Search Our Scientific Publications & Authors

AIAA J 2019 Aug 17;57(8):3322-3338. Epub 2019 Jul 17.

The University of Arizona, Tucson, Arizona 85718.

Active flow control (AFC) subscale experiments were conducted at the Lucas Wind Tunnel of the California Institute of Technology. Tests were performed on a generic vertical tail model at low speeds. Fluidic oscillators were used at the trailing edge of the main element (vertical stabilizer) to redirect the flow over the rudder and delay or prevent flow separation. Read More

AIAA J 2019 Jan;57(1):267-278

NASA Langley Research Center, Hampton, Virginia 23681.

Experimental measurements were performed on a swept flat-plate model with an airfoil leading edge and imposed chordwise pressure gradient to determine the effects of a backward-facing step on transition in a low-speed stationary crossflow-dominated boundary layer. Detailed hot-wire measurements were performed for three step heights ranging from 36 to 49% of the boundary-layer thickness at the step and corresponding to subcritical, nearly critical, and critical cases. In general, the step had a small localized effect on the growth of the stationary crossflow vortex, whereas the unsteady disturbance amplitudes increased with increasing step height. Read More

AIAA J 2019 Apr;57(4):1725-1735

NASA Langley Research Center, Hampton, VA, 23681-0001, United States.

Reduced-order modeling using the Volterra series approach has been successfully applied in the past decades to weakly nonlinear aerodynamic and aeroelastic systems. However, aspects regarding the identification of the kernels associated with the convolution integrals of Volterra series can profoundly affect the quality of the resulting reduced-order model (ROM). An alternative method for their identification based on artificial neural networks is evaluated in this work. Read More

AIAA J 2019 May 11;57(5):1793-1800. Epub 2019 Jan 11.

Princeton University, Princeton, NJ, 08540.

Femtosecond laser tagging is demonstrated for the first time in R134a (1,1,1,2-Tetrafluoroethane) gas, and in mixtures of R134a with small quantities of air. A systematic study of this tagging method is explored through the adjustment of gas pressure, mixture ratio and laser properties. It is found that the signal strength and lifetime are greatest at low pressures for excitation at both the 400 nm and 800 nm laser wavelengths. Read More

AIAA J 2018 6;56(11):4614-4620. Epub 2018 Sep 6.

NASA John H. Glenn Research Center, Cleveland, Ohio 44135.

AIAA J 2019 ;971

Applied and Computational Mathematics Division, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg MD, 20899, USA.

In computational materials science, coarse-graining approaches often lack a priori uncertainty quantification (UQ) tools that estimate the accuracy of a reduced-order model before it is calibrated or deployed. This is especially the case in coarse-grained (CG) molecular dynamics (MD), where "bottom-up" methods need to run expensive atomistic simulations as part of the calibration process. As a result, scientists have been slow to adopt CG techniques in many settings because they do not know in advance whether the cost of developing the CG model is justified. Read More

AIAA J 2018 Nov 30;56(11):4297-4311. Epub 2018 Sep 30.

NASA Langley Research Center, Hampton, VA 23681.

In this paper, we present a direct numerical simulation database of high-speed zero-pressure-gradient turbulent boundary layers developing spatially over a flat plate with nominal freestream Mach number ranging from 2.5 to 14 and wall-to-recovery temperature ranging from 0.18 to 1. Read More

AIAA J 2018 Jul 4;56(7):2604-2614. Epub 2018 Jun 4.

NASA Langley Research Center, Hampton, VA 23681.

The "Reshotko-Tumin transition criterion" based on optimal transient growth successfully correlates laboratory measurements of roughness induced transition over blunt body configurations. Even though transient growth has not been conclusively linked to the measured onset of transition, the above correlation denotes the only available physics-based model for subcritical transition in blunt body flows, since the latter do not support any modal instabilities at typical experimental conditions. Unlike other established models based on empirical curve fits that are valid for a specific subclass of datasets, the optimal-growth-based transition criterion appears to provide a reasonable correlation with measurements in various wind tunnel and ballistic range facilities and for a broad range of surface temperature ratios. Read More

AIAA J 2018 May 22;56(5):1867-1877. Epub 2018 Feb 22.

Purdue University, West Lafayette, Indiana 47907.

To study the azimuthal development of boundary-layer instabilities, a controlled, laser-generated perturbation was created in the freestream of the Boeing/U.S. Air Force Office of Scientific Research Mach 6 Quiet Tunnel. Read More

AIAA J 2018 Feb 30;56(2):510-523. Epub 2017 Nov 30.

NASA Langley Research Center, Hampton, Virginia 23681.

Supersonic boundary-layer receptivity to freestream acoustic disturbances is investigated by solving the Navier-Stokes equations for Mach 3.5 flow over a 7 deg half-angle cone. The freestream disturbances are generated from a wavy wall placed at the nozzle wall. Read More

AIAA J 2018 Feb 19;56(2):497-509. Epub 2017 Sep 19.

NASA Langley Research Center, Hampton, Virginia 23681.

A swept flat plate model with an imposed pressure gradient was experimentally investigated in a low-speed flow to determine the effect of a backward-facing step on transition in a stationary crossflow-dominated flow. Detailed hotwire measurements of boundary-layer flow were performed to investigate the upstream shift in transition due to a step height of 49% of the local unperturbed boundary-layer thickness. Increasing the initial stationary crossflow amplitude caused an upstream movement of the transition front for the backward-facing step case. Read More

AIAA J 2018 Jan 21;56(1):193-208. Epub 2017 Sep 21.

Flow Physics and Control Branch, NASA Langley Research Center, Hampton, VA 23681.

Boundary-layer transition in hypersonic flows over a straight cone can be predicted using measured freestream spectra, receptivity, and threshold values for the wall pressure fluctuations at the transition onset points. Simulations are performed for hypersonic boundary-layer flows over a 7-degree half-angle straight cone with varying bluntness at a freestream Mach number of 10. The steady and the unsteady flow fields are obtained by solving the two-dimensional Navier-Stokes equations in axisymmetric coordinates using a 5-order accurate weighted essentially nonoscillatory (WENO) scheme for space discretization and using a third-order total-variation-diminishing (TVD) Runge-Kutta scheme for time integration. Read More

AIAA J 2018 Jan;56(1):100-110

NASA Langley Research Center, Hampton, VA 23681.

A parametric experimental study was performed with sweeping jet actuators (fluidic oscillators) to determine their effectiveness in controlling flow separation on an adverse pressure gradient ramp. Actuator parameters that were investigated include blowing coefficients, operation mode, pitch and spreading angles, streamwise location, and size. Surface pressure measurements and surface oilflow visualization were used to characterize the effects of these parameters on the actuator performance. Read More

AIAA J 2017 Dec 6;55(12):4347-4356. Epub 2017 Sep 6.

NASA Langley Research Center, Hampton, VA, 23681.

This paper demonstrates a technique for locating the optimal control surface layout of an aeroservoelastic Common Research Model wingbox, in the context of maneuver load alleviation and active flutter suppression. The combinatorial actuator layout design is solved using ideas borrowed from topology optimization, where the effectiveness of a given control surface is tied to a layout design variable, which varies from zero (the actuator is removed) to one (the actuator is retained). These layout design variables are optimized concurrently with a large number of structural wingbox sizing variables and control surface actuation variables in order to minimize the sum of structural mass and actuator mass. Read More

AIAA J 2017 Dec 31;55(12):4142-4154. Epub 2017 Oct 31.

Senior Researcher, Advanced Measurements and Data Systems Branch, AIAA Associate Fellow. NASA Langley Research Center, Hampton, VA, 23681.

Femtosecond laser electronic excitation tagging (FLEET) velocimetry was used to study the flowfield around a symmetric, transonic airfoil in the NASA Langley 0.3-m TCT facility. A nominal Mach number of 0. Read More

AIAA J 2017 Dec 31;55(12):4274-4285. Epub 2017 Oct 31.

NASA Langley Research Center, Hampton, Virginia 23681.

This paper presents mechanisms to explain, as well as mathematics to model, time-averaged spatially resolved amplitude observations of number density and number density unsteadiness in a Mach 10 flow as it transitions from the freestream, through a bow-shock wave, and into the gas cap created by a blunt-body model. The primary driver for bow-shock unsteadiness is freestream unsteadiness or "tunnel noise." Primary unsteadiness is bow-shock oscillation. Read More

AIAA J 2017 Dec 6;55(12):4181-4192. Epub 2017 Sep 6.

HX5 Sierra, LLC, Cleveland, Ohio 44135.

The thrust of the dielectric barrier discharge plasma actuator is the plasma body force minus the wall shear force, and it equals the net induced momentum. Thrust measurement simplicity makes it a good metric of the aerodynamic performance for active flow control applications. Uncertainty and non-repeatability issues with conventional test setups motivated development of a novel suspended actuator test setup and a measurement methodology consisting of a burn-in procedure followed by frequency scans at constant voltages. Read More

AIAA J 2017 Nov;55(11)

Stanford University, Stanford, California 94305.

The performance of two wall models based on Reynolds-averaged Navier-Stokes is compared in large-eddy simulation of a high Reynolds number separating and reattaching flow over the NASA wall-mounted hump. Wall modeling significantly improves flow prediction on a coarse grid where the large-eddy simulation with the no-slip wall boundary condition fails. Low-order statistics from the wall-modeled large-eddy simulation are in good agreement with the experiment. Read More

AIAA J 2017 Sep 2;55(9):2859-2874. Epub 2017 Jun 2.

The advancement of flow measurement techniques continues to extend experimental boundaries and thus significantly contributes to improving our understanding of both basic and applied aerodynamics. This is particularly apparent in the case of particle image velocimetry (PIV), where its application has furthered the existing knowledge in several areas of helicopter rotor aerodynamics. The complex nature of helicopter rotor flows presents unique challenges to experimentalists, including transonic flow, concentrated vortices and dynamic stall. Read More

AIAA J 2017 Jul 25;55(7):2254-2268. Epub 2017 May 25.

Vantage Partners LLC, Cleveland, Ohio 44135.

The accurate measurement of power consumption by dielectric barrier discharge plasma actuators is a challenge due to the characteristics of the actuator current signal. Microdischarges generate high-amplitude, high-frequency current spike transients superimposed on a low-amplitude, low-frequency current. A high-speed digital oscilloscope was used to measure the actuator power consumption using the shunt resistor method and the monitor capacitor method. Read More

AIAA J 2016 Jun;Volume 55(No 1):49-56

Norfolk State University, Norfolk, Virginia 23504.

This paper describes a novel computational-fluid-dynamics-based numerical solution procedure for effective simulation of aerothermoacoustics problems with application to aerospace vehicles. A finite element idealization is employed for both fluid and structure domains, which fully accounts for thermal effects. The accuracies of both the fluid and structure capabilities are verified with flight- and ground-test data. Read More