Boundary Layer Suction Research Articles

Boundary layer modification by means of wall suction and the effect on the wake behind a rectangular forebody

The wake characteristics of a two-dimensional rectangular forebody with a smooth leading edge and a blunt trailing edge are investigated. Wall suction is applied along the forebody in order to modify the developing boundary layer. An initially laminar boundary layer subject to suction yields an asymptotic suction boundary layer at the trailing edge of the body, whereas a high enough suction coefficient relaminarizes an initially turbulent boundary layer. The critical suction velocity required to achieve this significant modification of the boundary layer properties is typically in the order of 1% of the free-stream velocity, where the critical suction coefficient depends on the Reynolds number. We show that a thinner boundary layer induces a higher vortex shedding frequency and a lower base pressure. Furthermore, the boundary layer state, laminar or turbulent, has a significant influence on the wake. For example, the Strouhal number based on the effective body thickness is being reduced by 25% from laminar to turbulent inlet conditions.

Corner Separation Control by Boundary Layer Suction Applied to a Highly Loaded Axial Compressor Cascade

Control of corner separation has attracted much interest due to its improvement of performance and energy utilization in turbomachinery. Numerical studies have been performed under both design and off-design flow conditions to investigate the effects of boundary layer suction (BLS) on corner separation in a highly loaded compressor cascade. Two new BLS slot configurations are proposed and a total of five suction slot configurations were studied and compared. Averaged static pressure rise, exit loss coefficient, passage blockage and flow turning angle have been given and compared systematically over a range of operation incidence angles. Distributions of significant loss removal, blade loading, exit deviation and total pressure loss at 3 degree and 7 degree incidence have also been studied. Under the same suction mass flows of 0.7% of the inlet mass flows, the pitchwise suction slot on the endwall shows a better optimal performance over the whole operation incidence among single suction slots. By using of the new proposed compound slot configuration with one spanwise slot on the blade suction side and one pitchwise slot on the endwall, the maximum reduction of total pressure loss at 7 degree incidence can be 39.4%.

Performance investigation of a supersonic air intake in the presence of the boundary layer suction

The performance of a supersonic mixed compression air intake has been investigated experimentally. The intake is of axisymmetric one and has been designed for a free stream Mach number of 2.0. The present work has two main goals, first to investigate the performance of the intake without boundary layer bleed at design and at off-design conditions and second to study the effects of a bleed slot on the intake performance. The intake has been tested at free stream Mach numbers of 1.8, 2.0, and 2.2 and at zero degrees angle of attack. Total pressure recovery, mass flow ratio, and flow distortion have been selected to assess the intake performance. The bleed slot is located upstream of the intake throat perpendicular to the compression ramp surface. The suction is applied by the natural pressure difference between the entrance and the exit faces of the bleed duct. Results show that applying the boundary layer suction upstream of the intake throat can considerably improve the intake performance at its design and off-design conditions while it does not affect the intake mass flow rate.

Aerodynamic Characteristics of an Airfoil with Boundary Layer Ingestion in Subsonic Flow

The influence of boundary layer ingestion (BLI) on the aerodynamics of airfoil RAE2822 in subsonic is investigated by numerical method. Based on the calculation, it is found that the mass flow ratio (MFR), intake height, cowl length and intake position in the chord can affect on the lift, drag and pitching moment remarkable. Considering a maximum lift-over-drag, intake position in the chord should be rearward as possible with a certain high MFR at a small angle of attack and a considerable intake height. There is a proper MFR which makes drag lowest.

Opportunities and challenges for distributed propulsion and boundary layer ingestion

Purpose – The performance benefits of boundary layer ingestion (BLI) in the case of air vehicles powered by distributed propulsors have been documented and explored extensively by numerous studies. Therefore, it is well known that increased inlet flow distortion due to BLI can dramatically reduce these benefits. In this context, a methodology that enables the assessment of different propulsion architectures, whilst accounting for these aerodynamic integration issues, is studied in this paper. Design/methodology/approach – To calculate the effects of BLI-induced distortion, parametric and parallel compressor approaches have been implemented into the propulsion system analysis. The propulsion architectures study introduces the concept of thrust split between propulsors and main engines and also examines an alternative propulsor configuration. In the system analysis, optimum configurations are defined using thrust-specific fuel consumption as figure of merit. Findings – For determined operating conditions, t...

A hybrid engine concept for multi-fuel blended wing body

Purpose – The purpose of this paper is to present a novel hybrid engine concept for a multi-fuel blended wing body (MFBWB) aircraft and assess the performance of this engine concept. Design/methodology/approach – The proposed hybrid engine concept has several novel features which include a contra-rotating fan for implementing boundary layer ingestion, dual combustion chambers using cryogenic fuel (liquefied natural gas [LNG] or liquid hydrogen [LH2]) and kerosene in the inter-turbine burner (in flameless combustion mode) and a cooling system for bleed air cooling utilizing the cryogenic fuel. A zero-dimensional thermodynamic model of the proposed hybrid engine is created using Gas Turbine Simulation Program to parametrically analyse the performance of various possible engine architectures. Furthermore, the chosen engine architecture is optimized at a cycle reference point using a developed in-house thermodynamic engine model coupled with genetic algorithm. Findings – Using LH2 and kerosene, the hybrid eng...

Drag-Reduction Mechanisms of Suction-and-Oscillatory-Blowing Flow Control

An active-flow-control study, using steady suction-and-oscillatory-blowing actuators, was conducted on an axisymmetric bluff-body model for a range of Reynolds numbers between and . Previous work on the same model demonstrated the experimental implementation and efficient drag reduction of the suction-and-oscillatory-blowing actuator system, including comparisons to computational fluid dynamics results. The current study presents a detailed analysis of the experimental data, coupled with a refined computational model toward a flow-physics understanding of the drag-reduction mechanisms of the suction-and-oscillatory-blowing active-flow-control system. The boundary-layer response was examined using time-averaged and phase-averaged hot-wire measurements conducted on the aft portion of the model where active flow control was applied. The drag-reduction behavior was scaled using multiple active-flow-control parameters associated with the unique and complex features of the suction-and-oscillatory-blowing active-flow-control system. The results show that the drag-reduction mechanisms associated with the suction-and-oscillatory-blowing actuation system include boundary-layer suction, wall-jet momentum addition, unsteady shear-layer excitation, the generation of thrust, and streamwise vortices.

Performances of Water Jet Propulsion with PD Pump

Water jet propulsion (WJP) with positive displacement pump (PD pump), in which the suction thrust of PD pump (Due to the effect of pump suction and the influence of the suction boundary layer) and the reaction thrust of water jet are used to push watercraft forward, has many advantages like high efficiency,low noise, high maneuverability, good adaptability to variable working conditions, simple transmission mechanism over traditional propulsion methods. A mathematical model is developed and the WJP theory with PD pump is analyzed. Numerical studies are carried out to study the suction thrust, reaction thrust, and propulsion efficiency of the thruster with Bernoullihyperbolic suction inlet-PD pump-Bernoulli hyperbolic nozzle. The results show that: 1) in the conditions of no cavitation in suction inlet and constant flow rate, the suction thrust is inversely proportional to the inlet diameter and reaches around 8.5% of the total thrust; 2) the reaction thrust increases with the decrease of the nozzle diameter; 3) the pump-suction coefficient v is proportional to the speed ratio μ; 4) the efficiency of WJP with PD pump is higher than with negative displacement pump under the same speed ratio. The related conclusions provide a basis for the design of high performance water propulsion unit.

Design considerations for the components of electrically powered active high-lift systems in civil aircraft

To address the challenges of future air traffic, such as more accessible air travel with better public acceptance, this research work focuses on efficient active high-lift systems (AHLS). This system applies boundary layer suction combined with a controlled Coanda jet. Electrically powered compressors are located in the wings at each flap to supply the suction and blowing air mass flow rate to the AHLS, thus coupling the two in a useful way. This concept provides flexibility and controllability of lift generation and is consistent with the trend towards an “all-electric aircraft”. General models for assessment of such systems in preliminary aircraft design are not available yet as they require interdisciplinary expertise. This paper provides general design considerations for the components of this type of electrically powered AHLS, comprising compressors, motors, power electronics, system peripherals, and generators. The conceptual design for those components results in models for preliminary aircraft design which allows estimating power consumption, size and mass.

Dynamics of flow control in an emulated boundary layer-ingesting offset diffuser

Dynamics of flow control comprised of arrays of active (synthetic jets) and passive (vanes) control elements , and its effectiveness for suppression of total-pressure distortion is investigated experimentally in an offset diffuser, in the absence of internal flow separation. The experiments are conducted in a wind tunnel inlet model at speeds up to M = 0.55 using approach flow conditioning that mimics boundary layer ingestion on a Blended-Wing-Body platform. Time-dependent distortion of the dynamic total-pressure field at the ‘engine face’ is measured using an array of forty total-pressure probes, and the control-induced distortion changes are analyzed using triple decomposition and proper orthogonal decomposition (POD). These data indicate that an array of the flow control small-scale synthetic jet vortices merge into two large-scale, counter-rotating streamwise vortices that exert significant changes in the flow distortion. The two most energetic POD modes appear to govern the distortion dynamics in either active or hybrid flow control approaches. Finally, it is shown that the present control approach is sufficiently robust to reduce distortion with different inlet conditions of the baseline flow.

Canonical exact coherent structures embedded in high Reynolds number flows.

The applications and implications of two recently addressed asymptotic descriptions of exact coherent structures in shear flows are discussed. The first type of asymptotic framework to be discussed was introduced in a series of papers by Hall & Smith in the 1990s and was referred to as vortex-wave interaction theory (VWI). New results are given here for the canonical VWI problem in an infinite region; the results confirm and extend the results for the infinite problem inferred the recent VWI computation of plane Couette flow. The results given define for the first time exact coherent structures in unbounded flows. The second type of canonical structure described here is that recently found for asymptomatic suction boundary layer and corresponds to freestream coherent structures (FCS), in boundary layer flows. Here, it is shown that the FCS can also occur in flows such as Burgers vortex sheet. It is concluded that both canonical problems can be locally embedded in general shear flows and thus have widespread applicability.

Periodically bursting edge states in plane Poiseuille flow

We investigate the laminar-turbulent boundary in plane Poiseuille flow by the method of edge tracking. In short and narrow computational domains we find for a wide range of Reynolds numbers that all states in the boundary converge to a periodic orbit with a period of the order of time units. The attracting states in these small domains are periodically extended in the spanwise and streamwise directions, but always localized to one side of the channel in the normal direction. In wider domains the edge states are localized in the spanwise direction as well. The periodic motion found in the small domains then induces a large variety of dynamical activity that is similar to that found in the asymptotic suction boundary layer.

Free-stream coherent structures in parallel boundary-layer flows

AbstractOur concern in this paper is with high-Reynolds-number nonlinear equilibrium solutions of the Navier–Stokes equations for boundary-layer flows. Here we consider the asymptotic suction boundary layer (ASBL) which we take as a prototype parallel boundary layer. Solutions of the equations of motion are obtained using a homotopy continuation from two known types of solutions for plane Couette flow. At high Reynolds numbers, it is shown that the first type of solution takes the form of a vortex–wave interaction (VWI) state, see Hall & Smith (J. Fluid Mech., vol. 227, 1991, pp. 641–666), and is located in the main part of the boundary layer. On the other hand, here the second type is found to support an equilibrium solution of the unit-Reynolds-number Navier–Stokes equations in a layer located a distance of $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}O(\ln \mathit{Re})$ from the wall. Here $\mathit{Re}$ is the Reynolds number based on the free-stream speed and the unperturbed boundary-layer thickness. The streaky field produced by the interaction grows exponentially below the layer and takes its maximum size within the unperturbed boundary layer. The results suggest the possibility of two distinct types of streaky coherent structures existing, possibly simultaneously, in disturbed boundary layers.

A class of exact Navier–Stokes solutions for homogeneous flat-plate boundary layers and their linear stability

AbstractWe introduce a new boundary layer formalism on the basis of which a class of exact solutions to the Navier–Stokes equations is derived. These solutions describe laminar boundary layer flows past a flat plate under the assumption of one homogeneous direction, such as the classical swept Hiemenz boundary layer (SHBL), the asymptotic suction boundary layer (ASBL) and the oblique impingement boundary layer. The linear stability of these new solutions is investigated, uncovering new results for the SHBL and the ASBL. Previously, each of these flows had been described with its own formalism and coordinate system, such that the solutions could not be transformed into each other. Using a new compound formalism, we are able to show that the ASBL is the physical limit of the SHBL with wall suction when the chordwise velocity component vanishes while the homogeneous sweep velocity is maintained. A corresponding non-dimensionalization is proposed, which allows conversion of the new Reynolds number definition to the classical ones. Linear stability analysis for the new class of solutions reveals a compound neutral surface which contains the classical neutral curves of the SHBL and the ASBL. It is shown that the linearly most unstable Görtler–Hämmerlin modes of the SHBL smoothly transform into Tollmien–Schlichting modes as the chordwise velocity vanishes. These results are useful for transition prediction of the attachment-line instability, especially concerning the use of suction to stabilize boundary layers of swept-wing aircraft.

Flow control of a boundary layer ingesting serpentine diffuser via blowing and suction

The use of serpentine boundary layer ingesting (BLI) diffusers offers a significant benefit to the performance of Blended Wing Body (BWB) aircraft. However, the inherent diffuser geometry combined with a thick ingested boundary layer creates strong secondary flows that lead to severe flow distortion at the engine face, increasing the possibility of engine surge. This study investigated the use of enabling active flow control methods to reduce engine-face distortion. An ejector-pump based system of fluidic actuators was used to directly manage the diffuser secondary flows. This system was modeled computationally using a boundary condition jet modeling method, and tested in an ejector-driven wind tunnel facility. This facility is capable of simulating the high-altitude, high subsonic Mach number conditions representative of BWB cruise conditions, specifically a cruise Mach number of 0.85 at an altitude of 12,000 m. The tunnel test section used for this experiment was designed, built, and tested as a validation tool for the computational methods.

Comoving frames and symmetry-related motions in parallel shear flows

AbstractParallel shear flows have continuous symmetries of translation in the downstream and spanwise directions. As a consequence, flow states that differ in their spanwise or downstream location but are otherwise identical are dynamically equivalent. In the case of travelling waves, this trivial degree of freedom can be removed by going to a frame of reference that moves with the state, thereby turning the travelling wave in the laboratory frame into a fixed point in the comoving frame of reference. We here discuss a general approach, the method of comoving frames, by which the symmetry related motions can also be removed for more complicated and dynamically active states and demonstrate its application for several examples. For flow states in the asymptotic suction boundary layer (ASBL) we show that in the case of the long-period oscillatory edge state we can find local phase speeds which remove the fast oscillations and reveal the slow vortex dynamics underlying the burst phenomenon. For spanwise translating states we show that the method removes the drift but not the dynamical events that cause the big spanwise displacement. For a turbulent case we apply the method to the spanwise shifts and find slow components that are correlated over very long times. Calculations for plane Poiseuille flow show that the long correlations in the transverse motions are not specific to the ASBL.

Numerical Studies on a Rotor with Distributed Suction for Noise Reduction

Minimizing the flow-induced noise is an important issue in the design of modern onshore wind turbines. There is a number of proven passive means to reduce the aeroacoustic noise, such as the implementation of serrations, porous trailing edges or the aeroacoustic airfoil design. The noise emission can be further reduced by active flow control techniques. In the present study the impact of distributed boundary layer suction on the noise emission of an airfoil and a complete rotor is investigated. Aerodynamic and aeroacoustic wind tunnel tests were performed for the NACA 64-418 airfoil and supplemented by numerical calculations. The aeroacoustic analyses have been conducted by means of the institute's Rnoise prediction scheme. The 2D studies have shown that noise reductions of 5 dB can be achieved by suction at moderate mass flow rates. To study the impact of three-dimensional effects numerical investigations have been conducted on the example of the generic NREL 5MW rotor with suction applied in the outer part of the blade. The predictions for the complete rotor provided smaller benefits compared to those for the isolated airfoil, mainly because the examined suction configurations were not optimized with respect to the extent of the suction patch and suction distribution.

Methodology to assess the performance of an aircraft concept with distributed propulsion and boundary layer ingestion using a parametric approach

The performance benefits of boundary layer ingestion in aircraft with distributed propulsion have been extensively studied in the past. These studies have indicated that propulsion system integration issues such as distortion and intake pressure losses could mitigate the expected benefits. This paper introduces and develops a methodology that enables the assessment of different propulsion system designs, which are optimized to be less sensitive to the effects of the aforementioned issues. The study models the propulsor array and main engine performance at design point using a parametric approach, and further at component level, the study focuses on identifying optimum propulsor configurations, in terms of propulsor pressure ratio and BL capture sheet height. At a system level, the study assesses the effects of splitting the thrust between the propulsor array and main engines. The figure of merit used in the optimization is the TSFC. The suitability of the concepts is further assessed using performance predictions for HTS electrical motors. For the purpose of this study, the NASA N3-X aircraft concept is selected as baseline configuration, where the different propulsion designs are tested. As the study focuses on performance assessment of the propulsion system, sizing implication issues and aircraft performance installations effects have not been included in the analysis. The results from the parametric analysis corroborated previous studies regarding the high sensitivity of the propulsion system performance to intake losses and BL inlet conditions. As the study found low-power consumption configurations at these operating conditions, this may be considered as a major issue. The system analysis from the study indicated that splitting the thrust between propulsors and main engines results in improved system efficiency with beneficial effects in fuel savings. When a 2% increase in intake pressure losses and a similar reduction in fan efficiency were assumed due to boundary layer ingestion, the study found an optimum configuration with 65% of thrust delivered by the propulsor array. To summarize, the present work built on past research further contributes to the field through the inclusion of the thrust split as a key variable in the propulsion system design. The thrust split, when introduced, enabled reduction of the detrimental effects of intake losses on the overall system performance. Additionally, as it reduces the power required for the propulsor array, it is expected to reduce the operating power of HTS and cooling systems and therefore improve the effectiveness of the concept.

Jet Engine Fuel Burn Reduction Through Boundary Layer Ingestion

This article explains various technical aspects of the boundary layer ingestion (BLI) concept. Using BLI, airliner designs featuring close-coupled, rear-mounted turbofans are being considered, with a fuselage sculpted to sweep a large part of the fuselage boundary layer into engine inlets for reduced fuel consumption. With an engine array fuselage-centered, rather than splayed out on wings, reduced rudder control is needed in the event of a single engine outage. This reduces the size of a BLI tail assembly, saving weight and reducing drag. A near-future goal of the BLI studies is to determine if modern engine front-mounted fans can be designed to operate efficiently and stably under BLI inlet conditions. The D8 design is aimed at the huge single-aisle, narrow-body market, now dominated by the Boeing 737 and Airbus 320 families. Airframe and engine designers strive to achieve 'clean' inlet flow conditions for jet engines.

Complexity of localised coherent structures in a boundary-layer flow.

We study numerically transitional coherent structures in a boundary-layer flow with homogeneous suction at the wall (the so-called asymptotic suction boundary layer ASBL). The dynamics restricted to the laminar-turbulent separatrix is investigated in a spanwise-extended domain that allows for robust localisation of all edge states. We work at fixed Reynolds number and study the edge states as a function of the streamwise period. We demonstrate the complex spatio-temporal dynamics of these localised states, which exhibits multistability and undergoes complex bifurcations leading from periodic to chaotic regimes. It is argued that in all regimes the dynamics restricted to the edge is essentially low-dimensional and non-extensive.