High-fidelity simulation of multi-physics flows

The multi-physics problems in fluid mechanics are computationally solved using direct numerical simulation (DNS) and large eddy simulation (LES). The range of the multi-physics reaches: (i) laminar-turbulent transition, (ii) self-sustained turbulence, (iii) viscoelastic fluid and (iv) two-phase fluid effects. The complex wall-geometry, which includes the (local) non-zero pressure gradient, is considered using either the curvilinear grid or the immersed boundary method.

Turbomachinery

High-fidelity simulation for turbomachinery rigs

Investigation of the turbulent boundary layer over an airfoil/cascade with compressibility
Numerical method
1. RANS for the industrial application
2. Eddy-resolving simulation (i.e., LES or hybrid RANS/LES) for examination of flow structures
3. High-order DG method (collaborated with NASA Ames Research center)
Physical interest
1. Flow separation in the downstream and/or blade-hub corner
2. Shock formation
3. Laminar-to-turbulent transition
Industrial interest
1. Blade loading
2. Efficiency and loss
3. High-fidelity database for the reference of RANS modeling
Related papers: Lee et al. (Stanford CTR summer program 2018), Min et al. (TurboExpo 2018), Xia et al. (TurboExpo 2020)

Casing treatment for the stall suppression

(figure from JHU)

Modeling of the Johns Hopkins' axial compressor (originally, NASA low-speed rig) with the axial casing groove
1. Sliding interface between the rotor and casing
2. Unsteady RANS
3. Hybrid RANS/LES using DDES model
Integration of the experimental data and DNS data for defining the RANS boundary conditions
Performance prediction of the newly-designed compressor at the research center
Funded by ONR
Collaborated with:
1. Prof. J. Katz, Johns Hopkins University
2. NASA Glenn Research Center

Aero-acoustic (noise) analysis in real engines and HVAC system

(figure from RTX Pratt & Whitney)

Modeling and analysis of the flow-induced noise in (proprietary projects)
1. Pratt & Whitney engines
2. Collins Aerospace HVAC system
Analysis of the broadband noise in a combustor
1. Funded by FAA

Turbulence: Modeling

Machine learning for turbulence modeling

Optimization of the k-w RANS model for transonic compressor cascade
1. Training the RANS model using experimental data
2. The Gaussian Process Regression with spatial kernels
3. Learning the error term from flow variables
Related work: Lee et al. (UTC Analytic Conference 2018)

Turbulence: Physics

Transitional and fully-turbulent boundary layer

Laminar-turbulence transition by homogeneous isotropic turbulent in free stream
Passive scalar transports in the fully-turbulent boundary layer
Temperature-dependent viscosity
Adverse pressure gradient
Related papers: Lee et al. (JFM 2013, IJHFF 2014, JoT 2010)

Turbulent flow over a rough surface

Curvilinear coordinate system for the wavy surface
Immersed boundary method for the square-rod roughness
Related papers: Lee et al. (PoF 2016), Nadeem et al. (IJHFF 2015)

Turbulent channel flow

Finite difference method
MPI-OpenMP hybrid parallelization
Related papers: Lee et al. (JFM 2014, PoF 2015), Hwang et al. (JFM 2016)

Turbulent pipe flow

Simulation in the cylindrical coordinates (radial grid)
Comparison to the channel flow at the same Reynolds number
Related papers: Lee et al. (PoF 2015), Ahn et al. (PoF 2015)

Complex flows

Viscoelastic fluid

Counter-intuitive characteristics to the ideal Newtonian fluid
Delay of the laminar-turbulence transition
Drag reduction of turbulent flows
Mathematical modeling by the FENE-P closure
Related papers: Page et al. (ICTAM 2016), Lee et al. (in preparation)

Open-channel flow with air-water interface

Coupled level-set and volume-of-fluid method for the interface tracking
Analysis of turbulent structures near the deformable interface
Froude number effect
Related papers: Lee et al. (JoT 2012a)

Diffusion, pressure gradient & more

Turbulent round jet

Cylindrical coordinates with the multigrid pressure solver
Inflow conditions from an auxiliary pipe flow simulation
Related papers: Lee et al. (in preparation)

Turbulent diffuser flow (diverging pipe)

Generalized cylindrical coordinates
Three angles of the divergence from 2 deg. to 8 deg.
Related papers: Lee et al. (JoT 2012b)

Homogeneous isotropic turbulence

3-D unsteady incompressible Navier-Stokes equation solver using the spectral method
Decaying turbulence from the initial random noise
For the free-stream turbulence of the by-pass transition simulation

Taylor-Couette flow

Cylindrical coordinates (radial grid)
Turbulent flows by the counter-rotating cylindrical walls

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