Location Of Boundary Layer Separation Research Articles

An insight into the vortex-induced vibration and near-wall vortex evolution of a roughed circular cylinder with truncated conical shape protrusions

The vortex-induced vibrations of a roughed circular cylinder with truncated conical shape protrusions to simulate the attachment of barnacles are numerically investigated in this paper. The coverage ratio (CR) of protrusions varies from 0% to 80% with an increment of 20%, and the simulation is conducted in the reduced velocity range of Ur = 1.43–11.00. The numerical results indicate that the boundary layer development of the cylinder is continuously disrupted by the protrusions, generating inter-rib vortices. Five near-wall vortex structures are identified, including the main vortex, merged main vortex, subordinate vortex, inter-rib quasi-stagnation vortex and dynamic inter-rib vortex. The evolution of the near-wall vortex is dependent on both the Ur and CR. As CR grows, the numbers of boundary layer separation and reattachment increase, and the associated points become to be more concentrated on the front surface of the cylinder. The vortex formation length and wake width are closely related to the location of boundary layer separation, significantly influencing the hydrodynamic coefficients. The emergence of merged main vortices leads to an increase in the vortex intensity, thereby affecting the hydrodynamic coefficients. The vibration response of the cylinder with protrusions of CR = 20% is significantly enhanced with the accompany of and broadened lock-in region, which is attributed to the transformation of the vortex shedding mode.

Study on flow separation and suction control of a compressor tandem cascade

To control the boundary layer separation on the front blade suction surface in a compressor tandem cascade at a high incidence angle, three suction schemes at different axial positions were introduced and the cascade performance improvement and separation flow control effects were investigated. Based on the experimental and numerical simulations, the aerodynamic performance of the prototype tandem cascade reaches the peak at the incidence angle of 5°. Continue to increase the incidence angle, the loss increases sharply, and the static pressure rise coefficient and diffusion factor begin to decrease. With the boundary layer suction on the suction surface of the front blade, fluid energy in the upstream boundary layer of the suction slot is increased, and the adverse pressure gradient downstream is decreased. By suction, the development and separation of the boundary layer are inhibited, and the loss in the tandem cascade is significantly reduced. The flow control effect of the suction scheme at the onset location of boundary layer separation is the best, and the flow loss can be reduced by about 34.9% with a suction flow rate of 1% at the incidence angle of 5°. The suction schemes have good flow control effects in a wide working range and broaden the available range of incidence angles of the tandem cascade. This study provides a reference for the design of tandem blades considering flow control and has a positive significance for improving compressor efficiency and pressure ratio.

Wind Effects on Dome Structures and Evaluation of CFD Simulations through Wind Tunnel Testing

In the study, a series of wind tunnel tests were conducted to investigate wind effects acting on dome structures (1/60 scale) induced by straight-line winds at a Reynolds number in the order of 106. Computational Fluid Dynamics (CFD) simulations were performed as well, including a Large Eddy Simulation (LES) and Reynolds-Averaged Navier–Stokes (RANS) simulation, and their performances were validated by a comparison with the wind tunnel testing data. It is concluded that wind loads generally increase with upstream wind velocities, and they are reduced over suburban terrain due to ground friction. The maximum positive pressure normally occurs near the base of the dome on the windward side caused by the stagnation area and divergence of streamlines. The minimum suction pressure occurs at the apex of the dome because of the blockage of the dome and convergence of streamlines. Suction force is the most significant among all wind loads, and special attention should be paid to the roof design for proper wind resistance. Numerical simulations also indicate that LES results match better with the wind tunnel testing in terms of the distribution pattern of the mean pressure coefficient on the dome surface and total suction force. The mean and root-mean-square errors of the meridian pressure coefficient associated with the LES are about 60% less than those associated with RANS results, and the error of suction force is about 40–70% less. Moreover, the LES is more accurate in predicting the location of boundary layer separation and reproducing the complex flow field behind the dome, and is superior in simulating vortex structures around the dome to further understand the unsteadiness and dynamics in the flow field.

Using baseball seams to alter a pitch direction: The seam shifted wake

An experimental investigation of how seams and their orientation relative to the spin axis and flight direction can alter the formation of a wake around a baseball was conducted. Particle Image Velocimetry (PIV) was used to examine the velocity field around a baseball in specific orientations and to find the boundary layer separation location, which is the location on the baseball where the wake begins to form. Certain orientations can advance the separation point on one side of the baseball, generating a pressure force on the baseball and modifying its flight path. Using this information as a guide, baseballs were launched 55 feet (a realistic pitching distance) in orientations designed to have an asymmetric separation point. These pitches were 90 mph at spin rates near 1200RPM with a vertical spin axis perpendicular to the initial flight direction. A Rapsodo 1.0 system was used to compare the pitch locations for different seam orientations. The results of this study showed a significant and repeatable difference in the path of the baseball depending on the orientation of the seams relative to the spin axis. This effect was more significant for baseballs with larger seams.

A computational study of vortex rings interacting with a constant-temperature heated wall

This study is motivated by understanding the connections between the vortical structures in impinging jets and the wall heat transfer. Of particular interest are: (1) examining how the stage of evolution of vortex pairing in the jet might influence the wall heat transfer, and (2) establishing correlations between the vortex characteristics and the Nusselt number (Nu) distribution. To this end, CFD simulations are conducted of three simplified model problems involving the interaction of isolated axisymmetric vortex rings with a flat, constant-temperature, heated wall. The cases represent three scenarios of vortex-wall interaction: before (Case I), during (Case II) and after (Case III) pairing. The results show that when two vortices concurrently interact with the wall and undergo pairing (Case II), a significant instantaneous enhancement in Nu is attained in comparison to that associated with a single vortex interacting with the wall (Cases I and III). However, Case II also leads to the largest subsequent decay in Nu enhancement due to the formation of a particularly strong secondary vortex. In all cases, a deterioration in Nu, relative to unsteady diffusion, is observed simultaneously with the enhancement. Notwithstanding this deterioration, the net effect of vortex-wall interaction on the heat transfer remains positive with Case II producing the highest heat transfer rate. An analysis is conducted to establish the connection between the instantaneous maximum and minimum Nu, the circulation and the radial and the wall-normal location of the core-centers of the vortices, the thermal boundary layer thickness, the boundary layer separation location and the wall shear stress.

Transient interaction between a reaction control jet and a hypersonic crossflow

This paper presents a numerical study that focuses on the transient interaction between a reaction control jet and a hypersonic crossflow with a laminar boundary layer. The aim is to better understand the underlying physical mechanisms affecting the resulting surface pressure and control force. Implicit large-eddy simulations were performed with a round, sonic, perfect air jet issuing normal to a Mach 5 crossflow over a flat plate with a laminar boundary layer, at a jet-to-crossflow momentum ratio of 5.3 and a pressure ratio of 251. The pressure distribution induced on the flat plate is unsteady and is influenced by vortex structures that form around the jet. A horseshoe vortex structure forms upstream and consists of six vortices: two quasi-steady vortices and two co-rotating vortex pairs that periodically coalesce. Shear-layer vortices shed periodically and cause localised high pressure regions that convect downstream with constant velocity. A longitudinal counter-rotating vortex pair is present downstream of the jet and is formed from a series of trailing vortices which rotate about a common axis. Shear-layer vortex shedding causes periodic deformation of barrel and bow shocks. This changes the location of boundary layer separation which also affects the normal force on the plate.

On vortex shedding from low aspect ratio dual step cylinders

A dual-step cylinder is comprised of two cylinders of different diameters. A large diameter cylinder (D) with low aspect ratio (L/D) is attached to the mid-span of a small diameter cylinder (d). The present study investigates the effect of Reynolds number (ReD) and L/D on dual step cylinder wake development for D/d=2, 0.2≤L/D≤3, and two Reynolds numbers, ReD=1050 and 2100. Experiments have been performed in a water flume facility utilizing flow visualization, Laser Doppler Velocimetry (LDV), and Particle Image Velocimetry (PIV). The results show that vortex shedding occurs from both the large and small diameter cylinders for 1≤L/D≤3 at ReD=2100 and 2≤L/D≤3 at ReD=1050. At these conditions, large cylinder vortices predominantly form vortex loops in the wake and small cylinder vortices form half-loop vortex connections. At lower aspect ratios, vortex shedding from the large cylinder ceases, with the dominant frequency in the large cylinder wake attributed to the passage of vortex filaments connecting small cylinder vortices. At these lower aspect ratios, the presence of the large cylinder induces periodic vortex dislocations. Increasing L/D increases the frequency of occurrence of vortex dislocations and decreases the dominant frequency in the large cylinder wake. The identified changes in wake topology are related to substantial variations in the location of boundary layer separation on the large cylinder, and, consequently, changes in the size of the vortex formation region. The results also show that the Reynolds number has a substantial effect on wake vortex shedding frequency, which is more profound than that expected for a uniform cylinder.

Effects of freestream turbulence on bypass transition of separated boundary layer on low-pressure turbine airfoils

This paper presents experimental studies on bypass transition of separated boundary layer on low-pressure turbine airfoils, focusing on the effects of freestream turbulence on the transition process. Hot-wire probe measurements are performed on the suction side of an airfoil in the low-pressure linear turbine cascade at several Reynolds number conditions. Freestream turbulence is enhanced by use of turbulence grid located upstream of the cascade. The results of this experimental study show that the location of boundary layer separation does not strongly depend on the freestream turbulence level. However, as the freestream turbulence level increases, the size of separation bubble becomes small and the location of turbulent transition moves upstream. The size of separation bubble becomes small as the Reynolds number increases. At low freestream turbulence intensity, the velocity fluctuation due to Kelvin-Helmholtz instability is observed clearly in the shear layer of the separation bubble. At high freestream turbulence intensity, the streak structures appear upstream of the separation location, indicating bypass transition of attached boundary layer occurs at high Reynolds number.

Numerical and experimental investigation of transverse injection flows

The flow field resulting from a transverse injection through a slot into supersonic flow is numerically simulated by solving Favre-averaged Navier–Stokes equations with κ − ω SST turbulence model with corrections for compressibility and transition. Numerical results are compared to experimental data in terms of surface pressure profiles, boundary layer separation location, transition location, and flow structures at the upstream and downstream of the jet. Results show good agreement with experimental data for a wide range of pressure ratios and transition locations are captured with acceptable accuracy. κ − ω SST model provides quite accurate results for such a complex flow field. Moreover, few experiments involving a sonic round jet injected on a flat plate into high-speed crossflow at Mach 5 are carried out. These experiments are three-dimensional in nature. The effect of pressure ratio on three-dimensional jet interaction dynamics is sought. Jet penetration is found to be a non-linear function of jet to free stream momentum flux ratio.

Review of tennis ball aerodynamics

The aerodynamics of a tennis ball are reviewed here with reference to several wind tunnel measurement efforts. Measurements for a wide variety of tennis balls, including the ‘oversized’ balls, are presented. Flow visualization results have shown that the separation location on a non‐spinning tennis ball occurred relatively early, near the apex, and appeared very similar to a laminar separation in the subcritical Reynolds number regime. The flow regime (boundary layer separation location) appears to be independent of Reynolds number in the range, 167,000 < Re < 284,000. Asymmetric boundary layer separation and a deflected wake flow, depicting the Magnus effect, have been observed for the spinning ball. Aerodynamic force (drag and lift) measurements for nonspinning and spinning balls are reviewed for a wide range of Reynolds numbers and spin rates. Relatively high drag coefficients (CD ≅ 0.6 to 0.7), have been measured for new nonspinning tennis balls. The observed (unexpected) behavior of the tennis ball drag coefficient is explained in terms of a flow model that includes the drag contribution of the fuzz’ elements.

Experimental Study of the Effect of Periodic Unsteady Wake Flow on Boundary Layer Development, Separation, and Reattachment Along the Surface of a Low Pressure Turbine Blade

The paper experimentally studies the effects of periodic unsteady wake flow on boundary layer development, separation and reattachment along the suction surface of a low pressure turbine blade. The experimental investigations were performed on a large scale, subsonic unsteady turbine cascade research facility at the Turbomachinery Performance and Flow Research Laboratory (TPFL), Texas A&amp;M University. The experiments were carried out at a Reynolds number of 110,000 (based on suction surface length and exit velocity) with a free-stream turbulence intensity of 1.9%. One steady and two different unsteady inlet flow conditions with the corresponding passing frequencies, wake velocities, and turbulence intensities were investigated. The reduced frequencies cover the entire operating range of LP turbines. In addition to the unsteady boundary layer measurements, blade surface measurements were performed at the same Reynolds number. The surface pressure measurements were also carried out at one steady and two periodic unsteady inlet flow conditions. The results presented in ensemble-averaged, and the contour plot forms help to understand the physics of the separation phenomenon under periodic unsteady wake flow. It was found that the suction surface displayed a strong separation bubble for these three different reduced frequencies. For each condition, the locations and the heights defining the separation bubble were determined by carefully analyzing and examining the pressure and the mean velocity profile data. The location of boundary layer separation was independent of the reduced frequency level. However, the extent of the separation was strongly dependent on the reduced frequency level. Once the unsteady wake started to penetrate into the separation bubble, the turbulent spot produced in the wake paths caused a reduction of the separation bubble height.

Design and Testing of a Winglet Airfoil for Low-Speed Aircraft

The PSU 94-097 airfoil has been designed for use on winglets of high-performance sailplanes. The design problem is difficult because the airfoil must operate over a wide range of Reynolds numbers, and this range includes values that are relatively low. To validate the design tools, as well as the design itself, the airfoil was tested in the Penn State Low-Speed, Low-Turbulence Wind Tunnel from Reynolds numbers of 2.4 × 10 5 to 1.0 × 10 6 . In addition to transition-free measurements, potential drag reductions using artificial turbulators were explored, although the benefits were found to be limited for this application. Finally, performance predictions from two well-known computer codes are compared to the data obtained experimentally, and both are found to generate results that are in good agreement with the wind-tunnel measurements. Nomenclature CP pressure coefficient, (pl - p∞ )/q∞ L. lower surface R Reynolds number based on free-stream conditions and airfoil chord S. boundary-layer separation location, xS/c T. boundary-layer transition location, xT/c U. upper surface c airfoil chord cd profile-drag coefficient cl section lift coefficient cm section pitching-moment coefficient about the quarter-chord point p static pressure, Pa (lbf/ft 2

Effect of natural ventilation on the boundary layer separation and near-wake vortex shedding characteristics of a sphere

Experiments were conducted in water and wind tunnels on spheres in the Reynolds number range 6 × 103 to 6.5 × 105 to study the effect of natural ventilation on the boundary layer separation and near-wake vortex shedding characteristics. In the subcritical range of Re (<2 × 105), ventilation caused a marginal downstream shift in the location of laminar boundary layer separation; there was only a small change in the vortex shedding frequency. In the supercritical range (Re > 4 × 105), ventilation caused a downstream shift in the mean locations of boundary layer separation and reattachment; these lines showed significant axisymmetry in the presence of venting. No distinct vortex shedding frequency was found. Instead, a dramatic reduction occurred in the wake unsteadiness at all frequencies. The reduction of wake unsteadiness is consistent with the reduction in total drag already reported. Based on the present results and those reported earlier, the effects of natural ventilation on the flow past a sphere can be categorized in two broad regimes, viz., weak and strong interaction regimes. In the weak interaction regime (subcritical Re), the broad features of the basic sphere are largely unaltered despite the large addition of mass in the near wake. Strong interaction is promoted by the closer proximity of the inner and outer shear layers at supercritical Re. This results in a modified and steady near-wake flow, characterized by reduced unsteadiness and small drag.

Measurements in Separated and Transitional Boundary Layers Under Low-Pressure Turbine Airfoil Conditions

Detailed velocity measurements were made along a flat plate subject to the same dimensionless pressure gradient as the suction side of a modern low-pressure turbine airfoil. Reynolds numbers based on wetted plate length and nominal exit velocity were varied from 50,000 to 300,000, covering cruise to takeoff conditions. Low and high inlet free-stream turbulence intensities (0.2 and 7 percent) were set using passive grids. The location of boundary-layer separation does not depend strongly on the free-stream turbulence level or Reynolds number, as long as the boundary layer remains nonturbulent prior to separation. Strong acceleration prevents transition on the upstream part of the plate in all cases. Both free-stream turbulence and Reynolds number have strong effects on transition in the adverse pressure gradient region. Under low free-stream turbulence conditions, transition is induced by instability waves in the shear layer of the separation bubble. Reattachment generally occurs at the transition start. At Re=50,000 the separation bubble does not close before the trailing edge of the modeled airfoil. At higher Re, transition moves upstream, and the boundary layer reattaches. With high free-stream turbulence levels, transition appears to occur in a bypass mode, similar to that in attached boundary layers. Transition moves upstream, resulting in shorter separation regions. At Re above 200,000, transition begins before separation. Mean velocity, turbulence, and intermittency profiles are presented.

Examination of the flow near the leading edge of attached cavitation. Part 1. Detachment of two-dimensional and axisymmetric cavities

The flow near the cavity detachment region of stable attached cavitation was examined using qualitative and quantitative flow visualization. The non-cavitating and cavitating flows around a hydrophilic brass and hydrophobic Teflon sphere and cylinder were examined. The location of non-cavitating boundary layer separation and cavity detachment was related to the free-stream Reynolds and cavitation numbers. The shape of the cavity near the detachment was greatly affected by the material of the cavitating object. The cavity interface on the hydrophilic test objects curved downstream to form a forward facing step. A region of recirculating fluid existed upstream of the cavity interface. The cavity detachment on the hydrophobic test objects was much closer to the location of boundary layer separation. The forward facing step and the recirculating region were nearly absent.The measured flow field near the surface of the brass sphere, cylinder, and hydrofoils under cavitating and non-cavitating conditions was used to calculate the position of two-dimensional laminar boundary layer separation. Thwaites' and Stratford's methods were used to predict the location of boundary layer separation upstream of the cavity detachment. The predictions compared well with the observed position of separation.

K-e and Reynolds Stress Turbulence Model Comparisons for Two-Dimensional Injection Flows

Two-dimensional steady e owe elds generated by transverse injection into a supersonic e ow are numerically sim- ulated by integrating the Favre-averaged Navier -Stokes equations. Fine-scale turbulence effects are modeled with compressible K-≤ and second-order Reynolds-stress turbulence models. These numerical results are compared to numerical results of the Jones -Launder K-≤ model and experimental data. The credibility of the Reynolds-stress turbulence model relative to experimental data and other turbulence models is demonstrated by comparison of surface pressure proe les, boundary-layer separation location, jet plume height, and descriptions of recirculation zones and e ow structureupstream and downstream of the jet. Results indicate that theReynolds-stress turbulence model correctly predicts mean e ow conditions for low static pressure ratios. However, it is also observed that, as the static pressure ratio increases, the boundary-layer separation point moves farther upstream of the jet and pre- dictions become lessconsistent with experimental results. The K-≤ results are lessconsistentwith theexperimental resultsthanthoseassociatedwiththeReynolds-stressturbulencemodel.Finally,unlikethe K-≤results,nonphysical vorticity phenomena upstream of the jet plume are not observed in the Reynolds-stress turbulence model results. This phenomenon is shown to coincide with strong gradients in the wall functions used to compute πt.

Link between flow separation and transition onset

The relationship between the location of boundary-layer separation and transition onset in compressible subsonic flow is studied. The flow separation considered in this study is caused by a localized adverse pressure gradient that results from a single roughness element (a hump) on an otherwise smooth flat plate. The mean flow is computed with the interacting boundary-layer (IBL) approach, and a quasiparallel stability analysis of the resulting profiles is performed. The N-factor criterion with N=9 is used to correlate transition. The effects of unit Reynolds number, hump location, hump length, and freestream Mach number on the relationship between separation and transition location are evaluated. Our results support the customary assumption that the separation location can be taken as the transition onset location in a certain parameter space; however, this assumption is not valid in other ranges of the parameter space, particularly at low unit Reynolds numbers, for short humps, and at relatively high freestream Mach numbers

Development of flow structures in the lee of an inclined body of revolution

Laser-induced fluorescence has been utilised for visualising the flow structures behind an inclined body of revolution in a large towing tank. The observations have focused on the effects of the incidence angle and the Reynolds number on the axial development of the large-scale structures within the separated region, as well as the location of boundary-layer separation and reattachment. At low angles of incidence and low Reynolds numbers the flow is symmetric, containing two large centres (foci) of coalescing stream surfaces as well as several secondary structures. The transition to an asymmetric flow is observed to begin first in the downstream sections with the gradual detachment of the left primary sector and an inward motion of the right-hand centre. While detaching, the primary left structure becomes elongated as the stream surfaces break and coalesce into two distinct centres. The resulting two foci start interacting further downstream and eventually recombine. When the left centre of coalescence is completely detached, an additional centre forms in its place and the right-hand structure starts lifting away from the surface. Evidence of asymmetry very close to the tip of the model at 45 degree inclination is also provided. A series of qualitative drawings of the flow topology is provided. They follow the evolution of the flow structures in the lee of the model.

Transition from laminar to turbulent flow in separated shear layers

The location of boundary layer separation may be predicted by either semiempirical criteria or by various computer codes written for such calculations; attention is presently given to the streamwide length from separation to transition, denoted by Re(sL). It is demonstrated that the unit Reynolds number, a feature of the transition phenomenon implicit in the data of Schmidt et al. (1985) and overlooked in previous investigations, must be considered in future work.

On the effects of boundary-layer transition on a cylindrical afterbody at incidence in low-speed flow

The experimentally observed changes in the location of boundary layer separation on the cylindrical afterbody of a tangent-ogive-cylinder, placed at incidence in a uniform flow, are related to the phenomenon of laminar-turbulent transition. Existing knowledge of the transition mechanisms which operate in general, three-dimensional; boundary layers, is described and simple prediction criteria are derived for this configuration. These criteria are compared with the currently available empirical methods for the prediction of critical Reynolds numbers for such flows. It is demonstrated that the methods which have been published to date are incomplete and the accompanying analyses are based upon superficial and erroneous physical arguments. The implications of boundary layer transition on the normal forces generated by a cylindrical afterbody are also discussed. Finally, the possibility of using transition fixing to establish ‘high Reynolds number’ separated flows in ‘low Reynolds number’ wind-tunnel facilities is briefly considered.