditional f-plane (e.g. inclined rotation at a general colatitude), promotes shear instability leading to a generation of strong turbulence even in the radiation zone. Both vertical (radial) and horizontal (latitudinal) shear instabilities have been studied under the combined effects of stable stratification, high thermal diffusivity, and the non-traditional f-plane approximation.
See also: Park et al. (2020, 2021), Park and Mathis (2025), Moisset et al. (2025).
Waves in planetary interior
Star-planet or planet-planet tidal interactions induce inertial or gravito-inertial waves in the interior of planets. In planetary convective zones where columnar flow structure is prevalent, we investigated how the tidally-induced inertial wave interacts with convective vortices. We also studied how the inertial wave extracts or loses its energy via the interaction with zonal shear flows in the planetary interior.
See also: Astoul et al. (2021), Dandoy et al. (2023)
High-speed boundary-layer flow and sensitivity analysis
Turbulence in high-speed compressible boundary layers augments both drag and heat transfer on the wall. Robust prediction on the onset of transition to turbulence is very important in high-speed boundary layers.
Using the PSE and adjoint technique, we evaluated the sensitivity to base-flow distortion in high-speed boundary-layer stability. The developed technique opens up the possibility of the method to be applied for optimal control of laminar-turbulent transition in supersonic and hypersonic flows.
See also: Park and Zaki (2019)
Stratified rotating flows
Vortices in stratified fluids are subject to an instability called the radiative instability induced by inertia-gravity wave emission and interaction with vortices at critical layers. We revealed that the radiative instability can be suppressed when the background (planetary) rotation is sufficiently fast in both cyclonic and anti-cyclonic senses. The regime of centrifugal instability is also unveiled when the rotation is weakly anti-cyclonic.
See also: Park and Billant (2012, 2013)
Bifurcation and chaos in non-linear dynamical systems
We have investigated various nonlinear dynamical systems and their bifurcation / chaotic behaviours; for instance, stability, bifurcation, Lyapunov exponents, chaotic attractors, characteristic chaotic structures, periodic orbits, etc. Non-linear dynamical systems research enables us to better understand complexity of real-world systems (e.g. convection in atmosphere and oceans) and our research has focused on considering more realistic conditions in non-linear dynamical systems and exploring how the realistic consideration (e.g. rotation, high-order modal interactions) alters the behaviours of non-linear dynamical systems.
See also: Park et al. (2015, 2016, 2021), Moon et al. (2017, 2019)
Turbulence structures
(1) Large-scale coherent structures in channel flows
Stability of large-scale coherent structures was investigated in a channel flow with vortex-induced streaky motions. The study has the potential to explain non-linear self-sustaining processes of turbulence.
See also: Park et al. (2011)
(2) Turbulent vortex shedding in real urban flow
In this example of large-eddy simulations of turbulence flow in a real urban area, we observed atmospheric turbulent vortices shed behind high-rise buildings and assessed how this vortex shedding affects pollutant dispersion in cities.
See also: Han et al. (2017)
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