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Two-dimensional rectangular bars have been used in an experimental study to control boundary layer transition and reattachment under low-pressure turbine conditions. Cases with Reynolds numbers (Re) ranging from 25,000 to 300,000 (based on suction surface length and exit velocity) have been considered at low (0.5%) and high (8.5% inlet) free-stream...
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... velocity profiles of Fig. 4 and the pressure profiles of Fig. 3 are in good agreement. Both show transition and reattachment at the same locations, and the measured static pressures agree with the local free-stream velocities of Fig. 4. The agreement between the pressure and velocity results was apparent in all cases. For brevity, the pressure profiles are not presented in the cases which follow. ...
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Citations
... However, their investigation also indicated a limited effect in the low Reynolds number flows. Volino, [53] examined the influence of turbulence produced by rectangular bars on reduction of the size of the separation bubble on suction sides of turbine blades. The research demonstrated promising results however, the efficiency of this method was affected by both the Reynolds number and the size of the rectangular bars. ...
... Scholars have conducted considerable research on constraining suction surface separation in high-lift low-pressure turbines. For example, active blowing devices, 7 vibration devices, 8 V-shaped grooves, ball sockets, 9 mix lines, 10 and other structures have been designed to augment blades. In recent years, unsteady wakes inherent in turbomachinery have been used to induce the transition of the separation boundary layer in advance and to constrain separation. ...
In a high-altitude cruising state, boundary layer separation exists in high-lift low-pressure turbines, and inflow conditions corresponding to different blade designs can directly affect the working efficiency of low-pressure turbines. In particular, the reduced frequency of wake and free-stream turbulence intensity in an inlet flow can greatly influence boundary layer separation and transition development. In this paper, the influence of different inflow turbulence intensities and reduced wake frequencies on the development of suction surface boundary layers in high-lift low-pressure turbines under the influence of upstream wakes is studied by numerical simulations and experiments. Due to the combination of inflow free-stream turbulence intensity and reduced wake frequency, many inflow conditions can be chosen in the design process, and the unsteady influence of upstream wakes complicates the boundary layer flow. In this paper, an RBF (radial basis function)-GA (genetic algorithm) machine learning method is used to explore the optimal inlet conditions corresponding to the minimum profile loss of the Pak-B profile. The search region of the free-stream turbulence intensity is 2%–4%, and the reduced frequency of the wake is changed by changing the flow coefficient, whose variation range is 0.7–1.3. It is found that the RBF-GA machine learning method can attain an inflow condition with a lower profile loss while using the same amount of computation and effort.
... Passive and active flow control strategies to extend the performance of aerothermal devices have been of significant interest in recent decades. Passive flow control techniques that mitigate separation are accompanied by penalties at the design condition, making them less palatable [2]. Active flow control techniques potentially allow for a minimal impact on the design operating condition while providing considerable potential performance benefits when actuated in conjunction with the appearance of bulk separation. ...
... At this point, it is worth noting that this effort is part of a larger initiative to investigate the feasibility of incorporating active flow control technologies in a high lift/high work low-pressure turbine stage. References [2,[4][5][6][7][8][9] are quite representative of the available literature on flow control technologies in that although many techniques have shown considerable promise in the laboratory over the years, there is very little regarding applications of flow control in the more complex flow fields associated with operating turbine stages. The approach is to investigate the feasibility of applying flow control to a turbine stage by conducting a series of experiments with geometries of increasing complexity, as laid out by technology readiness levels [12], and ultimately arriving at a demonstration of flow control in a transonic, rotating turbine. ...
A new class of power generation devices that experiences increased losses due to bulk flow separation in segments of their expected in-flight regime is emerging. As such, active flow control becomes increasingly relevant to mitigate these losses and reclaim the entire flight envelope. This study explores the effect of flow injection on transonic flows experiencing bulk separation. Reynolds-Averaged Navier-Stokes simulations of a 3D wall-mounted hump at low Reynolds numbers are conducted to assess the response of transonic bulk separation to flow injection. Unsteady simulations are performed to understand the differences between slot and discrete port injection and determine optimum forcing frequencies. Discrete ports require higher pressures to overcome the momentum deficit associated with the smaller injection area relative to the width of the domain. Steady and unsteady injection are found as viable strategies in mitigating the extent (or appearance) of bulk separation. Experiments are conducted with discrete injection for a range of Mach and Reynolds numbers. The response of the bulk separation to said injection is evaluated by analyzing both local pressure measurements and schlieren imaging. The study shows that the required pressure of injection is strongly correlated to the length scale of the uncontrolled separation. With Large Eddy Simulations, the flow separation and frequency content within the separated region can be reasonably predicted. This study aims to take further steps to establish guidelines for applying flow control to the emerging class of power generation devices experiencing losses from bulk separation.
... Passive and active flow control strategies to extend the performance of aerothermal devices have been of significant interest in recent decades. Passive flow control techniques that mitigate separation are accompanied by penalties at the design condition, making them less palatable [2]. Active flow control techniques potentially allow for a minimal impact while at the design operating condition while providing considerable potential performance benefits when actuated in conjunction with the appearance of bulk separation. ...
... At this point, it is worth noting that this effort is part of a larger initiative to investigate the feasibility of incorporating active flow control technologies in a high lift / high work Low-Pressure Turbine stage. References [2,[5][6][7][8][9][10] are quite representative of the available literature on flow control technologies in that although many techniques have shown considerable promise in the laboratory over the years, there is very little regarding applications of flow control in the more complex flow fields associated with operating turbine stages. Accordingly, the approach here is to investigate the feasibility of applying flow control to a turbine stage by conducting a series of experiments with geometries of increasing complexity [11] and ultimately arriving at a demonstration of flow control in a transonic, rotating turbine. ...
A new class of power generation devices that experiences increased losses due to bulk flow separation in segments of their expected in-flight regime is emerging. As such, active flow control becomes increasingly relevant to mitigate these losses and reclaim the entire flight envelope. This study explores the effect of flow injection on transonic flows experiencing bulk separation. Reynolds-Averaged Navier-Stokes simulations of a 3D wall-mounted hump at low Reynolds numbers are conducted to assess the response of transonic bulk separation to flow injection. Unsteady simulations are performed to understand the differences between slot and discrete port injection and determine optimum forcing frequencies. Discrete ports require higher pressures to overcome the momentum deficit associated with the smaller injection area relative to the width of the domain. Steady and unsteady injection are found as viable strategies in mitigating the extent (or appearance) of bulk separation. Experiments are conducted with discrete injection for a range of Mach and Reynolds numbers. The response of the bulk separation to said injection is evaluated by analyzing both local pressure measurements and schlieren imaging. The study shows that the required pressure of injection is strongly correlated to the length scale of the uncontrolled separation. With Large Eddy Simulations, the flow separation and frequency content within the separated region can be reasonably predicted. This study aims to take further steps to establish guidelines for applying flow control to the emerging class of power generation devices experiencing losses from bulk separation.
... On the one hand, their effect is based on a momentum transfer of the high-energy external flow into the boundary layer [3]. On the other hand, disruptive bodies can create flow instabilities that force a preliminary laminar-turbulent transition which is associated with increased momentum transfer [4]. ...
The use of passive flow control, e.g. by means of vortex generators attached to the blade or endwall surface, is one approach to reduce boundary layer separation induced aerodynamic losses in axial fans by separation point delay. To enhance the understanding of the involved flow phenomena, this work presents a detailed study of the flow on a flat plate using several conventional vortex generator geometries. The experiments are conducted in a open-loop cascade wind tunnel at a fixed turbulence intensity of 1.5%. The surface flow is investigated using oil flow visualization and infrared (IR) thermography utilizing the dependency of wall heat transfer on local flow characteristics. For an insight into the three-dimensional flow field, Particle Image Velocimetry (PIV) measurements were conducted on several planes parallel to the main flow direction.
... In order to effectively regulate the process of boundary layer separation and transition near the compressor blades, and to improve the aerodynamic characteristics under low Reynolds number conditions, scholars have adopted several active and passive techniques including blade surface layer suction, casing treatments, and pulsed vortex generator jets, ( Pacciani, et al., 2006;Volino, 2003;Bons, et al., 2012). Compared with the methods above, the surface roughness control method has shown comparative advantage in effectively affecting the development of the boundary layer without imposing additional complicated geometric shapes on the blade. ...
In the present study, a numerical simulation was conducted to investigate the influence of surface roughness on the aerodynamic performance of a 1.5-stage highly loaded axial compressor at low Reynolds number. It was especially considered how the roughness Reynolds number(k+) affected the change of the inlet and outlet conditions, the growth of the separation bubble (LSB), the status of the limiting streamline, the patterns of the wake. Regarding the roughness settings, five roughness magnitudes were mainly studied. The results showed that at low Re, surface roughness mainly improved the stage performance by reducing the length and width of the LSB, delaying the occurrence of three-dimensional flow separation, and increasing the turbulence level near the wall. However, it also aggravated the incoordination between the subsequent stages to a certain extent, which limited further improvement of the overall aerodynamic performance. Generally, with k+ increasing, the compressor aerodynamic performance improved, and achieved the best at k+= 137.8. The maximum increases in the total pressure ratio, peak efficiency, and chocked mass flow rate were approximately 4.01%, 5.34%, and 2.24% respectively.
... A mass of active and passive flow control strategies have been implemented to delay or prevent the boundary layer separation on the LPT blade's suction side (Sondergaard, 2008). Passive flow control is generally implemented by geometry modification of blade surfaces such as recessed grooves, dimples, turbulent trips, and micro-riblets and changing roughness (Lake et al., 1999;Lake et al., 2000;Volino, 2003;Robarge et al., 2004;Montis et al., 2011). Compared with the active control method, the passive way is cheaper and easier to apply in a real engine which is independent of extra energy and device. ...
... These studies have demonstrated the capability of the dimple to diminish the adverse effects of laminar flow separation. Furthermore, surface grooves also have been used to inhibit laminar separation, such as rectangular grooves (Volino, 2003) and U-shaped grooves (Robarge et al., 2004), which have taken a good effect too. ...
... It can be concluded that the dimples have the greatest potential in inhibiting flow separation compared with rectangular grooves and U-grooves when they are located at the same position and have a similar maximum depth and streamwise length. Furthermore, the dimples are unlikely to have the unacceptable penalties at high Reynolds number conditions, while the U-grooves and rectangular grooves have been proved to produce significantly higher losses at higher Reynolds numbers (Lake et al., 2000;Volino, 2003;Robarge et al., 2004). The blade recessed by dimples also tends to have higher strength than that recessed by spanwise grooves. ...
The boundary layer development on a low-pressure turbine blade surface modified by recessed dimples, U-grooves, and rectangular grooves has been investigated through the large-eddy simulations. The simulations are performed at a Reynolds number of 50,000 (based on the inlet velocity and axial chord length) and extremely low freestream turbulence conditions. The characteristic parameters of the boundary layer are used to estimate the development of the boundary layer, and spectral analysis has also been performed to identify the dominant frequency of shedding vortices. The results of simulations indicate that three surface modifications all reduce the profile losses by restraining the separation bubble size. However, the grooves and dimples show different mechanisms in inhibiting laminar separation. Grooves tend to promote the formation of spanwise vortices, which is more difficult to break into turbulence. A high-speed shedding vortex is induced by the particular 3D structure of dimple, and its shedding frequency is nearly twice the Kelvin–Helmholtz (K-H) instability frequency. The interaction between the shedding vortices and the K-H vortices promotes the breakdown process of the spanwise vortices, which leads to an earlier transition of the boundary layer at a low disturbance level. The current study reveals the different mechanisms of dimples and grooves and shows the great potential of dimples for flow control in low-pressure turbines. Besides, the flow structures inside the dimples with adverse pressure gradients are also explored.
... In order to further investigate and analyze the phenomenon and mechanism of the flow separation and transition on the turbomachinery blade surface at low Reynolds number and to improve the aerodynamic characteristics, scholars have adopted several active or passive techniques, including utilizing a flexible membrane [5], blade surface layer suction [6], casing treatments [7], pulsed vortex generator jets [8,9], and surface roughness control [10]. Compared with the methods above, the surface roughness control method has shown superiority in effectively controlling the development of the boundary layer without imposing extra complex geometric shapes on the blade. ...
In the present study, a numerical simulation was conducted to investigate the influence of surface roughness on the aerodynamic performance of a 1.5-stage highly loaded axial compressor at low Reynolds number. It was especially considered how the roughness Reynolds number (k+) affected the change of the inlet and outlet conditions, the growth of the separation bubble (LSB), the status of the limiting streamline, and the patterns of the wake. Regarding the roughness settings, five roughness magnitudes and four roughness locations were mainly studied. The results showed that at low Re, surface roughness mainly improved the stage performance by reducing the length and width of the LSB as well as the rotor tip vorticity, delaying the occurrence of three-dimensional flow separation and increasing the turbulence level near the wall. However, it also aggravated the incoordination between the subsequent stages to a certain extent, which limited further improvement of the overall aerodynamic performance. Generally, with k+ increasing, the compressor aerodynamic performance improved and achieved the best at k+=137.8. The maximum increases in the total pressure ratio, peak efficiency, and chocked mass flow were approximately 4.01%, 5.34%, and 2.24%, respectively. In addition, for all the four roughness locations, the roughness covering from the leading edge to 50% of the axial chord length on the suction surface had a relatively evident advantage in improving the compressor peak efficiency because of the better control of the LSB and wall shear stress.
... However, the operation at elevated altitudes is limited because of potential flow separation along diffusive passages, mitigating the overall power plant performance [3]. For this reason, over the last decades, both passive ( [4][5][6]) and active ( [7][8][9][10]) flow control techniques were introduced to abate the boundary layer detachment at reduced Reynolds numbers. The introduction of passive flow control elements alters the aerothermal performance at the design point, locally boosting the losses and cutting the power plant efficiency. ...
The boundary layer detachment limits compact power plants' operation at moderate Reynolds number in adverse pressure conditions. Flow detachment is promoted by the lack of momentum in the near-wall region when exposed to adverse pressure gradients. Transient flows or periodic flow perturbations may delay or prevent the flow detachment. The present investigation experimentally analyzes the behavior of separated flows based on an ad-hoc wall-mounted hump. The near wall flow region detachment and recirculated flow evolution under sudden flow release were experimentally characterized. The extension of the separated region and its dynamic development were monitored through surface pressure, temperature measurements, and hot-wire traverses. Comparing the 3D hump performance during steady state experiments against sudden flow acceleration runs, we report for the first time the temporal response of a diffusive passage exposed to a sudden flow acceleration and the impact of the acceleration on the boundary layer detachment. Due to the sudden flow release, the near-wall region can overcome the adverse pressure gradient. However, as the flow acceleration dilutes, the boundary layer detaches, and the recirculated flow region develops. The comparison of experimental results against 3D transient Computational Fluid Dynamics simulations, using the standard gas turbine industry approach, demonstrates the ability of Unsteady Reynolds Averaged Navier-Stokes models to predict the dynamic performance of this phenomenon.
... scitation.org/journal/phf it toward the suction side, and controlling the separation from the turbine blades. Lake et al. 7 introduced the use of surface dimples to reduce the size of the separated flow regions, and Volino 8 investigated the effect of rectangular bars on the suction side to promote boundary layer transition and encourage flow reattachment. The main weakness of this approach is that the geometrical modifications (e.g., the change in shape or the added bars) reduce performance under design conditions at high Reynolds numbers. ...
We present a stability-analysis-based optimization approach that minimizes the growth rate of the least stable mode associated with the flow structure governing flow separation. We compare this approach with a classic optimization approach that minimizes an integral function related to pressure loss. We analyze both approaches on a two-dimensional model that mimics the diffusing passage present in the aft portion of the suction side of a low-pressure turbine. This zone is prone to boundary layer detachment at low Reynolds numbers, while fully attached flow is present at higher Reynolds numbers. The goal of the optimization is to design a local blowing technique to prevent boundary layer detachment at low Reynolds numbers and thereby reducing the dynamic head pressure loss with a minimum energy input. We simulate the problem using the compressible Reynolds-averaged Navier–Stokes equations (with the k–ω turbulence model) and perform a stability analysis on the mean flow. We then use the stability information and associated sensitivity (through adjoints) to provide insights into local blowing and thereby guide the optimization. The two optimization strategies use the blowing location, blowing rate, and blowing angle as the optimization variables. The first strategy minimizes a classic global integral function, including input and output pressure losses. The second approach minimizes the amplification rate of the least stable eigenvalue resulting from a stability analysis. The targeted mode governs the detached recirculation, and stabilizing its amplification rate leads to attached flow and minimum losses. Indeed, both strategies lead to attached flows and similar optimal values for the blowing location and to enhanced performance. It is concluded that the stability-analysis-based optimization provides results comparable to those from a classic optimization approach and can be useful for cases where there is no clear integral functional to guide the optimization procedure (e.g., pressure loss).
