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A comprehensive review of all relevant experimental data was completed, including recent data for the drag coefficient for a sphere in supersonic and hypersonic flows. The primary characterization ...

The application of subsonic and supersonic vortex generators for flow control in an external compression, axisymmetric low-boom concept inlet was investigated experimentally and computationally. The low-boom inlet design is conceptually based on a previous scale model tested in the NASA 1×1 ft supersonic wind tunnel. It features a zero-angle cowl and relaxed isentropic compression centerbody spike, resulting in defocused oblique shocks and a weak terminating normal shock. A design of experiments computational methodology was used to select device size and location to determine the optimal choice of device geometry and placement, with the objective of reducing radial flow distortion for on-design conditions. The resulting test matrix of vortex generator geometry and placement was specified for model inlet testing conducted at the 8×6 ft supersonic wind tunnel at NASA Glenn Research Center in the Fall of 2010. Comparisons of Reynolds-averaged Navier–Stokes predictions with experimental data were subsequen...

Large-eddy simulations of a vortex generator embedded upstream of a normal shock boundary-layer interaction followed by a subsonic diffuser were conducted. In particular, the “ramped-vane” flow control devicewas placed in a supersonic boundary layer with a freestream Mach number of 1.3 and a Reynolds number of 2400 based on momentum thickness. The ramped vane had a height of 0:52 and generated strong streamwise vorticity that entrained the high-momentum flow to the near-wall region. This contributed to decreasing the shock-induced flow separation while significantly increasing the skin friction coefficient in the diffuser where a strong adverse pressure gradient was present. In addition, it was found that the high-momentum flow persisted far downstream of the shock interaction region, which yielded reductions of both the displacement thickness and the shape factors compared to the uncontrolled case.

Micro-vortex generators (μVGs) have the ability to alter the near-wall structure of compressible turbulent boundary layers to provide increased mixing of high speed fluid such that the boundary layer remains healthy even with some disturbance imparted to the flow. Due to their small size, μVGs are embedded in the boundary layer and may provide reduced drag when compared to traditional vortex generators. In particular, micro-ramps are of significant interest as they are cost-effective, physically robust, and do not require a power source. To examine their potential, a detailed computational study of micro-ramps in a supersonic boundary layer at M=1.4, 2.2 and 3.0 was undertaken. The moderate Reynolds number of 3,800 allowed the computations to use monotone integrated Large Eddy Simulations (MILES) which generally reproduced the experimentally obtained mean velocity profiles, although some differences were found with respect to grid resolution. The MILES results indicate that micro-ramps have greater impact at lower Mach number near the device where its influence decays faster than that for the higher Mach number cases. This may be due to the reduced aspect ratio of the turbulent structure at the lower Mach number such that their coherency is easily lost yielding the streamwise vorticity and the turbulent kinetic energy to decay quickly. The normal distance between the vortex core and the wall had similar growth indicating weak correlation with the Mach number; however, the spanwise distance between the two counter-rotating cores further increases with lower Mach number.

A methodology for simulating two-way multiphase coupling of mass, momentum, and energy was developed to investigate the effect of droplet mass and heat transfer on one-dimensional shock waves. The numerical approach employed a conservative formulation for the gas and a Lagrangian formulation for the particles. The approach was verified for one-way heat transfer, evaporation and condensation for low-speed flows, and for two-way shock attenuation for solid particles and small evaporating drops (for which breakup is not expected and internal temperature gradients are weak). Parametric studies were conducted to investigate the coupling physics, and, surprisingly, finite rate evaporation and two-way coupling were found to increase the rate of shock attenuation and reduce the postshock gas temperature for mass loadings as small as 0.5%. Larger drops led to long regions of nonequilibrium as did, unexpectedly, effects of evaporation.

A review of compressibility and rarefaction effects on spherical particle drag was conducted based on existing experimental data, theoretical limits, and direct simulation Monte Carlo method results. The data indicated a nexus point with respect to effects of Mach number and Knudsen number. In particular, it was found that a single drag coefficient (of about 1.63) is obtained for all particle conditions when the particle Reynolds number is about 45, and is independent of compressibility or rarefaction effects. At lower Reynolds numbers, the drag is dominated by rarefaction, and at higher Reynolds numbers, it is dominated by compressibility. The nexus, therefore, allows construction of two separate models for these two regimes. The compression-dominated regime is obtained using a modification of the Clift-Gauvin model to specifically incorporate Mach number effects. The resulting model was based on a wide range of experimental data and showed superior prediction robustness compared with previous models. For the rarefaction-dominated regime, the present model was constructed to directly integrate the theoretical creeping flow limits, including the incompressible continuum flow limit (Stokes drag), the incompressible weak rarefaction limit (Basset-Knudsen correction), and the incompressible free-molecular flow limit (Epstein theory). Empirical correlations are used to extend this model to finite particle Reynolds numbers within the rarefaction-dominated regime.

A LTHOUGH the drag and gravitational forces are typically the most important to particle motion, the lift force can often be significant and can even approach the magnitude of the other two forces in some circumstances for surface spin velocities on the order of the translational particle velocity. As such, there are a number of multiphase systems in which consideration of lift is vital. The lift has been noted to be important for lateral migration in tubes (Saffman [1]), effectiveness of microcentrifuges (Heron et al. [2]), and particle deposition in boundary layers (Young and Leeming [3]). Another flowfield for which particle lift effects are important is that of the rotating bioreactor, which allows animal tissue growth under idealized conditions, similar to that obtained in microgravity conditions [4]. In many of these conditions, the particle can be considered as spherical and solid and the surrounding fluid as incompressible and Newtonian, as will be the focus of this review. However, there have beenmany excellent reviews on particle lift that discuss other conditions. In particular, the review by Leal [5] discussed dynamics and theoretical results for nonspherical particles, deformable particles, and particles in non-Newtonian fluids. On the subject of clean bubbles, Drew [6], Magnaudet and Eames [7], and Tomiyama et al. [8] discussed theory and models for lift in shear, rotational, and straining flows. For noncontinuum conditions, Wang [9] investigated the effect of particle spin in free-molecular flow and obtained a lift force that acted in a direction opposite to that of drag in continuum creeping-flow conditions. Recently, Volkov [10] investigated the transition between the continuum and the freemolecular regimes for spinning particles to obtain the critical Knudsen number at which particle spin did not yield lift. To discuss solid particle lift for incompressible continuum Newtonian flow, it is helpful to first define the dimensionless parameters that influence lift, as well as the various forms of the lift coefficient. In general, the particle velocity v is defined as the translational velocity of the particle center of mass xp. The continuous-fluid velocity is generally defined in all areas of the domain unoccupied by particles. However, a hypothetical continuous-phase velocity can be extrapolated to the particle centroid and will be denoted as u and termed the “unhindered velocity.”The relative velocity of the particlesw is then based on the unhindered velocity (i.e., along a particle trajectory)

Passive methods of controlling shock/boundary-layer interactions (SBLIs) consist of a porous surface covering a cavity or a plenum located in the region of the SBLI. The present study focuses on the flowfield downstream of a Mach 1.42 SBLI controlled with various passive devices such as a conventional porous plate, a microporous plate, streamwise slots, a conventional mesoflap array, and a hybrid flap array. Qualitative analysis of the flowfield for the various control devices investigated was achieved with spark shadowgraph visualizations and surface oil-flow visualizations. Quantitative analysis was accomplished by measuring surface static pressure distributions and boundary layer velocity profiles. The flowfields downstream of the slot-controlled and hybrid flap array-controlled SBLIs were found to be highly three-dimensional, whereas the flowfields were predominantly two-dimensional for the remainder of the control devices. It was found that only the conventional mesoflap array had an improved total pressure recovery compared to the baseline solid wall.

A direct numerical simulation approach was employed along with a Lagrangian particle tracking technique to investigate dilute particle motion and dispersion in a horizontal turbulent boundary layer (Re τ = 270) with no streamwise pressure gradient. Particle inertias based on inner Stokes numbers St + (based on friction velocity) ranging from 10 -2 to 10 2 were investigated. The particles were injected near the wall at a height of four wall units (with elastic wall collision specified at one wall unit), and the terminal velocity was kept small so that particle-eddy interaction would be the primary dispersion

A trajectory control problem for simple micro air vehicles (MAVs) is introduced and studied. The MAVs are endowed with the ability to estimate their position and velocity and can control their drag coefficient. The objective of this work is to design a control law for the drag coefficient in order to land the MAV close to a desired streamwise location, after being dropped from an airborne carrier at low altitude. We use a simple model-predictive control law, relying on a two-dimensional time-averaged atmospheric velocity field, which is validated on an unsteady three-dimensional representation of an unstratified atmospheric boundary layer. The simulation results show that the proposed control law results in a significant (10-fold) improvement in the accuracy of the streamwise landing point location, with respect to the uncontrolled case. Similar results are obtained when errors are considered in the location of the release point and in the estimate of the mean flowfield.

Skin-friction measurements for a normal-shock/boundary-layer interaction with several recirculating flow control methods have been conducted in a planar Mach 1.4 wind tunnel. The skin friction has been measured along the spanwise centerline and downstream of the interaction using the laser interferometry skin-friction (LISF) technique, which optically detects the rate of thinning of an oil film applied to the test surface. Velocity profiles measured by laser Doppler velocimetry at the same locations as the LISF measurements in the interaction were also used to evaluate the skin friction. Comparison of the two measurement techniques for skin friction is found to show reasonable agreement. Various configurations of the mesoflap arrays of different shapes and thicknesses were examined, and the results were compared to cases of both a solid wall with no control mechanisms and conventional recirculating passive flow control with a porous plate. Of the various mesoflap arrays tested, one mesoflap array provided higher skin friction downstream of the interaction, and as such tends to have better recovery from flow separation. However, all values of skin friction for the mesoflap arrays and the porous plate were found to be lower than for the solid-wall reference case. As such, the flow downstream of these control systems can be more susceptible to separation for this particular condition, although the control systems may reduce viscous drag in external flows.

Aeroelastic mesoe aps for recirculating transpiration have been investigated in an effort to control shock/boundary-layer interactions (SBLIs) through passive cavity recirculation. The mesoe ap concept utilizes a matrix of small e apscovering an enclosed cavity that aredesigned to undergo local aeroelasticdee ection to achieve proper mass bleed or injection when subjected to gasdynamic pressure loading. Experiments were performed to investigate the applicability of the mesoe ap concept for oblique shock interaction by employing shadowgraph e ow visualizations, surface pressure measurements, and mean and e uctuating velocity measurements, along the spanwise midplane of the shock intersection. The experiments were conducted in a Mach 2.41 supersonic wind tunnel operating at a unit Reynolds number of 57 £106 mi1. With the thickest mesoe ap arrays in place, the leading shock formed at the location of the e rst e ap and the boundary-layer thickness at shock impingement was greaterduetoe owinjectionthroughtheupstreame aps.However,thethinnestmesoe aparraysyielded asomewhat reduced boundary-layer thickness downstream of the interaction as a result of the tangential bleeding by the last e aps. Stagnation pressure proe les for the thinnest arrays also showed improved recovery downstream of the SBLI as compared to thesolid-wall case. However, furtherstudy isneeded to investigate three-dimensional effectsand to determine whether this control strategy provides signie cant performance improvements for e ow conditions more consistent with actual inlets.

The main objectives were to establish and investigate discontinuous and continuous random walk models appropriate for free-shear e ows with regard to turbulent particle diffusion. The models were designed to capture the crossing trajectories effect, the continuity effect, and the inertial-limit effect, all for the case of heavy particles whose densities are much greater than that of the surrounding e uid. In addition, both techniques included an isotropic drift velocity to account for inhomogeneous turbulence. The computational efe ciency of the continuous random walk models is improved by utilizing local time stepping, which effectively e lters out high-frequency velocity e uctuations that do not have a signie cant ine uence on particle diffusion. The predictive performances of these two random walk models were examined through comparison with experimental data and idealized test conditions. The results indicate that both models agree well with experimental data for a nearly homogeneous turbulent wake and an inhomogeneous turbulent axisymmetric jet (although the continuous random walk model performs somewhat better for the inhomogeneous e ows ). It was also found that the proposed drift velocity models are important to ensure continuity when simulating particle diffusion with inhomogeneous turbulence.

A simple Eulerian model is developed to determine the qualitative mean transverse diffusion in a free shear flow of a particle with a drag coefficient inversely proportional to the particle Reynolds number. The model is based on integrating the particle equation of motion within a single time-varying eddy to yield a closed-form analytic expression for the turbulent diffusion. The description is based on an average local length scale and an average local timescale of the turbulence similar to previous Lagrangian stochastic diffusion models. The resulting expression indicates three important nondimenslonal parameters: a local Stokes number (ratio of particle response time to eddy lifetime), an eddy Froude number (ratio of rotational acceleration to gravitational acceleration), and a drift parameter (ratio of particle terminal velocity to turbulent rms fluctuations), where the third parameter is simply a function of the first two. The model yields a particle diffusion greater than that of a scalar for a local Stokes number of order unity, the value of which depends on the eddy Froude number

A nonequilibrium reacting flow methodology has been combined with a conservative, monotonic, two-phase, compressible flow solver to allow numerical simulations of gas detonations subject to inert particles. This flow solver incorporates unstructured dynamically adaptive meshes with the finite element method-flux-corrected transport scheme for both the gas and particle phases. A simple two-step induction parameter model was used to model the combustion of the gas phase coupled with a point-implicit scheme for the energy release equation. This combustion model was then used to simulate two-dimensional detonation macroscopic features of hypothetical fuel oxygen diluent mixture for a few sample cases. These simulations employed particles of various diameters and mass loadings resulting in both successful and failed detonations.

Passive acoustic excitation was employed to augment mixing in a bounded planar supersonic shear layer. It was found that acoustically reflective surfaces, both downstream and below the shear layer, were essential for this resonance excitation to occur, and an upstream acoustically reflective surface augmented this excitation. However, none of these surfaces directly contacted the shear layer. Based on total pressure measurements, as much as 100% augmentation in mixing layer growth rate was achieved with this passive excitation system even at pressure matched conditions. Shadowgraph images of the shear layer showed the presence of singular coherent large-scale structures, and the acoustic spectra measured below the shear layer indicated the presence of dominant frequency with overall high energy for the excited flow compared with a lower energy broadband spectrum for the unexcited flow. The results indicate that a combination of streamwise and transverse modes may be responsible for the excitation.

Experiments were performed using high-speed cinematography to spatially and temporally resolve compressible planar mixing layer structures in pressure matched and unmatched conditions. The unmatched conditions (overexpanded and underexpanded) were achieved by enforcing a static pressure difference between the supersonic planar jet exit (M ∼ 1.65) and the ambient quiescent flow below. The pressure mismatch resulted in the interaction of expansion-compression waves with the free shear layer. Temporally resolved planar images, autocorrelations, and temporal correlations were obtained for a portion of the flowfield just downstream of the expansion-compression interaction. The results showed increased fluctuations in shear layer structure angles for the unmatched pressure conditions as compared with those for the matched condition. The degree of angular fluctuations qualitatively correlated with a nondimensional pressure gradient parameter, and the time scale was consistent with the eddy passage frequency. The passive scalar convection speeds for the unmatched conditions were found to be significantly lower than for the matched cases.

The objective of this research was to study the time-evolving velocity field in a two-stream, turbulent, planar free shear layer using a cinematic partide image velocimetry technique. The water shear layer had a velocity ratio of 0.23 and a Reynolds number of 2.62 X 10 4 based on velocity thickness and velocity difference. The cinematic particle image velocimetry system employed an argon-ion laser, a scanning mirror, and a 35-mm movie camera. Experimental data obtained by this technique yielded a combined spatial and temporal evolution of the two-dimensional velocity and spanwise vorticity fields. The detailed velocity field structure of the shear layer was significantly different from previous lower Reynolds number flow visualizations in that the classical well-defined eddies and braids were replaced with complex three-dimensional agglomerated vortices of both signs. The velocity field evolution was also notably different from that of the passive scalar field, where the former exhibited stronger temporal variations and reduced spatial coherency. Temporal and spatial correlations yielded transverse distributions of convection velocities based on both streamwise velocity perturbations and vorticity.

This research employed an adaptive unstructured finite element grid with a nonlinear monotonic scheme to undertake high-resolution inviscid eddy simulations of spatially evolving supersonic free shear layers. The objectives were to both improve physical understanding and evaluate the simulation fidelity by direct comparisons with detailed experimental measurements. With a physically consistent perturbation model, the compressibility effects on the nature of the shear layer were investigated by testing three different convective Mach numbers, 0.35, 0.45, and 0.7, nonetheless holding velocity and density ratios fixed. Reduction in eddy coherency as compressibility increased was found to be related to modifications in the merging process (slapping vs rotational merges). Turbulence statistics of velocity and mixture fraction exhibited reductions in the peak values of turbulent intensities of transverse velocity and passive scalar as convective Mach number increases, whereas turbulent intensity of streamwise velocity peak values remain constant.

The formation of shocks within axisymmetric supersonic nozzles has received considerable attention in the past, both experimentally and computationally. The presence of undesirable oblique shocks can significantly alter the downstream flowfield, reduce the thrust efficiency, and affect both the external acoustic signature and base pressure. In this paper numerical simulations of axisymmetric supersonic nozzle flow have been accomplished using an axisymmetric version of the finite element method/flux corrected transport algorithm. The adaptive unstructured gridding and the conservative nonlinear FCT scheme predicted shock formation that agreed with experimental data.