Steady Suction Research Articles

RADIATION AND THERMAL DIFFUSION EFFECTS ON VISCO-ELASTIC FLUID PAST A POROUS PLATE WITH CHEMICAL REACTION AND HEAT GENERATION

A free convective two dimensional MHD boundary layer flow of a non-Newtonian (visco-elastic) fluid past a vertical porous plate have been thoroughly researched in this paper. The thermal diffusion Effects are also analyzed in presence of chemical reaction and radiation. A steady suction pressure is maintained at the plate. A singular perturbation method has been used to solve the problem. The analytical results are obtained for different values of the various dimensionless parameters entering into the flow problem and discussed quantitatively.

Transition delay in a Mach 6 boundary layer using steady blowing and suction strips

Direct numerical simulations (DNS) were carried out to investigate flow control for transition delay using steady blowing/suction strips at the wall of a flared cone at Mach 6 and zero angle of attack. For the numerical investigations of the transition control strategy, the flared cone geometry and the flow conditions of the experiments in the Boeing/Air Force Office of Scientific Research (AFOSR) Mach 6 Quiet Tunnel (BAM6QT) at Purdue University were chosen. For the DNS, transition was initiated by introducing random disturbances at the inflow of the computational domain, emulating ‘natural’ transition in wind-tunnel experiments caused by free-stream noise. In both wind-tunnel experiments and numerical simulations, streamwise ‘hot’ streaks were found on the surface of the flared cone, which are caused by a nonlinear interaction of an axisymmetric second-mode wave and a pair of oblique waves of the same frequency (‘fundamental resonance’). The objective of the flow control strategy proposed here is to delay the transition onset, and thus mitigate the negative consequences associated with the nonlinear transition stages, i.e. the development of hot streaks and large wall-pressure amplitudes that were observed in experiments and DNS. Our previous so-called ‘controlled’ transition simulations have shown that flow control using steady blowing and suction strips can lead to a significant delay of the hot streak development on the surface of the flared cone. The simulation results presented in this paper show that this flow control strategy remains effective, even in a natural transition scenario characterized by broadband disturbances.

Experimental study on pulsed suction under different momentum coefficients in variable compressor cascade with annular gaps

Variable Stator Vanes (VSVs) with penny cavities are crucial in the compressors of variable cycle engines, where complex interactions between annular and radial gaps can adversely affect aerodynamic performance. Unsteady active flow control techniques have significant potential for reducing secondary flow losses in compressors. This research investigates the internal flow characteristics of a variable compressor cascade with penny cavities using low-speed wind tunnel experiments and flow visualization techniques. This study is the first to experimentally explore the relationship between loss reduction and momentum coefficient using pulsed suction (PS) compared to steady continuous suction (SCS), evaluating the effects of various excitation frequencies. The findings show that at a momentum coefficient of 0.11 and an excitation frequency of 100 Hz, PS achieves optimal aerodynamic performance, reducing total pressure losses by 11.59 %, marking a 92.59 % improvement over SCS. As the momentum coefficient increases, the effectiveness of PS control significantly improves, whereas the changes in SCS effectiveness are less pronounced. The control effects of PS under varying excitation frequencies are complex and influenced by multiple factors. At higher momentum coefficients, the loss reduction effectiveness of PS is strongly correlated with excitation frequency, while at lower coefficients, performance changes due to frequency variations are minimal. This study provides valuable guidance for the design of modern multistage variable compressors.

MHD flow of a rotating micropolar fluid between two plates by using DTM

The primary purpose of this work is to investigate the influence of a magnetic field on a micropolar fluid, which is rotating between two horizontally arranged parallel plates. A uniform magnetic field acts perpendicularly to the porous plate, which consumes fluid with a steady suction velocity. The obtained higher-order nonlinear partial differential equations (PDEs) are converted into nonlinear ordinary differential equations (ODEs) by using similarity transformation. A semi-analytical methodology has been applied to obtain the result to the problem. The influence of various flow parameters of the flow field like coupling parameter, viscosity parameter, Reynolds number, porosity parameter, rotation parameter and magnetic field parameter has been examined and explained by graphs. A resemblance between the results obtained by solving by the differential transform method (DTM) and the outcomes of the former research works has also been displayed which shows the correctness and efficiency of the used analytical method. The numerical values of the coefficient of skin friction at the lower plate are also displayed in tabular form and compared with the literature.

Wake bi-modality: the effect of upstream boundary layer dynamics

The turbulent wake past a square-back Ahmed body in close proximity to the ground experiences random side-to-side switching between two asymmetric positions, a phenomenon known as bi-modality. It has been observed to be sensitive to the dynamics of the upstream boundary layers formed along the body surfaces. Close to the body fore end, these separate and reattach, with hairpin vortices emanating from the reattachment points and growing along the surfaces before breaking down upstream of the base. This study uses wall-resolved large eddy simulations to investigate the effect of using suction to suppress these upstream boundary layer separations on the wake bi-modality. It is seen that, in the unforced flow (in the absence of suction), the smaller top and side surface vortices resulting from breakdown interact as they convect downstream. Steady suction is confirmed to suppress the boundary layer separations on the different body surfaces. When the boundary layer separations on the two side (vertical) surfaces are suppressed, it is found that horizontal bi-modality is completely inhibited with weak vertical asymmetry preserved. The interaction of the small top/side surface vortices is interrupted, damping boundary layer fluctuations just upstream of the base. Applying suction on different combinations of side/top/bottom boundary layer separations is found to have different effects on the underbody flow and the wake vertical balance, with bi-modality suppression dependent on side surface suction. This confirms that bi-modality is triggered, at least in part, by boundary layer disturbances on the surfaces perpendicular to the switching direction.

The stabilizing effect of grooves on Görtler instability-induced boundary layer transition in hypersonic flow

Görtler vortex-induced hypersonic boundary layer transition controlled by grooves is investigated using direct numerical simulations and spatial bi-global stability analysis. In the simulations, Görtler vortices are excited by wall steady blowing and suction with spanwise wavelengths of 3 mm. It is found that when the wall is covered with grooves, the Görtler streaks keep more regular even at the end of the model. In addition, the skin friction coefficient is reduced efficiently. Furthermore, the wall-normal and spanwise velocity shear are both reduced, suppressing growths of secondary instabilities. In conclusion, grooves can delay Görtler vortex-induced transition by modifying the Görtler streaks structure and instability, which would shed light on hypersonic boundary layer transition control.

Mechanisms of oscillating suction in controlling the tip leakage flow in a high-load compressor cascade

To control tip leakage flow and reduce related loss in axial-flow compressors, the oscillating suction scheme was introduced in a high-load compressor cascade with a tip clearance, and its flow control mechanisms on the tip leakage flow were studied numerically. It is found that the flow control effect of oscillating suction is the best when the unsteady excitation frequency is equal or harmonic to the feature frequency of the flow field related to the flow separation. The loss of the cascade controlled by the oscillating suction with a reduced frequency of 0.39 and excitation amplitude of 10 kPa was reduced by about 3.4% compared with the steady suction scheme, and about 11.14% less than that of the non-suction scheme. Oscillating suction discretized the concentrated tip leakage vortex into a series of scale-reduced vortex structures by the periodic unsteady excitation, which can be identified by the analysis using proper orthogonal decomposition (POD). The feature frequencies became consistent with the oscillating excitation frequency or its multiple, and the fluctuation generated by the tip leakage flow was enhanced. The energy proportion of high-order POD modes was increased by oscillating excitation, which indicated the vortex structures with weak fluctuation intensity were more active. The mixing and energy exchange among the mainstream, separated flow, endwall secondary flow, and tip leakage flow were enhanced, so that a better flow control effect could be achieved. Furthermore, flow capacity of tip passage was modified, the overall aerodynamic performance of the compressor cascade was improved thereby.

Receptivity-orientated drag reduction of twin cylinders by steady leading-edge suction control based on adjoint method

This study investigates the drag reduction of two tandem square cylinders under steady suction control at Reynolds numbers 50–200. The position where the suction flow should be placed is determined by using a receptivity analysis based on the adjoint method, and we investigate how control affects the fluid force and flow structures. High-order dynamic mode decomposition (HODMD) is applied to analyze the dynamic coherence modes and uncover the underlying control mechanism. The adjoint modes show that the regions of maximum receptivity to momentum forcing are localized on each side of the up-cylinder (UC) near the leading edge (LE). Thus, the suction flow is placed on the LE. The drag can be significantly reduced at wide gap distances, especially for the co-shedding regime. Under suction flow control, the separation is suppressed near the LE, and the gap vortices are no longer fed by the vorticity generated by the separated shear layer; they only result from the trailing-edge separation, which weakens and shrinks. Subsequently, the interaction between the gap flow and the down-cylinder (DC) is weakened, which reduces the drag and lift forces. The decrease in drag exceeds 66.4% for the UC and reaches 81.6% for the DC. The fluctuating reduction in the lift for the UC (DC) exceeds 59.0% (75.7%). HODMD results show that, as the suction flow velocity increases, the LE suction flow modifies the local time-averaged modes rather than the global mode energy. Conversely, the dynamic mode energy decreases significantly, whereas the mode shape remains unchanged except for a phase shift.

Effective aero-optical suppression by steady wall blowing and wall suction schemes for supersonic turbulent boundary layer

AbstractAs a basic flow model for engineering applications, wall-bounded turbulent flow has been widely studied in the field of aero-optics, but the flow control methods that could effectively suppress aero-optical effects are relatively rare. As an urgent requirement in engineering application, the concept of the steady wall blowing and suction is proposed by the author. Firstly, the author briefly described the flow model and physical method. Secondly, the choice of disturbance type is given. Then, the results of wall blowing-suction, suction and blowing ways based on steady and unsteady disturbance are compared. Finally, it is concluded that employing the high steady wall blowing disturbance (A = 0.2) could realise aero-optical suppression by around 20%. Besides, the steady wall suction scheme contributes to about 70%–80% reduction effect within a wide amplitude range (A = 0.2–1.0), which suppresses this effect by maintaining laminar state downstream contrasted by the baseline case.

Study of pressure surge during priming phase of start transient in an initially unprimed pump-fed liquid rocket engine

In this paper, transient phenomenon during start up process of a pump fed liquid rocket engine is investigated through numerical simulation. The engine studied in this work is designed such that engine systems are not wetted with propellant until the engine is commanded to start. This is achieved by positioning the valves for propellant admission at the interface of test stand/flight stage and the engine. To evaluate engine performance during start transient for such systems, unsteady flow simulation was conducted using Method of Characteristics and equations for priming. The same has been reported in this work. The results indicated a brief period of abrupt pressure rise at pump upstream after opening of the propellant admission valves, during the process of priming of engine systems at valve downstream. The peak pressure obtained was significantly higher than the propellant tank pressure as well as the steady state pump suction pressure. The transitory pressure rise was found to occur due to flow resistance at impeller inlet caused by formation of a forced vortex for orienting the flow through impeller blades during off design transient regime. The maximum pressure at pump upstream, as computed from start transient simulation, was used as a design input for pump inlet feed lines. The engine was realized and subsequently qualified in a ground test facility. Hot test data obtained for pressure and flow rate during transient regime were found to be in good agreement with the simulation results.

Continuous subcortical language mapping in awake glioma surgery.

Repetitive monopolar short-train stimulation (STS) delivered from a suction probe enables continuous mapping and distance assessment of corticospinal tracts during asleep glioma resection. In this study, we explored this stimulation technique in awake glioma surgery. Fourteen patients with glioma involving language-related tracts were prospectively included. Continuous (3-Hz) cathodal monopolar STS (five pulses, 250 Hz) was delivered via the tip of a suction probe throughout tumor resection while testing language performance. At 70 subcortical locations, surgery was paused to deliver STS in a steady suction probe position. Monopolar STS influence on language performance at different subcortical locations was separated into three groups. Group 1 represented locations where STS did not produce language disturbance. Groups 2 and 3 represented subcortical locations where STS produced language interference at different threshold intensities (≥7.5 and ≤5 mA, respectively). For validation, bipolar Penfield stimulation (PS; 60 Hz for 3 s) was used as a “gold standard” comparison method to detect close proximity to language-related tracts and classified as positive or negative regarding language interference. There was no language interference from STS in 28 locations (Group 1), and PS was negative for all sites. In Group 2 (STS threshold ≥ 7.5 mA; median, 10 mA), there was language interference at 18 locations, and PS (median, 4 mA) was positive in only one location. In Group 3 (STS threshold ≤ 5 mA; median, 5 mA), there was language interference at 24 locations, and positive PS (median 4 mA) was significantly (p < 0.01) more common (15 out of 24 locations) compared with Groups 1 and 2. Despite the continuous stimulation throughout tumor resection, there were no seizures in any of the patients. In five patients, temporary current spread to the facial nerve was observed. We conclude that continuous subcortical STS is feasibly also in awake glioma surgery and that no language interference from STS or interference at ≥7.5 mA seems to indicate safe distance to language tracts as judged by PS comparisons. STS language interference at STS ≤ 5 mA was not consistently confirmed by PS, which needs to be addressed.

Suction and Pulsed Blowing Applied to Local Separation Downstream of Slat Cutout

An experimental study on the application of active flow control (AFC) to a .4 scale model of a swept wing in a landing configuration was conducted. The wing is fitted with an ultrahigh bypass ratio (UHBR) engine nacelle. The highly efficient UHBR engines are characterized by a large diameter that interferes with the flow around the wing, especially with slats deflected, degrading its performance. An innovative AFC device, creating steady suction and pulsed blowing (PB), was installed in the leading-edge region of the wing, above the nacelle, and its performance was experimentally evaluated. The effects of the suction and PB mechanisms were examined individually and simultaneously using relevant normalized parameters to pave the way to a full-scale wind-tunnel test. It was shown that the AFC actuators increased lift by up to 3%, redirected the flow to the desired downstream direction, and reduced the size of the separation zone created due to the implementation of the UHBR nacelle. The next step is validating the small-scale results of this study in a full-scale wind-tunnel tests that would be the last step before the technology is flight test ready.

Analysis of pulsed suction flow control behavior based on a nonlinear reduced-order model

The use of pulsed suction is an effective unsteady flow control method. However, compared with steady suction, which simply eliminates low-energy boundary layer, pulsed suction flow control presents unique phenomena and complex control mechanisms that are not explained fully. Thus, in this paper, pulsed suction flow control in a compressor cascade is investigated by numerical simulation. Numerical results show that the control performance depends on the suction frequency, momentum, and location, as any unsteady active flow control method. To further reveal the mechanism and explain the phenomena of pulsed suction, a nonlinear reduced-order model for pulsed suction flow control is established on the basis of the Navier–Stokes equation, Stuart vortex row model, Oseen vortex model, and Stuart–Landau equation. The entrainment degree and maximal Lyapunov exponent are used as indices to evaluate the flow losses and control performance in this model. By solving the equations of this model under different parameters, the model presents similar phenomena as found in the numerical simulations with various suction frequencies, momentum, and locations. Thus, it is innovative that the model with calibrated parameters has the ability to roughly predict control performance of unsteady pulsed suction in an efficient way. Also, based on this reduced-order model, one can explain that, unlike steady suction, chaos control, momentum transfer and lock-in mechanisms are especially relevant with the control physics of pulsed suction.

Characteristics of forced flow past a square cylinder with steady suction at leading-edge corners

We experimentally investigate the characteristics of a dynamic wake and of flow separation for a square cylinder with steady suction at its leading-edge corners. The wind tunnel experiments were conducted at a Reynolds number of 5946, and suction slots were manufactured symmetrically at the leading corners of the square cylinder. Steady suction was characterized with a suction momentum coefficient Cμ varying from 0.0227 to 0.3182. A time-resolved particle image velocimetry system was used to evaluate the control of leading-edge suction at different Cμ. Next, the measurements were analyzed by applying a proper orthogonal decomposition (POD) to study the control effectiveness. The POD results suggest that the first four modes of wake vortex shedding are transformed in controlled cases and that periodic Karman vortex shedding is suppressed. The results also show that, even with a very small momentum coefficient, the steady suction at the leading-edge corners stabilizes the cylinder wake. The wake region becomes longer and narrower in comparison with the baseline case. In addition, modifications of separation flow were visualized. At quite small Cμ, flow separation at the leading-edge corners is considerably suppressed. Upon increasing the suction momentum coefficient to 0.1364, flow separation at the leading edges is almost eliminated. Finally, we estimate the effect of drag reduction due to the leading-edge suction.

Numerical study on the steady suction active flow control of hydrofoil in the profile of the blended-wing-body underwater glider

The hydrodynamic performance of the blended-wing-body underwater glider can be improved by opening a hole on the surface and applying the steady suction active flow control. In order to explore the influence law and mechanism of the steady suction active flow control on the lift and drag performance of the hydrofoil, which is the profile of the blended-wing-body underwater glider, based on the computational fluid dynamics (CFD) method and SST k-ω turbulence model, the steady suction active flow control of hydrofoil under different conditions is studied, which include three suction factors: suction angle, suction position and suction ratio, as well as three different flow states: no stall, critical stall and over stall. Then the influence mechanism in over stall flow state is further analyzed. The results show that the flow separation state of NACA0015 hydrofoil can be effectively restrained and the flow field distribution around it can be improved by a reasonable steady suction, so as to the lift-drag performance of NACA0015 hydrofoil is improved. The effect of increasing lift and reducing drag of steady suction is best at 90° suction angle and symmetrical about 90° suction angle, and it is better when the steady suction position is closer to the leading edge of the hydrofoil. In addition, with the increase of the suction ratio, the influence of steady suction on the lift coefficient and drag coefficient of hydrofoil is greater.

Pulsed Suction Control in a Highly Loaded Compressor Cascade With Low Suction Flowrates

Abstract The influence of pulsed suction (PS) on flow separation and aerodynamic performance in a highly loaded compressor cascade is experimentally studied herein. The excitation frequency is investigated as it determines the effectiveness of PS in flow control. Low suction flowrates are examined to analyze the potential of PS in providing a satisfactory cascade performance. For comparison, the corresponding parameters of steady continuous suction (SCS) are studied as well. Oil-flow visualizations and steady and unsteady pressure data are used to characterize the control effects of SCS and PS. The experimental results validate the efficacy of PS in controlling flow separation, even at a reduced suction flowrate of 0.1%. It suppresses passage vortex (PV), improving aerodynamic performance. PS provides a better control effect than SCS at different excitation parameters, which can be attributed to twofold main reasons: first, at the same suction flowrate, PS has a larger suction momentum than SCS during the suction phase, resulting in a stronger suction force and having a more profound effect on the flow characteristics; and second, owing to the introduction of pulsed excitation to the suction, PS creates additional vortex structures that energize the boundary layer by transporting high momentum freestream fluid near the wall. PS is also effective at a higher incidence angle, but its control effect is reduced.

Performance Assessment of Fluidic Oscillators Tested on the NASA Hump Model

Flow separation control over a wall-mounted hump model was studied experimentally to assess the performance of fluidic oscillators (sweeping jet actuators). An array of fluidic oscillators was used to control flow separation. The results showed that the fluidic oscillators were able to achieve substantial control over the separated flow by increasing the upstream suction pressure and downstream pressure recovery. Using the data available in the literature, the performance of the fluidic oscillators was compared to other active flow control (AFC) methods such as steady blowing, steady suction, and zero-net-mass-flux (ZNMF) actuators. Several integral parameters, such as the inviscid flow comparison coefficient, pressure drag coefficient, and modified normal force coefficient, were used as quality metrics in the performance comparison of the AFC methods. These quality metrics indicated the superiority of the steady suction method, especially at lower excitation amplitudes that is followed by the fluidic oscillators, steady blowing, and the ZNMF actuators, respectively. An aerodynamic figure of merit (AFM) was also constructed using the integral parameters and AFC power usage. The AFM results revealed that, for this study, steady suction was the most efficient AFC method at lower excitation amplitudes. The steady suction loses its efficiency as the excitation amplitude increases, and the fluidic oscillators become the most efficient AFC method. Both the steady suction and the fluidic oscillators have an AFM &gt; 1 for the range tested in this study, indicating that they provide a net benefit when the AFC power consumption is also considered. On the other hand, both the steady blowing and ZNMF actuators were found to be inefficient AFC methods (AFM &lt; 1) for the current configuration. Although they improved the flow field by controlling flow separation, the power requirement was more than their benefit.

Ludwig Prandtl’s envisage: elimination of von Kármán vortex street with boundary-layer suction

The present study is a revisit to Ludwig Prandtl’s elimination of von Karman vortex street behind a circular cylinder by using steady suction in the boundary layer. We show in particular the full-time vortex evolutions and vortex dynamics in the wake. The wind tunnel investigations are conducted at the Reynolds number (Re) of 2.0 × 104. Slot suction is implemented symmetrically on both lower and upper boundary layers of the cylindrical test model to modify the flow separation process and to eliminate the downstream vortex street. The boundary layer suction is described by the dimensionless momentum parameter of suction relative to the free-stream airflow. A high-speed particle image velocimetry system is employed in the present study to visualize the flow patterns and capture the wake dynamics of the natural and modified cylinders controlled with boundary-layer suction.

CFD Investigation of Separation Control on a Vertical Axis Wind Turbine: Steady and Unsteady Suction

High-fidelity two-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) simulations are employed to investigate the influence of boundary layer suction through a slot located near the leading edge of a vertical axis wind turbine operating in dynamic stall. The analysis includes both steady and unsteady suction with different frequencies. The results shows that: (i) when the suction slot is located within the chordwise extent of the laminar separation bubble, dynamic stall can be avoided with minimal suction amplitude; (ii) the most promising suction location is the most upstream suction location studied, at 8.5%c where c is the blade chord length; (iii) the suction only needs to be applied during the azimuthal angles when dynamic stall occurs; (iv) the oscillation frequency of the suction velocity has insignificant influence on the obtained turbine power gain; (v) applying unsteady suction is interesting as it reduces the energy consumption of the suction system, thus the net power gain.

A comparative experimental analysis of two unsteady flow control methods in a highly loaded compressor cascade

A comparative experimental analysis is performed on pulsed suction (PS) and pulsed blowing (PB) used to control flow separations in a highly loaded compressor cascade. The effectiveness of the control methods is assessed via oil-flow visualization and steady and unsteady pressure measurements. Firstly, the control effect of the PS is evaluated by comparing the conventional steady continuous suction (SCS). A more efficient control effect is achieved by the PS compared to the SCS. Additionally, in order to further explore the potentials of the PS and PB and gain some insight into their controlling mechanisms, some important excitation parameters including excitation location, momentum coefficient and frequency are comparatively investigated in detail. It is found that the PS and PB are both able to improve the cascade performance by effectively suppressing the passage vortex. With the excitation location moving downstream, the almost opposite change trends for the PS and PB on the total pressure loss and energy efficiency are shown. The PS has an advantage over the PB in improving the cascade performance at the same average excitation momentum. But there is a slighter change of the losses for the PB cases at different excitation frequencies relative to the PS ones, indicating that the PB is more insensitive to the excitation frequency. Based on the optimal excitation parameters, the total pressure loss coefficients for the PS and PB are reduced by 11.3% and 10.3%, respectively. Furthermore, the effectiveness of the PS and PB is also corroborated at a larger incidence angle.