Oncoming Boundary Layer Research Articles

Scaling of the Wall Pressure Field Around Surface-Mounted Pyramids and Other Bluff Bodies

The turbulent flow around square-based, surface-mounted pyramids, of height h, in thin and thick boundary layers was experimentally investigated. The influence of apex angle ζ and angle of attack α was ascertained from mean surface flow patterns and ground plane pressure measurements taken at a Reynolds number of 3.3×104 based on h. For both boundary layer flows, it was found that the normalized ground plane pressure distributions in the wakes of all the pyramids for all angles of attack may be scaled using an attachment length (Xa′) measured from the upstream origin of the separated shear layer to the near-wake attachment point on the ground plane. It was also shown that this scaling is applicable to data reported in the literature for other bluff body shapes, namely, cubes, cones, and hemispheres. The ground plane pressure coefficient distributions in the upstream separated flow region, for all the shapes and angles of attack examined, were found to collapse onto two curves by scaling their streamwise location using a length scale (Xu), which is a function of the frontal projected width of the body (w′) and the height of the body. These two curves were for cases where δ∕h<1 (“thin” boundary layer) or δ∕h≥1 (“thick” boundary layer), where δ is the oncoming boundary layer thickness. Further work is required to provide a more detailed statement on the influence of boundary layer thickness (or state) on the upstream pressure field scaling.

Efficient treatment of complex geometries for large eddy simulations of turbulent flows

Incompressible turbulent flow over a backward facing step at Re h =5100 is investigated by large eddy simulations (LES). The ratio of the oncoming boundary layer thickness δ to the step height h was set to 1.2. Additionally channel flows at various Re τ numbers are presented for the validation of the numerical code. The results are compared with existing DNS and experimental databases. The present study focuses on different procedures for LES of engineering problems in complex geometries using structured rectangular grids. Two different methods that are able to treat complex geometrical configurations are implemented, examined and compared; namely the domain decomposition approach based on Schur’s complement and the immersed boundary method. In the present study both methods make use of a fast direct Poisson’s pressure solver based on a heavily modified version of the public domain package FISHPAK. The latter was optimised and fully parallelised for shared memory architectures, for solutions on rectangular grids stretched in one or two directions. The resulting code reaches performances of 1.0 μs/node/iter, allowing low cost computations on grids of the order of million points. The main objective of the present study was to investigate the potential of different methods for LES in complex geometrical configurations like bluff body flows and wakes. One of the main findings is that careful selection of numerical methods and implementation techniques can lead to accurate and very efficient codes, where the geometric complexity does not lead to algorithmic or numerical complexity.

Influence of wall proximity on vortex shedding from a square cylinder

Mean and fluctuating surface pressure data are presented for a square cylinder of side length D placed near a solid wall at Re D =18,900. One oncoming boundary layer thickness, δ=0.5 D was used. Measurements were made for cylinder to wall gap heights, S, from S/D=0.07 to 1.6. Four gap-dependent flow regimes were found. For S/D>0.9, the flow and the vortex shedding strength are similar to the no-wall case. Below the critical gap height of 0.3D, periodic activity is fully suppressed in the near wake region. In between, for 0.3 0.6 the influence of the viscous wall flow in the gap is not dominant and that, consequently, inviscid flow theory can describe changes in the mean lift as S/D decreases. For 0.3<S/D<0.6, the flow reattaches intermittently on the bottom face of the cylinder and viscous effects become important. Below the gap height of 0.4D, periodic activity cannot be observed on the cylinder.

An inclined rectangular jet in a turbulent boundary layer-vortex flow

A model test study was performed on streamwise vortices generated by a rectangular jet in an otherwise flat-plate turbulent boundary layer. The study was conducted in a low speed wind tunnel. The rectangular jet had a cross-section size of 28 mm by 5.5 mm. The oncoming boundary layer had a 99.5 percent thickness of 25 mm. The freestream speed of the oncoming flow was 20 m/s. Measurements were performed with a three-element LDA system. The effects of skew angle and streamwise development of vortex were investigated and the mean flow properties are presented. The study showed that the rectangular jet was able to produce a streamwise vortex of higher strength than that of a round jet, while at the same time keeping the same size and shape as that of a round jet. A 63% increase in the maximum vorticity was found. The 45∘ skew angle was identified as the optimal skew angle for vortex production.

Counter-Rotating Vortices Embedded in a Turbulent Boundary Layer with Inclined Jets

Counter-rotating vortex pairs in an otherwise e at-plate turbulent boundary layer were studied in a wind-tunnel model experiment. The vortices were produced with pairs of inclined round jets with nozzle exit e ush with the plate surface. The skew angle and pitch angle of the nozzle were 45 deg. The Reynolds number based on the momentum thickness of the oncoming boundary layer was 3 :8 ££ 10 3 . A velocity e eld survey was performed with a three-component laser Doppler anemometry system. The effects of jet speed on the streamwise development of the vortices were investigated. The e ow had an upwash region in between the vortices that were embedded in the turbulent boundary layer. The upwash was produced by the vortices and the interaction between diametrically opposed secondary, near-wall e ows. Velocity measurements indicated e ow separation and entrainment behind the jet exit, and this was corroborated by surface e ow observation. The vortex development was divided into two stages: a near-e eld stage where the wake of the jet plays an important role and a far-e eld stage where turbulent dissipation and diffusion are important.

On subsonic, supersonic and hypersonic inflectional-wave/vortex interaction

An unstable inflection point developing in an oncoming two-dimensional boundary layer can give rise to nonlinear three-dimensional inflectional-wave/vortex interaction as described in recent papers by Hall and Smith [1], Brown et al. [2], and Smith et al. [3]. In the current study on the compressible range the flow is examined theoretically just downstream of the linear neutral position, in order to understand how the interaction may be initiated. The research addresses both moderately and strongly compressible regimes. In the latter regime the vorticity mode, the most dangerous one, is taken as the wave part, causing the hypersonic interaction to become concentrated in a thin temperature-adjustment layer lying at the outer edge of the boundary layer, just below the free stream. In both regimes, the result is a nonlinear integro-differential equation for the wave-pressure which implies four different types of downstream behaviour for the interaction-a far-downstream saturation, a finite-distance singularity, exponentially decaying waves (leaving pure vortex motion) or periodicity. In a principal finding of the study, the coefficients of the equation are worked out explicitly for hypersonic flow, and in particular for the case of unit Prandtl number and a Chapman fluid, where it is shown that for sufficiently high wall temperatures the wave angle of propagation must lie between 45° and 90° relative to the free-stream direction and also no periodic solutions may occur then. The theory applies also to wake flows and others. Connections with experimental findings are noted.

Experimental Investigation of a Three-Dimensional Boundary Layer Flow in the Vicinity of an Upright Wall Mounted Cylinder (Data Bank Contribution)

The flow of a three-dimensional boundary layer approaching an upright wall mounted circular cylinder has been experimentally investigated by means of instantaneous flow visualization techniques using a laser sheet and time resolved measurements of the wall pressure, the gradients of which are related to the vorticity flux away from the wall. The mean separation point of the oncoming boundary layer is located on the plane of symmetry, 0.76 and 0.82 diameters upstream of the cylinder for the two investigated Reynolds numbers, based on the cylinder diameter, of 1.0 × 105 and 2.2 × 105, respectively. The present flow visualization studies have shown that there is always a primary vortex present in the flow which induces an eruption of wall fluid. Very often, this eruption results in the formation of counter rotating or mushroom vortices. A secondary vortex further upstream has been observed occasionally. This vortex, as well as the vortices formed by the fast eruption of wall fluid evolve quickly in time and space and therefore cannot be obtained from time-average measurements. The primary vortex consists of several large scale structures which have originated in the oncoming boundary layer and which have acquired substantial additional vorticity. Point measurements indicate that the r.m.s. pressure fluctuations increase as separation is approached and reach a maximum near reattachment. A low degree of space-time correlation and longer integral time scales were also observed downstream of separation. A bimodal probability density function of the fluctuating pressure was observed in the vicinity of the mean separation point, close to the corner region and in the wake of the cylinder. Quasi periodic vortex shedding from the cylinder with a Strouhal number 0.13 was also observed.

Experimental Investigation of a Turbulent Flow in the Vicinity of an Appendage Mounted on a Flat Plate

Experimental measurements were carried out in an incompressible three-dimensional turbulent shear layer in the vicinity of an appendage mounted perpendicular to a flat plate. The thickness of the turbulent boundary layer as it approached the appendage leading edge was 76 mm or 1.07 times the maximum thickness of the appendage. As the oncoming boundary layer passed around the appendage, a strong secondary flow was formed which was dominated by a horseshoe root vortex. This secondary flow had a major effect in redistributing both the mean flow and turbulence quantities throughout the shear layer, and this effect persisted to a significant degree up to at least three chord lengths downstream of the appendage leading edge.

Wind Tunnel Experiments on Cooling Tower Plumes: Part 2—In a Nonuniform Crossflow of Boundary Layer Type

The basic characteristics of plumes issuing into a boundary layer-type crossflow are reported. The flow can be considered as an interaction between two vorticity fields with different length scales and turbulence intensities. The large eddies of the oncoming boundary layer are responsible for the observed sudden changes in the plume direction. The type of structure emanating from the tower depends on the instantaneous velocity ratio. Mean velocities and normal velocity gradients are smaller than those in the case of uniform crossflow and therefore, the measured turbulence intensities are lower too. The cross-stream turbulence brings high-momentum fluid into the wake region and the velocity defect decays very rapidly. Dilution of the plumes takes place faster in the presence of external turbulence than in the case with uniform crossflow. The spreading rate is increased dramatically by the external turbulence, which causes different effects on the hydrodynamic and thermal fields.

Prediction of Film Cooling by a Row of Holes With a Two-Dimensional Boundary-Layer Procedure

The present paper describes predictions of film cooling by a row of holes. The calculations have been performed by a two-dimensional boundary-layer code with special modifications that account for the basically three-dimensional, elliptic nature of the flow after injection. The elliptic reverse-flow region near the injection is leapt over and new boundary-layer profiles are set up after the blowing region. They take into account the oncoming boundary layer as well as the characteristics of the injected jets. The three dimensionality of the flow, which is very strong near the injection and decreases further downstream, is modeled by so-called dispersion terms, which are added to the two-dimensional boundary-layer equations. These terms describe additional mixing by the laterally nonuniform flow. Information on the modeling of the profiles after injection and of the dispersion terms has been extracted from three-dimensional fully elliptic calculations for specific flow configurations. The modified two-dimensional boundary-layer equations are solved by a forward-marching finite-volume method. A coordinate system is used that stretches with the growth of the boundary layer. The turbulent stresses and heat fluxes are obtained from the k-ε turbulence model. Results are given for flows over flat plates as well as for flows over gas turbine blades for different injection angles, relative spacings, blowing rates, and injection temperatures. The predicted cooling effectiveness and heat transfer coefficients are compared with experimental data and show generally fairly good agreement.

Momentum Transfer in Turbulent Separated Flow Past a Rectangular Cavity

This paper presents results of an analytical and experimental investigation aimed at describing the turbulent momentum transfer mechanism in the separated-flow region of a rectangular cavity facing an oncoming turbulent boundary layer. A flow model of the mixing region in the slot postulated on the basis of eddy diffusion in free jets gives values of velocities and drag in good agreement with the measurements. The results further point to the significant effects exerted by the oncoming boundary layer on transfer rates from the slot. Flow-visualization pictures show some interesting vortex patterns inside the cavity when height-to-width ratio is varied between values of 1 and 3.