Professor F. Liu's areas of major research interests and expertise include:

The following is an incomplete description of active research projects in his research group. Please refer to list of publications for more details. 

Adaptive grid generation

Finite difference, finite volume and finite element methods all use a computational mesh to approximate a continuum problem. In areas of large gradient of velocity, pressure, temperature or other flow parameters, such as in the neighborhood of a shock wave, very small grid sizes are required. In other areas, such as in the far field where the flow is rather uneventful, fewer grid points can be used without degrading the computational accuracy. For many practical problems the precise location of shock waves or other flow features are not known a prior. It is highly desirable to have a computational method that can automatically detect the areas of high and low flow gradients and refine and coarsen the computational mesh accordingly. A grid generation method has been proposed that can provide precise control of the mesh size distribution. This research will develop the method for arbitrary computational domains and couple it with a flow solver for steady and unsteady flow calculations.

 
Multigrid method for solving the Reynolds-average Navier-Stokes equations with advanced turbulence models

We are interested in developing efficient numerical methods for simulating high Reynolds number flow with complex geometry. A staggered finite volume method with multigrid has been developed for the compressible Reynolds-averaged Navier-Stokes equations with algebraic and two-equations turbulence models. Multigrid method is applied to accelerate the convergence rate for both the solution of the Navier-Stokes and the turbulence model equations. Both upwind and central difference type algorithms are employed to discretize the differential equations in space. The resulting semi-discrete equations are marched in time by an explicit multi-stage method which are highly suitable for parallel processing.

 

Computation of Unsteady Flow in Turbomachinery Blade Rows

Almost all design methods for turbomachinery cascades such as the compressor and turbine blades in a jet engine have been based on steady flow calculations and experimental correlation. Yet, turbomachines are essentially unsteady machines. Indeed, they depend on flow unsteadiness to produce work. Unsteady flows are extremely important to the performance, heat transfer, aeroelasticity and noise of jet engines. We have develop a parallel, multigrid, multiblock Navier-Stokes code that can simulate unsteady flow through single and multiple turbomachinery blade rows. Such calculation take a lot of computer time and memory, even on massively parallel computers. Improved numerical algorithms are continuously sought to reduce the demand on computational resources while increasing the accuracy. At the same time, the computational code is used to study fundamental fluid flow, heat transfer, combustion, and fluid-structure interaction problems that exist in gas-turbine blade rows. 

 
Aeroelasticity

We are developing efficient numerical methods for calculating fluid/structure interactions. See The AGARD I-Wing 445.6
To be detailed.

 

Aerodynamic Optimization of Turbine Blade Rows via CFD and Control Theory

Current aerodynamic design procedures involve long hours of manual iteration by a designer. The optimality of a design rests heavily on the designer's knowledge and skills and is not in any way guaranteed. This usually leads to non-optimal machines with long design cycle and large developmental costs. With the advance of computational fluid dynamics and nonlinear optimization theory, this research attempts to replace this manual iteration by a systematic computer optimization procedure based on rigorous mathematical and fluid dynamics principles, that is, by solving the fundamental fluid dynamic equations and using advanced nonlinear optimization methods. By formulating an aerodynamic shape optimization problem into a nonlinear control problem, the computational effort in achieving an improved design can be dramatically reduced compared to conventional sensitivity analysis. Gradient information needed for optimization can be obtained by solving a linear adjoint equation to the original nonlinear governing equations, the Euler or the Navier-Stokes equations. The optimization theory is combined with a fast and accurate Navier-Stokes analysis code for turbomachinery flows. The proposed research will explore various two- and three-dimensional shape optimization of compressor and turbine blades.

 
Gas-Turbine Cycle Innovation: Turbine-Burner Engines

In a conventional gas-turbine engine, fuel is burned in the main  combustor before the heated high-pressure gas expands through the turbine. A turbine-burner concept is proposed in which combustion is continued inside the turbine  to increase the efficiency and the specific thrust of the turbojet engine or the specific power of a ground-based power gas-turbine. This concept includes not only continuous burning in the turbine (Continous Turbine-Burner: CTB) but also 'discrete' inter-turbine-burners (ITB) as an intermediate option. Thermodynamic cycle analyses are performed to compare the relative performances of the conventional engine and the turbine-burner engine with different combustion options for both turbojet and turbofan configurations and ground-based gas-turbine engines. Turbine-burner engines are shown to provide significantly higher specific thrust with no or only small increases in thrust specific fuel consumption compared to conventional engines. Turbine-burner engines also widen the operational range of flight Mach number and compressor pressure ratio.  Coupled with regeneration or other hybrid cycles, the CTB and ITB engines offer exceptional improvements in both efficiency and power density for ground-based gas-turbine engines. Due to reduced combustion temperature, the CTB and ITB engines also have the potential of reducing NOx emission. Recent studies  indicate potential longer endurance of turbine blades due to reduced turbine inlet temperatures during operation of a turbine-burner engine.

Like most innovations, there are also tremendous challenges for this technology to become a reality. Among them are the following that are of fundamental importance.

  1. Multi-objective, multi-design parameter cycle optimization and mission studies.
  2. Ignition and flame holding in a flow with high acceleration.
  3. Hydrodynamic stability of stratified flow with large transverse pressure gradient.
  4. Cooling of critical components. 
  5. Blade Aerodynamic Design and Optimization with heat release in the passage.  

Theoretical, computational, and experimental studies in the above topic areas are being performed at UCI. The turbine-burner engine has caught major attention from the Air Force Research Laboratory and recently NASA Glenn Research Center along with a number of engine companies. Realistic mission studies are being performed for commercial and military, subsonic and supersonic, and ground and aerial applications. Experimental studies are also being performed or planned at those facilities.

This work is in collaboration with Professor William A. Sirignano in the MAE Department. Professors Dimitri Papamoschou and Derek  Dunn-Rankin contribute in the experimental studies of this project. Funding has been received from the National Science Foundation, the California Energy Commission, and the US Air Force Research Laboratory (AFRL) via ISSI. 

Flame Propagation over Liquid Fuel pools under micro-gravity

To be detailed.

 
Stability of Leading-edge Separation Vortices over Slender Delta Wings and Conical Bodies at High Angle of Attack

To be detailed.


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