Unsteady Wake Effects Research Articles

Effects of Periodic Incoming Wakes on the Aerodynamics of a High-Speed Low-Pressure Turbine Cascade

The influence of unsteady wakes incoming from the upstream stages is of high relevance in modern high-speed, low-pressure turbines (LPT) operating at transonic exit Mach numbers and low Reynolds numbers for their potential to trigger transition and influence the separation of the boundary layer on the blade suction side. The aim of this paper is the experimental characterization of the influence of incoming wakes on the 2D aerodynamics of a high-speed LPT cascade operating at a low Reynolds number and transonic exit Mach number. A detailed analysis of the status of the flow along the blade under investigation and its impact on the profile loss are presented for a range of Mach numbers from 0.70 to 0.95 and Reynolds numbers from 70k to 120k under steady and unsteady inflow conditions. Tests were conducted at on- and off-design engine realistic conditions in the VKI S-1/C wind tunnel on the SPLEEN C1 transonic cascade. The wakes incoming from an upstream blade row have been replicated using a set of rotating bars, which shed wakes at an engine-representative reduced frequency (f+=0.95) and flow coefficient (Φ=0.80). A set of densely instrumented traversable blades were used to sample the surface pressure distributions. The development of the boundary layers along the blade suction side is examined through quasi-wall shear stress obtained with surface-mounted hot-film sensors. Wake traverses were carried out downstream of the cascade with a miniaturized L-shaped five-hole probe to characterize the blade losses. The introduction of periodic incoming wakes promotes variations in the flow topology over the blade. The effect on the suction side separation bubble is shown to depend on the exit flow conditions. At low Mach numbers, the incoming wakes determine a reduction in the size of the bubble; in contrast, this effect is not registered as the exit Mach number increases. Consistently, a high dependence of the unsteady wake effect on the profile loss on the exit Reynolds and Mach numbers is demonstrated.

Effects of Blowing and Density Ratios on Unsteady Film Cooling Effectiveness Using Fast-Response Pressure-Sensitive Paint

Abstract The transient effect of unsteady wakes on film cooling effectiveness using a fast response pressure sensitive paint (PSP) was studied. Testing was performed in a five-passage, low-speed wind tunnel using a scaled-up high pressure, blade profile. The blowing ratios studied include 0.4, 0.8, and 1.2 with a density ratio of 1.0 and 1.5 at each blowing ratio. An upstream wake generator produced Strouhal numbers of 0.1 and 0.3 while the wind tunnel inlet mainstream Reynolds number was held at a constant value of 300,000 for every test case. The results in this paper confirm the overall time averaged reduction in film cooling effectiveness with the addition of an unsteady wake while providing more detailed information on the transient response of the film cooling effectiveness, presented in the form of film cooling effectiveness RMS values. Results show that film cooling effectiveness RMS values increase on the leading-edge region of the blade and show little change on the pressure side of the blade with an increase in the Strouhal number. On the suction side, the area-averaged effectiveness RMS values decrease for the 0.8 and 1.2 blowing ratios and remain unchanged for the 0.4 blowing ratio. A change in the density ratio of the coolant from 1.0 to 1.5 has a minimal effect on area-averaged effectiveness RMS values for all areas of the blade.

Modelling marine turbine arrays in tidal flows

Tidal stream turbines operate in the harsh marine environment, subjected to turbulence, wave action and wakes from upstream devices when deployed in arrays. Turbines may be mounted on the sea bed or on floating platforms. Numerical models are invaluable to study individual and array performance and their interaction with environmental flows. To date, shallow water models and three-dimensional Reynolds averaged Navier–Stokes (RANS) simulations predominate the simulation of turbine arrays, while large-eddy simulation (LES) is becoming more widely used due to availability of high-performance computing resources. The accuracy of turbine performance and load prediction depends on the ability to represent turbulence in the tidal flow and unsteady wake effects. Inclusion of waves increases the modelling complexity and is particularly significant for floating platforms. In this Vision Paper, we provide a perspective on the numerical approaches currently used for modelling tidal flows and marine turbines, suggesting future challenges envisaged in this field.

Effects of forced frequency oscillations and unsteady wakes on the separation-induced transition in pressure gradient dominated flows

The individual and cumulative effects of forced frequency oscillations and unsteady wakes, on the separation-induced transition caused by an adverse pressure gradient on a flat plate geometry, are investigated. The flat plate is subjected to a streamwise pressure gradient via an inviscid contour wall, which is representative of the pressure gradient on the suction surface of a low pressure turbine. Periodic Gaussian wakes are imposed at the inflow to mimic the effect of upstream stator wakes. The transitional and turbulent regimes of the flow are investigated using instantaneous and time-averaged flow fields, turbulent kinetic energy (TKE) budgets and a disturbance enstrophy-based nonlinear receptivity analysis based on direct numerical simulations. The impression of the wake in the boundary layer convects 50%–70% slower than the wake. The case with oscillations and wakes is most effective in triggering early transition and reducing the region of separation, specifically the secondary separation bubble is completely suppressed. Enhanced near-wall mixing is observed with a shift in the inflection point of the velocity profile toward the wall. The budget of the TKE shows the dominance of the production term for all cases considered. The disturbance enstrophy transport equation shows the role of the vortex stretching, enstrophy diffusion and dissipation for flows with wakes. Finally, a linear spatio-temporal receptivity analysis is performed for velocity profiles in the attached and separated boundary layer. This provides a description of the effects of pressure gradient and unsteady wakes in the transition to turbulence.

Unsteady wakes-secondary flow interactions in a high-lift low-pressure turbine cascade

Detailed experimental measurements were conducted to study the interactions between incoming wakes and endwall secondary flow in a high-lift Low-Pressure Turbine (LPT) cascade. All of the measurements were conducted in both the presence and absence of incoming wakes, and numerical analysis was performed to elucidate the flow mechanism. With increasing Reynolds number, the influence of the incoming wakes on suppressing the secondary flow gradually increased owing to the greater influence of incoming wakes on reducing the negative incidence angle at higher Reynolds numbers, leading to a lower blade loading near the leading edge and suppression of the Pressure Side (PS) leg of the horseshoe vortex. However, the effect of unsteady wakes on suppressing the profile losses gradually became weaker owing to the reduced size of the Suction Side (SS) separation bubble and increased mixing loss in the free-flow region at high Reynolds numbers. Incoming wakes clearly improved the aerodynamic performance of the low-pressure turbine cascade at low Reynolds numbers of 25,000 and 50,000. In contrast, at the high Reynolds number of 100,000, the profile loss at the midspan and mass-averaged total losses downstream of the cascade were higher in the presence of wakes than in the absence of wakes, and the unsteady wakes exerted a negative influence on the aerodynamic performance of the LPT cascade.

Developments of a semiempirical dynamic stall model for unsteady airfoils

A new semiempirical dynamic stall model is presented to predict the aerodynamic coefficients of an airfoil in unsteady conditions. The model is mainly based on two modifications on the airfoil angle of attack which introduce an equivalent angle of attack to implement in the proposed aerodynamic coefficients. The first modification includes unsteady wake effects of the airfoil in the proposed model which are not considered in most of the semiempirical models. Hence, an effective angle of attack is introduced based on an unsteady aerodynamic theory using the Wagner function for pitching and plunging oscillations of the airfoil. The second modification takes into account time delay effects and dynamic effects due to the flow separation on the airfoil. These effects are included in the model by developing a semiempirical relationship between static and dynamic angle of attack published in the literature. Finally, the modifications form the equivalent angle of attack to implement in a set of equations based on Kirchhoff flow theory. The proposed approach dynamically approximates trailing edge separation point on the airfoil by a completely new way and contributes its effects in the aerodynamic coefficients. Moreover, apparent mass effects and the airfoil leading edge vortex contributions are sufficiently included in the model. The presented method effectively predicts unsteady lift and pitching moment coefficients of the airfoil undergoing stall conditions. Consequently, the proposed model is verified against experimental data under various test cases where obtained numerical results are in good agreement with the test data. Furthermore, the performance of the proposed model is compared with various dynamic stall models including Leishman–Beddoes model, ONERA model and Boeing–Vertol model. The comparison shows that the proposed model minimizes the number of the semiempirical parameters determined from an experiment in dynamic stall modeling compared with Leishman–Beddoes model and ONERA model. It also demonstrates that the presented model performs as well as other well-known models using the various approaches.

Combined effects of unsteady wake and free-stream turbulence on turbine blade film cooling with laid-back fan-shaped holes using PSP technique

Detailed film cooling effectiveness distribution for a gas turbine blade under the effects of unsteady wakes and oncoming free-stream turbulence intensities was obtained using pressure sensitive paint (PSP) technique. Tests were performed on a linear cascade at Reynolds number of 3.85 × 105 based on the blade chord at cascade exit. Upstream unsteady wakes were simulated using a spoke-wheel type wake generator. The test blade has three rows of compound angled cylindrical film holes at the leading edge, five rows of laid-back fan-shaped holes on the pressure surface and three rows of laid-back fan-shaped holes on the suction surface. The wake Strouhal number was varied from 0 to 0.36 and three mass flux ratios were determined. The oncoming free-stream turbulence intensities are 2.7% and 26.9%, respectively. Results show that the effect of the mass flux ratio on the film cooling effectiveness decreases under the high turbulence intensity and unsteady wake conditions. In most regions of the blade surface, the film cooling effectiveness decreases with the increase of wake Strouhal number, and the free-stream turbulence superimposed on the unsteady wake reduces the film cooling effectiveness further. The effect of the unsteady wake decreases under the high free-stream turbulence conditions.

Effects of unsteady wakes on heat transfer of blade tip and shroud

An experimental study has been conducted to investigate the heat-transfer characteristics of blade tips and shrouds with and without unsteady wakes. Depending on the presence of unsteady wakes, the local heat/mass-transfer coefficients of the tip and shroud were measured using the naphthalene sublimation method. Wakes from unsteady blades were modeled as wakes generated from moving cylindrical rod bundles. Test conditions were set to a Reynolds number of 100,000, based on an inlet velocity of 11.4 m/s and the axial chord length. The Strouhal number was varied from 0 to 0.22. For the case without unsteady wakes (St = 0), high heat/mass-transfer coefficients appeared in regions where various flow patterns, such as flow reattachment, swirling flow, and vortices, occurred. Unsteady wakes (St = 0.22) made high turbulence intensity of the tip leakage flow, and flow patterns ranging from flow reattachment to tip leakage vortex in the tip and shroud were changed and dispersed in the presence of unsteady wakes, thus changing the heat/mass-transfer distributions in these areas. Due to the high thermal load on the tip and shroud under unsteady wake conditions, more detailed cooling designs must be considered, ranging from film cooling holes to the installation of additional structures on the blade tip, especially in the leading edge region and mid-region I of the tip, and the mid-region I and trailing edge region of the shroud.

Physical mechanism of intermittency route to aeroelastic flutter

Intermittency has been observed in the response of aeroelastic systems in the presence of flow fluctuations. This study focuses on developing an understanding of the physical mechanisms that lead to intermittency in such systems. Specifically, the role of time scales of the input flow fluctuations is investigated. Numerical investigations reveal that flow fluctuations with predominantly long time scales in the pre-flutter regime lead to “on–off” type intermittency. On the other hand, rapid fluctuations constituting of small time scales lead to another qualitatively different intermittency, which is referred to in this paper as “burst” type intermittency. It is further shown that the unsteady wake effects play a crucial role in the burst type intermittency. Measures derived from time series analysis of the aeroelastic response are proposed to identify the different dynamical states quantitatively.

Effect of an unsteady wake on the external heat transfer of 2nd stage rotor

A rotor blade is affected by the unsteady wake from the stator in the previous stage. An unsteady wake causes differences in the flow and heat transfer distribution on the external surface of the rotor blade. Therefore, the effect of unsteady wake should be studied for optimal design of blade cooling. To investigate the effect of an unsteady wake, this study measured the local heat transfer distribution over the entire external blade surface using the naphthalene sublimation method. Several rods moved across the inlet of the cascade to make the unsteady wake and the Strouhal number was varied from 0.0 to 0.3. The heat transfer on the entire pressure-side surface in an unsteady wake increases due to the high turbulence. The unsteady wake flow disturbs the developing passage vortex and tip leakage flow. As a result, the heat transfer near the endwall and tip surfaces in the unsteady wake case is lower than that in the steady case.

State-Space Adaptation of Unsteady Lifting Line Theory: Twisting/Flapping Wings of Finite Span

In this paper, a low-order state-space adaptation of the unsteady lifting line model has been analytically derived for a wing of finite aspect ratio, suitable for use in real-time control of wake-dependent forces. Each discretization along the span has between 1–6 states to represent the local unsteady wake effects, rather than remembering the entire wake history which unnecessarily complicates controller design. Sinusoidal perturbations to each system degree of freedom are also avoided. Instead, a state-space model is fit to individual indicial functions for each blade element, allowing the downwash and lift distributions over the span to be arbitrary. The wake geometry is assumed to be quasi steady (no roll up) but with fully unsteady vorticity. The model supports time-varying surge (a nonlinear effect), dihedral, heave, sweep, and twist along the span. Cross-coupling terms are explicitly derived. This state-space model is then validated through comparison with an analytic solution for elliptic wings, an unsteady vortex lattice method, and experiments from the literature.

Numerical Simulation of the Wake Effect of a H-Type Wind Turbine on Downstream Wind Turbine

The unsteady wake effect of upstream wind turbine on the aerodynamic performance of downstream wind turbine is investigated using CFD simulations in this paper with two two-dimensional models of H-type wind turbine of three same blades. Dynamic grids based on the inertial coordinate system are employed and the SST turbulence model is used for turbulence modeling. The wind speed is 5.07 m/s and the tip speed ratio is 2.15. The distance between upstream and downstream wind turbine is 5D, 7D, 9D, 11D, 13D, 15D and 17D (D denotes the diameter of the wind turbine). The power coefficient of upstream and downstream wind turbine is comparative analyzed. The variation of the tangential force coefficient of single blade with azimuth and the velocity distribution of different locations of the wind turbine wake are discussed in detail with the distance between upstream and downstream wind turbine of 7D and 15D. In addition, two-dimensional unsteady viscous flow field through velocity contours in one circle is presented.

Experimental study of unsteady wake effect on a film-cooled pitchwise-curved surface

Unsteady wake interactions with the near wall flow field occur when a surface is exposed to fluid flow past upstream rotating bodies. In the case of gas turbines, understanding such interactions are essential to better design cooling schemes on endwalls. In view of this, an experimental study has been conducted to determine the heat transfer coefficient and film cooling effectiveness on a pitchwise-curved surface which is subjected to unsteady passing wakes generated using a wake rod in a wall-normal orientation. The mainstream Mach number was maintained approximately constant at 0.03. A single row of cylindrical film holes with pitch to diameter ratio of 3 and inclined at 35° to the test surface are used for discrete film injection. The coolant to mainstream mass flux ratio (M) is varied between 0.25 and 1. A spoke-wheel type wake generator is used to produce unsteady wakes at two wake Strouhal numbers (S=0.15, 0.3). Measurements are made for (i) steady mainstream flow (S=0) which serves as a baseline case, (ii) mainstream flow with unsteady wakes, (iii) steady mainstream flow with film injection (iv) both (ii) and (iii) combined. The unsteady passing wakes mitigated jet lift-off at high coolant to mainstream momentum flux ratios. The maximum increase in film effectiveness was measured to be ≅16.35% at the jet centerline for M=0.75, S=0.3 at x/D=2. At high coolant to mainstream mass flux ratios, a combination of increasing film jet turbulence and strong interaction with the mainstream, deteriorates film cooling effectiveness but increases the heat transfer coefficient. Heat transfer augmentation increased by ≅7.6% for the highest wake passing frequency (S=0.3) without film injection. A combination of unsteady passing wakes and film injection resulted in a maximum pitch-averaged and centerline heat transfer augmentation of ≅28% and 31.7% respectively.

Unsteady vortex lattice method coupled with a linear aeroelastic model for horizontal axis wind turbine

Many aerodynamic phenomena on wind turbines are extremely complicated, such as the ground boundary layer effects, directional and spatial variations in wind shear, unsteady yaw effects, and unsteady aerodynamics due to the platform motion of offshore floating wind turbines. Thus, it is unclear whether prediction of the aerodynamic load of a turbine using conventional blade element momentum theory is accurate. On the other hand, this method does not consider unsteady wake effect behind the rotor, the unsteady vortex lattice method can treat non-uniform flow effects derived from the trailing wake from the turbine. Moreover, the aeroelasticity of wind turbines is significant because of the recent scaling up of the turbine size. This induces unsteady aerodynamic phenomena because of fluctuations in the geometrical relative velocity and induced velocity. In the present work, we developed a fully coupled simulation code and analyzed the unsteady aerodynamics of wind turbines by substituting aerodynamic model from FAST's AeroDyn to the vortex lattice method. The developed code was validated with unsteady aerodynamic experimental Phase VI data from the National Renewable Energy Laboratory. Finally, the results show that the developed coupled code yields accurate values and good performance in the unsteady yaw case.

Effects of Unsteady Wakes on the Secondary Flows in the Linear T106 Turbine Cascade

In modern low pressure turbines the efforts to increase aerodynamic blade loading by increasing blade pitch and optimizing midspan performance in order to reduce weight and complexity can produce increased losses in the endwall region. Airfoils of high flow turning and high pressure gradients between the blades generate strong secondary flows which impair the global aerodynamic performance of the blades. In addition, the unsteady incoming wakes take influence on transition phenomena on the blade surfaces and the inlet boundary layer, and consequently affect the development and the evolution of the secondary flows. In this paper, the T106 cascade is used to identify the effect of unsteady wakes on the development of secondary flows in a turbine cascade. Numerical and experimental results are compared at different flux coefficients and Strouhal numbers, the relative differences and similarities are analyzed.

Effects of Unsteady Wake on Heat Transfer of Endwall Surface in Linear Cascade

The present study aimed to investigate the effect of an unsteady wake on the heat transfer for the endwall surface of a linear of cascade turbine blade. A naphthalene sublimation method was implemented to obtain the detailed heat/mass transfer distributions on the endwall surface. Tests were conducted on a five-passage linear cascade in a low-speed wind tunnel. The effects of unsteady wakes were simulated in the facility by a wake generator consisting of circular rods that were traversed across the inlet flow. The test conditions were fixed at a Reynolds number of 70,000 based on the inlet velocity and chord length. The flow coefficients were varied from 1.3 to 4.2 and the range of Strouhal number was 0.1–0.3. The results showed that the heat transfer distributions differed between steady and unsteady cases. The overall heat transfer for the unsteady cases was higher, and the heat transfer was enhanced with increasing the Strouhal number due to the resulting thin boundary layer and high turbulence intensity. Therefore, a cooling system for the endwall of a rotor should focus on reducing the high temperatures on the endwall surface induced by the unsteady wakes.

Effects of Periodic Unsteady Inflow on Film Cooling and Heat Transfer on Highly Loaded High Pressure Turbine Blades With Flow Separation

Film cooling experiments were run at the high-speed cascade wind tunnel of the University of the Federal Armed Forces Munich. The investigations were carried out on a linear cascade of highly loaded turbine blades. The main targets of the tests were to assess the film cooling effectiveness and the heat transfer in zones with main flow separation. The previous cascade was designed to have a large zone with flow separation on the pressure side starting at the leading edge and reaching up to approximately half of the axial chord. This cascade was changed for a new design with a larger pitch to chord ratio in order to set the focus on flow separation on the suction side. This increased pitch forces a massive separation on the suction side due to strong shocks. The flow separation is controlled with aid of vortex generating jets in order to reduce the total pressure loss caused by it. Film cooling is provided on the suction side upstream of the vortex generating jets. The measurements comprise of blade loading, profile loss, adiabatic film cooling effectiveness, and heat transfer coefficient under two Mach numbers at a Reynolds number of 390,000. In a previous publication detailed results with homogeneous inflow where shown. Now, the focus is set on the effects of periodic unsteady wakes resulting from bars moving upstream of the cascade. These moving bars create a periodic unsteady inflow similar to the interaction between stator and rotor in the machine. It is shown how these wakes have significant influence on the heat transfer in the acceleration region of the suction side and affect the adiabatic film cooling effectiveness upstream of the shock.

Unsteady Wake Effect on Film Cooled Rotating Blade

Vane-wake unsteady characteristics and its effects on rotor blade leading-edge were investigated with SST k-ω turbulence model in this paper. Conditions of different rotated speed (500rpm, 1000rpm, and 2000rpm), rotor blade surface static pressure and heat transfer coefficient distributions were obtained from spectrum analysis of rotor surface lift and drag coefficient under corresponding conditions. Results show that: Along with the increase of speed, wake vortex shedding frequency and energy entropy around rotor blade flow field increase, which leads to relatively high energy loss; pressure surface of rotor blade static pressure reduces with increasing speed, whereas, suction surface static pressure increases, and overall heat transfer performance of rotor blade surface increases.

Effect of Unsteady Wake on Flow Control of the Low-Pressure High-Lift Cascade with Synthetic Jet

The flow separation is susceptible to appear and is known to affect the aerodynamic performance of the low-pressure high-lift cascade. Large Eddy Simulation was adopted in the present work and the periodic moving bar was employed to simulate the unsteady wake upstream of the blade. The flow control of the synthetic jet with unsteady wake was investigated in detail. The upstream wake increased the turbulent level of the boundary layer of the cascade. The synthetic jet got a better control under the effect of the upstream wake.

Influence of Blade Geometric Parameters on Aeroelastic Response of a Helicopter Rotor System

Rotary wing aeroelasticity is a highly complex phenomenon involving coupling between flexible blade dynamics and unsteady aerodynamics including stall and unsteady wake effects. In this paper, a low-cost computational aeroelastic model including the structural coupling from geometric parameters and nonlinearities associated with structural modeling and dynamic stall, applicable to steady, level forward flight, has been developed. The differential equations of motion are solved in time domain in a sequential manner to obtain the response of all the blades in the rotor system, the dynamic inflow variables, and the sectional loads at every time step. A fourth-order Runge-Kutta integration scheme has been adopted for solving the differential equations. Iterations are carried out until convergence is achieved in blade response and helicopter trim. The effect of blade geometric parameters such as pretwist, hinge offset, and torque offset on aeroelastic response of a helicopter rotor system is investigated numerically. It is shown that the structural coupling from blade geometric parameters significantly influences the rotor blade response and loads.