Wall-Mounted Hemisphere Research Articles

The height ratio effects on the flow characteristics of a wall-mounted hemisphere immersed in the turbulent boundary layer

The effects of the height ratios on the flow characteristics of a wall-mounted hemisphere immersed in the turbulent boundary layer are investigated by particle image velocimetry technology (PIV). The Reynolds number based on the hemisphere diameter D is ReD=8160, and three height ratios (i.e., H/D=0.5, 0.4 and 0.3) are selected for comparison at δ/D=0.79, where δ is boundary layer thickness without the presence of hemisphere. The PIV measurements are carried out in x-y plane and x-z plane, and the flow characteristics behind the hemisphere and the distributions of the von Kármán vortices (vK vortices) are investigated. In x-y plane, the curl of the shear layer on the top of the hemisphere brings reverse flow of fluids, and the area of the recirculation region and the area fluctuations are decreased as H/D decreases from 0.5 to 0.3. In x-z plane, the recirculation regions are symmetrical about z/D=0, reflecting reverse flow of fluids induced by the curl of the shear layers on both sides of the hemisphere. Moreover, the maximal velocity deficit is decreased as H/D decreases in x-z plane. The vK vortices are formed from the shear layers on both sides of the hemisphere, and the formation frequencies are same for different H/D, and the value is about 0.35 Hz. The Strouhal number St of the vK vortex is calculated by the hemisphere base diameter, and St slightly decreases from 0.154 to 0.142 as H/D decreases from 0.5 to 0.3. The turbulent fluctuations behind the hemisphere are decreased as H/D decreases.

A novel algorithm for visualizing and quantifying vortices in complex 3D flows based on marching and converging vortex atoms

Numerous investigations aiming to reveal the underlying physics behind complex flows highlighted the important roles of vortices. This article proposes an integrative algorithm for visualizing and quantifying the vortices in three-dimensional flows. The algorithm not only extracts the vortex centerlines but also returns the vortex radii and circulations varying along the centerlines. The novel aspect of this algorithm is to represent the vortex field as a collection of discrete vortex atoms. By iteratively updating the positions of these vortex atoms, the algorithm manipulates them into marching toward the underlying vortex centerlines. The radii and circulations varying along the centerlines are estimated based on the vortex atoms converged on the vortex centerlines. The accuracy and robustness of the algorithm are first accessed by numerical tests based on a synthetic vortex ring. Subsequently, the algorithm is employed to investigate the complex vortices in a turbulent boundary layer, validating the scaling law of the vortices reported in the literature. At last, the algorithm is applied to the three-dimensional experimental data of the wake flow behind a wall-mounted hemisphere. It concludes that the algorithm can be used as an effective tool for analyzing vortices in complex flows.

Large-eddy simulation of a developing turbulent boundary layer over a wall-mounted hemisphere under the influence of a surrounding ditch: Flow characteristics and aerodynamic forces

In this study, we have investigated the interaction between a wall-mounted hemisphere and a developing turbulent boundary layer. The Reynolds number based on the hemisphere diameter and the freestream velocity is 50,000. Two configurations are considered, namely a plain hemisphere and a hemisphere with a surrounding ditch. The latter is imposed to eliminate the horseshoe vortex formed in front of the plain hemisphere. The incoming boundary layer is generated through a precursor simulation to have realistic turbulence in the upstream. Wall-resolved Large-eddy Simulation is carried out to characterize the flow features in the wake of the hemispheres. At first, the plain hemisphere mean flow and velocity fluctuations are comprehensively validated against a well-documented experimental study. In the case of the hemisphere with the ditch, it is shown that the latter produce coherent structures in the wake, termed as “ditch vortex.” The effects of the ditch vortex on the mean flow and velocity fluctuations are studied. The ditch is shown to have a substantial effect on both flow and turbulent statistics, particularly on the spanwise fluctuations. Further, the aerodynamic forces and the lateral force oscillations are compared and analyzed between the plain hemisphere and the hemisphere with the ditch. Spectral analysis is utilized to identify the dominant frequencies of the wake structures in both flow configurations. To examine the flow features in greater details, Proper Orthogonal Decomposition is performed on the symmetry plane to identify the energetic structures and their associated frequencies. The results suggest that the ditch shifts the separated shear layer and also increases the turbulent kinetic energy at higher elevations but, at the same time, greatly alleviates the spanwise velocity fluctuations near the bottom wall.

Turbulent coherent structures in channel flow with a wall-mounted hemisphere

Turbulent channel flows around a wall-mounted hemisphere numerically are investigated by large eddy simulation, and the Reynolds number based on the hemisphere’s diameter is 3 × 104. The statistical characteristics and turbulent structure evolution are revealed in the Eulerian frameworks and Lagrangian frameworks. The vortex identification and Dynamic Mode Decomposition (DMD) are used to study the evolution of turbulent structure in the Eulerian frameworks, and the finite-time Lyapunov exponents are applied to identify Lagrangian coherent structures (LCS) in the Lagrangian framework. It is found that the developing angle of the hairpin vortex is ∼7° at two frameworks. What is more, there are some hairpin vortices formed behind the hemisphere and some turbulent structures formed near the wall by DMD method. The correlation analysis is applied to investigate the angle variation and scale variation of turbulent structures, and it is observed that the angle of turbulent structures is negative at Y/d ≥ 1.2 and the spanwise length scales of turbulent structures increase as it moves downstream. By studying the LCS behind a wall-mounted hemisphere, there is formation of “kink” caused by viscous interaction between some hairpin vortex legs, which is the characteristic of hairpin vortex deformation. The comparisons of statistical characteristics between Eulerian frameworks and Lagrangian frameworks are conducted by the correlation analysis, the spectrum analysis, and the structure functions.

Unsteady Motion in the Supersonic Flow over a Wall-Mounted Hemisphere

Experiments were performed within Rutgers University’s Mach 3.4 wind tunnel to visualize the unsteady flow over a wall-mounted hemisphere using high-speed schlieren imaging and stereoscopic particle image velocimetry (SPIV). The hemisphere () was placed within the wind tunnel’s naturally developing turbulent boundary layer, resulting in a boundary-layer thickness-to-radius ratio of 0.43. The hemisphere flowfield demonstrates many features that are characteristic of a traditional shock/boundary-layer interaction, such as unsteady motion associated with the foot of the upstream separation shock and subsequent incipient separation. In addition, a strong reattachment shock emanates from the forward side of the hemisphere where flow reattachment occurs. Cross correlation of the high-speed schlieren data indicates that the upstream shock-dominated flow is not strongly correlated with the downstream wake flow. The upstream shock structure dominates; it wraps around the sides of the hemisphere, leading to a potentially far-reaching influence. The SPIV velocity fields reveal the presence of a highly unsteady wake structure with a separated shear layer and a downstream recompression shock. The observations are consistent with recent studies concluding that the wake flow comprises two recirculating lobes with symmetry along the spanwise centerline. Turbulence statistics of the hemisphere wake and separated shear layer were also analyzed.

Experimental Characterization of Supersonic Flow over a Wall-Mounted Hemisphere

Supersonic flow over a wall-mounted hemisphere was experimentally investigated to understand the shock-wave/boundary-layer interactions, flow separation, and other complex flow features associated with it. The experiments were carried out on a hemisphere mounted on the wind-tunnel floor at Mach 2 and a Reynolds number of 3.6 million. Measurements include oil flow visualizations, shadowgraph visualizations, and steady and unsteady pressure distributions on the floor and the hemisphere. Although the supersonic flow around the hemisphere and the shock/boundary-layer interactions associated with it share some similarities with well-documented supersonic flows over wall-mounted blunt bodies like cylinders and blunt fins, some marked differences unique to the hemisphere were noticeable in the present study. The triple point of the -shock structure formed ahead of the hemisphere due to the interaction between the separation shock and the bow shock made a significant excursion over the hemisphere. The excursion of the triple point over the hemisphere closely correlated with the movement of the separation shock rather than the bow shock. Strong vortex structures seemed to form at the foot of the reattachment bow shock repetitively and convect over the hemisphere. Unsteady pressure measurement results provided useful insights on the distribution of flow unsteadiness over the hemisphere.

Large-eddy simulation study of Reynolds number effects on the flow around a wall-mounted hemisphere in a boundary layer

Large-eddy simulations were used to investigate unsteady flows around a wall-mounted hemisphere as the Reynolds number (Re, based on the diameter of the hemisphere D) increased from 7 × 104 to 7 × 105. The hemisphere was immersed in a low-turbulence-intensity boundary layer with a thickness of δ/D = 0.5. Strong Re dependence was confirmed to be present even for the flow around a wall-mounted obstacle after systematic examination of aerodynamic forces, local pressures, and flow structures. Drag and lift crises were observed simultaneously, with the critical Re noted at approximately 3 × 105. As with circular cylinders and spheres, a laminar-turbulent transition and induced flow separation delay were observed in the supercritical Re regime. Flow separation occurred on the sides of the body later than on the top, regardless of whether Re was subcritical or supercritical. The spatial and temporal features of flow structures at different scales were described in detail based on the present high-resolution simulations. The coexistence of lateral oscillations and arch-type vortex shedding occurred throughout the subcritical and supercritical Re range. However, both of these motions diminished in scale and strength at supercritical Re. Flow motion frequencies were also quantified. The frequency ratio of arch vortex shedding to lateral oscillation was approximately 4 at subcritical Re but decreased to 3 at supercritical Re.

Time-Resolved Planar Velocimetry of the Supersonic Wake of a Wall-Mounted Hemisphere

Time-resolved particle image velocimetry was conducted at 40 kHz using a pulse-burst laser in the supersonic wake of a wall-mounted hemisphere. Velocity fields suggest a recirculation region with two lobes, in which flow moves away from the wall near the centerline and recirculates back toward the hemisphere off the centerline, contrary to transonic configurations. Spatio-temporal cross-correlations and conditional ensemble averages relate the characteristic behavior of the unsteady shock motion to the flapping of the shear layer. At Mach 1.5, oblique shocks develop, associated with vortical structures in the shear layer and convect downstream in tandem; a weak periodicity is observed. Shock motion at Mach 2.0 appears somewhat different, wherein multiple weak disturbances propagate from shear-layer turbulent structures to form an oblique shock that ripples as these vortices pass by. Bifurcated shock feet coalesce and break apart without evident periodicity. Power spectra show a preferred frequency of shear-layer flapping and shock motion for Mach 1.5, but at Mach 2.0, a weak preferred frequency at the same Strouhal number of 0.32 is found only for oblique shock motion and not shear-layer unsteadiness.

Enhanced injection method for synthetically generated turbulence within the flow domain of eddy-resolving simulations

The quality of eddy-resolving turbulence simulations strongly depends on appropriate inflow conditions. In most cases they have to be time-dependent and satisfy certain conditions for the first (mean velocities) and second-order moments (Reynolds stresses) as well as concerning suitable length scales. To mimic a physically realistic incoming flow, synthetically generated turbulent velocity fluctuations superimposed on the mean velocity field are a valuable solution. However, the resolution of the grid near the inlet has to be sufficiently fine to avoid excessive damping of the turbulence intensity. In order to circumvent this problem, the injection of synthetically generated inflow data not at the inlet itself but inside the flow domain near the area of interest, where the grid is typically much finer, is an elegant loophole. In the present study two different injection techniques based on a source-term formulation are analyzed and evaluated. In addition to these techniques the injected data are weighted by a Gaussian distribution defining the influence area. In the recent work the definition of the influence area is enhanced compared to the initial version of Schmidt and Breuer (2017) extending the application range. The case of a rather small influence area in comparison with the grid cell size is now tackled which is often relevant for industrial applications.The flow past a wall-mounted hemisphere is chosen as test case. The bluff body is exposed to a thick turbulent boundary layer at Re = 50,000. The generation of the turbulent velocity fluctuations in the present investigation relies on the digital filter concept, but the injection techniques evaluated are not restricted to this inflow generator. The synthetic turbulent velocity fluctuations are injected about one diameter upstream of the hemisphere. Wall-resolved large-eddy simulations are carried out for two grid resolutions and the corresponding results are analyzed and compared with the reference measurements by Wood et al. (2016). Finally, one injection technique is found to be clearly superior to the other, since it guarantees the correct level of the velocity fluctuations and the reproduction of the autocorrelations.

Unsteady Shock Motion in a Transonic Flow over a Wall-Mounted Hemisphere

Particle image velocimetry measurements have been conducted for a Mach 0.8 flow over a wall-mounted hemisphere with a strongly separated wake. The shock foot was found to typically sit just forward of the apex of the hemisphere and move within a range of about . Conditional averages based upon the shock foot location show that the separation shock is positioned upstream along the hemisphere surface when reverse velocities in the recirculation region are strong and is located downstream when they are weaker. The recirculation region appears smaller when the shock is located farther downstream. No correlation was detected of the incoming boundary layer with the shock position nor with the wake recirculation velocities. These observations are consistent with recent studies concluding that, for large, strong separation regions, the dominant mechanism is the instability of the separated flow rather than a direct influence of the incoming boundary layer.

Large eddy simulation of turbulent flow around a wall mounted hemisphere

Turbulent air flow around a wall-mounted hemisphere is investigated numerically for Reynolds numbers equal to 36,000 and 64,000. Large eddy simulation (LES) is used to simulate the flow field and results are compared with wind tunnel measurements and computations from RNG k–ε modeling. The shortcoming of the RANS model is reported and the LES advantages are highlighted for modeling of such external flows around a bluff body. Various subgrid scale models are considered and compared for LES simulation. Using the numerical simulation results, streamlines at different sections of the flow domain are presented and pressure and velocity distribution at different locations over the hemisphere are plotted and compared. It is shown that the LES model can be properly used to obtain the complex flow field behind the hemisphere where the RANS model fails to predict the flow field accurately. Among subgrid scale models the prediction of kinetic energy turbulence model shows better agreement with the experimental data.

Direct numerical simulation of particle transport by hairpin vortices in a laminar boundary layer

The transport of solid particles by coherent wall structures is studied here. This phenomenon is present in numerous environmental and engineering flows. The flow above a wall-mounted hemisphere is used for generating hairpin vortices in a laminar boundary layer in a controlled way. By means of direct numerical simulation (DNS) of the fluid flow and simultaneous Lagrangian tracking of particles, the influence of hairpin vortices on solid particles released in the wake of the obstacle is analyzed.

Density Measurements of a Turbulent Wake Using Acetone Planar Laser-Induced Fluorescence

The application of a planar density measurement technique for compressible flowfields based on acetone planar laser-induced fluorescence is presented. An error analysis indicates a minimum inherent uncertainty of in density measurements due to uncertainty in local pressure and a total experimental uncertainty of 8%, primarily driven by shot noise due to low-signal levels. The technique is demonstrated through the visualization of the separated shear layer and turbulent wake of a wall-mounted hemisphere at a freestream Mach number of 0.78 and a Reynolds number of approximately 900,000. The flow is marked by a large-scale flapping motion of the wake and low-density vortex cores, where density drops of up to 50% of the freestream density are detected. In addition, closeup images of the shear layer near the separation point reveal the formation of lambda shocks. The density fields are used to perform aerooptic distortion calculations through spatial integration of the density field. A correlation is found between the spatial scale of the distortion and the features present in the density field. These findings demonstrate the viability of acetone planar laser-induced fluorescence for conducting planar density measurements and providing a new method for the evaluation of aerooptic distortion in compressible flows.

Air flow aerodynamic on a wall-mounted hemisphere for various turbulent boundary layers

Abstract Air flow field around a surface-mounted hemisphere of a fixed height for two different turbulent boundary layers (thin and thick) are investigated experimentally and numerically. Flow measurements are performed in a wind tunnel using hot-wire anemometer and streamwise component of velocity fluctuation are calculated using a special developed program of the hardware system. Mean surface pressure coefficients and velocity field for the same hemisphere are determined by the numerical simulation. Turbulent flow field and intensity are measured for two types of boundary layers and compared at various sections in both streamwise and spanwise directions. Numerical scheme based on finite volume and SIMPLE algorithm is used to treat pressure and velocity coupling. Studies are performed for Reynolds number, Re H = 32,000. Based on the numerical simulation using RNG k – e turbulence model, flow pathlines, separation region and recirculation area are determined for the two types of turbulent boundary layer flows and complex flow field and recirculation regions are identified and presented graphically.