Laminar Flow Separation Research Articles

Theory-based prediction of separation angle and peak pressure for laminar separated hypersonic compression corner flows

Interactions between shock waves and the boundary layer will cause high pressure loads and severe heating locally on the material surface in hypersonic separation–reattachment flows. In this paper, theoretical modeling is employed to study the separation angle and peak pressure of hypersonic compression corner flows with separated regions at low-to-medium Reynolds number. According to the characteristics of wall pressure distribution and flow field structures, an equivalent double shocks inviscid flow model for peak pressure is introduced. Based on this model, combining the “free-interaction theory,” formulas for the separation angle and peak pressure with freestream parameters (Mach number and Reynolds number) and the geometry corner angle are derived, and the reasonability and effectiveness are also shown by the numerical data provided by the direct simulation Monte Carlo method. It is revealed that the separation angle is almost independent of the freestream Mach number, and the ratio of peak pressure to freestream pressure is approximately proportional to the square of the freestream Mach number under strong viscous interaction conditions. Moreover, it is indicated that present theoretical results can be extended to axisymmetric flows through the discussion of hypersonic separated flows generated by a hollow cylinder-flare geometry.

Axisymmetric Hybrid Nanofluid Flow Due to a Convectively Heated Stretching/Shrinking Disk

This significant study is designed to analyze the axisymmetric hybrid nanofluid flow with heat transfer on a convectively heated stretching/shrinking disk. The combination of metal (Cu) and metal oxide (Al2O3) nanoparticles with water (H2O) as the base fluid is used for the analysis. Similarity transformation is adopted to reduce the complexity of the PDEs into a system of ODEs. The utilization of suction in maintaining the steady flow solution for the shrinking disk case discloses the presence of dual solutions. Besides, an upsurge of Biot number and suction’s strength enhances the heat transfer operation. The application of Cu-Al2O3/water nanofluid can extend the range of solutions’ existence and consequently, decelerate the separation of laminar flow.

Dynamics of Shock Waves Interacting With Laminar Separated Transonic Turbine Flow Investigated by High-Speed Schlieren and Surface Hot-Film Sensors

Abstract The results presented in this paper are based on experimental investigations on a generic transonic low pressure turbine profile at high subsonic exit Mach numbers. Here, the flow on the suction side reaches a maximum isentropic Mach number of approximately 1.2 and features a large separation bubble in a transonic flow regime characterized by surface hot-film measurements. The measurements are supplemented by Schlieren images recorded with a high-speed camera at 19.2 kHz. A highly unsteady normal shock wave on the suction side is observable upstream of the trailing edge. It is interacting with laminar separated flow which is rarely documented in literature. The interaction of the normal shock with the boundary layer flow seems to amplifies the ongoing transition process over the separation bubble and the flow reattaches shortly downstream. A statistical analysis of the Schlieren images reveals characteristic low frequencies of the shock wave motions and a pulsation of the separation bubble. Additionally, the statistical information of the time-dependent signal from the surface hot-film sensors demonstrate the instabilities influencing the boundary layer linked to the unsteadiness in the main flow.

Numerical and experimental investigations into fluid resonance in a gap between two side-by-side vessels

When two vessels in a side-by-side configuration are under wave actions, fluid resonance occurs in a narrow gap between two vessels, termed ‘gap resonance’, leading to large free-surface responses. Large free-surface responses due to gap resonance can induce large relative motions and drift forces by both vessels, which influences operational safety. The present study aims at investigating the free-surface response near resonance in a narrow gap between two fixed, identical barges with square corners numerically and experimentally. Gap resonance is driven by three typical incident wave conditions including beam sea, quartering sea, and head sea. In light of the fact that the potential-flow theory overestimates the response in the gap, a potential-flow model with energy dissipation effects is developed based on the boundary element method. A dissipation surface is devised at the bottom opening as well as two end openings of the gap. Both linear and quadratic damping terms associated with laminar boundary layer wall friction and flow separation from sharp corners are accounted for. Physical experiments of gap resonance excited by irregular waves with different significant wave heights and peak periods are carried out and used for calibration and validation of the potential-flow model. Nonlinear correlation is observed between the incident wave amplitude and resonant gap response, which is predicted with satisfactory agreement by the developed potential-flow model. In-depth discussion on the mode shapes is made. It is found that the first mode is predominant in the beam sea, and the natural modes exhibit standing wave behaviors. Under quartering sea and head sea excitations, the second and third modes dominate over other modes. Waves propagate slowly along the gap in the head sea condition. Under the quartering sea excitation, the first and second modes propagate along the gap, but the third mode exhibits standing wave behavior, indicating the quartering sea response is a transition between head sea and beam sea. The significance of the present study is threefold. Firstly, the experimental measurements provide reference results for numerical simulation. Secondly, a simple but effective damping model to suppress unrealistic free-surface response near resonance is developed. Lastly, this work gives an insight into the spatial structure and dominance of natural modes under different wave conditions.

Mixed convective stagnation point flow of a hybrid nanofluid toward a vertical cylinder

PurposeThe purpose of this paper is to numerically analyze the stagnation point flow of Cu-Al2O3/water hybrid nanofluid with mixed convection past a flat plate and circular cylinder.Design/methodology/approachThe similarity equations that reduced from the boundary layer and energy equations are solved using the bvp4c solver. The duality of solutions is observed within the specific range of the control parameters, namely, mixed convection parameterλ, curvature parameterγand nanoparticles volumetric concentrationϕ1for alumina, while for copperϕ2. The stability analysis is also designed to justify the particular solutions’ stability. Additionally, the idea to obtain the solution for large value ofλandγis also presented in this paper.FindingsTwo solutions exist in opposing and assisting flows up to a critical valueλcwhereλclies in the opposing region. An upsurge of the curvature parameter tends to extend the critical value (delay the separation process), whilst increase the heat transfer performance of the working fluid. Meanwhile, the application of hybrid Cu-Al2O3/water nanofluid also can decelerate the separation of laminar boundary layer flow and produce higher heat transfer rate than the Cu–water nanofluid and pure water.Originality/valueThe results are new and original. This study benefits to the other researchers, specifically in the observation of the fluid flow characteristics and heat transfer rate of the hybrid nanofluid. Also, this paper features with the mathematical formulation for the solution with large values ofλandγ.

Numerical simulation of transitional flows using a blended IDDES and correlation-based transition model

In the present study, a hybrid RANS/LES model and a correlation-based transition model were coupled for simulating flow involving massive flow separation and laminar-turbulent transition including crossflow direction. For this purpose, blending of IDDES model and the γ−Reθt−CF+ transition model was accomplished in a tightly coupled manner. For validations, numerical simulations around a circular cylinder involving laminar flow separation without turbulent reattachment were conducted. Numerical simulations were also conducted for flows around an Aerospatiale A-airfoil at a larger Reynolds number accompanying turbulent flow reattachment. As an application of the present blended model for flows involving crossflow transition, calculations were performed for flows around a 6:1 prolate spheroid, and the results were compared with experiment and those without considering flow transition induced by the crossflow instability. Additional application was also made for flows around an ONERA M6 wing at a relatively high angle of attack, and the results were compared with those of the baseline models and experiment. It was founded that the results by the present blended model are in good agreements with experiment, and show improvements over other models by considering flow transition and unsteady flow feature simultaneously. It was concluded that the present blended model is useful for predicting flows involving massive flow separation and crossflow transition.

Laminar separated flows over finite-aspect-ratio swept wings

Abstract

REATTACHMENT OF A LAMINAR SEPARATED FLOW AT A HYPERSONIC VELOCITY OF THE FLOW

This paper describes a study of a laminar separated flow in a compression corner for the Mach number of the incident flow M = 6 and 8. The shock-wave structure of the flow in its reattachment region is considered. The formation mechanism of a high-pressure layer, which is a narrow gas region located above the boundary layer downstream from the attachment line, is investigated.

Control of laminar flow separation over a backward-facing rounded ramp with C-D riblets – The effects of riblet height, spacing and yaw angle

In the work presented in this paper, a section of convergent-divergent (C-D) riblets is applied upstream of a backward-facing rounded ramp in a fully developed laminar channel flow at a Reynolds number based on the channel height of 400. Numerical simulations are undertaken to examine the effects of riblet geometry and yaw angle on the strength of secondary flow produced by the riblets and the extent of flow separation zone. It is found that, in comparison with the baseline case with no riblets, flow separation is delayed and the reattachment occurs earlier leading to a smaller separation zone around the diverging line. The opposite phenomenon occurs around the converging line. Our results also show that for riblets with a given height a maximum strength of the secondary flow motion upstream of the ramp and a maximum reduction in the spanwise-averaged length of separation are obtained at s/h=4. At a given s/h=4, as the riblet height increases, the strength of the secondary flow motion generated by C-D riblets increases and a minimum riblet height of 3.75% of the channel height is required to produce a net reduction in the spanwise-averaged length of separation. For riblets with a given height and spacing, as yaw angle increases both the strength of secondary flow and net reduction of separation zone exhibit a parabolic trend and both of them peak at γ=45°. Overall, with the riblet setting tested here the benefit of suppressing the laminar separation bubble has not resulted in a net reduction in the total pressure losses compared to that in the baseline case. Nevertheless, an examination of the pressure losses from three different sections along the channel separately has produced some insights about the loss mechanisms and pointed to the possible ways by which the pressure losses could be reduced.

Empiricism in Microsampling: Utilizing a Novel Lateral Flow Device and Intrinsic Normalization to Provide Accurate and Precise Clinical Analysis from a Finger Stick.

Phlebotomy plays a key role in clinical laboratory medicine but poses certain challenges for the patient and the laboratory. Dried blood spots simplify collection and stabilize specimens effectively, but clinical reference intervals are based primarily on serum or plasma. We evaluated use of dried separated blood plasma specimens to simplify plasma sample collection via finger stick; however, this sampling technique posed substantial analytical challenges. We discuss herein our efforts to overcome these challenges and provide accurate and precise clinical measurements. Microsamples of whole blood were collected via finger stick using a collection device employing laminar-flow separation of cellular blood and plasma fractions with subsequent desiccation. Samples were analyzed on modern autoanalyzers with FDA-approved reagent and calibration systems, as well as commercially available reagents with laboratory-developed assay parameters. Measured analyte concentrations from extracted dried plasma samples were normalized to a coextracted endogenous analyte, chloride. Chloride normalization reduced variability incurred through extraction and undefined plasma volume. Excellent correlation of normalized measurements from dried finger-stick samples (whole blood and plasma) versus matched venous samples facilitated developing mathematical transformations to provide concordance between specimen types. Independent end-to-end performance verification yielded mean biases <3% for the 5 analytes evaluated relative to venous drawn samples analyzed on FDA-approved measurement systems. Challenges inherent with this microsampling technique and alternate sample matrix were obviated through capabilities of modern autoanalyzers and implementation of chloride normalization. These results demonstrate that self-collected microsamples from a finger stick can give results concordant with those of venous samples.

Effects of Reynolds number and loading distribution on the aerodynamic performance of a high subsonic compressor airfoil

The laminar-turbulent transition process on the compressor blade surface is often induced by the laminar separation flow at low Reynolds number ( Re). In the present study, numerical simulations were conducted to investigate the structure of the laminar separation bubble and its effects on the profile loss of a high subsonic compressor airfoil under different Re conditions, and the mechanism for the performance deterioration of compressor airfoil at low Re was clarified. Besides, the airfoil was redesigned to obtain a series of airfoils with different loading distributions, and the aerodynamic performance of these airfoils was compared and analyzed in detail. According to the simulation results, the laminar separation bubble mainly determined the loss generation process of a compressor airfoil. When Re decreased from 12 × 105 to 1.5 × 105, the laminar separation bubble on the suction surface grew thicker and the length was increased by 11.2% of the axial chord. As such, the reversed flow inside the laminar separation bubble became more obvious and the turbulence level downstream of the maximum thickness of laminar separation bubble was increased. Also, the growth in the turbulent boundary layer was enhanced, causing more serious flow blockage and wake mixing. According to the Denton's profile loss model, the larger trailing edge loss caused by the stronger displacement effect of laminar separation bubble was supposed to be the main reason for the performance deterioration of compressor airfoil under low Re conditions. The ultra-front loading distribution for airfoil has the possibility to suppress or even eliminate the negative effect of laminar separation bubble, and the profile loss was decreased by 26.7% at Re = 1.5 × 105; however, the less significant performance improvement was observed at some higher Re. Moreover, the ultra-front loaded airfoil was less sensitive to the inlet turbulence level and the superiority still holds even at some supercritical conditions.

Numerical investigation of the effects of plasma actuator on separated laminar flows past an incident plate under ground effect

Cylinders and flat plates are important models for studying flow separation. If a surface is in free flow on the upper and lower surfaces, vortices of equal strength are formed in the opposite direction, and this symmetry causes the average force oscillations to be zero, but when the surface approaches the surface-ground, this symmetry disappears. In other words, in addition to several factors such as boundary layer formation and object geometry, altitude also affects the flow field, known as the ground effect phenomenon. In the present work, the behavior and structure of fluid flow around a plate in and out of the boundary layer of the ground are investigated using a finite volume method. After ensuring the desired numerical solution method, the effects of applying the plasma actuator to control the flow structure behind a plate in ground effect is investigated. The plate is placed on the boundary layer zone with the attack angle of 20 degrees. It is found that the separated flow is dependent on the distance between plate and ground. Specifically, turning on the plasma actuator leads to a significant reduction in the separation zone behind the plates.

Transitional Flow on Model Propellers and Their Influence on Relative Rotative Efficiency

Unexpected low value of the relative rotative efficiency ηR is sometimes noted when scaling the towing tank model-test result with the ITTC-78 method to obtain the propulsive efficiency factors of propellers. The paper explains the causes of this phenomenon. The boundary layer state of three propellers was studied by a paint test and a RANS method. The paint tests showed that the propellers in behind conditions at low Reynolds number (Rn) are covered mainly with laminar flow, which is different from open water tests conducted at a high Rn. Apart from that a moderate difference in Rn between the open water and the self-propulsion test may lead to a low ηR value, the paper points out that flow separation in behind conditions could be another significant reason for the drop of ηR for some propellers. Therefore, two factors will lead to an unexpected decrease of ηR: (1) A slightly lower open water torque interpolated from an open water test carried out at a high Rn and (2) a slightly higher torque in a self-propulsion test due to laminar flow separation near the trailing edge. The phenomenon is caused by the Rn scaled effect and closely associated with design philosophy like the blade section profile, the chord length, and chordwise load distribution.

Heat transfer in supersonic separated flow of the compression corner

This article presents the study results for heat transfer in supersonic laminar separated compression corner flow. The study was carried out for free-stream Mach number M = 6 and the compression ramp angle in the range of φ = 20° – 40°.

Formation and turbulent breakdown of large-scale vortical structures behind an obstacle in a channel at moderate Reynolds numbers

This paper deals with experimental investigation and direct numerical simulation of three-dimensional separated laminar and transitional flows behind a semicircular spanwise rib on a bottom wall of a rectangular channel at Reynolds numbers of up to 480. Particular emphasis is given to the formation mechanism of quasiperiodic large-scale vortex clouds in the mixing layer behind the rib. Vortical structures near the channel axis are formed due to pairing of spiral vortices emerging close to the vertical walls when the corner boundary layers impact on the rib. The effect of the Reynolds number and normalized channel size on the spiraling motion, generation, and shedding of large-scale vortex clouds has been estimated.

Study on flow separation and transition of the airfoil in low Reynolds number

As typical flow characteristics in a low Reynolds number, laminar separation bubbles (LSBs) and transition to turbulence over airfoils have been extensively studied in recent years. In order to analyze their flow mechanism, numerical investigation using the finite volume method to solve the Reynolds averaged Navier-Stokes equations with a transition Shear Stress Transport (SST) four-equation transition model is performed in this work, combined with the experimental study facilitated by the oil film interferometry technique. Specifically, the transition SST four-equation transition model is solved to simulate the separation location and LSB structure at low Reynolds numbers on a Wortmann FX63-137 airfoil. Good agreement is obtained between the numerical simulation and experimental measurements regarding the separation, transition and reattachment location, aerodynamic coefficients, and overall flow structures. At higher Reynolds numbers of 200 000 and 300 000, similar bubble structures on the airfoil surface are observed, and the location of the bubble moves toward the leading edge of the airfoil by increasing the angle of attack. However, in Reynolds numbers ranging from 300 000 to 500 000, significant changes of the laminar flow separation structures emerge. The flow structure changes from the classical laminar separation bubble to the nonclassical separation flow structure that is composed of a major vortex 1(V1) and a minor vortex 2(V2). Due to the small distance between V1 and V2, it is difficult to distinguish the delicate structure of the two separation bubbles from the classical laminar separation bubble by the experimental method.

Numerical investigation on flow control effects of dynamic hump for turbine cascade at different Reynolds number and hump oscillating frequency

From takeoff to cruise, the operating Reynolds number (Re) of the low-pressure turbine (LPT) in an aero-engine decreases significantly, making the boundary layer near the aft portion of the blade surface susceptible to separation. This paper discusses the feasibility of using local dynamic surfaces as an active flow control method to suppress the laminar flow separation on the suction surface in PAKB LPT cascade at low Re. A local dynamic surface, shaped as a half-sinusoidal hump with an oscillation amplitude of 1 mm, was positioned just upstream of the peak velocity point. Unsteady Reynolds-Averaged Navier–Stokes (RANS) simulations were performed to investigate the aerodynamic performance of the cascades with and without dynamic hump. At first, the effect of Re was studied with the hump oscillating frequency at 200 Hz. Compared with uncontrolled cascade at various Re from 25,000 to 150,000, the profile losses of the cascade controlled by dynamic hump reduced significantly at low Re condition where the separation bubble was very large, and increased slightly at high Re condition where the separation bubble was small. Then the effect of the hump oscillation frequency was investigated under the Re of 25,000. It was found that the controlled cascade loss reached a minimum in a certain hump oscillating frequency range. At low Re of 25,000, the loss of controlled cascade was mainly composed of low-loss continuous laminar vortices that were attached to the suction surface. Continuous vortices with a certain spacing confined the low-energy fluid near the suction surface by energy exchange between the free flow and near-wall flow, which suppressed the large-scale flow separation.

Attached cavitation in laminar separations within a transition to unsteadiness

Attached sheet cavitation is usually observed in turbulent water flows within small laminar separation bubbles which can provide favorable conditions for inception and attachment of cavities. In the present study, viscous silicone oils are used within a small scale Venturi geometry to investigate attached cavitation into laminar separated flows for Reynolds numbers from 346 to 2188. Numerical simulations about single phase flows are performed with steady simulations for a Reynolds number range Re ∈ [50; 1400] and with unsteady simulations for Re ∈ [1000; 2000]. They reveal the emergence of two large laminar boundary layer separations downstream of the Venturi throat in addition to low pressure zones which can possibly induce both degassing or cavitation features. Experiments are performed with high-speed photography, and several multiphase dynamics are observed in these viscous flows, which are considered as quasisteady flows at low Reynolds numbers Re ≤ 1400. Degassing phenomenon with air bubble recirculation has been first observed at pressures far above liquid vapor pressure whereas typical attached cavities have been identified for low pressure conditions as “band” and “tadpole” cavities into the different separations of the laminar flows. For higher Reynolds numbers, a flow regime transition can be noticed in the wake of well-developed gas structures, characterized by wake instabilities, causing vortex cavitation above a critical Reynolds number associated with the bubble width Recb≃616. This regime transition can possibly occur either quasicontinuously in the wake of an attached “band” vapor cavity or intermittently behind a recirculating air bubble generated with degassing. This last phenomenon is associated in our study to classical “patch” cavitation.

Aerodynamic shape optimization of airfoils at ultra-low Reynolds numbers

The flow over NACA 0008 airfoil is studied computationally in the ultra-low Reynolds number regime Re ∈ [1000, 10000] for various angles of attack α ∈ [0°, 8°]. The laminar flow separation occurs even at low angles of attack in this Reynolds number regime. The lift curve slope is far reduced from the inviscid thin airfoil theory value of Cl,α = 2π. Significant increase in the values of drag coefficient is seen with a decrease in Re. Lift-to-drag ratios are consequently very low. An adjoint-based aerodynamic shape optimization methodology is employed to obtain improved aerodynamic characteristics in the ultra-low Re regime. Three different objective functions are considered, namely, (i) minimization of drag coefficient, Cd, (ii) maximization of lift coefficient, Cl, and (iii) maximization of lift-to-drag ratio, (Cl/Cd). Significant improvement in each of the objective functions is obtained.

Compact Version of the Quasi-Gasdynamic System for Modeling a Viscous Compressible Gas

We consider a compact version (the CQGD system) of the quasi-gasdynamic system. All algorithms that have been used to approximate the spatial derivatives in the Navier–Stokes equations can also be applied to the CQGD system. At the same time, the use of the CQGD system permits significantly improving the stability of explicit schemes, which is important for ensuring high-performance parallel computations. As examples of the use of algorithms based on the CQGD system, we present the results of computations of a laminar boundary layer on a plate and of a hypersonic laminar separated flow in a compression angle.