Krypton tagging velocimetry (KTV) is demonstrated in the Stevens Shock Tunnel to obtain streamwise and wall-normal velocity fluctuation profiles in high-speed flows over a hollow-cylinder-flare test article. Krypton was used as the tagged tracer in shock-tunnel-generated high-speed air flow, excited by a 212.56 nm wavelength beam at 100 kHz. Velocity fluctuations are obtained and match DNS values with good agreement. Tagging velocimetry techniques are compared to other experimental velocimetry techniques.

Experiments and numerical simulations of inertial particles in underexpanded jets are performed. The structure of the jet is controlled by varying the nozzle pressure ratio, while the influence of particles on emerging shocks and rarefaction patterns is controlled by varying the particle size and mass loading. Ultra-high-speed schlieren and Lagrangian particle tracking are used to experimentally determine the two-phase flow quantities. Three-dimensional simulations are performed using a high-order, low-dissipative discretization of the gas phase while particles are tracked individually in a Lagrangian manner. A simple two-way coupling strategy is proposed to handle interphase exchange in the vicinity of shocks. Velocity statistics of each phase are reported for a wide range of pressure ratios, particle sizes and volume fractions. An upstream shift of the Mach disk in the presence of particles reveals significant two-way coupling even at low mass loading. A semi-analytic model that predicts the extent of the Mach disk shift is presented based on a one-dimensional Fanno flow that takes into account volume displacement by particles and interphase exchange due to drag and heat transfer. The per cent shift in Mach disk is found to scale with the mass loading, nozzle pressure ratio and interphase slip velocity and inversely with the particle diameter.

This paper describes an updated nozzle centerbody, which acts as both a flow terminator and a debris blocker in the Stevens Shock Tunnel. A redesigned centerbody that slides on linear bearings was implemented to reduce tunnel turn-around time, increase plugging-time control, and improve shot-to-shot consistency in the facility. Pressure traces for shock tunnel experiments using the new centerbody design are presented to demonstrate comparable behavior to the previous version. Design features are discussed, and the new centerbody design will be further evaluated in future experimental work. A preliminary computational fluid dynamics analysis is also presented to explore and describe the plugging action of the debris blocker.

Acetone tagging velocimetry (ATV) with nominally 1 M_e=6.05 , T_w/T_r=0.65 , Re_θ =8925 , and Re_τ =408 . This single-laser tagging velocimetry experiment measured streamwise velocity and fluctuations, u and u'_RMS , had a repetition rate of 50 kHz, and did not require the use of an image intensifier. The ATV data are non-dimensionalized via the van Driest transformation and compares well with the literature down to y^+ ≈ 3 ; this is into the viscous sublayer. The Morkovin-scaled fluctuations matched the literature for y^+ > 10 ; however, below y^+ ≈ 10 , it appears that the noise introduced by the measurement technique, in addition to the velocity differences being small near the wall, rendered the ATV-measured Morkovin-scaled fluctuations below y^+ = 10 not useful.

No AccessTechnical NotesDebris Blocker and Flow Terminator for a Shock TunnelBen A. Segall, David Shekhtman, Ahsan Hameed, James H. Chen, Alex R. Dworzanczyk and Nicholaus J. ParzialeBen A. Segall https://orcid.org/0000-0002-2259-0625Stevens Institute of Technology, Hoboken, New Jersey 07030*Graduate Student, Mechanical Engineering, Castle Point on Hudson.Search for more papers by this author, David ShekhtmanStevens Institute of Technology, Hoboken, New Jersey 07030†Postdoctoral Researcher, Mechanical Engineering, Castle Point on Hudson.Search for more papers by this author, Ahsan Hameed https://orcid.org/0000-0001-9563-3959Stevens Institute of Technology, Hoboken, New Jersey 07030*Graduate Student, Mechanical Engineering, Castle Point on Hudson.Search for more papers by this author, James H. ChenStevens Institute of Technology, Hoboken, New Jersey 07030*Graduate Student, Mechanical Engineering, Castle Point on Hudson.Search for more papers by this author, Alex R. DworzanczykStevens Institute of Technology, Hoboken, New Jersey 07030*Graduate Student, Mechanical Engineering, Castle Point on Hudson.Search for more papers by this author and Nicholaus J. ParzialeStevens Institute of Technology, Hoboken, New Jersey 07030‡Associate Professor, Mechanical Engineering, Castle Point on Hudson; . Senior Member AIAA (Corresponding Author).Search for more papers by this authorPublished Online:31 Mar 2023https://doi.org/10.2514/1.J062348SectionsRead Now ToolsAdd to favoritesDownload citationTrack citations About References [1] Leyva I. A., “The Relentless Pursuit of Hypersonic Flight,” Physics Today, Vol. 70, No. 11, 2017, pp. 30–36. https://doi.org/10.1063/PT.3.3762 CrossrefGoogle Scholar[2] Hannemann K., “Short-Duration Testing of High Enthalpy, High Pressure, Hypersonic Flows,” Springer Handbook of Experimental Fluid Mechanics, edited by Tropea C., Yarin A. L. and Foss J. F., Springer, New York, 2007, pp. 1081–1125. Google Scholar[3] Danehy P. M., Weisberger J., Johansen C., Reese D., Fahringer T., Parziale N. J., Dedic C., Estevadeordal J. and Cruden B. A., “Non-Intrusive Measurement Techniques for Flow Characterization of Hypersonic Wind Tunnels,” Flow Characterization and Modeling of Hypersonic Wind Tunnels (NATO Science and Technology Organization Lecture Series STO-AVT 325), von Kármán Inst. NF1676L-31725, Brussels, Belgium, 2018, https://ntrs.nasa.gov/citations/20190029251. Google Scholar[4] Hertzberg A., Wittliff C. E. and Hall J. G., “Summary of Shock Tunnel Development and its Application to Hypersonic Research,” U.S. Air Force Office of Scientific Research TR 60-139, Arlington, VA, 1961. Google Scholar[5] Chue R. S. M. and Eitelberg G., “Studies of the Transient Flows in High Enthalpy Shock Tunnels,” Experiments in Fluids, Vol. 25, Nos. 5–6, 1998, pp. 474–486. https://doi.org/10.1007/s003480050253 CrossrefGoogle Scholar[6] Hanneman K. and Beck W. H., “Aerothermodynamics Research in the DLR High Enthalpy Shock Tunnel HEG,” Advanced Hypersonic Test Facilities, AIAA, Reston, VA, 2002, pp. 205–238. https://doi.org/10.2514/5.9781600866678.0205.0237 Google Scholar[7] Holden M. S., “Development and Code Evaluation Studies in Hypervelocity Flows in the LENS Facility,” Proceedings of the 2nd European Symposium on Aerodynamics for Space Vehicles, European Space Agency, ESTEC, Paris, Noordwijk, The Netherlands, 1994, pp. 319–334. Google Scholar[8] Holden M. S. and Parker R. A., “LENS Hypervelocity Tunnels and Application to Vehicle Testing at Duplicated Flight Conditions,” Advanced Hypersonic Test Facilities, AIAA, Reston, VA, 2002, pp. 73–110. https://doi.org/10.2514/5.9781600866678.0073.0110 Google Scholar[9] Lee J. K., Park C. and Kwon O. J., “Experimental Study of Shock Tunnel Flow with a Stationary Throat Plug,” Shock Waves, Vol. 22, July 2012, pp. 295–305. https://doi.org/10.1007/s00193-012-0370-2 CrossrefGoogle Scholar[10] Lee J. K., Park C. and Kwon O. J., “Experimental and Numerical Study of Stationary Throat Plug in Shock Tunnel,” Journal of Thermophysics and Heat Transfer, Vol. 29, No. 3, 2015, pp. 482–495. https://doi.org/10.2514/1.T4084 LinkGoogle Scholar[11] Lee J. K., Park C. and Kwon O. J., “Experimental Study of Moving Throat Plug in a Shock Tunnel,” Shock Waves, Vol. 25, July 2015, pp. 431–442. https://doi.org/10.1007/s00193-015-0573-4 CrossrefGoogle Scholar[12] Hornung H. G. and Parziale N. J., “Reflected Shock Tunnel Noise Control,” Proceedings of the 15th International Conference on the Methods of Aerophysical Research, Akademgorodok, Novosibirsk, Russia, 2010. Google Scholar[13] Hornung H. G., “Performance Data of the New Free-Piston Shock Tunnel at GALCIT,” Proceedings of 17th AIAA Aerospace Ground Testing Conference, AIAA Paper 1992-3943, 1992. https://doi.org/10.2514/6.1992-3943 Google Scholar[14] Shekhtman D., Hameed A., Segall B. A., Dworzanczyk A. R. and Parziale N. J., “Initial Shakedown Testing of the Stevens Shock Tunnel,” Proceedings of AIAA SciTech 2022, AIAA Paper 2022-1402, 2022. https://doi.org/10.2514/6.2022-1402 Google Scholar[15] Laufer J., “Some Statistical Properties of the Pressure Field Radiated by a Turbulent Boundary Layer,” Physics of Fluids (1958–1988), Vol. 7, No. 8, 1964, pp. 1191–1197. https://doi.org/10.1063/1.1711360 CrossrefGoogle Scholar[16] Schneider S. P., “Development of Hypersonic Quiet Tunnels,” Journal of Spacecraft and Rockets, Vol. 45, No. 4, 2008, pp. 641–664. https://doi.org/10.2514/1.34489 LinkGoogle Scholar[17] Parziale N. J., Shepherd J. E. and Hornung H. G., “Free-Stream Density Perturbations in a Reflected-Shock Tunnel,” Experiments in Fluids, Vol. 55, No. 2, 2014, Paper 1665. https://doi.org/10.1007/s00348-014-1665-0 CrossrefGoogle Scholar[18] Parziale N. J., Jewell J. S., Leyva I. A. and Shepherd J. E., “Effects of Shock-Tube Cleanliness on Slender-Body Hypersonic Instability and Transition Studies at High-Enthalpy,” Proceedings of AIAA SciTech 2015, AIAA Paper 2015-1786, 2015. https://doi.org/10.2514/6.2015-1786 Google Scholar[19] Jewell J. S., Parziale N. J., Leyva I. A. and Shepherd J. E., “Effects of Shock-Tube Cleanliness on Hypersonic Boundary Layer Transition at High Enthalpy,” AIAA Journal, Vol. 55, No. 1, 2017, pp. 332–338. https://doi.org/10.2514/1.J054897 LinkGoogle Scholar[20] Davies L., “The Interaction of the Reflected Shock with the Boundary Layer in a Shock Tube. Part 1,” Her Majesty’s Stationary Office, Ministry of Aviation: Aeronautic Research Council CP-880, London, July 1966. Google Scholar[21] Davies L. and Wilson J. L., “Influence of Reflected Shock and Boundary-Layer Interaction on Shock-Tube Flows,” Physics of Fluids, Vol. 12, No. 5, 1969, Paper I37. https://doi.org/10.1063/1.1692625 CrossrefGoogle Scholar[22] Chue R. S. M., Tsai C. Y. and Bakos R. J., “Driver Gas Contamination in a Detonation-Driven Reflected-Shock Tunnel,” Shock Waves, Vol. 13, March 2003, pp. 367–380. https://doi.org/10.1007/s00193-003-0221-2 CrossrefGoogle Scholar[23] Sudani N. and Hornung H. G., “Gasdynamical Detectors of Driver Gas Contamination in a High-Enthalpy Shock Tunnel,” AIAA Journal, Vol. 36, No. 3, 1998, pp. 313–319. https://doi.org/10.2514/2.383 LinkGoogle Scholar[24] Sudani N., Valiferdowsi B. and Hornung H. G., “Test Time Increase by Delaying Driver Gas Contamination for Reflected Shock Tunnels,” AIAA Journal, Vol. 38, No. 9, 2000, pp. 1497–1503. https://doi.org/10.2514/2.1138 LinkGoogle Scholar[25] Hannemann K., Schneider M., Reimann B. and Schramm J. M., “The Influence and Delay of Driver Gas Contamination in HEG,” 21st AIAA Aerodynamic Measurement Technology and Ground Testing Conference, AIAA Paper 2000-2593, 2000, pp. 1–14. https://doi.org/10.2514/6.2000-2593 Google Scholar[26] Olivier H., “The Aachen Shock Tunnel TH2 with Dual Driver Mode Operation,” Experimental Methods of Shock Wave Research, Springer, New York, 2016, pp. 111–129. https://doi.org/10.1007/978-3-319-23745-9_5 Google Scholar[27] Goozée R. J., Seal P. A. and Saxena D. R., “Simulation of a Complete Reflected Shock Tunnel Showing a Vortex Mechanism for Flow Contamination,” Shock Waves, Vol. 15, Nos. 3–4, 2006, pp. 165–176. https://doi.org/10.1007/s00193-006-0015-4 CrossrefGoogle Scholar Previous article Next article FiguresReferencesRelatedDetails What's Popular Articles in Advance CrossmarkInformationCopyright © 2023 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. All requests for copying and permission to reprint should be submitted to CCC at www.copyright.com; employ the eISSN 1533-385X to initiate your request. See also AIAA Rights and Permissions www.aiaa.org/randp. TopicsAerodynamicsAeronauticsAerospace SciencesFlow Control ValvesFlow Diagnostics and ControlFlow RegimesFluid DynamicsFluid Mechanics KeywordsFreestream Mach NumberFluid MechanicsFlow ConditionsPressure Relief ValveCenterbodyNozzle Flow TerminatorHypersonic Ground TestingShock TunnelsMach 6Flow QualityAcknowledgmentsBen A. Segall, David Shekhtman, Ahsan Hameed, James H. Chen, Alex R. Dworzanczyk, and Nicholaus J. Parziale were supported by U.S. Office of Naval Research and U.S. Air Force Office of Scientific Research (AFOSR) grants, including N00014-19-1-2523, N00014-20-1-2637, N00014-20-1-2682, N00014-20-1-2549, FA9550-18-1-0403, and FA9550-19-1-0182. We thank Eric Marineau and the U.S. Office of Naval Research for sponsoring the construction and development of the Stevens Shock Tunnel. We also thank AFOSR Program Manager Sarah Popkin for her support.PDF Received17 August 2022Accepted1 March 2023Published online31 March 2023

Tagging velocimetry is demonstrated in the Stevens Shock Tunnel to obtain velocity profiles in high-speed flows over a hollow-cylinder-flare test article. Acetone was used as the tagged tracer in shock-tunnel-generated high-speed air flow, excited by the fourth harmonic of the burst-mode Nd:Yag laser (266~nm) at 50~kHz. Freestream velocity is recorded and matches predicted values with good agreement. Additionally, turbulent boundary layer velocity profiles are reported.

Krypton Tagging Velocimetry (KTV) and Picosecond Laser Electronic Excitation Tagging (PLEET) velocimetry at a 100-kHz rate were demonstrated in Mach 18 flow conditions at the Arnold Engineering Development Center (AEDC) Tunnel 9 employing a burst-mode laser system and a custom optical parametric oscillator (OPO). The measured freestream flow velocities from both KTV and PLEET agreed well with the theoretical calculation. The increase in repetition rate provides better capability to perform time-resolved velocimetry measurements in hypersonic flow environments.

The Stevens Shock Tunnel is designed to replicate hypersonic, Mach 6 flow conditions. The tunnel produces a core flow of 0.3 m in the test section and a test time of at least 4 ms. This paper describes the tunnel’s operation and flow characterization for a range of run conditions: Mach 5.8-6.0, enthalpy 1.5 MJ/Kg, and unit Reynolds number 0.35-8.1 × 106 1/m (with a potentially lower limit available). This paper also describes the use of a nozzle centerbody flow terminator and debris blocker and tunnel cleaning to remove residual debris from a previous experiment. Reservoir, static, and Pitot pressure measurements are presented for a range of run conditions and compared to nozzle flow calculations.

In this paper, aQ-FLDI systemwas constructed to correlate disturbances inside and outside of the boundary layer over a blunted cone in high-enthalpy, hypersonic flow at T5, the freepiston-driven reflected-shock tunnel at California Institute of Technology. We present results for three shots representing a fully-turbulent case with a 5 mm nose-tip radius, an unstable case with a 5 mm nose-tip radius at moderate enthalpy (6 MJ/kg), and an unstable case with a 2 mm nose-tip radius at higher enthalpy (10.5 MJ/kg). In the fully-turbulent case, the spectra indicate the possibility of a region where there is a power-law variation in the spectrum. For the unstable case with a 2 mm nose-tip radius at higher enthalpy (10.5 MJ/kg), the FLDI detected the second mode instability at approximately 1 MHz. This agrees well with results obtained using the schlieren technique for experiments performed in this campaign at similar conditions. The mismatch when compared to stability calculations for this case could be attributed to run-condition calculation error. An unstable case with a 5 mm nose-tip radius at moderate enthalpy (6 MJ/kg) is also presented. In this case, there is more content measured outside of the boundary layer than inside. For this case, computations of the mean flow show that the entropy-layer had not been swallowed at the point of measurement suggesting the elevated energy content outside the boundary layer could be due to entropy layer instabilities or oblique waves not considered in the stability analysis.

In this work, we present the application of wavelet-based optical flow velocimetry (wOFV) to tagging velocimetry image data. wOFV is demonstrated to compare favorably to cross-correlation on experimental two-dimensional Krypton tagging velocimetry (KTV-2D) images from a Mach 2.75 turbulent shock wave-boundary layer interaction. Results from both methods show good agreement for the mean velocity field, while wOFV has several advantages compared to cross-correlation including increased spatial resolution as well as robustness and simplicity of implementation. The performance of wOFV on tagging velocimetry images is evaluated quantitatively using a set of simulated data from a turbulent boundary layer including images and specified velocity fields. wOFV is found to produce accurate results for turbulence statistics using write images with parallel 1D lines and is relatively insensitive to moderate amounts of noise. Additionally, it can accurately calculate two-dimensional velocity fields over the entire image domain for images containing sets of intersecting write lines, as well as derivative quantities such as vorticity, as long as the line spacing is sufficiently small.

A campaign of hypervelocity experiments was conducted with a blunted 5d-half-angle cone in the T5 shock tunnel. A modified Z-type schlieren setup was utilized to image the boundary-layer with total enthalpies around 10 MJ/kg and freestream-to-wall temperature ratios of approximately 5. Second-mode instabilities and transitional behavior in the boundary layer were identified imaged within the field of view between x/L = 0.57 and 0.73. Second-mode wavepackets manifested as alternating dark/light waves beginning right at the wall and permeating up to 30% of the boundary-layer height, d. Spectral analysis of the two lower-Reynolds-number shots demonstrated that these packets could be characterized by peak wavenumbers 315 - 365 /m at 10% d. In the highest-Reynolds-number shot, structures passing out of the boundary layer indicated the beginning of turbulent breakdown. For discrete examples where the onset of these structures was imaged, breakdown appeared to begin at x/L = 0.66. A Gabor-filter-based image analysis indicated that spectral content was transferred out of the boundary layer at disturbance Mach numbers 2.75-3.75. A larger range of wavenumbers, 305 - 415 /m, populated the near-wall content in this transitional shot, and high-wavenumber content up to 450 /m was identified outside the boundary layer. It is to be noted that this paper serves as a companion study to that of Ref. 1, which interrogated the same boundary layer with focused-laser differential interferometry (FLDI).

Results are presented for a Krypton Tagging Velocimetry (KTV) investigation of the freestream flow of the T5 reflected shock tunnel at Caltech. This KTV scheme utilizes two-photon excitation at 216.67 nm with a pulsed dye laser, followed by re-excitation at 769.4547 nm with a continuous laser diode. Results are presented for experiments performed in 97% N_2 / 3% Kr and 99% N_2 / 1% Kr gas mixtures at a unit Reynolds number of approximately 4×10^6 1/m and a reservoir mass-specific enthalpy of approximately 5 MJ/kg. Agreement between the KTV derived velocity measurement and the computational results is excellent, within the uncertainty of the experiment.

We apply Krypton Tagging Velocimetry (KTV) to measure velocity profiles in the freestream of a large, national-scale high-enthalpy facility, the T5 Reflected-Shock Tunnel at Caltech. The KTV scheme utilizes two-photon excitation at 216.67 nm with a pulsed dye laser, followed by re-excitation at 769.45 nm with a continuous laser diode. Results from a nine-shot experimental campaign are presented where N $$_2$$ and air gas mixtures are doped with krypton, denoted as 99% N $$_2$$ /1% Kr, and 75% N $$_2$$ /20% O $$_2$$ /5% Kr, respectively. Flow conditions were varied through much of the T5 parameter space (reservoir enthalpy $$h_R\approx 5-16$$  MJ/kg). We compare our experimental freestream velocity-profile measurements to reacting, Navier–Stokes nozzle calculations with success, to within the uncertainty of the experiment. Then, we discuss some of the limitations of the present measurement technique, including quenching effects and flow luminosity; and, we present an uncertainty estimate in the freestream velocity computations that arise from the experimentally derived inputs to the code.

This paper presents the application of wavelet-based optical flow velocimetry (wOFV) to tagging velocimetry image data. wOFV is demonstrated to compare favorably to cross-correlation on experimental two-dimensional Krypton Tagging Velocimetry (KTV-2D) images from a Mach 2.75 turbulent shock wave-boundary layer interaction. Results from both methods show good agreement for the mean velocity field, while wOFV has several advantages compared to cross-correlation including increased spatial resolution as well as robustness and simplicity of implementation. The performance of wOFV on tagging velocimetry images is evaluated quantitatively using a set of synthetic data including images and specified velocity fields. wOFV is highly accurate in regions of the images near the intersection of write lines, with median error vector magnitudes below 5%. Errors in the instantaneous measurements of the velocity fluctuations and vorticity are higher, with median values between 20 and 50%.

Krypton tagging velocimetry (KTV) requires high signal-to-noise ratio (SNR) to observe high-speed boundary layers and flow structures. In order to optimize the choice of laser excitation line for use in KTV (212.556 nm, 214.769 nm, 216.667 nm), a theoretical and experimental investigation of excitation processes was undertaken. This paper presents a multi-path, two-photon excitation, cross-section calculation, using an assumed finite basis of states consisting of 4p, 5s, 6s, 7s, 5p, 6p, 4d, 5d, and 6d orbitals. From the relative magnitudes of two-photon cross-sections for five Krypton lines, an excitation spectrum is constructed and compared against excitation spectrum data, with encouraging results. From this work and the successful comparison to experiment from our lab and those in the literature, we conclude that the optimal line is 212.556 nm for Kr-PLIF and single-laser KTV. For KTV where the read step in performed with a continuous wave (CW) laser diode, the 216.667 nm write-laser excitation is optimal.

In this work, we present an optimization process for the design of converging-diverging nozzle contours. The process is a brute-force algorithm that runs CFD simulations for various contours until the one with the most uniform flow properties is obtained. The contour is described using a Bezier curve, with control points serving as the independent variables in the optimization process. The function that is minimized is a penalty function that characterizes the non-uniformity of the flow properties. The optimization is carried out in three steps: contour definition, inviscid optimization, and viscous optimization. The inviscid optimization is performed with a coarse grid and a CFD model with no viscosity or turbulence. This serves to rapidly design a contour that is close to the desired performance. Using the result of the inviscid optimization as a starting point, further refinement is carried out in the viscous optimization, with a finer grid and a CFD model that accounts for all relevant physical phenomena. Different CFD solvers are used at each step, and results are presented for the design of a Mach 6 nozzle for the Stevens Shock Tunnel.

A high-speed, flat-plate flow is used as a benchmark problem for input-output (IO) two-dimensional stability analysis. This paper expands on low-level details necessary to analyze the stability of a general 2D-flow via IO analysis. We construct the global, analytical Jacobian from a discretized version of the linearized Navier-Stokes equations. The maximization of an energy-normed input-to-output gain subject to linear state dynamics degenerates into a singular-value-decomposition problem, which yields optimal input and output directions for disturbances and perturbed variables, respectively. The input-output analysis of a high-speed flow over a flat-plate is performed in MATLAB on the optimal output directions at two nondimensional frequencies, F = 1.6e-4 and F = 2.2e-4. Locations of instabilities, mode shapes, and growth rates of perturbed temperature were obtained along the length of the plate and compared to those in the literature.

This paper presents multi-path, two-photon excitation cross-section calculations for krypton, using first-order perturbation theory. For evaluation of the two-photon-transition matrix element, this paper formulates the two-photon cross-section calculation as a matrix mechanics problem. From a finite basis of states, consisting of 4 p , 5 s , 6 s , 7 s , 5 p , 6 p , 4 d , 5 d , and 6 d orbitals, electric dipole matrix elements are constructed, and a Green’s function is expressed as a truncated, spectral expansion of solutions, satisfying the Schrödinger equation. Electric dipole matrix elements are evaluated via tabulated oscillator strengths, and where those are unavailable, quantum-defect theory is used. The relative magnitudes of two-photon cross-sections for eight krypton lines in the 190–220 nm range are compared to experimental excitation spectra with good agreement. This work provides fundamental physical understanding of the Kr atom, which adds to experimental observations of relative fluorescence intensity. This is valuable when comparing excitation schemes in different environments for krypton fluorescence experiments. We conclude that two-photon excitation at 212.556 nm is optimal for single-laser, krypton tagging velocimetry or krypton planar laser-induced fluorescence.

The titular technique, KTV, is an important development in the field of laser diagnostics for supersonic and hypersonic flows, as it gives access to unexplored regimes. KTV is not plagued by the fundamental limitations of traditional tracer-particle techniques. The authors investigate the boundary-layer profiles that form over a sharp, hollow cylinder in supersonic flows of air and N${}_{2}$ via a single-laser scheme. With the use of high-repetition-rate lasers, this simple, cost-effective evaluation tool for large facilities will allow for time-resolved measurements of turbulent flows, for $e.g.$ the development of high-speed vehicles such as bullet trains.

As demonstrated by the 2014 MV Sewol incident, the prevention of top heavy ship capsize is necessary to protect life and property aboard a ship. The goal of this paper is to prevent the capsize of ships, which lack a restoring torque about the roll axis, by using a feedback-controlled pendulum actuator. A seven degrees-of-freedom (7DOF) model is developed for a ship equipped with a pendulum actuator. The model is used to conduct parameter analyses on the pendulum length, pendulum mast height, pendulum mass, ship center of mass (COM) height, and the pendulum controller's proportional feedback gain. The results of these analyses are depicted via time responses and phase plots. Key points for designing a pendulum actuator summarize simulation results, stating that the pendulum mass should be 3–7% of the total ship mass, and the pendulum moment of inertia should be 0.5–1.0 times the roll moment of inertia of the ship.