About

Hermann F. Fasel is a Professor of Aerospace and Mechanical Engineering and a member of the Graduate Faculty at the University of Arizona. His research focuses on high-speed boundary-layer instabilities, laminar-turbulent transition, and flow control in hypersonic flows. Fasel has contributed to the understanding of nonlinear transition stages in high enthalpy and hypersonic boundary layers, utilizing advanced computational tools such as direct numerical simulations and high-order-accurate flow solvers. His work includes investigations into boundary-layer stability, shock-wave interactions, and the development of transition prediction models for hypersonic flow regimes. Throughout his career, Fasel has been involved in numerous experimental and numerical studies related to flow phenomena relevant to high-speed aerodynamics. He has received recognition for his contributions to fluid dynamics, including being named an AIAA Fellow and receiving the AIAA Fluid Dynamics Award. His research aims to improve the understanding of flow stability and transition mechanisms in high-speed flows, which are critical for the design of hypersonic vehicles and aerospace applications.

Research topics

• Mechanics
• Physics
• Computer Science
• Mathematics
• Optics
• Geometry
• Materials science
• Aerospace engineering
• Mathematical analysis
• Computational science
• Electrical engineering
• Engineering
• Classical mechanics
• Quantum mechanics
• Composite material

Selected publications

• Numerical and Experimental Investigations of Transition in a Laminar Separation Bubble

  IUTAM bookseries · 2026-01-01

  book-chapterSenior author
  PublisherDOI

• Numerical Investigations of the Linear Stability Regime for a Separation Bubble on an Axisymmetric Compression Ramp at Mach 5

  2025-01-03

  articleSenior author

  Numerical investigations were carried in order to investigate the transitional shock boundary-layer interaction (SBLI) on a hollow cylinder flare geometry at Mach 5. The geometry and flow conditions were matched, as closely as possible, to those in the experiments at the Mach 5 Ludwieg Tube (LT5) at the University of Arizona (UA). In the literature, different mechanisms have been proposed to play a role in the transition process of the separated region of the SBLI, such as the shear layer as the the Görtler instability. Our previous results for low amplitude three-dimensional wave packets and various flare angles, showed evidence that both these mechanisms are present in the simulations. Estimations of the Görtler number along a streamline, showed that for all the flare angles investigated the flow is highly unstable with respect to the Görtler instability. Additionally, estimates available in the literature regarding the azimuthal wavenumber corresponding to the Görtler vortices match reasonably well with results from the wave packets. For the shear layer instability, a good match is obtained between local LST and low amplitude controlled forcing simulations.

  PublisherDOI

• Controlled Transition Simulations on an Axisymmetric 8-Degree Compression Ramp at Mach 5

  2025-07-16

  articleSenior author

  Numerical investigations are conducted to investigate the transitional shock-boundary-layer interaction (SBLI) on a hollow cylinder flare geometry at Mach 5. The flow conditions are matched as closely as possible to those used in experiments at the Mach 5 Ludwieg Tube (LT5) at the University of Arizona (UA). A variation of the geometry used in the experiments was used for the numerical simulations with a flare angle of 8 degree. This case was previously identified as only convectively unstable, with no signs of an absolute/global instability. The linear regime is investigated using local linear stability theory (LST) and low amplitude controlled forcing simulations. An oblique 1st mode/shear-layer mode is found to be amplified for a range of azimuthal wavenumbers. Subsequently, highly resolved direct numerical simulations (DNS) are performed in order to investigate the possibility of an oblique breakdown scenario, dominated by the 1st mode/shear-layer mode. Contours of the azimuthal averaged skin friction over time, show that the separation region for the transitional flow contracts compared to the laminar base flow. Pressure amplitude spectra show the amplification of the nonlinearly generated signature modes, consistent with an oblique breakdown scenario. Time averaged skin-friction coefficient and Stanton number, and their Fourier transformed amplitudes, indicate the formation of streaks that appear already inside the separation bubble. Towards the end of the computational domain, smaller structures can be observed in the instantaneous pseudo-Schlieren contours visualizations indicating that the late nonlinear stages of the transition process have been reached.

  PublisherDOI

• Numerical Investigation of Laminar-Turbulent Boundary-Layer Transition for an Ogive Geometry at Mach 7: Wind Tunnel and Flight Conditions

  2025-07-16

  articleSenior author

  Numerical investigations were carried out for an ogive geometry with a blunted nose tip at zero angle of attack for atmospheric flight conditions at �� = 7.1. These investigations are compared to those for the flow conditions of the hypersonic wind tunnel (H2K) experiments conducted by the German Aerospace Center (DLR), also at �� = 7.1. For this comparison, the Mach and unit Reynolds numbers for both flight and wind tunnel conditions are the same allowing for a direct comparison of how a change from wind tunnel to flight conditions affects the linear and nonlinear transition regimes. Linear Stability Theory (LST) calculations revealed that while both first and second mode waves experience significant amplification under wind tunnel conditions, only second mode waves are present under flight conditions. Similar to the investigations for the wind tunnel conditions, primary wave saturation and transition onset calculations were carried out for the flight conditions to determine whether large amplitude second mode waves can reach amplitudes sufficient for transition onset. Secondary instability calculations showed that the so-called fundamental resonance leads to strong secondary amplification, making this a relevant mechanism that could lead to transition for the ogive geometry under atmospheric flight conditions. Based on the primary and secondary instability investigations, high-resolution "controlled" transition simulations have been set up and are currently underway.

  PublisherDOI

• Experimental and numerical investigations of transition in a pressure-gradient-induced laminar separation bubble

  Journal of Fluid Mechanics · 2025-03-17 · 4 citations

  articleOpen accessSenior author

  A pressure-gradient-induced laminar separation bubble (LSB) was examined using wind-tunnel experiments, direct numerical simulations (DNS) and linear local/global stability analysis. The LSB was experimentally generated on a flat plate using the favourable-to-adverse pressure gradient imposed by an inverted modified NACA $64_3-618$ airfoil. Direct numerical simulation was performed using boundary conditions extracted from a steady precursor simulation of the entire flow field. Despite good agreement in the upstream boundary layer with the experiment, DNS exhibited an approximately 25 % longer mean separation bubble, attributed to an earlier onset of transition due to the free-stream turbulence (FST) in the experiment. Introducing a very low level of isotropic FST in the DNS, similar to that measured in the experiment, caused earlier transition, decreased the mean bubble length and led to a remarkably good agreement between the DNS and experiments. Differences were observed for the dominant frequencies in the experiment and DNS, but both were within the band of most amplified frequencies predicted by LST. Proper orthogonal decomposition confirmed that dominant coherent structures from DNS and experiments are related to the inviscid Kelvin–Helmholtz instability and have similar characteristics despite slight differences in frequency. Local and global stability and dynamic mode decomposition analysis corroborated the convective nature of the dominant two-dimensional (2-D) instability and showed that the LSB is globally unstable to a range of 3-D wavenumbers, in agreement with 3-D structures observed in experiments. Results confirmed the strong impact of very low FST levels on the LSB and indicate a close agreement of the time-averaged and instability characteristics between the experiments and DNS.

  PublisherOA PDFDOI

• Delay of the Hot Streak Development for a Flared Cone at Mach 6

  Journal of Spacecraft and Rockets · 2025-06-10

  articleSenior author

  Direct numerical simulations were carried out in order to explore flow control using steady blowing and suction (control) strips at the wall of a flared cone at Mach 6. The flared cone geometry and the flow conditions of the experiments in the Boeing/AFOSR Mach 6 Quiet Tunnel at Purdue University were used for the numerical investigations. The objective of the flow control strategy was to delay or mitigate the negative consequences associated with the nonlinear transition stages, such as the “overshoots” of skin friction and heat transfer and the development of “hot” streaks, which have been previously observed in experiments and simulations. A parameter study on the influence of the steady blowing and suction strips on the fundamental resonance revealed the most effective location and strength of the control strips to attenuate the growth rate of the secondary disturbance waves. Applying one control strip in a fundamental breakdown simulation resulted in significant delay of the “hot” streak development on the surface of the cone. With an additional blowing and suction strip, the streak onset was delayed so that they were no longer observable in the entire computational domain. The research demonstrates that a detailed understanding of the nonlinear stages of transition can inform the development of effective flow control methods. The presented flow control method is specific to a second-mode-dominated transition scenario (fundamental breakdown).

  PublisherDOI

• Numerical Investigation of Boundary-Layer Transition on a Sharp Cone at Mach 10

  2025-07-16

  articleSenior author

  Numerical investigations were carried out to explore the linear and nonlinear stability regimes for boundary layers on a straight (right) cone with a 7 degree opening half-angle and a ``sharp'' nose tip at Mach 10 and zero angle of attack. The cone geometry of the experiments in the Arnold Engineering Development Complex (AEDC) Hypervelocity Wind Tunnel No. 9 (Tunnel 9) is used for the numerical investigations. Primary instability calculations using Linear Stability Theory (LST) exhibit unstable first, second, and third mode waves. For the investigated flow conditions and geometry the axisymmetric second mode disturbances are the dominant primary (linear) instability. Primary wave saturation calculations including transition onset were carried out in order to investigate the possibility to align transition onset in the simulations with that observed in the experiments. By varying the forcing amplitude of the axisymmetric second mode waves a range of frequencies was identified for which transition onset obtained in simulations and observed in experiments can be aligned. Secondary instability investigations, focusing on the so-called fundamental resonance, were carried out for the primary wave frequencies identified in the transition onset simulations. The results showed that the fundamental resonance is indeed a viable nonlinear mechanism that may be responsible for transition in the AEDC T9 experiments. Based on the primary and secondary instability calculations high-resolution ``controlled'' transition simulations were carried out and demonstrate that fundamental breakdown can lead to sustained turbulent flow. The Stanton number development observed in experiments shows slight differences compared to that obtained in the controlled transition simulations.

  PublisherDOI

• Contributors

  Elsevier eBooks · 2025-08-22

  book-chapter
  PublisherDOI

• Hypersonic Transition Model for Second-Mode Instability

  Journal of Spacecraft and Rockets · 2025-07-17

  articleSenior author

  A transition model for second-mode instability is proposed and evaluated. A comprehensive database consisting of 768 boundary-layer solutions for various freestream conditions and geometries is constructed and analyzed using a correlation study. Based on the analysis, the inputs, output, and architecture of the machine-learning model are selected. Additionally, a novel correlation to estimate the onset of second-mode instability is proposed. The model performance is evaluated for different grid resolutions, unit Reynolds numbers, nose-tip bluntnesses, and a case with pressure gradient. Good agreement is obtained with experimental measurements and direct numerical simulation data. Key findings from the correlation analysis include a strong correlation between the local [Formula: see text] ratio (kinematic viscosity over speed of sound) and second-mode growth, indicating larger local growth at lower viscosity and higher speed of sound. The study also indicates that Menter’s local pressure gradient parameter might be sufficient for pressure gradient estimation in two-dimensional and axisymmetric flows. A deep neural network model with local Galilean-invariant inputs is shown to accurately estimate local second-mode growth. Extensive testing confirms the grid independence of the model and its ability to predict transition for different unit Reynolds numbers and nose radii. Excellent agreement with experimental measurements and direct numerical simulation data is also demonstrated for a flared cone geometry at Mach 6.

  PublisherDOI

• Numerical Investigation of the Linear and Nonlinear Transition Stages for a Sharp Cone at Mach 14

  2025-07-16

  articleSenior author

  Numerical investigations were carried out to explore the linear and nonlinear stability regimes for boundary layers on a straight (right) cone with a 7◦ opening half-angle and a “sharp” nose tip at Mach 14 and zero angle of attack. The cone geometry of the experiments in the Arnold Engineering Development Complex (AEDC) Hypervelocity Wind Tunnel No. 9 (Tunnel 9) was used for the numerical investigations. The flow conditions corresponding to the “sharp” cone experiments carried out at the T9 facility were considered. The primary instability regime was explored using Linear Stability Theory (LST) and revealed unstable first, second and third mode waves. The axisymmetric second mode waves are the dominant primary instability. Primary wave saturation calculations were carried out for a range of frequencies that resulted in substantial N-factors near the transition onset location observed in the experiments. These investigations were used to adjust the forcing amplitudes of the primary waves such that transition onset in the simulations aligns approximately with that observed in the experiments. Based on the primary instability and primary wave saturation calculations, secondary instability calculations investigating the so-called fundamental resonance were carried out. These investigations confirmed that a strong fundamental resonance is present for the investigated flow conditions and geometry, therefore making this a viable mechanism that might lead to transition in the experiments. Based on the primary and secondary instability investigations, high-resolution “controlled” transition simulations have been set up and are currently underway.

  PublisherDOI

Recent grants

• UNS: Collaborative Research: Numerical/Experimental Investigation of Solar Chimney Power Plants

  NSF · $205k · 2015–2020

Frequent coauthors

• Andreas Groß

  New Mexico State University

  105 shared

• Christoph Hader

  University of Arizona

  78 shared

• Shirzad Hosseinverdi

  University of Arizona

  34 shared

• Christoph Brehm

  University of Maryland, Baltimore

  31 shared

• Stefan Wernz

  31 shared

• Jayahar Sivasubramanian

  29 shared

• Jesse C. Little

  The Ohio State University

  25 shared

• Richard D. Sandberg

  University of Melbourne

  20 shared

Awards & honors

• AIAA Fellow (Spring 2021)
• AIAA Fluid Dynamics Award (Summer I 2019)
• Ludwig Prandtl Ring (Fall 2018)