Effects Of Adverse Pressure Gradient Research Articles

A study of the effect of curvature and pressure gradient on aerodynamics performance and turbulence structure of S-shaped compressor transition duct

ABSTRACT The overall performance of the high-pressure compressor in a turbofan engine entirely depends on the flow supplied by the upstream transition duct. A robust and efficient design of the transition duct conveys a highly uniform flow with minimum pressure loss. The present study aimed to determine the effect of adverse pressure gradient and curvature on the flow of the compressor duct. The results suggest that the outer wall curvature causes a significant decrease in from +0.19 to −0.6, and as a result, 75% length of the outer wall is suspended in the favourable pressure gradient. On the contrary, it increases from −0.54 to +0.14 along the inner wall. It implies that most of the inner wall lies in the adverse pressure gradient; consequently, the turbulence structure, mean flow behaviour, and Reynolds stresses are considerably modified. The presence of a strong adverse pressure gradient significantly enhances turbulence quantities along the inner wall. Additionally, the boundary layer parameters grow relatively thicker along the inner wall than on the outer wall. As a result, a considerable increase in the shape parameter (H = 2.12) is obtained.

On the effect of adverse pressure gradients on wall-pressure statistics in a controlled-diffusion aerofoil turbulent boundary layer

Wall-pressure and velocity statistics in the turbulent boundary layer (TBL) on a cambered controlled-diffusion aerofoil at $8^{\circ }$ incidence, a Mach number of 0.25 and a chord-based Reynolds number ${Re}_c=1.5\times 10^{5}$ are analysed at four locations on the suction side with zero and adverse pressure gradients (ZPG and APG), characterised by increasing Reynolds numbers based on momentum thickness, ${Re}_{\theta }=319$ , 390, 877 and $1036$ . The strong APG yields a highly non-equilibrium TBL at the trailing edge that significantly affects the turbulent flow statistics. Different normalisations of the full wall-pressure statistics involved in trailing-edge noise are analysed for the first time in such strong APG with convex curvature, and compared with available experimental and numerical data. Good overall agreement is found in the ZPG region, and most results obtained in previous APG TBL can be extended to the present highly non-equilibrium case. The presence of strong APG augments the intensity of wall-pressure fluctuations noticeably at low frequencies, shortens the streamwise and broadens the spanwise coherence of wall-pressure fluctuations in both time and space, and significantly reduces the convection velocity. The wall-pressure power spectral density are found to scale with the displacement thickness, the Zaragola–Smits velocity and the root-mean-squared pressure, the latter possibly being replaced by the local maximum Reynolds shear stress. The other two key parameters to trailing-edge noise modelling, the spanwise coherence length and the convection velocity, rather scale with displacement thickness and friction velocity, respectively.

Investigation of the unsteady flow in a transonic axial compressor adopted in the compressed air energy storage system

During operation, compressed air energy storage systems should respond rapidly to variations in power network demand, requiring that the compression system should always be in changeable off-design conditions. Compression systems with low flow rates confront difficulties such as diminished aerodynamic performance and increased flow losses. Given that the emergence of compressor instability is a process of the accumulation and intensification of flow unsteadiness. In this study, a numerical simulation was conducted for a transonic compressor to investigate the unsteady evolution characteristics of the tip leakage flow field, and it focused on the flow field of a condition where the flow unsteadiness was stimulated and further investigated the origin of flow unsteadiness, which was supposed to provide insights in developing flow control strategies and enable the application in a compressed air energy storage system under low mass flow rate conditions. The Fourier fast transformation (FFT) signal analysis showed that characteristic frequency of the unsteady tip flow field was 1863.3 Hz. Unsteady disturbances were predominantly distributed in the following three regions: the leakage vortex region of the blade suction surface, the location of the epitaxially detached shock wave at the entrance of passage, and the 5 %–30 % chord of the blade pressure surface. The high disturbance region of leakage vortex was generated by the periodic shift in leakage vortex intensity over time, the interaction between the main flow and various types of secondary flow due to leakage vortex breaking processes was the primary cause for the disturbance region of the epitaxially detached shock wave at the entrance of passage, and the high disturbance region of blade pressure surface was caused by the adverse pressure gradient effect and flow direction deflection of the airflow in the passage, as well as the impacted on the pressure surface of the adjacent blade. It was speculated that leakage vortex breakdown due to vortex wave interference was the key factor to induce the unsteadiness of the flow field. A new vortex structure was formed at the leading edge of adjacent blades after leakage vortex breakdown, which was defined as a “leading edge vortex”, and its series of evolutionary processes occurring timely were the main reasons for the unsteady flow in the tip flow field. This study laid a theoretical foundation for the research of the flow field control strategy of axial compressor operating in off-design conditions in a large-scale compressed-air energy storage system.

A driving mechanism of near-wall turbulence subject to adverse pressure gradient in a plane Couette flow

The effect of adverse pressure gradient (APG) on near-wall turbulence is studied, with a particular attention to the turbulence production mechanism. A plane turbulent Couette flow is considered for several values of constant APG in the lower wall region. A direct numerical simulation (DNS) in a large computational domain shows that turbulence near the lower wall continues to exist even at sufficiently large APGs. On increasing the APG, the cross-streamwise turbulence intensities increase, and the near-wall streaks gradually disappear. A linear analysis using the optimal transient growth indicates that the APG inhibits the generation of near-wall streaks due to the significant reduction of the mean shear in the region near the lower wall. The turbulent fluctuation dynamics beyond the linear regime is studied with a DNS in a minimal flow unit. The near-wall self-sustaining process involving streaks is significantly weakened or destroyed as APG increases, while the turbulent fluctuations become more isotropic and localised. Using a conditional averaging analysis, a new mechanism of near-wall turbulence production under strong APG is uncovered. This mechanism is initiated by the wall-normal nonlinear transport of an outer wall-normal velocity fluctuation to the near-wall region. The transported wall-normal velocity fluctuation is subsequently amplified via the Orr mechanism, resulting in the non-zero turbulence production involving spatially localised vortical structures. This mechanism is also confirmed by DNS of the flow in a large computational domain, where strong correlation between the wall-normal nonlinear transport and turbulence production is observed.

Boundary Layer Flashback Model for Hydrogen Flames in Confined Geometries Including the Effect of Adverse Pressure Gradient

Abstract The combustion properties of hydrogen make premixed hydrogen-air flames very prone to boundary layer flashback. This paper describes the improvement and extension of a boundary layer flashback model from Hoferichter et al. (2017, “Prediction of Confined Flame Flashback Limits Using Boundary Layer Separation Theory,” ASME J. Eng. Gas Turbines Power, 139(2), p. 021505) for flames confined in burner ducts. The original model did not perform well at higher preheat temperatures and overpredicted the backpressure of the flame at flashback by 4–5×. By simplifying the Lewis number-dependent flame speed computation and by applying a generalized version of Stratford's flow separation criterion (Stratford, 1959, “The Prediction of Separation of the Turbulent Boundary Layer,” J. Fluid Mech., 5(1), p. 1), the prediction accuracy is improved significantly. The effect of adverse pressure gradient flow on the flashback limits in 2 deg and 4 deg diffusers is also captured adequately by coupling the model to flow simulations and taking into account the increased flow separation tendency in diffuser flow. Future research will focus on further experimental validation and direct numerical simulations to gain better insight into the role of the quenching distance and turbulence statistics.

Evolution Characteristics of Separated Vortices and Near-Wall Flow in a Centrifugal Impeller in an Off-Designed Condition

Flow separation is undesirable and lowers the efficiency of centrifugal impellers. In this study, the evolution characteristics of separated vortices in a centrifugal impeller are studied under the off-designed flow rate condition. Unsteady Reynolds-Averaged Navier–Stokes (URANS) with standard k-ε turbulent model is applied to simulate the alternating stall in the six-blade centrifugal impeller. We present and analyze the distributions of pressure gradient (either adverse or favorable) and skin friction coefficients on both sides of the blade for the stalled and unstalled passages to study the relationship between pressure gradient and separation of boundary layer flow. The evolution of skin friction coefficient is also presented at various axial cross sections. Numerical results reveal that, for the stalled passage, the increase in adverse pressure gradient on the pressure surface near the middle of the blade (S/S0 = 0.4) is much larger than that of the suction surface during a vortex formation cycle. The skin friction coefficient on the pressure surface also increases in magnitude sharply and the variation shows a peak-valley trend, while the coefficient on the suction surface increases slowly. Comparing the distribution of skin friction coefficient on the pressure surface of the same blade at different axial cross sections, it is found that the skin friction coefficient notably increases at S/S0 = 0.6 on the middle axial cross section (Z/b2 = 0.5). For the unstalled passage, both the pressure and suction surfaces produce favorable pressure gradients. The skin friction coefficient on the pressure surface shows an increasing trend around S/S0 = 0.5, and a large vortex can be seen at the exit of the impeller. The variation of skin friction coefficient on the suction surface is relatively mild; thus, the flow is relatively stable. It is clarified that the effect of adverse pressure gradient and wall shear stress jointly cause separation of the boundary layer; thus, the separated vortices are generated in the rotating impeller and deteriorate the performance of the impeller.

A numerical study over the effect of curvature and adverse pressure gradient on development of flow inside gas transmission pipelines

In order to inspect the effects of adverse pressure gradient (APG) and curvature on the corrosion in the main gas transmission pipelines, the mean velocity and turbulence boundary layer on the walls of six different geometries including a straight pipe (A), a convex pipe (B1), a concave pipe (B2), a convex diffuser (C1), a concave diffuser (C2) and a straight diffuser (D) are calculated. The values of pressure gradient and curvature parameters are chosen 0.62 and 0.023, respectively. The results indicate that the corrosion in the concave curvature is greater than that the corrosion in the convex curvature, since the turbulence intensity increases in the concave curvature while it is suppressed in the convex curvature. In addition, when the boundary layer is exposed to APG and concave curvature simultaneously, the corrosion is greater than the situation when there is only APG or concave curvature. Ultimately, it was found that for the six studied geometries, there are two flow regimes (for velocities lower than 36 m/s) that the turbulence quantities are dependent on velocity. However, for velocities more than 36 m/s, the turbulence quantities and other affected quantities in the shear region are independent.

Effect of adverse pressure gradients on turbulent wing boundary layers

The characteristics of turbulent boundary layers (TBLs) subjected to adverse pressure gradients are analysed through well-resolved large-eddy simulations. The geometries under study are the NACA0012 and NACA4412 wing sections, at and angle of attack, respectively, both of them at a Reynolds number based on inflow velocity and chord length of . The turbulence statistics show that adverse pressure gradients (APGs) have a significant effect on the mean velocity, velocity fluctuations and turbulent kinetic energy budget, and this effect is more prominent on the outer region of the boundary layer. Furthermore, the effect of flow history is assessed by means of an integrated Clauser pressure-gradient parameter (Vinuesa et al., Flow Turbul. Combust., vol. 99, 2017, pp. 565–587), through the study of cases with matching local values of and the friction Reynolds number to isolate this effect. Our results show a noticeable effect of the flow history on the outer region, however the differences in the near-wall peak of the tangential velocity fluctuations appear to be mostly produced by the local APG magnitude. The one-dimensional power-spectral density shows energetic small scales in the outer region of APG TBLs, whereas these energetic scales do not appear in zero-pressure-gradient (ZPG) TBLs, suggesting that small scales near the wall are advected towards the outer layer by the APG. Moreover, the linear coherence spectra show that the spectral outer peak of high-Reynolds-number ZPG TBLs is highly correlated with the near-wall region (Baars et al., J. Fluid Mech., vol. 823, 2017, R2), unlike APG TBLs which do not show such a correlation. This result, together with the different two-dimensional spectra of APG and high-Reynolds-number ZPG TBLs, suggests different energisation mechanisms due to APG and increase in Reynolds number. To the authors’ knowledge, this is the first in-depth analysis of the TBL characteristics over wings, including detailed single-point statistics, spectra and coherence.

Empirical Model of Wall Pressure Spectra in Adverse Pressure Gradients

An empirical model of wall pressure spectra for adverse pressure gradient boundary layers is proposed. The effect of adverse pressure gradients on wall pressure spectra and the relationship between the boundary-layer parameters and the wall pressure are studied and demonstrated. A dataset for five experiments at four different test facilities, covering Reynolds number between and based on the boundary-layer momentum thickness, is selected to develop the new model. Goody’s model is served as the basis for the development. Predictions of the new model and comparisons with other published wall pressure spectral models for adverse pressure gradient boundary layers are made for the selected dataset. The new model shows good prediction accuracy for the selected dataset and a significant improvement compared to the other published models.

Karakteristik Aliran Fluida di Dalam Asymmetric Diffuser dengan Penambahan Vortex Generator

A numerical investigation of asymmetric diffuser which equipped with vortex generator is presented. The investigation is aimed to know fluid flow characteristic in asymmetric diffuser which equipped with vortex generator on the up side of its diverging wall and to determine the effectiveness of using vortex generator on diffuser performance. Numerical investigation method was done by using Computational Fluid Dynamics (CFD) method with Fluent 6.3.26 software and with software of GAMBIT 2.4.6 to designed geometry and discriminated (meshing) it. The procedures performed on numerical investigation were pre-processing, solving, and post-processing stages. The results show that the flow of fluid simulated shear stress transport (SST) k-ω did not suffer a deflection toward the straight wall shortly at the time when entering the diffuser. Standard k-ε predicted the flow was deflected when entering the diffuser. Addition of vortex generator was not able to reduce the amount of adverse pressure gradient effect that occurred within the diffuser so using vortex generator was not effective to improve diffuser performance.

Correlation-Based Riblet Model for Turbomachinery Applications

An empirical riblet model for manufactured V-shaped and trapezoidal riblets which is suitable for turbomachinery application is presented. The implementation of the riblet effect employs a correlation-based correction for the damping of the specific dissipation rate ω in the vicinity of the wall which has been previously presented by other researchers. In the current paper, the correlations are extended into the drag-increasing regime and are extended to account for the effect of misalignment of the riblets relative to the flow and for the effect of adverse pressure gradients. In order to account for the latter in modern, massive parallel Reynolds-averaged Navier–Stokes (RANS) codes, a local Clauser parameter has been newly derived. The model is implemented in a three-dimensional (3D) turbomachinery design code and validated with flat plate measurement data and a NACA6510 compressor cascade. The predictions of the experimental values are in very good agreement with the experimental data, showing the capability of the model for designing riblet structured turbomachinery blading.

Structure of Turbulent Flows Over Forward Facing Steps With Adverse Pressure Gradient

This paper reports an experimental study on the effects of adverse pressure gradient (APG) and Reynolds number on turbulent flows over a forward facing step (FFS) by employing three APGs and three Reynolds numbers. A particle image velocimetry (PIV) technique was used to conduct velocity measurements at several locations downstream, and the flow statistics up to 68 step heights are reported. The step height was maintained at 6 mm, and the Reynolds numbers based on the step height and freestream mean velocity were 1600, 3200, and 4800. The mean reattachment length increases with the increase in Reynolds number without the APG whereas the mean reattachment length remains constant for increasing APG. The proper orthogonal decomposition (POD) results confirmed that higher Reynolds numbers caused the large-scale structures to be more defined and organized close to the step surface.

Direct numerical simulation of the flow around a wing section at moderate Reynolds number

A three-dimensional direct numerical simulation has been performed to study the turbulent flow around the asymmetric NACA4412 wing section at a moderate chord Reynolds number of Rec=400,000, with an angle of attack of AoA=5∘. The mesh was optimized to properly resolve all relevant scales in the flow, and comprises around 3.2 billion grid points. The incompressible spectral-element Navier–Stokes solver Nek5000 was used to carry out the simulation. An unsteady volume force is used to trip the flow to turbulence on both sides of the wing at 10% of the chord. Full turbulence statistics are computed in addition to collection of time history data in selected regions. The Reynolds numbers on the suction side reach Reτ ≃ 373 and Reθ=2,800 with the pressure-gradient parameter ranging from β ≈ 0.0 to β ≈ 85. Similarly, on the pressure side, the Reynolds numbers reach Reτ ≈ 346 and Reθ=818 while β changes from β ≈ 0.0 to β≈−0.25. The effect of adverse pressure gradients on the mean flow is consistent with previous observations, namely a steeper incipient log law, a more prominent wake region and a lower friction. The turbulence kinetic energy profiles show a progressively larger inner peak for increasing pressure gradient, as well as the emergence and development of an outer peak with stronger APGs. The present simulation shows the potential of high-order (spectral) methods in simulating complex external flows at moderately high Reynolds numbers.

Effect of Adverse Pressure Gradient on a Fluctuating Velocity over the Co-Flow Jet Airfoil

Authors : Morteza Mirhosseini, Amir B. Khoshnevis Abstract : The boundary layer separation and new active flow control of a NACA 0025 airfoil were studied experimentally. This new flow control is sometimes known as a co-flow jet (cfj) airfoil. This paper presents the fluctuating velocity in a wall jet over the co-flow jet airfoil subjected to an adverse pressure gradient and a curved surface. In these results, the fluctuating velocity at the inner part increasing by increased the angle of attack up to 12o and this has due to the jet energized, while the angle of attack 20o has different. The airfoil cord based Reynolds number has 105.

Receptivity of Hypersonic Boundary Layers over Straight and Flared Cones

The effects of adverse pressure gradients on the receptivity and stability of hypersonic boundary layers were numerically investigated. Simulations were performed for boundary-layer flows over a straight cone and two flared cones. The steady and the unsteady flowfields were obtained by solving the two-dimensional Navier–Stokes equations in axisymmetric coordinates using the fifth-order-accurate weighted essentially nonoscillatory scheme for space discretization and using a third-order total-variation-diminishing Runge–Kutta scheme for time integration. The mean boundary-layer profiles were analyzed using local stability and nonlocal parabolized stability equations methods. After the most amplified disturbances were identified, two-dimensional plane acoustic waves were introduced at the outer boundary of the computational domain and time-accurate simulations were performed. The adverse pressure gradient was found to affect the boundary-layer stability in two important ways. First, the frequency of the most amplified second-mode disturbance was increased relative to the zero-pressure gradient case. Second, the amplification of first- and second-mode disturbances was increased. Although an adverse pressure gradient enhances instability wave growth rates, small nose-tip bluntness was found to delay transition due to the low receptivity coefficient and the resulting weak initial amplitude of the instability waves. The computed and measured amplitude-frequency spectra in all three cases agree very well in terms of frequency and the shape except for the amplitude.

Effect of Adverse Pressure Gradient and Different Vegetated Banks on Flow

AbstractThis study examines velocity components in 3D and shear stress distributions in flows with adverse pressure gradient over a gravel‐bed channel using three types of vegetation on banks, rice, straw and Typha stems. Results of this experimental study are compared with those for flows in the bare bank channel. Results show that the flows with adverse pressure gradient are not self‐similar in vegetated and bare banks channels. The logarithmic law and parabolic law can be used to well describe flow in the inner and outer regions, respectively, but with different relative flow depths. Shear stress distributions depend on nature of bank roughness and the secondary flow effects because of small values of the aspect ratio and adverse pressure gradient. Quadrant analysis is performed to investigate the effect of different vegetation covers on the Reynolds stress distributions at the central axis of channel and near the vegetated banks. Vegetation covers influence the contributions of different quadrants to the shear stress distribution, showing that sweeps and ejections are dominant events for all vegetated banks for near bed region. However, near the water surface the contributions of outwards and inwards for banks with Typha stems are more important than those for rice and straw. Instead of using a single value for friction factor, for different vegetated banks, a range of friction factor has to be applied in river restoration projects. Vegetated banks result in an increase of more than 60% in friction factor in comparison with that for bare banks under adverse pressure gradient flows. Copyright © 2015 John Wiley & Sons, Ltd.

LES of spatially developing turbulent boundary layer over a concave surface

We revisit the problem of a spatially developing turbulent boundary layer over a concave surface. Unlike previous investigations, we simulate the combined effects of streamline curvature as well as curvature-induced pressure gradients on the turbulence. Our focus is on investigating the response of the turbulent boundary layer to the sudden onset of curvature and the destabilising influence of concave surface in the presence of pressure gradients. This is of interest for evaluating the turbulence closure models. At the beginning of the curve, the momentum thickness Reynolds number is 1520 and the ratio of the boundary layer thickness to the radius of curvature is δ0/R = 0.055. The radial profiles of the mean velocity and turbulence statistics at different locations along the concave surface are presented. Our recently proposed curvature-corrected Reynolds Averaged Navier-Stokes (RANS) model is assessed in an a posteriori sense and the improvements obtained over the base model are reported. From the large Eddy simulation (LES) results, it was found that the maximum influence of concave curvature is on the wall-normal component of the Reynolds stress. The budgets of wall-normal Reynolds stress also confirmed this observation. At the onset of curvature, the effect of adverse pressure gradient is found to be predominant. This decreases the skin friction levels below that in the flat section.

Coupled Reynolds-Averaged Navier–Stokes/Large-Eddy Simulation for Outlet Guide Vane and Prediffuser Flows

The objective of the present study was the development/testing of a novel hybrid Reynolds-averaged Navier–Stokes/large-eddy simulation methodology for predicting the flow at the compressor/combustor interface of a gas turbine engine. The coupling between the flow through the outlet guide vanes and the downstream combustor prediffuser represents the first key component of this interface. Important flow characteristics include three-dimensional turbulent outlet guide-vane wakes, end-wall boundary layers, and both favorable and adverse pressure gradient effects. There is a risk of flow separation if diffuser length and area ratio are not appropriately chosen; alternatively, a significant opportunity exists to reduce system flow losses if this risk is managed properly. Outlet guide-vane/prediffuser flow thus represents a significant modeling challenge. In the present approach to Reynolds-averaged Navier–Stokes/large-eddy simulation computational fluid dynamics, the Reynolds-averaged Navier–Stokes solver is applied to the (attached) outlet guide-vane flow and the large-eddy simulation solver to the (close to separation) prediffuser. Interfacing a large-eddy simulation prediction with an upstream Reynolds-averaged Navier–Stokes prediction introduces the problem of generating a spatially and temporally correlated unsteady velocity field for large-eddy simulation inlet conditions, which is also consistent with the time-averaged Reynolds-averaged Navier–Stokes solution. A new Reynolds-averaged Navier–Stokes/large-eddy simulation coupling procedure has been developed. Experimental data for conventional and advanced outlet guide vane/prediffuser geometries are used to assess the success of the new coupled approach. The results show that the Reynolds-averaged Navier–Stokes/large-eddy simulation approach is able to provide a good balance of predictive capability, improving prediction of velocity profiles at prediffuser exit compared to Reynolds-averaged Navier–Stokes only solution, and computational cost; it offers an attractive option for outlet guide-vane/prediffuser flow prediction.

Effect of adverse pressure gradient on high speed boundary layer transition

The effect of adverse pressure gradients (APG) on boundary layer stability, breakdown, and heat-transfer overshoot is investigated. Flat plate isothermal boundary layers initially at Mach 6 with APG imposed through the freestream boundary condition are simulated using suction and blowing to produce boundary layer instabilities. The three different transition mechanisms compared are first mode oblique breakdown, second mode oblique breakdown, and second mode fundamental resonance. For all of the transition mechanisms, an adverse pressure gradient increases the linear growth rates and quickens the transition to turbulence. However, the nonlinear breakdown for all three transition mechanisms is qualitatively the same as for a zero pressure gradient boundary layer. First mode oblique breakdown leads to the earliest transition location and an overshoot in heat transfer in the transitional region. Both types of Mack second mode forcing lead to a transitional boundary layer but even with the increased growth rates and N factors produced by the adverse pressure gradient, the breakdown process is still more gradual than first mode oblique breakdown because the primary Mack second mode instabilities saturate and produce streaks that breakdown further downstream.

Numerical Simulation of Laminar Flow over Slotted Airfoil

In this paper mainly focused effect of fixed slot on performance of NACA0012 airfoil. Using slot lower surface high pressurized air passes through slot to energized upper surface. Analysis has been done on NACA0012 of 1m cord length at 25C of air with 5m/s air velocity. First plain airfoil at different angle of attack has been analyzed to find out stall condition and Cl, Cd . After that same parameter and physics, only change in geometry of airfoil with 15%C leading edge slot. Flow separation is adverse effect for performance of airfoil. Due to flow separation adverse pressure gradient effect reverse flow is there, we cause reduction of lift coefficient and increment of drag coefficient so flow separation necessary to reduce. There are different techniques to reduce flow separation but effect of slot on airfoil has been studied. Keyword: Angle of attack (AOA), plain airfoil, slotted airfoil, SST model,