Streamwise Position Research Articles

Statistical characterization of free-stream turbulence induced transition under variable Reynolds number, free-stream turbulence, and pressure gradient

In this work, the free-stream turbulence (FST) induced transition of a flat plate boundary layer is studied using particle image velocimetry (PIV) under variable Reynolds number (Re), FST intensity, and adverse pressure gradient (APG). Overall, 10 different flow conditions were tested concerning the variation of these parameters. The streak spacing and the probability density function (PDF) of turbulent spot nucleation are computed for all cases. The streak spacing is shown to be constant in the transition region once scaled with the turbulent displacement and momentum thickness, with resulting values of around 3 and 5, respectively. Nucleation events are shown to occur near the position where the dimensionless streak spacing reaches such constant values. The streamwise position where most turbulent spots are formed is strongly influenced by the FST intensity level. Additionally, the PDF of spot nucleation becomes narrower with increase in the APG, while FST has the opposite effect. A common distribution of all the PDFs is provided as a function of a similarity variable accounting for the streak spacing, the shape factor of the boundary layer, and the FST intensity.

INVESTIGATIONS ON THE CONTROL OF WAKES DOWNSTREAM OF A ROTARY OSCILLATING PLATE

The vortex wake of a flutter bridge deck can be simulated by the flow across a forced rotary oscillating plate. Two narrow strips of width ratio b/H = 0.33 are set symmetrically on the upper and lower sides of an oscillating plate of chord to thickness ratio B/H = 5, to suppress synchronized vortex shedding in the wake. The method of numerical simulation and experimental validation is used, and the ranges of amplitude and frequency of oscillation investigated are β = 0° ~ 10° and feH/V∞ = 0 ~ 0.0857 respectively, and the Reynolds number Re = V∞H/ V∞ = 2800, where V∞ is velocity of on-coming flow. Three kinds of stream-wise strip positions, i.e. the front edge, mid-chord and trailing edge of the plate are studied respectively, with transverse location y/H of the strip as varying parameter. The results of experiment demonstrate that, in a certain range of strip location y/H, and β = 0° ~ 7.5°, feH/V∞ = 0 ~ 0.08, the peak to peak ratio of power spectra of fluctuating velocities in the wakes with and without control can be much lower than 1, and the minimum is about 0.3. The results of simulation show that, in β = 0° ~ 7.5° and a certain range of feH/V∞, the root mean square values of fluctuating torque and lift of the plate can be considerably reduced, and the top reductions are 43% and 80% respectively, if the mid-chord strip position is in the vicinity of y/H = ±1. The 1st and 2nd eddy viscosity coefficients are introduced to link the normal and shear turbulent stresses in the wake with the gradients of amplitudes of the perturbation velocities, and a linear stability equation is derived. Stability analysis indicates that, the maximum amplification factor of perturbation ωi max can be drastically reduced, and the frequency range of perturbation with maximum growth rate is substantially narrowed by the control. The application of the strips alters the velocity profiles and promotes the eddy viscosity, therefore weakens the instability of the wake.

Relations between shear flow and vortices in the near wake of a high speed train

Two flow cases for high speed train models with different lengths have been numerically computed by performing the improved delayed detached-eddy simulation. Based on the Omega method and turbulence production (TP) distribution, the relations between the shear flow and vortices in the near turbulent wake of a high speed train have been comparatively analyzed. First, in the wake region immediately close to the tail, the boundary layer separation plays significant roles. And the mechanism makes shear deformation prominent in the region with the formed vortices. Moreover, the shear layers are pertinent to the boundary-layer thicknesses and help to the TP distribution. However, the shear-dominated region is very limited due to high dissipation. One the other hand, a vast majority of the vortices captured with the parameter Omega increasing in the downstream region away from the tail. And the TP distributions are stable at different streamwise positions, though obviously decreased. They are greatly attributed to the mean strain rate in the horizontal plane. Meanwhile, the vortical vorticity is thought to be the dominant component inside the vortex cores, although the shear becomes weaker. And thus the turbulence itself can be spatially sustained due to the relatively stable vortex structure. Moreover, the weak shear is believed to depend upon the interaction between the vortices and the ground.

Explicit Algebraic Reynolds-Stress Modeling of Pressure-Induced Separating Flows in the Presence of Sidewalls

AbstractReynolds-averaged Navier–Stokes (RANS) approach is used to simulate the steady-state of a family of pressure-induced turbulent separation bubbles in the presence of sidewalls. Different turbulence models are employed with a specific emphasis on the baseline explicit algebraic Reynolds stress model (BSL-EARSM) and the simulations are compared with experimental data. The separation and reattachment of a flat-plate turbulent boundary layer are generated through a combination of adverse and favorable pressure gradients (APG-FPG) by numerically reproducing the geometry of the wind-tunnel test section used for the experiments. Three cases are considered a large (LB) and a medium (MB) bubble presenting mean backflow, and a small bubble (SB) without mean-flow reversal. This is achieved by varying the streamwise position of the APG/FPG transition. Good agreement between the BSL-EARSM-computed solutions and the experimental results are obtained for wall-pressure and skin-friction distributions on the centerline plane of the test section as well as for the overall three-dimensional flow topology. However, both detachment and reattachment are predicted too early and the bubble length is slightly overestimated for cases LB and MB. For case LB, the streamwise Reynolds stress is estimated fairly well but its production is somewhat delayed. Normal and shear stresses are in good agreement with the experiments in the upstream part of the bubble but are significantly over-estimated in the reattachment region. The k−ω shear-stress transport (SST) model with the so-called reattachment modification performs better than the other tested linear-eddy-viscosity models but it is still unable to reproduce accurately the three-dimensional flow topology even for the “simplest” case SB. Overall, the results suggest that BSL-EARSM is the most suitable turbulence model for this flow configuration.

Windmilling Characteristics of a Contra-Rotating Fan

Abstract In the present experimental study, a low aspect ratio, low hub-tip ratio contra-rotating axial fan is investigated to understand its performance under windmilling conditions. Two configurations are tested; in the first configuration (event A), the front rotor of the contra-rotating fan is powered and the rear rotor is allowed to windmill; in the second configuration (event B), the rear rotor of the contra-rotating fan is powered and the front rotor is allowed to windmill. The spanwise distribution of the loading coefficient and the flow angles at different streamwise positions reveal the details of the flow development across the rotors. Though the average total pressure drops across the windmilling rotor for both the events, a small spanwise region behaves as a fan or a stirrer. Thus, a “neutral radius” on the windmilling rotor is identified for both events A and B. The neutral radius appears close to the tip for event A and it appears close to the hub for event B. Thus, the span regions close to the tip behave as a fan for event A and the span regions close to the hub behave as a stirrer for event B. It is also observed that the neutral radius shifts to a lower span location as the flow coefficient reduces. Thus, the flow coefficient is a significant parameter that decides the position of the neutral radius. Further, the unsteady pressure measurements recorded at the casing capture the fundamental phenomena during the stall inception. The paper thus relates the similarities and unveils the contrasting features of the windmilling events A and B. In summary, this paper discusses the performance, flow physics, and stall inception characteristics of a contra-rotating axial fan under windmilling conditions.

Topological modifications due to ramped vanes in a flare-induced shock–boundary layer interaction flowfield

Effect of ′ramped vane′-type vortex generators on a shock-induced flow separation in the vicinity of an axisymmetric compression corner was evaluated. Numerical simulations were performed at Mach 2 on a cone–cylinder–flare model with a flare angle of 24°. The undisturbed boundary layer thickness (δ) at the location of the compression corner was 5 mm. A single array of these vortex generators with a device height of 0.28δ (1.4 mm) was placed on the cylinder surface at different streamwise positions, viz. 5δ, 10δ and 15δ upstream of the compression corner, and their ability to manipulate the shock–boundary layer interaction flowfield was compared. The presence of these devices caused substantial changes in the interaction region and the separation bubble structure. The separation bubble transformed into a series of spade-shaped structures with pockets of attached flow in between them. The ramped vanes increased the separation length along the device centreline, but this effect was attenuated considerably, by bringing them closer to the interaction region. Moving the ramped vanes closer also had a collapsing effect on the spade-shaped structures, which simultaneously widened the attached flow zones in between them.

Investigation of influence of compact actuators on air cavity in water flow under recessed hull

Air cavity drag reduction is one promising method for reducing power consumption of ships. Its current practical applications are rather limited, owing largely to the fact that air cavity size and shape change drastically in response to variations in ship attitude, motions and speed, as well as sea conditions. This study explores how deployment of moveable hydrodynamic actuators near the air cavity on a small-scale simplified hull form can effectively increase the air cavity size in adverse hull positions. Experimentally investigated actuators included an adjustable plate in the front part of an air cavity, a stern spoiler, and a hydrofoil with regulated attack angle and streamwise position beneath the hull. In the cases of significant hull trims that are challenging for maintenance of long air cavities, optimal actuator placement increased cavity length by nearly 110% from its degraded state at negative trim and by 24% at positive trim. Actuator effects were more pronounced at higher water speeds.

Simulation of Surface Pressure Fluctuations on an Airbus-A320 Fuselage at Cruise Conditions

The fuselage surface pressure fluctuations on an Airbus-A320 aircraft at cruise conditions are simulated by solving a Poisson equation. The right-hand-side source terms of the Poisson equation, including both the mean-shear term and the turbulence–turbulence term, are realized with synthetic turbulence generated by the fast random particle-mesh method. The stochastic realization is based on time-averaged turbulence statistics derived from a Reynolds averaged Navier–Stokes simulation under the same condition as in the flight tests conducted with DLR, German Aerospace Center’s Airbus-A320 research aircraft. The fuselage surface pressure fluctuations are calculated at three streamwise positions from front to rear, corresponding to the measurement positions in the flight tests. Features of the pressure fluctuations relevant to the fuselage surface excitation are obtained and analyzed. A good agreement for one- and two-point spectral characteristics of the surface pressure fluctuations is found between the simulation and flight-test results.

Numerical study of the suction flow control of the supersonic boundary layer transition in a framework of gas-kinetic scheme

In this paper, we numerically study the supersonic boundary layer transition process and the suction flow control method by employing the DNS technology. By adopting the high resolution GKS method, the mechanism of the boundary transition and the statistical characteristics of the compressible turbulent boundary layer are investigated. Results indicate that the streamwise vortex and the three-dimensional shear layer with high instability are the main causes of the boundary layer transition. Suction control cases show that the laminar boundary layer suction can weaken the development of the unstable disturbance waves in the boundary layer, and delay the streamwise position of the local disturbance. Also, the space distributions of the vortex structure are weakened to delay the process of the transition.

Amplitude Scaling of Wave Packets in Turbulent Jets

This paper studies the amplitude of large-scale coherent wave-packet structures in jets, modeled by the parabolized stability equations (PSEs). Linear PSEs can retrieve the shape of the wave packets, but linearity leads to solutions with a free amplitude, which has traditionally been obtained in an ad hoc manner using limited data. We systematically determine the free amplitude as a function of frequency and azimuthal wave number by comparing the fluctuation fields retrieved from PSEs with coherent structures educed from large-eddy simulation data using spectral proper orthogonal decomposition. The wave-packet amplitude is shown to decay exponentially with the Strouhal number for axisymmetric and helical modes at both Mach numbers considered in the study: 0.4 and 0.9. Analytical fit functions are proposed, and the scaled wave packets provide reasonable reconstructions of pressure and velocity spectra on the jet centerline and lip line over a range of streamwise positions.

Using shock control bumps to improve engine intake performance and operability

AbstractShock control bumps can be used to control and weaken the shock waves that form on engine intakes at high angles of attack. In this paper, it is demonstrated how shock control bumps applied to an engine intake can reduce or eliminate shock-induced separation at high incidence, and also increase the incidence at which critical separation occurs. Three-dimensional Reynolds-average Navier–Stokes (RANS) simulations are used to model the flow through a large civil aircraft engine intake at high incidence. The variation in shock strength and separation with incidence is first studied, along with the flow distribution around the nacelle. An optimisation process is then employed to design shock control bumps that reduce shock strength and separation at a fixed high incidence condition. The bump geometry is allowed to vary in shape, size, streamwise position and circumferential direction around the nacelle. This is shown to be key to the success of the shock control geometry. A further step is then taken, using the optimisation methodology to design bumps that can increase the incidence at which critical separation occurs. It is shown that, by using this approach, the operating range of the engine intake can be increased by at least three degrees.

Eulerian–Lagrangian direct numerical simulation of preferential accumulation of inertial particles in a compressible turbulent boundary layer

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Measurements of pressure and velocity fluctuations in a family of turbulent separation bubbles

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Numerical investigations on the effect of convex-dimple streamwise arrangements on the flow and heat transfer characteristics of rectangular convex-dimple-grooved channels

In this article, the effects of convex dimple streamwise arrangements on the flow and heat transfer characteristics of protrusion-grooved channels are determined, and five streamwise positions of convex dimples are considered. In addition, research on the transverse-circular-protrusion-grooved channel is also performed to serve as a contrast. The results indicate that replacing the transverse-circular-protrusion with the convex dimple produces a counter-rotating vortex pair, and the streamwise arrangements of the convex dimple significantly affect the flow structure downstream of the convex dimples. As the convex dimple is moved downstream, the velocities near the center protrusion-grooved surface increase till a maximum occurs for the case of a convex dimple placed at the center between adjacent grooves, and then decrease. Compared with the case of transverse-circular-protrusion, the convex dimple at the center region can enhance the overall heat transfer rate, while if it approaches the adjacent grooves lower heat transfer rate appears. Besides, replacing the transverse-circular-protrusion with the convex dimple can significantly reduce the pressure loss penalty and enhance the thermal performance factor. In addition, the effect of streamwise arrangements of convex dimples on the friction factor is slight, but the case with the center convex dimple exhibits the best overall thermal performance.

Evaluation of Atmospheric Boundary Layer in Open-Loop Boundary Layer Wind Tunnel Experiment

The present work discussed the development of the quasi-atmospheric boundary layer in an open-loop boundary layer wind tunnel over a smooth surface. The working section of the wind tunnel which is 1 m high and 9 m long is divided into three parts of 3 m long each. A constant temperature anemometer (CTA) hot wire was used to measure the flow velocity inside the test section. The wind speed of the wind tunnel was set at 10 m/s. The measurement was performed in three streamwise positions located in the three respective parts. The profiles of the streamwise velocity, standard deviation, and skewness that were obtained at the three streamwise positions revealed that the boundary layer height was developing from the upstream to the downstream positions in the wind tunnel. Additionally, flow uniformity and turbulence intensity of the inflow condition that was obtained in the first part of the test section were 7.1% and 6.4%, respectively.

On the maintenance of an attached leading-edge vortex via model bird alula

Researchers have hypothesized that the post-stall lift benefit of bird's alular feathers, or alula, stems from the maintenance of an attached leading-edge vortex (LEV) over their thin-profiled, outer hand-wing. Here, we investigate the connection between the alula and LEV attachment via flow measurements in a wind tunnel. We show that a model alula, whose wetted area is 1% that of the wing, stabilizes a recirculatory aft-tilted LEV on a steadily-translating unswept wing inclined at post-stall incidences. The attached vortex is the result of the alula's ability to smoothly merge otherwise separate leading- and side-edge vortical flows. We identify two key processes that facilitate this merging: i) the steering of spanwise vorticity generated at the wing's leading edge back to the wing plane and ii) an aft-located wall-jet of high-magnitude root-to-tip spanwise flow (>80% that of the freestream velocity). The latter feature induces LEV roll-up while the former feature tilts LEV vorticity aft and evacuates this flow toward the wing tip via an outboard vorticity flux. We identify the alula's streamwise position (relative to the leading-edge of the thin wing) as important for vortex steering and the alula's cant angle as important for high-magnitude spanwise flow generation. These findings advance our understanding of the likely ways bird's leverage LEVs to augment slow flight.

Dynamics and scale analysis of the transient convective flow induced by cooling a Pr

The convective boundary layer flow as well as the subsequent intrusion flow induced by cooling an initially isothermal and quiescent fluid at Pr<1 using a linear thermal forcing is investigated with a scaling analysis in the present study. It is demonstrated that the thickness of the convective boundary layer flow does not depend on the streamwise position in both the initial growth and the fully developed states, whilst the characteristic velocity is constantly streamwise location-dependant in these two states. It is also shown that the oscillatory behaviour of flow parameters in the transitional state of the boundary layer flow is dramatically weaker than the classical homogenously thermal forcing problems. Four possible flow regimes are identified for the cold intrusion flow and it is revealed that their occurrence is unconditional, which dramatically differentiates from the flow associated with Pr>1 fluids. It is further found that the first flow regime, i.e. the unsteady viscous-buoyancy dominance, is ephemeral in nature and can therefore be ignored. The intrusion flow can thus be practically divided into three flow regimes: an unsteady inertia-buoyancy regime; a steady inertia-buoyancy regime; and a steady viscous-buoyancy flow regime. The scaling relations quantifying the convective flow in different flow states are obtained. Selected scales are validated against the DNS (Direct Numerical Simulation) results and a good agreement is achieved.

Analysis of the factors contributing to the skin friction coefficient in adverse pressure gradient turbulent boundary layers and their variation with the pressure gradient

This paper reports on a study of the various factors contributing to the skin friction in incompressible adverse pressure gradient turbulent boundary layer (APG-TBL) flows. Specifically, it deals with the contributions to the skin friction coefficient from the Reynolds stresses and the viscous effects and the role of the pressure gradient. The skin friction coefficient is calculated based on the theoretical decomposition for mean skin friction generation introduced by Renard and Deck (2016). This decomposition is compatible with spatially developing flows as it is applicable to every local streamwise position. The turbulent flows are generated through the direct numerical simulation of a TBL on a smooth flat plate with the desired farfield pressure gradient. It is observed that the Reynolds shear stress provides the dominant positive contribution to the skin friction coefficient for all the pressure gradient cases. However, with increasing adverse pressure gradient, the skin friction coefficient continues to decrease and approaches zero as the positive contribution from the Reynolds shear stress is diminished by the negative contribution of the pressure gradient. When the flow reaches the verge of separation, the predominant Reynolds shear stress contribution to the skin friction coefficient is from a spatially localized outer peak at an approximate height of the displacement thickness (y=δ1) which coincides with the inflection point of the mean streamwise velocity. Even though, the decompositions in Renard and Deck (2016) and Fukagata et al. (2002) give a different distribution for the skin friction coefficient in the zero pressure gradient (ZPG) and the mild APG cases, both of the identities capture the dominant outer peak of the Reynolds shear stress contribution when the flow reaches the verge of separation. This emphasizes the growing importance of the outer layer dynamics with increasing pressure gradient as it pertains to skin friction generation.

Space-time correlations of velocity in a Mach 0.9 turbulent round jet

In a turbulent jet, the numerical investigation of space-time correlations C(r, τ) at two-point and two-time of streamwise fluctuating velocities is presented along the nozzle lipline. Large-eddy simulation (LES) is performed for a Mach 0.9 turbulent jet issuing from a round nozzle. The turbulent boundary layer is well developed at the nozzle outlet, upon the inner wall, by adopting synthetic turbulent inlet boundary conditions. We study the cross correlations of streamwise fluctuating velocities at three particular streamwise positions, i.e., x = 0.71, 7.03, and 34.47r0, corresponding to different stages of jet development, where r0 is the radius of the nozzle. Present results show that the classical Taylor’s frozen-flow model is unable to predict C(r, τ) accurately in this strongly spatially developing shear flow since the distortion of the flow pattern is missing. The isocorrelation contours of C(r, τ) show a clearly elliptical feature, which is found to be well predicted by the elliptic approximation (EA) model [G.-W. He and J.-B. Zhang, “Elliptic model for space-time correlations in turbulent shear flows,” Phys. Rev. E 73, 055303 (2006)]. According to the EA model, C(r, τ) has a scaling form of C(rE, 0) with two characteristic velocities U and V, i.e., rE = (r − Uτ)2 + V2τ2. By examining LES data, it is found that the characteristic velocity U determined in LES is in general consistent with the theoretical Ut in the EA model, while the trend of V in LES also matches with that of the theoretical Vt. Additionally, it is interesting that the ratio of V to Vt is approximately a constant V/Vt ≃ 1.3 in the turbulent jet.

Influence of streamwise position of crescent-shaped block on flat-plate film cooling characteristics

Film cooling injection from discrete circular holes is widely used in gas turbines to reduce the heat load of the turbine blades. Placing a special designed structure upstream or downstream, the circular hole has been proven to be a new effective way to improve cooling effectiveness in recent years. In the present study, numerical investigations were performed on a row of circular holes over a flat plate with crescent-shaped blocks. Totally seven configurations without block, with blocks located at six streamwise positions including 6 times, 5 times, 4 times of hole diameter upstream, and 0.5 times, 1.5 times, 2.5 times of hole diameter downstream at blowing ratios of 0.5–1.5 were tested symmetrically. The Reynolds-averaged Navier–Stokes equations with the k–ω shear-stress transport model were solved. Flow fields, cooling effectiveness and aerodynamic losses were analyzed in detail. Although with different generation mechanism, additional vortex pair opposite to the counter-rotating vortex pair was generated by placing either an upstream or downstream block, which could finally improve the coolant lateral coverage and thus cooling effectiveness. The cooling performances and aerodynamic losses between configurations without block and with blocks at six streamwise positions were then compared. Finally, the optimal streamwise position of the block was recommended at various blowing ratios.