Current Research Topics
Past Research Topics
Attenuation of Hypersonic Second Mode Instability with Discrete Surface Roughness on Straight Blunt Cones
Summary
 The 'roughness effect' is a counterintuitive phenomenon wherein Mack's secondmode instability can be damped with surface roughness immersed within the hypersonic boundary layer and judiciously placed at the synchronization location of a desired cutoff frequency.
 Frequencies higher than the cutoff frequency are attenuated, while lower frequencies are amplified.
 Research on the roughness effect is limited to 2D flat plate geometries and lower Mach numbers. This theortical and computational investigation extends the research to higher Mach conical flow fields.
 Cones are common test articles in hypersonic research. The axisymmetric geometry is better suited to high Mach number shock tubes and blowdown wind tunnels.
 The investigation also seeks to study the longterm downstream growth of secondmode frequencies amplified by the roughness effect.

Very HighOrder MultiLayer Compact (MLC) Schemes with SpectralLike Resolution
Summary
 The MLC scheme is designed for solving complex flows with multiple temporal and spatial scales, such as hypersonic boundary layers, computational aeroacoustics, turbulence, etc.
 The multilayer framework leads to both very highorder accuracy and good spectral resolution within a compact stencil
 The directional MLC (DMLC) scheme and the weighted leastsquare MLC scheme improve performance in multidimensional flow simulations

Grid stencil and formula of the 7thorder MLC scheme 
Scheme Accuracy and Comparison
Left: Steady Couette flow simulation: temperature and velocity distribution with the seventhorder MLC scheme using 6 grid points. Right: 2D isentropic vortex simulation: comparison of vorticity distribution along the vortex centerline using different schemes
Supersonic Unstable Modes in Hypersonic Boundary Layers with Thermochemical Nonequilibrium Effects
Summary of Supersonic Mode
 Supersonic mode occurs when a disturbance propagates upstream supersonically w.r.t. the local mean flow
 Causes “Mach wave” like disturbances that are actually a form of sound radiation away from the boundary layer
 Sound radiation can be difficult to capture in traditional stability solvers like LST  requires special boundary conditions
 Supersonic modes have until recently thought to be negligible due to smaller amplification than Mack’s 2nd mode (the dominant instability mechanism in hypersonic flow)
 Recent results show supersonic modes in unexpected conditions with comparable or larger amplification than the 2nd mode
 Warrants revisiting conditions leading to supersonic mode and reevaluation of the mechanism of its creation
 Recent results show the supersonic mode can be created via a nonlinear modal interaction that is not accounted for in most LST solvers
 Requires combined LST and DNS (or other nonlinear stability technique) to study the supersonic mode accurately
Objectives
 Verify our thermochemical nonequilibrium Direct Numerical Simulation (DNS) and Linear Stability Theory (LST) tools are capable of capturing the supersonic mode
 Show supersonic mode exists in conical flow in addition to flat plate
 Development of LST shock boundary conditions
 Determine envelope of conditions leading to supersonic mode
 Mach number, wall temperature, nose bluntness, real gas effects, etc.
 Develop theoretical mechanism of supersonic mode creation
 Linear synchronization vs. nonlinear modal interactions
 Assess the supersonic mode’s impact on transition to turbulence
Direct Numerical Simulation Results
Instantaneous pressure pertubation featuring supersonic mode and Mack's 2nd mode resulting from broadband forcing
Linear stability theory (LST) analysis and direct numerical simulation (DNS) results of the phase speed (left) and growth rate (right) exhibiting the supersonic mode
Investigation of 3D Bluntness Effects on Hypersonic Transition
Investigation of Nose Bluntness Effects on Supersonic Modes
Simulation of a Compression Cone with a Freestream Impulsive Entropy Spot in Mach 6 Freestream


Entropy spot diagram 
Compression cone 
Objectives
 Development of computer code for simulating the interaction of a freestream hotspot with the bow shock and the boundary layer in high order of accuracy
 Simulation of the hotspot perturbed shock layer on compression cone under the effect of a freestream hotspot
 Analysis and investigation on the receptivity and instability growth of hotspot perturbed boundary layer based on the simulation results
 Comparison of numerical results with those obtained by Professor Schneider’s group in the Purdue Mach6 quiet tunnel
Mean Flow for Compression Cone Cases
Freestream Flow Conditions
 M = 6.0
 T = 52.8 K
 P = 610.8 Pa
 Twall = 300 K
 Re_u = 1.026e7 1/m


Geometrical parameters
 R_flared = 3.0 m
 cone halfangle = 2.0 deg
 cone length = 0.45 m
 grid resolution = 3720 x 240
 R=0.001 m


Numerical Study of Hypersonic Boundary Layer Instability Affected by 2D Surface Roughness
Objectives
 Our goal is to utilize numerical simulation to study the role of roughness on the stability of a hypersonic boundary layer, mainly focus on roughness effect on transition
 In the current project, we consider 2D roughness on a hypersonic flat plate at Mach 5.92
 The roughness is modeled as a hump and is treated by a high order cut cell method
 Pure mode S (slow acoustic wave), mode F (fast acoustic wave) perturbation at 100KHz, and wall normal velocity pulse which has a frequency range 1MHz are imposed into the mean flow separately
 The evolution of disturbance on the wall due to different roughness height and locations are studied using FFT


Discrete roughness element 
Four separate computational cases with one roughness element 
FFT results for 100 khz slow acoustic disturbance (Mode S) with different roughness heights


Roughness located upstream of synchronization point 
Roughness located downstream of synchronization point 
 Upstream of synchronization point, roughness amplifies disturbance
 The overall amplification of disturbance depends roughness height
 However, when roughness is placed downstream of synchronization
point, it damps disturbance with strength related to roughness height
Freestream waves receptivity to nonlinear breakdown over Mach 5.5 circular cone
Objectives
 develop a direct numerical simulation code to simulate hypersonic boundary layer flow from laminar to nonlinear breakdown in transition over blunt circular cone
 build a linear receptivity database for different types of freestream waves with a selected frequency specturm
 construct the inflow condition for subsequent nonlinear region using receptivity database and conduct the breakdown simulation
Linear receptivity simulations were conducted to obtain freestream waves database. The database were used as entrance conditions for subsequent nonlinear breakdown simulations.
3D nonlinear breakdown simulation: pressure disturbances contour at wall surface
Numerical Study of WaveEnergy Extraction Device with Electroactive Polymer
Objectives
 Collaborate with the Soft Material Research Laboratory of Prof. Pei (UCLA) to develop technology using electroactive polymer to extract and convert wave energy to electricity
 Implement a finite volume method with freesurface tracking done by Volume of Fluid method to simulate the dynamic interaction between energycontaining incident wave and the energy extracting device
 In preliminary numerical study, the targeted strain of 20 to 30 percent was achieved, suggesting dielectric elastomer may be practical for largescale power generation
Change from high to low capacitance state raises the potential of the charge of the film
Schematic of numerical wave tank
Effects of Graphite Ablation Induced Outgassing on Hypersonic Boundary Layer Stability
Objectives
 Study effects of graphite ablation on the receptivity process and linear growth of Mack's first and second modes in a high speed boundary layer
 Surface chemistry model to approximate ablation includes oxidation, recombination of atomic oxygen and sublimation of C_3, C_2 and C
 Develop nonequilibrium LST code for ablation gas model and compare stability results to DNS
Direct Numerical Simulation Code Validation
11 species ablation code validation to Keenan's computations. Left: surface mass fractions. Right: surface mass flux.
Surface chemistry model comparison
Direct Numerical Simulation Results
Instantaneous snapshot of oxygen (O2) and translationrotation temperature perturbation with 550 khz freestream fast acoustic forcing waves
Wall pressure perturbation amplitude for real gas simulation with ablation (left) and ideal gas simulation (right). Real gas flow is unstable while ideal gas is stable. Significant difference is shown between the two gas models.
Efficient Control of Hypersonic Boundary Layer Transition Using Surface Porous Coating
Motivations
 Porous coating significantly stabilizes Mack’s second mode (acoustics) whereas it moderately destabilizes the first mode (TS wave)
 In previous studies, porous coating covers either the entire flat plate or the surface around half the cone circumference
 No work is reported on the effect of porous coating location on the firstmode destabilization, and how to increase stabilization efficiency of porous coating


Felt metal coating  Regular porous coating 
Conclusions  There are three approaches to increase the stabilization efficiency of porous coating
 Put porous coating downstream of the synchronization point
 Use regular porous coating if one must put porous coating along the whole surface
 Design new porous coating of smaller admittance phase angle to attenuate first mode destabilization
Direct Numerical Simulation of Strong Shock and Turbulence Interaction Using ShockFitting Methods
Objectives
 To conduct extensive DNS studies on strong shock and turbulence interactions for perfect gas flow with mean Mach numbers ranging from 2 to 30
 To validate our new 3D highorder shockfitting code for nonequilibrium flow
 To conduct DNS studies on strong shock and turbulence interactions for nonequilibrium flow
Numerical Analysis


Inflow: Temporal DNS  DNS with shock fitting 
Variation of various turbulence statistics in simulation of decaying isotropic turbulence.
Conclusions
 Increasing shockstrength reduces the shock deformation
 The trend of maximum values of streamwise vorticity fluctuations versus shock Mach number reverses at M1 = 2.8 from increase to decrease
 The trend of maximum values of Reynolds stress R11 versus shock Mach number reverses at M1 = 8.8 from decrease to increase
Development and validation of new high order numerical method for hypersonic nonequilibrium flow simulations
Objectives
 Develop and validate new highorder shockfitting numerical method for hypersonic nonequilibrium flow simulations
 Conduct numerical simulation studies on transient growth of reentry vehicles, including the effects of small/finite surface roughness, the interaction of surface roughness and freestram disturbances
 Update and validation of the code with more advanced 5species models and the more realistic 11species air models are ongoing
 The simulation of the steady flow over 9degree halfangle cone shows that real gas effects have significant impacts on flow fields
Two Temperature Model and Governing Equations
 Translation and rotation energy modes are in equilibrium at translation temperature (T)
 Vibration and electronic energy modes are in equilibrium at vibration temperature (Tv)


Two temperature comparison along the stagnation line  Species densities along the stagnation line 
Motivations
 Receptivity studies have focused more on freestream disturbances than on wall disturbances.
 Wall disturbances, together with freestream disturbances, are the main disturbances that hypersonic vehicles experience under the real flight conditions.
 Blowing–suction is not only the most sensitive wall disturbance for hypersonic boundary layers, but is also widely used to control the boundarylayer transition.
Mode S is strongly excited only when the blowingsuction actuator is upstream of the synchronization point. When the blowingsuction actuator is downstream of the synchronization point (Case 7), there is very little excitation of mode S, despite the fact the actuator is still located within the unstable region of mode S.
Conclusions
 The synchronization point of mode F and mode S played an important role in the excitation of mode S.
 To excite strong mode S at a specific frequency, it is necessary to place the blowing–suction actuator upstream of the corresponding synchronization point.
 New control strategy of hypersonic boundarylayer transition can be developed based on such results.